Study On The Adsorption Of Hydrated Aluminum Ions On The Kaolinite Surface By Density Functional Theory

In ionic rare earth ores,rare earth is mainly adsorbed on the surface of clay minerals in the form of hydrated or hydroxyl hydrated ions.Rare earth ions can be exchanged and desorbed by leaching with leaching agent solution containing active cations.In this process,the impurity aluminum ions adsorbed on the surface of clay minerals will also be exchanged into the leaching mother solution.Qualified rare earth products can be obtained only after impurity removal.Therefore,in-depth investigation of the occurrence mechanism of Al³⁺and other impurity ions on the surface of clay minerals will help to provide theoretical guidance for the development of highly selective ionic rare earth extractants to reduce the cost of impurity removal and the loss of rare earth.At present,few domestic and foreign scholars have reported on the occurrence of impurity ions on the surface of clay minerals at the molecular and atomic levels,and neglected the important influence of ion hydration on their adsorption.Therefore,in order to reveal the micro occurrence mechanism of the impurity Al³⁺in the real environment on the surface of clay minerals in ionic rare earth ores,the structure and properties of hydration,monohydroxy hydration and dihydroxy hydrate of Al³⁺in the water environment are analyzed by using density functional theory,and the optimal configurations of the above three hydrated Al ions are calculated and obtained for the adsorption of the outer layer of kaolinite（001）and（00-1）and the inner layer（monodentate and bidentate）of kaolinite（001）respectively.Based on the analysis of electronic structure and properties,the micro adsorption mechanism of Al ions is revealed,and the DFT calculation results are verified and analyzed by adsorption test.The main findings are as follows:（1）The three highest coordination hydration structures formed by Al ions in water environment are Al（H₂O）₆³⁺、Al（OH）（H₂O）₅²⁺、Al（OH）₂（H₂O）₄⁺,and the order of binding energy is Al（H₂O）₆³⁺<Al（OH）（H₂O）₅²⁺<Al（OH）₂（H₂O）₄⁺.When hydration occurs,an ionic covalent bond is formed between Al atom and O atom,and the covalence is stronger.（2）The three hydrated Al ions are mainly adsorbed on the outer layer of kaolinite（001）and（00-1）in the form of hydrogen bond.The adsorption energy results show that the order of adsorption stability of the outer layer of the three hydrates is:Al（H₂O）₆³⁺>Al（OH）（H₂O）₅²⁺>Al（OH）₂（H₂O）₄⁺.Due to the three-dimensional effect of hydroxyl groups on the surface of kaolinite（001）,the adsorption of hydrated Al ions on（00-1）is more stable than that on（001）.（3）The three hydrated Al ions can be adsorbed on the inner layer of the deprotonated kaolinite（001）surface.The adsorption energy results show that the inner layer adsorption is more stable than the outer layer adsorption.The order of the inner layer adsorption stability of the three hydrates is consistent with that of the outer layer adsorption,and the bidentate adsorption is more stable than the monodentate adsorption.The adsorption energy of hydrated aluminum ions on the outer/inner layer of kaolinite surface is lower than that of hydrated rare earth ions,indicating that the adsorption of impurity ions on clay minerals is more stable in leaching.（4）The adsorption test results show that under the optimal adsorption conditions,the adsorption capacity of Al³⁺in kaolinite increases gradually with the increase of p H value.This is attributed to the fact that the degree of protonation of hydroxyl groups on the surface of kaolinite（001）increases with the increase of environmental p H value,which makes the adsorption form of hydrated aluminum ions gradually change from outer layer adsorption to more stable single tooth adsorption and double tooth adsorption,which is consistent with the law of adsorption energy of outer layer adsorption,single tooth adsorption and double tooth adsorption of hydrated aluminum ions.