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), 12832-12840 Zezhen Pan Instructor: Dr. Young-Shin Jun Environmental Nanochemistry 2016.04.11

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,

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

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, 1-100 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

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

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

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

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

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

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.

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

Mechanism: electrosteric stabilization II 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

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

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

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

Thank you and questions

Figure 7