# Thorium Recovery with Crown Ether-Polymer Composite Membranes
## 4 Conclusions  
The toxicity of thorium is only partly related to the fact that it is a radioactive element, because its half-life is extremely long. However, being a heavy element, its removal from aqueous systems is imperative.  
If we consider that it is desired to remove thorium from waters in isolated areas (for example, mining operations, mountain areas, etc.), one of the more accessible and efficient processes of doing so would be the membrane process. In this paper, nanofiltration is approached as a procedure for removing thorium from diluted synthetic aqueous solutions.  
The composite membrane that is proposed in this study is made of sulfonated polyether ether ketone (sPEEK) in which Amino-Benzo-15C5-Ether (ABCE) is incorporated.  
The performance of the nanofiltration process is compared between a sPEEK membrane and the composite membrane (ABCE-sPEEK), prepared by the same procedure.  
The membranes were characterized by scanning electron microscopy (SEM), energy-dispersive X-Ray spectroscopy (EDAX), thermal analysis (TG and DSC), and from the perspective of thorium removal performance.  
For all test parameters: pH and thorium ion concentration; working pressure; and stirring regime, the composite membrane (ABCE-sPEEK) has superior performance compared to the sPEEK membrane.  
Thorium retention for the composite membrane (ABCE-sPEEK) reaches a maximum of 98% compared to the sPEEK membrane which does not exceed 90%.  
The fluxes determined under the optimal conditions resulting from the study (pressure, stirring regime, concentration, and pH of the thorium solution) are a maximum of 16 L\(\cdot\)m\({}^{-2}\cdot\)h\({}^{-1}\) for the ABCE-sPEEK composite membrane, and 15 L\(\cdot\)m\({}^{-2}\cdot\)h\({}^{-1}\) for the sPEEK membrane.  
The results obtained during the nanofiltration of the solution containing thorium are in agreement with the initially expected requirements when working with the ABCE-sPEEK membrane: selectivity/retention over 90%, operation over the entire pH range (also covering the acidic environment), maximum working pressure of 9 atmospheres, the possibility of application in isolated places, no need to use reagents or auxiliary materials, the possibility of being scaled up by simply increasing the membrane surface, and a reduction in investments.  
The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app14219937/s1](https://www.mdpi.com/article/10.3390/app14219937/s1)]([https://www.mdpi.com/article/10.3390/app14219937/s1](https://www.mdpi.com/article/10.3390/app14219937/s1)), Figure S1. Fourier Transform Infra-Red (FTIR) spectrum at 500 \({}^{\circ}\)C.  
Conceptualization, G.N., A.C.N. and V.-A.G.; methodology, A.C.N., G.N., P.C.A. and L.M.; validation, L.M., S.-K.T. and A.R.G.; formal analysis, G.N., A.C.N., A.R.G. and L.M.; investigation, G.N., L.M., P.C.A., G.T.M., V.-A.G., A.C.N., A.R.G. and V.E.M.; resources, A.C.N., S.-K.T. and G.N.; data curation, L.M., A.C.N., A.R.G. and V.-A.G.; writing--original draft preparation, A.C.N., G.N. and V.-A.G.; writing--review and editing, G.N., A.C.N., A.R.G. and V.-A.G. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
The original contributions presented in the study are included in the article/Supplementary Material. Further inquiries can be directed to the corresponding authors.  
The authors are grateful to the Romanian Government for providing access to the research infrastructure of the National Center for Micro and Nanomaterials through the National Program titled \"Installations and Strategic Objectives of National Interest\". The authors gratefully acknowledge the valuable help and friendly assistance of Eng. [PERSON] for performing the microscopy analysis.