Study of factors affecting Ni2+ immobilization efficiency by temperature activated red mud

The waste red mud, remaining in remarkable quantities after the digestion of bauxite ores following the Bayer process, contains number of voluble minerals with excellent sorption properties towards aqueous heavy metals. Heating at 600 °C was found to be a favorable treatment for revalorization of rinsed red mud into an efficient Ni2+ sorbent (RBRM600). As potential practical application of RBRM600 for Ni2+ accumulation greatly depends on the solution composition and pH, the influence of these vital process variables was considered in this study. The initial pH rise from 2 to 3.5 caused the most evident increase in the amounts of Ni2+ removed; furthermore, the effect was more obvious for lower initial sorbate concentrations. Conversely, changes of the solution pH between 3.5 and 8 did not have a significant influence on the sorption. The increase of initial cation concentration caused the increase of the sorbed amount, following Langmuir isotherm model. The calculated maximum sorption capacity of 27.54 mg/g demonstrated capacity increase of approximately 20% in respect to inactivated rinsed red mud. Coexisting cations inhibited Ni2+ removal in the following order: Cu2+ > Pb2+ ≥ Zn2+ > Cd2+ ≫ Ca2+, whereas the presence of Na+ and K+ did not affect the process. The investigated anions caused decrease of Ni2+ removal efficiency in the order: EDTA > chromate > acetate > sulphate, however, fluoride slightly improved sorption. Low desorption of Ni2+ in various media showed that the heavy metal cations are tightly bound to RBRM600 surface, thus disposal or reuse of spent sorbent should be considered.