Role of Alginate in Calcite Recrystallization

Abstract

Recrystallization is one of the dominant processes active in early diagenesis. There is ample evidence in the literature that polysaccharides significantly inhibit growth of calcium carbonate, but little is known about their effect on recrystallization at low degrees of supersaturation. In this study, we monitored recrystallization in calcite saturated 0.1 M NaCl solutions using H14CO3– as a tracer. Alginate inhibits calcite recrystallization as expected, but for the same alginate concentrations inhibition is weaker than for growth. We explain these results by extending the interpretation of data from precipitation experiments and by understanding that alginate acts in two opposite directions. On the one hand, alginate pins steps, effectively inhibiting calcite growth. On the other hand, alginate promotes heterogeneous calcite nucleation and subsequent growth, decreasing the interface free energy and serving as an effective template for calcite nucleation and growth. To get closer to an assessment of the effective free surface energy, γ′, in this study, we measured the energy of the alginate–water interface, γaw by using vapor adsorption. The values of interfacial surface tension between pure calcite and water, γcw, and calcite with adsorbed alginate and water, γaw, were found to be equal to 76 and 115 mJ m–2. γaw > γcw, which contributes to a lower energy barrier for calcite nucleation on adsorbed alginate. The high value of γaw testifies that alginate molecules prefer to create new calcite–alginate interfaces than to maintain alginate–water interfaces. In growth experiments, we take into account only inhibition because inhibition and subsequent nucleation are separated by specific induction times. For recrystallization, the time scale for induction is much longer than for growth, and the two oppositely directed processes act almost together and simultaneously, resulting in weaker overall inhibition than for simple calcite growth alone.