稀土La^(3+)离子在高岭石表面吸附的原位AFM研究

In-situ AFM Study of the Adsorption Mechanism of La^(3+)on Distinct Surfaces of Kaolinite

离子吸附型稀土矿是我国优势矿产资源,该类型矿床中,稀土元素主要是以离子的形式吸附在高岭石、埃洛石和铁锰氧化物等次生矿物上。本文主要通过原子力显微镜(AFM)的微观区域的力学测量功能,结合DLVO理论,研究原位水环境下稀土La^(3+)对高岭石不同表面(Si面和Al面)Stern电位的影响,揭示高岭石对La^(3+)的吸附能力随Si面和Al面表面性质不同而变化。实验结果表明:当La^(3+)浓度从0增加至1.0 mmol/L时,高岭石Si面的Stern电位由−46 mV转变为4 mV,表明高岭石Si面对La^(3+)有吸附作用;而Al面的Stern电位(在28~31 mV之间波动)几乎没有变化,表明高岭石Al面对La^(3+)的吸附作用不强。该研究表明在pH=4~6的风化壳条件下,高岭石Si面对La^(3+)的吸附可能占主导作用,Al面的贡献相对较小。本研究从微观角度揭示了稀土元素在高岭石不同表面的吸附特征,明确了高岭石不同表面对稀土元素吸附的贡献,为阐明风化壳中稀土的富集机制提供信息。

The regolith-hosted ion-adsorption deposits of rare earth elements(REEs)are the dominant repositories discovered in China,in which hydrated or hydroxyl-hydrated REE ions were assumed to be adsorbed on secondary minerals such as kaolinite,halloysite,and Fe-Mn oxides.Due to the limitations of current characterization techniques,deep insights into the adsorption mechanism microscopically,especially the surface/interface interaction at the molecular scale,are still lacking.The results based on the conventional adsorption experiment reveal the mean value of the adsorption capacity macroscopically,which shows good statistical significance,but the results provide neither the adsorption information on specific surface or sites of clay minerals nor the microscopic adsorption mechanism of REEs on clay minerals on atomic-and molecular-scales.In addition,the electron microscopy analysis at nano-and/or micron-scales is mostly operated under high-vacuum conditions,which cannot characterize the occurrence information and interactions between REEs and the surfaces of clay minerals in natural aqueous environments.In this study,we report the application of force curve measurement by atomic force microscopy(AFM)and a combination of the classic DLVO theory to investigate the effects of La^(3+)on Stern potentials of distinct surfaces(Si-basal and Al-basal planes)of kaolinite under an in-situ aqueous environment.We demonstrate the variable adsorption capacity of kaolinite for La^(3+)with surfaces,which advances our understanding of the adsorption mechanism of REEs on distinct surfaces of kaolinite and provides information for elucidating the REE enrichment mechanism in weathering crusts.The results show that the Stern potential of the kaolinite Si-basal plane changes from−46 mV to 4 mV with the increment of La^(3+)concentration from 0 to 1.0 mmol/L,indicating that La^(3+)is adsorbed on the Si-basal plane of kaolinite.The Stern potential of the Al-basal plane decreases(from 28 mV to 31 mV)in a narrow range with increasing La^(3+)concentration,suggesting weak adsorption of La^(3+)on the Al-basal plane of kaolinite.In a weakly acidic(pH=4–6)environment of weathering crusts,we suggest that the Si-basal plane of kaolinite dominates the adsorption of REEs,rather than the commonly recognized Al-basal plane.This study reveals the microscopic adsorption characteristics of REEs on distinct surfaces of kaolinite and clarifies the contribution of each surface of kaolinite to the adsorption of REEs.