168
J. W.Morse
Second Acid Dissociation Constant:
HCO; H CO;- + H+
K1,c0 2 = (aCO~-aH+) I aHC0 3 = 4.68 x 10- 11 (7.5)
HS- HS 1 - + H+
K1,H 2 S= (aS2-aH+) I aHS- < 1 X 10 -17
(7.6)
In the above relationships, it is noteworthy that, while the solubilities of the gases
and first acid dissociation constants are similar for both systems, the second dissociation constant for H1S is much smaller than for carbonic acid. It is in fact rather poorly
known and may be orders of magnitude less than 10 -17 (Rickard et al. 1995). This presents major problems for understanding solid phase solubilities and the importance
of metal-S ligand complexes. Over the pH ranges commonly encountered in marine
sediments, bicarbonate is the dominant form of the carbon dioxide system, as is
bisulphite for the hydrogen sulphide system in alkaline waters. However, as near neutral pH values are approached, H1S becomes an increasingly important component of
the hydrogen sulphide system. It should also be kept in mind that the apparent constants for these systems in sea water are strongly influenced by temperature and salinity, and in the deep sea, by pressure.
7.2.2
Redox Reactions
Redox (oxidation-reduction) reactions are at the very heart of sedimentary carbon
and sulphur systems and their interrelationships. Complicating things or making them
interesting depending on your viewpoint, is the fact that many of these reactions are
mediated by consortiums of bacteria, which in many cases are just beginning to be
understood. In this section many of the basic redox reactions of general importance
will be presented. It should be carefully noted that often these are only very simple
approximations of what is occurring and should only be taken as "schematic" representations of processes. It will be useful to refer to Fig. 7.2 to keep the "big picture" in
mind. Some of these redox processes will be treated in considerably more detail later
in this chapter.
7.2.2.1
Important General (-0-5 Redox Reactions
The simple redox reaction (Eq. 7-7) for the CIS system that is most often used as an
example is the reaction of two organic-Cs with sulphate to produce hydrogen sulphide
and carbon dioxide system species. At temperatures typical of recent sediments, this
reaction takes place only via the
2CH 1 0 + SO~- ~ HS- + 2HCO; + H+
(7.7)
activity of sulphate reducing bacteria such as Desulfovibrio (It is important to note that
these organisms can only utilize relatively small organic compounds such as acetate
that are produced in sediments by the breakdown of more complex organic matter by
fermenters). Later, this important general reaction will be examined in a more com-
J. W.Morse
Second Acid Dissociation Constant:
HCO; H CO;- + H+
K1,c0 2 = (aCO~-aH+) I aHC0 3 = 4.68 x 10- 11 (7.5)
HS- HS 1 - + H+
K1,H 2 S= (aS2-aH+) I aHS- < 1 X 10 -17
(7.6)
In the above relationships, it is noteworthy that, while the solubilities of the gases
and first acid dissociation constants are similar for both systems, the second dissociation constant for H1S is much smaller than for carbonic acid. It is in fact rather poorly
known and may be orders of magnitude less than 10 -17 (Rickard et al. 1995). This presents major problems for understanding solid phase solubilities and the importance
of metal-S ligand complexes. Over the pH ranges commonly encountered in marine
sediments, bicarbonate is the dominant form of the carbon dioxide system, as is
bisulphite for the hydrogen sulphide system in alkaline waters. However, as near neutral pH values are approached, H1S becomes an increasingly important component of
the hydrogen sulphide system. It should also be kept in mind that the apparent constants for these systems in sea water are strongly influenced by temperature and salinity, and in the deep sea, by pressure.
7.2.2
Redox Reactions
Redox (oxidation-reduction) reactions are at the very heart of sedimentary carbon
and sulphur systems and their interrelationships. Complicating things or making them
interesting depending on your viewpoint, is the fact that many of these reactions are
mediated by consortiums of bacteria, which in many cases are just beginning to be
understood. In this section many of the basic redox reactions of general importance
will be presented. It should be carefully noted that often these are only very simple
approximations of what is occurring and should only be taken as "schematic" representations of processes. It will be useful to refer to Fig. 7.2 to keep the "big picture" in
mind. Some of these redox processes will be treated in considerably more detail later
in this chapter.
7.2.2.1
Important General (-0-5 Redox Reactions
The simple redox reaction (Eq. 7-7) for the CIS system that is most often used as an
example is the reaction of two organic-Cs with sulphate to produce hydrogen sulphide
and carbon dioxide system species. At temperatures typical of recent sediments, this
reaction takes place only via the
2CH 1 0 + SO~- ~ HS- + 2HCO; + H+
(7.7)
activity of sulphate reducing bacteria such as Desulfovibrio (It is important to note that
these organisms can only utilize relatively small organic compounds such as acetate
that are produced in sediments by the breakdown of more complex organic matter by
fermenters). Later, this important general reaction will be examined in a more com-