Gypsum Precipitation under Saline Conditions: Thermodynamics, Kinetics, Morphology, and Size Distribution

Abstract

Gypsum (CaSO4·2H2O) is the most common sulfate mineral on Earth and is also found on Mars. It is an evaporitic mineral that predominantly precipitates from brines. In addition to its precipitation in natural environments, gypsum also forms an undesired scale in many industrial processes that utilize or produce brines. Thus, better insights into gypsum formation can contribute to the understanding of natural processes, as well as improving industrial practices. Subsequently, the thermodynamics, nucleation and crystal growth mechanisms and kinetics, and how these factors shape the morphology of gypsum have been widely studied. Over the last decade, the precipitation of gypsum under saline and hypersaline conditions has been the focus of several studies. However, to date, most of the thermodynamic data are derived from experiments with artificial solutions that have limited background electrolytes and have Ca2+/SO42− ratios that are similar to the 1:1 ratio in the mineral. Moreover, direct observations of the nucleation and growth processes of gypsum are still derived from experimental settings that can be described as having low ionic strength. Thus, the mechanisms of gypsum precipitation under conditions from which the mineral precipitates in many natural environments and industrial processes are still less well known. The present review focuses on the precipitation of gypsum from a range of aspects. Special attention is given to brines. The effects of ionic strength, brine composition, and temperature on the thermodynamic settings are broadly discussed. The mechanisms and rates of gypsum nucleation and growth, and the effect the thermodynamic properties of the brine have on these processes is demonstrated by recent microscopic and macroscopic observations. The morphology and size distribution of gypsum crystals precipitation is examined in the light of the precipitation processes that shape these properties. Finally, the present review highlights discrepancies between microscopic and macroscopic observations, and studies carried out under low and high ionic strengths. The special challenges posed by experiments with brines are also discussed. Thus, while this review covers contemporary literature, it also outlines further research that is required in order to improve our understanding of gypsum precipitation in natural environments and industrial settings.

Editorial for Special Issue “Formation of Sulfate Minerals in Natural and Industrial Environments”

“…In this valuable contribution, the authors demonstrate the importance of small contamination particles (nm to microns), which drive the nucleation of gypsum, and whose presence is rarely, if ever, considered. This paper highlights the complexity of solution-based growth phenomena in sulfates, and clearly links to [8,9].…”

“…This nano-to-macro aspect, related to CaSO 4 , is further explored by Reiss at al. in [9], where a thermodynamic modelling approach to calcium sulfate crystallization is explored in addition to current experimental observations. This is undertaken with an emphasis on high-salinity environments, i.e., brines, and is thus relevant for industrial and geological settings on Earth (e.g., the Dead Sea), and other planets such as Mars.…”

Removal of High-Concentration Sulfate from Seawater by Ettringite Precipitation

“…e CaSO 4 precipitation method is one of the most widely applied methods in the industry to remove sulfate in waters through the formation of CaSO 4 or gypsum from Ca 2+ and SO 4 2− ; but the high solubility of CaSO 4 (1,200-2,000 ppm in low-salinity water and 5000∼7000 ppm in high-salinity water) results in insufficient removal of SO 4 2− and high Ca 2+ concentrations in treated water, which could significantly degrade the performance of prepared fracturing fluid [2,[18][19][20]. Another highly effective method is BaSO 4 precipitation, in which barium chloride or barium carbonate is used to form barium sulfate (pK sp � 10) with SO 4 2− .…”

Crystallization Kinetics of α-Hemihydrate Gypsum Prepared by Hydrothermal Method in Atmospheric Salt Solution Medium

“…According to the difference in hydration, calcium sulfate can be divided into dihydrate gypsum (CaSO 4 •2H 2 O), hemihydrate gypsum (CaSO 4 •0.5H 2 O), and anhydrite (CaSO 4 ) [1][2][3]. Phosphogypsum (PG) is mainly composed of calcium sulfate dihydrate (DH), which is a byproduct from the wet process of phosphoric acid production.…”