Anoxic photochemical oxidation of siderite generates molecular hydrogen and iron oxides

Abstract

Photochemical reactions of minerals are underappreciated processes that can make or break chemical bonds. We report the photooxidation of siderite (FeCO3) by UV radiation to produce hydrogen gas and iron oxides via a two-photon reaction. The calculated quantum yield for the reaction suggests photooxidation of siderite would have been a significant source of molecular hydrogen for the first half of Earth's history. Further, experimental results indicate this abiotic, photochemical process may have led to the formation of iron oxides under anoxic conditions. The reaction would have continued through the Archean to at least the early phases of the Great Oxidation Event, and provided a mechanism for oxidizing the atmosphere through the loss of hydrogen to space, while simultaneously providing a key reductant for microbial metabolism. We propose that the photochemistry of Earth-abundant minerals with wide band gaps would have potentially played a critical role in shaping the biogeochemical evolution of early Earth.

Keywords: Archean iron cycle; astrobiology; banded iron formations; photogeochemistry.

Fig. 1.

(A) Siderite was placed in a quartz tube and irradiated with a Xe lamp with 300 W output under anoxic conditions at pH 7.5–8.0. (B) Siderite oxidation is observed as the color changes upon UV irradiation over time (from top to bottom). The mineral turns brown (oxyhydroxide) and subsequently black (magnetite). Trapped hydrogen bubbles are marked with yellow wedges.

Fig. 2.

(A) The photochemical oxidation of siderite (Left) leads to a magnetic product after 1 d of illumination under 600 W (Right). Upon exposure to oxygen, siderite readily becomes nonmagnetic goethite (Fig. S2 and S3). (B) Powder XRD spectra of the magnetic product reveals it is maghemite/magnetite under anoxic conditions.

Fig. 3.

(A) Plot of the relative quantum yield of the photochemical oxidation of siderite as a function of wavelength. The maximal cross-section is at 267 nm, which closely corresponds to the energy gap between the Fe(3d) ground state and a C–O antibonding orbital excited state. Four measurements were used to fit to a Gaussian function. (B) Plot of the hydrogen production (µmol) in 24 h as a function of the photon flux (µmol quanta/m²⋅s). The experimental results revealed the rate of production of H₂ is proportional to the square of light intensity, and hence the yield increases linearly as a function of photon flux density. This analysis strongly suggests the overall reaction requires two photons to drive a two-electron transfer reaction mechanism. The estimated effective solar UV flux is ∼4.4 µmol quanta/m²⋅s at the ocean surface (dotted line) and the corresponding reaction yield based on the linear fit is ∼1.8 × 10⁻⁴. (C) Proposed mechanism of the two-photon oxidation of siderite. Upon UV irradiation, an electron from Fe(3d) state is transferred to a neighboring iron atom generating charge separation (I). This process is probably assisted by intermediate product (IIa and IIb) by simultaneous protonation of surface Fe(I). A second photon oxidizes the intermediate product to hydrido–Fe(III), and this highly reactive intermediate product (III) reacts with a proton to form a molecular hydrogen. The two-photon reaction yields an Fe(III)–oxyhydroxide precursor (IV) after losing CO₂ and adding hydroxyl groups. Since two photons are involved in the reaction, the probability of H₂ production scales linearly with increasing flux.