Effect of dissolved silicate on the degradation of sulfamethoxazole by nZVI@D201 nanocomposite

Dissolved silicate (Si), which is omnipresent in natural water and wastewater, is a key factor mediating the separation and transformation of contaminants by forming complexes on the surface of zero-valent iron (ZVI) and iron oxides. In this study, we prepared an nZVI@D201 nanocomposite and studied the redistribution of sulfamethoxazole (SMX) on nZVI@D201 in the pre-equilibration and Si-SMX simultaneous addition systems with 0.1 mM and 1.0 mM Si at an initial pH of 3.0–11.0. The results revealed that both Si pre-equilibration and Si-SMX simultaneous addition exhibited excellent SMX removal efficiency. Si pre-equilibration altered the adsorbed SMX to degradative SMX at an initial pH of 5.0–9.0. Moreover, the simultaneous Si addition at increasing concentrations decreased the degradative SMX. X-ray diffraction, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy were used to characterize nZVI@D201 after pre-equilibration and simultaneous addition of Si and SMX at an initial pH of 5.0. The results showed that Si pre-equilibration inhibited the transformation of ZVI to magnetite and decreased the crystallinity of the minerals. In addition, the simultaneous addition of Si and SMX impeded the transformation of nZVI to magnetite, whereas the formation of Fe-O-Si complexes hindered the crystallinity. Moreover, scavenger experiments and electron spin resonance tests proved that addition of Si didn’t change the reactive species formation and ·OH was the mainly species for SMX degradation. Based on the detected intermediates via ultra-high performance liquid chromatography-mass spectra, the possible degradation pathways of SMX were proposed.