Nitrogen oxide removal from simulated flue gas by UV-irradiated electrolyzed seawater: Efficiency optimization and pH-dependent mechanisms

UV/chlorine has attracted much attention in flue gas and water treatment fields in recent years. However, only a few studies highlighted gas pollutants removal performance of UV/chlorine method in real water. In this study, nitrogen oxide (NO x ) removal using UV-irradiated electrolyzed seawater (UV/electrolyzed seawater) was explored under simulated seawater environment in a semi-batch photochemical reactor. Comparison experiments between UV/H 2 O 2 and UV/chlorine in both freshwater and seawater systems were performed. Results showed that NO removal efficiency in UV/chlorine freshwater system was higher than those in UV/chlorine seawater system, due to radical scavenging effects of water matrices in seawater. Several strong reactive radicals (e.g., OH, Cl, OCl, ClOH − ) yielded from UV/chlorine system were considered to contribute to NO x removal, among which the role of OH radical was confirmed by radical-trapping experiments. The alkalinity of the seawater adversely impacted contributions of these reactive radicals to NO x removal, which caused a sharp drop in NO x removal efficiencies. Comparatively, the chloride ion in seawater had slight influences on NO x removal efficiencies of UV/electrolyzed seawater. Then, optimization of NO x removal efficiencies against three key variables (active chlorine concentration, UV irradiation dose, and initial pH) was investigated in simulated seawater using Design Expert Software. A regression model of NO x removal efficiency was obtained with satisfactory accuracy. Results indicated that active chlorine concentration and UV irradiation dose significantly influenced NO x removal efficiencies, compared with pH effects. Furthermore, NO x removal mechanisms involved in reactive radicals were discussed for UV/electrolyzed seawater method. Due to pH-dependent characteristics of both active chlorine species and carbonate species in electrolyzed seawater, NO x removal pathways by UV/electrolyzed seawater were different as the environment changed from acid to alkalinity.