UC Berkeley

Hypothesis

Calcium carbonate nucleation is often a complex and multistep process that is difficult to follow in situ. The time-resolved electrochemical and electrophoretic methods can provide a new insight into the nucleation pathway.

Experiments

Here, we used a combination of speciation calculations with time-resolved electrophoretic and potentiometric methods to monitor calcium carbonate precipitation from a slightly supersaturated solution.

Findings

After an initial mixing period of three minutes in which metastable CaCO₃ phases may have nucleated and subsequently dissolved due to locally-high supersaturations, bulk solution pH and Ca²⁺ concentrations stabilize before decreasing in tandem with the precipitation of a CaCO₃ phase. After an hour, the precipitate is dominated by calcite that grows at the expense of dissolving vaterite. The time-dependent electrokinetic potential shows analogous signatures of multistage nucleation process: initial rapid changes in ζ-potential are followed by much slower equilibration starting about one hour after reagents are mixed. The changes in ζ-potential, solution pH, saturation indexes, and particle morphology are consistent with vaterite to calcite transformation via dissolution of the former and recrystallization of the latter. These findings highlight the potential use of ζ-potential measurements for monitoring polymorphic transformations of carbonate phases in-situ.