Study On Degradation Of Diclofenac Sodium Using Fenton-like Technology Based On Calcium Peroxide

In recent decades,pharmaceuticals and personal care products have become a global concern.Diclofenac sodium（DCF）as one of the most used drugs in the world can be discharged into the environment through a variety of ways,posing a threat to human health and the ecological environment.Due to the recalcitrance and persistence of DCF in the environment,it is necessary to strengthen the removal of DCF in the wastewater treatment process.Fenton technology as a kind of advanced oxidation technology has been widely used in the removal of various refractory pollutants.However,the oxidant hydrogen peroxide（H₂O₂）in the traditional Fenton technology is unstable and easy to disproportionate,resulting in the catalyst precipitation and low utilization rate.In this study,from the perspective of an improved-Fenton system,nano-calcium peroxide（nCaO₂）was selected as the sustained-release H₂O₂ material.First,preparation methods of nCaO₂ were optimized,from the perspective of the ability of H₂O₂release.It could provide a theoretical basis for the subsequent research on the application of nCaO₂ in the Fenton-like system.Secondly,the newly prepared catalysts such as Fe²⁺complexed with ethylenediaminetetraacetic acid（EDTA）and nano-iron tetroxide（nFe₃O₄）were applied in the iron salt catalytic-Fenton system for DCF degradation,which solved the problem of low catalytic efficiency in the traditional Fenton system.Finally,the difference between nCaO₂ and H₂O₂-Fenton system was explored,which provided a theoretical basis for the feasibility of degrading pollutants based on nCaO₂-Fenton technology.The main conclusions are as follows.（1）nCaO₂ with a purity of 89.1%was prepared,under conditions of reagents addition of calcium nitrate 6.0 g,H₂O₂ 15.0 m L,water 10.0 m L,cetyltrimethylammonium bromide 1.0 g,and reaction time of 10 minutes.Characterized with scanning electron microscopy,the prepared nCaO₂ had smaller particle sizes of 200～400 nm than the commercially available CaO₂（particle size 2μm）.In addition,nCaO₂ had a higher specific surface area of 26.5 m²·g^-1 than those of the commercially available CaO₂(3.7 m²·g^-1).The dissolution process of nCaO₂ was influenced by the factors such as dosage,stirring speed,temperature and pH etc.The maximum H₂O₂release amount of 2.3 m M could be achieved under conditions of mixing speed 500 rpm,pH6.0,temperature25℃ and reaction time 300 min.This manifested that the release rate of H₂O₂from nCaO₂ was controllable,by adjusting the dissolution rate of nCaO₂ in water.The DCF removal rate was 97.5%at 180 min,when the prepared high-purity nCaO₂ was applied as the oxidant in the Fenton-like system.（2）An iron salt-catalyzed-Fenton system was established for DCF degradation,with the oxidant nCaO₂,the catalyst Fe²⁺and the chelating agent EDTA.The removal rate of DCF was97.5%under conditions of the initial DCF concentration 0.02 m M,the molar ratio of nCaO₂/EDTA-Fe²⁺/DCF 16/8-8/1,pH 6.0 and reaction time 180 min,which exerted significant improvements of 31.9%and 13.6%than those in the nCaO₂/Fe²⁺and H₂O₂/Fe²⁺/EDTA system,respectively.HCO₃^-and HA with high concentrations could severely inhibit the degradation of DCF.Low concentration of Cl^-had an inhibitory effect on DCF degradation,while high concentration of Cl^-had a promoting effect.NO₃^-and SO₄^2-showed no obvious influence on DCF degradation.Free radicals such as·OH,O₂^-·and ¹O₂ were observed in the nCaO₂/Fe²⁺/EDTA,nCaO₂/Fe²⁺and H₂O₂/Fe²⁺/EDTA systems,and·OH played a major role in the DCF degradation.The addition of EDTA would further promote·OH formation in the nCaO₂ system,and the nCaO₂ system could produce more O₂^-·than the H₂O₂ system.The possible degradation pathways of DCF were proposed with the intermediate products obtained by GC-MS,which included dechlorination,decarboxylation,hydroxylation,intramolecular condensation and C-N cleavage reactions.（3）Another iron salt-catalyzed-Fenton system was established for DCF degradation,with the oxidant nCaO₂ and the catalyst nFe₃O₄.The removal rate of DCF was 98.8%under conditions of the initial DCF concentration 0.02 m M,the nCaO₂ concentration 100.0 m M,the nFe₃O₄ dosage 0.1 g·L^-1,pH 6.0 and reaction time 180 min.Compared with the H₂O₂/nFe₃O₄system,the removal effect of DCF increased by 28.7%within 30 min.Instead of H₂O₂,the application of nCaO₂ could decrease·OH quenching on the surface of the catalyst.Moreover,the nFe₃O₄ catalytic system could effectively broaden the pH range of the reaction solution,which was same as the EDTA chelated Fe²⁺catalytic system.According to the iron ion leaching in the reaction system,it could be inferred that DCF degradation in the iron oxide catalytic system was mainly rate-limited by the interface reaction.Furthermore,nFe₃O₄ had good stability and was easily separated by the external magnetic field,and the DCF removal rate could reach 93.6%in the fourth cycle of repeated use.In this system,·OH played a major role in DCF degradation,according to the free radical quenching experiment and electron paramagnetic resonance detection.