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, based on the quotient of the constituent ion activity product {Ba2+}{SO4 2–} over the solubility product K sp (Ωbarite = {Ba2+}{SO4 2–}/K sp = 100, 500, or 1000–11,000 in steps of 1000), and ion activity solution stoichiometries (r aq = {Ba2+}:{SO4 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 Ωbarite of 1000 and using the forward detection angle. At this Ωbarite 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 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 r aq. The estimated initial bulk growth rates from batch DLS results show that BaSO4 crystals formed fastest at near-stoichiometric conditions and more slowly at nonstoichiometric 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. The combined DLS and TEM results imply that BaSO4 formation is influenced by solution stoichiometry and may aid to optimize antiscalant efficiency and regulate BaSO4 (scale) formation processes.
The effect of stoichiometry on the new formation and subsequent growth of CaCO 3 was investigated over a large range of solution stoichiometries (10 –4 < r aq < 10 4 , where r aq = {Ca 2+ }:{CO 3 2– }) at various, initially constant degrees of supersaturation (30 < Ω cal < 200, where Ω cal = {Ca 2+ }{CO 3 2– }/ K sp ), pH of 10.5 ± 0.27, and ambient temperature and pressure. At r aq = 1 and Ω cal < 150, dynamic light scattering (DLS) showed that ion adsorption onto nuclei (1–10 nm) was the dominant mechanism. At higher supersaturation levels, no continuum of particle sizes is observed with time, suggesting aggregation of prenucleation clusters into larger particles as the dominant growth mechanism. At r aq ≠ 1 (Ω cal = 100), prenucleation particles remained smaller than 10 nm for up to 15 h. Cross-polarized light in optical light microscopy was used to measure the time needed for new particle formation and growth to at least 20 μm. This precipitation time depends strongly and asymmetrically on r aq . Complementary molecular dynamics (MD) simulations confirm that r aq affects CaCO 3 nanoparticle formation substantially. At r aq = 1 and Ω cal ≫ 1000, the largest nanoparticle in the system had a 21–68% larger gyration radius after 20 ns of simulation time than in nonstoichiometric systems. Our results imply that, besides Ω cal , stoichiometry affects particle size, persistence, growth time, and ripening time toward micrometer-sized crystals. Our results may help us to improve the understanding, prediction, and formation of CaCO 3 in geological, industrial, and geo-engineering settings.
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