Speciation and Ecological Risk Assessment of Heavy Metal(loid)s in the Municipal Sewage Sludge of China

Abstract

To reveal the speciation and pollution status of heavy metal(loid) s (HMs) in the dewatered sewage sludge(SS) of municipal wastewater treatment plants(MWTPs) in China, SS samples were collected from 40 MWTPs located in different regions of China. The total concentrations and geochemical fractions of As, Cd, Cr, Cu, Ni, Pb, and Zn in the SS samples were analyzed. The ecological risks induced by HMs in the SS were assessed based on the toxicity characteristic leaching procedure(TCLP), risk assessment code(RAC), and potential ecological risk index(RI). The median values of HMs in SS followed the order Zn > Cu > Cr > Pb > Ni > As > Cd. The general attainment rates of HMs in SS were satisfactory(>90%). As showed a generally even distribution among all the fractions; Cd and Zn mainly existed in the reducible fraction; Cr mainly occurred in the residual fraction, followed by oxidizable and reducible fractions; Cu mainly occurred in the oxidizable fraction; Ni occurred mainly as exchangeable forms; and Pb mainly occurred in the reducible and residual fractions, with its exchangeable form being the lowest fraction. According to the RAC method, the environmental risks induced by HMs in the SS followed the order Ni > As > Zn > Cd > Cu > Cr > Pb. The ecological risk of Ni was high; Zn, As, and Cd had a medium level of risk; Cr and Cu had a low level of risk; and the risk of Pb was negligible. According to the RI method, Cd and Cu showed the highest potential ecological risk, indicating that these are the primary HM pollutants in the SS. However, if the agricultural application of SS is carried out in accordance with the national standards, the overall level of risk from soil HM pollution is considered relatively low.