Assessment of the environmental impacts of utilizing coal ashes and OPC for soil stabilization applications: Leachate analysis in response to rainfall interaction

The construction industry is a leading sector for coal ashes utilization, including in cement manufacturing and soil stabilization applications. In soil stabilization, coal ashes, alongside Ordinary Portland Cement (OPC), are the most commonly used materials. However, the environmental impact of coal ashes, particularly the leaching of heavy metals from stabilized soil when exposed to rainfall


Introduction
Although efforts to respond to climate change by increasing the proportion of power generation through renewable energy sources such as solar and wind are a strategy [51], power generation is still largely dependent on coal power plants [45].This comes from the fact that the electricity demand risen by urbanization and industrial sprawl is still largely dependent on the coal power plants [3].At the present, coal still the largest fuel resource and accounts for approximately 38 % of global energy production, which is related to the growing demand for energy consumption [28].However, it is important to note that fully utilizing the potential of renewable energy remains the ideal option for transitioning to clean energy sources.
In that context, burning coal in power plants produces residues called Coal Combustion Products (CCPs).CCPs are comprised of fly ash, bottom ash, boiler slag and flue-gas desulfurization gypsum.However, the substantial amounts of CCPs are coal ashes (fly ash and bottom ash) [27,3].On average, fly ash constitutes around 80-85 % of the total CCPs generated in a coal-fired power plant.It is the most abundant of the CCPs due to its fine particle size and the high quantity of unburned carbon it contains.Bottom ash accounts for about 10-20 % of the total CCPs produced in a power plant.As coarser and heavier particles, it settles at the bottom of the combustion chamber and is collected separately from fly ash.Approximately 600-800 million tons of coal fly ash is produced globally on an annual basis [69].A large portion of the coal ashes have been used for low value-added applications in the construction industry, such as in cement, soil application, brick, and road/dam, as well as for road base pavement, structural fill [42].Coal ashes have been used as a substitute for material in the construction industry, especially either as a raw material or as an additive in the cement industry [71].Nevertheless, according to [27] the actual amount of coal fly ash reused is still lesser than the amount produced with only up to 30 % reuse.Altough, it is important to not that the utilisation rate varies considerably between countries [3], as well as in the estmition in various studies.For example, according to the American Coal Ash Association, in 2017 around 38.2 million tons of fly ashes were generated in the United States, and 24.1 million tons of these were reused [45].This indicates that the current situation is still progressing toward full utilization of coal ashes.However, an environmental assessment should take place for individual applications to ensure that they have been utilized in a safe and sustainable manner.
In fact, given the variability in metal leaching data, a careful assessment of the targeted coal ashes before application is necessary; further, simultaneous leaching of metal/metalloid/toxic elements from coal ashes is a concern [69,71].In particular, certain elements can be released into the environment when coal ashes comes into contact with water.The borderless nature of the environment can facilitate the transfer of pollutants into groundwater and river systems, either in dissolved or particulate form, posing significant threats to aquatic organisms and potentially endangering human health [29].The leaching of trace metals from coal ashes can pose a substantial environmental risk (Komonweeraket et al., 2015), and environmental risk assessment of coal ashes disposal should not account merely for the leachability of constituents [65], but needs to assess the potnetil impacts to the environment of utlising coal ashes.
The construction industry is a leading sector for coal ash utilization, including cement manufacturing and soil stabilization.In soil stabilization, coal ashes play a pivotal role in enhancing the engineering and mechanical properties of soils, such as strength, bearing capacity, and durability.Indeed, soils with low shear strength and bearing capacity pose challenges not only for building construction and road infrastructure but also for resource extraction, disaster resilience, and regulatory compliance, both formal and informal [32,67].In particular, organic soil that is characterized by its problematic nature due to its high organic content, which leads to low strength and high compressibility, similar to other soils such as soft soil [32,57].Also, it is important to indicate that organic soil or peat deposits are distributed in many countries worldwide, occupying approximately 5 %-8 % of the Earth's land surface.A high distribution of peatlands can particularly be seen in the northern hemisphere, including Canada, the United States, Russia, Finland, Indonesia, China, Malaysia, and other countries [26].Thus, stabilization techniques have been adopted to improve soil engineering and mechincal propopeties.In the current research work, peat soil was stabilized by using fly ash, bottom ash, and Ordinary Portland Cement (OPC).Coal ashes are commonly used alongside OPC [25,31,41], offering a cost-effective solution as a by-product derived from coal power plants.And compared to the disposal of coal ashes, employing coal ashes in soil stabilization applications remains a better option from several perspectives [3].
However, it is imperative to use coal ashes safely and responsibly to mitigate negative environmental consequences.The extensive use of coal ashes in soil stabilization does raise concerns, particularly regarding the trace elements in fly ash, which can pose environmental risks by leaching into groundwater [75].Indeed, soil stabilization using coal ashes is currently widespread across various environmental conditions.Some projects are done near rivers, lakes, and wetlands, often lacking proper assessment or monitoring, including considering leaching potential, heavy metal content, and potential long-term effects on the surrounding environment.Yao et al. [71] and Zimar et al. [75] have emphasized that future research should aim for a understanding of the potential risks and benefits associated with coal ashes utlization.Nonetheless, Zimar et al. [75] have noted that there are a few studies focusing on potential environmental impacts of leaching of heavy metals related to coal ashes soil stabilization.Additionally, these few studies have primarily concentrated on pH behavior in the leachate and the pattern of elements under various pH values, as mentioned by [54], while there is a lack of studies simulating real-world conditions of soil stabilization and non-stabilization, particularly regarding interactions with rainfall.Therefore, this study aims to assess the environmental impacts of utilizing coal ashes and OPC in soil stabilization applications, with a focus on when the stabilized soil interacts with rainfall.
In fact, the environmental impact assessment of coal ash leachate should not only consider its disposal but also the subsequent interactions of its utilization with rainfall as supplemental cementitious materials or stabilization agents in soil stabilization applications.Thus, in the present study, coal ashes and OPC were utilized in peat soil stabilization, and assessments were conducted by simulating the environmental impacts associated with soil stabilization in the presence of rainfall under different conditions and A. Hauashdh et al. seasons.Through a robust methodology and cutting-edge analytical techniques that involve materials characterization, soil stabilization processes, leaching behavior, and environmental impacts of the stabilized soil due to rainfall, it leads to significant contributions to both environmental and engineering perspectives.

Methods
This section details the materials collection and preparation procedures.Subsequently, it outlines the experimental procedures, which are further divided into three subsections: materials characterization, mixing and stabilization, and environmental impact simulation.

Materials collection and preparation
In this study, peat soil was collected from Kampung Medan Sari, Pontian, Johor, Malaysia, and has been tested on-site by sampling according to the Von Post scale to determine which type of soil is considered.The test revealed hemic peat soil, which was subsequently used in the study.This approach ensures a representative sample of the entire peat deposit while accounting for potential spatial variations in peat properties.Then, several steps have been taken to ensure the homogeneity of the collected soil for subsequent mixing.Firstly, the site was cleared of small plants and marked as shown in Fig. 1.A depth of 50 cm was excavated to ensure the surface impurities, airborne dust, and residues from human activity were free, and the area was enclosed by using tape.Then peat was collected in a storage bin and covered with plastic bags to prevent moisture loss before finally being placed in a chamber at a temperature of 25 • C. The initiation of peat soil sample preparation involves separating fibers exceeding 2 cm in dimension from the collected peat before the mixing process.This step, as shown in Fig. 1, removes large fibers, clear roots, and other particles exceeding 2 cm.This ensures a more homogenous peat material when mixed with the stabilization materials, potentially leading to more consistent results in the stabilization process.Fly ash and bottom ash were collected from waste from coal-fired electric power generation at Tanjung Bin power plant, Johor, Malaysia, and OPC produced by Cement Industries of Malaysia (CIMA).Fly ash, bottom ash, and OPC were dried to ensure moisture-free storage and kept in container.In contrast to bottom ash, fly ash was a fine powder similar to OPC.Therefore, fly ash and OPC were directly included in the mixture without sieving.However, it is important to indicate that, in the present study, the particle size distribution has a passing percentage of 100 % for OPC from 10000 µm to 90 µm and for fly ash from 10000 µm to 71 µm.In the case of bottom ash, it partially passed at a particle size of 10,000 µm.This clear bottom ash has a larger particle size compared to fly ash and cement, which were originally fine powders, as seen in Fig. 2, which allow for fly ash and OPC to work as binders' materials and bottom ash as filler materials.Finally, peat soil, fly ash, bottom ash, and OPC are ready for mixing.The rainwater used in this study was collected from Parit Raja, Johor, Malaysia, by using an open plastic container in an open-space environment and was properly stored in a plastic bottle designed to keep water or rainwater naturally and in good condition in order to minimize changes in chemical composition.To ensure accurate representation of real-world conditions, rainwater collection methods aimed to preserve its natural chemical composition.This ensures that the rainwater used in the later experiments reflects the quality of rainwater that would actually fall on a soil stabilization site.Also, the rainwater was collected in a clean and quiet environment to minimize contamination and preserve its natural state, ensuring the collected rainwater remained unadulterated and reflecting its inherent properties.This approach reflects the natural interaction of rainwater with soil, simulates real environmental conditions more closely, and ensures the absence of external factors that would impact the leaching process later.Moreover, to ensure the initial quality of the collected rainwater accurately reflected its natural state, the concentration of some anions in the collected rainwater was detected using ion chromatography (IC).While the concentration of elements was determined using an atomic absorption spectroscopy instrument (AAS).Both instruments reported the concentration in parts per million (ppm), which is equivalent to milligrams per liter (mg/L).Further details about the collected rainwater's initial quality assessment can be found in Section 3.5.1.Following the collection and preparation of materials, as described previously, the experimental procedures can be outlined as follows: Fig. 1.Peat soil collection and preparation.

Experimental procedures 2.2.1. Materials characterization
Materials characterization is a critical step to understanding the properties, composition, and structure of the materials that will be used or utilized.Further, early characterization of materials and monitoring their properties are essential steps in identifying and controlling leaching, especially in the context of environmental impact assessment [54].In this study, related characteristics of peat soil-hemic, fly ash, bottom ash, and OPC-were identified as shown in Fig. 2. The characterization encompassed various parameters, including moisture content and specific gravity, which adhered to the standards specified in BS1377-2:2022, as well as loss on ignition

Mixing and Stabiliza�on
Environmental Impact Simula�on  and pH measurements following the guidelines of [10]+A1:2021.Fiber content assessment was conducted in accordance with ASTM [15], while vane shear testing was carried out as per [17].Moreover, X-ray fluorescence (XRF) was used to analyze the chemical composition of the raw materials and potentially help interpret and understand how the leachate's heavy metal and anion concentrations were impacted by these raw materials' compositions.Also, this study utilized the Field Emission Scanning Electron Microscopy (FESEM), JEOL JSM-7600 F, technique to analyze the microstructure and surface features of the soil-stabilized mixture under different conditions.The peat was analyzed for its microstructure in three stages: in its natural state, beginning stabilized, and after stabilized peat is exposed to rainwater.Finally, X-ray powder diffraction (XRD) was conducted to understand the changes made in the mineral phase of the stabilized peat over different curing times.It is worth noting that while some of these characteristics were examined for all materials used in this study, others were specifically conducted for the soil component, given its direct relevance.Furthermore, certain properties were continuously monitored after materials characterization, within mixing and stabilization processes and environmental impact simulation to observe and manage any potential changes including, Unconfined Compressive Strength (UCS), which was done according to [16] and FSEM and pH and moisture content.

Mixing and stabilization
Fly ash, bottom ash, and OPC were dried and then mixed with raw peat soil at its natural moisture content using a mixer machine for ten minutes, until stabilization materials were evenly distributed over the soil particles and thoroughly blended.The percentage of each component is as follows: Peat soil at its natural moisture makes up approximately 60.98 % of the total mixture mass, while dry fly ash constitutes about 13.41 %, dry bottom ash accounts for around 17.68 % of the mixture, and dry OPC represents approximately 7.93 % of the total mass.These percentages reflect the proportion of each component in the overall mixture by mass, based on the provided mass values for each component.The mixing ratios were designed to align with common percentages used in industry practices as well as by considering the percentage of fly ash, bottom ash, and OPC in previous studies, such as [41,58,72].Fly ash and OPC work as binders' materials and bottom ash as filler materials.The mixed samples were formed in a UCS mold (38 mm 76 mm), as shown in Fig. 2 producing samples of stabilized peat were placed in curing with temperature 27 • C. Lastly, samples of stabilized peat with 28 curing days begin to be simulated on the physical model, where normally peat soil stabilization takes a maximum of 28 days to complete [57,68].

Environmental impact simulation
The simulation aimed to recreate real-world conditions using a soil column, a cylinder with a 295 mm height and 90 mm diameters with 7 mm of thickness glass to endure internal pressure and featured four outlets for extracting soil leachate as shown in Fig. 2 in order to simulate the impacts of stabilized peat leachate on the environment under conditions similar to those at the actual soil stabilization site.The simulation using a physical column, which involved assessing the leaching by column or flow-through tests, provides an accurate assessment and valuable data on the release pattern of elements under uncontrolled pH among the available leaching methods as a medium for simulation close to real environmental conditions [29].A column leaching test is more representative of natural field conditions than other tests, such as batch test [7], because it more closely simulates the flow through the material.To simulate conditions of soil stabilization similar to those at the actual soil stabilization, peat soil was added to the column and compacted layer by layer.A 30 mm gap was left at the top of the soil column to allow for rainwater intake to work as surface which means rainwater was placed at the top of the soil column.In the leachate simulation: natural peat, leachate simulation was conducted within a column filled with natural peat soil.For the leachate simulation: natural peat + stabilized peat, it involved removing 90 mm of the natural soil in the column and replacing it with stabilized peat soil, and the excess that was removed was used to compact the empty spaces surrounding the stabilized peat soil, as shown in Fig. 2. In other words, stabilized peat, cured for 28 days, was positioned in the middle at the top of the column, while the natural peat was placed in the rest of the column, as shown in Fig. 2. The leachate was collected in four chemical flasks from four outlets strategically placed at different depths, ensuring that the overall interaction of the leachate represented the actual on-site conditions with the soil column.The leachate assessments were done in two seasons (dry and wet) with different conditions, and the column leaching tests and conditions are: ▪ Natural peat (Flush 1, distilled water), ▪ Natural peat (dry, rainwater), ▪ Natural peat (wet, rainwater), ▪ Natural peat (Flush 2, distilled water), ▪ Natural peat + stabilized peat (dry, rainwater), ▪ Natural peat + stabilized peat (wet, rainwater) * While the column leaching tests and conditions were conducted in the order listed above, it is important to note that the reporting of the findings was presented by comparing the simulations of similar conditions and seasons between natural peat soil conditions and natural peat + stabilized peat conditions to achieve the aim of the study.
In the wet season, each condition was simulated with 1500 mL of rainwater over a period of 20 days for the natural peat soil (wet) condition and 24 days for the natural peat + stabilized (wet) condition.In the dry season, 50 mL was used to simulate each condition over a period of 4 days for the natural peat + stabilized (dry) condition and 6 days for the natural peat + stabilized (dry) condition.As mentioned earlier, the leachate was collected in four chemical flasks from four outlets strategically placed at different depths, resulting in the collection of four leachate samples for each condition.To establish a consistent starting point for the experiments, the natural peat soil was flushed with 1500 mL of distilled water for two weeks (flush 1) before initiating the simulation.The same flushing procedure (flush 2) was repeated before simulating the natural peat soil combined with stabilized peat.This maintains data reliability by reducing interference and is an essential step in column leachate tests to ensure consistent and accurate results.All leaching tests were conducted under tropical weather conditions, where the ambient temperature remained stable throughout the experimental period, ranging between 27 • C and 28.2 • C.During each condition test, pH averages were recorded for leachate in each condition by measuring pH three times in each outlet leachate sample.The volume for each outlet and the moisture content of the raw soil in the column (without teaching or moving the stabilized model sample at the top of the column during measuring the moisture content) were recorded after finishing the leaching of each condition.The leachate-out samples were collected, underwent a filtering process using filter paper, were stored in a cold environment, and then prepared for leachate analytical processes, as shown in Fig. 2. Ion chromatography (IC) was employed to analyze the anion concentrations in the leachate using an ICS-2000 model from Dionex in the USA.While analyzing leachate heavy metal concentrations, the Inductively Coupled Plasma Mass Spectrometry (ICP-MS) model, specifically the PerkinElmer ELAN 9000 from the USA, was employed.The ICP-MS instrument had a limit for most elements below 10 ppb.This analysis was achieved by running the reference standard, involving the preparation of a solution based on the multi-element calibration standard references 3 for the ICP-MS instrument and the Dionex seven-anion standard II for the IC instrument.The software then plotted the calibration curve, and the concentration of samples was subsequently generated using this calibration curve as a reference.The IC instrument measured the concentration of anions in parts per million (ppm), equivalent to milligrams per liter (mg/L), and the ICP-MS instrument measured the concentration of heavy metals in parts per billion (ppb), equivalent to micrograms per liter (µg/L).In this study, the concentrations have been reported for anions in mg/L and for heavy metals in µg/L.

Physicochemical properties
Table 1 presents the physicochemical properties of mixture materials and provides an overview of various properties for four materials: peat soil, fly ash, bottom ash, and OPC.Peat soil exhibited a remarkably high organic carbon content of 89.97 %, as estimated by the loss-on-ignition (LOI) test.This value signifies a substantial abundance of organic matter within the soil.It can be noted that fly ash, bottom ash, and OPC have 0.48, 0.46, and 0.33 % of LOI, respectively.According to Rahman et al. [56], peat soil has a cumulative organic content (carbon content) that initially develops as a result of microbial activity.They found that after peat soil begins to be stabilized, the number of microbial colonies in stabilized peat decreases with an increase in stabilized peat strength.Furthermore, another study result showed that the application of coal fly ash to tropical peats can control the carbon mineralization process in peat, ultimately reducing the production of carbon dioxide and methane from the peats (Saidy et al., 2022).Furthermore, organic matter in soil inhibits cementation by absorbing calcium ions from cement hydrolysis, thereby limiting calcium hydroxide production [68].According to Khanday et al. [32], Ca 2+ ions released from the binder get consumed by organic matter in peat soil.The effectiveness of soil stabilization requires reducing carbon content while increasing calcium content to counteract this effect [68].In the context of moisture content, peat soil stands out with a high value of 678 %.This high moisture content in peat soil is attributed to its rich organic content, which increases the space between its particles, facilitating water absorption, and found to be acidic with a pH of 3.8.It is important to note that fly ash, bottom ash, and OPC were free from moisture.When considering specific gravity, OPC shows the highest value at 2.41, indicating greater density than the other materials.In contrast, peat soil records a specific gravity of 1.63, while fly ash and bottom ash have values of 2.67 and 1.61, respectively.Additionally, peat soil displays a notable 32.5 % fiber content, which is a significant feature in its composition.

Chemical compositions analysis
Peat soil exhibits a remarkably high organic carbon content, reaching 89.97 %, as estimated by the loss on ignition (LOI) test as reported in Table 1.This value significantly highlights the predominant presence of carbon in this material.While LOI offers instant estimation, it's important to remember that it also captures other volatile components.The high organic carbon content, primarily derived from cellulose and decomposed plant matter, suggests significant potential for carbon storage in this peatland.The remaining peat soil compositions analyzed using XRF are presented in Table 2. XRF analysis does not directly measure carbon content; thus, the measurements only include the percentage analysis of inorganic chemical compositions using XRF.compounds.In the case of OPC, the highest composition of CaO was 54.10 %.CaO is a critical component of OPC, as it is derived from the raw limestone used in the production process, making it a fundamental chemical component of cement.On the other hand, both bottom ash and fly ash display notable compositions of SiO 2 , with values of 44.40 % and 48.80 %, respectively.This high SiO 2 (silica) content is significant since it influences the reactivity and properties of these materials in various applications.Indeed, the high SiO 2 content in fly ash and bottom ash, making their binding and reactivity and influence on material properties play a significant role.Also, the composition of Fe 2 O 3 varies, with OPC at 5.32 %, fly ash at 6.21 %, and bottom ash at 8.51 %.While SO 3 compositions are noted, with OPC containing 2.72 %, fly ash containing 1.01 %, and bottom ash containing 0.25 %.These amounts of SO 3 play a crucial role in the setting properties of OPC and can influence its performance as a binder material [5,70].According to Renjith et al. [60], SO 3 content contributes to the improvement of strength when it mixes with other materials.Nonetheless, previous studies have limited the standard SO 3 content in cement to 4 %, as high SO 3 content can lead to delayed ettringite formation [48].Nevertheless, OPC and lime are manufactured materials, their chemical compositions can be controlled [72].While coal ashes chemical composition distribution and percentage depend on the nature of the coals used and the combustion conditions [1,59,72].Thus, the percentage of composition varies depending on the type of source that produces fly ash and bottom ash and the chemical content of the coal used by power plants may change over time.
Table 2 XRF analysis provides detailed insights into the chemical composition of peat soil, fly ash, bottom ash, and OPC.By quantifying the composition of these major chemical compounds present in these stabilization materials, including SiO 2 , Al 2 O 3, CaO, Fe 2 O 3 , and SO 3 .The levels of these compounds when they interact with water can influence leachate behavior, including the mobility of heavy metals and their concentrations, as well as their potential environmental implications.Indeed, heavy metals present in these stabilization materials, including the fly ash and bottom ash matrices, are released into the liquid phase during the leaching process [1].Indeed, there is a recognized relationship between the chemical composition of elements in coal ashes and their leaching behavior [69].Understanding the chemical composition of coal ashes is crucial for predicting the environmental mobility and bioavailability release of heavy and toxic elements within them [61].Consequently, a thorough understanding of the chemical composition of coal ashes is essential to understanding the leachate behavior.Likewise, understanding the chemical composition of soil and OPC is equally critical for similar reasons.Therefore, the XRF analysis conducted on material samples has revealed the chemical composition of the peat soil, fly ash, bottom ash, and OPC, providing a basis that would help in the interpretation of the following sections, which covered soil stabilization effectiveness, leachate analysis, and potential risks associated with utilizing coal ashes and OPC in soil stabilization applications, particularly in response to rainfall interactions.Further, the chemical compositions identified through XRF analysis contribute to understanding the stabilization mechanisms.For instance, the interaction of CaO with water in OPC leads to the formation of calcium hydroxide [60], which plays a crucial role in soil stabilization.Similarly, the presence of SO 3 and Fe 2 O 3 in various materials affects their performance as binding agents, influencing setting properties and strength development [5,70].

Compressive strength test
Fig. 3 shows the UCS for stabilized soil after 28 days and 14 days of curing.The results revealed that the UCS was 47 kPa and 32 kPa, respectively.The findings shows significant enhancement in strength over 14-and 28-days curing, with the UCS progressing from an initial measurement of 32 kPa to a markedly elevated 47 kPa.This observed strengthening of the peat emphasizes the pivotal role played by the mixture that utilized in the stabilization process.Initially, peat soil in its natural sates was characterized by a relatively modest natural strength of about 8.4 kPa, which was reported by Rahman et al. [57] for samples collected at Sepang Selangor, Malaysia, and by Rahman et al. [56] for peat collected from Pontian Johor, Malaysia, which was recorded at 4, 8, and 12 kPa for fibric, hemic, and sapric peat, respectively.In the present study, peat soil undergoes a significant transformation following the introduction of stabilizing agents and a curing period spanning 14-28 days, resulting in a significant UCS within the range of 32-47 kPa.However, even that stabilized peat still shows lower strength as compared to other types of stabilized soils, as a result of the high water holding capacity of peat [32].
The strength of stabilized soil is increasing after mixing it with fly ash and OPC, leading to improved soil strength properties.Coal ashes with OPC have the applicability to improve soil properties due to the fact that calcium in coal ashes has considerable cementitious properties in addition to pozzolanic properties, whereas they also have pozzolanic properties.SiO 2 in coal ashes reacts with calcium hydroxide released by the hydration of calcium oxide to produce calcium silicate hydrate [71].Therefore, due to their cementitious and pozzolanic properties, they lead to improved soil strength properties [3,75].Pozzolanic reactions involve the reaction of calcium hydroxide (lime) with silicate and aluminate minerals, facilitated by water.Consequently, this reaction results in the formation of calcium silicate hydrate (CSH) and calcium aluminate hydrate (CAH), making these hydrated compounds contribute to the development of strength in the stabilized soil using a calcium-based additive [60].Furthermore, cation exchange is also another mechanism of soil stabilization that works by replacing the cations that are naturally present in the soil with other cations, which can improve the soil's strength and stability [75].This process involves the reaction of soil with a cementitious additive, leading to the replacement of soil cations like Al 3+ and Na + by Ca 2+ from the additive.The replacement of cations contributes to an increase in the workability and strength of the soil-additive mixture.Nevertheless, the rate of this exchange depends on factors such as soil type, concentration of the solution, and temperature [62].As minerals form bonds between soil particles, this chemical process emerges, resulting in the stabilized soil gaining more strength.Firoozi et al. [23] pointed out that the mechanisms of soil stabilization include cation exchange, pozzolanic reaction, and carbonate cementation, which include oxide reacting with carbon dioxide from the atmosphere to form calcium carbonate precipitates, leading to cementing the soil particles.Overall, these reactions contribute to physical, chemical, mineralogical, and microstructural changes in the stabilized soils [8].
In a study done by Kolay et al. [35], stabilized peat with the addition of 15 % fly ash was recorded to have a strength equal to approximately 55 Kpa by the UCS test within 28 days of curing.While in the study by Rahman et al., [57] the stabilized peat samples cured at 28 days with 20 % OPC was recorded at 50 kPa by UCS test.Nevertheless, there is a notable variation in studies that focused on stabilized peat soil in terms of the strength that has been recorded for stabilized peat due to many factors, such as the types and doses of materials used to stabilize it and the geographic location of the peat.Thus, there is still a need for soil stabilization standards that  include determining the optimal strength as well as shear strength that should be achieved within soil stabilization work purposes based on the consideration of environmental assessments to ensure these practices are done in a sustainable and safe manner.Up to date, the standards are limited to the procedures for the design of stabilization, such as (ASTM: [18]).The challenge comes from the fact that the requirements for stabilization of specific materials may vary due to local conditions, the intended use of the stabilized material, or both, making establishing the standard in a way similar to concrete code difficult.Thus, it is crucial for state and local regulatory bodies to establish or refine standardized protocols and codes governing the use of diverse additives in soil stabilization to encourage industry to work to fulfill sustainable practice ( [4]; ASTM: D7762-11, 2021).

Vane shear strength
The vane shear strength works effectively to measure the shear strength of saturated soil with high moisture content and with low shear strength compared to unconfined test [2]; thus, in this study, the vane test shows the results of the shear strength of peat soil in its natural state and after being exposed to rainwater beside the stabilized peat soil in certain conditions.Initially, natural peat showed a shear strength ranging from 4-8 kPa.Following a curing period of 14 days and 28 days, stabilized peat demonstrated shear strengths of 32.5 kPa and 47 kPa, respectively, indicating a significant increase in strength compared to its natural state.In terms of the influence of rainwater on stabilized peat soil, exposure to rainwater in a soil column for 24 days resulted in a reduction of shear strength by approximately 17 kPa.The findings the highlight substantial impact of rainwater on the mechanical properties of stabilized peat soil, suggesting structural or compositional alterations due to exposure to wet conditions during the rainy season.To conclude, Fig. 4 presents four conditions: Natural/before stabilization (8 kPa): This represents the shear strength of untreated peat soil, reflecting its initial strength without any modifications.Stabilized peat (14 days of curing, 32.5 kPa): After 14 days of curing, the shear strength increased to 32.5 kPa, indicating improved cohesion and mechanical properties increase with the curing process, as pozzolanic activity is time-dependent [75].Stabilized Peat (28 days of curing, 47 kPa): Continuing the curing for 28 days further enhanced the shear strength of the stabilized peat to 47 kPa.Stabilized peat exposed to rainwater (24 days in the soil column, 30 kPa): The exposure to rainwater for 24 days resulted in a shear strength of 30 kPa, along with a subsequent loss of approximately 17 kPa.A condition of peat exposed to rainwater, distinct from the previously stabilized peat conditions, showed a shear strength reduction of approximately 17 kPa after exposure to rainwater.This reduction occurred because rainwater had an impact on the stabilized soil particles, including fly ash, bottom ash, and OPC, due to leaching, making the soil less resistant to shear.These findings highlights the impact of engineering and environmental factors, such as stabilization, curing, and rainwater exposure, on both the shear strength of peat samples and the stability of stabilized peat.Furthermore, this indicates the importance of analyzing the leachate of stabilized soil when it interacts with rainwater.

FESEM and XRD analysis 3.4.1. FESEM microstructural analysis
Fig. 5 presents FESEM images captured at a magnification of 10,000x, illustrating distinct stages in the transformation of peat soil.The microscopy examination was conducted on peat soil samples before stabilization in their natural state, after stabilization through a 28-day curing process, and subsequent exposure to rainwater during the wet season in a soil column for 24 days.Fig. 5-A clearly shows the particle and fiber arrangement, as well as the pores between particles in the non-stabilized peat sample in its natural state.Specifically, Fig. 5-A highlights the presence of voids between particles, emphasizing the inherent spacing within the peat matrix.In its natural state sample, peat soil interspersed with irregular particles and composed of fibers which are woody and porous in nature [35].Conversely, Fig. 5-B portrays the FESEM image of stabilized peat soil following a 28-day curing period.Here, a noticeable transformation occurs as the pores between particles become markedly smaller.Additionally, it has been noted that the particles exhibit close packing and strong bonding, corresponding to the enhancements in UCS value and shear strength.This enhancement has been linked to the expansion of hydrates within the gaps among the particles [36].As shown in Table 2, OPC, fly ash, and bottom ash all contain CaO, SiO₂, and Al₂O₃, with OPC distinguished by its high CaO content and both fly ash and bottom ash rich in SiO₂, followed by Al₂O₃.According to Zimar et al. [75], the CaO/SiO 2 or CaO/(SiO 2 + Al 2 O 3 ) ratio plays a critical role in pozzolanic reactions, governing the formation of CSH and CAH, which are primarily responsible for strength development.Jha and Sivapullaiah [30] mentioned that the changes in the composition of particle surfaces occur due to the coating of new minerals formed in the stabilized soil.The FESEM micrographs provide revealing visual representations of the ultrastructural differences between stabilized peat and its original state.The stabilized peat exhibits a more coherent and compact structure, while the original peat appears unstructured and sporadic.Furthermore, the clearly defined pores observed in Fig. 5-A, characteristic of natural peat, result from its fiber content, which creates gaps between particles.In Fig. 5-B, the effect of stabilization after 28 days of curing is evident, with the pores between soil particles significantly reduced due to the cementitious properties imparted by the addition of fly ash, bottom ash, and OPC.This contrasts with Fig. 5-A, which clearly shows the fragmented structure of the natural peat soil, along with distinctly visible voids between particles.Thus, pore size reduction in Fig. 5-B can be observed between the peat soil particles due to the stabilization materials' ability to bond soil particles.In fact, stabilization materials, which appear as small particles in Fig. 5-B, penetrate the gaps between soil particles or settle on their surfaces, effectively holding the soil particles together, as seen in Fig. 5-B.This appearance is not observed in natural peat soil, as shown in Fig. 5-A.This phenomenon is attributed to the pozzolanic reaction resulting from the incorporation of fly ash, bottom ash, and OPC, all of which possess cementitious properties, leading to chemical reactions and a reduction in water content [31,75].Further, coal ashes and OPC particles can also exchange their higher valence cations with the lower valence cations in the soil, producing an increase in the apparent cohesion between soil particles [31].To end, Fig. 5-C shows the FESEM image of stabilized peat soil after being placed in a soil column and exposed to rainwater for 24 days during the wet season.It reveals that the pores have been eliminated and the fibers have appeared clearly due to the leaching of stabilized peat particles caused by the flowing water.Fig. 5-C indicates that it is possible for the bonded particles to leach out of the soil over time when stabilized peat interacts with rainfall in the wet season.The observation indicates that the bonded particles in the stabilized peat soil have the potential to dissolve or degrade over time when exposed to flowing water, leading to changes in the soil structure.Therefore, this behavior could have negative consequences for the surrounding environment's components, making the environmental analysis and assessment of the stabilized soil leachate significant.This transformation is evident, emphasizing the effects of rainwater on the peat soil's microstructure.Therefore, FESEM analysis of peat soil under different conditions has revealed the microstructural features, surface properties, and binding Fig. 6.XRD analysis of the stabilized peat of 7 days of curing.
capacity of soil due to pozzolanic reactions, the formation of new minerals, cation exchange, soil strength improvements, and potential interactions with soil components under varying environmental conditions.These FESEM images provide valuable insights into the change's microstructure of peat soil, highlighting the impacts of chemical mechanisms and environmental factors on its ultrastructure.In particular, the influence of rainfall interaction on the microstructure of the stabilized soil indicates that bonded particles have the potential to dissolve or degrade over time, as observed in Fig. 5-C, which highlights the potential risks associated with utilizing coal ashes and OPC in soil stabilization applications.

XRD analysis
The chemical mechanisms of soil stabilization process include the formation of new minerals.Accordingly, XRD analysis examined the changes of mineral phases in stabilized peat during the curing process.Fig. 6 shows XRD results for the stabilized peat after 7 days of curing.As can be seen in Fig. 6, silicate minerals including pargasite, potassic-chloropargasite, richterite, and kaersutite were found to be the minerals in stabilized peat within 7 days of curing, making SiO 2 have an influence on these silicate minerals.Fly ash and bottom ash show notable compositions of SiO 2 , with values of 44.40 % and 48.80 %, respectively, in XRF results (Table 2), as can be seen in Table 2.The presence of SiO 2 in fly ash and bottom ash may indeed contribute to the formation or stabilization of silicate minerals, including pargasite, during the soil stabilization process.While this study utilizes raw fly ash and bottom ash, it has been reported in the literature that, after undergoing ferrous and non-ferrous metal recycling through magnetic and eddy current separation, received bottom ash can exhibit a dominance of silicate minerals, constituting approximately 15 % of the crystalline components, while received fly ash constituted 5 % of silicate minerals [64].
Pargasite has been observed as the most significant mineral in the stabilized peat soil sample, as indicated by its highest peak in the analysis.Considering that pargasite is a silicate mineral containing silicon, there is a potential influence of SiO 2 in fly ash and bottom ash that could contribute to the dominance of pargasite, as can be seen in Figs. 6 and 7.Moreover, within the 28-day curing period of the stabilized peat pattern, the same minerals that appeared for the stabilized peat after 7 days of curing appeared, including potassicpargasite as new as well as the dominant pargasite, which had the two highest peaks in the sample of stabilized peat soil within 28 days of curing.The formation of new minerals in stabilized organic or peat soil within the stabilization process was also observed by Pokharel and Siddiqua [55] and Jha and Sivapullaiah [30].Indeed, the analyses revealed that significant peaks of new minerals have been formed; this again confirms what has been reported in previous studies that stabilization soil led to the formation of new minerals, leading to some changes between patterns of minerals within 7 days of curing and 28 days as the minerals changed during the stabilization soil process.Jha and Sivapullaiah [30], reported that the changes in the composition of particle surfaces due to the formation of new minerals due to the soil stabilization process reactions.Therefore, it can be concluded that considerable changes were made to the mineral content of the stabilized peat over the curing time, and the high SiO 2 content in fly ash and bottom ash is significant as it influences the mineral phases of the stabilized peat during the curing process.These minerals may contribute to the overall volume and density of the stabilized soil matrix.

Rainwater initial quality assessment
In fact, Malaysia's year-round tropical climate, with steady temperatures and abundant rainfall, leads to relatively consistent environmental conditions, including minimal variation in rainwater characteristics across the country, as supported by previous studies.Ariffin et al. [6] investigated rainwater quality at 24-gauge stations across Malaysia.Their analysis classified most of the rainwater samples as good quality.However, areas with high levels of manufacturing activity may experience variations in rainwater quality due to industrial emissions and pollutants.This study also found some ions and elements were observed in the collected rainwater, such as nitrate (NO₃⁻) with a concentration of 0.44 mg/L and phosphate (PO₄ 3 ⁻) with a concentration of 0.15 mg/L.Additionally, the concentrations of sodium (Na) with a concentration of 0.14 mg/L, magnesium (Mg) with a concentration of 0.07 mg/L, potassium (K) with a concentration of 0.21 mg/L, and calcium (Ca) with a concentration of 3.00 mg/L.These initial quality assessments for these ions and elements were consistent with the range that has been reported by the study of Fazillah Abdullah et al. [22], who focused on the chemical composition of rainwater in Malaysia.In terms of rainwater pH, the mean pH of rainwater in Malaysia has been recorded at 5.146 by Ariffin et al. [6] and 5.27 by Fazillah Abdullah et al. [22].In the end, this study simulated the environmental impact by comparing leachate composition under natural peat soil conditions and combining natural and stabilized peat conditions for similar environmental conditions(e.g., temperature, rainwater amount) and experienced both wet and dry rain seasons.This approach facilitated the identification of impacts on leachate composition that could either be due to stabilization materials or rainwater.

Leachate attributes analysis
Table 3 presents the leachate attribute analysis, including leachate flowrate and leachate mass flux.Flow rate pertains to the discharge from the outlets of the soil column, while mass flux refers to the mass of soil particles that is carried along with leachate exiting the soil column.Table 3 illustrates the inflow and outflow of the soil column in (mL).In the dry season, each condition inflow is 50 mL, and in the wet season, it is 1500 mL of rainwater.In the dry season, the results show that the mass flux of flow out of the natural peat soil condition is 0.5 g over 4 days with a flowrate of 18.75 mL/day, while the mass flux of flow out of the natural peat soil + stabilized condition is 0.6 g over 6 days with a flowrate of 10 mL/day.The natural peat condition has a higher porosity than the natural peat soil + stabilized condition, which is why it took less time for the water to flow out in the natural peat condition, while the soil + stabilized condition reflects the impact of the stabilized soil mold sample at the top of the column to allow for the leachate to pass through it easily.It is also important to note that the soil has a high moisture content, which means the soil itself makes the outflow volume larger than the inflow.In the wet season, the total volume of natural peat soil is 1249 mL for 20 days, with a flow rate of 62.45 mL/day and a mass flux of 6.88 g for 20 days.In natural peat and stabilized peat, the volume is 1200 mL for 24 days, with a flow rate of 50 mL/day and a mass flux of 6.6 g for 24 days.During natural peat + stabilized (wet), it was 50 mL/day smaller than natural peat soil (wet) because the stabilized peat mold sample that was placed on top of the soil column was affected to decrease outflow as the void between particles on the stabilized peat sample is smaller than the void between particles on non-solidified peat.Particularly in the wet condition, the natural peat soil demonstrated a significantly higher flow rate and mass flux, indicating rapid leachate transport in saturated soils.4 presents the results concerning leachate pH and column soil moisture content.The average pH of the leachate reveals the level of pH of the leachate coming from all four column outlets.In the dry season, in natural peat soil condition, the leachate is slightly acidic with a pH of 5.3.While in a natural peat-stabilized condition, it is more acidic with a pH of 3.5.In the wet season, natural peat soil condition still maintain a slightly acidic leachate with a pH of 4.45, and natural peat + stabilized conditions exhibit a somewhat more acidic leachate with a pH of 4.8.This observed change in pH would be due to the soil cation exchange capacity; in fact, peat soil has a considerable cation exchange capacity [50].And according to Khanday et al. [32], in stabilized peat soil, the cation exchange involves the monovalent cations present in the soil being replaced by divalent cations present in stabilization agents.In cation exchange, when divalent cations are introduced and replace monovalent cations, it can affect the balance of ions in the soil leachate.The replacement of monovalent cations with divalent cations may contribute to an increase in pH.Hence, in the dry season, acidic cations like aluminum could be tightly bound to the soil particles, contributing to a lower pH leachate.Consequently, during the wet season, when flow increases, it can displace these acidic cations and replace them with divalent cations like calcium from the stabilization agents, resulting in a higher pH leachate.Paul and Hussain [52] observed that the pH in stabilized peat increased with a rise in cement content.According to that study, the hydration of CaO present in cement releases calcium ions (Ca 2+ ) into the pore fluid, consequently raising the pH value.In terms of moisture content, the overall moisture content of the raw soil in the column in dry natural peat soil condition is 455.5 %.While in the dry season, in natural peat and stabilized condition, the moisture content is increasing in the overall soil column by 464.28 %, which reflects the impact of the stabilized soil sample at the top of the column to allow for the rainwater to distribute more evenly throughout the surrounding area.Under wet condition, natural peat soil maintains a moisture content of 406.8 %.Meanwhile, natural peat + stabilized exhibits a moisture content of 445.45 %, showing the impact of the stabilized soil sample at the top of the column to allow for the rainwater to distribute more evenly throughout the surrounding area in both dry and wet seasons.

Analysis of leachate heavy metals
As pointed out earlier, the chemical mechanisms of soil stabilization commonly involve cation exchange, pozzolanic reactions, hydration reactions, and the formation of new minerals.All these interactions and reactions would influence the leachate of the stabilized soil.This comes from the fact that the complex effects of coal ashes on soil physicochemical properties, microbial activity, and soil agriculture characteristics make there a critical need for assessments [14].Table 5 presents the analysis of heavy metal concentrations in leachate from natural peat soil and natural peat + stabilized conditions during the dry and wet seasons as detected by ICP-MS.These metals include copper (Cu), aluminum (Al), manganese (Mn), iron (Fe), and zinc (Zn).In the dry season, the average Cu concentration was 24.27 µg/L under natural peat soil condition, while it increased to 32.23 µg/L under natural peat + stabilized condition.In the wet season, Cu concentrations ranged from 30.7 to 60.6 µg/L, with an average of 43.2 µg/L for natural peat soil condition.While under natural peat + stabilized conditions, Cu concentrations ranged from 43.8 to 65.9 µg/L, resulting in a slightly higher average of 52.5 µg/L.Cu in rainwater in Malaysia was reported to be, on average, equal to 30 µg/L [22].Nonetheless, Cu concentrations were higher in the leachate that included stabilized peat soil in both dry and wet seasons, and both the natural peat soil condition and the natural peat + stabilized condition showed increased copper concentrations compared to the dry season.According to Wang et al. [69] report, citing Rivera et al. [61], Cu in coal fly ash exists in several chemical species, mainly in aluminosilicate glass, tenorite, cuprite, and chalcopyrite.Of particular concern, Cu within aluminosilicate glass has the potential to leach out under circumneutral pH conditions or dissolve in alkaline environments [61].In addition, according to Mahedi and Cetin [46], an increase in cement content initially led to higher leached concentrations of Cr and Cu.It can be concluded that the increase in Cu concentration in the soil leachate is a complex phenomenon driven by the interaction between the organic content of the peat soil and the chemical components introduced during the stabilization process.Stabilizers such as OPC, along with CaO, play a role in promoting the release of Cu into the leachate.According to Izquierdo and Querol [29] and Tian et al. [63], Ca influences the makeup of leachate and significantly affects the potential for heavy metals to leach into the environment.Further seasonal variations impact Cu concentrations, with wet seasons generally resulting in higher levels of leaching.However, Cu concentrations are still lower than the limit of the EPA (U.S. Environmental Protection Agency) of the secondary maximum contaminant level in drinking water, which is equal to 1300 µg/L [20,46].
Given that the Cu concentrations are below this regulatory limit, it indicates that they are not expected to present direct health risks to humans.However, Cu can be toxic to some aquatic life at high enough levels, making understanding Cu concentrations and its bioavailability in natural environments is crucial [21].In fact, it is still important to consider potential ecological impacts on aquatic organisms and ecosystems, particularly when Cu concentrations approach levels known to cause toxicity to sensitive species.Nonetheless, according to the EPA [21], The toxicity of a chemical to an organism requires the transfer of the chemical from the external environment to biochemical receptors on or in the organism, and this transfer is not necessarily proportional to the total chemical concentration in the environment.Thus, 'bioavailability' is not simply related to the total amount of the chemical present but varies according to attributes of the organism, chemical, and exposure environment, resulting in the chemical being more or less 'bioavailable' [21]."For example, the free metal cation concentration in soil solution, which is heavily influenced by surface chemical and physical properties of the soil [40].Thus, the amount of "free" metal floating around in the soil leachate depends on the physical and chemical properties of the soil.In similarity, metal toxicity depends not only on the activity of free metals in the soil solution, but also on competition with other cations [66].This, in turn, controls how much of the metal is available for plants and organism to take in, which is called bioavailability.Therefore, it has been reported that the response of organisms was not consistently correlated with the concentration of heavy metals such Cu and Zn during the infiltration testing by Tsiridis et al. [65].According to Kim et al. [34], the bioavailability of heavy metals in soils involves three sequential stages: the environmental availability of heavy metals in the soil, their uptake by organisms (environmental bioavailability), and the subsequent accumulation and toxic effects within the organism (toxicological bioavailability).However, it is important to note that while there is research on the concentration and availability of heavy metals in soil itself based on mass, the existing literature offers limited information on the assessment of their bioavailability and toxicity to organisms in the surrounding environment, making further investigation into this area crucial.Top of Form In contrast, the situation with Al is more complicated.In the dry season, remarkably high concentrations of Al, reaching 376.2 µg/ L, were observed under natural peat soil condition.On the other hand, the average Al concentration in the natural peat and stabilized condition was lower, at 231.75 µg/L, indicating a potential reduction in Al leaching due to the stabilization process.During the wet season, under the natural peat soil condition, Al concentrations ranged from 99.1 to 474 µg/L, with an average of 235.4 µg/L, while in the natural peat + stabilized condition, Al concentrations ranged from 178 to 232 µg/L, resulting in a lower average of 195 µg/L.The concentration of Al in rainwater is typically very low and was not detected by Ariffin et al. [6] and commonly, the soil is naturally high in Al.This observed reduction in Al leaching in the natural peat + stabilized condition, even during the wet season, likely results from a combination of chemical interactions or unique behavior exhibited by Al in peat soils.These complex interactions collectively contribute to the lower Al concentrations observed in the leachate under natural peat + stabilized condition in both the dry and wet seasons compared to its concentration under natural peat soil condition in both seasons.Although the EPA does not establish a specific maximum contaminant level for aluminum in drinking water [12], only the secondary maximum contaminant level, which is equal to 50-200 µg/L for AI.However, its ecological impact can be significant, especially in certain environmental conditions, due to its ability to influence soil and ecosystem processes.As the Al in the soil is self-reduced by mixing peat soil with stabilizers into agents, the strength of soils significantly increased, making Al immobilize in stabilized soil, which contributed to reducing the Al concentration of the leachate.The reaction of Ca 2+ with Al in the context of stabilizing soil can lead to the formation of cementitious compounds such as CSH, CAH, and calcium alumino-silicate hydrates (CASH) [9].These compounds can indeed play a role in binding and immobilizing certain elements or substances, including Al, present in the soil.Then, this reaction results in a reduction of their leaching potential, contributing to improved stability and environmental conditions in the stabilized soil.Therefore, in this study, the involvement of Al in binder hydration process outcomes, including calcium aluminate gels, is therefore more likely to induce pozzolanic reactions, which are likely to be the mechanism for Al immobilization in stabilized soils.According to Komonweeraket et al. [38], the immobilization of certain elements of cementitious compounds can occur due to the inclusion of these elements in the form of calcium silicate gels and calcium aluminate gels as a result of pozzolanic reactions with silica and alumina to form these compounds.Thus, as a result of incorporating certain elements into these cementitious compounds, the pozzolanic reactions effectively immobilize these elements, reducing their mobility and potential for leaching into the surrounding environment.This interpretation of the immobilization of certain elements in stabilized soil has also been reported by Ge et al. [24] and Mahedi et al. [45].In addition, Zha et al. [73], observed that some heavy metals also can be significantly influenced by the strength of stabilized soil.This occurs as the strength of stabilized soil increases, leading to the immobilization of certain elements that can be found even in the soil itself, such as Al, making the leachate from stabilized soil demonstrate lower element concentrations when compared to natural peat.Therefore, stabilizing peat soil by using fly ash, bottom ash, and OPC can reduce the concentration of certain elements in the leachate due to the immobilization process, including Al, which was notable in this study.In addition, Al from coal fly ash leachate itself is known for its poor and limited leachability compared to other elements [29,49], due to the extremely slow dissolution rates of the glassy matrix and the crystalline aluminosilicate phases [29].This limited leachability of Al is actually a positive aspect for the environment.It translates to a lower risk of Al release into leachate from coal fly ash, thereby minimizing potential impact to environmental components.Top of Form In the case of Mn, during the dry season under natural peat soil conditions, Mn concentrations averaged 14.92 µg/L, exhibiting a relatively moderate range from 12.9 to 16.4 µg/L.However, in the presence of stabilized soil, the average Mn concentration was slightly higher at 24.67 µg/L, suggesting a potential increase in leaching due to stabilization.As the wet season followed, Mn concentrations surged.Under natural peat soil condition, levels ranged from 27.6 to 56 µg/L, with an average of 39.7 µg/L.Meanwhile, in the presence of stabilized soil, Mn concentrations were notably higher, varying from 43.12 to 88.5 µg/L, resulting in an average of 69.8 µg/L.Mn in rainwater in Malaysia was reported to be, on average, equal to 80 µg/L [22].The EPA has set a secondary maximum contaminant level for Mn in drinking water that is equal to 50 ug/L [20].However, its ecological impact can be significant, especially in certain environmental conditions, due to its ability to disrupt ecological processes, whose long-term effects still need to be investigated.In fact, according to Zhao et al. [74], the presence of Mn in coal can significantly increase its environmental mobility and potential toxicity, necessitating proper management strategies during coal washing or utilization to minimize its release into the environment.
The observed rise in Mn leaching, particularly in wet conditions with stabilized soil, may be attributed to a combination of chemical interactions or unique behavior within peat soils.These intricate processes collectively contribute to the higher Mn concentrations observed in the leachate under natural peat and natural peat + stabilized condition during both dry and wet seasons, contribute further to the higher Mn concentrations in the leachate in wet conditions.Mn concentrations slightly increased with addition rates of fly ash content, which was consistent with the findings reported by Mahedi et al. [45].Also, Cetin and Aydilek [11] reported that the leaching of Mn is influenced by the availability of Mn in the fly ash itself, rather than by other factors.According to Komonweeraket et al. (2015), the Mn(OH) 2 (s) minerals control the solubility of Mn metal in the solutions of the soil-fly ash mixtures.Stabilized soil with fly ash, bottom ash, and cement enhances the surface area of the stabilized soil due to the added materials.This provides more sites for Mn (OH) 2 (s) formation; potentially, rainfall flow will dissolve Mn(OH) 2 (s), releasing Mn ions into the leachate, leading to a higher measured concentration.Furthermore, Abd Manan et al. [1] reported that cation exchange capacity can cause the transfer of certain heavy metals from the fly ash and bottom ash matrix into the leachate.This suggests that ion exchange processes might also contribute to the release of Mn from these ashes.Zhao et al. [74] and Wang et al. [69] pointed out that coal fly ash can contain different Mn oxidation states, including Mn(II), Mn(III), and Mn(IV).According to these studies, Mn(II) is the most soluble form and typically exists within the glassy phase or interacts with ferromagnetic particles.The leachability of Mn differs depending on its location within fly ash, with Mn leaching from the glassy phase being superior compared to leaching from ferromagnetic particle [69].
In the case of Fe, during the dry season, under natural peat soil condition, Fe concentrations averaged 647 µg/L, with levels ranging from 538 to 834 µg/L.In the presence of natural peat + stabilized during the same season, the average Fe concentration was slightly higher at 710 µg/L, ranging from 645 to 855 µg/L.During the wet season, under natural peat soil condition, levels ranged from 290 to 855 µg/L, with an average of 543.2 µg/L.In contrast, in the presence of stabilized soil, Fe concentrations varied from 567 to 687 µg/L, with an average of 611.5 µg/L.The presence of stabilized soil appears to have an asserted impact on Fe leaching, resulting in substantially higher concentrations, especially during dry conditions.Thus, it can be noted that the presence of stabilized soil indicates a significant impact on Fe leaching, resulting in higher Fe concentrations in both dry and wet seasons.Fazillah Abdullah et al. [22] found that Fe in rainwater in Malaysia was reported to be, on average, equal to 70 µg/L.Although the presence of an increment of Fe in the stabilized soil was presented and may indeed be attributed to the chemical composition of fly ash, bottom ash, and OPC, which contain a percentage of Fe 2 O 3 , they contribute to the overall Fe content in the stabilized soil mixture.According to Komonweeraket et al. [38], the presence of Fe 2 O 3 has the potential to affect the leaching behavior of Fe.The dissolution and hydrolysis of oxides such as Fe 2 O 3 occur when soil is contacted with water.This process leads to the formation of metal hydroxide precipitates [37].As a result, the concentration of Fe in the leachate is expected to increase because the Fe 2 O 3 from the fly ash, bottom ash, and OPC dissolves into the water, contributing to the leachate.Additionally, Mahedi et al. [44] report that the solubility of Fe is controlled by the dissolution and precipitation of oxide and hydroxide minerals such as hematite (Fe₂O₃) and Fe(OH)₃.This established understanding highlights that the presence and stability of these iron-containing minerals can significantly influence the solubility and mobility of Fe within the stabilized soils.Based on XRF results in Table 2, the Fe 2 O 3 content in fly ash is 6.21 %, in bottom ash is 8.51 %, and in OPC is 5.32 %.This indicates that a combination of chemical interactions and alterations in soil chemistry plays a role in the observed higher Fe concentrations in the leachate when stabilized soil is present.To end, Fe 2 O 3 is a contributor to Fe concentrations in the leachate, and these conclusions are consistent with previous studies, supporting this interpretation.
Further, the increase in Fe concentrations can be attributed to the lower pozzolanic activities, which subsequently result in a poorer association of iron cations.This interpretation is consistent with the results reported by Mahedi and Cetin [46].Moreover, the reduced pozzolanic activity may lead to a less efficient binding of iron cations, suggesting that the hydration process in such mixtures is not optimal.This insight underscores the need for further investigation into the factors influencing pozzolanic activity and its impact on the incorporation of metal ions in stabilized soil using coal ashes and cement.
In the case of Zn, during the dry season, under natural peat soil condition, Zn concentrations averaged 343 µg/L, with levels ranging from 279 to 384 µg/L.In the presence of natural peat and stabilized conditions during the same season, the average Zn concentration increased notably to 510 µg/L, with values ranging from 455 to 573 µg/L.As the wet season, Zn concentrations increased, under natural peat soil condition, levels ranged from 338 to 907 µg/L, with an average of 521 µg/L and with natural peat + stabilized condition, Zn concentrations ranged from 407 to 931 µg/L, resulting in an average of 625 µg/L.According to Fazillah Abdullah et al. [22], the average concentration of Zn in rainfall in Malaysia, on average, equal to 300 µg/L.However, the presence of the stabilized peat soil appeared to have an impact on Zn leaching, resulting in higher concentrations, particularly during wet conditions.Although the specific Zn content in fly ash, bottom ash, and OPC is not provided in the XRF results as presented in Table 2, however, according to Liu et al. [43], coal contains an array of heavy metals including Zn.According to Komonweeraket et al. [37], Zincite (ZnO) and zinc hydroxide (Zn(OH) 2 ) have been identified as potential solid phases that could control the presence of Zn, and Zincite, a primary phase in fly ash, has been reported to regulate the leaching of Zn in fly ash leachate.Furthermore, according to Rivera et al. [61], the presence and chemical state of Zn in fly ash was associated with Fe(III) phases and oxide phases.This state of Fe(III), is commonly found in various iron compounds, including iron oxides like F e 2 O 3 (ferric oxide or hematite), which was identified in this study based on XRF results.Fe 2 O 3 is present in various samples: fly ash: 6.21 %, bottom ash: 8.51 %, OPC: 5.32 %.Thus, it can be possible that these materials contribute to the increased Zn concentrations in the leachate when stabilized soil is present, making the presence of Zn in the leachate a complex interplay of factors, including the chemical compositions of the materials used, their interactions with the peat soil, and the natural content of Zn in the soil itself.
The leachate from the soil stabilization site is anticipated to percolate into the groundwater as a result of interacting with rainfall through soil profiles, or it could leach directly into surface water when it is adjacent to bodies of water such as rivers.This could threaten aquatic life, [29], while the presence of heavy metals in the water is a significant concern due to their toxicity, as they persist in the environment, contaminate food chains, and ultimately lead to various health issues [28].According to Ismanto et al. [28], Malaysia lacks established groundwater quality standards; therefore, the groundwater quality status is determined based on the national guidelines for raw water quality as the benchmark.In this study, Class IV water serves as a relative benchmark to some extent for comparing leachate concentrations, as this leachate arises from the interaction of rainwater with the soil, potentially being absorbed by plantations.Class IV water is considered suitable for irrigation uses according to the national water quality standards for Malaysia [47].Accordingly, Cu recorded the highest average under natural peat and stabilized wet conditions, with Cu measuring at 52.5 µg/L.And, according to the Class IV water standard, the standard limit for Cu is 200 µg/L (equal to 0.2 mg/L), indicating that Cu in the leachate remains below this limit.While Al begins to immobilize, as explained earlier, the leachate of Al after the soil begins to stabilize lowers the natural soil leachate.In the case of Mn, the highest average was recorded also under natural peat and stabilized wet condition, with an average of Mn 69.8 µg/L, and the standard for water Class IV of the Mn limit is also equal to 200 µg/L, indicating that Mn in the leachate also remains below this limit.In the case of Fe, the highest average was recorded under natural peat +  stabilized dry condition, with an average of Fe 710 µg/L, and the standard for water Class IV of Fe limit equal to 1000 µg/L for leaf irrigation, indicating that Fe in the leachate also remains below this limit.With regard to Zn, the highest average was recorded also under natural peat and stabilized wet condition, with an average of Zn 625 µg/L, and the standard for water Class IV of Fe limit equal to 2000 µg/L, indicating that Fe in the leachate also remains below that limit.However, it is important to indicate that, in terms of long-term considerations regarding the leaching of these heavy metals, these metals could pose a threat to potential impacts on local groundwater and ecosystems as they have the potential to accumulate in considerable amounts.
In the end of this part, heavy metals Cu, Al, Mn, Fe, and Zn and concentrations in leachate from natural peat soil and natural peat + stabilized soil peat conditions during the dry and wet seasons were presented.It can be concluded that Cu, Mn, Fe, and Zn increased with stabilizing soil conditions, while Al got immobilized and ultimately decreased after peat soil began to stabilize.Although the most important of that, it has been found that Cu, Mn, Fe, and Zn increased in the presence of stabilized peat soil leachate.And considering the stabilization of soil in the site, the stabilization of peat soil takes place in a site with a considerable area, and in particular if the site of soil stabilization is located in an area characterized by rainfall over time.Thus, by considering the long-term effects, the leachate of these heavy metals that begins to increase due to soil stabilization has the potential to accumulate in a significant amount and can impact the groundwater and surrounding surface water, such as lakes and rivers.According to Kura et al. [39], heavy metals, including Zn, Cu, etc., can lead to groundwater contamination when present in extensive amounts.Also, elevated levels of these heavy metals may result in the accumulation of heavy metals in the environment, exacerbating pollution and environmental degradation, and they can be toxic to aquatic organisms at high concentrations as they disrupt ecological processes and interactions within terrestrial and aquatic ecosystems, making the long-term effects still need to be investigated.

Analysis of leachate anions
Table 6 presents an analysis of anion concentrations in leachate from natural peat soil under both natural peat and stabilized condition during both dry and wet seasons, as detected using IC.The anions under consideration include fluoride (F⁻) chloride (Cl⁻), nitrate (NO₃⁻), phosphate (PO₄ 3 ⁻) and sulfate (SO₄ 2 ⁻).These were the major anions that were detected by IC during the analysis of the leachate.Additionally, it is important to note that nitrite (NO₂⁻) and bromide (Br⁻) were also detected in small amounts in certain samples and under specific conditions and have also been reported at the end of this section, even though they were not reported in Table 6, as they were detected in small amounts in some samples.These ions have potential environmental consequences, including threats to groundwater, human health, and ecological impact on aquatic ecosystems, when their concentrations exceed the recommended safe level in environmental components, including soil and groundwater.
During the dry season under natural peat soil condition, the average F⁻ concentration was 0.874 mg/L, with levels ranging from 0.246 to 1.478 mg/L.In the presence of natural peat + stabilized condition, F⁻ concentrations were notably lower, with an average of 0.152 mg/L, ranging from 0.118 to 0.205 mg/L.This suggests a potential modest reduction in F⁻ leaching due to stabilization.As the wet season followed, F⁻ concentrations remained relatively low under natural peat soil condition, with an average of 0.195 mg/L and levels ranging from 0.150 to 0.248 mg/L.On the other hand, in natural peat + stabilized soil condition, concentrations increased slightly, with an average of 0.245 mg/L and levels ranging from 0.225 to 0.267 mg/L.The concentration of F⁻ in rainwater in Malaysia was reported to range from 0.03 to 5.37 mg/L by [22].To sum up, these findings indicate that, from the perspective of F⁻ leaching, the stabilized soil may not lead to significant variations in F⁻ concentrations, indicating that F⁻ levels can almost be maintained as in natural peat conditions.
After that, Cl⁻ during the dry season under natural peat soil condition, averaged 5.694 mg/L, displaying a range from 2.828 to 7.915 mg/L.Conversely, in the presence of natural peat + stabilized during the same season, Cl⁻ concentrations were notably higher, with an average of 9.103 mg/L, ranging from 7.531 to 10.613 mg/L.This indicates an increase in Cl⁻ leaching due to stabilization.During the wet season, Cl⁻ concentrations remained relatively low under natural peat soil condition, with an average of 4.776 mg/L and levels ranging from 2.292 to 10.55 mg/L.In contrast, with natural peat + stabilized condition, Cl⁻ concentrations increased substantially, with an average of 38.94 mg/L and levels ranging from 17.761 to 78.725 mg/L.These observations suggest that the presence of stabilizing agents may lead to increased Cl⁻ leaching, especially during wet conditions.The reasons for this increase in Cl⁻ concentrations could be complex and the high CaO content in OPC may lead to increased Cl⁻ solubility and leaching when mixed with peat soil.Nevertheless, the concentration of Cl⁻ in rainwater in Malaysia was reported with an average of 22.18 mg/L by [22].
In the case of NO₃⁻, in the dry season under natural peat soil condition, NO₃⁻ concentrations averaged 1.026 mg/L, ranging from 0.538 to 2.388 mg/L.In the presence of natural peat + stabilized condition during the same season, NO₃⁻ concentrations were comparable, with an average of 1.449 mg/L, ranging from 0.704 to 2.170 mg/L.During the wet season, NO₃⁻ concentrations were notably higher under natural peat soil condition, with an average of 3.269 mg/L and levels ranging from 0.473 to 9.173 mg/L.With stabilized soil during wet condition, NO₃⁻ concentrations decreased, with an average of 0.203 mg/L and levels ranging from 0.197 to 0.214 mg/L.The concentration of NO₃⁻ in rainwater in Malaysia was reported with average of 6.16 mg/L by [22].Considering the XRF results suggest that the presence of Ca and Al in the stabilization materials may have led to reduce NO₃⁻ leaching in in the presence of stabilized soil during the wet season.These elements likely played a role in absorbing or complexing NO₃⁻, making them less mobile in the stabilized soil.The fact that the stabilized soil showed small variations in NO₃⁻ concentrations indicates that NO₃⁻ levels can be maintained within an acceptable range, even under varying conditions.
PO₄ 3 ⁻ displayed that during the dry season under natural peat soil condition, PO₄ 3 ⁻ concentrations showed higher values, with an average of 13.26 mg/L and levels ranging from 13.195 to 13.314 mg/L and with not was not detected in SF2.In the presence of natural peat + stabilized condition during the same season, PO₄ 3 ⁻ concentrations remained low, with an average of 1.606 mg/L, ranging from 1.081 to 2.170 mg/L.Moving to the wet season, PO₄ 3 ⁻ concentrations were relatively consistent under natural peat soil condition, with an average of 13.68 mg/L and levels ranging from 2.354 to 21.563 mg/L.In natural peat + stabilized condition during the same season, PO₄ 3 ⁻ concentrations decreased, with an average of 6.745 mg/L and levels ranging from 2.986 to 10.54 mg/L.The concentration in PO₄ 3 ⁻ rainwater in Malaysia was reported with an average of 0.61 mg/L by Khoon et al. [33], although it was not found to be detected and reported by Fazillah Abdullah et al. [22] and Ariffin et al. [6].The presence of Ca, Al, and SiO 2 in the stabilized soil, as indicated by the XRF results, may lead to chemical interactions with PO₄ 3 ⁻.These interactions could include adsorption or complexation, which influence PO₄ 3 ⁻ mobility and leaching to be decreased in the presence of stabilized soil conditions in dry and wet seasons.
Also, it is important to indicate that NO₂⁻ was detected in SF3 only at wet season in natural peat condition with 0.271 mg/L, and Br⁻ was detected in all samples SF1-SF4 at wet season in natural peat condition + stabilized soil condition only and ranged from 0.213 to 0.251 mg/L with an average of 0.229 mg/L.It is important to note that, according to Khoon et al. [33], rainwater in Malaysia was found to contain NO₂⁻ with low values ranging equal to 0.037-0.168mg/L, and Br⁻ was also detected in one sample equal to 0.078 mg/L.This suggests that the presence of NO 2 and Br2 in the leachate at these low concentrations likely comes from rainfall.
SO₄ 2 ⁻ concentrations displayed stability and variation.During the dry season under natural peat soil condition, SO₄ 2 ⁻ concentrations were relatively stable, with an average of 1.291 mg/L and levels ranging from 0.944 to 1.722 mg/L.In the presence of natural peat + stabilized condition during the same season, SO₄ 2 ⁻ concentrations showed a significant rise in concentration, with an average of 19.120 mg/L and levels ranging from 11.911 to 28.699 mg/L.In the wet season, SO₄ 2 ⁻ concentrations under natural peat soil condition, with an average of 23.98 mg/L and levels ranging from 6.449 to 72.89 mg/L.In the presence of natural peat + stabilized condition during the same season, SO₄ 2 ⁻ concentrations were notably higher, with an average of 93.75 mg/L and levels ranging from 88.136 to 103.00 mg/L.The concentration of SO₄ 2 ⁻ in rainwater in Malaysia was reported with range from 0.01 to 187.41 with an average of 5.63 mg/L by [22].Also, it is important to indicate that the EPA has established a secondary maximum contaminant level for SO₄ 2 ⁻ in drinking water at 250 mg/L based on aesthetic effects [20].Nonetheless, this observation strongly indicates a significant increase in SO₄ 2 ⁻ concentration under stabilized soil conditions during both dry and wet seasons.This increase is a direct result of blending peat soil with fly ash, bottom ash, and OPC.The presence of CaO and Al 2 O 3 in these materials, primarily in the form of CaO and Al 2 O 3 , is likely responsible for facilitating the release or dissolution of sulfate ions into the soil.Moreover, the influence of these elements on SO₄ 2 ⁻ leaching is notable, particularly due to the presence of SO 3 in these materials, which further contributes to the observed trend.Furthermore, Mahedi and Cetin [46] found that SO₄ 2 ⁻ were controlled by both gypsum (CaSO 4• 2 H 2 O) and anhydrite (CaSO 4 ), both of which are rich in fly ash and cement, thus leading to leaching of SO₄ 2 ⁻ in significant amounts.In addition, Mahedi et al. [45] report that the solubility of SO₄ 2 ⁻ is controlled by the precipitation and dissolution reactions of these sulfate minerals across all pH values.Further, Tian et al. [64] found that sulfate mineral (15.5 %) in as-received fly ash (which was received after ferrous and non-ferrous metal recycling by magnetic and eddy current separation) had two main categories: CaSO 4 minerals such as gypsum with 6.3 % and other byproducts formed with CaSO 4 minerals, is it possible sulfate minerals would contribute to the leachate of SO₄ 2 ⁻.Therefore, considering the presence and solubility of these sulfate minerals, it is highly likely that the fly ash will contribute SO₄ 2 ⁻ to the leachate of stabilized peat.
The EPA has set a secondary maximum contaminant level of 250 mg/L for both Cl⁻ and SO₄ 2 ⁻ in drinking water, based on aesthetic effects [20].Although Cl⁻ and SO₄ 2 ⁻ concentrations in the analysis of this study were still lower than the limit of the EPA for the secondary maximum contaminant level in drinking water.It is also important to indicate that such Cl⁻ would interfere with the behavior of heavy metals, including interference in the determination of concentrations in soil solutions.According to Chang Chien et al. [13], Cl⁻, SO₄ 2 ⁻ is one of the most abundant anions and potentially affects the absorption of the toxic heavy metal of cadmium (Cd) by crop roots in soil and associated environments.Also, they further pointed out that the behavior of heavy metals in soil solutions, including mobility and bioavailability, can thus not be predicted and evaluated by their total concentrations but also based on the consideration of the effects of such chemicals as Cl⁻, SO₄ 2 ⁻, as most of the heavy metals in soil solutions and associated environmental solutions were in the form of free ions, complex ions, and neutral molecules.Additionally, Peng et al. [53] report that SO₄ 2 ⁻, NO₃⁻, and Cl⁻ have influenced the biological toxicity of heavy metals in water to varying degrees.Furthermore, Cl⁻ would impact the salinity level in cases where it exists in significant amounts and would likely contribute to an increase in salinity level on surrounding environmental components such as soil and water.
At the end of this section, it can conclude that the combined SiO 2 , which often referred to as silica, content from fly ash and bottom ash in the mixture is indeed relatively high.However, it is important to note that SiO 2 , when present in fly ash and bottom ash and utilized in soil stabilization, playing primary role as a crucial component contributing to pozzolanic reactions, this corresponds to what has been found by [19,25].Its primary function is to enhance the binding and stabilization of the soil rather than acting as a significant source of leachate elements.SiO 2 typically remains stable and does not leach out into the environment in significant quantities.Thus, SiO 2 itself does not typically leach into the environment in significant quantities, although it possible to influence the overall behavior of other elements and compounds in the soil, including heavy metals like Cu.In summary, SiO 2 plays a pivotal role in soil stabilization by improving soil properties, but it is not a major contributor to leachate elements.The overall leaching behavior of stabilized soil is determined by a combination of factors beyond SiO 2 content.

Challenges and perspectives
This section discusses the challenges in the research field of environmental assessment of soil stabilization applications using stabilization material agents, in particular the use of coal ashes alongside OPC.This section also explores potential avenues to address these challenges.
1. Lack of benchmarks: A major challenge is the absence of standardized guidelines for acceptable limits of toxic and heavy metals, as well as major anions, in leachate from soils.Existing benchmarks, such as those for industrial effluents and soil contamination standards, have limitations when applied to leachate.Industrial effluents are typically treated before release, while leachate from stabilized soil can directly infiltrate groundwater.Moreover, soil contamination benchmarks (expressed in mg/kg) are not directly comparable to leachate concentrations, as they are based on the contaminant mass per unit weight of soil, rather than the leachate resulting from interactions with water or rainfall.
To address this, the present study employed chemical composition averages of rainfall in Malaysia, which is relevant given that the leachate originates from rainfall.Secondary maximum contaminant levels for drinking water established by the EPA were also referenced to provide interpretive context.Furthermore, Class IV water quality standards for irrigation [47] were used as a relative benchmark due to potential leachate interaction with plantation soils.Additionally, leachate from non-stabilized peat soil was analyzed for comparison with stabilized peat soil leachate, providing an indicator of how stabilization impacts leaching behavior.The establishment of clear benchmarks would enhance the effectiveness of research on the environmental assessment of soil stabilization applications using alternative waste materials such as coal ashes.2. Long-term analysis: As previously mentioned, a critical long-term concern is the potential impact of heavy metal leaching from stabilized soil on surrounding groundwater and ecosystems.In fact, heavy metals can accumulate in environmental components like groundwater over time, posing a risk to ecological and potentially human health.Consequently, analyzing the potential longterm impacts required an extensive amount of time.Thus, real-world case studies with long-term monitoring of stabilized soil projects, to be conducted and supported by established environmental departments, could be highly valuable.These studies would involve establishing test plots for extended periods to analyze leaching behavior and monitor the quality of surrounding water and ecosystems under various environmental conditions.Long-term monitoring data would be crucial for assessing the potential impacts of leachate on groundwater and ecosystems, as it would facilitate the confirmation of possible long-term effects.3. Cross-disciplinary research field: The environmental assessment of soil stabilization applications is a dynamic research field that intersects with diverse disciplines such as environmental science, engineering, construction, and regulatory compliance.This multifaceted nature presents challenges, as each discipline has its own priorities (e.g., environmental considerations, engineering solutions, construction requirements, industry standards).Thus, collaborative research efforts among these fields can lead to the integration of environmental practices into engineering solutions, which is essential.In fact, cross-disciplinary research collaboration can bring collective expertise from these fields to advance holistic approaches to the development of comprehensive, sustainable soil stabilization practices.This approach balances engineering requirements with environmental conservation goals for a more sustainable future by considering the impact on environmental components and ecosystems, accounting for engineering aspects such as integrity and long-term durability and adhering to industry regulations and environmental standards.At last, crossdisciplinary research collaborations will be crucial for improving innovative and effective soil stabilization procedures and methods that meet both environmental and technical challenges.

Conclusions
The present study has assessed the environmental impacts of fly ash, bottom ash, and OPC on stabilized soil in response to rainfall interactions.The study starts by characterizing soil mixture materials, including their physicochemical and engineering properties.The findings of those characterizations revealed the enhancement of soil compressive and shear strength through overcoming periods, the formation of new minerals, and the improvement of the microstructure of stabilized soil surfaces due to the chemical mechanisms of the soil stabilization process, mainly pozzolanic reactions and cation exchange.Also, the findings showed the impacts on stabilized soil properties when they interact with rainfall, such as soil strength and microstructure.Then, the study revealed that heavy metal concentrations in leachate from peat soil exhibited complex dynamics influenced by soil stabilization processes, which can be summarized as follows: • Al concentrations decreased significantly in both dry and wet seasons after stabilization due to chemical immobilization.
• Cu, Mn, Fe, and Zn concentrations increased in both seasons following stabilization, due to some factors such as the metal content in stabilizing materials and chemical reactions.• Soil stabilization significantly impacts heavy metal leaching, with the impact being complex and influenced by both seasonal variations and specific metal characteristics.• The presence of metals within the stabilizing materials, along with chemical reactions, surface runoff, and enhanced particle transport by rainwater-particularly during wet seasons-contributes to these dynamics.
Subsequently, this study has also revealed the notable influence of soil stabilization processes on anion leaching patterns, which can be summarized as follows: • SO₄ 2 ⁻ concentrations increased dramatically in both dry and wet seasons due to the addition of stabilizing materials.
• Cl⁻ also concentrations showed a marked increase within the stabilized soil, particularly in the wet season, suggesting increased leaching under wetter conditions.• PO₄ 3 ⁻ concentrations decreased significantly in both seasons, indicating potential immobilization within the stabilized soil.
• F⁻ concentrations decreased significantly in the dry season and increased slightly in the wet season after soil stabilization.
• NO₃⁻ exhibited behavior with slight increases in the wet season within the stabilized soil.
The implications, recommendations for practice, and directions for future studies that have been drawn from this study can be summarized as follows: 1) Environmental implications: The findings have shown that when peat soil is stabilized by fly ash, bottom ash, and OPC, the concentration of heavy metals in the leachate can increase due to interactions with these stabilizing substances.This occurs due to the presence of heavy metals in the stabilizing materials, chemical reactions, or dissolution processes.Those interactions result in higher concentrations of the heavy metals (Cu, Mn, Fe, and Zn) in the leachate, which could have significant environmental implications, notably during heavy rain seasons.In contrast, the stabilization of soil using these stabilizing materials has led to the immobilization of Al in the leachate, resulting in a decrease in the Al concentration.Thus, Zn, Fe, Mn, and Cu have been significantly affected by the leachate with stabilized soil conditions in both seasons, in particular in the wet season.Further, the study findings also indicate that soil stabilization using stabilizing materials can have diverse effects on anion concentrations in leachate.While certain anions, like F⁻ and NO₃⁻, may show potential for reduced leaching, others, like Cl⁻ and SO₄ 2 ⁻, can experience increased concentrations.Therefore, environmental impacts need to be considered in the practical application process.2) Engineering implications: Stabilization of peat soil by utilizing fly ash, bottom ash, and OPC contributes to enhancing engineering and mechanical soil properties.However, it is very important to indicate that soil stabilization by using those agent stabilization materials has led to an increase in the concentration of certain heavy metals such as Cu, Mn, Fe, and Zn, particularly in wet seasons.Thus, from an engineering perspective, we should take into consideration those impacts, specifically if the soil stabilization sites are supposed to be exposed for heavy rain seasons or to be done around wetland such as surrounding rivers and lakes.To ensure the site where soil stabilization is done, have an efficient drainage system such as roads or other construction sites.
Therefore, to mitigate these potential impacts, regulatory measures and consideration of site environmental conditions before soil stabilization are essential.3) Recommendations for practice: To promote sustainable soil stabilization practices, particularly when utilizing coal ash and OPC, local regulatory bodies should establish or refine standardized protocols and codes.These guidelines should prioritize environmental considerations, including: determining the optimal mixture design of stabilizing materials based on local soil properties and environmental conditions; setting acceptable thresholds for heavy metals and major anions in leachate from stabilized soils, with particular attention to coal ash; conducting preliminary experiments to simulate on-site conditions and implementing real-time monitoring to evaluate leachate quality and identify necessary adjustments; and ensuring efficient drainage to minimize rainwater interaction and reduce leaching of contaminants.Lastly, collaboration between industry, government, researchers, and environmental organizations is essential for developing and implementing these recommendations.4) The directions for future studies: In this study, coal ashes and OPC effectively stabilized peat soil, and simulating and assessing environmental impacts under various conditions and seasons, particularly in the presence of rainfall, aimed to replicate real-world conditions by using the most commonly used mix design that has been employed in practical applications and used in previous studies, along with the optimal required curing time to get the soil fully stabilized.However, there is a possibility that different dosages of establishing agents and the curing period of stabilized soil may impact leachate concentration.Thus, future studies are encouraged to explore the relationship between the leachate concentration of stabilized soil, different dosages, and curing periods to know the possible impact of this relationship.Secondly, while this research has investigated the effects of chemical mechanisms in stabilized soil on leachate concentration, further study can employ geochemical modeling to understand how leachate can be predicted and controlled at specific ranges in stabilized soils.Thirdly, since this study has investigated the environmental assessment of the utilization of coal ashes with OPC in soil stabilization applications, similar applications of the utilization of coal ashes, OPC, and alternative waste materials in other related applications of the construction industry can be replicated, which will build upon this work with welcome new contributions.Finally, to further enhance the understanding of soil stabilization practices, future research should investigate the interaction of leachate with coal ash-OPC-stabilized mixtures across various commonly stabilized soil types, including clay, and study these effects in different climates to broaden the applicability of the findings.

Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Fig. 4 .
Fig. 4. Vane shear test result of peat soil in different conditions.

Table 1
The highest mass percentages in peat soil are CaO, Fe 2 O 3 , SO 3 , and Al 2 O 3 , with values of 37.40 %, 21.40 %, 17.00 %, and 4.62 %, respectively.This analysis provides a detailed breakdown of the major chemical compositions present in the soil sample, where CaO and Fe 2 O 3 are the most common Physicochemical properties of mixture materials.

Table 2
XRF results analysis of mixture materials.

Table 3
Leachate attributes analysis.

Table 4
Result of leachate pH and column soil moisture content.

Table 5
Leachate analysis of heavy metals concentration.

Table 6
Leachate analysis of anions concentration.