Reduction mechanisms of V5+ by vanadium-reducing bacteria in aqueous environments: Role of different molecular weight fractionated extracellular polymeric substances

Extracellular polymeric substances (EPS) are high-molecular polymers secreted by microbes and play essential roles in metallic biogeochemical cycling. Previous studies demonstrated the reducing capacity of the functional groups on EPS for metal reduction. However, the roles of different EPS components in vanadium speciation and their responsible reducing substances for vanadium reduction are still unknown. In this study, the EPS of Bacillus sp. PFYN01 was fractionated via ultrafiltration into six components with different kDa (EPS >100 , EPS 100 – 50 , EPS 50 – 30 , EPS 30 – 10 , EPS 10 – 3 , and EPS <3 ). Batch reduction experiments of the intact cells, EPS-free cells, the pristine and fractionated EPS with V 5+ were conducted and characterized. The results demonstrated that the extracellular reduction of V 5+ into V 4+ by EPS was the major reduction process. Among the functional groups in EPS, C=O/C-N of amide in protein/polypeptide and C O of carboxyl in fulvic acid-like substances might act as the reductants for V 5+ , while C O in polysaccharide molecules and P O in phosphodiester played a key role in the adsorption process. The intracellular reduction was via translocating V 5+ into the cells and releasing V 4+ by the intracellular reductases. The reducing capacity of the fractionated EPS followed a sequence of EPS <3 > EPS 10 – 3 > EPS 50 – 30 > EPS 100 – 50 > EPS 30 – 10 > EPS >100 . The small molecules of fulvic acid-like substances and amino acids were responsible for the high reducing capacity of EPS <3 . EPS >100 had the lowest reducing capacity due to its macromolecular structure decreasing the exposure of the reactive sites. In addition to reduction, those intermediate EPS components may also have supporting functions, such as connecting protein skeletons and increasing the specific surface area of EPS. Therefore, the diverse effects of the EPS components cannot be neglected in vanadium biogeochemical cycling.