Immobilization of Cd and phosphorus utilization in eutrophic river sediments by biochar-supported nanoscale zero-valent iron.

The Ulansuhai River is polluted by heavy metals and faces a serious problem of eutrophication. According to a previous study, biochar-supported nanoscale zero-valent iron composite (BC-nZVI) can be used to effectively immobilize heavy metals, converting Cd from labile to a stable fraction. The present study aimed to evaluate the immobilization of Cd in sediments of eutrophic rivers by BC-nZVI and investigate the effects of phosphorus on Cd immobilization in BC-nZVI immobilized sediments. The immobilization of Cd with BC-nZVI as a stabilizer at different KH2PO4 solution concentrations, the available phosphorus in the sediments, the total phosphorus in the overlying water, and the changes in the pH of the sediments were investigated. The changes of available phosphorus in sediments after the addition of BC, nZVI and BC-nZVI stabilizers were also studied. Results showed that the presence of phosphorus could promote the immobilization of Cd in sediments. The content of total phosphorus in overlying water was reduced, precipitates of phosphate and Cd were produced, and the available phosphorus in sediments was increased after the addition of BC-nZVI. The pH of sediments increased along with the increase in incubation time, which is beneficial for Cd immobilization. This study proved that (1) BC-nZVI can effectively immobilize Cd in eutrophic river sediments, (2) the presence of phosphorous in overlying water is conductive to the conversion of Cd from labile fractions to stable fraction in the sediment, and (3) adsorption and precipitation may be the main mechanisms in Cd immobilization.