The number of studies dealing with the use of activated carbons as redox mediators in biological systems has grown rapidly in recent years. However, evidence explaining the role of the surface chemistry of activated carbons in the biotransformation of recalcitrant pollutants under continuous conditions is rather meager at present. This study offers a discussion about the role of the chemistry of activated carbon fibers (ACFs) on the increased reduction of a model nitroaromatic compound 4-nitrophenol (NP), under continuous conditions and using novel UASB-packed reactors. ACFs with different physicochemical properties and different levels of redox-active groups were studied, including un-modified (AW), HNO3 treated (OX) and modified with anthraquinone-2,6-disulfonate (AQDS) ACFs. The results indicate that biofilm formation could cover up the effect of the different ACFs studied during the first days of the continuous experiment. However, with modifications in the concentration of ethanol (exogenous carbon source) the interaction bacteria-ACF surface was enhanced and resulted in increased biotransformation efficiencies of 1.47-, 1.97- and 2.11-fold for the reactors packed with AW, AQDS and OX materials, respectively, as compared with the control reactor, which lacked any carbonaceous support. We correlated the observed NP biotransformation efficiencies in the order OX > AQDS > AW with the content or redox-active groups such as carbonyl groups (quinone moieties) on the ACF surface in the order 1.3 > 1.0 > 0.78 milliequivalents/g, respectively. The present work proposes a novel treatment concept to enhance the reductive biotransformation of contaminants and provides a deep understanding on the complex subject of using activated carbons as redox mediators in biological systems. (C) 2015 Elsevier B.V. All rights reserved.