Uptake of trivalent chromium from aqueous solutions using protonated dry alginate beads

Abstract

The uptake of trivalent chromium by protonated dry alginate beads from aqueous solutions was investigated at 25 °C in batch-type experiments. The differences between the mechanisms associated with formation of alginate beads and with metal uptake were demonstrated. Uptake was coupled with a release of protons; this ion exchange was found to be the mechanism of the uptake. The uptake was strongly dependent on the Cr-bearing solution pH up to a value of around 4.5. Uptake reaches a value as high as 112 mg of chromium per g of alginate beads (dry wt.) at pH 4.5; this uptake is higher than that reported for several biosorbents. The experimental data fitted the Langmuir adsorption model; however, the maximum uptake computed from it was 32% lower than the value measured. SEM analysis of the cross section of beads after uptake experiments showed no evidence of precipitation of chromium within the alginate beads at the pH ranges tested. A residual concentration around 0.3 mg/L of chromium, which allows safe discharge of solutions having Cr(III), was reached when alginate beads were challenged with a solution initially having as low as 10 mg/L of the metal. EPMA-EDX analysis of Cr-loaded beads showed a uniform distribution of chromic species throughout the structure of alginate beads, regardless of the solution pH.