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Ayten Karaca, O.Can Turgay, Sevinç Arcak Soil Science Department, Faculty of Agriculture, Ankara University, Ankara ABSTRACT Ethylenediaminetetraacetic acid (EDTA), are persistent in the environment. The presence of EDTA in soil may alter the mobility and transport of Zn, Cd, and Ni in soils because of the formation of water soluble chelates, thus increasing the potential for metal pollution of natural waters. Mobility of metals is related to their extractability. To investigate metal extractability affected by EDTA, Zn, Cd, and Ni were added to 2 different type of soil samples (the fine-textured soil and the coarse-textured soil) at rates of 75, 1.5, and 4 mg kg-1 , respectively. Both natural and metal-amended soils were treated with Na2EDTA at rates of 0, 0.1, and 0.2 mg kg-1 . After 6 months of incubation, soil samples were extracted with 0.1N HCl, DTPA (diethylenetriamine-pentaacetic acid) and 1 M MgNO3 , the latter of which extracts the exchangeable form of metals. Results showed that Zn in all extraction increased with increasing rates of EDTA in the natural and metal-amended soils. In the natural soils, Ni in HCl extraction significantly increased with adding EDTA in the coarse-textured soils. In the metal-amended soils, Ni in all extraction increased with increasing rates of EDTA. In the natural soils, the presence of EDTA did not greatly affect extractibilty of Cd in all soils. In the metal-amended soils, Cd in all extraction increased with EDTA in the fine-textured soil. The presence of EDTA did not affect extractibilty of Cd in the coarse-textured soils. INTRODUCTION Synthetic chelating agents such as Ethylenediaminetetraacetic acid (EDTA) have been used in the past to decontaminate nuclear reactors and other material (Ayers, 1970; Piciulo, et al, 1986). EDTA is a powerful hexadentate chelating ligand. It is widely utilized for industrial purposes (e.g. metal treatment, photography, pharmaceutical products, industrial cleaning, textile, paper etc.), (Kari et al., 1995) when it is necessary to inactivate undesirable metal ions that could cause problems. It is estimated that approximately 20% of the total EDTA production are used in detergents and cleaning products (AIS, 1987). EDTA is not a phosphate substitute. The purpose of the EDTA content in laundry detergents is the stabilization of the perborate bleach, and there for it is used only in low levels (Alder et al., 1990). In agriculture, EDTA comlexes have been used for about 30 years as commercial soil amendments to improve micronutrient availability (Li and Shuman, 1996). The behavior of chelating agents in plants was described by Wallace et al, (1974). Attempts have been made to develop theoretical models from which the complexation of heavy metals by various chelates can be predicted and explained (Lindsay et al, 1967; Lindsay and Norvell, 1969; Halverson and Lindsay, 1977). EDTA forms highly stable water soluble complexes with a wide range of radionuclide and metal ions. It is persistent in the environment because it is resistant to decomposition by radiation (Kari et al., 1995) and rather slowly biodegradable in soil (Means et al. 1980, Bolton et al. 1993). Bolton et al. (1993) showed that only 15% of EDTA added to soil was degraded after 5 months, indicating that it could affect metals for a significant period of time. They also found that EDTA was not mineralized more rapidly or to a great extent in the surface soil than in the subsurface sediments. These results contrast with previous studies in which surface soils and organic C enriched soils had grater rates and amounts of EDTA (Tiedje, 1975, 1977). It is likely that chelate structure and/or the liability of the metal-chelate complex determine the resistance of the complex to mineralization. Albasel and Collenie (1985) and Yu and Klarup (1994) found that EDTA increased the yields in spite of the fact that it increases the soluble fraction of Fe, Zn, Mn, and Cu in soil. DTPA and 0.1N HCl extraction procedures are commonly employed to estimate the plant-available forms of micronutrients in soil. They are used equally to determine available forms of heavy metals such as Cd and Ni (Roca and Pomares, 1991). Shuman (1985) used 1M Mg(NO3)2 the exchangable fraction of metals in soil. This solution extracted the most labile form of metals in soil without disturbing other fractions. The presence of EDTA in soil may alter the mobility and transport of Zn, Cd, and Ni in soils because of the formation of water soluble chelates, thus increasing the potential for metal pollution of natural waters. Mobility of metals is related to their extractability. The aim of the experiment described here was to study the influence of EDTA on the extractability of Zn, Cd, and Ni in both natural and metal-amended soils. MATERIALS and METHODS Soil and Experimental Procedure : Two top-soils were taken from the experiment field of agriculture faculty of Ankara University. Soil samples were collected from the A horizon (0-20 cm) in uncultivated areas. Soil samples were air dried, sieved (2 mm), and stored. It were obtained pH of soil at 1:2.5 soil water suspension according to Richards (1954), organic material by using modified Walkley-Black Method (Jackson, 1958), grain size distribution by Bouyoucos (1951), and cation exchange capacity (CEC) was determined according to methods given in Black (1965). A solution containing soluble salts of Zn, Cd, and Ni were added a portion of each soil to investigate the extractability of metals in soils. The metals were applied in the form of ZnSO4.7H2O for Zn, CdCH2COOH for Cd, and Ni (NO3).6H2O for Ni at rates of 75, 1.5, and 4.5 mg kg -1 soil for Zn, Cd, and Ni, respectively. An aqueous Na2EDTA solution was applied at three rates (0, 0.1, 0.2 mg kg -1 soil) to 300 g of natural or metal-amended soil samples. After being mixed well, each sample was split into three 100-g replications and put into plastic pots. The pots were placed in the greenhouse and the water content of the soil was adjusted to 70% of field capacity. Throughout the six months, water losses exceeding 10% of the initial values were compensated for by addition of distilled water. Soil Extraction : Six months after the treatments were initiated, the soils were air-dried, ground and sieved through a 2-mm sieve. Both incubated and original soil samples were extracted for Zn, Cd, and Ni using the 0.1N HCl (Nelson et al, 1959), DTPA (0.005M DTPA+0.005M CaCl2+0.1M TEA (triethanolamine) pH 7.3), (Lindsay and Norvell, 1978), and 1 M Mg(NO3)2 (pH 7) (Shuman, 1985) solutions. The soil samples without treatments were digested in aqua regia ( Loon and Lichwa, 1973) to determine the total Zn, Cd, and Ni . All the solutions of Zn, Cd, and Ni were analyzed by atomic absorption spectrophotometry (AAS) with flame or graphite furnace when required. Minitab and Mstat computer programs were used for statistical analyses. RESULTS and DISCUSSION Physical and Chemical Characteristics of Soils : The soils were divided into two groups, based on soil texture. Averages of the clay percentage, organic matter content, pH, and CEC are 28.69 %, 1.78 %, 7.8, and 32.70 Cmol kg-1 for the fine-textured soil, and compared with 15.07 %, 2.5 %, 7.8, and 18.99 Cmol kg-1 for the coarse-textured soil. The fine-textured soil probably had greater adsorption capacity for metals than did the coarse-textured soil because of the amount of colloidal surface area. Higher total metal concentrations were found in the fine-textured soils than in the coarse-textured soil (Table 1). Compared with the total metal concentration, the proportion of metal concentrations in the HCl, DTPA, and Mg(NO3)2 extractions was relatively low in the fine-textured soil, suggesting that these metals in the fine-textured soils were less extractable in the natural soil profile. No significant relationship was found between either HCl, DTPA or Mg(NO3)2extractions and total concentration of any metal, indicating that metal extractability did not depend on the total metal contents.  Effect of EDTA on the Extractability of Zn : Extractable Zn concentration increased with increasing amounts of EDTA in both natural and metal-amended soils (Table 2). There was significant difference between rate of 0.2 mgkg-1 EDTA and zero EDTA rate, and between 0.1 mgkg-1 EDTA and the zero EDTA (P<0.05). The increase of Zn in the exractions with adding EDTA demonstrated that the presence of EDTA in soil elevated the extractability of this metal, probably because of an increase in metal solubility by forming soluble metal chelates. In the fine-textured soil, some treatments of EDTA did not show significant increases in the Mg(NO3)2 extractable Zn in the natural soil. The highest Zn concentrations were obtained with 0.1 N HCl and the lowest with Mg(NO3)2. DTPA extracted less Zn than 0.1 N HCl which found in other study (Brown et al, 1971). The extractable Zn was higher in the coarse-texture soil than in the fine-textured soil. Evidence of the importance of soil texture is hard to find in the literature. A high clay content is assumed to result in stronger adsorption and less plant uptake. Hansen and Tjell (1983) found that fine-textured soils reduce levels in soil solution and/or plant uptake. The adsorption of Zn by the soil can be influenced by the clay, CEC, organic matter and soil pH (Shuman, 1975). The results of the correlation of three extractants showed that 0.1N HCl, DTPA; and Mg(NO3)2 extractions were in agreement in all soils (Table 3). This proposals are supported by finding of Li and Shuman (1996). Effect of EDTA on the Extractability of Cd : The presence of EDTA did not greatly affect on the extractability of Cd in the natural soils. There was usually a significant difference between rate of 0.2 mgkg-1 EDTA and zero EDTA rate. However, there were fever instances of statistical significance between 0.1 mgkg-1 EDTA and 0.2 mgkg-1 EDTA and between 0.1 mgkg-1 EDTA and the zero EDTA (Table 2). In the metal-amended soils, Cd in all extraction increased with EDTA in the fine-textured soil (P<0.05). The presence of EDTA did not affect extractability of Cd in the coarse-textured soil. The HCl extractable Cd in the metal-amended soils was more than 50% of the total Cd (Table 1). The higher extractability with this extractant suggested Cd is a relatively mobile element (King, 1988). In the natural soil, extractable Cd was higher in the coarse-texture soil than in the fine-textured soil due to its high CEC and complexing ability. John (1974) found that as CEC increased, Cd bonding strength increased. Haas and Horowitz (1986) studied the adsorption of Cd to kaolinite in the presence of EDTA. They showed that EDTA had a negative effect on sorption of Cd. They attributed the latter to formation of an adsorbed organic layer on the clay surface and concluded that the effect of an organic ligand on Cd sorption varied with the nature of the metal-ligand and ligand-clay interactions. Thus, the strength of the Cd-ligand complex may be one of several parameters controlling the adsorption process. As the strength of the Cd-ligand bond increases, the ability of the added organic ligand to effectively compare for Cd also increases. Increased metal sorption may occur either by direct complexation of the metal or by the of a soil-ligand metal complex (Naidu and Harter, 1998). Elliott and Denney (1982) determined that EDTA had an inhibitory effect on the binding of Cd to whole soils. Similar results were reported by Fuji (1978).  The results of the correlation of three extractants showed that 0.1N HCl, DTPA, and Mg(NO3)2 extractions were in agreement in all soils (Table 3). This contradicts the results of Li and Shuman (1996), who reported that there was not a good correlation between Mg(NO3)2 extractable Cd and either Mehlich-1 or DTPA extractants in both natural and metal-amended soils.  Effect of EDTA on the Extractability of Ni : In the natural soils, Ni in all extraction increased with increasing rates of EDTA in the coarse-textured soils (Table 2). There was usually a significant difference between rate of 0.2 mgkg-1 EDTA and zero EDTA rate in the HCl and DTPA extractions (P<0.05). However, no statistical analyses were performed on the extractability of Ni in Mg(NO3)2 extraction. The presence of EDTA affect on the extratability of Ni in the fine-textured soil. There was a significant difference between rate of 0.2 mgkg-1 EDTA and zero EDTA rate. However, there was no significant difference between rate of 0.2 mgkg-1 EDTA and 0.1 mgkg-1 EDTA rate. In the metal-amended soils, the presence of EDTA significantly increased Ni in all extractions in both fine and coarse-textured soils (P<0.05). The results of the correlation of three extractants showed that DTPA and Mg(NO3)2 extractions were in agreement in the natural soils (Table 3). However, there were no significant correlations between DTPA and HCl and between HCl and Mg(NO3)2 extractants. In the metal-amended soils, HCl, DTPA, and Mg(NO3)2 extractions were in agreement. This proposals are supported by finding of Li and Shuman (1996). CONCLUSIONS The complexation of metals with EDTA reduced the activities of the free metallic ions in the soil solution, and, therefore, decreased toxicity of metals to plants. Adding EDTA increased the metal solubility, and raised the concentration of total cations in the soil solution. Therefore, EDTA in natural soils frequently enhanced the availability of micronutrients. The extractable Zn and Cd were higher in the coarse-texture soil than in the fine-textured soil. The adsorption of Zn and Cd by the soil can be influenced by the clay and CEC. The Mg(NO3)2 extractant was equivalent to HCl and DTPA in estimating the extractability of Zn, Cd, in all soils, but was less effective for Ni in the natural soils. REFERENCES AIS (Association International de la Savonnerie et de la Detergence, Bruxelles-Task Force). (1987). An assessment of the implication of the use of EDTA in detergent products. Albasel, N., Collenie, A. (1985). Heavy metals uptake from contaminated soils as affected by peat, lime and chelates. Soil Sci. Soc. Am. J. 49: 386-390. Alder, A.C., Siegrist, H., Gujer, W., Giger, W. (1990). Behavior of NTA and EDTA in biological waste-water treatment. Water Res. 24: 733-742. Ayers, J.A. (1970). Decontamination of nuclear reactors and equipment. The Ronald Press Company, New York. Black, C.A. (ed.).(1965). Methods of soil analysis, parts 1 and 2. Agronomy.9: 552-562. Am. Soc. Agron., Madison, WI. |