Degradation Of Diclofenac By HO^· And SO₄^·--based Advanced Oxidation Processes

As a class of emerging contaminants（ECs）,pharmaceuticals and personal care products（PPCPs）have been frequently detected in different environmental compartments,such as surface water,groundwater,soil,and sediment,worldwild.Their occurrence in the environment may have toxic effects on organisms and induce the development of drug resistant bacteria and resistant gene,threatening human health and the balance of ecosystem.PPCPs cannot be removed completely by conventional wastewater treatment processes and water purification processes due to their complex physico-chemical and antibiotic properties.Therefore,it is necessary to develop the other highly efficient,economic and green treatment technologies for the removal of PPCPs.In this study,diclofenac（DCF）was chosen as a model contaminant to investigate its degradation in water by four hydroxyl radical（HO^·）and sulfate radical(SO₄^·-)based advanced oxidation processes（AOPs）,respectively.The main contents are as followed:（1）The degradation of DCF by Fe²⁺catalyzed peracetic acid（PAA）was systematically investigated.The results showed that Fe（II）could effectively activate PAA to produce HO^·and CH₃CO₂^·,resulting in the rapid degradation of DCF.The degradation efficiency of DCF decreased gradually with the increase of solution p H,and it could be completely removed within 5 s at p H 5.The radical scavenging experiment indicated that HO^·,CH₃CO₂^·and CH₃CO₃^·were the main active radicals in Fe（II）/PAA system.As Fe（II）dosage or PAA concentration increased,the removal of DCF was enhanced,but excess Fe（II）or PAA could compete with DCF for HO^·,leading to the slow growth of DCF degradation rate.The presence of Cl^-,HCO₃^-and natural organic matter（NOM）could inhibit DCF removal,while NO₃^-,SO₄^2-,Ca²⁺and Mg²⁺had little effect on its degradation.The degradation mechanism of DCF in Fe（II）/PAA system was proposed including four different transformation pathways,i.e.,hydroxylation,C-N bond cleavage,decarboxylation and dehydrogenation.（2）In homogeneous Fe（II）/PAA system,Fe²⁺could not be cycled effectively.In order to solve this problem,Fe²⁺was loaded into artificial zeolite to produce Fe²⁺-modified zeolite（FMZ）,which was used to catalyze PAA,and the degradation of DCF by FMZ/PAA was systematically studied.The results showed that DCF could be degraded effectively in the heterogeneous FMZ/PAA system,which was mainly attributed to the role of HO^·,CH₃CO₂^·and CH₃CO₃^·generated by PAA activation.With the increase of solution p H,the degradation efficiency of DCF decreased gradually,while its removal rate enhanced with the increase in PAA dosage or FMZ dosage.Different water components had different effects on DCF degradation by FMZ/PAA.Among them,the presence of Cl^-,CO₃^2-and NOM could inhibit the elimination of DCF;Fe³⁺and Cu²⁺could promote DCF degradation;while NO₃^-,SO₄^2-,Ca²⁺and Mg²⁺had little effect on DCF removal.In FMZ/PAA system,the possible degradation mechanism of DCF included three different reaction pathways,i.e.,C-N bond cleavage,decarboxylation and formylation.（3）Using heat activated persulfate（PS）to produce SO₄^·-,and the degradation of DCF in this system was systematically investigated.The results showed that DCF could be degraded effectively by heat/PS,and its removal followed a pseudo first-order kinetic model.The degradation rate of DCF decreased gradually with the increase in solution p H.When p H<7,SO₄^·-was the main reactive radical in heat/PS system;while HO^·guadually became the dominant active species under alkaline p H conditions.With the increase of reaction temperature or PS dosage,the degradation efficiency of DCF enhanced gradually.In current experimental condition,the apparent activation energy of DCF degradation by heat/PS was calculated to be 101.4 k J/mol.Different water components had different effects on DCF degradation by heat/PS.Among them,Cu²⁺and CO₃^2-could promote DCF degradation;NOM could inhibit DCF removal;while no obvious effect of NO₃^-,SO₄^2-and Fe³⁺on DCF degradation was observed.Nine reaction products of DCF were detected in heat/PS system,and the possible degradation mechanism of DCF was thus proposed exhibiting five different transformation pathways,including hydroxylation,decarboxylation,formylation,dehydrogenation and C-N bond cleavage.（4）The degradation of DCF by UV activated peroxymonosulfate（PMS）was systematically studied.The results showed that compared with DCF photolysis by UV alone,its removal was significantly improved in UV/PMS system,which was mainly due to the role of SO₄^·-and HO^·.In UV/PMS system,the degradation efficiency of DCF increased gradually with the increase in solution p H,and its fastest removal rate was obtained at p H 11,because PMS could be activated to produce additional SO₄^·-and HO^·under strong base conditions.The increase of PMS dosage could improve DCF removal;while its degradation was inhibited with the increase of its initial concentration.The presence of NO₃^-and HCO₃^-could both promote DCF degradation by UV/PMS,which was due to the role of HO^·and carbonate radical(CO₃^·-),respectively.The presense of NOM could inhibit the elimination of DCF;while Cl^-and SO₄^2-could not affect DCF removal.According to the detected seven reaction products,the possible degradation mechanism of DCF in UV/PMS system was proposed including six different degradation pathways,i.e.,hydroxylation,decarboxylation,dechlorination-cyclization,formylation,dehydrogenation and dechlorination-hydrogenation.