Chloramination of Nitromethane: Incomplete Chlorination and Unexpected Substitution Reaction

Ozone is commonly used as a predisinfectant in potable water reuse treatment trains. Nitromethane was recently found as a ubiquitous ozone byproduct in wastewater, and the key intermediate toward chloropicrin during subsequent secondary disinfection of ozonated wastewater effluent with chlorine. However, many utilities have switched from free chlorine to chloramines as a secondary disinfectant. The reaction mechanism and kinetics of nitromethane transformation by chloramines, unlike those for free chlorine, are unknown. In this work, the kinetics, mechanism, and products of nitromethane chloramination were studied. The expected principal product was chloropicrin, because chloramines are commonly assumed to react similarly to, although more slowly than, free chlorine. Different molar yields of chloropicrin were observed under acidic, neutral, and basic conditions, and surprisingly, transformation products other than chloropicrin were found. Monochloronitromethane and dichloronitromethane were detected at basic pH, and the mass balance was initially poor at neutral pH. Much of the missing mass was later attributed to nitrate formation, from a newly identified pathway involving monochloramine reacting as a nucleophile rather than a halogenating agent, through a presumed SN2 mechanism. The study indicates that nitromethane chloramination, unlike chlorination, is likely to produce a range of products, whose speciation is a function of pH and reaction time.

[Determination of 25 disinfection by-products in drinking water using liquid-liquid extraction and gas chromatography]

OBJECTIVE

To establish a liquid-liquid extraction and gas chromatography method for the determination of 7 kinds of haloacetaldehydes, 7 kinds of haloacetonitriles, 7 kinds of halonitromethanes and 4 kinds of haloacetamides in drinking water.

METHODS

A liquid-liquid extraction gas chromatography technique was employed. Experimental parameters, such as capillary column type, inlet temperature, concentration of salting out reagent and sample pH were optimized to develop an analytical method, and then method validation was conducted.

RESULTS

HP-5 MS UI column(30 m×0.25 mm, 0.25 μm), inlet temperature at 180 ℃, addition of 8 g sodium chloride in 50 mL water sample and pH 4-5 were chosen as the final parameters. Good correlation coefficients were obtained in the linear range of 0.20-15 μg/L, with r greater than 0.999.Methods detection limits were between 0.008-0.088 μg/L. When spiked concentration was 1.0 μg/L for pure water and tap water, the recoveries were 81%-106% and 75%-117%, respectively, and relative standard deviations were both less than 4%. When spiked concentration was 12 μg/L for pure water and tap water, the recoveries were 92%-101% and 86%-106%, respectively, and relative standard deviations were less than 4% and 2%, respectively.

CONCLUSION

This method is simple, sensitive, and effective. It is suitable for simultaneous determination of 25 disinfection byproducts in drinking water.

Monitoring trihalomethanes and nitrogenous disinfection by-products in blending desalinated waters using solid-phase microextraction and gas chromatography

A simple and efficient method has been developed for the extraction and determination of 16 common volatile halogenated disinfection by-products (DBPs) (four trihalomethanes, six haloacetonitriles, and six halonitromethanes) in blending desalinated waters, using headspace solid-phase microextraction and gas chromatography with flame ionization detector (HS-SPME/GC-FID). After the optimization using factorial designs of the HS-SPME parameters (optimum: carboxen/polydimethylsiloxane such as fiber, extraction time of 60 min at 30°C, pH 7, addition of 40% (w/v) of sodium chloride, and desorption time of 2 min at 250°C), quantification limits ranged from 3.03 to 40.8 µg L^-1, and relative standard deviation (inter-day) were lower than 9.7% for all the target DBPs. Adequate relative recoveries (with the exception of chloronitromethane) were obtained even when spiking waters at low levels (25 µg L^-1), with values between 83.1% and 119% for ultrapure water, and between 87.4% and 115% for blending desalinated waters, supporting in this way the applicability of the method. The influence of various dechlorinating agents on the stability of 16 DBPs in water was evaluated, with ammonium chloride being the most suitable inhibitor of residual chlorine and carrying out the analytical determination of DBPs within 48 h after sampling. Different blending desalinated water samples collected in the South of Tenerife Island (Spain) were successfully analyzed.

Formation and speciation of nine haloacetamides, an emerging class of nitrogenous DBPs, during chlorination or chloramination

Haloacetamides (HAcAms) are an emerging class of nitrogenous disinfection by-products (N-DBPs) of health concern. However, there are very limited data on the formation and speciation of the nine bromine- and chlorine-containing haloacetamides (HAcAm9). In the study, their formation and speciation during chlor(am)ination were investigated for a group of waters with a range of specific ultraviolet absorbance at 254 nm (SUVA₂₅₄), dissolved organic nitrogen (DON), and bromide levels. The waters that were the least impacted by anthropogenic pollution had the lowest DON levels, the highest ratios of dissolved organic carbon (DOC) to DON, and exhibited the least HAcAm9 formation. DON/DOC may act as an indicator of HAcAm yields during chlorination. HAcAm9 exhibited more formation during chloramination in the low-SUVA waters with no bromide, relative to high-SUVA waters with bromide. The selected waters all formed primarily dihalogenated (di-) HAcAms, followed by trihalogenated (tri-) species and, to a much lesser extent, monohalogenated (mono-) HAcAms. Di-HAcAm formation had similar trends as that of HAcAm9; whereas chloramination formed more mono- and less tri-HAcAms than chlorination. Bromine utilization factors and bromine incorporation factor increased with decreasing and increasing bromide during either chlorination or chloramination, and bromine was easier to incorporate into tri-HAcAms during chloramination than chlorination.