Nitrate reduction through the visible-light photoelectrocatalysis and photoelectrocatalysis/reverse osmosis processes: Assessment of graphene/Ag/N-TiO2 nanocomposite

Abstract

Coupling photoelectrocatalysis with reverse osmosis (RO) provides a promising way to improve the performance of RO through the elimination of concentrated nitrate in retentate flow. However, poor visible-light response limits the performance of photoelectrocatalytic process. Hence, graphene/Ag/N-TiO2 was prepared as a good visible-light response through loading N-TiO2 nanoparticles, Ag particles and graphene nanoplatelets on Ti plate using electrophoretic deposition method. The prepared nanocomposite was characterized by X-ray diffraction (XRD), diffuse reflectance spectra (DRS), scanning electron microscopy (SEM), energy dispersive X-ray (EDX) and N2 adsorption/desorption isotherm. First, the effect of pH, bias potential, catalyst surface area and irradiation power on the nitrate reduction was assessed through the photoelectrocatalytic process. It was observed that almost 85 % of the nitrate was reduced through the photoelectrocatalytic process using the optimized conditions including neutral pH, potential of -1.5 V, three electrodes and three visible-light lamps. Nitrate removal efficiency was considerably higher than sorption (8%), photocatalysis (8%) and electrocatalysis (35 %) processes. Furthermore, 33 % of nitrate was reduced to NO2− and NH4+ using the optimized photoelectrocatalytic process after 120 min. This indicates more than 50 % of nitrate reduction products are volatile compounds such as N2 and N2O which can be get out of the water. Finally, the photoelectrocatalytic process using the graphene/Ag/N-TiO2 was integrated with RO to eliminate the nitrate in concentrated retentate of the RO through the reclamation. Results showed that the nitrate concentration was not increased over time on the contrary of observations made with RO.