Supporting information
Coupled Dissolution and Precipitation at the
Cerussite-Phosphate Solution Interface: Implications for
Immobilization of Lead in Soils
Lijun Wang,*,† Christine V. Putnis,*,‡ Encarnación Ruiz-Agudo,§ Helen E. King,‡,‖and
Andrew Putnis‡
†
College of Resources and Environment, Huazhong Agricultural University, Wuhan 430070, China
‡
Institut für Mineralogie, University of Münster, 48149 Münster, Germany
§
Department of Mineralogy and Petrology, University of Granada, Granada 18071, Spain
‖
Department of Geology and Geophysics, Yale University, New Haven, CT 06511, United States
* Corresponding authors. E-mail: [email protected] (Lijun Wang), Tel/Fax: +86-27-87288095;
E-mail: [email protected] (Christine V. Putnis), Fax: +49-251-8338397.
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Results and Discussion
For surface reactions, the proposed surface complexation model (SCM) for the
cerussite-water interface suggests that two primary hydration sites with a 1:1
stoichiometry, >PbOH0 and >CO3H0 exist on the (010) face (the notation “>” means
surface sites).1 According to this model, the complexes formed from the primary
species exposed to the aqueous solution include: >CO3- and >CO3Pb+ at the >CO3
sites; >PbO-, >PbCO3-, >PbOH2+ and >PbHCO30 at the >Pb sites. In this study, we
used the notation and values of complexation constants for reactions on the cerussite
surface given by Pokrovsky and Schott1 and So et al.2 In the reaction solution
containing 1 mM (NH4)2HPO4 at pH 7.7, phosphate primary speciation in solution is
dominated by HPO42- (88.9%) and H2PO4- (11.1%), whereas the concentration of
PO43- is very low (0.01%) according to our PHREEQC calculations. Phosphate
adsorbs onto the >Pb sites to mainly form >sPbHPO4- (17.9% of total strong cation
sites) and >PbHPO4- (0.3% of total weak cation sites) (Table S2).
Table S1. Surface complexation reactions and their stability constants at the
calcite/solution interface (modified from Pokrovsky et al., 2002 and So et al. 2011).
Reaction on the surface
Log K (25ºC)
>CO3H0 => CO3- + H+
-5.0
>CO3H0 + Pb2+ => >CO3Pb+ + H+
-2.4
>PbOH0 => >PbO- + H+
-11
> PbOH0 + H+ => >PbOH2+
8.30
+ CO32-+ 2H+ => >PbHCO30 +
H2O
19.65
> PbOH0 + CO32-+ H+ => >PbCO3- + H2O
14.15
>sPbOH0 => >sPbO- + H+
-11
> sPbOH0 + H+ => >sPbOH2+
8.3
> PbOH0
+ CO32-+ 2H+ => >sPbHCO30 +
H2O
19.65
> sPbOH0 + CO32-+ H+ => >sPbCO3- + H2O
14.15
> sPbOH0
2
> PbCO3-
+ HPO42-=> >PbHPO4-+ CO32-
-2.0
> PbCO3-
+ CaPO4-=> >PbPO4Ca + CO32-
-0.72
> sPbCO3-
+ HPO42-=> >sPbHPO4-+ CO32-
-0.17
+ CaPO4-=> >sPbPO4Ca +
CO32-
2.30
> sPbCO3-
Table S2. Surface speciation of cerrusite in equilibrium with a 1 mM (NH4)2HPO4
solution.
Mole fraction
>CO3>CO3H0
>CaOH2+
>CaOH0
Cation surface site (weak)
>CaHPO4>CaO>sCaOH2+
>sCaHPO4Cation surface site
(strong)
>sCaOH0
>sCaOAnion surface site
no background
electrolyte
NaCl 0.1
M
NaF 0.1
M
0,965
0,035
0,983
0,014
0,003
0
0,81
0,179
0,011
0
0,981
0,019
0,967
0,025
0,007
0
0,648
0,335
0,017
0
0,981
0,019
0,967
0,025
0,007
0
0,649
0,334
0,017
0
Table S3. Saturation indices (SI) with respect to different carbonate and phosphate
phases of a 1 mM (NH4)2HPO4 solution at pH 7.7 after equilibration with cerussite.
SI
no
Phase
background NaCl 0.1 M NaF 0.1 M
electrolyte
Cerrusite
0
0
0
Hydroxipyromorphite
20,05
32,17
32,07
Hydrocerrusite
1,6
1,36
1,35
Cl-Pyromorphite
30,69
Fl-pyromorphite
17,75
3
A
B
2
1
Figure S1. AFM (A) deflection and (B) height images of cerussite (010) surfaces
following 3 min of injecting H2O. Arrows 1 and 2 show deep and shallow pits,
respectively. AFM images, 5× 5 μm.
A
B
Figure S2. AFM images of needle-like Pb-P crystals formed on cerussite surfaces (A)
after 14 min of injecting 1 mM NaH2PO4 (pH 7.7), and (B) after 5 min injecting 1
mM NaH2PO4 (pH 5.1). AFM images, 5× 5 μm.
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A
B
Figure S3. AFM images of rounded Pb-P crystals formed on cerussite surfaces (A)
after 16 min of injecting 1 mM (NH4)2HPO4 + 0.1 M NaF (pH 7.7), and (B) after 7
min of injecting 1 mM (NH4)2HPO4 + 0.25 M NaF (pH 7.7). AFM images, 5× 5 μm.
B
A
C
5 μm
D
2 μm
E
1 μm
F
keV
Figure S4. (A) Cerussite (010) surface prior to reaction; (B) Cerussite (010) surface
reacting with 1 mM (NH4)2HPO4 + 0.1 M NaCl (pH 7.7) for 7 days, and (C) Cerussite
(010) surface reacting with 1 mM (NH4)2HPO4 (pH 7.7) for 7 days. (D-F) EDX
Spectrum taken from A to C indicating a corresponding phase.
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Figure S5. Raman spectra with background in the water region for unreacted cerrusite
(green), a sample reacted in pure 1 mM (NH4)2HPO4 solution (black), Cl-bearing
phosphate solution (red) and F-bearing phosphate solution (blue). Small but distinct
O-H stretch band can be observed above the background for the precipitates formed
from pure phosphate solutions.
B
Height images
Deflection images
A
Figure S6. AFM deflection and height images showing etch pits on cerussite (010)
surfaces in the presence of 1 mM (NH4)2HPO4 + (A) 1 mM or (B) 0.1 M NaF at pH
7.7. Images taken from 2 min after injecting reaction solutions. AFM images, 5×5 μm.
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B
A
D
C
E
New area
Figure S7. AFM images of chlorapatite surfaces reacted in (A) H2O, (B) 0.1-10 μM
Pb(NO3)2 (pH 6.0) for 25 min, (C) 100 μM Pb(NO3)2 (pH 6.0) for 15 min, and (D, E)
100 μM Pb(NO3)2 for another 60 min. (E, new area). The dotted ellipses in (C) and (E)
show very few precipitates formed on apatite surfaces, an arrow in (E) demonstrates
irregular etch pits on apatite surfaces. AFM images, 5×5 μm.
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H2O on calcite
14 min 10 μM Pb(NO3)2
5 ×5 μm
8 min
0.1 μM Pb(NO3)2
5 ×5 μm
10 ×10 μm
2.5 h 100 μM Pb(NO3)2
10 ×10 μm
13 min
1 μM Pb(NO3)2
5 ×5 μm
5 ×5 μm
Figure S8. AFM images showing spherical cerussite or hydrocerussite precipitates
quickly formed on calcite with increasing the concentration of the Pb(NO3)2 solution
at varying pH values (4.0-8.0).
Supporing references
(1)
Pokrovsky, O. S.; Schott, J. Surface chemistry and dissolution kinetics of
divalent metal carbonates. Environ. Sci. Technol. 2002, 36, 426–432.
(2)
SØ, H. U.; Postma, D.; Jakobsen, R.; Larsen, F. Sorption of phosphate onto
calcite; results from batch experiments and surface complexation modelling.
Geochim. Cosmochim Acta 2011, 75, 2911–2923.
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