17
also useful for the consideration of biological sorption.
He
describes three processes by which the materials from a dis-~
persion medium may be transferred to a mineral surface:
({1)
preformed cation-anion complexes or groups or parts of such
groups may be fixed to the mineral surface by the formation
of cation-anion bonds between the complex and the mineral surface;
(2)
ions may be transferred from ionic complexes in the
dispersion medium to the mineral surface as in simple base
exchange; and
(3) condensed films or ionic groups may be fixed
to the surface by Van der Waal's forces.
Except for
(3),
the
formation of chemical bonds accompanies the fixation of material,
suggesting the term chemisorption.
Absorption is defined as
chemisorption in which the added material becomes a regular part
of the mineral structure; adsorption occurs when the fixed
material is not accommodated into the mineral structure, but
occupies sites on growth surfaces,
imperfections,
dislocations,
and various interfaces of the mineral surface as surface films
or groups between adjacent structures.
In general,
trace ele-
ments which accumulate on mineral surfaces do not regularly
occupy lattice sites,
faces.
but are adsorbed to assorted crystal inter-
A trace element cation will occupy a lattice site only if
the cation has sufficient chemical similarity to one of the
mineral constituents that anions coordinated with it may be