Evaluating Mg(OH)2 as an ocean alkalinity agent in tropical river, estuary and saline water

Ocean Alkalinity Enhancement (OAE) is a proposed mechanism of atmospheric CO₂ removal, or negative emission technology. The addition of mineral particles to natural waters is one potential method of achieving OAE, yet there are substantial uncertainties concerning the efficiency of this process in terms of total alkalinity (TA) generation under natural conditions. Laboratory incubations are generally conducted under standardized conditions with filtered, deionized or sterile water which, whilst necessary to conduct reproducible mechanistic studies, is clearly not representative of most natural waters in which OAE might be deployed. In order to assess how variation in natural water properties affects the dissolution of minerals proposed as OAE agents, here we test the conversion of Mg(OH)₂ to total alkalinity (TA) in river water, estuarine water, coastal seawater and offshore seawater. We found that when added as milk of magnesia, a 10 g/L (final concentration) Mg(OH)₂ dose was efficiently converted to TA (>95% efficiency) in river water with low initial TA (mean TA = 239.44), river water with medium initial TA (mean TA = 1636.13), high salinity estuarine water (salinity 27), low salinity estuarine water (salinity 5.3), and seawater. However, when Mg(OH)₂ was applied to high TA river water as a single dose (10 mg/L), the TA increase was only 60% of the calculated addition. The effect of multiple small doses (2.5 mg/L) was also tested, with no significant difference in the TA conversion in most cases. Dry additions of Mg(OH)₂, rather than pre-mixed suspensions of milk of magnesia, were found to be inefficient TA sources, sometimes leading to negative TA changes- especially when using small incubation bottles (2 L). Overall it was demonstrated that Mg(OH)₂ can be used as an efficient TA source in most natural waters for final doses in the range 2.5-10 mg/L.