Removal Of Monovalent Salt Ions By Enhanced Capacitive Deionization

Saline wastewater has become an important concern in the field of water treatment after the pollution of COD,nitrogen,phosphorus and heavy metals.The removal of monovalent salts is one of the difficulties in saline wastewater treatment,due to the removal rate of monovalent salts is much lower than that of polyvalent salts.Capacitive Deionization(CDI)is a new desalination technology,it has many advantages compared with traditional desalination technologies,such as low energy consumption,environmentally friendly and so on.It is one of the hotspots in the field of desalination.This paper focuses on the development of CDI electrodes and reactors on the reactor platform with enlarged electrode area,activated carbon(AC)electrodes and carbon coated sodium titanium phosphate(NaTi2(PO4)3/C)electrodes were prepared by polyvinylidene fluoride(PVDF)coating method,two new CDI reactors:membrane capacitive deionization(MCDI)and hybrid capacitive deionization(HCDI)were designed and assembled respectively,to enhance the removal of monovalent salts.By discussing the influence of preparation conditions of activated carbon electrodes on the electroadsorption performance,it was found that under the same CDI experimental conditions,the ion adsorption capacity of pretreated AC electrodes was 1.7 times that of untreated ones.Therefore,pretreatment of activated carbon materials to remove ash could improve the desalination performance of the electrodes.In the process of electrode forming,the content of binder has a significant effect on the performance of CDI electrode.The experimental results showed that the activated carbon powder,conductive carbon black and PVDF were coated on the foam nickel on the 8:1:1 ratio,the prepared electrode had the best desalination performance.Moreover,the thickness of the nickel foam had little effect on the desalination performance of the electrode.MCDI unit was obtained by introducing anion and cation exchange membrane into CDI unit.The experimental results showed that the desalination effect of MCDI was obviously better than that of CDI unit,and the ion adsorption capacity of the former was about 3.3 times of that of the latter.The mechanism is that MCDI greatly improves the "co-ion effect" in the traditional CDI process,thus greatly improving the desalination performance of CDI.Compared with CDI,MCDI also had better cyclic stability.The optimum operation parameters of MCDI were obtained by single factor optimization experiments:the working voltage was 1.2 V,the flow rate was 20 mL/min,and the plate distance was 1.5 mm.To further enhance the ability of CDI to remove the monovalent salts,the mesoporous microcrystalline sodium titanium phosphate(NaTi2(PO4)3)and carbon coated sodium titanium phosphate(NaTi2(PO4)3/C)materials were successfully prepared by sol-gel method.The AC+//NaTi2(PO4)3/C_and AC+//NaTi2(PO4)3 group were desalted by means of sol-gel method.The ion adsorption capacity of the system was lower than that of AC+//AC-traditional CDI system.In view of this phenomenon,this paper puts forward the following possible reasons:a.NaTi2(PO4)3 material has a low specific capacity of sodium intercalation in aqueous electrolyte,almost 39%of its theoretical specific capacity of sodium intercalation;b.NaTi2(PO4)3 has a relatively small number of effective active components in its larger molecular weight electrodes,so the number of sodium ions that can be intercalated and removed is relatively small.c.NaTi2(PO4)3 has poor electrical conductivity,and carbon coating still can not meet the requirements of its application in HCDI desalination;d.Dissolved oxygen in water may affect the process of sodium insertion of NaTi2(PO4)3 electrode,thus reducing the actual specific capacity of NaTi2(PO4)3 material for sodium insertion.In summary,by constructing MCDI system and optimizing the process parameters of MCDI operation on the reactor platform with enlarged electrode area,the effect of salt removal was effectively strengthened.At the same time,the feasibility of NaTi2(PO4)3,a sodium ion battery material,applied in HCDI system was explored to further promote the development of HCDI technology.