[Developments in Clay Science] Developments in Palygorskite-Sepiolite Research Volume 3 ||...

Chapter 15

Developments in Clay Science, Vol. 3. DOI: 10.1016/B978-0-444-53607-5.00015-3# 2011 Elsevier B.V. All rights reserved.

Adsorption of Surfactants,Dyes and Cationic Herbicideson Sepiolite and Palygorskite:Modifications, Applications andModelling

Uri Shuali*, Shlomo Nir* and Giora Rytwo{{

*Department of Soil Science, The Robert H. Smith Faculty of Agriculture, Food and Environment,

The Hebrew University of Jerusalem, Rehovot, Israel{Department of Environmental Sciences, Faculty of Sciences and Technology, Tel-Hai Academic

College, Upper Galilee, Israel{MIGAL, Galilee Technology Center, Kyriat Shmona, Israel

1. INTRODUCTION

In 1984 was published the book ‘Palygorskite—Sepiolite: Occurrences,

Genesis and Uses’ (Singer and Galan, 1984).

While working on an update version of the book, Prof. Arieh Singer

passed away prematurely.

This chapter is dedicated to commemorate his contribution to the sepiolite/

palygorskite research and his legacy.

The current chapter focuses on the adsorption of organic molecules, such

as surfactants, and the modelling of the adsorption of cationic and neutral sur-

factants by these clays.

2. SURFACE-RELATED PHYSICO-CHEMICAL PROPERTIES

2.1. Formulae and Chemical Analyses

It was suggested that the ideal structural formulae of sepiolite are represented

by two models:

Si12Mg9O30(OH)6(OH2)4�6H2O (Nagy and Bradley, 1955)

Si12Mg8O30(OH)4(OH2)4�8H2O (Brauner and Preisinger, 1956)

351

Developments in Palygorskite-Sepiolite Research352

More recently, Santaren et al. (1990) suggested a formula based on the

Brauner and Preisinger (1956) model: Si12O30Mg8(OH,F)4(H2O)48H2O.

The ideal structural formula of palygorskite is represented by:

Si8Mg5O20(OH)4(OH2)4�4H2O (Bradley, 1940)

These clays are characterized by a porous structure, resulting from

repeated inversions of the silicate layer, in which the Si��O��Si tetrahedrons

serve as bridging groups between the alternating ribbons of the alumino–Mg

silicates. The Si��O bond in the bridging Si��O��Si group has a double-bond

character (Yariv, 1986). The two minerals differ in the frequency of this

inversion sepiolite having wider channels than palygorskite (0.37�1.06 vs.

0.37�0.64 nm), respectively (Singer, 1989); Kitayama and Michishita

(1981) found in sepiolite pore dimensions of 0.56�1.1 nm.

Representative data of chemical analysis are given in Table 1.

Recently, Garcia-Romero and Suarez (2010) investigated the chemical

composition of the two clays using analytical electron microscopy (AEM).

In contrast to the accepted claim that a compositional gap exists between

them, in which sepiolite occupies the most magnesic and trioctahedral

extreme and palygorskite occupies the most aluminic–magnesic and dioctahe-

dral extreme, their findings show that all intermediate compositions may exist

between the two pure extremes.

2.2. Crystallography

The cell-unit crystallographic data for these clays are summarized in Table 2.

Sepiolite is characterized by a strong X-ray reflection in the 110 plane at

d¼12 A and a back reflection towards the b axis at d¼27 A; the

corresponding values for palygorskite are 10.5 and 18 A. The information

concerning the X-ray spectra of the two clays was gathered and reported by

Caillere and Henin (1972) and Bailey (1980).

2.3. Surface Area and Porosity

Based on the models for sepiolite and palygorskite, the calculated specific sur-

face areas are 900 and 815 m2/g, respectively (Serna and van Scoyoc, 1979).

The experimental values are significantly smaller than the theoretical ones;

they depend on the nature of the test agents and the mathematical model for

calculations (Del Rey et al., 1985; Fernandez-Alvarez, 1978), and on the

degree of dehydration of the clays (Fernandez-Alvarez, 1978; Jimenez-Lopez

et al., 1978). Representative values of measured surface areas are:

Sepiolite: 230–320 m2/g. (Dandy, 2006; Radojevic et al., 2002; Ruiz-

Hitzky and Fripiat, 1976; Serratosa, 1978).

Palygorskite: 125–195 m2/g. (Barrer et al., 1954; McCarter et al. 1950).

TABLE 1 Chemical Analyses (%) of Sepiolite and Palygorskite.

Sepiolite, Vallecas, Spain

Palygorskite, Quincy,

Florida

Galan andCastillo (1984)

Campeloet al. (1987)

Shuali(1991)

Nathan(1969a)

Nathan(1969b)

Shuali(1991)

SiO2 63.10 62.0 55.26 48.61 55.35 59.38

Al2O3 1.08 1.7 1.52 7.71 8.91 8.02

MgO 23.08 23.9 23.08 8.88 10.62 8.40

Fe2O3 0.27 0.5 1.06 3.06 3.64 3.46

FeO n.d n.d n.d 0.16 – n.d

TiO2 – n.d – 0.40 – 0.22

MnO – n.d – 0.02 0.02 –

CaO 0.49 0.5 0.27 0.81 0.95 1.41

Na2O 0.09 0.3 0.1 n.d 0.07 <0.02

K2O 0.21 0.6 0.44 0.44 0.75 0.82

H2O (a) 10.88 10.5 13.48 5.88 10.43 10.75

H2O (b) n.d n.d 4.37 23.61 7.20 7.05

CO2 n.d n.d n.d 0.24 0.19 n.d

Cl n.d n.d n.d – 0.18 n.d

Total 99.20 100.0 99.58 100.4 98.99 99.69

n.d, not determined; –, traces, not found.Remarks: H2O (a) corresponds to weight losses at 100 �C (Shuali’s results: 180 �C); H2O(b) corresponds to weight losses at high temperatures (Shuali’s results: 1000 �C).

Chapter 15 Adsorption of Surfactants 353

Shuali (1991) measured their specific surface area at various dehydration

temperatures; his results are shown in Table 3.

The porosity spectrum of sepiolite or palygorskite is attributed to the con-

tribution of two sources:

(a) External porosity resulting from the inter-fibres/bundles spaces; the

effective diameters of these pores are >15 A.

(b) Structural porosity resulting from the repeated inversions of the silicate

layer: The two minerals differ in the frequency of this inversion, sepiolite

having wider channels than palygorskite (0.37�1.06 vs. 0.37�0.64 nm,

respectively; Singer, 1989). Their effective diameters are <15 A.

TABLE 2 Cell-Unit Crystallographic Data.

a b

c or c

sinb bSpace

Group

Sepiolite Nagy and Bradley(1955)

5.30 27.00 13.40 A/2m

Brauner and Preisinger(1956)

5.28 26.80 13.40 Pnan

Brindley (1959) 5.25 26.96 13.50 –

Zvyagin et al. (1963) 5.24 27.20 13.40 Pnan

Bailey (1980) 5.28 26.95 13.37 Pnan

Galan (quoted byJones and Galan,1988)

5.23 26.77 13.43 Pnan

Palygorskite Bradley (1940) 5.20 18.00 12.90 95.83� A2/m

Zvyagin et al. (1963) 5.22 18.06 12.75 90� P2/a

Christ et al. (1969) 5.24 17.87 12.72 95.78� Pn

5.24 17.83 12.78 90.96� P2/a

Bailey (1980) 5.20 17.90 12.70 107�

TABLE 3 Specific Surface Areas (m2/g) at Various Dehydration Temperatures.

T (�C) 25 135 180 250 350

Sepiolite 141.5�9.3 213.4�15.8 269.0�21.9 275.3�18.7 220.4�17.3

Palygorskite 92.3�6.9 139.4�7.1 158.3�8.3 141.6�7.8 127.4�6.1

T (�C) 450 550 700 900

Sepiolite 176.0�9.9 152.0�8.7 117.2�5.6 108.3�5.6

Palygorskite 81.4�6.3 74.5�5.2 67.4�5.1 60.8�4.9


Although several authors considered that the internal surface areas of the

structural pores are not accessible to branched molecules, it has been shown that

intra-pore adsorption of small- and short-branchedmolecules like pyridine and tri-

methylpyridine (Shuali, 1991; Shuali et al., 1989) or acetone (Kuang et al., 2006)

does occur.Ovarlez et al. (2009) showed that dyemolecules as large andhydropho-

bic as indigo can be incorporated inside sepiolite thus changing dramatically the

thermal stability of sepiolite and preventing nanoclay folding.


Based on these findings, Yariv and Cross (2002) suggested a new termi-

nology for the structural porosity which differs between channels and pores:

the term ‘pores’ is attributed to vacancies crossing along the clay crystal

(boarded by the inverted TOT units and open at the edges), whereas the term

‘channel’ is attributed to vacancies at the edges/broken parts of the clay

crystal.

2.4. Adsorption Sites and Ion Exchange Capacity

The silanol groups (Si��OH) present at external surfaces of the tetrahedral

sheet are usually accessible to organic species, acting as neutral adsorption

sites (designated N). The ‘N sites’ content of Vallecas–Vicalvaro sepiolite is

estimated to be in the order of 0.60 mmol/g (Ruiz-Hitzky, 1974; Ruiz-Hitzky

and Fripiat, 1976; Rytwo et al., 1998). Isomorphic substitutions, such as Al3þ

for Si4þ, are responsible for the charged adsorption site (P sites) and the cat-

ion exchangeability. The reported values for the cation-exchange capacity of

palygorskite or sepiolite vary between 0.03 mmol/g (palygorskite) and 0.1–

0.15 mmol/g sepiolite. According to Grim (1968) it is difficult to determine

exact values for palygorskite because of existence of montmorillonite impuri-

ties. Galan (1987) reported for sepiolite (Vallecas, pangel) CEC of

0.095 meqiv./g clay. Shuali (1991) reported the values 0.061 and

0.076 meqiv./g. (ammonium acetate method) for sepiolite (Vallecas) and

palygorskite (Quincy Florida), respectively. Lemic et al. (2005), who studied

the modification of sepiolite with various quaternary amines, found that their

adsorption capacities, calculated by fitting experimental data to the Lang-

muir–Freundlich equation, were 324%, 278% and 258% of the CEC for stear-

ylbenzyldimethylammonium (SBDMA; octadecylbenzyldimethylammonium),

distearyldimethylammonium (DDA; dioctadecyldimethyl) and hexadecyltri-

methylammonium (HDTMA; cetyltrimethylammonium), respectively.

2.5. Thermal Analysis

The thermal behaviour and stability of sepiolite and palygorskite were inten-

sively investigated, and results were summarized in several surveys (for

instance, Fenoll Hach-ali and Martin Vivaldi, 1968; Singer, 1989). During

the heating process, the following changes occur:

(a) Loss of adsorbed, bound and zeolitic water. The desorption of bound

water occurs in two stages, in which the first one is reversible, and at

its end, the clays pass a structural folding (into sepiolite- or palygors-

kite-anhydride)

(b) Dehydroxylation of the clays

(c) Collapse of the lattice and transformation into meta-clay state

(d) Crystallization into new phases


The first three stages are endothermic while the last is exothermic.

Shuali (1991), using DTA-TG-DTG-MS and DSC techniques, found in the

DTA curve of sepiolite (Vallecas, Spain) four endothermic peaks which were

associated with water loss:

(a) 135 �C(b) 315 and 360 �C (two small peaks) in air or 345 �C in nitrogen atmosphere

(c) 515 �C and

(d) 825 �C

The last endothermic peak was followed by an immediate exothermic peak

at 825 �C.For palygorskite (Quincy, Florida), he reported the existence of three

endothermic peaks which were associated with water loss: (a): 150 �C; (b):265 and 285 �C in air and nitrogen atmospheres, respectively; (c): 465 �C.

2.6. Infrared Spectroscopy

IR spectral data of sepiolites from different sites were reported by van der

Marel and Beutelspacher (1976): the characteristic peaks are 503, 534, 784,

1025, 1074, 1200, 3245 and 3620 cm�1. The characteristic peaks for paly-

gorskites are 482, 513, 648, 987, 1037, 1190, 3265, 3540 and 3620 cm�1.

Zeolitic water gives rise to absorption bands at 1670, 3250, and 3420

(sepiolites) and 1660, 3290 and 3400 cm�1 (palygorskites). Absorption peaks

for bound water in sepiolites or palygorskites are at 3568 and 3625 cm�1 and

at 3550 and 3585 cm�1, respectively (Hayashi et al., 1969; Mendelovici,

1973; Prost, 1973, 1975; Serna et al., 1975, 1976, 1977; Serna and van

Scoyoc, 1979). Prost and Serna et al. explain that the appearance of two

stretching bands in the spectra of bound water is due to the fact that the two

water molecules, which are bound to the octahedral cation exposed to the

pores, are not identical.

The clayey properties of sepiolite and palygorskite (relatively high surface

area, existence of the ‘N’- and ‘P sites’ and the thermal stability) together

with, acidic nature and molecular-sieving ability due to the structural pores/

channels, enable their use as excellent substrates for adsorption of neutral

organic molecules and organic cations (Alvarez, 1984; Shuali, 1991).

3. SURFACE MODIFICATIONS OF SEPIOLITE BYSURFACTANTS—LITERATURE SURVEY

The interactions between clay minerals and surfactants (e.g., quaternary

amines, organic dyes, silanes), which change the chemical nature of the

clayey matter, have been widely investigated and reviewed during the past

two decades (e.g., Yariv and Cross, 2002). Sepiolite and palygorskite, which


have large surface areas, zeolitic-like porous structure and a moderate cation-

exchange capacity (0.03–0.15 meqiv./g clay), have been found suitable for

such treatments. A representative list of studies dealing with the modifications

of these two clays, and their utilization in slow-release pesticide-formulations

or remediation of polluted water, is given in Tables 4 and 5.

The formation of organically modified sepiolite or palygorskite is carried

out by applying the surfactants as monomers, micelles or liposomes, using

the same techniques employed for the treatment of other clays, (e.g., Arrma-

gan et al., 2003; Lemic et al., 2005; Ozcan et al., 2007; Ozdemir et al., 2007;

Rytwo et al., 1998; Sanchez-Martin et al., 2006; Yildiz and Gur, 2007).

Rytwo et al. (2000, 2002) found that the adsorption on sepiolite of divalent

organic cations (e.g., paraquat, diquat, methyl green) was between 100%

and 140% of the CEC, and adsorption of monovalent organic cations (Rytwo

et al., 1998) reached more than 400% of the CEC. They proposed that in the

case of the divalent organic cations there is almost no interaction with the

neutral sites. In contrast monovalent organic cations are mainly bound to

the ‘N sites’. Arrmagan et al. (2003) who investigated the adsorption of neg-

atively charged azo dyes on natural sepiolite and hexadecyltrimethylammo-

nim (HDTMA) modified sepiolite, found that the adsorption on the natural

clay occurred through electrostatic attraction of the dye molecules onto oppo-

sitely charged sites while the adsorption onto the modified clay is governed

initially by electrostatic attraction onto the head groups of the already

adsorbed quaternary amine. Modified sepiolite yields adsorption capacities

of 169, 120 and 108 mg/g for ‘reactive yellow’, ‘reactive black’ and ‘reactive

red’, respectively.1 The molecular weights of these dyes were not provided

and therefore it is difficult to express the adsorbed amounts in units such as

moles per gram for the estimation of the degree of coverage; yet, the adsorbed

amount can imply adsorption above the ion exchange capacity (IEC) even if

taking values of 800 Da.

Characterization of the modified clays was carried out by means of XRD

(Dogan et al., 2008; Tartaglione et al., 2008), SEM (Tartaglione et al., 2008)

HRTEM (Tartaglione et al., 2008), zeta potential measurements (Alkan et al.,

2005a,b; Demirbas et al., 2007; Dogan et al., 2008; Sabah et al., 2007) FTIR

(Alkan et al. 2005b) and thermal analysis (Dogan et al., 2008; Lemic et al.,

2005).

3.1. Cationic Surfactants

Examination of the adsorption of cationic surfactants revealed that the mole-

cules are sorbed in multilayers, with the first layer formed by cation exchange

between magnesium and surfactant cations, whereas the other layers are

1. These azo dyes (manufactured by Everlight Chemical Industrial Corporation, Taiwan) are

known to contain anionic sulphonate groups.

TABLE 4 Sepiolite Modification by Surfactants.

Surface-treatment agents References Remarks

Cationic surfactants

Dodecylammonium (DDA, DA) Akcay and Yurdakoc (2000) Removal of phenoxyalkanoic acid herbicides

Dodecylethyldimethylammonium(DDEDMA)

Ozcan et al. (2005) Adsorption of nitrate ions

Ozcan and Ozcan (2008) Adsorption of Yellow 99

Dodecyltrimethylammonium(DDTMA)

Gok et al. (2008) Adsorption of naphthalene

Sabah and Celik (2002a)

Hexadecylpyridine (HDPy) Rodriguez-Cruz et al. (2008) Sorption of fungicides (penconazole and metalaxyl);significance of the long-chain organic cation structure

Sabah and Celik (2002a) Adsorption mechanism

Hexadecyltrimethylammonium(HDTMA)

Arrmagan et al. (2003) Adsorption of negatively charged azo dyes

Li et al. (2003) Removal of anionic contaminants

Adsorption mechanism

Hexadecyltrimethyl ammonium(HDTMA)

Lemic et al. (2005) Adsorption capacity and isotherm

Yildiz and Gur (2007) Adsorption of phenol and chlorophenols

Dihexadecyldimethylammonium(DHDDMA)

Rodriguez-Cruz et al. (2008) Sorption of fungicides (penconazole and metalaxyl);significance of the long-chain organic cation structure

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Octadecyltrimethylammonium(ODTMA)

Sanchez-Martin et al. (2003, 2006) Adsorption of pesticides, isotherms


Rodriguez-Cruz et al. (2007, 2008) Sorption of fungicides (penconazole and metalaxyl);significance of the long-chain organic cation structure

Dioctadecyldimethylammonium(DODDMA)

del Hoyo et al. (2008) Physico-chemical study


Anionic surfactants

Sodium dodecylsulfate (SDS) Ozdemir et al. (2007) Effect of adsorption parameters

del Hoyo et al. (2008) Physico-chemical study

Sanchez-Martin et al. (2008) Studying the effect of clay structure on adsorption capacity

Sodium dodecylbenzenesulfonate(SDBS)

Ozdemir et al. (2007) Effect of adsorption parameters

Neutral Surfactants

Triton�100 del Hoyo et al. (2008) Physico-chemical study

Rytwo et al. (1998), Shariatmadari et al.(1999)

Calculations estimate the contribution of different adsorptionsites capacity

15 Crown ether 5 (15 C-5) Sanchez-Martin et al. (2008) Effect of clay structure on adsorption capacity

Rytwo et al. (1998), Shariatmadari et al.(1999)

Calculations estimate the contribution of neutral adsorptionsites

Continued

Chapter

15

Adsorptio

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rfactants

359

TABLE 4 Sepiolite Modification by Surfactants.—Cont’d

Surface-treatment agents References Remarks

Cationic Dyes

Crystal violet (CV) Rytwo et al. (1998) Modelling

Shariatmadari et al. (1999) Calculations estimate the contribution of different adsorptionsites capacity

Eren et al. (2010) Adsorption, kinetics, thermodynamics

Methylene blue (MB) Rytwo et al. (1998) Calculations estimate the contribution of different adsorptionsites capacity

Shariatmadari et al. (1999) Modelling, contribution of different adsorption sites

Methyl green (MG) Rytwo et al. (2000, 2002) Adsorption, modelling

Thioflavin-T (TFT) Casal et al. (2001) Photo and thermal stabilization of pesticides (trifluralin, TFT).

Silanes

Dimethyloctadecylchlorosilane(DMODCS)

Alkan et al. (2005b) Modification, FTIR, z potential

Dimethyldichlorosilane (DMDCS) Alkan et al. (2005b) Modification, FTIR, z potential

3-aminopropyltriethoxysilane Alkan et al. (2005b) Modification, FTIR, z potential

Demirbas et al. (2007) Modification, adsorption of ions, electrokinetics

Triethoxy-3-(2-imidazolin-1-yl)propylsilane

Turan et al. (2008) Characterisation by FTIR, XRD, thermal analysis

Tartaglione et al. (2008) Thermal and morphological characterisations, (TGA, SEM,HRTEM)

Develo

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TABLE 5 Modifications of Palygorskite with Surfactants.

Surface-treatment Agents References Remarks

Cationic surfactants

Tetramethylamine (TMA) Chang et al. (2009) Anion–cation premodification by quaternary amines and SDS,sorption of p-nitrophenol

Hexadecylpyridine (HDPy) Rodriguez-Cruz et al. (2008) Sorption of fungicides (penconazole and metalaxyl); significance ofthe structure of long-chain organic cation

Hexadecyltrimethylammonium(HDTMA)

Li et al. (2003) Removal of anionic contaminants

Chen and Zhao (2009) Removal of congo red

Chang et al. (2009) Anion–cation modified, sorption of p-nitrophenol

Dihexadecyldimethylammonium(DHDDMA)

Rodriguez-Cruz et al. (2008) Sorption of fungicides (penconazole and metalaxyl); significance ofthe structure of long-chain organic cation

Chang et al. (2009) Anion–cation modified, sorption of p-nitrophenol

Octadecytrimethylammonium(ODTMA)

Sanchez-Martin et al. (2003, 2006,2008)

Sorption of pesticides, isotherms

Rodriguez-Cruz et al. (2007, 2008) Sorption and retention of pesticides

Huang et al. (2008) Selective adsorption of tannin from flavonoids

Neutral Surfactants

Triton X Shariatmadari et al. (1999) Estimates of the contribution of different adsorption sites

Sanchez-Martin et al. (2008) Studying the effect of clay structure on adsorption capacity

15 Crown ether 5 (15 C-5) Shariatmadari et al. (1999) Studying the contribution of different adsorption sites, modelling

Continued

Chapter

15

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361

TABLE 5 Modifications of Palygorskite with Surfactants.—Cont’d

Surface-treatment Agents References Remarks

Anionic Surfactants

Sodium dodecylsulfate (SDS) Sanchez-Martin et al. (2008) Influence of clay structure and surfactant nature

Cationic Dyes

Crystal violet (CV) Shariatmadari et al. (1999) Calculations estimate the contribution of different adsorption sitescapacity

Al-Futaisi et al. (2007) Adsorption isotherms, kinetics

Shariatmadari et al. (1999) Calculations estimate the contribution of different adsorption sitescapacity

Methylene blue (MB) Al-Futaisi et al. (2007), Shariatmadariet al. (1999)

Adsorption isotherms, kinetics

Anionic Dyes

Congo red Chen and Zhao (2009) Removal of anionic dye, adsorption kinetics, isothems

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formed by hydrophobic bonding between surfactant molecules [Lemic et al.

(2005) who studied the modification of sepiolite by octadecyltrimethylammo-

nium (applied as monomers or as micelle species), by dioctadecyldimethy-

lammonium and by hexadecyltrimethylammonium]. The large amount of

released magnesium ions, compared to the CEC of sepiolite, was attributed

to some dissolution of the minerals present. A kinetic study of the adsorption

process showed that the adsorption process reached equilibrium in less than

1 h. The thermal behaviour of the modified sepiolite provided evidence for

the multilayer adsorption. Similar behaviour was reported by Sabah and

Celik (2002a).

3.2. Anionic Surfactants

Adsorption isotherms of anionic surfactants exhibit three regions having dif-

ferent slopes. The first region is characterized by the complexation of the

anionic surfactants with Mg2þ ions at the octahedral sheet or hydrogen bond-

ing between the oxygen groups of anionic head groups of surfactant and Hþ

of the bound or zeolitic water. The second region is characterized by the

release of Mg2þ ions and their precipitation; the third region marks both the

beginning of a plateau and micellar dissolution of the precipitate. (Ozdemir

et al. (2007) studied the modification of sepiolite by sodium dodecylsulphate

(SDS) and sodium dodecylbenzenesulfonate (SDBS)).

3.3. Neutral Surfactants

Adsorption of neutral surfactants (Triton X-100) on sepiolite occurred both on

the surface and in the structural channels (del Hoyo et al., 2008 XRD, FTIR

and TA techniques). Changes observed in the wave numbers of the OH vibra-

tion modes of the clay mineral indicated interactions of surfactant with the sil-

icate through the functional groups of organic compound and the water

coordinated to the exchangeable cations of the clay minerals, by ion-dipole

or hydrogen bonding. Absorption bands corresponding to CH2 stretching

and bending modes of hydrocarbon chain groups of Triton X-100 exhibit

shifts to higher wave numbers. These displacements were related by del Hoyo

et al. (2008) to a reorganization of the organic molecules when interaction

with the adsorbent was established.

3.4. Pesticide Formulations

The efficiency of palygorskite and sepiolite, modified with a cationic surfactant

(hexadecyltrimethylammonium, ODTMA), in the adsorption of pesticides (pen-

conazole, linuron, alachlor, atrazine and metalaxyl), was compared to that of

kaolinite, montmorillonite, muscovite- or illite- modified by ODTMA. San-

chez-Martin et al. (2006) concluded that the efficiency of the ODTMA–clays


for adsorption of pesticides depends on the degree of saturation of adsorption of

the organic cation. Depending on the clay content soils saturated with ODTMA

provide natural barriers for decreasing the mobility of non-ionic pesticides,

depending on the degree of hydrophobicity of the pesticides. The significance

of the structure of long-chain organic cation in the sorption of pesticides was

also investigated by this group of researchers; they compared the efficiency in

sorption of the fungicides penconazole and metalaxyl by the above listed clays,

modified by ODTMA, hexadecylpyridinium (HDPy) or the two-chain cationic

surfactant dihexadecyldimethylammonium (DHDDMA; the bromide salt is also

designated in literature as DDAB; Rodriguez-Cruz et al., 2008). The results

obtained showed that the DHDDMA clays increased the sorption of the fungi-

cides, depending on the type of clay (i.e., higher for layered than for non-layered

clays), and on the fungicide hydrophobicity (higher for penconazole with

Kow¼3.72 than for metalaxyl with Kow¼1.75).

Modelling of adsorption was studied, for instance, by Rytwo et al. (1998),

Shariatmadari (1998), Shariatmadari et al. (1999), Sabah et al. (2002a,b),

Ozcan et al. (2007). This topic is treated intensively in the next part of the cur-

rent review.

4. MODEL EQUATIONS

The model presented here was developed by Rytwo et al. (1998) for sepiolite.

The same approach was described by Shariatmadari et al. (1999) for both

sepiolite and palygorskite. In developing the model for adsorption of cations

and neutral molecules to sepiolite the guiding information was that, it has

an open structure exhibiting a microfibrous morphology with a high specific

surface area (ca. 340 m2/g) and a large micropore volume (around 0.44 cm3/

g) due to the existence of intracrystalline cavities (tunnels).Sepiolite with Si12O30Mg8(OH,F)4(H2O)48H2O as unit cell formula (Brau-

ner and Preisinger, 1956; Santaren et al., 1990) is structurally formed by an

alternation of blocks and tunnels that grow up in the fibre direction (c axis).

Each structural block is constructed by two tetrahedral silica sheets enclosing

a central magnesia sheet similarly to other 2:1 silicates, such as talc, albeit in

sepiolite there are discontinuities of the silica sheets that give rise to structural

tunnels. Such arrangement determines that some silanol groups (Si��OH) are

present at the border of each block located at the ‘external surface’ of the sili-

cate (Alrichs et al., 1975). These Si��OH groups are usually accessible to

organic species, acting as neutral adsorption sites (designated as N). The N

sites content for the Vallecas–Vicalvaro (Spain) sepiolites can be estimated

at about of 0.60 mmol/g (Ruiz-Hitzky, 1974; Ruiz-Hitzky and Fripiat,

1976). Certain isomorphic substitutions in the tetrahedral sheet, such as

Al3þ instead of Si4þ, are responsible for the exchangeable cations that are

needed to compensate for the electrical charge and constitute the charged


adsorption sites (P sites). The cationic exchange capacity (CEC) for the con-

sidered sepiolites is in the range of 0.10–0.15 molc/kg.

The model employed (Rytwo et al., 1998) is an extension of that described

by Nir (1986) Margulies et al. (1988) and Rytwo et al. (1995), which accounts

for the ability of charged organic monovalent cations to adsorb to neutral

binding sites on the silicate layer. The rationale for such adsorption stems

from analysing results by Alvarez et al. (1987), where adsorption of the neu-

tral molecule Triton X-100 did not release exchangeable Mg2þ to the solution.

Consequently, it was deduced that TX-100 adsorbs to neutral sites denoted by

N.

Let Xiþ denote a monovalent cation that binds to a singly charged nega-

tive site, P�, on the surface of the silicate, creating the neutral complex PXi:

P� þ Xiþ , PXi ð1Þwith a binding coefficient, Ki, which satisfies:

Ki ¼ ½PXi�ð½P��½Xiþð0Þ�Þ

ð2Þ

in which [Xi(0)þ] is the concentration of the cation at the surface. The adsorp-

tion of another organic monovalent cation to the neutral complex creates a

charged complex P(Xi)2þ:

PXiþ Xiþ , PðXiÞþ2 ð3Þwith a binding coefficient

�Ki ¼ ½PðXiÞþ2 �ð½PXi�½Xiþð0Þ�Þ

ð4Þ

Such reaction was essential to explain adsorption at amounts higher than the

CEC of montmorillonite, or its charge reversal (Margulies et al., 1988).

When several organic monovalent cations interact with the clay, we can

also have the formation of mixed complexes, but this consideration was not

essential in this case.

The calculations employed the relation:

Xið0Þ ¼ XiY0Zi; ð5Þ

where Xi(0) is the molar concentration of cation i in its monomeric form close

to the mineral layer, Xi is the molar concentration in the equilibrium solution

(at infinite distance from the clay), Zi is the valence of the given ion, Y0 is aBoltzmann factor defined as Y0 ¼ e�

e f0kT in which the energy is given by the

product of the surface potential (’0) by the absolute magnitude of an elec-

tronic charge (e).For a negatively charged surface Y0>1, and the concentration of the cation at

the surface, Xi(0) may be significantly larger than Xi. When charge reversal


occurs, the surface potential is positive and Y0<1, reducing significantly

the concentration of non-adsorbed cations in the double layer region

below their equilibrium solution concentration. The excess concentration of

cation i in the double layer region above the equilibrium concentration is

calculated.

The intrinsic binding coefficients, in Equations (2) and (4), were deter-

mined for MB (Methylene Blue) and CV (Crystal Violet) in the case of mont-

morillonite (Rytwo et al., 1995) from adsorption data.

The extension of the model for sepiolite is required to consider the

reaction

Nþ Xiþ ¼ NXiþ ð6Þwith a binding coefficient, Kn,

Kn ¼ ½NXiþ�ð½N�½Xiþð0Þ �Þ

ð7Þ

Some organic cations (i.e. MB) can form dimers, trimers and even higher-

order aggregates in solution (Cenens and Schoonheydt, 1988; Spencer and

Sutter, 1979). Expressions for the general distribution of aggregates (Nir

et al., 1983) yield that the total concentration of primary molecules in solution

Xit, is given by

Xit½ � ¼ ½Xi�ð1� Kag½Xi�Þ2 ð8Þ

in which Kag is the corresponding coefficient (M�1) for aggregation in solu-

tion. Aggregation of dye molecules reduces the concentration, Xi, of dye

monomers. The adsorption of dimers or higher-order aggregates was ignored,

in order to reduce the number of parameters and was not needed for the sim-

ulation of the adsorption results. It may be noted that dye aggregation in solu-

tion can only have an influence when its total added amounts are above the

CEC of the clay, since below the CEC, essentially all the dye is adsorbed

(Rytwo et al., 1991, 1995; and results on sepiolite).The total site concentra-

tion, PT, equals the sum of concentrations of all sites, free and complexed.

In Equation (9) below, the sum on PX0 is the sum of concentrations of all neu-

tral complexes.

ssini

¼ P� �PPXþ

2 �PNXþ

P� þPPX0 þP

PXþ2

ð9Þ

The Gouy–Chapman equation yields:

s2 ¼ ekT2p

Xn1ðYZi

0 � 1Þ ¼ 1

G

XXiðYZi

0 � 1Þ ð10Þ


where e is the dielectric constant of the medium, n1 is the number of mole-

cules of each ion per unit volume in the equilibrium solution, and G depends

on T, the Avogadro number, and the system of units (Nir, 1984).

For the case of 1, 2, 3 and 4 valent cations, and mono and divalent anions,

the combination of Equations (9) and (10) gives a polynomial equation for Y0.The solution has been obtained by numerical procedures. In certain limiting

cases of no binding, where all ions have the same valency, analytical solutions

are also available.

The equations shown form a closed set. Thus, the mass balance of ion i

may yield the values Xi, if Y0 and P� are known. Equations (9) and (10)

may yield P�, if the different Xi, complexes and Y0 are known. By introdu-

cing the values of the binding coefficients, the equations can be solved itera-

tively by the following procedure:

1. An initial value for Y0 and P� is assumed.

2. The values of Xi are calculated by using Equations (1)–(7).

3. A new value of Y0 is obtained by using Equation (9).

4. A new value is obtained for P�, by subtracting from PT the bound sites.

5. Another iteration starts from stage 2.

The iteration steps may be continued until the desired degree of conver-

gence is reached. The determination of the binding coefficients that give the

best fit of the calculated adsorbed amounts to the experimental values has

been described in Nir et al. (1986), Hirsch et al. (1989), Rytwo et al. (1995,

1996b) and Rytwo et al. (1996a). Only one parameter, Kn was employed, fix-

ing the values of other parameters from the adsorption of CV and MB by

montmorillonite.

The concentration of neutral binding sites was determined by analysis of

the adsorption to sepiolite of two neutral molecules, TX100 (Triton X 100)

and 15C5 (15-Crown-5) according to the Langmuir equation.

5. RESULTS OF MODEL APPLICATION

Unless specified, we refer to results of Rytwo et al. (1998). In order to mini-

mize molecular aggregation in solution and at the same time reach large

adsorbed amounts relative to the CEC of the clay the concentration of the clay

was reduced to 0.2% (2g/l), which was still within the limits of sensitivity of

the measurements. Indeed, the largest adsorbed amount of TX100 was

0.315 molc/kg, that is, 25% more than the value in Alvarez et al. (1987).

The largest adsorbed amount of MB was 0.57 molc/kg clay which was 25%

more than in Aznar et al. (1992). The largest adsorbed amount of CV was

0.64 molc/kg, that is, 4.57-fold of the CEC of sepiolite.

The study of the adsorption of the neutral molecules to sepiolite was also

intended to provide an estimate for the amount of neutral sites, N, which

was required for calculating the adsorbed amounts of the organic cations


MB and CV. In calculations according to Langmuir equation, the experimen-

tal values were used to obtain the values of the binding constants, k, byassuming a wide range of N values. The value of N which gave the smallest

variation of k-values was chosen. Next, the value of k that gave the best fit

to all the experimental values was determined.

TX 100 adsorption yielded N¼3.4-fold of the CEC. In comparison, Shar-

iatmadari et al. (1999) deduced that N was about fourfold of the CEC of sepi-

olite but their assumed value of the CEC was about 0.786-fold of that in

Rytwo et al. (1998) which amounts to a similar estimate of N. In both cases,

the results imply that the simplest binding model, which assumes no coopera-

tivity in the binding of the neutral molecules to sepiolite, can adequately

explain the experimental data. The value of N for palygorskite was about two-

fold larger than the CEC of palygorskite, which was interpreted to reflect

smaller surface area in the latter case, that is, 222 versus 384 m2/g for sepio-

lite. Interestingly, the adsorbed amounts of MB and CV by sepiolite and paly-

gorskite showed a similar pattern; the adsorbed amounts to palygorskite were

about 10–15% smaller than to sepiolite. Shariatmadari et al. (1999) found that

no reduction of pH accompanied the adsorption of the neutral molecules, but

there was one unit decrease following MB or CV adsorption, indicating some

release of Hþ. In both cases, it could be deduced that the neutral adsorption

sites become more important as the dye adsorption approaches saturation. In

all cases, the model accounted well for the adsorbed amounts.

In both cases, FTIR results contributed to the interpretation of the experi-

mental adsorption studies and their modelling, and vice versa. The band repre-

senting the external neutral sites of sepiolite, OH vibrations of silanol groups

at 3716 cm�1 (Rytwo et al., 1998) or 3720 cm�1 (Shariatmadari et al., 1999)

decreased in intensity due to sorption of the neutral molecules as well as MB

and CV. The band at about 3680 cm�1 which corresponds to OH vibrations of

hydroxyl groups linked to Mg ions located in the interior of sepiolite blocks

remained unperturbed by the adsorption of MB or CV. Shariatmadari et al.

(1999) observed some perturbance of this band. In conclusion, the organic

cations mostly adsorbed to neutral sites in the outer surface, and to sites where

large structural defects occur.

Rytwo et al. (2002) determined experimentally adsorption of the divalent

organic cations paraquat (PQ), diquat (DQ) and methyl green (MG) on sepio-

lite and performed analysis with the adsorption model described above. The

largest amounts of DQ, PQ and MG adsorbed were between 100% and

140% of the CEC of sepiolite. Those amounts were considerably lower than

reported for the adsorption of monovalent organic cations. This outcome led

to the hypothesis that these divalent organic cations do not interact with the

neutral sites of sepiolite. This assumption was confirmed by infrared spectros-

copy (IR) measurements, which did not show influence in the peaks arising

from the vibrations of external Si��OH groups of the clay, when the divalent

organic cations were added, unlike changes which were clearly observed


when monovalent organic cations were added to the mineral. The model could

adequately simulate the adsorption of the divalent organic cations DQ and PQ,

without considering interaction of those ions with the neutral sites. The

model also yielded good fit for the results of competitive adsorption between

the monovalent dye MB and DQ. In competitive adsorption experiments, when

total cationic charges exceeded the CEC, monovalent organic cations were pref-

erentially adsorbed on the clay at the expense of the divalent cations. A similar

effect was observed in other clays, such as montmorillonite.

Interestingly, in an earlier study by Rytwo et al. (2000), it was shown that

the divalent organic cation, methyl green (MG) undergoes a slow transforma-

tion (6 h) to a monovalent cation, carbinol (MGOHþ) upon dilution of its

solution (10 mM), or upon addition of a buffer at neutral pH. Adsorption of

MG on sepiolite raised the possibility that a certain fraction of MG2þ trans-

formed into the monovalent form during the incubation period. The maximal

adsorbed amounts of MG2þ and MGOHþ were 0.09 and 0.30 mol/kg sepio-

lite, respectively.

In passing, we note that recent studies (Rytwo et al., 2009) indicate that at

least a small part of the large amount of CV adsorbed on sepiolite undergoes

partial degradation to Arnold base, whereas the products of the process remain

bound to the clay. Such a process was more extensively observed in Texas

vermiculite, but IR measurements indicate that it also occurs partly in

sepiolite.

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