Reduction of Cr(VI) into Cr(III) by Spirulina dead biomass in aqueous solution: Kinetic studies

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Spirulina sp., a blue green algae, enriched in protein is capable in Cr(VI) removal from aqueous effluent. This study explores the effect of chloride, sulfate and nitrate on Cr(VI) reduction to Cr(III) using Spirulina sp. biomass. The batch laboratory scale experiments are conducted with fixed initial pH of 0.5, biomass dose of 5 g L-1 and initial Cr(VI) of 200 mgL-1. The influence of various background anions are investigated varying the concentration from 100 to 400 mg L-1. 

The results show that the presence of anions inhibits Cr(VI) reduction rate to Cr(III) using Spirulina biomass. 

However, the effect is not appreciable at the lower concentration of the anions. The inhibitory effect of these ions on the 

reduction of Cr(VI) follows the order of nitrate>sulfate>chloride with almost similar results with Cl- and SO42-. The influence of anions on Cr(VI) reduction rate depends on biomass type and metal solution chemistry. The results obtained are possibly due to the combined effect of the (i) variation of frequency of contact of Cr(VI) with active biomass sites with different anions, (ii) competition of accessibility between the metal ions and background anions for the limited number of active sites and, (iii) different Cr(VI) species concentration based on the aquatic chemistry. The results show that Cr(VI) reduction efficiency is decreased by around maximum 2 and 10.6% in case of Cl- and SO42- respectively. The same is about 13.7% in presence of NO-3 under the test conditions. 

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