Applications of Equilibrium Thermodynamics - Elements of Environmental Engineering: Thermodynamics and Kinetics - page 158

TABLE 4.11
Surface Properties of Important Oxide Minerals Present in the
Environment

p K intr
s1

p K intr
s2

Surface Composition Surface Area pH pzc

− 3

Quartz SiO 2 0.14 7 2

Geothite FeO(OH) 46 6 9 7.5

Alumina Al 2 O 3 15 7 10 8.5

Iron oxide Fe(OH) 3 600 7 9 8

Gibbsite Al(OH) 3 120 5 8 6.5

Na-montmorillonite Na 3 Al 7 Si 11 O 30 (OH) 6 600-800 — — 2.5

Kaolinite Al 2 Si 2 O 5 (OH) 4 7-30 — — 4.6

Illite KAl 3 Si 3 O 10 (OH) 2 65-100 — — —

Vermiculite 600-800 — — —

Muscovite 60-100 — — —

Source: From Schwarzenbach, R.P. et al. 1993. Environmental Organic Chemistry . New York:
Wiley; Sparks, D. 1999. Environmental Soil Chemistry . NewYork: Academic Press.

φ sorb is the ratio of moles of metal adsorbed to the fixed initial moles added, and
φ soln =

− φ sorb is the fraction of metal unadsorbed. A plot of ln D versus pH is
calledthe Kurbatovplot .Thevaluesof a and b havephysicalsignificance.Ifhalfofthe
added metal is adsorbed, then D

1

= 1 and pH at this point is designated pH 50 =− a/b .

Then we have

d

φ sorb
dpH

b
4

pH 50 =

(4.69)

or

ln D = b · ( pH − pH 50 ) . (4.70)

Hence,

1

exp −

· pH

pH 50

φ sorb = . (4.71)

+ −

1 b

Thus, the evaluation of a and b from the Kurbatov plot gives us the fraction of
metal adsorbed at any given pH.
There are other aspects that should also be considered in understanding the
exchange of metal ions with clay minerals, since with surface exchange reactions, the
forces of bonding within the clay structure will also physically distort their size and
shape. In other words, extensive changes in the activity coefficients of the adsorbed
ionsarepossible.Expansionorcontractionofclayswillcomplicatethesimpleactivity
coefficient relationships. Such general relationships are, heretofore, lacking. General
trends have been noted and verified, and hence qualitative relationships are possible.
The simple models described above should serve to exemplify the general approach
to the study of exchangeable metal ions on soils and sediments.