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VI. Uses of pe–pH Diagrams

VI. Uses of pe–pH Diagrams

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166



R I C H M O N D J. BARTLETT AND BRUCE R. JAMES



a,



a



PH

Figure 1. A pe-pH diagram for 0, reduction reactions, including partially reduced

intermediates:superoxide (Oi), hydroxyl free radical (OH .), and hydrogen peroxide (H202).

Reduction of 0,to O2is also included for comparison with 0, reduction reactions. Activity

of oxygencontaining ionic or molecular species is 1 6 ‘ M.except that Pa is 2 1 Wa.



reduction of 0, is relatively insensitive to the 0, partial pressure in this

range.

and the hydroxyl free radical (OH-)are the most powerful

Ozone (0,)

oxidants among the oxygen species (Table I), and the latter may be formed

during stepwise, four-electron reduction of 0, to Oy, H,O,, and H,O

(Fridovich, 1978). The low and high positions of the pe-pH lines for

superoxide (0;) reduction to H,O, and for superoxide oxidation to 0,

indicate that both a powerful oxidlzing and reducing agent is formed in the

first step of reduction of 0,. The enzyme superoxide dismutase scavenges

0; in living cells using 0, as the terminal electron acceptor, but relatively

little is known about its reactivity in biological and chemical processes in

soils that may be pertinent to our understanding of the formation of highly

reduced components (e.g., soil organic matter) and highly oxidized species

(e.g., NO;) that coexist in soil at chemical equilibrium or quasi-equilibrium.



B. NITROGEN

SPECIES

Most reduction reactions of N species (Fig. 2 and Table I) are not

reversible, and therefore are not well defined by thermodynamic pe-pH



REDOX CHEMISTRY OF SOILS



167



a,



a



0



1



2



3



4



5



6



7



8



9 10



PH

Figure 2. A pe-pH diagram for nitrogen species half-reactions, including intermediates

formed or consumed in denitrification and dinitrogen fixation. Partial pressures of gaseous

intermediates are 0.01 kPa, except N, is 78 kPa; activity for NO;, NO:, and NH:is lo-' M.



relationships. The series of half reactions composing the process of denitrification, though, is instructive in that it identifies the wide range of pe for

reduction of each of the intermediates believed to form in the sequence of

electron acceptors used by microbes:



NO;

Step



- NO?



(1)



(2)



NO



+



(3)



N,O



-



N,



(4)



Step 1 of the sequence occurs at pe values less than those for reduction to

0, to H,O, whereas those for steps 2, 3, and 4 are increasingly higher. The

overall reduction of NO; to N,, though, is almost identical to that for the

0 2 / H , 0 couple. The overlap of pe values for the 0, and NO; reduction

intermediates indicates that denitrification and aerobic respiration may

occur at the same time under certain conditions when organic C is used as

the electron donor. They may not be mutually exclusive, as predicted from

log K values for the overall reactions, 0,to H,O and NO; to N, .



C. MANGANESE

OXIDE

SPECIES

Manganese exists in soils in the 11, 111, and IV valence states, and the

latter two are most stable as oxides or oxyhydroxides. Trivalent Mn may



168



R I C H M O N D J. B A R T L E T T A N D BRUCE R. JAMES



-15



0



1



2



3



4



5



6



7



8



9 10



PH

Figure 3. A pe-pH diagram for Mn oxides, trivalent Mn ions, and superoxide. Ion

activities and H 2 0 2concentrations are lo-' M,Pa is 21 Wa.



exist as a cation, especially if stabilized by ligands, such as pyrophosphate

or citrate. The pe-pH relationships of Fig. 3 predict that different valences

of Mn in Mn,O,, MnOOH, and Mn02 affect the pe at which Mn2+would

be expected to form at pH < 7, but they are all similar at pH values near 7.

The Mn3+/Mn2+line indicates that at approximately pH 4, Mnw is a

powerful oxidant similar to 0;and Mn,O, (Fig. 3) if in equilibrium with

Mn2+.At pH values near 6.5, Mnw in equilibrium with MnO, is a powerful reductant, similar to H2and 0;.

This powerful oxidizing ability of Mn3+ in equilibrium with Mn2+ may

be pertinent to anaerobic soils that are exposed to 0,, and in which Mn2+

is oxidizing to form Mn(II1, IV) oxides via Mn3+. In oxidized soils containing MnO, ,flooding and the process of becoming reduced may produce

Mn3+,which is a powerful reducing agent. The trivalent Mn species may be

ephemeral intermediates in such processes at redox interfaces, such as in

the rhizosphere of plant roots or between soil water and groundwater. As

the metal analog of superoxide in its oxidizing/reducing power and as a

free radical, Mn3+is appropriately referred to as the supermanganese ion.

Because many Mn(II1, IV) oxides are nonstoichiometric and no compound with the exact composition of MnO, is known (Arndt, 1981),

predictions of their redox properties as a function of mineralogy or valence



REDOX CHEMISTRY OF SOILS



169



PH

Figure 4. A pe-pH diagram for Mn3+, MnO,, and Mn,O,; as compared with reduction

values between pH 5 and 7 for Co, Cr, Se, As, V, and Pu.Activity for ionic species is lo-' M.



in heterogeneous soils may be hard to formulate. Despite the uncertainty

of thermodynamic predictions for the redox behavior of Mn, the chemistry

of this element is pertinent to a number of processes governing speciation

and valence state of trace elements and pollutants found in soils.

The pe-pH data indicate that oxides of Mn may oxidize Pu(1II) to

Pu(IV), V(II1) to V(V), As(II1) to As(V), Se(IV) to Se(VI), N(II1) to

N( V), and Cr( 111) to Cr( VI), because the pe for each of these couples falls

below that for Mn oxides (Fig. 4 and Table I). The oxidations of Pu(III),

As( 111), Se(IV), N( 111), and Cr( 111) all have been demonstrated to occur in

soils containing Mn oxides or by synthetic Mn oxides (Amacher and

Baker, 1982; Bartlett and James, 1979; Bartlett, 1981b; Blaylock and

James, 1992; Moore ef af.,1990).

The instability of Mn3+and its ability to dismutate, as do H202and OF,

mean that kinetic constraints may be particularly important in understanding the redox behavior of Mn in soils undergoing transitions between

anaerobic and aerobic conditions. The kinetic lability of these species is

poorly understood and new knowledge could contribute significantly to

predictions of bioavailability and toxicity of numerous plant nutrients and

pollutants in a range of types of soils from rice paddies and wetlands to

well-drained agricultural and forest soils.



I70



RICHMOND J. BARTLETT AND BRUCE R. JAMES



aJ



Q



0



1



2



3



4



5



6



7



8



9 10



PH

Figure 5. A pe-pH diagram for Fe(II1) oxides and dihydroxy species, Few/Fe2+in the

presence or absence of five organic complexing Ligands. and the HCrO;/Cr(OH), redox

couple. Activity of ionic species is IO-‘ M.



D. IRON SPECIES

Predictions of the redox behavior of Fe( 11) and Fe( 111) species indicate

that it falls below most Mn oxides species (lower pe values and less free

energy released per equivalent upon reduction), but intermediate hydrolysis products, such as Fe(OH)l, theoretically can oxidize Cr(111) to Cr( VI)

at pH <4 (Fig. 5 and Table I). In addition, complexation of Fe2+ by

organic ligands lowers the pe values at which Fe3+is converted to Fe2+and

the redox couples are similar to those of Fe( 111) oxides in the pH range of 5

to 7. This phenomenon suggests that Fe2+ becomes a more powerful

reducing agent when complexed, and may explain the ability of Fe to act as

a cofactor in enzymes involved in redox processes, such as peroxidases and

superoxide dismutases. These enzymes reduce or dismutate H20, and Oy.

The application of such concepts to abiotic redox processes in soils remains

a key area for future research.



E. CARBON

AND SULFUR

SPECIES

Reduced forms of C and S are normally viewed as reductants in soils,

either in chemical or biological processes. Thermodynamic predictions



171



REDOX CHEMISTRY OF SOILS



pyruvate/lactate



A dehydroascorbate/ascorbate



25



- - _- _- _



- _- - _- - _



-5

1-



0



1



2



3



4



5



6



7



8



9 10



PH

Figure 6. A pe-pH diagram for S, C, and Se species. Ion activity and molecular concentrations are lo-' M and Pa is 0.032 kPa.



support this idea for carbohydrates produced in photosynthesis, methane

from methanogenesis, and hydrogen sulfide from reduction of SO, (Fig. 6

and Table I). The reduction reaction of 0- and pquinone suggest that these

compounds may be reduced at higher pe values than are C02and SO,. The

low position of these lines, however, coincides with the Mn02/Mn3+couple at pH 7, suggesting that Mn3+ may act as a reducing agent for certain

organic species in near-neutral soils. Coupling of reduction of the organic

with oxidation of Mn may result in formation of free radical species. This

is pertinent to understanding the formation and persistence of organic

matter in high-pH soils that may contain reactive forms of Mn oxides.

Reactions of H2S and H,Se are predicted to be similar with respect to

SO, and S e O , formation (Fig. 6). Sulfidization has been studied as a

mechanism for precipitation of Fe and other heavy metals in tidal marshes

and natural or constructed wetlands (Rabenhorst and James, 1992; Rabenhorst et al., 1992; Hines et al., 1989; Kittrick et al., 1982), and selenide

formation may result in analogous products in sulfidic soils (Masscheleyn

et al., 1991).

Although SeO, and SO, are similar chemically, the oxidation of SeO, to

SeO, is predicted to occur at higher pe values than is the oxidation of H2S

to SO, (Fig. 6 and Table I). Blaylock and James (1992) observed that Mn

oxides in soils or in pure form will oxidize SeO, to SeO,, as predicted by

thermodynamics (Fig. 4). They also observed that adding reducing, phe-



172



RICHMOND J. BARTLETT AND BRUCE R. JAMES



nolic acids, such gallic and ascorbic acids, actually enhanced this oxidation. They hypothesized that partial reduction of MnO, in soils converted

the Mn oxide into a Mn(II1) form that was a more powerful oxidant for

SeO, than was MnO,. Such a hypothesis is supported by the relative

oxidizing power of MnO,, MnOOH, and Mn,O,, where the latter two

oxides contain Mn( 111) (Fig. 3).



VII. MEASUREMENT OF OXIDATION- REDUCTION

STATUS OF SOILS

The most common method for quantifying electron activity of soils and

natural waters is to measure the potential difference between a Pt indicator

electrode and a calomel or Ag/AgCl reference electrode, both connected to

a voltmeter of pH meter (Rowell, 1981 ;Bricker, 1982). In this method, the

Pt electrode is presumed to be inert and to not react chemically while

coming into equilibrium with electroactive species in soil solution and on

soil colloids. Recent advances in evaluations of the reliability of this potentiometric measurement have generally resulted in it being considered

unreliable for accurate assessments of redox status of soils, especially aerobic ones (Bartlett, 1981a). Other methods that employ analyses of soil

solution analytes indicative of redox status, along with thermodynamic

half-reactions, as discussed above, may prove more reliable for calculating

pe ranges for aerobic and anaerobic soil systems.



A. CONSTRUCTION

AND USEOF PLATINUM

ELECTRODES

Platinum and suitable reference electrodes are relatively easy and inexpensive to construct (Mueller et al., 1985; Farrell er al., 1991), but the

measurement technique may significantly alter measured voltages; several

aspects of electrode use and misuse with respect to the reliability of

recorded voltages for natural systems have been described (Bartlett, 198la;

Bricker, 1982; Matia et al., 1991).



B. INADEQUACIESOF PLATINUM

ELECTRODE

POTENTIALS

Assessing “electron activity” in soils relates strictly to an evaluation of

the ability of the electron to be transferred, to do thermodynamic work,

and not to its concentration in soil solution, as can be defined for H+.

Because of the nature of the electron and its differences from the H+, a



REDOX CHEMISTRY OF SOILS



173



number of caveats must be described and recognized when evaluating Pt

electrode potentials.

1. Dissolved Oxygen Status



A stable potential can be obtained for a Pt reference electrode pair

immersed in an oxygenated soil suspension, but it is unreliable as a measure of dissolved oxygen status (Bricker, 1982; Stumm and Morgan, 198l ).

The Pt surface may react with 0, to form ROH, which develops a potential with elemental Pt with a pe of 9.6 at pH 7 (Table I). In addition, the

measurement may not be that of the 0 2 - H 2 0 couple, but may be responding to 0, reduction intermediates, such as H,02and 0;-(Bricker,

1982). In addition, predicted pe values are relatively insensitive to changes

in dissolved O2 between 0.21 and 0.0021 atm (Table 11), the range of 0,

partial pressures in which aerobic respiration occurs (Russell, 1973). For

these reasons, Pt electrode potentials cannot be used reliably as a measure

of redox status for aerobic soils, but empirical values for pe (EMpe) may be

obtained for comparison purposes (Bartlett, 198la). Although more faith is

placed in measurements of soil pH, it also should be considered an empirical measurement because of uncertainty about the form of the hydrogen

ion in colloidal environments and about the behavior of the glass electrode

in such systems. For these reasons, both pe and pH measured with electrodes in soils may be very uncertain for accurate descriptions of the redox

status of soil environments containing air-filled pores.

2. Irreversibility of Redox Couples



Many of the important redox processes involving C, H, N, 0, and S (the

“light” elements, relative to the “heavy” metals) are irreversible in the

thermodynamic sense, and nonelectroactive gases and molecules may be

consumed or formed. As a result, potentials generated by redox couples for

these elements are difficult to obtain and interpret using a Pt electrode. In

addition, many of these reactions do not reach true chemical equilibrium,

and activities measured in soil solution may be kinetically constrained (Liu

and Narasimhan, 1989). Because the redox status of soils is often set by

“microbial potentials,” consuming or producing compounds or ions containing one or more of these elements may render Pt electrode measurements inaccurate.

3. Mixed Potentials



The goal of relating measured redox potentials to species and valence

states of various elements in soils requires that a given, singular redox



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