Page 239 - Hydrogeology Principles and Practice

HYDC06 12/5/05 5:33 PM Page 222

222 Chapter Six

−3
with a solid density of 2.65 g cm , an organic carbon of mixtures of organic compounds where co-solvents
−4
content of 1.5 × 10 kg kg −1 ( f = 0.00015) and alter the solubility characteristics of individual com-
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porosity of 0.35, that is contaminated with the aro- pounds and compete in sorption-desorption reactions.
matic amine 2,4,6-trichloroaniline. The log K value Overall in the above approach, it is assumed that
10 OW
for this compound found from laboratory experi- the higher the hydrophobicity of a contaminant,
mentation is equal to 3.7 (Worch et al. 2002) and the greater the adsorption behaviour. In advancing
this enables an estimate of the retardation factor as models to describe the sorption of hydrophobic organic
follows. chemicals to heterogeneous carbonaceous matter
From Table 6.4, the adsorption behaviour of in soils, sediments and rocks, Allen-King et al. (2002)
aromatic amines is described by the equation: considered more specifically the relative importance
of partitioning and adsorption of hydrophobic con-
log K = 0.42 log K + 1.49 eq. 6.23 taminants in the environment. Partitioning is likely
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to control contaminant solid-aqueous behaviour
By substituting log K = 3.7 in equation 6.23, the when the solid phase contains a substantial propor-
10 OW
resulting value for log K is: tion of ‘soft’ humic substances, such as in modern
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soils and sediments high in organic carbon content,
log K = (0.42 × 3.7) + 1.49 = 3.04 and/or concentrations of any adsorbing solutes are
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sufficiently high to effectively saturate all the adsor-
and: bent sites present. Under these circumstances, K
d
values estimated from K , K and total f values
OW OC OC
K = 1107 (eq. 6.22) are likely to produce a reasonable estimate
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of K , usually accurate to within a factor of 3. Such
d
From equation 6.22, the value of the partition coef- estimates can be successfully applied in circum-
ficient K is: stances such as the attenuation of hydrophobic pestic-
d
ides in agricultural soils and for chlorinated solvents
K = 1107 × 0.00015 = 0.17 mL g −1 near a source containing non-aqueous phase liquid.
d
Deeper in the subsurface environment, other types
and using equation 6.13, the retardation factor, R is: of organic matter with high adsorption capacities
d
are present, for example, thermally altered ‘hard’
( 0 35 2 carbonaceous material (pieces of coal, soot, char or
− . ) .65
1
R =+ .17 = .184

1
0
d
. 035 kerogen), and under these circumstances it is likely
that adsorption will contribute significantly to the
With a retardation factor of less than 2, the retarda- total sorption over a broad concentration range of
tion of 2,4,6-trichloroaniline by adsorption in the sand contaminants. Therefore, hydrophobic sorption of
aquifer is low and so it is likely that this type of aro- organic compounds in soils, sediments and rocks
matic compound, which is poorly biodegradable, will occurs as a combination of phase partitioning and
persist as a groundwater contaminant. surface adsorption, with the former typically more
The hydrophobic sorption model is recommended linearly dependent on aqueous concentration (Allen-
where the solubility of the organic compound is less King et al. 2002).
−3
than about 10 M. More soluble compounds, such as In the process of remediating a contaminated
methanol, show much less affinity for organic carbon aquifer, it should be noted that desorption involving
and preferentially partition to the aqueous phase. the partitioning of an organic compound back into
Also, if the fraction of organic carbon, f , is small, it the aqueous phase yields a different isotherm to the
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is possible that organic compounds will show a tend- sorption process. This so-called sorption ‘hysteresis’
ency to sorb to a small but significant extent on to is often attributed to slow processes of diffusion within
inorganic surfaces, particularly where a clay fraction sediment particles and soil aggregates. Usually, de-
presents a large surface area to the contaminant. A sorption exhibits a greater affinity for partitioning to
further limitation is the possible effect of the presence the solid phase than is the case during sorption.