Reductive sorption of hexavalent chromium (Cr(VI)) by vivianite under varying groundwater conditions

Abstract

Rapid industrialization in the past half century has led to a dramatic increase in the frequency of acid spill accidents in Korea. Acid spill accidents not only cause soil and water acidification issues but may also lead to secondary problems such as heavy metal (e.g. Cu, Cd, As, Cr(VI)) leaching from the affected soils. Among heavy metals, hexavalent chromium (Cr(VI)) is highly mobile, acutely toxic and mutagenic under many in situ groundwater conditions, while its reduced form Cr(III) is less mobile and less toxic. Monoclinic octa-hydrated ferrous phosphate, vivianite ($Fe^{2+}_3(PO_4)_2 \cdot 8H_2O$ ), is an iron-bearing soil mineral frequently found in natural and engineered phosphorous-enriched reducing environments. Vivianite has attracted attention as a chemical reagent for reductive remediation of redox-active chemical contaminants, as vivianite provides surface for adsorption of charged contaminants, while simultaneously donating electrons for reduction of the contaminants. Co-occurrence of reduction and adsorption on vivianite surfaces often leads to effective removal of redox-sensitive contaminants. In this study, the feasibility of using vivianite for reductive remediation of Cr(VI) has been examined in a series of laboratory experiments. The mechanisms of the reductive adsorption were investigated by experimentally isolating the adsorption and reduction processes. This study has investigated reductive sorption of Cr(VI) on a synthetic vivianite sample under varying groundwater conditions. Batch experiments were performed to examine the effect of pH and ionic strength on Cr(VI) sorption by vivianite: a 5 ppm Cr(VI) was reacted with a 6.5 g/L vivianite suspension buffered at pH 5, 7, and 9, using 0.05 M HEPES or tris buffer solution. The removal of aqueous Cr(VI) from solution was fastest at pH 5 and slowest at pH 9. The ionic strength had only minor effect on the removal kinetics, while phosphate hindered Cr(VI) removal among competing anions. Thermodynamic calculations of Cr aqueous speciation were performed using Visual MINTEQ 3.1 and its built-in thermodynamic database. Surface complexation modeling of the sorbed Cr species was conducted using PHREEQC 3.0 software for a mechanistic interpretation of the sorption process. Oxidation-reduction potential was measured to verify that iron sources were induced into anaerobic condition. Same amount of Cr(III) is tested to react with vivianite to check selectivity on reductive adsorption on vivianite compared to Cr(VI). Solubility and dissolution rate of ferrous and phosphate from vivianite at different experimental matrices are identified to support understanding of reactions. It is resulted that major reaction mechanism comes from the redox reaction between Cr(VI) and Fe(II) and Cr(III)/Fe(III) precipitataion on the surface of vivianite at specific pH range.