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).