Effects of electrolyte pHs, temperatures, potentials and oxalate ions on the electrocatalytic reduction of nitrates

Nitrate reduction reaction (NO 3 - RR), an emerging technology for clean and efficient nitrate removal, is of great significance to reduce water pollution. In this study, NO 3 - RR activity was assessed on the copper foils, and the effects of various variables including initial pHs, temperatures, and nitrate concentrations on the NO 3 - RR were investigated. The copper foils displayed excellent NO 3 - RR activity with a high ammonium selectivity of 45% at pH = 1 and a conversion rate of 55% at pH = 13. The conversion of nitrate to nitrite was the rate determining step at pH = 7 and pH = 13. Oxalic ions displayed significant inhibition effects on NO 3 - RR by copper foils in the acidic and alkaline electrolytes. In particular, the inhibition effect of oxalate ions on NO 3 - RR approached to 50% with a nitrate/oxalate ratio of 2:1 in acidic media, which is likely attributed to the formation of complexes between oxalate ions and the copper surface. Such an inhibition effect is not noted in the neutral electrolyte. Structural characterizations including atomic force microscopy, Raman spectroscopy and Fourier transform infrared spectroscopy elucidated that the oxalate ions occupied a large number of the copper sites on the surface of copper foil to form competitive adsorption with nitrate. The experimental parameters for NO 3 - RR by copper foils were comprehensively analyzed, affording new insights on the complex interactions between the copper, nitrate, and oxalate.