Geology Reference
In-Depth Information
state [C(−IV) as in methane, CH
4
] it is referred to as
'reduced carbon'.
Sulfur behaves in a rather similar fashion: depending on
the environment, it may occur:
These relationships are illustrated by the downward pro-
gression from Cu
2
O through Cu metal to Cu
2
S minerals in
Figure 4.1a.
The relative oxidizing or reducing character of a nat-
ural solution, determining the stability of minerals that
coexist with it, is expressed in terms of its redox (or
oxidation) potential,
Eh,
expressed in volts (V) or milli-
volts (mV). In practical terms, the
Eh
value of an aque-
ous environment (such as open seawater, lake sediment
or bogwater) is measured by inserting a platinum elec-
trode into the solution and reading the voltage it devel-
ops relative to a reference electrode.
10
High values
represent oxidizing conditions, whereas low or negative
values signify reducing environments (Figure 4.1b).
To quote Krauskopf and Bird (1995):
(a) in the elemental state S(0) as the 'native' sulfur often
deposited around volcanic vents;
(b) as 'oxidized sulfur' in sulfur dioxide, SO
2
[S(IV)], and
the sulfate ion, SO
4
2−
[S(VI)];
(c) as 'reduced sulfur' in H
2
S [hydrogen sulfide, S(−II)] and
in metal sulfides like galena (PbS).
The oxidation state of sulfur, like iron (Equation 4.7.2),
can be changed by interaction with organic matter. In the
diagenesis of sediment accumulating on the ocean floor,
for example, dissolved sulfate present in pore water
(Table 4.3) may be reduced by organic matter in the sedi-
ment to form crystals of pyrite (FeS
2
).
The stability of minerals containing these oxidation
states depends on the external conditions to which they
are exposed. Environments open to the atmosphere are
intrinsically oxidizing, because oxygen is good at drawing
electrons away from metal atoms, and such conditions
stabilize oxygen-bearing minerals like sulfates, hematite
(Fe
2
O
3
) and cuprite (Cu
2
O). Environments dominated by
organic matter, on the other hand, or those cut off from
the atmosphere (below the water table, for example), tend
to be reducing. These are conditions favourable to the
formation of sulfides and other oxygen-poor minerals.
Redox potential in many ways is analogous to pH. It meas-
ures the ability of an environment to supply electrons to an
oxidizing agent, or to take up electrons from a reducing
agent, just as the pH of an environment measures its ability
to supply protons (H
+
ions) to a base or to take up protons
from an acid.
10
The symbol
Eh
derives from the fact that the measured
potential
E
of the platinum electrode is by convention
expressed relative to a reference electrode called a
hydro-
gen electrode
.
a new problem recognized only recently: dangerously
high levels of easily mobilized arsenic.
Arsenic is a metalloid infamous for its toxicity. Its
inorganic solution geochemistry is summarized in
Figure 4.2. Most toxic trace metals occur in solution as
simple cations (like Cd
2+
, Cu
2+
, Pb
2+
, Zn
2+
) that are sol-
uble only in acid solution; in neutral and alkaline waters
these metals tend to precipitate out (as illustrated by
the stability fields of crystalline Cu minerals shown in
Figure 4.1a) or
adsorb
on to solid mineral surfaces. The
aqueous chemistry of arsenic, on the other hand, is
dominated by
oxy-anions
that remain soluble across
the pH and
Eh
range (Figure 4.2), including As(III)
arsenites and As(V) arsenates. Unlike Figure 4.1a, all
the species shown in Figure 4.2 are dissolved forms
of As. Arsenic differs from other oxy-anion-forming
elements like Cr, V, Mo and P in remaining soluble even
under reducing (low
Eh
) conditions, a fact that lies
behind arsenic's environmental mobility.
The high-As aquifers in southern Bangladesh con-
sist of micaceous sands, silts and clays, capped by
impervious clay that inhibits entry of air. Isolation
from the atmosphere, combined with an abundance of
solid organic matter, leads to highly reducing cond-
itions in these deeper aquifers that favour selective
mobilization of As. The aquifer sediments here are not
particularly As-rich (typically 10-30 ppm), but arsenic
is known to adsorb on to hydrous iron oxide minerals
present as ubiquitous coatings on sedimentary grains,
in which As is present at higher levels up to 500 ppm
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