Functionalized Nanomaterials for Remediation and Environmental Applications

Ayushi Jain, Shweta Wadhawan, Vineet Kumar, and Surinder Kumar Mehta

Introduction

The word ‘nano’ has been taken from the Greek word ‘nanos’ which means ‘small’ which is used for substances in the dimensions of a billionth of a meter (10~9) in size. The particles whose one dimension are in the range of l-100nm are known as nanoparticles. Nanoparticles have different physicochemical properties as compared to bulk materials. In the field of nanotechnology, nanostructures are gaining great recognition from researchers due to their unique properties in various fields viz. optical, phys- iochemical, and biological. Many researchers around the globe are working on the synthesis and functionalization of nanoparticles by various physical, chemical, and biological methods. The word functionalization means modification of synthesized nanoparticles for tuning their properties for various applications. For a specific application, a particular functionalization process is required. For example, for use in the field of biomedicine, NPs must outperform the traditional agents’ context to their minimum toxicity for in vitro applications. On the other hand, for in vivo applications, NPs must avoid non-specific interactions with plasma proteins, so as to reach their projected target efficiently. In addition to this, colloidal stability of NPs must also be maintained under various physiological conditions and a wide range of pH.

In this chapter, we have described the use of various functionalized nanoparticles in the remediation process. The term ‘remediation’ refers to the removal of harmful and toxic materials from the environment.

Use of Functionalized NPs in Remediation

Functionalized nanomaterials can be used for the remediation of wastewater and ground water.

Wastewater Remediation

Wastewater remediation can further be categorized into organic, heavy metal and, uranium contaminant remediation.

Organic Compounds

Organic water remediation involves dyes and pesticides as main contaminants.

8.2.1.1.1 Dye Remediation

Various kinds of organic effluents viz. dyes, pesticides, insecticides are discharged from industries into water. These contaminants cause water pollution, which imposes harmful impacts on living organisms and aquatic life. Therefore, the remediation of contaminated water is crucial to make it fit for human consumption and for aquatic organisms. Dyes are colored pollutants mainly discharged from textile and paper industries. Dye-contaminated wastewater is hazardous for the health of aquatic animals, as well as other living organisms who use this water for life purposes like drinking, bathing, washing etc [1]. Basic dyes cause skin irritation, allergic dermatitis, shock, increased heartbeat, vomiting, cyanosis, jaundice, tissue necrosis [2]. They may also cause mutations and sometimes even cancer in humans [3]. Aside from this, dyes cause reduced sunlight absorption into water resources which inhibit bacterial growth and are responsible for the inefficient biological degradation of pollutants [4]. Therefore, these toxic substances should be removed prior to their discharge into receiving water bodies [5]. For the past few years, nanoparticles have emerged as attractive materials for the removal of colored pollutants from wastewater, due to their very high specific surface area and high adsorption capacity. Nanoparticles remove dyes from waste water by adsorption of dyes on their surfaces.

Sometimes this adsorption is followed by the degradation of dyes into simple degraded by-products. Nanoparticles are more convenient for the removal process, as compared to other conventional adsorbents like charcoal in terms of their reusability and recyclability. But the problem with bare nanoparticles is their colloidal dispensability and difficult recovery from the solution after adsorption [6, 7]. To overcome this problem, NPs are functionalized with a polymer, surfactants, resin, or other macromolecules [8, 9]. The functionalization process not only enhances the surface area of the NPs and available adsorption sites, but also improves the mechanical, thermal, and chemical stability, which increases the practical applicability and reusability of the adsorbent. To make the nanoparticles more water dispersible and stable they are functionalized with other hydrophilic molecules or polymers like polyfacrylic acid), polyfamido acid), polyfamido amine), and hyperbranched polyglycerol (HPG) [10-15]. He et al. reported a method for the synthesis of multi-hydroxy hyperbranched polyglycerol (HPG) capped Fe,04 (Fe,04/HPG) nanoparticles for the removal of methylene blue, rhodamine B, and Congo red [16]. In addition to the above-mentioned synthetic polymer, some natural polymers or resin-like agar, chitosan cellulose, and cellulose derivatives are also used to functionalize the bare NPs for dye removal. The advantages of using natural polymers are that they are plentiful, inexpensive, biodegradable, and eco-friendly [17]. Also, they have a high density of hydroxyl groups that can easily interact and bind with other functional groups of nanoparticles as well as pollutants [17]. For example, Wang et al. [18] reported hydroxyethyl cellulose (НЕС) and hydroxyl propyl methyl cellulose (HPMC) as a stabilizer for the preparation of magnetic Nps (MNPs), which is used for dye discoloration. To prevent the agglomeration of MNPs in solution, naked MNPs are also functionalized with different surfactants. In a unique approach, functionalization of nanoparticles with magnetic nanomaterials is performed to enhance the recovery of nanoparticles after adsorption. For example, magnetic carbon nanomaterials have been prepared by magnetic functionalisation of CNT with Fe,04 for the elimination of methylene blue dye from water [19]. Furthermore, the functionalization and deposition of nanadsorbents onto conventional adsorbents like activated carbon also results in a drastic increase in reactive centers and adsorption capacity. In one of the studies, CuS nanoparticles were loaded onto the activated carbon for the effective adsorption of methylene blue and bromophenol blue [20].

Another approach to remediate dyes from wastewater is degradation of dyes using nanoparticles. NPs have gained a lot of interest due to their catalytic role in the degradation of organic dyes. NPs catalyse the degradation process by absorbing a photon of adequate energy which is equal to their band-gap energy

[21] . It results in excitation of an electron from the valence band to the conduction band of the photocatalyst, leading to creation of a hole in valence band. A photocatalyzed reaction is favored by preventing the excited electron and the hole recombination. The excited electron interacts with an oxidant to give a reduced product, and the hole interacts with a reductant to give an oxidized product. The photogenerated electrons reduce the organic pollutant or dissolved O, into a superoxide radical anion 0:_'

[22] . On the other hand, the photogenerated holes oxidize the organic pollutant to carbocation i.e. R+, or OH", and H,0 into ОН» radicals.

The »OH radical formed is a very strong oxidizing agent (standard redox potential +2.8 V) which can oxidize most azo dyes to the mineral end-products. Therefore for nanostructure to be a good photocatalyst it should have a small band gap and large surface area. Furthermore, to enhance their degradation capacity various functionalization techniques are available for the complete degradation of dyes to water and carbon dioxide. To improve the properties of a nanophotocatalyst, they are functionalized with different materials like graphene. It provides a high specific surface area, mechanical stability, and high mobility of electrons, which enhances the photocatalytic activity of nanoparticles. For example, CdO nanoparticles were functionalized with graphene for the degradation of methylene blue [23]. The photocatalytic activity was enhanced due to the increased amount of 02~ and »OH radicals in the solution containing dye. During the photo catalytic process, NPs can be used in suspension form as well as in immobilized form. For practical application, photocatalyst immobilized on a support is preferred, since it does not require separation and recycling of the photocatalyst from the solution. So, many researchers have focused on the immobilization of photocatalyst on various support materials. For example, the widely used nano photocatalyst TiO, has been immobilised on different inorganic supports such as MCM-41 [18], SBA-15 [24], Na-HZSM-5 [25], NaA and CaA zeolite [26], rice husk silica nanocomposite [27]. But there is a limitation with the use of the above-mentioned inorganic supports that photocatalysts i.e TiO, may not have even distribution on the support surface because of high aggregation tendency resulting, in agglomeration of the particles. So, to eliminate this problem, functionalization of these inorganic supports like silicates can be done with organic polymers. The various functional groups from organic molecules coordinate with photocatalysts leading to even distribution of nanophotocatalysts on the support surface [28]. In this context, water-soluble tannins with the polyhydroxy groups have been proven to be appropriate, since they can coordinate with many types of metal ions by their many phenolic hydroxyls. One of the scientific studies has shown that tannin can form bonds with -NH, groups of aminated mesopo- rous silica through the crosslinking of aldehyde. In this study, TiO, supported on oak gall tannin-immobilized hexagonal mesoporous silicate (TiO,-OGTHMS) were used for the catalytic degradation of DY86. They showed that immobilization of tannin on the surface of HMS results in uniform distribution of TiO, nanoparticles on the surface of OGT-HMS without agglomeration, which aggregated on the surface of tannin-free HMS. The photocatalytic performance was also enhanced with TiO,-OGTx-HMS photocatalysts, as compared to tannin-free Ti02-HMS [29].

After the photo degradation experiment, photocatalysts are difficult to separate from the solution due to their nano powder form. The separation process consumes a lot of money and time, as they require a very long time to settle, or sometimes centrifugation is employed for their separation. In order to overcome this problem, the magnetic property can be introduced into the photocatalyst by making their nanocomposites with magnetic materials. Once the catalyst gets magnetic behavior, it could be easily separated using a magnet or the magnetic particles settled at the bottom by joining together. Further, the incorporation of MNPs also enhances the photocatalytic activity due to better suppression of the photo-generated electron-hole recombination and the increased light absorption due to the surface plasmon effect of nanoparticles. In a study, Ni NPs were functionalized with Si02/Ti02 for the enhanced degradation of AB1 dye [30]. Similarly, the introduction of Ag nanoparticles helped to suppress the electron-hole pair recombination into Si02/Ti02 [31].

The photocatalytic efficiency of NPs can also be improved by other different approaches such as the modification of NPs by metal and non-metal doping and the use of coupled semiconductors. As the pollutant molecules should be adsorbed by the photocatalyst surface prior to photocatalytic reaction, the specific area of the surface and crystal defects play an important role in the degradation process. Doping or co-alloying of metal oxide NPs with metals (transition) and non-metals leads to an increase in the crystal defects and shifts the band gap energies towards a visible region, thus enhancing the photocatalytic activity. In addition to the band gap shift of the material, doping also leads to change in the structure and oxidation state of the nanomaterials. Dopants also create suitable trap states to capture these photo-generated electrons and holes, which prevent the recombination thus increasing the lifetime of the electron and hole, which enhances the degradation efficiency. Bhattacharya et al. demonstrated the enhanced photocatalytic activity of Ti02 nanoparticles towards rhodamine В by doping TiO, with a different concentration of Mo dopants [32]. Similarly, Mn-doped ZnO NPs show better degradation performance due to the creation of defect states by Mn which act as intermediate states for the excitation of electrons from the valence band to the conduction band [33, 34]. The introduction of some isoelectronic transition metal cations such as 4d (Nb5+ or Mo6+) and 5d (Ta5+ or W6+) on the surface of TiO, leads to mixing of their orbital with the 3d (Ti2+) orbital in conduction band. The introduction of anions such as N3~ or P’~ leads to mixing of 2p orbital of 02~ in the valence band. This mixing lowers the band-gap of the materials providing suitable band edges for many chemical reactions, like dye degradation or water splitting. Some TiO, coalloyed systems including Nb? and N3_ are found to have seven times better degradation efficiency towards methylene blue as compared to anatase TiO, and almost twice as compared to commercial P-25 [35]. In addition, Hoang et al. [36] demonstrated the co-incorporation of Та and N into TiO, rutile nanowires for photoelectrochemical water oxidation.36

To decrease the band gap and widen absorption range into a visible light region, two or more semiconductor materials can be coupled with each other. The coupled semiconductor materials possess different energy-level systems that increase the charge separation resulting in the suppression of the electron-hole pair recombination under irradiation. This provides enhanced photo catalytic activity [37-39]. These systems also exhibit higher degradation of organic pollutants. For example, Saravanan et al. [40] prepared coupled ZnO/CdO nanocomposites for the efficient photodegradation of methylene blue in visible light.

Furthermore, for the remediation of dyes and other organic pollutants containing waster another new alternative technology i.e. advanced oxidation process (AOP) has been applied. This technique provides better discoloration and degradation of organic pollutants due to their efficiency, low cost, small waste and sludge production, the lack of toxic reagents, and the simplicity of the technology. The electro-Fenton (E-Fenton) process is an efficient method among AOPs for the oxidation of dyes present in waste water [41-43]. It involves the formation of a strong oxidizing hydroxyl radical («ОН) in aqueous solution by the reaction between hydrogen peroxide (H,02) and iron ions as catalyst. In traditional Fenton processes, dissolved iron (Fe2+) is used, which leads to the production of large amounts of sludge at pH above 4 and the formation of a large amount of anions in the treated wastewater. To overcome this limitation, Fe-containing NPs are used at circum neutral pH without sludge formation. In this process, Fe2+ ions are immobilized on the nanocatalyst surface - due to this they are not involved in complexation reactions, even at high pH [44-46]. In particular, magnetite (Fe,04) has been employed for the oxidation of various organic compounds in the Fenton process. This activity is attributed to the fact that both Fe2+ and Fe3t ions are present in the octahedral sites of the Fe,04 crystal structure, which enhances the decomposition of the H,0, molecule, resulting in the formation of »OH radicals [47, 48]. Furthermore, its magnetism makes it easily separable at the end of the reaction. In addition to this, the efficiency of the oxidation of organic compounds is enhanced with the incorporation of other transition metals in the spinel systems, like Fe,_xCox04, Fe,_xCrx04 Fe3_xMnx04 [49, 50], and Fe,_xVx04 [51] during the heterogeneous Fenton process. This formation of substitutional solid solution results in an increase in the surface area of the NPs which increases the adsorptive removal of dye on NPs, as well as the conversion of the H,0, to «ОН, which is also enhanced due to a larger number of exposed active sites [52, 53]. For example, Barros et al. [54] described the use of substituted magnetic nanoparticles i.e. Fe,_xCux04 (0

8.2.1.1.2 Pesticide Remediation

Pesticides are the chemicals which are used to control or remove animal or plant pests and prevent disease caused by pests or insects. These can be classified either on the basis of their purpose or on the basis of different chemical compounds present in them. According to their mode of function, they can be classified as insecticides, herbicides, fungicides, etc. On the basis of the chemical compound, these involve arsenic, pyrethroid carbamates, organophosphates, organochlorides, coumarins, and nitrophenol derivatives. Pesticides can be classified on the basis of use and chemical structure (Figure 8.1). Pesticides pose hazardous effects on the central nervous system (CNS) in humans. Due to the high toxicity of these pesticides, environment protection agencies have set the maximum limits for their contamination level in drinking water. Pesticides mainly used for agricultural purposes either remain in soil (soil pollution) or they enter the water system, depending upon their solubility. Furthermore, degradation products of pesticide in animals, vegetables, and water sources undergo biomagnification at each level of the food chain. Once pesticides enter the environment through direct agricultural use or from industrial waste, they can undergo various changes into more toxic degradation products. Thus a persistent need for the development of the methods for pesticide remediation has attracted the attention of researchers (Figure 8.2).

Nanomaterials have emerged as the most promising tools for the removal and degradation of pesticides to remediate the environmental pollution, due to their unique physicochemical

Types of pesticides on the basis of their use

FIGURE 8.1 Types of pesticides on the basis of their use.

Classification of pesticides on the basis of their chemical structure

FIGURE 8.2 Classification of pesticides on the basis of their chemical structure.

properties. Pesticides can be degraded or removed as such from wastewater via magnetic separation or nanofiltration.

Pesticides can be degraded by photo catalytic degradation or advanced oxidation technologies. The use of nanoparticles in the above-mentioned techniques is to enhance the degradation capacity, due to their increased surface area. Furthermore, nanoparticles are functionalized to improve their catalytic, optical, magnetic, mechanical, thermal, and other properties. Depending upon the type of functionalizing material, these can be categorized into three classes i.e. ceramic nanocomposites (Al203/ SiO,), metal (Fe/MgO), and polymer nanocomposites (polyester/Ti02) [55, 56]. Many magnetic Fe20, nanoparticles functionalised with polystyrene for the efficient removal of different organochlorine pesticides [57] have been reported. In addition to polymers, zeolites have also been used to functionalise the metal nanoparticles (ZnO, TiO,) for the removal of different organophosphate pesticides [58, 59].

The functionalization of nanoparticles with biopolymers is emerging as a more favorable approach for the removal of organic and inorganic pollutants from the environment. Due to the hydrophilic nature of biopolymers, their applicability and separation in aqueous medium is stepped up. Ag and ZnO nanoparticles functionalized with biopolymer chitosan have been used for the removal of pyrethroid and triazines respectively, through adsorption [60. 61].

 
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