Kinetic iron stable isotope fractionation between iron (-II)and (-III) complexes in solution

Alan Matthews a;b;*, Xiang-Kun Zhu a, Keith O'Nions a

a Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, UKb Institute of Earth Sciences, Hebrew University of Jerusalem, 91904 Jerusalem, Israel

Received 29 November 2000; received in revised form 25 June 2001; accepted 9 July 2001

Abstract

Iron stable isotope fractionation between tris(2,2P-bipyridine) iron-II ([FeII(bipy)3]2�) and iron-III chloridecomplexes has been measured using plasma source mass spectrometry. The experimental protocol involves complexingFeII ion with 2,2P-bipyridine in a FeII/FeIII chloride solution and then separating the FeII and FeIII solution species in6 M HCl on an anion exchange resin. Large isotopic variations of O57Fe and O56Fe are experimentally measured in thetwo separated solution fractions, with isotopic fractionations increasing from v(FeII-FeIII) = 25 to 174 O units for57Fe/54Fe and 17 to 117 O units for 56Fe/54Fe. The increase in fractionations correlates with a decrease in the molefraction of FeII in the solution (Fe* = (FeII)/[(FeII)+(FeIII)]) that results from the dissociation and breakdown of[FeII(bipy)3]2� complex in 6 M HCl solution. The data variations are mainly ascribed to a kinetic fractionationoccurring during this dissociation reaction. Mass balance calculations, assuming that a Rayleigh law describes theoverall isotopic trends, suggest a kinetic fractionation of ca. 1.010 (V100 O units). The magnitude of this fractionationis attributed to the rupturing of the strong covalent bonds between 2,2P-bipyridine and FeII ion. The experimental dataconfirm that the coordination chemistry of iron exhibits a profound control on its isotopic behaviour and that kineticfractionations may play an important role in its isotope geochemistry, as was also found in the pioneering experimentalstudies of the sulphur isotopic system in solution. ß 2001 Elsevier Science B.V. All rights reserved.

Keywords: iron; stable isotopes; fractionation; ferric iron; ferrous iron

1. Introduction

Of the transitional metal elements, iron has per-haps the most complex and fascinating geochem-istry, including, at low temperatures, a major role

in natural redox processes and biological chemis-try. The fractionations of the stable isotopes ofiron (57Fe/54Fe, 56Fe/54Fe) among natural materi-als are known to vary by at least several permil[1^4] and thus provide a promising tool for inves-tigating iron geochemistry and biogeochemistry.Critical to such study is the ability to measurethe isotopic variations with su¤cient accuracy;this has now become possible through pioneeringstudies of the technique of multiple collector in-ductively coupled plasma source mass spectrome-try (MC-ICP-MS), which allows stable isotope

0012-821X / 01 / $ ^ see front matter ß 2001 Elsevier Science B.V. All rights reserved.PII: S 0 0 1 2 - 8 2 1 X ( 0 1 ) 0 0 4 3 2 - 0

* Corresponding author. Tel. : +972-2-658-4913;Fax: +972-2-566-2582.

E-mail addresses: alan@vms.huji.ac.il (A. Matthews),xiangkun.zhu@earth.ox.ac.uk (X.-K. Zhu),keith.onions@earth.ox.ac.uk (K. O'Nions).

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ratios of the heavier elements (e.g., Fe, Cu, Zn,Mo, Mg) to be measured at the O (0.1x) level [4^10]. The fractionation mechanisms controlling thenatural variations of iron isotope ratios are notyet adequately understood although initial resultssuggest that biogenic processes are important.

As with studies of the light stable isotopes ofcarbon and sulphur, where redox and biologicalprocesses are associated with both kinetic andequilibrium fractionation e¡ects [11], it is criticalthat the isotopic fractionation in the iron systemis experimentally studied. Laboratory experimentsinvolving bacterial reduction of ferrihydrite [2]and the dissolution of hornblende in the presenceof soil bacteria [12] resulted in formation of iso-topically lighter iron species, suggesting that mi-crobial processes favour kinetic fractionation. Sta-ble isotope fractionation theory [13^16] indicates

that the heavier isotope(s) will be concentrated inspecies with the stronger bonding framework.Calculations based on Mossbauer spectroscopy[17] and vibrational spectroscopy [18] show thatboth oxidation state and the nature of the coor-dinating ligand contribute to iron isotope frac-tionation. Limited progress on measuring the in-organic stable isotopic fractionation among thetransition group metal elements iron, copper andzinc has been made using ion exchange elutionexperiments, which suggest that small fractiona-tions occur among di¡erent solution species[8,19,20], and through precipitation experiments[20] which indicate that permil scale fractionationsexist between reduced and oxidised species.

In order to expand our understanding of frac-tionation properties of the iron isotope system, wereport here the results of an experimental study ofthe iron 57Fe/54Fe and 56Fe/54Fe fractionation be-tween FeIII and FeII complexes in solution. Theexperimental protocol involves complexing of FeII

ion in a ferrous/ferric chloride solution by chela-tion with the bidentate N-donor ligand 2,2P-bipyr-idine ([C6H5N]2) and subsequent separation of theFeII and FeIII solution complexes on an ion ex-change resin for isotopic analysis using plasmasource mass spectrometry.

2. Experimental protocol

The experimental protocol is based on themethod described by Popa et al. [21] for determin-ing the concentration of FeII ion in a mixed va-lence FeII/FeIII solution. The principal features ofthe experimental procedure are outlined in Fig. 1.

2.1. Initial FeII /FeIII solutions

The initial solutions were 1000 ppm Fe in 1 MHCl. To minimise the possibility that isotopicfractionation is an artifact of the preparation pro-cedure, three di¡erent FeII/FeIII chloride solutions(I, II and IV) and pure FeIII chloride solution (III)were prepared. The procedures and the accompa-nying solution ionic processes are summarised inTable 1. The iron starting material was pure ironmetal sponge (Koch-Light, 99.999%). Solution IFig. 1. Template illustrating the experimental protocol.

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was prepared by dissolving Fe metal in 6 M HClat 60³C and then diluting to 1 M HCl strengthwith Meq water at 60³C and allowing the dilutesolution to cool to room temperature in an air-tight Te£on container. Solution II was preparedby the much slower procedure (1 week) of directdissolution of Fe metal in 1 M HCl. Solution IVwas prepared by the reduction of the FeIII chlo-ride solution (III) in air-tight Te£on containersusing 1 M hydrazine in 1 M HCl at room temper-ature. The reduction of FeIII with hydrazine isvery sluggish at room temperature and times of6^15 days were allowed. Three solutions were pre-pared in this way, labelled IVa, b and c (refer tothe footnote to Table 1). The presence of FeII inthe three mixed valence solutions was veri¢ed bythe strong characteristic red coloration that fol-lowed the addition of 2,2P-bipyridine solution. Incontrast, FeIII solution III gave no coloration.Slight adjustment of the FeII/FeIII ratio in exper-imental solutions was made by adding smallamounts of 5 vol H2O2 solution in 1 M HCl to1 ml aliquots of the FeII/FeIII.

2.2. Complexation (chelation) of FeII with2,2P-bipyridine

The second experimental stage entailed the ad-dition of a stoichiometric excess of 1 wt% 2,2P-bipyridine solution to the 1 M HCl FeII/FeIII so-lutions (1:1 by volume giving a 0.5 M HCl solu-tion). The 2,2P-bipyridine exclusively complexes

with the FeII species in solution forming thedeep red coloured [FeII(bipy)3]2� complex ion.The red colour takes about 10 min to fully devel-op, but once developed remains stable for a peri-od of at least a year. The ion exchange separationwas performed at 6 M HCl acid strength, condi-tions under which FeIII is adsorbed on the columnas the anion species [FeCl4]3 (Cresin/Csoln

(FeIII)s 103 [22]), but positively charged FeII spe-cies are not (Cresin/Csoln (FeII)6 1). The solutionswith bipyridine were allowed to stand for 40 minbefore 7 M HCl was added to bring them to 6 MHCl acidity, giving a solution with 100 ppm Fe.After approximately 1.5^2 h, the red colorationstarted to noticeably disappear in the 6 M HClsolutions, eventually to be replaced by a pale yel-low solution characteristic of FeIII chloro-com-plexes. This suggested that decomposition (disso-ciation) of the [FeII(bipy)3]2� complex wasoccurring in 6 M HCl solution. The in£uence ofthis dissociation reaction on isotope fractionationwas checked in experiments in which the[FeII(bipy)3]2�^6 M HCl solution was allowed tostand for di¡erent periods of time prior to loadingon the resin (V5 and 15 min), and in which thesolution volume/resin volume ratio was varied.

2.3. Ion exchange separation andMC^ICP^MS spectrometry

The ion exchange separation was made on acolumn containing a strongly basic anion ex-

Table 1Initial solution preparation protocol

Solution Composition Preparation method Ionic speciation pathway

I FeII/FeIII in 1 M HCl Fe metal dissolved in 6 M HCl at 60³C. Dilute to1 M HCl with Meq water at 60³C. Cool to roomtemperature in air-tight Te£on

Fe0CFeIICFeIIICl34 CFeII+FeIII

II FeII/FeIII in 1 M HCl Fe metal dissolved in 1 M HCl at room temperatureover the period of 1 week

Fe0CFeIICFeII+FeIII

III FeIII in 1 M HCl Solution I oxidised with excess 20 vol H2O2 in1 M HCl at room temperature

FeII+FeIIICFeIII

IVa FeII/FeIII in 1 M HCl Solution III reduced with 1 M hydrazine in1 M HCl at room temperature

FeIIICFeII+FeIII

aSolution IV. Three solutions were prepared for isotopic separation: IVa = 4 ml solution III+3 ml 1 M hydrazine hydrochloridesolution in air-tight container for 6 days. IVb = 4 ml solution III+1.6 ml 1 M hydrazine hydrochloride in air-tight container for15 days. IVc = 4 ml solution III+2.4 ml 1 M hydrazine hydrochloride solution in air-tight container for 15 days.

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change resin (Bio-Rad AG1-X8, 200^400 mesh,chloride form). The standard procedure involvedloading 2 ml of the experimental solution in 1 mlaliquots onto a column containing 4 ml of resin.Several experiments were made using 2 ml resinvolume and/or 1 ml solution. The [FeII(bipy)3]2�

complex, which is not taken up by the resin, isseparated from the resin-bound ion [FeIIICl4]3 by£ushing with 1 ml aliquots of 6 M HCl. Elutionof the [FeIIICl4]3 ion from the resin was thenmade using 1 ml aliquots of 0.1 M HCl as theelutrient.

Following the ion exchange separation, the twosolution fractions (FeII and FeIII) were preparedfor mass spectrometric analysis by evaporating todryness, then treated to a series of heating andevaporation cycles with perchloric acid and hy-drochloric acid to remove all organic matter andoxidise all iron to FeIII. The solid residue fromthis treatment was then dissolved in 0.1 M HClfor mass spectrometric analysis. Isotopic analysiswas made on a Nu Instruments MC-ICP-MS.Iron concentration was determined from compar-ison of the height of the 54Fe peak of appropri-ately diluted solutions with that of a 20 ppm Festandard. Mass spectrometric procedures are asdescribed by Zhu et al. [4], with each sample anal-ysis being bracketed by two standard solutionanalyses. The samples and standards were intro-duced into the plasma through a modi¢ed CetacMCN 6000 desolvating nebuliser. The three iso-topes 54Fe, 56Fe and 57Fe were measured and57Fe/56/Fe and 56Fe/54Fe ratios are reported usingthe epsilon notation relative to the IRMM-14 ironreference standard [4] :

O57Fe �

�57Fe=54Fe�sample=�57Fe=54Fe�standard31:0� �

U104

Errors on replicate measurements are less than0.6 O units. Fractionations are expressed as thedi¡erence:

v�FeII3FeIII� � O57Fe�FeII soln�3

O57Fe�FeIII soln�W104 ln K �FeII3FeIII�

where K (FeII3FeIII) is the fractionation factor:

K �FeII3FeIII� � �1� O57Fe�FeII soln�=

104�=�1� O57Fe�FeIIIsoln�=104�

The FeII/FeIII ratio of the two solution frac-tions is expressed by the mole fraction:Fe* = (FeII)/[(FeII)+(FeIII)]. Analytical uncertaintyof Fe* is estimated at þ 0.03^0.04.

The e¤ciency of uptake of the FeIII ion by theresin was checked in experiments in which 2 mlaliquots of FeIII solution III at 100 ppm concen-tration (with and without the addition of 2,2P-bi-pyridine) were loaded onto 2 and 4 ml resins andthe extraction protocol followed with the additionthe resin was further eluted with 2 M HNO3 afterthe 0.1 M elution stage to check if there were stillresidual traces of FeIII remaining on the resin.The results showed that there was complete up-take of FeIII on the resin (i.e., none of the FeIII

species comes down with the 6 M HCl) and com-plete elution of this ion in 0.1 M HCl. The O57Feand O56Fe values of the 0.1 M HCl FeIII solutionfraction were identical to those of the initial solu-tion within experimental errors, indicating thatisotopic recovery is complete. The main sourceof error for mass balance comes from the inevi-table sample loss during sample treatment.

3. Results

The e¤ciency of the ion exchange separation ofthe two solution fractions on 4 ml and 2 ml resinvolumes is illustrated in Fig. 2. Two distinct peaksare observed on loading 2 ml solution into thecolumns, indicating complete separation of thesolution fractions. However, there are also varia-tions in the relative amounts of the two solutionfractions. A higher yield of the FeII solution wasobtained with the 2 ml resin (Fe* = 0.7) than withthe 4 ml resin (Fe* = 0.47). A repeat of this experi-ment with isotopic measurement of the two solu-tion fractions gave similar di¡erences in yield (Ta-ble 2). The FeII solution passes through the 2 mlresin (10 ml column with a funnel-shaped cross-

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A. Matthews et al. / Earth and Planetary Science Letters 192 (2001) 81^9284

section) in a shorter time than through the 4 mlresin (the FeII peak concentration appears at V14min, compared to V18 min in the 4 ml column).These observations indicate that decomposition

and oxidation of the FeII solution is occurringwithin the time span of column separation, aspreviously suggested by the weakening of thered colour of the solution on standing. This di¡er-ence is also re£ected in the 57Fe/54Fe and 56Fe/54Fe fractionations (Table 2). The 57Fe/54Fe frac-tionation on the 2 ml resin is 26 O units and 57 Ounits on the 4 ml resin. When 1 ml of iron solu-tion is loaded on the 2 ml resin (i.e., the samesolution volume/resin volume ratio as on the4 ml resin), the values of Fe* and the isotopicfractionation become closer to those of the 4 mlresin.

The full set of isotopic compositions and v val-ues for the di¡erent initial solution fractions ispresented in Table 3 (corresponding values offractionation factors expressed as 1000 lnK(FeII3FeIII) are also given). O57Fe values forthe two solution fractions are plotted againstFe* in Fig. 3a and the 57Fe/54Fe fractionationsare plotted in Fig. 3b. The O57Fe values showlarge variations, which are particularly markedfor the FeII solution fraction (18^132 O units).57Fe/54Fe fractionation factors vary from 25 to174 and show a progressive increase in with de-crease in Fe*. The isotopic variations are notdependent on the initial solution type: high frac-tionation factors are given by both initial solu-tions I and IV, and similarly the results for solu-tions I and II fall on the same general trend. Thelowest Fe* values and largest fractionations aregiven by experiments where the 6 M HCl initialsolutions were allowed to stand for ca15 min pri-or to loading on the 4 ml ion exchange resin.

Data trends similar to those for 57Fe/54Fe arefound for O56Fe values and 56Fe/54Fe fractiona-tions (Table 3). A plot of O57Fe values againstO56Fe values lies along a straight line de¢ned bythe equation: 0.6744 þ 0.0014 (Fig. 4). This line

Fig. 2. Plots showing the ion exchange separation of the FeII

solution fraction in 6 M HCl solution from the FeIII solutionfraction in 0.1 M HCl. The concentration ordinate expressesthe amount of iron in ppm determined in 1 ml aliquots ofsolution collected from the column. 2 ml of 100 ppm ironsolution was loaded on to 4 ml (a) and 2 ml (b) resins.

Table 2Solution volume/resin volume e¡ect on fractionation factors

Solution Solution volume Resin volume Fe*a v(FeII3FeIII) 57Fe/54Fe v(FeII3FeIII) 56Fe/54Fe(ml) (ml)

II 2 4 0.45 56.7 38.0II 2 2 0.65 24.8 16.7II 1 2 0.46 50.0 33.7aFe* = Fe(II)/(FeII+FeIII) ratio determined from mass spectrometry.

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A. Matthews et al. / Earth and Planetary Science Letters 192 (2001) 81^92 85

Tab

le3

Isot

ope

frac

tion

atio

nre

sult

s

Run

#In

itia

lso

luti

ona

H2O

2

adde

dbV

Soln

/V

Res

in

Tim

eF

eIIc

FeII

solu

tion

frac

tion

dF

eIII

solu

tion

frac

tion

dF

e*v

(FeII

3F

eIII )

57F

e/54

Fe

v(F

eII3

FeII

I )56

Fe/

54F

e10

3lnK

(FeII

3F

eIII )

57F

e/54

Fe

103

lnK

(FeII

3F

eIII )

56F

e/54

Fe

(Wl)

(ml)

(min

)O57

Fe

O56F

eO57

Fe

O56F

e

10I

^2/

432

44.6

29.9

336

.43

24.7

0.41

81.0

54.6

8.10

5.46

11I

52/

432

45.9

30.7

318

.93

12.5

0.41

64.8

43.2

6.47

4.31

12I

152/

432

47.0

32.7

323

.33

15.2

0.38

70.3

47.8

7.02

4.78

13I

302/

432

56.4

37.8

318

.83

12.4

0.31

75.2

50.2

7.50

5.01

20I

^2/

442

131.

788

.93

42.3

328

.40.

2817

4.0

117.

317

.33

11.6

921

I20

2/4

4210

9.9

74.2

336

.13

23.9

0.33

146.

098

.114

.54

9.78

22I

402/

442

97.7

66.0

332

.03

21.2

0.32

129.

787

.212

.92

8.70

26I

^2/

432

29.7

19.9

312

.83

8.3

0.52

42.4

28.2

4.24

2.82

27II

^2/

432

38.6

26.0

318

.23

12.0

0.45

56.7

38.0

5.66

3.79

28II

^2/

226

18.5

12.7

36.

43

4.0

0.65

24.8

16.7

2.48

1.66

29II

^1/

232

32.6

21.7

317

.53

12.0

0.46

50.0

33.7

5.00

3.37

23IV

a^

2/4

4212

1.9

82.8

340

.63

27.0

0.24

162.

510

9.8

16.1

810

.94

24IV

b^

2/4

4210

8.1

72.9

334

.23

22.8

0.30

142.

395

.714

.18

9.55

25IV

c^

2/4

4210

6.0

71.5

334

.23

23.6

0.30

140.

295

.113

.97

9.48

14II

I^

2/4

^^

31.

33

1.5

1.00

17II

I^

2/2

^^

1.3

0.8

1.00

aSo

luti

ons

I,II

,II

Ian

dIV

asde

¢ned

inT

able

1.b5

vol.

H2O

2ad

ded

to1

ml

init

ial

solu

tion

prio

rto

2,2P

-bip

yrid

ine.

c App

roxi

mat

eti

me

for

com

plet

eco

llect

ion

ofth

eF

eIIso

luti

onfr

acti

on,

com

men

cing

from

the

tim

eof

acid

i¢ca

tion

to6

MH

Cl.

dF

eIIan

dF

eIII

solu

tion

frac

tion

sco

llect

eddu

ring

elut

ion

wit

h6

MH

Cl

and

0.1

MH

Cl,

resp

ecti

vely

.

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A. Matthews et al. / Earth and Planetary Science Letters 192 (2001) 81^9286

de¢nes the mass fractionation relationship for thethree isotope system and is close to the simpleharmonic oscillator^mass fractionation relation-ship of O56Fe = 0.678UO57Fe calculated fromequations in Criss [16].

Isotopic fractionation accompanying the pas-sage of the FeII solution through the resin wasmeasured in an experiment in which this solutionwas collected in 1 ml fractions and the isotopiccomposition of each fraction measured. The re-sults for 57Fe/54Fe are presented in Fig. 5. Theyshow a variation from a value of O57Fe = +80units in the ¢rst solution fraction to 390 unitsin the fraction collected after 99% of the FeII so-lution had passed through the column. The bulkO57Fe of the FeII solution is ca. 30 O units. Theform of the isotopic pro¢le is similar to thoseobserved for copper and iron isotopes during elu-tion experiments [8,19,20].

4. Discussion

The overall isotopic scheme can most simply beregarded in terms of a small initial fractionation,over which is superimposed a large fractionationthat enriches the [FeII(bipy)3]2� in the heavier iso-tope. The key observation in this respect is thatthe fractionations increase with decrease in the

Fig. 3. (a) A plot showing the variation of O57Fe values ofthe FeII and FeIII solution fractions as a function of the FeII

mole fraction Fe*. (b) A plot showing the variation of 57Fe/54Fe fractionations with Fe*.

Fig. 4. O56Fe vs. O57Fe plot of the isotopic compositionaldata given in Table 3. The linear ¢t corresponds to the massfractionation relationship for the three isotope system.

Fig. 5. Plot showing the results of a 'chromatographic' ex-periment on an FeII solution fraction (run 30). 2 ml of solu-tion I was loaded onto a 4 ml resin. The FeII solution wascollected in ca. 1 ml sequential portions by £ushing with 1 mlaliquots of 6 M HCl. The plot represents the variation inO57Fe values of the solution aliquots and Fe fraction col-lected in each solution aliquot.

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A. Matthews et al. / Earth and Planetary Science Letters 192 (2001) 81^92 87

(FeII)/[(FeII)+(FeIII)] ratio. The case for this var-iation being related to a kinetic fractionation pro-cess will be presented in the following section.

4.1. Isotope fractionation mechanisms

Speciation calculations made using stepwiseformation constants for FeIII solution species giv-en by Bjerrum and Lukes [23] and for FeII speciesby Meites [24] show that initial solutions in 1 MHCl are dominated by [FeCl2]� and minoramounts of [FeCl]2� and [FeCl3]0 for FeIII speciesand roughly equal amounts of Fe2� and [FeCl]�

for FeII species. Iron isotopic fractionation inthese initial solutions should therefore be repre-sentative of this speciation. The ionic speciationchanges in 6 M HCl solution. [FeCl4]3 becomes asigni¢cant ion, whereas FeII complexes are notstable with respect to FeIII. The formation ofthe [FeII(bipy)3]2� inhibits this oxidation process,the chelation reaction e¡ectively `locking up' theFeII ion. [FeII(bipy)3]2� belongs to a class of com-plex ions in which the ligands bond to Fe in a`low spin' electronic structure (i.e., the six 3d elec-trons of FeII are spin paired in the lower energyt2g electron orbitals in the octahedral metal^li-gand structure). This property, shared by otherchelating ligands such as CN3 and 1,10-phenan-throline, results in the formation of a strong co-valent linkage between the central iron atom andnitrogen atoms of the chelating molecules, inwhich the back-bonding with empty ligand Z or-bitals contributes to the stabilisation of iron in thelow spin, low oxidation state FeII ion [25].

The experimental observations of this studymake it clear that the stabilisation of the[FeII(bipy)3]2� ion in 6 M HCl is a kinetic chem-ical e¡ect. Dissociation of this complex occurs in6 M HCl both prior to and during the ion ex-change separation of the FeII species. The varia-tion in Fe* thus re£ects breakdown of the[FeII(bipy)3]2� complex ion and the correspondingformation of FeIII ions. The dissociation of the[FeII(bipy)3]2� complex ion in 6 M HCl is consis-tent with studies of the kinetics of dissociation ofiron-2,2P-bipyridyl and 1,10-phenanthroline com-plexes in strong acid solution [26]. The markedincrease in fractionations with decreasing Fe* sug-

gests that this dissociation reaction strongly in£u-ences the isotopic fractionation. Dissociation re-actions are typically associated with kineticisotope fractionation [27,28].

The formation of FeIII as the result of break-down of the [FeII(bipy)3]2� ion involves twoconsecutive processes: (1) the dissociation of[FeII(bipy)3]2� to form non-chelated FeII ionicspecies; (2) the FeIICFeIII transformation andthe uptake of the FeIII as [FeCl4]3 on the resin.The mechanism of the dissociation of[FeII(bipy)3]2� in acid solution has been describedin a number of works [26,29^31]. The dissociationmechanism involves the creation of an intermedi-ate protonated species in which a nitrogen atomof one of the 2,2P-bipyridine molecules becomesprotonated and detached from the FeII ion. Theformation of this intermediate also involves race-misation of the protonated nitrogen away fromthe FeII ion [29]. The rupturing of one of theFe^N bonds destroys the stability of the [FeII

(bipy)3]2� ion and subsequent dissociation reac-tions are kinetically rapid. The covalent organo-metallic bonding between the ligands and the cen-tral iron atom in the [FeII(bipy)3]2� complex fa-vour that it will be enriched in the heavier isotoperelative to iron species in coordination that lackthis type of bonding. Thus, the isotopic fraction-ation between strongly bonded (higher vibrationalfrequency) [FeII(bipy)3]2� ion and the FeII speciesproduced by dissociation (dissociation-FeII) maybe relatively large, depending on the con¢gurationof the activated complex (see later discussion).The formation of an isotopically light dissociatedspecies should therefore enrich the residual[FeII(bipy)3]2�, which qualitatively agrees withthe increase in O57Fe and O56Fe with decreasingFe* (Fig. 3 and Table 3). The fractionation trendsresemble the kinetic fractionations observed in thepioneering sulphur isotope studies of Thode andcoworkers [11,28,32,33] on the reduction of sul-phate to sulphide in solution.

4.2. Quantitative modeling of isotopic fractionation

Quantitative interpretation of the fractionationsin the experimental system presents a number ofdi¤culties. The data indicate that at least two

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A. Matthews et al. / Earth and Planetary Science Letters 192 (2001) 81^9288

mass fractionation processes are operating: (1)fractionation of [FeII(bipy)3]2� during ion ex-change separation (chromatography) and (2) frac-tionation caused by the dissociation of[FeII(bipy)3]2�. The isotopic variation observedin a single [FeII(bipy)3]2� solution fraction (Fig.5) may represent chromatographic fractionationbetween dissolved and resin-bound species, as ob-served in elution experiments on iron and copperchloro-aquo complexes [8,19,20]. However, posi-tively charged solution species should not undergochromatographic interaction with the anion resin.An alternative explanation for the isotopic pro¢lecould be that some of the FeII ion created by thedissociation of [FeII(bipy)3]2� comes through inthis fraction as a more negative isotopic species.This would occur if the oxidation rate of this dis-sociation FeII and its uptake on the resin as[FeCl4]3 were slower than the rate of its elutionout of the column.

Despite these complications, the overallchanges of O57Fe should follow a form of Ray-leigh fractionation relationship if dissociation of[FeII(bipy)3]2� is the main control of the isotopicfractionation. This proposal is examined in a massbalance calculation based on the following as-sumptions. (1) An initial fractionation in 1 MHCl solution with Fe*0 representing the molefraction of FeII species in this solution and v0

the initial di¡erence in the O57Fe values of theFeII and FeIII species. The magnitude of theseparameters is not known, but must beFe*0 v 0.65 and v0 9 25 O units (Table 3). (2)The dissociation of [FeII(bipy)3]2� behaves ac-cording to a Rayleigh fractionation equation:R=R0 � f �K k31�, where R0 and R are the 57Fe/54Fe ratios of the [FeII(bipy)3]2� before and dur-ing dissociation, Kk is a kinetic fractionation fac-tor between the [FeII(bipy)3]2� and the dissociatedproduct species, and f is the fraction of the orig-inally formed [FeII(bipy)3]2� remaining in the so-lution at any time. The value of f at any time isgiven by the relation f = Fe*/Fe*0.

Since there is no independent knowledge of v0

and Kk, our analysis is limited to exploring thefeasibility of a range of parameters. For simplicitywe have assumed that Fe*0 = 0.7 and v0 = 10. Fig.6 presents a ¢t of the mass balance calculation to

the experimental O57Fe data assuming Kk = 1.010(vV100 O units). It can be seen that a reasonable¢t to the experimental data is obtained using thismodel, suggesting that it provides a good overallapproximation to the fractionation process.

The theoretic framework for calculating kineticfractionation factors has been given by Bigeleisen[34]. To a close approximation, kinetic isotopefractionation on dissociation is expressed by theequation:

k1=k2wm�m

� �1=2�1� L3L ��

where k1 and k2 are the rate constants for thelight and heavy isotopic species respectively, m*and m are the reduced masses across the bondbeing broken in the dissociation reaction, L andL� are the reduced partition function ratios forisotopic exchange [14]. The asterisk indicates theheavier isotopic species and superscript þ refersto the activated complex in the dissociation reac-tion.

This equation allows us to place limits on themagnitude of the kinetic fractionation in the[FeII(bipy)3]2� dissociation. Following Harrisonand Thode [32] we de¢ne two limiting alternatives

Fig. 6. Model ¢t of the mass balance Rayleigh fractionationcalculation to the experimental O57Fe vs. Fe* data. The Ray-leigh calculation is based on the assumption of an initialfractionation v0 = 10 O units between the FeII and FeIII solu-tions at Fe*0 = 0.7. On top of this is superimposed the Ray-leigh isotopic fractionation occurring as the result of the uni-directional dissociation of [FeII(bipy)3]2�. The kinetic frac-tionation factor used in the Rayleigh calculation isKk = 1.010.

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for the activated complex. If the activated com-plex is similar in structural con¢guration to thereactant [FeII(bipy)3]2�, then LWL� and the ki-netic fractionation is equal to the reduced massterm in the above equation. The critical bondbeing broken is the Fe^N bond, as discussed inSection 4.1, and the corresponding reducedmass term gives a kinetic fractionation factor ofk1/k2 = 1.0054 for the 57Fe/54Fe fractionation. Ifon the other hand the shape of the activated com-plex is similar to that of the dissociation product,then L� can be approximated by the reduced par-tition function of this dissociation species. Thisspeciation is clearly not known in this study.If we take the reasonable assumption that theL3L� term must be less than the di¡erence of1.015 between the L(57Fe354Fe) values for[FeII(CN)6]43 and [FeIIICl4]3 at 298 K (estimatedby dividing corresponding L(56Fe354Fe) values in[18] by 0.68), a maximum limit for the kineticfractionation factor is given k1/k2 = 1.020. TheKk = 1.010 used in the Rayleigh analysis is there-fore compatible with these theoretical limits.

5. Conclusions

The iron stable isotope fractionation betweenthe [FeII(bipy)3]2� and FeIII chloride complexeshas been measured after ion exchange separationusing plasma source mass spectrometry. Largeisotopic variations of O57Fe and O56Fe are exper-imentally measured in the two separated solutionfractions, and isotopic fractionations increasefrom 25 to 174 O units for 57Fe/54Fe and from17 to 117 O units for 56Fe/54Fe. The increase infractionation factors correlates with a decrease inthe mole fraction of FeII that results from thedissociation and breakdown of [FeII(bipy)3]2� insolution. The data indicate that mass fractiona-tion processes are expressed at two di¡erent lev-els : fractionation of [FeII(bipy)3]2� during the ionexchange separation process and a large overallkinetic fractionation associated with the dissocia-tion of [FeII(bipy)3]2� in 6 M HCl. This kineticfractionation is attributed to the rupturing of thestrong covalent Fe^N bonding between 2,2P-bi-pyridine and FeII ion in a stable 'low spin' t2g

electronic con¢guration. Barring an unspeci¢edchromatographic isotope separation e¡ect, theisotopic data can reasonably be modeled by amass balance calculation based on a Rayleighlaw, using a kinetic fractionation factor Kk =1.010.

The study thus provides experimental con¢rma-tion that signi¢cant non-biological iron isotopefractionations are observable among iron speciesin solution and that the type of metal^ligandbonding shown by iron will exert a critical controlon its stable isotope geochemistry. Metal^ligandbonding is a fundamental aspect of the geochem-istry of transition metal elements, iron, copperand zinc, and its potential importance in isotopicfractionation has been recognised in recent exper-imental studies [8,19,20]. This study favours astrong enrichment of the heavy isotope in ener-getically stable metal^ligand bonds with iron in alow spin FeII electronic con¢guration. Naturalphenomena where this stabilisation occurs includehaem iron, where the chelation of porphyrin, alsoa dianionic N-donor ligand, can stabilise FeII inboth low and high spin electronic states in elec-tron transferring proteins [35]). Pyrite, FeS2,among the iron sulphides, is unique in readilyforming a low spin electronic con¢guration forthe iron atom [36]. Although the system studiedin this work is non-biological, it is within the ¢eldof biologically mediated processes that the dataprobably have the most relevance. Uni-directionalkinetic fractionations play an important role inbiological evolution, as examples from carbon iso-tope geochemistry (e.g., the C3, C4 metabolicpathways) and sulphur isotope geochemistry(e.g., the bacterial reduction of sulphate) indicate.The experimental study presents a clear case thatkinetic iron isotope fractionation occurs in theiron system.

Acknowledgements

The authors would like to thank Dr N.S. Bel-shaw for his help and expertise in mass spectrom-etry and Dr J. Arden and S. Wyatt for advice andhelp with the chemical procedures. We are greatlyindebted to Prof. R.J.B. Williams, who contrib-

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uted much to the clari¢cation of key aspects inthe work. Fruitful discussions with Albert Galy,Francis Albarede, Peter Williams, Edwin Schau-ble and Max Coleman, and reviews by SimonSheppard, Ariel Anbar and an unknown reviewerwere in£uential in the revision of an earlier draft.This study was supported by a grant from theNatural Environment Research Council. A.M. ac-knowledges support of Israel Science FoundationGrant 455/00.[AH]

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