Application of NMR relaxometry for real-time monitoring of the removal of metal ions from water by synthetic clays†

The removal of paramagnetic metal ions with different charges and ionic radii (i.e. Gd³⁺, Cu²⁺, and Co²⁺) from aqueous solutions was carried out by using a Na⁺-exchanged synthetic saponite clay. Saponite, composed of sub-micrometer particles and characterized by high cation-exchange capacity, was prepared through a classical low-cost hydrothermal approach. The metal ion uptake tests were performed in water at pH = 5.5 and 3.0, and the capture process was monitored in real time by ¹H-NMR relaxometry. The experimental data were confirmed by the conventional ICP-OES technique. Details of the uptake process kinetics were extrapolated from the NMR analyses as well. Saponite showed good sorption capacity for all selected metal ions. The regeneration of the solid sorbent after metal uptake was also analysed, obtaining encouraging results.