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