Unveiling the Neglected Roles of Chloride and Sulfate in the Removal of Nitro Compounds by Sulfidated Zero-Valent Iron/Ferrous Ion Systems

Ferrous ions (Fe2+) can effectively promote the removal of pollutants by sulfidated zero-valent iron (S-ZVI), but the role of anions coexisting with Fe2+ was often ignored. This study systematically compared the performances of S-ZVI/FeCl2 and S-ZVI/FeSO4 systems for chloramphenicol (CAP) and nitrobenzene (NB) removal. The results showed that FeCl2 and sulfidation had a synergistic promoting effect on the reaction, but FeSO4 could not promote the removal of nitro compounds by S-ZVI. Liquid chromatography/mass spectrometry (LC/MS) and UV-vis analysis showed that only the nitro group was reduced during the removal of CAP and NB. The results of batch experiments showed that both NaCl and Na2SO4 could effectively promote the reaction, but the promotion effect of Na2SO4 weakened with the decrease of pH. X-ray photoelectron spectroscopy (XPS) analysis showed that Fe2+ could promote the adsorption of SO42- on the surface of S-ZVI. A possible mechanism was proposed: Fe2+ could maintain a low pH level because of hydrolysis, which promoted the adsorption of Cl- and SO42- on the surface of S-ZVI. Cl-promoted the reaction because of its pitting corrosion, while SO42- competed with nitro groups for the adsorption sites. Our findings suggest that the coexisting anions are quite essential in the removal of nitro compounds by S-ZVI/Fe2+ systems.