Degradation Of TCAA Based On Carbon Dioxide Radicals:Efficiency And Mechanism

Among the currently available disinfectants, chlorine is globally the most frequently used disinfectant in drinking water due to its high inactivation efficiency, low cost, and residual disinfecting capacity. In the chlorine disinfection process, some organic matter in water such as natural organic matter (NOM), react with disinfectants by oxidation, addition and substitution reactions to form chlorinated disinfection by-products (DBPs). Trihalomethanes (THMs) and Haloacetic acids (HAAs) are the two major categories of chlorinated DBPs in drinking water. HAAs cancer risk accounted for more than 91.9% of the total chlorinated DBPs. Therefore, developping technology which can effectively control HAAs in drinking water is very urgent. Previous research mainly focused on formation mechanism and removal of HAAs precursors. Little research on the removal of formed HAAs in drinking water has been found. This paper proposed the reduction technology based on the CO2.- radical to degrade TCAA in water, and discuss the effects of common drinking water background components, TCAA’s transformation mechanism and degradation kinetics model in detail.By comparing degradation efficiency of TCAA for different processes (UV, UV/TiO2、UV/TiO2/formate(FM)), UV/TiO2/FM process was found to be the most efficient (150 min,>99% degradation). To evaluating the potential for practical application for UV/TiO2/FM process, TCAA degradation was determined experimentally by quantifyng the effects of drinking water background components including dosage of FM, solution pH, temperature, HCO3-, DOM, NO3-. The degradation of TCAA by UV/TiO2/FM process was enhanced by the increased FM dosage at 0.5-5mM. With further increasing dosage of FM, degradation rate of TCAA dropped dramatically. Better elimination performance was obtained at low or high pH, the worst performance was received at pH 5.0. Low temperature is not conducive to the TCAA degradation. TCAA removal efficiency increased greatly as the temperature increased, and TCAA degradation by FM/UV/TiO2 process had an optimal temperature (25℃). Experimental data indicated that addition of HCO3-improved TCAA removal significantly. It is demonstrated that the removal efficiency of TCAA increased with HA concentration increase from 0 mg/L through 0.5 mg/L to 1.0 mg/L. But when HA concentration further increased, enhancement gradually became weak and even disappear. NO3- at concentration of 30-60mg/L inhibited the degradation rate, [NO3-]=120-240mg/L:photosensitive effect of NO3- slowly became dominant, [NO3-]=360mg/L:Significant enhancement effect was receivedCO2·- and eaq- initiate TCAA degradation in UV/TiO2/FM process. The EPR measurements approved the formation of CO2·- radical when the FM/TiO2 slurry was irradiated with 254nm UV light. High dechlorination rate (>60%) was observed after reaction started.The kinetic and transformant investigation lead to the suggestion that CO2·- as well as eaq- were involved in the TCAA degradation mechanism and C-Cl bond was attacked.Finally, the degradation kinetics of UV/TiO2/FM process was studied using kinetic competition method. As introducing Nitrobenzene (NB) as a probe compound, the reaction rate constant of TCAA with CO2·- was determined to be 1.8×108M-1s-1. Based on the reaction mechanism and steady-state assumption, kinetic model of TCAA degradation was deduced.