Research On Formation Process Of Trichloronitromethane, Disinfection By-product In Drinking Water And Its Control Strategies

The safety of drinking water has become one of the most serious concerns and it is well known that the disinfection by-products in drinking water cause great harm to human health due to the carcinogenesis, genetic toxicity and mutagenicity. Therefore, it is of great practical significance to probe and research on the disinfection by-products in drinking water.In this study, the quantitative analysis of trichloronitromethane (TCNM), one kind of nitrogen disinfection by-product in drinking water by gas chromatography/mass spectrometry (GC/MS) on the extracting agent of methyl tertiary butyl ether (MTBE) and using1,2-dibromopropane as internal standard, was introduced. Based on this analytical method, the formation process of TCNM and its influencing factors were evaluated with methylamine as the precursor. In addition, the technologies, such as activated carbon adsorption, iron reduction and advanced oxidation technologies were applies to control the TCNM, and then its degradation mechanism and dynamic behaviors were also discussed.According to the spiked recovery and accuracy, the measurement method for TCNM was highly accurate and the recovery rate was between97.3%and106%with relative standard deviation of1.43%-2.25%and limit of detection less than1μg/L.It was indicated from the formation process of TCNM that the TCNM amount produced under alkali condition was higher than those produced under the neutral and acid conditions, and the TCNM amount increased with the increase of pH value. It was also found that the TCNM amount increased with the increase of chlorine addition when the chlorine dosage was in the range of2-8mmol/L. However, TCNM amount reduced when the chlorine dosage was enhanced from8mmol/L to12mmol/L, in which the concentration of free chlorine was higher and methylamine turned into nitriles and aldehydes through other reactions. Temperature is another important factor to affect the TCNM formation from methylamine especially in the range of10-30℃and the higher temperature was, the more TCNM amount was produced. The formation process of TCNM from methylamine by chlorination corresponded with the mechanism of electrophilic reaction, in which HClO and ClO-could be used as electrophilic reagent to attack methylamine and then to form TCNM. In order to enhance the removal efficiency of TCNM by activated carbon, the modification of granule activated carbon by sodium hydroxide was applied with the higher adsorption capacity. The surface physical and chemical properties of activated carbon before and after modification were investigated separately by surface porosity detector, scanning electron microscope (SEM), fourier transform infrared spectrometry (FTIR) and Boehm functional group titration method. The results showed that the specific surface area of NaOH(30%, w/v)-GAC increased by9.47%compared to that of GAC, while the acidic groups (mainly carboxyl group, lactone group and phenolic hydroxyl group) on the surface of NaOH-GAC reduced by29.6%, which revealed that the adsorption capacity of TCNM by modified GAC was enhanced greatly.The results of adsorption tests also showed that when the initial concentration of TCNM solution was10μ/L,87%removal was achieved using NaOH-GAC with the addition of0.3g/L, which was1.71times higher than that by GAC. The adsorption process of TCNM by the modified GAC could be divided into three phases, which are rapid phase, slow phase and dynamic equilibrium phase. The time of equilibrium adsorption of TCNM by GAC was36h, while the time by NaOH (30%, w/v)-GAC was reduced to6h.Meanwhile, the addition of iron scraps enhanced the removal efficiency of TCNM in low concentration, and when the initial concentration of TCNM was5μg/L, TCNM removal was90.15%with the addition and reaction time of40g/L and180min, respectively. The concentration of iron scraps was of great impact on TCNM removal and the removal increased with the increase of iron scraps addition. However, TCNM removal did not change a lot with the variation of initial concentration of TCNM when it was in a low level. The TCNM reduction by iron scraps followed the first-order kinetic model.In the experiments of advanced oxidation processes, when the initial TCNM concentration of was20μg/L, after150min, we found that TCNM removal was enhanced by using hydrogen peroxide (H2O2) and it increased with the increase of reaction time and H2O2addition; the removal was39.54%when H2O2concentration were15mg/L; ozone (O3) has a great effect on TCNM removal and the removal was obviously improved with the increased ozone concentration, the removal reached35.30%when the ozone concentration were10.06mg/L, the application of UV light was also helpful to increase TCNM removal, the removal were43.53%when the UV light intensity were31μw/cm2. Furthermore, the research on TCNM removal by the UV-H2O2process and UV-H2O2-O3process was conducted, which performed better with higher TCNM removal than the individual process shown above. In the UV-H2O2process, when the UV light intensity was set at31μw/cm2, after reaction time of150min, removal of TCNM with the initial concentration of20ug/L was improved from82.26%to95.61%with the increase of H2O2addition from15mg/L to45mg/L. The enhancement of UV light intensity also contributed to the TCNM removal. In the UV-H2O2-O3process, the highest TCNM removal of97.28%with the initial TCNM concentration of20μg/L was obtained when the UV light intensity, H2O2addition and O3addition were only31μw/cm2,15mg/L and10.06mg/L, respectively due to the more hydroxyl radical generated in this process. The TCNM degradation by UV-H2O2-O3process accorded with the first-order kinetics model.