Nitrate reduction via micro-electrolysis on Zn-Ag bimetal combined with photo-assistance

A selective and efficient chemical reduction of nitrate to nitrogen gas using micro-electrolysis on ZnAg combined with a photo-assisted reduction in the presence of formic acid was investigated. The 99.58% removal of nitrate, 0.073 min^-1 of rate constant and 94.3% nitrogen selectivity were achieved in Zn-Ag/hv/HCOOH system under the initial pH 2.5, 13.8 mmol/L of formic acid and 60 g/L of Zn-0.06%Ag dose at 60 min. The Zn-Ag/hv/HCOOH system had the highest removal rate and nitrogen selectivity for nitrate reduction compared with the alone or two combinations of ZnAg bimetals, formic acid, and UV-A. Furthermore, the co-existence anions of HCO₃^- and CO₃^2- showed a negative effect on nitrate reduction while SO₄^2- had slightly promoted the reduction process. During the nitrate reduction by Zn-Ag/hv/HCOOH process, rapid reduction of NO₃^- to NO₂^- was primarily caused by ZnAg bimetal. Subsequently, the conversion of NO₂^- to N₂ was mainly owing to the produced CO₂^- by the reaction of formic acid and UV-A. The results suggested a novel strategy of chemical reduction combined with photoreduction for denitrification with high reaction kinetic as well as high nitrogen gas selectivity.