Capture of Mn2+ and Cu2+ by ion exchange resins and activated carbon: an NMR relaxometry study

Abstract Water pollution by heavy metal ions is a critical environmental concern. Ion exchange resins and activated carbon are commonly used for their removal. In this study, we assess the efficiency of ion exchange resins (Amberlite IR120, Amberlite IRC748, and Dowex Marathon MSC) and activated carbon for Cu 2+ and Mn 2+ removal using NMR relaxometry. Kinetic curves and adsorption isotherms were obtained from batch experiments by monitoring the relaxation of supernatant water. Additionally, the relaxation properties of water in the interporosity and intraporosity of loaded sorbent beds were investigated. The kinetics of Cu 2+ removal are similar across all sorbents, whereas for Mn 2 ⁺, Marathon MSC exhibits the fastest uptake. The maximum sorption capacities for Cu 2+ are 12.11 ± 0.28 mg/g for activated carbon, 77 ± 2.7 mg/g for Marathon MSC, 82 ± 3.6 mg/g for IR120, and 121.4 + 3.8 mg/g for IRC748, while for Mn 2 ⁺, they are 4.2 ± 0.32 mg/g for activated carbon, 48.2 ± 1.4 mg/g for IRC748, 50.3 ± 0.6 mg/g for IR120, and 58.1 ± 3.4 mg/g for Marathon MSC. Activated carbon exhibits the highest affinity for Cu 2+ ( K L = 0.41 ± 0.09 L/mg), whereas IRC748 shows the strongest affinity for Mn 2+ ( K L = 1.23 ± 0.25 L/mg). Relaxation studies on loaded sorbents exhibit multiexponential behavior. The relaxation rates of water in the intraporosity are strongly correlated with metal content for all resins, except for IRC748, whose chelating properties reduce the relaxation effect. A similar correlation is observed in activated carbon for water confined within mesopores. These findings confirm that NMR relaxometry could be useful to study paramagnetic ions removal by ion exchange resins and activated carbon.