Manganese Oxide-Mediated Reactions with Olivine Dissolution Products: A Double-Edged Sword for Ocean Alkalinity Enhancement

Olivine-based ocean alkalinity enhancement (OAE) is a promising carbon dioxide removal strategy, yet interactions with layered manganese oxides─ubiquitous minerals controlling trace metal biogeochemistry in marine sediments─remain poorly understood. We investigated these mechanisms using synthetic birnessite, a natural analogue of hexagonal layered Mn oxides, in controlled laboratory experiments in seawater under three scenarios reflecting different OAE deployment strategies: direct olivine-birnessite contact, exposure to simulated olivine leachate, and repeated alkaline inputs. Results revealed a dual role for birnessite. It accelerated olivine dissolution through proton-releasing cation exchange and surface-mediated Fe(II) oxidation. However, this proton generation consumed alkalinity, diminishing carbon sequestration efficiency. Regarding trace metals, birnessite efficiently scavenged Ni (>50%) and Co (>99%) but markedly enhanced Cr mobility (reaching ∼0.05 μmol kg^-1), likely via oxidation to more toxic Cr(VI). Crucially, sustained Fe(II) supply mitigated this risk by reducing >50% of Cr(VI) back to Cr(III). Birnessite maintained structural stability throughout. While natural sediment systems are expected to introduce additional complexities, our findings underscore potential environmental trade-offs: Cr(VI) accumulation could exceed ecological thresholds in poorly flushed environments. This study provides foundational mechanistic insights into olivine-sediment interactions, establishing key parameters for modeling OAE safety in complex marine environments.