HEAVY-METAL UPTAKE OF ALGINATE GELS STUDIED BY NMR MICROSCOPY

In recent years, heavy metal uptake by biopolymer gels such as calcium alginate or chitosan has been studied by various methods. The main mo tivation for these studies was the possible application of such materi als in environmental technology. Most of the work done on absorption p henomena in such materials was concerned with equilibrium concentratio ns of different heavy metal ions and their dependence on parameters su ch as temperature, pH and concentrations of competing ions. A few of t hese studies also referred to kinetic aspects. However, no experiments on the spatial distribution of heavy metals during the absorption pro cess are known to us. Therefore, we have introduced NMR microscopy int o this field. By this method, both spatially- and temporally-resolved data of the absorption process can be measured. As for the possible im age contrasts, there are mainly two mechanisms: changes in relaxation behaviour of water protons in the presence of heavy metal ions; and ch anges in magnetic susceptibility upon loading with paramagnetic ions. In this contribution, results of both imaging methods are presented. F rom these data, it is obvious that the ion intrusion does not follow t he ordinary diffusion equation. Instead, there are several different f orms of ion intrusion behaviour. In the case of rare earth ions, the s hrinking core model is found to give a satisfying description of the e xperiments.