Synthesis and adsorption performance of a novel pyridinium-functionalized hypercrosslinked resin for the removal of chromium(vi) ions

Abstract

This study presents the synthesis and characterization of a novel anion exchange hypercrosslinked resin (HPR1) designed for the removal of high-concentration hexavalent chromium Cr(vi) from aqueous solutions. The resin was synthesized via a one-step Friedel-Crafts alkylation reaction using a low-crosslinking copolymer of divinylbenzene and vinylbenzyl chloride (DVB-co-VBC), prepared by suspension polymerization with toluene as a porogen. The successful incorporation of pyridinium groups into the resin network was confirmed using elemental analysis, FTIR spectroscopy, and solid-state ¹³C NMR spectroscopy. The adsorption performance of HPR1 was evaluated at various pH (2, 4, and 6.5) and initial Cr(vi) concentrations. The nonlinear Langmuir isotherm model provided the best fit for the experimental data compared with tc Freundlich and Redlich-Peterson isotherms. Notably, the adsorption equilibrium was achieved within 4 min, with a maximum capacity of 207 mg g^-1 at pH 2. Kinetic studies indicated that the adsorption process was best described by a pseudo-second-order model, with higher rates observed at pH 4 than at pH 2. Additionally, intraparticle diffusion has been identified as the mechanism that controls the adsorption process. The high adsorption capacity of HPR1 at acidic pH values suggests its potential for treating industrial wastewater containing elevated concentrations of Cr(vi).

Scheme 1. Synthesis route of the anion exchange hypercrosslinked resin (HPR1).

Fig. 1. Scanning Electron Microscopy (SEM) Images of synthetized resins. (a) Spherical particles obtained through suspension polymerization of the R1 resin. (b) The surface of a single particles of the HPR1 resin (c) morphology of cleaved particles after the post-crosslinked of the HPR1 resin.

Fig. 2. FTIR spectra of the materials synthetized. (a) R1 poly(DVB-co-VBC) copolymer synthetized by suspension polymerization. (b) HPR1 resin obtained through the Friedel–Crafts alkylation reaction and subsequent formation of the pyridinium group (R–N⁺–Cl).

Fig. 3. Solid-state (118 MHz) ¹³C CP/MAS NMR spectrum. (a) Precursor copolymer synthetized from divinylbenzene (DBV) and vinylbenzyl chloride (VBC). (b) Anion exchange hypercrosslinked resin obtained in a single step.

Fig. 4. TGA curves of the R1 resin and HRP1 resin synthetized in a single step.

Fig. 5. Effect of initial concentration hexavalent chromium Cr(vi) concentration as a function of pH of the anion exchange hypercrosslinked resin (HPR1).

Fig. 6. Kinetic curves of the nonlinear pseudo-first-order and pseudo-second-order kinetic models at pH = 2 and pH = 4 of HPR1 resin.

Fig. 7. Linear form of the pseudo-second-order model for the HPR1 resin at pH 2 and pH 4 at room temperature of HPR1.

Fig. 8. Inter-particle diffusion of the anion exchange hypercrosslinked resin at pH = 2 and pH = 4 at room temperature.

Fig. 9. Adsorption equilibrium isotherms of the HPR1 resin analyzed using nonlinear Langmuir, Freundlich and Redlich–Peterson isotherm models. (a) At pH 2, the adsorption parameters were best fit by the nonlinear Langmuir isotherm model. (b) At pH 4, the data were also well described by the nonlinear Langmuir model. (c) At pH 6.5 the experimental data were best fit by the Redlich–Peterson isotherm model.