Ni2+ removal by ion exchange resins and activated carbon: a benchtop NMR study.

Heavy metal pollution in water is a critical environmental concern, demanding effective remediation techniques. Traditional methods, including ion exchange and adsorption, often rely on inductively coupled plasma (ICP) atomic emission spectroscopy/mass spectrometry (AES/MS) for the indirect and time-consuming measurement of residual metal concentrations. In contrast, this study employs innovative direct monitoring of nickel removal by benchtop NMR relaxometry using the paramagnetic properties of Ni²⁺. To prove the feasibility of the NMR follow-up of Ni²⁺ uptake, batch experiments were performed with Amberlite IR120, Amberlite IRC748, Dowex Marathon MSC, and activated carbon (AC), which were previously characterized by various techniques. The effect of contact time, pH, and Ni²⁺ concentration on removal efficiency were studied. Pseudo-first and pseudo-second order kinetic models were used. The Langmuir model effectively described the equilibrium isotherms. The longitudinal and transverse relaxation curves of the loaded resins were biexponential. For sulfonic resins, a strong correlation was observed between the relaxation rates of the fast-relaxing fraction and the Ni²⁺ content determined by ICP-AES/MS. For IRC748, the effect of Ni²⁺ loading on the relaxation rates was weaker because of Ni²⁺ complexation. The relaxation curves of loaded AC revealed multiple fractions. Centrifugation was employed to eliminate the contribution of intergranular water. The remaining intragranular water contribution was biexponential. For high Ni²⁺ loadings, the relaxation rates of the slow relaxing fraction increased with the AC Ni²⁺ content. These results mark the initial stage in developing a column experiment to monitor, in real-time, adsorbent loading by NMR relaxometry. [ABSTRACT FROM AUTHOR]