Role of Extracellular Polymeric Substances in a Methane Based Membrane Biofilm Reactor Reducing Vanadate

For the first time, we demonstrated vanadate (V(V)) reduction in a membrane biofilm reactor (MBfR) using CH₄ as the sole electron donor. The V(V)-reducing capability of the biofilm kept increasing, with complete removal of V(V) achieved when the influent surface loading of V(V) was 363 mg m^-2 day^-1. Almost all V(V) was reduced to V(IV) precipitates, which is confirmed by a scanning electron microscope coupled to energy dispersive X-ray spectroscopy (SEM-EDS) and X-ray photoelectron spectroscopy (XPS). Microbial community analysis revealed that denitrifiers Methylomonas and Denitratisoma might be the main genera responsible for V(V) reduction. The constant enrichment of Methylophilus suggests that the intermediate (i.e., methanol) from CH₄ metabolism might be used as the electron carriers for V(V) bioreduction. Intrusion of V(V) (2-5 mg/L, at the surface loading of 150-378 mg m^-2 day^-1) into the biofilm stimulated the secretion of extracellular polymeric substances (EPS), but high loading of V(V) (10 mg/L, at the surface loading of 668 mg m^-2 day^-1) decreased the amount of EPS. Metagenomic prediction analysis established the strong correlation between the secretion of EPS and the microbial metabolism associated with V(V) reduction, tricarboxylic acid cycle (TCA) cycle, methane oxidation, and ATP production, and EPS might relieve the oxidative stress induced by high loading of V(V). Colorimetric determination and a three-dimensional excitation-emission matrix (3D-EEM) showed that tryptophan and humic acid-like substances might play important roles in microbial cell protection and V(V) binding. Fourier transform infrared (FTIR) spectroscopy identified hydroxyl (-OH) and carboxyl (COO^-) groups in EPS as the candidate functional groups for binding V(V).