Geoscience Reference
In-Depth Information
Table 5.4
Adsorption of heavy metals on goethite as a function of pH
Metal
pH
4.7
5.2
5.5
5.9
6.4
7.2
7.5
8.0
Cu 17 55 75 90
Pb 43 56 75
Zn 13 22 68
Cd 23 44 53
Co 39 54 78
Data expressed as percent of initial amount of metallic cation solution. Reprinted from Quirk and
Posner (
1975
). Copyright 1975 with permission of Elsevier
valence. This is caused by the differential sizes and polarizability of the cations, by
the structural properties of the adsorbent surfaces, and by the differences in the
surface charge distribution. For example, ammonium ions (NH
4
+
) are sorbed
preferably over anhydrous H
+
or Na
+
in 1:2 clay minerals, because they may form
NH-oxygen links in the hexagonal holes of Si-O sheets, and they may also link to
adjacent oxygen planes in the interlayer space by an OH-N-HO bond. Selectivity
of the divalent alkaline earth cations is less pronounced. Trivalent cations, such as
Al
3+
, coordinated octahedrally
to water
molecules,
link more strongly than
hydrated Ca
2+
ions.
Cation selectivity on organic matter is related mainly to the disposition of the
acidic groups in the adsorbent. Multivalent cations adsorb preferentially over
monovalent cations, and transition metals adsorb preferentially over strong basic
metals. Organo-mineral complexes exhibit a CEC smaller than the sum of the
CECs of the components. This phenomenon is reflected in the pattern of cation
selectivity (Greenland and Hayes
1981
). Two aspects should be considered in the
cation exchange process: the number of exchange sites occupied by the cation
investigated and the selectivity of the cation relative to the concentration of the
exchanging cation.
Heavy metal cations participate in exchange reactions with negatively charged
surfaces of clay minerals, with Coulombic and specific adsorption being the
processes involved in the exchange. Metal cationic adsorption is affected by pH.
At low pH values (\5.5), some heavy metals do not compete with alkali metals
(e.g., Ca
2+
) for the mineral adsorption site. At higher pH values, heavy metal
adsorption increases greatly; an example of heavy metal adsorption on goethite as
a function of pH is given in Table
5.4
.
Cationic organic contaminants often compete with mineral ions for the same
adsorption site. At low pH, organic cationic molecules are adsorbed more strongly
on earth materials than on mineral ions of a similar valence. At moderate pH
values, however, mineral ions are favored over organic cations. In general, the
charge density of the adsorbing surface is a determining factor in adsorption of
cationic organic molecules, but their adsorption also is affected by the molecular
configuration (Mortland
1970
). Organic molecules may be adsorbed by clays via a
