Synthesis of linear alkylbenzene sulphonate intercalated iron(II) iron(III) hydroxide sulphate...

Synthesis of linear alkylbenzene sulphonateintercalated iron(II) iron(III) hydroxidesulphate (green rust) and adsorption of

carbon tetrachloride

K. B. AYALA-LUIS* , D. K . KALDOR, C . BENDER KOCH,

B . W. STROBEL AND H. C . B . HANSEN

Department of Natural Sciences, Faculty of Life Sciences, University of Copenhagen, Thorvaldsensvej 40,

DK-1871 Frederiksberg C, Denmark

(Received 11 June 2006; revised 26 March 2007)

ABSTRACT: Green rusts, GRs, can act as both sorbents and reductants towards selected

pollutants. Organo-GRs are expected to combine these properties with a high affinity for

hydrophobic substances. A novel organo-GR, GRLAS, was synthesized by incorporating a mixture

of linear alkylbenzenesulphonates (LAS) into the interlayer space of synthetic sulphate green rust,

GRSO4. Mossbauer analysis of GRLAS indicates that the structure of the organo-GR is very similar to

that of the initial GRSO4with regard to the FeII/FeIII ratio and local coordination of Fe atoms. X-ray

diffraction demonstrates that the GRLAS formed was well ordered, although a mixture of surfactant

was used for intercalation. The basal spacings of the GRLAS and the kinetics of the ion-exchange

process were dependent on the initial surfactant loading; basal spacings of ~2.85 nm were obtained at

LAS solution concentrations >10 mM. The ratio LASadsorbed/SO42�

desorbed significantly exceeded the

stoichiometric ratio of 2 during the initial part of the ion-exchange process (t = 5 h). However, this

ratio was reached progressively with time. GRSO4preferentially sorbed LAS homologues with long

alkyl chains over short ones. Carbon tetrachloride was successfully adsorbed into GRLAS. The

adsorption isotherm was linear with a distribution coefficient, Kd, of 505A19 litre kg�1.

KEYWORDS: green rust, phyllosilicates, layered double hydroxides, reactivity, carbonates, synthesis, organiccontaminants.

Surface modification using organic reactants is

widely used to optimize or adjust the reactivity of

mineral particles such as clay phyllosilicates and

carbonates (Domka, 1993; Deitsch et al., 1998;

Bergaya & Lagaly, 2001). For example, modified

clay minerals have been used successfully for the

removal of organic contaminants (e.g. pesticides,

heavy metals and volatile organic compounds) from

water and as additives for slow-release formulations

of pesticides (Gitipour et al., 1997; Bojemueller et

al., 2001; El-Nahhal et al., 2001; McLeod, 2001;

Nennemann et al., 2001; Yaron-Marcovich et al.,

2004).

Layered double hydroxides (LDHs), also called

hydrotalcite-like compounds or anionic clays,

represent another kind of material for which

surface modifications have been widely reported

(Newman & Jones, 1998; Pavan et al., 2000).

Structurally, LDHs belong to the pyroaurite-

sjogrenite group of minerals (Taylor, 1973). The

structure of this group consists of positively charged* E-mail: [email protected]: 10.1180/claymin.2007.042.3.04

ClayMinerals, (2007) 42, 307–317

# 2007 The Mineralogical Society

trioctahedral brucitic metal hydroxide sheets alter-

nating with interlayers of charge-compensating

anions and variable amounts of water. The positive

charge of the metal hydroxide sheets is due to the

partial replacement of divalent by trivalent metal

cations (Allmann, 1968; Taylor, 1973). Layered

double hydroxides have the general formula:

MxIIMy

III(OH)2(x+y)Ay/n·zH2O

where MII represents a divalent cation (e.g. Mg2+,

Ni2+, Fe2+) and MIII represents a trivalent cation

(e.g. Al3+, Fe3+) and A represents the intercalated

n-valent anion (e.g. Cl�, NO3�, SO4

2�) (Carrado &

Kostapapas, 1988).

The LDHs possess a pronounced anion exchange

capacity, which, in addition to their easy synthesis

and opportunities for surface modification through

the interlayer space, makes these compounds useful

as catalysts, selective sorbents, and hosts for

reactions in confined interlayer spaces (Miyata,

1983; Newman & Jones, 1998; Duan & Evans,

2006). However, due to the hydrophilic character of

the hydroxide layers, the adsorptive properties of

LDHs are restricted to anionic molecules and polar

compounds, e.g. aromatic and aliphatic carboxylate

anions, phosphonates, polymeric anions, anionic

surfactants and polyols (Esumi & Yamamoto, 1998;

Newman & Jones, 1998; Barriga et al., 2002).

Organo-LDHs with hydrophobic properties can

be generated through the incorporation of anionic

surfactants into LDHs. The presence of hydrophobic

layers permits the partitioning of non-ionic organic

molecules such as PAHs, pesticides and chlorinated

compounds into the organo-LDH interlayers (Esumi

& Yamamoto, 1998; Rives, 2001; You et al., 2002;

Zhao & Nagy, 2004; Wang et al., 2005). Hence,

these materials have a potential use as sorbents of

non-ionic organic pollutants.

The FeII-FeIII hydroxides, commonly designated

green rusts (GR), belong to the family of LDHs.

Green rusts may occur naturally as metastable

intermediates in Fe oxide-oxyhydroxide systems in

aquatic and terrestrial environments (Bernal et al.,

1959; Abdelmoula et al., 1998). The general

formula of LDHs applies also to GRs, with Cl�,

CO32� and SO4

2� as the most common interlayer

anions. The Fe(II)/Fe(III) ratio of the hydroxide

layer depends on the nature of the intercalated

anion, but typical ratios for synthetic materials are

1.8�3 (Genin et al., 2001). Green rusts are strong

reductants towards different classes of chemical

compounds such as halogenated ethanes and

ethylenes, chlorinated methanes, nitrates and

pesticides (Erbs et al., 1999; Lee & Batchelor,

2002; Legrand et al., 2004; O’Loughlin & Burris,

2004; Park et al., 2004). By analogy with other

LDHs, GRs are expected to intercalate anionic

surfactants into their structures. This may provide

an organo-GR which can serve both as a sorbent for

non-ionic compounds and as a strong reductant of

adsorbed and reducible compounds (e.g. chlorinated

solvents). Hitherto, the synthesis of organo-GRs has

not been reported, and thus the potential environ-

mental uses of organo-GR have never been

investigated.

The aims of this study were to synthesize and to

characterize an organo-GR by intercalation of a

sulphate-interlayered GR (GRSO4) with linear alkyl

benzene surfactants (GRLAS), and subsequently to

demonstrate the hydrophobic adsorption properties

of the GRLAS using CCl4 as a test compound.

MATER IALS AND METHODS

Anionic surfactants

A technical solution of linear alkylbenzene

sulphonates (LAS), which is a mixture of homo-

logues and isomers, was used as the anionic

surfactant (Fig. 1). The distribution of homologues

was of C10 & 23%; C11 & 47%; C12 & 27% and

C13 & 3% (determined by capillary zone

electrophoresis, CZE; see below). Aqueous

100 mM LAS solutions were prepared by neutraliza-

tion of linear alkylbenzene sulphonic acid (98%,

Alfa Aesar) with 1 M NaOH to pH 7.

Synthesis of GRSO4and reaction with LAS

The GRSO4(Fe4

IIFe2III(OH)12SO4·nH2O) was

synthesized according to the procedure described

by Koch & Hansen (1997). The procedure for

synthesis comprises the partial aerial oxidation of

FeSO4 solutions at constant pH 7.00, and the re-

dispersion of the washed precipitate in 200 ml of

Ar-flushed water. The formation of GRLAS was

achieved by transferring 50 ml of the freshly

prepared suspension of GRSO4into 100 ml glass-

infusion bottles. Different aliquots of 100 mM stock

solutions of Ar-bubbled LAS (pH 7) were added to

the reaction flasks and diluted to 80 ml with Ar-

flushed water to achieve initial LAS concentrations

in the range 0.01 to 100 mM. The reaction flasks

were wrapped in Al foil to avoid photochemical

308 K. B. Ayala-Luis et al.

side reactions and placed on a shaking table (200

rpm) for 6 days at room temperature. The ion-

exchange studies between LAS and sulphate anions

were carried out by reacting a GR with an initial

concentration of 2.23 mM with a 10-fold molar

excess of LAS.

The amount of FeII present in the GR was

calculated by: FeIIGR = FeII

tot � FeIIsol, where FeII

tot is

the total amount of Fe within GR and solution in

the reaction flasks and FeIIsol is the amount present

in solution (Hansen et al., 1996). For the

determination of FeIItot and FeII

sol, 4 ml of GR

suspensions were withdrawn from the reaction

mixtures using a 5 ml Ar-flushed polyethylene

syringe, 1 ml of the suspension was treated for 30

min with 20 ml of 0.1 M HCl to dissolve GR,

followed by filtration through a 0.22 mm Millipore

filter to remove insoluble Fe oxide products such as

magnetite (Hansen et al., 1996; Rives, 2001), and

FeII was determined in the filtrate (FeIItot). The

remaining 3 ml were passed through a 0.22 mm

Millipore filter and 1 ml of the filtrate was used for

FeII determination (FeIIsol).

Partitioning of CCl4 into GRLAS

The GRLAS was prepared by transferring 200 ml

of Ar-bubbled 20 mM LAS into a 300 ml reaction

flask containing freshly synthesized GRSO4. The

reaction flask was stirred constantly (100 rpm,

room temperature) until the intercalation of LAS

was complete (48 h). The GRLAS formed was

separated by centrifugation (3000 g, 15 min) and

salts were removed from the product by addition of

200 ml of Ar-bubbled water and further centrifuga-

tion (4300 g, 30 min, 3 times). An aliquot from this

freshly synthesized GRLAS was taken for Mossbauer

analysis. Cleaned GRLAS was re-dispersed in 150 ml

of Ar-bubbled water and transferred to 11614 ml

serum vials (2 mM GRLAS). The vials were sealed

with Teflon-coated rubber septa, which were kept in

place with Al crimp seals. Pure CCl4 (Merck, 99.8%,

solubility at 298 K: 5.29610�3 mol l�1) was added

to the GRLAS suspensions in variable amounts to

achieve initial solution concentrations ranging from

1.47 to 7.74 mM. The sealed serum vials were placed

on a shaking table (50 rpm) for 2 h at room

temperature. Aliquots of 6�7 ml of suspension

were withdrawn with a 10 ml polyethylene syringe

and passed through a 0.22 mm Millipore filter. The

filtrates were collected in 2 ml vials, which were

then sealed and the amount of CCl4 present in

solution was determined by gas chromatography

(GC). The total amount of FeII in the vials was

determined by treating the remaining reaction

mixture with 5�7 ml of 4 M HCl for 2 days.

The synthesis, redispersion of GRSO4and

reactions with LAS and CCl4 were carried out

under an atmosphere of Ar (99.9995%).

Analyses

The FeII concentration was quantified by a

modified phenanthroline method (Fadrus & Maly,

1975). Sulphate and LAS were determined by

capillary zone electrophoresis after Westergaard et

al. (1998) and Vogt et al. (1995), respectively. The

determination of sulphate comprises the addition of

10% v/v (20 mM) EDTA (pH 11) to the samples

before analysis to eliminate interferences from FeIII.

Both analyses were performed with a Beckman

P/ACE 5510 instrument equipped with a PDA-

detector. The homologue LAS species (C10�C13)

were separated in a 50 mm inner diameter (i.d.),

50/57 cm long fused silica capillary with an applied

voltage of 25 kV, and detected at 214 nm. C8

S

S

S

O3-

O3-

O3-

a

b

c

FIG. 1. Molecular structure of linear alkylbenzene

sulphonate isomers (LAS). Here C11 is presented with

the benzene ring attached to the alkyl chain at the

(a) 2-position, (b) 3-position and (c) 4-position.

Synthesis of synthetic green rust 309

(4-octylbenzenesulphonic acid, sodium salt, 97%,

Aldrich) was added to all samples as an internal

standard and determined with an electrolyte that

contained 100 mM sodium dihydrogenphosphate at

pH 6.8 and 30% v/v acetonitrile. Sulphate ions were

separated with a 75 mm i.d. and 50/57 cm long fused

silica capillary with an applied voltage of �30 kV

and determined by indirect detection at 254 nm. The

electrolyte contained 3 mM 1,2,4-benzenetricar-

boxylic acid and 0.02% v/v diethylenetriamine

adjusted to pH 5.8. Both determinations were

carried out using 20 nl sample introduction.

Standard solutions with 2�100 mM sulphate were

prepared from Na2SO4 stock solutions (99%, Merck).

CCl4 was determined by GC-FID using a

Shimadzu GC-16A coupled with a Shimadzu

Integrator C-R4A. A SPB-1 column (60 m60.53 mm i.d., 0.5 mm film thickness) was used

with N2 as carrier gas. The oven temperature

programme was 313 K for 10 min, 288 K/min

ramp to 473 K and hold for 7 min and sample

volumes were 4 ml injected at 523 K.

X-ray diffraction (XRD) was performed using a

Philips PW1710 goniometer and Fe-filtered Co-Karadiation (40 kV, 40 mA). Samples for XRD were

prepared by collecting the solids on a 0.22 mm

Millipore filter. Glycerol was used to preserve the

filtered samples against oxidation as described by

Hansen (1989). The glycerol paste was smeared on

to glass plates and scanned at a rate of 1º2y min�1

in the range 4�80º2y. Suspension samples for

Mossbauer spectroscopy were withdrawn quickly

and injected into Perspex capsules (diameter

15 mm, height 1.5 mm) which were dropped into

liquid N2 and stored at this temperature until

analysis. Mossbauer spectra were obtained at 80 K

using a constant acceleration spectrometer and a Rh57Co source. The isomer shifts are given relative to

the centroid of the room temperature spectrum of

a-Fe. The spectra were fitted using a simple

Lorentzian line shape. The absence of texture

effects were assumed in the fitting of the spectra.

All chemicals were of pro analysis quality or

better and deionized water was used throughout.

RESULTS

Formation and characteristics of GRLAS

The XRD pattern for the GRSO4is characterized

by a basal spacing of 1.06 nm (Fig. 2), which is in

good agreement with previously reported values

(Hansen et al., 1994). The intercalation process at

different initial concentrations of surfactant into the

GR interlayer spaces was followed after 5 days by

XRD (Fig. 2). Formation of GRLAS was detected

only in samples treated with initial LAS concentra-

tions >0.1 mM (0.12 mol LAS per mol GR). The

presence of GRLAS diffraction peaks was verified

rapidly after addition of LAS (30 min; data not

shown).

After 5 days of reaction, GRSO4could not be

detected in samples treated with surfactant concen-

trations >10 mM (12.05 mol LAS per mol GR),

whereas in samples treated with lower concentra-

tions (0.1�1 mM) two GR phases (GRSO4and

GRLAS) were observed (Table 1). The basal

spacings of the synthesized organo-GRs were

slightly smaller at low surfactant loadings

(0.1�1 mM), varying between 2.68 and 2.77 nm;

whereas values between 2.81 and 2.89 were

measured for GRLAS treated with loadings greater

than 10 mM (Table 1). The XRD patterns of GRLAS

consisted of sharp (FWHM: 0.24�0.53º2y) and

5 10 15 20 25

°2θ

Inte

nsity

a

b

2.62c

d

0.99e

2.77

1.46

2.59

1.36 1.050.92

1.06

1.06

1.06

0.54

0.54

0.54

0.54

FIG. 2. XRD patterns of GRSO4after 5 days of reaction

with: (a) 0.0 mM; (b) 0.01 mM; (c) 0.1 mM; (d) 1 mM;

and (e) 10 mM of LAS. d spacings in nm. [GR]initial =

0.83A0.11 mM.


symmetrical peaks, which provided evidence of a

well ordered material (Table 1, Fig. 2).

The Mossbauer spectrum of GRLAS was fitted with

a simple Lorentzian line shape (Fig. 3) and the

hyperfine parameters are summarized in Table 2.

Although the statistical quality of this fitting strategy

results in a poorer fit compared to the fit of the initial

GR, it is preferred because the deviation is very

minor close to the absorption peaks. This allows

direct comparison of parameters and shows that the

line shape does deviate from the assumed Lorentzian.

The Mossbauer parameters for GRLAS, synthesized

with an initially large amount of surfactant (10 mol

LAS per mol GR), do not differ significantly from the

TABLE 1. XRD parameters for organo-green rust (GRLAS) formed at different initial concentrations of surfactant

(LAS) at an initial GRSO4concentration of 0.83A0.11 mM (295 K).

————— 60 min ————— ————— 5 days —————

[LAS](mM)

d(nm)a

I003

(%)bFWHMc

(º2y)Phase d

(nm)aI003

(%)bFWHMc

(º2y)Phase

0 1.075 100 0.24 GRSO41.073 100 0.24 GRSO4

0.01 1.071 100 0.27 GRSO41.075 100 0.24 GRSO4

0.1 1.071 100 0.27 GRSO41.073 100 0.27 GRSO4

2.706 19 n.d.d GRLAS 2.684 31 n.d. GRLAS

1 1.074 34 0.31 GRSO41.066 27 0.27 GRSO4

2.767 100 0.27 GRLAS 2.742 100 0.27 GRLAS

10e 1.073 n.d. 0.35 GRSO42.885 100 0.35 GRLAS

2.813 n.d. 0.53 GRLAS

100e 1.072 n.d. 0.44 GRSO42.828 100 0.35 GRLAS

2.856 n.d. 0.35 GRLAS

a Calculated as d = [d003 + 26d006 + 36d009]/3b Relative intensity of the (003) reflectionsc FWHM: full width at half maximum. Measured from 003 reflections for GRSO4

and from 006 reflections fromGRLASd n.d.: not determinede Measurements carried out after 30 minReproducibility of XRD measurements: A0.003 nm

Velocity (mm/s)

Rel

ativ

e ab

sorp

tion

FIG. 3. Mossbauer spectrum of GRLAS measured at 80 K. Full lines represent the fit to the Lorentzian line shape.

[GR]initial = 2mM, LAS = 20 mM.


characteristic parameters of GRSO4(Hansen & Koch,

1997). Only a slight increase in the full width at half

maximum of the FeII and FeIII doublets is observed in

GRLAS (0.08 mm/s). Increasing the number of formal

fit components to two of each gives a statistically

better fit and an area ratio of 1.95 that compares well

with the typical ratio of 2 found in GRSO4(Hansen &

Koch, 1997). It is suggested that the physical

explanation for the increase in line width is related

to the increased mechanical flexibility of the

octahedral sheet in the GRLAS allowing local

adjustment of the coordination environment.

Figure 4 shows the kinetics of LAS adsorption

into GRSO4. The adsorption was faster for the long-

chain homologues (C12�13) than for the shorter

ones (C10�11). Complete adsorption of the initial

amount of surfactant was observed only for the long

LAS molecules (C12�13). The adsorption vs. time

curves of the LAS molecules are characterized by

an initial region which includes the first 10 h of

reaction. This region comprises the adsorption of up

to 90% of the starting amount of C12�13

molecules and of 40% for the C10�11 molecules.

With longer reaction times, the LAS molecules

show a difference in behaviour. For the long

molecules, complete adsorption occurs within 24 h

of reaction. The curves for LAS with shorter chains

show two additional regions. In the second region

(from 10 to 35 h), adsorption proceeds gradually

and more slowly than in the first region until no

more adsorption is observed (third region, t > 40 h).

The ion-exchange process between SO42� and LAS

occurs at a high rate within the first 20 h of reaction,

and then slows down (Fig. 5). At reaction times of

<5 h, the ratio between the amount of LAS adsorbed

and the SO42- desorbed (LASadsorbed (mM)/SO4

2�desorbed

(mM)) is significantly >2, the value required to

achieve charge neutrality in the GRLAS system.

However, at longer reaction times, this ratio gradually

decreases until it reaches a value close to 2.

Adsorption of CCl4 into GRLAS

The hydrophobic adsorption properties of the

new organo-GR were verified by the uptake of CCl4into GRLAS (Fig. 6). The adsorption coefficient

(Kd), as determined from a least squares linear

regression fit of the isotherm data, amounted to

505A19 litre kg�1.

TABLE 2. Mossbauer parameters for GRLAS and GRSO4at 80 K.

— GRLAS — — GRSO4

1 —Fe(II) Fe(III) Fe(II) Fe(III)

DEq (mm/s) 2.90(3) 0.47(3) 2.89(2) 0.43(2)d (mm/s) 1.23(3) 0.46(3) 1.27(2) 0.46(2)g (mm/s) 0.34(3) 0.36(3) 0.28(2) 0.28(2)A (%) 63.7(5) 36.3(5) 66.6(2) 33.3(2)

d: isomer shift; DEq: quadrupole splitting; g: full width at half maximumA: Relative area of componentsValues in parentheses are the uncertainty in the last digit1 Hansen & Koch (1997)[GR]initial = 2mM, LAS= 20 mM

0

20

40

60

80

100

0 10 20 30 40 50

Time (h)

Rel

ativ

e am

ount

sor

bed

(%)

C10C11C12C13Total LAS

FIG. 4. Kinetics of adsorption of LAS molecules to

GRSO4at 295 K. [LAS]initial = 2.25 mM. [GRSO4

]initial =

0.9 mM.


DISCUSS ION

The increase in basal spacing observed in the XRD

patterns of GRSO4confirms the intercalation of LAS

into the GR. Although the intercalation of anionic

surfactants into LDH has already been demonstrated

(Meyn et al., 1990; Esumi & Yamamoto, 1998;

Pavan et al., 1998), this is the first time that

intercalation of an anionic surfactant into GR has

been reported.

The formation of a well ordered GRLAS indicates

that the intercalated organic phase possessed a

constant thickness along the GRLAS structure,

although a technical grade surfactant was used for

the synthesis of the organo-GR. The aggregation of

organic molecules with different alkyl chain lengths

to form films of constant thickness was also

observed by Meyn et al. (1990), who intercalated

a similar, technical-grade alkylbenzenesulphonate

into different types of LDHs. The rigidity of the

metal hydroxide sheets, the high conformational

freedom of the alkyl chains and the presence of

holes in bimolecular films were suggested as the

driving forces for this behaviour (Meyn et al., 1990;

Lagaly & Dekany, 2005).

Based on the basal spacings, the intercalated

organic molecules in the LDHs form monolayers or

bilayers and can be oriented perpendicular or tilted

toward the hydroxide layer. The specific arrange-

ment of the surfactant molecules in the interlayer

space depends on the preparation method and on

the type of surfactant and LDH used (Meyn et al.,

1990; Clearfield et al., 1991; Newman & Jones,

1998). Depending on specific characteristics of the

organic layer (e.g. to achieve better packing,

optimal transport of smaller molecules within the

films), the organic molecules can self-assemble in

different arrangements by variation of the tilting

angle and conformational changes in the alkyl

chains (e.g. kink or gauche conformations) (Lagaly,

0.0

2.0

4.0

6.0

0 40 80 120 160

0.0

0.4

0.8

1.2

Time (h)

Mol

ar ra

tio L

AS s

orb/

SO4

deso

rb2–

[SO

4 ] de

sorb

, [LA

S]so

l (m

M)

2–

[SO4 ]desorb2–

[LAS]sol

[SO4 ]desorb2–

[LAS]sorb

FIG. 5. Temporal variation (right axis) in the concentration of sulphate desorbed, SO42�

desorb, and LAS present in

solution, LASsol; and (left axis) in the molar ratio LASsorb/ SO42�

desorb, after addition of LAS to GRSO4.

0 1 2 3 4

0

400

800

1200

1600

2000

[CCl4]aq,eq (mmol/l)

Sorb

ed C

Cl 4 (m

mol

/kg

GR

LAS)

Kd ≅ 505 l/kg

FIG. 6. Sorption isotherm for sorption of CCl4 to

GRLAS after 2 h of reaction at room temperature.

[GRLAS]initial: 2 mM. The slope of the regression line

(r2: 0.9903) equals the distribution coefficient (Kd).


1979; Dekany & Haraszti, 1997; Lagaly & Dekany,

2005). The observed dependence of the GRLAS

basal spacing on the surfactant loading seems to

correspond with this self-assembly mechanism. By

analogy with organo-silicates, large amounts of

organic molecules with alkyl chains can be placed

in the LDH-interlayers when the tilting angle

approaches 90 degrees (Lagaly et al., 1979).

Hence, at low surfactant loadings, smaller angles

of tilt are expected.

In addition, high surfactant loadings are related to

an increase in the absolute amount of long

molecules in the interlayer space, which tend to

be adsorbed rapidly (Fig. 4). The presence of LAS

molecules with long alkyl chains in the interlayer

space would imply fewer holes in the hydrophobic

interlayer space, and consequently less free space to

accommodate more long alkyl chains.

The similar quadrupole splitting and isomer shift

ratio of GR after the ion-exchange process indicates

that the replacement of sulphate ions by LAS has

not changed the local coordination environment of

the Fe nuclei (Koch, 1998), although a relatively

large initial amount (10 mol LAS per mol GR) was

used.

The preferential adsorption of longer-chain LAS

(Fig. 4) by GR can be explained by stronger lateral

van der Waals interactions between long alkyl chains

than between shorter chains (Somasundaran &

Fuersten, 1966). These interactions facilitate the

adsorption of the surfactant molecules in the

interlayer space, occupying the most accessible sites

within the GR interlayer spaces. The kinetic

adsorption patterns of molecules with shorter alkyl

lengths (Fig. 4) present three distinguishable regions

which may be explained in the following way: (1)

fast adsorption mainly onto the edge faces and partly

into the interlayer space; (2) adsorption into the

interlayer space, which with time becomes dominated

by the diffusion of the surfactant molecules into the

vacant spaces; and (3) the saturation of the ion-

exchange sites, which blocks further adsorption of

the surfactant (Pavan et al., 2000; Cases et al., 2002).

The initiation as well as the rate of the ion-

exchange process is dependent on the initial

concentration of LAS used (Table 1). Stronger

electrostatic interactions are expected between the

GR metal hydroxide layers and the sulphate anion

than with the monovalent sulphonate functional

groups of the surfactant molecules. Therefore, to

achieve the displacement of sulphate ions by the

surfactant molecules, an initial critical concentration

of surfactant is required to initiate the reaction.

Once the surfactants are positioned in the GR layers

via columbic interactions, resulting in swelling of

the interlayer space, further adsorption of LAS

takes place due to the action of additional forces,

i.e. van der Waals forces between alkyl chains and

p-electron polarization between the electrons of

aromatic nuclei (Pavan et al., 2000).

The decrease in the rate of adsorption for longer

reaction times (Fig. 5) can be related to the

progressive saturation of most accessible sites for

ion-exchange. The high LASadsorbed/SO42�

desorbed

ratio observed at the beginning of the reaction

indicates that LAS molecules may be adsorbed

partly as ion pairs with counter cations (e.g. Na+),

which are expelled from the interlayer space at later

stages of the adsorption.

GRSO4is known to be able to reduce CCl4 to

CHCl3 and other products (Erbs et al., 1999). We

observed that GRLAS was much more stable in the

presence of CCl4 than GRSO4(data not shown).

However, assuming that oxidation of GRLAS by

CCl4 takes place at the same rate as for GRSO4and

using the kinetics reported by Erbs et al. (1999), a

maximum of 16% of CCl4 could be reduced during

the 2 h of contact with the GRLAS. It is probable

that considerably less is reduced due to the lower

reactivity of GRLAS compared with GRSO4, and

hence the sorption data are affected only to a small

extent. Also the fact that a linear sorption isotherm

can be observed indicates that no significant

reduction of CCl4 takes place during the sorption

experiment.

The linear adsorption isotherm (Fig. 6) demon-

strates that the affinity of the sorbate for the

adsorbent does not change over the observed

concentration range, and the dominating mechanism

for CCl4 adsorption is the partitioning of the

organic molecule into the hydrophobic GRLAS

interlayers (Chiou et al., 1979). Highly linear

adsorption isotherms (Kd 176�1030 l kg�1) have

also been observed for the adsorption of other small

non-ionic organic compounds (e.g. trichloroethy-

lene, tetrachloroethylene) into organo-LDH inter-

calated with dodecylbenzenesulphonate and

dodecylsulphate molecules (You et al., 2002;

Zhao & Nagy, 2004). The Kd reported in the

present study (505A19 l kg�1) falls within the range

mentioned and is much higher than the value

observed in the adsorption of CCl4 into bentonite

intercalated with quaternary ammonium alkyl

cations, Kd 4.2�13.5 l kg�1 (Deitsch et al., 1998).


FUTURE PERSPECT IVES

Several cleaning techniques are currently used for

the remediation of polluted soils and water

resources (Hamby, 1996). Special attention is

given to the remediation of the contamination of

hydrophobic compounds like chlorinated solvents

and petroleum hydrocarbons (Sabatini et al., 2000).

Due to the immiscibility of these compounds with

water, the use of conventional remediation techni-

ques, e.g. pump-and-treatment, is limited (Sabatini

et al., 2000). Hence, the design of new materials

which are able to improve the remediation

efficiency is required. The successful synthesis of

organo-GR opens up the possibility for the design

of new and inexpensive materials which can be

used not only as sorbents of hydrophobic contami-

nants, but also as potentially strong reductants of

adsorbed reducible pollutants. Finally, in order to

produce an efficient organo-GR, further work needs

to be carried out to explore the possibility of

intercalating other families of organic molecules

into green rusts.

CONCLUS IONS

Organo-GR was successfully synthesized from

GRSO4and LAS. The quick (5 days) and complete

formation of GRLAS was observed when initial

loadings of surfactant were >10 mol LAS per mol

GR. The formation of a GRLAS material possessing

an organic interlayer with constant thickness along

the structure of the organo-GR is in good agreement

with the self-assembly capacity of alkyl chains

observed in other studies. The hydrophobic proper-

ties of the organo-GR were confirmed by the linear

adsorption isotherm of CCl4 in aqueous solution.

ACKNOWLEDGMENTS

Thanks are due to J.P. Ludwig for carrying out parts of

the syntheses and chemical analyses.

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