Low resolution benchtop nuclear magnetic resonance for the follow-up of the removal of Cu2+ and Cr3+ from water by amberlite IR120 ion exchange resin

Abstract

Water pollution by heavy metals is a major environmental concern. For example, copper and chromium are produced by metal and electronic industries and must be removed from wastewater. But Cu2+ and Cr3+ are also paramagnetic ions which previously allowed the use of Magnetic Resonance Imaging (MRI) and Nuclear Magnetic Resonance (NMR) relaxometry to follow their migration in several matrixes and their adsorption on different compounds. In this work, the removal of Cu2+ and Cr3+ from water by amberlite IR120 resin is studied with T2 relaxometry. The ion exchange process obeys a pseudo-second-order kinetics model, while the equilibrium isotherms can be fitted by the Langmuir theory providing values of 79 and 10.1 mg/g for the maximum metal content per gram of resin for Cu2+ and Cr3+ respectively. The equilibrium constants are 2.07 and 0.61 mM−1 for Cu2+ and Cr3+ respectively. pH does not influence the removal capacity of the resin in the studied range (pH 2–4.5). The longitudinal and transverse relaxation of the wet resin are shown to be biexponential. The relaxation rate of the fast relaxing water fraction of the wet resin is correlated with its Cu2+ or Cr3+ content. This paves the way of column experiments in which the gradual loading of the resin with Cu2+ or Cr3+ could be followed on-line by NMR relaxometry.