Unraveling the intrinsic activity origin of sludge-derived biochar for boosted chlorination of diclofenac: Key role of persistent free radicals

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 • , 1 O 2 , and O 2 •− . Further investigation revealed that enhanced DCF chlorination relied on both radical attack by •OH, ClO • , O 2 •− 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.