Behaviour of Fe-oxides relevant to contaminant uptake in the environment

Behaviour of Fe-oxides relevant to contaminant

uptake in the environment

S.L.S. Stipp a,*, M. Hansen a, R. Kristensen a,1, M.F. Hochella Jr. b, L. Bennedsen a,2,K. Dideriksen a, T. Balic-Zunic a, D. Leonard c,3, H.-J. Mathieu c

aNanoGeoScience, Geological Institute, Copenhagen University, Øster Voldgade 10, DK-1350 Copenhagen K, DenmarkbGeological Sciences, Virginia Polytechnical Institute, Blacksburg, VA, 24061 USA

cLMCH, Ecole Polytechnique Federale, Lausanne, Switzerland

Abstract

The behaviour of Fe-oxides was investigated during precipitation and co-precipitation, phase transformation and dissolution,

while their ability to adsorb and incorporate trace components was examined. Some samples were synthesised and studied

under controlled laboratory conditions and other samples were taken from experiments designed to test the effectiveness of

waste treatment strategies using iron. Surface-sensitive and high-resolution techniques were used to complement information

gathered from classical, macroscopic methods.

Adsorption isotherms for Ni2 + uptake on synthetic ferrihydrite (Fe5HO8�4H2O, often written simply Fe(OH)3), goethite

(a-FeOOH), hematite (a-Fe2O3) and magnetite (Fe3O4) were all similar, increasing as expected at higher pH. Desorption

behaviour was also similar, but one third or more of the Ni2 + failed to return to solution. In the past, ‘‘irreversible sorption’’ has

been blamed on uptake into micro-fractures or pores, but during examination (using Atomic force microscopy, AFM) of

hundreds of Fe-oxide particles, no evidence for such features could be found, leading to the conclusion that Ni2 + must become

incorporated onto or into the solids. When solutions of Fe(II) are oxidised in controlled laboratory conditions or during

treatment of ash from municipal waste incinerators, two-line ferrihydrite forms rapidly and on never-dried samples, AFM shows

abundant individual particles with diameter ranging from 0.5 to several tens of nanometers. Aging in solution at 70jC promotes

growth of the particles into hematite and goethite and their identification (by X-ray powder diffraction, XRPD, with Rietveld

refinement) becomes possible at the same aging stage as mineral morphology becomes recognisable by AFM. In other

experiments that were designed to mimic natural attack by organic acids, colloidal lepidocrocite (g-FeOOH) was observed in

situ by AFM, while reductive dissolution removed material on specific crystal faces. Lath ends are eroded fastest while basal

planes are more stable.

In order to help elucidate mechanisms of contaminant immobilisation by Fe-oxides, we examined samples from a reactive

barrier made with 90% quartz sand, 5% bentonite and 5% zero-valent iron filings that had reacted with a solution typical of

leachate from coal-burning fly ash using time-of-flight secondary ion mass spectroscopy (TOF-SIMS). Fe(0) oxidised to Fe(III),

while soluble and toxic Cr(VI) was reduced to insoluble Cr(III). Chemical maps show Fe-oxide coatings on bentonite; Cr is

0009-2541/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved.

PII: S0009 -2541 (02 )00123 -7

* Corresponding author. Tel.: +45-35-32-24-80; fax: +45-33-14-38-22.

E-mail address: [email protected] (S.L.S. Stipp).1 Present address: Niels Bohr Institute, Blegdamsvej, Copenhagen University, Denmark.2 Present address: Københavns Vand, Copenhagen K, Denmark.3 Present address: GE Plastics, Bergen op Zoom, Netherlands.

www.elsevier.com/locate/chemgeo

Chemical Geology 190 (2002) 321–337

associated with Fe-oxides to some extent but its association with Ca in a previously undescribed phase is much stronger. Other

samples taken from municipal waste incinerator ash that had been treated by aeration in Fe(II) solutions were examined with

transmission electron microscopy (TEM), selected area electron diffraction (SAED) and energy dispersive X-ray spectroscopy

(EDS). Pb and some Zn are seen to be dispersed throughout two-line ferrihydrite aggregates, whereas Sn and some Zn are

incorporated simply as a result of entrainment of individual ZnSn-oxide crystallites.

Geochemical speciation models that fail to account for contaminant uptake in solid solutions within major phases or as thin

coatings or entrained crystals of uncommon phases such as those described here risk to underestimate contaminant retardation

or immobilisation.

D 2002 Elsevier Science B.V. All rights reserved.

Keywords: Atomic Force Microscopy (AFM); Time-of-Flight Secondary Ion Mass Spectroscopy (TOF-SIMS); X-ray Photoelectron

Spectroscopy (XPS); Transmission Electron Microscopy (TEM); Energy dispersive X-ray Spectroscopy (EDS); Selected Area Electron

Diffraction (SAED); X-ray Powder Diffraction (XRPD); Rietveld refinement; Fe-oxide; Fe-hydroxide; Fe-oxyhydroxide; Ferrihydrite; Goethite;

Hematite; Magnetite; Lepidocrocite; Zero-valent iron; Precipitation; Transformation; Dissolution; Adsorption; Immobilisation; Incorporation

1. Introduction and background

Iron oxides are common in the environment, occur-

ring either naturally or as a result of human activities.

The most common Fe(III)-hydroxides, -oxides and

-oxyhydroxides include ferrihydrite (Fe5HO8�4H2O,

but often written as Fe(OH)3) which transforms to

hematite (a-Fe2O3) and/or goethite (a-FeOOH),

depending on solution composition, temperature and

pH. ‘‘Green rust’’ is the name given to the layered

Fe(II,III)-hydroxides which contain anions such as

CO32� , SO4

2� or Cl � in the interlayers. Depending

on the composition of the solid and solution, oxida-

tion can transform green rust to lepidocrocite (g-

FeOOH) or magnetite (Fe(II)Fe(III)2O4). Magnetite

can also be present as detrital grains or as relicts of

biological activity. Weathering can degrade magnetite

to maghemite (g-Fe2O3) and all of the Fe-oxides are

subject to attack and dissolution by organic acids and

ligands that are formed during breakdown of bio-

logical material. Especially near the water table,

seasonal fluctuation of the redox boundary promotes

alternation of oxidised and reduced forms (Cornell

and Schwertmann, 1996).

The red or brown colour that Fe-oxides impart to

soil often makes it look as if they dominate compo-

sition, but they are frequently present only in minor

concentrations, even when colour is intense. In rocks,

soil and sediments, Fe-oxides can be present as

discrete particles or as coatings on grains of quartz,

clay or other minerals. Fractures through rocks and

unconsolidated sediments are often observed to be

lined with red or brown Fe-oxides, such as seen in

Fig. 1. In such cases, where coatings cover particles or

fracture walls, Fe-oxides often make up b1 wt.% of

bulk composition but they shield the underlying

material from contact with the groundwater. Thus,

interaction with the major components, quartz, feld-

spar, clay, sulfides or other minerals is limited, mean-

ing that Fe-oxide mineralogy plays the dominant role

in controlling groundwater composition in these sit-

uations.

Macroscopic investigations have led to a broad

understanding of the uptake behaviour of Fe-oxides

(many examples cited in Stumm and Morgan, 1981

and Cornell and Schwertmann, 1996). More recently,

spectroscopic studies have been providing new infor-

mation about bonding environments for contami-

nants incorporated within the bulk as well as

adsorbed at the mineral/solution interface (for exam-

ple Ford et al., 1999; Manceau et al., 2000; Alcacio

et al., 2001; Randall et al., 2001). It is well known

that Fe-oxides, with their high surface area and

strong affinity, sequester cations such as the transi-

tion metals and radionuclides, in proportions that are

strongly a function of solution composition and pH.

Under the right conditions, Fe-oxides are also strong

adsorbers of anionic complexes such as AsO43 �

(Randall et al., 2001), CrO42� (Ding et al., 2000)

and PO43� (Russel et al. 1974; Persson et al., 1996;

Dideriksen and Stipp, submitted for publication) and

organic molecules such as pesticides (Piccolo et al.,

S.L.S. Stipp et al. / Chemical Geology 190 (2002) 321–337322

1994; Clausen and Fabricius, 2001; Dideriksen and

Stipp, submitted for publication) and humic or fulvic

acids (Zhou et al., 2001). Thus, the Fe-compounds

are useful for retarding transport or immobilising

contaminants in groundwater systems. For compo-

nents that are susceptible to biological breakdown,

Fe-oxides can serve as a substrate to make them

more available to bacteria. Because of these proper-

ties, as well as low cost, easy availability and lack

of health risk, Fe-compounds are frequently chosen

for use in treatment for drinking water and for waste

(for examples: O’Melia, 1972; Gould, 1982; Astrup

et al., 2000; Lundtorp et al., 2002).

Treatment of waste before disposal can help to

decrease the leaching of contaminants from the waste

facility. One method for such treatment makes use of

the precipitation of Fe-oxides to trap heavy metals.

In the Ferrox process (Lundtorp et al., 2002), ash

residue from municipal incineration is mixed with

FeSO4 solutions and aerated. Many of the contami-

nants, which were present in the original ash as salts,

are dissolved in the solution and then incorporated

into the solid Fe(III)-oxide precipitate as the solution

is bubbled with air. Their immobilisation in the solid

product prevents leaching to the groundwater

beneath the waste disposal site. In the design of

waste disposal sites, reactive barriers are often used

to insure that contaminants do not escape. One

simple, effective and inexpensive type of barrier is

made with zero-valent iron (Cantrell et al., 1995;

Powell et al., 1995). Redox-sensitive contaminants

are reduced while they oxidise the Fe(0). Some

organic molecules can be broken down in this way,

giving degradation products that are not dangerous

and some transition metals which are soluble and

toxic in oxidised form, such as Cr(VI), can be

reduced to a nonsoluble form, Cr(III), which precip-

itates. The Fe-(hydr)oxides produced in the barrier

material can also offer reactive surface area for

adsorption of other contaminants.

The potential of Fe-oxides to take up contami-

nants by adsorption or by incorporation within the

bulk, and likewise to release them again to solution,

depends on their behaviour during three major

stages: their initial formation by precipitation or

coprecipitation, their transformation to a more stable

phase, and their susceptibility to dissolve again or to

exsolve incorporated trace components. Improved

Fig. 1. Orange and red Fe-oxides line fractures in clayey till. (a)

Gedser Odde, Denmark; grass at the top, shovel at the bottom for

scale (photo by Knud Erik Klint, GEUS, Denmark). (b) Flakkeb-

jerg, Denmark; Fe-oxide-enriched zones where the water flows

(appear dark), running parallel to the long dimension of the sample

box; the box is f 15 cm long.

S.L.S. Stipp et al. / Chemical Geology 190 (2002) 321–337 323

understanding of the processes involved in each of

these stages will lead to diminished uncertainty in

models that assess the risk of contaminants in the

environment, and will promote development of strat-

egies for waste treatment or containment that are

more effective and less expensive.

There are still many unanswered questions about

processes in the Fe-oxide system. One is the problem

of defining the mechanism(s) responsible for ‘‘irre-

versible adsorption’’. Adsorption/desorption iso-

therms, which show a solid’s capacity for uptake of

a species and its release back to solution again, are a

well-used tool in macroscopic geochemical studies.

An example is presented in Fig. 2. Ni2 + has been

adsorbed to a set of synthetic Fe-oxides and then

desorbed again as pH was adjusted. Uptake and

release behaviour follows roughly the same pattern

for the four materials, with little effect from crystal

structure or Fe2 + /Fe3 + ratio in the solid. However,

only 40–70% of the initially ‘‘adsorbed’’ material is

given back to solution again when pH is decreased.

When an adsorbed component is not all released

again following reversal of the chemical conditions,

it is interpreted as being ‘‘irreversibly adsorbed’’.

However, it is often difficult to determine what this

means with respect to molecular behaviour. Is the

material no longer available for desorption at the

surface because it has moved into the bulk by some

means, such as into pore spaces or by solid-state

diffusion into the atomic structure? Or has it copre-

cipitated with the main phase during the continuous

dissolution/precipitation process we know as dynamic

equilibrium? Or has it precipitated as a separate phase

on the substrate in a layer that is only a couple of

atomic layers thick, and therefore ‘‘invisible’’ to X-

ray analysis? Or could it be that the contaminant is a

colloidal particle that is entrained in its entirety

during growth or recrystallisation of the substrate?

Is it possible to observe such coatings or separate

phases or pore spaces on or in colloidal-size Fe-oxide

particles?

The morphology of a solid controls its dissolution

and growth and the atomic structure on various crystal

faces determines preference for adsorption of contam-

inants. Particle size can also be a very important

parameter in determining the extent of contaminant

Fig. 2. Adsorption isotherms for Ni2 + on four synthetic Fe(III)-oxides (solid line) and desorption isotherms for the same samples (dashed line).

Solutions contained 25 Am Ni2 + in 4 mM KClO4; pH was adjusted with KOH and HClO4 and the time interval for equilibration at each pH step

was 2 h. Other details are described in Methods.


transport because if particles are small enough, they

can move with the groundwater, carrying adsorbed

contaminants. On scanning electron and transmission

electron microscopy (SEM and TEM) images, we

frequently see Fe-oxides as aggregates of smaller

grains. Are aggregates also the dominant form in

solution? Or is aggregation a result of surface tension

effects during drying of the sample, which is neces-

sary for analysis by SEM and TEM? Can a size range

be proposed for ferrihydrite under ambient conditions,

outside a vacuum chamber? Transformation of a

certain phase to one that is more stable alters mor-

phology because it requires a change in crystal struc-

ture. As ferrihydrite transforms to goethite and

hematite, are the initially precipitated particles dis-

solved and reprecipitated again? Or could the initial,

water-rich colloids that are amorphous to X-rays serve

as nuclei for growth of the dehydrated phases? What

is the relationship between dissolution and crystal

growth? Are dissolution rates specific to crystallo-

graphic directions? Is the rounding of corners and

roughening of edges, which is observed macroscopi-

cally during dissolution, also observed at nanometer

scale?

From macroscopic data alone, it is often difficult to

propose a unique conceptual model for what happens

at the molecular scale. Models of molecular behaviour

are particularly important for predicting the mobility

of components that are present (and also dangerous) in

trace concentrations. If we could define the mecha-

nism that holds a contaminant to a solid, we would

have a better hope of modelling it as ‘‘immobilised’’

with confidence.

The purpose of this article is to answer some of

the questions posed above, by supplementing what

is already known about the behaviour of Fe-oxides

and contaminant uptake with new information gath-

ered from near-surface-sensitive and high-resolution

techniques. We used synthetic samples in experi-

ments that were designed to investigate specific

processes under controlled conditions and we exam-

ined samples produced during experiments that had

been designed to refine and test the effectiveness of

waste treatment procedures. We studied the behav-

iour of Fe-oxides during precipitation, transforma-

tion and dissolution to clarify the processes

responsible for adsorption and incorporation of con-

taminants.

2. Experimental details

2.1. Methods

Samples and solutions were characterised using

classical methods such as X-ray powder diffraction

(XRPD) (such as Fig. 3), optical and polarised optical

microscopy, atomic absorption spectrometry (AAS),

and potentiometry (pH). In addition, several near-

surface-sensitive and high-resolution techniques were

applied. Atomic force microscopy (AFM), which uses

a sharp tip to feel the atomic-scale forces on a sample

while the sample is rastered beneath it, can make

images of the physical properties of a surface, with

resolution in x and y of about 2 A (2� 10� 10 m) and

in z, of a fraction of 1 A. Images highlight either

topography directly (height mode) or slope (deflection

mode) (such as Fig. 4). For height mode, features are

given a false colour that is proportional to their height

Fig. 3. X-ray powder diffraction (XRPD) patterns for (a) synthetic

two-line ferrihydrite, within a day after Fe(II) is oxidized. There are

very weak hematite reflections which represent the very beginnings

of transformation. (b) The same material after 10 weeks of aging in

water at 70jC; maxima represent goethite and hematite and the

broad background typical of ferrihydrite has disappeared.


above some baseline so that hills appear light and

valleys, dark. For deflection mode, apparent light and

shadow on feature edges enhance the roughness of the

topography. AFM is excellent for observing colloidal

particles, in situ, under air or solution. Time-of-flight

secondary ion mass spectroscopy (TOF-SIMS) col-

lects chemical information from the top two or three

atomic layers of a sample. Results in the form of

spectra or chemical maps are possible, where x and y

represent spatial location and gray scale brightness

represents higher relative surface concentration. Spa-

tial resolution is about 1 Am (10 � 6 m) and mass

resolution is better than 1 per mil of a mass unit,

allowing the discrimination of inorganic and organic

components with very similar mass numbers. X-ray

photoelectron spectroscopy (XPS) detects photoelec-

trons emitted from the top few nanometers of a sample

surface, allowing determination of chemical identity

and bonding relationships. Data are provided in spec-

tra where peaks can be fit according to information

from analysed standards or from ab initio calculations.

Transmission electron microscopy (TEM) uses a

finely focused, high-energy electron beam to interact

with an ultrathin sample, to produce images with sub-

nanometer resolution. It also provides structural infor-

mation from selected area electron diffraction (SAED)

and chemical analysis with energy dispersive X-ray

spectroscopy (EDS).

For XRPD, a Philips PW 3710 diffractometer with

Bragg–Brentano geometry was used. CuKa radiation

was applied together with a secondary-beam graphite

monochromator and a pulse-height discriminator for

filtering the Fe-fluorescence radiation. The AFM

images presented here were made in contact mode

with standard or sharpened Si3N4 tips, using piezo-

electric scanners that have total x,y range of 100, 12 or

1 Am on a Digital Instruments MultiMode Scanning

Probe Microscope. The experiments were made under

ambient conditions, either in air, under a film of water

adsorbed from humid air, or directly under solution.

Images were not filtered. TOF-SIMS chemical maps

were made on a PHI-Evans (Trift I) system. We used a69Ga + primary beam in pulsed, rastered mode. The

source was operated at a potential of + 25 kV vs.

ground with a sample bias of F 3 kV. Surface charg-

ing was inhibited by use of a low-energy (f 20 eV)

electron flood gun and mounting the samples under a

metal grid. Samples were not sputter-cleaned. XPS

spectra were collected from a Perkin-Elmer 5500

instrument using MgKa radiation with no compensa-

tion for charging and no sputter cleaning. Charge

referencing was made by assuming adventitious car-

Fig. 4. Atomic force microscopy (AFM) deflection mode images of (a) the original ferrihydrite taken soon after the precipitate formed.

Convolution of tip shape over the shape of the particles makes them appear much wider than they actually are. Fig. 5 explains how. (b) An

aggregate of hematite grains with ferrihydrite in the background and (c) several goethite crystals. Images (b) and (c) were taken after aging for

several weeks.


bon C1s binding energy at 285.0 eV. Peaks represent-

ing Fe-compounds were fit based on the theoretical

calculations of Gupta and Sen (1974, 1975) and the

experiments of McIntyre and Zetaruk (1977). A JEM-

3010 (JEOL) transmission electron microscope was

used in this study at a beam energy of 300 kV and

beam currents between 155 and 119 AA generated

from a LaB6 filament. The EDS on this TEM instru-

ment was an Oxford Link ISIS system (model 6636)

equipped with an atmosphere-thin window for light

element detection.

2.2. Materials

All solutions for sample synthesis and treatment

were made of reagent grade chemical products, in

clean glass or plastic ware, with deionised water

(V 0.05 AS/cm).

2.2.1. Samples produced and used in pure system

experiments

Fe-oxides were synthesised following the recipes

presented by Schwertmann and Cornell (1991) with

only one major difference: Solids were rinsed of salt

solutions by repeated centrifugation because spurious

results from material rinsed in dialysis bags suggests

they contribute to organic contamination on the Fe-

oxide surfaces (Clausen and Fabricius, 2001). Treat-

ing the dialysis bags by boiling in deionised water

before use decreases but does not prevent hydro-

carbon contamination (work in progress), which is

no great surprise because dialysis membranes are

composed of organic C. The synthetic solids were

stored in deionised water, in plastic bottles, in a

refrigerator. A small subsample of each was dried

and its structure and purity were verified with XRPD.

Samples produced were: ferrihydrite (F), goethite (G),

hematite (H), magnetite (M) and lepidocrocite (L).

The experiments to observe the adsorption/desorp-

tion of Ni2 + on Fe-oxides that were mentioned as an

example in the introduction were carried out on fresh

samples of the synthetic material that had never been

dried. Aliquots of Fe-oxide/water slurry were used

directly in the adsorption experiments. The quantity of

solid used was calibrated from the salt-free dry-mass

remaining after evaporation of a number of identical

volume aliquots. BET determinations were made on

the dried subsamples; pre-experiment surface areas

were: F = 190 m2/g; G = 40 m2/g; H = 45 m2/g; M= 15

m2/g.

For the adsorption experiments, pure water

(250 ml) was put into a reaction vessel where temper-

ature could be held constant at 20F 1.0jC. pH was

adjusted to 3 and Ni2 + was added to produce a

solution of 25 Amol/l. Fe-oxide (0.5 g) was added

as a slurry. Ionic strength was maintained at 0.004 M

with KClO4 and the samples were stirred very slowly.

pH was adjusted and held constant at one pH-step

increments in the range from 3 to 8 using 0.01 M

KOH and HClO4. Barrow et al. (1989) found 2 h to

be enough to bring the ‘‘fast’’ uptake process to

completion while limiting the extent of the ‘‘slow’’

uptake process, so we used 2h intervals between pH

steps. When pH 8 was reached, the solution was left

overnight (f 10 h) and the desorption experiments

were carried out the next day. The procedure and time

steps were the same. Solution samples were removed

and filtered at the end of each pH step and analysed

for Ni2 + with AAS. After the experiment, XRPD

verified no change in mineral composition; BET

results were within experimental uncertainty of the

original values.

For the investigations of Fe-oxide morphology,

ferrihydrite was stored in plastic bottles and exam-

ined directly after synthesis. The solids that were

produced during transformation in clean water at

pH = 7, at 70F 1.0jC, were examined as a function

of time. The relative proportions of F, G and H were

determined by XRPD with Rietveld refinement

(Young, 1993) and morphology was observed with

AFM.

In order to mimic the reductive dissolution of Fe-

oxides that often occurs at the redox boundary in soil,

we chose to use ascorbic acid as a model for the

natural organic acids that result from decay of bio-

logical material. We chose to observe dissolution of

lepidocrocite as a representative of the Fe-oxides

because it has orthorhombic symmetry and thus

presents a crystal structure distinctly different in the

three crystallographic directions. Other studies

(Larsen and Postma, 2001) show that lepidocrocite’s

rate of dissolution is independent of acid concentra-

tion when ascorbic acid is present at 10 mM or higher,

so we examined colloidal size particles while exposed

to such a solution in an AFM fluid cell, as a function

of time.


2.2.2. Samples produced during experiments to test

waste treatment methods

Some samples for this project were taken from

column experiments (Astrup et al., 2000) designed to

test a reactive barrier’s effectiveness for removing

Cr(VI) from leachate typical of rainwater passing

through ash from a coal-fired power generating plant.

The barrier material was made of 90% quartz sand,

5% bentonite and 5% Fe(0) filings. The zero-valent

iron had an average particle size of about 150 Am and

BET surface area of about 0.1 m2/g. Solutions were

made to represent a typical coal ash leachate (Hjelmar,

1990) with Cr(VI) composition of 25 ppm and 3000

ppm SO42� , 130 ppm Cl � , 730 ppm Na + , 900 ppm

K + and 250 ppm Ca2 + . As the leachate passed the

columns, the redox front could be observed first as a

dark green boundary, probably because of ‘‘green

rust’’, as Fe(0) oxidised to Fe(II), and then a red

one, as Fe(II) converted to Fe(III), reducing Cr(VI) to

Cr(III). A sample to represent the reacted barrier (RB)

was removed from the column and stored dry, under

N2 gas, in a glass bottle until analysis. Some sub-

samples were examined using optical microscopy.

Selected grains were removed with a needle and

analysed using XPS and TOF-SIMS.

Other samples were taken from the Ferrox ash

treatment process (FA) soon after formation. They

were stored dry in a glass bottle until analysis.

Subsamples were examined with optical microscopy,

XRPD and TEM. TEM samples were prepared in the

same way as described by Hochella et al. (1999, end

of p. 3396).

3. Results and discussion

3.1. Precipitation and coprecipitation

Fe is more soluble in the divalent than the metallic

or trivalent form. In a system open to air, oxidation

forms Fe(III) which leads to precipitation of ferrihy-

drite as the first solid. In both the pure system (sample

F) and during the Ferrox treatment of municipal waste

ash (sample FA), a red-brown, very fine-grained

precipitate formed. Both samples gave an XRPD

pattern typical of two-line ferrihydrite: two very broad

diffraction maxima. Fig. 3a shows the pattern from a

sub-sample of F. Some very weak peaks representing

hematite are visible within a day of precipitation. An

AFM image of a separate subsample of the synthetic

material, taken directly from solution soon after pre-

cipitation and mounted on the sample holder without

drying, is shown in Fig. 4a. The samples were imaged

under the thin film of water that adsorbs on solids

from humid air, in order to avoid artifacts from drying.

It is typical of hundreds of other images of similar

samples. Sometimes the tiny colloids are observed in

clusters, but most exist as separate, individual entities.

The height of the individual particles ranges from

about half to several nanometers. On materials that are

not elastic, AFM provides reliable height measure-

ments. We assume ferrihydrite particles are spherical,

as suggested by Cornell and Schwertmann (1996), but

on the images, widths are distorted by an artefact that

is always present in AFM imaging when the feature

being imaged is near the same size as the radius of

curvature of the tip. Fig. 5 explains how lateral

dimensions are increased by convolution of the tip’s

shape over the particle. A zoom-in view of one

ferrihydrite colloid (Fig. 5a) gives a cross-section

(Fig. 5b) where the expected spherical particle ends

up appearing as an oval (thin dotted line) because of

vertical exaggeration in the z direction. The flanks of

the cross-section are an artifact of the edges of the

AFM tip. Fig. 5c demonstrates how the signal regis-

tered during deflection of the tip (dark line) is con-

voluted by the shape of the tip as it feels the particle.

Thus, the particle appears much wider than it is. The

tip radius of curvature, Rt, and the particle radius, Rp,

are related through a function of the apparent width of

the particle, w*, measured along any plane parallel to

the substrate where the distance from the plane to the

top of the particle is h*, as shown. h* and w* are

measured directly from images and because they are

interrelated, one can choose any plane parallel to the

substrate to measure them from. We assume ferrihy-

drite to be spherical and inelastic, so height measure-

ments from many AFM images allow us to say that

for a fresh precipitate that has never been dried,

individual particles range in diameter from about 0.5

to several nanometers.

The reactive barrier material before reaction with

the artificial coal fly ash leachate was a light, sandy-

brown colour. After reaction, it was dark red–rusty

brown. Examination of a subsample of the reacted

material (RB) with an optical microscope showed


some dark particles (the iron filings) associated with

reddish powder that coated and cemented together

some smaller grains or that lay in hollows on some of

the larger, clear, quartz sand grains. Other quartz

grains were completely clear and colourless, with

appearance in the optical microscope that was

unchanged from before reaction. In spite of the

dramatic colour change of the bulk sample, XRPD

failed to show any difference in the mineral compo-

sition of the barrier and specifically, no new peaks of

crystalline Fe-oxides were detected. Neither could the

broad diffraction maxima representative of ferrihy-

drite be observed. However, analysis with XPS on

individually mounted grains provided clear evidence

of Fe-oxides on all grains examined, even those that to

the eye appeared clear and colourless. As expected,

there was a higher relative proportion of Fe on spectra

collected from grains that were dark in colour than

from those that were light. Fitting of the spectra gives

information about atomic bonding relationships in the

near-surface, allowing the possibility to determine,

semiquantitatively, the relative proportion of FeUOH

OH and FeUO bonds. Based on previous work

(Gupta and Sen, 1974, 1975; McIntyre and Zetaruk,

1977) and comparison with spectra from the barrier

material before exposure to the leachate, the data

suggest the development of some goethite, but most

of the Fe and O sub-peaks fit more closely to binding

energy regions expected for ferrihydrite. Although

time in the ultrahigh vacuum chamber and exposure

to the X-ray beam were both minimised, it cannot be

excluded that dehydration induced by analysis was

responsible for the formation of goethite. Further

studies of Fe-oxides with XPS should be made before

the possibility of analysis-induced transformation can

be discarded unequivocally. From the mineralogical

perspective, such a ‘‘dry’’ transformation, unaided by

recrystallisation in bulk solution, would be interesting.

XPS provided chemical evidence for Fe-oxide,

probably ferrihydrite, as coatings on the mineral

grains of the reactive barrier, even when such a

coating was invisible under an optical microscope,

such as on the grains of clear, colourless quartz. XPS

spectra average over a definable area of some fraction

of 1 mm2. Chemical or structural variation within the

analysed area is not observable with the spectrometer

configuration used and chemical information comes

from the near-surface, meaning that information depth

Fig. 5. (a) AFM top view and (b) cross-section through a

ferrihydrite particle. The particle is assumed to be roughly spherical

but vertical exaggeration makes it look like a disk (sketched) and

convolution by the tip’s apex, which is very much larger than the

particle, greatly increases the apparent particle dimensions. (c)

Schematic diagram to show the relationship between apparent

particle shape and the tip radius (after Garcia et al., 1997). The


is something less than about 10 nm. TOF-SIMS,

however, provides chemical maps of the material that

is ejected from the top few atomic layers of a sample

with lateral resolution of about a micrometer. Fig. 6 is

typical of a set of maps from one of the dark grains

from the RB sample. The patterns for Si and Al are

very similar; they represent clay particles covering the

quartz sand grains. K follows the Si and Al patterns,

suggesting that it is adsorbed on clay. Fe has a similar

but not identical pattern to those of K, Si and Al. Over

the middle of the map, the pattern reflects the other

elements, showing that Fe is also adsorbed on the

clay. Along the top of the image and at the bottom

right corner, intensity is high for Fe and low for the

other elements, indicating thicker Fe-oxide. Cr is

found associated with Fe in some locations, as

expected, where it probably is present as a mixed

FeCr-hydroxide. The striking feature of the Cr pattern,

however, is its similarity with that of Ca, indicating

the formation of Cr(III)Ca-oxide either as a coating or

as particles of a separate phase. A Cr(III)–Ca phase

has not been recognised in sediments contaminated by

Cr, but CaCr(III)2O4 has been documented as a

synthetic compound (JC-PDS file).

Leaching experiments with the reacted barrier

column material returned essentially none of the Cr

that had been taken up by the solid phase (Astrup et

al., 2000). In general, precipitation of thin coatings or

coprecipitation of a trace component within a major

phase are two ways that material taken up from

solution may be immobilised so that it is not released

again when chemical conditions are reversed.

In the Ferrox treated ash (FA), ferrihydrite precipi-

tated in the presence of transition metals such as Pb,

Zn, Sn, Cd, Hg, etc., that had dissolved from the salts

in the municipal waste residue. TEM of ultramicro-

Fig. 6. Time-of-flight secondary ion mass spectroscopy (TOF-SIMS) chemical maps of a sample of reacted barrier material (RB). Highest

relative concentration is represented as white. Image width is 80 Am in all cases and information depth is a few atomic layers.


tomed thin slices of the treated product showed areas

of 100 to several hundred nanometers in diameter. The

rounded area that takes up two thirds of the image in

Fig. 7 is one example. Texture in some of the area was

uniform and grainy (such as where the top line points)

and in other parts of the area, it was nonuniform with

elongated, ribbon-like, darker areas that were some-

times curved. Both in the uniform and the ribbon-like

locations, SAED confirmed two-line ferrihydrite and

EDS showed Fe, Pb, Zn and Ni to be present. The

signal for Ni arises from the TEM mounting grid so

we cannot say if there is also Ni in the sample. Based

on the observations of fresh, synthetic material with

AFM, and because of the homogeneity of composition

and structure, we interpret that these areas of more

than 100 nm diameter are aggregates of primary

ferrihydrite particles that either came together during

the Ferrox treatment process or during drying and

preparation of the TEM sample. Uptake of the tran-

sition metals, then, could either be by adsorption on

the surfaces of the primary particles so contaminants

are included at the particle borders in the aggregates,

or they could be dispersed randomly within the

primary particles, or both. A third form of uptake,

entrainment, is shown by the dark rectangular grains

observed frequently within ferrihydrite patches. These

consistently had concentrations of Zn and Sn in

proportions suggestive of ZnSnO3�4H2O, a high-tem-

perature oxide. We interpret that these grains were

present in the original waste residue. They were not

soluble in the Ferrox solution so they were captured in

the ferrihydrite aggregates.

In all of these examples of ferrihydrite formed

during precipitation, trace components that happen to

be present could be taken up by one or more of several

mechanisms: (i) adsorption at surfaces, leaving them

Fig. 7. A transmission electron microscopy (TEM) image of Ferrox-treated municipal waste incinerator residue (FA). The scale bar at bottom is

100 nm. The energy dispersive X-ray spectroscopy (EDS) spectrum at the top shows chemical composition within the ferrihydrite aggregate and

the spectrum at the bottom represents the dark, rectangular crystals.


available for desorption again; (ii) precipitation as a

separate phase, producing a thin coating or separate

particles; (iii) coprecipitation, allowing them to sub-

stitute within the atomic structure of a major or minor

phase; (iv) entrainment, where a preexisting particle is

incorporated into a growing one, simply by capture.

Geochemical models that omit solubility relationships

for solids which are present in very minor quantities, or

that fail to account for elements present in solid

solution, risk to underestimate the potential for con-

taminant immobilisation.

3.2. Transformation

With time and favourable conditions, Fe-oxides

transform from one phase to another. Reaction rate

and products formed depend on solution conditions,

activity of water and temperature. In synthetic systems

where other components are mostly absent, ferrihy-

drite transforms readily to hematite with some goe-

thite at neutral pH and principally to goethite at

pH >12 (Cornell and Schwertmann, 1996). We exam-

ined fresh ferrihydrite stored in a plastic bottle during

aging at 70 jC, pH= 7.0 as a function of time for

10 weeks. Within a day of formation, some of the

ferrihydrite had already begun to transform to hema-

tite (Fig. 3a) and by the end of the experiment,

hematite and goethite dominated (Fig. 3b). Fig. 4

shows typical images of the three compounds taken

with AFM during the early (Fig. 4a) and middle (Fig.

4b and c) stages of the experiment. Ferrihydrite occurs

as individual particles and clusters. Hematite appears

as single crystals or as aggregates and goethite forms

elongated crystals, occasionally with characteristic

twinning at 117.5j as seen on the left end of the

crystal in the upper right corner of Fig. 4c.

During the 10 weeks, as ferrihydrite transformed,

subsamples were removed, dried and analysed using

XRPD. Rietveld refinement was used to determine the

relative proportions of ferrihydrite, hematite and goe-

thite. In a pure system, after only a few hours at 70

jC, very small XRPD peaks representing goethite and

hematite were already visible (Fig. 3a). After 150 h,

about half of the ferrihydrite had transformed and by

250 h, evidence for ferrihydrite was no longer detect-

able in the powder patterns. From that time until the

end of the experiment, crystal dimensions simply

increased. During the 10 weeks, another set of sub-

Fig. 8. Data for length versus width of crystals imaged with AFM. Measurements from samples from day 1, day 8, week 6 and week 10 have

been combined. Particles starting as ferrihydrite simply grow larger as a function of time and become either hematite or goethite.


samples was examined periodically using AFM with-

out drying. Through the XRPD data and their mor-

phology, particles were identified on the images and

their length, width and height were measured from

cross-sections. Length as a function of width is plotted

in Fig. 8 for all time steps. Initial ferrihydrite particles

grew in diameter until they became recognisable by

XRPD as hematite. The particles continued to grow,

with no noticeable break in dimension, indicating that

ferrihydrite (amorphous to X-rays) becomes hematite

(recognisable by XRPD) simply when particle size is

large enough. Note that the scale on the two axes of

Fig. 8 is different so that the 1:1 line roughly follows

the limit of the hematite data. Hematite is expected to

be equant, but the grains in the images often have an

oblate appearance (Fig. 4b). Goethite crystals are

elongated and as expected, their length-to-width ratios

follow a different trend than hematite. However, the

smallest of the goethite crystals appears within the

range of the largest of the ferrihydrite particles also

suggesting that the first goethite to form does so from

a nucleus of the amorphous material that we label

ferrihydrite. Further studies in pure and contaminant

systems are in progress.

Lateral dimensions plotted in Fig. 8 are those

measured directly from cross-sections; they do not

account for interference by tip shape as explained in

Fig. 5. The tip makes all the particles appear as if they

are at least 25 nm wider than they are. One could

simply correct for the tip effect and shift all of the data

so that the smallest of the ferrihydrite particles is

represented at 0.5 nm length and width, which is

determined from the height of the smallest particles.

However, such a shift would introduce a small addi-

tional uncertainty because tip radius of curvature

cannot be assumed to be constant for all images.

Fresh tips are not all the same and during their

lifetime, they wear, break, or collect debris so that

in fact an adsorbed particle or macromolecule can act

as the imaging point.

During these studies of Fe-oxide colloidal par-

ticles, we imaged hundreds of samples and

observed thousands of particles. On individual

grains, we saw no evidence for fractures, pore

spaces or any other form of surface roughness that

could explain ‘‘irreversible adsorption’’. We did see

aggregates of particles (such as Fig. 4b) where

grain boundaries could account for transport at slow

rates, but the relatively large fraction of material

occurring as individual particles suggests that Fe-

oxide colloids in natural conditions may be less

aggregated than electron microscopy studies have

led us to believe up to now. The AFM studies

show that micro- or nano-fractures and pore spaces

do not exist.

3.3. Dissolution

In nature, Fe-oxides can be dissolved, especially in

the presence of organic acids and ligands that result

from the breakdown of biological material. Dissolu-

tion rates determined from macroscopic experiments

sum the effects of attack on all faces, whereas surface

complexation models often resort to more than one

type of site to develop empirical fits to data. With high

resolution AFM images taken as a function of time

during dissolution, one can determine rates specific to

a certain face. Fig. 9 shows four snapshots during a

dissolution experiment. Ascorbic acid, which serves

as a model compound to represent natural redox-

active acids, is oxidised while Fe3 + from lepidocro-

cite is reduced. The ligand aids removal of Fe2 + to

solution and crystal volume decreases. In Fig. 9a–d,

crystal length decreases visibly from 450 nm at the

beginning of the experiment to 270 nm, 2h later.

Width and height also decrease but at slower rates.

Particle length, width and height can be determined on

cross-sections of the images. By calculating the

change in volume at each time step, one can determine

Fe removed and thus calculate the overall rate of

dissolution for the crystal. For this series, the rate

corresponds within an order of magnitude to the

macroscopic rate determined for the same lepidocro-

cite and ascorbic acid by Larsen and Postma (2001).

Considering the volume of material that is present as

smaller (and thus more reactive) particles within the

sample, this closeness of results provides confidence

that rates of dissolution determined from individual

crystal directions is meaningful. Measurements from

cross-sections along the length and across the width of

the grain provide data representing the area of the

crystal remaining at each time step (Fig. 9e). The

sketch to the right of the plots shows the cross-

sections measured for each set of data and the direc-

tion of attack that was investigated. Dissolution rate at

the long ends of the crystal is clearly faster than that at


Fig. 9. A series of atomic force microscopy (AFM) images of lepidocrocite dissolving reductively in 10 mM ascorbic acid. (a) Particle length is

450 nm at 20 min after the experiment began, (b) length is 370 nm, 35 min, (c) 310 nm, 97 min, (d) 270 nm, 120 min. Dissolution rounds

corners and enhances features running parallel to the a-axis (long direction). (e) Area of the colloidal lepidocrocite remaining as a function of

time. Dissolution is favoured at the ends of the crystals, on the (100) faces and at the edges, on the (001) faces, in preference to the larger, tabular

(010) faces.


the sides, demonstrating that the directions of pre-

ferred growth for this mineral are also the directions

for preferred dissolution.

Crystal morphology developed during dissolution

is not necessarily the reverse of the morphology that

develops when a crystal is growing. Macroscopic

studies show that when growth rate is slow to

moderate, crystals often grow by ordered deposition

parallel to crystallographic planes, but during disso-

lution, corners round and edges become rough.

Similarly, freshly precipitated colloidal-size particles

of lepidocrocite have smooth and well-defined edges

and corners, whereas during dissolution corners

become rounded, indicating that even at nanometer

scale, dissolution is not the reverse of precipitation.

Just as dissolution may not be the reverse of crystal

growth, desorption may not be the reverse of adsorp-

tion, which has consequences for the behaviour of

contaminants. Material may adsorb preferentially on

a specific face or faces of lepidocrocite, or any other

Fe-oxide, and faces that adsorb preferentially are

likely to be more reluctant to give back their material

to solution during desorption. As dissolution pro-

ceeds, adsorbed material on the attacked face has the

possibility of being released again to solution, but it

might also inhibit dissolution on that face in prefer-

ence to others. Further, it is possible that adsorbed

contaminants remain on a surface even while dis-

solution is taking place. Although it may seem

counter-intuitive that a compound could be adsorbing

while a surface is under attack by dissolution, the

apparent conflict is resolved by remembering that

adsorption and dissolution are two separate pro-

cesses, each driven to reach their own equilibrium.

Equilibrium is simply the balance of competing

forward and back processes in the dynamic system.

Adsorption is the net balance (that we measure

macroscopically) of material coming on and off the

surface (which we cannot measure) at the molecular

level and dissolution is the net process that we

observe while material is both precipitated and

removed from the surface. Experimental (XPS and

AFM) evidence for adsorption of Ni2 + during dis-

solution of calcite is presented in Hoffmann and Stipp

(2001). Therefore, even for dissolution of lepidocro-

cite that has adsorbed a contaminant such as Ni2 + or

an organic compound such as a pesticide, the major-

ity of the contaminant might remain associated with

the surface of the Fe-oxide during the Fe-oxide’s

dissolution until suddenly, when surface area has

decreased to the point where all reactive sites are

full, the contaminant load begins to be released again.

Results of the dissolution experiments in the ideal

system presented here are serving as a base for

further studies in a system where contaminants are

also present.

4. Conclusions and implications

Ferrihydrite precipitates as discrete particles that

range in diameter from 0.5 to several nanometers.

Aging promotes growth, which appears to be simply

enlargement of particle dimensions. When they reach

tens of nanometers, particles are already recognisable

as goethite or hematite by XRPD. The continuous

range of particle sizes from ferrihydrite through to the

less-hydrated phases suggests that both hematite and

goethite grow from nuclei of primary ferrihydrite by

dissolution and reprecipitation in the process known

as Ostwald ripening.

Isotherms for adsorption of Ni2 + on ferrihydrite,

goethite, hematite and magnetite showed no signifi-

cant difference in behaviour. Desorption from the four

compounds also followed the same pattern but only

40–70% of the material was returned to solution. No

micro-pores or -fractures that could be responsible for

uptake were ever observed on any of the thousands of

Fe-oxide particles examined. Clusters formed, pro-

ducing grain boundaries which could allow absorp-

tion but the relative abundance of individual particles

observed in fresh material as well as in that aged for

up to 10 weeks suggests that slow transport along

grain boundaries could not be a plausible explanation

for the failure of Ni2 + to return to solution. Adsorp-

tion and incorporation as part of the crystal structure,

as solid–solution, seems more likely; this hypothesis

is currently being investigated.

Reductive dissolution of lepidocrocite colloids

could be observed in situ. Dissolution rate based on

the total volume of one colloid was within an order of

magnitude of rates determined in macroscopic experi-

ments, giving confidence that rates determined for

individual faces are meaningful. Dissolution pro-

ceeded fastest on the longest axis and most slowly

on the shortest, in the same sequence as material is


added during precipitation. However, crystal growth

results in smooth terraces and sharp edges, whereas

dissolution rounds corners, corrodes terraces and

removes material from defect areas such as twin

planes. Thus, even at the nanometer scale, dissolution

is not simply the reverse of precipitation.

When ferrihydrite precipitates in the presence of

other minerals, it may form separate particles or it

may produce coatings on surfaces already present.

Coatings of only a few atomic layers are thick enough

to interfere with the substrate’s interaction with sol-

ution. Because such coatings and separate phases are

below the range of resolution with conventional

techniques, there may be many that have escaped

our attention. Cr(VI) reducing to Cr(III) in the pres-

ence of reduced Fe and Ca, leads to formation of a

CaCr solid that has not previously been documented

in environmental systems and is not treated by geo-

chemical speciation or transport modelling. Entrain-

ment of separate crystals of ZnSnO3�4H2O and

incorporation of Pb and Zn within ferrihydrite clusters

are not scenarios typically described by conventional

modelling codes, either by solubility or by surface

complexing approaches. Models that omit solids

which might be present in very minor quantities, or

that fail to account for elements present in solid

solutions, risk to underestimate the potential for con-

taminant immobilisation. Observation of complex

systems with surface-sensitive and high-resolution

techniques has only begun to reveal uptake sites for

toxic materials present in very low concentrations. We

can expect this approach to deepen understanding in

the future.

Acknowledgements

Thanks to Birgit Damgaard who helped with the

Fe-(hydr)oxide synthesis and AAS analyses, Helene

Almind who collected the XRPD data, Britta Munch

who helped with the figures, Torben Egeberg who

kept the computer network running, Nicolas Xantho-

poulos for help with XPS, the Interdisciplinary Centre

for Electron Microscopy and Microanalysis, Univer-

sitat Munster for TEM support and colleagues on the

Ferrox and Reactive Barrier projects. We are grateful

for financial support from Denmark’s Natural Scien-

ces Research Council, the Carlsberg Fund, the

Groundwater Group of the Geological Survey of

Denmark and Greenland through SMP96 (The

Pesticide Project) and the J. William Fulbright Foreign

Scholarship Board. [EO]

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Behaviour of Fe-oxides relevant to contaminant uptake in the environment