| With the development of medicine and animal husbandry,the production and use of antibiotic drugs have increased dramatically.In recent years,a wide variety of antibiotics have been detected around the world in various water environments such as surface water,sewage plants,and groundwater.These antibiotics that migrate and remain in the aquatic environment increase the potential ecological risk and seriously affect the health of aquatic organisms and humans.The electrochemical process overcomes the shortcomings of traditional biological treatment and physicochemical technology,such as low processing efficiency and easy occurrence of secondary pollution,which also has a good application prospect in the removal of organics.The current electrochemical process still needs to develop low-cost and efficient electrode materials to meet the practical application requirements for organics removal.Among them,highly dispersed metal materials have become the focus of electrocatalytic removal of organics due to their excellent electrical conductivity and catalytic activity,but the effects of their regulating synthesis methods and morphological structure characteristics on electrocatalytic activity and selectivity still need further study.In response to the above problems,this paper synthesized highly-dispersed bimetallic cathode catalysts and atomically-dispersed cathode catalysts with different morphologies,sizes and coordination structures by means of control.Using chloramphenicol as the object pollutants,the catalytic performance of different cathode catalysts was compared;the mechanism of electrocatalytic reductionoxidation combined process to remove chloramphenicol and the mechanism of synergy between cathode and anode were also revealed.In order to enhance the electrocatalytic activity and stability,a simple impregnation method was used to synthesize highly-dispersed Ni(Fe)-Cu/graphene cathode catalysts.The morphology and size of catalysts could be adjusted by changing the Ni(Fe)doping and Ni(Fe):Cu ratio.When the ratio of bimetal in the Ni(Fe)-Cu/graphene cathode catalyst was Ni(Fe):Cu=0.5:0.5(wt%:wt%),the bimetallic particle size was the smallest(<5 nm),and the lattice doping between the bimetallics formed a stable alloy structure.Among them,the Ni-Cu/graphene cathode catalyst with a ratio of 0.5:0.5 not only showed significant hydrogenation activity at 0.75 V(vs RHE),but also acheived the highest H2O2 Faraday generation efficiency(85%)in the oxygen reduction reaction.Chloramphenicol can be rapidly converted by electrocatalytic hydrogenation reduction and indirect oxidation processes by using Ni-Cu/graphene cathode catalyst,and the conversion efficiency showed a linear growth trend within a certain range of chloramphenicol concentration(<50 mg/L).In order to further improve the catalytic activity of electrode materials and maximize the utilization of metal active sites,atomically-dispersed Fe/graphene cathode catalysts were synthesized by high temperature annealing.The N-doped Fe/graphene cathode catalysts(2.77~10.14 wt%)were mainly composed of co-existing coordination structure of Fe-O and Fe-N.The hydrogen adsorption potential of the atom-dispersed Fe/graphene cathode catalysts was about 0.8 V(vs RHE),belonging to the atomically adsorbed hydrogen with higher reactivity.In addition,the atomically dispersed Fe/graphene cathode catalysts achieved the highest H2O2 Faraday generation efficiency in the oxygen reduction reaction of about 85%,which was comparable to the Ni-Cu/graphene cathode catalyst.The adsorbed hydrogen and H2O2 generated on the electrode surface were captured and consumed for the Fe/graphene cathode catalyst to electrocatalyze the hydrogenation reduction and indirect oxidation of chloramphenicol,which promoted a positive shift in the initial overpotential.Atomically-dispersed Ni/graphene cathode catalyst was synthesized by chemical impregnation method to further improve the selectivity in electrocatalytic hydrogenation reduction and oxygen reduction reactions.The Ni species in the Ni/graphene cathode catalyst(1.5 wt%)synthesized by coordination protection was highly dispersed in the graphene sheet layer in the atomic state and exists in the Ni-O coordination structure.The Ni/graphene cathode catalyst had a higher hydrogen adsorption potential(1.0 V vs RHE)and hydrogen evolution potential than the Fe/graphene cathode catalyst,and the hydrogenation reduction efficiency was higher.In addition,the catalytic activities and selectivity of Ni/graphene cathode catalysts in oxygen reduction reaction were also better than that of Fe/graphene cathode catalysts,and its H2O2 Faraday generation efficiency was about 96%,which is also better than the optimized Ni-Cu/graphene cathode catalyst.The stable Ni-O coordination structure in the Ni/graphene cathode catalyst effectively improved the conversion of active hydrogen and H2O2,thereby promoting its conversion efficiency in the hydrogenation reduction and indirect oxidation of chloramphenicol,showing higher shift of overpotential compared with Fe/Graphene cathode catalyst.The optimized highly-dispersed Ni-Cu/graphene and atomically-dispersed Ni/graphene catalytic materials were used to prepare gas diffusion cathodes and then constructed a three-electrode system for electrochemical chloramphenicol removal.The optimal reaction conditions for chloramphenicol removal in bimetallic Ni-Cu/graphene cathode system and atomic Ni/graphene system were as follows: current density was 30 m A/cm2,electrolyte was 0.05 mol/L Na2SO4 solution(p H=7).Under those conditions,the TOC removal rates in the bimetallic Ni-Cu/graphene system after 4 h reached 96.6±0.7%(cathodic compartment 1),96.1±0.8%(cathodic compartment 2)and 98.9±0.7%(anodic compartment),respectively.The single-atom dispersed active sites and ultra-high H2O2 selectivity accelerated the conversion and removal of chloramphenicol in the atom-dispersed Ni/graphene system,and the TOC removal rate after 3 hours in the cathodic and anodic compartments reached 99.1±1.4%(cathodic compartment 1),98.9±1.5%(cathodic compartment 2)and 98.95±0.7%(anodic compartment),respectively.The final current mineralization efficiency of the bimetallic Ni-Cu/graphene system and atomic Ni/graphene catalytic system in anodic compartments were 61.9% and 82.1%,respectively,indicating that the use of atomically dispersed Ni/graphene catalytic cathode could not only shorten he reaction time for chloramphenicol mineralization,but also effectively reduced the energy consumption in removal.Judging from the measurement results of the intermediate products in the cathodic and anodic compartments,the reactions in the cathodic compartments mainly included nitro reduction,hydrodechlorination and subsequent indirect oxidation through oxygen reduction.The reaction pathway in the anodic compartments is relatively simple,mainly for the oxidation process of intermediate products.The formed primary intermediate product then gradually evolved into various small-molecule organic acids under the synergetic decomposition of the cathode and anode,and finally reached complete mineralization.The results showed that the highly-dispersed metal active sites in the electrode material are the key factors in the electrochemical removal process.Compared with the bimetallic Ni-Cu/graphene cathode,the single-atom Ni/graphene cathode,which was achieved by heteroatom doping,greatly improved the mineralization efficiency of chloramphenicol mineralization,and reduced the reaction energy consumption of the system.Single-atom materials also has better development prospects in the application of electrochemical removal of antibiotics. |
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