Clay Science9, 385-391 (1996)

BONDING

STATE X-RAY

OF SILICON IN NATURAL FERRIHYDRITES PHOTOELECTRON SPECTROSCOPY

BY

MITSUYUKISOMA1, HARUH1KOSEYAMA1,NAGANORI YOSHINAGA2,BENNY K.G. THENG3 AND CYRIL W. CHILDS4 1 National Institute for Environmental Studies , Tsukuba 305, Japan; 2 Faculty of Agriculture , Ehime University, Matsuyama 790, Japan; 3 Manaaki Whenua-Landcare Research, Private Bag 11-052, Palmerston North, New Zealand; 4 Department of Chemistry , Victoria University, PO Box 600, Wellington 1, New Zealand (Accepted April 15, 1996)

ABSTRACT

The bondingstate and distributionof Si in fivenaturalsamplesand one synthetic sampleof "coprecipitated"siliceousferrihydritehavebeen studiedby X-rayphotoelectronspectroscopy(XPS). For the natural samples,values of the Si 2s peak bindingenergyindicatethat three-dimensional polymerisationof SiO4tetrahedra is, at most,onlypoorlydeveloped.In contrast, the valuefor the synthetic"coprecipitated"samplesuggestsa significantly higherdegree of three-dimensional SiO4 polymerisation. For all of the naturalsamplesthe Fe/Siatomicratio determinedby XPSis comparableto, or slightlylargerthan, the averagebulkratio,indicatingthat the Si in these samplesis welldispersedthroughoutor that the outer layersof the ferrihydriteaggregatesare slightlydepletedin Si.For the syntheticsamplethe Fe/ Si XPS ratio is markedlysmaller than the correspondingbulk ratio indicating surfaceprecipitation of a Si-richphase.Syntheticsamplespreparedbycoprecipitation of Si and Fe maynot be goodmodelsfor naturalsiliceousferrihydrites. Key words: ferrihydrite,natural, synthetic, silicon,X-ray photoelectronspectroscopy,bondingstate. INTRODUCTION Ferrihydrite, a poorly crystalline iron oxide mineral, is an important component of many surficial environments. Samples of ferrihydrite from soils and related environments commonlycontain up to 9% Si but the location and function of Si have not been clearly defined (Childs, 1992 and refs. therein; Zhao et al., 1994;Manceau et al., 1995). The primary particles of natural siliceous ferrihydrites are spheroidal with diameters ranging from 2 to 7 nm, and individual spherules are composed of 20-60 (Fe, 0, OH, H2O) structural domains (Childs, 1992;Parfitt et al., 1992). Electron micrographs show that spherules are associated into aggregates of up to several hundred microns in size (e.g. Childs et al., 1982, 1986, 1990;Schwertmann and Taylor, 1989). Conventional and low-angle X-ray powder diffraction studies indicate that the Si in siliceous ferrihydrite is present as silicate that bonds to, and bridges, the surfaces of the structural domains within each primary particle (Parfitt et al., 1992).This model accounts

386

M. Soma et al.

for

(i)

even

the

apparent

though

effect

of Si content

1985);

(iii)

the

the

silicate

(Parfitt,

1989);

Here in five

depth)

the

of

are

with

in

those

Three

previously PC863

of

Tongariro

National

Kumamoto,

Japan

used.

These

were

New

Zealand,

(Lowe the

a sample

mol/l

X-ray

to

using Sample

energy the

powders

(84.0

Si 2s,

eV) of the

was

of

calculated

determined

source

placed

Table

those

Fe for

relatively overall All

2p3/2

atomic

goethite narrow

line of

XPS binding and

Fe

shape. the

natural

2p3/2 The

layers

a primary

particle. synthesised

be

width

reason

for have

al.

(1993).

a

and

this a

et

Vacuum

were

Hamilton, Conference seven

0.1 mol/l

lines

FeSO4

All

in

with

samples

Generators

with

an

were

onto

the

for

both Fe

ESCALAB

analyser tape

relative

of

0.99

for

the

samples

Fe

2p

line

eV

to

pass

fixed the

sample.

The

Si 2s and

2p

and

to

Au

Fe

of

a stainless

4f712 binding uncertainty

Fe 3p

5

energy

2p3/2. lines

in

The

Fe/

relative

to

respectively.

2s

peak

studied

shapes

for as

and

(e.g.

except

vs 5.6 eV)

exception Si

HF2,

DISCUSSION

minerals, (5.1

eV

factors

AND

FeOOH

line

ml

adhesive

and

and

Creek,

exhibited

from

Aso-Dani,

Clay

200

determined

10.2

HF1

International

investi-

PC991

from

Gibbons

stirring

5 mA)

is •}0.1

were

1783

which

evaporated

parameters

samples

(Defosse

analysis.

on

sensitivity to

energies other

Si

path

outer

1986);

samples,

10th

double-sided

energies

al.,

and

,and

Childs

kV,

were

vacuum

binding

experiment

1 summarises

nm

samples

9037,

of

(12

on

energies

RESULTS

observed

by

recorded

using

by

described

1982);

Bank

by

were

X-ray

al.,

the

synthesised

as

film

of

free

the

ferrihydrite et

natural

with

spectra

a gold

electron

et

ferrihydrite,

convenience

were

of

used

siliceous

Ferry

for

Electron

natural

nature

mean

only

size

who

the

the

is a few

"see"

(Childs

other

associated

was

A1Ka

holder.

values

Si ratio

an

of

probe

X-rays

to the

natural

Two

siliceous

pH7

to

will

(1990)

Zealand

powders

photoelectron

sample

tour A

at

form

of

respectively,

pattern,

Na2SiO3

crushed

instrument

steel

a field

transition

of time

METHODS

(Childs

1990).

from,

fraction over

Since

soft

XPS

et al.

New

Zealand

et al.,

1993).

diffraction

0.1

50 eV.

(Childs

Benjamin,

a small

continues

thermal

(XPS)

equal

AND

samples

New

significant

and

only

reaction

the

by

1994),

Vempati

Springs,

obtained

Percival,

X-ray

lightly

Park,

during

and

35 ml

Kokowai

the

routes.

characterised

from

slow

structure

(ii)

(Anderson

of ferrihydrite.

approximately

alternative

ferrihydrite

1992);

displace

Si on

sample

MATERIALS

gated:

a of

Swaffield,

to a depth

two

readily

spectroscopy

synthetic liberated

the

1993).

photoelectron

and

by

to

effect

Paterson

on

(Childs,

of ferrihydrites

solution

al.,

electrons

compared

confers

although

et

one

aggregates

laboratory

properties in

inhibitory

X-ray

silicate component

ferrihydrites

and

1980;

results the

adsorption

(Childs

used

that

structural

phosphate

(iv)

samples

of ferrihydrite Our

the of

natural

have

Rouxhet,

stability

essential

hematite

natural

("escape" and

on

and to

we

an

ability

from

ferrihydrites

in

chemical

it is not

well

related

Fig.

1)

sample as

minerals. are

1783

a slightly

all

The

similar

which different

has

to a

Fe

2p

eV

as

is unknown. binding

energy

of

about

152.8

Silicon in Natural

TABLE 1.

387

Ferrihydrites

XPS data for ferrihydrites

and related

minerals

n.a., not applicable to this work; n.d., not determined; a Childs et al. (1986); b Childs et al. (1982); CChilds et al. (1990); d Lowe and Percival (1993); a ratio used in synthesis, Childs et al. (1993); f Seyama and Soma (1985); g McIntyre and Zetaruk (1977); h Imai et al. (1991); ' Seyama and Soma (1987), Soma et al. (1992).

shown in Fig. 2 for sample 1783. This value is close to that of olivine, a neosilicate with isolated SiO4 tetrahedra, indicating the absence of a three-dimensional polymerised network of SiO4 tetrahedra in natural ferrihydrites (Seyama and Soma, 1985). If such a component does exist in our samples, it can be, at most, poorly developed. This interpretation is supported by infrared spectroscopic data (Childs et al. 1982, 1986) which also provides evidence for a progressive increase in the proportion of Si-O-Si linkages with an increase in the sample Si/Fe ratio. By comparison, the Si 2s peak binding energy for the synthetic sample (9037), at about 154.1eV (Fig. 2), is significantly larger, and is close to that of quartz (Table 1). This sample apparently contains a component with a well developed three-dimensional SiO4 network since, in general, the larger the Si 2s(2p) binding energy, the higher is the degree of three-dimensional SiO4 polymerisation (Seyama and Soma, 1985). For all of the natural samples the Fe/Si atomic ratio determined by XPS is comparable to, or (for PC863 and PC991) larger than, that of the bulk average (Table 1). We infer that the Si in natural ferrihydrites is well dispersed although the outer layers of the aggregates of PC863 and PC991 are somewhat depleted in Si. When a sub-sample of PC991 was thoroughly ground using a mortar and pestle the Fe/Si ratio determined by XPS decreased (from 7.5) to 5.4, approaching the bulk value of 4.4 (Table 1). In line with the suggestions of Parfitt et al. (1992), based on X-ray diffraction data for samples PC863, PC991 and 1783 (see Introduction), we propose that silicate in natural siliceousferrihydrites bonds to, and bridges between, surfacesof micro-crystallinedomains within each primary particle. Silicate bridges may likewise form between the surfaces of

388

M. Soma et al.

a

b

FIG. 1. The Fe 2p X-ray photoelectron spectra of natural ferrihydrites: (a) sample 1783 and (b) sample PC863.

primary particles making up an aggregate as Naito et al. (1992) have postulated for zinc acetate dispersed in fine SiO2 aggregates. Such bridges may have higher stability than silicate bound to only one surface and this could account for the apparent depletion of Si at the surface of aggregates of PC863 and PC991. Alternatively, the development of a relativelyiron-rich phase on siliceous ferrihydrite could account for the depletion. Such a process seems unlikely here because the waters associated with the ferrihydrites at the time of sampling contained considerable amounts (35-75 g/m3) of dissolvedsilica (Childs

Silicon in Natural

389

Ferrihydrites

a

b

FIG. 2. The Si 2s X-ray photoelectron spectra of ferrihydrites: (a) sample 1783 (natural) and (b) sample 9037 (synthetic).

et

al.,

1982,

1986,

1990).

Kinetic

factors,

however,

may

be

important

in

the

deposition

process. In

contrast

to

is markedly of

9037

during the

in Si may

spectrum the

peak

with

the

Earlier,

surface

of

Si-free In

determined eV,

samples

line of

by as with

ferrihydrite, to

Vempati

Si/Fe

et

in XPS atomic

al.,

may

measurements ratios •†0.10

to

(i)

(ii) our

indicates

XPS by

the

none

of

0 the

the

into

the

shown

has

only

more

in

the

Si

minerals. Si 2s binding

2s For

of

Si in onto

energy

equivalents

energies

1985),

Sithe

preparation

binding

their

a single

structure.

state

the

3

peak,

Soma,

silicate

during

Fig.

one

a silica-like

bonding

Si 2p

surfaces in

than

and

precipitating

silicate

silicate

particle

As

of

(9037)

enrichment

on

(Seyama

to

or

theirs,

translated of

presence

assigned

coprecipitating

be

1783

sample

surface

spectra.

sample

characterise

with

The

minerals

adsorbing

data

synthetic

1).

layers

is

for

in silicate be

the

silica-like by O

that

can

used

and

compare

1783

energy

synthesised

order

verified

for

for

(Table

of

whereas

energies

(1990)

ratio

value

is supported

shape

binding

XPS

development

peaks

binding

et al.

ferrihydrites

ferrihydrite.

the

two

higher

Vempati

to

has

basis

Fe/Si bulk

interpretation

asymmetric the

the

corresponding

ascribed

9037

On

containing

51.0

be

the

This

for

O species.

ferrihydrites,

than

preparation.

although

the

natural

smaller

of values,

by

adding

"coprecipitated" correspond

to

390

M. Soma et al.

a

b

FIG. 3. The O is X-ray photoelectron spectra of ferrihydrites: (a) sample 1783 (natural) and (b) sample 9037 (synthetic) .

our values for natural ferrihydrites(Table 1). Accordingly,the Si specieswith Si 2p bindingenergyof 102.8eV that Vempatiet al. havelikenedto Si in layersilicates,is not significantly presentin the samplesof naturalferrihydriteswe havestudied. Onlyone of the three Si 2p binding energies (101.6eV) that Vempatiet al. reported for silicate adsorbedonto ferrihydritecorrespondscloselyto the Si 2s bindingenergyof our natural ferrihydrites.The full-width-at-half-maximum valuesthat we observedfor the Si 2s lines of PC863,PC991and 1783were all 3.0eV. Thisis only slightlylarger than that for quartz (2.8eV), indicatingthat site variabilityis not a significantfactor. We conclude,firstly, that neither our syntheticsample 9037 nor the Si-containing ferrihydritessynthesisedby Vempatiet al. (1990)are goodmodelsof the naturalsiliceous ferrihydriteswe have examined. Secondly,the Si in natural siliceousferrihydritesis presentmainly as silicate bridgingthe surfacesof crystallinedomainswithin primary particles,and of primaryparticleswithinaggregates(Parfittet al., 1992;Childset al., 1993;Zhao et al., 1994). The silicatedoesnot forma three-dimensional networkstructure. Rather,it exists, at most, as a poorlydevelopedpolymerisedspecies. REFERENCES ANDERSON,P.R. and BENJAMIN,M.M. (1985) Environ. Sci. Technol., 19, 1048-1053.

Silicon

in Natural

Ferrihydrites

391

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