Preparation Of Montmorillonite-based Composites For Capacitive Deionization Performance

Capacitive deionization(CDI)is an emerging desalination technology that removes all kinds of anions and cations from brackish water.Compared with thermal distillation,electrodialysis and reverse osmosis,it has attracted much attention for its advantages of low cost,low energy consumption and environmental friendliness,and is especially suitable for treating brackish water with medium and low salt concentration.However,advanced and efficient electrode materials are the key factors for the development of CDI.Montmorillonite(MMT)is a natural silicate mineral,which has been used in adsorption,separation and catalysis because of its unique two-dimensional layered structure and cation exchange characteristics.After inorganic modification(acid modification,inorganic salt modification)and organic modification(cationic surfactant,anionic surfactant),montmorillonite has a large interlayer spacing,good thermal stability and adjustable adsorption sites,which can be used as a new catalytic material and adsorption material,and as a negative electrode material in a hybrid capacitor desalination device,it can efficiently adsorb and store cations.In this paper,the intercalation reaction of small organic molecules was used to expand the interlayer spacing of montmorillonite,and the intercalation composites of alkyl quaternary ammonium salt(MMT@STAB)and polyvinylpyrrolidone(MMT@PVP)were prepared,which optimized and improved the desalination performance of MMT materials.A hybrid capacitor deionization device was assembled with modified MMT as cathode material and activated carbon as anode.The effects of working voltage and brine concentration on desalination performance were systematically studied.The main studies are as follows:(1)Sodium-montmorillonite(Na-MMT)was used as the precursor,and alkyl quaternary ammonium salts with different chain lengths were used as the intercalation agent to prepare alkyl quaternary ammonium salt intercalation MMT composites.The composites were characterized by XRD,XPS,TG and BET.The results showed that alkyl quaternary ammonium cations entered the interlayer.The interlayer spacing of Na-MMT increased from 12.36 to 18.55,23.92 and 22.41,which provided favorable conditions for the storage and migration of sodium ions and improved the desalination performance.The electrochemical tests show that the MMT@STAB composite electrode material has higher specific capacitance(141.9 F g-1),longer charge and discharge time and lower equivalent resistance when the scanning rate is 1 mV s-1 in 1 M NaCl solution.The results of CDI performance test show that the desalination capacity of AC//MMT@STAB system is 112.8 mg g-1 at 1.2 V working voltage in NaCl solution with initial conductivity of 1000 mg L-1.In addition,the charge efficiency of the system is 93.16%,the energy loss is 0.31 kWh kg-1.It also has good cycle stability at 1.2 V working voltage.The larger interlayer spacing accelerates the movement rate of Na+,thus increasing the desalting quantity,and at the same time accelerating the transfer of electrons and increasing the desalting rate.(2)Sodium-montmorillonite(Na-MMT)was used as precursor and modified by polyvinylpyrrolidone(PVP)with different molecular weights,and MMT@PVP K24,MMT@PVP K30 and MMT@PVP K58 composites were synthesized.XRD results showed that the interlayer spacing of Na-MMT increased from 1.24 nm to 2.23 nm,and the BET results show that the specific surface areas of MMT@PVP K24,MMT@PVP K30 and MMT@PVP K58 composite electrode materials decrease from 6.8984 m2 g-1 to 6.8726 m2 g-1,3.8879 m2 g-1 and 4.4263 m2 g-1.which proves that the layers of electrode materials have regular structures and are more conducive to the shuttle of Ne+ between layers.The electrochemical tests show that the specific capacitance of MMT@PVP K30 composite electrode material is 124.7 F g-1 in 1 M NaCl solution with a scanning rate of 1 mV s-1,And it has long constant current charging and discharge time and lower equivalent resistance.The modified composite electrode material was used as the negative electrode and activated carbon as the positive electrode material to assemble a hybrid capacitor deionization device,and its desalination performance was studid.The results show that the MMT@PVP K30 electrode shows better desalination performance,which is 112.8 mg g-1,when the initial conductivity is 1000 mg L-1 NaCl solution and the working voltage is 1.2 V,and the AC//MMT@PVP K30 system has a faster desalination rate(6.84 mg g-1 min-1).After PVP replaces the impurity ions,the morphology of the material is more uniform,whici reduces the obstacle to Na+transportation,thus improving the storage capacity of ions.