The Cr(III) Exchange Mechanism of Macroporous Resins: The Effect of Functionality and Chemical Matrix, and the Statistical Verification of Ion Exchange Data

Abstract

This study focuses on investigating and comparing the influence of the surface functional groups and chemical matrices of macroporous resin on the Cr(III) exchange mechanism. The results discussed herein indicate that sulfonic resin removed Cr(III) ions with faster kinetics than carboxylic resin. Equilibrium was established within 15 and 7 min for the carboxylic and sulfonic resins, respectively, with a 99.5% removal efficiency at 333 K. The Langmuir exchange capacity was observed to be higher for the sulfonic resin (1.5 mmol∙g−1) than the carboxylic resin (0.80 mmol∙g−1) at 333 K. The adsorption isotherms obtained for the carboxylic and sulfonic resins were H and S types, respectively, representing a higher affinity of the carboxylic resin for Cr(III) removal at a low metal ion concentration. Additionally, it was noted that the carboxylic resin preferentially co-sorbed H+ and Cr(OH)2+ ions below Cr(III) concentrations of 6–8 mmol.L−1. The H+ ions co-sorption was almost negligible, whereas the Cr(III) exchange was 87 and 34.5% for the carboxylic acid resin and sulfonic acid resins, respectively. The data of the concentration studies were evaluated using non-linear forms of Freundlich, Langmuir, and Dubinin–Radushkevich adsorption isotherm models, and the kinetic data were analyzed using pseudo-Ist- and pseudo-IIst-order kinetic models. The activation energy Ea for Amberlite IRC-50 (Na+) was greater (22.4 kJ∙mol−1) than that of Amberlyst-15 (Na+) 17 kJ∙mol−1, indicating a higher energy barrier for the ion exchange reaction on carboxylic resin. As per the findings of a statistical error analysis (RMSE and SSE) and absolute average relative distribution (AARD) statistical model, a close agreement between the experimental and theoretical values suggested that the Langmuir isotherm was well-fitted to the current adsorptive investigations. The interaction of the COO− and SO3− functional sites of the resins for the exchange of Cr(III) ions was validated through an FT-IR analysis. The macroporous resins used in the current study for Cr(III) exchange showed promising performances compared to other resins. The current investigations revealed valuable insights for choosing macroporous resins as adsorbents in water filtration systems.