Asymmetrical Dependence of {Ba²⁺}:{SO₄^2–} on BaSO₄ Crystal Nucleation and Growth in Aqueous Solutions: A Dynamic Light Scattering Study

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+}{SO42–} over the solubility product Ksp (Ωbarite = {Ba2+}{SO42–}/Ksp = 100, 500, or 1000–11,000 in steps of 1000), and ion activity solution stoichiometries (raq = {Ba2+}:{SO42–} = 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 raq was best investigated at an initial Ωbarite of 1000 and using the forward detection angle. At this Ωbarite and set of raq, 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 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 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

Schematic of key processes affecting S, Fe, CH₄ and P in and below the SMT.

<p>The dotted lines indicate how the release of Fe²⁺ and PO₄ from Fe-oxides lead to Fe(II)-P formation.</p

Sediment depth profiles of CDB Fe, total Fe, total Mn and total S (in µmol/g).

<p>CDB-Fe is typically used as a measure of total Fe-oxide Fe in the sediment but may include vivianite-Fe <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0062386#pone.0062386-Nembrini1" target="_blank">[33]</a> and Fe-sulfides <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0062386#pone.0062386-Slomp3" target="_blank">[26]</a>.</p

Location map of the study sites in the Bothnian Sea.

<p>Site characteristics are given in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0062386#pone-0062386-t001" target="_blank">Table 1</a>. Simplified bathymetric overlay and bottom sediment type are adapted from <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0062386#pone.0062386-Korshuk1" target="_blank">[73]</a> and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0062386#pone.0062386-AlHamdani1" target="_blank">[72]</a>, respectively.</p

Porewater profiles of SO₄²⁻ and CH4 for sites US2 and US5b for June 2009.

<p>Porewater profiles of SO₄²⁻ and CH4 for sites US2 and US5b for June 2009.</p

Compilation of key sediment and porewater profiles for US5B.

<p>Depth profiles of sediment contents of CDB-Fe, total sulfur (S_tot), reactive Fe oxides (Fe_reac; see text) and Fe-bound P in units of µmol/g, and depth profiles of porewater concentrations of methane, sulfate and dissolved Fe²⁺ and PO₄ (in µmol/l) for site US5B. Data are for October 2008, unless indicated otherwise.</p

Porewater profiles of key components for June 2009 and October 2008.

<p>Units are in µmol/l. Note the different scales for group 1 and groups 2 and 3.</p

Characteristics of the 6 study sites based on conditions at the time of sampling in October 2008 and June 2009.

<p>O₂-pen.: oxygen penetration into the sediment as measured with micro-electrodes. Temp: temperature of the bottom water. Sediment accumulation rates (SAR) and the annually accumulated layer of sediment (AAL in cm/yr, at a depth of 10 cm) are based on ¹³⁷Cs dating and sediment porosity and density as described in the study of Mattila et al. <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0062386#pone.0062386-Mattila1" target="_blank">[27]</a>.</p

Depth profiles of sediment P, Fe and Mn (in µmol/g).

<p>Sediments for sites US5B and SR5 were extracted with CDB (filled circles) and ascorbate (open circles).</p

Sediment phosphorus speciation (in µmol/g) and organic carbon contents (C_org in wt%) for all sites.

<p>Sediment phosphorus speciation (in µmol/g) and organic carbon contents (C_org in wt%) for all sites.</p