Removal of hexavalent chromium by limonite in aqueous solutions

Cr(VI) is a toxic pollutant and its reduction to relatively less toxic Cr(III) can solve this problem to a greater extent. In the present study, coupled reduction-sorption of Cr(VI) in aqueous solutions using limonite was investigated as a function of pH, limonite dose and particle size. Results demonstrated that the smaller limonite particle size and low pH aqueous medium favored high Cr(VI) removal. Significant Cr(VI) removal (>55 +/- 1%) was achieved with 100-200 mesh, whereas only 25 +/- 0.7% and 10 +/- 0.5% removals were attained using 30-40 mesh and 20-30 mesh, respectively, after 2 h reaction. Acid pH proved beneficial and the complete Cr(VI) removal was observed at pH <4.0 compared to that of 10 +/- 0.5% at pH 9.0. Moreover, above 53 +/- 2.5% of Cr(VI) removal rates were maintained in the first three batch experiment runs and then drastically decreased to below 5 +/- 03% in experiment Run-7. The Brunauer-Emmett-Teller (BET) surface area and sorption capacity of limonite were 35.22 m(2) g(-1) and 10.03 mg g(-1), respectively. The reaction mechanism demonstrated that under acidic aqueous medium, the dissolved Fe(II) and S(II) on limonite surface reacted with Cr(VI) and formed Cr3S4 and Cr5Si3 precipitates. But the resultant precipitates on limonite surface hindered further Cr(VI) removal and passivated, affirmed in XRD and ESEM analyses. This study suggested that limonite can be used for the effective removal of Cr(VI) from contaminated water environments. (C) 2013 Elsevier B.V. All rights reserved.