An official website of the United States government
Nickel K- and L 2,3 -edge X-ray absorption spectra (XAS) are discussed for 16 complexes and complex ions with nickel centers spanning a range of formal oxidation states from II to IV. K-edge XAS alone is shown to be an ambiguous metric of physical oxidation state for these Ni complexes. Meanwhile, L 2,3 -edge XAS reveals that the physical d-counts of the formally Ni IV compounds measured lie well above the d 6 count implied by the oxidation state formalism. The generality of this phenomenon is explored computationally by scrutinizing 8 additional complexes. The extreme case of NiF 6 2− is considered using high-level molecular orbital approaches as well as advanced valence bond methods. The emergent electronic structure picture reveals that even highly electronegative F-donors are incapable of supporting a physical d 6 Ni IV center. The reactivity of Ni IV complexes is then discussed, highlighting the dominant role of the ligands in this chemistry over that of the metal centers.
more »
« less
The role of defects in Fe(II)-goethite electron transfer
Despite substantial experimental evidence for Fe(II)–Fe(III) oxide electron transfer, computational chemistry calculations suggest that oxidation of sorbed Fe(II) by goethite is kinetically inhibited on structurally perfect surfaces. We used a combination of 57Fe Mössbauer spectroscopy, synchrotron X-ray absorption and magnetic circular dichroism (XAS/XMCD) spectroscopies to investigate whether Fe(II)–goethite electron transfer is influenced by defects. Specifically, Fe L-edge and O K-edge XAS indicates that the outermost few Angstroms of goethite synthesized by low temperature Fe(III) hydrolysis is iron deficient relative to oxygen, suggesting the presence of defects from Fe vacancies. This nonstoichiometric goethite undergoes facile Fe(II)–Fe(III) oxide electron transfer, depositing additional goethite consistent with experimental precedent. Hydrothermal treatment of this goethite, however, appears to remove defects, decrease the amount of Fe(II) oxidation, and change the composition of the oxidation product. When hydrothermally treated goethite was ground, surface defect characteristics as well as the extent of electron transfer were largely restored. Our findings suggest that surface defects play a commanding role in Fe(II)–goethite redox interaction, as predicted by computational chemistry. Moreover, it suggests that, in the environment, the extent of this interaction will vary depending on diagenetic history, local redox conditions, as well as being subject to regeneration via seasonal fluctuations.
more »
« less
- Award ID(s):
- 1633098
- PAR ID:
- 10069277
- Date Published:
- Journal Name:
- Environmental science & technology
- Volume:
- 52
- Issue:
- 5
- ISSN:
- 1520-5851
- Page Range / eLocation ID:
- 2751-2759
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
The kinetics of model contaminant 4-chloronitrobenzene (4-ClNB) reduction by Fe( ii ) in aqueous suspensions containing either or both goethite (α-FeOOH) nanoparticles and kaolinite (Al 2 Si 2 O 5 (OH) 4 ) were quantified to elucidate the effects of nonreactive clay minerals on the attenuation of nitroaromatic groundwater contaminants by iron oxide nanoparticles. Increasing the amount of kaolinite in the presence of goethite decreased the reduction rate of 4-ClNB and competitive Fe( ii ) adsorption on kaolinite occurred. Cryogenic transmission and scanning electron microscopy (cryo-TEM and cryo-SEM) images did not reveal significant loss of accessible reactive surface area as a result of heteroaggregation. Sequential-spike batch reactors revealed that in the presence of kaolinite, 4-ClNB reduction rate decreased by more than a factor of three with extended reaction as a result of kaolinite dissolution and subsequent incorporation of Al and Si in goethite or on the goethite surface. The reactive sites residing on the {110} faces were comparatively more reactive in the presence of a large loading of kaolinite, resulting in shorter and wider goethite particles after reaction. These results elucidate the mechanisms by which nonreactive clays affect the reactions of Fe( ii )/iron oxides in groundwater systems and indicate that nonreactive clays are not passive components.more » « less
-
Reduction of nitroaromatic compounds (NACs), an important class of groundwater pollutants, by Fe( ii ) associated with iron oxides, a highly reactive reductant in anoxic aquifers, has been studied widely, but there are significant differences between the well-controlled, batch reactor conditions of the laboratory and the complicated conditions encountered in the field. Continuous flow column reactors containing goethite-coated sand and aqueous carbonate buffer were continuously exposed to 0.05 mM 4-chloronitrobenzene (4-ClNB) and 0.5 mM Fe( ii ) to emulate more realistic scenarios and to allow study of the oxidative growth of goethite particles using both saturated and unsaturated flow conditions. The experiments were designed to test how attachment to a surface affected particle growth and how particle growth affected the extent of reaction over time. After reaction, particles from different sections of each column were collected, and the goethite was detached from the sand grains for characterization using transmission electron microscopy. The amount of oxidative growth varied as a function of distance from the column inlet, with the most growth observed at the inlet end (bottom) of the column. Similar to previous work using batch reactors, newly oxidized Fe( iii ) was mostly added to the goethite particle tips, resulting in up to an 81% increase in length under saturated flow and a 50% increase in length under unsaturated flow after 220 pore volumes. With saturated flow, reactant concentrations and the extent of the reaction are important factors determining the extent of mineral growth. For unsaturated column conditions, however, flow path substantially impacts mineral growth in the column. Reactors sacrificed after 220 pore volumes under saturated flow conditions resulted in an overall 70% increase in goethite mass while the unsaturated flow column resulted in a 40% increase in goethite mass, more variable mineral growth as a function of distance from the inlet, and overall, 50% less 4-ClNB conversion. The results demonstrate that quantitative characterization of oxidative mineral growth of goethite nanoparticles attached to an underlying mineral is practical and elucidates the major variables impacting the reactivity of mineral nanoparticles in contaminated groundwater systems.more » « less
-
interactions between phosphate and various Fe (oxyhydr)oxides are poorly constrained in natural systems. An in-situ incubation experiment was conducted to explore Fe (oxyhydr)oxide transformation and effects on phosphate sorption in soils with contrasting saturation and redox conditions. Synthetic Fe (oxyhydr)oxides (ferrihydrite, goethite and hematite) were coated onto quartz sand and either pre-sorbed with phosphate or left phosphate-free. The oxide-coated sands were mixed with natural organic matter, enclosed in mesh bags, and buried in and around a vernal pond for up to 12 weeks. Redox conditions were stable and oxic in the upland soils surrounding the vernal pond but largely shifted from Fe reducing to Fe oxidizing in the lowland soils within the vernal pond as it dried during the summer. Iron (oxyhydr)oxides lost more Fe (− 41% ± 10%) and P (− 43 ± 11%) when incubated in the redox-dynamic lowlands compared to the uplands (− 18% ± 5% Fe and − 24 ± 8% P). Averaged across both uplands and lowlands, Fe losses from crystalline goethite and hematite (− 38% ± 6%) were unexpectedly higher than losses from short range ordered ferrihydrite (− 12% ± 10%). We attribute losses of Fe and associated P from goethite and hematite to colloid detachment and dispersion but losses from ferrihydrite to reductive dissolution. Iron losses were partially offset by retention of solubilized Fe as organic-bound Fe(III). Iron (oxyhydr)oxides that persisted during the incubation retained or even gained P, indicating low amounts of phosphate sorption from solution. These results demonstrate that hydrologic variability and Fe (oxyhydr)oxide mineralogy impact Fe mobilization pathways that may regulate phosphate bioavailability.more » « less
-
null (Ed.)Among high-valence metal oxides, LiCoO 2 and related materials are of environmental importance because of the rapidly increasing use of these materials as cathodes in lithium ion batteries. Understanding the impact of these materials on aqueous environments relies on understanding their redox chemistry because Co release is dependent on oxidation state. Despite the critical role that redox chemistry plays in cellular homeostasis, the influence of specific biologically relevant electron transporters such as nicotinamide adenine dinucleotide (NADH) and glutathione (GSH) on the transformation of engineered nanoparticles has not been widely considered previously. Here we report an investigation of the interaction of LiCoO 2 nanoparticles with NADH and GSH. Measurements of Co release using inductively coupled plasma-mass spectrometry (ICP-MS) show that exposing LiCoO 2 nanoparticles to either NADH or GSH increases solubilization of cobalt, while corresponding spectroscopic measurements show that NADH is concurrently oxidized to NAD + . To demonstrate that these effects are a consequence the high-valence Co(III) inLiCoO 2 nanoparticles, we performed control experiments using Co(II)-containing Co(OH) 2 and LiCoPO 4 , and dissolved Co 2+ /Li + ions. Additional experiments using molecules of similar structure to NADH and GSH, but that are not reducing agents, confirm that these transformations are driven by redox reactions and not by chelation effects. Our data show that interaction of LiCoO 2 with NADH and GSH induces release Co 2+ ions and alters the redox state of these biologically important transporters. Observation of NADH binding to LiCoO 2 using x-ray photoelectron spectroscopy (XPS) suggests a surface catalyzed reaction. The reciprocal reduction of LiCoO 2 to enable release of Co 2+ and corresponding oxidation of NADH and GSH as model redox-active biomolecules has implications for understanding the biological impacts of high-valence metal oxide nanomaterials.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- The DOI is not currently available.
