Decontamination of Cr(VI) from water using sewage sludge-derived biochar: Role of environmentally persistent free radicals

Biochar is a well-known material for pollutant removal owing to its low cost and rich surface functionality. A kind of highly active substance, called environmentally persistent free radicals (EPFRs), can be produced in the preparation process of biochar, playing an important role in the removal of pollutants. In this study, sludge-derived biochars (SBC 120 and SBC 270 ) were prepared by the hydrothermal carbonization under two temperatures (120 °C and 270 °C) to investigate their removal abilities of Cr(VI). The maximum removal amounts of Cr(VI) by SBC 120 and SBC 270 were 16.58 and 22.93 mg·g −1 , respectively. It was further revealed that the appearance of Cr(III), as a result of EPFRs on sludge-derived biochar (SBC) transferred electrons to Cr(VI) in neutral solutions. That is to say, oxygen-centered (O-centered) EPFRs on SBC 120 and carbon-centered (C-centered) EPFRs on SBC 270 all could be used as electron donors to Cr(VI) to make it become Cr(III). This study not only provides a theoretical basis for the mechanism of pollutants removal by sludge-derived biochar but also offers a new perspective on the direct effect of EPFRs on pollutants.