Towards a novel two-phase liquid-liquid bioreactor for microbial Cr(VI) removal from wastewaters

The scope of the present study was to identify the most effective and less toxic combination of various organic solvents and extractants for the extraction of Cr(VI) from synthetic wastewaters with extremely high chromium concentration (up to 1,000 ppm) in order to be further used in a novel two-liquid-phase bioreactor for microbial reduction of Cr(VI). In terms of organic solvents, hexane, heptane, chloroform, ethyl-acetate, and kerosene were tested, whereas Aliquat 336, TOPO, and TPB were used as extractants. The effect of the pH of the aqueous phase and of the ratio of organic to aqueous phase were studied in terms of Cr(VI) extraction. The extraction capacities of different combinations of solvents/extractants and the addition of 1-hexanol as a stabilizer for the most effective separation of phases were also assessed. Moreover, the toxicity of each solvent was assessed using acclimated mixed consortia. The acclimated, enriched culture was further used as inoculums for the start up of a sequential batch reactor for the reduction of Cr(VI) with quite promising results. When operating the bioreactor as a two-phase system, however, the microbial consortium finally collapsed. This was attributed to the toxic effect of the extractant Aliquat 336, when being in prolonged contact with the micro-organisms.