• higher efficiency of removal even at low concentration of adsorbents

• functionalization capability of nanomaterials leads to specific uptake

• low waste generation.

1. zeolites

2. dendrimers

3. metal-containing nanoparticles including metal oxides

4. carbon nanotubes.

16.4.1 Zeolites

Zeolites are microporous crystalline solids with well-defined structures. Generally they contain silicon, aluminium and oxygen in their framework and cations, water and/or other molecules within their pores. Many occur naturally as minerals and they are extensively mined in many parts of the world. Others are synthetic and are made commercially for specific uses. Synthetic zeolites are usually made from silicon-aluminium solutions or coal fly ash, and are used as sorbents in column filters. Zeolites are widely used as ion-exchange beds for domestic and commercial water purification, and water softening where alkali metals such as sodium or potassium prefer to exchange out of the zeolite, being replaced by the ‘hard’ calcium and magnesium ions from the water. Commercial wastewater containing heavy metals can also be cleaned up using such zeolites. Zeolites are generally used for the removal of metal contaminants. Natural zeolites from Mexico and Hungary have been shown to reduce arsenic from drinking water sources to levels deemed acceptable by the World Health Organization (Elizalde-González et al., 2001).

Zeolites made from coal fly ash can adsorb a variety of heavy metals including lead, copper, zinc, cadmium, nickel, and silver from wastewater. Under some conditions, fly ash zeolites can also adsorb chromium, arsenic and mercury. NaP1 zeolites (Na₆Al₆ Si₁₀O₃₂, 12H₂O) have a high density of Na (I) ion exchange sites. They can be inexpensively synthesized by hydrothermal activation of fly ash with low Si/Al ratio at 150 °C in 1.0–2.0 M NaOH solutions. NaP1 zeolites have been evaluated as ion exchange media for the removal of heavy metals from acid mine wastewaters. Alvarez-Ayuso et al. (2003) reported the successful use of synthetic NaP1 zeolites to remove Cr (III), Ni (II), Zn (II), Cu (II) and Cd (II) from metal electroplating waste-water. The adsorptive capacity of zeolites is influenced by several factors including their composition, the water pH, and the concentrations and types of contaminants. For example, the water pH influences whether the ash surface is positively or negatively charged. Also, because lead and copper are more easily adsorbed by fly ash, high concentrations of these metals decrease the amount of cadmium and nickel removed (Wang et al., 2004).

Zeolite-silver compound has been proven effective against microorganisms, including bacteria and mould. Additionally, the silver in this compound provides residual protection against regrowth of these biological contaminants. Zeolites do not adequately remove organic contaminants. Also, air moisture contributes to zeolites’ saturation and makes them less effective. Zeolites were reported to result in high flux reverse osmosis nanocomposite membrane (the utilities of having a membranous structure have been discussed in the last section) without compromise in selectivity (Jeong et al., 2007).

16.4.2 Dendrimers

Dendrons are dendritic wedges that comprise one type of functionality (such as chemical bonding) at their core and another at the periphery. To obtain a dendrimer structure, several dendrons are reacted with a multifunctional core to yield a dendrimer. Using different synthetic strategies, over 100 compositionally different dendrimer families have been synthesized and over 1000 differentiated chemical surface modifications have been reported (Bosman et al., 1999; Fischer and Vögtle, 1999; Tomalia and Majoros, 2003). One such dendrimer structure is shown in Fig. 16.2 (Jang et al., 2009).

Dendritic polymers exhibit many features that make them particularly attractive as functional materials for water purification. These ‘soft’ nanoparticles, with sizes in the range of 1–20 nm, can be used as high capacity and recyclable water soluble ligands for toxic metal ions, radionuclide and inorganic anions (Ottaviani et al., 2000). The environmental applications of dendrimers were first explored by Diallo et al. (2005). They reported the effective removal of copper from water via different generations of poly(amidoamine) (PAMAM) dendrimers. Later, Diallo et al. (2005) studied the feasibility of using dendrimer improved ultrafiltration to recover Cu (II) from aqueous solution. The dendrimer-Cu (II) complexes can be efficiently separated from aqueous solutions by ultrafiltration. Dendritic polymers can also be used as (i) recyclable unimolecular micelles for recovering organic solutes from water (Arkas et al., 2003) and (ii) scaffolds and templates for the preparation of redox and catalytically active nanoparticles. Dendritic polymers have also been used successfully as delivery vehicles or scaffolds for antimicrobial agents such as Ag (I) and quaternary ammonium chlorides (Balogh et al., 2001).

PAMAM-based silver complexes and nanocomposites have proved to be effective antimicrobial agents in vitro. Rether and Schuster (2003) made a water-soluble benzoylthiourea modified ethylenediamine core-polyamidoamine dendrimer for the selective removal and enrichment of toxicologically relevant heavy metal ions. They studied complexation of Co (II), Cu (II), Hg (II), Ni (II), Pb (II) and Zn (II) by the dendrimer ligand and using the polymer-supported ultrafiltration process.

One of the novel systems for encapsulating organic pollutants is cross-linked dendritic derivatives. In the research carried out by Arkas et al. (2005) for the preparation of ultra pure water, the amino groups of polypropyleneimine dendrimer and hyperbranched polyethylene imine were interacted under extremely mild conditions with 3-(triethoxysilyl) propyl isocyanate. They produced porous ceramic filters and employed these dendritic systems for water purification. In this experimental work, the concentration of polycyclic aromatic compounds in water was reduced to a few ppb’s by continuous filtration of contaminated water through these filters. Then, the filters loaded with pollutants were effectively regenerated by treatment with acetonitrile.

In another work, Arkas et al. (2006) developed a method that permits removal of organic pollutants by employing a simple filtration step, which can be easily scaled up. They used the long-alkyl chain functionalized polypropylene imine dendrimers, polyethylene imine hyperbranched polymers and β-cyclodextrin derivatives which are completely insoluble in water.

16.4.3 Metal-containing nanoparticles

Metal and metal oxide nanoparticles have been studied extensively for water treatment applications. Most notable among the metal nanoparticles are the noble metals (such as silver and gold which are treated as famous biocides) and nano zero-valent iron (NZVI) used for treatment of water containing pesticides. Among metal oxides, oxides of iron, aluminium, zinc, magnesium and titanium have been made use of significantly in groundwater remediation. We first discuss the category of metals and then case studies pertaining to metal oxides are discussed.

16.4.4 Carbon nanotubes

A carbon nanotube (CNT) is a one-atom thick sheet of graphite (called graphene) rolled up ino a seamless cylinder with diameter of the order of a nanometer and capped at both ends by hemispheres of fullerene. This results in a nanostructure where the length-to-diameter ratio exceeds 10,000. CNTs can be categorized by their structures as single-walled nanotubes (SWNT) and multi-walled nanotubes (MWNT). In the most general way, the CNT can be shown as composed of a concentric arrangement of several cylinders (Fig. 16.4). The high curvature of the graphene sheets increases the total energy of the tubules per carbon atom, but this is more than offset by a lowering of the energy because of the absence of dangling bonds at the edges of the graphene sheets. Such cylindrical carbon molecules have novel properties that make them potentially useful in a wide variety of applications in water and health care.

The as-grown or acidified CNTs have shown potential as a sorbing media for removal of various contaminants from water. The as-grown CNTs have got defect sites (originated during synthesis), which make them a suitable adsorbent for uptake of contaminants. However, it is noteworthy that such CNTs do not pose any selectivity towards uptake of specific contaminants. On the other hand, the surface area of as-grown CNTs lies in the range of 50–100 m²/g, which is not significant in comparison to activated charcoal or activated alumina (where the surface area is up to 1000 m²/g), which are being extensively used in water decontamination because of being cheaper and having easy availability. However, some of the work carried out by researchers using as-grown and functionalized CNTs are cited below.

Peng et al. (2005) developed a novel adsorbent, ceria supported on CNTs (CeO₂-CNTs), for the removal of arsenate from water. Under natural pH conditions, an increase from 0 to 10 mg/L in the concentration of Ca (II) and Mg (II) results in an increase from 10 to 81.9 and 78.8 mg/g in the amount of As (V) adsorbed, respectively. The adsorption was shown to be pH-dependent. The efficient regeneration of the loaded adsorbent was carried out and the adsorption mechanism was suggested.

Y. H. Li et al. (2003a) used aligned carbon nanotubes (ACNTs) for the removal of fluoride from water. The adsorption slightly depends on the solution pH value. The highest adsorption capacity of ACNTs occurs at pH 7 and reaches 4.5 mg/g at equilibrium fluoride concentration of 15 mg/L. Y. H. Li et al. (2003b) studied the removal of cadmium (II) with as-grown and surface-oxidized CNTs. Cadmium (II) adsorption capacities for three kinds of oxidized CNTs increase owing to the functional groups introduced by oxidation compared with the as-grown CNTs. The cadmium (II) adsorption capacity of the as-grown CNTs is only 1.1 mg/g, while it reaches 2.6, 5.1 and 11.0 mg/g for the H₂O₂-, HNO₃- and KMnO₄-oxidized CNTs, respectively, at the cadmium (II) equilibrium concentration of 4 mg/L. Adsorption of cadmium (II) by CNTs was strongly pH-dependent and the increase of adsorption capacities for HNO₃- and KMnO₄-oxidized CNTs is more obvious than that of the as-grown and H₂O₂-oxidized CNTs at lower pH regions. Analysis revealed that the KMnO₄-oxidized CNTs hosted manganese residuals, and these surely contributed to cadmium sorption to a yet-undefined extent.

Li et al. (2002) found that CNTs show exceptional adsorption capability and high adsorption efficiency for lead removal from water. The adsorption is significantly influenced by the pH value of the solution and the nanotube surface status, which can be controlled by their treatment processing. The adsorption isotherms are well described by both the Langmuir and Freundlich models.

The first data on multi-wall nanotubes (MWNTs) as sorbent for dioxin removal was reported by Long and Yang (2001). A technique based on temperature-programmed desorption (TPD) was used to study dioxin adsorption. The amount adsorbed on CNTs is 1034 higher than on activated carbon. Hence, significantly higher dioxin removal efficiency is expected with CNTs than with activated carbon. The strong interaction between dioxin and CNTs may be attributed to the unique structure and electronic properties of CNTs. The CNTs consist of hexagonal arrays of carbon atoms in graphene sheets that surround the tube axis. Strong interactions between the two benzene rings of dioxin and the surface of the CNTs are expected.

Srivastava et al. (2004) reported the fabrication of freestanding macroscopic hollow cylinders having radially aligned CNT (ACNT) walls, with diameters and lengths up to several centimetres. These cylindrical membranes are used as filters in the elimination of multiple components of heavy hydrocarbons from petroleum – a crucial step in post-distillation of crude oil – with a single-step filtering process, and the filtration of bacterial contaminants such as Escherichia coli or the nanometre-sized poliovirus (~ 25 nm) from water. These macro filters can be cleaned for repeated filtration through ultrasonication and autoclaving.

Techniques have been developed to synthesize CNTs in significant quantities using three different methods:

An overview of these different routes is summarized in Table 16.2 (Balasubramanian and Burghard, 2005).

Of the various methods of CNT synthesis, CVD has got tremendous potential for scaleup. In addition, the growth of CNTs can take place without having patterned substrate (with impregnation of catalyst particles). A special case of CVD is called spray pyrolysis, where liquid precursors like mixture of benzene and ferrocene can be used. The experimental setup is shown in Fig. 16.5 (Dasgupta et al., 2008). It consists of a sprayer, a container for the liquid precursor and a quartz tube (inner diameter 10 mm). The sprayer is made up of a pyrex nozzle (inner diameter 0.4 mm) and an outer pyrex tube with an exit diameter of 2 mm. The inner nozzle carries the liquid precursor and the outer one carries the nitrogen gas. The sprayer is attached to the quartz tube kept inside a resistive furnace. An optimized combination of composition and the flow rate of liquid precursor solution, the carrier gas flow rate, the diameter of inner and outer nozzles of sprayer and the temperature of growth helps in the synthesis of CNTs with particular length, diameter and alignment. Each of the parameters is critical toward the nature as well as yield of CNTs (Dasgupta et al., 2008). This synthesis route provides CNTs with a high degree of alignment because the catalyst particle (iron in the case of ferrocene precursor) is generated in situ. Also the method is economical with no requirement of substrate preparation as well as less involvement of trained manpower compared to the other two synthesis routes.

A self-standing tubular macrogeometry of aligned carbon nanotubes with very high surface area (~ 90 m²/g) was developed with optimization of spray pyrolysis parameters. The macrogeometry of aligned CNTs is shown in Fig. 16.6, and Fig. 16.7 shows the scanning electron microscope (SEM) image depicting the highly aligned CNTs along the thickness of cylinder. It has the usual advantages of a membrane where, based upon the principle of size exclusion, the water purification can take place.

A noteworthy breakthrough in the area of development of nanotube chemistry is the oxidation of CNT in concentrated nitric acid (Rosca et al., 2005). Such a drastic condition helps in opening of the CNT tips as well as oxidative etching along the sidewalls enabling the decoration of walls with various oxygen-containing groups (mainly carboxyl group). The incorporation of carboxyl group exposes various useful sites in CNTs for further modification as per requirements (ester or amide bond formations can take place). In addition, the formation of anhydride at the tube ends can take place through which the rings of CNTs are accessible (Sano et al., 2001). The most important implication of the introduction of a carboxyl group lies in the fact that the van der Waals forces existing between the individual CNTs are reduced and hence the CNTs can be made water soluble (as-grown CNTs are not soluble in any solvent) by addition/substitution of new moieties.

On the other hand, addition reactions help in direct coupling of functional groups onto the π-conjugated carbon framework. A series of addition reactions like fluorination, hydrogenation and cycloaddition can be possible. The fluorine atoms of fluorinated CNTs can be replaced through nucleophilic substitution reaction, and thus, functional groups of alcohols, amines and Grignard reagent, etc., can be successfully incorporated onto the CNT sidewall.

So far we have been concerned mainly with the functionalization of the sidewall and tip of CNTs. But the inner hollow cavity of CNTs offers tremendous opportunity to materials scientists, chemists and engineers to do excellent R&D in the nanoscale test tube. The critical issue is the wetting properties of the CNTs. The wettability determines what liquid would fill the tube by capillary action and cover the inner surface. The Young–Laplace relation relates the pressure difference AP across the liquid-vapour interface in a capillary to the surface tension of the liquid (γ) and contact angle (θ) between the solid and the liquid as shown by:

[16.1]

where r is the radius of curvature of the meniscus. The contact angle θ is an indicator of the strength of the interaction between the liquid and the solid interface relative to the cohesive forces in the liquid. If θ is smaller than 90°, the contact between the liquid and the surface is said to be wetting and ΔP is positive. Therefore the liquid will be pulled into the capillary spontaneously, as there is an energy gain in the wetting process. If θ is larger than 90°, the contact angle is said to be non-wetting and ΔP will be negative. Therefore, when θ > 90°, the only way to introduce liquid into a capillary is to apply pressure larger than ΔP.

Extremely high aspect ratios, molecularly smooth hydrophobic graphitic walls, and nanoscale inner diameters of carbon nanotubes give rise to the unique phenomenon of ultra-efficient transport of water through these ultra-narrow tubes. The idea of water occupying such confined hydrophobic channels is somewhat difficult to comprehend, though experimental evidence has confirmed that water can indeed occupy these channels (Naguib et al., 2004; Kolesnikov et al., 2006). The proposed water transport mechanism has a distinct similarity to the transport mechanisms of biological ion channels. In recent years, numerous simulations (Hummer et al., 2001; Kalra, 2003) of water transport through SWNT have suggested that fast molecular transport takes place, far in excess of what continuum hydrodynamic theories would predict if applied on this length scale. There have been many efforts to define the boundary between bulk water and confined water transport, and it was found sensible to set a threshold for the continuum treatment of liquid as around 7.5 nm.

A no slip boundary condition is typically used in continuum fluid dynamics. It constrains a fluid closest to a solid boundary to obtain the same tangential velocity as the solid. When the tangential velocity of the fluid differs from that of the solid, we usually say the surface imposes a slip boundary condition as depicted in Fig. 16.8 (Choi et al., 2011). The slip boundary condition helps us describe non-continuum behaviour of water transport inside the CNT in the framework of continuum dynamics. For example, slip length, L_s, may serve as a good indicator for the molecular interaction between water molecules and CNT via provision of information about the degree of departure the transport innately has from the hydrodynamic Hagen–Poiseuille flow. Also, the slip length indicator can compare with results of molecular dynamics (MD) simulations often used for the exact prediction of water flow under the CNT nanoconfinement.

Slip length, L_s, is convenient to explain the hydrodynamic boundary condition at the interface of fluid and wall, which is defined according to the Navier boundary condition:

[16.2]

where n and t denote normal and tangential directions of the wall, v_t is the velocity of a fluid tangential to the wall, and v_wall is the velocity of the wall. v_t,wall – v_wall is denoted as a slip velocity.

The solutions for the velocity and the corresponding volume flow rate in the flow direction, z, with respect to the distance from the centre, r, have a parabolic profile given by:

[16.3]

[16.4]

[16.5]

where μ, p and R represent viscosity, pressure, and tube radius, respectively.

There are two primary reasons a continuum theory of a molecular scale slip is investigated. On the one hand, it helps to understand the behaviour of a flow in a nanoscale tube. On the other hand, the slip length theory can serve as a reference to explain large water flow enhancements, compared with no-slip Hagen-Poiseuille formalism, observed by several experimental studies (Majumder et al., 2005a; Holt et al., 2006; Whitby et al., 2008) using CNT membranes.

Although the ultrafast water transport in the CNT could be explained by the slip length model to some extent, results of MD simulations, especially on the water configuration, are investigated as an effective way to understand many fundamental nanofluidic characteristics. In fact, lots of physical properties of water depend on the coherence and the number of hydrogen bonds. The hydrogen bond is useful in explaining water configuration or properties inside CNT, since water has a dipole and an electrostatic interaction via hydrogen bonding.

The effect of CNTs on bacteria and viruses has not received particular attention, probably due to the difficulty of dispersing CNTs in water. Surfactants or polymers such as polyvinylpyrolidone (PVP) or Triton-X are generally used to facilitate the dispersion. The few studies available credited SWNTs with antimicrobial activity towards Gram-positive and Gram-negative bacteria, and the damage inflicted was attributed to either a physical interaction or oxidative stress that compromise cell membrane integrity (Narayan et al., 2005; Kang et al., 2008). However, the degre of aggregation and the bioavailability of the nanotube will have to be considered to exploit the antimicrobial properties effectively.

It is noteworthy here that CNTs are the only nanostructured material bestowed with so many unusual (however essentially interesting) attributes which can serve as a wonderful global water filter. The critical factor is to put the CNTs in a membranous structure to exploit all their potential benefits, which are discussed in Section 16.7.

16.4.5 Other nanomaterials (nanoclays, micelles, magnetic nanoparticles)

Clays are alumino silicates with a planar silicate structure. There are three main categories of clay: kaolinite, montmorillonite–smectite and illite, amongst which the first two are the most widely studied (Pradeep and Anshup, 2009b). Usually the structure contains silicate sheets (Si₂O₅) bonded to aluminium oxide/hydroxide layers (Al₂(OH)₄) called gibbsite layers. The primary structural unit of this group is a layer composed of one octahedral sheet with one tetrahedral sheet (kaolinite is 1 : 1 clay mineral). The condensation of two sheets happens by coordination of an oxygen atom with one silicon atom in the tetrahedral sheet and two aluminium atoms in the octahedral sheet. Clays undergo exchange interactions of adsorbed ions with the outside too.

Although clays are very useful for many applications, they have one main disadvantage, they lack permanent porosity. To overcome this problem, researchers have been looking for a way to prop and support the clay layers with molecular pillars. Most of the clays can swell and thus increase the space in between their layers to accommodate the adsorbed water and ionic species. Ding et al. (1999) and Ooka et al. (2004) prepared different kinds of TiO₂ pillared clays from different raw clays and the adsorption and photocatalytic-decomposition performance were evaluated. It was found that surface hydrophobicity of pillared clays (especially TiO₂) largely varied with the host clay. Since the TiO2 particles in the pillared clays are too small to form a crystal phase, they presented poor photocatalytic activity.

Nanocomposite of iron oxide and silicate was also synthesized for degradation of azo-dye orange (II) (Feng et al., 2003). A new class of nano-sized large porous titanium silicate (ETAS-10) and aluminium-substituted ETAS-10 with different Al₂O₃/TiO₂ ratios were successfully synthesized by Choi et al. (2006) and applied to the removal of heavy metals, in particular Pb (II) and Cd (II).

Micelles are self-assembled surfactant materials in a bulk solution. Surfactants or ‘surface active agents’ are usually organic compounds that are amphipathic, meaning they contain both hydrophobic groups (tails) and hydrophilic groups (heads). Therefore, they are typically soluble in both organic solvents and water. Surfactant-enhanced remediation techniques have shown significant potential in their application for the removal of polycyclic aromatic hydrocarbon (PAHs) pollutants. Molecular self-assembly is gathering of molecules without guidance or management from an outside source. There are two types of self assembly: intramolecular and intermolecular. Attaching a monolayer of molecules to mesoporous ceramic supports gives materials known as self-assembled monolayers on mesoporous supports (SAMMS). The highly ordered nanostructure of SAMMS is the result of three molecular self-assembly stages. SAMMS of silica-based materials are highly efficient sorbents for target species, such as heavy metals, tetrahedral oxometalate anions and radionuclides (Mansoori et al., 2008).

Magnetic nanoparticles are generally studied as adsorbents and nanocatalysts for water treatment. One of the major applications of magnetic particles is in the area of magnetic separation. In this case, it is possible to separate a specific substance from a mixture of different other substances, called ‘magnetically assisted chemical separation’ (MACS). Hu et al. (2005) developed an innovative process combining nanoparticle adsorption and magnetic separation for the removal and recovery of Cr (VI) from waste-water. Chang et al. (2006) prepared the magnetic chitosan nanoparticles with an average diameter of 13.5 nm as a magnetic nano-adsorbent. Magnetic chitosan nano-adsorbent was shown to be quite efficient for the fast removal of Co (II) ions at the pH range of 3–7 and the temperature range of 20–45 °C.

Ngomsik et al. (2006) have studied the removal of nickel ions from the aqueous solution using magnetic alginate microcapsules. Also, magnetic particles in the microcapsules allowed easy isolation of the microcapsule beads from aqueous solutions after the sorption process. Mayo et al. (2007) also studied the effect of particle sizes in the adsorption and desorption of As (III) and As (VI). Different kinds of magnetic nanoparticles were also employed for the removal of organic pollutants, such as sorption of methylene blue on polycyclic acid-bound iron oxide from an aqueous solution (Mak and Chen, 2004).

• layer-by-layer deposition (Philips et al., 2006)

• self-assembly (Graveland and Kruif, 2006; Lorenceau et al., 2005)

• gas phase synthesis and sol-gel processing (Siegel, 1991, 1994; Uyeda, 1991)

• crystallization (Boanini et al., 2006)

• microbial synthesis (Bhainsa and Souza, 2006; Bhattacharya and Gupta, 2005)

• other methods (sonochemical processing, cavitation processing, microemulsion processing and high-energy ball milling).

• size of particles: the size of nanoparticles necessitates usage of sophisticated analytical tools.

• increased reactivity and conductivity: nanoparticles are more reactive and conductive than the same material in bulk.

• routes of exposure: because of their very small size, nanoparticles can be inhaled or ingested; in addition, they are capable of crossing the blood-brain barrier, which protects the brain against contamination (Oberdörster et al., 2004).

16.6.1 Evidence for toxicity of nanomaterials

Nanoparticles have the ability to induce lung injuries because of their small size, a large surface area, and an ability to generate reactive oxygen species (ROS).¹ Section 16.6.1 is adapted from Wani et al., 2011: Nanotoxicity: dimensional and morphological concerns, Advances in Physical Chemistry, Volume 2011, doi:10.1155/2011/450912. The short-term pulmonary toxicity studies in rats with ultrafine and fine carbon black, nickel and TiO₂ particles have established enhanced lung inflammatory strength of the ultrafine particles in comparison to fine-sized particulates of similar composition (Warheit et al., 2006; Grassian et al., 2007; Pettibone et al., 2008). Low toxicity nanoparticles such as carbon black and polystyrene stimulate the macrophages via reactive oxygen species and calcium signalling, to make proinflammatory cytokines such as tumour necrosis factor alpha (Brown et al., 2004). The cationic nanoparticles, including gold and polystyrene, have shown to cause haemolysis and blood clotting, while usually anionic particles are quite non-toxic. High exposures to diesel exhaust particles (DEPs) by inhalation caused altered heart rate in hypertensive rats interpreted as a direct effect of DEP on the pacemaker activity of the heart (Hansen et al., 2007). Exposure to singlewalled carbon nanotubes has also resulted in cardiovascular effects (Li et al., 2007). The nanoparticles inhaled can gain access to the brain by means of two different mechanisms, namely, transsynaptic transport after inhalation through the olfactory epithelium and uptake through the blood-brain barrier (Lockman et al., 2004; Jallouli et al., 2007). In vitro studies have shown that multiwalled carbon nanotubes are capable of localizing within and initiating an irritation response in human epidermal keratinocytes, which are a primary route of occupational exposure (Baroli et al., 2007; Zvyagin et al., 2008).

The change in the structural and physicochemical properties of nanoparticles with a decrease in size could be responsible for numerous material interactions that could lead to toxicological effects (Nel et al., 2006); for example, shrinkage in size may create discontinuous crystal planes that increase the number of structural defects as well as disrupt the electronic configuration of the material and give rise to altered electronic properties (Fig. 16.9). These changes could establish specific surface groups that could function as reactive sites. Chemical composition of the materials is particularly responsible for these changes and their importance. The surface groups can make nanoparticles hydrophilic or hydrophobic, lipophilic or lipophobic, or catalytically active or passive. These surface properties can lead to toxicity by the interaction of electron donor or acceptor active sites (chemically or physically activated) with molecular oxygen (O₂), and electron capture can lead to the formation of the superoxide radical, which generates additional reactive oxygen species (ROS) through Fenton chemistry (Fig. 16.9).

Various studies have been carried out to investigate the adverse effects of nanoparticle on the biological systems (N. Li et al., 2003; Hoshino et al., 2004; Li et al., 2007; Baker et al., 2008; Lyon et al., 2006; Peters et al., 2006) and the interaction of nanoparticles with biological systems is represented in Fig. 16.10. It has been demonstrated that carbon black nanoparticles produce their increased inflammatory effects via mechanisms other than the leaching of soluble components from the particle surface. Transition metals are an important source of free radicals, which are important in PM10-stimulated lung inflammation. Therefore, it is clear that nanoparticles may exert their increased proinflammatory effects, at least in part, by modulating intracellular calcium.

Nanomaterials themselves constitute a new generation of toxic chemicals. As particle size decreases, in many nanomaterials the production of free radicals increases, as does toxicity. Studies have shown that nanomaterials now in commercial use can damage human DNA, negatively affect cellular function and even cause cell death. There is a small but growing body of scientific studies (termed as nanotoxicology) showing that some nanomaterials are toxic to commonly used environmental indicators such as algae, invertebrate and fish species (Hund-Rinke and Simon, 2006; Lovern and Klaper, 2006; Templeton et al., 2006; Federici et al., 2007; Lovern et al., 2007). There is also evidence that some nanomaterials could impair the function or reproductive cycles of earthworms which play a key role in nutrient cycling that underpins ecosystem function (Fordsmand et al., 2008).

Studies demonstrated that when introduced into the lungs of rodents, certain carbon nanotubes cause inflammation, granuloma development, fibrosis, artery ‘plaque’ responsible for heart attacks and DNA damage (Donaldson et al., 2006; Lam et al., 2006; Muller et al., 2006). Two independent studies have shown that some carbon nanotubes can also cause the onset of mesothelioma – cancer previously thought to be only associated with asbestos exposure (Poland et al., 2008; Takagi et al., 2008). We now discuss in brief the toxicity of those nanomaterials (such as silver, CNT, TiO₂ and silica) that are being used substantially in water purification applications.

16.6.2 Toxicity of silver nanoparticles

Silver nanoaprticles (Ag NPs) are, due to their antimicrobial properties, the most widely used NPs in commercial products. Ag NPs are incorporated into medical products like bandages as well as textiles and household items. The toxicity of Ag NPs has also been shown by a number of in vitro studies (Kawata et al., 2009; Kim et al., 2009; Foldbjerg et al., 2009, 2011). Toxicological investigations of NPs imply that, for example, size, shape, chemical composition, surface charge, solubility, their ability to bind and affect biological sites as well as their metabolism and excretion influence the toxicity of NPs (Schrand et al., 2010; Castranova, 2011). The high surface area of metal-based NPs increases the potential that metal ions are released from these NPs (Bian et al., 2011; Mudunkotuwa and Grassian, 2011), yet it is not clear to what degree the toxicity of Ag NPs results from released silver ions and how much toxicity is related to the Ag NPs themselves. It is quite possible that free silver ions in Ag NP preparations play a considerable role in the toxicity of Ag NP suspensions. While the contribution of free silver ion to the measured toxicity of Ag NP suspensions is an important determinant for the toxicity, a combined effect of Ag ion and Ag NP appears for lower concentrations of Ag ions.

16.6.3 Toxicity of carbon nanotubes

Despite their many advantages, CNTs represent a hazard to the environment and human health. Like asbestos, the aspect ratio (length : diameter) and metal components of CNTs are known to have an effect on the toxicity of carbon nanotubes. Kim et al. (2011) evaluated the toxic potential of CNTs in relation to their aspect ratio and metal contamination, in vivo and in vitro genotoxicity tests were conducted using high aspect ratio (diameter: 10–15 nm, length: ~ 10 μm) and low aspect ratio multi-wall carbon nanotubes (MWCNTs, diameter: 10–15 nm, length: ~ 150 nm) according to OECD test guidelines 471 (bacterial reverse mutation test), 473 (in vitro chromosome aberration test), and 474 (in vivo micronuclei test). High aspect ratio MWCNTs were found to be more toxic than the low aspect ratio MWCNTs. Thus, while high aspect ratio MWCNTs do not induce direct genotoxicity or metabolic activation-mediated genotoxicity, genotoxicity could still be induced indirectly through oxidative stress or inflammation.

A recent in vivo cancer therapy study using CNTs originally designed as drug delivery enhancers was able to demonstrate that tumour cells respond to toxicity differently than do wild type cells (Liu et al., 2008). Lam et al. (2004) tested a variety of SWCNT samples with varying amounts of metal impurities and concluded that all SWCNT preparations induced dose-dependent lung granulomas in mice. Warheit et al. (2004) reported a mild and transient pulmonary inflammatory response in rats instilled intratracheally with SWCNTs, with subsequent development of multifocal granulomas in the lungs after 1 month in a mouse instillation study using highly purified SWCNTs.

Shvedova et al. (2005) found granulomas, lung fibrosis and a significant elevation in markers of toxicity in bronchoalveolar lavage (BAL) fluid and concluded that SWCNTs exerted greater toxicity on a mass basis than crystalline silica. A critical review of carbon nanotube toxicity and assessment of potential occupational and environmental health risks was provided by Lam et al. (2006), where the toxicological hazard assessment of potential human exposures to airborne CNTs and occupational exposure limits for these novel compounds are discussed in detail.

16.6.4 Toxicity of titanium dioxide and silica (SiO₂) nanoparticles

Studies with fine and ultrafine (< 100 nm) TiO₂ particles demonstrate some respiratory toxicity and epithelial inflammation of the lung in rodents (Ferin and Oberdörster, 1985; Ferin et al., 1991; Oberdörster et al., 1992; Bermudez et al., 2002, 2004; Warheit et al., 2005, 2006). Silica nanoparticles have been shown to have a low toxicity when administered in moderate doses (W Lin et al., 2006; Chang et al., 2007; Jin et al., 2007). Unfortunately, silica nanoparticles also tend to agglomerate and have been demonstrated to lead to protein aggregation in vitro at a dose of 25 μg/mL (Barik et al., 2008). Oxidative stress has been implicated as an explanation behind silica nanoparticles cytotoxicity both in vitro and in vivo (Chen and von Mikecz, 2005; Yang et al., 2009; Wang et al., 2009). All these studies have reported cytotoxicity and oxidative stress, as determined by increasing lipid peroxidation (LPO), reactive oxygen species (ROS), and decreasing cellular glutathione (GSH level), but no similarity exists regarding dose response.

Very little is known about the safety risks presented by engineered nanomaterials. Given their unique properties, particularly their increased reactivity and electrical conductivity, safety concerns are focusing on whether nanomaterials could cause fires or explosions. Because nanoparticles behave differently from larger particles, questions have arisen about whether they can pollute the water supply or damage crops during processes that release these particles into the air, soil or water. Again, studies in this area are in their infancy.

In the short term, the major health and safety risks will be to researchers in laboratories and production staff exposed during the manufacture of nanomaterials. People in these occupations must be aware of the potential hazards of using materials that have unknown properties, and they must take measures to mitigate their risks. However, their activities are contained and generally do not pose a threat to the public or to the environment.

Owing to the highly interdisciplinary nature of nanotechnology, it can be viewed as an enabling technology that is a sincere augmentation of the existing technologies in the field of water purification, textile, aerospace, health care and electronics. Keeping in view the unknown behaviour and fate of nanomaterials in the environment, nanotechnology may pose tremendous challenges to the existing waste management systems. Knowledge on the mobility, persistence and bioaccumulation potential in the environment is hardly available. Hence risk assessment on the possible impact of nanowastes is critical and needs to be made.

Regulators in the United States, the European Union and elsewhere around the world believe that nanoparticles represent an entirely new risk and that it is necessary to carry out an extensive analysis of the risk. Such studies then can form the basis for government and international regulations. As a proper and comprehensive risk and life cycle analysis, encompassing production, application and waste management strategies of nanomaterials, is of urgent need for a successful commercialization of a technology with proven societal benefits, otherwise environmental costs could be high and the technology as a whole could be distrusted or rejected by the public.

• 1.1 billion lack access to an ‘improved’ drinking water supply; many more drink water that is grossly contaminated.

• 4 billion cases of diarrhoea occur annually, of which 88% is attributable to unsafe water, and inadequate sanitation and hygiene.

• 1.8 million people die every year from diarrhoeal diseases, the vast majority children under 5.

• Lack of safe water perpetuates a cycle whereby poor populations become further disadvantaged, and poverty becomes entrenched.

• WHO estimates that 94% of diarrhoeal cases are preventable through modifications to the environment, including through interventions to increase the availability of clean water, and to improve sanitation and hygiene.

16.7.1 Sustainability of a water purification technology

There are thousands of types of water filters that have the capability to purify contaminated water. However, most filters are too expensive for the nations with the greatest need for potable water. Technologically advanced filters have no real application in countries without the capability to sustain them, which is why more basic filtering methods are needed to truly have an impact on the global clean water shortage. Such organizations as the UN and WHO are currently pushing the water filter industry to develop sustainable solutions to empower many rural nations with the ability to filter their own water in cheaper, more environmentally friendly ways. These sustainable technologies are innovative, simple, and incorporate combinations of basic science and local materials to create usable and efficient filters. Sustainable, or appropriate, technology is defined as, ‘Technology that can be made at an affordable price by ordinary people using local materials to do useful work in ways that do the least possible harm to both human society and the environment’ (Cunningham et al., 1999).

The following guidelines are suggested based on an evaluation of the sustainable technology definition, the ethical standards, and information from sustainable development of technologies (Skye McAllister, 2005). It is important to keep in mind that these steps are to be used in addition to traditional engineering standards and ethics.

• availability of nanomaterials

• integration of nanomaterials into water purification systems

• societal implications because of health and environment risks.

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