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　(SBC120　and　SBC270)　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　SBC120　and　SBC270　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　SBC120　and　carbon-centered　(C-centered)　EPFRs　on　SBC270　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.　©　2022　The　Chemical　Industry　and　Engineering　Society　of　China,　and　Chemical　Industry　Press　Co.,　Ltd