The mechanism of the initial steps of pyrite (100) surface oxidation was investigated in detail by means of density functional theory/plane-wave calculations. Pyrite oxidation is related to many environmental and technological issues, and its mechanism has not been completely understood. A chemical picture of the pyrite oxidation process in the presence of oxygen and water was proposed in the present investigation. The reaction steps of the oxidation mechanism can be separated into two types. Type I reactions present lower activation energies and are redox processes that involve oxidation of two Fe(II) sites on the surface to form predominantly the Fe(III)−OH − . This species is formed from hydrogen transfer between the adsorbed water to the adsorbed oxygen molecule on the Fe(II) sites. Type II reactions present higher activation energies and lead to the formation of a SO bond through the hydrogen atom transference from a water molecule to the Fe(III)−OH − species, forming Fe(II)−OH 2 . These reactions present higher activation energies. The determinant step of this oxidation mechanism involves the formation of two adsorbed hydroxide species (OH − ) on the surface. The hydroxides in the presence of water from the bulk liquid react to form two water molecules adsorbed on the surface and the first S−O chemical bond. Parallel reactions were investigated explaining the experimental detection of the O 2 − and OOH − species. Furthermore, the proposed mechanism explains the experimental observation that the oxygen present in the sulfate is mostly originated from water instead of an oxygen molecule. The present study strengthens the importance of the water/solid interface to understand the oxidation mechanism of pyrite in the presence of water at a molecular level.

In biological water oxidation, a redox-active tyrosine residue (D1-Tyr161 or Y Z ) mediates electron transfer between the Mn 4 CaO 5 cluster of the oxygen-evolving complex and the charge-separation site of photosystem II (PSII), driving the cluster through progressively higher oxidation states S i ( i = 0–4). In contrast to lower S-states (S 0 , S 1 ), in higher S-states (S 2 , S 3 ) of the Mn 4 CaO 5 cluster, Y Z cannot be oxidized at cryogenic temperatures due to the accumulation of positive charge in the S 1 → S 2 transition. However, oxidation of Y Z by illumination of S 2 at 77–190 K followed by rapid freezing and charge recombination between Y Z • and the plastoquinone radical Q A •– allows trapping of an S 2 variant, the so-called S 2 trapped state (S 2 t ), that is capable of forming Y Z • at cryogenic temperature. To identify the differences between the S 2 and S 2 t states, we used the S 2 t Y Z • intermediate as a probe for the S 2 t state and followed the S 2 t Y Z • /Q A •– recombination kinetics at 10 K using time-resolved electron paramagnetic resonance spectroscopy in H 2 O and D 2 O. The results show that while S 2 t Y Z • /Q A •– recombination can be described as pure electron transfer occurring in the Marcus inverted region, the S 2 t → S 2 reversion depends on proton rearrangement and exhibits a strong kinetic isotope effect. This suggests that Y Z oxidation in the S 2 t state is facilitated by favorable proton redistribution in the vicinity of Y Z , most likely within the hydrogen-bonded Y Z –His190–Asn298 triad. Computational models show that tautomerization of Asn298 to its imidic acid form enables proton translocation to an adjacent asparagine-rich cavity of water molecules that functions as a proton reservoir and can further participate in proton egress to the lumen.

Arsenopyrite is commonly present in mining tailings and, together with pyrite, is responsible for acid rock drainage (ARD) phenomenon. The mineral undergoes oxidation in contact with oxygen and water producing a solution containing acid and heavy metals and, hence, causing important environmental impacts. Density functional/plane wave calculations were carried out to investigate the oxidation of arsenopyrite aiming to understanding its intricate mechanism at a molecular level. Molecular oxygen is dissociatively adsorbed in the Fe−O−As adsorption sites leading to the oxidation of arsenic and iron sites in good agreement with the available experimental data. It avoids the many steps observed for the pyrite oxidation mechanism. The presence of water is extremely important for the next steps of the oxidation mechanism similarly to the oxidation mechanism of pyrite. The present work reinforces the fact that the removal of the humidity can inhibit the oxidation of the arseno(pyrites) in an aerobic condition.

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