Efficacy And Mechanism Of Electrocatalytic Oxidation Of PPCPs In Water With Boron-doped Diamond Electrode

With the development of global economy and the impact of human activities,a large amount of Contaminants of Emerging Concerns(CECs)will enter the sewage treatment system through various channels.Pharmaceuticals and Personal Care Products(PPCPs)are the most typical representatives of CECs,due to their strong environmental persistence and bioaccumulation,it's difficult for traditional sewage treatment process to remove it,which causes serious harm to the natural environment and human health.Therefore,it is of urgent practical significance to develop efficient and green advanced treatment technologies for water pollution.As an environmental friendly treatment method,Electrochemical Advanced Oxidation Process(EAOPs)have been favored by scientific researchers in the field of water treatment in recent years.The core of electrochemical oxidation technology lies in the strength of the electrode plate's oxidation ability,while the oxidation efficiency of different electrode materials are quite different,so it is particularly critical to choose a suitable electrode material.As a mixed material with high oxygen evolution potential,wide chemical potential window and low adsorption characteristics,Boron-doped Diamond(BDD)has gradually become a leader in the field of electrochemical wastewater treatment technology.In this paper,BDD was selected as electrode material to study the efficacy and mechanism of BDD electrochemical process for oxidative degradation of 8 typical PPCPs in water,and to investigate the effects of different electrolyte conditions and different water substrates on the degradation efficiency;the active species in the system were qualitatively and quantitatively identified by free radical quenching agent,trapping agent and other methods,and their contribution to pollutant degradation was clarified;finally,the mineralization pathway and mechanism were inferred by analyzing and detecting the intermediate products of degradation of two typical pollutants,Primidone(PMD)and Caffeine(CA).Because in the electrochemical oxidation water treatment process,Dissolved organic matter(DOM)contained in the water is often found to reduce the efficiency of the electrochemical oxidation process,therefore,Effluent organic matter(EfOM)from actcual sewage(NO.3 Sewage Treatment Plant of Xi'an City)was enriched,separated and characterized firstly in this paper,and the degradation efficiency of BDD electrochemical technology under the presence of EFOM was emphasized.The results of this paper can provide support and technical guidance for the theoretical research and practical application of BDD electrochemical technology in the treatment of new pollutants.The main research results of this paper are as follows:(1)Two EfOM components(ST XAD4 and ST XAD8)in Xi'an sewage treatment plant were obtained by enrichment and separation by resin adsorption,and UV-Vis scanning(UV-Vis),total organic carbon(TOC)detection,infrared spectrum scanning,organic element analysis and three-dimensional fluorescence analysis were used to obtain basic information of two EfOM such as the basic characteristics,structural composition and element types.The UV-Vis scan results showed that XAD4 has a stronger polarity,but less aromatic than XAD8,andUV254 and TOC results was combined to get their respective special ultraviolet absorbance,confirming that XAD4 and XAD8 are hydrophilic and hydrophobic EfOM,respectively.Infrared scanning spectroscopy showed that the types of functional groups contained in these two EfOM are almost the same,and O-containing functional groups occupies the main part,but XAD4 contains more proteins and small molecules that are easily soluble.The organic element analysis results showed that the N and S elements in the two EfOM are higher than the natural water EfOM,which also showed that XAD4 contains a high proportion of amino acids,peptide bonds and amides,and contains more O functional groups.Finally,through three-dimensional fluorescence spectroscopy analysis,it was found that humic acid is the main part of the two EfOM,and XAD8 contains a small amount of hydrophobic tryptophan-like proteins.(2)The experiment investigated the effect of BDD electrode on the removal of Pharmaceuticals and Personal Care Products(PPCPs)under the condition of NaCl electrolyte,and analyzed the effects of different factors on the degradation efficiency of the system.The results of the study showed that under the conditions of neutral pH and room temperature,the eight kinds of PPCPs were degraded to varying degrees under the oxidation of the BDD electrode at the same current(0.08 A).Except for the three substances PMD,Nitrobenzene(NB)and Carbamazepine(CBZ),the degradation rate of all other pollutants increased with the increase of NaCl concentration.After comprehensive consideration,10 mM NaCl was selected as the experimental conditions for the next stage.The addition of HCO3-had little effect on the degradation of NB,and the degradation efficiency was only about 3.0%,while the inhibition rate of other substances ranged from 18.2%to 97.2%.The results of DOM influence showed that CBZ and Sulfamethoxazole(SMX)were almost unaffected by EfOM,while the inhibitory effects of PMD and CA were obvious.In the comparison experiment of Mixed Metal oxide(MMO)and BDD electrodes,it was found that the chlorine production of MMO electrodes was about 10 times that of BDD electrodes under the same Cl-concentration,but in the case of the same chlorine production,the degradation effect of CA,1,4-Dimethyloxybenzene(DMOB),Gemfibrozzi(GEM),SMX and Norfloxacin(NOR)on the BDD electrode was better,and the degradation rates of the other three substances on the two electrodes were similar.The experimental results of the contribution of free radicals and non-radicals showed that PMD and CA were oxidized by some active chlorine and a small amount of·OH,the degradation of DMOB,GEM and SMX was affected by a large amount of·OH,and CBZ was affected by the free radicals adsorbed on the electrode plate.In addition,NB and NOR were only oxidized by·OH and active chlorine,respectively.(3)The experiment explored the removal efficiency of the BDD electrode on PPCPs degradation under different influencing factors when Na2SO4 was used as the electrolyte,mainly in the electrolyte type,water matrix influence,free radical contribution and BDD-Fenton combined technology part were analyzed.The results showed that under the same experimental conditions,CA,DMOB,CBZ,GEM,SMX and NOR all followed the degradation rule of kNaCl>kNClO4 under the three electrolyte types,but PMD and NB had better degradation effects under the Na2SO4 system.In the 10 mM HCO3-system,the degradation of seven kinds of PPCPs except SMX was inhibited due to the scavenging of free radicals by HCO3-.In contrast,SMX promoted degradation due to the catalysis of CO3·-.The results of DOM influence experiments showed that the inhibition rates reached 55.9%,66.6%,41.8%,and 13.1%,respectively.In terms of free radical species contribution experiments,it was found that PMD and NOR were affected by·OH and SO4·-,CA,NB,GEM and SMX were more contributed by·OH,DMOB is only affected by SO4·-?and CBZ degradation mainly comes from the effect of adsorbed free radical on oxidation of electrode plate.Some experimental results of BDD-Fenton combined use showed that Fe(II)of 20 ?M was the optimal concentration of activator in the experiment,and the degradation of target pollutants in acidic environment with pH=3 is significantly better than in neutral conditions,free radical steady-state concentration experiments also showed that after the addition of Fe(?),the concentration of·OH and SO4·-in the system increased significantly,fully demonstrating the success of the establishment of this combined technology.(4)In the series of PMD and CA intermediate product detection experiments,it was found that PMD produced maleic acid,acetic acid,succinic acid,and acetic acid in NaCl and Na2SO4,respectively,and the content of organic acids generated in NaCl was much less than that under Na2SO4 conditions.In the CA degradation system,the types of small-molecule acids increased significantly.Among them,maleic acid and acetic acid appeared in both electrolyte conditions and the content of maleic acid was stable.Under NaCl conditions,the acetic acid gradually decreased with the electrolysis time,followed by NaCl.Citric acid and lactic acid were detected in Na2SO4,and succinic acid was detected in Na2SO4,and their concentrations accumulated with the increase of electrolysis time.In the intermediate product detection experiment,PMD produces P-B1 under NaCl conditions,while under Na2SO4 conditions,through the combined action of·OH and SO4·-,three substances can be produced;in contrast,CA has both under two electrolyte conditions.The four intermediate products were formed,and were similar to previous studies.