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

Abstract

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.