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Interaction mechanisms between (nano)particles and natural organic matter 7

Chapter I - Introduction and thesis objectives

I.4 Interaction mechanisms between (nano)particles and natural organic matter 7

When (nano)particles interact with NOM the stability of the particles is strongly dependent on the particle intrinsic properties (size, surface charge, etc), the NOM physicochemical properties (structural charge, molecular weight, etc) and concentration (i.e. influence of the particle surface coverage, osmotic pressure, etc) and the water systems properties (pH, ionic strength, etc) (Colvin 2003, Ju-Nam and Lead 2008, Klaine et al. 2008, Nowack and Bucheli 2007, Wiesner et al. 2006).

At low NOM concentration, the adsorption of NOM onto the (nano)particle surface is expected to induce the destabilization of the (nano)particles especially by charge neutralization and bridging mechanisms whereas high NOM concentration usually tend to stabilize the (nano)particles through electrostatic repulsions (after charge inversion) and steric restabilization (Elimelech et al. 1995, Gregory 1987, 1996). Parameters such as the affinity of NOM for the (nano)particle surface, its chemical structure, electronic properties and molecular weight but also the (nano)particle-water interface conditions have key role on the resulting (nano)particle stability (Gregory 1973). All this parameter will strongly influence the layer thickness and the conformation adopted by the NOM adsorbed on the (nano)particle surface (coating).

Charge neutralization is often the predominant mechanism at low NOM concentration when NOM adsorption onto the oppositely charged (nano)particle strongly reduces the electrostatic repulsions between the (nano)particles (electrophoretic mobility close to zero) (Fig. 5a).

Fig. 5: Schematic representation of (nano)particles destabilization by charge neutralization (a) and patch mechanisms (b).

In presence of (nano)particles having a relatively low surface charge, highly structural charged NOM is known to induce local charge heterogeneity. Such "patches" interact with regions of opposite charge from different (nano)particles and promote the agglomeration via patch mechanisms (Fig. 5b). In such a destabilization mechanism the overall (nano)particle surface charge can differ from neutrality.

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Destabilization of (nano)particles in the presence of NOM is also possible due to bridging effects. The NOM properties and concentration strongly influence the bridging mechanism (Fig. 6a).

To optimize such mechanism NOM should indeed attach several points and be large enough to have free loops and tails to bind other (nano)particles. Such mechanism can be achieved via extended and linear NOM conformations such as polysaccharides. The (nano)particle surface coverage is also playing essential role and the highest probability of bridging is found for half surface coverage (Healy and Lamer 1964). Such mechanism can be largely influenced by several factors. At high ionic strength the (nano)particle surface charge is partially screened and the contact between (nano)particles is facilitated due to the reduction of the electrostatic repulsions. High ionic strength also modifies the NOM conformation (to coils) and reduces the NOM rigidity hence decreasing the importance of bridging mechanism. Optimal NOM bridging mechanism occurs for intermediate (nano)particle charge densities and ionic strength (Matsumoto and Adachi 1998).

Fig. 6: Schematic representation of polymer bridging and depletion agglomeration mechanisms.

Destabilization mechanism of (nano)particles in presence of NOM can also be induced when the conformational entropic restrictions of NOM are not compensated by the energy of adsorption (Jenkins and Snowden 1996). Such agglomeration mechanism, named depletion agglomeration mechanism (Fig. 6b), occurs when NOM has a stronger affinity for the water than for the (nano)particle surface, which can be uncoated or already covered by NOM.

An osmotic pressure, induced when NOM is excluded from the space between (nano)particles, promotes here (nano)particle agglomerate formation. Such scenario is favored for high molecular weight and high concentration of NOM and for (nano)particle diameters less than the effective NOM diameter. (Nano)particle and NOM sizes are key parameters influencing depletion agglomeration mechanism (Burns et al. 2000, 2002, Otsubo 1996, Sperry et al. 1981).

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Presence of NOM can not only promote the (nano)particle agglomeration but also favor their stabilization. Indeed when (nano)particles are in the presence of a relatively high amount of NOM, electrostatic and steric interactions between the coated (nano)particles can prevent agglomerates formation or promote the agglomerate fragmentation (Fig. 7).

Fig. 7: (Nano)particle steric effects and electrostatic repulsions.

Steric stabilizations depend on the NOM conformational, structural and electronic properties but also on the way they are coating the (nano)particle. If the NOM layer is thick enough it will not only sterically stabilize the (nano)particle. If the NOM carries structural charge, it will prevent the agglomeration due to important electrostatic repulsions between the NOM coating the (nano)particles. High NOM concentration is needed to undergo steric restabilization, but not to high on another hand, as non adsorbed NOM are favoring depletion agglomeration mechanisms.

I.5 Thesis objectives

The main objective of this thesis work is to evaluate the stability of TiO2 ENPs in the presence of two important natural organic matter compounds (humic and exopolymeric substances) by increasing step by step the dispersion complexity (chemical composition, component number, etc). The influence of water pH, ionic strength, electrolyte valency and concentration as well as the NOM concentration were systematically investigated.

Quantification of the energy involved during interaction process between ENPs and natural organic matter was also investigated using isothermal titration calorimetry. Such a novel approach is expected to have a high potential to contribute to a better understanding of the behavior of ENPs in the presence of NOM. Indeed such quantitative information is often missing when ENP interactions with aquagenic compounds have to be investigated.

Experimental results obtained will permit a better understanding of the interaction processes and mechanisms regulating the stability of ENPs in aquatic systems which constitutes an important step in the current knowledge on risk assessment associated to ENPs in the environment.

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This thesis work was supported by the Swiss National Foundation (200010_152647 and 200021_13240).

I.6 List of papers

Loosli, F., and Stoll, S., 2012, Adsorption of TiO2 nanoparticles at the surface of micron-sized latex particles. pH and concentration effects on suspension stability, published in Journal of Colloid Science and Biotechnology, v. 1, no. 1, p. 113-121

Loosli, F., Le Coustumer, P., and Stoll, S., 2013, TiO2 nanoparticles aggregation and disaggregation in presence of alginate and Suwannee River humic acids. pH and concentration effects on nanoparticle stability, published in Water Research, v. 47, no. 16, p.

6052-6063

Loosli, F., Le Coustumer, P., and Stoll, S., 2014, Effect of natural organic matter on the disagglomeration of manufactured TiO2 nanoparticles, published in Environmental Science:

Nano, v. 1 , p. 154-160

Loosli, F., Le Coustumer, P., and Stoll, S., 2015, Effect of electrolyte valency, alginate concentration and pH on engineered TiO2 nanoparticle stability in aqueous solution, published in Science of the Total Environment, DOI: 10.1016/j.scitotenv.2015.02.037

Loosli, F., Le Coustumer, P., and Stoll, S., 2015, Impact of alginate concentration on the stability of agglomerates made of TiO2 engineered nanoparticles: Water hardness and pH effect, published in Journal of Nanoparticle Research, v. 17 , p. 44

Loosli, F., Vitorazi, L., Berret, J.F., and Stoll, S., 2015, Towards a better understanding on agglomeration mechanisms and thermodynamic properties of TiO2 nanoparticles with natural organic matter, published in Water Research, v. 80 , p. 139-148

Not published yet:

Loosli, F., Vitorazi, L., Berret, J.F., and Stoll, S., 2015, Isothermal titration calorimetry as a powerful tool to quantify TiO2-Humic acids interactions, will be submitted to Environmental Science: Nano, (at the end of June)

Loosli, F. and Stoll, S., 2015, Effect of sodium dodecyl sulfate concentration, pH and divalent electrolytes on the stability of manufactured TiO2 nanoparticles. will be submitted to Journal of Colloid and Interface Science (first draft)

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