Citation: Castillo-Aguirre, A.; Maldonado, M.; Esteso, M.A. Removal of Toxic Metal Ions Using Poly(BuMA–co–EDMA) Modified with C-Tetra(nonyl)calix[4]resorcinarene. Toxics 2022, 10, 204. https://doi.org/ 10.3390/toxics10050204 Academic Editor: David R. Wallace Received: 12 March 2022 Accepted: 17 April 2022 Published: 20 April 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). toxics Article Removal of Toxic Metal Ions Using Poly(BuMA–co–EDMA) Modified with C-Tetra(nonyl)calix[4]resorcinarene Alver Castillo-Aguirre 1 , Mauricio Maldonado 1, * and Miguel A. Esteso 2,3, * 1 Departamento de Química, Facultad de Ciencias, Universidad Nacional de Colombia-Sede Bogotá, 30 No. 45-03 Carrera, Bogotá 111321, Colombia; aacastilloa@unal.edu.co 2 Universidad Católica de Ávila, 05005 Ávila, Calle los Canteros s/n, Spain 3 U.D. Química Física, Universidad de Alcalá, 28805 Alcalá de Henares, Madrid, Spain * Correspondence: mmaldonadov@unal.edu.co (M.M.); mangel.esteso@ucavila.es (M.A.E.) Abstract: A copolymer of poly(BuMA–co–EDMA) modified with C-tetra(nonyl)calix[4]resorcinarene was obtained via the impregnation method. The formation of the modified copolymer was confirmed and investigated using various techniques; in this way, the presence of calix[4]resorcinarene was confirmed by FT-IR spectroscopy and by high resolution transmission electron microscopy. The modified copolymer was used for the removal of highly toxic cations (Pb 2+ , Hg 2+ , and Cd 2+ ) from aqueous solutions. To perform the removal, we used the batch sorption technique and the effects of time of contact, pH, and volume of sample on the effective sorption were determined. The best results were observed for Pb 2+ extraction, which was comparatively more efficient. Adsorption–desorption experiments revealed that the modified copolymer could be used for several cycles without significant loss of adsorption capacity. Finally, the results showed that the modified copolymer application is highly efficient for the removal of lead ions from aqueous solutions. Keywords: toxic metal ions; poly(BuMA–co–EDMA); C-tetra(nonyl)calix[4]resorcinarene; impregna- tion method; water contamination 1. Introduction In recent decades, the environment has been a constant concern of all countries. In this context, the contamination of water due to human activities has been the focus of great attention, especially contamination by metal cations generated by various industrial processes. Therefore, it is necessary to have effective treatment methods to remove toxic heavy metals. In practice, physical treatment methods such as membrane filtration, reverse osmosis, coagulation, and adsorption have been conventionally used by different types of industries [1–3]. The removal of toxic metal ions such as lead(II), cadmium(II), and mercury(II), using polymeric materials modified with ligands is a promising approach to improve environmental quality, because of its high stability, high selectivity, and removal efficiency [4–6]. Taking the above into account, the modification of polymeric materials by means of organic ligands is efficient if there is a large surface area for the chemical or physical modification process. Polymeric materials with these characteristics are known as monoliths [7–9], which are polymers or copolymers obtained through a polymerization processes in the presence of a porogenic agent [10,11]; in this way, a highly porous material can be obtained that confers a large surface area to the polymer, which is convenient for the modification processes and also for the adsorption processes of heavy metal cations or other types of substances. The modification of polymeric materials with organic ligands can be performed by physical adsorption or by chemical reaction with a functional group of the polymer. There are several types of molecules that can be used to modify the surface of polymers, for example, the use of macrocyclic ligands such as crown ethers, cyclodextrins, calix[4]arenes, and calix[4]resorcinarenes. It is interesting to note that the use of calix[4]resorcinarenes as Toxics 2022, 10, 204. https://doi.org/10.3390/toxics10050204 https://www.mdpi.com/journal/toxics