Applications of Equilibrium Thermodynamics - Elements of Environmental Engineering: Thermodynamics and Kinetics - page 159

4.3.3 A DSORPTION OF O RGANIC M OLECULES ON S OILS AND S EDIMENTS
As is evident from Table 4.10, a typical sediment or soil will have an overwhelm-
ing fraction of sand, silt, or clay and only a small fraction of organic material. The
total organic fraction is characterized mainly by humic and fulvic acids. They exist
bound to the mineral matter through Coulombic, ligand exchange, and hydrophobic
interactions. Some of the species are weakly bound and some are strongly bound.
The weakly bound fraction enters into dynamic exchange equilibrium between the
solid (mineral) surfaces and the adjacent solution phase. In some cases, this relation-
ship can be characterized by a Langmuir-type isotherm (Thoma, 1992; Murphy et
al., 1990). The adsorbed concentration rapidly reaches a maximum. The differences
in the maxima for different surfaces are striking. The sorption of humic substances,
for example, will depend not only on the site concentrations but also on its relative
affinity with iron and aluminol hydroxyl groups. It is known that adsorption of DOCs
on hydrous alumina is a pH-dependent process with maximum adsorption between
pH 5 and 6, and decreasing adsorption at higher pH (Davis and Gloor, 1981). As a
consequence of this change, alumina coagulates more rapidly at pH values close to
the adsorption maximum. It is also known that increasing molecular weight of a DOC
leads to increased adsorption on alumina. In other words, adsorption of DOCs on
alumina is a fractionation process.An adsorbed organic film such as DOCs (humic or
fulvic compounds) can alter the properties of the underlying solid and can present a
surface conducive for chemical adsorption of other organic compounds (Figure 4.10).
Asdiscussedintheprevioussection,mineralmatterhasalargepropensityforwater
molecules because of their polar character. Nonpolar organic molecules (e.g., alkanes,
polychlorinated biphenyls, pesticides, and aromatic hydrocarbons) have to displace
the existing water molecules before adsorption can occur on mineral surfaces. For
example, on a Ru(001) metal surface, Thiel (1991) found that the adsorption of water
from the gas phase involved a bond strength of 46 kJ/mol for water and 37 kJ/mol
for cyclohexane. The adsorption bond strengths are comparable. However, the area
occupied by one cyclohexane molecule is equivalent to about seven water molecules.

(b)

16

(a)

100
90
80
70
60
50

15.8

OC on EPA 5
Sediment

HA on
haematite
HA on
Kaolinite

15.6

15.4

40
30
20
10
0

15.2

15

0 10 20 30 40

Dissolved organic carbon (mg/L)

50 60 70 80

0 0.5 1

HA in solution (umol/mL)

1.5 2 2.5

FIGURE 4.10 (a) Sorption of humic acid on hematite and kaolinite; (b) sorption of organic
carbon on EPA5 sediment.