Environmental Engineering Reference
In-Depth Information
Isolation steps included solvent-solvent and solid-phase
extraction. The other major approach to study Hg(II)-
DOM interactions involves studies of DOM fractions iso-
lated from a bulk DOM or OM pool. The isolation process
tends to favor separation of DOM into hydrophilic and
hydrophobic fractions that are then assessed for their abil-
ity to bind with Hg(II) by a variety of approaches. Haitzer
et al. (2003) separated hydrophobic organic acid fractions
from surface water using AMBERLITE™ XAD™-8 resin
and then used equilibrium dialysis to determine distribu-
tion coeffi cients for Hg-DOM interactions. Gaspar et al.
(2007) used several XAD resin techniques to isolate DOM
of varying hydrophobicities and then applied a modifi ed
version of the CLE-SPE method of Hsu and Sedlak (2003) to
determine equilibrium reactions between free ionic Hg
2
and the DOM isolates. Lamborg et al. (2003) developed an
Hg speciation technique based on the operational “reac-
tive Hg” assay. The authors describe the method as a tech-
nique for determining the labile fraction of Hg, which they
assume represents only complexes of Hg that have stability
constants that are too low to protect the Hg(II) from wet
chemical reduction.
Electrochemical techniques have also been described
to determine the concentration and speciation of Hg in
aqueous systems, but the sensitivity of these methods is
currently limited and hence cannot be used to determine
binding constants or concentrations at levels that other
techniques currently can achieve (Turyan and Mandler,
1994; Wu et al., 1997).
Phase-Speciation Methods
Natural waters contain a variety of colloidal particles,
including organic biopolymers and inorganic nanopar-
ticles that readily interact with many trace elements to
remove them from the solution phase (Honeyman and
Santschi, 1988; Buffl e and Leppard, 1995; Santaschi et
al., 2002). One of the most common techniques to isolate
colloids and colloidally bound metals from aqueous solu-
tions is cross-fl ow ultrafi ltration (Buffl e et al., 1992; Bues-
seler et al., 1996; Guo and Santschi, 2007). Cross-fl ow ultra-
fi ltration techniques have been described to determine
the phase speciation of Hg and MMHg in both marine
and freshwater systems (Stordal et al., 1996; Babiarz et al.,
2000; Guentzel et al., 1996; Choe and Gill, 2001). Table 3.6
is a brief summary illustrating the importance of colloi-
dal Hg and MMHg. Determinations of colloidal Hg using
ultrafi ltration techniques involve signifi cant attention to
operational characteristics, including concentration factors
and careful calibration of the membrane for size fraction-
ation (Babiarz et al., 2000; Choe and Gill, 2001). To date,
most ultrafi ltration studies have observed signifi cant levels
of colloidal Hg and MMHg in estuarine, marine, and fresh-
water systems.
table 3.6
Colloidal Mercury and Methylmercury Levels Observed in Natural Water Systems
Filter-passing
a
mean and/or
(range) (pM)
Colloidal Hg
b
mean and/or
(range) (pM)
Truly dissolved
c
mean and/or
(range) (pM)
% of fi lter-passing
phase that is
colloidal
System
Reference
Total Hg
Freshwaters
20.5
12
9.5
59
Babiarz et al. (2001)
Estuarine
3.3 (0.32-8.2)
(0.29-4.4)
(0.9-2.9)
57
20
Stordal et al. (1996)
San Francisco Bay
(1.8-7.8)
38
18 (fall)
Choe and Gill (2003)
57
10 (spring)
MMHg
Freshwaters
1.1
0.60
0.50
54
Babiarz et al. (2001)
Galveston Bay
(0.025-0.105)
(0-0.07)
(0.015-0.06)
52-60
Choe and Gill (2001)
San Francisco Bay
34
11 (fall)
Choe and Gill (2003)
56
15 (spring)
m fi lter.
b. Colloidal Hg is defi ned as Hg in the fi lter-passing fraction that has been retained by ultrafi ltration. The minimal size fraction isolated varies
with investigation, but is typically between 1 and 10 kDa.
c. The truly dissolved fraction is defi ned as Hg observed in the natural water sample that has passed through the ultrafi ltration system. Plus-
minus values are means
a. Filter-passing Hg is defi ned as Hg that is observed in a water sample that has been fi ltered with a 0.4- or 0.45-
SD.
