Research On Degradation Kinetics Of Some Pharmaceutical Contaminants In The UV/Chlorine System

It was pointed out from the national 14th Five-Year Plan Period and Long-Range Objectives Through the Year 2035 that we need take much attention on the treatment of emerging contaminants.Ministry of Ecology and Environment released the“Acting Plan for Emerging Contaminant Treatment(draft for comments)”in Oct.2021,which emphasized the enhancing sourc control,process control,and end-of-pipe control.Pharmaceutically active compounds(Ph CAs)are the representative emerging pollutants,which are widely detected in various environmental water bodies.It has attracted much attention because of its endocrine disrupting activity and inducing bacterial resistance.Ph CAs in environmental water bodies mainly derived from municipal sewage,aquaculture wastewater,and pharmaceutical wastewater.At present,Ph CAs is rarely included in the control index of wastewater treatment process.Even if Ph CAs in the effluent meet the relevant national discharge standards,a considerable amount of Ph CAs are still released into environmental water body,thus the advanced treatment of Ph CAs in the sewage effluent is urgent.Recent studies have demonstrated that the UV/chlorine technology is an effective method to treat organic contaminants in water.In addition to generating non-selective·OH,it can also produce a large number of reactive chlorine species(RCS)with high oxidation activity and selectivity,showing high reactivity to specific Ph CAs.At present,UV/chlorine technology has been widely used in the oxidation degradation of many Ph CAs in water.However,the mechanism of UV/chlorine oxidation degradation of most Ph CAs is very different,and the UV/chlorine oxidation degradation mechanism of most Ph CAs is still unclear,and a further research is needed.At the same time,the water quality parameters in water body significantly affected the generation and conversion process of·OH and RCS in the UV/chlorine system,which makes the degradation process of Ph CAs more complicated.Previous studies utilized kinetic models to investigate the effects of active free radicals in the UV/chlorine system on the degradation of pollutants,but they paid more attention to the contribution of·OH in the system,and the systematic studies of RCS including Cl·,Cl₂·^–,Cl O·is scarce.Based on these statements,we selected sulfapyridine(sulfonamide antibiotics),gemfibrozil(blood lipid regulator drugs),and naproxen(anti-inflammatory drugs)as model compounds to study the degradation mechanism in the UV/chlorine system,which considers the influence of the main water quality parameters in the water.At the same time,a kinetic model based on the possible active free radicals in the UV/chlorine system was constructed to systematically study the effect of·OH and RCS on the degradation of target substances.The degradation kinetics of pollutants in actual waters was predicted.The main results of the study are as follows:(1)The degradation process mechanism of sulfapyridine in the UV/chlorine system and the influence of water quality parameters were studied.Radical quenching experiments showed that·OH and Cl·were the main active free radicals for the oxidation degradation of sulfapyridine in the UV/chlorine system;steady-state photochemical experiments using small molecule model compounds containing the main active sites of sulfapyridine demonstrated that·OH is easy to attack the aniline and sulfonyl groups of sulfapyridine,and Cl·is more likely to react with aniline group;product degradation experiments found that·OH and Cl·induced sulfapyridine degradation undergoes oxidation,hydroxylation,polymerization,and addition reactions.(2)Due to co-existence of dissolved components in actual waters,the multivariate effects of important water quality parameters(p H,dissolved organic matter DOM,HCO₃^–,Cl^–)on degradation of sulfapyridine in UV/chlorine system are studied by response surface methodology(RSM).The results showed that at the significance level of p<0.05,in addition to single factor,the interaction between factors also affected the degradation of sulfapyridine;further optimization experiment showed that p H and DOM are the dominant factors affecting the degradation of sulfapyridine in the UV/chlorine system,and the overall contributions of the two factors on the degradation are up to 81.1%.Steady-state photochemical experiments showed that p H can affect the apparent quantum yield of free chlorine production·OH and RCS,which further affects the degradation of sulfapyridine;DOM has a quenching effect on the generation of active free radicals in the UV/chlorine system,thereby inhibiting sulfapyridin degradation.Although HCO₃^–and Cl^–have little contribution to UV/chlorine oxidation of sulfapyridine,they are still significant influencing factors(p<0.05).They also inhibited sulfapyridine degradation mainly by quenching the free radicals in the UV/chlorine system.(3)Based on the possible active free radicals in the UV/chlorine system,and considering the influence of the main water quality parameters in the water body,steady-state free radical kinetic model for predicting the degradation of gemifibrozil and naproxen in UV/chlorine systems was established.The predicted model showed that·OH,Cl·and Cl O·are the main active free radicals for the degradation of model compounds in pure water system;Cl^–promoted the degradation of gemfibrozil and naproxen by UV/chlorine oxidation,and found that Cl O·is the main active species for Cl^–enhancing the degradation;HCO₃^–also has a promoting effect on gemfibrozil and naproxen UV/chlorine oxidation.Although HCO₃^–quenches·OH and RCS,the reaction process produced CO₃·^-with high reactivity to the target(secondary reaction rate constants are 2.45×10⁷and 3.50×10⁷ L mol^-1 s^-1),which can compensate the degradation of the targets caused by·OH and RCS quenching,thereby exhibiting a promoting effect;DOM showed quenching effect on·OH and RCS,and then inhibited UV/chlorine oxidation degradation.Based on the influence of water quality parameters,the steady-state free radical kinetic model has successfully predicted the degradation kinetics of gemifibrozil and naproxen in three real water matrices.