Heavy metal contents and chemical speciations in sewage-irrigated soils from the eastern suburb of Beijing, China

Accumulation of heavy metals in agricultural sewage-irrigated soils has originated increasing concern. This study analyzed the total concentrations and chemical speciations of heavy metals including Cd, Cr, Cu, Zn and Ni in sewage-irrigated soils in the eastern suburb of Beijing, China. Here Tessier sequential extraction was adopted to analyze the chemical fractionations, and their concentrations were determined by ICP-MS. The results showed that there was remarkable buildup of Cd, Cr, Zn and Cu in sewage-irrigated topsoils compared to reference topsoils. Besides, the total Cd and a part of Ni were beyond Chinese agricultural soil environmental quality criteria. In sewage-irrigated soils, Cd, Cr, Ni, Zn and Cu were dominated by residual fractions and few was present in exchangeable and carbonate fractions as a result of higher soil pH. The order of Cu, Zn and Ni in sewage-irrigated topsoils in each fraction was residual > organic > Fe-Mn oxide > exchangeable > carbonate, for Cd in sewage-irrigated soils the order was residual > Fe-Mn oxide > organic > exchangeable > carbonate and for Cr in sewage-irrigated soils the order was residual > organic > Fe-Mn oxide > carbonate > exchangeable. However, for Cu and Ni in sewage-irrigated subsoils the order was organic > residual > Fe-Mn oxide > carbonate > exchangeable; for Zn the order was organic > residual > exchangeable > Fe-Mn oxide > carbonate, and for Cd the order was residual > exchangeable > organic > Fe-Mn oxide and carbonate. The mobility and bioavailability of the five metals declined in the following order: Zn, Cu, Ni, Cd and Cr. However, compared to other four metals, Cd was more mobile and bioavailable in the sewage-irrigated topsoils than in the reference topsoils. Higher Cd contents in sewage-irrigated soils may constitute potential risk on food security and human health.