Kinetic Study on the Reduction of Room-Temperature NO_x in Etching Waste Gas by Na₂S₂O₅ and Urea in Alkaline Pretreatment

Etching waste gases derived from photovoltaic cell production are difficult to treat at room temperature. Therefore, the use of wet denitrification with Na₂S₂O₅ is recommended to overcome this issue. The presence of O₂, Cl₂, and metals in gases leads to the consumption of 65% of SO₃^2– within 2 h. Although alkaline pretreatment adsorbs most of the gases and metals, the high ceiling temperature results in a short residence time and incomplete reaction with the lye. Therefore, in this study, urea was added to Na₂S₂O₅. After adding the optimum concentration of urea (0.5 mol L^–1) to a 1.0 mol L^–1 SO₃^2– solution, the anodic potential measured using cyclic voltammetry shifted from 0.051 to −0.119 V, the electron transfer rate k⁰ decreased from 4.89 × 10^–5 to 4.27 × 10^–5 cm s^–1, the exchange current density measured using the polarization curve decreased from 18.37 to 13.58 A cm^–2, and the reaction activation energy increased from 37 to 63 kJ mol^–1. Therefore, adding urea to generate hydroxyl functional groups can effectively block the free radical oxidation chain reaction of SO₃^2–, hindering its oxidization. In summary, alkaline pretreatment and urea addition can increase the effective reaction between SO₃^2– and NO_x and reduce the costs. Our study provides data to support the application of wet denitrification at room temperature.