Replicating whiting events: testing a process for carbon sequestration using algae, sunlight, nutrients, and calcium-rich water

Whiting events, which involve precipitation of calcium carbonate (CaCO₃), occur regularly in aquatic and marine systems when pH is high and calcium ions and carbonate ions (some from atmospheric carbon dioxide dissolution) are present, often during an algae bloom. These events may contribute to carbon sequestration on geological time scales when the CaCO₃ precipitate sinks and is buried in the sediment. Yet, the present-day need to remove and sequester excess carbon from the atmosphere is clear and must happen on shorter time scales. The goal of this research program was to mimic whiting events in a laboratory setting using freshwater and seawater microalgae systems and determine if the resulting process for precipitating CaCO₃ could be operationally net carbon negative or neutral. Algae with tolerance to high pH conditions were cultured in modified chemostats with 1 mm membranes to separate the algae culture from the high pH water that they produced. Seawater cultures have been maintained with a mean pH of 9.32 +/- 0.70 (1σ) (n = 28) for 1 month and freshwater cultures were maintained for 3.2 months with a mean pH of 10.8 +/- 0.5 (1σ) (n = 289). The high-pH water from freshwater cultures was combined with calcium-rich water in a series of experiments to determine if precipitation of CaCO₃ would occur. Precipitation of aragonite and calcite, two forms of CaCO₃, occurred in some, but not all, of the experiments. This presentation will describe the results of efforts that are underway to understand and control the precipitation process and to determine if a precipitation system based on algae, sunlight, nutrients, and calcium-rich water has the potential to remove and sequester carbon from the atmosphere.