Reinvestigation of the acetone degradation mechanism in dilute aqueous solution by the UV/H2O2 process

Article abstract of DOI:10.1021/es9808548

A reinvestigation of the UV/H₂O₂ treatment of acetone has revealed previously undetected intermediates (pyruvic acid, pyruvic aldehyde, and hydroxyacetone). The time profiles of the concentrations of all intermediates have been determined, and a detailed mechanism for the degradation steps accounting for all detected intermediates is proposed. A kinetic model was developed on the basis of the proposed mechanism, and the predicted patterns of the reactants and intermediates are in good agreement with the experimental data. The application of the UV/H₂O₂ process to the degradation of acetone results in the eventual mineralization of all organic compounds, as demonstrated by TOC measurements.

Full text of DOI:10.1021/es9808548

Environ. Sci. Technol. 1999, 33, 870-873
Reinvestigation of the Acetone
Degradation Mechanism in Dilute
Aqueous Solution by the UV/H O
2 2
Process
M I H A E L A I . S T E F A N * A N D
J A M E S R . B O L T O N
Department of Chemistry, The University of Western Ontario,
London, Ontario, Canada N6A 5B7
A reinvestigation of the UV/H O treatment of acetone has
2
2
revealed previously undetected intermediates (pyruvic
acid, pyruvic aldehyde, and hydroxyacetone). The time
profiles of the concentrations of all intermediates have been
determined, and a detailed mechanism for the degradation
steps accounting for all detected intermediates is
FIGURE 1. Decay of acetone and time profiles of main intermediates.
Solid lines represent the computer modeling patterns.
range, as determined by potassium persulfate actinometry
(3) and the spectral emission of the UV lamp.
proposed. A kinetic model was developed on the basis of
the proposed mechanism, and the predicted patterns of
the reactants and intermediates are in good agreement with
the experimental data. The application of the UV/H O
Analytical Determ inations. The acetone and hydroxy-
acetone concentrations were determined by gas chroma-
tography using a model 6890 Hewlett Packard gas chro-
matograph and an HP Carbowax (30 m × 0.53 mm, 0.25 mm
thickness) column. The injector and detector (FID) temper-
atures were 250 and 260 °C, respectively, and helium was
used as the carrier gas. By running a temperature program
isothermally at 40 °C for 5 min then to 150 °C (hold for 2 min)
at a rate of 20 °C/ min, acetone and hydroxyacetone were
eluted out of the column at retention times of 1.8 and 9.1
min, respectively.
Organic acids were identified and quantified by ion-
exchange chromatography (IC) performed with a Dionex DX-
100 ion chromatograph (conductivity detector). Different IC
conditions were developed, depending on the nature of the
acids analyzed. An IonPac AS 14 (4 × 250 mm) ion exchange
column preceded by an AG 14 guard column was used for
the analysis of acetic, formic, and pyruvic acids with a 3 mM
Na2B4O7 solution as the eluent (isocratic flow of 1.45 mL
min-1). The same column was employed for the analysis of
oxalic acid but with 9 mM Na2B4O7 as the eluent (isocratic
flow of 1.65 mL min-1). Since under the above conditions
glyoxylic acid coelutes with acetic acid, the IonPac AS 10 (4
× 250 mm) analytical column preceded by the AG 10 guard
column with 80 mM NaOH solution as eluent was used for
the analysis of the glyoxylic acid concentration.
2
2
process to the degradation of acetone results in the eventual
mineralization of all organic compounds, as demonstrated
by TOC measurements.
Introduction
Since acetone is found in many contaminated waters, a kinetic
and mechanistic study was previously undertaken by the
authors on the degradation of acetone and its reaction
intermediates by the UV/ H2O2 process until complete
mineralization was achieved (1). Our recent reinvestigation
of the acetone/ H2O2 system under UV light indicated that,
apparently due to an analytical problem, some of the
degradation intermediates (pyruvic aldehyde, pyruvic acid,
and hydroxyacetone) had been overlooked, and consequently
the proposed reaction mechanism was incomplete.
Since the above-mentioned paper has been published,
only one other paper has emerged in the literature on the
treatment of ketones in contaminated waters, but the process
used was UV/ O3 (2); however, it does not offer a detailed
mechanism of acetone degradation.
The present work presents a complete reaction scheme
for the treatment of acetone by the UV/ H2O2 process under
which any organic compound is eventually mineralized.
Aldehydes were quantitatively determined as hydrazones
by an HPLC method (4). Formaldehyde was also estimated
by Hantzsch reaction (5), and the results agreed very well
with the HPLC measurements. Hydrogen peroxide was
destroyed with 0.5% catalase aqueous solution (5 µL/ 20 mL
of sample) in those samples subjected to analysis for organic
acids and aldehydes. Hydrogen peroxide and the total organic
carbon have been analyzed as mentioned previously (1).
Experimental Section
Reagents and Materials. All chemicals (analytical reagent
grade) were used without any further purification, as
mentioned previously (1).
Apparatus. The Rayox reactor and the method for the
calculation of the fraction of light absorbed individually by
acetone and hydrogen peroxide have been described previ-
ously in detail (1). The total incident photon flux entering
the reactor from the UV-Vis 1 kW medium-pressure Hg lamp
was (2.18 ( 0.10) × 10-4 einstein s-1 within the 200-300 nm
Results and Discussion
Acetone Decay and Degradation Products. When a ∼1.1
mM acetone and 15 mM H2O2 aqueous solution was
irradiated, acetic, pyruvic, and oxalic acids and pyruvaldehyde
were identified and quantified as major degradation products
(Figure 1), whereas formic and glyoxylic acids, hydroxyac-
etone, and formaldehyde were considered as minor products
(Figure 2). A list for the rate constants for the reaction of •OH
* Corresponding author e-mail: mstefan@julian.uwo.ca; tel: (519)-
663-3178; fax: (519)661-3022.
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10.1021/es9808548 CCC: $18.00
1999 Am erican Chem ical Society
Published on Web 01/26/1999

SCHEME 1. Degradation Pathways of Acetone to the Primary
Intermediates
FIGURE 2. Time profiles of minor intermediates. Solid lines represent
the computer modeling patterns.
producing pyruvaldehyde; or fragmentation to the aceton-
yloxyl radical (4). The latter undergoes â-scission to form-
aldehyde and the acetyl radical (9), disproportionation in
the solvent cage, or fast isomerization by 1,2-H shift (water-
assisted process) to a carbon-centered radical (7), which
finally leads to pyruvic aldehyde (5) through HO2• elimination.
The presence of acetic acid at very early stages of
irradiation indicates that it is a primary product. It could be
formed by cross-termination reactions, as suggested by
Zegota et al. (8) and as shown below:
CH3C(O)O2• + •O2CH2COCH3 f
CH3COO- + H+ + CH3COCHO + O2 (1)
FIGURE 3. Total organic carbon (TOC) balance.
CH3C(O)O2• + CH3O2• f
radicals with the organic compounds present in the inves-
tigated system was given in the previous paper (1). The
missing value therein and known in the literature is that for
pyruvate ion {k ) 3.1 × 107 M-1 s-1 (6); pKa ) 2.93 (7)}.
The TOC balance (Figure 3) confirms that all significant
intermediates have been identified, quantified, and eventually
degraded.
CH3COO- + H+ + HCHO + O2 (2)
Reaction 1 should prevail over reaction 2 due to the higher
concentration of acetonylperoxyl radicals (2) as compared
with that of methylperoxyl radicals (11). The latter would
lead primarily to formaldehyde (11). Pyruvic aldehyde (5)
exists mostly in its hydrated form (12) and is assumed to
Reaction Mechanism during the UV/H2O2 Treatm ent of
Acetone. As noted in the Introduction, the reaction mech-
anism proposed in the previous paper (1) did not account
for the formation of pyruvic aldehyde, hydroxyacetone, and
pyruvic acid. We propose a new mechanism for the generation
of the acetone degradation products, as outlined in Schemes
1 and 2 and eqs 1 and 2. The degradation paths toward the
complete mineralization of some of byproducts were de-
scribed previously (1) and will not be repeated in this paper.
Pyruvic aldehyde (5), hydroxyacetone (6), formaldehyde
(8), and acetic acid (16) are the primary degradation
intermediates as generated from the acetonylperoxyl radical
(2). Zegota et al. (8) measured the bimolecular decay rate of
the acetonylperoxyl radical as 2k ) 8 × 108 M-1 s-1 in pulse
radiolysis studies. The formation of a tetroxide (3) is generally
accepted in peroxyl radical chemistry (9). It can convert
through various pathways, such as disproportionation (Rus-
sell mechanism), to pyruvic aldehyde (5) and hydroxyacetone
(6); H2O2 elimination by an electrocyclic process (10)
•
react very rapidly with OH radicals by H-abstraction at the
formyl group (Scheme 2), being oxidized to pyruvic acid (14).
Similarly, hydroxyacetone (6) should be oxidized to pyruvic
acid through pyruvic aldehyde as an intermediate.
Scheme 2 includes the proposed mechanism for the
degradation of pyruvic acid, mostly present in its ionized
form. Dark experiments conducted on a 0.25 mM pyruvate/ 5
mM H2O2 system indicated a significant conversion of
pyruvate to acetate, for which a rate constant of 0.11 M-1 s-1
was calculated. The reaction consists of the addition of H2O2
to the double bond of the carbonyl group leading to the
adduct 15, which decarboxylates by C-C bond cleavage.
Similar behavior has been observed with glyoxylic acid (1,
12, 13) and could explain the rapid decay of pyruvate as
compared to that expected only from the light-induced
oxidation.
Reactions with •OH radicals provide other routes of acetic
acid (16) generation from pyruvic acid. One of the two
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SCHEME 2. Degradation Pathways of the Intermediates
SCHEME 3. Simplified Reaction Scheme and Branching
Ratios Used in the Kinetic Modeling
constants for the reaction of the •OH radical with each
compound and also on their relative concentrations at a given
irradiation time.
Kinetic Model. A kinetic model was developed using the
same assumptions as in the earlier paper (1), except that the
calculation was performed on an Excel spreadsheet with time
intervals of 2 s. A value of G ) 1.88 × 10-4 einsteins s-1 and
a truncated reaction scheme as shown in Scheme 3 were
used. The rate constants were the same as those listed in the
earlier paper (1) with the following additions:
possible attack sites of •OH radicals, depicted in Scheme 2,
is at the methyl group generating the carbon-centered radical
18. â-Scission of radical 18 leading to ketene (19) should
prevail over reaction with oxygen (dashed arrow route in
Scheme 2), since ketomalonaldehyde (23) and ketomalonic
acid were not detected among the degradation products.
The ketene (19) instantly generates acetic acid by hydration.
Since acetic acid is rapidly generated from pyruvic acid and
builds up in the solution because of its slow oxidation rate
by the •OH radicals, it is hard to distinguish whether the low
levels of formaldehyde (maximum 0.02 mM) detected during
the irradiation of an aqueous solution of 0.7 mM pyruvate
and 8 mM H2O2 result from the â-scission of the oxyl radical
21 or from an acetate oxyl radical.
•OH + pyruvaldehyde 7.0 × 108 M-1 s-1 (6)
•OH + pyruvic acid 6.0 × 107 M-1 s-1
(assumed) pKa ) 2.93 (7)
Considering the behavior of ketomalonic acid in aqueous
solution (14), pyruvic acid should exist in solution to a certain
extent in its hydrated form. Therefore, H-abstraction from
the hydrated pyruvate leading to the oxygen-centered radical
17 is also possible, followed by C-C bond cleavage with the
generation of the acetate ion (16) and carboxyl anion radical.
The reaction schemes for the degradation of the acetic,
formic, glyoxylic, and oxalic acids to CO2 and H2O are given
in previous papers (1, 15) and references therein. However,
we should mention that in those reactions the organic acids
(except for acetic acid, pKa ) 4.75) are mostly in their ionized
forms considering the pH values measured during the
treatment. In the case of oxalic acid, the monoionized form
is preponderant, which reacts with the •OH radical by electron
transfer:
•OH + pyruvate 6.0 × 107 M-1 s-1 (assumed)
pyruvic acid + H2O2 (dark) 0.11 M-1 s-1 (experimental)
•OH + gyloxylic acid 1.5 × 108 M-1 s-1 (assumed)
•OH + hydroxyacetone 8.0 × 108 M-1 s-1 (assumed)
The model fits in the figures are very satisfactory. The
disagreement exhibited in the decay curves for oxalic and
glyoxylic acids may be due to the contribution of the
intermediates to the total light absorbed in the system by the
end of the experiment, which is not accounted by the model.
Note that the decays of acetone and H2O2 are very well
predicted without the assumptions of recycling of acetone
and a lower quantum yield (0.8 vs 1.0 used here) for H2O2
photolysis as were made in the previous paper (1).
-
HOOCCOO- + •OH f H2O + CO2 + •CO2
(3)
The calculated fractions of UV light absorbed by H2O2
and the quantified organic compounds at different irradiation
times show that H2O2 remains the main absorber during the
experiment, so that direct photolysis of acetone and of
degradation byproducts cannot be expected. Consequently,
the oxidation processes are strongly dependent on the rate
Acknowledgments
This work was supported financially by a Collaborative
Research and Development Grant jointly funded by the
Natural Science and Engineering Research Council of Canada
and Calgon Carbon Corporation, Markham, Ontario, Canada.
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(8) Zegota, H.; Schuchmann, M. N.; Schulz, D.; von Sonntag, C. Z.
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(9) Peroxyl Radicals, 1st ed.; Alfassi, Z. B., Ed.; John Wiley & Sons:
Chichester, England, 1997.
We are thankful to Dr. Stephen Cater, Dr. Ali Safarzadeh-
Amiri, and Mr. Keith Bircher of Calgon Carbon Corporation
for their helpful comments and support during this research.
(10) von Sonntag, C.; Schuchmann, H.-P. Angew. Chem., Int. Ed.
Engl. 1991, 30, 1229.
(11) Schuchmann, H.-P.; von Sonntag, C. Z. Naturforsch. 1984, 39B,
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(12) von Sonntag, C.; Schuchmann, H.-P. In Peroxyl Radicals; Alfassi,
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Received for review August 19, 1998. Revised manuscript
received December 15, 1998. Accepted December 21, 1998.
ES9808548
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