«By Yusuf Nur A thesis submitted to The University of Birmingham for the Degree of DOCTOR OF PHILOSOPHY School of Geography, Earth and Environmental ...»
On the other hand, naturally occurring organic materials may display disaggregation of NP such as nano-TiO2 and thus in the natural aquatic environment, the dispersion and mobility of TiO2 nanoparticles might occur to a much greater extent than predicted by laboratory measurements (Domingos et al., 2009b). Similar studies have indicated that Iron oxide NPs have been shown to change their solubility, stability, and aggregation behaviour in response to changes in pH and organic matter concentration with open, porous aggregates in the absence of standard Suwannee River humic acid (SRHA) and compact aggregates in the presence of SRHA (Baalousha et al., 2008). Organic substance may influence the behaviour of NPs in different ways. Humic substances are likely to form nanoscale coating on the surface of the solid NPs which stabilized the NPs through charge stabilisation (Tipping,
1981) while longer fibrils are more likely to form bridges between NPs and increase aggregation via bridging mechanisms (Buffle et al., 1998). Different environmental compartments differ in many aspects in terms of the aforementioned factors which determine the ultimate fate and behaviour of NPs. It is the task of the following subsections (2.2.1 and 2.2.2) to give clear introduction of the behaviour of NPs in both soil and water environments.
2.2.1 Nanoparticles in soil Apart from the intrinsic characteristics of the nanoparticles which determine their behaviour, there is an equally important number of soil conditions which will have impact on their fate and behaviour. Soils are complicated structures of solid, liquid, gas, organic and inorganic components which result in the overall properties of different types of soils. Soil chemistry, pH, type of minerals and organic contents of the soil seem to have the most effect on the aggregations and transport of NPs in soil environment (Kretzschmar et al., 1997). In order to
impacts on the stability of nanoparticles and their ecotoxicology, detailed studies are necessary. Although there is a relatively substantial number of ecotoxicology and behaviour studies of nanoparticles in aquatic environment (Nel et al., 2006, M.N, 2006, Zhang et al., 2008, Keller et al., 2010) currently, few studies have to date focused on the toxicity of NPs in soil environment (Yang and Watts, 2005, Tong et al., 2007) and their transport in terrestrial environment with the aim of providing to the science community relevant data for making acceptable prediction for the NPs behaviour and exposure pathways(Xu et al., 2008). This indicates that this field of research is in its early stage and little information is available for a profound understanding of the NPs fate and behaviour in soil.
2.2.2 Nanoparticles in water:
Despite the fact that nanotechnology is apparently quite a recent technology, the fate, behaviour and the impact of nanoparticles on aquatic environment is becoming an increasing concern that needs to be prioritized in order to facilitate quantitative ecological risk assessment which is not yet available for environmental chemists (Klaine et al., 2008).
Therefore, it has attracted many researches and has been relatively extensively studied.
Consequently, our knowledge in this rapidly growing field has increased in the last decennia to a point that many researches in this field have pointed out the possible ecotoxicologial impacts of NPs on the aquatic organisms (Lovern et al., 2007, Li et al., 2010a, Adams et al., 2006). The ways of production and applications of nanoparticles in a variety of fields including consumer products make it inevitable that nanoparticles and their by-products end up in the aquatic environment (rivers, lakes, ground water, coastal waters) (Christian G,
2004) through rain events, runoff, leaching, industrial waste and all urban water sewage.
by both their own properties and relevant environmental factors. As noted previously, dissolution, aggregation and subsequent sedimentation determine the ultimate fate of NPs.
The main sinks and receptors of aquatic NPs are therefore sediments in which there are living benthic organisms (Klaine et al., 2008). Different waters will affect similar NPs in different ways depending on a number of factors such as pH, ionic strength, inorganic species, organic matter and others. Seawater and freshwater are the extremes of the aquatic mesocosms in terms of water properties including ionic strength and total organic carbon(TOC) which are two important factors for the stabilisation or aggregation of NPs.
22.214.171.124 Fate and behaviour of NPs in freshwater Freshwater is a term generally used for the naturally - occurring water sources on the surface of the Earth such as lakes, ponds, rivers and icecaps and ground water like underground streams and aquifers. To define water as freshwater, it needs to have the concentration of dissolved salts less than 500 parts per million (ppm). General characteristics of different types of freshwater such as ionic strength, pH, natural organic matter, ionic composition and their combined effects will result in many transformations of nanoparticles such as reactions with biomacromolecules, redox reactions, aggregation, and dissolution which will alter the fate, transport, and toxicity of nanomaterials. (Lowry et al., 2012). Although many studies have focused on the individual effects of the above mentioned factors, only few studies have addressed the behaviour of NPs in more complex aquatic matrices (Keller et al., 2010, Findlay et al., 1996, Gao et al., 2009). The pH of the aquatic media may influence the surface charge of the particles through protonation and deprotonation processes in various ways. The pH around the zero point charge of the particles will reduce repulsion forces between particles which keep them separate and subsequently facilitate aggregation as pointed out by
the media will determine the overall ionic strength wherein NPs are exposed. Nanoparticles, which owe their stability to the charge stabilization caused by (Liu and Hurt, 2010) repelling surface charges such as citrate coated NPs will show more aggregation than sterically stabilized NPs where bulk uncharged organic materials are adsorbed on their surfaces. Under high ionic strength conditions, the electrical double layer around the charge NPs shrinks and zeta potential approaches to zero value which facilitates fast agglomeration (Elimelech et al., 1995).
Natural organic matter (NOM) is ubiquitous in the freshwater and it can either increase or decrease the stability of NPs depending on the type of organic materials as stated in the previous section (section 2.2). Nanoparticles entering fresh water will inevitable interact and being transformed by water abundant organic matter naturally. This includes proteins, polysaccharides, and humic substances (HS) (Lowry et al., 2012, Kiser et al., 2012). Initial interaction of the NPs and organic matter and their subsequent mobility and behaviour in the environment is mainly dependent on the nature of coating agents on the surface of the NPs (Saleh et al., 2008). Coating of organic materials on the surface of the nanoparticles will influence on the speciation of the nanoparticles. Liu and Hurt have, for example, shown the decrease of ionic silver Ag+ when citrate-capped Ag NPs were coated with humic or fulvic acids (Liu and Hurt, 2010). Since the ions show apparent toxicity on many organisms the decrease of ions will influence the toxicity of the NPs in aquatic environment. Recently published data aiming to understand the interaction of the organic compounds in the freshwater and the gold nanoparticles has indicated that the colloidal stability of the NPs in the absence of NOM is a function of capping agent, pH, ionic strength, and electrolyte valence. In the presence of NOM, the capping agent is a less important determinant of
water was investigated and the results have shown that sterically stabilized silver nanoparticles can persist longer as individual particles in natural water systems and thus will release silver more quickly and to a greater extent than would particles that aggregate quickly after entering the water (Li and Lenhart, 2012).This finding points out that sterically stabilsised NPs release more ions and thus are more hazardous to the aquatic organisms than charge stabilised NPs ( citrate capped NPs) which are more likely to aggregate in environmental relevant conditions.
Studies have shown that the size of small aggregates of NPs in waters with high total organic matter and low ionic strength will remain stable in water column and that no sedimentation takes place (Keller et al., 2010, Hyung et al., 2006). Aggregation or lack of it of nanoparticles determines the particles size and the fraction of ions in the water which are exposed to any aquatic organisms living inside the water column (Fabrega et al., 2011). In general aggregation is an important toxicity factor of all different types of nanoparticles (Wick et al., 2007). Stability of nanoparticles in the water column increases the toxicity of the NPs while the aggregation mitigates their potential toxicity due to decreasing available surface area of the NPs and their reduced mobility (Kvitek et al., 2008, Bradford et al., 2009).
126.96.36.199 Fate and behaviour of NPs in Marine water
and covers 72% of the Earth’s surface. Due to the weathering process of rocks on the Earth’s surface, it contains more dissolved ion and has lower freezing point and higher density than freshwater. It conducts electricity better and is more basic. Coastal runoff and atmospheric deposition may contribute to the contamination of marine environment with chemical wastes including NPs. The abovementioned properties will apparently affect the behavior of NPs
increase of salinity of freshwater by adding seawater will increase aggregation followed by sedimentation and therefore less nanoscale colloidal concentration are left in the water column (Stolpe and Hassellöv, 2007). Similar investigation has concluded that singly dispersed 20 nm core size, citrate capped AgNPs in seawater and waters with greater than 20 mmol L− 1 sodium chloride were unstable and formation of aggregates were recorded due to the charge screening by greater salt concentrations and the presence of divalent cations in the seawater (Chinnapongse et al., 2011). High aggregation rate and less dissolution of AgNPs in seawater as compared with deionised water was also reported (Chen and Zhang, 2013). The zeta potential of the AgNPs in the seawater was less negative than in the deionised water. Zeta potential is measure of stability of the NPs, the higher the absolute value of the zeta potential of the particles the higher the repulsion forces between them and less likely that they form aggregates. Similar results were reported for TiO2 nanoparticles in seawater for which rapid aggregation occurred in suspensions of TiO2 NPs to form micrometer size particle(Ates et al., 2013). Overall, the stability of NPs decreases in line with increasing salinity over the course of their transport along an estuary passing ultimately to the sea, due to a decrease in the repulsion forces by surface charge screening of counter-ions in the diffuse double layer(Lapresta-Fernández et al., 2012).
2.2.3 Ecotoxicology and the effect of nanoparticles on bacteria population Results from ecotoxicological studies have suggested possible ways that NPs can affect planktonic bacteria in different environmental mesocosms. A good summary of the performed researches on the toxicity of different types of the NPs on bacteria are given in Table 2-2 below adopted from the critical review of Kleine et al (Klaine et al., 2008).
Literature review in the table was updated with more recent literature for all nanoparticles in the table. Table 2-2 clearly outlines that different NPs have different toxic effects on the types of bacterial species studied. Some NPs like silver, carbon containing fullurenes and carbon nanotubes manifest antibacterial effects on bacteria while others like Silicon oxide and gold have mild or low toxicity. The toxicity of NPs are found to be mainly size or surface area dependent (Nel et al., 2006).
The data in Table 2-2 shows that silver is by far the most toxic nanoparticles in the list. All tested silver NPs have shown certain toxicity on the target bacterium while the toxicity of gold nanoparticles seems to be the lowest in the rank. Among the different types of the nanomaterials listed in Table 2-2 silver nanoparticles have so far been the most studied group in terms of ecotoxicology due to the long term known ant-microbial activity of the silver.
using different coating agents. The effects of these functionalised silver nanoparticles on different organisms were studied by a number of researchers and detailed review of the effect of silver NPs on different aquatic invertebrate, vertebrate and prokaryote species was given by Fabrega et al ( (Fabrega et al., 2011).
Although the effect of gold nanoparticles on the bacteria was not studied as extensively as silver NPs, there is an increasing list of researches that are aimed to investigate the possible effect of gold NPs of different sizes and surface coatings on the environmental bacteria.
Since the topic of this thesis is to study the effect of gold On Pseudomonas fluorescens the available literature of the effect of gold nanoparticles on bacterial populations was presented and summarised in separate Table 2-3 below.
Table 2-3: Toxicity of AuNPs on different strains of bacteria.
found the lack of significant toxic effects on many bacterial strains. In contrast to those reports, there are few groups which have reported that gold nanoparticles can be toxic to the target bacteria depending mainly on the coating agents of the NPs (Goodman et al., 2004, Hernández-Sierra et al., 2008). The abovementioned researches did not provide a clear census on the ecotoxicology of the gold NPs on the environmental bacteria and the need for further research is eminent in order to understand the effect of gold on bacterial population.
The following subsections will provide a short introduction of the forms of bacterial population and their role in the natural environment.