Enhanced immobilization of cadmium and lead adsorbed on crop straw biochars by simulated aging processes

Aging is an important natural process affecting the physiochemical properties of biochar, while mechanistic understanding of its effect on the adsorbed heavy metals is still lacking. After adsorption of Cd2+ and Pb2+, biochars produced from wheat straw (WS) and maize straw (MS) at 300 and 500 °C (denoted as WS300, WS500, MS300, and MS500, respectively) were subjected to 60 cycles of wet-dry or freeze-thaw aging. The results showed that simulated aging treatment transformed the Cd2+ and Pb2+ adsorbed on the low-temperature biochars from the readily and potentially bioavailable fractions into the non-bioavailable one, while the fractionation of Cd2+ and Pb2+ adsorbed on WS500 and Pb2+ on MS500 barely changed. Spectroscopic characterization revealed that simulated aging enhanced the complexation of Cd2+ and precipitation of Pb2+ on the biochars. These findings suggest that heavy metals could be effectively immobilized on low-temperature biochars amended to contaminated soils in the long term.