Manganese enhances the immobilization of trace cadmium from irrigation water in biological soil crust

Abstract

The effect of biological soil crust (BSC) in paddy field on the immobilization and removal of heavy metal from irrigation water is an important issue. BSC was cultured in solutions with different concentrations of manganese (Mn) salt and cadmium (Cd) sulfate for 15 days. We analyzed the Mn, Cd and Fe contents in the BSC and investigated the effects of Mn salt on the Cd distribution in different binding-forms in BSC as well. The results show that Mn salt was effective at enabling BSC to immobilize the Cd, and its removal efficiency from irrigation water improved with an increase in the Mn concentration used. The removal of 50.00 μg/L of Cd from irrigation water by BSC reached as high as 95.70% in present of 20.00 mg/L Mn. The highest obtained biological concentrated factor of BSC for Cd is ~2.7 × 104. The mainly Cd species (75%) in BSC is the non-EDTA extracted minerals. Based on the SEM-EDS and XPS analyses, it was reasonably inferred that the Mn ion was oxidized by Mn oxidizing bacteria (MOB), to yield the porous spongy-like birnessite with d-spacing of 2.31 Ǻ, while Cd was scavenged and immobilized in the crystal lattice. The MOB was identified as Bacillus. This study provides a potentially novel method to decontaminate irrigation water polluted with Cd by using BSC in presence of Mn.