The Effects of Iron on Optical Properties of Dissolved Organic Matter

Poulin, Brett Albert

Graduate Thesis Or Dissertation

The Effects of Iron on Optical Properties of Dissolved Organic Matter Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/jm214p573

Abstract

Although iron is a recognized source of spectroscopic interference in the analysis of dissolved organic matter (DOM), its effects on the absorption and fluorescence of DOM are poorly defined. Here, iron(II) and iron(III) titration experiments (0-1.5 mg L^-1) were performed with two DOM isolates and two surface water samples collected from environments spanning a range of DOM source materials. Changes in DOM UV-vis absorption and fluorescence properties were characterized. The effects of iron(II) on DOM UV-vis absorption coefficients were negligible. Additive effects of iron(III) on DOM UVvis absorption coefficients were observed for all samples independent of DOM composition. Iron(III) extinction coefficients were established at prominent UV-vis wavelengths utilized for DOM characterization. Consequently, UV-vis absorption by iron(III) increased the measured specific UV absorbance at λ = 254 nm (SUVA₂₅₄) and decreased the measured spectral slope ratio (S_R) and absorption ratio at λ = 250 to 365 nm (E₂:E₃) of all DOM samples. In contrast, both iron(II) and iron(III) quenched the fluorescence of the DOM isolates and surface water samples at pH 6.7. The degree and location of fluorescence quenching varied with the iron:DOC ratio and between DOM source materials. Regions of the fluorescence excitation-emission matrix (EEM) spectra associated with greater DOM conjugation were found to be more susceptible to iron quenching, and DOM fluorescence indices were sensitive to the presence of iron. The analysis of EEMs using a 7- and 13-component parallel factor analysis (PARAFAC) model showed low PARAFAC sensitivity to iron addition. Acidification of samples to pH ≤ 3 minimized quenching by iron.

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