CaSO₄ Mineralization in Carboxy- and Amino-Functionalized Reverse Micelles Unravels Shape-Dependent Transformations and Long-Term Stabilization Pathways for Poorly Hydrated Nanophases (Bassanite)

Abstract

Mineralization studies on calcium sulfates when compared to those on phosphates and carbonates are relatively rare despite the economic relevance of these sulfates. A series of recent findings, however, have renewed the interest in calcium sulfate mineralization. These studies, similar to those on calcium carbonates and phosphates, have shown a richer than previously thought scenario especially at the nanoscale. Here, we intentionally use confinement reactors with fast exchange rates (organically/inorganically functionalized reverse micelles) to rapidly establish pathways relevant to calcium sulfate mineralization in solution (including long-term stabilization). Furthermore, we rationalize all the results on the basis of an adsorption model with solubility inputs for inorganic additives. Thus, the typical bassanite nanofibers involving oriented attachment are observed either at weak-adsorption conditions or in the presence of less soluble inorganic dopants. At strong adsorption conditions, nanofibers are no longer observed, and long-term stabilization of nanobassanites is chemically possible if calcium/carboxylate bridging bonds are established during the first stages of nucleation and growth (aliquots after 30 min are similar to 24 h and remain preserved after 5 months). The results here presented are thus important, as they suggest specific conditions for the stabilization of biogenic bassanites. Furthermore, our results suggest that when oriented attachment is no longer energetically favored at intermediate stages (presence of rounded nanoparticles instead of nanorods), gypsum crystallization still can occur from small nanobassanites, however, through a classical dissolution−recrystallization mechanism.

Mineral Scaling on Reverse Osmosis Membranes: Role of Mass, Orientation, and Crystallinity on Permeability

“…Additionally, the enhanced trend of aspect ratios of the needle crystals suggested that the increasing oriented aggregation extent was occurring from the newest growing ends. This finding was consistent with previous studies, which demonstrated that needle gypsum crystals in bulk aqueous solutions were formed from the oriented attachment of nano-bassanite. ,,, Christie et al. also reported that the oriented growth of gypsum during membrane distillation was able to easily cover the membrane surface, eventually compromising the distillate quality .…”

“…Figure shows that the characteristic peak of gypsum at q = 0.82 was present on both bare and BSA-conditioned membranes, while the characteristic peak of bassanite at q = 2.25 was only observed on BSA-conditioned membranes, indicating that BSA could stabilize bassanite, which is a precursor for gypsum formation. − ,,,− This finding was consistent with the studies by Aizenberg et al, , where organic macromolecules with abundant carboxylic functional groups could stabilize the transition of amorphous CaCO 3 to crystalline calcite, thus affecting the growth orientation of crystals because amorphous materials are isotropic and moldable and can easily overcome the directional restrictions of crystals . Based on the literature, top (111) crystal facets of bassanite are enriched in Ca 2+ ions, any interactions with Ca (II) of the bassanite facets can inhibit oriented growth along the c-axis to form needle-like gypsum crystals . Therefore, it is suggested that these (111) crystal facets found on the bassanite might have been inhibited or blocked on the BSA-conditioned membrane, which would explain why their membrane surfaces presented a rod-shaped structure rather than a needle-like structure.…”

“…Calcium sulfate (CaSO 4 ) is usually chosen as the scalant in the above-mentioned studies because of its ubiquitous occurrence, and when crystallized as gypsum, it becomes a difficult scalant to be removed with the usual pH adjustment method . Gypsum crystal formation can typically follow two different scaling pathways: the first one is the classical pathway, which involves the dissolution of bassanite and subsequent recrystallization of gypsum; the second pathway is the non-classical pathway, in which gypsum is formed through the oriented attachment of amorphous precursors (i.e., bassanite), and subsequent crystal growth. − In the present study, such process was investigated by determining the calcium sulfate scaling on bare and BSA-conditioned membranes. BSA was chosen to condition the membrane because it was previously demonstrated to inhibit gypsum scaling and retard the flux decline. ,, In this study, three objectives were defined with bare and BSA-conditioned membranes, namely: (1) to establish systematic characterization methods for the amount, crystallinity, and orientation of mineral scales on reverse osmosis membranes; (2) to understand how bare and BSA-conditioned membranes would affect the amount, crystallinity, and orientation of mineral scales; (3) to reveal the relationship between the amount, crystallinity, and orientation of precipitates and the permeability of the membrane.…”

Calcium sulfate precipitation pathways in natural and engineered environments

“…But, taking into account the general nucleation mechanism proposed for calcium sulfate in section 2, we hypothesize that these organisms create a low water activity environment, making use of a high density matrix of organic molecules (such as proteins, peptides, or polysaccharides), and/or confinement promoting the formation of bassanite and avoid its further transformation to gypsum. Support for this hypothesis has been obtained from reported CaSO 4 mineralization experiments conducted in carboxy-and amino-functionalized reverse micelles (Tartaj et al, 2015). Strong binding between calcium and carboxylate groups lead to long-term (up to 5 months) stabilization of nano-bassanite.…”

Structural Characteristics of Amorphous Calcium Sulfate: Evidence to the Role of Water Molecules

“…Calcium sulfate is also biomineralized by deep-sea medusa to form the statoliths in the phase of bassanite . Aside from the significance of this mineral in a geological environment, the formation of scale in marine and fresh water treatment makes the antiscaling of calcium sulfate a technological problem in industry. − Thus, knowledge of the crystallization mechanisms of calcium sulfate is of fundamental importance to many fields. − …”