Environmental Engineering Reference
In-Depth Information
as they are taken up by organisms. Lindsay ( 1979 ) presents a series of empirical
equilibrium constants and phase diagrams for the reaction:
Soil-metal + xH +
soil-H + metal x+
(16.1)
Soil-metal and soil-H indicate metal ions and protons sorbed to exchange sites
H + and metal x+ represent protons and metal ions in solution.
Besides, Lindsay ( 1979 ) gives partition co-efficients and solubility data for a
range of metals, that allow predictions of conditions under which specific metal
ions are likely to reside in the pore water or on exchange sites in soils.
In contrast to cationic contaminants, contaminants present as anions such as
arsenate and chromate tend to be more significantly sorbed at lower pH, thus becom-
ing more bioavailable (but also more mobile in the soil) towards more neutral pH
conditions.
pH influences bioavailability via the generation of exchange sites as well as exert-
ing controls via solubility and competition for exchange sites between protons and
other cations. The charge of mineral surfaces may be permanent or pH-dependent.
The former, i.e. the permanent charge of minerals, results from structural imper-
fections in mineral structures. The latter, i.e. the pH-dependent charge of minerals,
results from dissociation (leading to negative surface charge) or protonation (leading
to positive surface charge) of hydroxyls associated with the surface of clay miner-
als, oxyhydroxides or organic matter. The relative importance of permanent and
pH-dependent charge of minerals varies between metals. Generally, permanent
charge is most important for so-called 2:1 clays such as vermiculite, which com-
prise a basic repeating structure of Al-octahedral layers with a Si-tetrahedral layer
sandwiched in between. pH dependent charge is more important for 1:1 clays such
as kaolinite (i.e. a clay with alternating layers of Al-octahedral and Si-tetrahedral
sheets) and oxyhydroxides. The pH at which the surface charge switches from
positive to negative (the point of zero charge, PZC) varies with sorbent, being
typically 7-9 for Fe-oxyhydroxides, 8-9.2 for Al-oxyhydroxides, 1.5-4.6 for Mn-
oxyhydroxides and 5-6 for clay minerals. These PZC values mean that in “typical”
soils with a slightly acidic pH, Fe- and Al-oxyhydroxides are important for retaining
bioavailable anions in the soil and Mn oxyhydroxides for retaining cations.
Surfaces of organic matter can have a negative charge through the dissociation
of carboxylic acid and phenolic acid groups and thus can provide exchange sites for
cationic metals. However, metals and organic matter also interact to form chelate
complexes in which the metal is sorbed to the organic matter through more than
one bond so that a ring structure is formed. As an illustration of the formation of
chelate complexes, cadmium is complexed with one deprotonated carboxylate group
(Fig. 16.1a ), and copper is complexed with one carboxylate and one neighbouring
phenolate group (Fig. 16.1b ):
The resulting ring structures are very stable. There is an element of covalency
in the bonds and thus metals held in this way are not readily bioavailable. Thus,
increasing organic matter often leads to a decrease in the bioavailability of metals.
Different metal cations show different tendencies to form complexes with organic
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