Mineral Formation during Ocean Alkalinity Enhancement Laboratory Experiments

Ocean alkalinity enhancement (OAE) is a potential ocean-based carbon dioxide (CO₂) removal approach that involves the addition of alkaline substances to the marine environment to increase seawater buffering capacity and allow it to absorb more atmospheric CO₂. Increasing seawater alkalinity can trigger mineral precipitation, consuming the added alkalinity and decreasing OAE efficiency. To explore mineral formation as a result of alkalinity addition, we present results from laboratory experiments conducted by adding alkalinity as an aqueous solution of either NaOH or Na₂CO₃ to unfiltered seawater collected from Vineyard Sound near Woods Hole, Massachusetts, USA. The seawater used in the experiments is characterized by an average total alkalinity (TA) value of 2158 µmol/kg and an average dissolved inorganic carbon (DIC) value of 2043 µmol/kg.  The amount of alkalinity added was 2000, 5000, and 10000 µmol/kg. The carbonate chemistry was monitored through time by measuring TA and DIC, which were used to calculate the saturation state Ω with respect to a number of minerals including carbonates and brucite. The amount and mineralogy of the precipitate through time were determined in order to monitor the mineralogical changes of the precipitated phases. Results show that mineral precipitation took place in all experiments where alkalinity was enhanced except in the experiment where 2000 µmol/kg was added as Na₂CO₃. In all experiments where precipitation was visually observed, TA and DIC decreased with time. In the NaOH experiments, TA decreases while DIC remained constant for a period of time, followed by the decrease of both TA and DIC in a 2:1 ratio. In the Na₂CO₃ experiments, TA and DIC decreased in a 2:1 ratio throughout the duration of the experiment. These trends are interpreted to reflect the initial precipitation of brucite followed by carbonate minerals in the NaOH experiments and the precipitation of only carbonate minerals in the Na₂CO₃ experiments. Raman Spectroscopy data confirmed the formation of brucite, aragonite, and vaterite in the NaOH experiments and aragonite in the Na₂CO₃ ones. Thermodynamic modeling results are consistent with these observations and show that alkalinity addition makes seawater supersaturated with respect to all the minerals that are observed to precipitate. Collectively, our data indicate that adding alkalinity to seawater induces the precipitation of various minerals and that the mineralogy of the precipitate is dependent on the form of alkalinity addition (i.e., as NaOH or Na₂CO₃). Moreover, the precipitate mineralogy changes through time, pointing to a dynamic system characterized by mineral precipitation, dissolution, and transformation. Importantly, determining what minerals form under what conditions is critical to evaluate the efficiency of OAE at sequestering CO₂.