The effects of hydraulic retention time (HRT) on chromium(VI) reduction using autotrophic cultivation of Chlorella vulgaris

Chromium is an acutely toxic heavy metal that is known to be a carcinogen. Of the two predominant forms of chromium, Cr(III) and Cr(VI), Cr(III) has only about one thousandth the toxicity of Cr(VI). Using microalgal biomass is one way to remove Cr(VI) from the environment. Four days of hydraulic retention time (HRT) was required to completely reduce 10 mg/L of Cr(VI) in the influent. Microalgal biomass is conventionally regarded as an adsorbent in most Cr(VI) reduction studies. However, this study found that Chlorella vulgaris had the potential to convert Cr(VI) to Cr(III) through the enzymatic route of chromium reductase although the measured chromium reductase activity of C. vulgaris was less than that reported values obtained in bacteria. X-ray absorption near-edge spectroscopy (XANES) analysis further showed the absorption edge of Cr(III) in Cr(VI)-treated C. vulgaris, supporting the assumption of Cr(VI) potentially being converted to less-toxic Cr(III).