Conversion of ozone into hydroxyl radical by granular activated carbon with and without biofilms: Implications for micropollutant abatement

Abstract

The transformation of ozone (O-3) into hydroxyl radical ((OH)-O-center dot) during the ozonation was evaluated in the presence of granular activated carbon (GAC) and biofilm-covered granular activated carbon (BGAC). While both GAC and BGAC accelerated O-3 decomposition, the manner in which they generated (OH)-O-center dot was different. GAC catalyzed the conversion of O-3 into (OH)-O-center dot, with increasing hydroxyl radical exposure (integral[(OH)-O-center dot]dt) by 24 -72 % depending on the GAC dose. Conversely, BGAC exhibited limited capacity for (OH)-O-center dot generation from O-3 decomposition, resulting in decreased integral[(OH)-O-center dot]dt by 14 -25 % at higher BGAC doses. This disparity is likely attributable to biofilms on BGAC surface obstructing catalytic sites and hinder O-3-to-(OH)-O-center dot conversion. These different behaviors influenced the degradation of contaminants during ozonation. Specifically, GAC enhanced the degradation of O-3-resistant contaminants, whereas BGAC inhibited it. Machine learning (ML) models were developed based on experimental data to predict oxidant exposures (R-2 > 0.99 for integral[O-3]dt and integral[(OH)-O-center dot]dt). The elimination of several contaminants in systems was successfully predicted by these ML models, coupled with a straightforward kinetic equation that included adsorption and oxidation parameters. Additionally, ozonation modified the catalytic properties of GAC and BGAC. Extended ozonation oxidized GAC surface, diminishing its capability to convert O-3 into (OH)-O-center dot. In contrast, oxidation of BGAC disrupted surface biofilms, thereby restoring its catalytic function.