An Introduction to Environmental Chemistry

available as electron acceptors in marine sediments, their importance is small in
comparison with SO 42 - , which is abundant in seawater (Table 6.1). At seawater pH
around 8, sulphate-reducing bacteria metabolize organic matter according to the
following simplified equation.

eqn. 6.12

This process is widespread in marine sediments but is most important in conti-
nental margin sediments, where organic matter accumulation is largest. Sulphate
reduction in sediments occurs at depths (varying from a few millimetres to metres
below the sediment/water interface) where seawater SO 42 - can readily diffuse, or
be pumped by the actions of sediment-dwelling organisms. The reaction yields
highly reactive hydrogen sulphide (HS-), most of which diffuses upward and is
reoxidized to SO 42 - by oxygenated seawater in the surface sediment. However,
about 10% of the HS-rapidly precipitates soluble Fe(II) in the reducing sedi-
ments to yield iron monosulphide (FeS).

eqn. 6.13

Iron monosulphides then convert to pyrite (FeS 2 ). At Eh below -250 mV (see Box
5.4) and at pH around 6, the conversion of FeS to FeS 2 may occur via oxidation
of dissolved FeS by hydrogen sulphide, for example:

eqn. 6.14

eqn. 6.15

Alternatively, under less reducing conditions, conversion may involve the
addition of sulphur (S) from intermediate sulphur species (e.g. polysulphides,
polythionates or thiosulphate (S 2 O 32 - )), which are the products and reactants in
microenvironmental sulphur redox cycling. The reaction involving S 2 O 32 - can be
summarized as:

eqn. 6.16

The sulphite (SO 32 - ) is subsequently oxidized to SO 42 -. Sedimentary pyrite, formed
as a byproduct of sulphate reduction in marine sediments, is a major sink for
seawater SO 42 -. The presence of pyrite in ancient marine sediments shows that
SO 42 - reduction has occurred for hundreds of millions of years. On a geological
timescale, removal of SO 42 - from seawater by sedimentary pyrite formation is
thought to be about equal to that removed by evaporite deposition (Section 6.4.2).
Compilations of pyrite abundance and accumulation rates are used to calculate
modern SO 42 - removal by this mechanism and to derive the estimate in Table 6.2.
Sulphate reduction (eqn. 6.12) also produces HCO 3 - , and this anion slowly dif-
fuses out of the sediment into seawater, accounting for about 7% of the HCO 3 -
flux to the oceans. The slow diffusion also means that HCO 3 - may build up to
sufficiently high concentrations in sediment pore waters for the ion activity
product of Ca^2 +(from seawater) and HCO 3 - to exceed the solubility product for

FeS()s +Æ+S O 2 23 - ()aq FeS 2 ()s SO 32 - ()aq

FeS()aq+Æ+H S 222 ()aq FeS()s H()g

FeS()saÆFeS()q

Fe()^2 aq+ +Æ+HS-()aq FeS()s H()+aq

S

The Oceans 207