The role of citrate and phthalate during Co(II) coprecipitation with calcite

The influence of citrate and phthalate on Co coprecipitation with calcite was investigated using a combination of batch experiments, Fourier-transform infra-red (FT-IR) spectroscopy, and thermogravimetric analysis (TGA) over a wide range of precipitation rates. Steady-state growth conditions (at constant [Ca], [Co], DIC, and pH) were generally achieved within 3-5 h, after which Co(II) partitioning into calcite was evaluated. Only minor differences are observed in the partition coefficient (Kd) trends with and without citrate and phthalate as a function of calcite precipitation rate except at very low rates. Slight inhibition of calcite growth is observed in the presence of citrate or phthalate, which can be attributed to adsorption at surface sites. TGA curves for samples coprecipitated with citrate show a significant mass loss between 375 and 550 °C, whereas the weight-loss curves for the Co-phthalate coprecipitates are indistinguishable from those of the organic-free Co coprecipitates. This indicates that citrate is incorporated into calcite during calcite crystallization, whereas phthalate is excluded. FT-IR spectra for the sample with citrate show a broad absorption in the range 3700-3100 cm−1, which is attributable to water molecules coordinated to citrate coprecipitated with calcite. The preferential incorporation of citrate over phthalate likely reflects differences in both aqueous speciation and conformation of the carboxylate groups. This new finding may provide new insight to the factors that control the behavior of macromolecules and their incorporation into the structure of calcium carbonate during biomineralization.