Organic Complexation of Fe(II) and Its Impact on the Redox Cycling of Iron in Rain

Abstract

More than 80% of the iron(II) present in a dilute (pH 4.5) H2SO4 solution was oxidized by hydrogen peroxide (3 microM) in 24 h, whereas in rainwater Fe(II) remained stable for days indicating that a complexed form of Fe(II) exists in rainwater that protects it against oxidation. When a rain sample was irradiated for 2 h with simulated sunlight, there was a 57 nM increase in Fe(II) resulting from photoreduction of organic Fe(III) complexes. Once irradiation ceased, the photoproduced Fe(II) rapidly oxidized back to its initial concentration of 32 nM prior to irradiation, but not to zero. These photochemical studies demonstrate that during the daytime when sunlight is present there are dynamic interconversions between complexed and uncomplexed Fe(II) and Fe(III) species in rainwater. During the night, after the photochemically produced Fe(II) is reoxidized to Fe(III), virtually all remaining Fe(II) is complexed by ligands which resist further oxidation. Rain samples oxidized under intense UV light lost their ability to stabilize Fe(II), suggesting the ligands stabilizing Fe(II) are organic compounds destroyed by UV-irradiation. Additional UV-irradiation studies demonstrated that on average 25% of the Fe-complexing ligands in rainwater are extremely strong and cannot be detected by spectrophotometric analysis using ferrozine. The stability of organically complexed Fe(II) has important implications for the bioavailability of rainwater-derived Fe in the surface ocean.

Effect of atmospheric organic complexation on iron-bearing dust solubility

“…The iron solubility (% SFe) is calculated as follows: % SFe = 100 × (DFe T /Fe tot ). For some experiments, the dissolved Fe(II) (DFe(II)) was determined by a modification of spectrophotometric ferrozine method (Kieber et al, 2005). Absorbance of the iron ferrozine complex was read at 562 nm using a 1 m liquid wave-guide capillary cell attached to a JAZ spectrometer (Ocean Optics, World Precision Instruments).…”

“…Nevertheless, recent works of Deguillaume et al (2010) show the daytime dissolved concentrations of Fe(II) into cloud droplets are controlled both by the initial DFe(II)/DFe T ratio upon dissolution of immersed particles and by the photochemical reactions into droplets. Moreover, Kieber et al (2005) observed that 25 % of the Fe(II)-complexing ligands in rainwater is strong enough to prevent and slow oxidation in seawater, suggesting that organic complexation enables iron to stabilize as Fe(II), probably via organic macromolecule binding ligands (Willey et al, 2008). This implies that the dissolved Fe(II) concentration could be significant in the aqueous phase even during the night due to the organic complexation.…”

“…Atmospheric chemical processes affect Fe speciation and solubility (Desboeufs et al, 2001;Hand et al, 2004), and hence its bioavailability in the ocean. Among these processes, organic complexation enables both enhancing the iron solubility, in particular with oxalate (Sulzberger and Laubscher 1995;Paris et al, 2011), and to stabilising iron in dissolved form after wet deposition in seawater (Kieber et al 2005). Thus, the atmospheric organic complexation of iron in cloud-or rainwater could be essential to estimate the supply of bioavailable iron to the ocean (Shi et al, 2012).…”

“…Thus, it appears that some of the most important organic compounds present in the aerosols and atmospheric aqueous phases are potentially preferred ligands of iron. The organic speciation of Fe(II) and Fe(III) in rainwaters has been only highlighted under natural conditions by thermodynamical calculation (Okochi and Brimblecombe, 2002) or by indirect ferrozine method (Kieber et al, 2005). However, recent works of Cheize et al (2012) showed by cathodic stripping voltammetry that 80 % or more of dissolved Fe(III) present in rain samples was bound with a natural strong organic binding ligand, whose stability constant is close to those of siderophores.…”

Dissolved iron in the vicinity of the Crozet Islands, Southern Ocean

“…To estimate soluble Fe inputs we used our field data (Section 3.2) and assumed a solubility for dry deposition of 8 % (Baker et al, 2006b) and 14 % for wet deposition Kieber et al (2005) have reported the active photochemical production of soluble Fe(II) and also higher Fe solubilties in N. Atlantic rains, while Halstead et al (2000) report similar solubilities to those of Jickells and Spokes (2001). The solubility of Fe in rainwater is clearly an important uncertainty in these calculations and merits further study.…”