Catalytic degradation and adsorption of metaldehyde from drinking water by functionalized mesoporous silicas and ion-exchange resin

Sulfonic acid functionalized mesoporous silicas with various loadings of acid functionality were synthesized, characterized and applied as heterogeneous catalysts for the degradation of metaldehyde, a persistent organic pollutant in water supplies. Nuclear magnetic resonance spectroscopy showed that acetaldehyde was the only by-product of catalytic degradation, and a detailed mechanism is proposed. Kinetic studies revealed that catalyst performance is related to the accessibility of metaldehyde to active sites, such that high sulfonic acid content is undesirable since it reduces pore size, and decreases pore volume and surface area. Acetaldehyde produced via catalytic degradation, was successfully removed via chemisorption on a second mesoporous silica adsorbent modified with amine functionalities. However, limited by the surface condensation reaction mechanism, mesoporous adsorbents are less desirable than macroporous materials, with respect to acetaldehyde removal, hence, a macroporous ion-exchange resin was employed, which showed much superior performance than the amine modified silica, with a maximum capacity up to 441 mg/g. A dual-stage method is proposed to completely remove metaldehyde from drinking water by initial degradation of metaldehyde, using sulfonic acid functionalized mesoporous silica, into a single by-product, acetaldehyde, removed via chemisorption on amine bearing macroporous ion-exchange resin. The results present a promising system for removal of metaldehyde from drinking water supplies, with potential application to other contaminants.