Phosphate Sorption and Desorption on Pyrite in Primitive Aqueous Scenarios: Relevance of acidic → Alkaline Transitions

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Phosphate (Pi) sorption assays onto pyrite in media simulating primeval aquatic scenarios affected by hydrothermal emissions, reveal that acidic conditions favour Pi sorption whereas mild alkaline media - as well as those simulating sulfur oxidation to SO2-4 - revert this capture process. Several mechanisms relevant to Pi availability in prebiotic eras are implicated in the modulation of these processes. Those favouring sorption are: (a) hydrophobic coating of molecules, such as acetate that could be formed in the vicinity of hydrothermal vents; (b) water and Mg2+ bridging in the interface mineral-aqueous media; (c) surface charge neutralization by monovalent cations (Na+ and K+). The increase of both the medium pH and the SO2-4 trapping by the mineral interface would provoke the release of sorbed Pi due to charge polarization. Moreover it is shown that Pi self-modulates its sorption, a mechanism that depends on the abundance of SO2-4 in the interface. The relevance of the proposed mechanisms of Pi capture, release and trapping arises from the need of abundant presence of this molecule for primitive phosphorylations, since - similarly to contemporary aqueous media - inorganic phosphate concentrations in primitive seas should have been low. It is proposed that the presence of sulphide minerals with high affinity to Pi could have trapped this molecule in an efficient manner, allowing its concentration in specific niches. In these niches, the conditions studied in the present work would have been relevant for its availability in soluble form, specially in primitive insulated systems with pH gradients across the wall.

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