Competitive removal of water-borne copper, zinc and cadmium by a CaCO3-dominated red mud

Batch experiments were conducted to investigate the competitive removal of water-borne Cu, Zn and Cd by a CaCO3-dominated red mud. The results show that the water-borne Cu had a higher affinity to the red mud, as compared to the water-borne Zn and Cd. The major mechanism responsible for the preferential retention of Cu by red mud was the formation of atacamite. It is likely that, initially, atacamite was formed mainly through the reaction between CuCl2 and NaOH. Reaction between CuCl2 and CaCO3 to form atacamite became more and more important with the gradual consumption of NaOH. Sequential extraction results show that the water-borne metals were preferentially associated with the NH2OH.HCl-extractable fractions at the early stage of the experiment. With increase in the saturation degree of binding sites on red mud particles by the metals, the proportion of HCH3COO-extractable Cu fraction increased accordingly. Water-borne Zn and Cd were also increasingly bound in the HCH3COO-extractable forms until the metal binding capacity of the red mud was nearly depleted. After the binding sites of red mud particles were saturated, part of the Zn and Cd previously retained by the red mud was displaced by water-borne Cu, resulting in the release of the previously immobilized Zn and Cd to the solution.