Effect of pH and Type of Stirring on the Spontaneous Precipitation of CaCO3 at Identical Initial Supersaturation, Ionic Strength and a(Ca2+)/a(CO32-) Ratio

CaCO3 precipitation is physical-chemical basis of biomineral formation of hard tissue (shells, skeletons) in marine calcifying organisms (=biomineralization). Processes controlling biomineralization are still not fully clarified, so the study of influence of pH on basic processes of CaCO3 precipitation should contribute to better understanding of biomineralization under climate change. This paper reports on the effect of initial pH (pH(0)) and type of stirring (mechanical and magnetical) on spontaneous precipitation and phase composition, size and morphology of spontaneously precipitated CaCO3 formed at the identical initial supersaturation, ionic strength and a(Ca2+)/a(CO32-) ratio. The initial pH varied in a range 8.50 <= pH(0) <= 10.50 and included values relevant for mimicking the conditions related to biomineralization in marine organisms. In all systems two CaCO3 polymorphs were found: calcite and/or vaterite. The increase of pH(0) favoured the formation of rhombohedral calcite no matter the type of stirring. This was exclusively influenced by the systems' pH(0) (other relevant initial parameters were identical). Furthermore, increase of pH(0) caused change of vaterite morphology from cauliflower-like spheroids to regular spherulites. The mechanically stirred systems produced larger calcite and vaterite particles and higher content of calcite.