Removal of hexavalent chromium using zero-valent magnesium in a continuous flow system

Zero-valent magnesium (ZVM) is a powerful and beneficial material for the reduction of Cr6+ to Cr3+. To the best of our knowledge, no work has been conducted on the removal of Cr6+ from water using ZVM in a continuous flow system. Therefore, the aim of this research was to study the reduction of Cr6+ in a column with a thin ZVM layer working in continuous mode. In particular, the process kinetics were investigated using different types of Mg0 particles (powder, grit and turnings) under several operating conditions. In this regard, the effects of the inlet Cr6+ concentration and pH of the inlet solution on the process performance were studied. In addition, the influence of co-existing ions was investigated, and the material longevity was assessed. The smallest ZVM particles (powder) showed the greatest reactivity, reaching efficiencies between 78 and 100 %, followed by magnesium grit and turnings. For all the materials applied, increasing efficiencies were detected with decreasing chromium concentrations. Acidic conditions of the influent promoted the pollutant removal and the highest removal rates were achieved at pH 3. However, magnesium powder was able to guarantee satisfactory efficiencies even at pH 5 and 7, with abatements from 63 to 92 %. The presence of sulfate and nitrate did not significantly affect the process performance. Based on the collected data, a modified first-order kinetic law was formulated, resulting in an equation useful for the design of continuous systems such as reactive permeable barriers. The exhaustion tests proved a satisfactory durability of the reactive material.