- Volatile hydrophobic compounds (HOCs) brought into soil and sediment
systems represent a serious threat to the environment. Dissolved
organic carbon (DOC) may exert an important influence over the total
aqueous solubility and mobility of organic pollutants through their
incorporation into micelles or the formation of soluble complexes.
To date, however, it is not clear whether DOC interacts with nonionic
volatile HOCs under natural conditions. Also, very little is known
about the influence of DOC-type and other environmental parameters.
A static headspace analysis technique was used in combination with
gas chromatography to determine the extent of interactions between
DOC and benzene, toluene, and chlorobenzene. Three different types
of DOC were compared: a natural water-soluble extract (WSE) obtained
from a Cumulic Humaquept (Labish Series), a commercial humic acid
(HA), and a high purity fulvic acid (FA). In factorial experiments,
the effect of type and concentration of DOC, solution pH, preparative
treatment of DOC solutions, and hydrophobicity of HOCs was examined.
DOC concentrations in the experiments ranged from 1.3 to 36.5
mmol-L⁻¹, the ionic background was made up by 0.10 M KC1, and the temperature was held constant at 25 °C. The solution pH in different
experiments was 4.0, 6.5, and 9.0. The acidity of classes of DOC
functional groups, the influence of solution pH on the degree of
DOC-protonation, and the maximum number of protons complexed per mol
DOC were determined for WSE, HA, and FA through continuous
potentiometric titrations. Basic solutions 9.36 mM in DOC were
titrated with HC1 under N₂ in a 0.1 M KC1 ionic background at 25 °C.
At DOC concentrations ≥ 9.3 mmol-L⁻¹, a small percentage of HOC
molecules associated with DOC. However, no statistically significant
linear relation between DOC concentration and DOC-HOC interaction was
observed. Differences in the association of benzene, toluene, and
chlorobenzene with DOC were inconsistent with different DOC-sources.
Toluene exhibited a higher affinity for unfiltered and aged WSE- and
HA-solutions when compared with freshly prepared, filtered solutions
of the same materials. Most HOC molecules associated with DOC at pH
6.5. HA and FA showed a significantly greater affinity for HOCs than
WSE. This reflects the acidic functional group properties of the
WSE displayed the greatest capacity to complex protons,
although the FA and HA functional groups were more acidic. WSE, FA,
and HA complexed a maximum number of 0.196, 0.158, and 0.136 mol
H-mol⁻¹ DOC, respectively. Formation functions of WSE, HA, and FA
calculated from titration data were fitted to a chemical model
through a non-linear least-squares minimization program in order to
obtain conditional protonation constants (cK) for classes of
functional groups. Log cK values were 5.52 and 9.12 for WSE, 4.87 and 8.77 for FA, and 4.75, 7.62, and 9.39 for HA. These values were
in agreement with protonation constants obtained through a graphical
procedure. A comparison of the data obtained for WSE, HA, and FA
from association and titration experiments shows that the capacity to
complex protons and the affinity for HOCs are inversely related.
The relatively high functional group content of WSE corresponds to a
relatively low hydrophobicity, as expressed in the small extent of
WSE-HOC interactions when compared with HA and FA.