Study On Performance And Mechanism Of Fluoride Removal From Water By Activated Carbon Based Capacitive Desalination

Endemic fluorosis is widely distributed in China and even in the world.The treatment of fluorine-containing water has been an important topic in the field of environmental protection all over the world.The development and application of new fluorine-removing technologies with friendly environment,low energy consumption and easy regeneration have been the research hotspots.In recent years,with the development of electrode materials for supercapacitors and the improvement of capacitor process design,capacitive desalination(capacitive deionization,CDI)technology has become a new desalination technology for seawater and brackish water.In this paper,the effect of the coating process on the preparation of activated carbon electrode and the use of CDI technology to remove fluorine ions in water under the Batch model was systematically investigated from the perspective of six factors included by the process parameters of electrode forming and operation parameters.The dynamic fitting of capacitive defluoridation process was carried out,and the Faraday reaction was analyzed.The function and mechanism of capacitive defluoridation have been mastered comprehensively,and a simplified kinetic model has been established,which can well fit the process of capacitive defluoridation and lay a foundation for the application of CDI technology in the field of fluorine removal.The paper mainly has the following research results:(1)When PVDF is used as a binder to prepare activated carbon electrodes,the porosity and electrochemical properties are reduced.When PTFE is used as a binder to prepare electrodes,the surface pore structure is relatively developed,and the adsorption effect on fluorine ions is better.The amount of binder is inversely proportional to the surface porosity of the activated carbon electrode.However,if the ratio is too low,the electrode will be de-powdered and the stability of the electrode will be weakened.The optimum ratio of powdered activated carbon,PTFE binder and graphite is 8:1:1.The electrode thickness is positively correlated with the total adsorption amount,but inversely related to the adsorption amount per unit mass electrode.The optimal electrode thickness selected in this experiment was 0.4mm.(2)If the flow rate is too large or too small,the desalination efficiency will decrease.If the flow rate is too large,the hydraulic retention time will be too short.If the flow rate is too small,the co-ion eviction effect will be greater than the adsorption force,and the adsorption amount will be reduced.The optimal flow rate selected in this experiment was 25 mL/min.In addition to the positive relationship between the amount of fluorine adsorption and the voltage,but the excessive voltage leads to the electrolysis of water The initial concentration of the solution increases the number of ion migration.If the concentration is too high,the ion effect will hinder the electrosorption process.(3)The capacitive defluoridation process follows a quasi-first-order kinetic process.The poor fit under micro-voltage(less than 0.6 V)is because the common-ion repulsion effect hinders ion adsorption.At high voltage(greater than 1.2 V),the decrease of fitting degree is due to non-capacitive adsorption phenomenon.Mesoporous activated carbon electrode has better performance of electric adsorption in removing fluorine.When the applied voltage is 1.5V,the flow rate is 25mL/min,and the raw water concentration is 2.5mm,the removal rate of fluorine ion is 31.49%,and the specific adsorption capacity is 3.84mg/g.The capacitive defluoridation process belongs to physical adsorption and mainly depends on the theory of electric double layer.However,the reduction of dissolved oxygen and the precipitation of hydrogen at the cathode,the oxidation of the carbon electrode and the electrolysis of water at the anode cause a decrease in energy efficiency.Thus,by combining of cathode and membrane,and developing metal oxide anode,the efficiency of the capacitive defluoridation will be improved.(4)The simplified kinetic model established by the improved Donnan model can well fit the activated carbon-based capacitive defluoridation process.