Uncovering the Role of Bicarbonate in Calcium Carbonate Formation at Near‐Neutral pH

Abstract

Mechanistic pathwaysrelevant to mineralization are not well-understood fundamentally,l et alone in the context of their biological and geological environments.Through quantitative analysis of ion association at near-neutral pH, we identify the involvement of HCO 3 À ions in CaCO 3 nucleation. Incorporation of HCO 3 À ions into the structure of amorphous intermediates is corroborated by solid-state nuclear magnetic resonance spectroscopy, complemented by quantum mechanical calculations and molecular dynamics simulations.W e identify the roles of HCO 3 À ions as being through (i)competition for ion association during the formation of ion pairs and ion clusters prior to nucleation and (ii)incorporation as as ignificant structural component of amorphous mineral particles.The roles of HCO 3 À ions as active soluble species and structural constituents in CaCO 3 formation are of fundamental importance and provide ab asis for ab etter understanding of physiological and geological mineralization.

Probing the effects of polymers on the early stages of calcium carbonate formation by stoichiometric co-titration

“…Key to the unambiguous detection of this precursor is the combination of a potentiometric and a turbidimetric probe, along with the new co-titration method that reduces the amount of spectating ions in favour of nucleation-relevant species. In the absence of PAA and in regular excess titrations, this liquid precursor is likely to form as well, [49][50][51]53 but does not prevail long enough for reliable detection due to limited kinetic stability (which may indeed be a function of stoichiometry, among other factors).…”

Hydrogen phosphates play a critical structural role in amorphous calcium phosphates

“…25 In the light of this work, the structural element of the PILP phase for calcium phosphate should be attributed to hydrogen phosphate. While the abundance of bicarbonates in ACC or Mg stabilized ACC was relatively minor (∼5%), 28,38 hydrogen phosphate was one of the major species in ACPs. This should not be surprising in view of their difference in p K a .…”

“…This inference is consistent with a recent finding that bicarbonate ions are a significant structural component of amorphous calcium carbonate (ACC). 28 The preparation of CaP-PILP has been reported in the literature. 24,27 To a solution mixture of NaH 2 PO 4 and polyacrylic acid (PAA), an equal volume of CaCl 2 was added with thorough mixing.…”

“…This inference is consistent with a recent finding that bicarbonate ions are a significant structural component of amorphous calcium carbonate (ACC). 28…”

Formation, Chemical Evolution, and Solidification of the Calcium Carbonate Dense Liquid Phase

“…What little HCO 3 the signal does exist exhibits the same temporal trend as is seen at low pH -i.e., broadening followed by resharpening, as well as continuous shifting to a lower frequency, combined with a large initial shift of the HCO 3 peak to a higher frequency -suggesting the formation of a small amount of a composite DLP with a mixed composition, such as (Ca•HCO 3 ) n •Ca•CO modi ed to the extent that the initial ratio of HCO 3 to CO 3 2is increased ( g. S5). This composite DLP is reminiscent of the condensate reported by Huang et al 24 , but those experiments were done at 10 to 50 times lower concentrations and mixing was performed via slow titration.…”