Effect of arsenate and molybdate on removal of selenate from an aqueous solution by zero-valent iron.

Zero-valent iron (ZVI) is an inexpensive agent that can remove many common environmental contaminants. We investigated the effects of arsenate [As(V)] and molybdate [Mo(VI)] on the removal of selenate [Se(VI)] (1000 microg/L) in a 5 mM Cl- and SO4(2-) solution by ZVI. Analysis of selenium (Se) species in a control experiment revealed that there was no detectable selenite [Se(IV)] in the solution and Se(IV) was the only dominant form of Se in the PO(4)3- extract, revealing that the removal of Se(VI) by ZVI appeared to be partly attributed to the reduction of Se(VI) to Se(IV) by Fe(II) oxidized from ZVI, followed by rapid adsorption of Se(IV) to Fe oxyhydroxes (Fe(OH)). Addition of As(V) in a range of 5 to 25 mg/L in the solution did not affect Se(VI) removal. As(V) was removed at much faster rates than Se(VI). It took 16 h for ZVI to completely remove As(V) from the solutions, whereas it needed 31 h to remove 99% of the added Se(VI). Addition of Mo(VI) to the solution significantly affects the removal of Se. After 16 h, 99%, 98%, and 73% of the added Mo(VI) was removed from the solutions with 10, 50, and 200 mg/L Mo(VI) added, respectively. Whereas about 18%, 38%, and 78% of the added Se(VI) still remained in the solutions, respectively. This study suggests that a sequence for rapid element removal by ZVI is As(V)>Mo(VI)>Se(VI) and that ZVI can be used to effectively remove a high concentration level of As(V), Mo(VI), and Se(VI) from water.