CRYSTAL FORMATION ONTO AN ORGANIC MATRIX: EFFECT OF PH ON NUCLEATION RATE AND POLYMORPHS OF CALCIUM CARBONATE

Organisms synthesize skeletal structures as mineral-organic composites, in a process known as biomineralization. Calcium carbonate (CaCO₃) is the most abundant biomineral in the ocean and typically occurs as the three most common polymorphs— aragonite, calcite, or vaterite. Progressive acidification of world oceans raises the question of how decreasing pH will impact the rate and types of CaCO₃ polymorphs that form.

We tested the hypothesis that environmental pH influences the type of polymorph that initially precipitates onto an organic matrix and the nucleation rate._. A series of experiments were conducted, with triplicate replications, using Type B gelatin as a simple model for an organic matrix. The gelatin was mounted in a flow-through cell using CaCl₂ and Na-HCO₃ solutions; and with the pH of each bicarbonate solution adjusted to 8.98, 9.32, or 9.90. The formation of crystallites were recorded with an optical microscope equipped with Leica Application Suite software using previous methods to determine the nucleation rate (Giuffre et al., 2013, PNAS; Hamm et al., 2014, PNAS). SEM and XRD were used to characterize the resulting polymorphs.

For the simple system that does not contain additives (controls), vaterite formation is predominant at all pH conditions, with variable amounts of aragonite and calcite. The rate of nucleation increases with solution pH where rates at pH = 8.98 are approximately 50% of rates at pH = 9.90.

With the introduction of magnesium chloride (MgCl₂) at a Mg:Ca ratio = 3:1, aragonite is the only polymorph to form. There is no evidence of calcite or vaterite for any solution pH. The rate of aragonite nucleation is approximately 2X faster than the control experiments for all pH conditions. The rate of aragonite nucleation is pH dependent with slowest rates at the lowest pH (8.98). Data analysis is underway to estimate an interfacial free energy for aragonite nucleation onto the organic substrate. Additional experiments will determine the effect of carboxylation and pH on polysaccharides with and without MgCl₂ present in the system.

The rate of CaCO₃ formation in the ocean, and the polymorphs that occur, may be compromised or inhibited as a result of the lowering pH levels in marine environments.

MMW acknowledges support by the Clare Boothe Luce Undergraduate Research Award.