Photo-assisted Fe2+ modified molybdenum disulfide activated potassium persulfate to degrade sulfadiazine: Insights into the degradation pathway and mechanism from density functional theory

Sulfadiazine (SDZ) entering the environment has an adverse impact on animal, plant, and human health, it is important to find effective ways to remove SDZ from the aquatic environment. This work comprehensively explored the degradation of SDZ by MoS2 in photocatalysis and Fe2+ activation of potassium peroxodisulfate (PDS) from both theoretical and experimental aspects. The PDS/Fe2+/MoS2 system had a faster PDS conversion rate and a higher degradation efficiency for SDZ compared with PDS/Fe2+ and PDS/MoS2. The degradation rate of the PDS/Fe2+/MoS2 system to SDZ reached 97.1% within 60 mins. The degradation rate is higher under acidic conditions compared with alkaline conditions. Free radical capture experiments and EPR tests showed that the degradation of SDZ was the result of a variety of free radicals, among which the SO4·−, 1O2 and h+ played a major role. The chemical morphology and composition of MoS2, as well as the changes in the concentration of Fe ions in different valence states were explored and the changes in the O-O and, S-O bond lengths, and differential charge density of PDS in the MoS2 and, Fe2+/MoS2 systems were calculated by DFT. The results showed that both MoS2 and Fe2+ significantly increased the conversion rate of PDS. The activation of PDS on the Fe-modified MoS2 surface had stronger active sites. MoS2 can promote the cycle of Fe3+/Fe2+ and the conversion of PDS. According to the intermediate products measured by LC-MS/MS, as well as the frontier molecular orbitals and Fukui function diagrams of SDZ and SDZ* calculated by quantum chemistry, four degradation paths of SDZ were speculated and confirmed the existence of the self-sensitized photodegradation process of SDZ in the system.