Enhanced nitrate removal by alcohol-involved nitrate photolysis-induced advanced reduction process

Organic radical-based advanced reduction processes (ARPs) are garnering increasing attention for pollutant removal. However, there have been few reports on the use of organics other than formic acid in constructing ARPs. This study successfully developed a novel alcohol-involved ARP induced by nitrate photolysis and investigated its performance in nitrate removal using various alcohols (UV/NO3–/alcohol). Five alcohols, namely methanol, ethanol, n-propanol, 1,3-propanediol, and glycerol, significantly enhanced nitrate removal by at least 30%. While the photolytic removal of nitrate was 20% higher under basic conditions (pH 10) compared to pH 3 and pH 7, the efficiency of nitrate removal in the UV/NO3–/alcohol system was nearly independent of the initial pH (p greater than 0.05). However, in general, the basic condition (pH 10) promoted total nitrogen (TN) removal and reduced the residual amount of organic nitrogen in the effluent for all five alcohols. Methanol and ethanol exhibited similar performance in the UV/NO3–/alcohol systems, showing improved nitrate and TN removal compared to the other three alcohols containing a 3-carbon chain under basic condition (pH 10). Among the alcohols, UV/NO3–/glycerol resulted in the highest production of ammonia, approximately five times that formed in the UV/NO3–/methanol system. The stoichiometry of the nitrate-to-alcohol molar ratio in the UV/NO3–/alcohol systems was investigated and determined as 0.5 at an initial pH of 10 based on the analysis of nitrate removal efficiency and residual concentration of various nitrogen species. Dissolved oxygen did not significantly affect the NO3– removal and conversion in all five UV/NO3–/alcohol systems and cosolutes generally inhibited the nitrate reduction. The outcome of present study expands the alternative organic additives of ARPs to alcohols and demonstrates the potential effectiveness of the ARP as a viable approach for the reductive removal of pollutants.