THE EFFECTS OF MG2+ AND H+ ON APATITE NUCLEATION AT SILICA SURFACES

Abstract

Apatite (Ca 10 [PO 4 ] 6 [OH] 2) precipitation from aqueous solutions involves multiple steps, such as nucleation of a precursor calcium phosphate phase, cluster aggregation, crystal growth, and transformation to apatite, which are affected by the presence of other ions in solution. I report the mechanisms by which two ions common in natural solutions, Mg 2 and H , affect heterogeneous calcium phosphate nucleation on the amorphous silica surface. Ab initio molecular orbital calculations of structures and ener-gies of model clusters were performed at the Hartree-Fock level using effective core potentials and valence double-basis sets with polarization functions. Solvation was represented by ex-plicit hydration and by a modified Born model. The model molecular clusters are considered to be reaction intermediates that closely follow transition states in elementary rate-deter-mining steps. Calculated reaction energies are then related to transition state energies by invoking Hammond's postulate. The elementary reactions are assumed to be Ca 2 and Mg 2 inner-sphere or outer-sphere surface complexation, followed by phosphate attachment at the reactive surface site. At the ambi-ent pH values of 6 to 9, the surface site is partially deprotonated and represented as Si 3 O 6 H 5 (H 2 O) 3 , where H 2 O represents the explicitly hydrating water molecules. The rate at which the hexaquo cation forms a surface com-plex decreases as Mg 2 outer sphere Mg 2 inner sphere Ca 2 inner sphere. Faster Mg 2 sorption is driven by gas-phase electrostatic attraction, despite slower Mg(H 2 O) 6 dehy-dration compared to Ca(H 2 O) 6 2 . Mg 2 sorbs rapidly, block-ing access of nucleating surface sites to Ca 2 . HPO 4 2 attaches directly to the Ca inner-sphere complex, forming the Si 3 O 6 H 5 CaHPO 4 (H 2 O) 3 critical nucleus. The outer-sphere Mg 2 surface complex has to convert to the inner-sphere complex for HPO 4 2 attachment to form Si 3 O 6 H 5 MgHPO 4 (H 2 O) 3 . HPO 4 2 attachment at the Ca 2 inner-sphere complex is faster than at the Mg 2 inner-sphere complex. Apatite nucleation is thus retarded but not entirely prevented from solutions with high Mg/Ca ratios, where Mg 2 would otherwise have had the thermodynamic advantage. The geochemical implication is that the Mg/Ca ratio of pore water does not need to be altered significantly from seawater com-position by ad hoc processes to allow authigenic apatite pre-cipitation. Protons catalyze apatite precipitation by affecting surface charge and surface tension. Apatite is close to its point of zero surface charge (pH 7 to 8) at the ambient circumneutral pH values. At low surface charge, Lippman's equation suggests a reduction in the interfacial tension between adjacent critical nuclei and solutions, thus promoting nucleation and growth. The broad similarity in chemical composition between ma-rine pore waters, from which authigenic apatite forms, and mammalian blood, from which apatite precipitates as bone and teeth, suggests that similar reaction mechanisms are involved in heterogeneous apatite nucleation in both systems.