Zinc Remediation by Zero-Valent Iron and the Associated Isotope Fractionation: Batch Experiments

Abstract

Zinc is a bio-essential micro nutrient which is essential to the health of humans and other organisms. However, high Zn concentrations, as has been observed in mining waste water and urban runoff, can be harmful. The major remediation methods used to reduce the mobility of Zn in groundwater flow systems include Zn precipitation and adsorption. Zero-valent Iron (ZVI) has been used as a remediation material in permeable reactive barriers (PRBs), and it can be used to reduce the concentration of Zn in waste water. Measurements of Zn isotope ratios, may provide information to trace Zn(II) migration and help define reaction mechanisms during remediation of Zn contamination using ZVI. Laboratory batch experiments, combined with traditional geochemical analysis, non-traditional stable isotope analysis and solid-phase analysis (XANES and EXAFS) were used to evaluate Zn removal mechanisms associated with ZVI in differing initial Zn-bearing solutions and with varying alkalinity concentrations. Decreasing concentrations of Zn were observed throughout all of the experiments. X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) analyses indicted the presence of Zn(II) on the solid phase with a coordination number of four, compared to a coordination number of six in the initial solutions. Models based on the measurements of EXAFS were used to assess the possible products on the solid phases. The results suggest that a combination of sorption and precipitation mechanisms dominated the removal of Zn for all of the aqueous solutions. The decline in dissolved Zn concentrations was accompanied by a decreasing value of δ66Zn in the experimental solutions, indicating preferential accumulation of 66Zn in the solid phase. The differences in dissolved Zn solutions and alkalinity did not significantly affect the extent of Zn isotope fractionation. The change in Zn concentration and δ66Zn can be fit with equilibrium fractionation models. The fractionation factors were similar for all batch experiments, reflecting the consistent change in coordination. The fractionation factors, which were calculated from the fitting process, cannot separate sorption-dominated and precipitation-dominated removal mechanism in these experiments.