Date of Graduation

12-2021

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Engineering (PhD)

Degree Level

Graduate

Department

Civil Engineering

Advisor

Julian L. Fairey

Committee Member

David G. Wahman

Second Committee Member

Wen Zhang

Third Committee Member

Justin Chimka

Keywords

Chloramine Chemistry, NDMA, Unified Model

Abstract

NDMA occurrence and formation pathways in drinking water systems are reviewed and NDMA yields are compared on the basis of disinfectant type, water chemistry, and precursor category. In chloramination, despite monochloramine being the predominant species between pH 7-9, evidence suggests that dichloramine is the primary species involved in NDMA formation. This is somewhat confounding as NDMA yields are maximal at pH 9, yet at pH 9 dichloramine decays faster than it forms and hence is present at trace levels; additionally, the proposed mechanism involves a spin-forbidden incorporation of dissolved oxygen as a triplet, which is presumably kinetically slow. This review reveals that kinetic data for NDMA formation is lacking, and its influence on chloramine chemistry has not been carefully considered.

In pH 7-10 waters amended with 10 μM total dimethylamine and 800 μeq Cl2.L–1 dichloramine (NHCl2), NDMA, nitrous oxide (N2O), dissolved oxygen (DO), NHCl2, and monochloramine (NH2Cl) were kinetically quantified. NHCl2, N2O, and DO profiles indicated reactive nitrogen species (RNS) formed during NHCl2 decomposition, including nitroxyl/nitroxyl anion (HNO/NO−) and peroxynitrous acid/peroxynitrite anion (ONOOH/ONOO–). Experiments with uric acid (an ONOOH/ONOO– scavenger) implicated ONOOH/ONOO– as a central node for NDMA formation, which was further supported by concomitant N-nitrodimethylamine formation. A kinetic model accurately simulated NHCl2, NH2Cl, NDMA, and DO concentrations and included (1) the unified model of chloramine chemistry revised with HNO as a direct product of NHCl2 hydrolysis, (2) HNO/NO− then reacting with (i) HNO to form N2O, (ii) DO to form ONOOH/ONOO–, or (iii) NHCl2 or NH2Cl to form nitrogen gas, and (3) NDMA formation via ONOOH/ONOO– or their decomposition products reacting with (i) dimethylamine (DMA) and/or (ii) chlorinated unsymmetrical dimethylhydrazine (UDMH-Cl), the product of NHCl2 and DMA.

The role of DO was further examined at pH 9 by assessing kinetic profiles of NHCl2 and NDMA under ambient DO (~280 μM) and low-DO (< 20 μM) conditions in the presence and absence of 10 μM TOTDMA. Uric acid completely shut down NDMA formation under the low-DO condition, validating ONOOH/ONOO− as the central node in NDMA formation. Yield experiments with initial NHCl2 of 200-, 400-, and 800 μeq Cl2.L–1 tracked the formation of NH3/NH4+, NH2Cl, N2O, N2, NO2−, and NO3−. NH3/NH4+ yields were 20–40% greater under the low-DO condition, implying a reaction occurred between NH3/NH4+ and ONOOH/ONOO− or its decomposition products. NH2Cl yields were 16–20% lower under the low-DO condition, revealing a previously unknown NH2Cl formation reaction. Under ambient DO conditions, about 80% of the nitrogen was accounted for compared to the low-DO conditions in which nitrogen recoveries were 90- and 100% in the absence and presence of 10 μM TOTDMA, respectively. An existing mechanistic model accurately predicted NH3/NH4+, NH2Cl, and N2 under ambient conditions but underpredicted N2O and overpredicted NO2− and NO3−. The results provide a framework to guide future experiments with ONOOH/ONOO− generators and revise the mechanistic model to better capture the nitrogenous end-products.

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