We report the synthesis of H2SO4-modified biochars (SBCs) as solid-acid catalysts to activate H2O2 at circumneutral pH under visible light radiation. Spent coffee grinds were pyrolyzed with TiO2 at 300, 500 and 600 °C followed by steeping in 5 M H2SO4 and were used for the Fenton-like degradation of methyl orange (MO). The catalytic activity of SBC depended on the pyrolysis temperature and correlated well with the surface acidity and persistent free radical (PFR) concentration. Results showed that a complete MO removal and a TOC reduction of 70.2% can be achieved with SBC500 under photo-Fenton conditions. However, poisoning of the Lewis acid sites on SBC by PO43- led to a dramatic decrease in the removal of MO with inhibition effects more pronounced than with radical scavengers, suggesting the key role played by acid-sites on the activation of H2O2. Finally, electron paramagnetic resonance (EPR) studies identified •OH as the key transient in the degradation followed by •O2- and 1O2. These findings suggest that H2O2 was likely adsorbed on the surface oxygenated functional groups before being decomposed by accepting electrons from the PFRs on the SBC surface.
Keywords: Biochar; Fenton-like oxidation; Free radicals; Persistent free radicals; Solid acid catalysts.
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