On the Use of Hydroxyl Radical Kinetics to Assess the Number-Average Molecular Weight of Dissolved Organic Matter

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(•)) quenching by DOM. HO(•) is often assumed to be relatively unselective, reacting with nearly all organic molecules with similar rate constants. Literature values for HO(•) reaction with organic molecules were surveyed to assess the unselectivity of DOM and to determine a representative quenching rate constant (k(rep) = 5.6 × 10(9) M(-1) s(-1)). This value was used to assess the average molecular weight of various humic and fulvic acid isolates as model DOM, using literature HO(•) quenching constants, kC,DOM. The results obtained by this method were compared with previous estimates of average molecular weight. The average molecular weight (Mn) values obtained with this approach are lower than the Mn 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(•), 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(•) 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(•) 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.

Molecular Composition and Photochemical Reactivity of Size-Fractionated Dissolved Organic Matter

“…Measurements of the molecular weight of SRFA determined with field flow fractionation (number-averaged: 1150 g/mol; weight-averaged: 1910 g/mol), 35 SEC (number-averaged: 1360 g/mol; weight-averaged: 2310 g/mol), 27 reactivity with hydroxyl radical (number-averaged: 760 g/mol), 36 and vapor pressure osmometry (number-averaged: 829 g/mol) 38 report number-averaged molecular weights near 1 kDa and weight-averaged molecular weights near 2 kDa. However, 87 ± 1% of SRFA DOC (2.6–35.3 mg-C/L) was retained by a 3 kDa membrane, and 22% was retained by a 5 kDa membrane at pH 7 (Figure S1d).…”

“…This agrees with previous observations of lower MW for PLFA compared to SRFA, as well as lower MW for DOM derived from autochthonous sources compared to allochthonous sources. 25,35,36,71 …”

“…This suggests that PLFA has a lower average molecular weight than SRFA, in agreement with comparisons based on radical reactivity and SEC. 36,71 …”

Trace Element Removal in Distributed Drinking Water Treatment Systems by Cathodic H2O2 Production and UV Photolysis

“… 36 Experiments were performed at a scan rate of 10 mV s –1 starting from the open-circuit potential (∼0 to −0.1 V) and scanned cathodically over a 2 V potential range. CVs were collected for electrolytes containing Suwannee River NOM (90 mg L –1 ; ∼ 45 mgC L –1 ) 37 in the presence of 1 mM Ca(Cl) 2 , Mg(Cl) 2, and/or Fe(Cl) 3 . Spectra were corrected for the background spectra of DMSO.…”

Effects of weak organic acids on the size distribution and size-dependent metal binding of humic substances as studied by flow field-flow fractionation

“…It is known that larger size humic molecules are less fluorescent compared with smaller size humic molecules [39]. Appiani et al [40] have also demonstrated that the average molecular weight estimated by means of the quenching rate constant of HS is smaller than the one that would be expected by using other techniques. These earlier studies provide potential support for the idea that the fluorescent fraction of HS would be distributed to the smaller sized region.…”