About the Author

About the Author Nashaat N Nassar Nashaat N Nassar is Assistant Professor in the Department of Chemical and Petroleum Engineering at the Schulich School of Engineering in the University of Calgary (AB, Canada). He received his BSc in chemical engineering from An-Najah National University (Nablus, Palestine) in 2000, his MSc in chemical engineering from McGill University (QC, Canada) in 2003, and his PhD in chemical engineering from the University of Calgary in 2008. His research interests are in the areas of nanotechnology and its applications for energy and the environment, especially in heavy oil upgrading and recovery, air pollution control and wastewater treatment; polymer processing; and nanocomposites. He is also an expert in chemical engineering education, especially in process development, effluent treatment processes and process design, as he taught more than 20 different courses at both graduate and undergraduate levels. He has won a number of awards and prizes. He has given more than 40 conference presentations and delivered a number of keynote speeches at local and international levels. He is an author/coauthor of more than 45 refereed scientific journal articles and book chapters, and he is a professional member of the Association of Professional Engineers and Geoscientists of Alberta.

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Chapter

3 The application of nanoparticles for wastewater remediation

Synthesis of nanomaterials 56 Application of nanotechnology in wastewater remediation 57 Nanoparticles as nanoadsorbents for organic & inorganic contaminants in wastewater 58 Nanoparticles as nanocatalysts for oxidizing & breaking down contaminants 60 Merging nanoparticles with membrane filtration techniques 60 Nanoparticles as nanosensors for rapid detection & identification of contaminants & pathogens61 Impacts of nanoparticles on health & the environment 62

Nashaat N Nassar Nanoparticles represent a promising new technology for wastewater remediation, not only because of their high treatment efficiency, but also for their cost–effectiveness, as they have the flexibility for in situ and ex situ applications. In this chapter, we briefly introduced the main synthesis techniques for nanoparticle formation, their potential benefits in environmental clean-up, and their recent advances and applications in wastewater treatment. These advances range from the direct applications of synthesized nanoparticles as adsorbents for removing toxic contaminants or as catalysts to oxidize and break down noxious contaminants in wastewater, to integrating nanoparticles into conventional treatment technologies, such as the composite photocatalytic membrane that combine the separation technology with photocatalytic activity. Finally, the impact of nanoparticles on the environment and human health is briefly discussed.

doi:10.4155/EBO.13.373

© 2013 Future Science

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Nassar There is no doubt that the demand for fresh water has increased tremendously, with agricultural, industrial and domestic sectors consuming 70, 22 and 8% of the available freshwater, respectively [1]. Consequently, huge amounts of wastewater effluent containing various types of toxic contaminants are produced. In addition, the need for clean water is increasing due to the increase of population, draughts and the contamination of conventional water sources. Therefore, improvement of the conventional treatment processes and recyclability of wastewater is essential for meeting the worldwide demand for clean water. A number of chemical, physical and biological conventional treatment methods have been developed for the removal of various contaminants from wastewater [2]; including coagulation–flocculation, precipitation, oxidation, ion-exchange processes, electrolytic methods, reverse osmosis and membrane filtration, adsorption onto activated carbon, low-cost adsorbents, and so forth. However, most of these techniques are found to be limited owing to their high capital and operating cost, ineffectiveness in meeting stringent environmental standard and the huge amount of sludge they could generate. Recently, the application of nanotechnology (i.e., the technology related to the preparation of materials at nanoscale, 1–100 nm) has emerged as a fascinating area of interest for removal of various contaminants from wastewater effluents [3–5]; indeed, nanoparticles are the drivers of the nanotechnology revolution. They display unique and unexpected properties that differ from their bulk material counterparts [6]. Furthermore, nanoparticles can be functionalized with various chemical groups to enhance their affinity toward a target contaminant [7,8]. Consequently, nanoparticles provide high capacity/ selectivity and recyclable ligands for various toxic contaminants in wastewater. Given their large surface area, and size- and shape-dependent catalytic properties, nanoparticles can also be employed as adsorbents and/or catalysts for efficient water clean-up. In this chapter, we briefly introduce the recent advances and applications of nanotechnology for wastewater treatment. These applications include the direct application of the synthesized nanoparticles as adsorbents for removing toxic contam Nanotechnology: a new area of science and inants, as catalysts to oxidize and break engineering that involves the discovery and down noxious contaminants in wastewater, ability to manipulate, control and apply nanoscale and integrating nanoparticles into material in a useful way. The name ‘nano’ comes from conventional treatment technology, such the size of molecules, which is measured in nanometers as the composite photocatalytic membrane or one billionth of a meter (i.e., 1 nm = 1 × 10 -9 m). that combines the separation technology Nanoparticles: small atomic clusters in the range of Nanoparticles could be employed as adsorbents/ catalysts for decomposing organic pollutants and adsorptive removal of heavy metals and salts.

1–100 nm with size-dependent properties.

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The application of nanoparticles for wastewater remediation of the membrane with the photocatalytic activity of the nanoparticles [9,10]. It is should be noted here that the application of nanotechnology is not limited to wastewater treatment and remediation, but also includes energy storage, production and conversion, minerals, agribusiness, health and medical devices, materials and manufacturing, electronics, and information and communication technology, and so forth [11]; nanotechnology can be easily incorporated with currently available technologies to enhance it. In addition, nanotechnology has the potential to solve many current problems by virtue of smaller, lighter, faster, stronger, and better-performing materials, components and systems [12]. Therefore, the applications of nanotechnology are growing across a number of industry sectors worldwide, and it is expected to impact on everyday life, as illustrated in Figure 3.1 [12]. Figure 3.1. The future applications of nanotechnology in everyday life. Nanoparticle paint to prevent corrosion

Thermochromic glass to regulate the influx of light

Piezo mats prevent annoying vibrations Hip joints made from biocompatible materials Helmet maintains contact with the wearer Intelligent clothing measures pulse and respiration Bucky-tube frame is as light as a feather, yet strong Fuel cells provide power for mobile phones and vehicles

Magnetic layers for compact data memory

Courtesy of the European Commission [12].


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Nassar Synthesis of nanomaterials Several approaches have been developed for the synthesis and structuring of nanomaterials [3], but the challenge remains in the control over the particle size, composition, extent of dispersion, stability and surface functionality [13]. The nanomaterial synthesis can be grouped into top-down and bottom-up approaches, as seen in Figure 3.2. Top-down approaches are defined as those by which nanomaterials are directly prepared from bulk materials via the generation of isolated atoms by using various distribution techniques that involve physical methods such as milling or grinding, laser beam processing, repeated quenching and photolithography [14]. Bottom-up methods involve molecular components as starting materials linked with chemical reactions, nucleation and growth processes, and then arranged and assembled to form larger-sized materials [15–17]. It should be noted here that the preparation methods play a vital key role in determining the size distribution, stability, morphology and surface chemistry of nanomaterials, and, consequently, their adsorption affinity and catalytic activity. Excellent review papers on the synthesis and applications of nanoparticles are available [4,5,18–21]. Several types of nanomaterials have been evaluated or employed for wastewater treatment including [13,22–24]: Metal-containing nanoparticles (e.g., metals, bimetallic and mixed metal oxide)

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Carbonaceous nanomaterials (e.g., fullerenes, single or multiwalls nanotube)

n

Nanoclay (e.g., zeolites and montmorillonite)

n

Dendrimers

n

Figure 3.2. The top-down and bottom-up approaches for the synthesis of nanomaterials. 100 µm 10 µm

Top-down

1 µm 0.1 µm 100 nm 10 nm

Nanoscale region

1 nm 0.1 nm 0.01 nm

Bottom-up

These nanomaterials have a broad range and unique physicochemical properties that make them particularly attractive for contaminant removal from wastewater. Figure 3.3 shows a schematic representation of the different type of nanomaterials being evaluated as potential materials for wastewater treatment. While these nanomaterials showed high treatment efficiency, their separation and recovery after the treatment processes tend to be costly and time consuming [8], particularly for ex situ application. Accordingly, more laboratory investigations and pilot-scale

0.001 nm

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The application of nanoparticles for wastewater remediation Figure 3.3. Nanomaterials being evaluated as functional materials for wastewater treatment. A

B

E

C

D

F

Nanoparticle

(A) The structure of C60 ‘Buckminsterfullerene’; (B) single-walled carbon nanotube; (C) multiwalled carbon nanotube; (D) nanoclay ‘structure of zeolite’; (E) dendrimer; and (F) metal, bimetallic or metal oxide nanoparticles.

testing will be needed to gain a clear understanding of recycling and regeneration potential to implement this economically beneficent technique into the existing wastewater treatment industry.

Application of nanotechnology in wastewater remediation Nanotechnology is playing or shall be playing a vital role in solving problems related to energy, water and health [5,6,12,25]. The main potential benefits of nanotechnology in the environmental clean-up include [3,5,6,13]: n Early treatment and remediation, hence time saving Stronger, lighter and more effective nanomaterials

n

Smaller size, which results in more accurate and more sensing and monitoring

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Cost-effectiveness, owing to the possibility of in situ application and consequently low energy requirement

n

Simplicity treatment and space saving

n

The application of nanotechnology in wastewater remediation can be summarized as follows [11]:


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Nassar Employing the nanoparticles as nanoadsorbents for adsorbing pollutants (usually metals) by sequestration

n

Applying nanoparticles as nanocatalysts to oxidize and breakdown contaminants

n

Merging nanoparticles with membrane filtration techniques

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Using nanoparticles as nanosensors for rapid detection and identification of pathogens and viruses

n

Nanoparticles as nanoadsorbents for organic & inorganic contaminants in wastewater Owing to their small size, high surface area and surface multifunctionalities, the nanoparticle surface poses a significant number of active sites for interaction with different chemical species, as shown in Figure 3.4 [26]. These features make the nanoparticles not just efficient adsorbents for various contaminants in wastewater, but also time effective, as the small size of nanoadsorbents enhances the degree of dispersion and surface accessibility, which in turn fastens the adsorption rate and facilitates the adsorption efficiency, owing to the dominated external adsorption [3]. Furthermore, metal and metal oxide nanoparticle surface functionality can be altered by pH of the solution, as the surface can undergo protonation or deprotonation, depending on solution pH and the point of surface Figure 3.4. The multifunctionalities of metal oxide nanoparticle surface. Phosphonic acid O P O

O O

HO

C

O Trimethoxy silane

Si O

O

O

O

Nanoparticle

O

O

O O

NH

Si

Amine

Carboxylic acid O

O

O C

S

S

Dopamine

O C Cysteine

Adapted from [26].

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The application of nanoparticles for wastewater remediation charge of the nanoparticle [3]. Accordingly, the surface of nanoadsorbents can be either acidic with a positive charge attracting anions, or basic with a negative surface potential attracting cations. For instance, adsorptive removal of heavy metals, such as nickel, cadmium, cobalt and lead onto Fe3O4 nanoadsorbents, was favored by the increase of pH, where the surface has a negative charge. The adsorption of the same heavy metals was retarded at lower pH values where the nanoadsorbent surface had a positive charge [27,28]. Therefore, the adsorption of metal ions or functionalized contaminants by nanoadsorbents depends significantly on the electrostatic interactions between the nanoadsorbents surface and the ions. One can anticipate that as the surface charge of the nanoadsorbents changes with the pH, the interaction with various contaminants is affected. This feature enhances the adsorptive tendency of nanoadsorbents towards organic compounds, whereby surface-modified nanoadsorbents with surfactant such as cetyltrimethylammonium bromide and hexadecyltrimethylammonium bromide would favor the removal of organic pollutants and enhance the removal of complex compounds such as As(III) [29,30]. In addition, the regeneration of the spent nanoadsorbents and recovery of the adsorbate would be possible by manipulating the solution pH. Several types of metals, bimetallic, metal oxides and mixed metal oxides, have been employed successfully for removing various contaminants from wastewater [3,4,6,8,13,19,20]. Due to their natural availability, magnetic property and surface multifunctionalities, iron-based nanoparticles represent the majority of the nanoadsorbents employed for environmental remediation [3]. Iron-based nanoadsorbents such as iron oxides, oxyhydroxides and hydroxides are the most widely used nanoparticles for wastewater treatment, as they showed higher adsorption affinity, capacity and faster adsorption rate in comparison to many other adsorbents [3]. Furthermore, owing to their magnetic properties, iron oxide nanoparticles (especially maghemite [g-Fe2O3] and magnetite [Fe3O4]) can be recovered easily after their use with a magnet. However, particle aggregation is a major drawback for nanoadsorbents and poses a serious problem that could challenge the viability of the use of nanoparticles in wastewater treatment. One approach of minimizing particle aggregation is to prepare them in situ, where the treatment is needed [5]. Furthermore, particle aggregation could also be minimized by using a capping agent, such as a surfactant [16], or incorporating nanoparticles into a support Nanoparticles offer a great opportunity for such as clay [25,31]. This would provide a mitigating fouling and improving membrane better dispersion ability of nanoparticles performance. The incorporation of nanoparticles in membrane structure serves to improve membrane and eliminate nanoparticle coaggregation. surface nanostructure and adhesion property.


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Nassar Furthermore, from a practical point of view, clay-supported nanoparticles would help the storage and pelletization of the nanoadsorbents [32].

Nanoparticles as nanocatalysts for oxidizing & breaking down contaminants Nanoparticles of metal, bimetallic and metal oxide are intrinsically active catalysts for oxidation reactions [6]. They pose high catalytic activity by which the contaminants can be oxidized to a less toxic product or converted into an environmentally neutral final product. Their catalytic activity stems from their nanoscale size and surface functionality. At nanoscale, materials show unique characteristics and possess a large surface-area-to-volume ratio. This gives the nanoparticles better catalytic potential and more active sites than classical catalysts. Nanoparticles are of particular interest for the degradation of organic contaminants [5]. An effective illumination is usually necessary for such treatment process. Semiconductor-based nanoparticles have emerged as a promising photocatalyst for contaminant photooxidation and degradation. In particular, TiO2 and different phases of iron oxides [8], zero-valent iron [5] and bimetallic nanoparticles [33] were successfully employed as both effective reductants and catalysts for a number of wastewater contaminants, such as toxic metal ions (e.g., Cr(VI), Ag(I) and Pt(II)), inorganic anions (e.g., nitrates) and chlorinated organic compounds (e.g., chlorinated alkanes and alkenes, chlorinated benzenes, pesticides). Like conventional catalysts, nanocatalyst application faces some challenges; including catalyst deactivation and environmental impact of photocatalytic processes [34]. These limitations should be taken into consideration for the application of nanocatalysts at the commercial scale. However, to date, no viable scale-up process or pilot plant has yet been developed successfully using this technology. Accordingly, several engineering and scale-up issues must be addressed before commercial process units can be realized [5,35]. Merging nanoparticles with membrane filtration techniques Membrane filtration plays a very important role in removing various contaminants and producing high-quality pure water [2]. Recently, membrane filtration is increasingly being utilized in drinking water and wastewater treatment due to its effectiveness in removing particulate matter, divalent and monovalent ions, and waterborne pathogens such as viruses and bacteria, which are hard to remove by conventional wastewater treatment processes [2] . Nanoparticles can be used as nanosensors for However, membrane fouling is still one of the detecting and identifying waterborne key challenges for the viability of membrane biological threat contaminants, such as viruses or pathogens.

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The application of nanoparticles for wastewater remediation filtration processes [2]. Membrane fouling can result in a shorter lifetime of the membrane, decline in clean water production, higher energy demand and, consequently, higher capital and operating costs. Therefore, developing a new technology or modifying the currently available membrane technology is of paramount importance for sustaining the application of membrane filtration in wastewater treatment plants and meeting the demand for clean water. Nanoparticles offer a great opportunity for mitigating fouling and improving membrane performance [36]. The incorporation of nanoparticles in membrane structure serves to improve membrane surface nanostructure and adhesion property. This results in the enhancement of the ‘molecular sieve’ property, better salt rejection selectivity, enhanced catalytic activity of membrane surfaces towards organic contaminants, and improvement in surface hydrophilicity in order to inhibit the attachment of microorganisms (e.g., viruses and bacteria) and to inactivate microorganisms that attach on the membrane surface [36]. The simplicity of this technique allows for the convenient modification of the existing membrane technologies in drinking water and wastewater treatment plants. Carbon nanotube filters were successfully synthesized for the effective removal of bacteria pathogens and poliovirus sabin 1 from contaminated water [37]. The nanostructure surface modification of nanofiltration membrane for improving salt rejection selectivity and microporousity were achieved [38,39]. More than 70% rejection to NaCl and less than 40% rejection to CaCl2 for the same aqueous solution were reported [39]. This monovalent/divalent selectivity is very crucial for minimizing membrane fouling by calcium carbonate or sulphate salts and maintaining the Na/Ca ratio to a proper level for agricultural use. Nanosilver impregnated polysulfone ultrafiltration membranes were found to be very effective against Escherichia coli K12 and Pseudomonas mendocina bacteria strain, and efficient for virus removal and biofouling resistance [40]. This was attributed to the release of Ag+ ions, which prohibited the attachment of bacteria to the membrane surfaces [40]. Recently, composite photocatalytic membranes that combine the separation process provided by the membrane and the photocatalytic activity of catalysts have been successfully fabricated with the capability of degrading organic contaminates, lowering the permeate turbidity (lower than 0.75 nephelometric turbidity units) and exhibited excellent antifouling ability [41,42].

Nanoparticles as nanosensors for rapid detection & identification of contaminants & pathogens Nanoparticles can be employed as nanosensors that detect the presence of particular contaminants in water such as viruses or pathogens through changes in either a physical property of the surrounding media or a


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Nassar magnetic property of the nanoparticles itself, in the case of magnetic particles [6]. For instance, nanosensors can be used for detecting and identifying waterborne biological threat contaminants, so that immediate water remediation can be taken. Furthermore, nanosensors could be employed in situ or on-site for water quality analysis [43]. Therefore, it fastens the detection and identification processes without the need for laboratory testing. Overall, nanosensors have proven to have high sensitivities, as low as 100 colony-forming units for E. coli, excellent selectivity and multifunctionality, time effectiveness and ease of execution [44]. It is worth noting here that a nanoscale sensing system that allows real-time detection of waterborne viruses and bacteria contaminants are being developed using fluorescent semiconducting nanoparticles combined with antibody fragments of targeted biological contaminants [43]. In addition, single and multiwall carbon nanotubes are being tested for contaminants detections [45].

Impacts of nanoparticles on health & the environment Nanotechnology, particularly nanoparticles, represent a promising new technology and potential benefits to enhance the economics of various areas ranging from environmental remediation to health [101]. Over the last decade, different nanomaterials have moved into the marketplace with direct and indirect applications in society [46]. However, we should not just consider the current and future importance of these nanoparticles; their potential benefits should be assessed in terms of lifecycle assessments and effects on health and the environment [47]. However, relatively little research has been published on the impact of man-made nanomaterial exposure on human health and the environment [48]. Furthermore, little information is available about the manufacturing, usage, reactivation/ regeneration and disposal of these nanomaterials. In a recent report, the UK Royal Society (London, UK) and the Royal Academy of Engineering (London, UK) concluded that various applications of nanomaterials pose no new health or safety risks [47]. The environmental hazards are limited to some nanoparticles, particularly those that are freely mobile and not incorporated into a material such as support, and may have negative impacts on health and the environment due to their size or particular chemical properties [47]. Therefore, the UK report of the Royal Society and the Royal Academy of Engineering recommended that in the specific case of free nanomaterials (e.g., nanopowder), existing regulatory frameworks need to be modified and the release of these nanomaterials into the environment should be avoided, until more is known about their environmental impacts [47].

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The application of nanoparticles for wastewater remediation Financial & competing interests disclosure The author has no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties. No writing assistance was utilized in the production of this manuscript.

Summary. Owing to their unique behavior, nanoparticles have promising potential for a wide range of industrial applications. Nanoparticles are emerging as a new alternate for removing different contaminants from wastewater. This is owing to their high surface area and, consequently, high surface functionality and reactivity compared with their bulk counterparts. Nanoparticles are successfully employed as effective oxidants, reductants and catalysts for a number of common contaminates in wastewater, including organic and inorganic compounds and toxic metal ions. These contaminants are turned into benign hydrocarbons or neutral byproducts. Incorporating nanoparticles with conventional membrane filtration technologies offers a great opportunity for mitigating fouling and improving membrane performance. Nanoparticles can be employed as nanosensors for detecting and identifying waterborne biological threat contaminants, so that immediate water remediation can be taken. The impact of nanoparticles on health and the environment has not been explored deeply. However, the environmental hazard of nanoparticles is limited to some nanoparticles, particularly those that are employed in free form and not incorporated into a support.

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