Study On The Performance Of Layered Double Hydroxide Based Functional Materials For Efficient Removal Of Aqueous Cr(Ⅵ)

Heavy metal pollution in water is one of the serious issues,which threatens the ecological security and human health.Among the heavy metals,hexavalent chromium（Cr（Ⅵ））is a primary carcinogen,which can cause permanent damage to humans and animals through food chains.How to removal Cr（Ⅵ）from aqueous environment has been the focus of extensive research.Layered double hydroxides（LDHs）,with exchangeability of anions,easy tenability of cations in the host layers,memory effect,and highly dispersed active sites,have been endowed with great potential in Cr（Ⅵ）adsorption and photocatalysis fileds.In this dissertation,we selected MgAl-LDH or ZnAl-LDH as the target matrix and prepared LDH-based adsorbents/photocatalysts,which can be obtained by different synthetic pathways,such as intercalation,amino-functionalization,assembly and calcination.The removal performance and reaction mechanisms of Cr（Ⅵ）on the as-synthesized materials was systematically evaluated by adsorption/photocatalytic experiments and characterizations.The results are shown as follows:（1）We prepared EDTA intercalated MgAl-LDH（LDH-EDTA）using a hydrothermal method,and then loaded Fe₃O₄ on the surface of LDH-EDTA using a solvothermal method to obtain Mag-LDH-EDTA.The technologies,such as scanning electron microscopy（SEM）with energy dispersive spectrum（EDS）,transmission electron microscopy（TEM）,Brunauer-Emmett-Teller（BET）surface area determination,X-ray diffraction patterns（XRD）,Fourier transform infrared spectroscopy（FTIR）,X-ray photoelectron spectroscopy（XPS）and Zeta potential were used to characterize LDH-EDTA and Mag-LDH-EDTA.The characterization results indicated that EDTA anions were intercalated into the interlayer of LDH,the crystal structure of LDH was remained,and the obtained composites had abundant of groups,such as hydroxyl,carboxyl.Though batch adsorption experiments,the adsorption process of LDH-EDTA and Mag-LDH-EDTA for Cr（Ⅵ）was less influenced by initial solution pH value and quickly reached equilibrium state at 50 min.The pseudo-second order model and the Langmuir equation well fitted to the adsorption kinetic and isotherm data.The maximum adsorption capacities of LDH-EDTA and Mag-LDH-EDTA for Cr（Ⅵ）were 47.62 and 32.46 mg/g,respectively.The adsorption mechanisms of Cr（Ⅵ）by LDH-EDTA and Mag-LDH-EDTA could be attributed as electrostatic attraction,surface complexation,anion exchange,and reduction reaction.（2）We synthesized EDTA intercalated MgAl-LDH（LDH-EDTA）,then applied acrylamide（AM）to graft LDH-EDTA by cross-linking method to yield the LDH-EDTA-AM composite,and characterized the crystal structure,micro-morphology,functional groups and surface chemical properties by series of characterization techniques.The adsorption behavior for aqueous Cr（Ⅵ）and CR were also systematically investigated by changing the experimental conditions in single and mixed adsorption systems.The characterization results proved that LDH-EDTA-AM had rich function groups（e.g.hydroxyl,carboxyl,amino）and exhibited hexagonal lamellar structure.The adsorptive efficiency on LDH-EDTA-AM for Cr（Ⅵ）and CR were relative high over a wide range of pH,and could achieve more than 90%within 30 min.The adsorption kinetic data of Cr（Ⅵ）and CR were coincident to the pseudo-second-order model.The adsorption isotherm data were fitted to the Langmuir and Freundlich isotherm equations for CR,and the Langmuir model for Cr（Ⅵ）.The maximum uptake quantities of Cr（Ⅵ）and CR on LDH-EDTA-AM were 48.47 and 632.9 mg/g,respectively.In mixed solutions,the chromate removal was stimulated by existed CR,while the adsorptive efficiency of CR was minially influenced by the coexisting Cr（Ⅵ）.The mechanisms for both hazardous pollutants adsorption by LDH-EDTA-AM involved electrostatic attraction,surface complexation,and anion exchange.In the Cr（Ⅵ）adsorption process,the reduction reaction was also taken placed.Wastewater simulation experiments showed that LDH-EDTA-AM had good adsoptive efficiencies for Cr（Ⅵ）and CR in the real contaminated water.（3）We prepared ZnAl-LDH and Fe₃O₄/ZnAl-LDH by simple coprecipitation and solvothermal method,and then synthesized Fe₃O₄/LDO/BiOBr composite by integrating Fe₃O₄/ZnAl-LDH with bismuth oxybromide（BiOBr）and calcination treatment.The characterization results indicated that Fe₃O₄/LDO/BiOBr had large specific area,strong magnetic property and enhanced visible light absorption ability.The Fe₃O₄/LDO/BiOBr-1.5exhibited the best photocatalytic activity for Cr（Ⅵ）than other photocatalysts.Specially,the photoreduction efficiency of 50 mg/L Cr（Ⅵ）was 98%under visible light irradiation within 30min at pH of 2.0.The magnetic Fe₃O₄/LDO/BiOBr-1.5 also exhibited excellent photocatalytic activity and photo-stability after four recycles,and can be employed as a promising photocatlyst for practical water remediation by efficient solar light harvesting.The photocatalytic mechanisms of Cr（Ⅵ）were speculated by transient photocurrent response（TPR）,electrochemical impedance spectroscopy（EIS）,photoluminescence（PL）and Mott-Schottky.The formation of heterogeneous structure can promote the charge separation and improve the transfer rate of charge.During the electrons transfer process,the Fe₃O₄ acted as an electron-bridge between BiOBr and LDO,and promoted the charge separation and transfer.The LDH-based adsorbents and photocatalysts have lots of advantages,such as simple preparation process,facile modification,environmental friendly and low price,and can efficiently remove Cr（Ⅵ）from aqueous solutions.Then this dissertation could provide some theoretical basis for functional modification of LDHs,and further materials in the field of Cr（Ⅵ）-containing actual wastewater treatment.