Kinetics And Mechanisms On The Degradation Of Diclofenac From Aqueous Solution By Mn(?) Generated In Electrochemical Activation Of Permanganate

In recent years,with the continuous improvement of the economy and living standards,pharmaceuticals and personal care products have received much persistent attention.Diclofenac is a typical over-the-counter drug in non-steroid anti-inflammatory drugs,which is widely utilized in anti-inflammatory,analgesic,anti-rheumatoid and antipyretic.It is often detected in the surface water,the effluent of the sewage sludge and the wastewater treatment plants.Diclofenac is potentially ecotoxic to the water environment,causing adverse reactions in mammals,vultures and some fishes,such as the damage of intestinal,hepar and kidney,even the concentration of diclofenac lower than 1.0?g L^-1.However,the conventional wastewater treatment technology is quite difficult to completely remove diclofenac.It is necessary to develop an effective,sustainable and energy-saving method to treat this emerging refractory trace organic pollutants in water.Electrolysis and permanganate oxidation are two technologies commonly utilized in water treatment,while they are hampered with the slow reaction rates.In order to improve the degradation of diclofenac,the electrolysis and potassium permanganate processes were combined.The feasibility and synergistic effect of the electrochemical activated potassium permanganate process on diclofenac removal was investigated when Mn²⁺was added and activated carbon fiber was used as cathode,respectively.Effect of operation parameters on diclofenac removal were studied.The energy consumption of diclofenac removal by the electrochemical activated potassium permanganate process was analyzed.The removal mechanism,reaction pathways and the toxicity of diclofenac degradation by the electrochemical activated potassium permanganate process in solution were also studied.The main conclusions of this study are as follows:?1?The addition of Mn²⁺in electrochemical activated permanganate process(E-PM-Mn²⁺)promoted the degradation of diclofenac and the mineralization of solution.In the presence of Mn²⁺,there was obvious synergistic effect between electrolysis(E-Mn²⁺)and permanganate oxidation.And with the increase of of Mn²⁺dosage,the synergistic effect between E-Mn²⁺and permanganate oxidation was significantly enhanced.The acid condition was beneficial to the removal of diclofenac in water by E-PM-Mn²⁺process.When the solution was neutral or alkaline,the degradation ratio of diclofenac was obviously decreased.For example,only 33.87%of diclofenac was removed in E-PM-Mn²⁺process at the initial solution pH of 11.The degradation ratio of diclofenac increased with increasing the current density from 0 to 171.6 A m^-2,while the energy consumption of in E-PM-Mn²⁺process for diclofenac removal also increased.Therefore,the removal of diclofenac and the energy consumption of E-PM-Mn²⁺process should be considered comprehensively in the selection of appropriate current density in subsequent practical water treatment.The degradation ratio of diclofenac in E-PM-Mn²⁺process increased with the increase of permanganate dosage.When the dosage of permanganate was higher than 100?M,the degradation ratio of diclofenac increased little.Thus 100?M of permanganate was the optimum dosage in diclofenac removal by electrochemical activated potassium permanganate process.The comparison of the removal efficiency of diclofenac in different water matrices showed that the removal of diclofenac was higher in surface water than tap water and ultrapure water,and the removal ratios of diclofenac in these three water matrices were above 96%,indicating that E-PM-Mn²⁺process could be utilized to diclofenac removal in actual water sources.?2?The removal of diclofenac was further promoted in electrochemical activated permanganate process with the activated carbon fiber as the cathode?E-ACF-PM?in comparison of E-PM-Mn²⁺process.Diclofenac and its intermediates was also effectively mineralized and the removal ratio of TOC was 82.71%.The synergistic effect of electrolysis?E?ACF??and permanganate oxidation was more significant than that in E-PM-Mn²⁺process.Diclofenac could be efficiently removed in E-ACF-PM process with the initial pH range from 3 to 10,which was related to the change of pH in solution during the reaction.The increase of current density and the addition of permanganate obviously enhanced the removal of diclofenac in E-ACF-PM process,and the optimum current density and the permanganate dosage were 57 A m^-22 and 100?M,respectively.The activated carbon fiber cathode could be reused as the removal ratio of diclofenac was still over 80%in E-ACF-PM process with the cathode reused for 20 times.The removal of diclofenac in surface water was higher than tap water and ultrapure water,and the E-ACF-PM process had relatively lower energy consumption in actual water source.The proper addition of the electrolyte in surface water could obviously reduce the energy consumption in E-ACF-PM process for diclofenac removal.The presence of humic acid significantly promoted the removal of diclofenac in E-ACF-PM process.Compared with the electrolysis and permanganate oxidation,water,the removal of carbamazepine,phenol,sulfanilamide,nitrobenzene and methyl blue were obviously enhanced in E-PM-Mn²⁺and E-ACF-PM processes.Electrochemical activated permanganate process could be utilized for the removal of organic pollutants in the actual water source.?3?The effect of excess methanol and tert-butanol on the removal of diclofenac in E-PM-Mn²⁺and E-ACF-PM processes was negligible,indicating that there was no hydroxyl radical,sulfate radical or singlet oxygen produced during diclofenac removal by electrochemical activated permanganate process.Through the analyses of spectral scanning during reaction,the effect of manganese dioxide and sodium pyrophosphate,it could be seen that the main oxidizing substance for the promotion of diclofenac removal in electrochemical activated permanganate process was actived Mn???.Compared with the instantaneous generation of Mn???in the homogeneous system,Mn???generated on the surface of cathode in electrochemical activated permanganate process was continuous.The amount of Mn???generated in E-ACF-PM was higher than E-PM-Mn²⁺and E-PM process,and the order was as follows:E-ACF-PM>E-PM-Mn²⁺>E-PM process.The addition of Mn²⁺promoted the generation and stabilization of Mn???for diclofenac removal in E-PM-Mn²⁺process.The use of activated carbon fiber as the cathode not only also increased a part of electroadsorption of diclofenac,but also enhanced the formation of Mn???for the rapid removal of diclofenac.Electrolysis,permanganate and Mn²⁺were essential factors for the generation of Mn???in electrochemical activated permanganate process and the rapid removal of diclofenac.The Mn intermediates on the cathode surface eventually entered the solution in the form of free Mn???or Mn²⁺.?4?Three possible degradation pathways were proposed in the removal of diclofenac by electrochemical activated permanganate process.The degradation of diclofenac included hydroxylation,dechlorination,decarboxylation,formylation,dehydrogenation,C-N cleavage and ring opening reaction.The toxicity changes of diclofenac solution and the solution during the reaction were assessed by the growth inhibition of microcystis aeruginosa.The results indicated that the toxicity of the solution can be effectively reduced after the degradation of diclofenac in electrochemical activated permanganate process compared with the intial diclofenac solution.Therefore,the potential ecological risk of diclofenac in the solution was reduced.