Enhanced selective oxidation capacity and utilization efficiency of Fe(VI) initiated by HBT for contaminant oxidation: Identification of HBT radicals and function.

Studies on the Ferrate [Fe(VI)] process have focused on enhancing contaminant oxidation by adding reductants to generate Fe(V/IV), yet have overlooked significant reactive iron species loss caused by reductants, which ultimately reducing Fe(VI) utilization efficiency. Here, the redox mediator 1-hydroxybenzotriazole (HBT) was introduced into the Fe(VI) system and significantly improved the utilization efficiency and selective oxidation capacity of Fe(VI). The amount of sulfamethoxazole (SMX) oxidized by each μmol Fe(VI), namely ΔSMX/ΔFe(VI), was 0.82 in the Fe(VI)/HBT system, far exceeding the value of 0.24 in the Fe(VI) system. Further research clarified that HBT primarily hindered Fe(VI)/Fe(V) self-decay and steered the effective transfer of oxidation capacity from Fe(VI) to the more reactive Fe(V/IV)/HBT species. In particular, HBT are capable of selecting pollutants for oxidization more rapidly and accurately than Fe(VI) due to their higher redox potentials and superior selectivity. Density functional theory calculations indicated that the ln(k) of compounds with an electrophilic index below 1.49 were dramatically increased under Fe(VI)/HBT process. More importantly, other N-conjugated hydroxyl compounds (e.g., violuric acid) enhanced Fe(VI)-mediated SMX degradation by 13-33-fold, suggesting the broad applicability of N-OH groups in activating Fe(VI). This study provides a new perspective on enhancing the oxidation capacity of Fe(VI), emphasizing the role of organic radicals in pollutant removal and the potential of N-OH group-containing natural organic matter to activate Fe(VI).