Effect of organic matter oxidation on the fractionation of copper, zinc, lead, and arsenic in sewage sludge and amended soils

Long-term land application of sewage sludge (SS) has caused concern over the potential release of trace metals into the environment following the degradation of organic matter (OM). This study was performed to assess the impact of OM degradation on the relative distribution of Cu, Zn, Pb, and As in SS and SS-amended soils. Three SSs of different ages and two soils treated with SS were subjected to incubation and direct chemical oxidation using diluted HO, followed by a sequential extraction. The majority of Cu, Pb, and As were bound to OM, whereas the majority of Zn was bound with Fe/Mn oxides for all three SSs. Incubation of SS for 6 mo did not result in a substantial decrease in OM content or a change in the relative distribution of Cu, Zn, Pb, and As. Direct OM oxidation to 30 and 70% by diluted HO resulted in a significant decrease in organically bound Cu but increased its exchangeable, carbonate-bound, and Fe/Mn-bound fractions. Oxidation of OM slightly decreased organically bound Zn but significantly increased exchangeable Zn in all SSs. Oxide- and carbonate-bound Zn also decreased following OM oxidation. Exchangeable fractions of As and Pb were minute before and after OM degradation, indicating that release into the environment would be unlikely. The relative distribution of Cu, Zn, Pb, and As in SS-treated soils was similar to that of SS, suggesting a dominant role of SS properties in controlling metal distribution following OM oxidation. Overall, OM oxidation increased the mobility and bioavailability of Zn and Cu, whereas it had less impact on Pb and As.