Chapter 1 Introduction
1.5 Fate and Behaviour of AgNPs in the Environment
1.5.1 Dissolution
Dissolution refers to the release of ions from the NPs, and the dissolution rate refers to how fast they dissolve to produce the ions. Dissolution has an impact upon NP transportation and their overall environmental fate (Miao et al, 2009) which is dependent on size, shape, and surface chemical properties (such as surface coatings) (Zhang et al,
19 | P a g e 2011 b, Misra et al, 2012). The ionic form of the NP will also have a different fate, effect, and transport compared to the original NP (Li and Lenhart, 2012). Other physicochemical parameters from the surrounding aqueous medium such as pH, ionic strength (Baalousha, 2009, Elzey and Grassian, 2010, Ma et al, 2012), dissolved oxygen (DO) content (Zhang et al, 2011) and ligands, for example natural organic matter (NOM) chlorine (Cl) and sulfur (S) will also affect the dissolution rate.
The DO content is particularly important as AgNPs released into aerobic aqueous environments, such as surface natural water, will be subsequently oxidised (Li and Lenhart, 2012). High DO content combined with low pH provides optimal conditions (figure 1.9) for oxidation processes (Liu and Hurt, 2010). Oxidation of AgNPs results in the release of Ag+ (Li and Lenhart, 2012) and will change the morphology of the NPs (Zhang et al, 2011). Therefore, the DO concentrations contribute to the dissolution via oxidation at the surface of the AgNPs as seen in figure 1.9.
Figure 1.9: An example of an uncoated silver nanoparticle with a surface oxide layer showing how dissolution and surface oxidation occurs. Picture extracted from(Li and Lenhart 2012).
A further influencing factor on AgNP dissolution is in the presence and absence of light.
Since Ag is light sensitive,photochemical oxidation in the presence of light can induce NP
20 | P a g e dissolution (Kim et al, 2008 and Wodka et al, 2010). Studies by Wodka et al (2010) deposited AgNPs on a titania surface to act as a photocatalyst and observed that in the presence of light oxidation was facilitated. Literature suggests that depending on the media in which NPs are dispersed, exposure to NOM increases dissolution by acting as ligand. NOM can also reduce the dissolution process via steric protection due to re-capping (Misra et al, 2012). Liu and Hurt (2010) conducted ion release studies on AgNPs exposed to natural sea water in the presence and absence of humic and fulvic acids. They discovered that increasing temperature resulted in increased dissolution. In addition, increased pH in the presence of humic and fulvic acids, resulted in reduced dissolution (Liu and Hurt 2010).
Aggregation and agglomeration of the NPs in suspension will affect the dissolution rate as the surface area and dynamics of the NP will change, leading to increased size and reduced surface area (Hotze et al, 2010, and Misra et al, 2012). Many studies have looked at the dissolution of AgNPs in simple aquatic media and aggregation of citrate coated AgNPs with the influence of NOM and ionic strength (Bae et al, 2013). Further studies have focused on the dissolution of AgNPs to help identify differences between NP and ion specific toxicity (Zook et al, 2011, Linlin and Tanaka, 2013). However, there is still an absence of information on the AgNP dissolution behaviour in the presence of NOM (Linlin and Tanaka 2013) in a synthetic water standard.
Dissolution rates are important in assessing NP transport and behaviour of AgNPs, as the ionic species have been demonstrated to form NP surface complexes, particularly with chloride, sulphate and sulphide species (Ha and Payer, 2011). AgNPs can also form complexes with these electrolytes under environmental conditions which will alter the
21 | P a g e dynamics of the NP species affecting the dissolution rate (Quadros and Marr, 2010, Levard et al, 2013). The dissolution of a particle is size dependant, the smaller the NP the higher the surface area and surface reactivity (Ma et al, 2012). Therefore, the tendency to dissolve increases, as particle size decreases (Misra et al, 2012). An important factor to consider is that AgNPs are typically synthesised with surface stabilisers, which can remain stable (depending on the type of coating) in solution and can reduce the dissolution rate, compared uncapped particles (Kvitek et al, 2008, Zhang et al, 2011b). Shape and crystalline structure will also affect how quickly the NPs dissolve.
1.5.2 Diffusion and Sedimentation
In stagnant water systems, NPs are transported by diffusion, and/or sedimentation.
Diffusion occurs due to the difference of NP concentration between the different water compartments (surface, middle or bottom), whereas sedimentation occurs following NP aggregation. Ficks Laws of diffusion, which was first described in 1855, states that particles of a high concentration move to areas of a low concentration gradient until they are evenly distributed (Fick, 1855, Gorban et al, 2011). Diffusion is one transport mechanism used to describe the movement of NPs when exposed to an aqueous suspension. The movement can be explained by a diffusion coefficient which depends on particle size and viscosity of the media (Eq 1.1).
[Eq 1.1]
Where D is the diffusion coefficient (m2s-1), R is the gas constant (L.kPa K mol), T is temperature, NA is Avogadro’s number, viscosity of the solution µ (Pa.s) and d is the
22 | P a g e particle diameter (m). Hinderliter et al (2010) described the sedimentation of NPs in cell culture systems. Together they produced a computer model that describes the speed at which NPs settle in order to predict the in vitro cellular dose of NPs as seen in figure 1.10.
According Stokes Law:
[Eq 1.2]
Where the sedimentation velocity of particles in solution is U (ms-1), g refers to the gravitational force (m s2), particle density is
ρ
p (km m3), fluid density isρ
f,
the particle diameter is d (m) and the viscosity of the solution media is termed µ (pa.s) (Hinderliter et al, 2010). The diffusion-sedimentation transport of NPs can be described by applying the partial differential equation (Eq 1.3) (Mason and Weaver 1924). The right hand side of the equation describes the diffusion and sedimentation of the particles as seen in Ficks Law (Socolofsky and Jirka, 2004) and is written as:[Eq 1.3]
Where c is the concentration of the solute (g mL-1), t is time (s), z is distance from the source (m), D is the diffusion coefficient, and U is the sedimentation velocity (Mason and Weaver, 1924). The boundary condition and the solution of the diffusion-sedimentation equation are presented in chapter 2.
23 | P a g e Figure 1.10: Diffusion and sedimentation transport processes of NPs, combined with the problems that affect transportation once exposed to an aqueous media. Figure courtesy of Hinderliter et al (2010).
As described in figure 1.10, the diffusion-sedimentation of the NPs will affect the transport process in terms of time and speed. The larger the NP the slower it will travel due to sedimentation. Equally, the smaller the NP, the faster it will move due to diffusion.
Simple ionic exposure solutions have revealed that when NPs are exposed to aqueous environments they tend to aggregate which significantly affects their sedimentation and diffusion rates (Keller et al, 2010). Sedimentation and diffusion will further depend on the physicochemical properties of the AgNPs. In simple ionic solutions, polymer coated AgNPs have been demonstrated to remain stable (Tejamaya et al, 2012). Therefore, the aqueous conditions need to favour aggregation of AgNPs in order to have effects upon the sedimentation and diffusion rates.
24 | P a g e Overall it is has been shown that the combined effects of adsorbed NOM (Li and Sun, 2011), pH, size, shape (Keller et al, 2010) and ionic strength (Li and Chen, 2012) all effect particle aggregation, which defines the diffusion/sedimentation rates and bioavailability of the nanoparticles. Eventually, in a natural water system sedimentation will determine the fate of these nanomaterials (Nowack and Bucheli 2007), especially aggregated particles, as they are less mobile and are more likely to be dispersed into sediments (Keller et al, 2010). Sedimentation creates potential problems as NPs can be taken up by small organisms feeding in the sediment, or accumulates in plants growing in this area (Nowack and Bucheli 2007).