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VII. Measurement of Oxidation–Reduction Status of Soils

VII. Measurement of Oxidation–Reduction Status of Soils

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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



174



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



couple is responsible for the equal cathodic and anodic currents present

when the applied potential is zero at the Pt electrode (equilibrium is

achieved). For example, a Fe2+-Fe3+ couple at activities greater than

M generates sufficient currents to obtain a measurable voltage for the

system. In heterogeneous soils, however, a mixed potential may be observed where anodic and cathodic currents are equal, but the redox

partners may not be in equilibrium with each other. Such a situation

disallows quantitative interpretation. An example would be the coupling of

an Fez+-Fe” couple with an 0 2 - H 2 0 couple (Stumm and Morgan,



1981).

The establishment of a measureable balance between the cathodic and

anodic current at the Pt electrode at the point of zero-applied voltage

Mya condition that may not exist for many

requires activities of >

may be common,

redox-active species of interest in soils. Activities <

especially for certain plant nutrients and soil pollutants. For this reason,

application of measured Pt electrode potentials to predicting soil composition may not be possible if activities of oxidants and reductants are low.

4. Coupling of pH and pe



Based on the complementary nature of pH and pe (Stumm and Morgan,



+



198I), summing “pe pH” to describe log K for soils is possible theoretically. Because the Pt electrode is an “all-purpose” electrode that responds

to pH as well as electron activity, pH should always be measured and

reported with pe. The negative slopes of the pe-pH (Figs. 1 -6) relationships of many of the reduction half-reactions also indicate that the energy

change associated with a particular reduction decreases with increasing pH.

Therefore, an 8, measurement cannot be used to predict the presence of a

particular redox couple unless pH is known. Because higher pe values at

lower pH values correspond to larger releases of free energy, reduction

reactions are expected to be favored by lower pH. That is, such systems are

ones in which “reduction is favored.” In contrast, loss of electrons from

reductants of a particular couple is favored at higher pH, or the system is

more “prone to oxidation.”



C. ALTERNATIVE

STRATEGIES

FOR MOREACCURATE

MEASUREMENT

OF SOILREDOXSTATUS

1.



Using Electrochemical Relations in Reverse



Given the uncertainty associated with measured Pt electrode potentials

in soils to quantify pe, actual measurements of reductant and oxidant



REDOX CHEMISTRY OF SOILS



175



activities, along with a reliable pH measurement, may be a better approach

(Stumm and Morgan, 1981). The activities are substituted into appropriate

half-reactions relating pe and pH, and pe is thereby obtained by calculation.

As shown in Table 11, the predicted pe by such a technique will be

inaccurate to different degrees in different half-reactions. For example, an

error of two log units for H,S or 0, partial pressures will only produce

errors of 0.3 to 0.5 pe units. In contrast, similar errors in measurement of

Mn2+or Fe2+will result in pe errors of 1.O to 3.0 pe units. Because Mn and

Fe are relatively easy to measure accurately by atomic absorption or

colorimetric methods, and mineralogy of associated oxides can be made

with infrared or X-ray techniques, assessing pe values for Mn and Fe

oxide-dominated systems could be reliable. Detailed research is needed to

prove this hypothesis. In contrast, dissolved gases are harder to measure

accurately, and qualitative estimates may be sufficient to obtain accurate

evaluations of pe.

If calculated pe values obtained with this “reverse electrochemical technique” are equal for two different reduction half-reactions, then chemical

equilibrium may be assumed to exist. If the pe values are unequal, then

disequilibrium and a metastable, kinetically limited soil system probably

exist. This latter condition is common in soils due to spatial heterogeneity

of soil solution, oxide mineralogy, and organic matter reactivity. New

thinking and hands-on research are needed to provide new ideas for evaluation of the “electron activity” for such soils.

2. Redox Ranges for Empirical pe Values



These limitations to assessing soil pe based on R electrode or reverse

electrochemical methods indicate that our sense of accuracy for soil redox

status must be modified. If we surrender in our efforts to conceptualize and

operationally define soil redox status, we will have lost a challenging

scientific crusade. Rather, we should retreat temporarily. This means that

while new ideas are being developed, we accept a lack of knowledge of pe

values more accurate than ranges bracketed by whole numbers.

Liu and Narasimhan (1989) have described “redox zones” in which a

range in gh or pe defines an electron activity condition. The oxygennitrogen range is defined by 8, values of 250 to 100 mV, the iron range

is 100 to 0.0 mV, the sulJhte range is 0.0 to -200 mV, and that for

methane-hydrogen is defined at 8 h <-200 mV.

Sposito ( 1989) proposed oxic soils as those with pe > 7, suboxic ones in

the range of pe between 2 and 7, and anoxic soils with pe < 2, all at

pH 7. These ranges correspond roughly to redox control by oxygennitrogen, manganese - iron, and sulfur couples.



+



+



+



+



+



+



176



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



Berner (198 1) proposed categories for redox named oxic, postoxic, suljidic, and methanic controlled by transformations of oxygen - nitrogen, iron,

sulfur, and methane- hydrogen, respectively. James ( 1989) has proposed

these names be assigned to ranges in EMpe (Bartlett, 198 1 a) of 7 to 1 3,

+ 2 to +7, - 2 to +2, and -6 to -2. The appropriateness of these

categories and names will require further evaluation of new operational

definitions for the concept “redox status” in soils.



+ +



VIII. FREE RADICALS IN REDOX PROCESSES



A. FORMATION

OF FREERADICALSIN SOILS

A free radical is an odd electron species, that is, an atom or group of

atoms either donating or accepting a single electron and ending up with an

unpaired electron. Because the spin of an odd electron is not cancelled by

an electron of opposite spin, the free radical generates a magnetic moment

that makes it paramagnetic. This means that it is attracted to an external

magnetic field and can be detected and studied by means of electron spin

resonance (ESR) spectrophotometry. Electron cleavage in its formation

results in a high-energy free radical with a strong tendency to lose some of

its energy by forming new chemical bonds. It acts either as a powerful

reducing agent by donating its unpaired electron or as a powerful oxidizing

agent by accepting a mate for its odd electron.

Equation (30) shows a simple straightforward reduction half-reaction in

which one molecule of dioxygen (0,)accepts four hydrogen atoms, consisting of four protons and four electrons, and forms two molecules of

water. Restricting conditions of interaction between the availabilities of

soil 0, and electron donors, for example, at an interface between oxygenated water and anaerobic soil in a rice paddy, or between a living root and

the rhizosphere soil surrounding it, tend to favor electron transfers in single

steps, one electron at a time. Whenever the transferred electrons are not in

pairs, free radicals will be formed. Equations (27) and (29) show formation

of oxygen free radicals. The products in Eqs. (28) and (30) are not free

radicals.

The following reactions are half-reactions showing odd and even electron additions to O,, and reactions showing transfer of electrons among

oxygen species:



+ + H+

0, + 2e- + 2H+



O2 e-



-



__+



H Q (protonated superoxide)

H,O,



(hydrogen peroxide)



(27)

(28)



REDOX CHEMISTRY OF SOILS



0,



+ 3e- + 3H+ H,O + OH (hydroxyl free radical)

0, + 4e- + 4H+

2H,O (water)

HO; + H,O,

H,O + 0, + OH.

-+



-



*



OH. + H 2 0 2 - H 2 0 + H Q



+



(30)

(31)

(32)

(33)



2H202 a 2 H 2 0 + 0,

0.



(34)



MnO,



OH



(29)



2HO; x H Z O Z 0,



o+

OH



177



+ 3H+



-0



+ Mn3? + 2H,O



(35)



OH



B. BEHAVIOR

OF SOILFREERADICALS

With one electron, a hydrogen atom is a free radical, a single protonated

electron in frantic search for an electron mate. It is the simplest and most

reactive (least stable) free radical. As H,, however, it is quite stable, because the electrons are paired.

Catalases or fresh recently oxidized Mn oxides at pH > 6 (Bartlett,

I98 la) will catalyze the oxidation of one of the oxygens in H,Oz by the

other and thereby will destroy the peroxide by dismutation [Eq. (34)]. In

acid media, oxidized Mn will oxidize H,O,. The dismutation of HO; to

H,O, and 0, by superoxide dismutase (SOD) enzymes, Eq. (33), followed

by H,O, dismutation to 0, and H,O or oxidation of H,O, to O,, make

aerobic life possible (Fridovich, 1975; Halliwell, 1974) by preventing the

formation of the biodestructive hydroxyl free radical by Eq. (3 1). Reforming HO; by Eq. (32) is prevented.

Oxygen free radicals are much more reactive than 0,. Probably free

radical mechanisms explain why kinetically very slow and seemingly unlikely redox transformations sometimes occur readily. The hydroxyl free

radical, (OH the superoxide free radical, -OF, and the supermanganese

free radical ( Mn33are close to being the most powerful oxidizing agents in

soil systems, and superoxide and supermanganese also are the most powerful reducing agents (Table I, Fig. 3, and see Sections VI,A and VI,C)

(Bartlett, 198 I a).

Oxygen free radicals are among the few species having the thermodynamic capability for oxidizing Mn(I1). This means that Mn is one of the

few elements that is capable of scavenging these radicals and protecting life

a),



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