Neonicotinoid pesticides are the most widely used insecticides in the world. The widespread application of neonicotinoids has led to their proliferation in waters across the U.S., including those used as drinking water sources. Although neonicotinoids exhibit relatively low toxicity towards mammals, formation of toxic metabolites is an exposure concern. Previous studies comparing neonicotinoid detection at the University of Iowa and Iowa City drinking water treatment plants (DWTPs) have identified granular activated carbon (GAC) as a possible method for removing imidacloprid, clothianidin, and thiamethoxam (common neonicotinoids) and the toxic metabolite, desnitro imidacloprid, from drinking water. The objective of the present study was to determine the efficacy of GAC removal of neonicotinoids and a neonicotinoid metabolite from drinking water. Adsorption isotherms were conducted with imidacloprid, clothianidin, thiamethoxam, and desnitro imidacloprid to assess their affinity towards GAC. Desorption tests were also conducted using five-year-old used GAC from the Iowa City DWTP to determine the likelihood of residual neonicotinoid desorption. Results indicate neonics (despite high polarity / solubility) readily adsorb to GAC and minimally desorb in buffered solutions at environmental pH. The greatest extent of neonicotinoid desorption occurred in solvents of medial polarity (acetonitrile, acetone, and dichloromethane). In these solvents, concentrations of up to 600 ng neonic/ g GAC were measured. These results suggest the long-term use of GAC as a viable option for neonicotinoid removal from drinking water. Kinetic isotherms of desnitro imidacloprid indicate that this metabolite can adsorb to GAC at similar rates as imidacloprid. However, when analyzing drinking water samples from the University of Iowa and Iowa City DWTPs, we observed that concentrations of desnitro imidacloprid were statistically similar (~ 20 ng/L) in DWTP influent and tap water samples. The prevalence of desnitro imidacloprid in drinking water may be due to its formation in the DWTP from abiotic and/or biotic reactions with imidacloprid. Over time, pipes and activated carbon filters are prone to biofilm formation, which may enhance the biotransformation of adsorbed or aqueous phase neonicotinoids into toxic metabolites. Studies are currently underway concerning the biotransformation of neonicotinoids in environmental and engineered systems containing electron transporters such as GAC.

Sorption of neonicotinoid insecticides to granular activated carbon during drinking water treatment

255th National Meeting (2018)

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