1. Deposition of Carboxymethylcellulose-Coated Zero-Valent Iron Nanoparticles onto Silica: Role of solution Chemistry and Organic Molecules
Julien Fatisson, Subhasis and Nathalie Tufenkji
Langmuir, 2010, 26 (15),
Zezhen Pan
Instructor: Dr. Young-Shin Jun
Environmental Nanochemistry

2. Introduction:
Nanoscale systems: commercial products, drug delivery and development of environmental solutions (application of nanosorbents for removal of contaminants from water)
Nanosized zero-valent iron (nZVI): soil decontamination, including chlorinated solvents, nitrates, mercury, arsenic
Limitation: transport of nZVI particles through soils toward contaminated zones as particles can aggregate because of attractive magnetic forces
Solutions: coating nZVI particles with polymeric materials, modifying with hydrophilic carbon, embedding into silica matrix
So several solutions have been proposed to enhance nZVI To stabilize the colloidal suspensions,

3. Introduction:
Limitation: many previously studied polymers (branched copolymers, polyacrylic aicd) are physically adsorbed onto already formed nZVI particles, weak bind and less effective for stabilization than a chemical bond
Solutions: applying covalently bound polymers such as the carboxymethylcellulose (CMC)
Approach for transport study:
Quarz crystal microbalance with energy dispersion:
(1) deposition behavior on silica
(2) effectiveness of polymer coatings in nZVI deposition onto silica
Transport and deposition of nZVI in model subsurface environments can be performed in packed bed reactors with glass beads, sand and soil

4. Objectives:
Examine the effect of using CMC as a surface-modifying polymer on nZVI transport and deposition onto a model sand surface
Study the deposition over a broad range of solution ionic strengths (IS, mM, at pH 7)
Understand the potential influence of common naturally occurring molecules in groundwater on nZVI transport and deposition (SRFA, rhamnolipid)
Verify observations in QCM-D by employing the prediction based on DLVO theory
SRFA (suwannee river fulvic acid)
R: biosurfactant

5. Materials Characterization of nZVI (bare and CMC-): TEM
TEM may lead to multiple artifacts regarding the aggregation
state of nanoparticles; electrolyte evaporation during
TEM sample preparation increases the nanoparticle concentration
locally and enhances particle aggregation
A mean diameter: 77 ± 15 nm

6. Materials Characterization of nZVI: DLS and NTA (Ionic strength)
bare nZVI
CMC-nZVI
? Not clear why NOM doesn’t affect hydrodynamic diameter at high IS for CMC-nZVI
? Did not explain the measurement by NTA clearly

7. Materials Electrokinetic: bare nZVI and CMC-nZVI
Under all conditions, ζ-potentials become less negative with increasing IS because of an increase in charge screening.
More negative in the presence of SRFA, suggesting that the fulvic acid readily associates with the bare particle surface.
Free CMC with nZVI and CMC-nZVI: comparable
Zeta potential calculated from EPM measurements
Zeta potential and DLS size: DLVO interaction energy

8. Deposition on Silica I: N2-purged BG electrolyte for base line
II: suspension of bare nZVI in electrolyte, a rapid, linear decrease in frequency, with a linear increase in dissipation, Deposition of bare nZVI onto clean silica-coated crystal
III: non-linear signals, ripening happened as nZVI particles deposit onto particles previously attached to silica
Zeta potential calculated from EPM measurements

9. Deposition on Silica Information from QCM-D:
Q: What can be obtained from measuring the change in f and D as a function of time?
A: Evaluate the change in deposited mass on the crystal surface
Deposition kinetics can be determined by evaluating the initial slope in the frequency and dissipation shifts (-fslope and Dslope )
Zeta potential calculated from EPM measurements

10. Effect of CMC
The deposition rate of nZVI particles increases with IS (diffuse double layer decreases because of more effective charge screening) higher ion content decreases the transport of nZVI particles through a sand column
CMC-nZVI particles have the lowest –fslope and Dslope, covalently bound CMC hindered deposition onto silica
thus attenuating
the electrostatic repulsion between nZVI particles and the
silica surface. Hence, increased nZVI deposition (larger values of
-fslope and Dslope) onto the silica surface is observed at higher IS.

11. Mechanism: electrosteric stabilization
CMC covalently bound nZVI has thicker CMC layer than CMC physisorption, and has enhanced steric repulsion;
Covalent CMC coating led to better particle surface coverage
Interaction forces
Deposition is unfavorable under studied conditions
Particle-surface interaction energy profiles:
- repulsive electrical double layer
- attractive van der Waals
- repulsive steric interactions
Zeta potential calculated from EPM measurements
UDLVO (DLVO interaction)=ULW + UEL

12. Mechanism: electrosteric stabilizationII
III
Substantial energy barrier
For bare nZVI (3 mM), an energy barrier is present , low values of –fslope and Dslope
The height of energy barrier decreases with increasing IS, deposition increases
CMC-nZVI, a substantial energy barrier
Still deposition on the silica surface (secondary energy minimum was not controlling)
DLVO not in agreement with QCM-D
Imperfection of the experiment system
Bare nZVI 3 mM an energy barrier is present limited particle deposition
Variation between model and experiments

13. Effect of Organic molecule
Additional stabilizing effect of rhamnolipid when added to the CMC-nZVI suspensions
- Particles are more negatively charged, greater repulsive electrosteric interactions
Stronger interaction between CMC and rhamnolipid
Small molecular weight to stabilize CMC-nZVI
Transport would be enhanced in natural environments where rhamnolipid is present
Zeta potential calculated from EPM measurements

14. Conclusion
The solution salt concentration was generally found to affect nZVI particle deposition
Covalent bound CMC has better effect to creat stable nZVI suspensions
Electrosteric repulsion can play a considerable role in nZVI colloid stabilization by CMC
NOM have varying effects on bare and CMC coated particles

15. From this study:
Zeta potential, DLS can be really important to combine theoretic and experimental system
Stability of deposited nanoparticles, whether it can be reversed

16. Thank you and questions

17. Figure 7