THE GEOCHEMISTRY OF NATURAL WATERS

1
THE GEOCHEMISTRY OF NATURAL WATERS

• REDOX REACTIONS AND PROCESSES - II
• CHAPTER 5 - Kehew (2001)
• S-O2-H2O

2
S-O2-H2O SYSTEM
The value of 44.8 given in the Appendix to Kehew
(2001) is incorrect.
3
Schematic map of the expected locations of sulfur
species in pe-pH space.
4
H2S/HS- BOUNDARY

• H2S ? HS- H
• We define the boundary to be where
• ?Gr ?GfHS- - ?GfH2S
• ?Gr (12.3) - (-27.7) 40.0 kJ mol-1

5
The vertical H2S0/HS- boundary. As usual, this
boundary will be truncated by another boundary
eventually, but we do not yet know where.
6
HS-/S2- BOUNDARY

• HS- ? S2- H
• We define the boundary to be where
• ?Gr ?GfS2- - ?GfHS-
• ?Gr (85.8) - (12.3) 40.0 kJ mol-1

7
Our pe-pH diagram with the HS-/S2- boundary added.
8
HSO4-/SO42- BOUNDARY

• HSO4- ? SO42- H
• We define the boundary to be where
• ?Gr ?GfSO42- - ?GfHSO4-
• ?Gr (-744.0) - (-755.3) 11.30 kJ mol-1

9
Our pe-pH diagram with all three vertical
boundaries. The next logical boundary to
calculate would be the HSO4-/H2S0 boundary.
10
HSO4-/H2S BOUNDARY

• HSO4- 9H 8e- ? H2S 4H2O
• We define the boundary to be where
• ?Gr ?GfH2S 4?GfH2O - ?GfHSO4-
• ?Gr (-27.7) 4(-237.1) - (-755.3) -220.80

11
Now we have enclosed a predominance field for
HSO4- on our pe-pH diagram. We can see that HSO4-
will be the predominant aqueous sulfur species
only in very acidic waters, such as those that
might result from acid-mine drainage.
12
SO42-/H2S BOUNDARY

• SO42- 10H 8e- ? H2S 4H2O
• We define the boundary to be where
• ?Gr ?GfH2S 4?GfH2O - ?GfSO42-
• ?Gr (-27.7) 4(-237.1) - (-744.0) -232.10

13
Our pe-pH diagram with the predominance field for
H2S0 filled in. The HS-/SO42- boundary is next!
14
SO42-/HS- BOUNDARY

• SO42- 9H 8e- ? HS- 4H2O
• We define the boundary to be where
• ?Gr ?GfHS- 4?GfH2O - ?GfSO42-
• ?Gr (12.3) 4(-237.1) - (-744.0) -192.10

15
Our pe-pH diagram with the predominance field for
HS- filled in. The last boundary is the one
between SO42- and S2-.
16
SO42-/S2- BOUNDARY

• SO42- 8H 8e- ? S2- 4H2O
• We define the boundary to be where
• ?Gr ?GfS2- 4?GfH2O - ?GfSO42-
• ?Gr (85.8) 4(-237.1) - (-744.0) -118.60

17
Our final pe-pH diagram. We can see from this
diagram that the predominant form of sulfur in
most natural waters will be sulfate. Under
extremely reduced conditions, H2S0 and HS- may be
important. However, we will rarely encounter
natural waters where HSO4- or S2- are
predominant. Finally, we note that this diagram
has been constructed for values of ?Saq
sufficiently low that native sulfur is not stable.
18
S(s)/H2S BOUNDARY

• S(s) 2H 2e- ? H2S
• We choose ?Saq a H2So 0.1 mol L-1
• ?Gr ?GfH2S (-27.7) -27.7

19
S(s)/HS- BOUNDARY

• S(s) H 2e- ? HS-
• We choose ?Saq a HS- 0.1 mol L-1
• ?Gr ?GfHS- 12.3 12.3

20
S(s)/SO42- BOUNDARY

• S(s) 4H2O ? SO42- 8H 6e-
• We choose ?Saq a SO42- 0.1 mol L-1
• ?Gr ?GfSO42- (-744.0) - 4(-237.1) 204.4

21
S(s)/HSO4- BOUNDARY

• S(s) 4H2O ? HSO4- 7H 6e-
• We choose ?Saq a HSO4- 0.1 mol L-1
• ?Gr ?GfHSO4- (-755.3) - 4(-237.1) 193.1

22
The pe-pH diagram for the S-O2-H2O system at a
total dissolved S concentration high enough to
yield a stability field for solid sulfur. Not
that the latter appears as a wedge along the
sulfate-sulfide boundary as expected because S(0)
is intermediate in oxidation state to S(-II) and
S(VI). This wedge pinches out (dissappears) at
lower total dissolved sulfur concentrations, but
expands at higher concentrations.