Divergent Responses of Manganese Oxides to Olivine Dissolution and Their Control on Trace Metal Fates under an Ocean Alkalinity Enhancement Scenario

Olivine-based ocean alkalinity enhancement (OAE) is a promising carbon dioxide removal technology; however, the geochemical fate of olivine-derived toxic metals, particularly nickel (Ni) and chromium (Cr) at marine sediment interfaces, remains poorly constrained. We employed dialysis-membrane microcosms to isolate olivine from Mn oxides, allowing only dissolved species to diffuse and react under three olivine loadings (1, 5, and 10 g L–1) in seawater for 60 days. Results revealed that (1) Ni was preferentially sequestered by the Mn oxides, with δ-MnO2 showing higher uptake capacity than birnessite, (2) Cr was co-sequestered with secondary Fe(III) (oxyhydr)oxides rather than adsorbing directly onto Mn oxides, and (3) synergistic Ni2+ isomorphic substitution and Fe(II)-driven Mn(III) reduction rearranged δ-MnO2 into a more stable hexagonal layered framework. Consequently, both δ-MnO2 and birnessite act as long-term Ni sinks, preventing Ni remobilization via desorption or diagenetic phase transitions. These findings support a proposed “spatial decoupling” OAE strategy, where olivine is pre-dissolved in land- or ship-based reactors to remove Cr via Fe co-precipitation. Subsequent discharge of the Ni-bearing alkaline effluent into shallow coastal regions allows benthic Mn oxides to scavenge Ni, reducing toxicity risks associated with direct mineral contact.