Effects of thermal modification of a biochar on persulfate activation and mechanisms of catalytic degradation of a pharmaceutical

Abstract

In this study, a biochar (BC) derived from the pruning wastes of apple trees was thermally modified and was evaluated as a catalyst to explore an innovative strategy of valorization. The BC was thermally modified at 400, 550, and 700 °C, denoted as BC400, BC550, BC700, respectively. The removal of 50 mg/L acetaminophen (ACT) with 1.0 g/L biochars was <9.0% except for BC700 (19.1%), without an oxidant. However, it was greatly enhanced in the presence of 0.5 g/L sodium persulfate. The surface area normalized ACT removal rate constant increased from 6.56 × 10−5 min−1·m−2 for BC to 28.4 × 10−5 min−1·m−2 for BC700, and it was well-correlated with indicators of the degree of graphitization (sp2/sp3-C content), defects (ID/IG), C/O ratio, and C=O content. They were increased from 72.9% to 96.1%, 2.24 to 2.86, 4.9 to 12.1, and a negligible level to 3.9%, respectively, for BC and BC700. The electron paramagnetic resonance spectra and the characteristics of the biochars indicated that the ACT removal was attributed predominantly to SO4•− generation accepting electrons from persistent free radicals and the graphitic structures. The signal of SO4•− was the highest and dominant for BC700, which had the most developed graphitic structures, defects, and graphitic C=O with abundant (de)localized π electrons and non-bonding electrons. The results suggest that biochars can be excellent, environmentally friendly catalysts and that the activation mechanisms can be controlled by simple thermal modification to customize the performance for a wide variety of applications in various media, including water, soil, and sediments.