Study On Reaction Mechanism,Products And Toxicity Of Diclofenac During Chlorine Dioxide And Ferrate(Ⅵ) Disinfection

In recent years, diclofenac(DCF) is one of the most commonly used non-steroidal anti-inflammatory drugs and emerging micro-organic contaminants, which has been frequently detected with high levels in the aquatic environment. Researches have shown that traditional flocculation, precipitation and filtration cannot remove DCF efficiently because of its low biodegradability and limited elimination capacity,leading to significant emissions to drinking water disinfection process. As two sorts of new multifunctional water treatment reagents and oxidants, chlorine dioxide(ClO2) and ferrate(Fe(Ⅵ)) are widely used for the disinfection of relatively high quality water including groundwater, surface water and drinking water or used as preoxidation of wastewater before conventional disinfection process. They have been proved to be promising disinfectants to remove both inorganic and organic contaminants. However, due to poor mineralization of water treatment, research on organic pollutants removal as well as byproducts formation is critically important to address the safety concerns of drinking water treatment, since some byproducts may be of similar or even higher toxicity compared with their parent compounds.To both investigate the removal efficiency and reaction kinetics and elucidate degradation mechanism and toxicity variation, we select DCF as target compound and ClO2 and Fe(Ⅵ) as two typical disinfectants respectively to carry out our systematic study, which provide theoretical basis and technical support for practical application in drinking water treatment. The primary conclusions achieved in this work are as follows:1. The reaction kinetics, removal efficiency and influence factors on the degradation of DCF by aqueous ClO2 were investigated under simulated water disinfection conditions. The results showed that DCF could be rapidly and completely oxidized in the presence of an excess of ClO2. All the reactionsfollowed second order kinetic model and measured rate constant was 1.50×103 M-1·s-1 under the condition of pH 7.0 and 298 K. The reaction rate constants and degradation rates were obviously affected by the ClO2 concentration and reaction temperature instead of pH. The activation energy, enthalpy and entropy were7.87 kJ·mol-1、5.38 kJ·mol-1、-165.75 J·mol-1·K-1, respectively, which indicated that DCF-ClO2 reaction can occur under conventional drinking water treatment conditions. Base on the experiments results with the addition of inorganic salt or organic matter, different coexistent matters were found to play different roles in the DCF removal behavior. The presence of NO3-, Cl-, Br-, Ca2+, Fe3+ or surfactant CTAB optimized and accelerated the degradation of DCF by ClO2 to a certain extent, while the inhibitory effect were observed instead in the presence of NH4+, NO2-, I-, Fe2+, humic acid and surfactant SDBS, Tween-80, the addition of SO42- and Mg2+ had little impact on the removal of DCF. Furthermore the mechanism of these influences were analyzed.2. The reaction mechanism, toxicity variation and TOC removal efficiency were elucidated during the oxidation of DCF by ClO2. Nine byproducts were identified by UPLC-MS and 1H-NMR. Three main reaction pathways were based on initial decarboxylation of DCF on the aliphatic chain, hydroxylation and chlorination of the phenylacetic acid moiety at the C-4 position. DCF was oxidized by Cl O2 dominantly and by O2·- partially. With the increase of ClO2 dosage, TOC removal efficiency continuously increased,but the inhibition of luminescent bacteria vibrio fischeri initially increased and subsequently decreased,suggested that more toxic intermediates were formed. It was inferred that toxicity was mainly contributed by phenolic derivative and aldehyde intermediate.3. The reaction kinetics, removal efficiency and influence factors on the oxidation of DCF by Fe(Ⅵ)were investigated. The study demonstrated that ferrate could be applied to effectively remove DCF in the disinfection of potable water. The reaction between DCF and Fe(Ⅵ) followed second-order kinetics. Thevalue of rate constant was measured to be 5.04 M-1·s-1, at pH 9.0 and 298 K, which is lower than that of ClO2. The concentration of Fe(Ⅵ), pH and temperature exhibited significant influences on the DCF-Fe(Ⅵ)reactivity. The rate constants and degradation rates gradually decreased as pH was increased from 7.0 to11.0, and the species-specific rate constants were 16.8 M-1·s-1 for HFeO4- and 3.61 M-1·s-1 for FeO42-respectively. Ea, △H and △S were determined to be 23.3 kJ·mol-1, 20.9 kJ·mol-1,-161.0 J·mol-1·K-1,respectively. The experiments of coexisting substances in nature water showed that the same environmental factor has different effect results on two oxidation reaction system. In the Fe(Ⅵ) system, the presence of NO3-, HCO3-, NH4+, Ca2+, Fe3+, CTAB and humic acid clearly initiated its promotion, whereas the presence of Cl-, SDBS and Tween-80 had some inhibition effect on the removal of DCF, the addition of SO42- and Mg2+ still conveyed little influences on the reaction. Furthermore, the causal effects of the relevant factors were interpreted.4. The reaction mechanism, toxicity variation and TOC removal efficiency were explored during the Fe(Ⅵ) oxidation process. Five primary products were identified by UPLC-MS combined with the structure characteristic of DCF and reactivity properties of Fe(Ⅵ). Fe(Ⅵ) rather than ·OH radical plays a dominant role during the oxidation process. The reaction mechanisms were proposed to be hydroxylation of DCF and degradation products and coupling reaction of organic radical intermidates. DCF oxidation occurred an the phenol moiety and yielded chlorophenol, quinine intermediates and other organic dimmers. As the reaction time was prolonged, the mineralization of solution was gradually improved, while there was a slight decrease of inhibition rate during the initial reaction period. Subsequently, the overall toxicity began to increase rapidly and then decreased slowly, reaching a maximum inhibition rate when DCF had just completely disappeared. These results suggested that higher concentrations of chlorophenol derivatives did not yield appreciable effect on the overall toxicity, relative to quinine dimmer. In contrast, the detoxicationand mineralization process in the Fe(Ⅵ) oxidation system was longer than in the ClO2 oxidation system due to the formation of higher molecular weight of disinfection byproducts.