The influence of {Ba2+}:{SO42-} on particle charge, size and morphology during BaSO4 Crystal Nucleation and Growth in Aqueous Solutions

Barite (BaSO4) is a major nuisance in terms of scale in oil and gas recovery and in the use of geothermal energy. Therefore, it is essential to study the physico-chemical paramaters influencing the particle charge, size, morphology and timing of formation, providing new insight into mitigating BaSO4 scale formation. In this study, the impact of solution stoichiometry (raq) at a fixed supersaturation (Ωbarite), upon the formation (i.e. nucleation + growth) of BaSO4 crystals in 0.02 M NaCl suspensions, on the development of particle size, charge and morphology was investigated using Dynamic Light Scattering (DLS), Mixed-Mode Measurement – Phase Analysis Light Scattering (M3-PALS) and Scanning Electron Microscopy (SEM). DLS batch experiments showed that the average particle size in all suspensions at Ωbarite = 1000 with varying raq of the largest population present grew from ~ 200 to ~ 700 nm within 10 to 15 minutes and grew fastest near the ideal 1:1 stoichiometric ratio and more slowly at non-stoichiometric conditions. Additional flow DLS measurements at the same initial conditions confirmed that BaSO4 nucleation kinetics were very fast and showed strong signs of aggregation of prenucleation clusters, which formed particles in the range of 200 – 300 nm. M3-PALS batch experiments  showed that the charge stayed negative for raq 1. At raq = 1, positive and negative populations of particles prevailed for 2.5 hours before circumneutrally charged particles remained. Moreover, SEM results showed that morphology is drastically affected by Ωbarite and raq

The Influence of {Ba2+}:{SO42-} Solution Stoichiometry on BaSO4 Crystal Nucleation and Growth in Aqueous Solutions

The impact of solution stoichiometry, upon formation of BaSO4 crystals in 0.02 M NaCl suspensions, on the development of particle size was investigated using Dynamic Light Scattering (DLS). Measurements were performed on a set of suspensions prepared with predefined initial supersaturation (Ωbarite = {Ba2+}{SO42-}/Ksp = 1000) and dissolved ion activity stoichiometries (raq = {Ba2+}:{SO42-} = 0.01, 0.1, 1, 10 and 100), at a pH of 5.5 to 6.0, and ambient temperature and pressure. At this Ωbarite and set of raq , the average apparent hydrodynamic particle size of the largest population present in all suspensions grew from ~ 200 nm to ~ 700 nm within 10 to 15 minutes. This was independently confirmed by TEM imaging. Additional DLS measurements conducted at the same conditions in flow confirmed that the BaSO4 formation kinetics were very fast for our specifically chosen conditions. The DLS flow measurements, monitoring the first minute of BaSO4 formation, showed strong signs of aggregation of prenucleation clusters forming particles with a size in the range of 200 – 300 nm for every raq. The estimated initial bulk growth rates from batch DLS results show that BaSO4 crystals formed fastest at near stoichiometric conditions and more slowly at non-stoichiometric conditions. Moreover, at extreme SO4-limiting conditions barite formation was slower compared to Ba-limiting conditions. Our results show that DLS can be used to investigate nucleation and growth at carefully selected experimental and analytical conditions. Additional SEM imaging on formed BaSO4 crystals for a range of initial conditions of Ωbarite (i.e. 31, 200, 1000 and 6000), raq (0.01, 0.1, 1, 10 and 100) and different background electrolytes (i.e. NaCl, KCl, NaNO3 , MgSO4 and SrCl2) confirms that {Ba2+}:{SO42-} impacts the growth rate significantly in different directions for the different background electrolytes at the different Ωbarite-values. Furthermore, the BaSO4 crystal morphology varies with raq and the type of background electrolyte. The combined DLS, TEM and SEM results imply that solution stoichiometry should be considered when optimizing antiscalant efficiency to regulate BaSO4 (scale) formation processes

CaCO3 nucleation and growth at different {Ca2+}:{CO32-} ratios in aqueous environments

CaCO3 precipitation is an important process in industrial applications as well as in nature. However, its precipitation is usually impure and despite the ample amount of information available on this topic, the mechanisms playing a role during CaCO3 precipitation are still not fully unraveled. We investigated the effect of stoichiometry on the new formation (nucleation) and subsequent growth of CaCO3 over a large range of supersaturation with respect to calcite (30 > 1000), stoichiometry effects the CaCO3 formation substantially and observed trends closely match those observed with DLS. The CaCO3 precipitation kinetics are more strongly affected by the absence of Ca2+ compared to the absence of CO32-. Our results imply that, besides Ω, stoichiometry effects initial precluster size and persistence, growth mechanism and ripening time towards μm-sized crystals.m-sized crystals. Ultimately, our findings may help to improve future predictions of, or optimize the physico-chemical conditions for CaCO3 formation in industrial and geo-engineering settings

Asymmetrical dependence of {Ba2+}:{SO42–} on BaSO4 crystal nucleation and growth in aqueous solutions: A dynamic light scattering study

The impact of solution stoichiometry, upon formation of BaSO 4 crystals in 0.02 M NaCl suspensions, on the development of particle size was investigated using dynamic light scattering (DLS). Measurements were performed on a set of suspensions prepared with predefined initial supersaturation, based on the quotient of the constituent ion activity product {Ba 2+}{SO 4 2-} over the solubility product K sp (Ω barite = {Ba 2+}{SO 4 2-}/K sp = 100, 500, or 1000-11,000 in steps of 1000), and ion activity solution stoichiometries (r aq = {Ba 2+}:{SO 4 2-} = 0.01, 0.1, 1, 10 and 100), at circumneutral pH of 5.5-6.0, and ambient temperature and pressure. DLS showed that for batch experiments, crystal formation with varying r aq was best investigated at an initial Ω barite of 1000 and using the forward detection angle. At this Ω barite and set of r aq, the average apparent hydrodynamic particle size of the largest population present in all suspensions increased from ∼200 to ∼700 nm within 10-15 min and was independently confirmed by transmission electron microscopy (TEM) imaging. Additional DLS measurements conducted at the same conditions in flow confirmed that the BaSO 4 formation kinetics were very fast for our specifically chosen conditions. The DLS flow measurements, monitoring the first minute of BaSO 4 formation, showed strong signs of aggregation of prenucleation clusters forming particles with a size in the range of 200-300 nm for every r aq. The estimated initial bulk growth rates from batch DLS results show that BaSO 4 crystals formed fastest at near-stoichiometric conditions and more slowly at nonstoichiometric conditions. Moreover, at extreme SO 4-limiting conditions, barite formation was slower compared to Ba-limiting conditions. Our results show that DLS can be used to investigate nucleation and growth at carefully selected experimental and analytical conditions. The combined DLS and TEM results imply that BaSO 4 formation is influenced by solution stoichiometry and may aid to optimize antiscalant efficiency and regulate BaSO 4 (scale) formation processes

Aragonite dissolution protects calcite at the seafloor.

peer reviewedIn the open ocean, calcium carbonates are mainly found in two mineral forms. Calcite, the least soluble, is widespread at the seafloor, while aragonite, the more soluble, is rarely preserved in marine sediments. Despite its greater solubility, research has shown that aragonite, whose contribution to global pelagic calcification could be at par with that of calcite, is able to reach the deep-ocean. If large quantities of aragonite settle and dissolve at the seafloor, this represents a large source of alkalinity that buffers the deep ocean and favours the preservation of less soluble calcite, acting as a deep-sea, carbonate version of galvanization. Here, we investigate the role of aragonite dissolution on the early diagenesis of calcite-rich sediments using a novel 3D, micrometric-scale reactive-transport model combined with 3D, X-ray tomography structures of natural aragonite and calcite shells. Results highlight the important role of diffusive transport in benthic calcium carbonate dissolution, in agreement with recent work. We show that, locally, aragonite fluxes to the seafloor could be sufficient to suppress calcite dissolution in the top layer of the seabed, possibly causing calcite recrystallization. As aragonite producers are particularly vulnerable to ocean acidification, the proposed galvanizing effect of aragonite could be weakened in the future, and calcite dissolution at the sediment-water interface will have to cover a greater share of CO2 neutralization.SERENAT

Recalibrating the calcium trap in amino acid carboxyl groups via classical molecular dynamics simulations

In order to use classical molecular dynamics to complement experiments accurately, it is important to use robust descriptions of the system. The interactions between biomolecules, like aspartic and glutamic acid, and dissolved ions are often studied using standard biomolecular force-fields, where the interactions between biomolecules and cations are often not parameterized explicitly. In this study, we have employed metadynamics simulations to investigate different interactions of Ca with aspartic and glutamic acid and constructed the free energy profiles of Ca2+–carboxylate association. Starting from a generally accepted, AMBER-based force field, the association was substantially over and under-estimated, depending on the choice of water model (TIP3P and SPC/fw, respectively). To rectify this discrepancy, we have replaced the default calcium parameters. Additionally, we modified the σij value in the hetero-atomic Lennard-Jones interaction by 0.5% to further improve the interaction between Ca and carboxylate, based on comparison with the experimentally determined association constant for Ca with the carboxylate group of L-aspartic acid. The corrected description retrieved the structural properties of the ion pair in agreement with the original biomolecule – Ca2+ interaction in AMBER, whilst also producing an association constant comparable to experimental observations. This refined force field was then used to investigate the interactions between amino acids, calcium and carbonate ions during biogenic and biomimetic calcium carbonate mineralisation

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

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 ∼1 million km3 of evaporites (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 SO4/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

Enhanced phosphorus recycling during past oceanic anoxia amplified by low rates of apatite authigenesis

International audienceEnhanced recycling of phosphorus as ocean deoxygenation expanded under past greenhouse climates contributed to widespread organic carbon burial and drawdown of atmospheric CO 2 . Redox-dependent phosphorus recycling was more efficient in such ancient anoxic marine environments, compared to modern anoxic settings, for reasons that remain unclear. Here, we show that low rates of apatite authigenesis in organic-rich sediments can explain the amplified phosphorus recycling in ancient settings as reflected in highly elevated ratios of organic carbon to total phosphorus. We argue that the low rates may be partly the result of the reduced saturation state of sediment porewaters with respect to apatite linked to ocean warming and acidification and/or a decreased availability of calcium carbonate, which acts as a template for apatite formation. Future changes in temperature and ocean biogeochemistry, induced by elevated atmospheric CO 2 , may similarly increase phosphorus availability and accelerate ocean deoxygenation and organic carbon burial

Sorption and catalytic oxidation of Fe(II) at the surface of calcite

The effect of sorption and coprecipitation of Fe(II) with calcite on the kinetics of Fe(II) oxidation was investigated. The interaction of Fe(II) with calcite was studied experimentally in the absence and presence of oxygen. The sorption of Fe(II) on calcite occurred in two distinguishable steps: (a) a rapid adsorption step (seconds-minutes) was followed by (b) a slower incorporation (hours-weeks). The incorporated Fe(II) could not be remobilized by a strong complexing agent (phenanthroline or ferrozine) but the dissolution of the outmost calcite layers with carbonic acid allowed its recovery. Based on results of the latter dissolution experiments, a stoichiometry of 0.4 mol% Fe:Ca and a mixed carbonate layer thickness of 25 nm (after 168 It equilibration) were estimated. Fe(II) sorption on calcite could be successfully described by a surface adsorption and precipitation model (Comans & Middelburg, GCA 51 (1987), 2587) and surface complexation modeling (Van Cappellen et al., GCA 57 (1993), 3505; Pokrovsky et al., Langmuir 16 (2000), 2677). The surface complex model required the consideration of two adsorbed Fe(II) surface species, >CO3Fe+ and >CO3FeCO3H0. For the formation of the latter species, a stability constant is being suggested. The oxidation kinetics of Fe(II) in the presence of calcite depended on the equilibration time of aqueous Fe(II) with the mineral prior to the introduction of oxygen. If pre-equilibrated for >15 h, the oxidation kinetics was comparable to a calcite-free system (t(1/2) = 145 +/- 15 min). Conversely, if Fe(II) was added to an aerated calcite suspension, the rate of oxidation was higher than in the absence of calcite (t(1/2) = 41 +/- 1 min and t(1/2) = 100 +/- 15 min, respectively). This catalysis was due to the greater reactivity of the adsorbed Fe(II) species, >CO3FeCO3H0, for which the species specific rate constant was estimated. (c) 2009 Elsevier Ltd. All rights reserved