# Thorium Recovery with Crown Ether-Polymer Composite Membranes
###### Abstract  
Thorium is a weak radioactive element, but the control of its concentration in natural aqueous systems is of great interest for health, because it is a toxic heavy metal. The present paper presents the recovery of thorium from diluted synthetic aqueous systems by nanofiltration. The membranes used for the nanofiltration of systems containing thorium species are composites containing 4\({}^{\prime}\)-Aminobenzo-15-crown-5 ether (ABCE) and sulfonated poly-etherether-ketone (sPEEK). The composite membranes (ABCE-sPEEK) 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. To determine the process performance, the variables were the following: the nature of the composite membrane, the concentration of thorium in the aqueous systems, the rotation speed of the stirrer, and the pressure and the pH of the thorium aqueous system. When using pure water, a permeate flux value of 12 L-m-2 h-1 was obtained for the sPEEK membrane, and a permeate flux value of up to 15 L-m-2 h-1 was obtained for the ABC-sPEEK composite membrane. The use of mechanical stirring, with a propeller stirrer, lead to an increase in the permeate flux value of pure water by about 20% for each of the studied membranes. Depending on the concentration of thorium and the pH of the feed solution, retentions between 84.9% and 98.4% were obtained. An important observation was the retention jump at pH 2 for the ABC-sPEEK composite membrane. In the paper, a thorium ion retention mechanism is proposed for the sPEEK membrane and the ABCE-sPEEK composite membrane. thorium; thorium removing; thorium recovery; composite membranes; crown ether; sulfonated poly-ether-ether-ketone (sPEEK) 2024 20  
However, what must be considered is that thorium is a heavy metal, with a potential toxicity similar to other heavy metals [5; 6]. A special feature of thorium is its presence in various surface waters [6; 7], soil [8], and it can even accumulate in various plants [8]. It is expected that thorium appears in mining areas in aqueous effluents [9], but also naturally in streams or rivers that pass-through areas where thorium is accumulated [9; 10]. At a common pH of surface waters [11], thorium precipitates and is found in sediments, but under acidic water conditions thorium is found in the ionic form Th\({}^{4+}\), or the hydroxyl forms Th(OH)\({}^{3+}\), Th(OH)\({}_{2}\)\({}^{+}\), Th(OH)\({}_{3}\)\({}^{+}\)[12; 13]. Thus, the recovery of thorium from aqueous systems containing thorium or other heavy metals becomes extremely important [14]. Classical separation processes (precipitation, sedimentation, flocculation, ion exchange) can be taken into account [15; 16], but modern extractive, adsorptive, and membrane processes can also be approached (Table 1) [17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33]. From a practical point of view, to obtain thorium-free drinking water, the use of emulsion membranes is excluded due to the need for reagents and solvents which would impurity the water and, at the same time, attempting recovery by breaking the emulsions raises problems difficult to solve in the isolated areas that we have in view [34; 35]. If an approach based on a membrane technique is desired, then one that uses hollow fiber membranes is viable [36; 37]. The beginning of an application with hollow fiber membranes occurs, most of the time and in the present study, with the study of separation with bulk or flat membranes [38; 39; 40]. An accessible and efficient procedure is nanofiltration, which uses nanoporous composite membranes and achievable pressures (4-8 atm) with usual means [41; 42; 43; 44; 45]. The proposed thorium separation process is expected to perform with a similar, complementary, or superior performance to those previously presented [17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33], according to Table 2.