| Capacitive deionization (CDI) as an emerging and environment-friendly water treatment technology, which has some advantages of low power consumption, low cost and no secondary pollution, is able to remove ions in solution by the electrostatic force and has attracted more and more attention to domestic and oversea scholars, compared to the traditional desalting technologies such as distillation and reverse osmosis.In this paper, to solve the problems on the shortage of the low desalting efficiency and the incomplete CDI mechanisms, an experimental device for CDI named flow through capacitor was firstly constructed. And the technical system for electrosorption desalination was improved by means of the optimization of operational parameters, the reinforcement of traditional desalting technology on CDI, and the discuss on mechanisms for enhancing desalting efficiency and decreasing energy consumption, which provided the basis and support for large-scale development and application of CDI technology.Activated carbon electrodes used in the CDI process were prepared by coating activated carbon powder (ACP), PVDF binder and graphite conductive agent to the graphite current collector as the mass ratio of 85:10:5. The specific surface area and mesoporous proportion of the coating materials ACP were 1770 m2/g and 35%, moreover, SEM images and EDX analysis for the prepared electrodes indicated the ACP was covered with polymer binder, and they were bound together effectively.According to response surface methodology, a quadratic optimization model was built under the Box-Behnken experimental design based on the response values of electrosorption capacity and energy consumption. The model was proved to be significant, accurate and reliable by the analysis of variance. After the optimization by RSM, the maximum electrosorption capacity of 10.53 mg/g and minimum energy consumption of 9.81 kWh/kg were given when the values of operational parameters of applied voltage, initial NaCl concentration, flow rate and plate distance were 1.57 V, 1000 mg/L,25 mL/min and 2 mm, and 1.38 V,900 mg/L,40 mL/min and 2 mm, respectively, in accordance with the predicted values by the quadratic response model, which indicated the process optimization based on RSM was feasible and effective. Besides, considering both the impacts of desalination efficiency and energy consumption, when the value ranges of cell voltage, initial NaCl concentration, flow rate and electrode plate distance were 1.3-1.6 V,800-1000 mg/L,25-40 mL/min and 2 mm, respectively, the experimental process for CDI was most cost-effective.The coating material of ACP was modified by reactivation with the activating agent of KOH at 850? for 1 h. The BET specific surface area and mesoporous proportion increased to 1902m2/g and 41%. In addition, the specific capacitance and electrosroption capacity of the AC electrodes prepared by the reactivated ACP rose to 97.71 F/g and 12.73 mg/g. The conventional CDI process was improved by introducing ion exchange membranes to form membrane capacitive deionization (MCDI), and the electrosorption capacity and desalination rate of the MCDI process were 13.78 mg/g and 41.34%, respectively, increasing by 30.8% compared to the common CDI process under the same conditions. The adsorption mechanism for MCDI process was not changed. In the end, the optimum effect which the electrosorption capacity and desalination rate were 14.75 mg/g and 44.26% can be acquired when both the reactivation of AC materials and MCDI system were employed, furthermore, the energy consumption also declined by 10% from 12.24 kWh/kg to 11.01 kWh/kg in the same case.The electrode potential was derived from a formula according to the relations among applied voltages, electrode plate distance, and current density, and then it is deduced that the lower applied voltages and larger plate distance can be employed to guarantee the desalting effects and reduce energy consumption if the initial concentration of feed water is higher; moreover, when the initial concentration of feed water is lower, the applied voltage and plate distance need to be increased and decreased respectively on the premise that no electrode redox reactions occur. The resistance and approximate equivalent circuit were analyzed by the Nyquist plot of AC electrode derived from electrochemical impedance spectroscopy (EIS), which proved the existence of polarization resistance caused by concentration polarization. The concentration polarization effect will be weaken by these measures such as increasing flow rate, strengthening the surface properties of electrode, increasing useful areas of electrode surface and shorting plate distance during the actual CDI process. Also, the resistance will be decreased and the desalting effect and energy consumption are able to be improved by the measures mentioned above. Besides, there were no obvious redox peaks in the CV curves, which indicated that ions in solution were adsorbed on the electrode surface mainly due to the electric field force rather than electrochemical redox.The adsorption mechanism of CDI was analyzed with the pseudo-first-order and pseudo-first-order kinetic model, and the Langmuir and Freundlich adsorption isotherm equations. The electrosorption of NaCl onto AC electrodes followed the pseudo first order kinetic model and Langmuir adsorption isotherm, which was monolayer coverage mainly driven by a physisorption process. The ionic migration was sped up and the desalting efficiency and electrosorption capacity were improved with the increase of applied potential and initial concentration.In conclusion, increasing the desalting efficiency and lowering the energy consumption are able to be achieved through the control and optimization of operational parameters, improvement of electrode materials and desalination by MCDI. And the analysis of mechanism for strengthening desalination in CDI process is good for the further application of CDI technology. |
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