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XI. Photochemical Redox Transformations in Soil and Water

XI. Photochemical Redox Transformations in Soil and Water

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have begun study of this phenomenon at the University of Vermont using

polyethylene beakers containing salt solutions varying in concentration

from 40 to 200pM, exposed to sunlight through a glass window or to a

daylight fluorescent tube for periods of one or more hours. We measured

changes in solutions in Fe2+,Cr(VI), organic carbon, nitrate, nitrite, and

pH that occurred in the solutions as a result of their exposure to light. The

equations that follow are based on hypotheses developed to explain the

results of some of our experiments (R. J. Bartlett, unpublished data).

+ 2H,O1lghiHO; + 3H+ + 3Fe(OH),

Cr3++ 2H20 + HO; ~ighiHCr0;+ 4H+

KHC20, + 4Fe(OH)f + 2H,O llghi

HO; + 4H+ + 4Fe(OH), + KC20;

KC,O; + HO; + HCrO; + 3H+ llghi

Cr3++ 3H20 + 30, + 2C02 + KOH






Amazingly, a significant fraction of the Fe(II1) in a dilute solution of

FeCI,, exposed for 6 hr to through-glass sunlight, was reduced to Fez+ [ Eq.

(37)]. The original solution, consisting only of 200 p M FeCI, in distilled

water, was assumed to be largely Fe(OH)f, based on the pH. We concluded

( 1 ) that the electron donor for the Fe reduction was water, because no other

reduced species was known to be present, and (2) that the water oxidation

product probably was the protonated superoxide free radical. The superoxide radical was identified as the likely product by its behavior as both an

oxidant, in oxidizing Cr(II1) [Eq. (38)], and as a reductant, reducing

tetrazolium blue or methylene blue (Afanas’ev, 1989). Both phenomena

were prevented by pretreatment of the solution with superoxide dismutase,

made from horseradish.

The protonated superoxide product in Eq. (37) became the superoxide

reactant in Eq. (38), and the protonated superoxide and the oxalate free

radical, products in Eq. (39), were considered to be reactants in Eq. (40).

The light-induced reducing activity of the solutions for Fe was greatly

enhanced by the presence of oxalate [Eq. (39)], citrate, or phenolic compounds extracted by water from dried soil samples. The organics disap

peared from solution by apparent oxidation in the presence of light. The

presence of these organics was accompanied by light-induced reduction of

Cr( VI) [ Eq. (40)], and also by reduction of both nitrate and nitrite (equations not shown).

Although Fe(0H)t theoretically can oxidize Cr at pH < 4 (Fig. 5 and

Table I), it did not do so in the absence of light. Chromium was not

oxidized at all when organics were present. More easily oxidized than



Cr( HI), the organics were oxidized first. The order of oxidation apparently

was related to ease of oxidation.

It appears probable that these photochemical phenomena are of significance in waters above the soil surfaces in wetlands or paddies as well as on

unshaded moist surfaces of better drained soils. Toxic and allelopathic

organic substances could be destroyed or rendered harmless or even be

enhanced in reactivity by this process, and both CO, and 0, may be

evolved. Redox metals may be oxidized or reduced. Preliminary results

show denitrification greatly enhanced by light as well.



Next to putting the O2 into the atmosphere, covering surfaces of the

earth with humified soil has to be the second most astoundingly important

contribution of Mn to life on our planet. Development of soil organic

matter seems to be a process that takes place in the absence of light,

beneath the soil surface, and although oxidized Fe is an essential link in the

process of humus stabilization, the role of Mn redox is paramount in the

domain of darkness. Manganese ushers atmospheric oxygen from the

open-air soil pores to the poorly ventilated interior pore regions where

Fe(I1) and organic building blocks of polymers would tend to remain

reduced, were it not for Mn.

In observing profiles of acid forest soils in the northeastern United States

and southeastern Canada over a period of 20 years, we noticed a relationship between the Mn oxide content of the horizons of the mineral soil, as

measured in the field using tetramethylbenzidine (see Section XIV,D), and

the organic matter distribution in the profile. Spodosol profiles that had

well-developed E horizons (leached, bleached, low-organic horizons underlying organic layers) generally were almost devoid of Mn oxides. Profiles that had A horizons, mull types, with stabilized intermixing of mineral

and organic material, without E horizons beneath them, invariably displayed positive tests for Mn oxides throughout the profile.

In a subsequent study by Bartlett (1990a), chemical analyses of 4 1

pedons with spodic horizons directly under E horizons and 37 showing

spodic horizons directly beneath A horizons, including oxalate and pyrophosphate extractions, Mn electron demand (see Section XIVJ), H,02

dismutation (see Section XIV,J), leached columns, and flooded soil equili-



brations (see Section XIV,B), resulted in the conclusion that Mn oxides are

associated with absence of a developing E horizon and that Mn presence

strongly favors the formation of a mull type A horizon as the uppermost

mineral horizon of a Spodosol.



In the first part of a simple demonstration to illustrate the formation of

low-organic-matter E horizons in acid parent materials, dissolve a pinch of

FeSO, in a water extract of partially decomposed leaves (ordinary black tea

will do), and pour the solution through some light-colored fine sand from

an E horizon. The water coming through is still tea-colored, and the pale E

horizon retains its bleached E character. This could be considered an

extreme example of the white sand - black water phenomenon described by

Jenny (1980), i.e., a pale mineral E horizon disassociated from organic

matter that has been mobilized from the soil profile, giving color to the

leaching waters. In soils developed by the podzolization process in average

or low-Mn parent materials, Mn(I1) and Fe(I1) both will tend to be reduced by organic residues in the acid environment, and both will be

leached downward. Humus accumulation in the surface mineral soil layer

will be minimal, and eventually it will develop into an E horizon.

The organic matter distribution picture in acid soils would be skewed in

the extreme without mention of Al. In very acid soils containing only trace

amounts of Mn, the humified organic matter, mostly fulvic acids, is precipitated by Al, and is almost entirely in the B horizon, beneath the E.

Spodosol criteria are based on the amounts of accumulated Al-bound

organic matter in the B horizon. In sandy Ac;..ods, developed in poorly

drained parent materials in warm climates, both Fe and Mn tend to be

reduced and move completely out of the upper profile, and E horizons

above the Al -organic accumulation frequently are more than a meter

thick. Such profiles always will be practically devoid of Mn.




1. A Horizon Development

In the second part of our demonstration, add a few drops of fresh

synthetic MnO, suspension to the FeSO,/tea solution, and almost instantly

the Fe( 11) is oxidized, and inky black precipitated Fe( 111)-tannins cloud



the once-clear tea. Pouring the suspension through the pale-colored sand

causes the sand to turn black as it adsorbs the organic precipitate. The

filtrate coming through the sand will be clear and colorless, and the fine

sand is on its way to becoming an A horizon.

Topsoil-as in “all that’s between us and starvation is 6 inches of

it”-is the generic name for A horizon material of a generic soil. Evidence

seems to point to Mn as the key to the formation of A horizon topsoil in

both acid and nonacid soils. Iron and Mn are both important in formation

of A horizons in acid soils and in poorly drained soils, but in soils that

rarely reduce it (well-drained nonacid soils), Fe is much less important

than Mn in A development.

When atmospheric 0, oxidizes Fe( 11), protons are released by hydrolysis

of the resulting Fe(III), and the pH is depressed, causing this oxidation

process to be self-limiting [Eq. (4 l)]. When Mn oxidizes the Fe(11) instead,

protons are consumed as the Mn is reduced, and half as many protons are

released by the overall Fe oxidation/hydrolysis [Eq. (42)]. But when Mn

oxidizes organic substances, the pH tends to rise because electrons are

consumed during the Mn reduction [ Eq. (43)].


2Fe2+ 5H,O


+ 1/20,

+ + MnO,

4H+ + (COO-), + MnO,

2Fe2+ 4H,O

+ 4H+

2Fe(OH), + MnZ++ 2H+

2C0, + MnZ++ 2H,O





Addition of Eqs. (42) and (43) will show that the net effect of oxidizing

both Fe and the organic acid together by Mn is still one of H+ consumption

and pH rise, despite hydrolysis of the Fe(II1). If the Fe(II1) has been

complexed by fulvic acids as it formed, the pH effect, because of proton

release, might have been similar to that of hydrolysis. Loss of organic

matter and consumption of protons during the oxidiation would increase

both pe and pH in the environment of the Mn(II), and this should favor

reoxidation of the Mn( 11).

The Mn oxide-mediated synthesis of the A horizon in an acid soil

involves two roles for Mn: the oxidation of Fe(I1) and the oxidative

polymerization of humus components. Iron catalyzes the oxidative decomposition of phenols and other ready electron donors in soils (Section

IX,B). By oxidizing the Fe(II), Mn oxide interrupts the reaction of Fe( 11)

and 0, with the easily oxidizable organics and prevents their degradation.

The Fe(1II) and the phenolics form a black resistant precipitate that may

become part of metastable humus. Besides oxidizing Fe, the Mn oxides

may also partially oxidize some of the phenolics to free radicals, raw

materials for instigation of oxidative polymerization.



2. Oxidative Polymerization by Mn Oxides

Synthetic, amorphous MnO, causes the browning at room temperature

of a glucose-glycine solution and results in the formation of a stable dark

reddish brown gellike polymer similar to that obtained by boiling the same

sugar-amino acid mixture (Bartlett, 1988). At neutral pH, Mn( IV) causes

almost instant browning of phenolic compounds. Shindo and Huang

( 1982, 1984) showed that browning of hydroquinone by MnO, was related

to the formation of colors with spectra suggesting polymerization, and

Shindo (1990) investigated synthesis of humic acids from phenolic compounds by Mn( IV). Iron oxides produced little browning effect. Pohlman

and McColl(I988) showed that polyhydroxyphenolic acids with para and

ortho phenolic OH groups were rapidly oxidized by Mn oxides, with

spectral evidence indicating that the reaction led to polymeric humic

products by way of benzoquinone derivatives. The effects of Fe and Mn

oxides, and other soil minerals, on abiotic catalysis of oxidation and

browning and subsequent polymerization of phenolic compounds are discussed in a review by Wang et al. ( 1986).

Equation (35) shows the induction by MnO, of a semiquinone free

radical and also a Mn3+ free radical in the same reaction. By readily

accepting from or donating electrons in single electron steps to organic

species, Mn( 111) has the potential for creating semiquinone and organic

acid free radicals from fulvic acids, tannins, and simpler phenolic acids

(Bartlett, 1988). Like Mn( III), which may have helped form it, an organic

free radical can act as either a reducing agent or an oxidizing agent by

either donating its odd electron or by accepting a mate for it. Thus organic

free radicals serve as connecting links in inducing polymerization of humic

substances to form stable humus (Bartlett, 1986, 1988; Schnitzer and

Khan, 1972; Senesi and Schnitzer, 1978). Schnitzer and Khan (1972)

suggested that unstable free radicals induce humification, and the presence

of stable free radicals denotes that humification already has taken place.

Suflita ef al. ( I98 1) proposed a mechanism whereby a hydroxyl free

radical dehydrogenates a phenolic substrate, forming phenoxy free radicals, which, they suggested, will spontaneously couple among themselves

or attach to the polymeric structure of soil humus.

3. Stabilization of Organic Matter in Acid Soils

In acid soils developed from high-Mn parent materials, under podzolization perhaps less severe than in Spodosols that develop without A horizons, small amounts of Mn may resist reduction and leaching and will



persist in the profile. Some of this Mn could eventually be oxidized (probably by scavenging free radicals in rhizospheres) and then catalytically

oxidize Fe(I1) held in the reduced state by acid organic matter. Freshly

oxidized Fe will be in highly reactive amorphous oxide forms that become

attached to organic surface microsites. The result is precipitation and

stabilization of the Fe - organic matter and subsequent development of an

A horizon, perhaps over an E horizon or as an intermediate A/E horizon,

although usually, if a well-expressed A horizon is present in a Spodosol, the

E will be absent. In effect, the Mn acts as an oxygen pump, moving 0,

from the soil pores and stabilizing that oxygen as oxides on the soil surfaces

that would be highly reductive, were it not for the Mn. By preventing

mobilization of organic matter in the surface mineral horizon, presence of

Mn in the A horizon will prevent the formation of an E horizon beneath it.

It is apparent that Mn in the parent material of a developing acid soil

detracts from expression of spodic character in the soil profile, but the

effects may not be enough to prevent the soil from qualifying as a Spodosol.

4. Stabilization of Organic Matter in Near-Neutral Soils

Although it may not be possible to say which is adsorbed by which, there

appears to be an almost universal association between oxidized Mn and the

organic matter in A horizons. Freshly formed Mn oxides have negative

charges and measurable CECs and thus show strong tendencies to adsorb

Mn( 11) cations. Electrophoretic mobility observations show that adsorption of Mn(I1) will reverse the charge on a Mn oxide from negative to

positive (Bartlett, 1988). The positive charge induced by adsorption of

Mn(11) seems to be a likely source of the attraction by oxides to negatively

charged organic substances (Fig. 7). The adsorption onto organic matter

sometimes causes the oxides of the heavy metal Mn to float on top of the

solution in a centrifuge tube. The presence of Mn oxides, as part of the

humified organic matter, contributes greatly to the stabilization of the

overall humus system.

In parent materials containing quantities of base-forming elements sufficient to maintain surface horizon pH values above about 5.5, reduction of

Fe by organics will not occur. Manganese will be readily reoxidized after it

is reduced by oxidizing organic ligands. The freshly formed Mn oxides, and

especially the activated MnH supermanganese free radical, will catalyze

polymerization of organic substances to form humic materials, and a

strongly developed mull type of A horizon material will be developed

immediately under the litter layer. The resulting soil will not be a Spodosol.



Soils developed under conditions of very high base saturation and/or

from calcareous parent materials are noted for their degree of development

of mull-type surface horizons. The A horizons are dark in color, high in

organic carbon, and have strongly expressed structure related to their high

humus content (Lutz and Chandler, 1946). It is easy to see how Mn oxides

would be maintained in a high state of oxidation in such soils and would

have maximum contribution to A horizon development. Rapid rates of

reoxidation of Mn( 11) following free radical-forming oxidations of organic

acids and phenolic compounds by Mn oxides would lead to rapid rates of

oxidative polymerization of humus components. Leaching losses of Mn( 11)

would be almost nonexistent.

5 . CO, from Added Organics Versus Organic Matter Stability

In laboratory experiments, evolved C 0 2 was used as a negative index of

organic matter stabilization from added organics. Organic substances, such

as organic acids, phenolic compounds, and cellulose, added to soil samples,

were assumed to have become stabilized as soil organic matter after rate of

CO, evolution settled to the level that it was before the organic additions

(Bartlett, 1990b). Compared to low-Mn oxide soil samples, those with

high-Mn oxides behaved as follows: ( 1 ) High-Mn samples evolved more

CO, when cellulose, glycine, and citric and gallic acids were added at field

capacity moisture. Mn oxides appeared to stimulate microbial activity. (2)

With flooding and organic additions, high-Mn samples remained aerobic

and at first evolved less CO, . Low-Mn samples formed Fe(I1) and evolved

more CO, . (3) High-Mn oxides increased CO, when microbial respiration

was the predominating oxidizing vector. (4) However, high-Mn oxides

decreased CO, evolution when microbial activity was suppressed in biologically relatively inert E horizon material or in the presence of Cr(V1)

formed by the oxidation of Cr( 111) by Mn oxides.








Not surprisingly, a wetland is frequently wet, but it owes its characteristics not to water directly but to the effects of water-filled soil pores in



restricting the supply of oxygen to solid surfaces in the soil body. Wetland

soils are presently legally defined by agreement among the Soil Conservation Service (SCS), The Fish and Wildlife Service, the Environmental

Protection Agency (EPA), and the Army Corps of Engineers. The SCS

considers that a “hydric” soil identifies a wetland, and to be hydric, a soil

must have a presence of free water and virtual absence of O2 in it or on it

for extended periods, but not necessarily continuously. Anaerobic conditions must develop in the upper profile during these periods. This means

there must be organic residues present, and temperatures need to be of the

degree and duration required for microbial synthesis of reactive electron

donors. Soil conditions must favor the production and regeneration of

hydrophytic vegetation. Soils will tend to have high accumulations of

organic matter, gley mottles near the surface, a predominance of gray or

black colors below, and Fe2+ will form extensively during anaerobic periods. Unpolluted sediments at the bottom of a cold lake with low biological activity may not be a wetland.

Large areas of soils in the United States, developed on level lacustrine,

marine, and ground moraine parent materials and also large areas of peat

and muck soils, have been artificially drained by ditching or tiling to create

highly productive and valuable agricultural land. Drainage of these lands

was encouraged by the Federal government for many years and has been

accomplished to a considerable extent in the major humid agricultural

regions of the country. Recently, tough new laws have been passed to

prevent further drainage of wetlands. Consequently, emphasis is on diverting areas of precious welldrained farmland for use in development.




There are many valid reasons, often related to redox, for preserving any

unique ecosystem in its natural state, particularly if it has plants, animals,

and soils uniquely adapted to it. Preserving any soil-wetland or upland

-means saving a portion of our biological heritage. Soils being used in

productive agriculture have already been destroyed as parts of natural

ecosystems, but their continued preservation as viable cropland soils for

producing food is of extreme importance to the survival of humankind.

Because of its substantial accumulation of humus, the wetland soil is a

caricature or an exaggeration of a “normal” soil. Draining a wetland soil

will release C02and will increase the greenhouse effect and also entropy of

the earth more than disturbing a well-drained soil.

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XI. Photochemical Redox Transformations in Soil and Water

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