Rinsed and thermally treated red mud sorbents for aqueous Ni2+ ions

Red mud, an abundant industrial waste from alumina industry, represents a rich source of various metal oxides, mainly of Fe2O3, Al2O3 and SiO2. In this study, the rinsed Bosnian red mud (RBRM) was evaluated as an economical, composite sorbent for aqueous Ni2+ ions. The process was investigated as a function of pH, contact time and initial metal concentration. The investigated mineral mixture exhibited a high acid neutralising capacity, and its most important role in cation immobilization was observed in the initial pH range 2–8. The initial metal ion concentration strongly influenced the sorption kinetics and equilibrium times. Addition of 5 g/L of RBRM caused 100% removal from the solutions of 10−4 to 5 × 10−4 mol/L, whereas with the further increase of Ni2+ concentration to 8 × 10−3 mol/L, the removal efficiency decreased to 26%. The maximum sorption capacity of 0.372 mmol/g, at initial pH 5, was determined using Langmuir theoretical model. The possibility of sorption efficiency improvement by annealing RBRM powder was investigated in the range 200–900 °C and the relationships between temperature, red mud physicochemical and sorption properties were established. The optimum heating temperature was found to be 600 °C, due to water exclusion from gibbsite and bayerite phases, leading to improved porosity and surface area, as well as increased pH value and sorption efficiency. The stability of the sorbed cation was assessed by leaching experiments in distilled water and acidic TCLP2 solution.