Study On The Characteristics And Mechanism Of Capacitance Deionization Of Heteroatomic Doped Carbon Electrode

Capacitive decapacitive(CDI)technology is widely used in desalination of alkaline water and reuse of waste water.It has the advantages of high efficiency,low energy consumption,environmental friendliness and high utilization of water.The key to CDI technology is the selection of electrode materials,among which carbon materials(such as activated carbon and derived carbon materials,carbon fiber and graphene,etc.)are the most widely studied.However,the uneven pore size distribution and poor hydrophilicity are causing of the low specific capacitance.The high price was due to the complex preparation.And the low material recycling rate are limiting the utilization of carbon-based materials in CDI technology.Therefore,the research on the theory and application of CDI based on new carbon-based electrode materials possess not only great scientific value but also broad application prospect and practical significance.In this paper,we are focused on the preparation of new carbon-based materials and the study of its own heteroatomic effect,systematically investigating the relationship between the CDI performance of the electrode and the structure,morphology as well as heteroatomic effect of the material.Meanwhile,the electrode materials were further optimized with the best CDI performance.Finally applied to low salinity dyeing tail water treatment.The main tasks are as below:1.Anew hexagon carbon-based material(HAT)was prepared by in situ synthesis method,and then a HAT-CN material was prepared by removing C2N2 based on π-electron recombination condensation and used as the electrode for membrane capacitive deionization(MCDI).The effects of carbonization temperature on the morphology,specific surface area and pore diameter distribution,element ratio,electrochemical and electroadsorption properties of HAT were studied.Results show that the carbonization temperature of HAT at 550℃ show the C2N-type measurement through the characterization.It can be found that the larger specific surface area(771.2 m2/g),abundant pore structure,good electrical conductivity,and high N content(36%)were provided plenty of space and the activity of charge storage site to obtain the higher salt ions adsorption.Meanwhile,the initial concentration of 500 mg/L in NaCl solution has good electric adsorption(24.66 mg/g),far higher than that of activated carbon(11.14 mg/g).In addition,the MCDI performance of HAT-CN-500 at different initial voltage and salt ion concentration was investigated.The results show that the ion transport speed and ion removal rate increased with the increase of NaCl concentration.Finally,HAT-CN-500 shown good regeneration performance in 30 electroadsorption cycles.The results show that each atom of in-situ N-doped material has a dangling bond in C2N,and the size of the gap allows to anchor several interacting Na+ or Cl’ atoms.Compared with the original carbon materials,heteroatomic doping can enhance the polarization level of the structure and increase the specific binding sites of ions,providing an abundant of space and active sites to store charge for salt ion adsorption.At the same time,in-situ heteroatomic doping introduced hydrophilic functional groups,it can improve the hydrophilicity of materials,and further improve the electroadsorption capacity.2.Based on excellent electroadsorption performance of C2N materials.C2N@rGO material was synthesized by a simple hydrothermal method,used as a flow-electrode in FCDI system for the first time.Characterization proved that C2N@rGO had a large specific surface area(812.3 m2/g)and micropore content,as well as a high N content(～29.5%)and adjustable graphitization structure,which provided the basis for the excellent electrochemical and desalination properties of the prepared electrode.FCDI results showed that C2N@rGO-4%had higher salt ion removal amount(180.72 mg/g),salt ion removal rate(0.3043 mg/(min·cm2》and salt ion removal efficiency(49.93%).In addition,it was found that the performance of FCDI depends partly on the synergy effect between the porous structure and the chemical components.Meanwhile,the experiment also proves that adding a small amount of rGO(4%)in C2N@rGO can obtain the good dispersing performance,effectively solve the problem of the flow-electrode channel blocking during the flow capacitive deionization(FCDI)process.Meanwhile,when adding a little rGO,the rGO can forming a "electric bridge" to improve the utilization rate and addition amount of C2N particles,and greatly improve the effect of salt ion removal.3,Although FCDI technology has good deionizing performance,the problem of flow electrode recovery in FCDI process is seldom studied.In order to recycle the flow electrode,we were used self-assembly method to prepared the MPZS,which is consisted by the transition metal(Fe)and nonmetallic atom(N,P,S),and it synthesized by the Poly-cyclotriphosphazene-4,4’-sulfur-bisphenol(PZS)coated nano-magnetic sphere.The effect of different carbonization temperatures on the structure,morphology,specific surface area,magnetism and element(N,P,S)content of MPZS on the performance of FCDI was studied.The results show that the CMPZS electrode inherits all the advantages of PZS at the carbonization temperature of 500℃,that is,it has good wettability,low impedance(0-50 ohm),high specific capacitance(211.2 F/g),long-term colloidal stability and good water dispersion due to the heteroatom influence.At the same time,because the transition metal atoms are uniformly wrapped in the carbon sphere,it has a good superparamagnetism and magnetic intensity(54.99 emug-1)to realize high magnetic recycling(10 time),and abundant reactive sites to obtain fast charge transport.Finally,the ion removal rate of CMPZS in practical dyeing wastewater treatment is as high as 50.34-98.86%,indicating its great potential in the field of deionization.4.For the comprehensive treatment of low-salt textile printing tail water,the ion exchange membrane in the process of MCDI or FCDI will be blocked due to a small amount of organic matter in tail water.A GDI cathode was prepared by cross-linking carbonized bacterial cellulose(CBC)with polymers containing-COOH and-SO3H groups,respectively.Meanwhile,CDI anode was prepared by cross-linking CBC with the cationic polymer polyvinyl alcohol(PEI),and then the anode and cathode were combined to form an asymmetric non-membrane capacitive deionization(p-CDI)unit.The removal effect of different functional groups in the cathode on salt ions and the selective removal effect of NO2-were studied.The results show that the CBC modified by-SO3H has higher capacitance and negative potential than that by-COOH,so that the CBC modified by-SO3H has better total ion removal ability(38.55%)and total ion removal(14.56 mg/g),as well the selective removal rate(71.01%)of NO2-was greatly improved at applied voltage of 1.0 V.At the same time,the total ion removal rate increases with the increase of voltage and decreases with the increase of initial salt ion concentration.Then,the synchronization effect between the electric field and charged functional groups in the polymer,the effect of ion exchange,the different ion hydration radius and ion migration rate on the selective removal of ions were investigated.The anion removal sequence was NO2->SO42->NO3->F-≈Cl’.Finally,in the tail water treatment of low-concentration printing and dyeing wastewater,the ion removal rate of p-CDI can reach 60.94-84.26%,and the removal rate of NO2-can reach 90.60-97.30%.In brief,the results show that CDI has a good application prospect in removing nitrite and treating printing and dyeing wastewater.