How the conditioning and storage of samples for SEM-EDX analysis may influence the interpretation of diffusion mechanisms in the sorption of metals ions by extractant impregnated resins

Abstract

BACKGROUND Modeling mass transfer steps is important for optimizing the use of sorbents such as extractant impregnated resins (EIR). However, mathematical fit of kinetic profiles requires physical characterization for validating mechanism hypotheses. SEM-EDX is a very useful tool for achieving this objective. RESULTS In this work a special attention has been focused on the simultaneous modeling of the diffusion mechanisms (using homogeneous diffusion and shrinking core models associated to resistance to film diffusion, to intraparticle diffusion and to chemical reaction rate) and the SEM-EDX analysis of the diffusion profiles for gold sorption on ionic liquid/impregnated resins (tetraalkylphosphonium IL immobilized in Amberlite XAD resins). Some discrepancies in the conclusions raised by the two approaches led to carefully investigate the effect of conditioning and storage of resins before SEM-EDX analysis. The self-diffusion of the metal ion in the EIR causes misinterpretation of diffusion profiles when inappropriate conditions are used. CONCLUSION SEM-EDX can be efficiently used for improving the understanding of diffusion mechanisms in the study of metal uptake kinetics in EIRs providing that the analysis of the samples is carried out within 24 h and the sample is freeze-dried (to limit metal-ion self-diffusion in the extractant phase). This study can obviously be extended to other sorption systems.