Abstract
Oxidation and reduction kinetics of iron (Fe) and proportion of steady-state Fe(II) concentration relative to total dissolved Fe (steady-state Fe(II) fraction) were investigated in the presence of various types of standard humic substances (HS) with particular emphasis on the photochemical and thermal reduction of Fe(III) and oxidation of Fe(II) by dissolved oxygen (O2) and hydrogen peroxide (H2O2) at circumneutral pH (pH 7–8). Rates of Fe(III) reduction were spectrophotometrically determined by a ferrozine method under the simulated sunlight and dark conditions, whereas rates of Fe(II) oxidation were examined in air-saturated solution using luminol chemiluminescence technique. The reduction and oxidation rate constants were determined to substantially vary depending on the type of HS. For example, the first-order rate constants varied by up to 10-fold for photochemical reduction and 7-fold for thermal reduction. The degree of variation in Fe(II) oxidation was larger for the H2O2-mediated reaction compared to the O2-mediated reaction (e.g., 15- and 3-fold changes for the former and latter reactions, respectively, at pH 8). The steady-state Fe(II) fraction under the simulated sunlight indicated that the Fe(II) fraction varies by up to 12-fold. The correlation analysis indicated that variation of Fe(II) oxidation is significantly associated with aliphatic content of HS, suggesting that Fe(II) complexation by aliphatic components accelerates Fe(II) oxidation. The reduction rate constant and steady-state Fe(II) fractions in the presence of sunlight had relatively strong positive relations with free radical content of HS, possibly due to the reductive property of radical semiquinone in HS. Overall, the findings in this study indicated that the Fe reduction and oxidation kinetics and resultant Fe(II) formation are substantially influenced by chemical properties of HS.
Highlights
Iron (Fe) is an essential micronutrient for various metabolic processes of organisms such as photosynthesis, respiration, processing of intracellular reactive oxygen species and nutrient acquisition [1]
The Fe(III) thermal reduction rate constants showed the similar trend except for DHA: for example, the measured rate constants ranged from 5.3 × 10−6 to 3.5 × 10−5 s-1 at pH 7.0 (i.e., 6.6-fold change depending on the type of standard humic substances (HS)), which were higher than or comparable to those at pH 8.0 (Fig 1b)
The Fe(III) reduction rate constants determined in this study were reasonably comparable to the reported values for Fe(III)Suwanee River fulvic acid (SRFA) complex at pH 8.0 in previous study (e.g., 1.3 × 10−5 s-1 for the thermal reduction and 1.7 × 10−4 s-1 for the photochemical reduction in the presence of solar simulator, Fujii et al [52])
Summary
Iron (Fe) is an essential micronutrient for various metabolic processes of organisms such as photosynthesis, respiration, processing of intracellular reactive oxygen species and nutrient acquisition [1]. Iron redox kinetics and molecular composition of standard humic substances of Fe in natural waters is tightly regulated by Fe redox transformation kinetics and resulting chemical speciation, which can be affected by the physicochemical factors such as light, pH, reactive oxygen species (ROS), dissolved oxygen (O2) and organic and inorganic ligands [2,3]. Inorganic Fe(III) is readily removed from the solution phase by precipitation of hydrolysis species, resulting in extremely low concentration of dissolved inorganic Fe at circumneutral pH (e.g., ~10−11 M at pH 7.5–9, [12]). Due to the low Fe(III) solubility and external input such as atmospheric dust deposition, concentrations of dissolved Fe in surface waters of the remote open oceans are as low as 0.03–1.0 nM [13]. Fe is a limiting factor for the primary production in one-third of the world ocean where macronutrient concentrations are perennially high [4]
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