Coupling Acid Neutralization and Resource Recovery to Scale Ocean Alkalinity Enhancement.

Ocean alkalinity enhancement (OAE) is a promising carbon dioxide removal (CDR) strategy with the theoretical potential to sequester gigatonnes of atmospheric CO2 each year. Electrochemical OAE, where seawater alkalinity is increased by removing hydrochloric acid (HCl), has attracted particular interest, but the viability of this approach at climate-relevant scales hinges on the development of low-cost, carbon-neutral strategies for HCl disposal. Here, we demonstrate a two-stage process that helps address this challenge by first neutralizing HCl using common mafic and ultramafic rocks and then recovering valuable products from the resulting solutions. This process can yield coproducts such as amorphous silica and nickel/cobalt-bearing mixed hydroxide precipitates while converting HCl into a saline solution that can be discharged to the ocean without reversing alkalinity gains. A techno-economic case study in Washington state, USA, estimates the net cost of acid neutralization at <$200 per tonne of CO2 removed via OAE, with the potential for future net profitability with certain process improvements.