Alkaline materials for coastal ocean alkalinity enhancement: A comparative study of natural silicates and industrial byproducts

Coastal ocean alkalinity enhancement (OAE) is a promising ocean-based carbon dioxide removal (CDR) approach for mitigating climate change and counteracting ocean acidification. However, uncertainties persist regarding the efficacy and environmental safety of alkaline materials under realistic coastal conditions. This study comparatively investigated the CO₂ sequestration potential, geochemical processes, and environmental impacts of four alkaline materials-natural silicates (olivine, basalt) and industrial byproducts (fly ash, steel slag)-through laboratory incubations with natural seawater (filtered and unfiltered) and in situ deployments off the East China Sea. Over 31-day incubations, basalt induced negligible alkalinity release, while olivine showed limited alkalinity enhancement (12 μmol/kg/day) compared to theoretical estimation, projecting a CO₂ sequestration rate of 0.57 ± 0.06 Tg/month for a hypothetical coastal deployment in China. Notably, fly ash exhibited faster alkalinity release (30 μmol/kg/day) and the highest projected CO₂ uptake (1.24 ± 0.05 Tg/month). In contrast, steel slag caused rapid pH increase and alkalinity consumption via secondary carbonate precipitation, representing a distinct carbon sequestration pathway. Heavy metal pollution index (HPI) assessments indicated low overall contamination risks of the materials, particularly in unfiltered seawater that better resembles natural conditions, though Ni release from olivine remains a concern. Streamlined life cycle analysis (S-LCA) highlighted fly ash's advantages due to the avoidance of upstream carbon emissions (as a byproduct) and waste valorization potential, resulting in superior net CO₂ removal efficiency. This work provides critical insights into material-specific trade-offs, suggesting fly ash as a promising candidate for short-term coastal OAE deployment that balances CO₂ sequestration efficiency, manageable environmental risks, scalability, and affordability.