Hydrochar Production Methods: Comparative Insights into Hydrothermal and Microwave Processes

This study compares hydrochar production from agricultural waste using conventional hydrothermal carbonization (HTC) and microwave-assisted carbonization methods. Wheat straw (HWS), rice straw (HRS), and bagasse (HBG) were used as feedstocks. Microwave-assisted carbonization resulted in higher yields and distinct chemical structures compared to conventional HTC. Microwave hydrochars (m-HWS, m-HRS, m-HBG) showed lower surface areas but increased pore volumes and thermal stability. They also exhibited enhanced heavy metal adsorption capacities, particularly at higher pH levels. These findings highlight the advantages of microwave-assisted carbonization for producing hydrochar with superior properties, offering insights for optimizing production processes and expanding applications.


Introduction
The increasing generation of agricultural and industrial waste has necessitated the development of effective and sustainable waste management strategies.Among these, the conversion of method, which promises higher efficiency and enhanced material properties through rapid heating and energy transfer [5][6][7] .
This study aims to systematically compare the properties and performance of hydrochar produced by conventional HTC and microwave-assisted methods.By focusing on three common biomass sources-wheat straw (HWS), rice straw (HRS), and bagasse (HBG)-this research seeks to elucidate the differences in yield, elemental composition, surface area, and functional groups between the two production techniques.Moreover, the study investigates the thermal stability and adsorption capacities of the resulting hydrochars, providing a comprehensive understanding of their potential applications.
Through detailed characterization and comparative analysis, this research contributes to the growing body of knowledge on biomass conversion technologies, highlighting the transformative effects of microwave irradiation on hydrochar properties.The findings of this study will inform future efforts to optimize hydrochar production processes, enhance material performance, and expand the practical applications of hydrochar in various environmental and industrial contexts.

Methodology
For Hydrothermal carbonization via conventional oven Agricultural residues, including rice straw, wheat straw, and bagasse, were obtained from local field and farms in India for the synthesis of hydrochar.Approximately 60 grams of naturally dried agricultural waste separately, were combined with 400 ml of deionized (DI) water.This mixture was then introduced into a 500 ml stainless steel autoclave.The autoclave was subsequently subjected to heating, maintaining a temperature of 300 °C for a duration of 5 hours, while the pressure, monitored by a pressure gauge, was maintained at approximately 1000 psi.Following this controlled thermal treatment, the reactor was cooled to room temperature, allowing for the collection of the solid hydrochar product derived from the agricultural waste.Further, the agricultural waste hydrochar, was rinsed with DI water and was then dried at 80 °C in an oven.Once dried, the sample further processed through grinding and sieving to achieve a consistent particle size fraction ranging from 0.5 to 1.0 mm.To eliminate any remaining impurities, such as ash, rinsed by DI water, followed by another round of drying at 80 °C.The resulting hydrochar samples, now purified and free from contaminants, were stored for future experimental use.
For the production of modified hydrochar, Approximately 3 grams of the prepared hydrochar samples were immersed in a solution consisting of 20 ml of 10% hydrogen peroxide (H2O2) for a duration of 2 hours at room temperature, maintained at 22 °C with a permissible deviation of ±0.5 °C.After the reaction, the modified hydrochar underwent thorough rinsing with DI water and was subsequently dried at 80 °C.The resulting modified hydrochar samples were carefully stored for future experimentation and applications.
For MHTC, around 1 g of Agriculture waste was loaded into the glass vial and 5 mL of distilled water was added, corresponding to the water to biomass ratio same as the amount found in previous work.The optimization of reaction conditions was based on the type of raw biomass used.The use of seaweed rich in cellulose necessitated the optimization of conditions to enhance cellulose conversion to hydrochar.Cellulose is a complex carbohydrate that is difficult to break down.Therefore, high temperatures and high pressures are typically required to effectively convert cellulose to hydrochar and longer reaction times are also needed to ensure complete conversion 8 .The sealed glass vial was put into the microwave, stirring started and the reactor was heated up to the set temperature of 200 °C, corresponding to the temperature as found in previous work within 20 min.Thereafter, the HTC process was maintained at the reaction temperature for a certain reaction time.During the process, the pressure has been measured and recorded by the machine.After the reaction, the reactor was cooled down to 60 °C, and then the liquid and solid products were separated by vacuum filtration with F1001 grade filter paper, size 125 mm.The liquid products were stored in the refrigerator at 4 °C for further analysis, and the solid char was dried in a conventional heating oven at 105 °C for 3 h and stored at room temperature 6 .The hydrochar produced by microwave hydrothermal treatment was designated as MHC, and hydrochar produced by conventional heating (with a reaction time of 4 h) was designated as HC.

Characterisation of the Hydrochar prepared by MHTC and HTC
The investigation into the chemical composition and physical attributes of hydrochars and their efficiency derived from Wheat Straw (HWS), Rice Straw (HRS), and Bagasse (HBG) unveils nuanced insights into their potential applications across various domains Table 1 9,10 .Overall, FTIR serves as an indispensable tool in the comprehensive characterization of hydrochars, facilitating a deeper understanding of their chemical composition, structural properties, and potential applications in fields ranging from environmental remediation to sustainable resource management.
FTIR analysis was conducted to investigate the surface functional groups of Hydrochar prepared using both the Hydrothermal Carbonization (HTC) method and microwave-assisted methods at varying power levels.The results, illustrated in Fig. 1, revealed notable differences in the detected functional groups between the two preparation methods.Specifically, Hydrochar prepared via microwave-assisted methods (mHWS, mHRS, and mHBG) exhibited fewer surface functional groups compared to those prepared using the HTC method (HWS, HRS, and HBG).For instance, peaks corresponding to aliphatic or aromatic -OH stretching (3421 cm −1 ) and aliphatic C-H antisymmetric stretching (2919/2850 cm −1 ) gradually diminished in intensity for HWS, HRS, and HBG, ultimately disappearing entirely in mHWS, mHRS, and mHBG, indicative of the decomposition and dehydration of aliphatic groups.This observation aligns with previous findings and suggests a transformative effect of microwave irradiation on hydrochar surface chemistry.Additionally, the peaks at 1625 cm −1 (associated with carboxyl or carbonyl C--O) and 1061 cm −1 (linked to C-O) weakened with increasing microwave power, indicating the decomposition of oxygen-containing functional groups-a trend consistent with the total acidity measurements of Hydrochar (Table 1).Interestingly, the peak at 787 cm−1, derived from bending vibrations of aromatic C-H, exhibited an increase in intensity with rising microwave power, suggesting an augmentation in the aromaticity of Hydrochar under microwave-assisted conditions.These findings underscore the nuanced effects of preparation methods and microwave power levels on the surface chemistry and functional group composition of Hydrochar, elucidating crucial insights for its application in various fields, including environmental remediation and resource management.

BET Analysis
The Specific Surface Area (SSA) and Pore Volume (PV) increased with the process selection from Hydrothermal Carbonization to Microwave assisted method for the preparation of Hydrochar (Table 1).This might be caused by an increase in the volatile organic component loss, leading to the formation of porous structure in the case of Microwave assisted method comparing to Hydrothermal Carbonisation method 11,12 .The results showed that mHBG had the highest SSA (156.09m2 g−1), which was 60.5 times higher than the value found for HWS.On the other hand, the PV of mHRS (0.0790 cm3 g−1) was marginally higher than the 0.0741 cm3 g−1 of mHBG.This could be because, according to the Hydrochar skeleton collapsed at high pyrolysis temperatures, which corresponds to mHBG in the current investigation.The maximum hydrochar yield of 74.66% was achieved with HWS, while hydrochar yields fell as microwave power increased.This was due to the biomass heating rate being quicker and the greater output of bio-oil and syngas under high power 13 .The oxygen-containing functional groups are main source of surface acidity of carbon materials 14 , moreover play an important role for metal ion binding with Hydrochar thus influencing adsorption ability.It could also be seen in Table 1 that there is significant decrease in the total acidity in Microwave assisted hydrochars, this was because oxygencontaining functional groups are easily decomposed at high temperatures.
thermal properties and decomposition behavior of hydrochar, providing valuable insights for optimizing synthesis protocols and tailoring hydrochar properties for specific applications in environmental and industrial settings.

Adsorption of heavy metals
The pH of the solution is often strongly connected to the surface charge of the adsorbent and the speciation of metal ions in the aqueous solution 16,17 .The Fig. 3 displays the heavy metal adsorption capabilities on several types of Hydrochar in the initial solution's pH range of 2-7.Hydrochar made from various agricultural wastes had a similar tendency of adsorbing Pb2+, Cd2+, and Cu2+ as pH increased.The adsorption capabilities of heavy metals decreased with decreasing pH.The explanation most likely suggested that metal ions and plentiful H+ compete for adsorption sites on the surface of hydrochar 18,19 .The explanation most likely suggested that metal ions and plentiful H+ compete for adsorption sites on the surface of hydrochar.However, the majority of the functional groups on the hydrochar surface which are essential for the adsorption of heavy metals-were protonated by H+ in the solution.Consequently, an positively charged.The adsorption capacity of biochar increased significantly as pH increased.
For Pb 2+, Cd 2+ , and Cu 2+ , the maximum adsorption capacities were reached at pH increases of 6 and 5, respectively, at which point the adsorption quantities were Pb 2+ with 139.44 mg g-1, Cd 2+ with 52.92 mg g-1, and Cu 2+ with 31.25 mg g -1 .Previous researches have revealed similar results about the maximum adsorption capabilities for different plant-based hydrochars.Increased pH resulted in the loss of protons by oxygen-containing functional groups, a decrease in the positive charge on the surface of adsorbents, a weakening of the competition between protons and heavy metal ions on adsorption sites, and the release of adsorption sites, which increased adsorption capacity.The adsorbing capabilities of Cd 2+ , Pb 2+ and Cu 2+ declined during a further pH rise to 7, which was partly due to metal ions starting to precipitate as hydroxides.Since Pb(OH)2 and Cd(OH)2 were found to begin precipitating at pH 5.5 and 6.5, respectively, the starting pH of the solution in this investigation was lower than 7.When comparing the adsorption capacities of biochar prepared under different microwave powers, it was noted that mHRS showed higher adsorption performance for heavy metals even than mHBG.

Conclusion
This study provides a comprehensive comparison of conventional hydrochar and microwave hydrochar derived from various agricultural wastes, specifically wheat straw (HWS), rice straw (HRS), and bagasse (HBG).Microwave hydrochar generally demonstrated higher yields, enhanced elemental oxygen content, and higher atomic O/C and H/C ratios compared to conventional hydrochar.Despite a lower BET surface area in microwave hydrochar, the structural and compositional alterations brought by microwave-assisted methods have significant implications for their surface chemistry and functional group composition.FTIR analysis highlighted that microwave-assisted hydrochars exhibit fewer surface functional groups, indicating significant decomposition and dehydration under microwave conditions.Additionally, microwave hydrochar exhibited higher specific surface area (SSA) and pore volume (PV) due to the formation of a more porous structure, attributed to the loss of volatile organic components.
Thermal Gravimetric Analysis (TGA) revealed distinct stages of decomposition, with microwave hydrochars showing improved thermal stability and higher degrees of carbonization.The study also demonstrated the impact of solution pH on the adsorption capacities of heavy metals by hydrochars, with microwave hydrochar showing enhanced adsorption performance at higher pH levels.

Future Recommendations
Future research should explore the optimization of microwave power and process parameters to maximize the beneficial properties of hydrochar, such as yield, specific surface area, and adsorption capacity.Investigating the use of different catalysts during microwave-assisted hydrochar production could further enhance the material properties and performance.
Additionally, a more detailed analysis of the mechanisms underlying the decomposition of surface functional groups and the formation of porous structures in microwave hydrochar would provide valuable insights for tailoring hydrochar for specific applications.Further studies should also examine the long-term stability and reusability of microwave hydrochar in environmental applications, particularly in the adsorption of various contaminants.Finally, expanding the range of biomass sources and examining their effects on the properties and performance of microwave hydrochar would contribute to the development of more versatile and efficient carbon materials.

Fig. 1 FTIR
Fig.1 FTIR analysis of Hydrochars prepared by both conventional and microwave assisted methods

Fig 2 .
Fig 2. TGA analysis of Hydrochar produced by conventional method and Microwave assisted method

Table . 1 Physio-chemical properties oh Hydrochars prepared by conventional and microwave assisted method FTIR Analysis
Moving to m-HRS, it presents a similar trend in composition modifications, featuring a carbon content of 82.56%, hydrogen content of 48.34%, oxygen content of 6.32%, and nitrogen content of 39.25%.These values reflect alterations intended to enhance nitrogen incorporation, with the resultant O/C ratio of 0.38 and H/C ratio of 0.81 indicative of modifications influencing the relative proportions of carbon, hydrogen, and oxygen.The specific surface area of m-HRS is . The hydrochars prepared by microwave assisted approach, likely subjected to various treatment protocols, exhibit distinct chemical compositions and physical properties comparing to classical hydrothermal technology.Beginning with m-HWS, it demonstrates a significant carbon content (C %) of 85.4% alongside a hydrogen content (H %) of 51.88%, indicating a substantial alteration from its raw form.Moreover, m-HWS displays an oxygen content (O %) of 5.89% and a nitrogen content (N %) of 38.22%, illustrating potential modifications aimed at enhancing nitrogen incorporation.The resultant O/C ratio of 0.22 and H/C ratio of 0.74 suggest a composition skewed towards carbon and hydrogen, with notable nitrogen enrichment.Additionally, m-HWS exhibits a specific surface area (BET) of 11.76 m2/g, implying a reduction from its untreated counterpart, possibly due to structural alterations induced by modification processes.facilitatesthe identification of key functional groups such as hydroxyl (OH), carbonyl (C=O), and carboxyl (COOH) groups, which play crucial roles in determining the material's reactivity and adsorption capacity.By analyzing FTIR spectra, researchers can discern changes in functional