Study On Capacitive Deionization With Single-pass Mode

Freshwater crisis has become an urgent problem in not only China but also the whole world.Undoubtedly,utilizing the seawater and brackish is a feasible way to provide sustainable clear water for human beings.Traditional desalination technologies,such as reverse osmsis,multi stage flash,multi effect distillation and electrodialysis have issues in maintenance,complex pretreatment,high temperature and high energyconsumptionIn contrast,capacitive deionization(CDI)with a simple operating pattern,low costs of materials,low energy requirements is a potential alternative desalination technology.Capacitive deionization stack was operated by batch mode and single pass mode respectively to investigate the not only the differences and similarities but also the advantages and disadvantages of them,making contribution to the practical use of capacitive deionization.The relationship between faradic reactions and electro-adsorption in capacitive deionization using activated carbon electrodes was investigated.Meanwhile,the sustainability of hardness removal in capacitive deionization using CaCl2 and MgCl2 solution was studied in this study.Different faradic reactions could appear with the increase of applied voltage.The adsorption of anions promoted the reduction of DO and H2O2 while adsorption of cationscontributed to the oxidation of carbon itself and water electrolysis.The oxidation of Cl-is not the main reason for the fluctuations of pH and the rate of reduction of DO in the inner eletrodes is too fast could be contribute to the decrease of pH.The capacitive deionization showed a great sustainability of hardness removal,indicating that the Ca2+ and Mg2+ would not lead to the scaling of carbon electrodes.Based on pseudo-first-order and pseusdo-second-order equations,the adsorption/desorption process of activated carbon and influence of flow rate,concentration of influent,electrode thickness and applied voltage were studied.It was found that there is an optimum flow rate for desorption,beyond or below which the desorption rate could decrease.With the increase of influent concentration,adsorption rate increased but the adsorption capacity would not obviously increase while the desorption rate decreased.Increasing the thickness of electrodes led to the decrease of adsorption and desorption rate.Compared with lower applied voltage,both of adsorption rate and desorption rate decreased which could be attributed to the minimization of the plate distance.Kinetics analysis indicated that the adsorption/desorption processinactivated carbon electrodes followed pseudo-first-order kinetics model.