Supporting Information for: Oxygen Isotope Evidence for Mn(II)-Catalyzed Recrystallization of Manganite ( -MnOOH)

Transcription

1 Supporting Information for: Oxygen Isotope Evidence for Mn(II)-Catalyzed Recrystallization of Manganite ( -MnOOH) Andrew J. Frierdich 1,*, Michael J. Spicuzza 2, and Michelle M. Scherer 3 1 School of Earth, Atmosphere & Environment, Monash University, Clayton, VIC 3800, Australia 2 Department of Geoscience, University of Wisconsin, Madison, WI, 53706, United States 3 Department of Civil and Environmental Engineering, University of Iowa, Iowa City, IA, 52242, United States. *Corresponding author: Tel.: (+61) ; Fax: (+61) ; andrew.frierdich@monash.edu S1

2 MANGANITE SYNTHESIS Initial attempts to prepare manganite from previously reported methods 1 (i.e., titration of a 0.06 M MnSO4 and ~0.18 M H2O2 solution with NH4OH at 60 C) proved unsuccessful as the solid product consisted primarily of hausmannite (Mn3O4), demonstrating incomplete oxidation of Mn(II). The formation of hausmannite was prevented by first precipitating solid Mn(OH)2 followed by oxidation at 22 C. Briefly, addition of 300 ml of 0.2 M NH4OH was added to 1 L of 0.06 M MnSO4 over 1 min during vigorous stirring. This was followed by oxidation of the pale-pink suspension by drop-wise addition of 30 ml of 30% H2O2 over 3 min. The resulting black-brown suspension was placed into a closed container and heated at 70 C for 24 hr. The solid was then washed by centrifugation of the suspension, decantation of the supernatent, and resuspension of the solid in 18.2 M cm water (hereon water). These steps were repeated five times. The washed solid was then dried overnight at 70 C and then ground to a fine powder. S2

3 ESTIMATE OF THE PERCENT OF ATOMS AT THE SURFACE OF MANGANITE Calculation of Mn atom density on the (100) surface 1 1 Mn atoms + 6 1/2 Mn atoms = 4 Mn atoms 4 Mn atoms/(5.7 Å 5.24 Å) (10Å/nm) 2 = Mn/nm 2 Number of Mn atoms at the surface in a gram of manganite manganite specific surface area = 61 m 2 g Mn nm m 2 g -1 (10 9 nm m -1 ) 2 = Mn surface atoms g -1 for (100) surface Number of Mn atoms in one gram of manganite 1 mole Mn 1 mole manganite Mn atoms mole manganite 88 g mole Mn = atoms Mn/g Percentage of Mn atoms at the surface Mn surface atoms/g atoms Mn/g 100 = 12% surface Mn for (100) surface If this procedure is repeated for a the (010) and (001) surfaces of manganite for manganese and oxygen similar results are obtained and yeild an average of 11% surface atoms for manganite (SI Table S1). Similar results would be obtained for Ni-substituted manganite since they have the same specific surface area and the substitution of nickel (at 2 mol%) for manganese would have a minimal effect on the unit cell parameters. Unit cell parameters for manganite from Buerger, S3

4 TEST OF MANGANESE(III) REDUCTION BY HEPES BUFFER HEPES (4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid) is capable of reducing Mn(IV) to Mn(III). 3-5 It is unclear, however, if HEPES mediates the reduction of Mn(III) to Mn(II). This is difficult to assess because Mn(II) released from Mn(III)-bearing minerals could be the result of an external reductant (i.e., HEPES) or originate from the disproportionation of Mn(III). 4 If Mn(II) release from Mn(III) oxides occurs from reduction by HEPES then it is expected that the amount of Mn(II) release would vary as a function of the HEPES concentration. Hence, we conducted additional experiments by reacting MnOOH in ph 7.5 solutions having 0, 25, and 100 mm HEPES and measured Mn(II) release as an indicator of Mn(III) reduction. All experiments were conducted in an identical manner as those described in the main text except the 0 and 100 mm HEPES reactions were in natural abundance water. Additionally, the ph for the 0 mm HEPES experiment was manually adjusted with NaOH. Our findings reveal that the amount of Mn(II) released to solution from suspensions of MnOOH in the absence of HEPES is identical (within error) to the amount of dissolved Mn found in 25 mm HEPES buffered solutions (Figure S3). In the presence of 100 mm HEPES, there are greater amounts of Mn(II) release from MnOOH compared to the 0 and 25 mm HEPES solutions (cf., ~3 M vs 1-2 M). This still corresponds to a mere ~0.01% dissolution of the solid. Detectable concentrations of Mn in solution for these control experiments are nearly threeorders of magnitude lower than the concentrations of Mn(II) present in solution for our Mn(II) reactions. These results thus suggest that for the conditions used in this study, HEPES is not a significant reductant of Mn(III). However, since slightly higher Mn(II) concentrations were observed in the presence of 100 mm HEPES, and given the high sorption capacity of manganite for Mn(II), minor reduction at the lower concentrations used in our oxygen exchange experiments cannot be entirely ruled out as there may be Mn(III) reduction to Mn(II) which never desorbs from the manganite surface. S4

5 DISSOLVED MANGANESE FROM MANGANITE DISPROPORTIONATION Disproportionation of Mn(III) in manganite is a potential source of dissolved Mn(II) according to the following reaction: 2ΜnΟΟΗ + 2Η + MnΟ2 + Μn Η2O (eq S1) An equilibrium expression can be written as: K = [Mn 2+ ][H+] -2 (eq S2) where the equilibrium constant, K, for this equation is , assuming the Mn(IV) oxide is pyrolusite ( -MnO2) and Mn(III) oxide is manganite ( -MnOOH). 6 In this example, activities are assumed to be equal to concentrations. Rearranging this equation, taking the log of both sides, and given that ph = -log [H + ], we yield: log[mn 2+ ] = logk -2pH (eq S3) At ph 7.5 with a logk of 9.13, the concentration of Mn 2+ is predicted to be ~1.3 M. The equilibrium constant, K, for eq 1 can also be calculated from the Gibbs free energy of reaction ( G 0 rxn), which can readily be determined from tabulated thermodynamic data. 7 G 0 rxn = -RTln(K) (eq S4) Where R is the gas constant and is equal to J K -1 mol -1 and T is absolute temperature (i.e., in Kelvin). From this a logk value of 9.3 can be substituted into eq 3 which ultimately yields a Mn 2+ concentration of ~2.0 M at ph 7.5. These dissolved manganese values are in excellent agreement with our MnOOH control experiment (SI Table S2) suggesting that manganese in solution is the result of MnOOH disproportionation rather than reduction of Mn(III) by HEPES. Prior work has reported similar concentrations of dissolved Mn(II) for MnOOH in HEPES-free ph 7.5 suspensions. 8 S5

6 EFFECT OF Mn(II) SORPTION ON OXYGEN ISOTOPE VALUES OF MANGANITE Measurements of dissolved Mn(II) show that a significant fraction of added Mn(II) is taken up by manganite (Table 1, main text). Here, we seek to determine what effect Mn(II) sorption may have on the oxygen isotope composition of manganite. The concentration of sorbed Mn(II) ([Mn(II)]sorb) can be calculated by subtracting the actual aqueous Mn(II) concentration at time, t ([Mn(II)aq]t) from the initial aqueous Mn(II) concentration ([Mn(II)aq]0), i.e., at time t=0 (eq S5). [Mn(II)] sorb = [Mn(II) aq ] 0 [Mn(II) aq ] t (eq S5) From Table 1 in the main manuscript we know that [Mn(II)aq]0 = mm. Since all of the observed sorption occurs within the first day and [Mn(II)aq]t remains constant (within error) for the remainder of the reaction, [Mn(II)aq]t is taken as the average of Mn(II)aq values from times 1 day to 150 day, yielding [Mn(II)aq]t = mm. Substituting these values into eq S5 yields [Mn(II)] sorb = = mm After separating manganite from the aqueous component via filtration, we passed 10 ml of water through the filter to wash the solid. Analysis of this wash solution revealed that approximately 16% of the sorbed Mn(II) was removed and hence indicates that only 84% of the initial sorbed Mn(II) would be present on the solid during analysis. If octahedral Mn(II) binds to manganite as a bidentate complex then each mole of sorbed Mn(II) would bring 4 moles of oxygen (as H2O) to the solid. Therefore, the total amount of oxygen added to the solid from sorbed Mn(II) would be 0.84 X mmol Mn(II) sorb L Each reactor contained 20 mg of manganite which amounts to 20 mg MnOOH 4 mmol O 1 mmol Mn(II) sorb 0.01 L = mmol O 1 mmol MnOOH mg MnOOH 2 mmol O = mmol O 1 mmol MnOOH Therefore, the sorption of Mn(II) to manganite in our system could change the oxygen isotope composition of manganite by 3.6% (see below). S6

7 0.017 mmol O 100 = 3. 6 % mmol O mmole O This value is significantly less than the 10% difference in the amount of exchange that we observe between our control experiment and Mn(II) reaction experiment. It is also a minor contribution when compared to the 28% total amount of oxygen exchange in manganite that occurs in our study. Importantly, this value likely represents a maximum contribution from sorbed Mn(II) as Mn species with higher coordination should be more stable upon sample drying (solid samples were stored in a vacuum desiccator prior to analysis). Since Mn(II) has a low hydration energy it may also form a dehydrated, or partially dehydrated, surface complex or even a surface precipitate as previously observed for Mn(II) sorption on calcite. 9 Such processes would further reduce the contribution of sorbed Mn(II) to the oxygen isotope values of manganite. S7

8 TABLES Table S1. Summary of calculations of percentage of manganese and oxygen atoms at the surface of manganite. See section above for details on the calculations. Element Manganese Oxygen Surface Index (100) (010) (001) (100) (010) (001) Surface Atoms in Unit Cell Atom Density (atoms nm -2 ) BET Surf. Area (m 2 g -1 ) Surf. Atoms in 1 g MnOOH Total Atoms in 1 g MnOOH Percent of Atoms at Surface Average Average 11% 11% S8

9 Table S2. Extrapolated oxygen isotope compositions of manganite after reaction with Mn(II)aq in 17 O-enriched Fairbanks (FB) or Houston (H) water and calculated mineral-water fractionation factors. Note that the level of 17 O enrichment, as illustrated by 45 [CO2]calc, is identical within error for FB ( ) and H ( ) waters. FB H 18 OMnOOH a ± ± Owater Omin-water 5.23 ± ± 0.64 Weighted Average 3.97 ± 1.45 a Reported 18 O values (VSMOW) for manganite exchange experiments are from extrapolations to complete exchange (see Figure 4 in main manuscript). b Errors reported at the 2 level. c Error for weighted average value is the square root of the sum of the square of the errors. S9

10 FIGURES Figure S1. SEM images of manganite samples following the initial synthesis (i.e., t=0 at top), suspension in water for 150 days (i.e., 150 d control in middle row), and reaction in a 1 mm Mn(II) solution for 150 days (bottle row). Two magnifications are shown for each of the three samples. Mean particle sizes (length by width) in nm for the t=0 sample, 150 d control, and 150 d Mn(II) reaction are 565 (±218) by 31 (±6), 324 (±123) by 33 (±8), and 424 (±181) by 37 (±6), respectively. A total of 48 particles were counted for each sample. S10

11 Figure S2. Comparison of XRD patters over the (-1 1 1) reflection to compare manganite after the initial synthesis (i.e., t=0, solid line), following suspension in water for 150 days (i.e., 150 d control, red dashed line), and after reaction in a 1 mm Mn(II) solution for 150 days (blue dotted line). S11

12 Figure S3. Temporal aqueous manganese concentrations measured in solution following suspension of MnOOH in solutions containing varying amount of HEPES. Reaction conditions: ph 7.5, 25 mm KBr, 2 g/l MnOOH. S12

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