Non-quinone redox mediators enhanced perchlorate bioreduction: Effect, structure-activity relationship and mechanism

Abstract

• Perchlorate bioreduction rates were enhanced 1.40 to 1.64-fold by NQ-RMs. • Electrochemical and chemical properties of NQ-RMs was related to its enhanced effect. • NR and TPPS improved electrochemical activity and accelerated electron transfer. • NR and TPPS promoted cell metabolism activity. • NR and TPPS enhanced perchlorate reductase activity. The effect of various non-quinone redox mediators (NQ-RMs) on the enhancement of perchlorate bioreduction were investigated in this study, with analysis of the structure-activity relationship and mechanism. In the presence of Netural Red (NR), Riboflavin, Meso-tetrakis (4-sulfonatophenyl) porphyrin (TPPS), Meso-Tetraphenylporphyrin (TPP) and 5,10,15,20-tetrakis (4-hydroxyphenyl) porphyrin (THPP), perchlorate bioreduction rates were enhanced 1.40, 1.23, 1.67, 1.57 and 1.64-fold at 48 h, respectively. Structure-activity relationship analysis showed that NQ-RMs with increasingly negative redox potentials and greater redox reversibility were more conducive to perchlorate bioreduction. According to inductive effect, conjugated effect and Mulliken population analysis, substituents can affect the perchlorate bioreduction efficiency of NQ-RMs with similar chemical structures. Cyclic voltammetry and Tafel plot analysis demonstrated that the electron transfer rate was accelerated by NR and TPPS. The increased extracellular polymeric substance (EPS) secretion and electron transfer system activity proved that cell metabolic activity was promoted by NR and TPPS. Moreover, perchlorate reductase activity was enhanced by NR and TPPS. As a result, the possible mechanism for enhancement of perchlorate bioreduction by NR and TPPS is the acceleration of electron transfer, promoting metabolic activity and enhancing perchlorate reductase activity. This study proposed a strategy for screening of NQ-RMs and provided reference for improving the efficiency of perchlorate pollutants bioreduction.