Design Of Porous Carbon Electrodes And Their Mechanism Study Of Capacitive Deionization

Freshwater scarcity and water pollution are two of the most important environmental problems faced by human beings in the 21st century.Capacitive deionization（CDI）has attracted wide attention as an emerging desalination technology due to its low energy consumption,high removal efficiency,low cost,and non-secondary pollution.In addition to brackish water desalination,CDI has bee explored for treatment of water contamination.It has been reported that CDI is an effective technology for ionic contaminants removal from wastewater,such as nitrate,fluoride,sulfate,phosphate,which exist at low concentrations but have adverse impacts on the environment and human health.However,the perchlorate（ClO₄^-）removal mechanism by CDI is still not clear.The characteristics of porous carbon electrode materials pay a key role in the desalination performance of CDI.However,fabricating eco-friendly electrode materials with low cost for efficient CDI performance is still a huge challenge.Among the various configurations within the CDI cell,Flow-electrode CDI（FCDI）has attacted huge interest due to its facile electrode configuration,outstanding adsorption capacity,continuous ion removal and high water recovery rate.Nevertheless,it is the difficult points and the hot topics to optimize flow electrode configuration.In view of the above key scientific issues,the applications and related mechanism of CDI in treatment of ClO₄^-containing saline wastewater were explored and a one-dimensional CDI process model for dual-anions was developed to quantitatively describe the electrosorption kinetics of perchlorate and chloride.Meanwhile,co-friendly electrode material with low cost for efficient CDI performance is prepared,and the desalination performance and related mechanism were analyzed.Finally,the desalination performance and related mechanism of FCDI,and the applications of FCDI in treatment of iodide-containing saline wastewater were explored by adding iodide-based redox couple to the flow electrodes.The details are summarized as follow:（1）Using the activated carbon cloth as the electrodes,we investigated the effects of various operating parameters（eg.charging voltage,initial p H,and initial ClO4^-concentration）on dynamic electrosorption processes of both perchlorate and chloride were examined.Meanwhile,a one-dimensional CDI process model for dual-anions was developed to quantitatively describe the electrosorption kinetics of perchlorate and chloride,and an excellent agreement between the modeling results and the experimental data was observed.Experimental results revealed a favorably strong preferential adsorption of ClO₄^-over Cl^-in the studied CDI system under various conditions.Moreover,only slight discrepancy between adsorption-desorption cycles was found,demonstrating the good regeneration of electrodes and operational stability of the CDI cell.Finally,the scale-up studies indicated that the CDI stack with multiple pairs of electrodes could achieve a much more superior ion removal performance than the CDI cell with only one pair of electrodes.To conclude,these results showed that the CDI system was effective for perchlorate selective removal in the presence of other major ions and had potential in treatment of perchloratecontaminatedbrackish water.（1）Phosphorus-doped porous carbon derived from pinecones（PPCP）,an abundant biomass waste,was readily prepared by using H₃PO₄ as the activating agent at activation temperature（700-900℃）.The effect of activation temperature on the morphology characteristics,and electrochemical and brackish water desalination performance of PPCP samples were investigated.Results indicated that the PPCP800electrode exhibited the highest specific capacitance owing to the relatively high specific surface area and high degree of graphization,superior porous structure,the highest phosphorus content and lowest charge transfer resistance.Meanwhile,the PPCP800 electrode showed excellent desalination performance with a high deionization capacity（SAC）of 14.62 mg g^-1 at an applying voltage of 1.2 V in 1000mg L^-1 Na Clsolution and the fastest salt removal rate.Furthermore,the PPCP800electrode displayed great reproducibility in long-term consecutive operation.Therefore,the PPCP800 electrode might be a promising candidate for capacitive deionization applications.（3）The three different morphologies of W₁₈O₄₉@C（HSC,UNC and NBC）as the cathode electrode materials of hybrid CDI（HCDI）,were prepared by a simple hydrothermal method.The HSC,UNC and NBC showed excellent desalination ability with high SAC of 26.3,32.25 and 23.8 mg g^-1,respectively at an applying voltage of1.2 V in 1000 mg L^-1 Na Clsolution,which were better than the most reported TMOS materials.In addition,the UNC electrode material showed the most efficient CDI performance owing to its highest specific surface area,superior microporous structure and the highest carbon content.Furthermore,the UNC electrode displayed great reproducibility in long-term consecutive operation.Therefore,W₁₈O₄₉@C materials might be a promising candidate for capacitive deionization applications.（4）The effects of the electrolyte concentration and amount of iodide-based redox couple on FCDI desalination performance were investigated.The results showed that the appropriate amount of introduced iodide-based redox couple into the cathodic flow electrode can provide a highly efficient FCDI desalination performance with a stable behavior in salt electrosorption and desorption over multi-cycle consecutive runs.The enhanced FCDI desalination performance could ascribe to the reversible redox reaction of iodide-based redox couple and the maintenance of the electro-neutrality of the global electrolyte.Although possible minor release or leakage through the cation-exchange membrane and into the treated water occurred,which makes the proposed approach not quite suitable for potable water production.The results showed that the total iodine in the feed water decreased from 10 mg L^-1 to 5.5 mg L^-1 after 30-min electosorption.Therefore,this system has huge application prospects in treatment of iodide-containing saline wastewater without causing secondary pollution.