Evaluation of the interaction mechanisms between red muds and heavy metals

This paper investigated the heavy metal adsorption of non-treated (RMnt) and acid-treated red muds (RMa), bauxite ore processing waste, in order to evaluate how efficient they are in reducing metal solubility and bioavaliability in polluted soils. Red mud samples were artificially polluted with solutions containing increasing concentrations of Pb, Cd and Zn. Cancrinite and hematite were the main phases of the red muds, and were also the components which adsorbed most heavy metals. The results showed that the RMnt adsorption capacity for the three heavy metals was Zn ≥ Pb > Cd. Acid treatment with HCl decreased the red mud's capacity to adsorb the heavy metals by 30%.
In order to study the different heavy metal–RM interaction mechanisms, all samples after artificial contamination were treated with solutions with gradually increasing extraction capacity. H2O and Ca(NO3)2 treatments only extracted very low concentrations of Pb, Cd and Zn, while EDTA treatment extracted the most adsorbed heavy metals from the sorbent particles. In particular the water-soluble and exchangeable metal fractions were higher in the RMa than they were in the RMnt, while the concentrations of Pb, Cd and Zn extracted with EDTA were lower.
The results showed that red muds can be used successfully to reduce the solubility and bioavailability of heavy metals in polluted soils.