(712d) Continuous Kinetics of Siderophore-Mediated Olivine Weathering

Earth’s oceans have capacity to permanently sequester >1000 Gt of anthropogenic CO₂. The rate of marine carbon sequestration is determined by the weathering of silicate minerals. In this process, dissolved silicate ions accept protons from carbonic acid and stabilize bicarbonate ions in water for >10⁴ years. Olivine is the most abundant silicate mineral in the Earth’s crust and stabilizes about one mass of CO₂ per mass of dissolved rock (Eq. 1). Natural rock weathering occurs over geological time scales, however (>10³ years). Technologies to accelerate the rate of rock dissolution are needed to sequester meaningful quantities of CO₂ during this century.

Eq. 1 Mg_1.8Fe_0.2SiO₄ + 4H₂O + 4CO₂ --> 1.8Mg²⁺ + 0.2Fe²⁺ + H₄SiO₄ + 4HCO₃^-

Olivine dissolution in seawater is governed by iron, which is present in nearly all minerals. In aerobic conditions at circumneutral pH, iron spontaneously oxidizes and precipitates back on the mineral surface, severely restricting mineral dissolution. The insolubility of iron also presents a challenge for microorganisms, for which iron is an essential micronutrient. To overcome this, bacteria secrete siderophores—a diverse class of secondary metabolites that chelate and solubilize ferric iron. Siderophores have also been shown to accelerate the olivine dissolution at neutral pH. Despite these initial findings in batch reactions, however, the kinetics of olivine weathering in continuous flow systems is not understood.

We developed a continuous reactor platform for measurement of olivine dissolution over medium time scales (days to weeks). We originally used this platform to evaluate growth of microorganisms directly on the mineral substrate. Now, we present a fundamental characterization of the interaction of soluble siderophores with olivine sand. We assessed the kinetics and thermodynamics of siderophore-mediated mineral dissolution on both short and medium time-scales. We calculated the siderophore requirement for optimal acceleration of mineral dissolution and carbon sequestration. Finally, we performed a basic technoeconomic analysis of biological weathering, predicting the quantity of cells and siderophores that would be required for economic carbon sequestration. These findings will inform the scale up and deployment of mineral reactors.