Removal of metaldehyde in aqueous solutions by heterogeneous photocatalysis and adsorption processes

Metaldehyde has been detected in surface water and drinking water in the UK, exceeding the EU and UK standard of 0.1 μg L-1. Conventional water treatment methods are not effective due to the physicochemical properties of metaldehyde while newly proposed methods are under research and cannot be applied at an industrial scale. This thesis investigated the removal of metaldehyde from aqueous solutions by heterogeneous photocatalysis using nanoparticle photocatalysts and adsorption processes using carbon materials, aiming to provide a feasible solution to the problem. Powdered activated carbon (PAC) proved to be the most effective material to remove metaldehyde from water. It has a relatively large specific surface area of 962.4 m2 g-1 and a pore size distribution in the micro-/meso-pores range that favours adsorption of metaldehyde. Adsorption of metaldehyde onto PAC was explained by the Langmuir isotherm model, with a highest maximum adsorption capacity (qm) of 28.33 mg g-1, and by the pseudo-second order kinetic model, with highest adsorption rate (k2) of 0.16 g mg-1 min-1. Low initial concentration of metaldehyde solution led to lower qm of PAC for metaldehyde due to low driving force for mass transfer and competition with water molecules. Humic acid, which represents natural organic matter in water, has little effect on adsorption of metaldehyde onto PAC. For water samples collected from various stages at Walton-on-Thames Water Treatment Works, the best treatment stage for dosing PAC to remove metaldehyde was apparently after the ‘static flocculation’ stage. Desorption of metaldehyde from used PAC was observed, suggesting that constant monitoring of metaldehyde is essential for adjusting dosage and recycling of PAC. Lastly, low temperature on-site regeneration of used PAC may be possible.