Ozonation has been widely used as a pre-disinfectant in potable reuse treatment trains. In our previous work, we demonstrated a significant amount of nitromethane formation from ozonating wastewater effluent. Nitromethane was found to be a ubiquitous ozone byproduct that can persist through most of the advanced treatment units now employed in potable reuse, including reverse-osmosis and ultraviolet light advanced oxidation processes, and detectable nitromethane was found in many reuse plants’ final effluent. Upon secondary disinfection with chlorine or chloramines, nitromethane can be converted into halonitromethanes, including the genotoxin chloropicrin. This is especially alarming for expansion of direct potable reuse, because a secondary disinfection step is required by drinking water regulations prior to distribution. <br/>Nitromethane has been proven to be the key intermediate toward chloropicrin during subsequent secondary disinfection of ozonated wastewater effluent with chlorine. The kinetics of nitromethane chlorination was studied by previously. However, utilities are increasingly switching from free chlorine to chloramines as a secondary disinfectant because it can suppress the formation of regulated trihalomethanes and haloacetic acids. The reaction mechanism and kinetics of nitromethane chloramination remain unclear.<br/>In this work, the transformation products from nitromethane chloramination were studied under various conditions. The expected principal product was chloropicrin, based on well-studied reactions between nitromethane and free chlorine, and because chloramines are commonly to assumed to react similarly, although more slowly, compared to free chlorine. Inconsistent yields of chloropicrin were observed at acidic, neutral, and basic conditions, and surprisingly, transformation products other than chloropicrin were found. Monochloronitromethane and dichloronitromethane were detected at basic pH, and significant mass balance was missing at neutral pH, which was later attributed to nitrate formation, from a newly-identified pathway of monochloramine reacting as a nucleophile though a presumed S<sub>N</sub>2 mechanism. The reaction mechanism of nitromethane reacting with chloramines were investigated, and a comprehensive kinetic rate model was developed under a range of conditions.

Kinetics and mechanism of nitromethane chloramination

ACS Spring 2022

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