Role of schwertmannite or jarosite in photocatalytic degradation of sulfamethoxazole in ultraviolet/peroxydisulfate system

Ultraviolet (UV)/peroxydisulfate (PDS) system has been employed for degrading organic pollutants in water, but the insufficient production of reactive species in the system limits the mineralization of organic pollutants. This study evaluated for the first time the effectiveness of integrating iron hydroxysulfates, including schwertmannite (Sch) and jarosite (Jar), with UV/PDS system for sulfamethoxazole (SMX) degradation in water. The optimal conditions identified for 40 mu M SMX degradation were PDS 2 mM, photocatalysts 1 g L-1 and pH 3 in both of Sch/ UV/PDS and Jar/UV/PDS systems. A synergetic effect between UV/PDS and Sch/UV processes was found in Sch/ UV/PDS system with synergistic factor of 1.15, which drastically enhanced the production of reactive species (center dot OH and SO4 center dot-) and consequently improved SMX degradation and TOC mineralization. Additionally, schwertmannite exhibited higher photocatalytic activity than jarosite in UV/PDS system for both SMX degradation and TOC mineralization. In Sch/UV/PDS system, equivalent to Fe(OH)2+ on schwertmannite can be efficiently photolyzed to equivalent to Fe2+ and center dot OH under UV irradiation with center dot O2 - generation, in which the produced reactive species contributed to a relatively small fraction of SMX degradation. More importantly, the regeneration of equivalent to Fe2+ accelerated PDS activation to SO4 center dot-/center dot OH, which was responsible for the degradation of most SMX. However, the photoreduction of equivalent to Fe3+ to equivalent to Fe2+ on jarosite by UV irradiation was insignificant, thus leading to the poor PDS activation to SO4 center dot-/center dot OH and a lower SMX degradation rate. The degradation pathways of SMX in Sch/UV/PDS system were further proposed based on the identified degradation intermediates of SMX. Besides, schwertmannite exhibited a good adaptability in wide pH (2-9) range, good stability, and recyclability for catalyzing SMX degradation in UV/PDS system. Therefore, integrating schwertmannite with UV/PDS system is an attractive method for efficiently removing organic pollutants in water.