Ocean alkalinity enhancement (OAE) is proposed as a mechanism for atmospheric CO₂ removal technology. Adding mineral particles to natural waters is one potential method for achieving OAE. However, under natural conditions, there is considerable uncertainty regarding the efficiency of this process in terms of total alkalinity (TA) generation. To assess how changes in the properties of natural water may affect the dissolution of minerals intended for use as OAE agents, we conducted experiments on the conversion of Mg(OH)₂ to TA in river water, estuarine water, coastal seawater, and offshore seawater. To evaluate the efficiency of OAE with the addition of different concentrations of Mg(OH)₂, we conducted 50,000-liter and 55-liter scale culture experiments in Daya Bay. We found that when added as milk, a 10 g/L (final concentration) Mg(OH)₂ dose efficiently converted to TA (>90% efficiency) in river water with low initial TA (mean TA = 239.44 µmol L^-1), river water with medium initial TA (mean TA = 1636.13 µmol L^-1), 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 TA conversion in most cases and maintaining high levels of efficiency (>90%). In the 50,000-liter water tanks during the wet season, the trends of added TA of 300g and 600g both showed a decrease, with the TA addition efficiency dropping from highs of 71.07% to 27.98% and 59.86% to 26.09%, respectively. Our experimental results confirm the utilization of Mg(OH)₂ as an OAE agent for bolstering carbon sequestration but show that water body selection, seasonal considerations, and dosing strategies  affect the efficiency of the process.
 

Ocean alkalinity enhancement,Carbon sink,Magnesium hydroxide