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

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.

Differential Precipitation of Mg(OH)2 from CaSO4·2H2O Using Citrate as Inhibitor—A Promising Concept for Reagent Recovery from MgSO4 Waste Streams

In hydrometallurgical processing and acidic wastewater treatment, one of the neutralizing agents employed is MgO or Mg(OH)2. At the end of this process, the resulting solution, which is rich in SO42− and Mg2+ is treated with lime to remove (or minimize the amount) of these ions via the precipitation of Mg(OH)2 and CaSO4·2H2O (gypsum). In our work, an attempt was made to separate the two solids by increasing the induction time of the gypsum precipitation, thus regenerating relatively pure Mg(OH)2 which could be reused in wastewater treatments or hydrometallurgical processing circuits, and in this way, significantly enhancing the economic viability of the process. During our experiments, the reaction of an MgSO4 solution with milk of lime prepared from quicklime was studied. The effects of a range of organic additives, which can slow down the precipitation of gypsum have been assessed. The process was optimized for the most promising inhibiting agent—that is, the citrate ion. The reactions were continuously monitored in situ by conductometric measurements with parallel monitoring of solution pH and temperature. ICP-OES measurements were also carried out on samples taken from the reaction slurry. The composition of the precipitating solids at different reaction times was established by powder XRD and their morphology by SEM. Finally, experiments were carried out to locate the additive after the completion of the precipitation reaction to get information about its potential reuse.

Flow and reaction along the cement-rock interface during CO2 injection. Laboratory experiments and modeling.

<p>The interface between reservoir/cap rocks and the Portland cement around boreholes is a possible leakage pathway during deep geological injection of CO<sub>2</sub>. To study the alteration of cement and rock, laboratory experiments involving flow along this interface were performed. Cylindrical cores of about 5 cm in length and 2.5 cm in diameter and composed of half-cylinders of cement and rock (sandstone, limestone, marl) were used. They were reacted with a synthetic sulfate-rich saline groundwater under (a) atmospheric conditions (10<sup>-3.4</sup> bar CO<sub>2</sub>, 25ºC, pH 6.2) and (b) supercritic conditions (130 bar CO<sub>2</sub>, 60ºC, pH about 3) in flow-through reactors. Tracer (LiBr) tests were performed prior to the injection of the saline solution in the atmospheric experiments to characterize cement diffusivity. The evolution of solution chemistry at the outlet was monitored over time. Rock and cement were analyzed at the end of the experiments (SEM, XRD, profilometry).</p><p>In the atmospheric experiments pH increased up to about 11 (tracer tests) and 8 (groundwater injection, brucite precipitation). Calculated outlet pH was about 4 under supercritic conditions. Major-element concentrations showed little change during the atmospheric experiments, while Ca excess and S deficit were observed under supercritic conditions. Intense brucite precipitation was observed on the cement surface after the atmospheric experiments, while an apparently amorphous red-colored phase precipitated under supercritic conditions. Rock surfaces evidenced calcite dissolution in the supercritic experiments, while alteration was little in the atmospheric experiments. Some gypsum precipitation was also observed. Interface aperture increased during the supercritic experiments.</p><p>2D reactive transport modeling (CrunchFlow) was used to interpret the results. Phase reactivities (surface areas), and in some cases diffusion coefficients (rock and cement), were adjusted to fit models to measurements (solution and solid). Under atmospheric conditions, brucite precipitation (and decrease in porosity) results from the mixing by diffusion of the Mg in the input solution and the alkalinity in the cement. Ca from portlandite dissolution and sulfate from the input solution drives the precipitation of gypsum. For the supercritic experiments, model results show intense dissolution of portlandite, ettringite, siliceous hydrogarnet and hydrotalcite, extending for about 3 mm into the cement and causing an increase in porosity. The Ca released precipitates as calcite, with carbonate provided by the CO<sub>2</sub>-rich input solution. As the portlandite front moves into the cement, calcite dissolves next to the interface and some of the Ca precipitates as gypsum. Coupled calcite dissolution and gypsum precipitation also occurs, to a lesser extent, in the rock side. The calculations also result in the precipitation of small amounts of ferrihydrite, gibbsite and boehmite, which could correspond to the observed red-colored precipitates. Importantly, the adjusted values of the reactive surface areas for the different experiments point to a larger reactivity of the cement under supercritic conditions.</p>

Paratethys pacing of the Messinian Salinity Crisis: low salinity waters contributing to gypsum precipitation?

<p><strong>During the so-called Messinian Salinity Crisis (MSC: 5.97-5.33 Myr ago), reduced exchange with the Atlantic Ocean caused the Mediterranean to develop into a “saline giant” wherein ~</strong><strong>1 million km<sup>3</sup> of evaporites </strong><strong>(gypsum and halite) were deposited. Despite decades of research it is still poorly understood exactly how and where in the water column these evaporites formed. Gypsum formation commonly requires enhanced dry conditions (evaporation exceeding precipitation), but recent studies also suggested major freshwater inputs into the Mediterranean during MSC-gypsum formation. Here we use strontium isotope ratios of ostracods to show that low-saline water from the Paratethys Seas actually contributed to the precipitation of Mediterranean evaporites. This apparent paradox urges for an alternative mechanism underlying gypsum precipitation. We propose that Paratethys inflow would enhance stratification in the Mediterranean and result in a low-salinity surface-water layer with high Ca/Cl and SO<sub>4</sub>/Cl ratios. We show that evaporation of this surface water can become saturated in gypsum at a salinity of ~40, in line with salinities reported from fluid inclusions in MSC evaporites.</strong></p>

Evaluation of secondary mineral precipitation by reactive transport modeling at the Ketzin CO2 storage site, Germany

<p>One of the major keys to the success of the carbon capture and storage (CCS) is understanding the geochemical effects that CO<sub>2</sub> has on the storage reservoir. The injection of CO<sub>2</sub> into the reservoir disturbs geochemical equilibrium as it induces acid-generation reactions with subsequent CO<sub>2</sub>-brine-mineral interactions, including dissolution of certain host minerals and precipitation of secondary minerals. The mineral precipitation, especially precipitation of carbon-bearing minerals in geological formations, is generally a favorable for CO<sub>2</sub> trapping mechanism that ensures long-term geologic CO<sub>2</sub> sequestration. These precipitates, however, may clog the wellbore and its surroundings, followed by loss of injectivity.</p><p>The current study is dedicated towards a better understanding of the geochemistry of the geological CO<sub>2</sub> storage based on the Ketzin CO<sub>2</sub> pilot site. The Ketzin CO<sub>2</sub> storage site, the first on-shore geological CO<sub>2</sub> storage site in the European mainland, is demonstrated a safe and reliable CO<sub>2</sub> storage operation after injection of about 67-kilo tons of CO<sub>2</sub> and offers the unique opportunity to work on data sets from all storage life-cycle (Martens et al., 2014). Through both field measurement and modeling studies, this contribution aims to explore the secondary mineral precipitation mechanisms and identify the major influential factors during the CO<sub>2</sub> sequestration. This approach supports the H2020 project SECURe establishing best practice in baseline investigations for subsurface geoenergy operations, underpinned by data of pilot and research-scale sites in Europe and internationally. The secondary minerals solubility was investigated as a function of the reservoir temperature, pressure, and CO<sub>2</sub> concentration, which occurred in the reservoir. Special focus is set to sulfate minerals, as field evidence exists that gypsum precipitates as a result of reservoir exposition to CO<sub>2</sub>. Batch modeling was performed using the PHREEQC code version 3 (Parkhurst and Appelo, 2013) with the Pitzer database (pitzer.dat). The coupling interface OGS#IPhreeqc (He et al., 2015) applied reactive transport modeling, and the coupled reactive-transport processes in the reservoir with complex chemistry can be modeled. Our results suggest that the gypsum precipitation was found to increase as CO<sub>2</sub> concentration ascends. However, no significant porosity and permeability alterations are observed since the gypsum precipitation acts as a Ca<sup>2+</sup> sink and leads to further carbonate dissolution. The results highlight the high reactivity of the near-well zone due to CO<sub>2</sub> injection and emphasize the need to be monitored in the injection well to avoid the potential formation of gypsum, which could lead to well clogging.</p><p>He, W., Beyer, C., Fleckenstein, J.H., Jang, E., Kolditz, O., Naumov, D., Kalbacher, T., 2015. A parallelization scheme to simulate reactive transport in the subsurface environment with OGS#IPhreeqc 5.5.7-3.1.2. Geosci. Model Dev. 8, 3333-3348.</p><p>Martens, S., Möller, F., Streibel, M., Liebscher, A., 2014. Completion of Five Years of Safe CO2 Injection and Transition to the Post-closure Phase at the Ketzin Pilot Site. Energy Procedia 59, 190-197.</p><p>Parkhurst, D.L., Appelo, C.A.J., 2013. Description of input and examples for PHREEQC version 3: a computer program for speciation, batch-reaction, one-dimensional transport, and inverse geochemical calculations, Techniques and Methods, Reston, Virginia, USA, p. 519.</p>