Technical note: Effects of iron(II) on fluorescence properties of dissolved organic matter at circumneutral pH

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 mgL(-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 mgL(-1). Furthermore, the quenching effect was nonlinear and proportionally stronger at Fe(II) concentrations below 100 mgL(-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 mgL(-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 mgL(-1)) on EEMs.