In natural and engineered systems, nanoparticles can form in solution as homogeneous precipitation and on substrates (e.g., rocks, membranes, catalysis support, equipment and facilities) as heterogeneous precipitation. Nanoparticle precipitation starts with nucleation, followed by subsequent particle growth. The homogeneous and heterogeneous nucleation and growth processes affect the physicochemical properties of nanoparticles (e.g., size, composition, structure, and reactivity). Such processes also control the fate and transport of aqueous contaminants, and affect the safety and efficiency of many subsurface operations (e.g., oil production, geologic carbon sequestration, managed aquifer recharge) and membrane water treatment processes. Despite the importance, the mechanisms and rates of nanoparticle nucleation and growth, especially in the presence of substrates, were not well understood. For example, Fe(III) hydroxide nanoparticles, which can sequestrate aqueous metal cations through structural incorporation, surface adsorption, and surface precipitation, are an essential carrier for heavy metals in many natural and engineered aqueous environments. Using synchrotron-based grazing-incidence small angle X-ray scattering technique (GISAXS), homogeneous (in solution) and heterogeneous (on quartz and corundum) nucleation and growth of Fe hydroxide were quantified in 10-4 M Fe3+ solution in the presence of various metal ions of Cu2+, Pb2+, Al3+, Cr3+ or Mn2+ at pH = 3.8 ± 0.1. The interfacial interactions among aqueous ions, substrate surfaces, and nanoparticles were explored with quartz crystal microbalance dissipation (QCM-D), dynamic light scattering (DLS), and Fourier transform infrared spectrometry (FTIR). Pb(II) was significantly enriched on Fe hydroxide surfaces, thus inhibited particle growth. Aqueous Cr(III)/Fe(III) ratios were found to control the chemical composition (x) of (Fex, Cr1-x)(OH)3 precipitates, and affect their homogeneous and heterogeneous precipitation rates in different ways. The different adsorption behavior of aqueous ions onto quartz and corundum also affected the surface charge of the substrates and thus affected nanoparticle precipitation. We also studied the heterogeneous nucleation and growth of BaSO4, as a representative sparingly-soluble salt whose bonding between constituent ions is dominantly ionic. The controlling mechanisms of BaSO4 nucleation and growth were also explored, which were partly different from those of covalently bonded Fe hydroxide nanoparticles.
Bio: Dr. Yandi Hu obtained her Ph.D. in 2013 from Washington University in St. Louis, and she is currently an assistant professor in the department of Civil & Environmental Engineering at University of Houston. Her main research areas are environmental nanochemistry and geochemistry. Some specific interests include: nucleation and growth of iron hydroxide nanoparticles and heavy metal immobilization, surface and subsurface geochemical reactions related to geologic CO2 sequestration, radioactive waste immobilization, and hydraulic fracture operations.
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