On the
Use of Hydroxyl Radical Kinetics to Assess
the Number-Average Molecular Weight of Dissolved Organic Matter
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Abstract
Dissolved organic matter (DOM) is
involved in numerous environmental
processes, and its molecular size is important in many of these processes,
such as DOM bioavailability, DOM sorptive capacity, and the formation
of disinfection byproducts during water treatment. The size and size
distribution of the molecules composing DOM remains an open question.
In this contribution, an indirect method to assess the average size
of DOM is described, which is based on the reaction of hydroxyl radical
(HO<sup>•</sup>) quenching by DOM. HO<sup>•</sup> is
often assumed to be relatively unselective, reacting with nearly all
organic molecules with similar rate constants. Literature values for
HO<sup>•</sup> reaction with organic molecules were surveyed
to assess the unselectivity of DOM and to determine a representative
quenching rate constant (<i>k</i><sub>rep</sub> = 5.6 ×
10<sup>9</sup> M<sup>–1</sup> s<sup>–1</sup>). This
value was used to assess the average molecular weight of various humic
and fulvic acid isolates as model DOM, using literature HO<sup>•</sup> quenching constants, <i>k</i><sub>C</sub>,<sub>DOM</sub>. The results obtained by this method were compared with previous
estimates of average molecular weight. The average molecular weight
(<i>M</i><sub>n</sub>) values obtained with this approach
are lower than the <i>M</i><sub>n</sub> measured by other
techniques such as size exclusion chromatography (SEC), vapor pressure
osmometry (VPO), and flow field fractionation (FFF). This suggests
that DOM is an especially good quencher for HO<sup>•</sup>,
reacting at rates close to the diffusion-control limit. It was further
observed that humic acids generally react faster than fulvic acids.
The high reactivity of humic acids toward HO<sup>•</sup> is
in line with the antioxidant properties of DOM. The benefit of this
method is that it provides a firm upper bound on the average molecular
weight of DOM, based on the kinetic limits of the HO<sup>•</sup> reaction. The results indicate low average molecular weight values,
which is most consistent with the recent understanding of DOM. A possible
DOM size distribution is discussed to reconcile the small nature of
DOM with the large-molecule behavior observed in other studies