Precipitation of calcium carbonate in different carrageenan gelling environments was initiated by fast mixing of calcium chloride and sodium carbonate solutions. Carrageenan concentration and the gelling mechanism, controlled by the addition of different cross-linking cations, calcium or sodium, were varied in order to elucidate the mechanism and dynamic of solid phase formation in highly supersaturated precipitation systems in which initial formation of three calcium carbonate polymorphs and amorphous calcium carbonate is possible. The analysis of the samples isolated 5 days after initiating the precipitation showed that calcite was the only solid phase obtained in the whole range of carrageenan concentrations investigated. At lower carrageenan concentrations, the calcite aggregates were observed, while at higher concentrations rhombohedral crystals of uniform size distribution were found. Spherical imprints on the surfaces of calcite particles precipitated at lower gel concentrations were observed, thus pointing out the initial and simultaneous precipitation of stable calcite and metastable calcium carbonate phases, vaterite and possibly amorphous calcium carbonate. Time resolving precipitation experiments within gelling media of low concentration confirmed such an assumption and implied the solution mediated mechanism of transformation of metastable vaterite, which was in close contact with calcite crystals. Reference precipitation systems, performed at apparently similar initial supersaturation and hydrodynamic conditions but without the presence of carrageenan, showed a comparable pattern of stepwise solution mediated transformation of metastable amorphous calcium carbonate and vaterite into calcite
Mineralization of Calcium Carbonates in Gelling Media
FALINI, GIUSEPPE;
2011
Abstract
Precipitation of calcium carbonate in different carrageenan gelling environments was initiated by fast mixing of calcium chloride and sodium carbonate solutions. Carrageenan concentration and the gelling mechanism, controlled by the addition of different cross-linking cations, calcium or sodium, were varied in order to elucidate the mechanism and dynamic of solid phase formation in highly supersaturated precipitation systems in which initial formation of three calcium carbonate polymorphs and amorphous calcium carbonate is possible. The analysis of the samples isolated 5 days after initiating the precipitation showed that calcite was the only solid phase obtained in the whole range of carrageenan concentrations investigated. At lower carrageenan concentrations, the calcite aggregates were observed, while at higher concentrations rhombohedral crystals of uniform size distribution were found. Spherical imprints on the surfaces of calcite particles precipitated at lower gel concentrations were observed, thus pointing out the initial and simultaneous precipitation of stable calcite and metastable calcium carbonate phases, vaterite and possibly amorphous calcium carbonate. Time resolving precipitation experiments within gelling media of low concentration confirmed such an assumption and implied the solution mediated mechanism of transformation of metastable vaterite, which was in close contact with calcite crystals. Reference precipitation systems, performed at apparently similar initial supersaturation and hydrodynamic conditions but without the presence of carrageenan, showed a comparable pattern of stepwise solution mediated transformation of metastable amorphous calcium carbonate and vaterite into calciteI documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
