CHROMIUM(VI) TOXICITY TO LUMINESCENT BACTERIA

The toxicity of chromium(VI) to luminescent bacteria was determined us ing the Microtox(R) bioassay. The aim of this study was to assess the influence of pH on the toxicity of Cr(VI) samples because its speciati on depends on this parameter. A Microtox test basic protocol, modified to assure the same pH in all the cuvettes, was used to determine the toxicity of the Cr(VI) compounds. In the range studied, 4.6 to 9.3, th e observed EC50 values increased with the pH (31-1,184 mg/L Cr(VI)). T hese toxicity results were related to the percentage of the different Cr(VI) species. Hydrogenchromate anion was found to be the most toxic species. In general, only slight differences in toxicity were found wh en the Microtox test basic protocol was used. Nevertheless, when 20.4% (w/v) sucrose was used as an alternative medium to 2% (w/v) NaCl, the results obtained could not be compared because the sensitivity of the bacteria in this medium was found to be different. The influence of a n exposure-time effect was also evaluated. In 2% (w/v) NaCl medium, th e toxicity increased with time, which indicates a slow response of the bacteria to the toxin.