January 15, 2025
Journal Article

Mixing and dilution controls on marine CO2 removal using alkalinity enhancement

Abstract

Marine CO2 removal using enhanced-alkalinity seawater discharge was simulated in the estuarine waters of the Salish Sea, Washington, US. The high-alkalinity seawater would be generated using bipolar membrane electrodialysis technology to remove acid and the alkaline stream returned to the sea. Response of the receiving waters was evaluated using a shoreline resolving hydrodynamic model with biogeochemistry, and carbonate chemistry. Two sites, and two deployment scales, each with enhanced TA of 2,997 mmol/m3 and a pH of 9 were simulated. The effects on air-sea CO2 flux and pH in the near-field as well as over the larger estuary wide domain were assessed. The large-scale deployment (addition of 164 Mmoles TA/yr) in a small embayment (12.5 km2), resulted in removal of 2,066 T of CO2 (45% of total simulated) at rate 3,756 mmol/m2/yr, higher than the ideal global scale target of 63 mmol/m2/yr. It also reduced acidity in the bay, ?pH ˜ +0.1 pH units, which is comparable to the historic anthropogenic acidification in the Salish Sea. The mixing and dilution of added TA with distance from the source results in reduced CO2 removal rates. But even at a lower rate, spread over nearly 1,329-fold larger area covering the rest of the Salish Sea and continental shelf regions, additional 2,176 T CO2/yr (47%) was removed with the potential for more removal occurring beyond the region modeled. In supersaturated estuarine environments, the CO2 removal from reduction of outgassing may be significantly higher and account for as much as 90% of total CO2 removal. Of the total, only 375 T CO2 /yr (8%) was from the open shelf portion of the model domain. With shallow depths limiting vertical mixing, nearshore estuarine waters may provide a more rapid removal of CO2 using alkalinity enhancement relative to deeper oceanic sites.

Published: January 15, 2025

Citation

Khangaonkar T.P., B.R. Carter, M.R. Premathilake, S. Yun, W. Ni, M. Stoll, and N.D. Ward, et al. 2024. Mixing and dilution controls on marine CO2 removal using alkalinity enhancement. Environmental Research Letters 19, no. 10:Art. No. 104039. PNNL-SA-203413. doi:10.1088/1748-9326/ad7521

Research topics