Geology Reference
In-Depth Information
Table 1 Select physical-chemical property data for four relatively volatile
chemicals and two semi-volatile insecticides: p,p
0
-DDT and chlorpyr-
ifos (a currently used pesticide). Note that the chemicals are arranged
in order of volatility (Data sourced from Mackay
14
)
Henry's Law
constant (H)
(Pa m
3
mol
1
)
Vapour
pressure (P)
(Pa)
Molecular mass
(g mol
1
)
Aq. solubility
(S) (mol m
3
)
Chemical
Chloroform
119.4
23,080
68.7
336
n-hexane
86.2
20,200
0.11
18,364
Benzene
78.1
12,700
22.8
557
Phenol
94.1
70.6
871
0.081
Chlorpyrifos
350.6
0.00015
0.0011
0.14
p,p
0
-DDT
354.5
0.00002
0.0000087
2.3
Temperature is a key parameter for controlling H or K
aw
,thereby
strongly influencing air-water partitioning of chemicals in the environment.
A good deal of effort is expended in deducing temperature-dependent H for
contaminants of concern, which is not an easy task for low-polarity
hydrophobic organic chemicals (HOCs) with poor solubility.
Figure 3 shows a schematic diagram of the typical apparatus used to
measure H for these types of chemical. In essence the major component is
the reactor vessel, which contains a large volume of deionised water
(
B
0.5-10 L) spiked with a known concentration of the HOC in question.
The chemical is introduced by first dissolving the chemical in an organic
solvent which is miscible with water (e.g. acetone), and an aliquot added
to the water in the reactor vessel. Care must be taken that the concen-
tration does not exceed the aqueous solubility of the chemical and usually
the concentration in the vessel is only a few percent of the aqueous
solubility, resulting in a very low concentration,
B
1-2 mgL
1
.Clean,
water-saturated air (or N
2
) is then bubbled or sparged through the reactor
vessel and the exiting air passed through a vapour-trap to collect the
chemical present in the air stream. The chemical quantity on the vapour
trap can be turned into a time-averaged air concentration (C
a
), by
calculating the volume of air that has passed through the trap. H is
calculated by ratioing C
a
to the average water concentration in the reactor
vessel (C
w
) (see Equation 6.10) determined from analysis of water samples
taken at the start and end of the experiment. Each gas-stripping run will
be replicated to assess the precision, and new experiments conducted at
different temperatures to provide the regression parameters to derive
temperature-dependent values of H. Artefacts are always present, not
least keeping the reactor vessel at a uniform temperature, but also
preventing water from condensing in the vapour trap (particularly at
