The influence of citrate anion on Ni(II) removal by raw red mud from aluminum industry

The removal of Ni(II) from aqueous solutions by raw red mud powder was studied in the absence and in the presence of citrate ligand, as a function of process parameters. Using lower initial Ni(II) concentration (10(-3) mol/L) detectable amounts were found only at initial pH 3, whereas for higher concentration (2 x 10(-3) mol/L) removal increased simultaneously with pH in the range 3-4 and at pH > 8. Process efficiency was suppressed in the presence of citrate. However, at Ni/Citrate molar ratios 1:0.25 and 1:0.5, 10-20% decrease was noticed in acidic media, while the negative influence diminished with the rise of pH. At higher ligand rates, formation of stable aqueous complex between Ni(II) and citrate significantly reduced metal removal in the whole tested initial pH range (3-10). Due to instantaneous pH rise above 8, removal of Ni(II) from 10(-3) mol/L solution was completed in 5 min. Kinetic data demonstrated slower Ni(II) uptake using both solutions of higher initial Ni(II) concentrations and Ni/Citrate solutions. At initial pH 5, estimated capacity towards Ni(II) was found to be 27.4 mg/g, without citrate. For Ni/Citrate molar ratios 1:0.25 and 1:0.5, capacities decreased slightly to 25 mg/g and 21 mg/g, whereas at equimolar and higher concentrations, citrate significantly inhibited Ni(II) immobilization (7.6 mg/g and 2.5 mg/g, respectively for Ni/Citrate ratios 1:1 and 1:2). The results demonstrated beneficial effect of red mud high alkalinity on Ni(II) removal. The sorbent satisfactorily immobilize nickel from solutions having initial Ni/Citrate molar ratios lower than 1:1, and neutral to alkaline initial pH. FUR analysis confirmed that red mud can act as a removal agent for Ni(II), Ni/Citrate complex and free citrate ligand. (C) 2012 Elsevier B.V. All rights reserved.