## 2 Results and Discussion
### Characteristics of Synthesized REEIIPs and Hybrid Materials
This phenomenon was confirmed by means of SEM analysis and, in particular, by X-ray micro-tomography. The decrease in coercivity might also be due to a reduction in insulation between nanoparticles due to mutual interactions during the production of hybrid membranes [47]. At the same time, the recorded values of coercivity for any of the membranes were not smaller than 22.86 Oe, which was indicative of weakly ferromagnetic properties. The largest decrease in the value of coercivity was recorded for the Gd-selective hybrid membranes, likely due to dispersion effects reducing anisotropy and domain wall pinning. However, in the case of Nd- and Pr-sensitive membranes, these decreases were much smaller, which might indicate a better insulation of nanoparticles, resulting from the greater MWCNT functionalization by NdIIPs and PrIIPs and their greater interaction with the polymer matrix. So, it could be seen that the hybrid structures allow the tuning of magnetic softness (lower coercivity) while enhancing overall magnetic responsiveness, desirable for magnetically driven separations or targeting [48; 49]. The obtained hybrid adsorptive membranes were tested for their separation properties using a Sterlitech HP4750 (Sterlitech Corporation, Auburn, WA, USA) high-pressure stirred cell kit (pressure of nitrogen: 20-60 bar, temperature 20 \({}^{\circ}\)C, pH 7.5 of feed, which is synthetic coal fly ash extract, time 4-6 h) [50]. The recovery of adsorbed REE ions was performed in the same setup by the application of 50 mL of 1 M HCl after the separation process. It was found that the retention ratio increased with MWCNT-REEIIP addition, ultimately reaching 94.35%, 92.12%, and 90.11% for Nd, Pr, and Gd, respectively. While the recovery was around 89% and after 5 adsorption-desorption cycles was still 87%. The obtained membranes show a high rejection of Na, Mg, Ca, Al, Fe, and Si ions, which are components of the matrix in the extracts of coal fly ashes. The results obtained are comparable with usually used membrane techniques (recovery of REE ions over 89%). But it should be noted that the usually used membranes were applied for REE recovery from synthetic solutions, consisting of only a few REE ions and without matrix elements [8; 16; 51; 52; 53]. In the case of the usage of a feed consisting of REEs and matrix elements, such as Fe, Si, and Al, it was necessary to apply additional steps such as precipitation of the matrix ions, separation of monovalent ions, a microfiltration step, and finally separation of the rare earth elements by means of nanofiltration [54]. Our proposed solution enables the recovery of REEs from multicomponent, complex solutions corresponding to the composition of real coal fly ash extracts in the same reaction system. Furthermore, the use of highly selective and specific hybrid membranes enabled the separation of REEs with a high retention coefficient. Therefore, it should be stated that the tested hybrid membranes have the potential for further use in the recovery of rare earth  
Figure 8: Magnetic hysteresis loops of polymers and REE-sensitive adsorptive membranes: (**a**) modified chitosan Schiff base, (**b**) copolymers of ternary complex and styrene-DVB, (**c**) Nd-sensitive adsorption membranes with various amounts of MWCNT-NdIIP additives, (**d**) Pr-sensitive adsorption membranes with various amounts of MWCNT-PrIIP additives, (**e**) Gd-sensitive adsorption membranes with various amounts of MWCNT-GdIIP additives, (**f**) comparison of various REEIIPs with a 2 wt.% addition of the appropriate MWCNT-REEIIPs. elements from real coal fly ash extracts, and after introducing appropriate modifications, they will enable the production of single-component REE solutions in the future.