# Rare Earth Elements Uptake by Synthetic Polymeric and Cellulose-Based Materials: A Review
## 7 Synthetic Polymeric Materials for REE Uptake
### Loaded Synthetic Polymeric Materials  
Synthetic organic polymers supported on silica have been studied for RE uptake. Poly-diglycolamide supported on silica particles shows similar capture capacities for REEs to commercial ion exchange resins, although it seems more selective toward REEs at low pH values [35]. Another silica-based material with polyacrylamide also shows similar capture capacities for Tb (III) to commercial resins at pH 6.0 [36]. Poly (acrylamide-sodium 4-styrenesulfonate) supported in silica composites shows up to 136 mg/g capture capacity for REEs at neutral pH, and it seems selective toward light REEs [37]. Poly (acrylamide-acrylic acid-sodium 4-styrenesulfonate) supported in silica composites shows 206 mg/g capture capacity for Gd (III) at neutral pH due to mixed functionalities sulfonate and carboxylate groups present in the composite [38]. Interestingly, most silica-supported materials evaluated in this work show high capture capacities for REEs at pH values near neutral, which allows both chelating and ion exchange effects from carboxylic acid and nitrogenated groups present in those materials. These functional groups are also found in polymeric materials already mentioned. It seems that silica does not influence the REE capture capacity, and it only works as support media. Nevertheless, it should be pointed out that silica is an inexpensive, versatile material as a support due to its high porosity, high surface area, chemical stability, mechanical properties, and the highly advanced development of techniques associated with producing silica particles with different morphologies, sizes, and porosities, which makes silica highly attractive as support material [46; 47; 48; 49]. Fe\({}_{\text{S}}\)O\({}_{4}\) composites have been reported for REE capture as a convenient alternative to silica particles due to their magnetic properties. Poly(itaconic acid) supported in sepiolite (Fe\({}_{3}\)O\({}_{4}\)) showed up to 178 mg/g capture capacity for REEs, and they seemed selective toward heavy REEs at low acidic to neutral pH values [39]. 2-Ethlyhexyl phosphonic acid mono-2-ethylhexyl (PC88A)-doped silica functionalized \(\beta\)-cyclodextrintrin supported in Fe\({}_{3}\)O\({}_{4}\) performs better than commercial resins capturing REEs, having up to 64 mg/g capture capacity for REEs. It showed selectivity toward REEs in the presence of other metal cations, such as Na (I), Mg (II), Ca (II), Ni (II), and Zn (II), at low acidic pH values [40]. Poly (methyl methacrylate-glycidyl methacrylate-ethylenimine)-grafted-Fe\({}_{3}\)O\({}_{4}\) microspheres produced by means of a novel irradiation-induced grafting method showed capture capacity up to 189.8 mg/g for Ce (III) at pH 6.0 [41]. Although the performance of these particles is promising, the acidic conditions found in AMD or mining wastewaters decrease the amount of Ce (III) captured to approximately 40 mg/g [41] due to the protonation of amino coordinating groups present in the material. In general terms, the usage of Fe\({}_{3}\)O\({}_{4}\) particles in composites has some advantages: they share some properties with silica particles, such as high porosity and high surface area, and their magnetic properties allow easy recovery of the composites from aqueous media, which are especially useful features in batch applications [50]. Additionally, effects in surface morphology have been observed, which favors interaction with functional groups associated with Fe\({}_{3}\)O\({}_{4}\)-based composites. It has been reported that Fe\({}_{3}\)O\({}_{4}\)-based composites with organic functional moieties or inorganic modifiers usually have high heavy metal uptake [51; 52; 53; 54], which is also observed with REE capture, as shown in selected works. Furthermore, the use of polysaccharide derivates and composites has also been reported for REE capture. Chitosan-grafted poly(acrylic acid) supported in attapulgite composites shows a high capture capacity for REEs, up to 333 mg/g for La (III) at low-acidic to neutral pH values [42].