An assessment of ocean alkalinity enhancement using aqueous hydroxides: kinetics, efficiency, and precipitation thresholds

<p>Ocean alkalinity enhancement (OAE) is a promising approach to marine carbon dioxide removal (mCDR) that leverages the large surface area and carbon storage capacity of the oceans to sequester atmospheric <span class="inline-formula">CO₂</span> as dissolved bicarbonate (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M2" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msup><msub><mi mathvariant="normal">HCO</mi><mn mathvariant="normal">3</mn></msub><mo>-</mo></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="38pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="f7f7a137c195a83f06f2ec9ae074733a"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-21-3551-2024-ie00001.svg" width="38pt" height="15pt" src="bg-21-3551-2024-ie00001.png"/></svg:svg></span></span>). One OAE method involves the conversion of salt in seawater into aqueous alkalinity (NaOH), which is returned to the ocean. The resulting increase in seawater pH and alkalinity causes a shift in dissolved inorganic carbon (DIC) speciation toward carbonate and a decrease in the surface ocean <span class="inline-formula"><i>p</i></span><span class="inline-formula">CO₂</span>. The shift in the <span class="inline-formula"><i>p</i></span><span class="inline-formula">CO₂</span> results in enhanced uptake of atmospheric <span class="inline-formula">CO₂</span> by the seawater due to gas exchange. In this study, we systematically test the efficiency of <span class="inline-formula">CO₂</span> uptake in seawater treated with NaOH at aquarium (15 L) and tank (6000 L) scales to establish operational boundaries for safety and efficiency in advance of scaling up to field experiments. <span class="inline-formula">CO₂</span> equilibration occurred on the order of weeks to months, depending on circulation, air forcing, and air bubbling conditions within the test tanks. An increase of <span class="inline-formula">∼0.7</span>–0.9 mol DIC per mol added alkalinity (in the form of NaOH) was observed through analysis of seawater bottle samples and pH sensor data, consistent with the value expected given the values of the carbonate system equilibrium calculations for the range of salinities and temperatures tested. Mineral precipitation occurred when the bulk seawater pH exceeded 10.0 and <span class="inline-formula">Ω_aragonite</span> exceeded 30.0. This precipitation was dominated by <span class="inline-formula">Mg(OH)₂</span> over hours to 1 d before shifting to <span class="inline-formula">CaCO_3,aragonite</span> precipitation. These data, combined with models of the dilution and advection of alkaline plumes, will allow the estimation of the amount of carbon dioxide removal expected from OAE pilot studies. Future experiments should better approximate field conditions including sediment interactions, biological activity, ocean circulation, air–sea gas exchange rates, and mixing zone dynamics.</p>