Inorganic adsorbents

Inorganic adsorbents used for REEs adsorption include mineral oxides and hydroxides, clay minerals, and nanomaterials. Mineral oxides and hydroxides are adsorbents with reactive sites containing oxygens (O2-) or hydroxides (-OH) that interact with cations and anions. The mineral adsorbents used for REEs adsorption are silica, alumina, iron (hydro)oxides, Mn oxides, and TiO2.
Silica has been studied as an adsorbent for Gd(III), Y(III) (Kosmulski, 1997b), and Yb(III) (Marmier et al., 1999). These investigations focused on surface complexation modeling and % uptake and did not report maximum uptake in mg/g, distribution (partitioning) coefficient, or separation factor. Alumina was studied for adsorption of Gd(III), Y(III) (Kosmulski, 1997a), Yb(III) (Marmier et al., 1997), and Eu(III) (Wang et al., 2006). TiO2 was studied for adsorption of Y(III) (Zhang et al., 2004), Nd(III) (Ridley et al., 2005; Zhang et al., 2004), and Eu(III) (Tan et al., 2009). The qm for Eu(III) was 1.5 mg/g with TiO2 and 2.7 mg/g in the presence of fulvic acid (Tan et al., 2009).
Other researches involving inorganic adsorbents are Yb(III) adsorption on hematite (Marmier et al., 1997), La(III) onto hematite (Nicolas Marmier & Fromage, 1999), REEs onto Mn oxide, Fe oxy-hydroxide (Davranche et al., 2004, 2005; Ohta & Kawabe, 2001), and REEs on amorphous ferric hydroxide (Quinn et al., 2006, 2007). In addition to oxides and hydroxides, other minerals have been used as adsorbents as well. Adsorption of Nd(III) onto kaolinite (Aja, 1998), Ln(III) onto kaolinite and smectite (Coppin et al., 2002), Eu(III) onto montmorillonite (Kowal-Fouchard et al., 2004), REEs on kaolin (XIAO et al., 2016), REEs adsorption onto Kaolinite (Feng et al., 2021), and Eu(III) on halloysite and kaolinite (Zhou et al., 2022) are such studies involving inorganic mineral adsorbents. Kaolin showed a saturated adsorption capacity of 1.731, 1.587, and 0.971 mg/g for La(III), Nd(III), and Y(III), respectively (XIAO et al., 2016). Mosai et al. (2019) used natural zeolite and bentonite to recover REEs from aqueous solution and reported qm values of 0.09-0.24 mg/g and 1.07-2.87 mg/g for adsorption of different REEs onto zeolite and bentonite, respectively.
Most of the research mentioned above focuses on understanding the mechanism of REE adsorption onto the inorganic adsorbent and uses surface complexation models. The mechanism of adsorption is required for a better understanding of natural as well as engineered systems. However, since the adsorption capacity of these unmodified adsorbents is low (i.e., around 0.01-0.1 mmol/g) (Ehrlich & Lisichkin, 2017), it is impractical to employ these adsorbents for REE separation at an industrial scale.