Nanoparticulate iron oxide minerals in soils and sediments: unique properties and contaminant scavenging mechanisms
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Journal of Nanoparticle Research (2005) 7: 409–433 Springer 2005
DOI 10.1007/s11051-005-6931-x
Nanoparticulate iron oxide minerals in soils and sediments: unique properties
and contaminant scavenging mechanisms
Glenn A. Waychunas1, Christopher S. Kim1,2,3 and Jillian F. Banfield1,2
1
Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA (Tel.:
+1-510-495-2224; E-mail: GAWaychunas@lbl.gov); 2Department of Earth and Planetary Sciences,
University of California at Berkeley, Berkeley, CA 94720, USA; 3Department of Physical Sciences,
Chapman University, Orange, CA 92866, USA
Received 27 April 2005; accepted in revised form 3 May 2005
Key words: aggregation/growth, contaminant, ferrihydrite, goethite, hematite, oxide, schwertmannite,
sorption colloids, water quality
Abstract
Nanoparticulate goethite, akaganeite, hematite, ferrihydrite and schwertmannite are important constituents
of soils, sediments and mine drainage outflows. These minerals have high sorption capacities for metal and
anionic contaminants such as arsenic, chromium, lead, mercury and selenium. Contaminant sequestration is
accomplished mainly by surface complexation, but aggregation of particles may encapsulate sorbed surface
species into the multigrain interior interfaces, with significant consequences for contaminant dispersal or
remediation processes. Particularly for particle sizes on the order of 1–10 nm, the sorption capacity and surface
molecular structure also may differ in important ways from bulk material. We review the factors affecting
geochemical reactivity of these nanophases, focusing on the ways they may remove toxins from the envi-
ronment, and include recent results of studies on nanogoethite growth, aggregation and sorption processes.
Introduction biological activity (Barker et al., 1998), resulting in
thre concentrated FeOX phases found in nearly all
Fe oxides (hematite, magnetite), oxyhydroxides surficial soils and sediments (White & Brantley,
(goethite, akaganeite, lepidocrocite, feroxyhyte) 1995). One of the main reactions for producing
and hydrous oxides (ferrihydrite, hydrohematite, FeOX phases in the environment is the oxidation of
maghemite) [in this paper together abbreviated dissolved iron. Leached Fe(II) in reducing, anae-
‘‘FeOX’’ ] have important roles in biogeochemical robic conditions has a relatively high solubility in
processes at the Earth’s surface. This is due to their pH 7 waters, with the saturation concentration of
wide occurrence, tendency to nucleate and grow on the Fe(II) hexaaquo species approaching 0.5 M at
the surfaces of other phases, important redox room temperature. However, dissolved Fe con-
capabilities, and relatively high reactivity. As the centrations drop below 10)12 M if the Fe(II) is
most abundant crustal transition metal (repre- oxidized to the hexaaquo Fe(III) or polymerized
senting 6% of the chemical composition of the species (Baes & Mesmer, 1976). This reaction,
Earth’s crust), Fe is commonly leached from min- which occurs when ground water reaches the sur-
erals by both inorganic weathering processes and face (or due to mixing of subsurface fluids) and
410
abundant dissolved O2, results in the precipitation the coupled action of iron-reducing bacteria and
of FeOX and is commonly observed at natural reoxidation.
springs ½FeðIIÞðaqÞ þ 1=4 O2 þ 5=2 H2 O ! FeðIIIÞ Microorganisms can couple the reduction of
ðOHÞ3 þ 2Hþ . Iron oxyhydroxides can also form ferrihydrite, goethite and other FeOX phases to
under anaerobic conditions, for example, when iron the oxidation of organic carbon. The resultant
oxidation is coupled to nitrate reduction (Straub Fe(II) can be sequestered in FeS phases or is free
et al., 1996). This reaction, and iron oxidation in to be transported and eventually oxidized. Thus,
microaerophilic environments, is often catalyzed by FeOX colloids can reform, sink, form sediments,
microorganisms. Consequently, the mineral prod- and the process is repeated. All such nanopartic-
ucts are frequently associated in the environment ulate phases are expected to possess properties
with cell surfaces and extracellular polymers (e.g., markedly different from larger crystallites for the
Chan et al., 2004). following reasons:
Another process generating FeOX is the (1) They have large surface area/volume ratios
oxidation of pyrite (FeS2). In pyrite-rich rocks and hence present reactive surface areas dispro-
such as those associated with ore deposits, this portionately large for their volume fraction. For
can lead to environmental contamination known example, ferrihydrite is commonly found to have
as acid mine drainage. Oxidation of sulfide surface areas on the order of 50–200 m2/g. If a
generates solutions that are quite acidic (pH 2 sediment were to consist of such ferrihydrite and
or less) and that carry high concentrations spherical sand grains of 1 mm in diameter, then
of both dissolved Fe(II) and Fe(III). The the available reactive surface area of the ferrihy-
reaction ½FeS2 þ 7=2 O2 þ H2 O ! FeðIIÞðaqÞþ drite would equal that of the sand’s at a concen-
þ
2SO24 þ 2H generates aqueous Fe(II), and tration of only about 0.002% by volume (assuming
Fe(III) is formed by reaction with dissolved ideal dispersal).
oxygen. However, low solubility of O2 in these (2) The structure of their surfaces may differ
liquids and the slow rate of inorganic oxidation from those of larger crystallites, leading to altered
from ferrous ion to ferric ion by oxygen allow reactivity or varied crystal chemistry. This is a
Fe(II) concentrations on the order of 0.5 M. The complex issue encompassing many factors. For
Fe(II) can be oxidized by mixing of mine waters example, oxide nanocrystals may possess con-
with surface oxygenated water, or catalyzed by tracted or expanded surface layers compared to
microbial activity (Edwards et al., 2000) to pro- bulk phases (Cheng et al., 1993; Tsunekawa
duce large quantities of nanoparticulate FeOX. et al., 2000a, 2000b), they may have surface
Fe(III) phases which form from solution begin stoichiometry different from larger crystallites,
as small clusters of octahedral Fe (O, OH, OH2)6 the surfaces may be corrugated or otherwise
units that evolve into larger polymeric units with reorganized to reduce the effects of enhanced
time, eventually reaching colloidal sizes (Combes ‘‘curvature’’ (perhaps creating new types of sur-
et al., 1989). The driving force for aggregation face ‘‘sites’’ or a higher proportion of edge/cor-
and/or crystal growth of these particles is the ner/step site locations), and there can be
decrease in surface energy. However, for ferrihy- enhanced structural disorder at the surface
drite, goethite and possibly other Fe phases, the (Zhang et al., 2003; Gilbert et al., 2004). The
surface energy appears to be low enough redox potential of surface Fe on nanoparticles
(Navrotsky, 2001) that nano- to micro-size parti- may be size-dependent, or there may be a change
cles are metastable with respect to further growth, in proton-affinity of a coordinating oxygen atom
and crystallites larger than a few microns are (Boily et al., 2001). This can be due to the shift-
exceptional. Similarly, aggregates of small parti- ing of molecular orbital/band gap energy levels
cles may be metastable with respect to larger due to strain effects (well known in strained-layer-
crystallites or other Fe phases and persist for long superlattices, Osbourn, 1983), to quantum con-
periods. It is thus possible to have significant finement effects (Brus, 1983), or to stoichiometric
concentrations of nanophase FeOX in sediments changes. Recently quantum confinement effects
due to rapid formation with slow growth or have been observed in a-Fe2O3 nanorods, with a
transformation kinetics. A process that may affect band gap change from 2.1 eV in the bulk to
Fe(III) phase development is the cycling created by 2.5 eV in the nanorods (Guo et al., 2003).
411
The net effect of these factors is that nanocrys- surface complexes that are chemisorbed. Inner-
tallites may have dramatically different properties sphere complexes share some of their first
compared to larger crystallites, and this will be coordination shells with surface oxygen or anionic
especially true when considering reactions at species. For cations the shared atom is in the
nanoparticle surfaces such as sorption, surface ligation sphere of the cation, while for anions it
precipitation, redox reactions, and catalysis. We would involve the oxygens of the anion itself.
expect that most of these properties will modify Outer-sphere complexes are bound to a surface by
the binding of contaminants with FeOX nano- weaker electrostatic attraction and have interven-
phases, and thus lead to major effects on con- ing water molecules between themselves and the
taminant or nutrient transport and dispersal in the surface. They are thus farther from the sorbing
environment. surface. One can readily imagine intermediate
These considerations lead to a set of obvious cases of binding by hydrogen bonds that are
questions: What are the volumes of naturally weaker than inner-sphere binding, but stronger
occurring FeOX nanoparticles in specific envi- than outer-sphere binding.
ronments, and what specific phases are present? Sorption processes are generally approached
What are the nanoparticle size ranges in natural from different theoretical ends, with surface
systems? How do nanoparticle FeOX properties complexation models and electrical double layer
and structure change as a function of size? What (EDL) theory at one end, and molecular
are the lifetimes for FeOX nanoparticles prior to descriptions (MD-simulated or EXAFS-mea-
aggregation or growth/transformation into larger sured) at the other. The EDL and complexation
particles? What types of processes occur during modeling approach can be compared with labo-
aggregation, and how does this affect growth and ratory sorption experiments to understand the
reactive properties? near equilibrium behavior of sorption as a func-
If we could answer all of these questions we tion of solution pH, ionic strength and sorption
would be able to confidently predict the effect of density, but molecular details such as the types
FeOX nanoparticles on geochemical systems. and numbers of surface sites must be assumed
However, to date, there has been little study of (Davis & Kent, 1990). Molecular approaches
such environmental nanoparticles, and few model reveal the local nature of sorption sites, but not
studies to characterize nanophases in the labora- the larger chemical picture. Hence the approaches
tory. In this paper, we present new results on are highly complementary. Both molecular and
synthetic goethite nanoparticles from 3 to EDL/complexation modeling approaches suggest
75 nm in average size, briefly review results on ways in which sorption can be affected by
other model FeOX nanoparticle studies, and nanoparticle size.
speculate on the impact of Fe nanophases on
aqueous water geochemistry and water quality.
Consideration of the full gamut of nanoparticle Surface ‘‘curvature’’ and EDL failure
property changes with size is beyond the scope of
this work, but we can consider many of the For nanometer-sized particles it is no longer
important factors within the confines of one very possible to have ‘‘flat’’ surfaces or crystal faces of
important environmental process: sorption. any appreciable magnitude. The surfaces of these
particles must contain atomic steps that produce
an irregular outline. In this case, the electrostatic
Sorption conditions that define a classical EDL model
break down, and local fields vary markedly with
Sorption processes on FeOX phases are due to the proximity to the nanoparticle. Hence with
formation of chemical bonds with surface atoms, a decreasing particle size, EDL-predicted sorption
process known as chemisorption. The bonds can models may fail. From a molecular standpoint,
be covalent, ionic, or mainly involve hydrogen surface curvature may allow the approach of
bonds. Though there is variation in the bonding more surface sorbants than for a flat interface.
nomenclature used in the literature, we can speak This can lead to sorption densities higher than
generally of inner-sphere and outer-sphere types of normally predicted, or to changes in the nature of
412 the surface complexation geometry. The situation Different surface sites and site occupations is akin to the planar surface of a larger crystal possessing a high density of growth steps and Recent work has shown that the surfaces of irregular terminations, with much favored com- hematite and corundum in equilibrium with water plexation at these defect areas. Alternatively, the are altered with respect to the ideal terminated increased surface disorder likely in such non- structure (Eng et al., 2000; Trainor et al., 2004). In planar surfaces may result in sites less favorable the case of hematite, studies utilizing surface X-ray for sorption due to steric or electrostatic imped- diffraction show that the (0001) and (10–12) sur- ances, yielding lower sorption densities than faces are depleted in the octahedral Fe ions which predicted. Another effect of changes in the EDL in the normal structure would share coordination could be changes in the potential across the polyhedron faces with lower Fe sites. This double layer, which will affect electron transfer gives rise to a surface termination with reduced across the potential and thus redox activity. Fe concentration on the (0001) face, and ‘‘groves’’ Besides surface curvature and irregularity, quan- of depleted Fe sites on the (10–12) and other tum confinement effects may also affect this ‘‘r-plane’’ type surfaces. These surface adjustments potential (see below). Changes in nanoparticle thus allow active sites for surface complexation shape may also lead to variations in colloidal that would not occur on the so-called bulk struc- stability (Qu et al., 2004). ture terminated surfaces (Figure 1). Other possible Figure 1. Top: r-plane (10-1l) type fully oxygenated ideal termination of a 3 ·3 · 0.5 nm hematite particle. Bottom: same plane with Fe ions removed from the surface that share polyhedral faces with other polyhedra in the next lower layer. This type of surface change from ideal oxygen-terminated surface to reconfigured surface is observed on single hematite crystals equilibrated with neutral to acidic aqueous solutions, and indicates far more numbers and types of complexation attachment sites than would be expected in the ideal structure. In nanoparticles, additional reconfiguration, distortion and corrugation may occur to minimize surface energy or react to changes in structure.
413
surface changes involve reduced coordination molecular orbital energies. Probably the most
numbers of the metal ions, which has been sug- important effect of QC on sorption is an induced
gested for ferrihydrite (Zhao et al., 1994), and on change in the rate of electron transfer from nano-
nanohematite surfaces (Chen et al., 2002). In particle surface to sorption complex or aqueous
nanoparticles, other kinds of surface readjust- solution, i.e. rate of redox reaction, as size
ments may be required to minimize the surface decreases. This has been little studied in FeOX
energy, e.g., reconstructions into a denser atomic nanoparticles, but significant effects are observed
structure, quasi-amorphous disordering (Gilbert in semiconductor nanoparticles (Korgel &
et al., 2004), or complete phase transformation Monbouquette, 1997; Hoffman & Hoffman, 1993)
(see below). Thus both the types and proportions and TiO2 nanoparticles (Toyoda & Tsuboya,
of surface sites may vary with nanoparticle size, 2003). Because the wavefunction of an excited
leading to a dependence of complexation density electron is the size of the nanoparticle surface, all
and attachment energy on size. It is important to charge carriers in a semiconducting nanoparticle
note that such changes may either enhance or will be able to interact with the entire surface. This
diminish the stability of a particular surface sorp- can markedly increase quantum yields for electron
tion complex depending on whether the sites transfer, perhaps even to unity (Grätzel, 1989). An
generated by such surface alterations are more or important consideration for FeOX nanoparticles is
less hospitable to sorption. the possible interplay between local electronic
states of the surfaces and the ‘‘bulk’’ electron states
Surface contraction or expansion that are affected by QC. For particular types of
surface topologies, QC effects may be overwhelmed
Most crystal surfaces undergo relaxation to some by surface states (e.g., ‘‘dangling bonds’’, or local
degree. As mineral surfaces in equilibrium with a areas of unsatisfied charge, etc.). For semiconduc-
moist atmosphere tend to be oxygen terminated, tor nanoparticles, surface complexes may affect the
the relaxation is a response of the oxygen atoms to confinement, creating changes in physical proper-
the absence of overlying metal cations. The loss of ties. This might be exploited to ‘‘tune’’ a nano-
the cations produces an underbonding of the particle for particular reaction affinities.
surface oxygens, which can be compensated in part
by contraction or shortening of the lower oxygen– Surface energy, phase stability and transformations
metal bonds (Brown & Shannon, 1973). Such con-
traction redistributes surface charge and affects Critical nuclei have surface energy just offsetting
electrostatic bonding. In nanoparticles the dia- bulk energy such that any increase in size will tip
meters of the particles are small enough that con- the energy balance in favor of the bulk energy and
traction effects can propagate through the entire lead to greater thermodynamic stabilization, while
particle and alter the cell dimensions (Tsunekawa a decrease in size will lead to a larger relative
et al., 2000a). This effect will alter the bulk energy surface energy that will be destabilizing. The de-
of the nanoparticle and other thermodynamic stabilizing effect can be altered if the energy com-
properties, and can be a factor in phase transfor- ponents are changed by a phase (structural)
mations. Sorption is affected by surface bond transformation. For example, in TiO2, the stable
length contraction via weakening of surface oxy- bulk phase for large crystallites is rutile, but the
gen–complex binding, but this effect may be subtle anatase polymorph is found to be stable at nano-
and less important than changes in the nanoparticle meter sizes. The rutile structure has the smaller
phase diagram (temperature–composition–size). bulk energy/unit volume ratio but higher relative
surface energy per unit surface area. Hence at large
Quantum confinement effects enough surface area, a rutile nanoparticle is less
stable than an anatase nanoparticle. A generalized
Quantum confinement (QC) effects occur when the diagram of this behavior is shown in Figure 2
wave function of an excited electron or coupled (after Navrotsky, 2001), demonstrating that a size
electron–hole pair (exciton) carrier is near the size axis is required in any phase diagram involving
of the nanoparticle. Size then controls the nanometer scale materials. At extremely high
wavefunction extent and affects band gaps and surface areas, an amorphous phase may be more
414 Figure 2. Model phase diagram for a nanoparticle system where surface and bulk energy contributions to the total particle free energy change considerably with respect to one another as a function of particle size. For large crystallites the stable poly- morph is a. With decreasing particle size the surface energy contribution increases so that at point A the b polymorph, with lower surface energy per unit area, is favored. With further size reduction, eventually the beta phase is unstable with respect to an amorphous structure having lower surface energy per unit area (point B). All nanoparticles are metastable with respect to coarsening and adopting the alpha structure. [after Navrotsky, 2001] stable at nanometer dimensions than any crystal- surface energies), sorption may be driven in much line polymorph, essentially leading to a ‘‘glass the same way as growth or aggregation. This may transition’’ at low temperatures. These energy lead to a much larger degree of contaminant considerations dictate that a crystallite may sequestration on nanoparticle surfaces than would undergo one or more structural transformations as be expected from standard laboratory uptake it grows within the nanoscale regime. Phase experiments using coarser phases (Zhang et al., transformations will alter the relative proportions 1999). Such lowering of surface energy by com- of sorption sites and may lead to desorption of plexation effects is possibly due on the molecular initially adsorbed species, or conversely, to level to reductions in surface strain, distortion or enhanced uptake. A secondary effect is a likely other relaxation process due to the surface bind- change in PZC for the nanoparticle, which may ing. This type of relaxation has been observed in also change sorption density at specific pH values. arsenate binding to ferrihydrite (Rea et al., 1994). Another way that surface energy can affect sorption is by a direct effect on sorbate distribu- Stoichiometry changes tion coefficients between solution and nanoparticle surfaces. If there is a sufficient energetic advantage Work on single crystal surfaces shows that the for sorption (i.e., in cases of high nanoparticle stoichiometry of hematite must change as a
415
function of crystallite size due to the removal of sorption has been obtained from IR measure-
Fe(III) ions and addition of H+ at the surfaces ments utilizing the stretching modes of the com-
(Trainor et al., 2004). For example, on the (0001) plexes. This latter method is sensitive but can
surface, removal of the Fe ions whose polyhedra suffer from interpretation problems in as much as
share faces with deeper Fe polyhedra reduces the both sorption geometry and protonation changes
surface layer composition to FeO3. Protonation to can lead to similar spectral changes (Myneni
balance charge would then create an Fe(OH)3 et al., 2004). Other types of indirect structural
surface layer. Nominally anhydrous a-Fe2O3 thus characterization methodologies are useful but not
adds an increasing hydrous or hydroxyl compo- definitive in identifying complexation site and
nent with decreasing size. For heterovalent FeOX geometry (Sposito, 1986). A recent review of
solids like magnetite (Fe(II)Fe(III)2O4), both EXAFS analyses of sorption complex geometries
added hydration and a change in the ratio of metal (Brown & Sturchio, 2002) includes a very com-
valences may occur. One way in which valences plete reference list for EXAFS studies of com-
may be changed is due to surface contraction, case plexation. Table 1 comprises a subset of these
(3) above. Magnetite has both Fe(III) and Fe(II) results covering the complexes and geometries for
occupying octahedral FeO6 sites. Near the surface the FeOX compounds that have been studied to
in an anhydrous case, contraction of Fe–O dis- date.
tances ought to favor the shorter Fe(III)–O dis- A perusal of these complexation results shows
tances over Fe(II)–O, and hence the surface may that in many cases there are multiple types of
be enriched in Fe(III). The picture is complicated complexation geometries for the same sorbate,
by the effect of surface coordinated water which presumably due to differences in the exact experi-
may have a stabilizing effect with an opposite bond mental preparation conditions, variations in the
length dependence. Electron delocalization on the crystal form of the substrate, or difficulties in
vacuum surface can also affect valence states spectral interpretation. A suggestive comparison is
(Wiesendanger et al., 1994). For nanoparticles between results for ferrihydrite (hydrous ferric
featuring surfaces enriched in Fe(III), the amount oxide, or HFO) and hematite or goethite. HFO,
of Fe(II) could be dramatically decreased with a also called ‘‘2-line’’ ferrihydrite, is poorly crystal-
large resulting effect on physical properties such as lized having coherence sizes of about 2–5 nm in
magnetism and electrical conductivity. Changes in synthetic material (Janney et al., 2000). Hence we
stoichiometry will also affect the energy needed to expect that comparing sorption on ferrihydrite to
remove or add electrons to the conduction band, sorption on hematite and goethite (with crystallites
and thus change redox properties. The direct more commonly in the micron size range and lar-
effects of such changes on sorption have not been ger) might demonstrate the effects of reduced scale.
evaluated. For example, Cd(II) substitutes in the goethite
structure and also forms inner-sphere complexes
Surface complexation geometry sharing edges with FeO6 polyhedra (Spadini et al.,
1994; Manceau et al., 2000). However in HFO,
A large number of studies have attempted to there appears only to be edge-sharing inner-sphere
characterize the sorption of metal ions and complexation. With Pb(II), an inner-sphere bid-
oxyanions on FeOX using a variety of methods. entate edge-sharing complex is formed on goethite
EXAFS spectroscopy has been widely used (Bargar et al., 1997), but an inner-sphere multi-
because of its element selectivity – the structure nuclear complex is formed on HFO (Manceau
near a particular sorbing complex can be directly et al., 1992). For Se(VI) an outer-sphere complex
probed regardless of the presence of other types forms on goethite (Hayes et al., 1987), but an
of complexes. EXAFS spectroscopy can be inner-sphere bidentate complex forms on HFO
applied to distinguish inner- vs. outer-sphere (Manceau & Charlet, 1994). For arsenate
surface adsorption complexes, monodentate vs. (AsO4 Þ3 , there are variations among types of
bidentate linkage to the substrate, mononuclear inner-sphere complexes between the phases,
vs. multinuclear metal surface complexes, and including monodentate, bidentate, mononuclear
adsorbed vs. (co)precipitated metal species. Less and binuclear species (Waychunas et al., 1993).
direct structural information for oxyanion Finally, with Zn(II), the type of complex416
Table 1. EXAFS-derived complexation geometry for selected species on FeOX substrates
Magnetite Goethite Hematite Ferrihydrite/HFO Akaganeite Schwert
mannite
Cr(VI) sorbs as Cr(III) Cr(VI) inner-sphere Cr(VI) (0001) plane Cr(III) nonepitaxial Cd(II) Cd(II)
precipitate nonepitaxial monodentate and epitaxial precipitate or precipitate (Cr,Fe)OOH inner-sphere inner-sphere
CrOOH bidentate multinuclear complex
(100) plane amorphous Cr(III) nonepitaxial Cr(III) (0001) plane Ni(II) Ni coprecipitate
precipitate (Cr,Fe)OOH precipitate (Cr,Fe)OOH inner-sphere multinuclear
I)inner sphere Cu(II) inner-sphere Pb2+ inner-sphere Cu(II) inner-sphere bidentate
multidentate polymer bidentate, mononuclear
Zn(II) inner-sphere 4- or Lu(III) lattice substitution Zn(II) inner-sphere bidentate
6- coordinated bidentate 4-coordination (pH 6)
outer-sphere 6-coordination
As(V) (AsO4)3) inner-sphere As(V) (AsO4)3) (0001) As(V) (AsO4)3) inner-sphere
bidentate, minor plane bidentate, mononuclear bidentate
monodentate and binuclear (1–102) plane
bidentate, mononuclear
and binuclear
As(III) (AsO3)3) inner- Pb(II)/malonate Ternary Se(VI) (SeO4)2) inner-sphere
sphere complex bidentate
bidentate
Se(VI) (SeO4)2) U(VI) (UO2)2+/ Se(IV) (SeO3)2) inner-sphere
outer-sphere Carbonate Ternary bidentate
complex
Se(IV) (SeO3)2) Sr(II) outer-sphere
inner-sphere
bidentate
Sr(II) inner-sphere Cd(II) inner-sphere (edge sites)
bidentate; outer-sphere
Cd(II) lattice substitution; Pb(II) inner-sphere
inner-sphere bidentate bidentate, multinuclear
mononuclear
Au(III) inner-sphere bidentate Lu(III) lattice substitution
Hg(II) inner-sphere U(VI) (UO2)2+ inner-sphere bidentate
bidentate
Pb(II) inner-sphere bidentate
mononuclear
Lu(III) lattice substitution
U(VI) (UO2)2+ inner-sphere
bidentate
Np(V) (NpO2)+ inner-sphere417
(octahedral or tetrahedral) appears to be depen- oriented aggregation of nanoparticles occurs after
dent both on substrate and pH (Trivedi et al., sorption, then sorbed species may be trapped
2001; Waychunas et al., 2002). These differences within the bonded crystallites (Banfield et al.,
confirm that complexation is highly substrate (and 2000). These species cannot be desorbed in the
therefore surface)-dependent, and thus suggest normal sense, but will be released only if enough of
that large changes in the type of complexation the particle is dissolved that the sorbate is brought
could occur in nanoparticulate phases. once again to the solid/water interface. It is not
uncommon to find mineral crystals with extremely
thin layers of impurities, nano-scale zones, or ele-
Other differences between nanoparticles ment clustering that can be detected with TEM
and bulk phases techniques e.g., in arsenic-containing pyrite
(Savage et al., 2000). Such layers or clusters pre-
Other important factors in understanding the serve the host structure, but have local composi-
extent of nanoparticle uptake and retention of tion exceeding the bulk solubility for the impurity.
species include reaction kinetics, sorption revers- How do these impurity regions, often just a few
ibility, and sequestration of sorbates. Reaction atomic layers in size, form initially? One possibility
rates for sorption as a function of substrate is that such impurities were once sorbed species
dimension in natural systems have not been studied forced into structural incorporation by either ori-
to our knowledge. The basic energetic factors ented aggregation or by an unusual burst of
considered earlier will affect kinetics as the relative growth. Nanoparticles are profound subjects for
numbers of attachment sites and the energy of such encapsulation effects, as their maximum
binding affects the probability that a given solution sorption behavior may coincide with their maxi-
complex will be sorbed and remain attached to the mum tendency for aggregation, i.e., near their
surface. Kinetics are also affected by competition surface point of zero charge (PZC), where electro-
among sorption sites, diffusion rates on the surface static repulsion of particles is minimized.
and through the nearby aqueous solution, and by Beyond possibly fostering sorption, surface
counterion (charge balancing species) transport. precipitation and encapsulation effects, nanopar-
Hydrological behavior within pores among nano- ticles themselves may be vehicles for transport of
particles is expected to be quite different than in pollutant and nutrients (e.g., sorbed phosphate on
larger scale systems, and flow rates may be extre- FeOX). This is because, unlike larger bulk parti-
mely low. This is due in part to nanoscale structure cles which eventually settle out of the aqueous
in the solution near the particle, affecting the phase, they may readily be carried within hydro-
effective viscosity and rates of ligand exchanges. logic systems for indefinitely long distances. Hence
Hence the reactivity of the relatively large surface understanding pollutant dispersal by nanophases
areas within a set of nanoparticles will be crucially involves not only an understanding of their unu-
affected by the degree of aggregation and the type sual chemistry, surface structure and reactivity,
of pore system that develops. but also the factors driving or hindering aggrega-
Reversibility of sorption reactions is strongly tion (or attachment to other larger substrates).
dependent on the degree of surface precipitation. This aspect of nanomineral behavior requires an
Even a few interacting sorbed complexes will integration of knowledge that we currently do not
require larger proportional energies for desorption possess. However more and more is being learned
due to breaking of intercomplex bonds and entropy about specific nanoparticle growth.
changes. Added to this is the tendency for surface
‘‘templating’’ or catalytic effects where the critical
size for a precipitate nucleus is reduced on the sur- Nanoparticle growth
face. If these effects are heightened on nanoparticle
surfaces there may frequently be a large hysteresis in An expanding body of evidence indicates that
sorption/desorption cycles, favoring development nanoscale phases may grow through the oriented
and long-term stability of additional precipitates. attachment [OA] of individual nanoparticles (Penn
Also related to sorption reversibility is the issue et al., 1998; Banfield et al., 2000; Gilbert et al.,
of ‘‘internal’’ sequestration or encapsulation. If 2003; Guyodo et al., 2003; Huang et al., 2003;418
Penn, 2004; Tsai et al., 2004; Zhang & Banfield, them into the solid phase. The reductions in
2004). Oriented attachment is a specialized form of mobility, toxicity, and bioavailability associated
aggregation that differs dramatically from classical with sorption combined with the enhanced levels
models of ripening-based growth from solution of uptake expected of nanophases provide con-
commonly associated with the growth of macro- siderable incentive to further explore the role that
scale particles. Just as sorption may be influenced nanoparticulate minerals play in the attenuation of
by surface, structural, and quantum confinement contaminant species in the environment.
changes at nanoparticle surfaces, so may these The following section provides brief summaries
factors play a role in the tendency for growth via of our recent studies on the aggregation-based
OA. Most likely, some continuum between the two growth of goethite at the nanoscale and the effects
growth processes occurs in natural systems (Penn of particle size and growth on the sorption and
et al., 2001), with size serving as the determining precipitation of toxic heavy metals. Nanoparticles
factor for which process dominates. Aggregation- of goethite have been identified as the dominant
based growth at the nanoscale gives way to rip- reactive oxyhydroxide phase in lake and marine
ening-based growth at larger sizes, with the tipping sediments (van der Zee et al., 2003). Heavy metal
point dependent on the particular phase, solution contamination is a pervasive environmental
chemistry, and particle concentration of the system problem in numerous areas including abandoned
being considered. and inactive mines, industrial sites with legacies of
The exact effects of aggregation-based nano- unregulated dumping, geological areas naturally
particle growth on the retention of sorbed species, enriched in metals that have been disturbed by
whether toxic metal cations, inorganic anion human development, and bay/delta regions suf-
ligands, or organic contaminants, are largely fering from the accumulation of decades of pol-
unknown but may include the following: luted runoff and sediments. Therefore, a detailed
understanding of the mechanisms by which heavy
• Migration to other mineral/water interfaces on
metals interact with goethite nanoparticles and the
the aggregate, thus persisting as surface sorbed
extent and permanence of these interactions is
species
essential in assessing the prospects of nanoparticle
• Encapsulation into the aggregating matrix as
use in environmental cleanup strategies.
trapped species
The brief summaries of various projects on
• Diffusion into a nanoparticle’s original pore
nanoscale goethite that follow represent work that
structure or new pore spaces created by aggre-
either has been recently completed or is still
gation
ongoing. As such, these sections focus on our lat-
• Incorporation into the newly-formed aggregates
est results, forgoing detailed explanation of
as structural impurities
experimental methods or analytical techniques
• Formation of localized nano- or micro-precip-
used (to be published elsewhere), in favor of pre-
itates within the aggregating particles
senting initial conclusions we have drawn from the
• Desorption and re-release of the sorbed species
data.
back into solution
Depending on the type of sorbed species as well as Macroscopic/spectroscopic characterization
the nanoparticulate sorbent phase, different of nanogoethite growth
responses to nanoparticle aggregation are likely to
occur in varying proportions. Therefore, under- Nanoscale iron oxyhydroxides have been observed
standing the behavior of sorbed species under the to grow through oriented attachment of individual
conditions of substrate growth has significant nanoparticles in both natural (Banfield et al.,
implications for their long-term stability. This is of 2000; Chan et al., 2004) and synthetic (Penn et al.,
particular interest when considering the remedia- 2001; Guyodo et al., 2003) settings. Determining
tion of contaminants, particularly those present as the precise mechanism(s) of growth in this size
dissolved species in surface or groundwaters, regime has important implications for both our
through sorption- or (co)precipitation-based evolving understanding of aggregation-based
treatment strategies which effectively remove the growth as well as the practical applications of
pollutants from the aqueous phase and sequester this growth process towards issues such as419
contaminant sequestration. In order to minimize function of aging time (Figure 4) shows an
the complexities inherent in the natural environ- expected inverse relationship between the two,
ment, it is useful to develop a model system that with surface area decreasing as the average particle
allows us to study the effects of individual vari- size increases over time. Additionally, it appears
ables (in this case, aging time) on particle size, that there are two distinct stages of growth in the
morphology, growth rate, and structure. system, with a rapid increase in particle size
In our current work, the formation of goethite between 0 and 4 days followed by slower growth
nanoparticles was induced from iron-rich solutions from 4 to 32 days; this implies a shift in the growth
using a microwave-based synthesis method mechanism between the two stages, perhaps from
(Guyodo et al., 2003). The nanoparticle suspen- oriented aggregation to ripening as discussed
sion was dialyzed to remove excess dissolved iron earlier.
and then aged in an oven at 90C for durations X-ray diffraction patterns collected from the
ranging up to 33 days. Although this method of aged samples using high-flux synchrotron-based
aging and growth is clearly accelerated relative to X-rays (Figure 5) suggest that the nanoparticles do
natural growth rates in order to facilitate the not undergo a phase change with aging, which
generation of a wide range of differently-sized would be observed by changes in peak positions;
particles in a realistic time frame, comparisons of instead, the samples can be consistently identified
the particle aggregates with those of natural FeOX as goethite throughout the aging process, with
samples appear similar enough to presume that the changes in peak intensity/width attributed to
methods of growth were not fundamentally altered changes in particle size (progressively larger par-
through this heating process. Removing aliquots ticles resulting in sharper and more well-defined
of the aging suspension at various times allows us peaks). Some of this peak shape change is the
to collect nanogoethite samples representing dif- result only of particle size change and can be
ferent aging times. These samples were then char- quantified by the Scherrer relation (Warren, 1968).
acterized by an array of analytical techniques to However, there also appears to be an effect of
identify trends in specific characteristics as a decreasing degree of disorder as particle size
function of aging/growth, namely: increases, due most likely to the smaller propor-
tion of surface-associated atoms. EXAFS results
– High-resolution transmission electron micros-
support the notion of a progressive and continu-
copy (TEM) (morphology, size)
ous increase in the structural ordering of the
– Dynamic light scattering (DLS) (average parti-
nanoparticles with aging (Figure 6). This is most
cle size)
clearly evidenced in the Fourier transforms of the
– BET surface area analysis (reactive surface area)
spectra, which show an increasing intensity of the
– X-ray diffraction (XRD) (phase identification)
second-neighbor peaks consistent with an
– Fe K-edge extended X-ray absorption fine
increased Fe–Fe coordination number and/or
structure (EXAFS) spectroscopy (average local
degree of structural ordering around the average
atomic structure)
Fe atom in the sample. The trend observed in the
TEM analysis of samples of varying aging times Fourier transforms could be interpreted as either a
reveals that the initial nanoparticles are 3–5 nm in result of the increasing proportion of Fe atoms
size and oblong in shape (Figure 3a); with addi- associated with the bulk and the declining pro-
tional aging, direct evidence for oriented aggre- portion of surface-terminated Fe atoms with
gation of individual nanoparticles can be aging, or an actual phase transition from poorly-
observed, with the aggregates assuming a more ordered ferrihydrite to goethite. The XRD data,
rod-like morphology (Figure 3b). In the most aged however, seem to support the former explanation,
sample studied (33 days), an abundance of these with the structural classification of the nanoparti-
rod-shaped particles dominates, with dimensions cles as goethite upon initial formation and
of roughly 10 · 100 nm (Figure 3c); however, throughout the aging process.
several of the initial nanoparticles and smaller To summarize, the application of multiple
aggregates remain as well. complementary techniques to characterize
A combination plot showing changes in surface nanoparticulate goethite during the aging process
area and average hydrodynamic diameter as a (engendered by heating the suspension at 90C)420
Figure 3. High-resolution (500,000·) transmission electron microscopy (TEM) images of synthetic goethite nanoparticles that
have aggregated from more oblong shapes (a, showing two such particles connected through oriented attachment as viewed by
their lattice fringes) to more rod-like shapes (b, where 3–4 nanoparticles have aggregated to create a different, more elongated
morphology compared to the initial particles). After 33 days of aging, many such rods have formed (c) although many of the
initial oblong nanoparticles remain.
yields a series of observable trends that distinguish Clear understanding of these trends in goethite
the mechanisms of growth in the initial stages nanoparticle growth can help us interpret similar
following nanoparticle formation: processes in natural systems, and provide a basis
for the interpretation of studies involving obser-
– Continuous increase in particle size
vations of nanoparticle growth and the influence
– Two distinct stages of growth: a rapid growth
of particle size and aging on the sorption of heavy
stage within the first 4 days corresponding
metals in solution.
primarily to nanoparticle aggregation followed
by a slower growth stage more likely to reflect
traditional ripening-based growth mechanisms SAXS/WAXS studies of aggregation-based
– Change in particle morphology from oblong to growth of goethite nanoparticles
rod-shaped, initially driven by the oriented
aggregation of oblong particles The previous characterization techniques all
– Increased degree of structural ordering required the ex situ analysis of aged nanogoethite421 Figure 4. Combined results from initial dynamic light scattering (DLS) and BET surface area measurements of aged goethite nanoparticles, showing an inverse relationship between particle diameter and surface area. samples which were typically dried prior to anal- SAXS/WAXS patterns were also collected on the ysis, introducing the possibility of induced particle same ex situ samples analyzed in earlier charac- aggregation. Synchrotron-based small- and wide- terization studies; These samples were analyzed in angle X-ray scattering (SAXS/WAXS) provides a their original suspensions, minimizing any addi- method of characterization that allows real-time, tional aggregation that would have been induced in situ observation of changes in nanoparticle size, by drying. size range, and particle structure in solution during The processed in situ SAXS data show that at the aging process. the outset of the aging process, most nanoparticles For this work, goethite nanoparticles were syn- fall within a size range of 2–6 nm diameter thesized using the microwave method cited earlier (Figure 7a), corroborating the TEM images of the and analyzed at the 5-ID-D Dow-Northwestern- initial nanoparticles. With progressive aging, this Dupont (DND) beamline at the Advanced Photon dominant population of nanoparticles begins to Source, Argonne National Laboratory using a shrink (Figure 7b) while a population of slightly simultaneous SAXS/WAXS configuration larger particles featuring a broader size distribu- equipped with a heating stage. A suspension of the tion (>5 nm) starts to increase (Figure 7c). SAXS initial 3–5 nm particles was placed in a glass cap- data from the ex situ samples show the continua- illary and the temperature increased to 70C for tion of this trend (Figure 8), with the initial pop- continuous aging; the 90C temperature used in ulation of nanoparticles continuing to decline over the previous aging experiments could not be time in favor of a much broader distribution of achieved due to capillary strength and evaporation particle sizes ranging from roughly 8–50 nm. effects. SAXS/WAXS patterns were collected on These results provide evidence for aggregation- the aging suspension every 15 min for a period of based growth of the initial nanoparticles, 12 h. To cover a wider range of aging times, demonstrated by the direct decline in the initial
422 Figure 5. Synchrotron-based X-ray diffraction patterns of goethite nanoparticles after variable aging times. Over time the evolution of peaks clearly associated with goethite (as indicated by the Miller indices in the 25-day aged sample) can be observed. Data collected at beamline 7.3.3 of the Advanced Light Source. population of nanoparticles with aging. In con- process, but rather increase in size and structural trast, ripening-based growth of the nanoparticles ordering with time and in suspension. Ongoing would yield a shifting of this initial peak to greater work on this project involves repeating the SAXS/ and broader size ranges, as observed in other WAXS experiments in the presence of various studies (Lo & Waite, 2000), rather than a direct metals to determine the effects of the contaminants decline in the population. The fact that this type of on particle growth and size distribution. broadening and shifting appears to occur in the second population of particles suggests that ripening-based growth is perhaps a significant Particle size effects on the sorption of heavy metals mechanism facilitating the continued growth of the nanoparticle aggregates. The previous studies aimed to thoroughly char- The WAXS patterns of selected ex situ samples acterize the structure, morphology, size, and are presented in Figure 9. While considerable growth mechanisms of nanoscale goethite in a background subtraction issues clearly remain due model system setting. The subsequent series of to aqueous solution scattering, the evolution of experiments examine the effects of nanogoethite progressively sharper peaks corresponding to particle size on the sorption of heavy metals and crystalline goethite can be easily discerned. These metalloids. Specifically, As(V), Cu(II), Hg(II), and results are comparable to the ex situ XRD patterns Zn(II) have been selected for study due to the shown in the previous section, and reinforce the known hazards they pose to humans as well as notion that the nanoparticles do not undergo their prevalence in many areas of metal contami- phase transitions during the aging (or drying) nation, such as abandoned mine regions.
423
60 35
goethite
30
50
goethite
25d
20d 25 25d
40
20d
14d
20
8d
14d
30
4d 8d
15
4d
2d
20
10 2d
1d
0d 1d
10
5
ferrihydrite 0d
ferrihydrite
0 0
1 3 5 7 9 11 0 1 2 3 4 5 6
k(Å-1)
R+² (Å)
Figure 6. Fe K-edge EXAFS spectra and Fourier transforms of goethite nanoparticles after variable aging times. The con-
tinuous trends observed with aging could either be a result of a phase transition from ferrihydrite (bottom spectrum) to
goethite (top spectrum) or indicate an increasing degree of structural order over time.
Nanogoethite suspensions were aged for dura- speciation as a function of particle size. Presented
tions expected to yield particles of 5, 25, and are data for Hg(II) uptake, showing that the
75 nm average size based on earlier DLS analysis average interatomic distances between the metal
of aged samples. These differently-sized suspen- contaminant and surface iron atoms of the goe-
sions were then separately exposed to 0.5 mM thite nanoparticles appear to increase significantly
initial concentrations of the aforementioned ele- at the smallest particle size (5 nm) by 0.2 Å rel-
ments at pH 6 using previously-established pro- ative to the 25- and 75-nm particles (Figure 10).
cedures (Kim et al., 2004a, 2004b) for macroscopic This finding suggests a substantial degree of dis-
uptake experiments. After 24 h reaction time, tortion of the Fe(O,OH)6 octahedra which com-
the suspensions were centrifuged and the result- prise the 5-nm goethite structure, and that
ing pellets prepared for analysis by As K-edge, Cu corresponds with the oblate morphology and
K-edge, Hg LIII-edge, or Zn K-edge EXAFS increased degree of surface curvature seen in TEM
spectroscopy. EXAFS spectroscopy is the tech- images of the smallest nanoparticles compared
nique of choice for determining the speciation of with the more regular, rod-shaped particles formed
sorbed and/or co-precipitated As, Cu, Zn, and Hg through oriented aggregation.
associated with metal oxide substrates, as has been To assess the effects of particle size on the extent
demonstrated in numerous studies (Waychunas of heavy metal sorption, sorption isotherms were
et al., 1993; Bochatay et al., 1997; Trivedi et al., generated for the uptake of Hg(II) onto 5-nm,
2001; Waychunas et al., 2002; Roberts et al., 2003; 25-nm, and 75-nm goethite nanoparticles over a
Kim et al., 2004a, 2004b). Our preliminary range of initial concentrations from 5 to 1000 lM
EXAFS studies show subtle differences in metal and a constant solids concentration of 2.3 g/l. As424 Figure 7. (a) Processed small-angle X-ray scattering (SAXS) data for the in situ aging of an initial 5-nm nanoparticle sus- pension at 70C for 12 h, with expanded views showing (b) the decline in particles sized 2–6 nm and (c) the increase in particles >5 nm. expected, when uptake is expressed in absolute which shows distinct changes in both morphology terms (total lmol sorbed), the smallest particles, and structural ordering with increasing size and featuring the highest reactive surface area aging time. Referring to Figure 12, greater (300 m2/g), are shown to sorb the most Hg(II) disordering of the Fe(O,OH)6 octahedra at the over the entire concentration range (Figure 11a). surfaces of the 5-nm nanoparticles may explain In contrast, the largest particles, with lower sur- both the shift in second-neighbor metal-iron dis- face area (130 m2/g), have considerably less Hg(II) tances as well as the lower surface coverages due to uptake. However, once normalized for surface the reduced strength or energetic favorability of area, the trend is reversed, with the smallest 5-nm the binding sites available for metal uptake. That particles displaying lower surface coverages at is, although the smallest nanoparticles can sorb the each initial Hg(II) concentration relative to the greatest amount of Hg(II) on a per mass basis, larger particles (Figure 11b). Hence in this case the both the geometry of the sorption complexes and smallest nanoparticle surfaces appear to have the degree of surface loading are altered from fewer and somewhat different sorption sites com- larger particles with more ordered surfaces. Fur- pared to larger particles. thermore, charge distribution has been shown to These differences can be explained in the context vary at the nanoscale as a function of particle of the earlier nanogoethite characterization work, aspect ratio (Rustad & Felmy, 2005), with greater
425
Figure 8. Processed small-angle X-ray scattering (SAXS) data for ex situ samples of different aging times.
charge accumulation on more acicular particles; More work is needed to quantify macroscopic
this may also influence the formation of sorption uptake with different metals, and also study the
complexes on particles over a range of size and desorption of metals with time as a function of
morphology. These observations raise issues con- particle size. These studies clearly demonstrate
cerning the long-term stability of surface com- that nanoparticles display altered metal sorption
plexes formed on these smallest nanoparticles. properties relative to larger counterparts.
14
(130) (111)
12
(001) (021)
10
(121) (140)
8
(221) (151) (002)
6
33d
14d
4
8d
2
4d
0
1d
-2
-4 0d
-6
-8
10 15 20 25 30 35
Q (nm-1)
Figure 9. Wide-angle X-ray scattering (WAXS) data for samples of different aging times.426
Figure 10. Hg LIII-edge EXAFS and corresponding Fourier transforms of Hg(II) sorbed to goethite nanoparticles of 5, 25,
and 75 nm average diameter. Guidelines on the Fourier transforms reveal differences in the most distant (Hg–Fe) interatomic
distances when Hg(II) is sorbed to the smallest 5-nm particles.
Retention of heavy metals during aggregation-based kinetics of the nanoparticles themselves. To begin
nanoparticle growth to assess these effects, macroscopic uptake exper-
iments were conducted under similar conditions to
As discussed earlier, the effects of aggregation- those in the work just described. However, instead
based nanoparticle growth in the presence of and/ of introducing the metals to the nanoparticles after
or following the sorption of metals to their they had been aged to the desired size, they were
surfaces may have significant implications for both introduced into the initial nanoparticle suspension
the fate of the contaminants as well as the growth which was subsequently allowed to age at 90C
(a) (b)
100 10
1
10
0.1
5 nm 5 nm
1
25 nm 25 nm
0.01
75 nm 75nm
0.1 0.001
0 200 400 600 800 1000 0 200 400 600 800 1000
Initial Hg concentration (u) Initial Hg concentration (u)
Figure 11. Macroscopic uptake data of Hg(II) onto goethite nanoparticles of different sizes, showing (a) uptake expressed in
terms of total lmol sorbed and (b) uptake normalized for surface area and expressed as lmol/m2.427 Figure 12. Schematic diagram showing distortion of Fe(O,OH)6 octahedra, the building blocks of the goethite structure, thought to occur as particle size decreases and surface curvature/disorder increases. with periodic sample aliquots collected over the presence of Zn(II). A clear trend in both the course of 24 h. EXAFS spectroscopy of the EXAFS and Fourier transforms can be observed resulting solids was then conducted in order to with time; in particular, the substantial increase determine if the mode of uptake had changed over in the amplitude of the second-neighbor features this short time span. in the Fourier transform (representing Zn–Fe Figure 13 shows the results of one of these sets and/or Zn–Zn interactions) indicates greater of metal/aging experiments, conducted in the structural ordering of the Zn and/or a larger Figure 13. Zn K-edge EXAFS and Fourier transforms of 5-nm goethite nanoparticles aged in the presence of Zn(II) over the course of 24 h. The significant increase in the amplitude of the second-neighbor peaks in the Fourier transforms indicates an increase in structural order consistent with structural incorporation and/or (co)precipitation of Zn.
428 average number of neighboring iron or zinc oxyhydroxide formation or when exposed to iron atoms. This suggests that Zn has either become oxyhydroxides that are allowed to undergo phase more structurally incorporated within the sub- transitions from ferrihydrite (HFO) to goethite strate or has formed a secondary precipitate or hematite (a-Fe2O3). Ion size, preferred coor- phase as a direct result of the nanoparticle dination chemistry, and steric effects may dis- growth conditions. It is worth noting that the tinguish between metals that are more likely to spectra of Zn(II) sorbed to goethite nanoparticles form (co)precipitates and those that are not. that have been allowed to aggregate for the same period of time, but in the absence of metals, does not at all resemble that of the aged sample shown Nanoparticle studies of other FeOX materials in Figure 13. Hence the dramatic changes observed are almost exclusively a byproduct of Nanoparticle hematite has been studied for many the aggregation-based growth process of nano- years because of its interesting magnetic properties particles in the presence of metals. This process is (e.g., Bodker & Morup, 2000; Gee et al., 2004). considerably more relevant to natural systems, Hematite is a weak ferromagnet between the Morin where iron oxyhydroxide formation and growth and Neel transition temperatures (260 and occurs not in isolation but in the presence of a 956 K, respectively, in bulk), and is subject to host of organic and inorganic species, including superparamagnetic relaxation effects as particle metals, that may influence particle growth rates size decreases. The mode of preparation, cell and mechanisms as well as the fate of the metal dimensions and stoichiometry all affect magnetic contaminants during the growth process. properties (Dang et al., 1998). The Morin transi- Whether the changes observed in the EXAFS tion temperature is found to decrease by more than data correspond to the structural incorporation of 40 K with decreasing particle size, and there are Zn within the goethite structure or some form of also continuous changes in magnetic anisotropy. Zn (co)precipitation, each represents a different Study of nanoparticle hematite structure by X-ray mode of uptake from the inner-sphere surface absorption near-edge structure (XANES) spec- adsorption observed in macroscopic studies (Triv- troscopy suggests that surface sites are distorted edi et al., 2001; Waychunas et al., 2002). While the with respect to bulk sites, and may even have lower differences in the permanence and reversibility of oxygen coordination (Chen et al., 2002), though metal sorption as a result of these different this is not observed on large single crystal surfaces sequestration mechanisms are not yet known, (Trainor et al., 2004). Quantum confinement has expectations of improved metal pollutant con- been demonstrated in hematite nanorods (Guo tainment through such processes may underscore et al., 2004) using resonant inelastic X-ray scat- the potential of nanoparticle aggregation as a via- tering (RIXS). Specifically, the band gap in 50 nm ble strategy for the removal of metals from solution rods was found to increase from the 2.1 eV in bulk and storage in the solid phase. hematite, to 2.5 eV. Perhaps of more importance to From our other metal/aging experiments, sim- environmental chemistry, are studies of aqueous ilar differences are observed among EXAFS photochemical effects. Lindgren et al. (2002) found spectra of Hg(II) sorbed to the goethite nano- that hematite nanorods were more efficient at water particles before and after 24 h of aging, while oxidation than bulk hematite, though the material As(V) and Cu(II) exhibit only subtle changes may have practical limits due to slow reaction (not shown). This element-specific behavior is kinetics at the water interface. Madden and consistent with a host of mostly macroscopic Hochella (2005) observed related phenomena in studies (Spadini et al., 1994; Ford et al., 1997, nanoparticulate hematite, with a doubling of oxi- 1999; Martinez & McBride, 1998, 2001; Kart- dation rate for 9 nm particles compared to bulk. As hikeyan et al., 1999a, 1999b; Trivedi et al., 2001; photochemical water oxidation results in Fe Ford, 2002; Waychunas et al., 2002) which indi- valence reduction at the particle surface, there will cate evidence for structural incorporation of be increased aqueous ferrous ion and probable certain metal cations (e.g., Cu(II) Mn(II), Ni(II), changes in sorption processes. Such work suggests Zn(II)) and minimal incorporation of others that similar processes may be important in other (e.g., Cd(II), Pb(II)) either during initial iron nanosize FeOX materials.
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