Assessment of metaldehyde removal by GAC using the RapStech service


Severn Trent Water Ltd


Feb – Sept’ 08

Metaldehyde is a recently emerging problem and traces of the molluscicide have been detected in raw waters. Metaldehyde is thought to be less easily adsorbed onto Granular Activated Carbon (GAC) which is used for treatment.

WatStech Ltd has conducted an assessment for metaldehyde removal using Rapid Small Scale Column Tests (RSSCTs). This research was to give an indication of the approximate adsorption capacity of metaldehyde onto Virgin GAC the profile of breakthrough, using site water and laboratory water spiked with metaldehyde.

Supplementary research was carried out by WatStech Ltd, which involved the determination of the adsorption capacity of metaldehyde and mecoprop using single solute batch adsorption isotherms by various types of media (spent, regenerated & virgin GACs and zeolite, which is a microporous alumino-silicate mineral). The adsorption capacities were evaluated using the Freundlich model. Isotherms can be a useful tool in comparing GAC treatment for adsorption capacity for a particular chemical, but isotherm data is indicative only of the performance under static conditions.

WatStech Ltd also investigated the pH point of zero charge (pHpzc) which can assist in the determination of the GAC surface charge thereby assessing its effect on organics adsorption. Activated carbons are known to have amphoteric character and thus depending on the pH of the solution, their surfaces might be positively or negatively charged. However, factors such as the physicochemical properties of the organic solutes are also known to affect the adsorption capacity of GACs, often expressed as the log Kow value.

These GAC analysis techniques have been used to determine the adsorption of metaldehyde onto GAC and assist with better understanding of the removal of this pesticide from drinking water.