Benchtop NMR relaxometry to monitor Ni (II) removal by resin and activated charcoal

Water pollution by heavy metals is a major environmental concern. In this context, removal of heavy metal from wastewater by ion exchange resins or adsorbents is often necessary. However, current techniques to study adsorption efficiency are indirect and sometimes destructive. Some heavy metal ions like Ni(II) have paramagnetic properties that can affect the NMR relaxation times T1 and T2 of water protons, which can be measured by benchtop NMR relaxometry. Therefore, T1 and T2 can be used to monitor the removal of paramagnetic heavy metals by a sorbent. In this work, the removal of Ni(II) by Dowex Marathon MSC resin and commercial activated charcoal was studied.
Batch experiments were carried out to study the ion exchange/adsorption isotherms: a sample containing 10 mg of resin or 50 mg of activated charcoal was put in contact with aqueous solutions containing different concentrations of Ni(II), before being shaken by a vortex mixer. Once the equilibrium reached, T1 of the solution was measured which allowed the determination of the amount of adsorbed metal. The study of the loaded sorbent was also carried out.
The equilibrium isotherms of Ni(II) were satisfactorily described by the Langmuir model for both studied sorbent. The longitudinal and transverse relaxation of the wet resin is shown to be biexponential. The relaxation rates of the fast relaxing water fraction of the Ni(II) loaded wet resin can be correlated with the metal content obtained independently by AES after acidic digestion of the samples. Loaded activated charcoal, due to its important microporosity but also because of a partial desorption of Ni(II) ions, exhibits a more complex relaxation behavior.
Based on these results, a column experiment will be set up inside the NMR coil in order to follow the loading of the sorbent in real-time through the evolution of the NMR signal.