Simultaneous sorption of Cd, Cu, Ni, Zn, Pb, and Cr on soils treated with sewage sludge supernatant

Disposal of sewage sludge creates the potential for heavy metal accumulation in the environment. This study assessed nine soils currently used as Dedicated Land Disposal units (DLDs) for treatment and disposal of municipal sewage sludge in the vicinity of Sacramento, California. Adsorption characteristics of these soils for Cd, Cu, Ni, Zn, Pb, and Cr were studied by simultaneously mixing these elements in the range of 0-50 mu mol L(-1) with sludge supernatant and reacting with the soil using a soil:supernatant ratio of 1:30, pH = 4.5 or 6.5, and constant ionic strength (0.01 M Na-acetate). The concentration of metals in the supernatant was determined after a 24 hr equilibration period. Adsorption isotherms showed that metal sorption was linearly related to its concentration in the supernatant solution. The distribution coefficient K-d (K-d = concentration on solid phase/concentration in solution phase) was computed as the slope of the sorption isotherm. The distribution coefficients were significantly correlated to soil organic matter content for Ni, Cu, Cd, and Pb at pH 4.5 and for Ni, Cu, Zn, and Cd at pH 6.5. There was also a correlation between K-d and soil specific surface area but no relationship to other soil properties such as CEC, clay content, and noncrystalline Fe and Al materials. Therefore, soil organic carbon and surface area appear to be the most important soil properties influencing metal adsorption through formation of organo-metal complexes. The K-d values for all elements were higher at pH 6.5 than at 4.5. Selectivity between metals resulted in the following metal affinities based on their Kd values: Pb>Cu>Zn>Ni>Cd approximate to Cr at pH 4.5 and Pb>Cu approximate to Zn>Cd>Ni>Cr at pH 6.5.