Predicted Solution Phase Forms of Heavy Metals in Sewage Sludge‐Treated Soils

Abstract

AbstractChemical speciation of selected elements in the soil solution of sewage sludge‐treated soils was studied with aid of the computer program, GEOCHEM. Soil solutions were obtained by the saturation extract method from reconstructed soil profiles where anaerobically digested sewage was incorporated into the top 15 cm of the soil and the soils were leached for 25 months. Total concentrations of Cd, Cu, Ni, and Zn in the soil solutions were elevated by the sewage sludge addition, but in most cases were within ranges found in a survey of soils untreated with sludge. Concentrations of the metals were so low that their contribution to metal movement over the duration of the study was negligible. Concentration changes in solution metals across the sludge‐soil interface in most columns was dramatic. For Cd, Ni, and Zn between 50 and 60, 60 and 70 and 60 and 70%, respectively, of the total metal in solution was found to be in the free ionic form. Copper was almost exclusively in organically compelxed forms in sludge‐soil layers, and below these layers the form of Cu shifted to the free ionic form as the pH decreased. Activities of the metals indicated undersaturation with respect to established minerals in soils.