Sorption of Zn, Cd and Cr on calcite. Application to purification of industrial wastewaters

Batch and column sorption studies of Zn, Cd and Cr were conducted on calcite to determine its retention capacity for these elements commonly present in industrial effluents and to explore its behaviour as a purification filter in a continuous flow system. The Langmuir model was found to describe the sorption processes best, and the results show that calcite is suitable for the purification of industrial wastewaters down to the concentration limits prescribed by current legislation, with sorption capacities depending on the metal sorbed in the order Cr>Zn>Cd. X-ray photoelectron spectroscopy (XPS) analyses were performed with the samples: calcite–Cr, calcite–Zn, calcite–Cd and pure calcite to determine the sorption mechanisms of these metals on calcite. The results show that Cd precipitates on calcite surface after a first step of Cd adsorption via exchange with Ca, the Zn retention on calcite occurs by precipitation as hydrozincite, Zn5(OH)6(CO3)2, and Cr precipitates on calcite as a coating of oxide hydrocarbonate.