Benchtop NMR Relaxometry for monitoring Cu2+ removal using ion exchange resins and commercial activated charcoal

Water pollution by heavy metals is a major environmental problem [1]. To address this issue, the removal of heavy metals from wastewater often requires the use of ion exchange resins or adsorbents. However, current methods for assessing ion exchange/adsorption efficiency are often indirect and destructive. Some heavy metal ions, such as Cu2+, exhibit paramagnetic properties that influence the NMR relaxation times T1 and T2 of water protons. Benchtop NMR relaxometry can thus provide a means to monitor the removal of paramagnetic heavy metals by sorbents thanks to the measurement of T1 and T2 [2-3].
This research focuses on studying the removal of Cu2+ using Amberlite IR120; Amberlite IRC748; Dowex Marathon MSC resin and commercial activated charcoal (AC). Batch experiments were conducted to investigate ion exchange and adsorption isotherms. Samples containing 5.5 mg of IR120/MSC resin; 5 mg of IRC748 resin or 45 mg of AC were exposed to 350 µL of aqueous solutions with varying Cu2+ concentrations and then shaken until equilibrium was reached. Upon reaching equilibrium, T1 measurements of the solution were carried out to determine the amount of adsorbed metal. Additionally, the study of the loaded resin was also carried out using a larger amount of resin/AC, which was first dried and then rehydrated before being analyzed.
The equilibrium isotherms for Cu2+ were satisfactorily described by the Langmuir model for resins and by the Freundlich model for AC. The longitudinal and transverse relaxation of the wet resin exhibited biexponential behavior. The relaxation rates of the fast-relaxing water fraction of Cu2+ loaded sulfonic resins showed an excellent correlation with their Cu2+ content determined independently via Atomic Emission Spectroscopy. The impact of Cu2+ loading on relaxation rates in the case of IRC748 was less pronounced due to Cu2+ complexation. Loaded activated charcoal, due to its heteroporosity displayed a more complex relaxation behavior. The relaxation curves of loaded AC exhibit more than two fractions. To eliminate the contribution of intergranular water, centrifugation was employed. The residual contribution from intragranular water was biexponential. The relaxation rates of the slowest relaxing fraction increased with the Cu2+ content in AC.
In the future, it will be interesting to carry out a so-called NMR column experiment in order to follow the loading of adsorbent in real-time through the measurement of the NMR signal inside the column.