Mutual effects of selenium and chromium on their removal by Chlorella vulgaris and associated toxicity

The release of selenium (Se) and chromium (Cr) into the environment from anthropogenic activities has posed a hazard to aquatic ecosystems. In this study, we used Chlorella vulgaris for Se/Cr bioremediation and evaluated their mutual effects on the removal efficiency. Our results found C. vulgaris highly effective in removing selenite-Se(IV) (49.5 ± 1.9%), selenate-Se(VI) (93.0 ± 0.5%), chromic nitrate-Cr(III) (89.0 ± 3.2%) and dichromate-Cr(VI) (88.1 ± 1.3%) over a 72 h period. Cr(VI) significantly impeded Se removal, particularly for selenate, due to competition between both for algal uptake, whereas Cr(III) obviously enhanced Se removal, increasing Se volatilization by ~29%. Similarly, Se significantly increase Cr removal rates, with a maximum of 94.6 ± 0.2% for the algal co-exposed to Se(IV) and Cr(III). To reduce residual pollutants in the alga, we applied combustion as a post-treatment to burn off >99% of the biomass Se for all Se treatments, whereas most of the biomass Cr (54.7-81.6%) remained in the ash at significantly higher levels (~7430 μg Cr/g DW). For toxicity, our speciation analysis found organo-Se (SeCys and SeMet) dominant in the alga exposed to Se, particularly selenite. No Cr(VI) but Cr(III) forms were detected in all Cr-exposed alga. Elemental Se disappeared from all Se-exposed alga in the presence of Cr(VI), while Se resulted in the emergence of Cr-acetate in all Cr(III)-treated alga. After combustion, mineral Se, particularly elemental Se dominated most of the ash; likewise, elemental Cr, along with Cr₂O₃, was found in all the ash. Overall, our research would contribute to developing a low ecotoxic algal treatment system for Se/Cr contaminated water.