# The Role of Clean Coal Technologies in the Development of Renewable Energy Sources
## 1 Introduction
### Membrane Techniques
As part of the work carried out on the recovery of REEs from synthetic coal fly ash extracts, selective adsorption membranes were created based on modified chitosan (Schiff's base) with the addition of a polymer imprinted with an appropriate REE ion, in this case Pr. The obtained experimental data were analyzed using two computer applications, namely REE 2.0 and REE_isotherm 1.0. Adsorption membranes based on a complex polymer matrix imprinted with praseodymium ions were obtained by casting from a polymer solution. In the first synthesis, a Schiff base based on chitosan was synthesized, from which a solution was prepared in acetic acid, an appropriate amount of praseodymium nitrate (III) was added to it, and then stirred. In the second synthesis, an ion-imprinted copolymer was obtained in the process of copolymerization of the complex of Pr(III), 5,7-dichloroquinoline-8-ol and 4-VP, and styrene-DVB as a monomer and cross-linking agent. The obtained polymer was crushed and dried in an oven at 60 \({}^{\circ}\)C for 5 h. In the last stage, 30 wt% praseodymium-imprinted polymer was added to the obtained solution of modified chitosan and dispersed using a mechanical homogenizer, then mixed using a magnetic mixer and ultrasound. After adding the glutaraldehyde solution, it was mixed, poured onto a leveled PTFE Petri dish and the solvent was evaporated at 27 \({}^{\circ}\)C in a vacuum dryer. Then, the membrane was washed with water and leached with HCl solution to remove REE ions and obtain specific cavities. The last step was to immerse the membrane in a 1 M sodium hydroxide solution. The obtained membranes were stored in deionized water before use. The membranes thus obtained were subjected to Pr ion adsorption tests, taking into account the influence of pH on the adsorption process, kinetic tests, adsorption isotherms, selectivity tests and reuse of the tested adsorption materials. In this way, the basic parameters were determined, such as q\({}_{t}\), i.e., the amount of Pr ions adsorbed at time t [mg/g], adsorption capacity at equilibrium q\({}_{e}\) [mg/g], K\({}_{d}\), i.e., the partition coefficient and selectivity coefficients k. In turn, experiments related to the reuse of adsorption materials consisted in repeating five subsequent sorption-desorption cycles after regenerating the adsorption membrane using HCl. The influence of pH on the adsorption process (Figure 4) was tested for the range between 2.0 and 9.0 and pH = 7.5 was selected as optimal for further research, taking into account the possibility of the precipitation of Pr hydroxides in an alkaline environment. The study of the adsorption kinetics of Pr ions on the synthesized material (Figure 5) showed that changes in adsorption capacity over time were characterized by two main stages, namely a fast initial stage in which over 80% of praseodymium ions were adsorbed within the first 90 minutes and the slow second stage of reaching the equilibrium state. The adsorption process can be controlled by several mechanisms, which were analyzed using the REE 2.0 application. The obtained results are presented in Table 1.  
It was found that the experimental data showed the best fit to the pseudo-second-order Lagergren model, which indicates the presence of chemical coordination processes of praseodymium ions in the structure of the tested membrane. Moreover, the good fit to the Elovich model (R\({}^{2}\) > 0.85) also indicates the existence of chemisorption, and the differences between the experimental q\({}_{e}\) values and those obtained from simulations were much smaller for the pseudo-second-order Lagergren model and the chemisorption-diffusion model (R\({}^{2}\) > 0, 95). In the case of the intraparticle model, multi  
Figure 4: The effect of pH on the adsorption capacity of Pr ions. Source: own. Figure 5: Changes of adsorption capacity over time for synthesized membrane. Source: own. (R\({}^{2}\) < 0.61), which may indicate the influence of two or more mechanisms on the adsorption process.