Regulating the surface structure of sludge biochar by mixed-acid washing to enhance the activation of peroxymonosulfate for sulfamethoxazole removal

The exploitation of efficient sludge-derived biochar (SBC) is of great significance for the oxidative degradation of pollutants and resource recovery of waste. This study innovatively synthesized deashed biochar (DBC) by pyrolyzing municipal sludge followed by an acid washing process using a mixture of 10% hydrofluoric acid and 1 M hydrochloric acid. The mixed-acid treatment removed inorganic minerals from biochar, obviously improving the specific surface area (from 81.909 m²/g to 405.794 m²/g), average pore size (from 5.513 nm to 7.412 nm), and surface functional group content of the biochar. The DBC was utilized to activate peroxymonosulfate (PMS) for degrading sulfamethoxazole (SMX). It demonstrated markedly improved performance compared to the SBC. 59.1% of SMX was adsorbed in initial 30 min, and > 95% of SMX was degraded within the subsequent 20 min following PMS addition. Furthermore, DBC had high catalytic activity across a broad pH range, while its performance was hindered by HCO₃⁻. Mechanism analysis indicated that the effective degradation of SMX in SBC800/PMS and DBC/PMS systems was mainly achieved through a non-radical pathway dominated by singlet oxygen (¹O₂). The CO, graphitic N, and pyridinic N on DBC induced the formation of electron-deficient site C⁽⁺⁾, facilitating effective extraction of electrons from PMS, thus enhancing the production of ¹O₂. This study provides a new perspective for addressing the issue of low catalytic activity in biochar derived from high-ash sludge.