The rate constants for oxidizing ADNTs are not known, but their range can be estimated from
known compounds and structure activity relationships (SARs). For example, the rate constant for
electron-withdrawing nitro groups and one electron-donating methyl group, so a rate constant for
ozone oxidation of ADNT should be in the 104 to 107 M1 s1 range. Estimation of the hydroxyl
radical oxidation rate constant is possible because the related compound, p-nitroaniline, is known
to have a hydroxyl radical rate constant of 1010 M1 s1 (Buxton and Greenstoch 1988). Therefore,
the rate constant for ADNT oxidation by hydroxyl radical is expected to be in the 109 M1 s1 range.
In peroxone oxidizing systems, oxidant concentrations range from 104 M for ozone to 1014 M for
hydroxyl radical. We have used these values as a guide for our rate determination experiments.
When rate constants exceed 104 M1 s1, half-lives become extremely short (≈ 1 s) and it is diffi-
cult to measure oxidant and substrate loss. This problem can be solved by competitive oxidation
of the substrate (S) in the presence of a reference compound (R) with a known rate constant for
oxidation by a specific oxidant. In this case, the rate constant for the oxidation of the substrate is
kS = kR ln [(So)/(St)]/ln [(Ro)/(Rt)]
(4)
where kS and kR are the rate constants for the substrate and reference chemicals, respectively. For
this study, we have used this approach to measure rate constants for the oxidation of 2-ADNT and
4-ADNT.
OBJECTIVES
The objectives of this study were to examine the kinetics of oxidative transformation of 4-
ADNT and 2-ADNT by the peroxone advanced oxidation process, identify stable end-products,
and attempt to understand the oxidative pathway. We are interested in establishing rates of trans-
formation and stable end-products resulting from the process and the mechanism by which these
transformations occur.
EXPERIMENTAL METHODS
Ozone oxidations
Ozone oxidations were conducted in aqueous solutions adjusted to pH 5.0 with phosphate
buffer. Because the acid dissociation constants (pKas) of 2-ADNT and 4-ADNT are 1.23 and 0.59
(Glover et al. 1977), respectively, the amines are in their free form at this pH. To inhibit concurrent
hydroxyl radical transformations, some studies were conducted in the presence of t-butyl alcohol
as a hydroxyl radical scavenger (Hoigne and Bader 1983). Ozone was generated from a Welsbach
ozone generator and bubbled into water. Ozone concentration was determined by a standard col-
orimetric technique with indigo (Bader and Hoigne 1981).
An ozone stock solution was added in 20-M increments to a rapidly stirred stock solution of
ADNT (150 M) and a reference chemical (nitrite or resorcinol). After each addition, the ADNT
and resorcinol were analyzed by high-performance liquid chromatography (HPLC), and nitrite
and nitrate by ion chromatography (Thayer and Huffaker 1980). Aldehydes and ketones were
analyzed as their 2,4-dinitrophenylhydrazones according to the methods of Kieber and Mopper
(1990). Carboxylic acids were analyzed by ion chromatography with a conductivity detector and
by ion chromatography with UV detection as described below.
Ion chromatography with UV detection
Instrument: HP1081B liquid chromatograph
Column: Supelcogel C-610H (sulfonated polystyrene divinylbenzene), 7.8 300 mm
2
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