Reduction of Chromium(VI) mediated by zero-valent magnesium under neutral pH conditions

Abstract

In an effort to assess the potential use of ZVMg in contaminant treatments, we examined Cr(VI) reduction mediated by ZVMg particles under neutral pH conditions. The reduction of Cr(VI) was tested with batch experiments by varying [Cr(VI)]0 (4.9, 9.6, 49.9 or 96.9 μM) in the presence of 50 mg/L ZVMg particles ([Mg0]0 = 2.06 mM) at pH 7 buffered with 50 mM Na-MOPS. When [Cr(VI)]0 = 4.9 or 9.6 μM, Cr(VI) was completely reduced within 60 min. At higher [Cr(VI)]0 (49.9 or 96.9 μM), by contrast, the reduction became retarded at >120 min likely due to rapid ZVMg dissolution in water and surface precipitation of Cr(III) on ZVMg particles. Surface precipitation was observed only when [Cr(VI)]0 = 49.9 or 96.9 μM and increased with increasing [Cr(VI)]0. The effect of dissolved oxygen was negligible on the rate and extent of Cr(VI) reduction. Experimental results indicated that Cr(VI) was reduced not directly by ZVMg but by reactive intermediates produced from ZVMg-water reaction under the experimental conditions employed in this study. In addition, the observed rates of Cr(VI) reduction appeared to follow an order below unity (0.19) with respect to [Cr(VI)]0. These results imply that ZVMg-mediated Cr(VI) reduction likely occurred via an alternative mechanism to the direct surface-mediated reduction typically observed for other zero-valent metals. Rapid and complete Cr(VI) reduction was achieved when a mass ratio of [ZVMg]0:[Cr(VI)]0 ≥ 100 at neutral pH under both oxic and anoxic conditions. Our results highlights the potential for ZVMg to be used in Cr(VI) treatments especially under neutral pH conditions in the presence of dissolved oxygen.