Fundamentals of Sample Preparation for Environmental Analysis - Fundamentals of Environmental Sampling and Analysis - page 166

resultant leachate for both Cr(VI) and total Cr. The difference between the two
values approximates Cr(III).

2. Mercury (Hg)

Mercury (Hg) is a nonessential element for both animals and humans and all forms of
inorganic and organic Hg are considered to be toxic. The most notorious is, perhaps,
the methyl-mercury because of its high toxicity and bioaccumulation in the food
chain in aquatic systems. The digestion method discussed here is for the total
mercury using H 2 SO 4 -HNO 3 in the presence of KMnO 4 -K 2 S 2 O 8 (potassium
permanganate-potassium persulfate). Both KMnO 4 and K 2 S 2 O 8 are oxidizing agents
where KMnO 4 is used to maintain the oxidizing condition for Hg 2þ . The excess
KMnO 4 is removed with hydroxylamine hydrochloride (a weak reducing agent).
Mercury is a unique element from the analytical standpoint because this is the
only metal in an elemental state that is a volatile liquid at room temperature, and can
be purged directly from the sample for analysis by a specific atomic absorption
called cold vapor atomic absorption spectroscopy (Chapter 9). Because of this, the
acid digested sample containing Hg 2þ is further reacted with stannous chloride
(SnCl 2 ) to produce free Hg (Hg 0 ):
Hg 2þ þSn 2þ ! Hg 0

þSn 4þ

ð7:7Þ

3. Arsenic (As) and Selenium (Se)

Species of As and Se Both arsenic (As) and selenium (Se) are metalloids. They
are nonessential for plants but As is essential for several animal species and Se is
essential to most animals. Arsenic, in inorganic form, has two oxidation states, i.e.,
arsenate, designated as As (V), and the trivalent arsenite, designated as As (III).
Arsenite is many times more toxic than arsenate. The predominant As (V) species in
water is H 2 AsO 4 at pH 3-7 and HAsO 2 4 at pH 7-11. Under reducing conditions, the
major As (III) species are HAsO 2 (aq) or H 3 AsO 3 . Organic As can also arise from
industrial discharges, pesticides, and biological actions of inorganic As. Unpolluted
fresh water normally does not contain organic arsenic compounds, but may contain
inorganic arsenate and arsenite.
Inorganic selenium (Se) exists predominately as selenate ion (SeO 2 4 )
designated as Se(VI), and selenite ion (SeO 2 3 ) designated as Se (IV). Other
common aqueous species include Se 2 , HSe , and Se 0 . Se derived from microbial
degradation of seleniferous organic matter includes selenite, selenate, and the
volatile organic compounds dimethylselenide and dimethyldiselenide. Nonvolatile
organic selenium compounds may be released into water by microbial processes.

Digestion for total As and Se Total arsenic or selenium can be measured by
one of the following two digestion methods:

Total recoverable As/Se: H 2 SO 4 -HNO 3 -HClO 4 digestion is effective in
destroying organics and most particulates in untreated wastewaters or solid