Electrochemical reduction of nitrate in the presence of silver-coated polyvinyl alcohol beads as a spatially suspended catalyst

Abstract

Increasing concentrations of nitrate in water sources are a growing problem, requiring the development of techniques to remove them. In the present study, nitrate removal was investigated in undivided and divided electrolytic cells in the absence and presence of Ag0-coated polyvinyl alcohol (Ag-PVA) beads. In the undivided cell with graphite anode and iron cathode, nitrate removal was limited to 13 and 28 % in the absence and presence of 6.67 mM Ag-PVA, respectively, with negligible Total Nitrogen (TN) removal in both case. In a divided cell, nitrate and TN removal were 70 and 12 %, respectively, in the absence of Ag-PVA, increasing to 85 and 52 %, respectively, in the presence of 6.67 mM Ag-PVA. Nitrate removal in the divided cell in the presence of Ag-PVA using different cathode materials followed an order: Fe > Ti ≈ Cu > SS > Tin-coated Fe (Fe/Sn). The highest conversion to N2 was obtained with a Fe cathode with negligible nitrite formation. Among different concentrations of Ag-PVA, the highest nitrate removal and selectivity for N2 was achieved at 6.67 mM. Increasing the current density to 15 mA/cm2 increased nitrate reduction with improved selectivity for N2. It appears that the intercalation of nascent hydrogen on Ag0 leads to the formation of reductive species that enhance nitrate reduction. Ag-PVA beads could be reused up to 8 times. This study introduces novel, easily recoverable, and highly reusable Ag-PVA beads as a catalyst for the efficient removal of nitrate from water.