Aquatic systems are one of the environmental compartment where the estimated release of most used ENMs reaches the level of 30 000 metric tons/year in 2010 (Keller et al., 2013). The fate and behaviour of ENM will depend on various factors including water chemistry and intrinsic ENM properties. The effect of these factors on ENM fate are presented below in more details.
I.2.1 Water chemistry
ENMs can enter the aquatic system as either individual particle or aged materials or aggregates. Depending on water pH, ionic composition (ion valence), water hardness, ionic strength, presence of natural organic matter (NOM) and inorganic colloids (ICs), the behaviour of ENMs can drastically change and lead to various transformations (Fig. I.1).
NOM is one of the important water component that defines the stability of ENMs in aquatic system. NOM is a general name to denote all types of organic constituents in water, such as humic substances, polysaccharides, peptidoglycanes, hemicellulose, pectic compounds such as microbial cell walls and extracellular products, sugars etc (Buffle et al., 1998). For example, humic substances represent an assembly of organic macromolecules with heterogeneous functional groups such as hydroxyl OH), methyl (-CH3), carboxyl (-COOH), and are present in the aquatic environment in the form of fulvic acids (FAs), humic acids (HAs) and humin. NOM plays also an important role in reducing the aggregation rates, stabilisation and dispersion of colloidal particles and engineered nanomaterials (Liu et al., 2012). For example, the adsorption of NOM on CeO2
nanoparticles (NPs) was found to reduce the aggregation by increasing the electrostatic repulsions between particles (Quik et al., 2010). The coating of magnetic γFe2O3 NPs by HAs was found to stabilise these NPs due to the repulsive electrostatic and steric-electrostatic interactions (Ghosh et al., 2011). Even more, adsorption of HAs led to the disagglomeration of already formed TiO2 nanoparticle agglomerates (Loosli et al., 2013).
Another major type of natural water compounds which has an effect on the stability of ENMs is inorganic colloids (ICs) (Buffle, 2006). The most common ICs are clays, aluminosilicates and iron oxihydroxides (Eyrolle and Charmasson, 2004, 2001; Filella, 2007). ICs are compact compounds with high size polydispersity, different mineralogy, and varied concentration in natural water from a few to hundreds mg/L (Eyrolle and Charmasson, 2004). In Rhône river water, ICs are mainly present as clay minerals, calcite,
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quartz, muscovite and iron oxide and their concentration changed from 3 to 40 mg/L (sampling of Rhône water near Arles) (Slomberg et al., 2016). Another study indicates the proportions of Fe and Al observed in the colloidal phase in the Rhône river water reached 42% and 35%, respectively (Eyrolle and Charmasson, 2004). The size, including aggregation state, as well as surface chemistry of natural ICs define the availability of IC surface and possible interactions with ENMs (Slomberg et al., 2016).
Fig. I.1. Schematic representation of physicochemical processes and parameters influencing ENM fate and behaviour in aquatic systems.
Ionic strength and electrolyte ion valency are other parameters that influence the stability of ENMs in aquatic system. Ionic strength takes into account the charge and the number of ions which are present in solution. Electrolytes modify the thickness of the double-layer and affect the electrostatic interactions, hence influencing ENM behaviour.
The aggregation kinetics of CeO2 NPs was investigated as a function of ionic strength at pH 5.7 (Zhang et al., 2012). Authors found that the critical coagulation concentration (CCC) in KCl was equal to 40 mM. Divalent ions, such as Ca2+ and Mg2+, were found more effective for destabilisation and aggregation. The CCC for CeO2 NPs (at pH 5.6) was approximately equal to 9.5 mM for CaCl2 (Li et al., 2011). The fast aggregation was explained by an increase of the inverse of the Debye length which resulted in a decrease of repulsive electrostatic energy.
6 I.2.2 ENM intrinsic properties
Intrinsic properties of ENM such as size, aggregation state, shape and surface charge, are interconnected and play a key role in the environmental identity of ENMs.
The aggregation state of ENMs is directly related to the particle size as well as to the particle reactivity. The aggregation state also affects the behaviour of NPs in the environment and determines their toxicity (Rodea-Palomares et al., 2011; Röhder et al., 2014; Safi et al., 2010; von Moos and Slaveykova, 2013). It is known that smaller particles are more reactive due to the higher proportion of active centres on the surface compared to the bulk materials (Andreescu et al., 2014), and more toxic for the organisms (Cong et al., 2011; Fabrega et al., 2011; Moos et al., 2014). The aggregation state can be affected by the environmental conditions. Even if ENMs enter the aquatic environment in agglomerated state the presence of organic substances at natural concentration can lead to important surface changes and redispersion. TiO2 NPs that where in contact at environmentally relevant concentration of HAs were disagglomerated and changed their surface charge towards further stabilisation (Loosli et al., 2014, 2013). Zinc oxide NPs were also partially disagglomerated due to the coating with Suwanee River HAs (Mohd Omar et al., 2014).
In aqueous solution particles acquire a surface charge, most commonly, due to the presence of surface chemical groups which can be ionised in the presence of water. Such acidic or basic surface groups can release or gain protons (H+) depending on the pH of the solution (Gregory, 2005). As an example, the surface of metal oxide nanoparticle can be considered (Fig. I.2). At low pH value surface charge of oxide is positive and at high pH is negative, passing by the point of zero charge (PZC).
Surface deprotonation can be described by the following chemical equilibria (Oriekhova and Stoll, 2016a):
M-OH2+ + OH– → M-OH + H2O (I.1) M-OH + OH– ↔ M-O– + H2O (I.2)
There are two situations when the surface charge on metal oxide NPs is equal to zero. First, in absence of both positive and negative charges referring to the PZC. The second situation, when there is an equal amount of positive and negative charges, is called the isoelectric point (IEP) (Jolivet et al., 2000). It is referring to the situation when positive
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and negative charges compensate each other and very often is related to the specific adsorption of positive or negative species. These two concepts are crucial to control the stability of ENMs in aquatic systems as well as for the development of the ENM elimination processes and interpretation of aggregation results.
Fig. I.2. Schematic representation of ENM ionisation processes on the surface of metal oxide nanoparticles in aqueous solution (Adapted from (Oriekhova and Stoll, 2016b)).