# Rare Earth Elements Uptake by Synthetic Polymeric and Cellulose-Based Materials: A Review
## 9 Conclusions and Perspective  
REEs are present in many applications (semiconductors, luminescent molecules, catalysts, batteries, and many others), and mining of REEs has increased considerably in the last decade, which is producing many environmental and human health concerns due to REE mobility in the environment from mining wastewaters and AMD.  
In this review, we discussed the recovery of REEs from mining wastewaters and AMD as secondary sources using adsorbents and ion exchange materials from synthetic and natural source materials. Considerations should be taken when evaluating and proposing new materials for REE recovery from secondary sources, mainly referred to acidic pH conditions found near mining facilities and AMD, and the stability of materials under those conditions. Evaluating the selectivity of novel materials for REE capture and other possible elements that could compete with REE uptake should be taken in consideration. Furthermore, based on the evidence presented in this review, combining different functional groups in the same polymeric material or combining different polymers should be considered due to changes in selectivity toward REEs and/or improving performance of existing polymers and materials in terms of REE uptake. Carbonyl and carboxylic moieties are useful for REE capture, and their incorporation should be also considered in novel materials for REE recovery. Although the presence of amino-containing moieties could influence REE capture from aqueous media, AMD and mining wastewaters are acidic in nature. Under these conditions, nitrogen-containing groups are found protonated, which decreases the amino-containing groups' ability to coordinate REEs, making them less suitable for REE recovery from those sources.  
According to the reported literature, cellulosic materials could have better performance capturing REEs than commercial ion exchange resins. Cellulose can be chemically modified and increase its removal capacity of REEs. In addition, the composite materials of biopolymers, such as cellulose, and synthetic polymers increase the capture capacity of REEs. Considering the evidence presented in this review, the development of this novel class of materials in recovery systems for REEs is important and could be considered, for example, in preconcentration steps.  
In conclusion, recovering REEs from secondary sources using cellulose-based materials is a useful approach for the application of a circular economy in REE mining, and it could reduce the potential environmental impact and human health effects caused by REE exposure due to the liberation of these elements in localities near REE sources and mining facilities.  
Conceptualization, G.S.; investigation, G.S.; resources, J.S.; writing--original draft preparation, G.S.; writing--review and editing, G.S. and J.S.; supervision, J.S.; project administration, J.S.; funding acquisition, J.S. All authors have read and agreed to the published version of the manuscript.  
This research was funded by FONDECYT (grant number 1191336) and ANID, PCI (grant number NSFC190021).  
Not applicable.  
The authors declare no conflict of interest.