Supporting Information:
Assessing Interactions of Hydrophilic Nanoscale TiO2 with Soil Water
John H. Priester1,2,3, Yuan Ge1,2,3, Vivian Chang1, Peter K. Stoimenov4, Joshua P. Schimel2,3,5,
Galen D. Stucky3,4,6 and Patricia A. Holden1,2,3*
1
Bren School of Environmental Science & Management, 2Earth Research Institute, 3UC Center
for the Environmental Implications of Nanotechnology, 4Department of Chemistry and
Biochemistry, 5Ecology Evolution & Marine Biology, 6Department of Materials, University of
California, Santa Barbara, CA 93106
*
Corresponding Author. Email: holden@bren.ucsb.edu Tel: 805-893-3195 FAX: 805-893-
7612
Table of Contents
1. Estimation of Extreme High Concentrations of nTiO2 in U.S. Agricultural Soil by 2025
2. Prediction of Soil SSA upon Addition of High nTiO2
3. Calculated Effects of Extreme High nTiO2 on Soil Water Holding
4. Saturated Salt Solution Recipes Used for Isopiestic Soil Equilibration
5. Prediction of SSA for nTiO2-Amended Soil when nTiO2 is Agglomerated to Assumed
Sizes
Figure S1. Mean flux of water into, or out of, soil versus soil water potential for control
(without nTiO2, open symbols) and nTiO2 amended (closed symbols) soils. Fluxes are
calculated for each of 3 independent replicate soil samples; error bars are not visible due to low
standard error of the mean.
Figure S2. Predicted soil SSA (with the addition of P25 nTiO2) versus the number of particles
per agglomerate. The inset shows the particle/agglomerate range from 0 – 150. The addition of
agglomerates containing greater than 100 primary particles results in a predicted soil SSA value
less than the measured starting value of 15.19 m2 g-1 (dashed line).
Figure S3. Predicted soil SSA increase (%) with the addition of nTiO2 versus starting soil SSA
(this study, as in Tables S2 and S3) for various theoretical soils with different textural makeups
(See Table S4 in this Supporting Information). The predicted percentage SSA increase was
calculated assuming the addition of 0.5 g of spherical nTiO2 (diameter = 25 nm, density = 3.95 g
1
cm-3) to 25 g of soil (20 mg g-1). Major soil classes and the experimental soil are indicated by
arrows, as defined in Table S4.
Table S1. Measured study soil properties.
Table S2. Estimated predictions of soil SSA before and after nTiO2 addition, and of the percent
increase in SSA and H2O content from nTiO2 addition, for various textural compositions of
theoretical soils. The with-nTiO2 and % SSA increase values were calculated by assuming the
addition of 0.5 g of spherical nTiO2 (diameter = 25 nm, density = 3.95 g cm-3) to 25 g of soil (20
mg g-1), as per Section 2 of this Supporting Information (“Prediction of Soil SSA upon Addition
of High nTiO2”). The clay (assumed mixture of kaolinite and illite, as per the Discussion in the
manuscript), silt, and sand fractions were assumed to have SSAs of 67.6, 1.11 and 0.04 m2 g-1,
respectively, using published values 9. The % H2O increase values were calculated assuming a
uniform water film coating with a thickness of 3.75 nm (the average predicted water film
thickness using the relationship of Tuller and Or 5, as per Section 3a in this Supporting
Information). Soil classifications were assigned according to the USDA textural classification
triangle: http://soils.usda.gov/technical/aids/investigations/texture/ . A graphical representation
of Table S4 values are in Figure S2 of this Supporting Information.
References
2
1. Estimation of Extreme High Concentrations of nTiO2 in U.S. Agricultural Soil by 2025
In Robichaud et al. 1, Figure 4 conveys an upper cumulative nTiO2 global burden of 12E6
metric tons by the year 2025. Since nTiO2 is used mainly in advanced applications including
catalysts, coatings, consumer goods 2, we assumed here that it accumulates mainly in the
developed world, in soils. Further, although the practice of biosolids disposal to soils is highly
uneven in the developed world, we assumed that all cumulative nTiO2 resides in arable lands of
the developed world whose area in yr. 2025 is estimated to be 6.1E8 ha (6.1 million km2), as per
Bruinsma 3. We then assumed that the soil depth over which nTiO2 accumulates is 5 cm, i.e.
midway between the soil surface and a common shallow rooting depth of 10 cm. We applied a
concentration factor of 50 for the U.S. since, in accordance with the U.S. Environmental
Protection Agency, 50% of U.S. biosolids are land applied, and this practice only occurs on 1%
of agricultural lands (http://water.epa.gov/polwaste/wastewater/treatment/biosolids/genqa.cfm).
Thus, of all U.S. biosolids, only half enters soil, but that is concentrated by a factor of 100 onto
U.S. arable lands. We then assume that there are 10× “hot spots” where nTiO2 is hyperaccumulated in soil, i.e. due to uneven distribution. With these assumptions, the concentration of
nTiO2 on a soil volumetric basis could be as high as 0.02 g cm-3. Assuming a soil bulk density of
1.3 g cm3, the nTiO2 on a mass concentration basis in soil could be as high as 0.015g nTiO2 g-1
soil, or 15 mg nTiO2 g-1 soil. We rounded to 20 mg nTiO2 g-1 soil.
2. Prediction of Soil SSA upon Addition of High nTiO2
We calculated the expected increase to soil SSA from the addition of 20 mg g-1 nTiO2 (to
25 g of dry soil, the mass used in our experiments), as follows. The addition of nTiO2 at a
concentration of 20 mg g-1 to 25 g of soil results in an added nTiO2 mass of 0.5 g. Using the
density of P25 nTiO2 of 3.95 g cm-3 (calculated using values for anatase and rutile TiO2; 3.9 and
4.17 g cm-3, respectively 4), the total nTiO2 volume added to the soil was 0.13 cm3. Assuming
spherical geometry, and a diameter of 25 nm, the volume of a single P25 particle is calculated to
be 8.18 × 10-18 cm3. Dividing the total nTiO2 volume by the theoretical particle volume, the
estimated number of particles added to the soil would be 1.55 × 1016. A spherical particle with a
diameter of 25 nm would have a surface area of 1.96 × 10-15 m2. Multiplying the number of
particles and the surface area gives the total added surface area estimate of 30.38 m2. The
measured SSA for our starting soil was 15.19 ± 0.27 m2 g-1 (Fig. ). Multiplying this value by the
3
starting dry soil mass, 25 g, yields a starting surface area of 379.75 m2. Adding the starting
surface area and the added surface area from P25 TiO2, the expected final surface area is 410.13
m2. Dividing this area by the total (soil plus nTiO2) mass, 25.5 g, gives the final expected SSA
of 16.08 m2 g-1. The latter value is lower than the measured SSA for starting, and ending, soil
amended with nTiO2 (Table S2 and S3). The difference could be attributable to a range of
particle sizes, even if the mean diameter remains at 25 nm.
As a check, the SSA for P25 nTiO2 alone can also be estimated. The expected increase in
surface area from P25 (30.38 m2) divided by the mass of nTiO2 added to soil (0.5 g) yields an
SSA value for nTiO2 of 60.76 m2 g-1. This value is very close to the measured SSA value for
P25 of 59.72 ± 3.33 m2 g-1, suggesting that our assumptions about particle morphology are valid.
Essentially, if the measured SSA value is used, the particle diameter (again assuming spherical
geometry) is calculated to be 25.4 nm.
3. Calculated Effects of Extreme High nTiO2 on Soil Water Holding
a. Predicted Water Mass from Tuller and Or 5:
As outlined by Tuller and Or 5, the thickness of an adsorbed water film in dry soil can be
related to water potential using the following equation:
Asvl
h = (6πgρ
1
3
),
wΨ
where h is the water film thickness, Asvl is the Hamaker constant for solid-vapor interactions, g is
the gravitational constant, ρw is the density of water, and Ψ is the soil water potential. Using two
values of measured soil water potential in this experiment, -2.0 and -36.9 MPa respectively, the
predicted range of water film thickness is calculated to be 2.1 – 5.4 nm. Using the mean
measured increase in SSA for nTiO2 amended soil relative to control soil, 3 m2 g-1 (Table S3,
Supporting Information), and a soil mass of 25 g, the additional volume of water in the attached
film is expected to be 0.16 – 0.41 cm3, which corresponds to 160 – 410 mg (assuming a water
density of 1000 mg cm-3). While the above equation is generally only valid under dry conditions
(Ψ < -10 MPa), even under the driest conditions measured in this experiment we would expect
160 mg of additional water in the nTiO2 amended soil, owing to the measured SSA increase of 3
4
m2 g-1. This translates to a gravimetric water content expected of 7.84 × 10-2 g H2O g-1 dry soil
which is a 9% increase in water content.
b. Predicted Water Mass Increase from Ketteler et al. 6:
The mean surface area increase from the addition of nTiO2 soil across all treatments was
3.03 m2 g-1 (Table S3, Supporting Information). Multiplying by the dry mass of soil used for
each replicate, 25 g, the total mean surface area increase was 75.75 m2 per experimental
weighing bottle. Converting to cm2, the value becomes 7.575 × 105. According to Ketteler et al.
6
rutile TiO2 has a binding capability in one monolayer of 5.2 × 1014 molecules cm-2. While our
nTiO2 was a mixture of the anatase and rutile forms (with 81% being anatase), others7 have
reported that H2O binds similarly to both forms, thus the values given by Ketteler et al. 6 are used
for the following calculations. Multiplying the monolayer binding site density, 5.2 × 1014 cm-2,
by our mean surface area increase from nTiO2, 7.575 × 105 cm2, gives a total H2O molecule
value of 3.94 × 1020. Knowing that 18.016 g of H2O (1 mole) equals 6.022 × 1023 molecules,
one monolayer of H2O bound to the mean increased surface area due to nTiO2 in our weighing
bottles would equal approximately 11.8 mg. While this value is greater than the 0.1 mg
sensitivity of the balance used in this experiment, it is less than the mean calculated standard
error (across all treatments) of 47.5 mg per weighing bottle (Table 1). Ketteler et al.6 however,
reported approximately 5 – 12 monolayers of H2O binding to TiO2 in their experiments at
relative humidities between 75 – 100% (our measured water potential range corresponded to 76 –
98% relative humidity at 15 oC), which would correspond to expected mass increases of 59 – 142
mg in our experiment. These values are above both the resolution limit of the digital balance
using to weigh the soils and the standard error in our measurements.
c. Conclusion
Based on the calculations in parts a. and b. above, increased water content due to soil
water adsorbing to nTiO2 was expected to be measurable in our experiment.
4. Saturated Salt Solution Recipes Used for Isopiestic Soil Equilibration
As described in the Methods, saturated salt solutions (SSS, Figure 1 in manuscript) were
contained beneath a perforated aluminum platform, such that the headspace in the desiccator
equilibrated to the associated relative humidity (RH), and RH-controlled water vapor was
exchanged with the soil. Each salt (Fisher Scientific, reagent grade or better) was added to 200
5
ml of H2O at a salt mass twice that required to achieve a saturated solution. The saturation
values for NaCl, KCl, KNO3 and K2SO4 were 36.0, 35.5, 76.6 and 12.0 g 100 mL-1 H2O,
respectively at 25 oC 4. Thus, the masses of NaCl, KCl, KNO3 and K2SO4 within the water in the
desiccators were 144.0, 142.0, 153.2, and 48.0 g, respectively.
5. Prediction of SSA for nTiO2-Amended Soil when nTiO2 is Agglomerated to Assumed
Sizes
Using the hydrodynamic diameter (i.e. agglomerate size) value reported by Horst et al. 8
for P25 nTiO2 in water at a pH of 6.5 (our measured pH value was 6.44 – 6.45), 2,000 nm, the
estimated change in total SSA can be calculated as above. Again a spherical nanoparticle
geometry is assumed, and a particle density of 3.95 g cm-3 is used. Under these conditions, the
estimated SSA after the addition of nTiO2 to soil would actually decrease to a value of 14.91 m2
g-1. It is likely, however, that even if agglomerates of P25 nTiO2 exist in the soil, at least some
of the surfaces would still be available to interact with water. Therefore, the particle diameter for
the purposes of water binding is likely somewhere between 25 nm and 2,000 nm. Another useful
exercise is the calculation of the particle diameter required to maintain the starting SSA of 15.19
m2 g-1 when a concentration of 20 mg g-1 is added to the soil. Under the same assumptions as
above (spherical geometry and a density of 3.95 g cm-3), particles with a diameter of 100 nm
would not change the SSA from the starting value of 15.19 m2 g-1.
To further, and more fully, evaluate the impact of nTiO2 agglomeration on soil SSA for
nTiO2-amended soil (to 2%, as used here), we extended the calculations across a range of
agglomerate sizes (accounted for by particle number per agglomerate), and have plotted the
relationship between soil SSA and agglomerate size (Figure S1). As shown in the Figure S1
inset, with only ca. 50 particles theoretically agglomerating—and assuming that internal surface
area in the agglomerate excludes water—there is no expected effect to soil SSA.
6
Table S1. Measured study soil properties.
Characteristic
Saturation % (SP)
pH
Sand (%)
Silt (%)
Clay (%)
Estimated Soluble Salts (EC) (dS m-1)
Cation Exchange Capacity (CEC) (meq per 100g)
B, saturated paste extract (meq L-1)
Ca, saturated paste extract (meq L-1)
Ca, exchangeable (meq per 100 g)
Cl, saturated paste extract (meq L-1)
Cu, DTPA extraction (ppm)
Cu, total (ppm)
Fe, DTPA extraction (ppm)
Fe, total (ppm)
K, exchangeable (ppm)
K, exchangeable (meq per 100 g)
Mg, saturated paste extract (meq L-1)
Mg, exchangeable (meq per 100 g)
Mn, DTPA extraction (ppm)
Mn, total (ppm)
Na, saturated paste extract (meq L-1)
Na, exchangeable (ppm)
Na, exchangeable (meq per 100 g)
P, extractable (ppm)
Zn, DTPA extraction (ppm)
Zn, total (ppm)
HCO3-, saturated paste extract (meq L-1)
CO32-, saturated paste extract (meq L-1)
Total C (%)
Organic Matter, loss on ignition (LOI) (%)
Total N (%)
NH4+, extractable (ppm)
NO3-, extractable (ppm)
Mean*
46.00
6.45
51.00
27.00
22.00
0.28
24.44
0.06
1.41
12.79
0.59
2.05
33.00
42.70
25400.00
438.5
1.12
1.69
10.47
22.00
737.00
0.15
13.50
0.06
27.10
1.30
56.50
1.85
<0.1
3.11
6.03
0.27
5.88
0.40
SE*
NA**
0.01
0.00
0.00
0.00
0.00
0.09
0.00
0.00
0.06
0.01
0.05
0.00
0.30
500.00
0.50
0.00
0.01
0.02
0.40
6.00
0.00
0.50
0.00
0.40
0.00
0.50
0.05
NA
0.00
0.01
0.00
0.02
0.01
*
Each sample (except saturation % and CO3) was measured in duplicate. The Means and
standard errors of the duplicates are shown.
**
NA = not available; a single measurement was made.
7
Table S2. Estimated predictions of soil SSA before and after nTiO2 addition, and of the percent
increase in SSA and H2O content from nTiO2 addition, for various textural compositions of
theoretical soils. The with-nTiO2 and % SSA increase values were calculated by assuming the
addition of 0.5 g of spherical nTiO2 (diameter = 25 nm, density = 3.95 g cm-3) to 25 g of soil (20
mg g-1), as per Section 2 of this Supporting Information (“Prediction of Soil SSA upon Addition
of High nTiO2”). The clay (assumed mixture of kaolinite and illite, as per the Discussion in the
manuscript), silt, and sand fractions were assumed to have SSAs of 67.6, 1.11 and 0.04 m2 g-1,
respectively, using published values 9. The % H2O increase values were calculated assuming a
uniform water film coating with a thickness of 3.75 nm (the average predicted water film
thickness using the relationship of Tuller and Or 5, as per Section 3a in this Supporting
Information). Soil classifications were assigned according to the USDA textural classification
triangle: http://soils.usda.gov/technical/aids/investigations/texture/ . A graphical representation
of Table S4 values are in Figure S2 of this Supporting Information.
Soil
Classification
%
Clay
%
Silt
%
Sand
SSA (m2/g) SSA
w/nTiO2
(m2/g)
% SSA
Increase
% H2O
Content
Increase
Clay
Silty Clay
Sandy Clay
Silty Clay Loam
Clay Loam
Sandy Clay Loam
Sandy Clay Loam
(Study Soil)
Loam
Silt Loam
Sandy Loam
Silt
Loamy Sand
Sand
70
50
40
35
35
30
22
15
45
5
55
35
10
27
15
5
55
10
30
60
51
47.5
34.3
27.1
24.3
24.1
20.4
15.2
47.7
34.7
27.7
24.9
24.7
21.2
16.0
0.4
1.3
2.1
2.6
2.7
3.5
5.3
2.4
3.3
4.2
4.7
4.7
5.6
7.5
20
10
10
5
5
5
40
65
20
85
10
5
40
25
70
10
85
90
14
7.5
7.0
4.3
3.5
3.5
14.8
8.5
8.0
5.4
4.6
4.5
6.0
12.9
13.9
23.7
29.6
30.1
8.1
15.1
16.2
26.2
32.2
32.7
8
Figure S1. Mean flux of water into, or out of, soil versus soil water potential for control
(without nTiO2, open symbols) and nTiO2 amended (closed symbols) soils. Fluxes are
calculated for each of 3 independent replicate soil samples; error bars are not visible due to low
standard error of the mean.
9
Figure S2. Predicted soil SSA (with the addition of P25 nTiO2) versus the number of particles
per agglomerate. The inset shows the particle/agglomerate range from 0 – 150. The addition of
agglomerates containing greater than 100 primary particles results in a predicted soil SSA value
less than the measured starting value of 15.19 m2 g-1 (dashed line).
10
Figure S3. Predicted soil SSA increase (%) with the addition of nTiO2 versus starting soil SSA
(this study, as in Tables S2 and S3) for various theoretical soils with different textural makeups
(See Table S4 in this Supporting Information). The predicted percentage SSA increase was
calculated assuming the addition of 0.5 g of spherical nTiO2 (diameter = 25 nm, density = 3.95 g
cm-3) to 25 g of soil (20 mg g-1). Major soil classes and the experimental soil are indicated by
arrows, as defined in Table S4.
11
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2.
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Ketteler, G.; Yamamoto, S.; Bluhm, H.; Andersson, K.; Starr, D. E.; Ogletree, D. F.; Ogasawara, H.;
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Walle, L. E.; Borg, A.; Johansson, E. M. J.; Plogmaker, S.; Rensmo, H.; Uvdal, P.; Sandell, A., Mixed
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Horst, A. M.; Ji, Z. X.; Holden, P. A., Nanoparticle dispersion in environmentally relevant culture
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