Preparation Of Modified Bentonite Electrode For Capacitive Desalination

With the development of economy and the increase of population,water resource shortage has become one of the important problems to be solved in many countries in the world.At present,fresh water resources are not enough to meet the daily consumption of people around the world.Therefore,desalination technology has become one of the technologies that researchers need to pay special attention to.Traditional desalination technologies,such as distillation and reverse osmosis,are difficult to be applied on a large scale due to their high cost,complex process and secondary pollution.Therefore,it is necessary to develop a desalination method which is both energy saving and environmental protection.Capacitive Deionization（CDI）is a new seawater desalination technology,which has the advantages of low energy consumption,simple operation and no secondary pollution.The core of CDI technology is the selection of electrode materials.However,traditional carbon electrode materials have low adsorption capacity,serious co-ion effect and poor cycling stability,while Faraday materials have problems such as slow adsorption and desorption rate and poor electrical conductivity.Ion exchange materials such as Bentonite（Bent）are widely used in water treatment because of their high ion exchange capacity,low cost and good stability.In this study,Bent was activated by acid and modified by surface,and then prepared into electrode for the study of desalting performance.The specific research contents are as follows:（1）Acid-modified bentonite（H⁺-Bent）electrode was prepared by acid etching to activate Bent.The H⁺-Bent electrode was used as the positive electrode of the capacitor for desalting test.The results showed that when the charging voltage was 1.3 V and the electrode material ratio was 8:1:1,the electrode had a higher desalting capacity(about 11 mg g^-1),which was about 2 times that of the activated carbon（AC）electrode.After 50 cycles of adsorption-desorption,H⁺-Bent showed better cyclic stability,with a volume retention rate of 85%,much higher than that of AC（about 30%）.The feasibility of electrode CDI desalting in actual water was further investigated.The results show that H⁺-Bent electrode also has a good treatment effect on the actual water body,and the adsorption capacity retention rate reaches about 80%after 50 cycles of charge and discharge.This study confirmed the feasibility of H⁺-Bent electrode in desalting direction and its good cycling stability.（2）Based on the poor conductivity of Bent materials,this study prepared polydopamine（PDA）in situ on the surface of Bent.After high temperature treatment,the Bent was doped with C and N to improve its conductivity and adsorption capacity.By controlling the polymerization time and carbonization temperature of PDA on Bent surface to optimize the loading capacity,the appropriate NC@Bent composite material was obtained.The successful synthesis of NC@Bent composites was demonstrated by scanning electron microscopy（SEM）and transmission electron microscopy（TEM）.When the polymerization time is 30 h and the carbonization temperature is 700℃,the electrode material can not only maintain good conductivity,but also maintain good ion transport ability.The corresponding 700-NC@Bent-30 electrode shows high desalting capacity(～19 mg g^-1)and charge efficiency（64%）in the CDI test.The feasibility of electrode CDI desalting in actual water was further investigated.Through six absorption and desorbination tests,700-NC@Bent-30electrode achieved simultaneous purification of various ions in the actual water body,in which the final removal rates of K⁺,Ca²⁺,Mg²⁺,Cl^-and SO4^2-reached 94.5%,94.6%,97.9%,71.7%and90.9%,respectively.After repeated cycles,the morphology and structure of the electrode did not change significantly,indicating that the electrode has a good stability of adsorption and desorption cycle.This study proves that ion-exchange materials can be used as CDI electrodes with high adsorption capacity,good stability and low cost.（3）In order to further improve the desalting performance of 700-NC@Bent-30 electrode,the Rocking chair CDI（RCDI）device was used to test the desalting performance of 700-NC@Bent-30electrode.The results show that the 700-NC@Bent-30 electrode has a high desalting capacity(82mg g^-1)and a high desalting rate(0.031 mg g^-1 s^-1)at the current density of 8 m A g^-1.After 50 cycles of adsorption and desorption,the electrode still has nearly 90%capacity retention rate,indicating that 700-NC@Bent-30 electrode has a good adsorption and desorption cycle stability.Ion-exchange electrode 700-NC@Bent-30 is not only cheap and readily available,but also has stable desalinization performance.This study confirms the feasibility of ion-exchange electrode in RCDI device,which makes NC@Bent electrode have great application prospect in brine desalination.