Bergaya F. Handbook of Clay Science

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References 621

Handbook of Clay Science

Edited by F. Bergaya, B.K.G. Theng and G. Lagaly

Developments in Clay Science, Vol. 1

623

Chapter 11

CLAYS, ENVIRONMENT AND HEALTH

F. BERGAYA

, B.K.G. THENG

AND G. LAGALY

CRMD, CNRS-Universite

d’Orle

ans, F-45071 Orle

ans Cedex 2, France

Landcare Research, Palmerst on North, New Zealand

Institut fu

r Anorganische Chemie, Universita

t Kiel, D-24118 Kiel, Germany

Because of their large propensity for adsorbing and immobilizing extraneous

species, clays and clay minerals can serve as materials for pollution control (Chapter

11.1), carriers of pesticides (Chapter 11.2), liners in waste disposal (Chapter 11.3)

and barriers in nuclear waste management (Chapter 11.4). The peculiar surface

properties of clays, such as the large speciﬁc surface area, layer charge, and swelling

capacity also make these materials generally beneﬁcial to human health. The ability

of clays and clay minerals to serve as active principles, or excipients, in pharma-

ceutical formulations, spas, and aesthetic medicine is described in Chapter 11.5.

Being good adsorbents and mucostabilizers, clay minerals are also useful as drugs in

the treatment of intestinal disorders (Chapter 11.6).

DOI: 10.1016/S1572-4352(05)01019-6

Handbook of Clay Science

Edited by F. Bergaya, B.K.G. Theng and G. Lagaly

Developments in Clay Science, Vol. 1

625

Chapter 11.1

CLAYS AND CLAY MINERALS FOR POLLUTION CONTROL

G.J. CHURCHMAN

, W.P. GATES

, B.K.G. THENG

AND

G. YUAN

School of Earth and Environmental Sciences, University of Adelaide, Glen Osmond ,

S.A. 5064, Australia

Centre for Green Chemistry, School of Chemistry, Monash University, Clayton, VIC

6800, Australia

Landcare Research, Palmerston North, New Zealand

A widespread recognition of the need to develop technologies for pol lution con-

trol has arisen in only relatively recent times. This has been a result of increased

awareness of the effects of pollution on the health and longevity of both human

beings and the earth’s fragile ecosystems. Therefore it is not surprising to ﬁnd that

clays were not used for environmental protect ion to any great extent until quite

recently. Thus, such an authoritative volume on the practical applications of clays as

Grim’s Applied Clay Mineralogy (Grim, 1962) included only three uses that could be

called ‘environment al’ among the 34 miscellaneous uses that he identiﬁed, and none

among the 5 major uses that were enumerated. The uses of clays with arguably

environmental aspects that Grim (1962) identiﬁed over four decades ago were those

for radioac tive waste disposal, ﬂoor adsorbents, and water clariﬁcation. The major

uses that were identiﬁed for clays at that time were for ceramics, foundry-moulding

sands, engineering properties, petroleum discovery and recovery, and the reﬁning

and preparation of organic materials.

Because of the ubiquitous and widespread occurrence of clays in soils and se-

diments, it is not surprising that clays were long used for the control of toxic ma-

terials albeit at a small scale or only locally. For example, ‘Lemnian earth’, known

only by its location, was used medicinally in ancient Greece and Turkey to counter

the effects of snake-bites and poisons, and to cure festering wounds (Robertson,

1986; see also Chapter 11.5). Likewise, Sudanese villagers along the Nile long used a

local clay—recently identiﬁed as a bentonite—to purify the river’s turbid water. As a

consequence, these people suffer fewer gastrointestinal disturbances than other

DOI: 10.1016/S1572-4352(05)01020-2

communities nearby. Experiments showed that viruses (Lund and Nissen, 1986),

parasites (Olsen, 1987) and many types of bacteria (Madsen and Schlundt, 1989) are

removed from the water through addition of the clay, probably acting as both an

adsorbent and a ﬂocculant for the disease-bearing organisms.

The undoubted usefulness of clays for pollution control and environmental pro-

tection arises from many of the same characteristics that made them so useful in-

dustrially and otherwise in human societies for many centuries (e.g., Mackenzie,

1979; Robertson, 1986). Table 11.1.1 gives some of the particular or proposed ap-

plications of clays for pollution control and environmental protection, delineates

properties of clays that make them useful for these applications , and outlines the

requirements for pre-treatment that enhance their effectiveness in each case. Table

11.1.1 also refers to other sections of this chapter where further details of each

application are given. Nonetheless, it is important to realise that categorisation,

whether within sections of this chapter or into other parts of Chapter 11 is nec-

essarily arbitrary. This is because those properties of clays that are important for

pollution control per se, such as uptake capacity, also make clays most suitable for

speciﬁc applications. These include their use for pesticide application and control of

excess pesticides (see Chapter 11.2), in liners for waste disposal (see Chapter 11.3)

and nuclear waste management (see Chapter 11.4) and also in their application for

health (see Chapter 11.5) and to aid the delivery of drugs (see Chapter 11.6). This

chapter will focus on the use of natural, mined clays, largely to the exclusion of

‘synthetic’ clay minerals (see Chapter 4).

Table 11.1.1 and the related detai led discussions specify certain technical char-

acteristics of clays such as their charge, uptake abilities, high-surface areas, colloidal

or swelling capacities, among others. Clays are also attractive in comparison with

other environmental materials because they are widespread, generally easil y mined,

and relatively inexpensive. Clays are also naturally oc curring materials, generally

non-toxic, and have a consider able capacity to adsorb water. Hence, aside from their

use for controlling contaminants, they are also useful as adsorbents for water in such

modern, and generally urban, applications as in pet litter, although their function in

that case may extend beyond that of the simple uptake of water.

11.1.1. CONTROL OF HEAVY METAL CATIONS AND SIMPLE CATIONS

As cation exchangers, clays are effective for the control of cations in solution,

although, where onl y ion exchange is involved, their effectiveness is dictated by the

imperative of attaining equilibrium between ions in solution and those on exchange

sites. However, clays are most useful for the control of cationic pollutants when the

mechanism of uptake extends beyond that of just cation exchange. This is partic-

ularly the case for heavy metal ions, on which the following discussion is focussed.

The control of heavy metal ions in anionic form is related to that for anions in

general (see Section 11.1.4).

Chapter 11.1: Clays and Clay Minerals for Pollution Control626

A. Practical Applications

As liners in waste repositories clays are used for the control of heavy metal ions, often

among other pollutants. Otherwise, clay minerals in soils, along with metal hydr(ox-

ides) and organic matter, control the concentrations of heavy metal ions in surround-

ing and leaching solutions. The relative contribution of each of these soil components

to heavy metal ion uptake can vary with the particular heavy metal ion (Elliott et al.,

1986; Lumsdon et al., 1995; Tiller, 1996), and solution pH (Metwally et al., 1993;

Table 11.1.1. Applications of clays for pollution control and environmental protection

Contaminants

for control

Status (actual or

potential use)

Pretreatments

required

Relevant clay

properties

Further description

Heavy metal

cations and

simple cations

Actual, mainly

passive, use (e.g., in

soils, liners)

Mostly none,

some organic

and inorganic

modiﬁcation

Charge,

surface area,

reactive

surface groups

11.1.1

Organic and

biological

cations

Potential for water

and wastewater

treatment, pesticide

control

Generally

none, except

cation

saturation

Charge,

surface area,

especially

interlayer

11.1.2, also 11.2

Non-ionic

organic

molecules

Actual, for water

and wastewater

treatment;

potential, for

pesticide control,

waste liners

Cation

saturation,

organic or

inorganic

modiﬁcation

Charge 11.1.3, also 11.2

and 11.3

Anions Actual, for water

and wastewater

treatment;

potential, for

pesticide and

nutrient leaching

control

Appropriate

organic

modiﬁcation

Charge 11.1.4, also 11.2

Turbidity and

residual

treatment

chemicals

Actual, for

treatment of

potable water and

some wastewaters

and sewage

Generally

none

Colloidal,

from size and

charge;

charge,

surface area

11.1.5

Leachates Actual, for waste

liners and

radioactive waste

storage

Generally

none, except

cation

saturation

Swelling,

charge,

surface area,

reactive

surface groups

11.5 and 11.6

11.1.1. Control of Heavy Metal Cations and Simple Cations 627

Lumsdon et al., 1995), among other environmental factors. For example, organic

materials removed more Cu

from solution than pedogenic oxides, and each of these

removed more than a montmorillonite (McLaren et al., 1981) from concentrations

similar to those in soil solution. By contrast, McLaren et al. (1986) found that a

pedogenic oxide was much more effective than organic materials in removing Co

from solution, with a montmorillonite being the least effective. Furthermore, com-

binations of soil components, notably clay–organic matter complexes, can strongly

inﬂuence heavy metal ion uptake (Lumsdon et al., 1995; Petrovic

et al., 1999). Sim-

ilarly, heavy metal ion adsorption by clay minerals in soil may occur via hydr(oxides)

and/or organic matter coating their surfaces (Swift and McLaren, 1991; Jackson,

1998).

B. Mechanisms of Heavy Metal Ion Uptake

In summarising the uptake of trace and heavy metal ions by clay minerals, Tiller

(1996) has concluded that these minerals have a stronger afﬁnity for heavy metal ions

than for alkali and alkaline earth cations. Adsorption of heavy metal ions by clays is

a complex process, reﬂecting their strong tendency to form covalent bonds (Jackson,

1998). The extent of uptake is not simply a function of the cation exchange capacity

(CEC) of the clay minerals. This is because heavy metal ion uptake involves a variety

of processes, including surface complexation—which can be either direct (‘inner-

sphere’) or ind irect (‘outer-sphere’), simple ion exchange, and surface precipitation

(Swift and McLaren, 1991; Scheidegger and Sparks, 1996; Stumm and Morgan,

1996; Jackson, 1998). Some common types of complexes at the mineral/solution

interface are illustrated in Fig. 11.1.1.

Surface comp lexation, constituting speciﬁc adsorption, occurs on edge sites. It

involves the formation of direct bonds between the metal cations, and surface OH

groups and O atoms, that are intermediate in strength between ionic and covalent

bonds (McBride, 1991; Swift and McLaren, 1991; Jackson, 1998; Wu et al., 1999).

The process is not completely reversible, although this may reﬂect kinetic effects

rather than true irreversibility (McBride, 1991; Scheidegger and Sparks, 1996).

Adsorption of cationic heavy metal ions can occur at different sites on the alu-

minosilicate structure (Inskeep and Baham, 1983), and the site involved may vary

with each particular heavy metal. Using models for adsorption–desorption of copper

and cadmium by montmorillonite, Undabeytia et al. (1998, 2002) found that these

metal ions can be adsorbed on both edge and interlayer sites. For each of the metal

ions the preferred site may depend, in different ways and to different extents, on such

factors as ionic strength, pH, and the anions that are present in solution. Cadmium is

mostly adsorbed as the uncomplexed cation on planar (interlayer) sites over a wide

range of concentrations, and also in potentially comp lexing chloride solutions

(Undabeytia et al., 1998). On the other hand, adsorption of copper is affected by pH

and by the presence of Cl



, when CuCl

could form (Undabeytia et al., 2002).

Unlike that of Cd

, desorption of Cu

shows hysteresis. This would indicate that

Chapter 11.1: Clays and Clay Minerals for Pollution Control628

adsorption occurs on both edge and planar sites, the former being relatively irre-

versible while the latter is reversible.

Metal ion hydroxide phases may form as precipitates on mineral surfaces espe-

cially when the concentrations of the heavy metal ion are higher than tho se found in

most natural waters, and the pH is close to neutral or alkaline (Jackson, 1998). As

Tiller (1996) has pointed out, however, it is difﬁcult to explain the results of

solid–solution interactions in terms of precipitation, rather than simple ion ex-

change, adsorption of the hydroxo-ion, hydrolysis of the simple ion in solution

concomitant with its adsorption, or surface complexation. According to McBride

(1991), many so-called adsorption isotherms combine the effects of chemisorption

and precipitat ion with those of nucleation of metal hydroxides at surfaces, while

Scheidegger and Sparks (1996) suggested that there is a continuum between surface

complexation (understood as adsorption) and surface precipitation. The application

of high-resolution electron microscopy (HRT EM) and X-ray absorption ﬁne struc-

ture spectroscopy (EXAFS) has led Scheidegger et al. (1996) to conclude that Ni

Fig. 11.1.1. The location of inner- and outer-sphere complexes and a diffuse ion relative to an

aluminosilicate layer. From Sposito (1992).

11.1.1. Control of Heavy Metal Cations and Simple Cations 629

preferentially forms a mixed hydroxo-Ni precipitate at the edge surface of pyrophyl-

lite particles, even at low Ni concentrations.

The ambient pH often has a strong inﬂuence on the extent of adsorption because

it affects the nature of the heavy metal cation (Jackson, 1 998; Garcia Sanchez et al.,

1999). Genera lly, high pH favours adsorption through the production of hydroxy

cations (Jackson, 1998). For example, lead is only retained strongly by clays as long

as the pH is high enough to ensure that precipitation of lead occurs (Yong et al.,

1990) although in the case of mercury, an increase in pH causes uptake to decrease

(Farrah and Pickering, 1978). High pHs can also result in greater speciﬁcity of clay

minerals for heavy metal ions relative to alkaline earth ions, probably because of the

much greater tendency of heavy metal ions to hydrolyse (McBride, 1991).

In the last several years, synchrotron-b ased studies aided our understanding of

surface complexation and precipitation reactions (see Chapter 12.3). In particular,

they shed light on the mechanisms by which surface-induced precipitation can result

in the formation of more phases, including oxides, layered double hydroxides

(LDHs), and new phyllosilicates (see Chapter 12.3).

C. Relative Afﬁnities of Clays for Different Heavy Metal Ions

The preference of bentonites for heavy metal ions that adsorb by cation exchange

decreases in the order Cu

>Pb

>Zn

>Cd

>Mn

(Lagaly, 1995). From a

survey of the literature prior to 1961, Tiller (1996) has deduced that the relative

bonding energies to several minerals of some common heavy metals follows the

sequence: Cu

>Zn

>Co

>Ni

ﬃMn

which approximates the hydrolysis

constants of the metal cations. This suggests the important role played by hydroxo-

cations in adsorption. However, Tiller (1996) has also observed that the order is not

invariant in that Co

and Ni

could change places. In agreement with McBride

(1991), Tiller (1996) concluded that the mechanism of uptake was far from certain.

Jackson (1998) has summarised the literature on the relative afﬁnities of different

clay minerals for the same divalent cations, and given the following sequen ces:

montmorillonite Ca

2þ

4Pb

2þ

4Cu

2þ

4Mg

2þ

4Cd

2þ

4Zn

2þ

illite Pb

2þ

4Cu

2þ

4Zn

2þ

4Ca

2þ

4Cd

2þ

4Mg

2þ

kaolinite Pb

2þ

4Ca

2þ

4Cu

2þ

4Mg

2þ

4Zn

2þ

4Cd

2þ

As already noted, the adsorption capacity of different clay minerals for heavy

metal ions does not necessarily follow that of their CEC. Furthermore, their relative

afﬁnities can vary with solution concentration of heavy metal ion. For example, illite

is more effective than montmorillonite at high concentrations of Cu

, but the order

reverses at low concentrations (Swift and McLaren, 1991). For a single montmo-

rillonite, selectivities may also vary with saturating cation. According to Lagaly

(1995), the selectivity of montmorillonite for cations generally depends on the

Chapter 11.1: Clays and Clay Minerals for Pollution Control630