Assessment of Heavy Metal Equilibria in Sewage Sludge‐Treated Soil

Abstract

AbstractThe objective of this research was to determine the effects of sewage sludge on speciation of metals in the soil solution. Sewage sludge was applied to an acid soil (pH ≈ 5) as a single treatment in 1971, or as annual treatments from 1971 through 1974 in amounts totaling from 50 to 800 t/ha. Surface soil samples were obtained from the plots in 1977 and the soil solutions were analyzed for Zn, Cd, Cu, Ni, Pb, Mn, Fe, Ca, Mg, Al, Na, K, PO4, SO4, Cl, pH, and organic C. The metal and ligand species in the soil solutions were calculated with the computer program GEOCHEM. Application of sludge increased soil solution concentrations of Zn, Mn, and Cd, while Cu, Ni, and Pb were present in most solutions at levels below the detection limits of the analytical methods used. Cadmium, zinc, and manganese in the soil solution appeared to exist predominately as the free ion. Complexation by inorganic and organic ligands ranged from 9 to 37%, 3 to 22%, and 3 to 31% of the total soluble Cd, Zn, and Mn, respectively, depending on the model used to represent metal interaction with soluble organic C. Cadmium was complexed with organic ligands to a greater extent than were zinc or manganese. Sulfate, phosphate, and chloride formed inorganic complexes with Cd, Zn, and Mn, but they constituted < 10% of the total metal present. Speciation of Zn and Cd into free metal ion and complexed forms was essentially the same in the soil solution, regardless of metal concentrations in solution and rate or frequency of sludge application. The solubility and DTPA extractability of Cd were increased to a greater extent by a single application of sludge than by multiple, annual additions.