Preparation Of Potential-responsive Ni-based Layered Double Hydroxides/Oxides Composites For Inorganic Anion Separation

Inorganic anionic contaminants are a class of main pollutants in water.If their level in water exceed the standard,it will cause serious harm to environment and human beings.Therefore,it is of great significance to explore cost-effective methods and techniques for the removal of inorganic anionic contaminants from wastewater.Traditional ion exchange and adsorption have been regarded as attractive methods owing to their simplicity,high efficiency,and recyclability of adsorbents.However,several inherent limitations of them restrict their industrialized application.Such as,the adsorption rate is slow,the regeneration process could produce a large amount of secondary wastes.Electrochemically switched ion exchange?ESIX?,as an environmentally benign ion separation technology,could separate the target ions with low-concentration rapidly but without secondary pollution.For selective separation of target ions,designing ESIX film materials with specific structure is a key research issue for this technology.As a typical class of anion exchange materials,layered double hydroxides?LDHs?have some unique advantages in the treatment of anionic contaminants,such as large specific surface area,high anion exchange capacity and good thermal stability.Therefore,calcined Ni Co-LDO was designed as electroactive ion exchange film for phosphate ion(PO₄^3-)separation.Furthermore,Ni Co-LDH nanosheets coated on conducting polypyrrole?PPy?nanofiber was prepared to enhance the conductivity of electrode and to separate perchlorate?Cl O₄^-?.Lastly,a novel CNTs@Ni Mn-LDH composite consisting of CNTs core and Ni Mn-LDH coated layer was fabricated to realize the continuous anions separation and the removal of fluoride ions?F^-?.The detailed research contents and main conclusions can be found as follows:?1?Ni Co-layered double oxide?Ni Co-LDO?was fabricated via in-situ calcination of Ni Co-LDH and served as ESIX film for the removal of PO₄^3–.A quick uptake and release rate of PO₄^3–onto the Ni Co-LDO electrode was obtained by adjusting redox state of film.The morphology,crystal structure and elemental compositions of film was characterized by SEM,TEM,XRD and XPS analyses.The experiments revealed that,the ion exchange quantity of the Ni Co-LDO film for PO₄^3–removal by ESIX process was about twice over that of by IX.Moreover,the Ni Co-LDO film exhibited a high selectivity towards PO₄^3–.In addition,after5 uptake/release cycles,ion exchange quantity still retained 92%of its initial value.Coupled with XPS analysis,it was found that ESIX process of Ni Co-LDO for PO₄^3–removal included an irreversible“memory effect”structure recovery process and two reversible processes,which were the redox reaction of metal ion in lamellar and ligand exchange between PO₄^3–and O–H groups.?2?Ni Co-LDH nanosheets coated on conducting PPy nanowire was controllably fabricated by using UPED method and served as a novel ESIX hybrid film for the removal of Cl O₄^-anions based on proton-ligand effect.It is expected that the space among Ni Co-LDH nanosheets of the shell could act as the reservoir for the anions while the PPy core served as the potential-induced element for proton-ligand.The effects of pulse potential during Ni Co-LDH deposition and initial p H of the wastewater on the Cl O₄^-removal performance of this core-shell hybrid film were investigated.It was found that Cl O₄^-adsorption onto PPy@Ni Co-LDH followed the pseudo-second-order model,and the hybrid film fabricated with-1.5 V pulse potential showed an excellent performance for the rapid removal of Cl O₄^-with a high selectivity,and the Cl O₄^-adsorption quantity reached as high as 302 mg g^-1.In a wide p H range?3?10?,the hybrid film removed Cl O₄^-efficiently.The proton-ligand effect in PPy@Ni Co-LDH was proved by using XPS analysis and density functional theory.?3?Ni Mn-LDH nanosheets coated on conductive CNTs were successfully fabricated via a UPED method for efficient water defluoridation.It is found that the Ni Mn-LDH nanosheets with large surface areas can be grafted uniformly on the skeleton of conductive CNTs to facilitate the electron transport.Compared to IX process,faster and more efficient reversible uptake/release of F^-ions was realized by modulating the positive and negative charges on the electrical double layer?EDL?of the composite,coupling with ligand exchange at the active sites of Ni Mn-LDH during the electrochemical redox process.Especially,in a broad p H range?4?10?,the electroactive CNTs@Ni Mn-LDH can remove F^-effectively.Moreover,the F^-adsorption onto CNTs@Ni Mn-LDH followed the pseudo-second-order model and intra-particle diffusion model,with the maximum F^-adsorption quantity of 135.1 mg/g from the Langmuir model.After 5uptake/release cycles,ion exchange quantity still retained 92.4%of its initial value.Furthermore,the EDL ion transport coupled with ligand exchange mechanism in CNTs@Ni Mn-LDH was verified by XPS analysis.In addition,combining CNTs@Ni Mn-LDH in ESIP membrane separation system can realize a continuous F^-separation.The continuous runs implied that the flux of F^-by CNTs@Ni Mn-LDH was twice over that of by CNTs only while the time of the concentration reached even below WHO standard was a quarter of that of by CNTs only.