Glyphosate is the most commonly used herbicide in the U.S. Roughly 250 million pounds are applied each year in the U.S. with more than 3.5 billion pounds being applied since 1974. During the same time, more than 19 billion pounds have been applied globally. The International Agency for Research on Cancer (IARC) classifies glyphosate as a Group 1A carcinogen and it has an MCL of 0.7 mg/L in drinking water under EPA’s Safe Drinking Water Act. Its nitrosated analogue, N-nitrosoglyphosate (NNG), can occur in commercial glyphosate preparations as a byproduct and can form in soils where glyphosate is present at low pH (&lt;6). Toxicity of NNG has not been evaluated by EPA due to the low reported prevalence as a byproduct in commercial formulations of glyphosate. The aim of this study was to determine whether NNG could form at conditions relevant to drinking water treatment where glyphosate may be present as a contaminant, or where glyphosate and nitrite may be present as co-contaminants due to agricultural runoff, and to determine whether NNG was present in advanced treated recycled water. We hypothesized that disinfection with monochloramine, and disinfection with free chlorine in the presence of nitrite, would lead to NNG formation from glyphosate. NNG was formed in the mg/L range upon the reaction of glyphosate with nitrite-N above 2 mg/L and at glyphosate concentrations above 100 mg/L at pH 8 in three days, and the initial NNG production rate was 0.384 ppb/min at 20 mg/L of nitrite-N and 100 mg/L of glyphosate. The presence of monochloramine or free chlorine did not greatly impact the yield of NNG in the presence of nitrite. However, greater concentrations of nitrite resulted in greater concentrations of NNG. In formation potential tests utilizing monochloramine and glyphosate at pH 8, NNG did not form in the absence of nitrite indicating the reaction is nitrite-dependent. At a detection limit of 0.5 mg/L, NNG was not detected in advanced treated recycled water or secondary wastewater effluent. Because N-nitrosamines typically occur at the ng/L concentration in potable waters, future work is aimed at reducing the method detection limit, determining occurrence in agricultural run-off, recycled wastewater for agricultural irrigation and conventionally-treated drinking water and determining the influence of other water quality parameters on the formation of NNG.

Formation of N-nitrosoglyphosate from glyphosate and nitrite at neutral pH and occurrence in recycled wastewater

ACS Fall 2021

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