Development Of Polymer-based Nanocrystalline Layered Double Hydroxides Composites For Enhanced Water Defluoridation

Layered double hydroxides(LDHs)has attracted considerable attention in the recent decade for their great potential in water treatment.However,very few scaled-up applications is available mainly because that the existing LDHs are easy to aggregate and to lose activity.To solve these issues,a new defluoridation adsorbent was successfully fabricated by impregnating LDHs inside porous polymer beads binding ammonium groups in this study.Besides the preparation and defluoridation of nanocrystalline LDHs based composites,the invigorating effects of functional groups and nanopores of the carrier on the dispersion of LDHs particles and the formation of nanocrystalline was further discovered,and the defluoridation performance and mechanism of the composite were thoroughly studied.Furthermore,the regeneration process was developed,the effect of co-existing anions on defluoridation performance was analyzed,and the practical defluoridation performance in treating groundwater polluted by fluoride was also evaluated in this thesis.The adsorbents of LDHs containing aluminium used to remove fluoride from water have attracted wide interest due to their high specific surface area(20-120 m2/g)and high exchange capacity(3.0-4.8 meq/g).Furthermore,activated aluminum oxide,widely applied in defluoridation,was employed for reference.In this study,a series of Li/Al LDHs and Mg/Al LDHs were firstly synthesized by co-preciptation and characterized by X-ray diffraction(XRD)and Fourier transform infrared spectroscopy(FTIR).Effect of different metal molar ratio on the adsorption capacity of fluoride was examined by using Li/Al LDHs and Mg/Al LDHs as sorbents.Results indicated that the highest capacity was reached at the molar ratio of Li/Al and Mg/Al of 3 for both LDHs.The capacity of fluoride absorption onto LDHs was higher than that onto activated alumina under the tested pH value,and the applicable range of pH(4-8)was wider than that of activated alumina(5-6),which indicated LDHs was more suitable for removing fluoride from water.Furthermore,a series of Li/Al/La LDHs and Mg/Al/La LDHs with different contents of La were prepared by adding La in the preparation of the Li/Al LDHs and Mg/Al LDHs.The capacity of fluoride adsorption onto the LDHs was improved largely after La doping,with much wider pH range.XRD result showed that the peak pattern remained unchanged after fluoride adsorption,which indicated the adsorption of fluoride did not destroy the lamellar structure of LDHs.The capacity of fluoride adsorption on the LDHs was affected by coexisting anions with different electric charge,suggesting an ion exchange process between LDHs sorbents and fluoride.XPS result showed that the excellent performance of La doped LDHs was mainly due to the ligand exchange between Al,La in the laminate and fluoride.The adsorption kinetic and isotherm of Li/Al/La LDHs and Mg/Al/La LDHs indicated that the second-order kinetic model and Freundlich model fitted the adsorption process well,and the calculated equilibrium capacities were 46.2 and 40.6 mg/g,respectively.The defluoridation capacity of Li/Al/La LDHs was higher than that of Mg/Al/La LDHs.To further solve the issues of engineering applications of LDHs,a novel nano-hybrid sorbent,namely LA-LDH@201,was prepared by impregnating Li/Al LDHs nanoparticles into the porous polymer with anion exchange groups(D201).The average size of the loaded LDHs was around 23 nm.The mechanical strength and chemical stability of LA-LDH@201 was improved significantly over the unloaded Li/Al LDHs,which may attribute to the cross-linked polymeric host serving as a networking shield to inhibit the leaching of the loaded metals.A novel composite named LLA-LDH@201 was fabricated by utilizing lanthanum to modify LA-LDH@201,and characterized by XRD and SEM-EDS.The result indicated that some proportion of La was inserted into the lamellar structure of LDHs.LLA-LDH@201 showed excellent sorption performance even coexisting with other competing anions.Such performance was mainly due to three factors:(1)ion exchange between interlayer anions and fluoride,(2)ion exchange between anion group of D201 and fluoride,and(3)ligand exchange between Al,La in the sorbent and fluoride.LLA-LDH@201 was further tested in fixed-bed runs in treating fluoride contaminated groundwater sampled from Yunnan Province.The results showed that the breakthrough capacity of LLA-LDH@201 was 255 BV,and the exhausted LLA-LDH@201 can be regenerated efficiently under room temperature for reuse.LLA-LDH@201 was then used to treat fluoride contaminated groundwater sampled from Shandong Province.LA-LDH@201 exhibited satisfactory defluoridation performance even at high concentration of SO42-and HCO3-.In conclusion,the nano-hybrid sorbent loaded with Li/Al/La LDHs(LLA-LDH@201)is a promising composite in defluoridation from water for scale-up applications.