Appendix A

Supplementary material

Thiocyanate adsorption on ferrihydrite and its fate

during ferrihydrite transformation to hematite and

goethite

Hong Phuc Vu*, and John W Moreau

School of Earth Sciences, University of Melbourne, Victoria, 3010, Australia

* Corresponding author: email: hong.vu@unimelb.edu.au; telephone: +61 3 9035 6769; fax: +61

3 8344 7761

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Fig. S1. Stability of SCN- at 75 oC in the presence of 0.01 M NaNO3. These data showed that

SCN- was stable at 75oC up to approximately 34 days. Data points and error bars represent the

mean and standard deviation, respectively, of duplicate experiments.

Text S1. Fitting adsorption isotherms to the Langmuir model

Fitting the adsorption isotherm at pH 3, pH 4 and pH 5 to the Langmuir adsorption model using

the linearized approach (Sparks, 2003) and the non-linearized approach (Bolster, 2010; Bolster

and Tellinghuisen, 2010) yielded similar results. The parameters of the fittings are presented in

Table S1. Initially, the data at pH 5 did not fit well to the Langmuir model using the non-

linearized approach, and this was probably due to an outlier at SCN- concentration of 689.4 mg/L

(Fig. S2). Removing this outlier improved the quality of the fit significantly (Fig. S2 and S3),

and therefore the result of this fitting was selected. For other systems (pH 3 and pH 4), the data

fit well to the Langmuir model (Fig. S4).

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Table S1. Parameters of Langmuir fitting for the adsorption isotherms at pH 3, pH 4 and pH 5

using the linearized and non-linearized methods.

Fig. S2. Fitting of adsorption isotherm at pH 5 to the Langmuir model using the non-linearized

method. Data points and error bars represent the mean and standard deviation, respectively, of

duplicate experiments.

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pH Linearized Non-linearizedb K b K

pH 3 166.7 0.00344 163.9 0.00377pH 4 114.9 0.00253 117.1 0.00236pH 5 65.36 0.00205 65.9 0.00192

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Fig. S3. Fitting of adsorption isotherm at pH 5 to the Langmuir model using the non-linearized

method with the outliner at SCN- concentration of 689.4 mg/L was removed (note that the

quality of the fitting was significantly improved compared to Fig. S2). Data points and error bars

represent the mean and standard deviation, respectively, of duplicate experiments.

Fig. S4. Fitting of adsorption isotherm at pH 3 (a) and pH 4 (b) to the Langmuir model using the

non-linearized method. Data points and error bars represent the mean and standard deviation,

respectively, of duplicate experiments.

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Text S2. Surface coverage calculation

Calculation for the surface loading on the ferrihydrite followed the approach in (Langmuir,

1997):

(mol sites/L) = [Ns(sites/m2) x SA(m2/g) x Cs(g/L)]/[NA(sites/mol sites)] (1)

where is the concentration of adsorbing surface sites, SA is the ferrihydrite surface area = 200

m2/g (Vu et al., 2010), NS is the surface site density = 2.27 sites/nm2 (Liger et al., 1999), CS is the

solid/liquid ratio of 2 g/L (this study) and NA is Avogadro’s constant (6.023x1023). The

calculation resulted in a maximum adsorption capacity of the ferrihydrite is 1.5 mM.

Table S2. Amounts of SCN- and SO42- adsorbed in co-adsorption experiment at pH 3. Initial

concentration of SCN- was 0.86 mM, solid/liquid ratio was 2 g/L, and the total volume was 50

ml. All experiments were conducted at room temperature, and 0.01 M NaNO3 was used as an

electrolyte.

Initial [SO42-] (mM) Adsorbed SCN- (mM) Adsorbed SO4

2- (mM) Sum (mM)

0 0.48 - 0.48

0.52 0.53 0.15 0.68

1.04 0.46 0.46 0.92

2.08 0.25 0.99 1.24

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Fig. S5. ATR-FTIR spectra of 0.34 M NaSCN solution (a) and SCN- adsorbed ferrihydrite at

different electrolyte concentrations: 0.01 M NaNO3(b), 0.1 M NaNO3 (c), and 1 M NaNO3 (d). All

spectra were collected in absorption mode with pure ferrihydrite as the background for the SCN-

adsorbed ferrihydrite samples, and air as the background for the aqueous SCN- standard.

Text S3. ATR-FTIR spectrum of adsorption sample at 1 M NaNO3

The spectrum of the sample at 1 M NaNO3 had some features similar to that of the NaNO3

standard (Fig. S5 and S6). This is probably due to the fact that samples were not washed and

NaNO3 (used as the electrolyte in samples) re-crystallised (as white crystals observed in this

sample) after the drying stage.

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Fig. S6. ATR-FTIR spectrum of NaNO3 salt. The spectrum was collected in absorption mode

with air as the background.

Fig. S7. ATR-FTIR spectrum of 0.34 M NaSCN solution. The spectrum was collected in

absorption mode with air as the background.

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Text S4. XRD characterisation of minerals

XRD data revealed that in the presence and absence of SCN -, ferrihydrite crystallised to a

mixture of hematite and goethite at pH 5 and 75oC. There were two stages of crystallisation for

ferrihydrite, with stage one (0 - <1 day) featuring the presence of ferrihydrite, shown as a broad

hump in the spectra. Stage two was characterised by the formation of both hematite and goethite

until the end of the crystallisations (12 days). Notably, although ferrihydrite was the starting

material for recrystallisation, no diffraction peaks were observed for this mineral. This

observation is possibly due to the fact that ferrihydrite is a poorly ordered phase and the sample

holder gave a strong background. The presence of SCN- slightly inhibited the transformation of

ferrihydrite as evidenced by the slightly earlier appearance of hematite and possibly the larger

amount of hematite formed (between 1 day and 2 days, Fig. S8 and S9).

Fig. S8. X-ray powder diffraction patterns from solid phases during the crystallisation of

ferrihydrite in the presence of SCN- at 75oC and pH 5 and 0.01 M NaNO3 was used as an

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electrolyte. HM = hematite and GT = goethite. Background is the diffraction pattern of a glass

slide used as the sample holder.

Fig. S9. X-ray powder diffraction patterns from solid phases during the crystallisation of

ferrihydrite in the absence of SCN- at 75oC and pH 5 and 0.01 M NaNO3 was used as an

electrolyte. HM = hematite and GT = goethite. Background is the diffraction pattern of a glass

slide used as the sample holder.

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Fig. S10. TEM images of the starting material (two-line ferrihydrite; 3a), intermediate phase, and

end products of the transformations (after 1, 4 and 12 days; 3b-d) at 75oC and pH 5, in the

absence of SCN- and 0.01 M NaNO3 was used as an electrolyte. Insets in S10a and 10d are

selected area electron single diffraction patterns of ferrihydrite, hematite and goethite, showing

the d-spacings (for ferrihydrite) and zone axis projections (for hematite and goethite). FHY =

ferrihydrite, GT = goethite and HM = hematite.

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Fig. S11. An example of an energy dispersive X-ray spectroscopy spectrum of the iron oxides.

C and Cu peaks are from the transmission electron microscope sample support grid.

Fig S8c

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Fig. S12. Selected area single diffraction patterns of ferrihydrite (a and f) and hematite (b, d and

g) goethite (c, e and h), as insets in Fig. 6a, 6d and 6e, and Fig. S10a and S10d.

References

Bolster, C.H., 2010. Sorption Isotherm Spreadsheet. http://www.ars.usda.gov/pandp/docs.htm?docid=14971, (Accessed on 08/01/2014).

Bolster, C.H., Tellinghuisen, J., 2010. On the Significance of Properly Weighting Sorption Data for Least Squares Analysis. Soil Science Society of America Journal 74, 670-679.

Langmuir, D., 1997. Aqueous Environmental Geochemistry. Prentice Hall.

Liger, E., Charlet, L., Van Cappellen, P., 1999. Surface catalysis of uranium(VI) reduction by iron(II). Geochimica Et Cosmochimica Acta 63, 2939-2955.

Sparks, D.L., 2003. Environmental Soil Chemistry. Academic Press, California, USA.

Vu, H.P., Shaw, S., Brinza, L., Benning, L.G., 2010. Crystallization of Hematite (alpha-Fe(2)O(3)) under Alkaline Condition: The Effects of Pb. Cryst. Growth Des. 10, 1544-1551.

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