Effect Of Bromine Ion On Kinetics Of Degradation Of Organic Micropollutants In Ultraviolet/Persulfate Oxidation System

Organic micropollutants in water pollutants include a wide range of pollutants,high toxicity and difficulty in removal.The deterioration of the ecosystem and damage to biodiversity will not only endanger human health,but also threaten the sustainable development of global water resources.Advanced Oxidation Process（AOPs）based on Ultraviolet（UV）photolysis and photochemical reactions has been used to degrade various OMPs present in water.The UV/Persulfate（PS）process removes many OMPs by producing sulfate free radicals(SO₄^·-).Bromine ions（Br^-）are common in urban sewage,natural water and other media.Reactive Bromine Species（RBS）can be reactive with SO₄^·-and hydroxyl radicals（HO^·）,and this reaction pathway may affect the degradation mechanism and performance of these free radicals in removing target pollutants.However,at present,the internal mechanism of oxidative degradation of pollutants by UV/PS/Br^-in complex environment has not been fully studied.This study systematically investigated the effect of Br^-on the degradation of Ornidazole（ONZ）and Sulfamethazine（SMZ）in UV/PS oxidation system.The effects of process parameters（PS dosage,p H）and water background composition（Cl^-,HCO₃^-,Natural Organic Material（NOM））on the degradation of ONZ and SMZ by UV/PS/Br^-oxidation system were investigated.A kinetic model was established based on the steady-state concentration of free radicals on the degradation of target pollutants in UV/PS/Br^-oxidation system,and the contribution of different active substances(such as SO₄^·-,HO^·,Br^·,Br₂^·-,Br OH^·-,Cl^·,Cl₂^·-,CO₃^·-,etc.)to the degradation of target OMPs in different oxidation systems was analyzed using the kinetic model.The results show that the steady-state dynamic model can effectively predict the degradation of target organic compounds under specific experimental conditions.The main research results and conclusions are as follows:（1）When the concentration of Br^-increased from 0μM to 800μM,the presence of Br^-inhibited the degradation of ONZ in UV/PS oxidation system,and the apparent degradation rate decreased by 89.5%.For SMZ,the presence of Br^-effectively promoted the degradation of SMZ,and the apparent degradation rate increased by33.7%.The process of UV/PS/Br^-oxidative degradation of ONZ and SMZ conforms to pseudo-first-order kinetics.（2）UV/PS/Br^-oxidation system can effectively remove ONZ and SMZ.When PS concentration increased from 300μM to 700μM,the degradation rates of ONZ and SMZ increased by 64.9%and 59%,respectively.The change of p H in the system did not significantly affect the degradation rates of ONZ and SMZ.The presence of chloride（Cl^-）can significantly inhibit the UV/PS/Br^-oxidation on the degradation of ONZ,when the concentration of Cl^-increased from 0 m M to 10 m M,ONZ degradation rate reduced from 1.00×10^-3 s^-1 to 0.28×10^-3 s^-1,but has no effect on the degradation of SMZ and.The presence of 1m M bicarbonate（HCO₃^-）had a slight inhibitory effect on the degradation of the two target pollutants compared with no addition of HCO₃^-,and the degradation rate gradually increased as the concentration of HCO₃^-increased to10m M.NOM the increase of concentration of UV/PS/Br^-degradation rate from 2.18×10^-3s^-1 to 1.09×10^-3s^-1,but not influence on SMZ.（3）According to the simulation results of the kinetic model,the kinetic model in this study can well simulate the degradation rates of different process parameters（PS dose,p H）and background components（Cl^-,HCO₃^-,NOM）in the UV/PS/Br^-system as a whole.In the ONZ oxidation system,the concentration of each radical in the UV/PS/Br^-oxidation system increases with the increase of PS concentration.The change of p H from 5 to 7 and then to 9 did not cause a change in the concentration of SO₄^·-,but resulted in a 37.2%and 37.5%decrease in the concentration of Br^·and Br OH^·-,respectively.In addition,when p H was 9,the concentration of HO^·increased by about 1664 times,and the contribution of HO^·was the most prominent.Cl^-in ONZ system with the increase of the amount of the Cl₂^·-the contribution value increased from 0 to 3.23×10^-4s^-1,but can severely inhibit Br^·and Br OH^·-,the contribution of the two values with the loss of the Cl^-dosing quantity tends to 0,to curb the overall evaluation.When the dosage of HCO₃^-increased from 0 m M to 10 m M,the Br₂^·-contribution value decreased by about 73.6%,and when the dosage of HCO₃^-increased from 1 m M to 10 m M,the Br₂^·-contribution value decreased by about 67.3%,but the concentration of Br OH^·-increased significantly,and the contribution value increased by about 1.2 times,and the total degradation rate stabilized.NOM addition leads to the decrease of ONZ degradation rate,which can be attributed to the molar absorption coefficient of NOM,which competes for UV photons and thus affects the concentration of free radicals in the system.On the other hand,NOM captures free radicals,which will lead to a series of chain reactions.For SMZ oxidation system,the apparent degradation rate increases due to the increase of PS concentration is also because the increase of PS concentration directly leads to the increase of SO₄^·-concentration,which leads to the increase of secondary free radicals.The change of p H did not significantly affect the contribution value of each radical.The existence of Cl^-significantly reduces the Br OH^·-concentration,its contribution to the value by 2.94×10^-3s^-1 to 5.31×10^-8s^-1,the contribution values of Br₂^·-with the increment of Cl^-adding increased from1.95×10^-9s^-1 to 1.29×10^-3s^-1,It is worth mentioning that the presence of Cl^-and chlorine-containing active substances(such as Cl^·and Cl₂^·-)did not significantly degrade in the simulation.Similar to the effect of the presence of Cl^-,Br OH^·-contribution value with the increase of HCO₃^-dosing quantity by minimum 2.94×10^-3s^-1 to 8.32×10^-5s^-1,at the same time the Br₂^·-contribution of value with the increase of Cl^-dosing quantity increased from 3.69×10^-9s^-1 to 1.16×10^-3s^-1,The presence of HCO₃^-promotes the generation of a certain concentration of CO₃^·-in the process of UV/PS/Br^-oxidative degradation of ONZ and SMZ.However,due to the small concentration value of CO₃^·-and the second-order reaction rate constant with target organic matter,the contribution value of CO₃^·-in ONZ and SMZ is about 10^-10s^-1 level,so as to be ignored.In SMZ oxidation system,NOM capture on free radicals did not significantly change the contribution values of SO₄^·-and secondary bromine containing species,which also resulted in the UV/PS/Br^-degradation efficiency in SMZ system remained basically unchanged,which was consistent with the trend of apparent degradation rate of specific experimental values.