Removal of sulfamethoxazole by ferrous iron activation of persulfate: Optimization of dosing strategy and degradation mechanism

Abstract

In this paper, the degradation of sulfamethoxazole (SMX) was investigated using the ferrous iron (Fe2+) activation of persulfate (PS) (the Fe2+/PS process). The influence of the initial concentration of both PS and Fe2+ was investigated. It was found that increasing the PS concentration resulted in a higher SMX degradation efficiency. The influence of inhibiting reactions was found to increase with increasing Fe2+ concentration. In order to minimize these inhibiting reactions, different dosing strategies were applied. It was found that the SMX degradation efficiency could be enhanced significantly when changing from direct dosing (total amount of Fe2+ dosed at the start) to sequential dosing (dosing that same total amount but divided over specific time intervals) and even more when using continuous dosing (dosing the same total amount but continuously over 30 min reaction time). The contribution of different reactive species in this process was also investigated. It was found that hydroxyl radicals (•OH) were mainly responsible for the degradation of SMX during direct dosing, while using continuous dosing of Fe2+, the contribution of Fe(IV) and sulfate radicals (•SO4-) became more important (reduction of •OH contribution from 89 to 71%). Some degradation products formed during the SMX degradation process were identified and the difference in reaction mechanism between •OH on the one hand and Fe(IV) and •SO4- on the other hand was elucidated. At last, a comparison of different sulfate radical based advanced oxidation processes (SR-AOP) is performed by comparing the difference in SMX degradation efficiency, reactive species contribution and the formed degradation products. In most investigated processes, similar degradation products have been found, however, the large •OH contribution in the Fe2+/PS process resulted in distinct degradation products.