Solid Phase Forms of Heavy Metals in Sewage Sludge‐Treated Soils

Abstract

AbstractA sequential extraction procedure was used to fractionate Cd, Cu, Ni, and Zn in sludge‐treated soils into the designated forms of exchangeable, adsorbed, organically bound, carbonate, and residual. Samples were obtained from a soil column study where anaerobically digested sewage sludge in either liquid or air‐dried form was mixed into the top 15 cm of three reconstructed soil profiles and leached for 25 months with Colorado River water. The applied sewage sludge and uncontaminated soil samples were also sequentially extracted. The metals added into the soil had not moved out of the sludge‐soil layers during the course of leaching. The chemical forms present in the sludge‐soil layers at the termination of the leaching process were compared with those initially present in the soil and sludge. Most of each of the metals studied in the columns were found in the organically bound, carbonate, or residual forms, with the relative distribution among these forms depending on whether samples were taken in or below the sludge‐soil layer. Less than 36% of each metal in the sewage sludge was in the residual form, while, except for Cd, the soils contained > 65% of each metal studied in this form. It appeared that Cd, Ni, and Zn were all shifting to the residual form. The chemical forms of Cu had not changed significantly during the study. The occurrence of metals in the stable organically bound, carbonate, and residual forms in the sludge, coupled with a shift toward the more stable form (residual) after soil incorporation, contributed to the lack of metal movement in the soil profiles.