US EPA

1439915 

Journal Article 

REDOX POTENTIALS AND REDOX REACTIONS IN DEEP GROUNDWATER SYSTEMS 

Grenthe, I; Stumm, W; Laaksuharju, M; Nilsson, AC; Wikberg, P 

1992 

Yes 

Chemical Geology
ISSN: 0009-2541 

98 

1-2 

131-150 

WOS:A1992JG40800008 

Both laboratory investigations and field studies of deep
groundwater systems indicate that stable and reproducible redox potentials can be measured with a
precision of +/- 25 mV. The E(h) data measured in the field are in good agreement with the half-
cell reaction: ""Fe(OH)3""(s) + 3H+ +e- half arrow right over half arrow left Fe2+ 3H2O involving
hydrous ferric oxide and Fe2+ in solution, as indicated both by the slope of a plot of Eh vs.
(3pH + log [Fe2+]), and the value of the solubility product for ""Fe(OH)3""(s), calculated from
these data. The solubility product: pK(s) = - log[Fe3+][OH-]3 = 40.9 +/- 1.1 falls in the range
37.3 < pK(s) < 44.1, as previously given by D. Langmuir and D.O. Whittemore for amorphous ferric
hydroxide and crystalline goethite, respectively. The measurement and interpretation of field
redox data from borehole investigations are complicated by the mixing of waters of different
origin. Most groundwaters studied are not characteristic of the geochemistry of the particular
section sampled; they are mixtures of water of different origin and may contain significant
amounts of surface water (indicated by the tritium content) and drilling water used to cool the
drill-bit. The initial oxygen content in these components is rapidly reduced by Fe(II) minerals
with the formation of Fe(OH)3(s), resulting in mixed waters which are anoxic and containing
dissolved Fe(II). Surface-mediated redox reactions seem to play an important role both in the
reduction of oxygen and of uranium and other trace elements.