Although calcium oxalates are relevant biominerals, their formation mechanisms remain largely unresolved. Here, we investigate the early stages of calcium oxalate formation in pure and citrate-bearing solutions. Citrate is used as a well-known oxalate precipitation inhibitor; moreover, it resembles the functional domains of the biomolecules that modulate biomineralization. Our data suggest that calcium oxalate forms after Ca2+ and C2O4 2− association into polynuclear stable complexes that aggregate into larger assemblies, from which amorphous calcium oxalate nucleates. Previous work has explained citrate inhibitory effects according to classical theories. Here we show that citrate interacts with all early stage CaC2O4 species (polynuclear stable complexes and amorphous precursors), inhibiting calcium oxalate nucleation by colloidal stabilization of polynuclear stable complexes and amorphous calcium oxalate. The control that citrate exerts on calcium oxalate biomineralization may thus begin earlier than previously thought. These insights provide information regarding the mechanisms governing biomineralization, including pathological processes (e.g., kidney stone formation).

This study reports evidence for barite (BaSO 4 ) formation from aqueous solution via non-classical pathways. Our observations support the occurrence of a liquid-liquid separation in the absence of any additive as the initial stage of the crystallization process. The first solid (primary) particles or nuclei seem to form within the initially formed liquid-like precursor phase. TEM and SEM observations of the nanostructure evolution of samples quenched at successive stages of crystallization indicate two levels of oriented aggregation of nanosized solid particles. The first is the aggregation of crystalline primary nanoparticles of ca. 2-10 nm length to give larger but still nanometer sized particles (ca. 20-100 nm length). For the first time, clear evidence of crystallographically oriented aggregation of secondary, nanometer-sized particles to form a barite single crystal is reported. During the second aggregation step of these secondary nanoparticles, most of the porosity in the largest, micron-sized aggregates is annealed, resulting in perfect single crystals. Once an amount of 50 ppm of additive, in our case a maleic acid/allyl sulfonic acid copolymer with phosphonate groups, is present in the solution the dense liquid precursor phase seems to be stabilized, forming a PILP (polymer induced liquid precursor) and secondary nanoparticles are temporarily stabilized against recrystallization. Growth by classical monomer addition, ripening processes or nanoparticle attachment also seems to contribute to barite formation during the latest stages of the processes.

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The efficiency of Pb uptake by CaCO3 is different for calcite and aragonite due to surface passivation.

Barium sulphate (BaSO 4 ) precipitation has been suggested to occur by non-classical pathways that include the formation of a dense liquid precursor phase, nucleation of primary nanoparticles and two levels of oriented aggregation resulting in micron-sized barite single crystals. In this study we build from this previous knowledge and explore how Ba 2+ and SO 4 2− ions associate in solution prior to nucleation of a solid phase and the effects of poly(acrylic acid) (PAA) in these pre-nucleation and post-nucleation stages. With this aim, in situ potentiometric experiments and transmission electron microscopy (TEM) observations of timeresolved quenched samples were carried out. Additional bulk precipitation experiments in which supersaturation was achieved by rapid mixing of Ba-and SO 4 -bearing solutions were performed. The resultant precipitates were characterized by scanning electron microscopy (FESEM and ESEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and thermogravimetric (TG) analyses. Our study provides evidence that barium sulphate precipitation occurs via the formation of ion associates in solution (ion pairs and/or clusters), that are significantly destabilized in the presence of PAA. This is associated with a noticeable delay in nucleation in the presence of PAA. Thus, our results provide indirect evidence that suggests that prenucleation ion associates must form prior to solid BaSO 4 nucleation. Alternatively, BaSO 4 mineralization in the presence of PAA seems to occur by a different route that consists in the formation of Ba-PAA globules in solution followed by an amorphous hydrated BaSO 4 phase that transform into crystalline barite nanoparticles. PAA seems to stabilize this amorphous phase, which nevertheless also forms in the absence of PAA. Finally, single micron-sized crystals are formed by the oriented attachment of distinguishable smaller subunits, thus forming mesocrystals in the presence and absence of PAA.