Adsorbent properties of red mud and its use for wastewater treatment

Abstract

With a view to simultaneously conserving the environment and an important natural resource, we assessed the feasibility of using red mud (RM), a residue from bauxite refining, for wastewater treatment. Moistened mixtures of RM and 8% (w/w) CaSO 4 form aggregates which are stable in aqueous media. Batch experiments examining adsorption of phosphorus (P) by these aggregates for contact times of 3, 6, 24 and 48 h indicated that adsorption occurred by both fast and slow reactions. Likewise, comparison of experimental breakthrough curves (BTCs) for P retention on a column of RM aggregates (residence time 3 h) with MCMFIT-simulated BTCs indicated that fast and slow adsorption reactions were occurring. Actual P retention was predicted, taking into account both kinetics of fast and slow adsorption reactions. The RM aggregates had maximum adsorption capacities (evaluated from fits (0.90<R 2<0.99) of the Langmuir isotherm to batch adsorption data for contact time 48 h) for Cu 2+, Zn 2+, Ni 2+ and Cd 2+ of 19.72, 12.59, 10.95 and 10.57 mg g −1, respectively. In continuous adsorption experiments in which secondary effluent from an urban sewage treatment plant was percolated through RM aggregates packed into columns, purification efficiencies for P, Ni 2+, Cu 2+ and Zn 2+ were 100, 100, 68 and 56% respectively. Aggregated RM is suitable for treatment of wastewaters, in particular those whose principal contaminants are P or heavy metals.