hydrolysis of its formate esters does not occur within the pH
range generated under our experimental conditions.
Degradation Pathways for Form ic, Acetic, Glycolic, and
Oxalic Acids. Formic acid is a degradation product both of
1,4-dioxane and of some intermediates generated during the
UV/ H2O2 treatment of the investigated system. The rate
on the •OH rate constants for the reaction of individual
compounds, the relative concentrations of intermediates and
the oxygen level in the solution.
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 Oxidation Technologies, Markham,
Ontario, Canada. We are very grateful to Dr. Dave Budac for
carrying out the mass spectroscopic measurements. We
extend our thanks to Dr. Stephen Cater, Dr. Ali Safarzadeh-
Amiri and Mr. Keith Bircher, P. Eng., of Calgon Carbon
Oxidation Technologies for their helpful comments and
support during the conduct of this research. The comments
of the reviewers were highly appreciated.
•
constants of HCOOH/ HCOO- for reaction with OH radicals
are as high as that for 1,4-dioxane. At very early irradiation
times (1-3 min), the 1,4-dioxane concentration is still high
•
enough to scavenge most of the OH radicals generated by
the direct photolysis of H2O2, but then, formic acid becomes
•
an effective scavenger of OH radicals. Literature reports
(21, 57, 58) have emphasized the role of oxygen on the rate
and the mechanism of the photooxidation of formic acid in
aqueous solution in the presence of hydrogen peroxide. In
our system, where the oxygen concentration is strongly
depleted to ∼2.0 × 10-5 M after the first few minutes of
irradiation while the H2O2 concentration is still high (∼0.015-
0.012 M), the formoyl radicals (47) generated in the H-
abstraction reaction are undoubtedly involved in competitive
reactions as shown in Scheme 6.
The generation of hydroxyl radicals in anaerobic systems
through the reaction of •COOH radicals with hydrogen
peroxide plays an important role in enhancing the rate of
organic pollutant removal.
Experiments carried out on the degradation of acetic acid
(12) by the UV/ H2O2 process for similar concentration
conditions as in the present study indicated only small
amounts of glyoxylic and oxalic acids as intermediates.
Therefore, we assume that the route involving the alkoxyl
radical (50) is followed preponderantly (Scheme 7).
The same reaction products of acetic acid degradation
either in pulse radiolysis studies (59) or by a UV/ H2O2
treatment (23) have been reported in the literature.
Test experiments carried out on the oxidation of glycolic
acid (21), initiated by •OH radical attack, indicated a very fast
decay of glycolic acid with the generation of formic (23),
glyoxylic (40) and oxalic (41) acids as byproducts toward a
complete mineralization. The amount of oxalic acid gener-
ated was high, an observation which supports the above
statements regarding the degradation pathways of acetic acid.
The reaction steps approaching the degradation of these acids
are given in Scheme 8.
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•
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All the observed intermediates in the 1,4-dioxane treat-
ment by the UV/ H2O2 system in aqueous solution are weak
absorbers of UV light. Considering the initial H2O2 con-
centration and its decay during the irradiation period, one
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9
1 5 9 4 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 32, NO. 11, 1998