Heterogeneous Nucleation, Growth, and Recrystallization of Nanoparticles under Environmentally Relevant Conditions

The kinetics of heterogeneous nucleation, growth, and recrystallization (i.e., Ost-wald ripening) of environmentally abundant nanoparticles (i.e., ferrihydrite, barite, and lead phosphate) on substrates were quantified using in situ grazing incidence small angle X-ray scattering (GISAXS). To explore the mechanisms, the interactions among nanopar-ticles, substrates, and ions were investigated. Ferrihydrite nanoparticles can sequester various metal ions (i.e., Cu2+, Pb2+, Cr3+, Ni2+, Cd2+, Al3+, and Mn2+), and form impure ferrihydrite nanoparticles. The sequestration mechanisms (i.e., surface enrichment and structural incorporation) of these metal ions with ferrihydrite at acidic pH condition were identified, which were related to the metal ion hydrolysis. Also, these metal ions showed selective adsorption behaviors onto differ-ent substrates (i.e., quartz and corundum), which were controlled by the hydrolysis of these metal ions and the dielectric constants of the substrates. The electrostatic interac-tions between particles and substrates governed the heterogeneous nucleation, growth, and recrystallization (i.e., Ostwald ripening) processes of ferrihydrite.
Barium sulfate (BaSO4) is a common scale-forming mineral in natural and engi-neered systems. Under [Ba2+]/[SO42-] = 1, little amount of BaSO4 formed on bare and –COOH coated glasses; Meanwhile, BaSO4 heterogeneous nucleation was significantly promoted on–SH and mixed –SH & COOH coatings. We also found that heterogeneous precipitation rate increased under [Ba2+]/[SO42-] > 1. These observations suggest that thermodynamic favorability for nucleation is governed by substrate-water interfacial en-ergy; while given favorable thermodynamics, rate is governed by ion attachment to sub-strates and incipient nuclei. Phosphate is added to Pb-contaminated soils to induce lead immobilization through lead phosphate precipitation. Here, heterogeneous lead phosphate nucleation on coatings was measured from mixed Pb(NO3)2 and Na2HPO4/NaH2PO4 solutions at pH 7 with varied ionic strengths (IS = 0.58, 4, and 11 mM). Raman spectroscopy identified the homogeneous precipitates in solution as hydroxylpyromorphite (Pb5(PO4)3OH). The smallest lead phosphate nuclei (4.5 ± 0.5 nm) were observed on –COOH coatings, which resulted from the highest level of lead and phosphate ion adsorption on –COOH coatings. The IS of the solution also affected the sizes of the heterogeneous precipitates on –COOH coating, with smaller nuclei (1.3 ± 0.4 nm) forming under higher IS (4 and 11 mM).