Removal efficiency and mechanism of nitrate in the medium pressure ultraviolet/formate system
In this study,the nitrate removal efficiency and mechanism in the combination of medium pressure ultraviolet and formate(MPUV/HCOO-)system was investigated.The results showed that the nitrate removal efficiency in the MPUV/HCOO-system was significantly higher than that of MPUV photolysis alone.When the initial NO3-concentration was 0.1 mmol·L-1 and the HCOO-dosage was 1 mmol·L-1,the pseudo-first-order degradation rate of NO3-in the MPUV/HCOO-system(5.7×10-3 min-1)was 5.7 times higher than that of MPUV photolysis alone(1.0×10-3 min-1).The addition of HCOO-increased the NO3-reduction efficiency from 13.7％to 44.1％after 120 minutes.Increasing MPUV intensity and weakly alkaline pH condition were conducive to nitrate degradation.This study indicated that the reduction efficiency of NO3-was collectively enhanced by the direct photolysis of NO3-by MPUV and the reduction process mediated by carbon dioxide radical anion(CO2-)generated from HCOO-.Using the laser flash photolysis technology,the second order rate constants of the reaction between CO2-and nitrate or CO2-and nitrite were first quantified as 1.57×106 L·(mol·s)-1 or 9.12×107 L·(mol·s)-1,respectively.The MPUV/HCOO-system could lead to the transformation from NO3-to NO2-,supplemental hydrogen peroxide promoted the formation of CO2-,which could react with NO2-and produce gaseous nitrogen(such as nitrogen gas and nitrous oxide)discharged from water.In wastewater treatment,the MPUV/HCOO-system could be constructed with the medium pressure ultraviolet disinfection and the residual formate as the carbon source of biological treatment,which is expected to strengthen the deep denitrification of sewage and has a good application prospect.
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