Removal of Cr (Ⅲ) from aqueous solution by using bauxite residue (red mud): Identification of active components and...

Bauxite residue is the by-product of the aluminium industry with an annual output of more than 200 million metric tons in China. Its treatment is still a big problem because more than 96% of that is stockpiled on land causing environmental pollution and threatening the human health. This study used bauxite residue to remove Cr (Ⅲ) from aqueous solution and analyzed the removal mechanism. The removal time was dependent on the initial concentrations of Cr (Ⅲ) and different active components acted on different reaction period. Reaction time increased from <5 min to >2 h with an increase of Cr (Ⅲ) concentration from 5 to 100 and 170 mg/L. The existing forms of adsorbed-Cr were iron oxide-bound Cr (40.80%-87.85%), sulfide-bound Cr (4.04%-20.28%) and residue (6.60%-33.72%). All the components started to react as soon as bauxite residue was added. Cr did not precipitate in the presence of high alkalinity bauxite residue due to the slow release of alkalinity maintaining pH < 6, thus producing Cr(OH)²⁺, Cr₂(OH)₂⁴⁺ and Cr₃(OH)₄⁵⁺ by hydrolysis without precipitation. Fe₂O₃ and Al-containing components were the main active phases for Cr (Ⅲ) removal, with the reaction time lasting more than 2 h and producing Ca₆Al₄Cr₂O₁₅, AlCr₂, (Si, Al)₂O₄, Fe(Cr, Al)₂O₄, FeCr₂Si₃O₁₂, MgCr_0·1Fe_1·9O₄ and MgCr_0·4Fe_1·6O₄. Finally, bauxite residue was granulated and used for column tests. Cr (Ⅲ) wastewater (1 and 50 mg/L) was treated and the effluent can meet the first level of the Shanghai standard (0.1 mg/L) defined by Integrated Wastewater Discharge Standard (DB 31/199-2009).