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

The photochemical production of reactive species, such as triplet dissolved organic matter (3DOM) and singlet oxygen (1O2), contributes to the degradation of aquatic contaminants and is related to an array of DOM structural characteristics, notably molecular weight. In order to relate DOM molecular weight, optical properties, and reactive species production, Suwannee River (SRFA) and Pony Lake fulvic acid (PLFA) isolates are fractionated by sequential ultrafiltration, and the resultant fractions are evaluated in terms of molecular composition and photochemical reactivity. UV–visible measurements of aromaticity increase with molecular weight in both fulvic acids, while PLFA molecular weight fractions are shown to be structurally similar by Fourier-transform ion cyclotron resonance mass spectrometry. In addition, Bray–Curtis dissimilarity analysis of formulas identified in the isolates and their size fractions reveal that SRFA and PLFA have distinct molecular compositions. Quantum yields of 3DOM, measured by electron and energy transfer probes, and 1O2 decreased with molecular weight. Decreasing [3DOM]ss with molecular weight is shown to derive from elevated quenching in high molecular weight fractions, rather than increased 3DOM formation. This work has implications for the photochemistry of waters undergoing natural or engineered treatment processes that alter DOM molecular weight, such as photooxidation and biological degradation.