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Interactions of CeO2 nanoparticles with natural colloids and electrolytes
impact their aggregation kinetics and colloidal stability
Xing Li1, Erkai He2, Miaoyue Zhang2, Willie J.G.M. Peijnenburg3, Yang Liu4, Lan Song5,
Xinde Cao1, Ling Zhao1, Hao Qiu1,*
1 School of Environmental Science and Engineering, Shanghai Jiao Tong University,
Shanghai, 200240, China
2 School of Environmental Science and Engineering, Sun Yat-sen University, Guangzhou,
510275, China
3 National Institute of Public Health and the Environment, Center for the Safety of Substances
and Products, Bilthoven 3720 BA, The Netherlands
4 Faculty of Environmental Science and Engineering, Kunming University of Science and
Technology, Kunming, 650500, China
5 School of Environmental Science and Engineering, Southern University of Science and
Technology, Shenzhen 518055, China
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Text S1: Materials
Kaolin preparation: Briefly, a slurry of kaolin fines in Millipore water (18.2
MΩ·cm) was sonicated using a bath sonicator for 30 min. Then, the < 2
μm colloid fraction was separated from larger particles by precipitation
during 1 h. To obtain Na or Ca-saturated kaolinite, kaolinite was soaked in
0.5 M NaCl and CaCl2 solution, respectively, followed by washing with
Millipore water [1]. After soaking and washing four times, the solution was
freeze-dried at -50 ºC.
Goethite preparation: One L of 0.48 M NaHCO3 solution was added to equal
volumes of 0.40 M Fe(NO3)4 solution with vigorously stirring (the dropping rate was 4.5 mL
min-1). During this process, the pH of the Fe(NO3)4 solution increased from 1.0 to 2.4. Then,
the resulting suspension was heated using a microwave to boiling, and immediately cooled to
20 °C by ice bath. Subsequently, the suspension was dialyzed against Milli-Q water for 3
days.
Conversion of resulting hydroferrite particles to goethite NPs: Firstly, the suspension pH
was adjusted to 12 using 5 M KOH, followed by heating for 24 h at 90°C. The goethite
particles were washed with water and centrifuged three times. The obtained particles were
freeze-dried, grinded, and passed through a 45-μm sieve to obtain the NPs.
Text S2: DLVO calculations
The total interaction energy between particles (V TOT) can be defined as the sum of the
attractive van der Waals interaction (VVDW) and the repulsive electrostatic double layer
interaction (VEDL) [2]:
V TOT=V VDW +V EDL (1)
The VDW interaction for sphere-plate geometry between CeO2 NPs and the surface of
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minerals is calculated using the following equations [3]:
V VDW=−A132 r
6 h(1+ 14 hλ
) (2)
where A132 is the Hamaker constant, r is the initial radius of CeO2 NPs (137 nm), h is the
separation distance between particles, and λ is the characteristic wavelength of CeO2 NPs
(λ = 100 nm).
The Hamaker constant between two different particles is calculated as
A132=(√A11−√ A33 ) (√ A22−√ A33 ) (3)
Where A11 and A22 are the Hamaker constant of particle a and particle b
respectively in vacuum. A33 is the Hamaker constant of dispersion
medium. The Hamaker constants of CeO2 NPs, kaolin, and goethite were 5.57 ×
10-21，3.7× 10-21，and 1× 10-21 J [4-6]. The Hamaker constant for HA-coated CeO2
NPs was 6.610-21 J according to Li and Chen (2012) [7].
The EDL interaction for sphere-plate geometry between CeO2 NPs and the surface of
minerals can be calculated as follows:
V EDL=πr ε0 εr {2 φ1φ2 ln [ 1+exp (−kh )1−exp (−kh ) ]+(φ1
2+φ22 ) ln [1−exp (−2 kh ) ]} (4)
Where ε0 is the permittivity of free space (8.854×10-12 C/V/m), εr is the dielectric constant of
water (78.5)
φ1andφ2 are the surface potentials of CeO2 NPs and minerals. κ is the Debye reciprocal
length and can be calculated as follows due to the same magnitude of charge of the anions
and cations in this study [8]:
κ=√ e2 Σi
ni z2
εkT (5)
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where e is the elementary charge, n is the number concentration of ions in the bulk
suspension; z is the valence of ion in the bulk suspension, k is the Boltzmann constant, and T
is the absolute temperature.
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Table S1 Information about water samples.
Water samples Location AbbreviationLake water from Shanghai N 31°01′21.45″
E 121°25′27.72″LW-S
Lake water from Guangzhou N 23°08′1.57″ E 113°20′31.33″
LW-G
Pond water from Shanghai N 31°01′40.14″ E 121°25′43.60″
PW-S
Pond water from Huanggang N 30°19′43.20″ E 115°27′28.82″
PW-H
River water from Nanchang N 28°41′56.58″ E 115°52′20.32″
RW-N
River water from Huanggang N 30°13′21.31″ E115°21′53.52″
RW-H
Groundwater from Huanggang
N 30°18′34.08″ E 115°21′50.68″
GW-H
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Table S2 Characteristics of water samples from Shanghai, Guangzhou, Nanchang, and Huanggang.
Parameters LW-S LW-G PW-S PW-H RW-N RW-H GW-HpH 5.35 7.98 7.80 7.41 7.51 7.40 6.87
Dissolved oxygen (mg/L)
5.80 5.85 6.37 8.55 8.81 8.55 -
Conductivity (µS)
348 128.4 463 442 204 442 410
TOC (mg/L) 9.24 11.66 5.44 14.58 6.72 12.62 2.13Li+ (mg/L) 0.56 - 0.55 0.58 0.60 0.60 1.34Na+ (mg/L) 32.21 3.97 36.82 10.04 8.68 21.71 20.90K+ (mg/L) 5.68 3.6 7.14 3.30 7.23 5.20 12.78
Ca2+ (mg/L) - 8.82 37.28 16.38 12.50 35.73 39.74Mg2+
(mg/L)6.23 3.32 9.18 4.37 5.60 13.45 11.04
F- (mg/L) 1.58 0.97 1.36 1.13 1.03 0.92 1.38Cl- (mg/L) 80.00 7.52 42.43 16.35 17.52 24.80 55.95Br- (mg/L) 1.70 1.62 1.77 - 1.56 1.76 1.72
NO3- (mg/L) 2.60 2.49 3.20 6.24 5.87 6.94 3.64NO2- (mg/L) - - 12.26 2.61 2.47 6.76 12.78
SO42-
(mg/L)38.90 7.11 51.23 11.23 13.90 39.84 12.54
IS (mM) 3.34 1.16 5.40 2.04 2.05 4.80 4.84
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Fig. S1 TEM images of CeO2 NPs (A), kaolin (B), and goethite (C).
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Fig. S2 Zeta potential of CeO2 NPs, kaolin, and goethite as a function of pH.
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Fig. S3 Aggregation kinetics of CeO2 NPs, kaolin, and goethite at various NaCl and CaCl2
concentrations.
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Fig. S4 Heteroaggregation kinetics of CeO2 NPs and kaolin, CeO2 NPs and goethite, and
CeO2 NPs and humic acid at various NaCl and CaCl2 concentrations.
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Fig. S5 Estimated energy barrier of CeO2 NPs in the presence of kaolin, goethite and humic
acid (1 mg C/L).
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Fig. S6 Fluorescence excitation-emission matrix spectra of HA aggregates in the presence of
different concentrations of electrolyte solution. CHA=1 mg C/ L.
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