Investigation of the Coupled Effects of Molecular Weight and Charge-Transfer Interactions on the Optical and Photochemical Properties of Dissolved Organic Matter

Abstract

We studied the formation of photochemically produced reactive intermediates (RI) from dissolved organic matter (DOM). Specifically, we focused on the effects of variable molecular weight and chemical reduction on the optical properties of DOM (absorbance and fluorescence) and the formation of singlet oxygen ((1)O2), DOM triplet excited states ((3)DOM*), and the hydroxyl radical ((•)OH). The data are largely evaluated in terms of a charge-transfer (CT) model, but deficiencies in the model to explain the data are pointed out when evident. A total of two sets of samples were studied that were subjected to different treatments; the first set included secondary-treated wastewaters and a wastewater-impacted stream, and the second was a DOM isolate. Treatments included size fractionation and chemical reduction using sodium borohydride. Taken as a whole, the results demonstrate that decreasing molecular weight and borohydride reduction work in opposition regarding quantum efficiencies for (1)O2 and (3)DOM* production but in concert for fluorescence and (•)OH production. The optical and photochemical data provide evidence for a limited role of CT interactions occurring in lower-molecular-weight DOM molecules. In addition, the data suggest that the observed optical and photochemical properties of DOM are a result of multiple populations of chromophores and that their relative contribution is changed by molecular-weight fractionation and borohydride reduction.

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

“…14 The relationship between 3 DOM production efficiency and molecular weight has also been expressed as linear regressions of 1 O 2 quantum yields with E 2 : E 3 and slopes reported here agree with published results. Comparisons of diverse DOM samples have reported slopes of Φ 1O2 with E 2 : E 3 ranging from 0.00196 to 0.0159 13,14,16,31 that bound our observed overall slope for both SRFA and PLFA (Φ 1O2 = 0.0137 × E 2 : E 3 − 0.0455). Slopes of Φ 3DOM and f TMP with E 2 : E 3 in ultrafiltration-fractionated DOM samples have not been previously reported.…”

“…14 Similarly, inverse relationships between Φ 1O2 and apparent MW in ultrafiltration-fractionated DOM samples have been observed in wastewater effluent, 31 Mississippi River DOM, 43 and SRFA. 14 The relationship between 3 DOM production efficiency and molecular weight has also been expressed as linear regressions of 1 O 2 quantum yields with E 2 : E 3 and slopes reported here agree with published results. Comparisons of diverse DOM samples have reported slopes of Φ 1O2 with E 2 : E 3 ranging from 0.00196 to 0.0159 13,14,16,31 that bound our observed overall slope for both SRFA and PLFA (Φ 1O2 = 0.0137 × E 2 : E 3 − 0.0455).…”

“…In DOM fractionated using ultrafiltration, quantum yields of 3 DOM 14,42 and 1 O 2 31,43 are consistently enhanced in low MW fractions. Conversely, 3 DOM and 1 O 2 steady-state concentrations have been observed to increase, 32 decrease, 17,44,45 or remain constant 31,46 with molecular weight.…”

“…The production of reactive species varies with DOM optical properties, 13–16 molecular weight (MW), 14,17 and composition. 18,19 These structural variations derive from disparities in source (i.e., allochthonous or autochthonous) 20 and physical/chemical processing (e.g., irradiation or ozonation).…”

“…27 The specific-UV absorbance at 254 nm (SUVA 254 ) correlates with DOM aromaticity, as determined by the relative signal intensity in the aromatic region (110–160 ppm) of the 13 C NMR spectrum, and with molecular weight. 27–30 Quantum yields of 3 DOM 14,15 and 1 O 2 13–16,31 are consistently shown to increase with E 2 : E 3 , while reactive species steady-state concentrations generally increase with solution absorbance (i.e., the concentration of dissolved organic carbon; [DOC]). 17,32 …”