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.
Ozonation
is widely used in wastewater reclamation treatment trains,
either for micropollutant control or as a disinfectant and preoxidant
in certain reuse processes. We recently found that ozonation of secondary
effluent produces nitromethane, which can be efficiently transformed
to genotoxic halonitromethanes by chlorination. In this work, the
fate of nitromethane through water reuse treatment trains was characterized
by analyzing samples from five reuse operations employing ozone. Nitromethane
was poorly (<50%) rejected by reserve osmosis (RO), not removed
by, and in some cases, increased by ultraviolet/advanced oxidation
processes (UV/AOP). Sufficient nitromethane remained after advanced
treatment that when chlorine was added to mimic secondary disinfection,
halonitromethane formation was consistently observed. In contrast,
biological activated carbon removed most (>75%) nitromethane. Bench-scale
experiments were conducted to verify low removal by RO in clean systems
and with wastewater effluent and to quantify the kinetics of direct
and indirect photolysis of nitromethane in UV/AOP. An explanation
for increasing nitromethane concentration during AOP is proposed.
These results indicate that nitromethane presents a unique hazard
to direct potable reuse systems, due to its ubiquitous formation during
wastewater ozonation, poor removal by RO and UV/AOP, and facile conversion
into genotoxic halonitromethanes upon chlorine addition.
Surface functionalization of quantum dots (QDs) is one of the most important aspects of designing and preparing the desired QDs for intended optical and biomedical applications. In this paper, we synthesized aqueous-phase Cu2S quantum dots coating by three different stabilizers, i.e. mercaptoacetic acid, mercaptopropionic acid and glutathione (GSH). Different stabilizers can influence the coordination modes between Cu(+) on the surface of Cu2S and S(2-) of the ligand. The Cu2S QDs were characterized by UV-vis spectroscopy, energy dispersive spectrometry, transmission electron microscopy, Fourier transform infrared spectroscopy, x-ray diffraction and Raman spectra. Then, we performed a systematic study to evaluate the colloidal stability and in vitro toxicity of the formulations of Cu2S QDs with different stabilizers. Our results show that Cu2S QDs modified with different stabilizers have distinct functional groups on their surface and these groups make Cu2S produce different vibrations according to Raman spectra. The Cu2S-GSH exhibit the best colloidal stability in all pH buffer solutions and the lowest toxicity compare to the other two stabilizers. These properties make the Cu2S-GSH quantum dots a candidate for bioapplications in the future.
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