Characteristicsand Mechanism For Degradation Of Emerging Micropollutants In Water By UV/ozone Process

With the rapid development and improvement of people's living standard, the environmental pollution has been an issue of great concern, especially for the safety and sanitation of drinking water. However, there are no specific unites in the conventional drinking water treatment processes for the removal of emerging contaminants such as endocrine disrupting chemicals (ECDs), pharmaceutical and personal care products (PPCPs), microcystins (MCs) etc., so these emerging contaminants cannot be degraded by conventional drinking water treatment processes and then may enter into the drinking water . In addition, improving and promoting reclaimed water recycling is an effective method to relieve the stress of water source shortage. Based on the stated above, developing and designing efficient, easy, and economic advanced treatment technologies are in urgent need.Degradation of target compounds by ozonation (O₃), UV photolysis (UV), and combined use of ozone and UV (UV/O₃) was investigated, and the efficiencies of biotoxicity removal were also evaluated. The influences of operational parameters and water qualities on the three processes were also investigated. Finally, assuming that hydroxyl radicals are in a unsteady state, a kinetic model for the UV/O₃ system, containing water simulating the natural water, was established with matlab.Ozonation showed a satisfied performance on the removal of selected organic pollutants (sulfamethoxazole (SMX), galaxolide (HHCB), bisphenol A (BPA), and microcystin LR (MC-LR)), with removal rates over 90% in 20 min. The second-order rate constants of ozone with the four chemicals were on the order of 102¹06 M-1·s-1 at neutral condition. The UV process only degraded SMX and MC-LR rapidly, but showed slow degradation rates for HHCB and BPA due to their low molar extinction coefficients and(or) quantum yields, with removal rates less than 20% in 20 min.The influences of operational parameters and water qualities on the O₃ and UV processes were studied. Increasing the ozone or UV dose can improve the degradation of organic pollutants, while increase in the initial concentration of target compounds would worsen the degradation efficiency. Essentially, the ratio of ozone (UV) dose to concentration of target compounds determined the degradation efficiency. For O₃ process,the solution pH exerted effect on the degradation from two aspects. First, the solution pH can change the proportion of direct ozonation and indirect ozonation involved in the degradation process. Second, the solution pH can influence the dissociation state of target compounds which show different reactivity towards ozone, especially for the organic compounds with high reactivity towards ozone. As to UV process,the solution pH made its contribution mainly by changing the dissociation state of target compound. Bicarbonate ion, as a widely accepted hydroxyl radical quencher, showed different performance on different target compounds. For the compounds with high reactivity towards ozone such as SMX,BPA and MC-LR, the presence of bicarbonate ion in the range of 0～8 mM was favored to stabilize the aqueous ozone residual and thus benefit the removal of pollutants. However, when the pollutants reacted slowly with ozone, such as HHCB, the presence of bicarbonate ion would inhibit the degradation process. On the contrary, there was little effect in the presence of bicarbonate ion for UV process. Similar to the case of bicarbonate ion, the presence of humic acid (HA) in the range of 0～5 mg·L^-1 showed different performance on different target compounds. To SMX, BPA, and MC-LR,HA was an inhibitor. In the case of HHCB, low level of HA promoted the degradation but the high level of HA suppressed the degradation. HA at all the concentration levels (0～5 mg·L^-1) improved the degradation efficiencies of target compounds. The influences of operational parameters and water qualities on the UV/O₃ process was similar to that on O₃ process, but the UV/O₃ process seemed to more resistant to fluctuation of water quality.one point should be mentioned, in the investigated pH range(5.0～9.0), best performance was obtained in the neutral condition, which was very advantageous to practical application.The biotoxicity evolution of target compounds during the O₃, UV, and UV/O₃ processes was investigated with daphnia magna acute immobilisation test, vibrio fischeri toxicity test, yeast estrogen screen, and protein phosphatase 2A inhibition assay. Based on the results of TOC and intermidiates analysis, it can be concluded that a large amount of intermidiates were formed during the O₃ and UV processes. Unforunately, these intermidiates kept partial or intact functional groups contined in the parent moleceulars, which resulted in deviation between the concentration removal rates and biotoxicity rates. The UV/O₃process, with high mineralization ability, can effectively reduce the accumulation of intermidiates and thus decreased the biotoxicity meanwhile.The established kinetic model for the UV/O₃ system, containing water simulating the natural water, can predict the concentration variation of SMX well, but yield a unsatisfied prediction for the decay and accumulation of H₂O₂ on the early stage of reaction. In addition, the model underestimated the concentration of hydroxyl concentration by 20%～35% compared to the experimental data.