BACKGROUND Microbial reduction of Cr(VI) has been demonstrated as an environmentally friendly remediation protocol in recent years. In this study, the catalytic capability of soluble quinone compounds [i.e. anthraquinone-2,6-disulphonate (AQDS), 2-hydroxy-1,4-naphthoquinone (HNQ) and ubiquinone (CoQ)] as well as an immobilized HNQ-graphene oxide (GO) composite were evaluated for their ability to mediate Cr(VI) bioreduction by Shewanella xiamenensis. Additionally, the mechanism for Cr(VI) bioreduction was rigorously explored. RESULT At a test concentration of 0.02 mmol L-1, HNQ exhibited the highest efficiency for catalysing Cr(VI) bioreduction compared to AQDS and CoQ. During a 48 h test period, the highest removal efficiency (100%) of Cr(VI) (50 mg L-1) was exhibited in the presence of 0.015 mmol L-1 HNQ compared to biotic supplementations with 0.005, 0.010 and 0.020 mmol L-1 of HNQ. Furthermore, supplementation with 13.0 mg HNQ-GO composite greatly enhanced Cr(VI) bioreduction compared to the biotic assay without any quinone compounds. Specifically, complete reduction of Cr(VI) was found in the first reaction cycle (following 72 h), compared to < 26% Cr(VI) reduction observed in the control group under the same reaction time. Moreover, HNQ-GO composite was recovered and reused up to three times resulting in a similar to 20% loss of catalytic efficacy during the third use. In the presence of HNQ-GO composite, Cr(VI) bioreduction resulted predominantly from the enhancement of extracellular electron transfer mediated by HNQ and partly due to intracellular electron transfer processes occurring in the special metal respiratory system. CONCLUSION Our findings provide a promising alternative technology for remediating Cr(VI)-polluted wastewater. (c) 2018 Society of Chemical Industry