Effects of anion species and concentration on the removal of Cr(VI) by a microalgal isolate, Chlorella miniata

The presence of nitrate, chloride and sulfate anions on the removal of Cr(VI) by Chlorella miniata was investigated. Results of kinetic studies indicated that the equilibrium time in each anion system increased with increases of the initial salt concentration, and the inhibitory order was NO₃^- > Cl^- > SO₄^2- for Cr(VI) removal and was SO₄^2- > Cl^- ≈ NO₃^- for the biosorption of the bioreduced Cr(III). The inhibitory effect caused by different anions was attributed to biosorption mechanism and metal speciation. Since both biosorption and bioreduction were involved in Cr(VI) removal, the presence of anions could compete with Cr(VI) for the adsorption sites, and the affinity of anions to the algal biomass followed the order of NO₃^- > Cl^- > SO₄^2-, which was consistent with their inhibitory order on Cr(VI) removal. Speciation results also indicated that the formation of CrO₃SO₄^2- in the sulfate system made it easier to be adsorbed on the biomass than HCrO₄^-. The biosorption-bioreduction model further suggested that the bioreduction rate constant k decreased with increases of anion concentrations ranging from 0 to 0.5 M, and followed the order of SO₄^2- > Cl^- > NO₃^-. The biosorption constant b also decreased with anion concentrations in the range of 0-0.2 M, suggesting that this parameter was more sensitive to anion effects than the k values. The higher b values in the sulfate than in the nitrate and chloride systems indicated that Cr(VI) in the sulfate system was more easily adsorbed on the algal biomass. These findings demonstrated that the presence of anions significantly affected the removal of Cr(VI) by C. miniata. Since chloride, nitrate and sulfate ions are commonly found in industrial wastewater, it may be necessary to eliminate these ions prior to chromium removal.