Facilitated Fe(II) Oxidation but Inhibited Denitrification by Reduced Graphene Oxide during Nitrate-Dependent Fe(II) Oxidation

Microbially driven nitrate-dependent Fe(II) oxidation (NDFO) plays an important role in anaerobic iron transformation and occurs in both natural and engineered environments. Different factors including pH, oxyanions, humics, and mineralogical nucleation sites have been found to impact the kinetics and products of NDFO process. Considering that the increasing production and application of engineered nanomaterials led to their inevitable release into NDFO-occurring environments, the impacts of engineered nanomaterials on NDFO process deserved investigation. Here, the influences of reduced graphene oxide (rGO), a typical engineered nanomaterial, on NDFO process of Acidovorax sp. strain BoFeN1 were studied. It was found that the Fe(II) oxidation rate increased with increasing rGO concentrations (0.5-100 mg/L), and around 12.5-62.5% increases in pseudo-first-order rate constant of Fe(II) oxidation were observed. Moreover, rGO-mineral composites were formed, and cell encrustation was mitigated in the presence of rGO. In comparison to BoFeN1 cells, rGO with larger surface area and lower zeta potential promoted Fe(II) oxidation by facilitating Fe2+ adsorption and nucleation and growth of biomineralization products. However, although rGO promoted chemodenitrification, the whole nitrogen transformation process including nitrate reduction, nitrite accumulation, and gaseous nitrogen production was restricted, probably due to the inhibition of cell growth and biological denitrification by rGO. This study underlines the impacts of rGO on Fe/N transformation via participation in NDFO process in natural environment and the application of NDFO in wastewater treatment, and suggests the potential role of engineered nanomaterials in influencing NDFO process.