Comparison of chromium III and VI toxicities in water using sulfur-oxidizing bacterial bioassays

• Toxicities of Cr III and Cr VI were compared on sulfur oxidizing bacterial bioassays in fed-batch and batch tests.
• Cr III was not toxic even at higher concentrations while Cr VI was toxic to SOB at lower concentrations.
• Cr (III) was non toxic due to the structural similarity with Fe3+ and absence of iron uptake pathway.
• The toxicity of Cr (VI) was mainly due to its structural similarity with SO42−.
• An EC50 of 2.7 mg/L and 1.5 mg/L were measured for fed-batch and batch SOB tests.

The toxicities of Cr (III) and Cr (VI) in water were evaluated using sulfur-oxidizing bacterial (SOB) bioassays both in batch and fed-batch conditions. Two days were enough for a quick buildup of SOB consortium in the master culture reactor (MCR). At concentrations up to 100 mg L−1, Cr (III) was found to be nontoxic in both conditions, while Cr (VI) at very low concentrations (0.1–2 mg L−1) was very toxic to the SOB. Literature review suggested that the nontoxic nature of Cr (III) might be due to the absence of the iron uptake pathway in Acidithiobacillus caldus (the predominant bacteria in our reactors), which is required for Cr (III) uptake. The 2-h median effective concentration (EC50) values obtained for Cr (VI) in the batch and fed-batch tests were 2.7 mg L−1 and 1.5 mg L−1, respectively.