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224 Handbook on the Physics and Chemistry of Rare Earths enriched in LREEs relative to the parent granite when the dominant REE minerals being leached are LREE-rich fluorocarbonates, as shown by exam- ples of Heling (Bao and Zhao, 2008). 4.2.6 Adsorption of REE on Clays Adsorption of REE in soil and weathered rocks is dependent on nature of adsorption materials, and the pH and ionic strength of the solution as reviewed by Laveuf and Cornu (2009). Weathered granites contain various fine-grained weathering products (minerals, amorphous materials, and organic matter) that can adsorb REEs. Previous studies on ion-adsorption ores have shown that kaolinite and halloysite (kaolin group) are most likely to adsorb REE3+ ions because they are predominant clays in the ores from southern China (Bao and Zhao, 2008; Chi and Tian, 2009; Chi et al., 2012; Murakami and Ishihara, 2008; Wu et al., 1990, 1993a; Yang, 1987) and from Thailand (Imai et al., 2013; Sanematsu et al., 2013, 2015). Kaolinite (or hal- loysite) has two types of cation exchange resulting from isomorphous substi- tution of Al3+ for Si4+ (permanent charge) and basal surface (variable charge). Ferris and Jepson (1975) and Ma and Eggleton (1999) documented that exchangeable cations of kaolinite occur mostly on the edges and on the basal surface of the mineral. In addition, permanent negative charge from the iso- morphous substitution is not very significant in exchangeable cations. Cation exchange capacity is similar for kaolinite and halloysite, and tube hal- loysite may have a higher capacity (Joussein et al., 2005; Ma and Eggleton, 1999). The cation exchange capacities of kaolinite and halloysite increase with decreasing particle size because of the edge and surface effects. This is strongly dependent on the pH of the solution (Joussein et al., 2005; Ma and Eggleton, 1999). Other clay minerals such as illite, vermiculite, montmollironite, and amor- phous materials are also capable of exchanging REEs because they have per- manent charge and/or pH-dependent surface charge (variable charge), which can be expressed as the pH of the point of zero charge (PZC). The surface of minerals and amorphous materials are more positively charged below their own PZCs and they are more negatively charged above the PZCs. Generally silicates and silica have higher PZCs and metal oxides have lower PZCs (Kosmulski, 2009, 2011). PZC of kaolinite is less than 3.6 (Kosmulski, 2009, 2011; Schroth and Sposito, 1997), suggesting that kaolinite is negatively charged in ion-adsorption ores with low pH of 4.1–6.0 (Wei et al., 2001; Wu et al., 1990; Yang and Xiao, 2011) and is capable of adsorbing REEs. Although halloysite lacks data for PZC, it is likely similar to the PZC of kao- linite because of the almost same composition and crystal structure. Cation exchange capacities of 2:1 layer clays such as illite and montmorillonite are generally higher than those of kaolinite and halloysite (eg, Carroll, 1959; Laveuf and Cornu, 2009) because they have both permanent and variable charges. Illite, which is also found in ion-adsorption ores, may adsorb REEsPDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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