Transformation of N-Methylamine Drugs during Wastewater Ozonation: Formation of Nitromethane, an Efficient Precursor to Halonitromethanes

Abstract

Potable reuse of wastewater is expanding, and ozonation for water reuse is becoming more common, either as a preoxidant before membranes or as part of ozone/biological activated carbon (O3/BAC) systems. However, previous research has demonstrated that ozone drastically increases the formation potential of genotoxic halonitromethanes (HNMs), including during O3/BAC. Chloropicrin, the most common HNM, is synthesized by chlorinating nitromethane, suggesting that nitromethane may be the immediate precursor of chloropicrin, although nitromethane is unlikely to occur naturally in wastewater. We hypothesized that wastewater ozonation forms nitromethane, which would be the key intermediate toward HNMs. Ozonation of wastewater effluent was shown to form abundant nitromethane, which explained the majority (in one case, all) of subsequent chloropicrin formation. Next, we investigated a suspected category of nitromethane precursor: stimulant drugs, such as ephedrine and methamphetamine, and certain antidepressants. These drugs all feature N-methylamine functional groups, and certain N-alkylamines have been shown to produce primary nitroalkanes upon ozonation. Ozonation of N-methylamine drugs ubiquitously formed nitromethane, typically at >50% yield. Subsequent chlorination converted nitromethane to chloropicrin. The reaction mechanism was investigated to understand the variation in nitromethane yield between different precursors. These results suggest that nitromethane fate during reuse and nitromethane control should be investigated.

Formation and Fate of Nitromethane in Ozone-Based Water Reuse Processes

“…Speciation swiftly shifted after chlorine addition (post RO) toward the more toxic halogenated nitromethanes. The molar sum of the measured halonitromethanes post UV/AOP corresponds to 93.6% of the nitromethane present before chlorine addition, consistent with nitromethane serving as the primary precursor to halonitromethanes, as observed previously in secondary effluent …”

“…The molar yield of chloropicrin from nitromethane in chlorinated samples from plants A, B, and D ranged from 2 to 40% (Figure S11), which is smaller than that observed previously in DI water, , likely due to scavenging of chlorine by other species present in wastewater. The concentration trends of chloropicrin across the treatment trains generally mirrored the concentration profile of nitromethane prior to chlorination (data from plant D shown in Figure ), and although the yield was not 100%, the trend is consistent with our hypothesis that halonitromethanes as a class of byproducts generally derive from halogenation of nitromethane.…”

“…It was recently demonstrated that secondary amines in water can be ozonated to form various nitroalkanes . Based on existing research on hydroxyl radical reaction mechanisms, − we hypothesized that hydroxyl radical can fragment higher-molecular-weight nitroalkanes formed during wastewater ozonation to nitromethane rather than directly oxidizing amines to nitromethane because hydroxyl radical is not involved in established pathways for producing nitro compounds with ozone. , …”

“…Nitromethane and halonitromethanes were measured via static headspace gas chromatography/triple quadrupole mass spectrometry (GC/MS) as described previously . More details are provided in Text S1 and Table S2.…”

“…Recent research indicates that ozone oxidizes N -methylamines present in secondary wastewater effluent and generates abundant nitromethane (up to ∼35 μg/L) and that nitromethane is the key intermediate toward halonitromethanes during subsequent chlorination . Halonitromethanes are highly genotoxic − and have been observed in disinfected wastewater and recycled wastewater, particularly in systems using ozone as a primary disinfectant or preoxidant, followed by chlorine or chloramines as a final disinfection step. − In one study on wastewater reuse with ozone/BAC, ozonation was found to increase the halonitromethane formation potential of wastewater by 8–10-fold, but the identity of the halonitromethane precursors remained unknown .…”