Effectiveness of olivine dissolution in beach simulations for ocean alkalinity enhancement – insights from flow-through experiments

Abstract. Carbon Dioxide Removal (CDR) is required to mitigate climate change and to keep global warming below 1.5 to 2 °C. Ocean alkalinity enhancement (OAE) in the coastal environment is a promising and relatively low-cost technique, that could enhance marine CO₂ sequestration via silicate weathering. The high-energy environment in the surf zone is especially promising as constant grain collision provides a natural grinding mechanism, potentially enhancing alkaline mineral dissolution. In this study, we experimentally investigated the dissolution of dunite, an olivine-rich ultramafic rock, in natural Atlantic seawater using flow-through reactors. In the experiment, pure dunite (from now on referred to as olivine; forsterite endmember), beach sand (from the coast of West Brittany) and a mixture of olivine and beach sand (olivine/sand) was studied under turbulent and stagnant conditions, in order to identify the effect of grinding on mineral dissolution and alkalinity generation. We see, that alkalinity release was highest in all turbulent reactors compared to stagnant conditions, with the highest alkalinity release observed in the turbulent olivine/sand mixture (11.5 mmol mol^-1 olivine d^-1). The effective sand grinding and especially the hardness of the quartz grains, composing 49 wt.% of the sand, enhance olivine abrasion and rounding, and thus foster dissolution. This effect diminishes over experimental time, when an apparent steady state is reached after ca. 74 hours, where a constant alkalinity release from the turbulent olivine/sand treatment is attained with 1.1±0.6 mmol mol^-1 olivine d^-1; within error to turbulent treatments of pure olivine (1.0±0.2 mmol mol^-1 olivine d^-1). With respect to potentially toxic trace metals (Ni, Cr), we observed high concentrations in the first 24 hours for the olivine/sand treatment when mineral dissolution rates were high (Ni: 359 nmol L^-1; Cr: 171 nmol L^-1), but these decreased quickly to background seawater values (Ni: 18 nmol L^-1; Cr: 19 nmol L^-1). In addition, our results show that the natural alkalinity release from sand was not diminished by olivine addition, and that the probability of secondary mineral precipitation and by this CO₂ release is mostly low, due to high dilution rates with ambient seawater. A cooperative alkalinity release by sand and olivine is expected in natural conditions where similar high dilution rates of reaction products will prevail. Our study shows that ephemeral peaks of high critical element (e.g. Ni) concentrations due to high mineral reaction rates in the beginning of the experiment can be avoided, thus, olivine addition to surf zones could be an efficient marine CDR technique for OAE applications in coastal environments.