Ocean Acidification

Highlights

• Mineralization of CaCO₃ is possible via in situ alkalinization with bipolar membranes.
• A pH between 9.6 and 10 is most effective for carbonate capture from seawater.
• We extract 60% (real seawater) to 85% (synthetic seawater) of the DIC from the feed.
• Requires 318 kJ mol⁻¹ CaCO₃(s), less than half of that for ex situ mineralization.
• Fouling at the membrane surface should be mitigated for practical deployment.

Abstract

Bipolar membrane electrodialysis (BPMED) can provide a sustainable route to capture the oceanic-dissolved inorganic carbon (DIC) using an electrochemical pH-swing concept. Previous works demonstrated how gaseous CO₂ (through acidification) can be obtained from ocean water, and how carbonate minerals can be provided via ex situ alkalinization. In this work, we present, for the first time, the in situ mineralization via the alkalinization route using both real and synthetic seawater. An in situ pH-swing, inside of the BPMED cell, allows reducing the energy consumption of the oceanic-DIC capture. We demonstrate that, by accurately controlling the applied current density and cell residence time, the energy required for the process can be indeed lowered through facilitating an optimized pH in the cell (i.e., base-pH 9.6–10). Within this alkaline pH-window, we capture between 60% (for real seawater) up to 85% (for synthetic seawater) of the DIC from the feed, together with minor Mg(OH)₂ precipitates. The CaCO₃(s) production increases linearly with the applied current density, with a theoretical maximum extraction of 97 %. The energy consumption is dominated by the ohmic losses and BPM-overpotential. Through tuning the current density and flow rate, we optimised the energy consumption by applying a mild in situ pH-swing of ca. pH 3.2 – 9.75 (for real seawater). As a result, aragonite was extracted by using of 318 ± 29 kJ mol⁻¹ CaCO₃(s) (i.e., ca. 0.88 kWh kg⁻¹ CaCO₃(s)) from real seawater in a cell containing ten bipolar – cation exchange membrane cell pairs, which is less than half of the previously lowest energy consumption for carbonate mineralization from (synthetic) seawater.

Sharifian R., Boer L., Wagterveld R. M. & Vermaas D.A., 2022. Oceanic carbon capture through electrochemically induced in situ carbonate mineralization using bipolar membrane. Chemical Engineering Journal 438: 135326. doi: 10.1016/j.cej.2022.135326. Article.