Nonetheless, there are some recent studies involving novel inorganic adsorbents that show promising results. Multiple Zirconium organophosphonate (ZrP) with different Zr:P ratios were prepared using low-temperature hydrothermal reactions of zirconium propoxide with amino tris(methylene phosphonic acid) (ATMP). The adsorbents showed selectivity for lanthanide elements over mono- and divalent metal ions. The sorption capacities of 37, 56, and 60 mg Eu/g were reported for ZrP-0.65, ZrP-0.71, and ZrP-0.76 (legend 32, Figure 2), respectively (Veliscek-Carolan et al., 2014). In another study, Attallah et al. (2016) synthesized a number of aluminum silicotitanates (ATS 1-31) and obtained 99% La (III) and Eu (III) adsorption. Inorganic titanium phosphate showed Sc(III) uptake capacities of 26.1, 8.25, and 3.3 mg/g for amorphous, α-, and γ-TiP, respectively. In chromatographic separation using amorphous TiP, almost all Sc(III) were eluted using a mixed acid comprising of nitric and phosphoric acid, with Sc(III)/Fe(II) and Sc(III)/Al(III) enrichment factor of 9 and 265, respectively (Zhang, Koivula, et al., 2017). Titanium phosphate prepared from acidic wastewater of titanium dioxide achieved maximum Ce(III) adsorption capacity (qm) of 158.0 mg/g and was selective for Ce in presence of Na, K, Ca, and Mg ions (Zhaowang Liu et al., 2021). Another inorganic adsorbent, layered titanium (IV) butyl phosphate (TiP), showed Dy(III) uptake of more than 25 mg/g and a maximum separation factor of 2065 for Dy(III)-Nd(III), 11.5 for Dy(III)-Tb(III), and around 25 for La(III)-Ce(III) in respective binary element solutions (Zhang et al. 2018). Adsorptions conducted at different pH resulted in selectivity for different Ln(III) with the highest distribution coefficient (Kd) of around 105 ml/g (legend 17-20, Figure 2). The adsorbent TiP showed selectivity for Yb(III) and Lu(III) at pH 2.6. The distribution coefficient (Kd) of different REEs measures the selectivity of the adsorbent for a particular REE in competitive adsorption (Figure 2).
With another inorganic adsorbent amorphous zirconium phosphate (am-ZrP), maximum adsorption of 27.5 mg/g and 38.3 mg/g for Nd and Dy were obtained (Xu et al., 2018). The SF for Dy(III)/Co(II), Nd(III)/Co(II), Dy(III)/Nd(III) were 1811, 958, and 1.9, respectively (Xu et al., 2018). A microporous titanosilicate, ETS 10, had adsorption capacities (qm) of 48.8, 68.1, 99.0, 124.7, and 122.7 mg/g, respectively for Y(III), Nd(III), Eu(III), Tb(III), and Dy(III) (Thakkar et al., 2019). Magnetic hydroxyapatite composite (CaHAP/NF) derived from calcium hydroxyapatite [Ca10(PO4)6(OH)2], and nickel ferrite [NiFe2O4] was synthesized by a coprecipitation method had capacities of 137.35 and 130.43 mg/g for Eu(III) and Tb(III) (Attia et al., 2019). Adsorbent synthesized through modification of titanium oxide modified with arsenate (4As-TiO2) and further doping with Nd (Nd/4As–TiO2) reached maximum capacities of 65 and 37 mg/g mg/g for Y(III) at pH 9, respectively with the possible mechanism of Y(III) surface precipitation in alkaline medium and Y(III) surface complexation in neutral medium (Vasylyeva et al., 2021).
Many metal-oxides based nanoparticles have been tested for REE adsorption. Fe(III)–Ti(IV) mixed oxide nanoparticles showed capacities of 229, 181, and 126 mg/g for Ce(III), Nd(III), and Gd(III), respectively (Metwally & Rizk, 2014). Nano maghemite showed Y(III) uptake capacity of 13.5 mg/g (Dubey & Grandhi, 2016). A magnetic nanoparticle (MNP) CoxMn1-xFe2O4(x = 0.2) had adsorption capacity of 125 mg/g and 25 mg/g for La(III) and Ce(III) while CoxMn1-xFe2O4(x = 0.8) had adsorption capacity of 189 mg/g and 294 mg/g for La(III) and Ce(III) (Ghobadi et al., 2017). In another study, MNP MnFe2O4 prepared via co-precipitation and further modified with graphene oxide (MnFe2O4-GO) showed high capacities for La(III) and Ce(III) adsorption (Ghobadi et al., 2018). The adsorption capacity of MnFe2O4-GO for La(III) was 1001 mg/g and Ce(III) was 982 mg/g at pH 7, both higher than MnFe2O4 (La(III) = 785 and Ce(III) = 770 mg/g) (Table 1). Other Magnetic nano-MnFe2O4 composites with Al2O3 and SiO2-chitosan showed similar high adsorption capacities (Liu et al., 2021) (Table 1). Other high-capacity REEs adsorbents are listed in Table 1 (adsorbents are discussed in their respective sections).
Table 1. Some adsorbents with high adsorption capacity (qm >350 mg/g) for various REEs (III).