Photooxidation of neonicotinoids (imidacloprid) in aqueous solution by vacuum UV induced advanced oxidation process

Anthropogenic activities have caused pollutions of water sources with organic micro pollutants most of which are recalcitrant and not easily degradable in nature. In recent years, such pollutants have been frequently detected in various water sources and are considered to be one of the major threats to water quality. Among organic micro pollutants, imidacloprid which is the first introduced neonicotinoid insecticide and the most commonly used one, receives a high attention since its widespread presence in water bodies and persistence to conventional biological and chemical water/wastewater treatment methods. Imidacloprid pollution of aquatic systems has been found in several countries. Advanced oxidation processes (AOPs) has been successfully applied to oxidize recalcitrant organic pollutants in water and wastewater treatments. Vacuum ultraviolet (VUV) based AOP has gained attention in recent years to treat organic contaminants in water. Unlike to other AOPs, VUV irradiation is capable of generating hydroxyl radicals in water without addition of oxidants and catalysts. Primary goal of this doctoral study was to investigate effectiveness of VUV process on the degradation of imidacloprid in water. Additionally, effects of various experimental parameters, flow rate, initial pH of solution, initial imidacloprid concentration, presence of inorganic ions (HCO3-, CO32-, NO3-), water matrix, presence and absence of dissolved oxygen on the VUV induced photooxidation of imidacloprid was investigated. Independent performance verification of the VUV process was executed by discoloration of commercially available reactive textile dyes (Synozol Red KH-L and Synozol Yellow KH-L) in water. Effects of sleeve materials (clear fused quartz and high purity synthetic quartz) were also investigated during the discoloration study. The results showed that imidacloprid and reactive textile dyes were rapidly degraded by the VUV process. pH of the experimental solution played an important role in the degradation of imidacloprid by the VUV process. Significant decrease (15.33%) in the rate of degradation of imidacloprid was observed at basic pH=11 condition. Presence of inorganic ions (HCO3-, CO32-, and NO3-) also noticeably impacted the decomposition of imidacloprid via VUV photons. Among the experimental parameters water matrix affected the VUV photooxidation of imidacloprid the most due to presence of natural organic matters (NOM). Nevertheless, complete imidacloprid reduction was attained in less than 5 minutes of reaction time even in the presence of inorganic ions and natural organic matters. It was found that presence of dissolved oxygen did not have a significant impact on the degradation and mineralization processes. Kinetic analyses showed that degradation of imidacloprid by the VUV process under the all tested experimental conditions followed a pseudo first order reaction kinetic. Observed reduction rate constants of imidacloprid (Co = 5 mg/L) depending on the experimental conditions varied between 1.3877 min-1 and 1.9213 min-1. Almost 80% of 10 mg/L imidacloprid was mineralized by the VUV process within 2 hours of irradiation. LC/MS Q-TOF analyses revealed the generation of several byproducts during the VUV induced photooxidation of imidacloprid. Separate hydrolysis experiments revealed that imidacloprid was very stable under acidic (pH=3) and natural (original, pH=6.469) conditions during 104 days of hydrolysis time. Hydrolytic degradation of imidacloprid significantly increased under tested basic (pH=11) condition. First order kinetic pattern with the hydrolytic rate constant of 0.0083 day-1 and half-life of 83.51 days was observed at the tested alkaline solution. Additionally, it was found that Synozol Red KH-L and Synozol Yellow KH-L were also effectively removed by the VUV process. Textile dye experimental results showed that purity of the quartz sleeve used in the VUV photooxidation system had significant impact on the discoloration efficiency. Keywrods: Organic micro pollutants, Imidacloprid, Neonicotinoids, VUV irradiation, Advanced Oxidation Process, Photooxidation, Hydroxyl radicals, Hydrolytic degradation, Synozol Red KH-L, and Synozol Yellow KH-L