Value adding red mud waste: High performance iron oxide adsorbent for removal of fluoride

A novel method for the synthesis of iron oxide based sorbents from bauxite residue (red mud) was developed by way of a sequential digestion and selective precipitation process. The red mud was consecutively digested with phosphoric acid and hydrochloric acid, precipitated by sodium hydroxide addition, and aged for 72 h at pH 3-3.5. This process allowed the production of a relatively pure crystalline material, akaganeite (β-FeOOH), comprised of high surface area (223 m2/g). The performance of the synthesised material as a fluoride sorbent was compared with a commercial adsorbent (granular ferric hydroxide). Equilibrium isotherms revealed that both sorbents reduced the amount of fluoride in solution to <1.5 mg/L. Significantly, the red mud derived akaganeite had a higher maximum adsorption capacity than the commercial granular ferric hydroxide (11.40 and 9.23 g/kg, respectively). Performance differences were suggested to relate to the presence of active sites which comprised of iron and aluminium and/or titanium species.