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℃ and 270℃) 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.
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