Freezing-accelerated removal of chromate by biochar synthesized from waste rice husk

Abstract

The application of biochar has been considered a promising method for remediating contaminated water, as biochar exhibits a redox activity for environmentally relevant redox reactions. Although the mechanisms of various redox reactions by biochar in water have been widely investigated, investigations of reaction in ice have not been attempted. In this study, the freezing-accelerated removal of chromate (Cr(VI)) by biochar synthesized from waste rice husks (RH-BC) was investigated in water (25 °C) and ice (-20 °C). The reduction of Cr(VI) with RH-BC was insignificant in water, whereas an enhanced reduction efficiency of Cr(VI) was observed in ice due to the freeze concentration phenomenon. The enhanced redox reaction between Cr(VI) and dissolved organic matter (DOM) is primarily responsible for the accelerated Cr(VI) reduction in ice, wherein DOM is a primary component of RH-BC. Experiments on various conditions of pH and RH-BC concentrations reveal that Cr(VI) is heavily reduced at low pH values and an aggregation of RH-BC in ice can inhibit the reduction efficiency of Cr(VI) due to a decrease in active sites. The removal of Cr(VI) by RH-BC was successfully achieved with real Cr(VI)-contaminated wastewater in ice; this elucidated the environmental relevance of freezing-assisted Cr(VI) removal in cold regions.