An Introduction to Environmental Chemistry

126 Chapter Four

Box 4.15 Use of clay catalysts in clean up of environmental contamination

The interaction between some organic

contaminants and mineral surfaces has

recently attracted attention as a way of

cleaning up contaminants in natural waters.

The large cation exchange capacity of

smectite clay minerals (Section 4.5.2), in

particular, has prompted research into their

use as a catalyst, i.e. a substance that alters

the rate of a chemical reaction without itself

changing. Clay catalysts have potential

applications as adsorbents to treat

contaminated natural waters or soils.

The compound 2,3,7,8-tetrachlorodibenzo-

p-dioxin is one of the most toxic priority

pollutants on the US Environmental

Protection Agency’s list. Dioxin compounds

act as nerve poisons and are extremely toxic.

There is no lower limit at which dioxins are

considered safe in natural environments.

The destruction of dioxins by biological,

chemical or thermal means is costly, not least

because their low (but highly significant)

concentrations are dispersed in large volumes

of other (benign) material. Thus large

volumes of material must be treated in dioxin

destruction processes.

It is desirable to concentrate soluble

contaminants like dioxin by adsorption on

to a solid before destruction. The optimal

solid adsorbent should be cheap, benign,

recyclable and easy to handle and have a

high affinity for—and be highly selective to

• the contaminant. Finely ground activated
  carbon and charcoal have been used as
  adsorbents but they suffer from oxidation
  during thermal destruction of the
  contaminant, making them non-recyclable
  and expensive.
  Smectite clay catalysts are potential
  alternative adsorbents, although some
  modifications of the natural mineral are
  necessary. Interlayer sites in smectite
  dehydrate at temperatures above 200°C,
  collapsing to an illitic structure. Since the ion-
  exchange capacity of smectite centres on the
  interlayer site, collapse must be prevented if
  clay catalysts are to be used in thermal
  treatments of chemical organic toxins. The
  intercalation of thermally stable cations,
  which act as molecular props or pillars, is one

T

T

O

T

T

O

T

T

2 : 1package O

Al 13 O 4 (OH) 28

pillars or props

3+

Fig. 1Schematic diagram showing props or pillars in

the interlayer position in smectite clay. T,

tetrahedral sheet; O, octahedral sheet.

method of keeping the interlayer sites open

in the absence of a solvent like water (Fig. 1).

Various pillaring agents can be used, for

example the polynuclear hydroxyaluminium

cation (Al 13 O 4 (OH)^328 +), which is stable above

500°C.

The pillar also increases the internal

surface area of the interlayer site, making it

more effective as an adsorbent. Moreover, by

introducing cationic props of different sizes

and spacings (spacing is determined by the

radius of the hydrated cation and the

charge), it is possible to vary the size of

spaces between props. It is thus possible to

manufacture highly specific molecular sieves,

which could be used to trap large ions or

molecules (e.g. organic contaminants) whilst

letting smaller, benign molecules pass

through.

Smectite clays do not have a strong

affinity for soluble organic contaminants and

this is improved using a surface-active agent

(surfactant). A surfactant is a substance

introduced into a liquid to affect (usually to

increase) its spreading or wetting properties

(i.e. those properties controlled by surface

tension). Detergents and soaps are examples

of surfactants. A soap molecule has two

features essential for its cleansing action: a

long, non-polar hydrocarbon chain and a

(continued)