The Application Of Layered Double Hydroxides-Based Materials In Environmental Sorption And Photoelectrocatalysis

Layered doubled hydroxide(LDHs)is a kind of anionic clay material with unique layer structure.It can be controlled in the metal cations of the laminate and the anion species in the interlayer.It has high structural characteristics such as high dispersion and high specific surface area,so it has superior application development potential in the fields of catalysis,adsorption,photoelectrochemistry and biomedicine.At the same time,LDHs also have some problems which limit their application.For example,it is difficult for LDHs to adsorb and remove hydrophobic compounds or macromolecular organic substances.Due to the powder property,it is poor in recycle and reuse.By combining LDHs with other functional materials,not only can the performance of LDHs be improved,but also some defects in the functional materials themselves can be improved.The prepared multifunctional composite materials are beneficial to broaden their practical applications.The research focus of this paper is mainly on the development of LDHs-based functional composites on the basis of traditional LDHs materials by combining them with functional materials to improve their application in water environment adsorption and photoelectrocatalysis.The details are described as followings:1.Calcium-based LDHs have good dissolution and reconstitution properties in aqueous solution,Calcium aluminum layered double hydroxides modified with sodium dodecyl sulfate(SDS)(Ca/Al-DS LDH)was prepared to investigate the unique dissolution behavior in heavy metal ion solution.Ni(?)was selected as the target metal cation and its removal kinetics could be well described by the pseudo-second-order kinetic model.It was found that Ca/Al-DS LDH has a good removal effect on metal ions in addition to self-dissolution in metal ion solution.The maximum removal capacity of Ni(?)reached 2.45 mol/g at the LDH dosage of 0.5 g L-1.The XRD and element analysis results indicate that a new Ni/Al-DS LDH phase and CaCO3 were formed as well as a portion of Ca/Al-DS LDH remained in the final products after the removal process.Therefore,the removal of Ni(II)was contributed to the surface complexation.isomorphic substitution and sorption on the CaCO3.Therefore,this work not only provided a detailed understanding of the auto-dissolving behavior of organic-inorganic composite calcium-aluminum layered double hydroxide in metal ion solution,but also proposed a novel strategy to generate organic intercalated LDHs by isomorphic substitution provides a new way for the preparation of LDHs.2.A series of novel thermo-responsive composite sorbents,were prepared by free-radical copolymerization of N-isopropylacrylamide(NIPAm)and the silylanized Mg/Al layered double hydroxides(SiLDHs),named as PNIPAm-co-SiLDHs.For keeping the high affinity of Mg/Al LDHs towards anions,the layered structure of LDHs was assumed to be reserved in PNIPAm-co-SiLDHs by the silanization of the wet LDH plates as evidenced by the X-ray powder diffraction.The sorption capacity of PNIPAm-co-SiLDH(13.5 mg/g)for Orange-II from water was found to be seven times higher than that of PNIPAm(2.0 mg/g),and the sorption capacities of arsenate onto PNIPAm-co-SiLDH are also greater than that onto PNIPAm,for both As(III)and As(V).These sorption results suggest that reserved LDH structure played a significant role in enhancing the sorption capacities.NO3-intercalated LDHs composite showed the stronger sorption capacity for Orange-II than that of CO32-.After sorption,the PNIPAm-co-SiLDH may be removed from water because of its gel-like nature,and may be easily regenerated contributing to the accelerated desorption of anionic contaminants from PNIPAm-co-SiLDHs by the unique phase-transfer feature through slightly heating(to 40?).These recyclable and regeneratable properties of thermo-responsive nanocomposites facilitate its potential application in the in-situ remediation of organic and inorganic anions from contaminated water.3.A new type of triadic photoanode is developed by rationally designing the electrode structure.The NiFe-layered double hydroxide(NiFe-LDH)serves as a water oxidation catalyst(WOC)to accelerate the transportation of the photo-generated holes from the BiVO4 photoelectrode to the electrolyte for improved water oxidation reaction,while the layered reduced graphene oxide(rGO)nanosheets serve as the electron conductor for suppressing the electron-hole recombination.On the other hand,rGO can increase the electrodeposion potential of NiFe-LDH(-0.1 V vs RHE)which can greatly protect the BiVO4 electrode since a lower potential would reduce its PEC activity.Both experimental findings and theoretical calculations reveal that rGO plays an important role in the system.This electrode exhibits a significantly higher photocurrent density than those of the pristine BiVO4,BiVO4/rGO,and BiVO4/NiFe-LDH electrodes,producing a photocurrent of 3.26 mA cm-2 at 1.23 V vs RHE under AM 1.5 G illumination and showing excellent stability.The improved PEC performance is attributed to the accelerated charge separation/transfer between the photoanode/electrolyte interfaces and surface water oxidation reaction due to the synergistic effect of rGO and NiFe-LDH.The composite structure design demonstrated in this work provides a new method to develop highly efficient photoelectrodes for solar fuel production with excellent long-term stability.4.In order to solve the problem of low light absorption efficiency and low charge separation efficiency of BiVO4 photoanode,the translucent BiVO4 photoanodes(t-BVO)were prepared by adjusting the thickness of the precursor BiOI film in the preparation process of BiVO4.Further by adding nitrogen-doped graphene quantum dots(NGQDs)the.Jmax of the BiVO4 photoelectrod could be increased.The surface-loaded Ni/Fe-LDH film is used as a surface oxygen evolution co-catalyst to increase the charge transport efficiency in the system.The well-designed t-BVO/NGQDs/NiFe-LDH composite photoelectrode has excellent photocurrent density and stability,and which could provide a new idea for designing and developing high-efficiency photoelectrode for photoelectrochemical decomposition of water without bias.