Economical　and　eco-friendly　chlorine-based　advanced　oxidation　processes　(AOPs)　have　been　urgently　demanded　to　enhance　the　green　abatement　of　emerging　micropollutants.　Herein,　sludge-derived　biochar　(SBC)　was　innovatively　introduced　to　activate　free　available　chlorine　(FAC)　to　enhance　the　elimination　of　diclofenac　(DCF)　among　pH　5.0–11.0.　Pyrolysis　of　municipal　sludge　endowed　biochar　with　superior　structure,　various　chemical　compositions,　and　electronic　properties,　among　which　carbon-centered　persistent　free　radicals　(PFRs)　were　verified　as　the　dominant　active　sites　for　enhanced　chlorination　based　on　correlation　analysis.　Electron　spin　resonance　and　electrochemical　tests　firstly　verified　that　PFRs　could　mediate　electron　transfer　toward　FAC　and　dissolved　oxygen　to　generate　multiple　reactive　species　of　•OH,　ClO•,　1O2,　and　O2•−.　Further　investigation　revealed　that　enhanced　DCF　chlorination　relied　on　both　radical　attack　by　•OH,　ClO•,　O2•−　and　direct　electron　transfer.　Meanwhile,　the　regeneration　of　PFRs　mediated　by　FAC　and　electron-rich　DCF　greatly　promoted　the　reusability　of　SBC800.　SBC800/FAC　system　was　much　superior　to　sole　chlorination　in　terms　of　toxicity　reduction　and　anti-interference　capacity.　This　study　innovatively　introduced　SBC　as　a　green　and　sustainable　catalyst　to　enhance　chlorination　efficiency,　and　provided　new　insights　into　the　activation　potential　and　mechanism　of　PFRs　for　chlorination.　©　2024　Elsevier　B.V.