Calcium sulfate precipitation pathways in natural and engineered environments

Abstract

The solution-mediated formation of calcium sulfate minerals, i.e. gypsum, anhydrite and bassanite, is a common process in both natural and engineered settings. It plays a key role in the global sulfur cycle and serves as an indicator of past environmental conditions on Earth and Mars. Products relying on the crystallization of these minerals have been employed since antiquity, and today they are an essential part of a wide array of industrial applications. Accordingly, the fundamental aspects of calcium sulfate mineralization have been the focus of intensive research during the past century. However, a recent flurry of studies addressing alternative, i.e. nonclassical, nucleation and growth mechanisms has spurred a revisit of the precipitation pathway of the most common phase, gypsum. The newly obtained data sketch a far more complex picture of the mineralization process than previously assumed. This has important consequences for the interpretation of calcium sulfate deposits, both from a geochemical and industrial point of view. In order to shed light on this issue, we discuss in this review both recent and long-standing observations of abiotic formation routes of calcium sulfate minerals as a function of the physicochemical solution properties. By integrating both the classical and non-classical perspectives on crystallization we put forward a unified model for calcium sulfate crystallization. Using this model, we (re)-evaluate the phase stability and transformations taking place in the CaSO 4-H 2 O system. Next, we look into the formation of calcium sulfate minerals occurring in close association with the biosphere. Employing the abiotic case scenario as a benchmarking tool, the possible influence and/or control exerted by biological activity (and its byproducts) on the precipitation pathway is critically reviewed. Finally, we point out the central issues that need to be resolved if we wish to fully understand and control the formation of calcium sulfate solids in natural and engineered environments.

Gypsum Crystallization during Reverse Osmosis Desalination of Water with High Sulfate Content in Presence of a Novel Fluorescent-Tagged Polyacrylate

“…At the same time our results conflict somehow with the conclusions of numerous reports on static gypsum crystals formation in supersaturated aqueous solutions [24][25][26][27][28][29][30][31][32][33]. All these studies are built on the grounds of homogeneous nucleation scenario, excluding the possibility of "nanodust" presence.…”

“…This solution was filtered with 220 nm filter and a "purified" solution demonstrated still 1540 particles bigger than 100 nm in 1 mL. A homogeneous scenario was unlikely to take place in [24][25][26][27][28][29][30][31][32][33] as an energy barrier for crystals nuclei formation is much lower for heterogeneous scenario, than for homogeneous one [23].…”

“…The present study is focused on the scale inhibitor visualization during RO treatment of model water sample, with high sulfate content, in the presence of a fluorescent antiscalant -1,8-naphthalimide-tagged polyacrylate, PAA-F1, Figure 1. The gypsum scale was taken as a model of a sparingly soluble salt due to: (i) its importance for the RO and other water treatment technologies [10][11][12][13][14][15][16]; (ii) its poor dependence on pH; (iii) its easily detectable crystal shapes; and (iv) the nucleation of gypsum has been investigated extensively in the past [10,11,14,[16][17][18][24][25][26][27][28][29][30][31][32][33]. On the other hand PAA-F1 is expected to be a better antiscalant for CaSO4·2H2O deposits relative to HEDP-F.…”

Nucleation Pathway of Calcium Sulfate Hemihydrate (Bassanite) from Solution: Implications for Calcium Sulfates on Mars

“…Based on our previous (also in situ) measurements of the precipitation pathway of gypsum and the outcome of CaSO 4 precipitation experiments, we hypothesized that all calcium sulfate phases could share a common precipitation pathway 1,2 . The in situ data of the precipitation pathway of bassanite presented in this work, allow us to compare the crystallization process of bassanite with that of gypsum.…”

“…The strong band at 980 cm -1 corresponds to v 1 mode of aqueous sulfate, whereas the peak at 1010 cm -1 is a v 1 band of the growing solid phase. Each spectrum corresponds to a time step of 120 s. All spectra were baseline-corrected and a constant offset in intensity was introduced for clarity; B) Integrated areas of both v1 bands from (A) as a function of time; C) Integrated area of the v 1 band in aqueous sulfate from Na 2 SO 4 in 4.3 M NaCl at 90°C, as a function of its concentration; D) Comparative plot of reaction progression from the small-and wide-angle scattering data as well as the Raman pattern showing correspondence between kinetic pathways (see main text for an explanation of the kinetic proxies). Generic model of the steps leading to the formation of crystalline CaSO 4 phases based on the results of in situ scattering and vibrational spectroscopic experiments.…”