A novel akaganeite sorbent synthesised from waste red mud: Application for treatment of arsenate in aqueous solutions

This study presents a novel akaganeite sorbent derived from waste bauxite residue (RMA) for the treatment of arsenic contaminated waters. The performance of the developed sorbent was compared against a commercial material (granular ferric hydroxide (GFH)) in order to contextualise the sorption capacity. A combination of sorption equilibrium and kinetic studies were used to assess sorbent performance, where arsenic concentration, solution pH, and the presence of co-existing ions were considered. The pseudo first order model optimally described the kinetic data, and the RMA material exhibited faster sorption kinetics than GFH. The Langmuir model gave the best approximation of the equilibrium sorption data, and the maximum capacities calculated were 27.8 and 22.5 mg/g for RMA and GFH, respectively. The RMA adsorbent was largely unaffected by solution pH, which contrasted with the GFH material. This difference was attributed to the release of both chloride and protons from the RMA material upon exposure to aqueous solutions. Both materials exhibited similar performance in the presence of co-existing ions, where phosphate and carbonate inhibited arsenic uptake on both sorbents. X-ray photoelectron spectroscopy indicated that surface hydroxyl groups were involved in the arsenic sorption mechanism, which was proposed to be through the formation of inner sphere complexes. Treatment of a tap water sample spiked with arsenic showed that the RMA material may be a suitable candidate for the treatment of real arsenic contaminated waters.