Preparation Of Biochar Electrode Modified With Ammonium Ferric Citrate And Mechanism Of Capacitive Deionization And Desalting

Biomass comes from a wide range of sources around the world.Its internal tubular bundle structure and the skeleton remaining after carbonization form a biochar material with a multi-level pore structure and a huge specific surface area.The modified biochar can greatly improve the adsorption performance,used to adsorb pollutants in water.Capacitive deionization（CDI）is an emerging water treatment technology in modern times.By making adsorption materials into electrodes,applying a certain voltage to the adsorption electrodes,an electric field force is formed between the cathode and anode electrodes,which can promote the orientation of charged particles.When the charged particles move to the vicinity of the adsorption electrode,an electric double layer can be formed on the surface of the adsorbed material in the electrode,thereby capturing the charged particles.Compared with other water treatment technologies,capacitive deionization technology has the advantages of energy saving,high efficiency and environmental friendliness.In summary,the biochar modification technology can be combined with the capacitive deionized water treatment technology to prepare a modified biochar with excellent capacitive deionization and desalination performance.In this study,the abundant wheat straw was used as the raw material for biochar.Firstly,a series of biochars were prepared at different carbonization temperatures,through the characterization and analysis of the physicochemical properties and electrochemical properties of the biochars,the optimal carbonization temperature of wheat straw biochars as adsorption materials was obtained.Then,using ferric ammonium citrate solution rich in carboxyl group,hydroxyl group,ammonium group and iron ion as the modification reagent,a method of low temperature pre-carbonization followed by impregnation,and then high temperature secondary carbonization modification method was used to prepare a modified supported biochar,It not only modified the biochar with ammonia,introduced nitrogen by doping,but also loaded iron oxides on the surface of the biochar,and enriched the oxygen-containing functional groups on the surface of the biochar,which provided more active sites for the adsorption of the biochar.The optimal impregnation concentration of ferric ammonium citrate solution was obtained through characterization analysis and electrochemical test;This study also independently designed and built a capacitive deionization desalination test device that can be combined with CDI and MCDI,under two capacitive deionization desalination models,the designed modified biochar was made into an adsorption electrode,and the electrode was subjected to a capacitive deionization desalination test,and the experimental analysis of different influencing factors was carried out.The optimal capacitive deionization desalination conditions under the two desalination models were obtained.The main findings and conclusions are as follows:（1）Biochar retains the multi-layer bundle structure of straw fibers,which is layered and slit-type non-uniform pores.The optimum carbonization temperature of biochar as an electro-adsorbent material is 600 ℃.At this time,the carbonization of biochar is nearly complete,the stability is good,and at the same time,the structure of abundant functional groups is also retained.In addition,biochar also showed good electric double layer supercapacitive performance,with the most prominent electrochemical performance at WB-600 ℃.When the carbonization temperature is greater than or equal to 600 ℃,the functional groups on the surface of the biochar are further broken and reorganized,resulting in a decrease in the electrochemical performance.（2）The biochar was modified with ferric ammonium citrate solution.After characterization and analysis,iron oxide crystals such as Fe₃O₄,Fe₂O₃ and Fe⁺³O（OH）were successfully loaded on the surface of biochar,and iron oxide crystals such as Fe₃O₄,Fe₂O₃ and Fe⁺³O（OH）were successfully loaded into the biochar.Nitrogen was doped,and more CN bonds and CO bonds were introduced,which enriched the functional groups on the surface of biochar.The optimal impregnation concentration of ferric ammonium citrate modified biochar is 1 mol/L,and when it exceeds 1 mol/L,the load will agglomerate.The electrochemical performance of WB-ACFe1.0 biochar is the most outstanding.Under the condition of current density of 1 A/g,the specific capacitance is up to 244.93 F/g,which is 3.77 times higher than that of unmodified biochar,and at 10 A/g At the current density of g,it still has a retention rate of 77.53 %,showing a good rate capability.（3）Under the classical CDI capacitive deionization desalination model,the optimal adsorption voltage of WB-ACFe1.0 biochar is 1.3 V,the optimal electrode spacing is 1.5 mm,and the optimal initial salt solution concentration is 300 mg/L.Under these conditions,the first capacitive deionization desalination amount was 10.41 mg /g,and after 10 adsorption and desorption desalination,the capacitance deionization desalination amount was 9.34 mg /g,and the decay rate was 10.28 %.After 8 cycles of adsorption,desorption and desalination tests,the carbon material reached a stable stage,and the electro-adsorption desalination capacity has almost no attenuation,and has good adsorption and desorption cycle performance.（4）Under the MCDI capacitive deionization desalination model,the optimal adsorption conditions of WB-ACFe1.0 biochar are as follows: the electrode spacing is1.5 mm,and the adsorption voltage is 1.3 V.At the same time,MCDI also has good capacitive deionization for high-concentration brine.esalination performance.When the initial concentration of the salt solution was 400 mg/L and 500 mg/L,the capacitance deionization efficiencies were 53.81 % and 45.81 %,respectively,and the capacitance deionization and desalination capacities of the modified biochar per unit mass were13.85 mg/g and 14.72 mg/g,respectively,the desalination efficiency was improved compared with CDI,and even exceeded the limit adsorption capacity of biochar in the CDI desalination model of 12 mg/g.（5）According to the kinetic simulation analysis,under the best experimental conditions: the initial concentration of the salt solution is 300 mg/L,the adsorption voltage is 1.3 V,the electrode spacing is 1.5 mm,and the initial concentration of the salt solution is 300 mg/L,the modified biochar is stable in CDI.The capacitive deionization desalination process under the two electroadsorption desalination models and MCDI followed a pseudo-second-order kinetic model.（6）When the initial concentration of the salt solution is 300 mg/L,the adsorption voltage is 1.3 V,and the electrode spacing is 1.5 mm,500 m L of saline wastewater is treated.When the adsorption equilibrium is reached,the average energy consumption per unit mass of Na Cl in the entire adsorption process is 0.566 CDI.W h/g,MCDI was0.0392 W h/g.