Aggregation and Deposition Behavior of Carbon-Based Nanomaterials
in Aquatic Environments
Menachem ElimelechDepartment of Chemical EngineeringEnvironmental Engineering Program
Yale University
2007 NSF Nanoscale Science and Engineering Grantees Conference, Arlington, VA, December 3-6, 2007
NSF BES 0504258NSF BES-0646247
Engineered Carbon-Based Nanomaterials
Exponential growth in production and potential applications Unique properties (shape, surface charge, reactivity)Environmental and health impacts are not known
SWNT MWNT
100 nm
C60Nanoparticles
Aggregation and Deposition Behavior Determines Fate and Transport
Influences rate of settling and transport
Removal from aqueous phase
May influence reactivity and toxicity
Mineral Surfaces
Sediment
Aggregation
Deposition/ Attachment
NOM, biomolecules, minerals/suspended solids
IonsNa+
Cl-Ca2+
Mg2+
Nanomaterials
Complex Interactions in Aquatic Systems
Aggregation Kinetics of Fullerene Nanoparticles
Fullerene Nanoparticles
200 nm
Buckminsterfullerene C60 nC60
Ordered Structure of C60 Molecules
10 nm
Buckminsterfullerene C60 nC60
Two Synthesis MethodsSonicated C60 Nanoparticles (Son-C60)
Dissolve fullerene in
toluene
Sonicate with water and
ethanol for 3 hr
Filter with 0.45 then 0.2
µm filters
Fullerene (99.9% purity)
Aqueous C60 Nanoparticles (Aq-C60)Stirring fullerene in deionized water for 40 days before filtration
Physical CharacterizationSon-C60 Aq-C60
RH = 50.5 nm RH = 83.1 nm
20 nm 50 nm
Physical CharacterizationSon-C60 Aq-C60
10 nm10 nm
Electrophoretic Mobility (EPM) in KCl
Negatively charged Aq-C60 more negatively charged than Son-C60
EPM becomes less negative as KCl concentration increases
1 10 100
-4
-3
-2
-1
0
Son-C60 pH 5.5 Aq-C60 pH 5.5
EP
M (1
0-8 m
2 /Vs)
KCl Conc. (mM)
ALV Light Scattering SetupDynamic light scattering to derive hydrodynamic radiusYAG laser with wavelength of 532 nmScattered light intensity measured at 90° from incident beam
Time-Resolved Dynamic Light Scattering
( )0→
⎟⎠⎞
⎜⎝⎛∝
t
hA dt
tdrk
fastA
A
kkW,
/1 ==α0 400 800 1200
60
70
80
90
200 mM NaCl 145 mM NaCl 100 mM NaCl 80 mM NaCl
Hyd
rody
nam
ic R
adiu
s (n
m)
Time (s)
kA, fast
kA
Initial aggregation kinetics:
Attachment Efficiency or Inverse Stability Ratio:
Aggregation Kinetics in KCl
0.01 0.1 10.01
0.1
1
Son-C60
Aq-C60
Inve
rse
Sta
bilit
y R
atio
, 1/W
KCl Conc. (M)
pH 5.550 mM
200 mM
Classic slow and fast aggregation regimesAq-C60 are much more stable (CCC of 200 mM)
Aggregation Kinetics in KCl and CaCl2
0.01 0.1 10.01
0.1
1
Son-C60
Aq-C60
Inve
rse
Sta
bilit
y R
atio
, 1/W
KCl Conc. (M)10-3 10-2 10-10.01
0.1
1
Son-C60
Aq-C60
Inve
rse
Sta
bilit
y R
atio
, 1/W
CaCl2 Conc. (M)
pH 5.5 pH 5.550 mM
200 mM
4.3 mM
6.0 mM
Increased Stability in Humic Acid with NaCl
0 800 1600 2400 3200
60
80
100
120
No humic acid 1 mg/L TOC 5 mg/L TOC
Hyd
rody
nam
ic R
adiu
s (n
m)
Time (s)
NaCl concentration = 650 mMpH 8
Chen and Elimelech, J. Colloid Interface Sci. 2007, 309, 126-134
Diffusion-limited
Increased Stability in Humic Acid with NaCl and MgCl2
0 800 1600 2400 3200
60
80
100
120
No humic acid 1 mg/L TOC 5 mg/L TOC
Hyd
rody
nam
ic R
adiu
s (n
m)
Time (s)0.01 0.1
0.1
1
No humic acid 1 mg/L TOC
Atta
chm
ent E
ffici
ency
, αMgCl2 Concentration (M)
NaCl concentration = 650 mMpH 8
CCC increases in humic acid
pH 8
Chen and Elimelech, J. Colloid Interface Sci. 2007, 309, 126-134
Diffusion-limited
Steric Stabilization with Humic Acid in NaCl and MgCl2
0.0
-0.5
-1.0
-1.5
-2.0
-2.5
EP
M (1
0-8 m
2 /Vs)
No humic acid Humic acid
10 mM 3 mM 3 mMNaCl CaCl2 MgCl2
Electrophoretic mobility (EPM) similar with and without humic acid
Indication of steric stabilization
pH 8
Enhanced Aggregation at High CaCl2 Concentrations
0 400 800 1200
60
80
100
120
1 mg/L TOC No humic acid
Hyd
rody
nam
ic R
adiu
s (n
m)
Time (s)
Diffusion-limited
Enhanced aggregation
CaCl2 concentration = 40 mMpH 8
Enhanced Aggregation at High CaCl2 Concentrations
1E-3 0.01 0.1
0.1
1
10 No humic acid 1 mg/L TOC
Atta
chm
ent E
ffici
ency
, αCaCl2 Concentration (M)
0.0010 400 800 1200
60
80
100
120
1 mg/L TOC No humic acid
Hyd
rody
nam
ic R
adiu
s (n
m)
Time (s)
Diffusion-limited
Enhanced aggregation
pH 8
At 1 mg/L TOC, enhanced aggregation occurs above 10 mM CaCl2
Humic Acid Clusters Bridge Fullerene Nanoparticles
200 nm
100 mM MgCl2
500 nm
40 mM CaCl2
Humic Acid Clusters Bridge Fullerene Nanoparticles
500 nm
40 mM CaCl2
Aggregation Kinetics of Multi-Walled Carbon Nanotubes (MWNTs)
MWNT Sample Preparation
10 mg MWNTs added to 100 mL DI water
Sonicated for 30 minutes using ultrasonicating probe
Supernatant collected
Re-sonicated for 5 more cycles (30 minutes each) to obtain final sample
TEM Images of Untreated and Treated MWNTs
100nm 500nm
Tubes are bundled and long before treatmentSonication debundles and reduces average length
Untreated Treated
Diameter and Length Distributions of Treated MWNTs
Diameter and length distributions are obtained with TEMAverage diameter 18 nm and average length 1.5 μm
2 4 6 8 100
5
10
15
20
25
% M
WC
NT
Length (μm)10 20 30 40 50
0
2
4
6
8
10
% M
WC
NT
Diameter (nm)
Aggregation Kinetics with Monovalent Salt (NaCl)
10-3 10-2 10-1
0.01
0.1
1
A
ttach
men
t Effi
cien
cy, α
NaCl Conc. (M)
Aggregation Kinetics with CaCl2and MgCl2
10-5 10-4 10-3 10-2 10-10.01
0.1
1
Atta
chm
ent E
ffici
ency
, α
MgCl2 Conc. (M)
10-5 10-4 10-3 10-2 10-10.01
0.1
1
Atta
chm
ent E
ffici
ency
, α
CaCl2 Concentration (M)
Classic aggregation behavior with slow and fast regimes
Deposition Kinetics of Fullerene Nanoparticles
Aggregation and Deposition Behavior Determines Fate and Transport
Mineral Surfaces
Sediment
Aggregation
Deposition/ Attachment
Quartz Crystal Microbalance (QCM)
Quartz crystal
AC
Flow Cell: Sauerbrey relationship:
nfC
m nΔ−=Δ
0 10 20 30 40 50-25
-20
-15
-10
-5
0
Freq
uenc
y S
hift
(Hz)
Time (min)
Deposition starts
Silica coating
0 10 20 30 40 50-30
-25
-20
-15
-10
-5
01 mM
10 mM
30 mM
Fr
eque
ncy
Shi
ft (H
z)
Time (min)
Deposition Kinetics
0 10 20 30 40 50-30
-25
-20
-15
-10
-5
01 mM
10 mM
30 mMPLL
Fr
eque
ncy
Shi
ft (H
z)
Time (min)
Attachment Efficiency
fastD
DD k
k
,
=α
Favorable (fast) deposition:
Quartz crystal
Pre-adsorption of +ve charged poly-L-lysine (PLL) on silica surface
PLL
kD, fast
kD
Deposition attachment efficiency:
Influence of NaCl and CaCl2 on Deposition Kinetics
Below CCC, as electrolyte concentration increases – faster deposition through charge shielding
Towards CCC and above – significant drop in deposition rate
0.01
0.1
1
0.0001 0.001
Dep
ositi
on R
ate
(Hz/
min
)
CaCl2 Concentration (M)
0.01
0.1
1
Attachm
ent Efficiency, α
D1E-3 0.01 0.1 1
0.01
0.1
1
Dep
ositi
on R
ate
(Hz/
min
)
NaCl Concentration (M)0.001
CCC
Simultaneous Deposition and Aggregation at Higher Ionic Strength
0 20 40 60 80 100
-2.0
-1.5
-1.0
-0.5
0.0
0.5
Freq
uenc
y S
hift
(Hz)
Time (min)
Non-linear deposition behavior
At 100 mM NaCl:
Bigger aggregates formed at later stages –deposition rate decreases even more
Quartz crystal
Aggregate formation –lower convective-diffusive transport towards silica surface
Release of Deposited Nanoparticles
0 100 200 300
-30
-20
-10
0
10FEDCBA
Fr
eque
ncy
Cha
nge
(Hz)
Time (min)
0 100 200 300-60
-40
-20
0FEDCBA
Freq
uenc
y C
hang
e (H
z)
Time (min)
A – Baseline (pH 5.7)
B – Deposition at 30 mM NaCl and 0.6 mM CaCl2 (both αD ~ 1)
C – Rinsing with respective electrolytes
D – Rinsing with 1 mM NaCl
E – Rinsing with DI water
F – Rinsing with DI water (pH 12.3)
Release at Stage F – sudden increase in surface potential of C60 nanoparticles and silica surface
NaCl
CaCl2
Concluding RemarksEelectrostatic interactions control the aggregation and deposition behavior of carbon-based nanomaterials (CBNs)Humic substances stabilize CBNs by electrosteric repulsion Under solution chemistries of natural waters, CBNs are stable and thus expected to be mobile
Acknowledgments
NSF BES 0504258NSF BES-0646247
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