Dissolution and mineralization of ion exchange resins: differentiation between heterogeneous and homogeneous (photo-)Fenton processes

Abstract

Fenton and photo-Fenton processes lead under defined experimental conditions to the oxidative degradation and complete apparent dissolution of ion exchange resins (IER) based on copolymers of sulfonated styrene and divinylbenzene, as well as to the mineralization of the dissolved organic fragments. Using the optimal experimental design methodology (OED), the initial Fe(II)-concentration ([Fe(II)]₀) was found to control the time needed to completely degrade the IER into soluble fragments, whereas the H₂O₂ concentration was of minor impact. The photo-Fenton process enhanced primarily the rate of mineralization compared to the dark reaction. The results of process modeling for Fenton and photo-Fenton processes, investigations on the evolution of sulfate (SO₄²⁻), CO₂, formic and oxalic acids, as well as the comparison between results of photo-Fenton and VUV-photolysis experiments confirm: (i) the existence of two distinct Fenton processes taking place at the surface of the IER beads and in the aqueous bulk, (ii) the desulfonation as the reaction triggering the oxidative degradation and apparent dissolution of IER, (iii) the release of Fe(III) into the aqueous medium and its subsequent reduction, as well as the recycling of the Fe(II)-complexes at the surface of the IER, (iv) the low reactivity of HO˙ generated in solution toward the solid organic substrate, and (v) the important effect of Fe(III)-complexation by oxalic acid. Results also support the hypothesis that formic and oxalic acids are of different origin, and their probing might prove useful for other degradation processes as well.