Different arsenate and phosphate incorporation effects on the nucleation and growth of iron(III) (Hydr)oxides on quartz

Bridging molecular-scale interfacial science with continuum-scale models

Solid-water interfaces are crucial for clean water, conventional and renewable energy, and effective nuclear waste management. However, reflecting the complexity of reactive interfaces in continuum-scale models is a challenge, leading to oversimplified representations that often fail to predict real-world behavior. This is because these models use fixed parameters derived by averaging across a wide physicochemical range observed at the molecular scale. Recent studies have revealed the stochastic nature of molecular-level surface sites that define a variety of reaction mechanisms, rates, and products even across a single surface. To bridge the molecular knowledge and predictive continuum-scale models, we propose to represent surface properties with probability distributions rather than with discrete constant values derived by averaging across a heterogeneous surface. This conceptual shift in continuum-scale modeling requires exponentially rising computational power. By incorporating our molecular-scale understanding of solid-water interfaces into continuum-scale models we can pave the way for next generation critical technologies and novel environmental solutions.

Arsenic Removal via the Biomineralization of Iron-Oxidizing Bacteria Pseudarthrobacter sp. Fe7

Arsenic (As) is a highly toxic metalloid, and its widespread contamination of water is a serious threat to human health. This study explored As removal using Fe(II)-oxidizing bacteria. The strain Fe7 isolated from iron mine soil was classified as the genus Pseudarthrobacter based on 16S rRNA gene sequence similarities and phylogenetic analyses. The strain Fe7 was identified as a strain of Gram-positive, rod-shaped, aerobic bacteria that can oxidize Fe(II) and produce iron mineral precipitates. X-ray diffraction, X-ray photoelectron spectroscopy, and energy-dispersive X-ray spectroscopy patterns showed that the iron mineral precipitates with poor crystallinity consisted of Fe(III) and numerous biological impurities. In the co-cultivation of the strain Fe7 with arsenite (As(III)), 100% of the total Fe and 99.9% of the total As were removed after 72 h. During the co-cultivation of the strain Fe7 with arsenate (As(V)), 98.4% of the total Fe and 96.9% of the total As were removed after 72 h. Additionally, the iron precipitates produced by the strain Fe7 removed 100% of the total As after 3 h in both the As(III) and As(V) pollution systems. Furthermore, enzyme activity experiments revealed that the strain Fe7 oxidized Fe(II) by producing extracellular enzymes. When 2% (v/v) extracellular enzyme liquid of the strain Fe7 was added to the As(III) or As(V) pollution system, the total As removal rates were 98.6% and 99.4%, respectively, after 2 h, which increased to 100% when 5% (v/v) and 10% (v/v) extracellular enzyme liquid of the strain Fe7 were, respectively, added to the As(III) and As(V) pollution systems. Therefore, iron biomineralized using a co-culture of the strain Fe7 and As, iron precipitates produced by the strain Fe7, and the extracellular enzymes of the strain Fe7 could remove As(III) and As(V) efficiently. This study provides new insights and strategies for the efficient remediation of arsenic pollution in aquatic environments.

(Na, Pb)-Jarosite nucleation and growth on anglesite: Implications for inhibition of Pb releasing

The mobility and bioavailability of Pb can be significantly reduced by Pb-bearing minerals encapsulation in jarosite-group minerals, especially in sulfate-rich environments. However, the kinetic pathways and mechanisms of jarosite-group minerals formations on Pb-bearing mineral surfaces are not well understood. Here, time-resolved heterogeneous (Na, Pb)-jarosite nucleation and growth on anglesite were explored to gain insights into the encapsulation mechanisms. The initial dissolution of anglesite were clearly distinguished, and for the first time, the facet-specific heterogeneous nucleation of (Na, Pb)-jarosite on anglesite was demonstrated. Density functional theory calculations revealed higher adsorption energies and electronic interactions of FeSO4+ complex on anglesite (020), (140), (110) facets, attributed to the preferential nucleation of (Na, Pb)-jarosite on these facets, which resulted in effective passivation of the facets resistant to dissolution. An interpretation was proposed where (Na, Pb)-jarosite grew via a particle-attachment pathway involving the formation of amorphous intermediate, and subsequently, it transformed to the crystalline phase by solid-state conversion. These observations might improve the mechanistic understanding of interface interactions between slightly soluble Pb-bearing minerals and iron minerals, with implications for Pb immobilization in sulfate-rich environments.

A coupled electrochemical process for schwertmannite recovery from acid mine drainage: Important roles of anodic reactive oxygen species and cathodic alkaline

The increasing need for sustainable acid mine drainage (AMD) treatment has spurred much attention to strategic development of resource recovery. Along this line, we envisage that a coupled electrochemical system involving anodic Fe(II) oxidation and cathodic alkaline production will facilitate in situ synthesis of schwertmannite from AMD. Multiple physicochemical studies showed the successful formation of electrochemistry-induced schwertmannite, with its surface structure and chemical composition closely related to the applied current. A low current (e.g., 50 mA) led to the formation of schwertmannite having a small specific surface area (SSA) of 122.8 m² g^-1 and containing small amounts of -OH groups (formula Fe₈O₈(OH)_4.49(SO₄)_1.76), whereas a large current (e.g., 200 mA) led to schwertmannite high in SSA (169.5 m² g^-1) and amounts of -OH groups (formula Fe₈O₈(OH)_5.16(SO₄)_1.42). Mechanistic studies revealed that the reactive oxygen species (ROS)-mediated pathway, rather than the direct oxidation pathway, plays a dominant role in accelerating Fe(II) oxidation, especially at high currents. The abundance of •OH in the bulk solution, along with the cathodic production of OH^-, were the key to obtaining schwertmannite with desirable properties. It was also found to function as a powerful sorbent in removal of arsenic species from the aqueous phase.

Oxide- and Silicate-Water Interfaces and Their Roles in Technology and the Environment

Interfacial reactions drive all elemental cycling on Earth and play pivotal roles in human activities such as agriculture, water purification, energy production and storage, environmental contaminant remediation, and nuclear waste repository management. The onset of the 21st century marked the beginning of a more detailed understanding of mineral aqueous interfaces enabled by advances in techniques that use tunable high-flux focused ultrafast laser and X-ray sources to provide near-atomic measurement resolution, as well as by nanofabrication approaches that enable transmission electron microscopy in a liquid cell. This leap into atomic- and nanometer-scale measurements has uncovered scale-dependent phenomena whose reaction thermodynamics, kinetics, and pathways deviate from previous observations made on larger systems. A second key advance is new experimental evidence for what scientists hypothesized but could not test previously, namely, interfacial chemical reactions are frequently driven by "anomalies" or "non-idealities" such as defects, nanoconfinement, and other nontypical chemical structures. Third, progress in computational chemistry has yielded new insights that allow a move beyond simple schematics, leading to a molecular model of these complex interfaces. In combination with surface-sensitive measurements, we have gained knowledge of the interfacial structure and dynamics, including the underlying solid surface and the immediately adjacent water and aqueous ions, enabling a better definition of what constitutes the oxide- and silicate-water interfaces. This critical review discusses how science progresses from understanding ideal solid-water interfaces to more realistic systems, focusing on accomplishments in the last 20 years and identifying challenges and future opportunities for the community to address. We anticipate that the next 20 years will focus on understanding and predicting dynamic transient and reactive structures over greater spatial and temporal ranges as well as systems of greater structural and chemical complexity. Closer collaborations of theoretical and experimental experts across disciplines will continue to be critical to achieving this great aspiration.

Process-Specific Effects of Sulfate on CaCO3 Formation in Environmentally Relevant Systems

Additives, such as ions, small molecules, and macromolecules, have been found to regulate the formation of CaCO₃ and control its morphologies and properties. However, a single additive usually affects dominantly one process in CaCO₃'s formation and is seldom found to significantly affect multiple CaCO₃ formation processes. Here, we used in situ grazing incidence X-ray techniques to observe the heterogeneous formation of CaCO₃ and found that a series of formation processes (i.e., nucleation, growth, and Ostwald ripening) were modulated by sulfate. In the nucleation process, increased interfacial free energy and bulk free energy cooperatively increased the nucleation barrier and decreased nucleation rates. In the growth process, sulfate reduced the electrostatic repulsion between CaCO₃ precursors and nuclei, promoting CaCO₃ growth. This influence on the growth counteracted the inhibition effect in the nucleation process, causing a nearly 100% increase in the volume of heterogeneously formed CaCO₃. Meanwhile, adsorbed sulfate on CaCO₃ nuclei may poison the surface of smaller CaCO₃ nuclei, inhibiting Ostwald ripening. These revealed sulfate's active roles in controlling CaCO₃ formation advance our understanding of sulfate-incorporated biomineralization and scaling phenomena in natural and engineered aquatic environments.

Sea-level-rise-induced flooding drives arsenic release from coastal sediments

Sea-level rise (SLR) has a vital influence on coastal hydrogeological systems, biogeochemical processes, and the fate of coastal contaminants. However, the effects of SLR-induced perturbations on the mobilization of coastal pollutants are not fully understood. In this study, the impact of SLR-induced flooding on the concentration and speciation of arsenic and selected hazardous chemicals is investigated using exceedingly contaminated sediments (5-6% As) collected from an urban coastal site in Wilmington, DE, USA. The release of contaminants from sediments was monitored before, during, and after flooding with different intensities (bottom shear stresses) through laboratory-based erosion chamber experiments. Significantly increased release of As (up to 150%) and NO₃ (up to 50%) from sediments at shear stress levels typically measured in estuaries were found. The release of toxic chemicals from contaminated coastal sediments is thus not restricted to extreme flooding events but can occur throughout the year. The results also suggest that the dissolved concentrations of pollutants continue to be considerably high even after the flooding. SLR-induced flooding can hence increase the release of contaminants not only during erosion events but over longer timescales. The release mechanism proposed here contributes to improving the risk assessment of coastal water pollution as climate change and SLR continue to occur.

Sulfate-Controlled Heterogeneous CaCO3 Nucleation and Its Non-linear Interfacial Energy Evolution

Unveiling the effects of an environmental abundant anion "sulfate" on the formation of calcium carbonate (CaCO3) is essential to understand the formation mechanisms of biominerals like corals and brachiopod shells, as well as the scale formation in desalination systems. However, it was experimentally challenging to elucidate the sulfate-CaCO3 interactions at the explicit first step of CaCO3 formation: nucleation. In addition, there is limited quantitative information on the precise control of nucleation kinetics. Here, heterogeneous CaCO3 nucleation is monitored in real time as a function of sulfate concentrations (0-10 mM Na2SO4) using synchrotron-based grazing incidence X-ray scattering techniques. The results showed that sulfate can be incorporated in the nuclei, resulting in a nearly 90% decrease in the CaCO3 nucleation rate, causing a 120% increase in the CaCO3 nucleus size, and inhibiting the vaterite-to-calcite phase transformation. Moreover, this work quantitatively relates sulfate concentrations to the effective interfacial energies of CaCO3 and finds a non-linear trend, suggesting that CaCO3 heterogeneous nucleation is more sensitive at a low sulfate concentration. This study can be readily extended to study other additives and obtain quantitative relationships between additive concentrations and CaCO3 interfacial energies, a key step toward achieving natural and engineered controls on CaCO3 nucleation.

Interfacial and Activation Energies of Environmentally Abundant Heterogeneously Nucleated Iron(III) (Hydr)oxide on Quartz

Poorly crystalline iron(III) (hydr)oxide nanoparticles are ubiquitous in environmental systems and play a crucial role in controlling the fate and transport of contaminants. Yet, the thermodynamic and kinetic parameters, e.g., the effective interfacial (α') and apparent activation (E_a) energies, of iron(III) (hydr)oxide nucleation on earth-abundant mineral surfaces have not been determined, which hinders an accurate prediction of iron(III) (hydr)oxide formation and its interactions with other toxic or reactive ions. Here, for the first time, we report experimentally obtained α' and E_a for iron(III) (hydr)oxide nucleation on quartz mineral surfaces by employing a flow-through, time-resolved grazing incidence small-angle X-ray scattering (GISAXS). GISAXS enabled the in situ detection of iron(III) (hydr)oxide nucleation rates under different supersaturations (σ, achieved by varying pH 3.3-3.6) and temperatures (12-35 °C). By quantitative analyses based on classical nucleation theory, α' was obtained to be 34.6 mJ/m² and E_a was quantified as 32.8 kJ/mol. The fundamental thermodynamic and kinetic parameters obtained here will advance our fundamental understanding of the surface chemistry and nucleation behavior of iron(III) (hydr)oxides in subsurface and water treatment systems as well as their effects on the fate and transport of pollutants in natural and engineered water systems. The in situ flow-through GISAXS method can also be adapted to quantify thermodynamic and kinetic parameters at interfaces for many important solid-liquid environmental systems.

Ferrihydrite Transformation Impacted by Coprecipitation of Phytic Acid

Phytic acid is a common phosphate monoester that is present in soils due to the deposition of plant-derived materials. Thus far, its interaction with dissolved Fe and Fe minerals has not been as extensively investigated as phosphate, although it is expected be highly reactive due to its multiple phosphate functional groups. In this study, the effects of phytic acid on the formation of iron oxyhydroxide was investigated at near neutral pH as a function of the phytic acid/Fe ratio (0.05-0.5) and aging time using zeta potential measurements, X-ray diffraction, Fe K-edge X-ray absorption spectroscopy, and scanning electron transmission spectroscopy. It was found that an iron(III) phytate-like precipitate was formed when the phytic acid/Fe ratio was as low as 0.05. On increasing the ratio to 0.5, the quantity of iron(III) phytate-like precipitate increased to ∼60% in the ferrihydrite background. Interestingly, 10 month aging at 22 °C or hydrothermal treatment at 70 °C for 60 h did not transform the background ferrihydrite into goethite or hematite, suggesting the adsorbed phytic acid played an important role in inhibiting the transformation of ferrihydrite. The adsorption and incorporation of phytic acid into the Fe(III)O₆ polymers should be useful in understanding the complex phosphorus, iron, and hard acid chemistry in a terrestrial environment.

Dissolved Organic Matter Affects Arsenic Mobility and Iron(III) (hydr)oxide Formation: Implications for Managed Aquifer Recharge

During managed aquifer recharge (MAR), injected water significantly alters water chemistry in an aquifer, affecting arsenic mobility. To elucidate the effects of dissolved organic matter (DOM) on arsenic mobilization during MAR, this bench-scale study examined arsenic mobilization from arsenopyrite (FeAsS, an arsenic-containing sulfide) in the presence of Suwannee River natural organic matter, humic acid, and fulvic acid (SRNOM, SRHA, and SRFA), alginate (Alg), polyaspartate (PA), and glutamate (Glu). Suwannee River DOM (SRDOM) decreased arsenic mobility in the short term (<6 h) via inhibiting arsenopyrite oxidative dissolution, but increased arsenic mobility over a longer experimental time (∼7 days) via inhibiting secondary iron(III) (hydr)oxide precipitation and decreasing arsenic adsorption onto iron(III) (hydr)oxide. In situ grazing incidence small-angle X-ray scattering measurements indicated that SRDOM decreased iron(III) (hydr)oxide nucleus sizes and growth rates. A combined analysis of SRDOM and other proteinaceous or labile DOM (Alg, PA, and Glu) revealed that DOM with higher molecular weights would cause more increased arsenic mobility. These new observations advance our understanding of the impacts of DOM in injected water on arsenic mobility and secondary precipitate formation during MAR, and in other systems where interactions between DOM, arsenic, and iron(III) (hydr)oxides take place.

Structural Match of Heterogeneously Nucleated Mn(OH)2(s) Nanoparticles on Quartz under Various pH Conditions

The early nucleation stage of Mn (hydr)oxide on mineral surfaces is crucial to understand its occurrence and the cycling of nutrients in environmental systems. However, there are only limited studies on the heterogeneous nucleation of Mn(OH)₂(s) as the initial stage of Mn (hydr)oxide precipitation. Here, we investigated the effect of pH on the initial nucleation of Mn(OH)₂(s) on quartz. Under various pH conditions of 9.8, 9.9, and 10.1, we analyzed the structural matches between quartz and heterogeneously nucleated Mn(OH)₂(s). The structural matches were calculated by measuring the lateral and vertical dimensions using grazing incidence small angle X-ray scattering and atomic force microscopy (AFM), respectively. We found that a poorer structural match occurred at a higher pH than at a lower pH. The faster nucleation under a higher pH condition accounted for the poorer structural match observed. By fitting the structural match using classical nucleation theory, we also calculated the interfacial energy between Mn(OH)₂(s) and water: γ_nf = 71 ± 7 mJ/m². The calculated m values and γ_nf provided the variance of interfacial energy between quartz and Mn(OH)₂(s): γ_sn = 262-272 mJ/m². This study provides new qualitative and quantitative information on heterogeneous nucleation on an environmentally abundant mineral surface, quartz, and it offers important underpinnings for understanding the fate and transport of trace ions in environmental systems.

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 Earth's 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.

Enhanced Colloidal Stability of CeO2 Nanoparticles by Ferrous Ions: Adsorption, Redox Reaction, and Surface Precipitation

Due to the toxicity of cerium oxide (CeO2) nanoparticles (NPs), a better understanding of the redox reaction-induced surface property changes of CeO2 NPs and their transport in natural and engineered aqueous systems is needed. This study investigates the impact of redox reactions with ferrous ions (Fe2+) on the colloidal stability of CeO2 NPs. We demonstrated that under anaerobic conditions, suspended CeO2 NPs in a 3 mM FeCl2 solution at pH 4.8 were much more stable against sedimentation than those in the absence of Fe2+. Redox reactions between CeO2 NPs and Fe2+ lead to the formation of 6-line ferrihydrite on the CeO2 surfaces, which enhanced the colloidal stability by increasing the zeta potential and hydrophilicity of CeO2 NPs. These redox reactions can affect the toxicity of CeO2 NPs by increasing cerium dissolution, and by creating new Fe(III) (hydr)oxide reactive surface layers. Thus, these findings have significant implications for elucidating the phase transformation and transport of redox reactive NPs in the environment.