Effect of Ferrihydrite Crystallite Size on Phosphate Adsorption Reactivity. Xionghan Feng *,,, Donald L. Sparks

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1 1 2 Supporting Information Effect of Ferrihydrite Crystallite Size on hosphate Adsorption Reactivity Xiaoming Wang, Wei Li *,,, Richard Harrington,, Fan Liu, John B. arise,, Xionghan Feng *,,, Donald L. Sparks Key Laboratory of Arable Land Conservation (Middle and Lower Reaches of Yangtse River), Ministry of Agriculture, College of Resources and Environment, Huazhong Agricultural University, Wuhan , China Environmental Soil Chemistry Group, Delaware Environmental Institute and Department of lant and Soil Sciences, University of Delaware, Newark, Delaware, 19716, United States Department of Geosciences, Stony Brook University, Stony Brook, New York, 11794, United States Department of Chemistry, Stony Brook University, Stony Brook, New York, 11794, United States Tel: ; fax: ; fxh73@mail.hzau.edu.cn (X.F.) Tel: ; fax: ; weili@udel.edu (W.L.) Summary-total number of SI pages equals 16. The SI section contains nine Figures and two Tables. 23

2 S1. Characterization of ferrihydrites The crystal structures of the ferrihydrites were identified by XRD measured with a Bruker D8 ADVANCE X-ray diffractometer operated at 40 kv tube voltage and 40 ma tube current with a scanning rate of 1 min -1 at a step size of The morphologies and diameter of the particles were observed using high-resolution transmission electron microscopy (HRTEM). HRTEM was operated at 200 kv (JEM-2100F). N 2 adsorption-desorption at 77 K (Quantachrome Autosorb-1, JEDL-6390/LV) was used to measure the specific surface area (SSA BET ) and pore characteristics of the samples. rior to the adsorption experiments the samples were degassed at 50 C for 16 h under vacuum. Acid-base titrations of the ferrihydrites and KCl blanks at 25 C were carried out to obtain the absolute proton adsorption on the samples as a function of ph at a given salt concentration. For titrations, 1 g L -1 ferrihydrite suspensions were prepared in 50 ml of 0.01 M KCl and 0.1 M KCl solutions under nitrogen at 25 C. The ph of the suspensions was equilibrated with 0.1 M HCl to 4 under continuous N 2 bubbling overnight. After that, the suspensions were titrated with 0.1 M K until ph around 10 and backtitrated with 0.1 M HCl until ph around 4 in order to check reversibility. Titrants were added in minimum aliquots of 0.02 ml HCl or K. After addition of titrant the rate of drift was measured over a 90 s interval. Electrode readings were acceptable when the drift was less than 0.3 mv min -1, but a maximum time of 10 min was set for two successive additions of titrant. After each cycle, the ionic strength was increased to the next higher lever by adding KCl. After salt addition the cell was equilibrated for about 10 min under nitrogen before continuation of the titration. For all KCl concentrations a blank titration (no

3 ferrihydrite) was also performed. These blank titrations were fit to theoretical titration curves using the improved Davies equation (Davies, 1962) for the calculation of the mean ionic activity coefficient. roton binding-ph curves of the ferrihydrites were calculated by subtracting the calculated blank titration values from the measured titration points of the sample. The relative proton adsorption on the ferrihydrite observed at other salt concentration was made absolute by carrying out a ph 5-stat titration with a salt solution. This provides relative proton adsorption on the sample as a function of the salt concentration at ph 5. Combination of the results at the different salt concentration with the ph 5-stat provided the absolute proton adsorption at the different salt concentrations. Then, the absolute position of the curves was obtained by assuming that the common intersection point of the titration curves at different concentrations is the point of zero charge (ZC) (Tan et al., 2008; Weng et al., 2005). ATR-IR spectra of adsorbed phosphate on the ferrihydrite surface (wet samples) were studied with a Bruker Vertex70 spectrophotometer using a constant phosphate loading level of 1.61 mmol L -1 at ph 4.5. The experiments employed an ATR-IR flow-cell setup similar to those described in other studies. A horizontal 45 o ZnSe ATR crystal was coated with g of dispersed ferrihydrite and then placed inside the flow cell. The cell was placed on the horizontal ATR sample stage inside the IR spectrometer and connected to N 2 -purged reaction vessel containing 500 ml of 0.01 M KCl background electrolyte solution, which was adjusted to ph 4.5. A peristaltic pump was used to pass solute from the reaction vessel through the flow cell at 0.5 ml min -1. The flow cell effluent was collected as waste. Experiments were started by pumping the background electrolyte

4 through the flow cell at a flow rate of 0.5 ml min -1, and allowing the ferrihydrite deposit to equilibrate with the background solution. Background spectra, consisting of the absorbance of the ZnSe crystal, the ferrihydrite deposit and the electrolyte solution, were collected regularly during this equilibration period; after about 1 h, successive background spectra showed no further changes, indicating that the ferrihydrite deposit had equilibrated with the background solute. The background spectrum was collected at 1 h as the average of 512 scans at a 4 cm 1 resolution, and phosphate was injected into the reaction vessel to start the adsorption experiment at a flow rate of 0.5 ml min -1. The sample spectras at different times were also collected as an average of 512 scans at 4 cm 1 resolution. All obtained successive spectras involved subtracting the background spectrum from the sample spectrum. The FTIR spectra of adsorbed phosphate on the ferrihydrite surface (dried sample) were investigated using an initial 1.61 mmol L -1 at ph 4.5. At different reaction times (5 min, 1 h, 4 h and 24 h) samples were withdrawn, and prepared for the IR study. The infrared spectra of the samples were obtained with a Bruker Vertex70 spectrophotometer using KBr pellets. 84

5 S2. Rietveld fitting The difference between the observed and the fitted patterns and R wp values (table S2,) indicated that the XRD data can be fitted well by the Michel (2007a) model, and also confirmed that the crystalline structure of ferrihydrites from 2LFh_1 to 6LFh_4 with different crystallite size was similar (Figure S2). The crystallite size obtained from the fitting was in the order 1.6, 2.6, 3.4 and 4.4 nm for 2LFh_1 to 6LFh_4, respectively. The cell parameters of a were: 5.776, 5.652, and Å; of c: 9.256, 9.180, and Å; and of v: 267.4, 254.0, and Å 3, respectively, which showed that there was no significant correlation between cell parameters and crystallite size Figure S2. The Rietveld fitting results. The black blank circles represent the observed data, the red solid lines are the fitting profile. The difference between the observed and the fitting patterns is plotted below as black solid lines

6 100 Table S2. The cell parameters obtained by Rietveld fitting for the ferrihydrites. Samples a c Crystallite size Cell volume v R wp (Å) (Å) (nm) (Å 3 ) (%) 2LFh_ LFh_ LFh_ LFh_

7 S3. TEM results The TEM images of the ferrihydrites show that the particles have uniform morphology and poor crystallinity, existing as particle aggregats composed of tiny spherical particles with a size of ~1.5-5 nm. The particle size of ferrihydrites tends to increase gradually with increasing crystallinity, which is consistent with the refinement results. a b c d Figure S3. The TEM images of the ferrihydrites: (a) 2LFh_1, (b) 5LFh_2, (c) 5LFh_3, (d) 6LFh_4. 110

8 S4. N 2 adsorption-desorption The N 2 adsorption isotherms of the ferrihydrites show that the N 2 adsorption amount sharply increases at low relative pressures (/ 0 < 0.1), which is indicative of microporosity (< 2 nm) (Gregg and Sing, 1982). When / 0 is above 0.4, the amount of N 2 adsorbed for 2LFh_1 and 5LFh_2 increases slowly and the desorption curves have no obvious hysteresis loop with increasing / 0, while those of 5LFh_3 and 6LFh_4 increase more quickly and a H2 hysteresis loop emerges, caused by capillary condensation. Consequently, the isotherms of the ferrihydrites belong to type IV with slit shaped pores (Cornell and Schwertmann, 2003). The micro-pore volume (V micro ) for ferrihydrites, calculated by the DR (Dubinin-Radushkevich) method from N 2 adsorption isotherms for / 0 of about , are of the order 0.137, 0.109, 0.096, cm 3 g -1 for 2LFh_1 to 6LFh_4, respectively. The SSA calculated by the BET method follows the order 427, 367, 301, 234 m 2 g -1 for 2LFh_1 to 6LFh_4 in the range of / 0 of , respectively. Based on the the olanyi potential theory of adsorption (olanyi, 1914), Dubinin and Radushkevich (1947) postulated that the fraction of the adsorption volume V occupied by liquid adsorbate at various adsorption potentials ε can be expressed as a Gaussian function: 128 RT lgv = lgv 2.303( ) lg( / ) β E In the equation V 0 represents micropore volume, E 0 is the so-called characteristic energy of adsorption and β is the affinity coefficient. This shows that the micropore volume V 0 and E 0 parameters can be calculated from a

9 132 linear fit of the isotherm data plotted as lgv vs. [lg( 0 /)] 2. The intercept of the fitted 133 straight line gives lgv 0 while its slope m can be used to calculate E E 0 = RT m β 135 Therfore, we used the program Quantachrome AS1win to calculate the micropore 136 volume by the DR method from the N 2 adsorption isotherms of the ferrihydrites Figure S4. The N 2 (77K) adsorption-desorption isotherms (black lines are adsorption 139 curves; red lines are desorption curves). 140

10 S5. The correlation analysis of crystallite size or SSA BET and structural and adsorption parameters a b 143 c Figure S5. The correlation analysis of crystallite size and SSA BET, micropore volume (V micro ) and proton adsorption (Q H ) at ph 4.5 for ferrihydrite (a); the correlation analysis of crystallite size or SSA BET and adsorption kinetic parameters (k 1, k 2, k 3 are the adsorption rate constants at 0-20 min, min and min, respectively; Q 1, Q 2, Q 3 and S 1, S 2, S 3 are the amounts of phosphate adsorbed and - released at 0-20 min, min and min, respectively) (b and c). 151

11 152 S6. roton adsorption a b Figure S6. The proton adsorption capacity for ferrihydrites at 0.01 and 0.1 M KCl (a) and the proton adsorption density for ferrihydrites at 0.01 M KCl (b)

12 158 S7. hosphate adsorption isotherms Figure S7. hosphate adsorption isotherms at ph 4.5 and 0.01 M KCl and their fit with the Freundlich model with normalized SSA BET. 162

13 163 S8. ATR-FTIR patterns Figure S8. ATR-FTIR spectra of phosphate adsorbed on different size ferrihydrites at ph 4.5 at different times

14 S9. IR patterns The bands at 580 cm -1 and 465 cm -1 for ferrihydrites are less intense after phosphate adsorption, indicating that groups are replaced and phosphate inner-sphere complexes are formed. The band at ~1036 cm -1 ascribed to a --Fe vibration. for phosphate adsorbed on ferrihydrite can be a b Figure S9. The IR spectra of phosphate adsorbed on 2LFh_1 at ph 4.5 at different times (a), and IR spectra for the different size ferrihydrites before and after phosphate adsorption for 30 hrs (b). 179

15 S10. The adsorption procedure Diprotonated bidentate binuclear complexes are considered during the whole reaction. Within the fast adsorption stage, phosphate preferentially exchanges with the Fe- 1/2+ 2, and H 2 is released in an extremely short time. The Fe- 1/2+ 2 and Fe- 1/2- sites with high affinity are then exchanged and - is released in ~20 min. Subsequently, the Fe- 1/2+ 2 and Fe- 1/2- sites with low affinity, impacted by the decreasing surface positive charge due to adsorbed phosphate, are exchanged. - is released in about 240 min during the slow adsorption stage. Finally, the Fe- 1/2+ 2 and Fe- 1/2- sites in the micropores are exchanged with phosphate that diffuses into the micropore, and - is released within the long diffusion adsorption stages up to around 1800 min H s 2H 2 + H H s-20min H min H H H 2 4 H min Figure S10. The whole reaction process of phosphate adsorption on ferrihydrites. H H H 2 H H 2 2 H H H H 192

16 S11. The calculated numbers of different surface groups from potentiometric titration results and parameters in kinetics Table S11 Samples N(Fe 2 1/2+ ) N(Fe 1/2- ) N(Fe 2 1/2+ -) A N(Fe 2 1/2+ -) AB N(Fe 2 1/2+ -) N(Fe 1/2- -) Ratio AB (μmol g -1 ) (μmol g -1 ) (μmol g -1 ) (μmol g -1 ) (μmol g -1 ) (μmol g -1 ) 2LFh_ LFh_ LFh_ LFh_ N SSA 10 N(Fe ) Q 24 1/2 s1 BET 2 = + H 2Nav N SSA /2- s1 BET N(Fe ) = - QH 2Nav N(Fe -) = 2b 1/2+ 2 A N(Fe -) = S + 2(Q -b - S ) = 2Q -2b - S 1/2+ 2 AB N(Fe -) = 2b + S + 2(Q -b - S ) = 2Q - S 1/ /2- N(Fe -) = S S11. Where Q 3 and S 3 are the amount of phosphate adsorbed and - released, respectively, at a reaction time of 1800 min during the kinetics; b 2 is the linear equation intercept; N s1 is the singly-coordinated surface group (~6.0 ± 0.5 nm -2 ); SSA BET is specific surface area; and N av is Avogadro s number ( mol -1 ). N(Fe 1/2+ 2 ) and N(Fe 1/2- ) are the number of groups of Fe 1/2+ 2 and Fe 1/2- at ph 4.5 in 0.01 M KCl, respectively.

17 N(Fe 1/2+ 2 -) A and N(Fe 1/2+ 1/2+ 2 -) AB are the number of groups of Fe 2 exchanged with phosphate in the A and AB stage, respectively. N(Fe 1/2+ 2 -) and N(Fe 1/2- -) are the number of groups of Fe 1/2+ 2 and Fe 1/2- exchanged with phosphate during the kinetic process, respectively. Ratio AB (N(Fe 1/2+ 2 -) AB /N(Fe 1/2- -)) is the ratio of Fe- 1/2+ 2 to Fe- 1/2- reacted with phosphate during the AB process. LITERATURE CITED (1) Davies, C. W. Ion Association; Butterworth Scientific ublication: London, (2) Tan, W. F.; Koopal, L. K.; Weng, L..; an Riemsdijk, W. H.; Norde, W. Humic acid protein complexation. Geochim. Cosmochim. Acta 2008, 72, (3) Weng, L..; Koopal, L. K.; Hiemstra, T.; Meeussen, J. C. L.; Van Riemsdijk, W. H. Interactions of calcium and fulvic acid at the goethite-water interface. Geochim. Cosmochim. Acta 2005, 69, (4) Gregg, S. J.; Sing, K. S. W. Adsorption, surface area, and porosity. Academic ress: London, (5) Cornell, R. M.; Schwertmann, U. The Iron xides: structure, properties, reactions, occurrences and uses. Wiley-VCH: Weinheim, Germany, 2003.