Geoscience Reference
In-Depth Information
to have a limited number of strong Hg binding sites. Haitzer et al. (
2002
) suggest
that the binding of Hg to DOM under natural conditions (very low Hg-DOM
ratios) is controlled by a small functional fraction of DOM molecules containing a
thiol functional group. Therefore, Hg-DOM distribution coefficients used in
modeling biogeochemical behavior of Hg in natural systems must be determined at
low Hg-DOM ratios.
Contaminants bound to colloids also may lead to an increase in the apparent
solubility of the compounds. Most colloidal phases are effective sorbents of low-
solubility contaminants, due to their large surface area. For example, Fig.
8.21
depicts the solubilization of p-nitrophenol into hydrophobic microdomains, which
defines the trace metal level in the groundwater of a coastal watershed (Sanudo-
Wilhelmy et al.
2002
). The authors emphasize that the (heavy) metals contained in
the colloidal size fraction in some instances may reach more than 50 % of what is
considered ''dissolved'' metal; this should be considered to properly understand
the cycling of metals and carbon in the subsurface water.
Babiarz et al. (
2001
) examined total mercury (Hg) and methylmercury
(Me-Hg) concentrations in the colloidal phase of 15 freshwaters from the upper
Midwest and southern United States. On average, Hg and Me-Hg forms were
distributed evenly between the particulate (0.4 lm), colloidal, and dissolved
(10 kDa) phases. The amount of Hg in the colloidal phase decreased with
increasing specific electric conductance. Furthermore, experiments on freshwater
with artificially elevated electric conductance suggest that Hg and Me-Hg may
partition to different subfractions of colloidal material. The two colloidal Hg
phases act differently with the same type of adsorbent. For example, the colloidal
phase Hg correlates poorly with organic carbon (OC), but a strong correlation
between Me-Hg and OC was observed.
Once reaching a water system, the components of a crude oil or a petroleum
hydrocarbon are ''truly dissolved'' at a molecular level or ''apparently soluble'' at
a colloidal level when droplets characterized by radii of tens to hundreds of
microns are formed. The apparent solubility of PAHs from oil in an aquatic system
is reported by Sterling et al. (
2003
), who consider that the colloidal concentration
of a given hydrocarbon contaminant in aqueous phase, C, is described by the
equation
C ¼ /X
=
v
ð
8
:
6
Þ
where / denotes the volume fraction of oil emulsion in water (vol. emulsion/vol.
water), X is a (dimensionless) chemical mole fraction in the organic phase, and v is
average molar volume of component in the petroleum product.
Figure
8.22
highlights the fraction of naphthalenes present in water due to
colloidal entrainment. As the compound molecular weight increases, the relative
fraction of the compound in the colloidal phase increases. This occurs regardless
of the level of entrainment. Sterling et al. (
2003
) suggest that this behavior has
