Understanding the role of natural clay minerals as effective adsorbents and alternative source of rare earth elements: Adsorption operative parameters

To better understand the influence of structure and surface properties of NCMs towards REE, requires individual research for each NCM. The adsorption/extraction for light (La3+) and heavy (Yb3+) REE of kaolinite (Kao), montmorillonite (Mt), muscovite (Ms), illite (Ilt), were systematically investigated and compared. Additionally, all the NCMs were fully characterized by XRD, XRF, XPS, Zeta potential and nitrogen adsorption-desorption isotherms to build the relationship between adsorption/extraction mechanism and minerals' property. Our experiments show that the Mt. exhibits highest adsorption and regeneration efficiencies for both La3+ and Yb3+ and decrease in the order of Mt > Ms > Ilt > Kao, while Kao has highest extractions efficiencies for both REE in the order of Kao > Ilt > Mt > Ms 89% for La3+ and 85% for Yb3+ were achieved from Kao and the lowest extractions were obtained from Ms. (63% for La3+ and 57% for Yb3+). The lack of Ms. on both reuse and extraction characteristics is believed to be related to presence of iron oxide associated with Ms. In addition, the important role of the pH in extraction of REE from NCMs was evidenced, when REE-NCMs come into contact with the NH4+ solution, the pH is rapidly increased over initial pH solution for both Mt. and Ms., thus leading to the decrease of the availability of ion-exchangeable REE with NH4+ions. The results illustrated that the structure and surface properties of NCMs are also the key factors that affect the rare earth leaching, thus identifying the types of NCMs and associated impurities in clay materials are important, either for getting the best leaching system or in developing a new one.