19 research outputs found
Structural Transformation of Birnessite by Fulvic Acid under Anoxic Conditions
The structure and MnĀ(III) concentration
of birnessite dictate its
reactivity and can be changed by birnessite partial reduction, but
effects of pH and reductant/birnessite ratios on the changes by reduction
remain unclear. We found that the two factors strongly affect the
structure of birnessite (Ī“-MnO<sub>2</sub>) and its MnĀ(III)
content during its reduction by fulvic acid (FA) at pH 4ā8
and FA/solid mass ratios of 0.01ā10 under anoxic conditions
over 600 h. During the reduction, the structure of Ī“-MnO<sub>2</sub> is increasingly accumulated with both MnĀ(III) and MnĀ(II)
but much more with MnĀ(III) at pH 8, whereas the accumulated Mn is
mainly MnĀ(II) with little MnĀ(III) at pH 4 and 6. MnĀ(III) accumulation,
either in layers or over vacancies, is stronger at higher FA/solid
ratios. At FA/solid ratios ā„1 and pH 6 and 8, additional hausmannite
and MnOOH phases form. The altered birnessite favorably adsorbs FA
because of the structural accumulation of MnĀ(II, III). Like during
microbially mediated oxidative precipitation of birnessite, the dynamic
changes during its reduction are ascribed to the birnessite-MnĀ(II)
redox reactions. Our work suggests low reactivity of birnessite coexisting
with organic matter and severe decline of its reactivity by partial
reduction in alkaline environment
Sulfate Local Coordination Environment in Schwertmannite
Schwertmannite, a
nanocrystalline ferric oxyhydroxy-sulfate mineral,
plays an important role in many environmental geochemical processes
in acidic sulfate-rich environments. The sulfate coordination environment
in schwertmannite, however, remains unclear, hindering our understanding
of the structure, formation, and environmental behavior of the mineral.
In this study, sulfur K-edge X-ray absorption near edge structure
(XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopic
analyses in combination with infrared spectroscopy were used to determine
the sulfate local atomic environment in wet and air-dried schwertmannite
samples after incubation at various pHs and ionic strengths. Results
indicate that sulfate exists as both inner- and outer-sphere complexes
in schwertmannite. Regardless of the sample preparation conditions,
the EXAFS-determined SāFe interatomic distances are 3.22ā3.26
Ć
, indicative of bidentate-binuclear sulfate inner-sphere complexes.
XANES spectroscopy shows that the proportion of the inner-sphere complexes
decreases with increasing pH for both wet and dried samples and that
the dried samples contain much more inner-sphere complexes than the
wet ones at any given pH. Assuming that schwertmannite is a distorted
akaganeĢite-like structure, the sulfate inner-sphere complexation
suggests that, the double chains of the edge-sharing Fe octahedra,
enclosing the tunnel, must contain defects, on which reactive singly-Fe
coordinated hydroxyl functional groups form for ligand exchange with
sulfate. The drying effect suggests that the tunnel contains readily
exchangeable H<sub>2</sub>O molecules in addition to sulfate ions
Xāray Absorption Spectroscopic Quantification and Speciation Modeling of Sulfate Adsorption on Ferrihydrite Surfaces
Sulfate adsorption on mineral surfaces
is an important environmental
chemical process, but the structures and respective contribution of
different adsorption complexes under various environmental conditions
are unclear. By combining sulfur K-edge XANES and EXAFS spectroscopy,
quantum chemical calculations, and surface complexation modeling (SCM),
we have shown that sulfate forms both outer-sphere complexes and bidentateābinuclear
inner-sphere complexes on ferrihydrite surfaces. The relative fractions
of the complexes vary with pH, ionic strength (<i>I</i>),
and sample hydration degree (wet versus air-dried), but their structures
remained the same. The inner-sphere complex adsorption loading decreases
with increasing pH while remaining unchanged with <i>I</i>. At both <i>I</i> = 0.02 and 0.1 M, the outer-sphere complex
loading reaches maximum at pH ā¼5 and then decreases with pH,
whereas it monotonically decreases with pH at <i>I</i> =
0.5 M. These observations result from a combination of the ionic-strength
effect, the pH dependence of anion adsorption, and the competition
between inner- and outer-sphere complexation. Air-drying drastically
converts the outer-sphere complexes to the inner-sphere complexes.
The respective contributions to the overall adsorption loading of
the two complexes were directly modeled with the extended triple layer
SCM by implementing the bidentateābinuclear inner-sphere complexation
identified in the present study. These findings improve our understanding
of sulfate adsorption and its effects on other environmental chemical
processes and have important implications for generalizing the adsorption
behavior of anions forming both inner- and outer-sphere complexes
on mineral surfaces
Coupling Molecular-Scale Spectroscopy with Stable Isotope Analyses to Investigate the Effect of Si on the Mechanisms of ZnāAl LDH Formation on Al Oxide
While silicate has been known to affect metal sorption
on mineral
surfaces, the mechanisms remain poorly understood. We investigated
the effects of silicate on Zn sorption onto Al oxide at pH 7.5 and
elucidated the mechanisms using a combination of X-ray absorption
fine structure (XAFS) spectroscopy, Zn stable isotope analysis, and
scanning transmission electron microscopy (STEM). XAFS analysis revealed
that ZnāAl layered double hydroxide (LDH) precipitates were
formed in the absence of silicate or at low Si concentrations (ā¤0.4
mM), whereas the formation of ZnāAl LDH was inhibited at high
silicate concentrations (ā„0.64 mM) due to surface-induced Si
oligomerization. Significant Zn isotope fractionation (Ī66Znsorbedāaqueous = 0.63 Ā± 0.03ā°)
was determined at silicate concentrations ā„0.64 mM, larger
than that induced by sorption of Zn on Al oxide (0.47 Ā± 0.03ā°)
but closer to that caused by Zn bonding to the surface of Si oxides
(0.60ā0.94ā°), suggesting a presence of ZnāSi
bonding environment. STEM showed that the sorbed silicates had a close
spatial coupling with Ī³-Al2O3, indicating
that >SiāZn inner-sphere complexes (ā>ā
denotes
surface) likely bond to the Ī³-Al2O3 surface
to form >AlāSiāZn ternary inner-sphere complexes.
This
study not only demonstrates that dissolved silicate in the natural
environment plays an important role in the fate and bioavailability
of Zn but also highlights the potential of coupled spectroscopic and
isotopic methods in probing complex environmental processes
Synthesis of Birnessite in the Presence of Phosphate, Silicate, or Sulfate
Layered
manganese (Mn) oxides, such as birnessite, are versatile materials
in industrial applications and common minerals mediating elemental
cycling in natural environment. Many of birnessite properties are
controlled by MnĀ(III) concentration and particle sizes. Thus, it is
important to synthesize birnessite nanoparticles with controlled MnĀ(III)
concentrations and sizes so that one can tune its properties for many
applications. Birnessite was synthesized in the presence of oxyanions
(phosphate, silicate, or sulfate) during reductive precipitation of
KMnO<sub>4</sub> by HCl and characterized using multiple synchrotron
X-ray techniques, electron microscopy, and diffuse reflectance UVāvis
spectroscopy. Results indicate that all three anions decrease MnO<sub>6</sub> sheet sizes, attributed to oxyanion adsorption on edges of
the sheets, inhibiting their lateral growth. As a result of decreased
sizes, sheets undergo significant structural contraction. Meanwhile,
MnĀ(III) concentration significantly increases with increasing oxyanion/Mn
ratio. The increased MnĀ(III) concentration, along with the decreased
size, enlarges both direct and indirect band gaps of birnessite. Phosphate
imposes the strongest influence, followed by silicate and then by
sulfate, consistent with their decreasing adsorption affinity. Reacting
with 1 M KOH solution effectively removed the adsorbed oxyanions while
leading to increased sheet sizes, probably due to oriented attachment-driven
particle growth mechanisms. The results have important implications
for developing highly performed birnessite materials, for example,
small size MnĀ(III)-rich birnessite for photochemical and catalytic
applications, as well as for understanding chemical compositional
variations of naturally occurring birnessite
Impacts of Ionic Strength on Three-Dimensional Nanoparticle Aggregate Structure and Consequences for Environmental Transport and Deposition
The
transport of nanoparticles through aqueous systems is a complex
process with important environmental policy ramifications. Ferrihydrite
nanoparticles commonly form aggregates, with structures that depend
upon solution chemistry. The impact of aggregation state on transport
and deposition is not fully understood. In this study, small-angle
X-ray scattering (SAXS) and cryogenic transmission electron microscopy
(cryo-TEM) were used to directly observe the aggregate structure of
ferrihydrite nanoparticles and show how the aggregate structure responds
to changing ionic strength. These results were correlated with complementary
studies on ferrihydrite transport through saturated quartz sand columns.
Within deionized water, nanoparticles form stable suspensions of low-density
fractal aggregates that are resistant to collapse. The particles subsequently
show limited deposition on sand grain surfaces. Within sodium nitrate
solutions the aggregates collapse into denser clusters, and nanoparticle
deposition increases dramatically by forming thick, localized, and
mechanically unstable deposits. Such deposits limit nanoparticle transport
and make transport less predictable. The action of ionic strength
is distinct from simpler models of colloidal stability and transport,
in that salt not only drives aggregation or attachment but also alters
the behavior of preexisting aggregates by triggering their collapse
Quantification of Coexisting Inner- and Outer-Sphere Complexation of Sulfate on Hematite Surfaces
Sulfate
adsorption on hematite surfaces controls sulfate mobility
and environmental behavior but whether sulfate forms both inner- and
outer-sphere complexes and the type of the inner-sphere complexes
remain contentious. With ionic strength tests and S K-edge X-ray absorption
near-edge structure spectroscopy, we show that sulfate forms both
outer- and inner-sphere complexes on hematite surfaces. Both S K-edge
extended X-ray absorption fine structure spectroscopy and the differential
pair distribution function analyses determine the SāFe interatomic
distance (ā¼3.24 Ć
) of the inner-sphere complex, suggesting
bidentate-binuclear complexation. A multivariate curve resolution
(MCR) analysis of the attenuated total reflectionāFourier-transform
infrared spectra of adsorption envelope samples shows that increasing
ionic strength does not affect the inner-sphere but decreases the
outer-sphere complex adsorption loading, consistent with the ionic
strength effect. The extended triple layer model directly and successfully
models the MCR-derived inner- and outer-sphere surface loadings at
various ionic strengths, indicating weaker sulfate inner-sphere complexation
on hematite than on ferrihydrite surfaces. Results also show that
sample drying, lower pH, and higher ionic strength all favor sulfate
inner-sphere complexation, but the hematite particle size does not
affect the relative proportions of the two types of complexes. Sulfate
adsorption kinetics show increasing ratio of exchanged OH<sup>ā</sup> to adsorbed sulfate with time, attributed to inner- and outer-sphere
complexation dominating at different adsorption stages and to the
changes of the relative abundance of surface OH<sup>ā</sup> and H<sub>2</sub>O groups with time. This work clarifies sulfate
adsorption mechanisms on hematite and has implications for understanding
sulfate availability, behavior and fate in the environment. Our work
suggests that the simple macroscopic ionic strength test correlates
well with directly measured outer-sphere complexes
Determination of the Three-Dimensional Structure of Ferrihydrite Nanoparticle Aggregates
Aggregation
impacts the reactivity, colloidal stability, and transport
behavior of nanomaterials, yet methods to characterize basic structural
features of aggregates are limited. Here, cryo-transmission electron
microscope (cryo-TEM) based tomography is utilized as a method for
directly imaging fragile aggregates of nanoparticles in aqueous suspension
and an approach for extracting quantitative fractal dimensions from
the resulting three-dimensional structural models is introduced. The
structural quantification approach is based upon the mass autocorrelation
function, and is directly comparable with small-angle X-ray scattering
(SAXS) models. This enables accurate characterization of aggregate
structure, even in suspensions where the aggregate cluster size is
highly polydisperse and traditional SAXS modeling is not reliable.
This technique is applied to study real suspensions of ferrihydrite
nanoparticles. By comparing tomographic measurements with SAXS-based
measurements, we infer that certain suspensions contain polydisperse
aggregate size distributions. In other suspensions, fractal-type structures
are identified with low intrinsic fractal dimensions. The fractal
dimensions are lower than would be predicted by simple models of particle
aggregation, and this low dimensionality enables large, low-density
aggregates to exist in stable colloidal suspension
Phosphorus Speciation and Solubility in Aeolian Dust Deposited in the Interior American West
Aeolian dust is a significant source
of phosphorus (P) to alpine
oligotrophic lakes, but P speciation in dust and source sediments
and its release kinetics to lake water remain unknown. Phosphorus
K-edge XANES spectroscopy shows that calcium-bound P (CaāP)
is dominant in 10 of 12 dust samples (41ā74%) deposited on
snow in the central Rocky Mountains and all 42 source sediment samples
(the fine fraction) (68ā80%), with a lower proportion in dust
probably because acidic snowmelt dissolves some CaāP in dust
before collection. Iron-bound P (FeāP, ā¼54%) dominates
in the remaining two dust samples. Chemical extractions (SEDEX) on
these samples provide inaccurate results because of unselective extraction
of targeted species and artifacts introduced by the extractions. Dust
releases increasingly more P in synthetic lake water within 6ā72
h thanks to dissolution of CaāP, but dust release of P declines
afterward due to back adsorption of P onto Fe oxides present in the
dust. The back sorption is stronger for the dust with a lower degree
of P saturation determined by oxalate extraction. This work suggests
that P speciation, poorly crystalline minerals in the dust, and lake
acidification all affect the availability and fate of dust-borne P
in lakes
In Situ Structural Characterization of Ferric Iron Dimers in Aqueous Solutions: Identification of Ī¼āOxo Species
The structure of ferric iron (Fe<sup>3+</sup>) dimers in aqueous
solutions has long been debated. In this work, we have determined
the dimer structure in situ in aqueous solutions using extended X-ray
absorption fine structure (EXAFS) spectroscopy. An Fe K-edge EXAFS
analysis of 0.2 M ferric nitrate solutions at pH 1.28ā1.81
identified a FeāFe distance at ā¼3.6 Ć
, strongly
indicating that the dimers take the Ī¼-oxo form. The EXAFS analysis
also indicates two short FeāO bonds at ā¼1.80 Ć
and ten long FeāO bonds at ā¼2.08 Ć
, consistent
with the Ī¼-oxo dimer structure. The scattering from the FeāFe
paths interferes destructively with that from paths belonging to FeĀ(OH<sub>2</sub>)<sub>6</sub><sup>3+</sup> monomers that coexist with the
dimers, leading to a less apparent Fe shell in the EXAFS Fourier transform.
This might be a reason why the characteristic FeāFe distance
was not detected in previous EXAFS studies. The existence of Ī¼-oxo
dimers is further confirmed by MoĢssbauer analyses of analogous
quick frozen solutions. This work also explores the electronic structure
and the relative stability of the Ī¼-oxo dimer in a comparison
to the dihydroxo dimer using density function theory (DFT) calculations.
The identification of such dimers in aqueous solutions has important
implications for iron (bio)Āinorganic chemistry and geochemistry, such
as understanding the formation mechanisms of Fe oxyhydroxides at molecular
scale