Utilizing Coal Ash and Humic Substances as Soil Ameliorant on Reclaimed Post-Mining Land

Coal ash and humic substances can be used as soil ameliorant in the reclamation of formerly mined land. Due to its high pH and nutrients content, coal ash can be used to improve the chemical properties of the soil, such as increasing of pH, and increasing the levels of nutrients availability in the soil. Humic substances may also be used to complement, as they can increase the release of nutrients from the coal ash. Thus, the objective of this study was to assess the influence of coal ash and humic substances on soil chemical characteristics, nutrient absorption, and plant growth. This study was conducted in two locations - in a nursery area, involving two treatment factors: coal ash at different dosages (0, 200, and 400 g polybag-1), and humic material also at varying dosages (0, 0.04, and 0.08 g C polybag-1); and in a post-mining field using similar treatments: coal ash dosage (0, 2.5, and 5.0 kg planting-1 hole) and humic material dosage (0, 0.56, and 1.12 g C planting hole-1). The results showed that coal ash and humic materials significantly increased the soil pH, available P, and exchangeable K, Ca and Mg. Coal ash also contained a number of heavy metals but in quantities that are far below the limits set by both Indonesian Government Regulation and the US Environmental Protection Agency (USEPA). The above soil amelioration effects mean that. applicaton of coal ash and humic substances can significantly increase the growth of Jabon trees in the reclaimed post-mining land.[How to Cite: Ade MO, Sudarsono, Iskandar and DT Suryaningtyas. 2014. Utilizing Coal Ash and Humic Substances as Soil Ameliorant on Reclaimed Post-Mining Land. J Trop Soils 19: 161-169. Doi: 10.5400/jts.2014.19.3.161] [Permalink/DOI: www.dx.doi.org/10.5400/jts.2014.19.3.161]  


INTRODUCTION
Coal is one of the primary natural resources in Indonesia that has been increasingly utilized as fuel for electricity-generating thermal power plants. In the process of burning coal, among the resulting byproducts are bottom ash and fly ash, or collectively referred to simply as coal ash. In 2006, the total production of coal ash from the all thermal power plants in Indonesia amounted to over 2 million tons; this volume nearly doubled (3.3 million tons) in 2009 (Aziz et al. 2006). Such large quantities of coal ash generated annually in Indonesia continue to accumulate as solid landfill waste, and its utilization potential remains to be tapped to optimum benefit. To cite an example, PT Newmont Nusa Tenggara (PTNNT), one of the largest companies in Indonesia that is engaged in open mining of gold and copper, operates a coal-fired power plant. PTNNT's power plant produced 21672.87 tons of coal ash in 2011-2012 which has not been fully taken advantage yet (PTNNT 2012).
One promising prospect for coal ash utilization is in the reclamation of post mining land. Reclamation is necessary towards reclaiming the functional qualities of post-mined areas as such lands need to be remediated for example, by reforestation, but which is constrained by their generally low soil fertility condition, especially in terms of nutrient content essential for plant growth. Hence, the soil need to be improved first, and one way is by the application of soil ameliorants, such as coal ash. Aside from the fact that coal ash is already abundantly available in situ, it possesses a high pH (11-12) (Singh et al. 2011) and contains considerable amounts of chemical compounds: 52.00% SiO 2 , 31.86% Al 2 O 3 , 11.85% SO 3, 4.89% Fe 2 O 3 , 2.68% CaO, and 4.66% MgO (Wasim 2005).
Recent studies have demonstrated that coal ash can play a vital role in soil amelioration, which can lead eventually to successful resforestation of exhausted and degraded wastelands, particularly exmining land. For instance, McCarty et al. (1994) Singh et al. (2011 Sharma et al. (2006) found that applying coal fly ash substantially raised mineral soil pH. Iskandar et al. (2008) also reported that coal fly ash applied into peat soil increased pH, Available-162 AM Oklima et al.: Coal Ash and Humic Substances as Soil Ameliorant P, and exchangeable K, Na, Ca, and Mg. It has also been shown that fly ash from spent coal can adsorb such metals as Zn 2+ , Cd 2+ , Ni 2+ , Cu 2+ and Cr 6+ from liquid wastes as effectively as active carbon (Bayat 2002).
Ironically, in Indonesia, the utilization of coal ash is still very limited, and it is partly because of the existence of Government Regulation No. 85/ 1999 which categorizes it as hazardous and toxic substance (Category B3) and imposes rigid restrictions in its handling. Hence, before it can be legally utilized, burned coal residues must undergo a Toxicity Characteristic Leaching Procedure (TCLP) in order to determine its toxicity properties. In contrast, other countries like Finland (Korpijärvi 2012), Australia (ADAA 2009) and Pathan et al. (2002), along with Israel (Cohen et al. 2001) have already been widely utilizing coal ash in various ways of economically viable and environmentally friendly application.
Another potential soil ameliorant is humic substances, which comes as a result of organic matter decomposition. According to Suganya and Sivasamy (2006), humic substances can maintain soil humidity, improve efficiency of water uptake, and increase the nutrient potential of sandy soils. It also has the capacity to form complex bonds with metallic ions (Stevenson 1982;Santos 2007).
This study was conducted to look into the technical suitability of utilizing coal ash and humic substances as soil amelioration agents, and to examine their relative influences on the soil properties of previously mined land. Further, this study would evaluate the effects of coal ash and humic substances application on the growth of Jabon seedlings in the forest nursery, and trees planted in the reclaimed post-mining land.

Site Description
This study was conducted over a period of one year (December 2012-December 2013) in the forest nursery and land reclamation site (with geographic coordinates 116 o 44'0" -116 o 57'0" East longitude and 8 o 30'0"-9 o 3'0" South latitude) within the mining concession area of PTNNT in Sumbawa Barat Regency, Nusa Tenggara Barat Province in eastern Indonesia.
The materials used in this study consisted of coal ash obtained from the landfill of the PTNNT coal-fired power plant, humic substances, seedlings of Jabon, soil media, and various chemical reagents for laboratory analysis. Field equipment included planting tools and measuring devices, while the main laboratory instrument that was used was an Atomic Absorption Spectrophotometer (AAS).

Nursery Trial (NT)
For the nursery (greenhouse) portion of this study, a 2-factorial Completely Randomized Design was used with factor 1 being coal ash at different dosages (0, 200, and 400 g polybag -1 , which can correspondingly be scaled up to 0, 40, and 80 Mg ha -1 ); and factor 2 being humic substances at varying dosages (0, 0.04, and 0.08 g C polybag -1 ), or corresponding to 0, 15, and 30 liter ha -1 . Each treatment was replicated three times (3 × 3 × 3 = 27 polybags). The potting medium consisted of 10kg air-dried soil for each polybag. Humic substances was diluted 100 times prior to application. For this study, Jabon seedlings were used as indicator plant.

Field Trial (FT)
A 2-factorial Randomized Block Design was used in the field part of this study, with factor 1 being coal ash at different dosages (0, 2.5, and 5.0 kg/planting hole, or by extrapolation, corresponding to 0, 40, and 80 Mg ha -1 ; and factor 2 being humic substances at varying dosages (0, 0.56, and 1.12 g C/planting hole, or corresponding to 0, 15, and 30 L ha -1 ). Field blocking was randomized following eastwest sunlight exposure. Each treatment was 3 replicated (3 × 3 × 3 = 27 polybags), and each treatment replication consisted of 4 plants (27 × 4 = 108 plants) with plant spacing of 2 × 3 m. Thus, the total study land area covered in this study was 648 m 2 (108 plants × 6 m 2 ). The soil ameliorant (material) was applied in the field into each planting hole measuring 0.5 × 0.5 m × 0.5 m. The concentrated humic substances contained 3.23% C, was first diluted 100 times before field application. Again, Jabon was used as indicator plant.

Statistical Analysis
The measurement parameters and their respective method of data analysis that were used in this study are summarized in Table 1. The statistical analysis employed a significance level of 5%, and the iterative paired tests used DMRT (Duncan's Multiple Range Test).

Analysis of Chemical Properties of Coal Ash and Humic substances
The chemical composition and total metal content of coal ash are summarized in Table 2, while  Table 1. Measurement of soil and plant parameters and means of analysis.
Note: exch denotes "exchangeable" Table 3 shows the results of the coal ash toxicity test (TCLP), and humic substances composition. The results of the TCLP test on coal ash, as summarized in Table 3, indicate that the levels of heavy metals present in the coal ash are all far below the standard tolerance limits set under Indonesian Government Regulation (PP No. 85/ 1999), which are more stringent than that of the United States Environmental Protection Agency (USEPA). This finding has an important implication which is that the PTNNT coal ash is environmentally suitable for use as soil ameliorant.
On the other hand, the humic substances material that was used in this study was extracted from lignite coal. By its nature, humic substances possesses a highly functional group that can influence the degree of nutrient release from coal ash or mineral soil. Earlier, Ahmad (2011) highlighted that humic substances from lignite humic acid and peat humic acid can accelerate the release of elements from diorite porphyry, basalt porphyry, and trachite porphyry rocks. The properties of the humic substances solution in this study are shown in Table 4.

Suitability of Coal ash as Soil Ameliorant
Shown in Table 2, coal ash is highly alkaline and contains a number of nutrient elements which can be utilized as ameliorant to improve the chemical properties of soil. At the same time, although the TCLP test results in Table 3 suggest that coal ash contained several heavy metals, they occurred in levels which were far below the limits set by both Indonesian Government Regulation (PP No.85/ 1999) and the US Environmental Protection Agency (USEPA) and therefore, it could be safely utilized as soil ameliorant.

Soil pH
As depicted in Figure 1, the application of different dosage levels of coal ash (F) and humic substances (B) significantly raised soil pH in both nursery trial (NT) and field trial (FT) using Jabon tree seedlings.  Data analysis (Figure 1) reveals that the application of coal ash significantly altered soil alkalinity in both nursery and field trials using Jabon plants. However, the addition of humic substances did not exhibit any significant effect. In the nursery trial (NT), coal ash application raised soil pH from 6.30 (moderately acidic) into 7.97 (neutral), reaching up to 8.21 (moderately alkaline). The same pattern was observed in the field trial (FT): soil pH went up from 5.49 (acidic) into 7.54 (neutral), until 7.97 (moderately alkaline). Further, application of both coal ash and humic substances did not show any significant combinations on soil pH in the nursery trial (NT), but turned out to be significantly interactive in the field trial (FT). In short, the application of soil ameliorants can significantly improve the chemical properties of the recipient soil.
The resulting changes in soil pH (i.e., from acidic into neutral, and even into moderately alkaline) due to the application of coal ash (Table 2) appear consistent with the findings of earlier studies: in mineral soil (McCarty et al. (1994), and peat soil (Iskandar et al. 2008). Table 5 shows that the levels of organic-C, total-N and available-P in the soil were not significantly influenced by the addition of the soil ameliorants in the nursery trial (NT), but coal ash did increase both organic-C and total-N levels in the field trial (FT), although not high enough to alter the chemical properties of the soil and facilitate nutrient availability for plant growth. Interactively, the addition of both coal ash and humic substances did not influence organic-C and total-N levels in either nursery or field trial. In other words, it can be summed up that the total of the organic-C and total-N levels originally present in the soil, plus those contained in the soil ameliorants that were applied at given dosages in this study, were way too low to manifest any significant effect. Table 5 discloses that the application of coal ash and humic substances significantly increased the level of soil available-P in the nursery trial (NT): coal ash increased available-P from 2.47 mg kg -1 (F0B0: no coal ash applied, no humic substances applied), or "very low level", into 7.08 mg kg -1 (F1B1 -200 g coal ash polybag -1 , 0.04 g C humic substances polybag -1 applied), or "low level", reaching up to 9.55 mg kg -1 (F2B2: 400 g/polybag coal ash, 0.08 g C humic substances polybag -1 applied). In contrast, in the field trial (FT), the effect was not significant enough, although, in absolute terms, the resulting available-P level came out to be somewhat higher than that of Control (no soil ameliorant applied).

Exchangeable Cation (Exch.-K, Exch.-Ca and Exch.-Mg), Cation Exchange Capacity (CEC), and Micro-Element (Fe and Cu) Content
Shown in Tables 6 and 7, the application of coal ash significantly increased the level of exch.-K in both nursery (NT) and field (FT) trials, and that the two soil ameliorants brought about interacting effect in raising exch.-K in the field trial. The soil ameliorants likewise significantly raised exch.-Ca in the nursery (NT) and field (FT) trials, but there was no observed interaction between the two ameliorants in the nursery trial (NT). Both ameliorants did not significantly affect exch.-Mglevel in the nursery trial (NT), but they exhibited significant interaction effect in the field trial (FT).
In the nursery trial (NT), base cations levels of the exch.-K, exch.-Ca and exch.-Mg in the Jabon seedlings rose in direct proportions with higher additions of soil ameliorants. To illustrate, the level of exch.-K went up from 0.09 cmol + kg -1 to 0.12 cmol + kg -1 ; similarly, exch.-Ca increased from 1.82 cmol + kg -1 to 2.21 cmol + kg -1 , and reached up to 3.62 cmol + kg -1 . However, the soil ameliorants did not significantly affect the exch.-Mg level in the soil. In the same manner, the soil ameliorants significantly influenced soil CEC: rising from 7.80 cmol + kg -1 F0B0 (no coal ash applied, no humic substances Table 5. Effect of coal ash (F) and humat substances (B) application at various combinations of dosage levels on organic-C, total-N, and available-P content of the soil.  applied, no humic substances applied), up to 15.60 cmol + kg -1 at F2B0 (400 g polybag -1 applied, no humic substances applied). Moreover, the addition of coal ash raised available Fe in both nursery (NT) and field (FT) trials, with significant combinations effects. In contrast, the level of available Cu did not exhibit any significant reaction to the treatment in the nursery trial (NT). Table 7 shows the positive influence of the soil ameliorants on the levels of exch.-K and exch.-Ca , however, a reverse reaction was observed in the case of soil exch.-Mg level in the field trial (FT). Exch.-K went up a bit from 0.15 cmol + kg -1 to 0.17 cmol + kg -1 , while exch.-Mg rose in much higher proportion from 5.27 cmol + kg -1 at F0B0 (no coal ash applied, no humic substances applied) to 8.48 Table 6. Effect of coal ash and humic substances application at various combinations of dosage levels on the level of exchangeable cation, cation exchange capacity (CEC), and micro-element content of the soil in the Nursery Trial (NT). cmol + kg -1 at F1B0 (2.5 kg planting hole -1 applied, no humic substances applied). On the contrary, the exch.-Ca level exhibited a slight lowering from 3.79 cmol + kg -1 at F0B0 to 3.57 cmol + kg -1 at F1B0. In the field trial (FT), similar favorable effects were observed on soil CEC: its value increased from 12.60 cmol + kg -1 at F0B0 (no coal ash applied, no humic substances applied) to 15.80 cmol + kg -1 at F1B0 (2.5 kg planting -1 hole coal ash applied, no humic substances applied), which falls under the category of "moderate/average" under standard analytical result criteria (BPT 2005). Both independent and combinations positive effects of the two soil ameliorants were also recorded on available Fe levels in the soil, however, the reverse effect was found in the case of available Cu level. The analysis of data further demonstrated a distinguishable trend of exch.-Mg>exch.-Ca>exch.-K concentration in both nursery and field trials. According to Iskandar et al. (2008), the application of coal ash into peat soil could increase base cation availability including that of K, Na, Ca, and Mg. Other researchers, like Lee dan Bartlett (1976) and Reynolds et al. (1995) also found that the addition of lignite humic acid and lignite humic salt accelerated the release of phosphorous, potassium, and iron.

Response of Jabon Plant Growth to Coal Ash and Humic substances Application
The empirical consequence of the effects of the applied soil ameliorants particularly on the chemical properties of the soil was expected to be manifested by the response in the growth of the beneficiary Jabon plants in this study. These effects are depicted in Figures 2 and 3. Plant growth observations in the nursery trial (NT) covered the duration of 0-12 weeks after planting (WAP), while    in the field trial (FT), the monitoring period covered 0-20 WAP. Plant response parameters that were monitored were height and diameter growth rates. As shown in Figure 2, the addition of the two soil ameliorants (coal ash and humic substances) significantly increased the height growth of the Jabon seedlings in the nursery trial (NT), meaning the plants became appreciably taller and grew faster in height. However, as can be expected during this very early stage of physiological tree growth, the ameliorants did not pose any significant effect on the girth growth of the Jabon seedlings. Figure 3 depicts the impact of soil ameliorant application on the height and diameter growth of the beneficiary Jabon plants in the field trial (FT). As clearly shown, the Jabon trees were much taller and bigger in girth, and they continued to grow at much faster rates compared to the untreated (Control) group of plants in this study. In addition, and more importantly, it can also be noted that even without using humic substances, the sole application of coal ash, which is otherwise considered a hazardous and toxic waste material on mining landfills, could increasingly hasten Jabon tree growth, more so at higher ameliorant dosages.

CONCLUSIONS
This study has established fairly that the coal ash in the PTNNT mining landfill is highly alkaline and contains appreciable amounts of chemical elements that can favorably influence the chemical properties of the soil in reclaimed post-mining land. Although the coal ash also contains several heavy metals, they occur in levels which are far below the limits set by both Indonesian Government Regulation (PP No.85/1999) and the US Environmental Protection Agency (USEPA) and therefore, can be safely utilized as soil ameliorant.
In a nutshell, the following insights can be concluded from this study: Firstly, coal ash significantly raised field soil pH -from 5.49 (acidic) to 7.97 (moderately alkaline). However, by itself alone, humic substances did not exhibit any significant effect. Secondly, in this study, the sum of the organic-C and total-N levels originally present in the soil, and those contained in the soil ameliorants that were applied were way too low to manifest any significant effect. Thirdly, the application of coal ash and humic substances increased the level of available-P in the nursery -from 2.47 mg kg -1 (very low level) to 9.55 mg kg -1 (low level). In contrast, their effect in the field trial did not turn out to be significant enough. Fourthly, the application of coal ash significantly increased the level of exch.-K and exch.-Ca, but not exch.-Mg. The soil ameliorants also significantly influenced soil CEC: rising from 7.80 up to 15.60 cmol + kg -1 . Moreover, the addition of coal ash raised available Fe. In contrast, the level of available Cu did not exhibit any significant reaction to the treatment.
The practical consequence of the effects of the applied soil ameliorants particularly on the chemical properties of the soil would be manifested by the growth response in the beneficiary Jabon (Anthocephalus chinensis) plants. Thus, the addition of the two soil ameliorants (coal ash and humic substances) significantly increased the height growth of the Jabon seedlings in the nursery. However, the ameliorants did not pose any effect on the girth growth of the Jabon seedlings. In the field, due to the soil ameliorant application, the beneficiary Jabon trees grew much taller and bigger in girth, and continued to grow at much faster rate. More importantly, even without using humic substances, the sole application of coal ash, which is otherwise considered a hazardous and toxic waste material on mining landfills, can increasingly hasten Jabon tree growth, more so with higher dosages.