Degradation Of Metoprolol By UV Combined With TiO₂ Photocatalysis And UV Chlorine Processes

In recent years,the use of drugs and personal care products（PPCPs）is increasing with the development of society.Conventional sewage treatment processes can not effectively remove PPCPs,because its residues may be a threat to ecological health.In this subject,Metoprolol is selected as the representative of PPCPs in water,which has the characteristics of large usage,low removal rate,resistance to hydrolysis,strong harmfulness and limited biological metabolism.Photocatalytic oxidation based on the surface reaction of catalysts represented by TiO₂ when exposed to ultraviolet radiation is one of the most promising advanced oxidation technologies at present.UV/Cl process uses ultraviolet irradiation of chlorine to produce hydroxyl radicals（·OH）and chlorine-containing radicals（·Cl）to efficiently degrade various organic pollutants.In this experiment,by exploring the reaction mechanism of Metoprolol in UV-LED/TiO₂ and UV-LED/Cl two new AOPs systems,the effects of various influencing factors on the degradation of MTP by the two processes were investigated,the intermediates in the degradation process were identified and the degradation mechanism was explored.The main conclusions are as follows:（1）The photocatalytic degradation of MTP by UV/TiO₂ was remarkable,and the removal rate reached 96.5%after 60 minutes.The photocatalytic activity of mixed crystal TiO₂（Degussa 25）was the highest,and the removal rate of MTP increased at first and then decreased with the increase of nano-TiO₂ dosage,decreased with the increase of ultraviolet irradiance,initial mass concentration of MTP,humic acid and tert-butanol,and increased with the increase of pH.NO₃^-,SO₄^2-and Cl^-all inhibited the photocatalytic MTP of UV/TiO₂,and the inhibitory effects of the three anions were from strong to weak.HCO₃^-and CO₃^2-can slightly promote the photocatalysis of MTP by UV/TiO₂.Ca²⁺had almost no effect on the degradation of MTP by UV/TiO₂,while Fe³⁺promoted the degradation of MTP to a certain extent,while Mg²⁺slightly inhibited the degradation of MTP,while Cu²⁺significantly inhibited the degradation of MTP by UV/TiO₂.（2）UV/Cl can effectively remove MTP,and the degradation rate is up to 95.2%within30 minutes.At the beginning of the experiment,the effect of one-time addition of chlorine（3.0mg/L）on the degradation of MTP by UV/Cl was better than that in batches.The removal rate of MTP increased at first and then decreased with the increase of chlorine dosage,and decreased with the increase of initial mass concentration of MTP and tert-butanol concentration.Low concentration of humic acid（1.0mg/L）slightly promoted the degradation of MTP,and the higher the concentration of humic acid,the more obvious the inhibitory effect on the reaction.Five common anions(NO₃^-,Cl^-,SO₄^2-,HCO₃^-and CO₃^2-)in water all inhibit the degradation of MTP by UV/Cl in different degrees.Ca²⁺had almost no effect on the degradation of MTP by UV/Cl,while Fe³⁺slightly promoted the degradation of MTP,while the existence of Mg²⁺and Cu²⁺significantly inhibited the degradation of MTP by UV/Cl.（3）The change of TOC of MTP is very slow in the two processes.In the process of UV-LED/TiO₂ photocatalysis and UV-LED/Cl degradation,TOC decreases continuously under the condition of UV irradiation,indicating that the complete mineralization of MTP can be achieved by prolonging the UV irradiation time.It was found that MTP produced a variety of intermediates with different structures in the two degradation processes.The main pathways in the photocatalytic degradation of MTP include the hydroxylation of aromatic rings,the shortening of methoxy groups on the side chain,and the cleavage or addition of OH on the amine side chain.In the process of UV/Cl degradation,the main ways of MTP degradation include chain reactions initiated by·Cl and·ClO substitution,removal of methoxy groups on the side chain,dealkylation caused by electron transfer on the amine side chain and so on.