Abstract

Environmental mineralogy and Nanogeosciences are rapidly expanding in technological directions that allow for the detection, characterization, and understanding of non-crystalline and poorly crystalline phases, crystalline-amorphous mixed phases, and nanosized materials. In this regard, non-classical nucleation in water-based solutions is an exciting emerging field that presents an alternative option to the classical view of the crystallization process in the aqueous system (different stages of crystallization proceeding via attachment of basic monomers like atoms, ions or molecules). The precipitation of hydrobasaluminite (Al4SO4(OH)10 • 12-36H2O) can be considered as the perfect proxy for the study of non-classical nucleation pathways in water-based solutions with special emphasis in amorphous to nanocrystalline transitional phases. To achieve the main goals presented in this research proposal, the integration of highly advanced and complementary analytical techniques is required. In this study, the inorganic polymers generated during the incipient stages of hydrobasaluminite aqueous synthesis will be performed using electro-spray ionization coupled to a time-of-flight mass spectrometer tandem (ESI-TOF/MS). ESI working conditions were as follows: cylinder voltage of –4500 V, capillary voltage of –4000 V, injection flow rate of 30 µL/min, drying gas temperature of 350 ºC and nebulizer pressure in the probe of 80 psi. TOF–MS accurate mass spectra were recorded across a range of 50–1000 m/z. This technique was applied to determine the characteristic oligomers in four initial solutions of Al2(SO4)3 • 18H2O with concentrations ranging from 0.002 to 0.02M. Initial solutions pHs were risen by steps (concomitantly inducing hydrobasaluminite precipitation) adding portlandite, Ca(OH)2(s), according to a well established hydrobasalumite synthesis protocol (Adams and Rawajfih, 1977). Changes in the initial oligomers composition and distribution were monitored analyzing several solutions aliquots by ESI-TOF/MS until mineral precipitation ceased. Among the multiples compounds detected, 38 cationic polymeric species were assigned as the main oligomers involved in hydrobasaluminite precipitation (some representative examples are shown in Figure 1). Our results place ESI-TOF/MS as an invaluable analytical technique to study the incipient nucleation mechanisms of poorly crystalline minerals, opening a novel and promising research field within mass spectrometry and environmental nanogeosciences.