Kinetics of CO2 reduction across Fe(Ni)S precipitates: the role of pH gradients.

Alkaline hydrothermal vents have been proposed as geochemical reactors that could sustain prebiotic carbon fixation via natural pH and redox gradients. Here, we investigate the kinetics of CO2 reduction to formic acid across Fe(Ni)S precipitates in a microfluidic analogue of a hydrothermal vent. By combining controlled experiments with a minimal kinetic model, we demonstrate that formate production depends sensitively on both the absolute pH of the ocean-side compartment and the magnitude of the pH differential across the mineral interface. The model, which incorporates electron transfer through the conductive precipitate, successfully reproduces the observed exponential dependence of formate concentration on the pH gradient. Comparison with batch experiments further illustrates how the spatial separation of reactants and the local chemical environment modulate reaction rates. Our results highlight the central role of pH gradients in driving early carbon fixation and provide a quantitative framework for predicting reaction yields in prebiotic geochemical systems. This approach also lays the groundwork for future studies incorporating spatially resolved mineral structures, electrochemical circuit limitations, and multi-carbon product formation.