The comparison of Se(IV) and Se(VI) sequestration by nanoscale zero-valent iron in aqueous solutions: The roles of solution chemistry

[Display omitted] •Removal of Se(IV)/Se(VI) by NZVI were compared in DI water and groundwater.•The effects of HA, cations and anions on the removal of Se(IV)/Se(VI) were compared.•Removal of Se(IV) and Se(VI) were examined at different solution pH.•The mechanisms for the Se removal by NZVI under various conditions were probed. The sequestration of Se(IV) and Se(VI) by nanoscale zero-valent iron (NZVI) particles were compared under different solution conditions. Firstly, the comparison was conducted at three pH values (4.0, 6.0 and 8.0) in deionized water. Generally, the removal of Se(IV)/Se(VI) by NZVI was more rapid under acidic conditions and the removal efficiency of Se(IV) was much higher than that of Se(VI). Moreover, the pH variation exhibited much larger influence on the sequestration of Se(VI) than that of Se(IV) by NZVI. The spectroscopic analysis showed that both the Se(IV) and Se(VI) were reduced to Se0 and Se2−, while NZVI was transformed into iron (hydr)oxides. When the selenium-NZVI reactions occurred in synthetic groundwater, all the reaction systems were inhibited in varying degrees. The individual effects of humic acid (HA) and typical inorganic ions were also examined. It seems that HA could substantially hinder the sequestration of Se(IV) compared with that in deionized water, while sulfate (SO42−) and bicarbonate (HCO3−) inhibited the Se(VI) removal significantly. Notably, the presence of cations (i.e., Na+ or Ca2+) ions did not cause obvious interference to the Se(IV)/Se(VI) removal by NZVI, while the presence of Ca2+ could alleviate the adverse effect of HA on Se(IV) removal to some degree.