Use of Multiparametric Techniques To Quantify the Effects of Naturally Occurring Ligands on the Kinetics of Fe(II) Oxidation

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A multifactorial experimental design was employed to quantify and rank the effects of a series of ligands on the rate of Fe(II) (18 μM) oxidation in a system containing chloride, sulfate, carbonate/bicarbonate, fluoride, and natural organic matter (NOM) at pH 8.34 ± 0.13. Several factors and combinations thereof correlated with the rate of Fe(II) oxidation at the 95% level of confidence. Presented in decreasing order of significance, those factors were carbonate/bicarbonate, NOM, sulfate, chloride, the sulfate/fluoride interaction, and fluoride. The center point of the experimental design was repeated with different organic matters substituted, including Nordic Reservoir NOM, fulvic and humic acids; Suwannee River NOM, fulvic and humic acids; and Pony Lake fulvic acid. Despite the widely differing geographical origins of these organic materials, their overall impact on the oxidation rate of Fe(II) was consistent, with the observed rate varying no more than a factor of 2 as a function of different organic matters (on a milligrams of carbon per liter basis). The utility of the pentafactorial response surface model (based on Nordic Lake NOM) to predict Fe(II) oxidation rates was evaluated for different natural water samples, including two seawater and one freshwater.