Scholarly article on topic 'Transport Behavior of Engineered Nanosized Photocatalytic Materials in Water'

Transport Behavior of Engineered Nanosized Photocatalytic Materials in Water Academic research paper on "Environmental engineering"

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Journal of Nanomaterials
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Academic research paper on topic "Transport Behavior of Engineered Nanosized Photocatalytic Materials in Water"

Hindawi Publishing Corporation Journal of Nanomaterials Volume 2013, Article ID 856387,13 pages

Review Article

Transport Behavior of Engineered Nanosized Photocatalytic Materials in Water

Guang'an He,1 Hong Liu,1,2 Rui Chen,1 and Chuan Wang2

1 School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, China

2 Key Laboratory of Reservoir Aquatic Environment, Chinese Academy of Sciences, Chongqing Institute of Green and Intelligent Technology, Chongqing 401122, China

Correspondence should be addressed to Hong Liu;

Received 19 June 2013; Revised 1 July 2013; Accepted 1 July 2013

Academic Editor: Jiaguo Yu

Copyright © 2013 Guang'an He et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Engineered nanoparticles (ENPs) possess unique properties and are employed in many sectors, and thus their release into environment remains. The potential risks of ENPs have been confirmed by an increasing number of studies that necessitate a better knowledge to the fate and transport of ENPs. One important application ofENP is photocatalysis for production of H2 as energy and pollutant decomposition. Engineered photocatalytic nanoparticles (PCNPs) can also easily enter the environment with the rapid increase in its manufacture and use. This review focuses on the transport of PCNPs in water by addressing the important factors that determine the transport of PCNPs, such as particle size, pH value, ionic strength (IS), ionic valence, and organic matter. The transport of PCNPs in natural water systems and wastewater systems is also presented with an attempt to provide more abundant information. In addition, the state of the art of the detection technologies of PCNPs has been covered.

1. Introduction

Engineered nanoparticles (ENPs) with sizes at least in one dimension smaller than 100 nm have various unique properties, such as special optic, electric, thermotic, and magnetic properties [1]. They can be used for biomedical, pharmaceutical, catalytic, cosmetic, electronic, energy, environmental, and material applications and certainly have attracted intensive attention as one of the most promising technologies in the 21st century [2, 3]. Due to the existing and many potential applications of this technology, there has been a global increase in investment in nanotechnology research and development [4]. The market value for nanotechnology-related products in 2011-2015 is estimated to be more than USD 1 trillion per year [5]. It is particularly noted that owing to their excellent optical properties, a considerable portion of ENPs can be used as photocatalytic materials that can also be named as photocatalytic nanoparticles (PCNPs), which pose to play an important role in air purification, wastewater treatment, photocatalytic disinfection, photocatalytic degradation, dye-sensitized solar cells, and hydrogen generation [6-13].

The forecasted huge increase in the manufacture and use of ENPs results in their inevitable release into the environment. However, their unpredictable effects should not be ignored [14-19], and their adverse impacts on human health and ecology have been confirmed [5, 20-30]. Therefore, the fate and potential harmful effects of ENPs play an important role in evaluating the safety of ENPs in the environment [31]. To obtain detailed information on the above, a better understanding of the transport of ENPs is initially required. This review focuses on the transport behavior of PCNPs.

1.1. Transport Processes of the PCNPs. Each stage of the life cycle (production, transport and storage, use, and disposal) of PCNPs may lead to a potential exposure of human being [2, 3], once released into environment including air, water, and soil. The transport processes of PCNPs in the environment are depicted in Figure 1. Likewise, this transport behavior also occurs in aerosol, and the PCNPs are suspended in the air for a long period of time and then transport over a long distance by the aid of wind. They will then precipitate on the surface of water and be deposited on land by gravity. When the PCNPs enter the water, they may not only be stabilized or transported

Suspension A


Figure 1: Transport processes of the PCNPs in the environment.

through the water flow but also can be aggregated and settled to sediments. The behavior in water highly depends on the physical and chemical properties of the PCNPs, as well as the conditions of water chemistry such as pH, ionic strength (IS), and content and property of dissolved organic matter (DOM). PCNPs may be blocked by soil particles and remain in the soil for a long period of time, or break through the soil matrix and reach groundwater. Among the environmental media, water is the connection between air and soil. Hence, the transport of PCNPs in water becomes the most crucial process. It is critically of importance in enhancing the knowledge on the environmental behaviors of PCNPs.

1.2. Types of the PCNPs. The matrix of photocatalyst embraces the catalyst buck, the support, and dopant as well. Each of the three ingredients shares its intrinsic role in the catalytic reactions, and in some cases a synergetic effect emerges. Interestingly, the three ingredients can be well fabricated at the scale of nanosize for an improved activity on the whole. Table 1 shows some common types of PCNPs, with each kind being different in morphology. They can be spherical, tubular, or irregularly shaped and may exist in fused, aggregated, or agglomerated forms. Obviously, the diversity in the physical structure may lead to differences as PCNPs transport in water.

1.3. Detection Technology of the PCNPs. Nowadays, the detection technology used for monitoring ENPs is being rapidly developed [69], and many analytical and characterization methods have been adopted to disclose the transport behavior of PCNPs in water (Table 2). The most popular approaches are microscopic methods such as transmission electron microscope (TEM) and scanning electron microscope (SEM), for the observation of the morphology. Light scattering technology such as dynamic light scattering (DLS) is employed to quantify the particle size. These measures can also be used to disclose the transport behavior of PCNPs in water. Using these methods, we can obtain intuitional

Table 1: Ingredients of the of photocatalyst matrix at the nanoscale.

Ingredients Type Examples

Catalyst bulk Oxides TiO2 [32, 33], ZnO [34, 35], CeO2 [36, 37] Fe2O3 [38-41], Fe3O4 [42] Goethite/Hematite [43]

Sulfides ZnS, PbS [44], HgS [45]

Carbons Fullerene [46-48]

Metals Pt, Au [49, 50], Ag [51, 52], Cu [53], Pd [54]

Dopant Zero-valent iron (ZVI) [55]

Oxides NiO [56], MnO2 [57,58]

Carbons Carbon nanotubes (CNT) [59-64]

Support Oxides SiO2 [65, 66], Al2O3 [67, 68]

Table 2: Analysis and characterization methods of PCNPs.

Type Examples

Microscopy TEM, SEM, AFM

UV-Vis, FTIR, AAS, Raman spectroscopy ICP-OES, fluorescence spectroscopy Light scattering DLS

Light diffraction XRD

Computer technology Molecular simulations

information about the morphology and size of PCNPs. X-ray diffraction (XRD) is another useful tool for studying PCNPs as it can show more abundant information on the solid surface. At the same time, atomic force microscope (AFM) and molecular simulation technology are used to study PCNPs in order to detect and predict the forces between nanoparticles in water and determine how they affect the transport of ENPs in such medium [70-74]. Spectroscopy instruments (e.g., ultraviolet-visible spectrophotometer [UV-Vis], atomic absorption spectrometer [AAS], and inductively coupled plasma optical emission spectrometer [ICP-OES]) are also commonly used to detect the change in the concentration of PCNPs in water [75-80]. Some other spectroscopy technologies (e.g., Raman spectroscopy, Fourier transform infrared spectroscopy [FTIR], and fluorescence spectroscopy) are very helpful in providing more details about PCNPs [81-84]. Although remarkable progress has been made, the development of analysis and characterization techniques for ENPs is still challenging [85]. For instance, the precise size of PCNPs in water is hard to be identified; only the average hydrodynamic radium or electron microscope picture can be used to evaluate the aggregation and stabilization of PCNPs in water. The detection techniques discussed above are offline which can only reveal the state of PCNPs in water after several minutes or hours, but some processes of PCNPs in water may be quickly, so they can hardly monitor the instantaneous state (about several milliseconds or less) of PCNPs in water. Generally, more sensitive and efficient techniques are in great need to shed a light onto the transport behavior of PCNPs in water. It is necessary to develop more online detection methods which can monitor the state of

Surface of water

Water flow

Table 3: Reported size distribution of the selected ENPs as ingredients of photocatalyst matrix in water.

Gravity effect"

Short distance <->

Bottom of water

Large particle Small particle

Figure 2: Different transport behaviors of PCNPs in different sizes.

PCNPs in water at any time, and more combination methods which can analyze the physical properties of PCNPs and chemical conditions of water simultaneously are strongly recommended.

2. Factors Influencing the Transport Behavior of PCNPs in Water

2.1. Particle Size. The size of PCNPs not only affects the pho-tocatalytic activity significantly but also plays an important role in defining their transport and fate in aquatic environments. Figure 2 shows the different transport behaviors of PCNPs in different sizes. PCNPs with relatively large size tend to settle quickly by the aid of gravity, and their transport will be greatly restricted. Meanwhile, PCNPs with smaller size pose to remain in the water for a long period of time through the diffusion effect and may transport over a long distance by water flow. Several studies have confirmed that as PCNPs are released into water, they may aggregate. Table 3 shows the reported size distribution of the selected PCNPs in water. It can be seen that the hydrodynamic particle size is much larger than the individual particle size in the dry phase. The aggregation reduces the overall specific surface area of the PCNPs and the interfacial free energy, and thus limiting the reactivity. In addition, some of the PCNPs are ultrasonicated before their hydrodynamic size is determined, indicating that aggregation is a common process for PCNPs in water and that ultrasonication can do little to completely break the aggregates in water.

The sunlight and UV irradiation can prevent PCNPs especially fullerene from aggregation due to surface oxygenation and hydroxylation through photochemical transformation in the presence of dissolved O2 [86-88], so the size of PCNPs can remain stable [89-92].

In order to improve the photocatalytic activity and increase the light availability, the composite PCNPs that combine two or more PCNPs are synthesized extensively

ENPs Individual particle size (nm) Hydrodynamic particle size (nm) References

Ag 20-30 ~100 Gao et al. [93]

Al 41 215 ± 99 Darlington et al. [94]

Cu 26.7 ± 7.1 94.5-447.1 Griffitt et al. [95]

NiO 10-20 750 ± 30 Zhang etal. [96]

CeO2 (rods) (67 ± 8) x (8 ± 1) 231 ± 16 Keller et al. [76]

PbS 14.4 ± 1 240 Liu et al. [97]

TiO2 15-27 90-290 Jiang et al. [98]

ZnO 13 36-413 Xie et al. [78]

SiO2 14 205 Adams et al. [99]

Fe2O3 9.2 46.2 Baalousha et al. [100]

Fe3O4 <10 120 Illes and Tombecz [101]

AI2O3 60 763 Wang et al. [102]

ZnS 2.1 ~12.5 Slaveykova et al. [103]

[32, 33, 36, 37, 49, 52]. The sizes of these composite PCNPs are different from the size of those original PCNPs, and it can conclude that they can exhibit dissimilar transport behaviors to the original PCNPs.

2.2. pH. pH is a major factor determining the zeta potential of colloids. Zeta potential is the potential difference between the dispersion medium and the stationary layer of fluid attached to the dispersed particle. The value of zeta potential can be related to the stability of colloidal dispersions. The zeta potential indicates the degree of repulsion between adjacent, similarly charged particles in colloidal dispersions. For particles that are small enough, a high zeta potential will confer stability. This means that colloidal dispersions can resist aggregation. When the potential is low, attraction exceeds repulsion, and the dispersion will break, and floccu-lation takes place. Therefore, colloids with high zeta potential (negative or positive) are electrically stabilized, while colloids with low zeta potential tend to coagulate or flocculate.

The pH can significantly affect the zeta potential of colloids. A number of studies have proved that as the pH is at the point of zero charge (pHpzc) or isoelectric point, the colloidal system of PCNPs exhibits the least stability, and the sedimentation or aggregation rate increases [76, 101, 104-106]. Figure 3 depicts the relationship of aggregation and repulsion of PCNPs in different pH. At the pHpzc, the zeta potential is zero; the PCNPs can aggregate to become bigger particles due to the weak repulsion effect. As the pH continues to decrease until it becomes lower than the pHpzc, the surface of PCNPs colloids is positively charged, and the PCNPs prefer to remain small because of the strong electrostatic repulsion. Similar situation happens when the pH continues to increase until it becomes above the pH , the surface of PCNPs colloids is negatively charged, and the size of PCNPs is also stable.

Table 4: Reported pH of selected PCNPs.

PCNPs Particle size (nm) pHpzc References

TiO2 3.6, 8.1 1.8, 6.2 Guzman et al. [105]

TiO2 35 4.5 Fang et al. [107]

TiO2 50 ± 5 5.9 Yang and Xing [108]

Hematite 12, 32, 65 7.8, 8.2, 8.8 He et al. [109]

Hematite 12.5, 29.4, 51.4, 66.5 7.5, 7.7, 7.9, 8.5 Zeng et al. [110]

M2O3 60 7.9 Ghosh et al. [111]

MnO2 50 2.4 Feitosa-Felizzola et al. [112]

CeO2 157.4 >7.0 Necula et al. [113]

HgS Unavailable 4.0 Ravichandran et al. [114]

Q Negative charge © Positive charge

Figure 3: The relationship of aggregation and repulsion of PCNPs in different pH.

Different sizes of the same kind of PCNPs or similar size of different kinds of PCNPs may display a distinct difference in pHpzc (Table 4). Hence, at the same pH, different PCNPs have different surface charges. Keller et al. have reported that in the same simple solution with low IS and free of natural organic matter (NOM), TiO2 nanoparticles are negatively charged, while ZnO nanoparticles are positively charged, and CeO2 nanoparticles approach pHpzc [76].

IS can affect the aggregation of PCNPs at their pHpzc. French et al. have observed that at very low IS, the aggregation of TiO2 is not remarkable as the pH comes close to pHpzc [115].

When organic matters are present, they will coat the PCNPs and remarkably vary their surface properties, and thus served to reduce the charge of PCNPs and increase their size. As a result, the PCNPs become electrostatically or sterically stabilized, and the pH has little effect on the zeta potential of PCNPs [75, 116, 117]. PCNPs may be modified by organic matters in a specific pH value. The functional groups of PCNPs may be dissolved into solutions when

the pH is changed. Therefore, this can affect the stabilization and aggregation of PCNPs [118-120].

The pH value can also influenced the hydroxyl radicals production rate of PCNPs significantly, which may affect both the photocatalytic behavior and transport behavior of PCNPs in water. Xiang et al. have characterized the hydroxyl radicals produced by various semiconductor photocatalysts and indicate that the acidic pH environment of the solutions is beneficial to enhancing the formation rate of hydroxyl radicals [121].

2.3. IS. IS is another important factor influencing the stabilization and aggregation of PCNPs. Many studies have revealed that an increase in IS compresses the electric double layer on the surface of PCNPs [75,104,116,117,119], thereby decreasing the electrostatic repulsion between two particles with the same charge. The energy barrier will then decrease, and the attachment probability becomes closer to unity [5,122,123].

The critical coagulation concentration (CCC), known as a threshold electrolyte concentration, represents the minimum amount of electrolyte needed to completely destabilize the suspension [124]. It provides a useful parameter of colloidal stability for PCNPs and hence can be used to predict the transport behavior in water [79].

The classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory [125] describes the forces between charged surfaces interacting through a liquid medium. It combines the effects of the Van der Waals attraction with the electrostatic repulsion due to the so-called double layer of counterions. It is widely used as a model to predict the stabilization and aggregation of ENPs in water with electrolytes, and many extended models have been established based on the DLVO theory [126].

For spherical particles (e.g., TiO2, ZnO, fullerene), an increase in IS induces aggregation, and the particles size grows much bigger [76,115,127] (Figure 4(a)). Some similar phenomena have been found in tubular particles (e.g., CNT), which become unstable and precipitate when IS increases [106]. When PCNPs are coated by charged macromolecules (e.g., citrate acid), they may become electrostatically stabilized. At low IS, PCNPs can still be stable due to charge neutralization, which must be broken only at high IS (Figure 4(b)). When PCNPs are coated by noncharged polymers (e.g., PVP), they will be highly sterically stabilized (Figure 4(c)). The lack of effect of IS on the aggregation of sterically stabilized PCNPs was also observed [116].

2.4. Ionic Valence. The Schulze-Hardy rule [128] states that the CCC, varying as the inverse sixth power of the counterion charge, plays a significant role in destabilizing the stability of colloidal [129]. According to this rule, multivalent and divalent ions may be more efficient in neutralizing the charge on the surface of PCNPs than monovalent ions. With respect to charge screening, compared with monovalent ions, multivalent and divalent ions have a higher charge density and can induce aggregation at lower IS through more efficient double-layer compression. Alkali metal cations (e.g., Na+, K+) and alkaline earth metal cations (e.g., Mg2+, Ca2+) are

Low IS

High IS

IS increase

Electrostatically stabilized

PCNPs coated by charged macromolecules

PCNPs coated by noncharged polymers

Sterically stabilized PCNPs

+/_ Positive/negative charges

Sterically stabilized

Figure 4: The different aggregation behaviors of PCNPs (a), PCNPs coated by charged macromolecules (b), and PCNPs coated by noncharged polymers (c) in low IS and high IS.

most commonly used as model monovalent and divalent cations because of their ubiquity in aquatic environment.

Many experimental results have evidenced that divalent cations are much more efficient in enhancing the aggregation of bare PCNPs in water [79, 115, 130]. When a negative-charged organic matter is coated on the surface of PCNPs, cations with different valence will affect the aggregation process of PCNPs through different ways. For monovalent cations, they will be attracted by the negative charge of the coated organic matter of PCNPs (Figure 5(a)). The aggregation process is controlled by electrostatic and Van der Waals interactions between the particles, which do not lead to enhanced aggregation. It is similar to the conventional particle-particle collision aggregation process. When the divalent cations are attracted by the negative charge of the coated organic matter of PCNPs, they (especially Ca2+) can form complexlywith the coated organic matter and bridge the PCNPs (Figure 5(b)). The aggregation process is controlled by sterical interaction, which can lead to notable aggregation. When coated by the noncharged organic matter, PCNPs will remain stable. Both monovalent and divalent cations cannot enhance the aggregation of PCNPs.

2.5. Organic Matter

2.5.1. NOM. As ubiquitous components of aquatic systems, NOM is known to greatly influence the aggregation and

stabilization of PCNPs by adsorbing to the particulate surface. It is considered to be a crucial factor affecting the transport of PCNPs in water and has been well investigated [79,104,131133]. Suwannee River humic and fulvic acids (SRHA and SRFA, resp.), which can be obtained from the International Humic Substances Society (IHSS), are usually used as the standard models of NOM.

The chemical nature and structure of NOM serve to determine whether colloids will be stabilized or destabilized. Deonarine et al. have measured many parameters of NOM (e.g., aromatic carbon content, aliphatic carbon content, molecular weight, carbonyl and carboxyl content, etc.) that determine the relationship between the growth rate of ZnS-NOM particles and the specific NOM parameters [134]. They have found that an increase in the molecular weight of NOM leads to slower growth rate of ZnS-NOM particles. They have also revealed that aromatic carbon content shows the strongest linear relationship with the growth rate of ZnS-NOM particles, and a similar phenomenon is observed in multiwalled carbon nanotubes [119]. The factors which affect the aggregation behavior of PCNPs as NOM exists are presented in Table 5. Both high molecular weight and aromatic carbon content can reduce the aggregation of PCNPs. The former is attributed to the sterical repulsion, while the latter is due to the n-n interaction. With the addition of NOM, the negative surface charges of PCNPs increase significantly, and thus their propensity to aggregate

(M) w M

Table 5: The factors affect the aggregation behavior of PCNPs as

NOM exists.

^^ Coated organic matter @

Divalent cation

o.*o (TO

(M+ Monovalent cation C- Negative charge

Figure 5: Schematic diagrams for the mechanism of monovalent cations (a) and divalent cations (b) affecting the aggregation process of PCNPs coated by negative-charged organic matter.

is reduced. On the other hand, the negative charges that NOM imparts to nanoparticles could be neutralized by cations (especially divalent cations). It has been proven that Ca + induces the aggregation of NOM-coated ENPs [135]. NOM may be degraded by PCNPs when light irradiates and leads to aggregation of PCNPs as well, and this will eminently affect the transport of PCNPs in water.

2.5.2. Surfactants and Polymers. Many approaches have been used to obtain the surfactant-stabilized suspensions of dispersed CNTs [136-139]. For nonionic surfactants, higher molecular weight may lead to better dispersion of ENPs [140142]. Consequently, PCNPs can be well dispersed by surfactants, as they are generally insoluble in water. Hydrophobic interaction may play an important role in the stabilization of PCNPs suspensions for nonionic surfactants. The ability of nonionic surfactants to disperse PCNPs appears to be mostly dependent on the size of the hydrophilic group. Higher molecular weight suspends more PCNPs because of enhanced sterical stabilization with longer polymeric groups. For ionic surfactants, the addition of inorganic electrolyte can reduce the stabilization of suspensions of PCNPs. Bouchard et al. have indicated that CNTs dispersed by ionic surfactants are unstable in Ca + dominated systems at low surfactant concentrations and would likely aggregate and settle out of suspension [143].

Factors Cause

Reduce aggregation High NOM molecular weight High NOM aromatic Sterical repulsion n-n interaction

carbon content

Induce High cation concentration Light irradiation Charge neutralization

aggregation NOM degradation

Most polymers are commonly stable; hence, they may be used as a capping agent to improve the stability of PCNPs. Othman et al. use polyacrylic acid (PAA) and ammonium polymethacrylate (Darvan C) to disperse TiO2 nanoparticles, and more dispersed and stabilized aqueous TiO2 suspension is prepared successfully [144]. When they are coated on the surface of PCNPs, the latter can remain stable in water and may be stopped for further aggregation. This indicates that they can stay in water for a long period and is hard to be removed.

2.5.3. Biomacromolecules. Due to the potential harm ofENPs to human, animals, and plants, it is necessary to study the effect of biomacromolecules (e.g., polysaccharide, protein) on the transport of PCNPs in water. Saleh et al. have indicated that the presence of biomacromolecules significantly retards the aggregation rate of single-walled carbon nanotubes, and this is attributed to the sterical repulsion originating from the adsorbed macromolecular layer [145]. However, there is still lack of studies that focus on the effect of biomacromolecules, specifically on the transport of PCNPs.

3. Transport of PCNPs in Natural Water Systems

After PCNPs are released into the aquatic environment (e.g., rivers, lakes, seas, groundwater, stormwater, etc.), most of them have been shown to aggregate once they are hydrated, which results in the efficient removal of the small particles [146]. The formation of aggregates of PCNPs in natural water systems can be considered as physical processes, that is, Brownian diffusion, fluid motion, and settlement under gravity.

The aggregation and stabilization of PCNPs in natural water systems also depend on the above-discussed factors that influence the transport of PCNPs in water. The interactions between PCNPs or PCNPs and NOM play a crucial role in the transport of PCNPs in natural water systems. Mosley et al. have studied the forces between colloid particles in natural water system [70]. Under the conditions of low IS, the interparticle forces are dominated by electrostatic repulsion. However, at high IS, they are dominated by sterical repulsion forces, and electrostatic forces are largely absent. In addition, adhesive bridging between the surfaces of PCNPs

by adsorbed NOM creates a strong energy barrier to the spontaneous disaggregation of colloid aggregates.

Most studies use laboratory-made water or artificial natural water instead of real natural water to study the aggregation and stabilization of PCNPs. Nevertheless, the results of such studies may probably assist in predicting the transport of PCNPs in real natural water systems. Keller et al. and Ottofuelling et al. have presented the aggregation and stabilization of metal oxide (MeO) nanoparticles in different natural water systems [76,147]. They revealed that PCNPs in water systems with low IS and high NOM content (mesocosm freshwater) will remain stable, and the sedimentation rate of MeO nanoparticles is very slow. On the contrary, PCNPs in water systems with high IS and low NOM content (seawater) lead to a rapid sedimentation rate of MeO nanoparticles. This is consistent with the results of many studies that used laboratory-made water systems or artificial natural water systems. Obviously, natural water systems are complicated due to their diversity and randomicity, and it will be more complicated if photocatalytic reactions occur on the PCNPs in water. Therefore, the transport of PCNPs in natural water systems is influenced by the combined interactions of many factors. It is therefore a challenge to depict the detailed process of transport of PCNPs in natural water systems.

4. Transport of PCNPs in Wastewater Treatment Systems

There is a lack of knowledge regarding the transport of PCNPs in engineered systems, including wastewater treatment systems. The rapid increase in the production of PCNPs has created a demand for particle removal from industrial and communal wastewater streams. A common process wastewater treatment plants (WWTP) with advanced treatment includes the quite a few units. PCNPs involved within the wastewater will first be primarily treated. However, most PCNPs, except those surface-coated or functionalized, may still remain in the effluents because of their small size. Jarvie et al. have indicated that silica nanoparticles, which are surface-coated or functionalized, undergo rapid flocculation in wastewater and can be efficiently removed through primary treatment of WWTPs [148].

When PCNPs enter into an aeration tank, they will probably be absorbed on the activated sludge (AS). Recently, some PCNPs especially metal and metallic oxides nanopar-ticles are found in the AS [149-153]. Zou et al. establish an effective method to produce high-purity nano-SiO2 by recovering silicon from sewage sludge [154]. Gomez-Rivera et al. have revealed that CeO2 nanoparticles can be removed by the AS and suggest that it will be expected to provide extensive removal of CeO2 nanoparticles [155]. Whereas, it is doubtful that AS treatment is the best way to remove the nanoparticles in WWTPs. Recently, only a few pieces of evidence have shown that some types of PCNPs decrease the activity of AS [156-158]. However, it can be deduced that PCNPs may be harmful to the microorganisms in the AS due to their potential toxicity. Some PCNPs can dissolve and release metal ion which can cause metal toxicity. Liu et al. and Mu et al. have found out that when the ZnO nanoparticles

are absorbed on the AS, they will release Zn2+ which can exhibit great toxicity [159,160]. Ag nanoparticles are excellent antimicrobial materials as we known; therefore when Ag+ is released, it will probably enter the cells of microorganisms directly and inactivate cellular enzymes and DNA [149]. Because of the harmful impact of metal ions, some efforts are made to prevent the metal ion from dissolving by reassembling the nanoparticles into microspheres [161-165]. Due to their perfect photocatalytic property, PCNPs may also generate reactive oxygen species (ROS) which can kill the microorganisms by strong oxidation effect.

After secondary treatment, most of the PCNPs will be flocculated and removed from effluents. They may settle in finished biosolids after the solids handling process of WWTPs and can reenter the environment when the finished biosolids are used as fertilizers, incinerated, placed in landfills, or dumped in oceans [166]. After tertiary treatment, there is still a small proportion of PCNPs that remain in the treated effluent and enter the surface water environment, potentially disrupting numerous biological ecosystems.

5. Concluding Remarks

Increasing presence of PCNPs in the environment remains a challenge due to the rapid development of nanotechnology. It is essential to assess the potential risks of PCNPs that has been widely tested and employed as photocatalyst for environmental clean-up. Disclosure of the transport behavior of PCNPs in water is of particular importance for the prediction of associated environmental risk.

PCNPs could be intentionally or unintentionally discharged in each stage of the life cycle. They will then transport within the environment media. The transport of PCNPs in water poses as the most important process, and more studies should be conducted to explore such area. As PCNPs transport in water, they can aggregate and settle down or may be stabilized in water for a long period of time. The dramatic difference in the transport behaviors of PCNPs depends on several key factors that govern their stability and mobility as colloidal suspensions or their aggregation into larger particles and deposition in aquatic systems, such as particle size and water composition (e.g., pH, IS, ionic valence, and organic matters). However, only a few studies tackle the issue: which factor is predominant? Also, there is a lack of models that serve to predict the transport of PCNPs in water combining all the factors.

Most PCNPs will transport in natural water system once they are released into aquatic environment. The aggregation and stabilization of PCNPs in natural water systems highly depend on the water chemistry. Numerous studies have centered on the transport of PCNPs in laboratory-made or artificial water systems, which provide information on how to assess the transport behavior of PCNPs in real natural water system. However, there is still a lack of studies involving real natural water systems to determine the process of transport of PCNPs.

Wastewater treatment serves as an efficient process of preventing the release of PCNPs in water. Some studies have

indicated that most of the PCNPs can be removed through primary and secondary treatments and that only a little may still be left in water effluents after the tertiary treatment. The removed PCNPs may eventually become biosolids and return to the environment if the sludge is improperly disposed. Due to the potential toxicity of PCNPs to the microorganism of AS using conventional technology, more novel technologies should be put forward to solve such problem. During the actual process of transport, photocatalytic reaction of organic degradation under the illumination of sun light may occur, which serves to add complexity of the transport behavior, while this reaction may bring advantage by degrading more organic pollutants. Lack of relevant information in this issue requires the investigators to pay attention to this field.


This work was financially supported by the National Natural Science Foundation of China (no. 21067004).


[1] S. J. Klaine, P. J. J. Alvarez, G. E. Batley et al., "Nanomaterials in the environment: behavior, fate, bioavailability, and effects," Environmental Toxicology and Chemistry, vol. 27, no. 9, pp. 18251851, 2008.

[2] D. Lin, X. Tian, F. Wu, and B. Xing, "Fate and transport of engineered nanomaterials in the environment," Journal of Environmental Quality, vol. 39, no. 6, pp. 1896-1908, 2010.

[3] B. Nowack and T. D. Bucheli, "Occurrence, behavior and effects of nanoparticles in the environment," Environmental Pollution, vol. 150, no. 1, pp. 5-22, 2007.

[4] K. A. D. Guzman, M. R. Taylor, and J. F. Banfield, "Environmental risks of nanotechnology: national nanotechnology initiative funding, 2000-2004," Environmental Science and Technology, vol. 40, no. 5, pp. 1401-1407, 2006.

[5] E. Navarro, A. Baun, R. Behra et al., "Environmental behavior and ecotoxicity of engineered nanoparticles to algae, plants, and fungi," Ecotoxicology, vol. 17, no. 5, pp. 372-386, 2008.

[6] T. E. Agustina, H. M. Ang, and V. K. Vareek, "A review of synergistic effect of photocatalysis and ozonation on wastewater treatment," Journal ofPhotochemistry and Photobiology C, vol. 6, no. 4, pp. 264-273, 2005.

[7] J. Zhao and X. Yang, "Photocatalytic oxidation for indoor air purification: a literature review," Building and Environment, vol. 38, no. 5, pp. 645-654, 2003.

[8] U. I. Gaya and A. H. Abdullah, "Heterogeneous photocatalytic degradation of organic contaminants over titanium dioxide: a review of fundamentals, progress and problems," Journal of Photochemistry and Photobiology C, vol. 9, no. 1, pp. 1-12, 2008.

[9] X. Hu, G. Li, and J. C. Yu, "Design, fabrication, and modification of nanostructured semiconductor materials for environmental and energy applications," Langmuir, vol. 26, no. 5, pp. 30313039, 2010.

[10] D. S. Bhatkhande, V. G. Pangarkar, and A. A. Beenackers, "Photocatalytic degradation for environmental applications—a review," Journal of Chemical Technology and Biotechnology, vol. 77, no. 1, pp. 102-116, 2002.

[11] K. Kabra, R. Chaudhary, and R. L. Sawhney, "Treatment of hazardous organic and inorganic compounds through aqueous-phase photocatalysis: a review," Industrial and Engineering Chemistry Research, vol. 43, no. 24, pp. 7б83-7б9б, 2004.

[12] J. G. Yu, M. Jaroniec, H. G. Yu, and W. H. Fan, "Synthesis, characterization, properties, and applications of nanosized photocatalytic materials," Journal of Nanomaterials, vol. 2012, Article ID 783б8б, 3 pages, 2012.

[13] M. Ye, Y. Yang, Y. Zhang, T. Zhang, and W. Shao, "Hydrothermal synthesis of hydrangea-like F-doped titania microspheres for the photocatalytic degradation of carbamazepine under UV and visible light irradiation," Journal of Nanomaterials, vol. 2012, Article ID 583417, 8 pages, 2012.

[14] R. F. Service, "Calls rise for more research on toxicology of nanomaterials," Science, vol. 310, no. 5754, p. 1б09, 2005.

[15] A. S. Barnard, "Nanohazards: knowledge is our first defence," Nature Materials, vol. 5, no. 4, pp. 245-248, 200б.

[16] A. Seaton and K. Donaldson, "Nanoscience, nanotoxicology, and the need to think small," The Lancet, vol. 3б5, no. 94б3, pp. 923-924, 2005.

[17] D. B. Warheit, "Nanoparticles: health impacts?" Materials Today, vol. 7, no. 2, pp. 32-35, 2004.

[18] R. F. Service, "Nanotechnology grows up," Science, vol. 304, no. 5б78, pp. 1732-1734, 2004.

[19] G. Brumfiel, "Nanotechnology: a little knowledge...," Nature, vol. 424, no. б94б, pp. 24б-248, 2003.

[20] M. R. Wiesner, G. V. Lowry, R Alvarez, D. Dionysiou, and P. Biswas, "Assessing the risks of manufactured nanomaterials," Environmental Science and Technology, vol. 40, no. 14, pp. 433б-4345, 200б.

[21] V. L. Colvin, "The potential environmental impact of engineered nanomaterials," Nature Biotechnology, vol. 21, no. 10, pp. 11бб-1170, 2003.

[22] G. Bystrzejewska-Piotrowska, J. Golimowski, and P. L. Urban, "Nanoparticles: their potential toxicity, waste and environmental management," Waste Management, vol. 29, no. 9, pp. 25872595, 2009.

[23] A. Nel, T. Xia, L. Mädler, and N. Li, "Toxic potential of materials at the nanolevel," Science, vol. 311, no. 57б1, pp. б22-б27, 200б.

[24] P. H. M. Hoet, I. Bruäske-Hohlfeld, and O. V. Salata, "Nanoparticles—known and unknown health risks," Journal of Nanobiotechnology, vol. 2, no. 1, article 12, 2004.

[25] G. Oberdärster, E. Oberdärster, and J. Oberdärster, "Nan-otoxicology: an emerging discipline evolving from studies of ultrafine particles," Environmental Health Perspectives, vol. 113, no. 7, p. 823, 2005.

[26] K. L. Dreher, "Health and environmental impact of nanotechnology: toxicological assessment of manufactured nanoparticles," Toxicological Sciences, vol. 77, no. 1, pp. 3-5, 2004.

[27] E. J. Petersen, L. Zhang, N. T. Mattison et al., "Potential release pathways, environmental fate, and ecological risks of carbon nanotubes," Environmental Science and Technology, vol. 45, no. 23, pp. 9837-985б, 2011.

[28] B. I. Escher and K. Fenner, "Recent advances in environmental risk assessment of transformation products," Environmental Science and Technology, vol. 45, no. 9, pp. 3835-3847, 2011.

[29] C. Levard, E. M. Hotze, G. V. Lowry, and G. E. Brown, "Environmental transformations of silver nanoparticles: impact on stability and toxicity," Environmental Science and Technology, vol. 4б, no. 13, pp. б900-б914, 2012.

[30] W. Fan, M. Cui, H. Liu et al., "Nano-TiO2 enhances the toxicity of copper in natural water to Daphnia magna'" Environmental Pollution, vol. 159, no. 3, pp. 729-734, 2011.

[31] J. R. Lead and K. J. Wilkinson, "Aquatic colloids and nanoparticles: current knowledge and future trends," Environmental Chemistry, vol. 3, no. 3, pp. 159-171, 2006.

[32] X. Yu, J. Yu, B. Cheng, and M. Jaroniec, "Synthesis of hierarchical flower-like AlOOH and TiO2/AlOOH superstructures and their enhanced photocatalytic properties," The Journal of Physical Chemistry C, vol. 113, no. 40, pp. 17527-17535, 2009.

[33] Q. Xiang, J. Yu, B. Cheng, and H. C. Ong, "Microwave-hydrothermal preparation and visible-light photoactivity of plasmonic photocatalyst Ag-TiO2 nanocomposite hollow spheres," Chemistry—An Asian Journal, vol. 5, no. 6, pp. 1466-1474, 2010.

[34] M. M. Uddin, M. A. Hasnat, A. J. F. Samed, and R. K. Majumdar, "Influence of TiO2 and ZnO photocatalysts on adsorption and degradation behaviour of erythrosine," Dyes and Pigments, vol. 7, no. 75, pp. 207-212, 2007.

[35] S. Chakrabarti and B. K. Dutta, "Photocatalytic degradation of model textile dyes in wastewater using ZnO as semiconductor catalyst," Journal of Hazardous Materials, vol. 112, no. 3, pp. 269278, 2004.

[36] G. Li, D. Zhang, and J. C. Yu, "Thermally stable ordered mesoporous CeO2/TiO2 visible-light photocatalysts," Physical Chemistry Chemical Physics, vol. 11, no. 19, pp. 3775-3782,2009.

[37] M. Faisal, S. B. Khan, M. M. Rahman, A. Jamal, K. Akhtar, and M. M. Abdullah, "Role of ZnO-CeO2 nanostructures as a photo-catalyst and chemi-sensor," Journal of Materials Science and Technology, vol. 27, no. 7, pp. 594-600, 2011.

[38] D. Sun, T. T. Meng, T. H. Loong, and T. J. Hwa, "Removal of natural organic matter from water using a nano-structured photocatalyst coupled with filtration membrane," Water Science and Technology, vol. 49, no. 1, pp. 103-110, 2004.

[39] J. S. Jang, J. Lee, H. Ye, F.-R. F. Fan, and A. J. Bard, "Rapid screening of effective dopants for Fe2O3 photocatalysts with scanning electrochemical microscopy and investigation of their photoelectrochemical properties," The Journal of Physical Chemistry C, vol. 113, no. 16, pp. 6719-6724, 2009.

[40] F. Chen and J. Zhao, "Preparation and photocatalytic properties of a novel kind of loaded photocatalyst of TiO2/SiO2/y-Fe2O3" Catalysis Letters, vol. 58, no. 4, pp. 245-247,1999.

[41] B. Pal, M. Sharon, and G. Nogami, "Preparation and characterization ofTiO2/Fe2O3 binary mixed oxides and its photocatalytic properties," Materials Chemistry and Physics, vol. 59, no. 3, pp. 254-261, 1999.

[42] W. S. Tung and W. A. Daoud, "New approach toward nanosized ferrous ferric oxide and Fe3O4-doped titanium dioxide photo-catalysts," ACS Applied Materials and Interfaces, vol. 1, no. 11, pp. 2453-2461, 2009.

[43] Y.-H. Chen and F.-A. Li, "Kinetic study on removal of copper(II) using goethite and hematite nano-photocatalysts," Journal of Colloid and Interface Science, vol. 347, no. 2, pp. 277-281, 2010.

[44] T. Torimoto, T. Sakata, H. Mori, and H. Yoneyama, "Effect of surface charge of4-aminothiophenol-modified PbS microcrys-tal photocatalysts on photoinduced charge transfer," Journal of Physical Chemistry, vol. 98, no. 11, pp. 3036-3043, 1994.

[45] Z. Wang, S. Zhu, S. Zhao, and H. Hu, "Synthesis of core-shell Fe3O4@SiO2@MS (M = Pb, Zn, and Hg) microspheres and their application as photocatalysts," Journal of Alloys and Compounds, vol. 509, no. 24, pp. 6893-6898, 2011.

[46] S. Muthu, P. Maruthamuthu, and P. R. V. Rao, "Fullerenes as photocatalysts: studies on decomposition of water and oxalic acid," Fullerenes, Nanotubes, and Carbon Nanostructures, vol. 1, no. 4, pp. 481-497, 1993.

[47] J. Vakros, G. Panagiotou, C. Kordulis et al., "Fullerene C60 supported on silica and y-alumina catalyzed photooxidations of alkenes," Catalysis Letters, vol. 89, no. 3-4, pp. 269-273, 2003.

[48] M. D. Tzirakis, J. Vakros, L. Loukatzikou et al., "y-alumina-supported [60]fullerene catalysts: synthesis, properties and applications in the photooxidation of alkenes," Journal of Molecular Catalysis A, vol. 316, no. 1-2, pp. 65-74, 2010.

[49] J.-J. Wu and C.-H. Tseng, "Photocatalytic properties of nc-Au/ZnO nanorod composites," Applied Catalysis B, vol. 66, no. 1-2, pp. 51-57, 2006.

[50] H. Li, Z. Bian, J. Zhu, Y. Huo, H. Li, and Y. Lu, "Mesoporous Au/TiO2 nanocomposites with enhanced photocatalytic activity," Journal of the American Chemical Society, vol. 129, no. 15, pp. 4538-4539, 2007.

[51] Z. Xiong, J. Ma, W. J. Ng, T. D. Waite, and X. S. Zhao, "Silver-modified mesoporous TiO2 photocatalyst for water purification," Water Research, vol. 45, no. 5, pp. 2095-2103, 2011.

[52] R. Georgekutty, M. K. Seery, and S. C. Pillai, "A highlyefficient Ag-ZnO photocatalyst: synthesis, properties, and mechanism," The Journal of Physical Chemistry C, vol. 112, no. 35, pp. 1356313570, 2008.

[53] G. Colon, M. Maicu, M. C. Hidalgo, and J. A. Navio, "Cu-doped TiO2 systems with improved photocatalytic activity," Applied Catalysis B, vol. 67, no. 1-2, pp. 41-51, 2006.

[54] T. Sreethawong and S. Yoshikawa, "Comparative investigation on photocatalytic hydrogen evolution over Cu-, Pd-, and Au-loaded mesoporous TiO2 photocatalysts," Catalysis Communications, vol. 6, no. 10, pp. 661-668, 2005.

[55] W.-P. Hsieh, J. R. Pan, C. Huang, Y.-C. Su, and Y.-J. Juang, "Enhance the photocatalytic activity for the degradation of organic contaminants in water by incorporating TiO2 with zero-valent iron," Science of the Total Environment, vol. 408, no. 3, pp. 672-679, 2010.

[56] S. F. Chen, S. J. Zhang, W. Liu, and W. Zhao, "Preparation and activity evaluation of p-n junction photocatalyst NiO/TiO2" Journal of Hazardous Materials, vol. 155, no. 1-2, pp. 320-326, 2008.

[57] M. Xue, L. Huang, J.-Q. Wang et al., "The direct synthesis of mesoporous structured MnO2/TiO2 nanocomposite: a novel visible-light active photocatalyst with largepore size," Nanotech-nology, vol. 19, no. 18, Article ID 185604, 2008.

[58] L. Zhang, D. He, and P. Jiang, "MnO2-doped anatase TiO2—an excellent photocatalyst for degradation of organic contaminants in aqueous solution," Catalysis Communications, vol. 10, no. 10, pp. 1414-1416, 2009.

[59] W.-C. Oh, A.-R. Jung, and W.-B. Ko, "Characterization and relative photonic efficiencies of a new nanocarbon/TiO2 composite photocatalyst designed for organic dye decomposition and bactericidal activity," Materials Science and Engineering C, vol. 29, no. 4, pp. 1338-1347, 2009.

[60] Y. Yu, J. C. Yu, C.-Y. Chan et al., "Enhancement of adsorption and photocatalytic activity of TiO2 by using carbon nanotubes for the treatment of azo dye," Applied Catalysis B, vol. 61, no. 1-2, pp. 1-11, 2005.

[61] L. Jiang and L. Gao, "Fabrication and characterization of ZnO-coated multi-walled carbon nanotubes with enhanced photocatalytic activity," Materials Chemistry and Physics, vol. 91, no. 2-3, pp. 313-316, 2005.

[62] Y. Yu, J. C. Yu, J.-G. Yu et al., "Enhancement of photocatalytic activity of mesoporous TiO2 by using carbon nanotubes," Applied Catalysis A, vol. 289, no. 2, pp. 186-196, 2005.

[63] Y.-J. Xu, Y. Zhuang, and X. Fu, "New insight for enhanced photocatalytic activity of TiO2 by doping carbon nanotubes: a case study on degradation of benzene and methyl orange," The Journal of Physical Chemistry C, vol. 114, no. 6, pp. 2669-2676, 2010.

[64] R. Leary and A. Westwood, "Carbonaceous nanomaterials for the enhancement of TiO2 photocatalysis," Carbon, vol. 49, no. 3, pp. 741-772, 2011.

[65] C. Hu, Y. Z. Wang, and H. X. Tang, "Preparation and characterization of surface bond-conjugated TiO2/SiO2 and photocatalysis for azo dyes," Applied Catalysis B, vol. 30, no. 3-4, pp. 277285, 2001.

[66] C. Hu, Y. Tang, J. C. Yu, and P. K. Wong, "Photocatalytic degradation of cationic blue X-GRL adsorbed on TiO2/SiO2 photocatalyst," Applied Catalysis B, vol. 40, no. 2, pp. 131-140, 2003.

[67] V. Loddo, G. Marcl, L. Palmisano, and A. Sclafani, "Preparation and characterization of Al2O3 supported TiO2 catalysts employed for 4-nitrophenol photodegradation in aqueous medium," Materials Chemistry and Physics, vol. 53, no. 3, pp. 217-224, 1998.

[68] A. S. K. Sinha, N. Sahu, M. K. Arora, and S. N. Upadhyay, "Preparation of egg-shell type Al2O3-supported CdS photocat-alysts for reduction of H2O to H2," Catalysis Today, vol. 69, no. 1-4, pp. 297-305, 2001.

[69] G. C. Bushell, Y. D. Yan, D. Woodfield, J. Raper, and R. Amal, "On techniques for the measurement of the mass fractal dimension of aggregates," Advances in Colloid and Interface Science, vol. 95, no. 1, pp. 1-50, 2002.

[70] L. M. Mosley, K. A. Hunter, and W. A. Ducker, "Forces between colloid particles in natural waters," Environmental Science and Technology, vol. 37, no. 15, pp. 3303-3308, 2003.

[71] J. R. Lead, D. Muirhead, and C. T. Gibson, "Characterization of freshwater natural aquatic colloids by atomic force microscopy (AFM)," Environmental Science and Technology, vol. 39, no. 18, pp. 6930-6936, 2005.

[72] P. K. Naicker, P. T. Cummings, H. Zhang, and J. F. Ban-field, "Characterization of titanium dioxide nanoparticles using molecular dynamics simulations," Journal ofPhysical Chemistry B, vol. 109, no. 32, pp. 15243-15249, 2005.

[73] Q. Chen, C. Saltiel, S. Manickavasagam, L. S. Schadler, R. W. Siegel, and H. Yang, "Aggregation behavior of single-walled carbon nanotubes in dilute aqueous suspension," Journal of Colloid and Interface Science, vol. 280, no. 1, pp. 91-97, 2004.

[74] S. Kerisit and C. Liu, "Molecular simulations of water and ion diffusion in nanosized mineral fractures," Environmental Science and Technology, vol. 43, no. 3, pp. 777-782, 2009.

[75] D. P. Stankus, S. E. Lohse, J. E. Hutchison, and J. A. Nason, "Interactions between natural organic matter and gold nanoparticles stabilized with different organic capping agents," Environmental Science and Technology, vol. 45, no. 8, pp. 3238-3244, 2011.

[76] A. A. Keller, H. Wang, D. Zhou et al., "Stability and aggregation of metal oxide nanoparticles in natural aqueous matrices," Environmental Science and Technology, vol. 44, no. 6, pp. 19621967, 2010.

[77] J. M. Pettibone, D. M. Cwiertny, M. Scherer, and V. H. Grassian, "Adsorption of organic acids on TiO2 nanoparticles: effects of

pH, nanoparticle size, and nanoparticle aggregation," Langmuir, vol. 24, no. 13, pp. 6659-6667, 2008.

[78] B. Xie, Z. Xu, W. Guo, and Q. Li, "Impact of natural organic matter on the physicochemical properties of aqueous C60 nanoparticles," Environmental Science and Technology, vol. 42, no. 8, pp. 2853-2859, 2008.

[79] K. A. Huynh and K. L. Chen, "Aggregation kinetics of citrate and polyvinylpyrrolidone coated silver nanoparticles in monovalent and divalent electrolyte solutions," Environmental Science and Technology, vol. 45, no. 13, pp. 5564-5571, 2011.

[80] W. Vogelsberger, J. Schmidt, and F. Roelofs, "Dissolution kinetics of oxidic nanoparticles: the observation of an unusual behaviour," Colloids and Surfaces A, vol. 324, no. 1-3, pp. 51-57, 2008.

[81] V. L. Pallem, H. A. Stretz, and M. J. M. Wells, "Evaluating aggregation of gold nanoparticles and humic substances using fluorescence spectroscopy," Environmental Science and Technology, vol. 43, no. 19, pp. 7531-7535, 2009.

[82] D. A. Heller, P. W. Barone, J. P. Swanson, R. M. Mayrhofer, and M. S. Strano, "Using Raman spectroscopy to elucidate the aggregation state of single-walled carbon nanotubes," Journal of Physical Chemistry B, vol. 108, no. 22, pp. 6905-6909, 2004.

[83] S. Liufu, H. Xiao, and Y. Li, "Adsorption of poly(acrylic acid) onto the surface of titanium dioxide and the colloidal stability of aqueous suspension," Journal ofColloid and Interface Science, vol. 281, no. 1, pp. 155-163, 2005.

[84] T. Rudalevige, A. H. Francis, and R. Zand, "Spectroscopic studies offullerene aggregates," Journal ofPhysical ChemistryA, vol. 102, no. 48, pp. 9797-9802,1998.

[85] A. Jillavenkatesa and J. F. Kelly, "Nanopowder characterization: challenges and future directions," Journal of Nanoparticle Research, vol. 4, no. 5, pp. 463-468, 2002.

[86] W.-C. Hou and C. T. Jafvert, "Photochemical transformation of aqueous C clusters in sunlight," Environmental Science and Technology, vol. 43, no. 2, pp. 362-367, 2009.

[87] Y. S. Hwang and Q. Li, "Characterizing photochemical transformation of aqueous nC60 under environmentally relevant conditions," Environmental Science and Technology, vol. 44, no. 8, pp. 3008-3013, 2010.

[88] W.-C. Hou and C. T. Jafvert, "Photochemistry of aqueous C60 clusters: evidence of1O2 formation and its role in mediating C60 phototransformation," Environmental Science and Technology, vol. 43, no. 14, pp. 5257-5262, 2009.

[89] X. Qu, Y. S. Hwang, P. J. J. Alvarez, D. Bouchard, and Q. Li, "UV irradiation and humic acid mediate aggregation of aqueous fullerene (nC60) nanoparticles," Environmental Science and Technology, vol. 44, no. 20, pp. 7821-7826, 2010.

[90] C. W. Isaacson and D. C. Bouchard, "Effects of humic acid and sunlight on the generation and aggregation state of aqu/C60 nanoparticles," Environmental Science and Technology, vol. 44, no. 23, pp. 8971-8976, 2010.

[91] Q. Li, B. Xie, S. H. Yu, and Y. Xu, "Kinetics of C60 fullerene dispersion in water enhanced by natural organic matter and sunlight," Environmental Science and Technology, vol. 43, no. 10, pp. 3574-3579, 2009.

[92] X. L. Qu, P. J. J. Alvarez, and Q. L. Li, "Impact of sunlight and humic acid on the deposition kinetics of aqueous fullerene nanoparticles (nC60)," Environmental Science & Technology, vol. 46, no. 24, pp. 13455-13462, 2012.

[93] J. Gao, S. Youn, A. Hovsepyan et al., "Dispersion and toxicity of selected manufactured nanomaterials in natural river water

samples: effects of water chemical composition," Environmental Science and Technology, vol. 43, no. 9, pp. 3322-3328, 2009.

[94] T. K. Darlington, A. M. Neigh, M. T. Spencer, O. T. Nguyen, and S. J. Oldenburg, "Nanoparticle characteristics affecting environmental fate and transport through soil," Environmental Toxicology and Chemistry, vol. 28, no. 6, pp. 1191-1199, 2009.

[95] R. J. Griffitt, J. Luo, J. Gao, J.-C. Bonzongo, and D. S. Barber, "Effects of particle composition and species on toxicity of metallic nanomaterials in aquatic organisms," Environmental Toxicology and Chemistry, vol. 27, no. 9, pp. 1972-1978, 2008.

[96] Y. Zhang, Y. Chen, P. Westerhoff, K. Hristovski, and J. C. Crittenden, "Stability of commercial metal oxide nanoparticles in water," Water Research, vol. 42, no. 8-9, pp. 2204-2212, 2008.

[97] J. Liu, D. M. Aruguete, M. Murayama, and M. F. Hochella Jr., "Influence of size and aggregation on the reactivity of an environmentally and industrially relevant nanomaterial (PbS)," Environmental Science and Technology, vol. 43, no. 21, pp. 81788183, 2009.

[98] J. Jiang, G. Oberdörster, and P. Biswas, "Characterization of size, surface charge, and agglomeration state of nanoparticle dispersions for toxicological studies," Journal of Nanoparticle Research, vol. 11, no. 1, pp. 77-89, 2009.

[99] L. K. Adams, D. Y. Lyon, and P. J. J. Alvarez, "Comparative eco-toxicity of nanoscale TiO2, SiO2, and ZnO water suspensions," Water Research, vol. 40, no. 19, pp. 3527-3532, 2006.

[100] M. Baalousha, A. Manciulea, S. Cumberland, K. Kendall, and J. R. Lead, "Aggregation and surface properties of iron oxide nanoparticles: influence of pH and natural organic matter," Environmental Toxicology and Chemistry, vol. 27, no. 9, pp. 18751882, 2008.

[101] E. Illes and E. Tombacz, "The effect of humic acid adsorption on pH-dependent surface charging and aggregation of magnetite nanoparticles," Journal of Colloid and Interface Science, vol. 295, no. 1, pp. 115-123, 2006.

[102] H. Wang, R. L. Wick, and B. Xing, "Toxicity of nanoparticulate and bulk ZnO, Al2O3 and TiO2 to the nematode Caenorhabditis elegans" Environmental Pollution, vol. 157, no. 4, pp. 1171-1177, 2009.

[103] V. I. Slaveykova, K. Startchev, and J. Roberts, "Amine- and carboxyl- quantum dots affect membrane integrity of bacterium Cupriavidus metallidurans CH34," Environmental Science and Technology, vol. 43, no. 13, pp. 5117-5122, 2009.

[104] R. F. Domingos, N. Tufenkji, and K. J. Wilkinson, "Aggregation of titanium dioxide nanoparticles: role of a fulvic acid," Environmental Science and Technology, vol. 43, no. 5, pp. 1282-1286, 2009.

[105] K. A. D. Guzman, M. P. Finnegan, and J. F. Banfield, "Influence of surface potential on aggregation and transport of titania nanoparticles," Environmental Science and Technology, vol. 40, no. 24, pp. 7688-7693, 2006.

[106] D. Lin, N. Liu, K. Yang, L. Zhu, Y. Xu, and B. Xing, "The effect of ionic strength and pH on the stability of tannic acid-facilitated carbon nanotube suspensions," Carbon, vol. 47, no. 12, pp. 28752882, 2009.

[107] J. Fang, X.-Q. Shan, B. Wen, J.-M. Lin, and G. Owens, "Stability of titania nanoparticles in soil suspensions and transport in saturated homogeneous soil columns," Environmental Pollution, vol. 157, no. 4, pp. 1101-1109, 2009.

[108] K. Yang and B. Xing, "Sorption of phenanthrene by humic acid-coated nanosized TiO2 and ZnO," Environmental Science and Technology, vol. 43, no. 6, pp. 1845-1851, 2009.

[109] Y. T. He, J. Wan, and T. Tokunaga, "Kinetic stability of hematite nanoparticles: the effect of particle sizes," Journal of Nanoparticle Research, vol. 10, no. 2, pp. 321-332, 2008.

[110] H. Zeng, A. Singh, S. Basak et al., "Nanoscale size effects on uranium(VI) adsorption to hematite," Environmental Science and Technology, vol. 43, no. 5, pp. 1373-1378, 2009.

[111] S. Ghosh, H. Mashayekhi, B. Pan, P. Bhowmik, and B. Xing, "Colloidal behavior of aluminum oxide nanoparticles as affected by pH and natural organic matter," Langmuir, vol. 24, no. 21, pp. 12385-12391, 2008.

[112] J. Feitosa-Felizzola, K. Hanna, and S. Chiron, "Adsorption and transformation of selected human-used macrolide antibacterial agents with iron(III) and manganese(IV) oxides," Environmental Pollution, vol. 157, no. 4, pp. 1317-1322, 2009.

[113] B. S. Necula, I. Apachitei, L. E. Fratila-Apachitei, C. Teodosiu, and J. Duszczyk, "Stability of nano-/microsized particles in deionized water and electroless nickel solutions," Journal of Colloid and Interface Science, vol. 314, no. 2, pp. 514-522, 2007.

[114] M. Ravichandran, G. R. Aiken, M. M. Reddy, and J. N. Ryan, "Enhanced dissolution of cinnabar (mercuric sulfide) by dissolved organic matter isolated from the Florida Everglades," Environmental Science and Technology, vol. 32, no. 21, pp. 33053311, 1998.

[115] R. A. French, A. R. Jacobson, B. Kim, S. L. Isley, L. Penn, and P. C. Baveye, "Influence of ionic strength, pH, and cation valence on aggregation kinetics of titanium dioxide nanoparticles," Environmental Science and Technology, vol. 43, no. 5, pp. 13541359, 2009.

[116] A. M. El Badawy, T. P. Luxton, R. G. Silva, K. G. Scheckel, M. T. Suidan, and T. M. Tolaymat, "Impact of environmental conditions (pH, ionic strength, and electrolyte type) on the surface charge and aggregation of silver nanoparticles suspensions," Environmental Science and Technology, vol. 44, no. 4, pp. 12601266, 2010.

[117] B. J. R. Thio, M. O. Montes, M. A. Mahmoud, D. W. Lee, D. X. Zhou, and A. A. Keller, "Mobility of capped silver nanoparticles under environmentally relevant conditions," Environmental Science and Technology, vol. 46, no. 13, pp. 6985-6991, 2012.

[118] N. B. Saleh, L. D. Pfefferle, and M. Elimelech, "Aggregation kinetics of multiwalled carbon nanotubes in aquatic systems: measurements and environmental implications," Environmental Science and Technology, vol. 42, no. 21, pp. 7963-7969, 2008.

[119] H. Hyung and J.-H. Kim, "Natural organic matter (NOM) adsorption to multi-walled carbon nanotubes: effect of NOM characteristics and water quality parameters," Environmental Science and Technology, vol. 42, no. 12, pp. 4416-4421, 2008.

[120] K. L. Chen and M. Elimelech, "Relating colloidal stability of fullerene (C60) nanoparticles to nanoparticle charge and elec-trokinetic properties," Environmental Science and Technology, vol. 43, no. 19, pp. 7270-7276, 2009.

[121] Q. Xiang, J. Yu, and P. K. Wong, "Quantitative characterization of hydroxyl radicals produced by various photocatalysts," Journal of Colloid and Interface Science, vol. 357, no. 1, pp. 163-167, 2011.

[122] K. L. Chen and M. Elimelech, "Aggregation and deposition kinetics of fullerene (C60) nanoparticles," Langmuir, vol. 22, no. 26, pp. 10994-11001, 2006.

[123] N. Saleh, H.-J. Kim, T. Phenrat, K. Matyjaszewski, R. D. Tilton, and G. V. Lowry, "Ionic strength and composition affect the mobility of surface-modified fe0 nanoparticles in water-saturated sand columns," Environmental Science and Technology, vol. 42, no. 9, pp. 3349-3355, 2008.

[124] K. L. Chen, B. A. Smith, W. P. Ball, and D. H. Fairbrother, "Assessing the colloidal properties of engineered nanoparticles in water: case studies from fullerene C60 nanoparticles and carbon nanotubes," Environmental Chemistry, vol. 7, no. 1, pp. 10-27, 2010.

[125] B. Derjaguin and L. Landau, "Theory of the stability of strongly charged lyophobic sols and of the adhesion of strongly charged particles in solutions of electrolytes," Progress in Surface Science, vol. 43, no. 1-4, pp. 30-59,1993.

[126] J. N. Ryan and M. Elimelech, "Colloid mobilization and transport in groundwater," Colloids and Surfaces A, vol. 107, pp. 1-56, 1996.

[127] J. Brant, H. Lecoanet, and M. R. Wiesner, "Aggregation and deposition characteristics of fullerene nanoparticles in aqueous systems," Journal of Nanoparticle Research, vol. 7, no. 4-5, pp. 545-553, 2005.

[128] W. B. Hardy, "A preliminary investigation of the conditions which determine the stability of irreversible hydrosols," Journal ofPhysical Chemistry, vol. 4, no. 4, pp. 235-253,1900.

[129] M. Sano, J. Okamura, and S. Shinkai, "Colloidal nature of singlewalled carbon nanotubes in electrolyte solution: the Schulze-Hardy rule," Langmuir, vol. 17, no. 22, pp. 7172-7173, 2001.

[130] K. L. Chen and M. Elimelech, "Influence of humic acid on the aggregation kinetics of fullerene (C60) nanoparticles in monovalent and divalent electrolyte solutions," Journal of Colloid and Interface Science, vol. 309, no. 1, pp. 126-134, 2007.

[131] H. Hyung, J. D. Fortner, J. B. Hughes, and J.-H. Kim, "Natural organic matter stabilizes carbon nanotubes in the aqueous phase," Environmental Science and Technology, vol. 41, no. 1, pp. 179-184, 2007.

[132] K. J. Wilkinson, J.-C. Negre, and J. Buffle, "Coagulation of colloidal material in surface waters: the role of natural organic matter," Journal of Contaminant Hydrology, vol. 26, no. 1-4, pp. 229-243,1997.

[133] R. L. Johnson, G. O. Johnson, J. T. Nurmi, and P. G. Tratnyek, "Natural organic matter enhanced mobility of nano zerovalent iron," Environmental Science and Technology, vol. 43, no. 14, pp. 5455-5460, 2009.

[134] A. Deonarine, B. L. T. Lau, G. R. Aiken, J. N. Ryan, and H. Hsu-Kim, "Effects of humic substances on precipitation and aggregation of zinc sulfide nanoparticles," Environmental Science and Technology, vol. 45, no. 8, pp. 3217-3223, 2011.

[135] Y. Zhang, Y. Chen, P. Westerhoff, and J. Crittenden, "Impact of natural organic matter and divalent cations on the stability of aqueous nanoparticles," Water Research, vol. 43, no. 17, pp. 4249-4257, 2009.

[136] J. I. Paredes and M. Burghard, "Dispersions of individual singlewalled carbon nanotubes of high length," Langmuir, vol. 20, no. 12, pp. 5149-5152, 2004.

[137] H. Wang, W. Zhou, D. L. Ho et al., "Dispersing single-walled carbon nanotubes with surfactants: a small angle neutron scattering study," Nano Letters, vol. 4, no. 9, pp. 1789-1793,2004.

[138] M. F. Islam, E. Rojas, D. M. Bergey, A. T. Johnson, and A. G. Yodh, "High weight fraction surfactant solubilization of singlewall carbon nanotubes in water," Nano Letters, vol. 3, no. 2, pp. 269-273, 2003.

[139] L. Jiang, L. Gao, and J. Sun, "Production of aqueous colloidal dispersions of carbon nanotubes," Journal ofColloid and Interface Science, vol. 260, no. 1, pp. 89-94, 2003.

[140] V. C. Moore, M. S. Strano, E. H. Haroz et al., "Individually suspended single-walled carbon nanotubes in various surfactants," Nano Letters, vol. 3, no. 10, pp. 1379-1382, 2003.

[141] V. Y. Starchenko, L. A. Bulavin, Y. P. Boiko, V N. Moraru, and N. I. Lebovka, "Conductometric and gravimetric studies of the kinetics of graphite sedimentation in aqueous dispersions," Colloid Journal, vol. 67, no. 6, pp. 755-759, 2005.

[142] O. Matarredona, H. Rhoads, Z. Li, J. H. Harwell, L. Balzano, and D. E. Resasco, "Dispersion of single-walled carbon nanotubes in aqueous solutions of the anionic surfactant NaDDBS," Journal ofPhysical Chemistry B, vol. 107, no. 48, pp. 13357-13367, 2003.

[143] D. Bouchard, W. Zhang, T. Powell, and U.-S. Rattanaudompol, "Aggregation kinetics and transport of single-walled carbon nanotubes at low surfactant concentrations," Environmental Science and Technology, vol. 46, no. 8, pp. 4458-4465, 2012.

[144] S. H. Othman, S. A. Rashid, T. I. M. Ghazi, and N. Abdullah, "Dispersion and stabilization of photocatalytic TiO2 nanoparticles in aqueous suspension for coatings applications," Journal of Nanomaterials, vol. 2012, Article ID 718214,10 pages, 2012.

[145] N. B. Saleh, L. D. Pfefferle, and M. Elimelech, "Influence of biomacromolecules and humic acid on the aggregation kinetics of single-walled carbon nanotubes," Environmental Science and Technology, vol. 44, no. 7, pp. 2412-2418, 2010.

[146] C. R. O'Melia, "Aquasols: the behavior of small particles in aquatic systems," Environmental Science and Technology, vol. 14, no. 9, pp. 1052-1060,1980.

[147] S. Ottofuelling, F. von der Kammer, and T. Hofmann, "Commercial titanium dioxide nanoparticles in both natural and synthetic water: comprehensive multidimensional testing and prediction of aggregation behavior," Environmental Science and Technology, vol. 45, no. 23, pp. 10045-10052, 2011.

[148] H. P. Jarvie, H. Al-Obaidi, S. M. King et al., "Fate of silica nanoparticles in simulated primary wastewater treatment," Environmental Science and Technology, vol. 43, no. 22, pp. 86228628, 2009.

[149] Y. Yang, C. Q. Zhang, and Z. Q. Hu, "Impact of metallic and metal oxide nanoparticles on wastewater treatment and anaerobic digestion," Environmental Science: Processes & Impacts, vol. 15, no. 1, pp. 39-48, 2013.

[150] B. Kim, C.-S. Park, M. Murayama, and M. F. Hochella Jr., "Discovery and characterization of silver sulfide nanoparticles in final sewage sludge products," Environmental Science and Technology, vol. 44, no. 19, pp. 7509-7514, 2010.

[151] E. Lombi, E. Donner, E. Tavakkoli et al., "Fate of zinc oxide nanoparticles during anaerobic digestion of wastewater and post-treatment processing of sewage sludge," Environmental Science and Technology, vol. 46, no. 16, pp. 9089-9096, 2012.

[152] R. Kaegi, A. Voegelin, B. Sinnet et al., "Behavior of metallic silver nanoparticles in a pilot wastewater treatment plant," Environmental Science and Technology, vol. 45, no. 9, pp. 39023908, 2011.

[153] M. A. Kiser, D. A. Ladner, K. D. Hristovski, and P. K. Westerhoff, "Nanomaterial transformation and association with fresh and freeze-dried wastewater activated sludge: implications for testing protocol and environmental fate," Environmental Science and Technology, vol. 46, no. 13, pp. 7046-7053, 2012.

[154] J. L. Zou, Y. Dai, B. J. Jiang et al., "Recovery of silicon from sewage sludge for production of high-purity nano-SiO2," Chemosphere, vol. 90, pp. 2332-2339, 2013.

[155] F. Gomez-Rivera, J. A. Field, D. Brown, and R. Sierra-Alvarez, "Fate of cerium dioxide (CeO2) nanoparticles in municipal wastewater during activated sludge treatment," Bioresource Technology, vol. 108, pp. 300-304, 2012.

[156] A. Garcia, L. Delgado, J. A. Tora et al., "Effect of cerium dioxide, titanium dioxide, silver, and gold nanoparticles on the activity

of microbial communities intended in wastewater treatment," Journal of Hazardous Materials, vol. 199-200, pp. 64-72, 2012.

[157] Z. Liang, A. Das, and Z. Hu, "Bacterial response to a shock load of nanosilver in an activated sludge treatment system," Water Research, vol. 44, no. 18, pp. 5432-5438, 2010.

[158] H. Mu and Y. Chen, "Long-term effect of ZnO nanoparticles on waste activated sludge anaerobic digestion," Water Research ,vol. 45, no. 17, pp. 5612-5620, 2011.

[159] G. Liu, D. Wang, J. Wang, and C. Mendoza, "Effect of ZnO particles on activated sludge: role of particle dissolution," Science of the Total Environment, vol. 409, no. 14, pp. 2852-2857, 2011.

[160] H. Mu, Y. Chen, and N. Xiao, "Effects of metal oxide nanoparticles (TiO2, Al2O2, SiO2 and ZnO) on waste activated sludge anaerobic digestion," Bioresource Technology, vol. 102, no. 22, pp. 10305-10311, 2011.

[161] C. Wang, X. Zhang, H. Liu, X. Li, W. Li, and H. Xu, "Reaction kinetics of photocatalytic degradation of sulfosalicylic acid using TiO2 microspheres," Journal of Hazardous Materials, vol. 163, no. 2-3, pp. 1101-1106, 2009.

[162] X. Z. Li, H. Liu, L. F. Cheng, and H. J. Tong, "Kinetic behaviour of the adsorption and photocatalytic degradation of salicylic acid in aqueous TiO2 microsphere suspension," Journal of Chemical Technology and Biotechnology, vol. 79, no. 7, pp. 774781, 2004.

[163] R. Razali, A. K. Zak, W. H. A. Majid, and M. Darroudi, "Solvothermal synthesis of microsphere ZnO nanostructures in DEA media," Ceramics International, vol. 37, no. 8, pp. 36573663, 2011.

[164] C.-L. Kuo, T.-J. Kuo, and M. H. Huang, "Hydrothermal synthesis of ZnO microspheres and hexagonal microrods with sheetlike and platelike nanostructures," Journal of Physical Chemistry B, vol. 109, no. 43, pp. 20115-20121, 2005.

[165] W. Lu, S. Gao, and J. Wang, "One-pot synthesis of Ag/ZnO self-assembled 3D hollow microspheres with enhanced photocat-alytic performance," The Journal of Physical Chemistry C, vol. 112, no. 43, pp. 16792-16800, 2008.

[166] M. A. Kiser, P. Westerhoff, T. Benn, Y. Wang, J. Pérez-Rivera, and K. Hristovski, "Titanium nanomaterial removal and release from wastewater treatment plants," Environmental Science and Technology, vol. 43, no. 17, pp. 6757-6763, 2009.

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