Investigations Of Removal Processes And Mechanisms Of Uranium(Ⅵ) And Chromium Ⅵ)using Functionalized Phyllosilicates

Synthesis of advanced materials for effective application has heightened due to the continuous development of nanotechnology and microstructure research.The affinity and applicability of existing elements have greatly promoted the synthesis of nanoparticles with different properties.Phyllosilicates with tunable capability has gained attention owning to their layered structure,exchangeable interlayer cations,large surface area and adsorptive sites.This notwithstanding,their applicability is restricted due to limited properties and functionalities.Herein in this thesis,chemical precipitation or Pickering emulsion was used to synthesize different phyllosilicate-based functionalized composite structures by employing both natural and synthetic phyllosilicates coupling with organotrimethoxysilane or non-metallic photocatalyst semiconductor.The characteristics of the synthesized structures were examined analytically.Immobilization capability of the synthesized structures were investigated systematically to establish the optimal conditions by modulating the reaction system where hexavalent uranium and chromium are used as target ions in single-phase studies.Montmorillonite supported nanosize zero-valent iron（n ZVI/Mt）functionalized with thiols（SH）was synthesized via a two-step chemical precipitation method.A resultant foliated structure with enhanced distribution of n ZVI on the Mt support template after the functionalization was observed through morphology analysis.The synthesized composite structure was stable under a wide pH as it maintained a negative surface charge ranging from-29 m V（under strong acidic condition）to-34 m V（under alkaline condition）.Thiol groups were determined to be present on the surface of the sample that provided a ligand-like activity.The n ZVI/Mt-SH composite structure displayed an enhanced immobilization capability with the amount of Fe in the sample having significant influence.Protonation of n ZVI/Mt-SH surface under acidic condition and increase in hydroxyl groups under alkaline condition results in the optimal removal（100%）occurring at near neutral pH condition at 30 min of contact time.It was determined that the mechanism for the removal of uranyl ions by the n ZVI/Mt-SH composite structure was by reductive immobilization through the transformation of U（VI）to U（IV）.Meanwhile,adsorption（through ion exchange and surface complexation）and precipitation occurred in the process.Inspired by the above findings,organo-functionalized synthetic magnesium phyllosilicate（Mg-SH）was synthesized and used as a supporting template for n ZVI by a two-step precipitation method.n ZVI/Mg-SH structural analysis showed that n ZVI was successfully distributed on the surface of Mg-SH.n ZVI/Mg-SH complex possessed a large specific surface area（66.76 m2/g）,while the specific surface areas of Mg-SH and n ZVI were 64.10 m2/g and 55.89 m2/g,respectively.It was observed that the composite structure was highly reactive and thus capable of rapid removal of uranium ions,with 100%efficiency observed after 30 min,compared to 68.69%（Mg-SH）and 93.94%（n ZVI）of the reaction time.Deprotonation of the sample surface reduced the negative charge on its surface,leading to a decrease in the removal rate under acidic conditions,while deprotonation promoted the best removal by electrostatic attraction under near-neutral conditions.At different salt concentrations,n ZVI/Mt-SH proved to be effective for the removal of uranium ions with efficiencies exceeding 80%.The removal process was determined to be an endothermic process with a removal capacity of more than 212 mg/g at 50°C.The removal pathway was determined to be a reductive immobilization following a pseudo-second order kinetic process.The results indicate that Mg-SH can be used as a successful support template for n ZVI and that the composite can be effectively used for the removal of U（VI）ions from aqueous solutions.Interest in the synergistic effect of natural and synthetic phyllosilicate inspired the synthesis of Halloysite nanotubes（HNTs）and thiol-functionalized magnesium phyllosilicate composite for uranyl ions uptake.The natural-synthetic composite phyllosilicate displayed a mixed bulky ball-like and tubular morphology with a crystalline structure and an increased surface area.Zeta potential was determined to be negative under both acidic and alkaline condition.Elucidation of HNT/Mg-SH efficiency was conducted under varying conditions.pH～7 was determined as the optimal pH condition.Increased hydroxyl radicals under alkaline condition caused electrostatic repulsion limiting the uptake of uranyl ions.The reaction process was determined to be endothermic and the mechanism for uptake was established through the chemisorption process,the main route for the uptake of U（VI）ions was by the adsorption through surface complexation with Al,Si and S being the major participating elements in the reaction system.Finally,with inspiration from the use of trap materials to enhance the photo-catalytic ability of photocatalysts,Mt based-photoresponsive microcapsules were constructed by Pickering emulsion method.Graphitic carbon nitride（g-C₃N₄）was used as the metal-free photocatalyst semi-conductor.Mt served as a trap for photo-generated holes（h⁺）to enhance the transfer of electrons（e^-）for surface reactivity.The constructed microcapsule had a hollow core-shell structure microcapsules with diameter ranging up to 50μm with Mt as forming the predominant shell material.It was found that at higher g-C₃N₄ to Mt ratio,increased photo-generated electron-hole pairs resulted in 100%removal efficiency of Cr（VI）ions via monolayer chemical adsorption.In summary,a series of composite materials synthesized using functionalized natural and synthetic phyllosilicates are presented in this thesis.The physico-chemical properties of the composite structures were systematically analyzed using appropriate scientific analytical technology.Using different target ions in a single-phase system,the efficacy of the synthesized composites for environmental application were investigated under modulated conditions.Finally,the mechanisms and reaction process were elucidated.These studies would contribute to the expansion of phyllosilicate application research in environmental remediation by providing a scientific foundation and technical reference for the systematic synthesis of advanced materials.