Decoupling mechanism of Acid Orange 7 decolorization and sulfate reduction by a Caldanaerobacter dominated extreme-thermophilic consortium.

The biological treatment of textile wastewater discharged from the dye baths and rinsing processes are challenged by both high temperatures of 50-80 °C and sulfate reduction. At present, most studies report azo dyes can be removed under mesophilic conditions, but the sulfate reduction is inevitable, consuming extra electron donors and producing undesirable sulfide. In this work, a Caldanaerobacter (> 97%) dominated extreme-thermophilic consortium (EX-AO7) was enriched using xylose as the substrate. The typical sulfate-reducing enzymes such as sulfite oxidase and sulfite reductase were not identified in enriched EX-AO7 by the metagenomic analysis. Then, the decolorization and sulfate reduction were expectedly decoupled by enriched EX-AO7 in extreme-thermophilic conditions, in which no sulfide was detected during the AO7 decolorization process. AO7 of 100 and 200 mg/L could be totally decolorized by EX-AO7. However, when 400 mg/L AO7 was added, the residual AO7 concentration was 22 ± 19 mg/L after 24 h, which was mainly due to the toxicity of AO7. Dosing zero-valent iron (ZVI) could also promote AO7 decolorization by 1.7 times since the addition of ZVI could provide a proliferative environment for EX-AO7 growth. Thereby, our work provides a new paradigm to promote the AZO dyes decolorization and minimize sulfate reduction.