First-principles Study On The Adsorption Of Hydrated Rare Earth Ions On The Surface Of Kaolinite

Ionic rare earth ore is a kind of rare earth mineral resource with unique strategic value in China.The rare earth elements are mainly adsorbed on the surface of clay minerals such as kaolinite in the form of hydration or hydroxyhydrate,and a certain concentration of electrolyte solution can be used to exchange the rare earths.Although the ionic rare earth ore resources have been discovered and exploited for decades,the current state of the rare earth elements on the surface of clay minerals is still unclear.The structure and properties of rare earth hydrated ions and their microscopic adsorption structures on the surface of clay minerals,as well as the effects of impurity defects commonly found in clay minerals on the adsorption of rare earth hydrates,are controversial.These problems constrain the efficient exploitation of ionic rare earth ore.In this paper,the first-principles method based on plane wave pseudo-density functional theory was used,the effects of five common impurity defects such as magnesium,calcium,vanadium,iron and gallium on the electronic structure and properties of kaolinite?001?aluminum hydroxyl surface were investigated.The coordination structure and properties of hydration,monohydroxy hydration and dihydroxy hydrate of La???,Lu???and Y???ions were studied,and their outer-layer and inner-layer adsorption mechanisms on the surface of kaolinite?001?were also studied.The effects of magnesium,calcium,vanadium,iron and gallium defects on their chemisorption behavior were investigated.The main findings are as follows.?1?The ideal?001?aluminum hydroxy surface density of kaolinite is mainly contributed by 2p orbital near the Fermi level.Magnesium and calcium impurity defects enhance the covalentity of the kaolinite surface,significantly enhance the contribution of the surface 2p orbital at the Fermi level and the activity of the hydroxyl O atom,thereby enhancing the adsorption performance of kaolinite?001?aluminum hydroxyl surface.Gallium defects have little effect on surface covalency and density of states.The vanadium and iron defects slightly weaken the covalentity of the kaolinite surface,and at the same time,the contribution of the surface state density to the Fermi level changes from 2p to a small amount of 3d orbital,and the hydroxyl O atom activity is weakened,thereby weakening the adsorption performance of the aluminum hydroxyl surface.?2?The highest coordination hydrate structures that may exist in the first hydrate layer of La???,Lu???and Y???ions in water are La?H₂O?₁₀³⁺,Lu?H₂O?₉³⁺and Y?H₂O?₁₀³⁺,respectively.The highest coordination of monohydroxy hydrated structures are La?OH??H₂O?₈²⁺,Lu?OH??H₂O?₇²⁺and Y?OH??H₂O?₉²⁺,respectively.The highest coordination dihydroxy hydration structure are La?OH?2?H₂O?7⁺,Lu?OH?2?H₂O?6⁺and Y?OH?2?H₂O?7⁺.When hydrated,the rare earth and the O atom in the water molecule orbital hybridization and form a strong ionic coordination bond,all of which are obtained from the 2p orbital of the O atom by the rare earth d orbital,but the La-O and Y-O coordination bonds are dominated by the bond-forming effect,while the Lu-O bond is dominated by the reverse bond.The order of bond length is Lu-O<Y-O<La-O.The order of stability is hydrated rare earth ions<monohydroxy hydrated rare earth ions<dihydroxy hydrated rare eart h ions,which indicates that the rare earth entering the water body tends to bind more hydroxyl groups to form a hydroxy hydrated structure.?3?The rare earth hydrated ions are outer-layer adsorbed on the kaolinite?001?aluminum hydroxyl surface and the silicon oxide surface by hydrogen bonding.Most of the rare earth hydrated ions can maintain the original coordination number after outer-layer adsorption.The adsorption energies of La?H₂O?₁₀³⁺,Lu?H₂O?₉³⁺and Y?H₂O?₁₀³⁺on the aluminum hydroxyl surface are-275.32,-290.13 and-262.66 kJ·mol^-1,respectively.Adsorption energy on the silicon oxide surface are-473.37,-492.30 and-495.63 kJ·mol^-1,respectively.The adsorption energies of La?OH??H₂O?8²⁺,Lu?OH??H₂O?7²⁺and Y?OH??H₂O?₉²⁺on the aluminum hydroxyl surface are-256.75,-246.73 and-242.44 kJ·mol^-1,respectively.Adsorption energy on the silicon oxide surface are-440.29,-449.85 and-465.89 kJ·mol^-1,respectively.The adsorption energies of La?OH?2?H₂O?7⁺,Lu?OH?2?H₂O?6⁺and Y?OH?2?H₂O?7⁺on the aluminum hydroxyl surface are 191.94,-191.58 and-195.57 kJ·mol^-1,respectively.Adsorption energy on the silicon oxide surface are-378.21,-347.39 and-359.34 kJ·mol^-1,respectively.Since the H atom on the surface of aluminum hydroxyl has a certain repulsion effect on the coordinated water molecules in the hydrated rare earth ions,the outer-layer adsorption of kaolinite?001?silicon oxide to the rare earth hydrated ions is stronger than that of the aluminum hydroxyl surface.The outer-layer adsorption of hydrated rare earth ions are the most stable,followed by monohydroxy hydrated rare earth ions,and the dihydroxy hydrated rare earth ions are the most unstable.?4?The rare earth hydrated ions are inner-layer adsorbed on the kaolinite?001?aluminum hydroxyl surface by chemical bonding and hydrogen bonding,the dominant adsorption sites of La?OH?2?H₂O?6⁺and Lu?OH?2?H₂O?5⁺are Ol position,Y?OH?2?H₂O?6⁺is the Ot position,while other rare earth hydrated ions are the Ou position.When Lu?OH?2?H₂O?5⁺is adsorbed,in addition to bonding with the deprotonated O atom on the surface of kaolinite,Lu also forms a bond with a nearby unprotonated O atom to form a bidentate adsorption structure.And other rare earth hydrated ions form a single-tooth adsorption structure.When La,Lu and Y bond with O atoms on the surface of kaolinite,orbital hybridization occurs.The rare earth d orbitals acquire electrons from the 2p orbital of O atom,but the La-O and Y-O bonds are mainly composed of bonding,while the Lu-O bond is mainly composed of anti-bonding.After the inner-layer adsorption of La?H₂O?₉³⁺,Lu?H₂O?₈³⁺,Y?H₂O?₉³⁺,La?OH??H₂O?₇²⁺,Lu?OH??H₂O?₆²⁺,Y?OH??H₂O?₈²⁺,La?OH?₂?H₂O?₆⁺,Lu?OH?2?H₂O?5⁺and Y?OH?2?H₂O?6⁺,the coordination number are generally reduced due to the effect of steric hindrance,and the adsorption energies are-742.83,-755.31,-744.52,-710.20,-663.44,-745.42,-703.14,-663.86 and-659.61 kJ·mol^-1,respectively.The chemisorption energies are much lower than the outer-layer adsorption energies,which indicate that the rare earth hydrated ions mainly undergo chemisorption on the kaolinite?001?surface.The inner-layer adsorption of hydrated rare earth ions is the most stable,followed by monohydroxy hydrated rare earth ions,and the dihydroxy hydrated rare earth ions are the most unstable.?5?The presence of impurity defects affects the inner-layer adsorption configuration and coordination number of rare earth hydrated ions on the?001?aluminum hydroxyl surface of kaolinite,and has a significant effect on the adsorption energy.Among them,vanadium impurities increase the adsorption energy of all rare earth hydrated ions;magnesium and calcium impurities greatly reduce the adsorption energy of all rare earth hydrated ions;The gallium impurity can reduce the adsorption energy of rare earth hydrated ions except Y?OH??H₂O?8²⁺;The iron impurities increase the adsorption energy of the dihydroxy hydrated rare earth ions and Lu?OH??H₂O?6²⁺,and the adsorption energy of the rare earth hydrated ions decreases slightly.This indicates that kaolinite with high vanadium impurity is not conducive to enrichment of rare earth hydrated ions,and kaolinite with high content of magnesium and calcium impurities is beneficial to the enrichment of rare earth hydrated ions,which is more conducive to the formation of high-grade ionic rare earth ore,but it increases the difficulty of exchange of rare earths.The research results of this thesis not only have important reference value for the ore-forming theory of ionic rare earth ore and the solution chemistry and coordination chemistry theory of rare earth,but also lay a foundation for the theoretical study of ionic rare earth ore ion exchange process.