Enhanced Crystallographic Incorporation of Strontium(II) Ions into Calcite via Preferential Adsorption at Obtuse Growth Steps

Abstract

Sr-containing calcium carbonates were precipitated from solutions containing Ca(OH)2, SrCl2 and Na2CO3 in a reactor where constant solution composition was maintained. The total concentration of divalent ions was same in all experiments, but the Sr/Ca ratio was varied between 0.002 and 0.86, and the pH value was between 12.02 and 12.25. All solutions were oversaturated with respect to calcite (SIcalcite = 1.2-1.5). Calcite was the only product formed at low Sr/Ca ratios, but at Sr/Ca ≥ 0.45 strontianite was detected in some systems. Sr-rich precipitate was observed in both a surface layer on (6.9-6 µm) rhombic calcite seed crystals and as smaller (> 3.64-1.96 µm) calcite crystals that were elongated along their C-axis. The degree of crystal elongation increased with the Sr/Ca ratio in those crystals. Precipitates recovered from low Sr/Ca ratio experiments exhibited an XRD spectrum identical to that of rhombic calcite, however the peaks attributed to Sr-containing calcite shifted progressively to lower 2 values with increasing solution Sr/Ca ratio, indicating increased lattice volume. Sr K-edge EXAFS analysis of the precipitates showed that the shift in morphology and lattice volume is accompanied by a change in the local coordination of Sr 2+ in calcite. The Sr-O bond lengths were similar to the Ca-O bond lengths in calcite, but Sr-O coordination increased from 6 fold in crystals containing 0.21 Wt. % Sr, to 8 fold in crystals containing 9.47 Wt. % Sr, and the Sr-Ca coordination decreased from 6 and 6 (for the first and second Sr-Ca shells respectively) to 4 and 1. It is suggested that Sr 2+ undergoes preferential incorporation at obtuse (+) growth sites on the calcite surface due to its large ionic radius (1.13 Å), and this increases the growth rate parallel to the C-axis, resulting in the observed elongation in this direction.

The Combined Effects of Sr(II) and Poly(Acrylic Acid) on the Morphology of Calcite

“…Still, the disruption did not generate new types of crystal surfaces, preserving the overall rhombohedral morphology of the crystals. We note here that the crystallization method (carbon dioxide diffusion at pH 6–9) in the present study is quite different from that in the previous report on Sr(II) (homogeneous precipitation at pH > 12 with calcite seed crystals; Sr/Ca ratio up to 20 times higher), making direct comparisons implausible [12].…”

“…Alternatively, Sr incorporation in calcite could alter the lattice parameters of the growing crystal layers to induce lattice strain that may lead to the compartmentalized surface features [19]. In fact, the {104} XRD peak was shifted down 0.06° and 0.08° from that of neat calcite for Sr/Ca = 1:100 and 1:25 (Figure S1), respectively, which corresponded to the 2.9 wt% and 3.8 wt% Sr incorporation based on the analysis by Hodkin et al [12]. Still, the disruption did not generate new types of crystal surfaces, preserving the overall rhombohedral morphology of the crystals.…”

“…This behavior was also mimicked by the combination of Mg(II) and the synthetic macromolecules of poly(acrylic acid) and poly(methacrylic acid) [11]. In the present study, we examined whether such an enhancing effect could be found in another inorganic ion, specifically Sr(II), which has been recently reported to affect the morphology of calcite [12].…”

Therapeutic Applications of Phytoplankton, with an Emphasis on Diatoms and Coccolithophores

“…A possible pathway for introducing new therapeutic functionalities into coccolith plates can be achieved in vivo by specific optimization of coccolithophore growth parameters (light, pH, temperature, culture media composition) . This enables the coccolithophore cell to produce coccolith with specific ions or molecules in their composition, specific size, morphology, or porosity …”

“…Sr enhancement of coccolithophore was obtained by controlling the Sr in the culture media which allowed the coccolithophore cell to facilitate the metal ion transfer from the interior of the cell into the final crystalized coccolith plates . Similarly, the growth media of the same E. huxleyi coccolithophores was modified with different Zn/Ca ratios and the calcium and zinc uptake inside the cells and final coccolith was characterized .…”