Experimental determination of pyrite and molybdenite oxidation kinetics at nanomolar oxygen concentrations

Constraining the rate at which sulfide minerals undergo oxidative weathering at low atmospheric O ₂ is crucial for understanding the evolution of the Archean and Proterozoic biosphere when O ₂ was a trace atmospheric gas. However, recent studies attempting to constrain sulfide oxidation rates, atmospheric O ₂ sinks, and trace metal delivery to seawater under Archean conditions are limited by the need to extrapolate from experimental pyrite oxidation kinetics determined at much higher O ₂ levels. Extrapolation of those data sets to Archean levels of O ₂ (<10 ⁻⁵ present atmospheric level or PAL prior to 2.4 Ga) leads to more than an order of magnitude uncertainty in sulfide mineral oxidation rates, hampering efforts to quantify oxidative weathering under early Earth conditions. To quantify sulfide oxidation kinetics at low pO ₂ , we conducted aqueous pyrite and molybdenite oxidation experiments at ∼2–1200 nM dissolved O ₂ and pH values 1.83, 5.08, and 8.58. Our experimental approach used LUMOS O ₂ sensors to extend the pO ₂ range explored by oxidation experiments down to 10 ⁻⁵ PAL pO ₂ , the limit of the sensors, which is up to three orders of magnitude lower than the pO ₂ range explored in previous work. From these experiments, we use 28 independent rate measurements to derive a new rate law for the oxidation of pyrite as a function of pO ₂ : r _pyrite =10 ^{-8.83(±0.27)} O ₂ ^0.50±0.04 H ^{+ -0.25±0.02} , Where r _pyrite is the rate of oxidation in mol/m ² sec, and the activities of dissolved [O ₂ ] and [H ⁺ ] are in (mol/L). Our results most closely match the previous rate law presented by Williamson and Rimstidt (1994), but indicate a stronger pH dependence than previous studies. We also present the first kinetic rate law for molybdenite oxidation at low O ₂ , based on 13 independent rate measurements: r _molybdenite =10 ^-8.3(±2.3) O ₂ ^0.5±0.3 , Where r _molybdenite is the rate of oxidation in mol/m ² sec, and the activity of dissolved [O ₂ ] is in (mol/L). We find molybdenite oxidation to be nearly as rapid as pyrite oxidation even at low concentrations of dissolved O ₂ (equivalent to <10 ⁻⁵ PAL pO ₂ ), in contrast to previous work which argued for a threshold effect for molybdenite oxidation. Both pyrite and molybdenite oxidation kinetics exhibit a constant half-order dependence on dissolved O ₂ down to nanomolar levels of O ₂ . We show that this behavior is best explained by a reaction mechanism in which O ₂ undergoes dissociative adsorption to the sulfide mineral surface. This mechanism helps to resolve major uncertainties regarding the reaction mechanism of O ₂ with pyrite and molybdenite mineral surfaces and provides a strong theoretical basis for the robust extrapolation of present results to higher and lower O ₂ concentrations.