Environmental Engineering Reference
In-Depth Information
used for validation purposes. Sample contamination issues
are not as big a concern because sample preparation and
handling steps are minimal before the irradiation of the
sample. NAA has often been used as a reference method
against which other methods were checked and compared
(Ja ´imovi ´ and Horvat, 2004). A major limitation in the
widespread adoption of activation techniques for Hg analy-
sis is that the technique requires very expensive facilities
and well-trained personnel. NAA procedures also tend to
be lengthy and they cannot be adapted for use in the fi eld.
In recent years, however, signifi cant improvements of ana-
lytical methods in terms of specifi city and sensitivity have
been achieved. This has allowed the determination of Hg
speciation in many environmental compartments.
Occurrence of Organo-Mercury Species
in Natural Waters
Concentrations of organo-Hg compounds (predominantly
MMHg) in water samples are generally very low (at the
nanogram per liter level or below), so that accurate anal-
ysis requires great care in sample handling and analysis.
The theoretical approach via stability calculations can be
of great help in making rough estimates of the predomi-
nant Hg species under various conditions. Mercury com-
pounds occurring in natural waters (see Figure 3.1) are
most often defi ned by their ability to be reduced to elemen-
tal Hg. In lake waters, MMHg species account for 1-30%
of total Hg. Most of the MMHg is probably associated with
DOM. Thiol groups (-RSH) have been shown, however, to
have a higher capability to bind MMHg in comparison
with ligands containing oxygen and nitrogen donor atoms
and the inorganic ions (CN
-
, Cl
-
, OH
-
). Monomethylmer-
cury compounds in surface runoff waters, soil pore waters,
and groundwaters are similar to the species in lake waters
and are generally quite strongly associated with DOM (see
section on “Determination of Organo-Hg Compounds
in Aqueous Media”). Dimethylmercury has rarely been
reported in surface waters except in the deep ocean (Mason
and Fitzgerald, 1993; Cossa et al., 1994; Vandal et al., 1998;
Horvat et al., 2003) and during some seasons in the slurry
of salt marshes (Weber et al., 1998). Monomethylmercury
concentrations in seawater are generally lower than in
lake waters. The presence of organo-Hg species, including
dimethylmercury, was also detected in geothermal gases
and waters (Hirner et al., 1998).
ELECTROCHEMICAL METHODS
The use of electrochemical methods for total mercury anal-
ysis of environmental waters is limited, primarily because
of its lack of sensitivity. Detection limits are typically in
the low nanomolar range. Another limitation is that many
of the electrochemical approaches used for other trace-
element determinations use mercury-based electrodes (e.g.,
hanging drop mercury electrodes or thin fi lm Hg elec-
trodes). Electrochemical methods developed for mercury
analysis of water typically use noble metal electrodes
(Turyan and Mandler, 1994; Wu et al., 1997; Falter et al.,
1999; Bonfi l et al., 2000; Giacominoa et al., 2008). One
important advantage of electrochemical methods is that,
for example, using anodic stripping voltammetry (ASV),
it is possible to separate Hg(I) and Hg(II) in aqueous solu-
tions. However, the sensitivity is poor as compared with
other techniques for determination of total Hg (Sipos et al.,
1980; Švarc-Gaji ´ et al., 2009).
PHOTO-ACOUSTIC SPECTROSCOPY
Mercury is fi rst preconcentrated on a gold trap and after
thermal release it is quantifi ed by measuring the sound
produced from fl uorescent quenching when the sample
vapor is irradiated with a modulated Hg vapor lamp. The
detection limit is 0.05 ng. The method has been success-
fully used for detection of ultratrace levels of Hg in air and
snow (de Mora et al., 2005; Patterson, 1984).
Sampling and Storage
The same techniques described previously for collection
of inorganic Hg samples are also applicable to organo-Hg
samples. Samples may be collected in precleaned glass or
Tefl on, but it is essential that all residual oxidative acid
used in the cleaning process be leached from the walls of
Tefl on bottles prior to use for collection of samples. This
can be accomplished by fi lling the bottles with dilute
(0.5%) HCl and warming overnight. For the analysis of
organo-mercurials, preservation with oxidative reagents
(as advised for total Hg analysis) should be avoided, since
organo-mercurials are converted into inorganic Hg. Stabili-
zation by HNO
3
results in a decrease in MMHg, while Hg(II)
remains stable in the presence of HNO
3
(Leermakers et al.,
1990; Parker and Bloom, 2004). Hydrochloric acid is the
most appropriate acid for storing aqueous MMHg solutions
(Ahmed et al., 1987). Sulfuric acid is usually used for pres-
ervation of MMHg solutions in seawater (Leermakers et al.,
ATOMIC EMISSION SPECTROMETRY
Several types of plasma sources, including direct current,
inductively coupled, and microwave-induced gas (helium
and argon) plasmas have been used for the determination
of Hg (Fukushi et al., 1993). These methods are very sen-
sitive, but as compared with AAS and atomic fl uorescence
spectrometry (AFS) they are too complex and expensive for
routine work.
Determination of Organo-Mercury Species
There are no promulgated standard methods for the analy-
sis of organo-Hg compounds in environmental samples.
Most of the methods come from published papers detail-
ing methods developed for a specifi c analyte and matrix.
