Copper Solubility in Myersville B Horizon Soil in the Presence of DTPA

Organic ligands greatly affect the speciation and solubility of heavy metals in soils, depending on complex interactions of pH, ligand-tornetal ratio, and the order of application of metals and ligands to soils. To understand how these variables influence the fate of metals added to soil, we investigated the solubility of Cu(II) in Myersville B horizon soil (Ultic Hapludalf) in the presence of diethylenetriaminepentaacetic acid (DTPA), a strong complexing agent. Copper solubility was studied as a function of soil pH, individual and relative concentrations of Cu + and DTPA, and the order of application of these components to the soil. In the presence of DTPA (1.0 and 10 mmol kg" soil), Cu(II) solubility (0.1 and 1.0 mmol kg' soil) increased with an increase in pH (4 to 7). However, this occurred only when DTPA concentration > Cu(II) concentration, implying that the relative concentrations of DTPA and Cu(II) were critical in controlling Cu(II) solubility. The order of application of Cu(II) and DTPA to the soil also strongly influenced Cu solubility. The three orders of application of these components were (i) application of premixed stock solutions of Cu(II) and DTPA in specified ratios and concentrations to the soil, (ii) equilibration of Cu(II) with the soil followed by application of DTPA, and (Hi) equilibration of DTPA with soil followed by Cu(Il) application. Of these three, the highest Cu solubility was observed in the third case when DTPA > Cu(IT). This was attributed to surface coordination of DTPA, which prevents Cu from binding with soils. Also, an increase in background electrolyte concentration was accompanied by a decrease in the pH and, therefore, a change in Cu(II) solubility. These results have several implications in the field of contaminant and nutrient management in soils. T TOXICITY AND BiOAVAiLABiLiTY of certain heavy metals such as Cu, Fe, Zn, and Mn depends on their activity and speciation in soils (Lehman and Mills, 1994; Buffle et al., 1990; Adriano, 1986; Brown et al., 1985). The ultimate fate of these metals is dependent on their dynamic interactions with various soil components, especially the organic fractions (Harter and Naidu, 1996; McLean and Bledsoe, 1992; Evans, 1989; McBride, 1989). Solubility of heavy metals in soils and mineral oxide surfaces as a function of pH is often dictated by the presence of organic and inorganic ligands (Harter and Naidu, 1996; Sparks, 1995; McBride, 1989). Sorption of heavy metals onto mineral oxide surfaces and soils typically increases with an increase in pH in the presence of inorganic ligands. Organic ligands such as acetate, oxalate, and nitrilotriacetate have been found to promote sorption at low pH (Elliott and Huang, 1980, 1979), and it has been found that the presence of ligands such as EDTA and DTPA enhances heavy metal solubility with a corresponding increase in pH (Harter and Naidu, 1996). V.M. Vulava, Inst, of Terrestrial Ecology (ITO), Swiss Federal Inst, of Technology (ETH), Grabenstrasse 3, CH-8952, Schlieren, Switzerland; A. Torrents, Dep. of Civil Engineering, and B.R. James, Dep. of Natural Resource Sciences and Landscape Architecture, Univ. of Maryland, College Park, MD 20742. This work was performed at the Univ. of Maryland. Joint contribution from the Colleges of Engineering and Natural Resource Sciences, Univ. of Maryland. "Corresponding author ([email protected]. ethz.ch). Published in Soil Sci. Soc. Am. J. 61:44-52 (1997). This study concerns itself with metal-organic interactions in soils. Heavy metals have been known to be incorporated into soils due to various waste disposal practices, environmental pollution, or fertilizer application. Inputs of organic acids in soils are seasonal in nature due to plant growth or due to waste disposal practices. Metals are commonly added to agricultural production systems as chelates (Mortvedt et al., 1991). Organic-rich metal wastes might enter the soil as components of sewage effluent or sludge. Organic acids from root exudates come in contact with metal-rich soils (Mortvedt et al., 1991). Weathering of soil minerals by organic acids added to metal-rich soil systems are commonly encountered (Stumm and Wollast, 1990). Finally, the addition of metals to organic-rich systems in which the organic acids have already reacted with the soil could affect the fate of the metals in that soil. Thus, the solubility and degree of complexation of metals in any soil system may depend on whether organic ligands or metals react with the soil first, i.e., the order of application of the components to the soil. This behavior may be simulated to some extent using a laboratory soil system in the presence of low molecular weight organic acids (such as EDTA and DTPA). These organic acids have functional groups similar to those found in humic and fulvic acids (such as carboxylic acid and amino groups). The following have been found to affect the sorption behavior of metals onto mineral oxide surfaces in the presence of organic acids: pH, heavy metal and organic acid type (Elliott and Huang, 1980, 1979; Davis and Leckie, 1978; Vuceta and Morgan, 1978), the relative concentrations of the metals and organics (Elliott and Huang, 1979), their individual concentrations (Basta and Tabatabai, 1992a,b) the ionic strength (Elliott and Huang, 1985), and the order of application of each of these components to the medium (Bryce et al., 1994). The results from this study are specific to Cu solubility in Myersville B horizon soil in the presence of DTPA. Copper is a very important element found in terrestrial systems. It is an essential micronutrient for plants, and is also required in metabolic processes of various organisms (Flemming and Trevors, 1989; Adriano, 1986). Though Cu is considered nontoxic to mammals, it has been found to be extremely toxic to aquatic biota at higher concentrations (Flemming and Trevors, 1989; Adriano, 1986). Copper is also a high-priority pollutant on U.S. Environmental Protection Agency's assessment lists (Lehman and Mills, 1994). A very strong chelating agent, DTPA has been commonly used in the assessment of long-term availability of metals exposed to plants in soils (O'Connor, 1988; Lindsay, 1979). Myersville B horizon soil contains large amounts of Si, Al, and Fe oxides that Abbreviations: AEC, anion-exchange capacity; CEC, cation-exchange capacity; DTPA, diethylenetriaminepentaacetic acid; EDTA, ethylenediaminetetraacetic acid; NOM, natural organic matter; ZPC, zero point of charge.