Aluminum Detoxification by Humic Substance Extracted from Compost of Organic Wastes

Aluminum Detoxification by Humic Substance Extracted from Compost of Organic Wastes ( S Winarso, E Handayanto and A Taufiq):  Humic substance could control the activity of metal as well as exchangeable aluminum (Al exch ) on acid soil. The humic substance extracted from compost of agricultural waste is rarely conducted. The objective of research was to evaluate the effect of humic substance concentration and sources to pH and Al suspension. The research had been conducted in the Soil Laboratory of Agricultural Faculty of Jember University from April to July 2009. The humic substance used in these researches was extracted from compost of rice straw (RS), soybean straw (SS), cassava starch solid waste (CSSW), and empty bunch of oil palm tree (EBOPT). The humic substance concentration treatments were 0, 10, 20, 30, 40, 50, 75, 100, 150, 200, 250, and 300 ppm. Result of the analysis showed that characteristic of humic substance was difference depending on the source material used. The humic substance extracted from SS contain total base cations (K, Na, Ca, Mg) higher (1.17%) compared with CSSW (1.07%), OPT  (0.87%), and RS (0.69%). While the humic substances extracted from RS contain total organic acid (Acetic, Fumeric, cetoglutamic, Sucsinic, Propionic, Butiric, Ocsalic, and sitric acid) higher (157 ppm) compared with EBOPT (129 ppm), SS (115%), and CSSW (108%). Among these organic acids, acetic acid was highest concentration (> 34.51% of total acid). The addition of humic substance increased pH and decreased aluminum concentration of the suspension. The higher pH increment and Al concentration reduction took placed in the suspension treated with humic substance from SS compost. The correlation between decreasing Al concentration with humic substance concentration which explained chelation was higher (r = 0.97) than decreasing Al concentration with increasing pH which explained precipitation (r = 0.93). Based on these research results, it can be concluded that humic substance extracted from agricultural waste have a good prospect as a soil ameliorant to increase soil pH, detoxify soil aluminum, and increase soil P availability on acid soil.


INTRODUCTION
Previous researches showed that humic substance affect soil characteristic, water, and environment. The role of humic substance in the soil that affect or control the metal activities and other pollutant (like pesticide) were studied by previous researchers, for example Cu (Karlsson et al. 2008), Uranium (Singhal et al. 2006), Hg and Pb (Cruz-Guzma et al. 2003), and pesticide (Li et al. 2003).
Aluminum (Al) content in the soil is very high, but the solubility is very low. The Al solubility will increase as the soil acidity increase (Lindsay 1979). Aluminum concentration on Ultisol Kentrong Banten was more that 6.02 cmol kg -1 (Winarso 2009), and this concentration could be toxic to plant. High Al concentration not only has a negative effect on plant root growth, but also potentially retard nutrient absorbtion, like Mg (Sverdrup and Warfvinge 1993).
Humic substance decreases Al concentration in acid soil throught chelation prosess. Winarso et al. (2009) reported that humic substance extracted from rice straw compost decreased exchangeable Al on Ultisol through precipitation due to soil pH increased . and/or through chelation by functional group, especially carboxilate.
The objective of the research was to evaluate the effect of humic substance addition, that was extracted from compost of rice straw, soybean straw, cassava starch solid waste and oil palm tree, to pH and Al suspension.

Extraction of Humic Substance
The humic substance used in the research was extracted from the compost of rice straw (RS), soybean straw (SS), cassava starch solid waste (CSSW) and empty bunch of oil palm tree (EBOPT). Composting these organic materials and extracting the humic substance were conducted without any chemical addition that might affect humic substance characteristic. Aerobic composting was conducted by controling aeration and water content condition. The humic substance was extracted mechanically by pressing the composted material. Chemical extraction by addition of NaOH solution did non execute to prevent out side material to become part of humic substance compound. Characteristic of humic substance was based on organic acids analysis (acetic acid, citric acid, oxsalic acid, propionic acid, butiric acid, sucsinic acid, fumiric acid, and cetoglutamic acid ) using HPLC at Laboratory of PT Bogor Agro Lestari; pH (pH meter Jenway); C-organic (Walkey and Black method); N (Kjeldahl method); P, K, Na, Ca, Mg, Fe, Cu, Zn, and Mn (wet ashing with HNO 3 + HClO 4 , measur ed using Spectrophotometer for P and Atomic Absorption Spectrophotometer for the other ones).

Experiment Setup
The research consisted of two experiments. The first experiment aimed to evaluate the effect of addition of humic substance extracted from some agricultural wastes on suspension pH. The experiment was conducted at Soil Laboratory of Agricultural Faculty of Jember University on April to July 2009. The humic substance concentration used were 0, 10,20,30,40,50,75,100,150,200,250, and 300 ppm with total suspension volume of 100 ml. The suspension was shaken two hours a day during a week in ambient temperature, and then the suspension pH was measured.
The second experiment aimed to evaluate the effect of addition of humic substance extracted from some agricultural wastes on pH and Al suspension. The experiment was conducted in the same place and time as the first one. T he humic substance concentration used were 0,50,75,100,150,200,250, and 300 ppm. Aluminum used to express metal come from AlCl 3 pure analysis (pa) with concentration of 2 mM. The suspension was shaken two hours a day during a week in ambient temperature, and then the suspension pH was measured directly. Aluminum was measured based on titration method from N KCl extraction.

Data Analysis
All of humic substance concentration treatments were replicated 3 times and the value were average. Regression equations and pattern of two variables relationships were analyzed simplify with Microsoft Office Excel 2007.
Acetic acid, Fumiric acid, Cetoglutamic acid, Sucsinic acid, Propionic acid, Butiric acid, Oxalic acid, and Citric Acid are organic acid that contain double as well as single chain carboxylic functional group (-COOH) (Hart et al. 2003). If this functional group dissosiates it's proton (ion H + ), it becomes negative (anion compound) and it can chelate Al in the soil suspension (Essington and Anderson 2008).
The highest acetic acid concentration (carbon alifatic chain) (Table 1) indicates that decomposition process does not complete yet. Ait Baddi et al. (2003) reported that aromatic degree of humic substance extracted from compost of olive mill waste increased with increasing period of decomposition or the compost maturity. After 12 months decomposition, humic acid from this humic substance contains high aromatic structure and the reverse for alifatic structure. Winarso (2009) reported that humic substance from rice straw composted during a year contains sucsinic acid (aromatic and dicarboxcilate) higher (20.9%) compared with other organic acid.
The existing of organic acids containing functional group was in line with the hyphotesis from Stevenson (1982) and Buffle et al. (1977) about the model of humic acid and fulvic acid structure that are fraction of humic compound. Until the moment, there is no aggrement about humic acid and fulvic acid structures. Beside that, procedure of humic substance fractionation to humic acid, fulvic acid and humin; extraction and purification recommended by "The International Humic Substances Society" are based on chemically dillution process in the acid-base system. The pH of humic substance originated from compost of many agricultural wastes varied from pH 7.6 to 8.7, except from CSSW which pH value of 4.6 (Table 1). High pH seems correlated with high Ca, Mg, K and Na content (Table 1). Among the cation, Ca content was dominant (>70% of the total base cation) followed by Mg, K and Na. In case of Ca and Mg content, humic substance originated from compost of many agricultural wastes contains Ca and Mg ten times higher compared with K-Humat Plus 26% (produced comercially by Omnia Australia). It also contains other macro and micro nutrient (N, P, Fe, Zn, Mn, Mo, and B).
The composition of basic cation in the humic substance originated from compost of many agricultural wastes was better than the comercial one. The basic cation in the comercial humic substance dominated by Na rather than K, Ca, and Mg (Cerdan et al. 2007). Na is non esential element for plant, and on high concentration in the soil it can destroy plant root due to plasmolysis. Beside that, it also can disperse soil coloids and therefore soil become sensitive to erosion and compaction.
Based on these characteristic, the humic substance originated from compost of many agricultural wastes might have multiple function: it could be as fertilizer, acid soil ameliorant, and for amelioration of soil contaminated by metal around the mining area.

Aluminum Detoxification by Humic Substance
Addition of humic substance originated from compost of RS, SS and EBOPT increased pH suspension as humic substance concentration increased (Figure 1). Higher pH increament took place at concentration below 50 ppm, while pH level at concentration above 50 ppm to 350 ppm was relativly constant (pH about 8.5). Addition of humic substance originated from compost of CSSW could not change suspension pH, since it had low pH (pH = 4.6). Humic substance originated from compost of SS had higher pH (8.7) and therefore increased pH higher than the other one. It means that humic substance originated from compost of SS, RS and EBOPT can be used as acid soil amandement like Ultisol and Oxisol.
Addition of humic substance originated from SS at concentration of 200 ppm caused initial Al concentration in the suspension as much as 2 mM became undetactable (value = 0) (Figure 2). Significant decreased in Al concentration also took placed by addition of humic substance originated from the other sources. Aluminum concentration reduction was caused by precipitation as Al(OH) 3 due to pH increament, and/or chelation (linkage by more than one deprotonized functional group, especially carboxilate and some phenolate or other ones). This phenomenon indicated that humic substance originated from compost of agricultural wastes is highly potential to netralize Al.
The correlation value between extracted Al with humic substance concentration indicated that reduction in Al concentration was caused by chelation by functional groups of humic substance and then precipitated due to pH increment (Figures 2 and 3). Among the correlation values, correlation value between concentration of humic substance from compost of SS with extracted Al was higher (r = 0.97). Relationship between humic substance concentration and extracted Al was quadratic, and equation with higher determination coefficient was from RS, i.e. y p =7E-06 X 2 -0.004 X + 0.808; R² = 0,947. Skyllberg (1999) stated that complex Al-organic matter valid at pH < 4.2 and in the unsaturated Al(OH) 3 solution.
Addition of 2 mM AlCl 3 in the water decreased solution pH to nearly 4. Addition and humic substance concentration increment increased pH, except in the treatment with addition of humic substance originated from compost of SS (Figure 3). This result was in line with the previous research without any Al addition Al (Figure 1). Figure  Concentrations of Humic Substance (ppm) y = 2E-05x 2 -0.007x + 0.998 R 2 = 0.908 from suspension containing humic substance originated from some compost sources and 2 mM AlCl 3 . The equation between these two variables was quadratic, but had lower coefficient determination (R 2 ) compared with R 2 between humic substance and Al ( Figure 2). The highest correlation value took place between humic substance originated from SS compost (r = 0.93), followed by humic substance originated from EBOPT compost (r = 0.90), RS compost (r = 0.58) and CSSW compost (r = 0.45). Based on these explanations, correlation value, and relationship between Al and humic substance concentration indicated that reduction of Al concentration was primarily due to chelation by functional groups of humic substance rather than by precipitation due to pH increment. This close relationship between humic substance concentrations with Al measured at pH in the range of 4 to 5.5. Winarso (2009) showed that Al detoxification on Ultisol (Al -exch undetected) by humic substance and CaCO 3 effective on pH at about 5.5.
The results indicated that Al chelation and precipitation by humic substance originated from some agricultural wastes compost can be used to control metal element on land metal-contaminated. So that, it needs further research to know effective concentration on certain pH and certain metal element, since low humic substance concentration will promote metal mobilization especially in the water canal.

CONCLUSIONS
Characteristic of humic substance was difference depending on the source material used. The humic substance extracted from soybean straw contains total basic cations (K, Na, Ca, Mg) higher (1.17%) than that extracted from cassava starch solid waste (1.07%), empty bunch of oil palm tree (0.87%), and rice straw (0.69%). Humic substance extracted from rice straw contains total organic acid (Acetic acid, Fumeric acid, Cetoglutamic acid, Sucsinic acid, Propionic acid, Butiric acid, Oxalic acid, and Citric acid) higher (157 ppm) compared with the other. Among these organic acids, acetic acid was the highest concentration (>40% of total acid) except in the humic substance from rice straw compost (34.51%).
The addition of the humic substance decreased aluminum concentration of the suspension. Higher reduction of Al took place in the suspension treated with 200 ppm humic substance from soybean straw compost. Al detoxification by humic substance in various pHs below 5.5 levels primarily due to chelation rather than precipitation.
Further research to identify optimum concentration of humic substance to detoxify metal element in the soil and phosphorous desorption in acid soil. ACKNOWLEDGMENT Thanks a lot to Muhhamad Ilham on his help in conducting this research. This paper is part of applied research result founded by KMNRT 2009.