Mn-modified biochars for efficient adsorption and degradation of cephalexin: Insight into the enhanced redox reactivity

Mn-modified biochars (BCs) were developed by pre-treatment of feedstock (MBCs) or post-modification of biochar (BCM), for simultaneous adsorption and degradation of a model pollutant, cephalexin. The apparent removal rates of cephalexin in the presence of MBCs (2.49 - 6.39 × 10−2 h−1) and BCM (13.3 × 10−3 h−1) were significantly higher than that in the presence of biochar prepared under similar conditions (4.2 × 10−3 h−1). While the •OH generated from the activation of dissolved O2 by the persistent free radicals (PFRs) and phenolic -OH on BC could cause degradation of cephalexin, its removal was drastically enhanced through direct oxidation by the MnOx and related Mn species on Mn-modified BCs. The removal of cephalexin by MBCs decreased as the solution pH was raised from 5.0 to 9.0, which supports the critical role played by Mn3O4 in its oxidation. Removal of cephalexin in the presence of MBCs and Mn3O4 was enhanced with the introduction of Mn(II) ions, suggesting that the Mn3O4 present on MBCs facilitates the re-oxidation of Mn(II) to highly reactive Mn(III). While MnO2 anchored on BCM also enhanced the cephalexin oxidation, the active sites of BC and MnO2 were partially destroyed during post-modification of BC, compromising the redox cycling of Mn(II)/Mn(III) and the generation of •OH. As a result, the performance of BCM in oxidizing cephalexin was inferior to that of MBCs. These findings shed new light on the development of environmentally benign sorbents capable of simultaneously adsorbing and oxidizing organic pollutants.