Hexavalent chromium reduction by zero-valent magnesium particles in column systems

The discharge of hexavalent chromium in aquatic environments represents an issue of great concern. The chemical reduction of Cr⁶⁺ to Cr³⁺, which is less mobile and harmful, is a suitable approach for chromium removal. In this regard, in comparison to other reactive metals, the use of zero-valent magnesium (ZVM) has several advantages. Nevertheless, this element has been scarcely investigated in the decontamination of water and wastewater. In particular, no studies have been conducted by applying Mg⁰ particles fixed in column systems for Cr⁶⁺ reduction. In the present study, a wide experimental investigation was carried out to analyse the chromium abatement through zero valent magnesium particles in a packed batch column. The effects of pH, initial Cr⁶⁺ concentration and temperature were investigated. The experimental results proved that the process performances were mainly affected by pH values. High efficiencies were detected at pH 3, while unsatisfactory abatements were observed at pH 5 and under uncontrolled pH conditions. At pH 3, the process performance worsened with the rise in the initial chromium concentration. In particular, a complete abatement was detected in 180 min by treating solutions with initial Cr⁶⁺ concentrations up to 40 mg/L. The effect of temperature was negligible at pH 3 and under uncontrolled pH, while the increase from 20 to 30 and 40 °C produced a significant improvement in the removal yields at pH 5. By means of a kinetic analysis a theoretical law able to accurately describe the experimental removal trends was identified. Furthermore, a mathematical relation between the observed kinetic constants and the magnesium to initial chromium amount ratio was defined. Finally, the reaction pathways were proposed, and the reaction products identified.