Development of a novel dual-stage method for metaldehyde removal from water

A dual-stage method was developed to first efficaciously degrade metaldehyde into acetaldehyde, before subsequent removal of acetaldehyde using an amine functionalized ion-exchange resin. A range of Macronets, with different surface areas, pore volumes, pore size distributions and extents of functionalisation were evaluated for the depolymerisation of metaldehyde. NMR spectroscopy was used to confirm the compete degradation of metaldehyde by the selected Macronets. Kinetic studies showed that the rates of catalytic degradation were primarily determined by the porous structure of the materials rather than the extent of surface functionalization, since high levels of acid surface groups were observed to decrease the porosity significantly. The rate constants obtained show excellent correlation with non-micro pore volumes, which are of import as the only pores that are accessible to hydrogen bonded metaldehyde molecules; Macronet MN502 exhibits the largest non-micro pore volume and, hence, demonstrated the best kinetic performance. The effect of competing ions on catalytic performance was also studied and the results demonstrate that competing ions compromise the performance of the proposed system to some extent, however, it is notable that a good level of performance is maintained even for competing ion concentrations as high as 100 times that of metaldehyde. Isothermal studies of acetaldehyde onto ion-exchange resin A830, including kinetic evaluation, showed that acetaldehyde could be chemically adsorbed by the resin. Consequently, a dual-column system was proposed, which was determined to effectively degrade metaldehyde (MN502) and remove the resulting by-product, acetaldehyde, via a second fixed bed absorber (A830); this method could easily be adapted to existing facilities in water treatment works, making it very cost-effective and of great practical interest.