A proposed abiotic reaction scheme for hydroxylamine and monochloramine under chloramination relevant drinking water conditions.

Drinking water monochloramine (NH2Cl) use may promote ammonia-oxidizing bacteria (AOB). AOB use (i) ammonia monooxygenase for biological ammonia (NH3) oxidation to hydroxylamine (NH2OH) and (ii) hydroxylamine oxidoreductase for NH2OH oxidation to nitrite. NH2Cl and NH2OH may react, providing AOB potential benefits and detriments. The NH2Cl/NH2OH reaction would benefit AOB by removing the disinfectant (NH2Cl) and releasing their growth substrate (NH3), but the NH2Cl/NH2OH reaction would also provide a possible additional inactivation mechanism besides direct NH2Cl reaction with cells. Because biological NH2OH oxidation supplies the electrons required for biological NH3 oxidation, the NH2Cl/NH2OH reaction provides a direct mechanism for NH2Cl to inhibit NH3 oxidation, starving the cell of reductant by preventing biological NH2OH oxidation. To investigate possible NH2Cl/NH2OH reaction implications on AOB, an understanding of the underlying abiotic reaction is first required. The present study conducted a detailed literature review and proposed an abiotic NH2Cl/NH2OH reaction scheme (RS) for chloramination relevant drinking water conditions (µM concentrations, air saturation, and pH 7-9). Next, RS literature based kinetics and end-products were evaluated experimentally between pHs 7.7 and 8.3, representing (i) the pH range for future experiments with AOB and (ii) mid-range pHs typically found in chloraminated drinking water. In addition, a (15)N stable isotope experiment was conducted to verify nitrous oxide and nitrogen gas production and their nitrogen source. Finally, the RS was slightly refined using the experimental data and an AQUASIM implemented kinetic model. A chloraminated drinking water relevant RS is proposed and provides the abiotic reaction foundation for future AOB biotic experiments.

Direct gas injection method: a simple modification to an elemental analyzer/isotope ratio mass spectrometer for stable isotope analysis of N and C from N2O and CO2 gases in nanomolar concentrations.

RATIONALE: Stable isotope analyses of trace amounts of nitrous oxide gas require special instrumentation and laborious sample preparation methods that have hindered many laboratories from measuring this potent greenhouse gas. A simple modification to an Elemental Analyzer (EA) coupled to an Isotope Ratio Mass Spectrometry (IRMS) setup that allows users to measure the N and C isotopic ratios of nitrous oxide (N(2)O) and carbon dioxide (CO(2)) by injecting the gases directly into the EA is described. METHODS: The standard EA was fitted with a gas injection port and a home-made packed column filled with Hayesep Q polymer. A gas mixture of 3.1% N(2)O in helium (He) was injected directly into the EA. This method allowed large volumes of sample to be injected without saturating the column. RESULTS: The use of the home-made column resulted in better resolution of sample peaks and allowed smaller concentrations of the analyte to be injected. This study showed that this method produced accurate and reproducible stable isotope measurements with sample injection volumes ranging from 100 to 5000 µL, containing between 20 and 1000 nmol of analyte. CONCLUSIONS: This simple, inexpensive method can be useful for the laboratories that do not have access to more advanced and expensive interfaces to analyze nanomolar quantities of N(2)O and CO(2) from microbiological and ecological studies and offers a simple alternative for in-house measurements of these trace gases.