Abstract
Abstract. Modern fluorescence spectroscopy methods, including excitation–emission matrix (EEM) spectra parsed using parallel factor analysis (PARAFAC) statistical approaches, are widely used to characterize dissolved organic matter (DOM) pools. The effect of soluble reduced iron, Fe(II), on EEM spectra can be significant but is difficult to quantitatively assign. In this study, we examine the effects of Fe(II) on the EEM spectra of groundwater samples from an anaerobic deltaic aquifer containing up to 300 mg L−1 Fe(II), located a few kilometres from the ocean and adjacent to the Fraser River in Richmond, British Columbia, Canada. We added varying quantities of Fe(II) into groundwater samples to evaluate Fe(II)–DOM interactions. Both the overall fluorescence intensity and the intensity of the primary peak, a humic-like substance at excitation and emission wavelengths of 239 and 441–450 nm (peak A), respectively, decreased by approximately 60 % as Fe(II) concentration increased from 1 to 306 mg L−1. Furthermore, the quenching effect was nonlinear and proportionally stronger at Fe(II) concentrations below 100 mg L−1. This nonlinear relationship suggests a static quenching mechanism. In addition, DOM fluorescence indices are substantially influenced by the Fe(II) concentration. With increasing Fe(II), the fluorescence index (FI) shifts to higher values, the humidification index (HIX) shifts to lower values, and the freshness index (FrI) shifts to higher values. Nevertheless, the 13-component PARAFAC model showed that the component distribution was relatively insensitive to Fe(II) concentration; thus, PARAFAC may be a reliable method for obtaining information about the DOM composition and its redox status in Fe(II)-rich waters. By characterizing the impacts of up to 300 mg L−1 Fe(II) on EEMs using groundwater from an aquifer which contains similar Fe(II) concentrations, we advance previous work which characterized impacts of lower Fe(II) concentrations (less than 2 mg L−1) on EEMs.
Highlights
Fluorescence spectroscopy has been widely used to characterize the properties of dissolved organic matter as it is highly sensitive to the structures and functional chemistry of aquatic organic matter (Baker and Spencer, 2004; Fellman et al, 2010; Helms et al, 2008; Stedmon and Bro, 2008; Weishaar et al, 2003)
60 % of the fluorescence intensity found in the 1 mg L−1 Fe(II) experimental solution was quenched in the 306 mg L−1 Fe(II) experimental solution
The fluorescence intensity decreased more rapidly at lower Fe(II) concentrations: as Fe(II) increased from 1 to 101 mg L−1 the OFI decreased by ∼ 40 % and as Fe(II) increased from 101 to 306 mg L−1 the OFI decreased by an additional ∼ 20 %
Summary
Fluorescence spectroscopy has been widely used to characterize the properties of dissolved organic matter as it is highly sensitive to the structures and functional chemistry of aquatic organic matter (Baker and Spencer, 2004; Fellman et al, 2010; Helms et al, 2008; Stedmon and Bro, 2008; Weishaar et al, 2003). Jia et al.: Technical note: Effects of iron(II) on fluorescence properties of DOM emission matrix (EEM) is prepared by systematically repeating the measurements at a range of different excitation and emission wavelengths These measurements are highly sensitive to the structures and functional chemistry of aquatic organic matter, which determine the unique pattern of peaks present within the EEM spectra (Aiken, 2014; Coble, 1996; Coble et al, 2014; Fellman et al, 2010). Parallel factor analysis (PARAFAC) is a commonly utilized statistical means of compartmentalizing EEM spectra into discrete peaks that may be compared to broad organic matter classes (Bro, 1997; Chen et al, 2010; Jaffé et al, 2014; Murphy et al, 2013; Stedmon and Bro, 2008)
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