Application of the Fenton process to the dissolution and mineralization of ion exchange resins

Oxidative degradation of used ion exchange resins (IER) by the Fenton process is an attractive alternative to their deposition as hazardous waste. The process is leading to the dissolution of the polymer beads producing low molecular weight carboxylic acids and subsequently to their mineralization. Therefore, process development is focused for economic reasons at a fast dissolution of the polymer material, while keeping mineralization to a minimum. The optimal experimental design methodology (OED) was used to investigate the effects of primary reaction parameters (Fe(II)- and H2O2-concentrations and temperature) and to find the best parameter ranges for process optimization. The initial Fe(II)-concentration ([Fe(II)]0) controlled the rate of dissolution, whereas H2O2-concentration and reaction temperature (40–60 °C) were found to be of minor impact. Spectrophotometric analysis demonstrated that an increase in [Fe(II)]0 led to a fast decrease of the dissolution time (tdis) as long as the concentration of Fe(II/III) was smaller than that needed for saturation of the IER. When the concentration of Fe(II/III) was higher than the capacity of the IER, efficient mineralization was found to take place. Oxidative degradation leading to the dissolution of the polymer beads is therefore most efficient as long as the catalyst is bound to the surface, whereas mineralization is most effective once the organic material is dissolved. This hypothesis is supported by REM images taken under different experimental conditions, as well as by the linear relation between the extent of mineralization and tdis, in spite of the variation of the accessible surface of the polymer beads. An increase of the content of cross-linking agent affected adversely the oxidative degradation of the IER, mainly because it implies a diminution of the concentration of sulfonic acid groups and, hence, a decreased efficiency of iron complexation on the polymer beads.