Highly-efficient persulfate activation by nitrogen-doped biochar derived from dewatered sewage sludge for 2,4-dichlorophenol removal: Process and mechanism

Sewage sludge represents a promising substrate, meriting investigation for its nutrient content and potential for repurposing. Nitrogen-doped biochar (NSC) was synthesized from dewatered sludge over a range of pyrolysis temperatures, spanning 400 to 800 ℃. This study explores the efficacy of NSC in enhancing the removal of 2,4-dichlorophenol (2,4-DCP) through the integrated processes of adsorption and persulfate (PS) activation, marking its inaugural investigation in this work. The adsorption behavior was best described by both the pseudo-second-order kinetic model and the Freundlich isotherm model. Under conditions with an unadjusted pH value, [PS] at 2 mM, [NSC] at 5 g/L, and [2,4-DCP] at 100 mg/L, the NSC800 (pyrolyzed at 800 ℃)/PS system achieved a complete removal of 2,4-DCP within 120 minutes. The results from radical quenching tests, electron paramagnetic resonance (EPR), and electrochemical analysis indicated that 2,4-DCP degradation occurred via both radical and non-radical mechanisms. Among the nitrogen species, graphitic N was identified as the predominant PS activation site. The NSC800/PS system demonstrated notable catalytic efficiency and stability in the removal of 2,4-DCP, maintaining its efficacy in the presence of Cl-, NO3-, H2PO4-, and CO32-. Significantly, the optimal reaction stoichiometric efficiency (RSE = 72.059%) was observed at the minimal PS concentration. This research introduces a novel methodology for municipal sludge valorization, proposing an economical biochar designed for the combined adsorption and catalytic degradation of organic pollutants.