Environmental Engineering Reference
In-Depth Information
Soil mineral
Adsorbed
pollutant
Soil porewater
Soil mineral
Soil mineral
Soil organic matter
FIGURE 4.8
Partitioning of a solute between soil and porewater. Various niches are shown
where pollutants reside in the soil pore.
(Thiel, 1991). The strongly oriented water molecules at the surface have properties
different from that of bulk water. The near-surface water molecules are sometimes
referred to as
vicinal water
. Drost-Hansen (1965) has summarized the properties of
vicinal water.
Typically, a metal oxide such as SiO
2
is strongly hydrated and possesses Si-OH
(silanol) groups. The surface charge density in this case is dependent on the pH of
the adjacent porewater. Metal ions can enter into exchange reactions with the silanol
groups forming surface complexes (Stumm, 1993).
The adsorption of compounds on soil or sediment surfaces occurs through a variety
of mechanisms, each of which can be assumed to act independently. The overall
adsorption energy is given by
n
Δ
Δ
G
ads
=
G
n
,
(4.58)
where each
G
n
represents the contribution from the specific
n
th mechanism. These
mechanisms include (i) interactions with the electrical double layer, (ii) ion exchange
including protonation followed by ion exchange, (iii) coordination with surface metal
cations, (iv) ion-dipole interactions, (v) hydrogen bonding, and (vi) hydrophobic
interactions. The first two mechanisms are of relevance only for the adsorption of
ionizable species. Coordination with surface cations is important only for compounds
(e.g.,amines)thatareexcellentelectrondonorscomparedtowater.Ion-dipoleinterac-
tions between an uncharged molecule and a charged surface are negligible in aqueous
solution. Only compounds with greater hydrogen bonding potential than water can
havesignificanthydrogenbondingmechanisms.Mostneutralnonpolarcompoundsof
hydrophobic character will interact through the last mechanism, namely, hydrophobic
interactions.
Δ
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