Chapter 28 3+
Use of Model-Generated Fe Ion Activities To Compute Eh and Ferric Oxyhydroxide Solubilities in Anaerobic Systems 1
1
1,3
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Donald L. Macalady , Donald Langmuir , Timothy Grundl , and Alan Elzerman 2
1
Department of Chemistry and Geochemistry, Colorado School of Mines, Golden, CO 80401 Department of Environmental Systems Engineering, Clemson University, Clemson, SC 29634
2
Redox conditions, pH and ferric oxyhydroxide solubilities limit dissolved iron in most natural waters. Laboratory studies were performed in anaerobic systems in which Fe(III) activities were calculated using the computer model P H R E E Q E (46) with revised formation constants for iron hydroxy and chloride complexes. Lab and field Eh measurements using Pt or wax-impregnated-graphite (WIG) electrodes can provide nernstian potentials in the presence of measurable Fe(II) at pH's as high as 6.6. This allows calculation of F e 3 activities under a wide variety of conditions without difficult measurements of trace (< 10 M) Fe(III) concentrations. Also calculated are ferric oxyhydroxide solubilities, expressed as pQ = -log[Fe ][OH ] , which range in general from about 37 to 44. Laboratory studies in ferric/ferrous chloride solutions between pH 2 and 7 indicate an overall stoichiometry of 1/(3.06) +/- 0.15, and pQ values from about 38 to 41 for ferric oxyhydroxides after 12-200 hours. The above conclusions were supported by analyses and modeling of groundwater geochemical field data collected at Otis Air Force Base, M A and near Leadville, CO. +
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3+
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3
The measurement of dissolved Fe(III) and calculation of the activities of aqueous ferric species have challenged geochemists and others in their efforts to understand the chemistry of iron in natural waters (1-5). A new method for determining the activities of dissolved Fe(III) species in anaerobic systems containing Fe(II) is described in this paper (see also 6), along with conclusions about the behavior of redox electrodes in such systems and solubilities of associated ferric-oxyhydroxide solids. Redox conditions in dramatically different anaerobic aquifers, at Otis Air Force Base, M A and near Leadville, CO are also discussed. The ultimate goal of this research is to develop a methodology for predicting the redox behavior of anaerobic aquifer systems, particularly toward introduced chemicals. The approach described herein is an effort to understand in detail the behavior of one couple, 3
Current address: Department of Geosciences, University of Wisconsin-Milwaukee, Milwaukee, WI 53207 0097-6156/90/0416-0350$06.00/0 ο 1990 American Chemical Society
Melchior and Bassett; Chemical Modeling of Aqueous Systems II ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
28. MACALADY ET AL.
Computing Eh and Ferrie Oxyhydroxide Solubilities
[Fe(III)/Fe(II)]. Future investigations of additional couples will hopefully provide a more comprehensive understanding of the important parameters affecting redox transformations of anthropogenic chemicals in groundwaters.
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R E D O X POTENTIAL M E A S U R E M E N T S The redox potential (ORP) for a single redox couple is related to the activities of species in solution through the Nernst equation (7) which has the limitations inherent in any thermodynamic relationship. If a measured ORP (corrected to the standard hydrogen electrode, SHE) corresponds to one computed from the experimental activities of a particular redox couple, electrode behavior is said to be nernstian with respect to that couple. The complex and generally non-nernstian behavior of redox electrodes in natural systems has been discussed by many authors (8-11). Problems include mixed potentials (12-15), poisoning of platinum redox electrodes (16), lack of internal redox equilibrium (8,15,16), and lack of electrochemical equilibrium (17). Several reviews of the use of redox electrodes in geochemical studies have been published (18-20). Redox electrode behavior in natural systems has been reviewed by Morris and Stumm (8), who conclude that few of the redox couples important in natural waters have sufficient electrode exchange currents to provide nernstian electrode response. Exceptions include the ferrous/ferric iron couple, perhaps Μ η ^ / Μ η θ 2 (21), and certain redox couples involving native sulfur, hydrogen sulfide and polysulfides (22-24). For the iron couple (8), Fe(IIl) and Fe(II) ion activities of 10"^ M or greater are said to be necessary for nernstian response, as is the absence of trace dissolved oxygen. Nernstian behavior of redox electrodes in natural systems dominated by iron has been reported or assumed by many authors (25-27, for example). Doyle (26) concluded that the apparently nernstian response of Pt electrodes above a pH of about 4.2 in systems containing ferrous iron is due to the presence of ferric oxyhydroxide solid phases, such as lepidocrocite and maghemite, coating the Pt electrode. This coating, not one or more of the hydrolysis species of F e ^ , provides the exchange current necessary for stable and reproducible electrode potentials. The above conclusions have particular relevance to the findings reported herein, and are discussed in some detail below. +
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F O R M A T I O N CONSTANTS OF IRON H Y D R O X I D E A N D C H L O R I D E C O M P L E X E S Equilibrium and rate constants for the hydrolysis and chloride complexation of Fe(III) and Fe(II) ions are necessary in a detailed study of iron redox chemistry. Table I lists an internally consistent set of values for the relevant equilibrium constants. Their accuracy is discussed later in the context of a brief sensitivity analysis of the data. The rates of iron hydrolysis and chloride complexation reactions are also mentioned.
SOLUBILITIES OF T H E FERRIC O X Y H Y D R O X I D E S The solubility products of stoichiometric ferric oxyhydroxides can be expressed in terms of ion activities as: 3+
ρΚ,,ρ = -log[Fe ][OH-]
3
(1)
Melchior and Bassett; Chemical Modeling of Aqueous Systems II ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
351
352
CHEMICAL MODELING OF AQUEOUS SYSTEMS II TABLE I Cumulative Formation Reactions and Constants at 25°C and Zero Ionic Strength for Complexes Important in This Study
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REACTION
1) 2) 3) 4) 5) 6) 7)
Fe-3"*"+ + F e3 + + F e3 ++ + F e3 + + F e2++ + Fe3 + ++ Fe3 ++ + 3
3 +
3
J _ r
2
3 + 3
H 0 2H 0 3H 0 4H 0 CF CF 2CF 2
2
2
2
ρ β
Sources
2.19 5.67 12.56 21.6 0.51 1.48 2.13
(28,29)
η
= = = = = = =
2 +
Fe(OH) Fe(OH) Fe(OH) ° Fe(OH) " FeCl+ FeCl + FeCl
+
2
3
4
2
+
2
+ + + +
H+ 2H+ 3H + 4H +
(I)
#
(29,31,32) (29,30) (29,30)
In ρ β , the η denotes the number of ligands in the complex formed. #This value was computed from ^ ( m o l a l ) = 2.4 χ 10" measured in 0.68 m NaClO^ solutions by Byrne and Kester (1). Baes and Mesmer (28) suggest (1=0) < 10~ , which is consistent with the value tabulated above. η
14
12
Published values for pK based on field and laboratory solubility measurements range from about 37 for freshly precipitated, amorphous, colloidal-sized material, to about 43.5 for more crystalline phases (33-34). In low temperature aqueous systems, goethite (α-FeOOH) is probably the most stable well-crystallized oxyhydroxide, with hematite (
