Unpaired electrons-induced geochemical activity of native sulfur in energy-triggered ring-opening process

Abstract

Native sulfur (denoted as S0) has been regarded as the crucial species in the biogeochemical cycle of sulfur, and the opening of its eight-membered rings is critical to its activation. However, due to the extreme difficulty in detecting transient ring-opening species, the activation process and inducing factors have yet to be revealed. This study investigates the external (incident optical energy) and internal factors (impurity elements) that trigger the opening of S0 ring, and the underlying activation mechanisms by combining optical measurements, theoretical calculations, and photochemical experiments. Synchrotron radiation in situ X-ray absorption near edge spectra confirms that S0 can undergo ring-opening reactions under light irradiation below ∼575 nm, which results from electron transition via indirect bandgap. Density functional theory calculations and ab initio nonadiabatic molecular dynamics reveal that photo-assisted electron transition evokes ring opening in the femtosecond scale, resulting in a new chain-like molecular configuration with changed charge density and electronic structure. Compared with the ring-like structure, the ring cleavage can provide two unpaired electrons on the terminal S atoms as reactive sites for reducing bicarbonate to formate. The room-temperature bandgap of four S0 samples with different substituting concentrations of impurity elements (mainly As and Se, up to 282.0 and 19.3 μg/g, respectively) decreases linearly with the increase of doping amount. It confirms that As and Se can add 4p orbitals into the S 3p-dominated valence and conduction band of S0, thus reducing the energy required for electron transition and promoting the ring-opening reaction. S0 sample with 282.0 μg/g of As would produce much less HCOOH (3.3 nM·g·h−1·m−2) from carbonates compared with the few-As sample (34.0 nM·g·h−1·m−2), attributed to the new three bonds of As with only one active unpaired electron left after ring opening. Uncovering the critical roles of ring-opening reactions in improving the chemical activities of S0 under the impact of natural light and impurity elements can significantly deepen the understanding of molecular crystalline mineral S0 and the involved biogeochemical sulfur cycle.