Bioavailability and Speciation of Cd, Pb, Cu, and Zn in Sewage Irrigated Soils in the Southeastern Suburbs of Beijing

The bioavailability of trace metals in sewage irrigated soils in the southeastern suburbs of Beijing, China was examined by using in vitro digestive fluid extraction method. A sequential chemical extraction (SCE) procedure was used to measure the speciation of trace metals in soil samples, including Cd, Pb, Cu, and Zn. The average concentrations of Cd, Pb, Cu, and Zn were 2.23, 37.9, 59.9, and 127.0 mg kg-1, respectively, indicating that the heavy pollution of soils was caused by Cd, Cu, Pb, and Zn which were mainly distributed in the residual phase, whereas Cd was mostly in the carbonate phase. The extraction efficiencies of Cd, Pb, Cu, and Zn quantified by the in vitro digestive fluid extraction method, were lower than those by the SCE procedure, and the extraction efficiencies of both methods followed the same pattern: Cd >Zn>Pb>Cu. The amount of trace metals extracted by using these two methods were positively correlated. Therefore, the in vitro digestive fluid extraction method can be used to examine the bioavailability of soil-bound trace metals.