Transformations of AgNPs will highly depend upon the local environment in which they are released. Depending on the aqueous conditions, the physicochemical properties of AgNPs will change in terms of surface structure, reactivity and composition (Levard et al, 2013). It is likely that AgNPs in the environment will become associated with NOM, or form complexes to sulfide (Lowry et al, 2012) and chloride compounds (Ha and Payer 2011). Therefore, the effects of the environmental conditions to which AgNPs are exposed needs to be assessed to help determine the overall transportation and fate of AgNPs.
1.6.1 Effects of Environmental Water Chemistry
Ionic strength of the aquatic system in which the NPs are dispersed is particularly important (Jiang et al, 2009). Increased ionic strength will result in the reduction in the diffuse double layer and alters surface charge, enabling NP aggregation (Navarro et al, 2008, Romer et al, 2011). Additionally, the presence of monovalent and divalent cations has been demonstrated to have a destabilizing effect on the surface charge of the NPs (Li and Sun, 2011), particularly that of divalent cations such as, calcium (Ca2+) and
30 | P a g e magnesium (Mg2+) increase aggregation (Zhang and Oyanedel-Craver, 2012). Changes in pH will change in the NP surface charge and impact the rate at which particles aggregate.
Acidic conditions have been shown to increase the rate off aggregation by reducing the surface charge density in citrate stabilized AgNPs (El Badawy et al, 2010).
In aqueous environmental systems, AgNPs form complexes with electrolytes such as sulphide groups and chloride by acting as an electron donor (Blaser et al, 2008, Allen et al, 2010, Quadros and Marr, 2010, Levard et al, 2013). Several studies have identified the effects of simple solution chemistry on particle behaviour and differences in capping agents (Romer et al, 2011 and Tejamaya et al, 2012). Other studies have concentrated on the effects of NOM on AgNP behaviour in presence of simple electrolytes (Akaighe et al, 2012, Bea et al, 2013). Until recently, studies were mainly unavailable that combined the complexity of a natural environmental system to observe the environmental transformations to AgNPs. Lowry et al, (2012) observed the changes and transportation of AgNPs when spread on terrestrial soils and surface natural waters using mesocosm studies in a natural wetland environment. The researchers identified around 50% of the AgNPs exposed to terrestrial soils and sediments were transformed to silver sulfhydryl compounds (Ag2S), which reduced the amount of ionic silver in solution. Therefore, sulfidation of AgNPs was shown to reduce dissolution (Lowry et al, 2012).
Less complex studies have been conducted to identify the role of chloride in environmentally relevant conditions and its effects of AgNP transformations. AgNP transformations are hard to assess as there are many configurations of AgCl complexes that can be formed depending on the ratio of chlorine to silver in solution, which reduce
31 | P a g e the toxicity and availability of the NP (Blaser et al, 2008). Ag can be oxidised in the presence of chloride in solution to form AgCl (Ha and Payer, 2011):
Ag+Cl- = AgCl +e-
[Eq 1.6]
Levard et al, (2013) attempted to assess the effects of chloride on AgNPs and discovered that the presence of chloride in small amount reduces AgNP dissolution.
1.6.2 Effects of Natural Organic Matter (NOM)
The presence of NOM can have effects on surface charge, particle aggregation, stability and influence the fate and transport of NPs (Manciulea et al, 2009). NOM describes all naturally occurring organic materials in the environments and can be distinguished by several different types of compounds. The first can be described as biopolymers, produced as biological waste by bacteria and algae. The second type is known as humic substances (Diegoli et al, 2008). Humic substances (HS) can be sub categorised into humic acids (HA) and fulvic acids (FA). HA are organic matter of soils and peats. FA is produced by the degradation process of plants and has higher oxygen contents than those of HAs (Adegboyega et al, 2013). The last sub-category refers to non-humic substances (Buffle et al, 1998).
Approximately 30-50% of NOM in natural aquatic systems is FA and has 50-60%
total carbon content. FA is a mixture of high molecular weight organic matter which has several functional groups which can interact with metallic cations to increase their presence (Akaighe et al, 2012). The additional presence of the oxygen functional groups with the combination of high carbon content will influence the overall fate and behaviour of NPs as they move through natural environments (Li and Sun, 2011). It has been seen that NOM has the ability to coat the NP surface and enhance stability and persistence
32 | P a g e (Cumberland and Lead 2009, Bae et al, 2011). Adsorption of the NOM on to the particle surface can take place by several different interactions such as hydrophobic interactions, electrostatic interactions and hydrogen bonding depending on the surface charge (Navarro et al, 2008).
Studies by Diegoli et al (2008) and Baalousha et al (2009) reported findings of surface interactions between nanomaterials and humic substances, and found the presence of humic substances resulted in the changed behaviours of the NPs and increased size. Surface adsorption of NOM is seen in figure 1.12 where HA is used as an example (Bae et al, 2011).
Figure 1.12: An example of NPs released into the environment and possible interactions with constituents found in natural aqueous environments. Suspended APs are exposed to a) humic acids, b) bacteria and other living organisms, and c) salt. Each exhibit a different behavioural reaction for the NPs. Humic acids adsorb on to the particle surface, reducing aggregation, whereas high salt concentrations induce aggregation. The humic acids NPs are attracted to the surfaces of living organisms and bacteria. Picture extracted from Bae et al (2011).
33 | P a g e Figure 1.13: A TEM image of fullerene aggregates in the presence of 1 mg L-1 humic acid in a high ionic solution (40 mM) CaCl2. The arrows indicate the humic acids linking the aggregates together. Picture source: Chen and Elimelech (2007).
Re-coating NPs reduces aggregation as the presence of the NOM on the particle surface increases the electrostatic repulsion. Increased stability of NOM coated NPs has also been demonstrated when in the presence of monovalent electrolyte environments such as chlorine (Cl-) (Li and Sun, 2011). In complex mixtures that contain divalent electrolytes, such as Ca2+ and Mg2+, NOM can either enhance the stability at low electrolyte concentrations, or decrease the stability at high electrolyte concentrations (Adegboyega et al, 2013). An example of destabilisation by humic acids in a high ionic solution is seen in figure 1.13.
As different aquatic systems have higher concentrations of NOM than others, once NPs are released into natural waters they will interact with NOM which will have a major effect upon their transport, fate and behaviour (Navarro et al, 2008). NP interactions with NOM such as polysaccharides with a higher molecular weight will cause the particles to be removed into the sediment.
34 | P a g e