Doyoon Kim

Heterogeneous Nucleation and Growth of Nanoparticles at Environmental Interfaces

Mineral nucleation is a phase transformation of aqueous components to solids with an accompanying creation of new surfaces. In this evolutional, yet elusive, process, nuclei often form at environmental interfaces, which provide remarkably reactive sites for heterogeneous nucleation and growth. Naturally occurring nucleation processes significantly contribute to the biogeochemical cycles of important components in the Earths crust, such as iron and manganese oxide minerals and calcium carbonate. However, in recent decades, these cycles have been significantly altered by anthropogenic activities, which affect the aqueous chemistry and equilibrium of both surface and subsurface systems. These alterations can trigger the dissolution of existing minerals and formation of new nanoparticles (i.e., nucleation and growth) and consequently change the porosity and permeability of geomedia in subsurface environments. Newly formed nanoparticles can also actively interact with components in natural and engineered aquatic systems, including those posing a significant hazard such as arsenic. These interactions can bilaterally influence the fate and transport of both newly formed nanoparticles and aqueous components. Due to their importance in natural and engineered processes, heterogeneous nucleation at environmental interfaces has started to receive more attention. However, a lack of time-resolved in situ analyses makes the evaluation of heterogeneous nucleation challenging because the physicochemical properties of both the nuclei and surfaces significantly and dynamically change with time and aqueous chemistry. This Account reviews our in situ kinetic studies of the heterogeneous nucleation and growth behaviors of iron(III) (hydr)oxide, calcium carbonate, and manganese (hydr)oxide minerals in aqueous systems. In particular, we utilized simultaneous small-angle and grazing incidence small-angle X-ray scattering (SAXS/GISAXS) to investigate in situ and in real-time the effects of water chemistry and substrate identity on heterogeneously and homogeneously formed nanoscale precipitate size dimensions and total particle volume. Using this technique, we also provided a new platform for quantitatively comparing between heterogeneous and homogeneous nucleation and growth of nanoparticles and obtaining undiscovered interfacial energies between nuclei and surfaces. In addition, nanoscale surface characterization tools, such as in situ atomic force microscopy (AFM), were utilized to support and complement our findings. With these powerful nanoscale tools, we systematically evaluated the influences of environmentally abundant (oxy)anions and cations and the properties of environmental surfaces, such as surface charge and hydrophobicity. The findings, significantly enhanced by in situ observations, can lead to a more accurate prediction of the behaviors of nanoparticles in the environment and enable better control of the physicochemical properties of nanoparticles in engineered systems, such as catalytic reactions and energy storage.

The role of confined collagen geometry in decreasing nucleation energy barriers to intrafibrillar mineralization

Mineralization of collagen is critical for the mechanical functions of bones and teeth. Calcium phosphate nucleation in collagenous structures follows distinctly different patterns in highly confined gap regions (nanoscale confinement) than in less confined extrafibrillar spaces (microscale confinement). Although the mechanism(s) driving these differences are still largely unknown, differences in the free energy for nucleation may explain these two mineralization behaviors. Here, we report on experimentally obtained nucleation energy barriers to intra- and extrafibrillar mineralization, using in situ X-ray scattering observations and classical nucleation theory. Polyaspartic acid, an extrafibrillar nucleation inhibitor, increases interfacial energies between nuclei and mineralization fluids. In contrast, the confined gap spaces inside collagen fibrils lower the energy barrier by reducing the reactive surface area of nuclei, decreasing the surface energy penalty. The confined gap geometry, therefore, guides the two-dimensional morphology and structure of bioapatite and changes the nucleation pathway by reducing the total energy barrier.Nucleation in highly confined gaps shows distinctly different behavior from nucleation in extrafibrillar spaces. Here, using in situ X-ray scattering and classical nucleation theory, the authors show how confined geometry reduces energy barriers to intrafibrillar mineralization of collagen.

Designing the crystalline structure of calcium phosphate seed minerals in organic templates for sustainable phosphorus management

Global phosphorus (P) should be managed more sustainably to secure food, energy, and water for a growing population. Despite the abundance of calcium in most environments, we have not fully utilized its thermodynamic stability to form calcium phosphate minerals (CaP) for aqueous P management. In this study, we showed that the energy barriers to CaP nucleation can be reduced by seeding reactive CaP nuclei in calcium alginate beads. The CaP nucleation kinetics enhanced by seeds effectively immobilized aqueous P into the macroscale beads, which can be reused as a slow-release fertilizer. Given that more developed CaP crystalline seeds have a lower solubility than does an amorphous structure, equilibrium P concentration was regulated successfully by the seed crystallinity during both the removal and release processes. A simultaneous seed nucleation during alginate gelation enabled control of the degree of the seeds’ crystallization without using any hazardous substance or additional energy input. Poorly crystalline hydroxyapatite CaP seeds effectively decreased aqueous P concentration from 200 to 22.7 μM within one day at a final pH 7.2 (96.4 mg P g−1 dry seed). After P recovery, the beads were moved to a P-deficient environment to be evaluated as a slow-release fertilizer. Utilizing the thermodynamic stability of CaP at neutral pH, this approach highlights a potential application of naturally abundant biomaterials for sustainable P management.

Co-effects of UV/H2O2 and Natural Organic Matter on the Surface Chemistry of Cerium Oxide Nanoparticles

The widespread industrial applications of cerium oxide (CeO2) nanoparticles (NPs) have increased their likelihood of entering natural and engineered aqueous environments. This study investigates the surface chemistry changes of CeO2 NPs at pH 5.4 in the presence of both UV/H2O2 and natural organic matter (NOM). These conditions are relevant to advanced oxidation processes (AOPs). The results indicated that NOM stabilized CeO2 NPs in solution through surface complexation between the COO− functional groups of NOM and the CeO2 surfaces, reversing the zeta potential of CeO2 from 39.5 ± 2.7 mV to −38.3 ± 1.8 mV. UV/H2O2 treatment reduced the colloidal stability of CeO2 NPs, increased the percentage of Ce3+ on the surface from 17.8% to 28.3%, and lowered the zeta potential to close to neutral (3.8 ± 3.4 mV). With UV/H2O2 and NOM together, NOM coated on CeO2 NPs acted as a protective layer, making the direct reactions between reactive oxygen species (ROS) and CeO2 and their impacts on the colloidal stability insignificant in a short reaction period. During the UV/H2O2 treatment, the adsorption of superoxide radicals (O2˙−) dominated in neutralizing the surface charge of CeO2, leading to decreased electrostatic repulsive forces between nanoparticles and a higher extent of sedimentation. These new findings provide important implications for understanding the colloidal stability, sedimentation, and surface chemical properties of CeO2 NPs in aqueous systems where NOM and ROS are present.

The Effects of Phosphonate-Based Scale Inhibitor on Brine–Biotite Interactions under Subsurface Conditions

To explore the effects of scale inhibitors on subsurface water-mineral interactions, here batch experiments on biotite dissolution (0-96 h) were conducted in solutions containing 0-1.0 mM diethylenetriaminepenta(methylene)phosphonate (DTPMP, a model scale inhibitor), at conditions simulating subsurface environments (95 °C and 102 atm CO2). The phosphonate groups in DTPMP enhanced biotite dissolution through both aqueous and surface complexations with Fe, with more significant effects at a higher DTPMP concentration. Surface complexation made cracked biotite layers bend, and these layers detached at a later stage (≥44 h). The presence of DTPMP also promoted secondary precipitation of Fe- and Al-bearing minerals both in the solution and on the reacted biotite surfaces. With 1.0 mM DTPMP after 44 h, significant coverage of biotite surfaces by precipitates and less detachment of cracked layers blocked reactive sites and inhibited further biotite dissolution. Furthermore, adsorption of DTPMP made the reacted biotite basal surfaces more hydrophilic, which may affect the transport of reactive fluids. This study provides new information on the impacts of phosphonates in brine-mineral interactions, benefiting safer and more environmentally sustainable design and operation of engineered subsurface processes.