Reversible surface sorption induced elec

Reversible surface-sorption-induced electron-transfer oxidation of Fe(II) at reactive sites on a synthetic clay mineral

a b d e A. Ge´hin a , J.-M. Grene`che , C. Tournassat c,* , J. Brendle´ , D.G. Rancourt , L. Charlet

a LGIT, Universite´ Grenoble-1, F 38041 Grenoble, France b LPEC, Universite´ du Maine, F 72085 Le Mans, France

c BRGM, Environment and Process Division (EPI/MIS), 3 avenue Claude Guillemin, F 45060 Orle´ans, France d LMPC, ENSC, F 68093 Mulhouse, France e Department of Physics, University of Ottawa, Ottawa, Ont., Canada K1N 6N5

Received 6 April 2006; accepted in revised form 23 October 2006

Abstract The sorption of 57 Fe(II) onto an Fe-free, mineralogically pure and Ca-saturated synthetic montmorillonite sample (structural formu-

la: Ca 0.15 (Al 1.4 Mg 0.6 )(Si 4 )O 10 (OH,F) 2 ), was studied as a function of pH under strictly anoxic conditions (N 2 glove box atmosphere, O 2 content <1 ppm), using wet chemistry and cryogenic (T = 77 K) 57 Fe Mo¨ssbauer spectrometry. No Fe(III) was detected in solution at any pH. However, in pH conditions where Fe(II) is removed from solution, a significant amount of surface-bound Fe(III) was produced, which increased with pH from 0% to 3% of total Fe in a pre-sorption edge region (i.e. at pH < 7.5 where about 15% of total Fe is sorbed) to 7% of total Fe when all Fe is sorbed. At low pH, where the pre-sorption edge plateau occurs (2 < pH < 7.5), the total sorbed-Fe amount remained constant but, within this sorbed-Fe pool, the Fe(III)/Fe(II) ratio increased with pH, from 0.14 at pH 2 up to 0.74 at pH 7. The pre-sorption edge plateau is interpreted as cation exchange on interlayer surfaces together with a sorption phenomenon occurring on highly reactive (i.e. high affinity) surface sites. As pH increases and protons are removed from the clay edge surface, we propose that more and more of these highly reactive sites acquire a steric configuration that stabilizes Fe(III) relative to Fe(II), thereby inducing a Fe to clay particle electron transfer. A sorption model based on cation exchange combined with surface complexation and electron transfers reproduces both wet chemical as well as the Mo¨ssbauer spectrometric results. The mechanism is fully reversible: sorbed-Fe is reduced as pH decreases (Mo¨ssbauer solid-state analyses) and all Fe returned to solution is returned as Fe(II) (solution analyses). This would not be the case if the observed oxidations were due to contaminant oxidizing agents in solution. The present work

shows that alternating pH may induce surface redox phenomena in the absence of an electron acceptor in solution other than H 2 O.

1. Introduction et al., 1998a,b, 2002; Hofstetter et al., 1999, 2002, 2006; Pe- cher et al., 2002; Strathmann and Stone, 2003 and referenc-

Aqueous ferrous iron is a major cation and an impor- es therein). At equilibrium, dissolved Fe(II) is controlled by tant reductant in a variety of natural anoxic environments

siderite (FeCO 3(s) ), green rusts and Fe-rich calcite in sul- (eutrophic lakes, ocean and swamp sediments, hydromor-

ﬁde-poor, carbonate-rich surﬁcial environments, and by phic soils, anoxic groundwaters and ocean basins), as well

FeS x (where1 < x < 2; pyrite for x = 2) in sulﬁde-rich envi- as in engineered systems, i.e. zero-valent iron permeable

ronments ( Berner, 1971 ). In the former case, Fe(II) concen- reactive barriers, or PRBs, and radioactive waste reposito-

M, depending on pH ry sites (e.g. Wehrli, 1990; Klausen et al., 1995; Charlet

trations range from 10

to 10

and P CO2 ( Emerson, 1976; Postma, 1982; Criaud and Fou- illac, 1986a,b ), while in the latter case, Fe(II) concentrations are often in the 10 –10 M range ( Emerson et al.,

* Corresponding author. Fax: +33 2 38 64 30 62.

1980; Balzer, 1982; Davison et al., 1999; Bott, 2002 ). In

E-mail address: c.tournassat@brgm.fr (C. Tournassat).

low P CO2 , low P H2S environments, Fe(II) solubility is

doi:10.1016/j.gca.2006.10.019

A. Ge´hin et al. 71 (2007) 863–876

controlled by the precipitation of Fe(OH) 2(s) , Fe 3 O 4 (mag-

oxidation of the suspension prior to its transfer in the glove

netite), green rust, or Fe 3 PO 4(s) . In granular, zero-valent

box.

iron PRBs and their outﬂows, Fe(II) concentrations typi- cally lie between 10

and 3 · 10

M( Naftz et al.,

2.2. Clay material preparation

2000 ). However, many surfacic systems are not at equilibrium and are subject to cyclic redox ﬂuctuations ( Pere-

The synthesis of montmorillonite having the theoretical tyazhko and Sposito, 2005 ), e.g. as a result of seasonal

formula: Na 0.30 [(Al 1.70 Mg 0.30 )Si 4 O 10 (OH,F) 2 ], nH 2 O was climatic variations ( Thompson et al., 2006 ). The migration

performed in acidic and ﬂuoride medium under hydrother- of Fe(II) ions in aqueous environments is, as with most

mal conditions ( Reinholdt et al., 2001 ). A hydrogel having divalent cations, retarded by sorption on minerals with a

a SiO 2 :0.212 Al 2 O 3 :0.075 MgO:0.075 Na 2 O:0.05 HF:96 large surface area and speciﬁc sorption properties such as

H 2 O molar composition was prepared starting from water, clay minerals ( Drever, 1971; Liger et al., 1999 ).

hydroﬂuoric acid, sodium and magnesium acetate, silica The clay sorption capacity has been well established for

and aluminum oxide. This mixture was matured for 2 h

a wide range of potential pollutants in Na + and Ca 2+ back- at room temperature before being hydrothermally treated ground ionic media (e.g. Fletcher and Sposito, 1989; Char-

in a PTFE lined stainless steel autoclave at 493 K for let et al., 1993; Zachara et al., 1993; Zachara and Smith,

72 h under autogenous water pressure. After crystalliza- 1994; Baeyens and Bradbury, 1997; Bradbury and Baeyens,

tion, the product was separated by ﬁltration and thorough- 1997, 1999, 2002; Akcay, 1998; Turner et al., 1998 ). How-

ly washed with distilled water. The pH of the supernatant ever, the inﬂuence of other background ionic media such as

was 3.5–4.5. After drying for one night at 338 K, the sam- Fe(II)-containing water is poorly documented. Previous

ple was placed in a controlled humidity chamber (P/ studies of Fe(II) sorption processes on montmorillonite

P 0 = 80%, where P represents the saturated vapor pressure have shown that Fe(II) can sorb onto clay minerals in cat-

over the sample at 298 K and P 0 the saturated aqueous va- ion exchange position with a similar clay aﬃnity for Fe(II)

por pressure at 298 K). Ca-montmorillonite was obtained and Ca(II) ( Kamei et al., 1999; Charlet and Tournassat,

by saturation of the prepared Na-montmorillonite with a 2005 ). In the present study, new chemical and spectromet-

0.05 M aqueous solution of CaCl 2 . After treatment, the sol- ric experiments are proposed to characterize the mecha-

id suspension was argon-degassed, before being placed in nism of Fe(II) speciﬁc sorption on clay minerals.

the glove box.

We investigated the speciﬁc sorption of Fe(II) on a synthetic Fe-free clay mineral analogue of montmorillonite.

2.3. Clay material characterization

The starting solid synthetic sorbant was devoid of any Fe, which is otherwise present in natural samples, either

For XRD analysis, suspension aliquots were poured within the clay mineral or as accessory trace minerals. All

through a Millipore ﬁlter (0.4 lm) and the clay cake was the sorption experiments were carried out in strict anoxic

then deposited on a glass slide. The resulting oriented prep- conditions designed to prevent oxidation and to keep iron

arations were then dried at room temperature. XRD pat- as ferrous iron. Solution chemistry and Mo¨ssbauer spec-

terns were then recorded using a Bruker D5000 trometry were used concomitantly. Results were analyzed

diﬀractometer (CuKa 1,2 radiation: k = 0.15418 nm). The in terms of (i) surface complexation and (ii) steric-driven

X-ray diﬀraction patterns of the synthetic Ca-montmoril- reversible Fe oxidation in which electron transfer occurs

lonite ( Fig. 1 , grey line) show only basal 00‘ reﬂections, between the sorbed-Fe centre and highly reactive clay edge

due to the oriented preparation of the samples. It compares functional groups.

well with a Ca-exchanged natural montmorillonite (MX80

2. Materials and methods

2.1. Chemicals All solutions and suspensions were prepared with

boiled, argon-degassed Millipore Milli-Q 18 MX water. NaOH and HCl stock solutions were made from Titrisol

ampoules. The CaCl 2 and FeCl 2 stock solutions were pre-

pared from analytical grade salts using the method described in Charlet and Tournassat (2005) . The latter

involves the dissolution of 100 mg of 57 Fe(0) in concentrat-

ed HCl (0.1 mol l Fig. 1. X-ray diﬀraction patterns of the Ca 0.3 [(Al 1.4 Mg 0.6 )Si 4 O 10 (OH,F) 2 was placed in the glove box and diluted in deionized water ]

57 synthetic montmorillonite (grey diﬀractogram) and the Ca 0.14 (Al Fe(II) 1.61

Mg 0.24 Fe III 0:15 Fe 0:02 II ) (Si 3.98 Al 0.02 )O 10 (OH) 2 MX80 montmorillonite (black

stock solution. The very acidic conditions prevented the

diﬀractogram).

Fe electron transfer at montmorillonite reactive sites

sample, Fig. 1 , solid line). The d 001 value is the same for the ment. As a control of the sorption reproducibility, the sus- synthetic and the natural Ca-montmorillonite and equals

pension was titrated once again with NaOH after the ˚ . The reﬂections close to 4.45–4.48 A˚ (020, 110) 15.27 A

desorption experiment.

and 2.52–2.58 A ˚ (130, 200) correspond to hk0 bands of dioctahedral smectites.

2.5. 57 Fe Mo¨ssbauer spectrometry

The cation exchange capacity of the synthetic montmorillonite were measured with the Cs–Li method described

Mo¨ssbauer spectrometry was performed on a solid frac- by Anderson and Sposito (1991) at pH 7. At this pH, the

tion obtained from these sorption experiments at ﬁve diﬀer- permanent structural charge (Cs) was found to be

ent pH values during the sorption experiment and one pH

0.44 eq/kg and the variable charge (Li) 0.19 eq/kg, value during the desorption experiment. At these pH val- accounting for a total CEC of 0.63 eq/kg at pH 7.

ues, once the pseudo-equilibrium was reached, small aliquots (a few mL) of suspension were ﬁltered (Millipore

2.4. Fe(II) sorption and desorption experiments ﬁlter 0.02 lm) and deposited on Mo¨ssbauer sample holders (diameter 1.5 cm). The sample holders were capped and

Sorption experiments of Fe(II) on the synthetic mont- then sealed with an epoxy resin. They were stored in the morillonite were followed by both solution chemistry and

glove box before being taken out, immediately frozen in li- Mo¨ssbauer spectroscopy. The chemistry experiments were

quid nitrogen and transported in a Dewar ﬂask ﬁlled with conducted in a N 2 atmosphere glove box (Jacomex) in

liquid N 2 to the Mo¨ssbauer exchange-gas-bath cryostat.

The Mo¨ssbauer spectra were recorded at 77 K using a continuously with a Jacomex O sensor. The O

which the oxygen partial pressure (pO 2 ) was monitored

constant acceleration spectrometer and a Co source dif- never exceeded 1 ppm in the glove box atmosphere, corre-

2 2 content

fusing into a rhodium matrix. Velocity calibrations were

sponding to a maximum solute O 2 concentration of

carried out using a-Fe foil at room temperature (RT,

0.13 lmol/L. 295 K). All isomer shifts are reported relative to the a-Fe In these experiments, the speciﬁc sorption of Fe(II) was

spectrum obtained at RT.

studied as a function of pH on a short reaction time scale. Two ﬁtting models were considered to describe the Cation exchange between Fe(II) ions and the clay surface

Mo¨ssbauer spectra (which show a quadrupolar structure).

The ﬁrst one (MOSFIT: Teillet and Varret unpublished which limits cation exchange by mass eﬀect and maintains

was minimized by using a 0.05 M CaCl 2 ionic background,

program) consists in using a discrete number of indepen-

a constant total normality of the suspension. Experiments dent quadrupolar doublets of Lorentzian lines where the were carried out in closed 350 ml glass reactors with three

line width at half-height C (mm s ), the centre shift d inputs, one for the pH electrode and the two others for

(mm s ) and the quadrupole splitting DE Q (mm s ) were stock solution/suspension addition and sample extraction,

reﬁned using a least-squared ﬁtting procedure. The second respectively. An aliquot of clay stock suspension was added

modeling approach consists in using the Voigt-based meth- to CaCl 2 ionic background solution samples. The suspen-

od of Rancourt and Ping (1991) for quadrupole splitting sion was acidiﬁed with HCl to pH 2 overnight (approxi-

distributions (QSDs) with linear coupling to slave centre

shift distributions ( Rancourt, 1994a,b; Rancourt et al., solution aliquot was added and the sorption experiments

57 Fe(II) stock

1994; Evans et al., 2005 ) as implemented in the commercial

www.isapps.ca/recoil ). in order to enhance the Mo¨ssbauer signal. The sorption experiments proceeded by increasing the pH incrementally

were started. Isotopically pure 57 Fe(II) (> 97%) was used

3. Results

with successive additions of NaOH until all 57 Fe(II) was

sorbed. After each addition of NaOH, the suspension

3.1. Experimental verification of the anoxic conditions was allowed to equilibrate. Once the pH drift was less than

required for the study of Fe(II) sorption on montmorillonite

0.02 pH unit/10 min, a 10 ml sample of suspension was ﬁl- tered through a 0.22 lm pore size membrane and analyzed

In order to test the anoxic conditions, ferrous hydroxide for iron concentration by induced coupled plasma atomic

(a very unstable solid phase when put in contact with gas- emission spectrometry (ICP-AES). A blank sample without

eous dioxygen) was precipitated in the glove box at pH montmorillonite was also prepared using the same experi-

12.5, aged for 6 months and then analyzed by Mo¨ssbauer mental procedure with 10 mM ferrous chloride in a CaCl 2 spectrometry. The color of the precipitate (white) remained

0.05 M ionic background. The Ferrozine method ( Viollier unchanged over the 6 months that the presently reported et al., 2000 ) was also used to measure separately Fe(II)

sorption experiments lasted. On the other hand, when fer- and Fe(III) concentrations in solutions up to 30 lM after

rous hydroxide is precipitated under atmospheric oxygen a dilution and to check that no oxidation of Fe(II) to Fe(III)

green color is observed. In addition, the Mo¨ssbauer spec- occurred in solution.

trum ( Fig. 2 ) is composed of a single ferrous doublet, The desorption experiment was performed with the

allowing us to conclude that no oxidation occurred within same procedure using HCl instead of NaOH and using the suspension obtained at the end of the sorption experi-

Fe. We conclude, therefore, that the sorption experiments,

A. Ge´hin et al. 71 (2007) 863–876

100% of the iron in solution was in the ferrous oxidation state from pH 2 to 8.5. For pH > 8.5, more than 99% of the total iron was sorbed and the solute iron concentration was below the detection limit (1 lM). In conclusion, the Ferrozin method analysis demonstrates that the totality of iron in solution was in the Fe(II) form, whatever the pH and the sorption events of the sample.

Without montmorillonite, the aqueous ferrous iron dis- appears from solution at a pH value about 9. The position of this edge is in full agreement with the precipitation of an

amorphous ferrous iron hydroxide –Fe(OH) 2 – as shown by the calculated solubility curve in Fig. 3 . This result, togeth-

Fig. 2. Mo¨ssbauer spectrum of ferrous hydroxide Fe(OH) 2 at 77 K.

er with the Fe(OH) 2 aging test, is good evidence of the well

Hyperﬁne parameters: d = 1.29 mm s , the isomer shift with respect to

metallic a-Fe(0) at room temperature; DE Q = 3.09 mm s , the quadru-

maintained anoxic conditions during the experiments. For

polar splitting; C = 0.32 mm s , the full width at half height.

the experiment with synthetic montmorillonite, a ‘‘sorption edge’’ was observed at a pH of about 7, preceded at lower pH by a sorption plateau. The Fe(II) and Fe(III) concen-

the preparation and the transport of the samples were car- trations in solution were also measured during increasing ried out in the most strict anoxic conditions that can be

or decreasing pH cycles (between 2 and 9). The reproduc- achieved in a laboratory. Thus, no Fe(II) was accidentally

ibility of the data demonstrates the total reversibility of

the sorption phenomena. Therefore, a thermodynamics our measurement methods.

oxidized by atmospheric O 2 , within the detection limits of

equilibrium approach can be used with conﬁdence to describe the sorption phenomena.

3.2. Solution analyses Fig. 3 shows the results of iron solution analyses during

3.3. 57 Fe Mo¨ssbauer spectrometry

the adsorption experiments. Fe(II) sorption results obtained with the Ferrozine method ( Viollier et al., 2000 )

The paramagnetic Mo¨ssbauer spectra are shown in Figs. compare well with the values obtained by ICP-AES mea-

4 and 5 together with the hyperﬁne parameters ( Table 1 ) surements. The Ferrozin method results indicated a Fe(III)

obtained with the MOSFIT ( Fig. 4 ) and the QSD modeling concentration below the detection limit. This means that

approach ( Fig. 5 ), assuming two Gaussian QSD

Fig. 3. Sorption edge, as a function of pH, of Fe(II) sorbed on synthetic montmorillonite ([SM] = 10 g clay L , [Fe(II)] t = 630 lM) in a 0.05 M CaCl 2 background (open circles: ﬁrst increasing pH sorption edge, total Fe measured by ICP-AES; closed circles: ﬁrst decreasing pH sorption edge, total Fe measured by ICP-AES; open squares: second increasing pH sorption edge, Fe(II) measured by the Ferrozin method; closed squares: second decreasing pH sorption edge, Fe(II) measured by the Ferrozine method). The blank experiment results (without montmorillonite) are shown for comparison (open

triangles: 100 lM of Fe(II) in a 0.05 M CaCl 2 background). The dashed line is the computation of the Fe(OH) 2 solubility curve in the condition of the blank experiment using the PHREEQC2 chemical speciation code with the Llnl.dat database ( Parkhurst and Appelo, 1999 ). Arrows indicate Mo¨ssbauer sampling events (white arrows = increasing pH/sorption, black arrows = decreasing pH/desorption).

Fe electron transfer at montmorillonite reactive sites

Fig. 4. Mo¨ssbauer spectra at T = 77 K shown together with hyperﬁne contributions obtained with the MOSFIT modeling approach ( Table 1 ). On each spectrum, the pH of the suspensions is given. The paramagnetic doublets D 1 and D 2 with a large quadrupole splitting are characteristic of 57 Fe(II) surface

species while the doublet D 3 , with a weak quadrupole splitting, is due to 57 Fe(III) surface species.

components in a ferrous QSD and a single Gaussian QSD contribution to the Mo¨ssbauer spectra originates from component for the ferric QSD. According to the values of

the solid phase. Thus, we conclude that the hyperﬁne struc-

center shift, the D 1 and D 2 doublets are clearly attributed

ture is related to adsorbed iron.

The presence of Fe(III) signals in the Mo¨ssbauer spectra species. Both models allow us to obtain unambiguously the

to Fe(II) species while the D 3 doublet is assigned to Fe(III)

is unexpected. It cannot be explained by an accidental oxi- Fe(III)/(Fe(II) + Fe(III)) ratio, i.e. the Fe(III) percentage

dation given all the care that was taken to avoid an acci- of the total iron present in the solid sample, assuming equal

dental oxidation and, above all, the complete reversibility Mo¨ssbauer recoilless fractions for ferric and ferrous iron. It

of the oxidation processes. Synthetic montmorillonite, as is also important to emphasize that the values of some line

a material, is not likely to be considered as the oxidation widths appear to be larger than those expected for a highly

agent since its elemental constituents Si(IV), Al(III) and crystalline structure with no chemical disorder.

Mg(II) are not redox reactive in our experimental The values of the hyperﬁne parameters do not corre-

conditions.

spond a priori to Fe present in any expected solid phases The comprehension of this surface chemistry is compli- (e.g. ferrous hydroxide). During the pre-sorption edge

cated by the fact that the exact structure of the surface of ( Fig. 3 ; pH 4.01, 5.01, 6.10 and 7.08), the Fe(III) compo-

synthetic montmorillonite is not known. There are proba- nent area increases with pH up to 43–44% of total-

bly a high number of diﬀerent types of sorption sites in- sorbed-Fe at pH 7.08. At the end of the main adsorption

volved in Fe(II) sorption, as for natural montmorillonite edge, it goes down to 7–8% of total-sorbed-Fe. When the

( Tournassat et al., 2004 ). This can be intuited by looking pH returns to 4.91, the Mo¨ssbauer spectrum is very similar

thoroughly at the hyperﬁne parameters given by the MOS- to the one observed before at pH 5.01 with the same rela-

FIT approach. One notes that the Fe(II) quadrupole split- tive area of the total Fe(II) and Fe(III) components. How-

ting and centre shift values evolve with pH, providing ever, the isomer shifts and quadrupole splitting value are

evidence of changes in the sorbed-Fe(II) structural environ- signiﬁcantly diﬀerent. Because the initial aqueous solu-

ment. These changes can be due either to structural chang- tion containing iron is ﬁltered after reaction, the only

es due to protonation/deprotonation of the sorbed iron or

A. Ge´hin et al. 71 (2007) 863–876

Fig. 5. Mo¨ssbauer spectra at T = 77 K shown together with hyperﬁne contributions obtained with the RECOIL modeling approach ( Table 1 quadrupole splitting distribution QSD1 with a large quadrupole splitting is characteristic of 57

). The Fe(II) surface species while the quadrupole splitting

distribution QSD2, with weak quadrupole splitting, is due to 57 Fe(III) surface species.

to a change in site population occupancy or both of these values and having a more narrow width. (2) The next incre- eﬀects. Moreover, the pH 4.91 (return) and pH 5.01 (rais-

ments in pH, namely pH 5.01, 6.10 and 7.08 QSDs are very ing) spectra are signiﬁcantly diﬀerent, despite arising in

similar to each other and gradually evolve by narrowing samples that have the same sorbed-Fe(II) and sorbed-

and shifting to larger QS average values as pH increases. Fe(III) contents. This result provides strong evidence that

(3) The last increment in pH, namely pH 8.66, shows a the mechanism responsible for Fe(II) uptake and partial

QS shift towards low values together with a more narrow oxidation is due to several sites acting at diﬀerent pH.

width. (4) As stated previously with the MOSFIT ap- Mo¨ssbauer spectra were also ﬁtted using the method of

proach, the pH 4.91 (return) and pH 5.01 (raising) QSD Rancourt and Ping (1991) based on QSDs in order to try to

are signiﬁcantly diﬀerent, despite arising in samples that attribute the spectral changes in terms of local distortion of

have the same sorbed-Fe(II) and sorbed-Fe(III) contents. sorbed-Fe(II) environments ( Rancourt, 1994a,b; Rancourt

Such characteristics are correlated to the hyperﬁne results et al., 1994; Evans et al., 2005 ). We next discuss the Fe(II)

obtained using the MOSFIT description. QSDs, keeping in mind that each QSD characterizes a dis-

The ﬁrst Fe(II) binding sites (i.e., that binds at low pH) tribution of sorbed-Fe(II) local environments ( Rancourt,

can be attributed mainly to cation exchanged Fe(II) in the 1994a, 1998 ) on a given montmorillonite sample at a given

interlayer (Fe 2+ and FeCl + ). However, the signal cannot be pH. A Fe(II) QSD, therefore, characterizes the distribution

attributed entirely to this species given its diﬀerence with of montmorillonite surface sites that accommodate Fe(II),

previously published hyperﬁne parameters for exchanged including possible inner sphere coordinating anions on the

Fe(II) at lower pH ( Charlet and Tournassat, 2005 ). Other solution side. Fig. 6

sites that bind Fe at low pH are then needed and are the from the T = 77 K spectra of samples corresponding to

A shows the Fe(II) QSDs extracted

so-called ‘‘high energy’’ sites in the following chemical the speciﬁc pH values depicted in Fig. 3 .

modeling approach, those demonstrating the highest aﬃn- The sorbed-Fe(II) extracted QSDs allow the following

ity towards Fe(II). The non-reversible QSD change in main observations ( Fig. 6 ): (1) The starting pH 4.03 Fe(II)

going from pH 5.01 to pH 8.66 (raising) appears to be pre- QSD is signiﬁcantly diﬀerent from all the other Fe(II) QSD

dominantly due to pH-induced changes in the Fe(II) local curves, being weighted more strongly towards larger QS

environments, probably including surface annealing

869 Table 1

Fe electron transfer at montmorillonite reactive sites

Mo¨ssbauer hyperﬁne parameters of the spectra presented in Figs. 4 and 5 pH

Site

MOSFIT

QSD method

ÆDæ r D RA (%) 4.03 Fe(II)

RA (%)

ÆCSæ

D 1 1.36 0.31 3.35 50 QSD1

1.34 3.38 0.11 24 D 2 1.31 0.40 2.92 39 1.34 3.11 0.40 64

Fe(III)

0.44 0.98 0.44 12 5.01 Fe(II)

D 3 0.41 0.44 0.83 11 QSD2

D 1 1.35 0.38 3.38 34 QSD1

1.32 3.21 0.39 72 D 2 1.30 0.48 2.91 50 1.32 2.22 0.48 11

Fe(III)

0.46 0.83 0.56 16 6.10 Fe(II)

D 3 0.45 0.61 0.66 16 QSD2

D 1 1.33 0.28 3.37 21 QSD1

1.30 3.19 0.33 64 D 2 1.30 0.44 2.91 56 1.30 2.61 0.27 12

Fe(III)

0.42 0.73 0.46 24 7.08 Fe(II)

D 3 0.43 0.57 0.70 23 QSD2

D 1 1.34 0.33 3.27 18 QSD1

1.31 3.13 0.29 31 D 2 1.29 0.44 2.87 39 1.31 2.67 0.70 25

Fe(III)

0.47 0.70 0.40 44 8.66 Fe(II)

D 3 0.48 0.49 0.63 43 QSD2

D 1 1.27 0.27 3.10 45 QSD1

1.27 3.10 0.18 44 D 2 1.27 0.38 2.75 48 1.27 2.79 0.35 48

0.43 0.70 0.40 8 4.91 Fe(II)

Fe(III)

D 3 0.39 0.36 0.55 7 QSD2

D 1 1.29 0.38 2.96 49 QSD1

1.29 3.00 0.71 17 D 2 1.28 0.32 2.52 34 1.29 2.76 0.33 64

0.45 0.80 0.45 19 The temperature of the analyses was 77 K. The pH of the suspension is given for each spectrum. Two methods of model ﬁtting were used: the MOSFIT

Fe(III)

D 3 0.42 0.40 0.68 17 QSD2

program (unpublished program, Teillet and Varret, Universite´ du Maine, Le Mans, France) and the QSDs method of Rancourt and Ping (1991) . d (mm s ) isomer shift with respect to metallic a-Fe(0) at room temperature; DE Q (mm s ) quadrupole splitting, RA (%) relative abundance; C (mm s ) full

width at half maximum. ÆCSæ (mm s ) average centre shift, ÆQSæ (mm s ) the average quadrupolar splitting, r D (mm s ) the Gaussian width (std. dev.) of the QSD component.

around the newly complexed Fe(II). The proposed struc- of sorbed-Fe redox stabilization, in the case of sorption-in- tural annealing would produce the non-reversibility in local

duced reduction of Fe(III) bacterial cell wall complex environment populations. The gradual changes in QSD on

where the electron is transferred from the host cell to the increasing pH through 6.01–8.66 are probably due to the

sorbed-Fe(III). The same basic concept applies here for combined eﬀects of more and more low-aﬃnity sites being

sorption-induced Fe(II) oxidation. In both cases, the diﬀer- occupied and pH-induced structural annealing.

ence in Fe(II) versus Fe(III) complexation (or coordination or steric or ligand ﬁeld stabilization) energy is involved and

3.4. Discussion on the oxidation mechanism and modeling plays a dominant role. In both cases, the host is a high sur- approach

face charge aqueous particle (bacterium or montmorillonite) with many diﬀerent surface complexation sites,

We have established that, under strict anoxic conditions, including a variety of multi-dentate sites.

a charge transfer must occur so that some sorbed Fe(II) The overall reaction could be described in terms of the ends up being oxidized into sorbed Fe(III) atoms. The

following elemental steps. First, aqueous Fe(II) is sorbed at a speciﬁc surface site with high aﬃnity for Fe amount of observed Fe(III) exceeds by far the detected 2+ as shown

by the presence of speciﬁcally sorbed Fe(II) at pH 4 (see tion can not exceed 0.13 lmol/L in the suspension whereas

or expected limits of any chemical oxidants: O 2 concentra-

Mo¨ssbauer results):

at least 7 lmol/L Fe(II) was found to be oxidized into

Fe(III) (sample at pH 4). Furthermore, the observed elec- þ aq þ sBðOH Þ 3 () sBðOH ÞðO Þ 2 Fe þ 2H aq tron transfer is reversible. This reversibility implies that

FeðIIÞ

ð1Þ the phenomenon is not due to an accidental contamination by oxidant. This is corroborated by a Mo¨ssbauer blank test

where s„(OH ) 3 represents not the montmorillonite solid, carried out using the same handling procedure, but with a

but the explicitly mentioned oxo or hydroxo coordinating highly reactive synthetic ferrous hydroxide instead of the

surface functional group. Note the formal minus three Fe(II)-montmorillonite system. In this test, no Fe(III)

charge of the reactive site does not preclude the overall was detected ( Fig. 2 ).

charge of the site, as it is also controlled by the coordinated At ﬁrst sight, such electron transfer is surprising. Ran-

structural cations. Next, an electron transfer step is consid- court et al. (2005) have recently described a mechanism

ered, which oxidizes the sorbed Fe(II)

A. Ge´hin et al. 71 (2007) 863–876

Here, we bias our choice among possible structural-chemical adjustments to involve a surface proton loss. This is because we believe that sorbed Fe(III) will be favored, compared to sorbed Fe(II) with the first coordination shell proton removed for two reasons: (1) at the s„OH group, the H + –Fe(III) repulsion will be greater than the H + –Fe(II) repulsion and (2) the spherically symmetric and predomi- nantly ionic bonding Fe(III) will be able to more easily adjust a three surface-bound coordinating O chelate than the largely covalent bonding ferrous cation that will more easily adjust solution-side first shell anions than surface- bound ones to its covalent (bond angle and distance) requirements. This choice is also consistent with our obser- vation that the fraction of sorbed Fe present as Fe(III) increases with increasing pH in the sorption plateau region ( Figs. 3 and 7 ). Of course, conversely, it is also clear that some surface complexation sites will have first shell coordinating anions that sterically favor Fe(II) and that these sites will form relatively stable Fe(II) complexes, which would be statistically the last ones to oxidize upon introduction of dissolved oxygen in the system. All our discussion refers to a sorbant that displays a large array of complexation sites, as previously described to be the case for montmorillonite ( Tournassat et al., 2004 ).

Since the synthetic montmorillonite is assumed to be a perfect insulator, the question of the nature of the oxidant site remains and one can not exclude the possibility of redox reaction with the only one electron acceptor in the system i.e. water. The stabilization of sorbed Fe(III) is then

Fig. 6. (A) Evolution of the Fe(II) component from RECOIL ﬁts of the full Mo¨ssbauer signal: Fe(II) Mo¨ssbauer spectrum (left side) and of the

perhaps linked to the reduction of water via the following

quadrupolar splitting distribution given as distribution probabilities (p(D)

reaction, then replacing Eq. (3) :

in mm s ) as a function of quadrupolar splitting (D in mm s ). (B)

Evolution of the overall average quadrupolar splitting ÆŒDŒæ of the Fe(II) 3þ ÞðO

sBðe

2 Fe þ 2H 2 O

site (ﬁlled triangles) and of the overall standard deviation width of the

þ entire Fe(II) quadrupolar splitting distribution QSD (ﬁlled circles) as a

() sBðO Þ 3 Fe ðOH Þ 2 ðH 2 Þ 0:5 þ 2H aq ð4Þ

function of pH.

Reactions (2) and (3) or (2) and (4) together represent our proposed reversible anoxic-environment sorbed-Fe(II) oxi-

sBðOH 3þ ÞðO Þ

dation mechanism. Such oxidation will advance if there is a In reaction (2), one electron is transferred from the sorbed

2 Fe () sBðe

ÞðO Þ 2 Fe ð2Þ

Fe(II) to a nearby surface or near-surface site on or in the montmorillonite. The complex that receives the electron is denoted (e )* and will be some relatively receptive center, as determined by local bond valence and local electronega- tivity conditions. Reaction (2) alone is probably not ener- getically favored (given the ionization energy of Fe(II)), but we imagine it to be accompanied by a local structural and/or chemical adjustment that greatly stabilizes sorbed Fe(III), a small ion, relative to sorbed Fe(II), a large ion, such that the net change in free energy per sorption/oxidation event is negative for the given sorption-site/electron- acceptor-site complex. The required structural-chemical adjustment can be represented as

Fig. 7. Evolution of the percentage of Fe(III) on the montmorillonite surfaces as a function of pH. Open circles: experimental Mo¨ssbauer data

sBðe þ ÞðO Þ

2 Fe () sBðe

Þ 3 Fe þH aq

( Table 1 ). Solid line: surface complexation model computation using the parameters described in Tables 2–4 . The point at pH 8.66 corresponds to a

Fe electron transfer at montmorillonite reactive sites

net free energy gain per Fe oxidation event. Since one or terms of reaction with a neutral s„ 3+ (OH )

3 site. Eq. (1) is two protons are produced, part of the free energy change

then transformed into:

involves the proton activity (pH). Increasing pH drives

the overall reaction to the right, as observed. The other

log K sorp ð5Þ dation reaction (reactions 1 and 2 together) is equal to the diﬀerence in binding (and ionic) free energy between the

s 1 s component of the change in free energy for the overall oxi- 2 () sB ðOH ÞðO Þ

These reactions for sites s 1 and s 2 occur at a pH value lower than where the main edge of the curve is observed ( Fig. 3 s„(OH ). )(O )

2 Fe(II) and s„(e )*(O ) 3 Fe(III) complex-

es, including the Fe ionization energy. This diﬀerence, DF s , They are therefore very poorly constrained concerning the Fe depends on bonding strengths and has electrostatic, vibra- 2+ /H + balance and the sorption constant. However, the

tional, entropic, and steric contributions. The steric contri- choice of Fe /H balance for Fe sorption reaction on bution is largely the so-called ligand ﬁeld stabilization

‘‘weak’’ sites is crucial in order to describe as well as possi- energy. DF

ble the slope of the sorption edge at pH 7–8. It was found s , therefore, is strongly dependent on the local that a balance of 3H structure of the speciﬁc bonding site, including its position + for one Fe 2+ was the best solution

in nanovoids, local electronegativities, exact coordinating corresponding, for example, to a hydroxylated bidentate anion positions and stiﬀnesses, etc. We propose that the

complex („w 2 O 2 FeOH ) or a bihydroxylated mon- montmorillonite edge sites include a population of sites

odentate complex ðBwOFeðOHÞ 2 Þ. The arbitrarily select- that have DF < 0 and that these sites, therefore, cause

ed model corresponds to a hydroxylated bidentate

the net observed Fe(II) to Fe(III) oxidations.

complex:

The numerical applicability of such a model was tested

2BwOH þ Fe 2þ þH 2 O

using a very simple complexation model. As already stated,

log K sorp ð6Þ the discussion was in the context of a sorbant that displays

() Bw w

2 O 2 FeOH þ 3H

a large array of complexation sites. A model with two Eqs. (2) and (4) were then combined into a single equation ‘‘strong’’ sites (s 1 ,s 2 ) was then chosen where surface pro-

identical for the three strong sites:

cesses are possible following Eqs. (2) and (4) . A one strong

sB 3þ ðOH

Þ 2 Fe site model was also tested but was proved to be ineﬃcient 2þ ÞðO þ 2H 2 O

in reproducing the experimental data. In addition, cation

3 Fe ðOH Þ 2 ðH 2 Þ 0:5 þ 2H aq log K oxid exchange and sorption on a weak site (w) were also consid-

ð7Þ ered. Fe–Ca and Fe–Na cation exchange selectivity con-

stants in 0.1 mol/L chloride anionic background were Here, we have arbitrarily chosen to keep H 2 on the Fe sorp- taken from the literature ( Charlet and Tournassat, 2005 ,

tion site and to consider that it is not further released in Table 2 ).

solution given the absence of H 2 concentration mea- Given our ignorance of the sorption sites localization

surements.

and the common use of a non-electrostatic model for clay The total amount of strong sites can be deduced from minerals ( Bradbury and Baeyens, 1997, 1999, 2005b; Tour-

experimental results by considering that weak sites did nassat et al., 2004 ), we express Fe(II) sorption equations in

not oxidize sorbed Fe(II) and that strong sites completely oxidized Fe(II) into Fe(III) at pH 8.66. Considering Fe dilution steps during the experiment (by alkali solution

Table 2

additions), one calculates that the sorption of Fe(II) at

Cation exchange parameters for Ca–Fe and Na–Fe exchange in 0.1 mol l

chloride anionic background

kg clay . Seven percentage of this amount was oxidized and

Exchange reaction

log K GT a

clay .

BXNa þ H þ () BXH þ Na þ

0.0 The amount of strong sites can be also be deduced from the

BXNa þ CaCl þ () BXCaCl þ Na þ

2.5 diﬀerence between the experimental curve and the modeled

BXNa þ FeCl þ () BXFeCl þ Na þ

2.3 amount of Fe(II) that underwent cation exchange given

2BXNa þ Ca 2þ () BX 2 Ca þ 2Na þ

that no parameter is needed to be adjusted for cation ex-

Fe þ 2Na

2BXNa þ Fe 2þ () BX 2 þ

0.4 change reaction modeling ( Fig. 8 ). This diﬀerence led to a

The CEC value was considered to be 0.63 eq/kg at pH 7 according to the Cs–Li method results. The permanent structural charge (Cs result) was

s 1 ; s 2 clay , in agreement with previous 2+

value. The log K sorp constant were adjusted so that Fe

found to be 0.44 mol/kg.

a Note that the selectivity coeﬃcients are given in the Gaines and

start to be adsorbed below pH 2 in order to explain the ﬂat

Thomas convention ( Sposito, 1981 , log K GT ) and not in the Vanselow

shape of the sorption edge curve between pH 2 and pH 6.

convention (log K v ) as claimed previously in Charlet and Tournassat,

The relative amounts of sites s 1 and s 2 were then ﬁtted

2005 . This error originated from a misunderstanding of the term ‘‘mole

thanks to the evolution of the percentage of Fe(III) on

fraction’’ for surface species in the PHREEQC User’s guide ( Parkhurst and Appelo, 1999 ). Regarding in detail Eq. (15), p14 of this guide, it

the montmorillonite surfaces as a function of pH ( Fig. 7 ).

appears that the ‘‘mole fraction’’ is in fact an equivalent fraction,

One cannot fail to observe that the strong site density in

transforming the believed Vanselow selectivity coeﬃcient into a Gaines

this model is very similar to the density of the so called

and Thomas selectivity coeﬃcient.

strong sites in the Bradbury and Baeyens studies

A. Ge´hin et al. 71 (2007) 863–876

uptake is equivalent between Fe(II) exchanged under the form of Fe 2+

the signal) and strongly sorbed in Fe(II) or Fe(III) form tion of sorbed Fe(II) decreases drastically and the uptake

is dominated by ‘‘weak’’ sorption sites. Once the main contributors of Fe(II) uptake are known, it is possible to create a Mo¨ssbauer spectra decomposition model, taking into account all of this information ( Table 5 ).

First, the spectrum at pH 4.03 was decomposed into ﬁve doublets corresponding to exchanged Fe 2+ , exchanged FeCl + , Fe(II) sorbed on strong site 1, Fe(II) sorbed on strong site 2 and Fe oxidized on strong sites. Mo¨ssbauer parameters for Fe 2+ and FeCl + in cation exchanged position were taken from Charlet and Tournassat, 2005 . Fig. 9 (bottom) shows that this model allows reproducing the Mo¨ssbauer spectrum with the proportion between dif- ferent Fe sorption sites comparing well with those given by the chemical sorption model ( Table 5 , comparison of

Fig. 8. (A) Comparison of experimental sorption data with the

the two last rows). Spectra at higher pH were then ﬁtted

proposed model. Full line: total sorbed Fe(II). Black short dash line:

by adding ﬁtting component when necessary. Again, the re-

cation exchanged Fe(II). Black long dash line: Fe(II) sorbed on weak

sult from this ﬁt compares well with the result from the

sites. Blue dash-dot line: Fe(II) sorbed on strong sites. Red dash line:

chemical model ( Fig. 9 and Table 5 ) giving sense to our

Fe(III) sorbed on strong sites. (B) Details in the pH < 7.5 zone:

modeling approach. At pH 8.66, the need to use at least

decomposition of strong sites s 1 and s 2 contributions. Blue dash-dot

line: Fe(III) sorbed on strong sites s 1 and s 2 . Pink and green dash lines:

3 Mo¨ssbauer components is likely to represent the com-

Fe(II) sorbed on strong sites s 1 and s 2 . Colors on Fig. 8B refer to the

plexity in structure and chemistry of the clay edge sites.

colors used in Fig. 9 .

One can also note that the component weak 3 ( Table 5 , pH 8.66) is very similar to the one used for exchanged

Fe 2+ and for Fe(OH) 2(s) ( Fig. 2 ): it is then probable that (2 mmol/kg, Bradbury and Baeyens, 1997, 1999, 2002,

part of speciﬁcally but weakly sorbed Fe(II) forms outer- 2005a,b ). The complete set of modeling parameters is given

sphere complexes, surface or bulk precipitate at pH 8.66. in Tables 2–4 .

The two latter interpretations are preferred given the exper- Figs. 7 and 8 show that both solution analysis and spec-

imental conditions and the Fe(OH) 2(s) solubility curve trometric results are well reproduced by the model. In the

( Fig. 3 ).

pre-edge pH region (from pH 2 to pH 7.5), the Fe(II) The nature of the strongly sorbing sites is still unknown.

possible to draw some hypotheses based on the knowledge

Table 3

Site amounts for edge sorption sites and exchange sites (in mmol/kg clay )

of the structure and chemistry of the montmorillonite edg-

es. Analyzing the edge structure proposed by Tournassat

Name of site

Site amount (mmol/kg

clay

et al. (2004) , one notes that tridendate sorption sites as pro-

s 1 3+ (OH ) 2 „ 3+

posed in this paper can be constituted by the combination

„ (OH ) 3 2.8

w „ OH

80 of the following elementary sites:

X (exchange sites)

630 (pH 7)–440 (pH 2)

• One Me Oh –O site associated to one Me Oh –O–Me T site

The CEC value was measured to be 0.63 eq/kg at pH 7 according to the

Cs–Li method results ( Anderson and Sposito, 1991 ). The permanent

(with the same Me OH cation, Me OH = Al or Mg) and

structural charge was found to be 0.44 mol/kg. This last value was applied

either one Me Oh –O–Me OH site or one Me Oh2 –O–Me T

to the model, since speciﬁc sorption and oxidation start at low pH.

site;

Table 4 Fe aﬃnity constants for strong and weak sites and Fe(II) oxidation constants on strong sites

Reaction log K

2BwOH þ Fe þH 2 O () Bw 2 O 2 FeOH þ 3H

873 Table 5

Fe electron transfer at montmorillonite reactive sites

Mo¨ssbauer hyperﬁne parameters adjusted with constraints from the chemical modeling approach pH

Site (color in Fig. 9 )

Comparison with chemical modeling (%) 4.03 D exch FeCl+(red)

RA (%)

D exch Fe2+(blue)

D strong_red1(pink)

D strong_red2(green)

D strong_ox(cyan)

5.01 D exch FeCl+(red)

D exch Fe2+(blue)

D strong_red2(green)

D strong_ox(cyan)

6.10 D exch FeCl+(red)

D exch Fe2+(blue)

D strong_red2(green)

D strong_ox(cyan)

7.08 D exch FeCl+(red)

D exch Fe2+(blue)

D strong_ox(cyan)

D strong_red2

D weak1(black)

8.66 D exch FeCl+

D exch Fe2+

D strong_ox(cyan)

D weak1(black)

D weak2(black)

D weak3(black)

The temperature of the analyses is 77 K. The pH of the suspension is given for each spectrum. d (mm s ) isomer shift with respect to metallic a-Fe(0) at room temperature; DE Q (mm s ) quadrupole splitting, RA (%) relative abundance; C (mm s ) full width at half maximum. The subscripts exch FeCl+, exch Fe2+, strong_red, strong_ox and weak refer to the modeling sorption site given in Table 4 .

• Two associated Me Oh –O sites associated to either one above requirement in tridentate sites are present in the fol- Me Oh –O–Me OH site or one Me Oh2 –O–Me T site;

Oh –O site associ- • Two Me Oh –O–Me T sites associated to one Me Oh2 –O–

ated to one Mg Oh –O–Si site and one Me Oh –O–Me OH site Me T site.

or one Me Oh2 –O–Me T kg for two associated Mg Oh –O sites associated to either

By using equal probability combination rules as given in one Mg Oh –O–Me OH site or one Me Oh2 –O–Me T site and Tournassat et al. (2004) with the structural formula of our

Oh –O–Si sites associated to one synthetic montmorillonite and with a edge speciﬁc surface

Me Oh2 –O–Me T site. Assuming the ﬁrst reactive site conﬁg- area of 8 m 2 /g ( Tournassat et al., 2003 ), one can calculate

uration given above, one obtains the following reaction that Mg cations in octahedral position and fulﬁlling the

O -0.67

Mg OH 2 +0.33

+ Fe +2

Mg OH -0.67

O -0.67

Mg OH -0.67

Fe +2 +H 2 O

Mg OH -0.67

Fe +2 (OH) - +H + (9)

OH 2 +0.83 .

OH 2 +0.83

A. Ge´hin et al. 71 (2007) 863–876

O -0.67

O -0.67 .

Mg OH -0.67

Fe +2 (OH) -

Mg OH -0.67

Fe +3 (OH) 2 -2

Reaction (8) produces a near-neutral complex (the for- face species in the face of an instability provoked by mal charge is here +0.5, but a more accurate account of

hydrolysis.

bond valences might yield even a neutral surface complex). At the moment it is not possible to say if the good agree- Once the pH increases, the Fe(II) atom of the complex is

ment between the ﬁtted amount of strong site in this study hydrolyzed producing a charge imbalance of the surface

and the calculated amount of most electronegative sites complex (reaction (9)). This imbalance is further compen-

after the structural model by Tournassat et al. (2004) is sated by the oxidation of sorbed Fe(II) into sorbed Fe(III)

as a coincidence or not. In a future study, molecular (reaction (10)). The oxidation reaction can then be seen as

dynamics combined with electronic structure calculations one way the system responds to maintain a stable sur-

will help to conﬁrm (or inﬁrm) the mechanism that we

Fig. 9. Mo¨ssbauer spectra at T = 77 K shown together with hyperﬁne contributions obtained using the MOSFIT approach combined to constrains from chemical sorption model. Hyperﬁne parameters are given in Table 5 together with the color/site relation. Wide black continuous line is the sum of all of the contributions to the spectra.

Fe electron transfer at montmorillonite reactive sites

propose on these types of sites and to reﬁne the H + /Fe sto- ANDRA (French National Radioactive Waste Manage- chiometry of the reaction.

ment Agency), (1) in the framework of its program on the geochemical behavior of bentonite engineered barriers

4. Conclusions under the supervision of Dr. N. Michau and (2) in the framework of the 6th PCRD Euratom FUNMIG pro-

Based on wet analytical chemistry and Mo¨ssbauer gram under the supervision of Dr. S. Altmann. Support spectrometry, we have shown that Fe(II) has a peculiar

to DGR was provided by the Natural Sciences and Engi- behavior when sorbed to the surfaces of an iron-free syn-

neering Research Council of Canada. Three anonymous thetic montmorillonite. Fe(II) undergoes classical uptake

reviewers and A.E. Pr. Sverjensky are acknowledged for processes by cation exchange (in the form of Fe 2+ but

their comments on the manuscript. Finally, we thank also FeCl + ) and speciﬁc sorption. The main sorption

Pr. Garrison Sposito for helpful discussion on the oxidation mechanism.

is quantitatively dominated by cation exchange processes. In addition, Fe(II) is sorbed at very low pH on speciﬁc

Associate editor: Dimitri A. Sverjensky

sites and undergoes partial to total oxidation as a function of pH in absence of any oxidant in the solution

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