<p>Organic nitrogen compounds are
important in environmental systems because they are prevalent in natural waters
but are also components of polymers within membrane filters that are used for
water treatment. In both of these cases, these compounds can be exposed to free
chlorine during disinfection, which can trigger a set of reactions that can
form a host of different halogenated by-products. When such by-products form
during water treatment disinfection, these by-products, known as nitrogen-based
disinfection by-products (N-DBPs), can be highly toxic and affect human and
ecosystem health. Alternatively, when such reactions occur during membrane filtration,
the organic nitrogen compounds, which are embedded within the upper layer
polymer structure of the membrane filter, can degrade when free chlorine is
applied. Therefore, this research was aimed at exploring the chemistry behind
how specific types of organic nitrogen compounds which are found in these
applications, such as tertiary amines and amides, react with free chlorine. It
particularly focused on assessing the kinetics and by-product formation of
these reactions under variable water quality conditions (e.g., pH, halide
concentrations, and precursor doses).</p>
<p> </p>
<p>More specifically, in the first phase of this work, the roles
of tertiary amines in enhancing disinfection by-product (DBP) formation, such
as trihalomethanes (THMs) and haloacetic acids (HAAs), during chlorination of
aromatic compounds were studied. The results indicated that in synthetic
solutions, chloroform (CHCl<sub>3</sub>) and trichloroacetic acid (TCAA) were
enhanced by up to 20× with tertiary amines at low dose ([tertiary amine]<sub>0</sub>
= 0.5×[aromatic compound]<sub>0</sub>). The enhancement effect was also
dependent on the aromatic compound type, tertiary amine type and dose, and
water conditions such as pH and bromide concentrations. Thus, THMs and HAAs
were predicted to be enhanced when the aromatic compound reacted with R<sub>3</sub>N-X<sup>+</sup>
(X=Br or Cl) and was not outcompeted by aromatic compound or tertiary amine
reaction with free chlorine or bromine alone. In the second phase of this work,
the reaction kinetics, by-product formation, and overall mechanisms of a
polyamide-based monomer with chlorine were evaluated under varying water
conditions. The current known mechanism, Orton Rearrangement, was reevaluated,
and new mechanisms were proposed, where it was found that N-halogenation and
ring halogenation were two independent pathways. The ability to choose either
pathway was highly dependent on the water quality condition of the aqueous
solution. The roles of different chlorinating/brominating agents were also
investigated where certain species-specific rate constants were obtained. For
the N-halogenation pathway, only chlorination and no bromination occurred in
which the reactivity of the chlorinating agents likely decreased such that ClO<sup>-</sup>>HOCl.
However, for the ring halogenation pathway, both chlorination and bromination
occurred in which the reactivity of the chlorinating and brominating agents
decreased such that Cl<sub>2</sub> >HOCl, and BrCl > BrOCl > Br<sub>2</sub>
> Br<sub>2</sub>O > HOBr, respectively. Overall, this study suggests that
a number of unique reactions can occur for various types of organic nitrogen
compounds which: (i) allow them to affect water quality by enhancing DBP
formation, (ii) but, when integrated into a polymer matrix used for water
treatment, can induce reactions that lead to permanent structural damage of the
polymer. In all cases, the extent of these reactions is strongly governed by
the surrounding water matrix.</p>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/7404197 |
| Date | 17 January 2019 |
| Creators | Kun Huang (5929778) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/Organic_Nitrogen_Reactivity_with_Free_Chlorine_Effects_on_Disinfection_by-product_Formation_and_Polyamide_Membrane_Stability/7404197 |
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