Atom exchange between aqueous Fe(II) and structural Fe in clay minerals

Transcription

1 Supporting Information Environmental Science and Technology Atom exchange between aqueous Fe(II) and structural Fe in clay minerals Anke Neumann 1,, Lingling Wu 2, Weiqiang Li 2, Brian L. Beard 2, Clark M. Johnson 2, Kevin M. Rosso 3, Andrew J. Frierdich 1,2, and Michelle M. Scherer 1 1 Civil and Environmental Engineering, The University of Iowa, Iowa City, Iowa 2242, USA 2 Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 376, USA 3 Physical Sciences Division, Pacific Northwest National Laboratory, Richland, Washington 9932, USA anke.neumann@ncl.ac.uk Pages: 13 Tables: 2 Figures: 4

2 S2 CALCULATIONS S1 Methods S1.1 Fe isotope analysis Typically, each sample from the experiments of this study was dissolved in.2 ml 7M HCl, and loaded onto a column that contained.2 ml anion exchange resin (Bio-Rad AG 1X4 2 4 mesh), matrix elements such as (Na +, Ca 2+, Al 3+ ) were eluted using 2.1 ml 7M HCl. Following that, Fe was washed off the resin completely using 2.1 ml. M HCl. The collected Fe was then dried down, and re-dissolved in 7M HCl, and the purification procedure was repeated. Fe content of samples before and after the purification was measured using a Ferrozine method 1 to ensure that there was no loss of Fe during the ion exchange chromatography. Details of the procedure can be found in Beard et al. (23) 2. S2 Calculations S2.1 Correction of Fe isotope fraction for 8 Ni interference The fraction of Fe isotope i was calculated as given in equation 1 in the main manuscript. We also calculated the Fe isotope fraction corrected for 8 Ni interference by taking into account the counts of 6 Ni as well as the natural abundance of 8 Ni (68%) and 6 Ni (26%): f i Fe = i counts 4 counts + 6 counts + 7 counts + 8 counts counts The corrected Fe isotope fraction was usually within the associated standard deviations obtained for the uncorrected Fe isotope fraction from triplicate reactors. (S1) S2.2 Fe isotope fraction at equilibrium The Fe isotope fraction at equilibrium, ( f i Fe) eq, was calculated taking into account the initial amount and isotope composition of aqueous Fe(II) (n aq, ( f i Fe) aq ) and of structural Fe(III) in clay mineral NAu 1 (n s, ( f i Fe) s ) according to 3 : ( f i Fe) eq = n aq ( f i Fe) aq + n s ( f i Fe) s n aq + n s (S2) S2.3 a and b parameters From the unit cell formula (NAu 1: Na.3 (Al. Mg.2 Fe 1.84 )(Si 3.49 Al.1 )O 1 (OH) 2 ) 4, NAu 2: Na.36 (Al.17 Mg.3 Fe 1.77 )(Si 3.78 Al.8 Fe. )O 1 (OH) 2 ) ) and the known cation-oxygen S2

3 S2 CALCULATIONS Table S1: Calculation of Fe isotope fractions at equilibrium for the reaction of aqueous Fe(II) with clay mineral NAu 1 and NAu 2 at ph 6. and ph 7. according to equation S2. initial at equilibrium aqueous solid aqueous = solid Fe(II) f 7 Fe f 6 Fe f 4 Fe Fe(III) f 7 Fe f 6 Fe f 4 Fe f 7 Fe f 6 Fe f 4 Fe NAu 1, ph (.2).928(.3).48(.3).32(.2) 12.3(.8).23(.1).486(.2).92(.) NAu 1, ph (.4).91(.4).74(.4).6(.7) 12.44(.47).23(.1).486(.2).92(.) NAu 2, ph (.2).928(.3).48(.3).32(.2) 12.43(.22).23(.1).484(.2).92(.) NAu 2, ph (.4).91(.4).74(.4).6(.7) 12.22(.3).23(.1).484(.2).92(.) S3

4 S2 CALCULATIONS distances for Al (1.94 Å), Fe (2. Å), and Mg (2.12 Å), the average cation-oxygen distance (M O) in the octahedral sheet of the unit cell can be calculated 6 : and M O(NAu 1) = =.(Al O) +.2(Mg O) (Fe O) (1.94 Å) +.2(2.12 Å) (2. Å) M O(NAu 1) = 2.4 Å (S3) M O(NAu 2) = =.17(Al O) +.3(Mg O) (Fe O) (1.94 Å) +.3(2.12 Å) (2. Å) M O(NAu 2) = 2.4 Å (S4) Using this average M O distance, the b parameter of the octahedral sheet unit cell of the dioctahedral clay minerals NAu 1 and NAu 2 can be calculated according to 6 : b = 3 2 M O = Å The a parameter of the same unit cell then results from 6 : b = 8.6 Å (S) a = b 3 = 8.6 Å 3 a = 4.99 Å (S6) A clay mineral particle sized (..) µm (dimensions in a- and b-direction of the octahedral sheet within a single clay platelet, consisting of one octahedral sheet enclosed by two tetrahedral sheets) would therefore accommodate approximately 88 unit cells (see also Figure S1 for illustration: unit cells, based on b and a parameter). The circumference would be made up of 2 (a + b) = 2 ( ) unit cells = 316 unit cells. These numbers would be much smaller for a clay mineral particle of only (..) µm, with a S4

5 S2 CALCULATIONS b = 8.6 Å. µm 78 u.c. a = 4.99 Å. µm 12 u.c. clay mineral particle (a) octahedral sheet unit cell of dioctahedral clay mineral NAu 1 (b) unit cells in clay mineral particle Figure S1: Schematic illustration of a) octahedral sheet cation occupancy and dimensions of the unit cell of dioctahedral clay mineral NAu 1 with filled circles representing cations and empty squares representing octahedral vacancies, and b) the estimated number of unit cells in a clay mineral particle of (..) µm. size of 8 1 = 8 unit cells and thus only 37 unit cells making up the outer layer, which is exposed to the surrounding solution. S2.4 Depth of exchange To estimate the number of unit cells in which Fe atoms were exchanged with Fe atoms from the aqueous phase, we can assume that all unit cells contain only Fe based on (a) the unit cell formula, from which we can estimate that 9 92% of the octahedral places are filled with Fe, and (b) the average M O distance, which is very close to the Fe O distance (2.4 Å vs 2. Å, respectively), suggesting limited contribution of other cations to the overall octahedral structure. Thus, we can use the measured extent of Fe exchange (1% and 2% for NAu 1 and NAu 2, respectively) and the calculated number of unit cells within a clay mineral particle of (..) µm and (..) µm (88 and 8, respectively) to estimate the number of unit cells that need to have exchanged their Fe atoms with the aqueous solution: number of exchanged unit cells = extent of Fe exchange total number of unit cells in clay mineral particle S

6 S2 CALCULATIONS As a conservative estimate, we can then calculate how many outer layers (unit cells in the circumference) need to be replaced in order to accommodate this number of exchanged unit cells: penetration depth = number of exchanged unit cells number of unit cells in circumference This estimate of the penetration depth of Fe atom exchange into the clay mineral particle can then also be converted to units of Å by taking into account the a and b parameter of the unit cell. The results for the calculations are given in Table S2 for the range of observed Fe atom exchange, particles sized (..) µm and (..) µm as well as converted to Å for exchange in a-direction and b-direction. Table S2: Estimated number of exchanged unit cells and penetration depth (in units of unit cells and Å) for. µm and. µm particles for the observed extent of exchange of 1% (NAu 1) and 2% (NAu 2). number of penetration depth exchanged unit cells unit cells Å Å (a-direction) (b-direction). µm particle 1% Fe atom exchange % Fe atom exchange µm particle 1% Fe atom exchange % Fe atom exchange S2. Time estimates for atom diffusion To estimate the time t Fe atoms would need to diffuse through the mineral lattice to account for the observed 1% and 2% Fe atom exchange, we use the calculated distances in Å (L) reported in Table S2. Since the extrapolated diffusion coefficients D differ over three orders of magnitude (1 19 cm 2 s 1 for montmorillonite,1 21 cm 2 s 1 for kaolinite 7 ), we use the simple relation to estimate the time for diffusion 7 : S6

7 S2 CALCULATIONS L = D t (S7) t = L2 D and thus (S8) The times estimated for the different extent of Fe atom exchange, particle size and diffusion coefficients are reported in Table S3. The general relationship between diffusion coefficient D and time to penetrate particles sized. µm (average depth 28 Å) and. µm (average depth 18 Å) to account for 1 2% Fe atom exchange is given in Figure 3 in the main manuscript and the times estimated for D values of 1 19 cm 2 s 1 and 1 21 cm 2 s 1 are plotted as markers. Table S3: Time estimates for. µm and. µm particles, 1% and 2% Fe atom exchange into a- and b- direction. L D t Å cm 2 s 1. µm particle 1% Fe atom exchange 2% Fe atom exchange. µm particle 1% Fe atom exchange 2% Fe atom exchange years months years months years year years years months day months days months days years week S7

8 S3 GRAPHS AND TABLES S3 Graphs and Tables clay mineral Fe exchange (%), from f 7 Fe 2 1, NAu-1, ph 6., NAu-1, ph 7., NAu-2, ph 6., NAu-2, ph :1 line 2 clay mineral Fe exchange (%), from f 6 Fe (a) Extent of exchange calculated from f 6 Fe vs f 7 Fe (b) Extent of exchange calculated from f 4 Fe vs f 7 Fe clay mineral Fe exchange (%), from f 4 Fe 2 1, NAu-1, ph 6., NAu-1, ph 7., NAu-2, ph 6., NAu-2, ph 7. 1 clay mineral Fe exchange (%), from f 7 Fe 2 1, NAu-1, ph 6., NAu-1, ph 7., NAu-2, ph 6., NAu-2, ph :1 line 2 clay mineral Fe exchange (%), from f 4 Fe 1:1 line 2 clay mineral Fe exchange (%), from f 6 Fe (c) Extent of exchange calculated from f 6 Fe vs f 4 Fe Figure S2: Extent of exchange calculated a) from aqueous f 6 Fe (x-axis) and f 7 Fe (y-axis), b) from aqueous f 4 Fe (x-axis) and f 7 Fe (y-axis), and c) from aqueous f 6 Fe (x-axis) and f 4 Fe (y-axis) for NAu 1 (black markers) and NAu 2 (red markers) for both ph 6. (circles) and ph 7. (squares). Open and filled markers represent data resulting from analysis with quadrupole inductively couple mass spectrometry (Q ICP MS) and multi collector ICP MS (MC ICP MS), respectively. All data fall very close to the 1:1-line (dashed), indicating an apparent stoichiometry of 1:1 for Fe atom movement out of and into solution, which is consistent with Fe atom exchange. S8

9 S3 GRAPHS AND TABLES clay mineral Fe exchange (%) 1 NAu-1, ph 6. NAu-1, ph 7. SWa-1, ph time (days) 6 Figure S3: Extent of Fe isotope exchange between aqueous Fe(II) and structural Fe in clay mineral NAu 1 (black squares) at ph 6. (dashed line) and ph 7. (solid line) and SWa 1 (blue triangles) at ph 6.9, calculated from aqueous phase 7 Fe according to equation 2 in the main manuscript. Experiments were carried out with aqueous Fe(II) highly enriched in 7 Fe (open markers) and with low 7 Fe-enriched aqueous Fe(II) (filled markers). S9

10 S3 GRAPHS AND TABLES 1. f 7 NAu-1 1. Fe f 7 NAu-2 Fe.8.8 (f 6 Fe) eq.6 (f 6 Fe) eq.6 f i Fe f i Fe.4 f 6 Fe.4 f 6 Fe (f 7 Fe) eq (f 7 Fe) eq.2.2.,,, ph 6.,,, ph 7..,,, ph 6.,,, ph time (days) (a) NAu time (days) (b) NAu 2 Figure S4: Fractions of 7 Fe (black markers) and 6 Fe (red markers) in the aqueous phase at ph 6. (circles) and ph 7. (squares) during the reaction of aqueous Fe(II) enriched in 7 Fe and depleted in 6 Fe with (a) clay mineral NAu 1 and (b) clay mineral NAu 2 containing 7 Fe and 6 Fe in their natural abundance. Good agreement of isotope measurements with a quadrupole inductively coupled mass spectrometer (Q ICP MS, open markers) and with a multi-collector ICP MS (MC ICP MS, filled markers) were achieved. Dashed lines indicate the calculated isotope equilibrium fraction for 7 Fe (black) and 6 Fe (red). For calculation of the dashed lines representing the Fe isotope fractions at isotopic equilibrium (( f i Fe)eq), see subsection S2.2. S1

11 S3 GRAPHS AND TABLES years time (s) years 1 year. µm particle month 1 week 1 day. µm particle D (cm 2 s -1 ) Figure S: Relationship between diffusion coefficient D and time to penetrate particles sized. µm (average depth 28 Å, black) and. µm (average depth 18 Å, red) to account for 1 2% Fe atom exchange. The markers indicated the times estimated for D values of 1 19 cm 2 s 1 and 1 21 cm 2 s 1 for each particle size and the shaded areas indicate the time range for lowest to highest penetration depth for each particle size and extent of exchange (for penetration depths see Table S3). S11

12 S3 GRAPHS AND TABLES Table S4: Comparison of isotope data for the reaction of aqueous Fe(II) with clay mineral NAu 1 and NAu 2 at ph 6. and ph 7. for measurements with quadrupole inductively couple mass spectrometer (Q ICP MS) and multi collector ICP MS (MC ICP MS). Numbers in parentheses are standard deviations from analysis of duplicate (MC ICP MS) or triplicate reactors (Q ICP MS). Q ICP MS MC ICP MS time f 7 Fe exchange f 6 Fe exchange f 7 Fe exchange f 6 Fe exchange 7 Fe 6 Fe 7 Fe 6 Fe days % % % % NAu 1, ph (.3).48(.3) (.) 4.(.2).16(.4) 4.(.2).87(.) 4.(.).13(.) 4.(.) 1.86(.) 4.(.2).167(.4) 4.(.2).8(.1) 4.2(.).142(.1) 4.2(.) (.7) 4.9(.2).241(.7).(.2).772(.3).(.1).2(.3).(.1) NAu 1, ph 7..91(.4).74(.4) (.18) 9.7(1.).33(.17) 9.7(1.).74(.1) 1.9(.).279(.9) 1.9(.) 1.691(.11) 7.(.).28(.1) 7.6(.).744(.6) 8.(.2).241(.) 8.(.2) (.12) 9.6(.6).377(.14) 9.9(.7).634(.4) 8.2(.1).34(.3) 8.2(.1) NAu 2, ph (.3).48(.3) (.2) 1.6(.1).281(.1) 1.7(.1).72(.) 1.(.3).264(.) 1.(.3) NAu 2, ph 7..91(.4).74(.4) (.2) 14.2(1.4).384(.2) 14.3(1.4).627(.14).4(.9).31(.13).4(.9) S12

13 REFERENCES References [1] L. Stookey. Ferrozine a new spectrophotometric reagent for iron. Anal. Chem., 42: , 197. [2] B. L. Beard, C. M. Johnson, J. L. Skulan, K. H. Nealson, L. Cox, and H. Sun. Application of fe isotopes to tracing the geochemical and biological cycling of fe. Chem. Geol., 19(1-4):87 117, 23. [3] R. M. Handler, B. L. Beard, C. M. Johnson, and M. M. Scherer. Atom exchange between aqueous Fe(II) and goethite: An Fe isotope tracer study. Environ. Sci. Technol., 43(4): , 29. [4] J. L. Keeling, M. D. Raven, and W. P. Gates. Geology and characterization of two hydrothermal nontronites from weathered metamorphic rocks at the Uley Graphite Mine, South Australia. Clays Clay Miner., 48():37 48, 2. [] W. P. Gates, P. G. Slade, A. Manceau, and B. Lanson. Site occupancies by iron in nontronites. Clays Clay Miner., (2): , 22. [6] A. Meunier. Crystal structure species crystallisation. In Clays, pages 1 6. Springer, Berlin, 2. [7] T. K. Kyser and R. Kerrich. Retrograde exchange of hydrogen isotopes between hydrous minerals and water at low temperatures. In H. P. Taylor, J. R. O Neil, and I. R. Kaplan, editors, Stable Isotope Geochemistry: A Tribute to Samuel Epstein, Special Publication No. 3, pages The Geochemical Society, S13