Nanoscale Iron Oxide Research Articles

Engineering Nanoscale Iron Oxides for Uranyl Sorption and Separation: Optimization of Particle Core Size and Bilayer Surface Coatings.

Herein, we describe engineered superparamagnetic iron oxide nanoparticles (IONPs) as platform materials for enhanced uranyl (UO22+) sorption and separation processes under environmentally relevant conditions. Specifically, monodispersed 8-25 nm iron oxide (magnetite, Fe3O4) nanoparticles with tailored organic acid bilayered coatings have been systematically evaluated and optimized to bind, and thus remove, uranium from water. The combined nonhydrolytic synthesis and bilayer phase transfer material preparation methods yield highly uniform and surface tailorable IONPs, which allow for direct evaluation of the size-dependent and coating-dependent sorption capacities of IONPs. Optimized materials demonstrate ultrahigh sorption capacities (>50% by wt/wt) at pH 5.6 for 8 nm oleic acid (OA) bilayer and sodium monododecyl phosphate (SDP) surface-stabilized IONPs. Synchrotron-based X-ray absorption spectroscopy shows that iron oxide core particle size and stabilizing surface functional group(s) substantially affect U(VI)-removal mechanisms, specifically the ratio of uptake via adsorption versus reduction to U(IV). Taken together, tunable size and surface functionality, high colloidal stability, and favorable affinity toward uranium provide distinct synergistic advantage(s) for the application of bilayered IONPs as part of the next-generation material-based uranium recovery, remediation, and sensing technologies.

Electroactive nanofluids with high solid loading and low viscosity for rechargeable redox flow batteries

A typical dispersion of iron oxide nanoparticles in alkali electrolyte is limited to 20 wt% solids, above which it has a paste-like consistency (>370 cP), incompatible with flow applications. The formulation of stable electrochemically active colloids of nanoscale iron oxide (30–50 nm) with up to 70 wt% solids, low viscosity ( 2 weeks at rest), and 55% enhancement in thermal conductivity is reported herein. A thin surface coating allows for good particle dispersion, which is shown to be crucial for these enhanced features. These suspensions of electrochemically active nanoparticles (nanoelectrofuels) can undergo electrochemical charge and discharge in fluidized format through particle–electrode impact events with potential for application in redox flow batteries. The surface coating is found to partially suppress electrochemical access to the nanoparticle in a fluidized format but has no detrimental effects on discharge capacity in the solid state. This approach is also shown to suppress a parasitic nanoparticle agglomeration process, which is otherwise dominant during electrochemical cycling of pristine iron oxide. A dissolution re-precipitation mechanism is proposed to offer insight into this auxiliary benefit. This study provides the first insight into the feasibility of adopting electrochemically active nanofluids as high energy density redox flow battery electrolytes.

Rust Formation Mechanism on Low Alloy Steels after Exposure Test in High Cl- and High SOx Environmen.

Exposure tests were performed on low alloy steels in high Cl− and high SOx environment, and the structure of the rust were analyzed by TEM (Transmission Electron Microscopy) and Raman Spectroscopy. In the exposure test site, the concentrations of Cl− and SOx were found to be high, which caused the corrosion of the steels. The conventional weathering steel (SMA: 0.6% Cr-0.4% Cu-Fe) showed higher corrosion resistance as compared to the carbon steel (SM), and Ni bearing steel exhibited the highest one. Raman spectroscopy showed that the inner rust of Ni bearing steel was mainly composed of α-FeOOH and spinel oxides. On the other hand, SMA contained β- and γ-FeOOH in inner rust, which increased the corrosion. TEM showed that nano-scale complex iron oxides containing Ni or Cr were formed in the rust on the low alloy steels, which suppressed the corrosion of steels in high Cl− and high SOx environment.

Toxicological implications of released particulate matter during thermal decomposition of nano-enabled thermoplastics

Nano-enabled thermoplastics are part of the growing market of nano-enabled products (NEPs) that have vast utility in several industries and consumer goods. The use and disposal of NEPs at their end of life has raised concerns about the potential release of constituent engineered nanomaterials (ENMs) during thermal decomposition and their impact on environmental health and safety. To investigate this issue, industrially relevant nano-enabled thermoplastics including polyurethane, polycarbonate, polypropylene containing carbon nanotubes (0.1 and 3% w/v, respectively), polyethylene containing nanoscale iron oxide (5% w/v), and ethylene vinyl acetate containing nanoscale titania (2 and 5% w/v) along with their pure thermoplastic matrices were thermally decomposed using the recently developed lab based Integrated Exposure Generation System (INEXS). The life cycle released particulate matter (called LCPM) was monitored using real time instrumentation, size fractionated, sampled, extracted and prepared for toxicological analysis using primary small airway epithelial cells to assess potential toxicological effects. Various cellular assays were used to assess reactive oxygen species and total glutathione as measurements of oxidative stress along with mitochondrial function, cellular viability, and DNA damage. By comparing toxicological profiles of LCPM released from polymer only (control) with nano-enabled LCPM, potential nanofiller effects due to the use of ENMs were determined. We observed associations between NEP properties such as the percent nanofiller loading, host matrix, and nanofiller chemical composition and the physico-chemical properties of released LCPM, which were linked to biological outcomes. More specifically, an increase in percent nanofiller loading promoted a toxicological response independent of increasing LCPM dose. Importantly, differences in host matrix and nanofiller composition were shown to enhance biological activity and toxicity of LCPM. This work highlights the importance of assessing the toxicological properties of LCPM and raises environmental health and safety concerns of nano-enabled products at their end of life during thermal decomposition/incineration.

Thermal conductivity, viscosity, and electrical conductivity of iron oxide with a cloud fractal structure

In the present study, nanoscale iron oxide was synthesized using a hydrothermal method; XRD analysis revealed that all the produced crystals are iron oxide. FESEM microscopic imaging showed that particles are on the scale of nano and their morphology is cloud fractal. To study the laboratory properties of thermal conductivity, viscosity, and electrical conductivity of the nanoparticles, they were dispersed in ethylene glycol-based fluid and the nanofluid was in a two-step synthesis during this process. The experiments were carried out with a weight fraction between 0 and 2 % at temperatures between 25 and 45 °C. According to the results of the experiments, increasing the density of nanoparticles in the fluid increases thermal conductivity, as it was predicted in all theoretical models. On the other hand, nano viscosity increases as the weight fraction increases while it decreases as temperature goes up. Electrical conductivity also increases with raising the temperature and weight fraction. Theoretical models were studied to predict Thermal conductivity, viscosity, and electrical conductivity of the nanofluid.

Synthesis and Characterization of Iron Oxide/Carbon Microspheres Composite Material

Different carbon sources were chosen to synthetize microspheres by hydrothermal method. X-ray diffraction indicated a highly disordered carbon structure. SEM images showed that the carbon microspheres had perfect spherical morphology, smooth surface and uniform particle size of 1-2 μm. Nanoscale iron oxide was synthesized on the microspheres support via hydrothermal synthetic route using Fe (NO)3 and NaOH as reagents. The nanoparticles were α-Fe2O3, and uniformly loaded on the surface of the carbon microspheres with average diameter around 200 nm.

Rapid and effective removal of Cr(VI) from aqueous solutions using the FeCl3/NaBH4 system

A new emergency treatment technology for rapid and effective removal of hexavalent chromium [Cr(VI)] from aqueous solutions by direct addition of ferric chloride (FeCl3) and sodium borohydride (NaBH4) in sequence to Cr(VI) solutions under magnetic stirring conditions was proposed in this study. Various reaction conditions, such as solution pH and Fe(III)/Cr(VI) and FeCl3/NaBH4 molar ratios were investigated. The results showed that 97.6–99.9% of 50mg/L Cr(VI) can be removed from aqueous solutions within 3min in the initial pH range of 3.5–6.0 with Fe(III)/Cr(VI) and FeCl3/NaBH4 molar ratios of 1.0:1 and 1:3.0 respectively at 298K, which was significantly better than that of FeCl3 and NaBH4 alone. This indicated that a synergistic effect existed. Jar tests demonstrated that Cr(VI) was reduced to Cr(III) mainly by NaBH4 under a strong acidic condition in which the hydrogen ions were largely provided by the hydrolysis of FeCl3. In addition, co-precipitation of ferric and trivalent chromic ions occurred. The adsorption of ferric compounds also contributed to the loss of a relatively higher concentration of Cr(VI). Experimental results and especially theoretical calculations of redox potentials suggested that the expected nanoscale zero-valent iron (nZVI) or ferroferric oxide (Fe3O4) was formed only with the addition of excess FeCl3/NaBH4. X-ray photoelectron spectroscopy (XPS) indicated that these precipitates were complex Fe–Cr oxides/hydroxides. The results demonstrated the potential use of the FeCl3/NaBH4 system in copping with Cr(VI) pollution accidents.

Effect of changing the nanoscale environment on activity and stability of nitrate reductase.

Nitrate reductase (NR) is employed for fabrication of nitrate sensing devices in which the enzyme in immobilized form is used to catalyze the conversion of nitrate to nitrite in the presence of a suitable cofactor. So far, instability of immobilized NR due to the use of inappropriate immobilization matrices has limited the practical applications of these devices. Present study is an attempt to improve the kinetic properties and stability of NR using nanoscale iron oxide (nFe3O4) and zinc oxide (nZnO) particles. The desired nanoparticles were synthesized, surface functionalized, characterized and affixed onto the epoxy resin to yield two nanocomposite supports (epoxy/nFe3O4 and epoxy/nZnO) for immobilizing NR. Epoxy/nFe3O4 and epoxy/nZnO support could load as much as 35.8±0.01 and 33.20±0.01μg/cm(2) of NR with retention of about 93.72±0.50 and 84.81±0.80% of its initial activity respectively. Changes in surface morphology and chemical bonding structure of both the nanocomposite supports after addition of NR were confirmed by scanning electron microscopy (SEM) and fourier transform infrared spectroscopy (FTIR). Optimum working conditions of pH, temperature and substrate concentration were ascertained for free as well as immobilized NR preparations. Further, storage stability at 4°C and thermal stability between 25-50°C were determined for all the NR preparations. Analytical applications of immobilized NR for determination of soil and water nitrates along with reusability data has been included to make sure the usefulness of the procedure.

Nanoscale zerovalent iron (nZVI) at environmentally relevant concentrations induced multigenerational reproductive toxicity in Caenorhabditis elegans

Nanoscale zerovalent iron (nZVI) is widely used with large scale for environmental remediation for in situ or ex situ applications. The potential impact of nZVI on biota at environmentally relevant concentrations needs to be elucidated. In this study, the reproductive toxicities of three irons species: carboxymethyl cellulose (CMC)-stabilized nZVI, nanoscale iron oxide (nFe3O4), and ferrous ion (Fe(II)aq) in the soil-dwelling nematode Caenorhabditis elegans were examined. In addition, the generational transfer of reproductive toxicity of CMC-nZVI on C. elegans was investigated. The results showed that CMC-nZVI, nFe3O4, and Fe(II)aq did not cause significant mortality after 24 h exposure at the examined concentrations. Reproductive toxicity assays revealed that CMC-nZVI, nFe3O4, and Fe(II)aq significantly decreased offsprings in parental generation (F0) in accompany with the increased intracellular reactive oxygen species (ROS). Furthermore, the reproductive toxicity of CMC-nZVI at environmentally relevant concentrations was transferrable from the F0 to the F1 and F2 generations, but then recovered in the F3 and F4 generations. Further evidence showed that total irons were accumulated in the F0 and F1 generations of C. elegans after CMC-nZVI parental exposure. This study demonstrated that environmentally relevant concentrations of CMC-nZVI induced multigenerational reproductive toxicity which can be ascribed to its high production of ROS in F0 generation, toxicity of Fe(II)aq, and iron accumulation in C. elegans. Since nZVI is widely used for environmental remediation, considering the multigenerational toxicity, this study thus implicates a potential environmental risk of nZVI-induced nanotoxicity in the environment.

The Effect of Ce Added to Carburizing Paste on Phase Composition of Carburized Surface Layers

Investigations on carburized layers phase composition are usually made ​​by optical microscopy, electron microscopy and X-ray diffraction. A microstructure consisting in different proportions of superior bainite, inferior bainite, martensite and retained austenite was found after carburizing in paste of the 21NiCrMo2 steel which has appropriate bainitic hardenability. The investigations carried out on 21NiCrMo2 steel samples, carburized in paste with additions of about 10% Ce, highlighted a significant change of phase proportions ratio in the carburized layer resulting in an important increasing of the lower bainite ratio. It was also observed, that the carburized layer case depth obtained in the carburizing paste with Ce additions does not significantly differ in relation to that attained in the carburizing paste without Ce additions but the microhardness increases considerably. The reason is connected also to the presence of Ce carbide identified by X-ray diffraction in the carburized layer. The Ce hard affinity towards oxygen and its influence on bainitic hardenability increasing is well known and adopted, however neither Ce transfer mechanism from carburizing paste to the carburizing layer and nor the effects of its high affinity to oxygen (for example) are not addressed in this particular case. The present study proposes a possible transfer mechanism of Ce in the carburized layer and an explanation of the consequences of Ce hard affinity towards oxygen on the phase composition of the carburized layer. Nanoscale iron oxides such as wüstite, magnetite and maghemite were identified in the surface of the carburized layer by Mössbauer spectrometry.

Assessing hydrodynamic effects on jarosite dissolution rates, reaction products, and preservation on Mars

AbstractJarosite flow‐through dissolution experiments were conducted in ultrapure water (UPW), pH 2 sulfuric acid, and saturated NaCl and CaCl2 brines at 295–298 K to investigate how hydrologic variables may affect jarosite preservation and reaction products on Mars. K+‐based dissolution rates in flowing UPW did not vary significantly with flow rate, indicating that mineral surface reactions control dissolution rates over the range of flow rates investigated. In all of the solutions tested, hydrologic variables do not significantly affect extent of jarosite alteration; therefore, jarosite is equally likely to be preserved in flowing or stagnant waters on Mars. However, increasing flow rate did affect the mineralogy and accumulation of secondary reaction products. Iron release rates in dilute solutions increased as the flow rate increased, likely due to nanoscale iron (hydr)oxide transport in flowing water. Anhydrite formed in CaCl2 brine flow‐through experiments despite low temperatures, while metastable gypsum and bassanite were observed in batch experiments. Therefore, observations of the hydration state of calcium sulfate minerals on Mars may provide clues to unravel past salinity and hydrologic conditions as well as temperatures and vapor pressures.

Improving steel fiber reinforced concrete pull-out strength with nanoscale iron oxide coating

Fiber-reinforced concrete (FRC) has been extensively used all over the world for over 40years. Steel fiber, being the first and most commonly employed reinforcement fiber, is always favored as the uniformly dispersed reinforcement for concrete due to its improvement towards mechanical strength, failure mode and bonding toughness. This article depicts a nanoscale reinforcement of iron oxide coatings on steel fibers prepared by electrodeposition method. The results indicate that pristine Fe2O3 particles, Fe2O3 nanorods and nanosheets were synthesized under different depositing conditions. The nanorod and nanosheet coated steel fibers were embedded in cement mortar for fiber pull-out tests. Corresponding pull-out tests revealed that iron oxide coating showed a strength enhancement up to 82% compared to the conventional fiber reinforced cement mortar samples.

Facile synthesis of magnetite iron oxide nanoparticles via precipitation method at different reaction temperatures

The nano-scale magnetite iron oxide particles have been synthesised by a facile precipitation method. Magnetite iron oxide nanoparticles were synthesised in a bath with electrolytes composed of 0·10 M of iron (II) chloride with 0·45 M of sodium hydroxide at different reaction temperatures under oxidising environment. In the present study, the influence of reaction temperatures (30, 45 and 80°C) on the morphology, particle size and crystallinity of the iron oxide particles were investigated in detail. Based on the Malvern Zetasizer analysis, the iron oxide particles with variable size from ∼250 to ∼70 nm could be achieved when increasing the reaction temperature up to 80°C. The magnetite phase of iron oxide particles was determined by using X-ray diffraction analysis. In addition, field emission scanning electron microscopy micrographs were further affirmed that our synthesised iron oxide particles were in nano-scale with a spherical shape. It was found that the high reaction temperature is helpful in controlling the formation of uniform magnetite iron oxide nanoparticles.

Nanoscale iron oxides loaded granular activated carbon (GAC-NSIO) for cadmium removal

A new hybrid material was prepared by loading the nanoscale iron oxide particles within the pores of granular activated carbon (GAC-NSIO) and used as adsorbent to remove Cd(II) in aqueous phase. The new material is characterized by the pHpzc, TEM, XRD, BET, and XPS analytical techniques to deduce the potential adsorptive mechanism, and further it along with GAC were assessed in the removal of Cd(II) from aqueous solution for various physico-chemical factors iron-loaded content, pH, humic acid, isotherm, kinetics, and thermodynamics, under the batch reactor studies and rapid small-scale column tests also appended dealing with the breakthrough volume. Characterization of adsorbent indicated that the amorphous nanoscale iron oxides were successfully immobilized on GAC. Adsorption mechanism mainly included surface complexation, Donnan membrane effect, and electrostatic attraction. The optimum iron-loaded content of GAC-NSIO was 17%. The adsorption process was more favorable at high pH, and iron oxide was almost no leaching (<0.25%) in the pH range of 3–6. Adsorption isotherm fitted well with Langmuir and Freundlich models. And adsorption kinetics fitted well with pseudo-second-order and intraparticle diffusion models. Results indicated that loaded nanoscale iron oxides significantly increased adsorption capacity of Cd(II) though the adsorption rate declined slightly and the adsorption process was endothermic. The maximum adsorptive capacity of GAC-NSIO (7.84 mg/g) increased by 700%, compared to that of GAC (0.98 mg/g). The presence of humic acid could greatly impact Cd(II) adsorption which promoted adsorption at low concentrations (1–10 mg/L) and inhibited adsorption at high concentrations (10–300 mg/L). Rapid small-scale column tests were conducted to obtain Cd(II) breakthrough profile. The results corresponded well with those from batch tests. Results demonstrate that the GAC-NSIO is a promising adsorbent for the removal of Cd(II) in contaminated water environment.

Removal of water contaminants by iron oxide nanomaterials.

In recent years, there has been increasing concern about the usage of a broad range of organic substances, heavy metals and aromatic compounds in the aquatic environment due to their wide distribution and potential adverse health effects. The presence of toxic contaminants in water effluent, even at very low concentrations, is extremely harmful and undesirable. Various treatment processes have been investigated to reduce these toxic pollutants from wastewater. Because of the chemical stability of the contaminants, these technologies have proved to be ineffective for handling waste effluents. Nanotechnology offers the possibility of efficient removal of pollutants as nanoparticles have a smaller size and higher adsorptive surface area. From the past few years, nanoscale iron oxides such as magnetite, maghemite, and hematite have been used for the separation and removal of organic and inorganic contaminants. In this review we summarize the use of iron oxide nanomaterials performed over the last few years for the removal of dyes, heavy metals and aromatic compounds.

Effects of Mesoporous Silica Coating and Postsynthetic Treatment on the Transverse Relaxivity of Iron Oxide Nanoparticles

Mesoporous silica nanoparticles have the capacity to load and deliver therapeutic cargo and incorporate imaging modalities, making them prominent candidates for theranostic devices. One of the most widespread imaging agents utilized in this and other theranostic platforms is nanoscale superparamagnetic iron oxide. Although several core-shell magnetic mesoporous silica nanoparticles presented in the literature have provided high T2 contrast in vitro and in vivo, there is ambiguity surrounding which parameters lead to enhanced contrast. Additionally, there is a need to understand the behavior of these imaging agents over time in biologically relevant environments. Herein, we present a systematic analysis of how the transverse relaxivity (r2) of magnetic mesoporous silica nanoparticles is influenced by nanoparticle diameter, iron oxide nanoparticle core synthesis, and the use of a hydrothermal treatment. This work demonstrates that samples which did not undergo a hydrothermal treatment experienced a drop in r2 (75% of original r2 within 8 days of water storage), while samples with hydrothermal treatment maintained roughly the same r2 for over 30 days in water. Our results suggest that iron oxide oxidation is the cause of the r2 loss, and this oxidation can be prevented both during synthesis and storage by the use of deoxygenated conditions during nanoparticle synthesis. The hydrothermal treatment also provides colloidal stability, even in acidic and highly salted solutions, and a resistance against acid degradation of the iron oxide nanoparticle core. The results of this study show the promise of multifunctional mesoporous silica nanoparticles but will also likely inspire further investigation into multiples types of theranostic devices, taking into consideration their behavior over time and in relevant biological environments.

The zerovalent iron nanoparticle causes higher developmental toxicity than its oxidation products in early life stages of medaka fish

Nanoscale zerovalent iron (nZVI)-mediated oxidation reaction is increasingly being used for enhanced treatment of water or wastewater processes; however, the fate and eco-toxicological effects of nZVI in the surface aquifer remain unclear. We investigated bioaccumulation and lethal-to-sublethal toxic effects on early life development of Japanese medaka (Oryzias latipes) with 7-day exposure to 25–200 mg/L of well-characterized solutions containing carboxymethyl cellulose (CMC)-stabilized nZVI (CMC-nZVI), nanoscale iron oxide (nFe3O4) or ferrous ion [Fe(II)aq]. The CMC-nZVI solution had the greatest acute mortality and developmental toxic effects in embryos, with lesser and the least effects with Fe(II)aq and nFe3O4. The toxicity of CMC-nZVI was ascribed to its high reactivity in the oxygenic solution, which led to a combination of hypoxia and production of reactive oxygen species (ROS) and Fe(II)aq. nFe3O4 (50–100 mg/L) was more bioavailable to embryos and bioaccmulative in hatchlings than suspended CMC-nZVI. The antioxidant balance was differentially altered by induced intracellular ROS in hatchlings with all 3 iron species. We revealed causal toxic effects of nZVI and its oxidized products in early life stages of medaka fish using different organizational levels of biomarker assays. The toxicity results implicate a potential eco-toxicological impact of nZVI on the aquatic environment.

Stabilization or Oxidation of Nanoscale Zerovalent Iron at Environmentally Relevant Exposure Changes Bioavailability and Toxicity in Medaka Fish

Nanoscale zerovalent iron (nZVI)-based nanotechnologies are increasingly being used for environmental remediation; however, the fate and ecotoxicologic effects of nZVI remain unclear. Larvae of medaka fish (Oryzias latipes) underwent 3-14 days' aqueous exposure to thoroughly characterized solutions containing carboxymethyl cellulose (CMC)-stabilized nZVI, bare nZVI, nanoscale iron oxide (nFe(3)O(4)) or ferrous ion [Fe(II)(aq)] at μg/L-mg/L levels to assess the causal toxic effect(s) of iron nanoparticles (NPs). Acute larval mortality was decreased in the order of Fe(II)(aq) > CMC-nZVI > nZVI > nFe(3)O(4). CMC-nZVI (100 mg/L) increased hypoxia and reactive oxygen species (ROS) and Fe(II)(aq) production, thus increasing mortality and oxidative stress response as compared with unstabilized nZVI. Additionally, nFe(3)O(4) and nZVI were more bioavailable than suspended CMC-nZVI or Fe(II)(aq). Antioxidant activities were significantly altered by induced intracellular ROS levels in larvae with subchronic exposure to nFe(3)O(4) or Fe(II)(aq) at environmentally relevant concentrations (0.5-5 mg/L). We report on different organizational biomarkers used for rapidly assessing the lethal and sublethal toxicity of nZVI and its stabilized or oxidized products. The toxicity results implicate a potential ecotoxicological fate and impact of nZVI on the aquatic environment.

Amorphous iron-(hydr) oxide networks at liquid/vapor interfaces: In situ X-ray scattering and spectroscopy studies

Surface sensitive X-ray reflectivity (XR), fluorescence (XF), and grazing incidence X-ray diffraction (GIXD) experiments were conducted to determine the accumulation of ferric iron Fe (III) or ferrous iron Fe (II) under dihexadecyl phosphate (DHDP) or arachidic acid (AA) Langmuir monolayers at liquid/vapor interfaces. Analysis of the X-ray reflectivity and fluorescence data of monolayers on the aqueous subphases containing FeCl3 indicates remarkably high levels of surface-bound Fe (III) in number of Fe3+ ions per molecule (DHDP or AA) that exceed the amount necessary to neutralize a hypothetically completely deprotonated monolayer (DHDP or AA). These results suggest that nano-scale iron (hydr) oxide complexes (oxides, hydroxides or oxyhydroxides) bind to the headgroups and effectively overcompensate the maximum possible charges at the interface. The lack of evidence of in-plane ordering in GIXD measurements and strong effects on the surface-pressure versus molecular area isotherms indicate that an amorphous network of iron (hydr) oxide complexes contiguous to the headgroups is formed. Similar experiments with FeCl2 generally resulted with the oxidation of Fe (II)–Fe (III) which consequently leads to ferric Fe (III) complexes binding albeit with less iron at the interface. Controlling the oxidation of Fe (II) changes the nature and amount of binding significantly. The implications to biomineralization of iron (hydr) oxides are briefly discussed.

Influence of size, morphology, surface structure, and aggregation state on reductive dissolution of hematite nanoparticles with ascorbic acid

Nanoscale iron (oxyhydr)oxide minerals are omnipresent in natural environments and past studies have found that their size, morphology, surface structure, and aggregation state influence chemical reactivity. In this study, we have investigated the influence of these factors on the reductive dissolution of hematite with ascorbic acid using two types of nanoparticles with average diameters of 6.8±0.8nm and 30.5±3.5nm, referred to as Hem-7 and Hem-30, respectively, in this paper. Hematite nanoparticles were synthesized by forced hydrolysis of ferric nitrate and characterized with powder XRD, TEM, and BET surface area measurements. Reductive dissolution experiments were conducted at 23.0±0.5°C and pH 3.35±0.03 in the absence of light under nitrogen. Aqueous [Fe(II)] was measured by the ferrozine assay. High-resolution TEM (HRTEM) revealed that the Hem-7 crystals are pseudo-hexagonal plates and the Hem-30 crystals are rhombohedral. Initial and steady state dissolution rates were determined from batch experiments and compared after surface area (SA) normalization: the initial rates of Hem-7 and Hem-30 are 9.11±2.24 and 4.48±1.62 (10−7molm−2h−1), and the steady state rates are 1.94±0.53 and 1.29±0.36 (10−7molm−2h−1), respectively. These results suggest that both initial and steady state rates of Hem-7 dissolution are faster than the steady state rate of Hem-30, although the differences between the steady state rates of Hem-7 and Hem-30 are within one standard deviation. HRTEM observation of individual crystals and aggregates of Hem-7 reveal that this hematite is defect-free and no preferential etching occurred. Additional TEM measurements indicate that previous to dissolution, Hem-7 is present as both dispersed particles and as aggregates. Dispersed particles dissolve initially before aggregates, which influences the dissolution rate. The Hem-30 hematite has nanoscale surface steps and internal defects, and its dissolution initiates from the steps, defects, or sharp edges of the crystals. In addition, HAADF-STEM (high angle annular dark field – scanning transmission electron microscope) tomography was employed to observe the three-dimensional structures of individual particles and aggregates. This three-dimensional tomography reveals that aggregates of nanoparticles dissolve heterogeneously and holes form on the surface of crystals. This study directly shows the importance of size, surface roughness, defects, crystal morphology and aggregation states on dissolution rates of nanoparticles.