The Influence of {Ba²⁺}:{SO₄^2-} 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 (Ω_barite = {Ba²⁺}{SO₄^2-}/K_sp =
1000) and dissolved ion activity stoichiometries (r_aq = {Ba²⁺}:{SO₄^2-} = 0.01, 0.1, 1, 10 and 100), at a pH of 5.5
to 6.0, and ambient temperature and pressure. At this Ω_barite and set of r_aq , 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 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 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 Ω_barite (i.e. 31, 200, 1000
and 6000), r_aq (0.01, 0.1, 1, 10 and 100) and different background electrolytes (i.e. NaCl, KCl, NaNO₃ , MgSO₄
and SrCl₂) confirms that {Ba²⁺}:{SO₄^2-} impacts the growth rate significantly in different directions for the
different background electrolytes at the different Ω_barite-values. Furthermore, the BaSO₄ crystal morphology
varies with r_aq 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.