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sphere of water and hence, cannot approach the surface closer than the
radius of the hydrated ion. Adsorption is favoured by ions having a high
charge density, i.e., trivalent ions in preference to univalent ones.
Additionally, an entropy effect promotes the physical adsorption of
polymeric species, such as Al and Fe oxides, because a large number of
water molecules and monomeric species is displaced.
Chemisorption or specific adsorption involves greater forces of at-
traction than physical adsorption. As hydrogen bonding or p-orbital
interactions are utilised, the adsorbed species lose their hydrated spheres
and can approach the surface as close as the ionic radius. Whereas
multilayer adsorption is possible in physical adsorption, chemisorption
is necessarily limited to monolayer coverage.
As outlined previously, hydrated oxide surfaces have sites that are
either negatively charged or readily deprotonated. The oxygen atoms
tend to be available for bond formation, a favourable process for
transition metals. Several mechanisms are possible. An incoming metal
ion, M z1 , may eliminate an H 1 ion as
X O H þ M z þ Ð X O M ð z - 1 Þþ þ H þ
Alternatively, two or more H 1 ions may be displaced, thereby forming a
chelate as
—X - O - H
—X - O
+M z +
M ( z −2)+ + 2H +
—X - O - H —X - O
A metal complex, ML n z1 , may be co-ordinated instead of a free ion by
displacement of one or more H 1 ions in a manner analogous to the
above reaction. In addition, the metal complex might eliminate a
hydroxide group giving rise to a metal-metal bond as
X O H þ ML z n Ð X ML ð z - 1 Þ n þ OH
It should be noted that all of these reactions are equilibria for which an
appropriate equilibrium constant can be defined and measured. This
data can then be incorporated into the speciation models discussed in
Section 4.2.5.
4.3.2.3 Ion Exchange Reactions (see also Chapter 5). Both mineral
particles and particulate organic material can take up cations from
solution and release an equivalent amount of another cation into
solution. This process is termed cation exchange and the cation
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