Metaldehyde removal from drinking water by adsorption onto filtration media: mechanisms and optimisation

Abstract

a Trace micropollutants should be removed during drinking water production without increasing the disinfection-by-product formation potential or energy demand of the treatment process. We demonstrate the efficacy of different filtration media to remove metaldehyde through controlled batch experiments on water augmented with metaldehyde. Equilibrium concentrations of metaldehyde and surrogate organics were successfully described by the Freundlich isotherm. Metaldehyde can be attenuated to varying degrees with activated carbon and sand with an active and inactive biofilm with k f values ranging from 0.006-0.3. The presence of the active biofilm improved metaldehyde adsorption by sand media, due to additional biosorption mechanisms, a greater surface area or biodegradation. Baseline levels of competing natural organic matter surrogates (NOM) reduced overall adsorption efficacy but increasing concentrations of NOM did not impact metaldehyde removal efficacy in a significant way. Biological activated carbon was identified as the most suitable adsorbent of metaldehyde (94% removal) but sand with an acclimated biofilm was capable of acting as a bio-adsorbent of metaldehyde even under environmentally relevant concentrations (41% adsorption from 0.002.5 mg L −1). Moreover, we observed that thermal hydrolysis of metaldehyde occurred at 60°C, suggesting that thermal regeneration of GAC for this pesticide was possible at relatively low temperatures. Biological adsorption and thermal hydrolysis approaches presented herein offered a way forward to increase efficiency and cost effectiveness of existing treatments for metaldehyde. IntroductionIn Europe, water treatment works (WTW) are required to meet the European Union drinking water standard of 0.1 μg L −1 for individual pesticides. This includes the molluscicide, metaldehyde, which is an uncharged, low M w , highly soluble compound. Such characteristics make it difficult to remove in traditional drinking water treatment processes as evidenced by the fact that it is responsible for most pesticide related drinking water failures in the UK. Water impactOur study is topical, as it provides insights into adsorption of metaldehyde onto different sorbents. Bio-active carbon had the greatest metaldehyde removal through biosorption to biofilms which along with thermal hydrolysis represent strategies for improving the performance and resilience of filtration media to polar pesticides. overall metaldehyde adsorption capacity. 7,8 The impact of background interferences, on the metaldehyde adsorption, remains poorly quantified, in particular, the role of organic and inorganic compounds which are typically present in source waters. This is important as these compounds compete for adsorption sites in adsorptive media. Slow sand filters (SSFs) could provide an effective treatment for metaldehyde by promoting coupled adsorption and biosorption. 8 The adsorption efficiency of SSFs or microbial

Investigation of metaldehyde removal by powdered activated carbon from different water samples

“…11,12 Recently, there has been a study of metaldehyde adsorption with the presence of serine, leucine, and resorcinol onto different filtration media including different sands, GAC, and biological activated carbon (BAC); results have shown that BAC is very effective for metaldehyde removal. 13 This suggests that BAC filtration might be a feasible option for water treatment plants. However, there was observed desorption of metaldehyde from BAC back into the water, indicating that increased regeneration of BAC is required to maintain the effectiveness of metaldehyde removal which limits its economical application.…”

“…However, there was observed desorption of metaldehyde from BAC back into the water, indicating that increased regeneration of BAC is required to maintain the effectiveness of metaldehyde removal which limits its economical application. 13 Our previous research has shown that PAC, an alternative adsorbent to GAC, can remove metaldehyde very efficiently even in the presence of natural organic matter (NOM). 14 In fact, it was reported that PAC has a much higher adsorption capacity (28 mg g −1 ) for metaldehyde 14 13 Busquets et al explained that the adsorption of metaldehyde is affected by the pore size distribution of the carbon materials and a higher adsorption capacity was observed in microporous carbon materials.…”

“…13 Our previous research has shown that PAC, an alternative adsorbent to GAC, can remove metaldehyde very efficiently even in the presence of natural organic matter (NOM). 14 In fact, it was reported that PAC has a much higher adsorption capacity (28 mg g −1 ) for metaldehyde 14 13 Busquets et al explained that the adsorption of metaldehyde is affected by the pore size distribution of the carbon materials and a higher adsorption capacity was observed in microporous carbon materials. 15 Since the pore size distribution of the PAC used in our previous research is dominated by micropores, 14 the PAC may be an effective adsorbent for small molecular contaminants such as metaldehyde.…”

“…Moreover, Rolph et al argued that BAC could have low adsorption capacity for metaldehyde because of prior exposure and regeneration cycles. 13 In comparison, the PAC is unused (virgin) when applied to water samples and its unoccupied adsorption sites are available for metaldehyde.…”

“…The working concentration of metaldehyde solution in this study was 5 μg L −1 since it is close to the detected concentration of metaldehyde in surface water (8 μg L −1 ) by the UK Environment Agency and representative of the metaldehyde concentration in raw water during the peak season. 1,13,16 This study investigates the effect of PAC dosage and water quality on the removal of low concentration of metaldehyde from water. It also identifies the best treatment stage to dose PAC for removing metaldehyde at WTWTW, illustrating the potential application of PAC in a real water treatment plant to remove metaldehyde with consideration of possible regeneration of PAC at low temperature.…”