Occurrence of GRs
The redox chemistry of iron has a strong influence on the fate and distribution of numerous natural and xenobiotic compounds. The nature of the Fe-bearing minerals can modify considerably the oxido-reduction potentials where the transition from ferric to ferrous state occurs.
Some oxido-reduction reactions in soils are homogeneous in aqueous solution, but many are heterogeneous and involve both solids and the solution. For this reason, different redox reactions occurring in the soil cannot be simply ordered according to the standard potentialE0. Moreover, stoichiometric coefficients of oxidized and reduced species and Hþ are variable from one reaction to another, so that the order of reactions changes with pH. Sposito (1981) proposed to compute the electron potential (pe ) at a given pH and for a given value ofẵAred =ẵAox, the ratio of the activities of
0 1
1
Mg (OH)2
0
X2 mole fraction Fe (OH)3 X1 mole fr
action Fe ( OH)
2 X
3 mol e fract
ion Mg (OH
2) Excluded
domain Allowed
domain
N P
M A
2 3
3 4 1
2 1
3 1 4
1 4
1 4 1 3 1 2 2 3 3 1 4
3
1 2
2 3 3 4
D
C
B 0 1
Fe (OH)2 [Fe (OH)2] + [OH−]−
Figure 15 Range of variation in the composition of fougerite in the ternary diagram Fe(OH)2—Mg(OH) 2 —Fe(OH) 3 system, adapted from Bourrie´ et al. (2004 ). Fougerite is stable in the narrow domain between solid line AD and dashed line MN; it starts forming, for example, at point N and oxidizes from N to P, where it transforms into Fe (III) oxyhydroxide.
Geochemistry of Green Rusts and Fougerite 271
Table 10 Variations with pH of the ‘‘critical’’ value of pefor the main oxido-reduction reactions in water
‘‘Critical’’peat
Reactions pHẳ4 pHẳ7 pHẳ9
1/4 O2(g)ỵHỵỵe ⇆1=2H2Oðl:ị 16.6 13.6 11.6
1=8 NO3 ỵ5=4Hỵỵe ⇆1=8NHỵ4 ỵ3=8H2Oðl:ị 9.24 5.4 2.85
NO3 ỵ2Hỵỵ2e ⇆NO2 ỵH2Oðl:ị 7.3 4.3 2.3
gFeOOHlepidocrociteỵ3Hỵỵe ⇆Fe2ỵỵ2H2Oðl:ị 11.2 2.7 3.41
aFeOOHgoethiteỵ3Hỵỵe ⇆Fe2ỵỵ2H2Oðl:ị 10.0 1.0 5.0
1/8 SO42þHþþe ⇆1/8 S2þ1/2H2O(l.) 2.2 5.2 7.2
the oxidized and reduced species. Accordingly, for homogeneous reactions, the ẵAred =ẵAox ratio is taken as 106 conventionally. For heterogeneous reactions, where the oxidized species is a solid and dissolves during reduc- tion, [Ared] is taken as 107, with the assumption that the solid phase is pure and its activity [Aox] is equal to 1. The value ofpeobtained is designated as
‘‘critical’’pe. Forpesmaller than the ‘‘critical’’ value, the reduction reaction is quasi-complete.
From the thermodynamic data (Bratsch, 1989) of the most stable species, it is possible to establish a scale of sequential reduction potentials of O(0)/O (-II), N(V)/N(-III), Fe(III)/Fe(II), and S(VI)/S(-II). The classical order is obtained and steps larger than 100 mV (1.5peunits) separate the successive redox couples (Table 10).
In addition, in this sequence the position of Fe(III)/Fe(II) is based upon the assumption that redox potential is controlled by a ferric oxide/Fe2þ couple at equilibrium. But in soil, different Fe-minerals are observed indeed. In strong reducing conditions (i.e., pe<2), Fe-sulfides precipi- tate; in oxidizing conditions (i.e., pe > 10), Fe-oxides such as goethite, lepidocrocite, or hematite predominate. In moderately reducing conditions (i.e.,2<pe<9), aqueous Fe is controlled by the equilibrium with mixed Fe(II)–Fe(III) hydroxides (Ponnamperuma, 1972; Lindsay, 1979) belonging to the GRs group (Trolardet al., 1997; Bourrie´et al., 1999). When GRs are present in a milieu, they can control the redox potential at equilibrium rather than the more stable ferric iron species. The exact nature of the GR depends on the nature and abundance of anions. Ponnamperuma (1972), Olowe and Ge´nin (1991), Refait and Ge´nin (1993), and Drissiet al., (1994) have measured the respective thermodynamic data of the natural hydroxy- hydroxide Fe3(OH)8, the synthetic GR compounds of hydroxysulfate, hydroxychloride, and hydroxycarbonate. The ‘‘critical’’pedata (Table 10) were calculated for redox couples different from the compounds cited above and under the assumptions exposed in the preceding paragraph.
The sequences are equally calculated at pHẳ6 and pHẳ8.5, which are the most frequent edges of pH observed in soils.
Figure 16shows that the reduction of Fe oxides into aqueous Fe2þ is possible throughout a large range of redox potential, as underlined by Stumm and Sulzberger (1992). This fact must be kept in mind to predict the interaction between GRs and some anions in soils, such as nitrate, selenate, or chromate (Ge´ninet al., 2001).
In the recent literature, interactions between GRs and several chemical species have been studied in the laboratory or in the field. The principal effect of GRs is the reduction of these species, with the oxidation of GRs into ferric oxides, that is, magnetite, lepidocrocite, or goethite.
Geochemistry of Green Rusts and Fougerite 273
11.1. GRs and nitrogen
While in solution NO3 cannot be reduced by Fe2þ,Hansenet al., (1996;
2001) showed that in abiotic conditions GR1(Cl) and GR2(SO4) reduced stoichiometrically nitrates into ammonium in several hours. They observed that reduction rates increased with increasing nitrate concentrations up to some threshold concentrations, depending on the initial conditions of the reaction, above which no further increase in reduction rates took place. The kinetics of the reaction depend on the type of the interlayer anion, the layer charge, and the relative content of Fe(II) in the hydroxide layers.
In addition, Huang and Zhang (2004) observed that pH modified the rate of reduction by a direct implication of Hþ in the redox reaction following first-order kinetics and Hþ ions affected the nitrate adsorption onto reactive sites of iron grains. Thermodynamic calculations (Trolard and Bourrie´, 1999) showed that the reduction of nitrate proceeds largely before the reduction of ferric oxides, following the classical sequence when GRs were absent. When GRs were present, and irrespective of the nature of
1.0 Eh (V)
(pH7)
Ox Red
FeIII(phen)33+ FeII(phen)23+ Fe2+(10−7M)
Fe2+(10−7M) Fe2+(10−7M) 0.8
CI − GR(s)
SO4− GR(s) CO3− GR(s)
FeIIIOH2+
(≡FeIIIO)2FeIII+(s) (≡FeO)2FeII(s) FeIIOH+ FeIIsal
Fe2SiO4(s) FeIIEDTA2− FeIIporph
Fe2+(10−5M) Fe2+(10−5M) Fe2+(10−5M) FeCO3(s)*
Fe3O4(s) FeIIIEDTA−
FeIIIporph
aFeIIIOOH(s) aFeIIIOOH(s) aFeIIIOOH(s)
Oxferredoxin Redferredoxin aFe2O3(s)
Fe3O4(s) Ferrihydrite(s)
FeIIIsal
Limit of water stability O2/H2O(I)
Limit of water stability H2O(I)/H2
Fe3+ Fe2+
0.6
0.4
0.2
−0.2
−0.4 0
Figure 16 Scale of ‘‘critical’’ values ofEhfor major Fe redox couples, at pHẳ7 and
*valid forẵHCO3 ẳ103M. Phen, phenanthroline; sal, salicylate; porph, porphyrine;
GR, green rust.
274 Fabienne Trolard and Guilhem Bourrie´
80 70 60 50 40 30 20 10 0
S O N D
Time (month)
Fe(II)⫻10−6M NO3−⫻10−6M
NO3−/NO2−
J F M
O2/H2O
NO−3/NO−2 760
Eh (mV) Eh (mV)
O2/H2O Fougerite / Fe2+
FeOOH/Fe2+
760 400 256
Winter
−88 256
Summer
FeOOH/Fe2+
−88
Figure 17 Seasonal dynamics of aqueous Fe(II) and nitrate in soil waters in Brittany, modified from Jaffrezic (1997 ), showing first the disappearance of nitrate before the release of Fe(II), then the coexistence of Fe(II) and N(V) (see text). The scales of ‘‘critical’’Ehwere calculated for pHẳ6.
the compensating anion, the redox sequence was determined by the pH of the milieu. For example, at pH ẳ6, the reduction of GRs precedes the reduction of nitrate, whereas at pH ẳ 8.5, the reduction of GRs follows the reduction of nitrate.
These results imply that nitrate and GRs can compete as electron acceptors for microflora in anaerobic conditions. At pH ẳ 6, the redox buffering by GRs protects nitrate and thus bypasses denitrification. At pHẳ 8.5, in presence of GRs, denitrification is possible. However, in this case, as we have shown above, nitrate can be spontaneously reduced by GRs into NHþ4.
In the field, such evidence can be observed on Fe(II) and NO3 dynamics in soil solution (Jaffrezic, 1997,Fig. 17). At first, at the end of summer, no Fe(II) is present in solution. At the beginning of autumn, nitrate concentra- tion decreases to zero before Fe(II) is released in solution. This is the classical scheme (Fig. 17, left). Then, in winter, fluctuations of Fe(II) are observed, due to partial entry of oxygen from rainwater. This results in the formation of fougerite. When fougerite is reduced, as electron acceptor, it competes with nitrate and coexistence of N(V) and Fe(II) is observed, which is contrary to the classical scheme, but well explained, if the reaction fougerite/Fe(II) is considered in the scale of ‘‘critical’’Eh(Fig. 17, right), as predicted byTrolard and Bourrie´ (1999).
Strictly speaking, this reduction is not denitrification, as ammonium formed must undergo nitrification before any biotic denitrification, and this is not a depolluting process, as ammonium is more toxic than nitrate.
But this process can be used to prevent N loss from the soil as nitrate to the groundwater and surface water, or to the atmosphere as N2and N2O.
Ammonium is readily fixed on clay minerals and can be later nitrified to nitrate or released in solution after cation exchange, thus being available to plants.
11.2. Reaction mechanisms
The layered structure of the mineral explains the particular reactivity of GRs in abiotic conditions.Figure 18summarizes, for example, the steps of reduction of nitrate by SO4-GR: the first step is an anion exchange between nitrate and sulfate in the interlayer of the GR, and GR2 transforms into GR1. In these conditions, a minimum of 8 divalent Fe2þ are immediate neighbors of NO3. The reduction of NO3 into NHþ4 can, thus, proceed spontaneously with simultaneous transfer of 8 electrons from Fe2þto N. In the layer, the Fe(II)–OH–Fe(II) bonds transform into Fe(III)–O–Fe(III) by oxolation, releasing 8e and 8Hþ; 4 additional Hþ equilibrate the two sulfate anions released; 8e and 10Hþ are necessary for each NHþ4 (Table 10), so that there is a net release of 2 protons. The GR oxidizes into hematite.
276 Fabienne Trolard and Guilhem Bourrie´
The half-reactions and the net reaction can be written as follows:
2FeII4FeIII2 ðOHị12SO4 ! 6Fe2O3;hematiteỵ6H2O þ2SO24 þ8eþ12Hþ NO3 þ8eþ10H!NHþ4 þ3H2O
FeII/FeIII layer
FeII/FeIII layer
SO42−
Fe3+
+4H+ N−III
H H
H H
H H
H H
SO42−, H2O interlayer
Fe2+
NO32−
N+v Fe3+
Figure 18 Reaction scheme for the abiotic reduction of nitrate by sulfate-GR: top:
initial state of the interlayer in GR-SO4; middle: first step, exchange of sulfate by nitrate and collapse of the interlayer; bottom: initiation of reduction of nitrate by eight neighboring Fe(II) ions, and final state of ammonium production before ammonium release to solution.
Geochemistry of Green Rusts and Fougerite 277
2FeII4FeIII2 ðOHị12SO4ỵNO3 !6Fe2O3;hematite
þ 9H2Oþ2SO24þNHþ4 þ2Hþ:
11.3. GRs and selenium
Other works documented the reduction of selenate Se(VI) by GRs and underlined that it was the only relevant abiotic reaction of reduction pathway in natural environments, soils, and sediments. In contrast, Se(IV) reduction by GRs was less inhibited kinetically and was induced by a variety of abiotic and bacterial processes (McNeal and Balistrieri, 1989). They showed that SO4-GR reduced selenate Se(VI) to Se(0) or Se(-II) (Myneni et al., 1997; Refait et al., 2000). These reactions induced an increase in
80Se/76Se ratio, which shifted by 7.36 0.24 % of dissolved selenate as lighter isotopes were preferentially consumed during reduction by SO4-GR (Johnson and Bullen, 2003). Abiotic selenate reduction by GRs induced much greater isotopic fractionation than does bacteria selenate reduction.
11.4. GRs and metals
Many metals or metalloı¨ds, for example, Cu, Hg, Ag, Au, Cr, and As, have multiple valence states within the range of redox conditions encountered in surface or near-surface environments and may interact with Fe.O’Loughlin et al., (2003) reported experiments where aqueous solutions of AgCH3-
COO, AuCln(OH)4n, CuCl2, or HgCl2were added to hydroxysulfate GR suspensions. Soluble Ag(I), Au(III), and Cu(II) species are often relatively mobile, thus reduction to comparatively insoluble Ag(0), Au(0), and Cu(0) phases can also be expected to reduce the mobility of silver, gold, and copper. However, unlike Ag(0), Au(0), and Cu(0), Hg(0) is relatively soluble in aqueous solution. Moreover, Hg(0) has a significant vapor pressure and can be lost through volatilization.
Results showed that Ag(I), Au(III), Cu(II), and Hg(II) were readily reduced to Ag(0), Au(0), Cu(0), and Hg(0), respectively. The resulting solids from Ag (I)-, Au(III)- and Cu(II)-amended GR suspension observed by transmission microscopy were submicron-sized particles of Ag(0), Au(0), and Cu(0).
The reduction of Hg(II) to Hg(0) by GR may actually increase the overall mobility of mercury; indeed the release of Hg(0) vapor to the atmosphere is a significant component of the global cycling of mercury (Steinet al., 1996; Schlu¨ter, 2000).
Chromium displays several oxidation states from 0 toþVI but of these, Cr(III) and Cr(VI) are by far most common in nature. Cr(III) is much less soluble, and is considered as nontoxic, than Cr(VI), which is toxic. This
278 Fabienne Trolard and Guilhem Bourrie´
suggests that in Cr-polluted groundwater and soil, reduction of Cr(VI) to Cr(III) is, therefore, desirable. Loyaux-Lawniczak et al., (1999, 2000 ) stud- ied the interactions between Cr(VI) and GRs, namely, GR-SO4 and GR- Cl. They showed that GRs proved to be very reactive; their interaction with potassium chromate solution leads to the rapid and complete reduction of Cr(VI) into Cr(III). The nature of the initial GR proves to have no influence on the process, an outcome mainly due to the structural properties which define these compounds.