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.