The Growth of Gypsum in the Presence of Hexavalent Chromium: A Multiscale Study

Abstract

Abstract:The sorption of dissolved inorganic pollutants into the structure of minerals is an important process that controls the mobility and fate of these pollutants in the Earth's crust. It also modifies the surface structure and composition of the host mineral, affecting its crystallization kinetics. Here, we investigate the effect of hexavalent chromium, Cr(VI), on the nucleation and growth of gypsum by conducting two types of experiments: (i) in situ atomic force microscopy (AFM) observations of the growth of gypsum {010} surfaces in the presence of Cr(VI) and (ii) gypsum precipitation experiments by mixing aqueous solutions containing variable amounts of Cr(VI). Gypsum precipitation is progressively delayed when occurring from solutions bearing increasing Cr(VI) concentrations. Chemical analyses of gypsum precipitates show that gypsum incorporates small Cr(VI) amounts that correlate with the content of this ion in the aqueous solution. Gypsum cell parameters variation reflects this incorporation. At the molecular scale, Cr(VI) induces a slowdown of step advance rates on gypsum {010} surfaces accompanied by the roughening of nanostep edges and the so-called "template effect". This effect involves the reproduction of the original nanotopography after the completion of individual advancing monolayers and appears as a general nanoscale phenomenon occurring during growth of solid solutions from aqueous solutions even in the case of compositionally-restricted solid solutions.

Competitive TcO4–, IO3–, and CrO42– Incorporation into Ettringite

“…Additionally, this finding is consistent with (i) the results from differential PDF analysis above where calcite accounts for most of the difference between pure and doped-ettringite phases and (ii) thermodynamic equilibrium calculations using PHREEQC that suggest that CrO 4 2– favors incorporation into a calcite–CaCrO 4 solid solution rather than an ettringite–Cr-ettringite solid solution (see the Supporting Information for PHREEQC details and results). Several studies have shown that calcite can readily accommodate CrO 4 2– in the CO 3 2– site. − Therefore, these results suggest that, when IO 3 – and CrO 4 2– are co-mingled in a system where ettringite and calcite are readily formed, (i) IO 3 – will partition primarily into the ettringite phase and CrO 4 2– into the calcite phase and (ii) the presence and concentration of oxyanions will influence the mineralogical composition of the solid phase.…”

Substitution of Calcium with Ce, Nd, Er, and Tb in the Structure of Microcrystals of Calcium Sulfates with Controlled Hydration Water: A Proposed Mechanism

“…In Figure (a) the step bunches or macro-steps are visible on the surface of the crystals as dark lines, indicating that the growth is directed by advancing the steps. As the macro-steps have a slower speed than the elementary steps, , those are generally observed in the middle of the primary step. Figure (b), on the other hand, shows that the presence of REE ions in the solution have caused a dramatic change on the step motion and therefore the final morphology of the crystals; the REE slowed the velocity of the steps or even blocked their advance.…”

The Effects of Inorganic Additives on the Nucleation and Growth Kinetics of Calcium Sulfate Dihydrate Crystals

“…It is well-known that both inorganic , and organic additives , affect the nucleation, crystallization, and morphologies of gypsum crystals. To date, primarily the role that elements like Cr 3+ , Cu 3+ , Cr 6+ , Al 3+ , and Fe 3+ have on gypsum growth from solution have been studied. − In contrast, a mechanistic understanding of the effect that major ions in, for example, brines or formation waters (e.g., Na + , K + , Li + , Cl – , or Mg 2+ ) has on gypsum crystallization is still lacking. Existing data from studies that address the crystallization of calcium sulfate phases in the presence of these ions are highly discrepant, and whether these ions become structurally incorporated or only surface adsorbed into the growing gypsum is still debated.…”