Water Treatment Devices Based on Zero-Valent Metal and Metal Oxide Nanomaterials

CHAPTER 5

Water Treatment Devices Based on Zero-Valent Metal and Metal Oxide Nanomaterials Vicente de Oliveira Sousa Neto1, Tiago Melo Freire2, Gilberto Dantas Saraiva3, Celio Rodrigues Muniz3, Marcony Silva Cunha4, Pierre Bası´lio Almeida Fechine5 and Ronaldo Ferreira do Nascimento2 1

Laboratory of Study and Research in Removal of Pollutants by Adsorption (LERPAD), Department of Chemistry, State University of Ceara´ (UECE-FECLESC), Quixada´, Brazil 2Trace Analysis Laboratory (LAT), Department of Analytical and Physical Chemistry Chemistry, Federal University of Ceara´-UFC, Fortaleza, Brazil 3Department of Physics, State University of Ceara´ (UECE-FECLESC), Quixada´, Brazil 4Group of Theoretical Physics, State University of Ceara´, Fortaleza, Brazil 5Advanced Materials Chemistry Group (GQMat), Department of Analytical and Physical Chemistry Chemistry, Federal University of Ceara´-UFC, Fortaleza, Brazil

5.1 Introduction In general, nanomaterials are described as structured materials of sizes between 1 and 100 nm. Their nanoscale gives these materials different properties such as mechanical, electrical, optical, and magnetic properties when compared to their conventional sizes. Over the past few decades many scientific works have focused on the development of nanomaterials for specific application in the treatment of water and wastewater. This choice can be explained by the technological potential that can be developed from these nanometric compounds. In particular, nanomaterials have large surface areas due to their small size that promotes an increase in adsorption capacity and provides good catalytic activity. Another important property of nanomaterials is their mobility in aqueous media, making them potential adsorbents for the removal of pollutants such as heavy metals, organic pollutants, inorganic anions, and bacteria. This chapter aims to address nanomaterials based on zero-valent metals (ZVMs) and metal oxides. Further attention is focused on how these materials are synthesized and applied in the treatment of water and wastewater.

Nanomaterials Applications for Environmental Matrices. DOI: https://doi.org/10.1016/B978-0-12-814829-7.00005-7 © 2019 Elsevier Inc. All rights reserved.

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5.2 Nanomaterials: Nanoparticles Applied to Water and Wastewater Treatment 5.2.1 Zero-Valent Metal Versus Nanoscale Zero-Valent Metal Several researchers have shown that ZVMs have the potential to be used as water decontaminants. This chapter addresses the applicability of ZVMs as decontaminants for toxic chemical compounds of environmental interest. Special attention will be given to the chemistry involved as well as the synthesis of these materials. The use of ZVM particles to remove organic pollutants from soil and water has received much attention from researchers in various areas with a focus on the preservation of natural resources. There are two reaction mechanisms involving ZVMs: (1) oxidative mechanisms; and (2) reducing mechanisms. In the literature, it can be observed that most studies on the degradation of organic chemicals by ZVM particles use the reductive method. One aspect that is important and should be considered when using nanoparticles is related to their size. From kinetic studies, it is known that dimensional reduction considerably increases reagent reactivity due to the increase of contact surface. Therefore reducing the size of these particles to the nanoscale vastly increases their reactivity. Compared with ZVMs, nanoscale zero-valent metals (nZVMs) have higher efficiency due to their enormous effective surface area which is capable of potentiating their chemical reactivity due to the increase of the contact surface, hence promoting highly significant kinetic gain (Singh et al., 2012). In the next section, details about nanoscale ZVMs based on Fe0, Zn0, and Ti0 with a focus on the synthesis, action mechanism, and applicability of these nanomaterials in the treatment of water and wastewater will be given. 5.2.1.1 Nanoscale Zero-Valent Iron Nanoscale zero-valent iron (nZVI) is promising for pollution abatement due to its high reactivity and catalytic capability (Lefevre et al., 2016; Saif et al., 2016). For in situ remediation, nZVI presents better mobility and reactivity than micron-sized materials, therefore, yielding much higher efficiency. nZVI is one of the most widely used nanomaterials for soil and groundwater remediation. Some scientific studies have shown the excellent performance of nZVI in wastewater remediation (Fu et al., 2014; Grieger et al., 2010; Mueller et al., 2012) as well as how it improved the process of anaerobic digestion (Carpenter et al., 2015). nZVI has been successfully applied in the removal of pollutants with various natures such as halogenated compounds (e.g., chlorinated organic solvents, organochlorine pesticides, polychlorinated biphenyl, etc.,), nitrate, phosphate, polycyclic aromatic hydrocarbons, and heavy metals (Fu et al., 2014; Mueller et al., 2012; Tosco et al., 2014). nZVI has attracted the interest of most research involving ZVMs because it presents characteristics such as nontoxicity, shows abundance in nature, is cheap, easy to produce,

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Figure 5.1 General schematic presentation of nZVI. Reproduced with permission from Stefaniuk, M., Oleszczuk, P., Ok, Y.S., 2016. Review on nano zerovalent iron (nZVI): From synthesis to environmental applications. Chem. Eng. J. 287, 618632.

and its reduction process requires little maintenance. From the electrochemical point of view, nZVI is a reactive metal with a standard redox potential (Eo) of 20.44 V. In Fig. 5.1 a general schematic presentation of nZVI is shown. 5.2.1.2 Synthesis of Nanoscale Zero-Valent Iron Nanoscale valent iron can be obtained by either physical methods or chemical methods. The main physical methods include grinding or attrition, repeated quenching, abrasion, and lithography (Shan et al., 2009). However, these methods present some drawbacks that need to be considered such as difficulty in controlling particle shape and planned size which , in turn, affects the design of the final product (Stefaniuk et al., 2016). 5.2.1.2.1 Chemical Methods

(1) Borohydride reduction method: Several chemical methods were proposed and developed to obtain nZVI. The first work about obtaining nZVI with the specific purpose of environmental remediation occurred in the 1990s. Wang and Zhang (1997) proposed a route of synthesis of nZVI employing sodium borohydride (NaBH4) as a reducing agent, according to the reaction as shown in Eq. (5.1): 2 o 2FeðH2 OÞ31 6 ðaqÞ 1 6BH4 ðaqÞ 1 6H2 OðlÞ -2FeðsÞ k 1 6BðOHÞ3ðaqÞ 1 21H2ðgÞ

(5.1)

The authors applied the obtained nZVI for the dechlorination of trichloroethenes (TCEs) and polychlorinated biphenyls (PCBs). However, the method proposed by Wang and Zhang (1997) only presents applicability at the laboratory scale and contains serious limitations since nanoparticles are extremely polydispersed, ranging from tens to hundreds of nanometers in size. Dimensional variation easily promotes agglomerates. Costly reagents

190 Chapter 5 and the production of large volumes of hydrogen are also disadvantages of the method because they prevent possible industrial application on a large scale. Fig. 5.2 shows an experimental apparatus used to obtain nZVI (Sun et al., 2006). Bae et al. (2016) studied the effect of NaBH4 on the properties of nZVI. According to their study, the addition of NaBH4 to the nZVI suspension promoted a reduction of the initial size of the ZVI particles (60100 nm), resulting in the formation of much smaller particles (1540 nm) due to the chemical etching of the outer surface. In Fig. 5.3 the effect of the reducing agent, NaBH4, on the particle size of nZVI is illustrated (Bae et al., 2016). (2) Carbothermal reduction: In this chemical method, both iron oxide nanoparticles (Bystrzejewski, 2011) and Fe21 ion hydrated in salt form (Hoch et al., 2008) can be

Figure 5.2 Experimental apparatus used to obtain nZVI. Reproduced with permission from Sun, Y.-P., Li, X.-q, Cao, J., Zhang, W.-x, Wang, H.P., 2006. Characterization of zero-valent iron nanoparticles. Adv. Colloid Interface Sci. 120 (1), 4756.

Figure 5.3 Effect of NaBH4 on the particle size of nZVI. Reproduced with permission from Bae, S., Gim, S., Kim, H., Hanna, K., 2016. Effect of NaBH4 on properties of nanoscale zero-valent iron and its catalytic activity for reduction of p-nitrophenol. Appl. Catal. B Environ. 182, 541549.

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used as the starting material. They are reduced to nZVI under elevated temperatures in the presence of reducing gases, such as H2, CO2, and CO. The normally employed source of carbon monoxide is from the thermal decomposition of carbon-based materials (carbon black, biochar, and carbon nanoparticles). Fe0 is formed as a result of a high-temperature endothermic reaction ( . 500 C) in which only gaseous products are present (Crane and Scott, 2012). Such reactions can proceed according to Eqs. (5.2)(5.6). This reactive characteristic allows its production on an industrial scale in a continuous process. FeðC2 H3 O2 Þ2 1 C-Fe0 2CH2 CO 1 COm 1 H2 O

(5.2)

Fe3 O4 1 2C-3Fe0 1 2CO2

(5.3)

2C 1 Fe3 O4 -3Fe0 1 2CO2 m

(5.4)

4CO 1 Fe3 O4 -3Fe0 1 4CO2

(5.5)

C 1 2FeðC6 H5 O7 Þ  3H2 O-2Fe0 1 2C3 H6 O 1 6CO2 m 1 COm 1 5H2 O

(5.6)

This method entails low-cost production since the carbon-based materials used as raw materials (e.g., carbon black and biochar) are cheap and highly available. In addition, the material can be obtained at low-cost from the fossil fuel industry. (3) Green synthesis: Green synthesis is an environment-friendly and low-cost method. In particular, it is interesting to mention the use of plant extracts and other natural products as reducing agents for nZVI and various other preparations possessing Fecontaining nanoparticles. In terms of advantages, green synthesis does not make use of high temperatures, pressure, or additional energy demand and is easy to implement on a large scale (Machado et al., 2013). However, the reduction of iron ions is often incomplete and, thus, will generate a significant number of by-products, such as iron oxides. Unfortunately, so far, the production processes to appropriately direct its technological application are not sufficiently understood. Ebrahiminezhad et al. (2018) proposed a green synthesis from the extract of the leaves of the Mediterranean cypress (Cupressus sempervirens). According to the authors, the synthesis involved in obtaining the extract and its reaction with ferric chloride hexahydrate (1.0 mol L21) under controlled conditions for 24 hours. At that time the sample was centrifuged and the formed black precipitate was washed until the complete removal of the unreacted substances and phytochemicals was achieved. The black precipitate was oven dried at 50 C for 48 hours. In Fig. 5.4, a schematic illustration of the green synthesis of nZVI is shown. According to the authors, the experimental data suggest that C. sempervirens is a plant with good performance and high efficiency for use as a green reducing agent. The authors also observed that after the addition of the iron solution to the leaf extract there was a color

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Figure 5.4 Schematic illustration of the green synthesis of nZVI from Cupressus sempervirens leaf branches extract. Reproduced with permission from Ebrahiminezhad, A., Taghizadeh, S., Ghasemi, Y., Berenjian, A., 2018. Green synthesized nanoclusters of ultra-small zero valent iron nanoparticles as a novel dye removing material. Sci. Total Environ. 621, 15271532.

change during the reaction. This is considered qualitatively as a preliminary indication of the formation of metallic nuclei. Indirectly, it suggests that the extract of the plant has a high reduction potential that is attributed to its phytochemical component. In a previous work, Ebrahiminezhad et al. (2017) reported the bioreductive capacity of the extract of this plant against silver ions. (4) Ultrasound-assisted method: This method was first proposed by Tao et al. (1999). The synthesis is a combination of both a chemical process through the reduction of iron by sodium borohydride and a physical process such as the choice of ultrasound frequency employed to homogenize the size of the grains of nZVI. Similarly to the chemical method, the application of ultrasound while employing sodium borohydride as a reductant can be represented by the reaction in Eq. (5.7): 2 o 1 4Fe12 ðaqÞ 1 BH4 ðaqÞ 1 3H2 OðlÞ -4Fe 1 H3 BO3ðaqÞ 1 7HðaqÞ

(5.7)

However, it is necessary to consider that nZVI oxidation can simultaneously occur to the reduction reaction which negatively affects the yield of the process as shown in Eq. (5.8): 12 Feo 1 2H1 ðaqÞ -FeðaqÞ 1 H2ðgÞ

(5.8)

To overcome this reactive difficulty, the authors added ammonium hydroxide by chemically blocking the secondary reaction by removing H1 from the reaction medium. The reaction then proceeds according to Eq. (5.9): 2 o 1 4Fe12 ðaqÞ 1 BH4ðaqÞ 1 7NH4 OHðlÞ -4Fe 1 H3 BO3ðaqÞ 1 7NH4ðaqÞ 1 4H2 OðlÞ

(5.9)

Fig. 5.5 shows a schematic illustration of the ultrasonic method applied by Tao et al. (1999).

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Figure 5.5 Schematic illustration of the ultrasonic system proposed by Tao et al. (1999). Reproduced with permission from Tao, N.R., Sui, M.L., Lu, J., Lua, K., 1999. Surface nanocrystallization of iron induced by ultrasonic shot peening. Nanostruct. Mater. 11 (4), 433440.

(5) Electrodeposition: Electrodeposition consists of the deposition of metals (in an aqueous medium) on the electrode surface. All the theory involved at the industrial scale has already been consolidated and its application in the synthesis of nZVI has been studied as an inexpensive alternative that is simple and fast compared to chemical reduction (Chen et al., 2004; Yoo et al., 2007). Basically a current is applied and the nZVI particles are gradually deposited on the surface of the cathode. However, there is a natural tendency of nZVI to form aggregates and a cationic surfactant must be employed in the chemical bath to promote the dispersion of the nanoparticles. Ultrasonic waves may also be employed as a dispersing agent (Chen et al., 2004). In Fig. 5.6 a schematic diagram of this method is shown. 5.2.1.2.2 Physical Methods

(1) Physical methods of obtaining nZVI have some advantages when compared with chemical methods, including simple operation, easy separation of products, no use of toxic products, etc. Precision ball milling method: This is a rather simple mechanical method in which grinding with metal balls is employed to reduce the size of the particles. Li et al. (2009) demonstrated that precision ball milling was efficient in obtaining nZVI with uniform size and high surface area. The authors used micro iron particles as raw materials and reported that the method did not produce secondary pollution, it was nontoxic, and a suitable method for industrial-scale production. Despite this, the method requires specialized equipment and high energy demand to reach nanoscale. According to Stefaniuk et al. (2016) the material obtained by the

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Figure 5.6 Schematic diagram of electrochemical method. nZVI produced by electrodeposition using ultrasonic waves as a dispersing agent. Reproduced with permission from Chen, S.-S., Hsu, H.-D., Li, C.W., 2004. A new method to produce nanoscale iron for nitrate removal. J. Nanopart. Res. 6 (6), 639647.

precise milling method is characterized by higher reactivity relative to chloroorganic contaminants such as TCE. However, the authors also emphasize that nZVI particles obtained with this method have an irregular shape, which is associated with their deformation and cracking resulting from contact with steel shot. Moreover, they display a strong tendency to aggregate. (2) Gas condensation processing: In the gas condensation method (GPC), the gaseous iron particles under high temperature are condensed under inert atmosphere using liquid nitrogen as a refrigerant. This method has an interesting advantage in allowing efficient particle size control. However, disadvantages include the need for very restrictive operational conditions, an intense demand for energy, and a low yield that limits its application on an industrial scale. 5.2.1.3 Mechanism of Removal of Pollutants by Nanoscale Zero-Valent Iron Currently, the mechanism of the removal of pollutants by nZVI is related to the direct transfer of electrons from the nZVI to dissolved oxygen in the aqueous medium with the consequent formation of H2O2 in situ. The presence of Fe and H2O2 (Fenton mixture) in the aqueous medium leads to the formation of radical •OH, which has the capacity to effectively oxidize organic matter. 21 Fe0ðsÞ 1 O2ðgÞ 1 2H1 ðaqÞ -FeðaqÞ 1 H2 O2ðaqÞ

(5.10)

21 Fe0ðsÞ 1 H2 O2ðaqÞ 1 2H1 ðaqÞ -FeðaqÞ 1 2H2 OðlÞ

(5.11)

31  2 Fe21 ðaqÞ 1 H2 O2ðaqÞ -FeðaqÞ 1 OHðaqÞ 1 OHðaqÞ

(5.12)

Eqs. (5.10)(5.12) show a set of steps that result in the formation of the hydroxyl radical, •OH. Experimentally, the oxidation of Fe0 to Fe12 by O2 is widely known. The kinetics of this stage is extremely favorable and fast.

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5.2.1.4 Zero-Valent Zinc Zero-valent zinc (ZVZ) is well known as an important metal, which according to the literature achieves maximum efficient reduction. ZVZ acts as a reductive reagent for the dechlorination of chlorinated aliphatic compounds. such as 1,1,1trichloroethane,2,4,6-trichlorophenol (Wang et al., 2008; Fennelly and Roberts 1998; Choi and Kim, 2009). Bokare et al. (2013) reported the reductive dechlorination of octachlorodibenzo-p-dioxin (OCDD) using nanosized zero-valent zinc (nZVZ). This recalcitrant toxic compound is prevalent in soil and sediment and has been detected in groundwater and surface water. The authors studied the degradation of the OCDD in aqueous solutions using nZVZ, which shows efficiency in the degradation of OCDD into lower chlorinated congeners. ZVZ has been successfully applied as an effective reducing agent in the removal of environmental contaminants such as halogenated organic compounds (Tratnyek et al., 2010), bromates , nitrates, pesticides, and metals, etc. Regarding the elimination of bromate in water, it is important to note that for ZVZ to be an efficient reductive, it needs to be prepared by acid-washing treatments in order to remove the outer layer of zinc oxide from zinc powder (Lin et al., 2017; Fu et al., 2016; Wang et al., 2016). This process leads to changes in the morphology of the zinc powder. Fig. 5.7 shows the morphology of pristine zinc particles before and after acid-washing treatments. It has been observed that although the ability of ZVZ to degrade chlorinated solvents has been described in the literature under certain restricted conditions, the advantages of the ZVZ over ZVI under such conditions were not sufficient to arouse interest in the field application of ZVZ for contaminated water remediation. Therefore its application is still limited. Wen et al. (2014) investigated the use of ZVZ to enhance the ozonation degradation of di-n-butyl phthalate (DBP) through a semicontinuous reactor in aqueous solution. According to their results, the combination of ozone (O3) and ZVZ showed a synergetic effect, where the process of the degradation efficiency of DBP augmented gradually with the increase of the ZVZ dosage. The mechanism to explain the enhancement effect was proposed according to Fig. 5.8. It suggests that the introduction of ZVZ promotes the utilization of O3, which enhances the formation of superoxide radical by reducing O2 via one-electron transfer. As a conclusion, the O3/ZVZ process enhanced the ozonation degradation of DBP in the aqueous medium. 5.2.1.5 Nanoscale Zero-Valent Iron Applications The application of nanoparticles/nanomaterials depend on their morphology, superficial area, resistance, conductivity, electronic properties, structural, catalytic, photocatalytic, magnetic, and other unique properties that define their usage in different field applications (Adusei-Gyamfi and Acha 2016; Cui and Lieber 2001; Gudiksen et al., 2002;

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Figure 5.7 (A) Illustrative preparation of ZVZ by acid-washing zinc particle and SEM images of (B), (C) pristine zinc particle and (D, E) acid-washed ZVZ using 1 M HCl at different magnifications. Reproduced with permission from Lin, K.Y.A., Lin, C.H., Lin, J.Y., 2017. Efficient reductive elimination of bromate in water using zero-valent zinc prepared by acid-washing treatments. J. Colloid Interface Sci. 504, 397403.

Tian et al., 2007; Wang 2006; Aarthi and Madras 2007; Al-Abadleh and Grassian 2003). Various applications of nanomaterials require chemical modification in order to tune/control their physicochemical properties. One way of achieving this control is by carrying out doping processes through which atoms and molecules interact (covalently or noncovalently) with the nanomaterial surface (Bystrzejewski, 2011). The study of the intrinsic properties of a nanomaterial and the interaction that occurs between pristine samples and doped samples is vital to advancing the scientific understanding of a nanomaterial’s properties and the development of suitable applications (Zhang et al., 2003; Rodrigues et al., 2008). Adusei-Gyamfi and Acha (2016) reported a review about carriers for nZVI and their synthesis, application, and efficiency. This study reports the effective application of nZVI nanoparticles in environmental remediation. The high surface area, high reactivity, fast

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Figure 5.8 The proposed reaction mechanism for DBP degradation in O3/ZVZ process. Reproduced with permission from Wen, G., Wang, S.-J., Ma, J., Huang, T.-L., Liu, Z.-Q., Zhao, L., et al., 2014. Enhanced ozonation degradation of di-n-butyl phthalate by zero-valent zinc in aqueous solution: performance and mechanism. J. Hazard. Mater. 265, 6978.

kinetics, small particle size, and magnetic ability are the main properties of ZVI nanoparticles that promote this material for environmental applications (Wang et al., 2014). Sun et al. (2006) reported their results on the characterization process of ZVI nanoparticles prepared with the ferric iron reduction via the sodium borohydride method. The authors developed an important iron nanoparticle with considerable attention placed on its potential applications in groundwater treatment, environmental, and other applications. The characterized iron nanoparticles show a core of ZVI as well as a shell of mainly iron oxides (FeO) (Fig. 5.9). According to Sun et al.’s study, this iron nanoparticle may prove to be useful for the separation and transformation of many contaminants. Boparai et al. (2011) investigated the kinetics and thermodynamics of cadmium ion removal by adsorption onto nZVI particles. In this study, the authors investigated the removal of Cd21 by nZVI particles. According to them, the experimental results suggested that nZVI is an excellent adsorbent for removing cadmium from contaminated water sources.

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Figure 5.9 A coreshell structure for iron nanoparticles in aqueous solution. The core is made of metallic iron while the shell consists mostly of iron oxides and hydroxides. Thus iron nanoparticles exhibit characteristics of both iron oxides (e.g., as a sorbent) and metallic iron (e.g., as a reductant). Reproduced with permission from Adusei-Gyamfi, J., Acha, V., 2016. Carriers for nano zerovalent iron (nZVI): synthesis, application and efficiency. RSC Adv. 6 (93), 9102591044.

The use of nZVIs in environmental treatment is highly effective, has excellent availability on an industrial scale, and presents a reasonable cost, especially when applied in soil or groundwater purification with better results compared to macro iron particles used previously (Elliott and Zhang, 2001). However, it is important to note that nZVI shows disadvantages, such as aggregation, oxidation, and separation when applied to water and wastewater treatment (Singh and Misra, 2015; Chen et al., 2011). Other alternatives are the process of encapsulation in matrix and emulsification (Stefaniuk et al., 2016). 5.2.1.6 Silver Nanoparticles Nanoparticles (NPs) are a class of materials which have one dimension less than 100 nm at least. A review on the application of nanomaterials in environmental science and engineering has been published by Shan et al. (2009). As is well-known, nanomaterials are appropriate for examining important environmental issues concerning organic pollutants, remediation of polluted soils or water, green chemistry, and photocatalytic degradation (Aarthi and Madras 2007; Anandan et al., 2007). A general review on nanoparticles of different characterizations relates to the type, classification, characterization, and fabrication method. Herein, the application of nanoparticles in environmental water treatment is described since according to the World Health Organization (WHOGeneva, 1996) the main indication of water contamination is the presence of bacteria (Jain and Pradeep, 2005). Water treatment can be performed in different manners. In particular, silver (Ag) applied in water treatment as a disinfectant and

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toxicity remover has been considered in the specialized literature. Some authors described the use of both ionic silver and silver nanoparticles (AgNP) as well as copper and silver. The studies address the antimicrobial properties and toxicity of silver. In this context, the toxicity of silver related to human exposure from inhalation, ingestion, and injection has been reported (Fewtrel, 2014). Silver nanoparticles are applied in different manners in environmental water treatment. For instance, they have been incorporated into cellulosic materials such as filter paper, cotton fabric, and cellulose gels (Mpenyana-Monyatsi et al., 2012; Ferraria et al., 2010; He et al., 2003; Ifuku et al., 2009; Maneerung et al., 2008; Tang et al., 2009; Zhu et al., 2009). Dankovich and Gray (2011) report a bactericidal paper impregnated with silver nanoparticles (see Fig. 5.10) for POU water treatment. In their study, they describe the use of a paper sheet containing silver nanoparticles in which pathogenic bacteria were deactivated (Dankovich and Gray, 2011). The application of Ag in ionic form seems to support the disinfection of potable water. In this sense, many studies have reported the efficacy of silver ions. For instance, Hwang et al. (2007) reported the efficacy of ionic silver derived from AgCl against 3 3 106 CFU mL21 of Pseudomonas aeruginosa, Stenotrophomonas maltophilia, and Acinetobacter. Cunningham et al. (2008) used flow cytometry to examine the minimum inhibitory concentration (MIC) of AgNO3 on Escherichia coli. Silver nanoparticles application showed the potential use of AgNP for drinking-water disinfection in conjunction with filtration. Some examples of silver nanoparticle applications include the antibacterial effectiveness of polymer microspheres containing silver nanoparticles: (1) to incubate various bacteria (E. coli, P. aeruginosa, Bacillus subtilis, and Staphylococcus aureus), as investigated by Gangadharan et al. (2010); (2) the bacterial contamination in water was

Figure 5.10 Schematic presentation of the disinfection process of blotter paper containing silver nanoparticles. Reproduced with permission from Dankovich, T.A., Gray, D.G., 2011. Bactericidal paper impregnated with silver nanoparticles for point-of-use water treatment. Environ. Sci. Technol. 45 (5), 19921998.

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Figure 5.11 A schematic diagram showing the synthesis of thin film nano-templated composite (TFNt) membrane. A polysulfone substrate was first coated with polydopamine, followed by in situ reduction of silver by immersing it into an AgNO3 solution. An interfacial polymerization of PIP and TMC onto the PDA/Ag treated substrate forms the TFNt membrane, in which AgNPs is uniformly distributed in its composite rejection layer. Reproduced with permission from Yang, Z., Wu, Y., Guo, H., Ma, X.-H., Lin, C.-E., Zhou, Y., et al., 2017. A novel thin-film nano-templated composite membrane with in situ silver nanoparticles loading: Separation performance enhancement and implications. J. Membr. Sci. 544, 351358.

studied by Dankovich and Gray (2011) where the paper was impregnated with nanosilver in order to reduce bacterial contamination; (3) Loo et al. (2013) explored the use of AgNPc in cryogels as a possible POU treatment. Yang et al. (2017) reported a novel silver nanoparticle device based on a thin-film nanotemplated composite membrane, which is applied as an antibacterial (E. coli) water filter with a long sterilization property (see Fig. 5.11).

5.3 Metal Oxide Over the past few years, the use of nanomaterials for water treatment has increased significantly. Many of these are based on carbon, metal oxides, and ZVMs due to some properties such as reactivity, specific surface area (SSA), and adsorption capacity, along with decreasing the size of the materials (Hua et al., 2012; Das et al., 2017). Among these, metal oxide nanoparticles (MONs) such as iron oxide, titanium oxide, copper oxide, zinc oxide, and manganese oxide have been cited as powerful tools for water pollutant removal due to their high SSA and specific affinities (Pathania and Singh, 2014). Furthermore, MONs show minimal environmental impact and low solubility (Sadegh et al., 2017). The shape and size of these materials are parameters that directly affect their efficiency (Cao et al., 2016). Thereby, an efficient synthetic method to obtain nanoparticles with narrow size distribution and shape control has been an important topic over the past years (Su, 2017). Among the many methods of MON synthesis, the protocol of coprecipitation,

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thermal decomposition, hydrothermal, and solgel methods are techniques widely used due to their low-cost and easily protocol (Zhang et al., 2017). In the following section, the synthetic routes, characterizations, applications, and mechanisms of the main metal oxides in water treatment are discussed.

5.3.1 Synthetic Routes 5.3.1.1 Chemical Precipitation/Coprecipitation Coprecipitation is probably the simplest, most efficient, and oldest method for preparing MONs and their composites (Avgouropoulos et al., 2005; Reddy et al., 2012; Freire et al., 2017). In general, this process can be summarized in three steps: (1) the preparation of a solution in the liquid phase with chemical composition; (2) thermal treatment that directly influences the crystallinity, morphology, and structure of the nanoparticles (Gnanaprakash et al., 2007); (3) the addition of a precipitation agent (e.g., NaOH, NH4OH, Na2CO3, or urea), which may vary the sizes of the nanoparticles (Mascolo et al., 2013). This method is mainly applied to prepare magnetite nanoparticles by a stoichiometric mixture of ferrous and ferric salts in an aqueous medium. The general reaction of the precipitation of magnetite may be written as Eq. (5.13). 31 2 Fe21 ðaqÞ 1 2FeðaqÞ 1 8OHðaqÞ -Fe3 O4 1 4H2 O

(5.13)

The process of precipitation is complex since various processes are allowed to occur simultaneously, including initial nucleation, growth, coarsening, agglomeration, and ripeness (that may take hours to days) (Mirzaei and Neri, 2016). In this method, the control over nanoparticle size, morphology, and composition is limited by kinetic growth. In order to help in the control of size and morphology, a capping agent such as chitosan (Freire et al., 2016), polyethylene glycol (Masoudi et al., 2012), cellulose (Xiong et al., 2014), polyvinylpyrrolidone (Koczkur et al., 2015), or cetyltrimethylammonium bromide (Hoang Duy et al., 2014) can be added. Although the coprecipitation method is widely applied for iron oxide synthesis, it has also been found to be efficient in preparing many other metal oxides, such as ZnO (Thorat et al., 2012), SnO2 (Rashad et al., 2014), TiO2-ZrO2 (Afanasiev, 2008), and MnO2 (Kanha and Saengkwamsawang, 2017). In addition, it is also possible to produce core/shell nanoparticles from this method (Zhou, 2016). 5.3.1.2 Hydrothermal/Solvothermal Synthesis The hydrothermal and solvothermal approaches are normally conducted in steel pressure vessels (i.e., autoclave) with Teflon liners under controlled pressure and/or temperature. The synthesis temperature is often dependent on the solvent used in the process because it is generally elevated to above the boiling point of the solvent in order to reach pressure vapor saturation (Chen and Mao, 2007). Thus hydrothermal/solvothermal methods are among the most convenient and practical techniques for metal oxide formation because they

202 Chapter 5 enable the avoidance of special instruments, complicated processes, and cumbersome preparation conditions (Mirzaei and Neri, 2016). In the past few years, this approach has been successfully applied for the preparation of different nanoparticle types with specific size and shape (Gao et al., 2012). The preparation of MONs through hydrothermal/solvothermal methods can involve the processes of hydrolysis, oxidation, and thermolysis (Rajamathi and Seshadri, 2002). According to the literature, prepared MONs involve most of these process (Lu et al., 2014). The thermolysis route is based on the decomposition of an organometallic complex into metal oxide by heating and increased pressure in the system. Thimmaiah et al. (2001) developed a promising route for the synthesis of MONs containing only one or two metal atoms. They showed that the decomposition of a single or multiple transition metal cupferron complexes in toluene under solvothermal synthesis in the presence of nonpolar amine provides the formation of the corresponding oxide nanoparticles. Many MONs can be produced by hydrolysis, since it is expected that many metals will be solvated by a shell of water molecules (Rajamathi and Seshadri, 2002). In addition, this route provides an easy way to modify the shape and size of nanoparticles due to a large dielectric constant of water, which makes it a good solvent for many structure-directing molecules (Yoshimura, 2013). 5.3.1.3 SolGel The solgel method is the most exploited method for the synthesis of metal oxide and metal oxide nanocomposites (Akpan and Hameed, 2010). This method is based on the hydrolysis, condensation, and polymerization reactions of metal precursors, and after polymerization gel formation is observed, which is composed of three-dimensional metal oxide networks (Rao et al., 2012). Thus the solgel method represents a powerful and versatile strategy for the preparation of many different porous materials with different 3D structures (Sumida et al., 2017). The optimization of the procedure can be carried out by the modification of the parameters, such as pH, time, temperature, nature, and concentration of the salt precursors, the nature of the solvent used, and by incorporation of surfactants in the solution phase (Reddy et al., 2012). The addition of surfactants is one of the main factors that modifies surface morphology and surface charge, except for crystal structure, which is not altered. The use of an aqueous or a nonaqueous solvent in solgel syntheses is a versatile method for the production of nanoparticles from polar and nonpolar precursors. In the aqueous solgel process, the oxygen requirements for the formation of metal oxide is provided by water molecules, while for a nonaqueous process the oxygen is supplied by a solvent (ketone, ethers, alcohols, or aldehydes) or by an organic constituent of a precursor (alkoxides or acetylacetonates) (Rao et al., 2012).

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This process can be summarized in six steps: (1) the formation of a stable metal precursor solution referred to as “sol”; (2) the formation of a “gel” through a polycondensation reaction; (3) the aging of the gel for hours or days, resulting in the expulsion of the solvent, i.e., Ostwald ripening, and the formation of a solid mass; (4) the drying of the gel of any liquids; (5) dehydration and surface stabilization; and (6) heat treatment of the gels at high temperatures to generate crystalline nanoparticles (Mirzaei and Neri, 2016). By understanding the solgel process steps, various strategies can be defined in order to control the size, shape, and morphology of MONs (Sumida et al., 2017). 5.3.1.4 Microwave-Assisted Synthesis Commonly, the synthesis of MONs requires the use of high temperature, which can be obtained using isomantles, oil baths, or hot plates. However, this traditional method of heating is rather slow and inefficient for transferring energy to a chemical mixture due to convective currents and thermal conductivity, among other factors. On the other hand, microwaves have attracted much attention owing to their efficient internal volumetric heating (i.e., the system is uniformly heated) by direct coupling of the molecule precursors with microwave energy (Baghbanzadeh et al., 2011). In comparison with other methods, the main advantage of microwave irradiation is the decreased synthesis time of MONs. For example, Raj et al. (2017) showed that the formation of NiO nanoparticles by microwave synthesis needs only a few minutes for the desired reaction product, whereas other methods such as calcination take a few hours. In addition, microwave-assisted methods can be easily scaled up without suffering thermal gradient effects (Panda et al., 2006). Thus it is no surprise that the number of published papers involving microwave-assisted synthesis has increased over the past few years. Since microwave irradiation can be used together with various synthesis methods, it is relatively easy to find many types of synthetic routes to prepare MONs with different structures by changing the microwave exposure time or the power of the microwave irradiation (Rana et al., 2016). In the literature there are good review articles concerning the microwave-assisted synthesis of nanoparticles (e.g., Mirzaei and Neri, 2016; Baghbanzadeh et al., 2011; Zhu and Chen, 2014). 5.3.1.5 Microemulsion The microemulsion technique is based on the ternary mixture of an organic phase (oil), a surfactant, and an aqueous phase. Above a critical concentration of the surfactant, known as the critical micellar concentration (CMC), the formation of aggregates of surfactant molecule denominated micelles occurs. This method has been widely used for the synthesis of various nanoparticles, such as MONs, semiconductor quantum dots, and polymeric nanoparticles (Mirzaei and Neri, 2016).

204 Chapter 5 A microemulsion is a thermodynamically stable dispersion of two immiscible liquids in the presence of a surfactant in which small droplets are formed with narrow distribution sizes in a continuous phase (Landfester, 2009). Different types of microemulsion such as waterin-oil, oil-in-water, and water-in-supercritical CO2 have been developed and applied to control the size and morphology of nanoparticles (Pecher and Mecking, 2010). Microemulsion methods have also been used for the synthesis of MONs applied mainly as heterogeneous catalysts (Ganguli et al., 2010). Malgras et al. (2014) synthesized mesoporous TiO2 from a water-in-oil microemulsion and observed an increase of 240% in the active surface area. Complex nanostructure (nonnoble) metal oxides can also be synthesized by the solgel method in reverse microemulsion (Zarur and Ying, 2000). Iron oxides have also been synthesized by solgel in a reverse microemulsion and applied in the degradation of orange II under visible light (Jiang et al., 2015). Interestingly, this mixture of technology provides a general route to the production of materials with large surfaceto-volume ratios and ultrahigh component dispersion (Zarur and Ying, 2000). Another important procedure for the production of MONs is based on the mixing of two microemulsions containing appropriate reactants (Rao et al., 2012). This last procedure has been well reviewed by Ganguli et al. (2010).

5.3.2 Applications 5.3.2.1 Iron Oxides MONs based on iron are promising for the removal of toxic heavy metal ions and organic pollutants from wastewater, due to their natural abundance, low-cost, easy separation, enhanced stability, environment-friendly properties, and strong adsorption capacity (Hua et al., 2012). In general, iron oxide nanoparticles are applied in contaminated water treatment in three modes: (1) as a photocatalyst to degrade or convert organic pollutants into less toxic form; (2) as an adsorbent for removing pollutants from contaminated water; or (3) as a heterogeneous Fenton catalyst for the mineralization of various organic pollutants (Zhang et al., 2014). Among these application modes, iron oxides are more commonly used in Fenton reactions (Nidheesh, 2015). A variety of iron oxides are used as starting materials for water treatment, including goethite (α-FeOOH) (Tang et al., 2018), hematite (α-Fe2O3) (Liu et al., 2018), amorphous hydrous iron oxide (Hua et al., 2017), maghemite (γ-Fe2O3) (Jiang et al., 2013), magnetite (Fe3O4) (Vojoudi et al., 2018), and mixed iron oxides (Fontecha-Ca´mara et al., 2016). 5.3.2.1.1 Goethite

Goethite is easily found as a major component in many ores, sediments, and soils in humid and semiarid regions. In addition, it is one of the most thermodynamically stable iron oxyhydroxides (Liu et al., 2014). It crystallizes in an orthorhombic structure with the Pbnm

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space group and has been confirmed as a hexagonally close-packed array of O22 and OH2 with Fe31 in the center of an octahedron. It is characterized by octahedral double chains and parallels to the [0 0 1] direction. These chains are formed by edge-sharing linked to the neighboring double chains by corner-sharing (Liu et al., 2014; Alexandrov and Rosso, 2015). With a complicated surface structure due to the distribution of water/hydroxyl groups, the presence of water interfacial tension between the planes of goethite and two types of terminal hydroxyl, a hydroxo group and a aquo group, has been confirmed (Ghose et al., 2010; Boily, 2012). Moreover, water interfacial molecules generally had smaller selfdiffusion coefficients and weaker hydrogen bonding interactions (Liu et al., 2014). Thus these properties together with a high SSA imply that goethite has potential as an adsorbent or catalyst. As a catalyst, goethite is used due to its chemically active surface in water, ability to operate in a wide pH range, positive performance with sunlight, high thermodynamic stability, low cost, and environment-friendly properties (Rahim Pouran et al., 2014). Tang et al. (2018) promoted the degradation of 2,4-dinitrophenol under anoxic conditions using nitrate as the electron acceptor and goethite as the catalyst. They observed that the degradation of 2,4dinitrophenol was improved by goethite due to the complete removal of the intermediate product, showing that goethite might be used as an catalyst for denitrification. Qian et al. (2017) synthesized a Fenton-like catalyst composed of α-FeOOH supported in mesoporous carbon and evaluated its efficiency against phenol degradation assisted with visible light. The authors observed that the fresh catalyst presented a phenol removal of 52% accompanied by an efficiency of mineralization of 14%. Interestingly, the reuse of the catalyst showed much higher phenol oxidation and mineralization efficiency than the fresh and homogeneous Fenton system (FeSO4/H2O2). Fig. 5.12 shows a schematic illustration of photocatalysis in a heterogeneous Fenton process under visible light irradiation. In this scheme, the α-FeOOH/MesoC is excited in order to generate e2/h1 pairs. This pair plays important roles, since e2 are responsible for the reduction of Fe(III) to Fe(II) present in the structure of the goethite. Then, Fe(II) accelerated the generation of OH∙ and OH2 via route 2 and, at the same time, H2O2 is also catalyzed into OH∙ and OH2 via route 4. The h1 also reacts with H2O to produce OH∙ via route 5. From the results obtained, the authors concluded that photocatalysis improves the results obtained by the Fenton process, leading to higher catalytic activities and mineralization efficiency. In general, goethite can adsorb both organic and inorganic compounds due to its large SSA and predominant face which are associated with the preparation method (Liu et al., 2014). Perelomov et al. (2011) studied the adsorption of Cu21, Pb21, and Zn21 in the presence and absence of organic acid at different pH values. They showed that in pH constant, the contents adsorbed cations and decreased in the sequence Cu . Pb . Zn. They also observed

206 Chapter 5

Figure 5.12 Schematic illustration of photocatalysis promoted heterogeneous Fenton process under visible light irradiation on the α-FeOOH/MesoC composite. Reproduced with permission from Qian, X., Ren, M., Zhu, Y., Yue, D., Han, Y., Jia, J., et al., 2017. Visible light assisted heterogeneous fenton-like degradation of organic pollutant via α-FeOOH/mesoporous carbon composites. Environ. Sci. Technol. 51 (7), 39934000.

that the simultaneous addition of organic acid and metal cations increases the adsorption capacity of goethite due to the formation of stable, negatively charged, metal-complexes that improved the electrostatic attraction between adsorbed and adsorbate. Munagapati et al. (2017) investigated the adsorption of methyl orange on goethite chitosan beads and verified that the process is controlled by electrostatic attraction and the best fit was achieved with the Langmuir isotherm equation. 5.3.2.1.2 Hematite

Iron oxide is a natural compound which can be found as amorphous Fe2O3 as well as four crystalline polymorph structures (α, β, γ, and ε) which have been well reported (Cornell and Schwertmann, 2003). Among the four crystalline structures, α-Fe2O3 is one of the most stable and versatile. It can be used effectively as a metal ion adsorbent or as a photocatalyst due its stability, large surface area, high surface-to-volume ratio, and favorable optical properties even after losing its reactivity as a heterogeneous catalyst. It can be used as a starting material for pig iron production (Shen et al., 2016; Bhatia et al., 2017). The synthesis method of hematite is very important since it determines its effectiveness as an adsorbent of metal ions and a catalyst for the degradation of pollutants. Datta et al. (2016) synthesized hematite with a 3D rod/flower-like structure and applied it as a Fenton catalyst. A 3D nanoassembly of the nanoparticles showed the highest rate constant reported for the decomposition of H2O2 (1.43 3 1021 min21) and a superior efficiency for the degradation of aromatic compounds and chlorinated pollutants in contaminated water. In terms of the photodegradation of organic pollutants, Wang et al. (2010a) showed that hematite is the more active iron oxide at the beginning of the reaction since the results for the half-life (t1/2) of the degradation of sulfadiazine follows the order hematite . maghemite . goethite. However, goethite was found to be the most effective synthesis method.

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In a review article, it was shown that lead ion (Pb(II)) has the highest affinity toward hematite compared with other heavy metal ions such as Cd(II), Cu(II), and Zn(II) (Bhatia et al., 2017). Another important inorganic pollutant is Cr(IV), which has a strong migration capability and oxidation ability. Xiao et al. (2015) related the size-dependence of Cr(IV) adsorption on a hematite surface. They observed that Cr(IV) adsorption increases with a decrease of particle size due to the increase of SSA and surface free energy. Moreover, they showed that Cr(IV) replaced the binding site of OH on the hematite surface and would form inner-sphere complexes with hematite itself. Besides good adsorption of Cr(IV), hematite can also act as a catalyst in the reduction of Cr(IV) by citric acid (Gao et al., 2018). In this process, the Fe atoms of the hematite surface play an important role since it is assumed that they transport electrons from citrate to Cr(IV) through Fe(III)Fe(II) cycling. Fig. 5.13 shows an illustrative scheme of this process. 5.3.2.1.3 Magnetite and Maghemite

Magnetite is an iron oxide formed by Fe31 and Fe21 cations that can be represented by the chemical formula (Fe31)tet(Fe31Fe21)octO4, although it is more commonly represented by the chemical formula Fe3O4. The presence of ferrous ions in their structure ensures unique redox properties. In the Fenton process, iron ions play the role of a catalyst in the degradation of hydrogen peroxide to produce highly active species, mainly nonselective ∙OH radicals with an oxidation potential of 2.8 V (Nidheesh, 2015). This production of ∙OH radicals can be summarized by Eqs. (5.14 and 5.15): H2 O2 1 Fe21 -Fe31 1 OH2 1  OH

k 5 63 M21 S21

(5.14)

Figure 5.13 Ilustrative scheme of the reduction of Cr(IV) by citric acid catalyzed by hematite nanoparticles. Reproduced with permission from Gao, W., Yan, J., Qian, L., Han, L., Chen, M., 2018. Surface catalyzing action of hematite (α-Fe2O3) on reduction of Cr(VI) to Cr(III) by citrate. Environ. Technol. Innov. 9 (Supplement C), 8290.

208 Chapter 5 H2 O2 1 Fe31 -Fe21 1 H1 1 HOU2

k 5 0:002 2 0:01 M21 S21

(5.15)

From the constant rates of this reaction, it is possible to see that the ferrous ions are more reactive for the generation of radicals than the ferric ions are. Thus magnetite has gained considerable attention due to its unique characteristics, such as the presence of ferrous ions and of octahedral sites in its structure, thus improving hydroxyl radical production. Its magnetic properties allow for easy separation from the reactional system, a higher dissolution rate compared to other iron oxides, and higher electron mobility in its spinel structure (Rahim Pouran et al., 2014). He et al. (2014) synthesized magnetite having a quasi-spherical morphology and high SSA, and investigated its catalytic activity in a Fenton process against catechol and 4-chlorocatechol. The authors saw that the pollutants utilized were oxidized within 3 hours of H2O2 addition and were mainly attacked by the surface generated ∙OH and HO:2 /O:2 2 in accordance with the EleyRideal mechanism. The degradation of 4-chlorocatechol was faster than that of catechol, although only a 40% mineralization was observed. The presence of ferrous ions also limits the application of magnetite due to its easy oxidation to ferric ions (Freire et al., 2016). Thus magnetite is generally coated with an inorganic or organic shell to prevent its oxidation and to improve its adsorption capacity as well as its catalytic activity (Kharisov et al., 2012). Vojoudi et al. (2018) utilized a modified mesoporous magnetite nanostructure as an efficient adsorbent in a homemade device (Fig. 5.14). In this device, the column is filled with magnetite-coated silica and used to remove organic pollutants from aqueous solutions. In this work, the authors observed that the adsorption of Everzol blue dye was highly dependent on the solution pH, contact time, flow rate, and adsorbent dose. However, in specific conditions, it was possible to obtain a 100% removal efficiency of Everzol blue. Yan et al. (2017) synthesized Fe3O4-polyvinyl alcohol/chitosan composite fibers and evaluated their adsorption capacity against Cr(IV). This composite was more efficient in acidic environments and the adsorption mechanisms are mainly dominated by electrostatic attraction between adsorbent and adsorbate. In comparison to magnetite, maghemite is less active in the Fenton process, nevertheless it has great affinity for Cr(IV) from water (Hanna et al., 2008; Hua et al., 2012). Jiang et al. (2013) showed that the adsorption of Cr(IV) on the surface of hematite is spontaneous and highly favorable in both acid and neutral conditions with maximum removal observed at pH 4. Adsorption of multiple heavy metals on the hematite surface was performed by Koma´rek et al., 2015 under pH values 3, 4.5, 6 for Cr(VI) and Pb(II); 6, 7, 8 for Cd(II) (Koma´rek et al., 2015). It was observed that the adsorption of Cd(II), Pb(II), and Cr(IV) follow the expected trends, that is, increased adsorption of Cd(II) or Pb(II) with increasing pH, and decreasing Cr(IV) adsorption. There is no significant influence of Cd(II) or Pb(II) competition on the maximum sorption of other metals at low pH.

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Figure 5.14 Schematic operation of the homemade device for the adsorption of Everzol blue from aqueous solutions. Reproduced with permission from Vojoudi, H., Badiei, A., Amiri, A., Banaei, A., Ziarani, G.M., Schenk-Joß, K., 2018. Efficient device for the benign removal of organic pollutants from aqueous solutions using modified mesoporous magnetite nanostructures. J. Phys. Chem. Solids 113 (Supplement C), 210219.

5.3.2.1.4 Transition Metals Substituting Iron Oxide

In many crystalline structures of iron oxide, Fe atoms can be substituted isomorphically by other transition metals, which have similar ionic radius and oxidation state. The physicochemical properties of these materials depend on type, amount of loaded transition metal, synthetic route, and sites occupied by the transition metal (Zhong et al., 2012, 2013). Jacobs et al. (1994) reported that the cations of octahedral sites are the most responsible for catalytic activity and are totally localized on the surface of the crystal in the spinel structure. In magnetite, the Fe atom that is substituted by a transition metal is localized, preferably on an octahedral site, and the substitution of Fe on a tetrahedral site occurs only if the concentration of the imported active cation is high (Magalha˜es et al., 2007; Liang et al., 2012). Review articles have shown that various transition metals substituting iron oxide have been synthesized (Rahim Pouran et al., 2014). Among these, special attention has been given to those with a spinel structure, commonly called ferrites, due to their excellence magnetic properties, simple chemical composition, and wide applications in several areas, which include water and wastewater treatment, biomedical applications, as a catalyst, and in electronic devices (Kefeni et al., 2017). Particularly in water and wastewater treatment, ferrites are applied mainly as adsorbents of pollutants, photocatalysts for organic pollutant degradation, and the detection of different types of metal ions. Zhong et al. (2013) investigated the effect on the UV/Fenton catalytic activity of the isomorphous substitution of transition metals (Ti, Cr, Mn, Co, and Ni) in magnetite. They

210 Chapter 5

Figure 5.15 Illustrative scheme of the mineralization of tetrabromobisphenol from a photo-Fenton reaction catalyzed by a transition metal substituting iron oxide. Reproduced with permission from Zhong, Y., Liang, X., Tan, W., Zhong, Y., He, H., Zhu, J., et al., 2013. A comparative study about the effects of isomorphous substitution of transition metals (Ti, Cr, Mn, Co and Ni) on the UV/Fenton catalytic activity of magnetite. J. Mol. Catal. A Chem. 372, 2934.

observed that the incorporation of transition metals improved the degradation efficiency of tetrabromobisphenol following the order: Co , Mn , Ti  Ni , Cr. These substitution cations participated in the decomposition of H2O2 through the HaberWeiss mechanism and improved the separation and efficiency transfer of the photogenerated electrons and holes. Fig. 5.15 illustrates the participation of transition metals in the process. The adsorption on the surface of ferrites is influenced by various factors, such as particle size, type, and synthesis method of the adsorbent, pH of the wastewater, initial concentration of the pollutant, adsorbent dosage, temperature of the system, and charge of the adsorbate (Bhatia et al., 2017). In ferrites, the surface charge and hydroxy groups play an important role in the adsorption mechanism, which is mainly dominated by ion exchange and the formation of outer-sphere complexes and/or inner-sphere surface complexes depending on the pH medium (Kefeni et al., 2017). However, it was shown that adsorption processes involving ferrite can also be dominated by electrostatic force and physisorption (Yang et al., 2014; Neyaz and Siddiqui, 2015). In aqueous solution, ferrites and contaminants can have a positive, neutral, or negative surface charge, depending on the pH of the system (Yang et al., 2014). In general, the existence of an ideal pH with a maximal adsorption is evident. For example (Tu et al., 2014), determined that the maximal adsorption of Mo(VI) on CuFe2O4 occurred at pH 2.75 due to attraction between the positively charged CuFe2O4 surface and the negatively charged molybdate species present in the system. Other parameters affecting the adsorption process of ferrites have been well reviewed by Kefeni et al. (2017).

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5.3.2.2 Titanium Dioxide Titanium dioxide can be mainly found in three different crystalline structures: anatase, brookite, and rutile (Lan et al., 2013). In these three forms, the Ti41 atoms are coordinated with six oxygen (O22) atoms, forming a TiO6 octahedral (Pelaez et al., 2012). Fig. 5.16 shows the three main phases, that is, (1) anatase, (2) rutile, and (3) brookite structure. Anatase and brookite phases are metastable and can be irreversibly converted to a stable rutile upon heating (Kaplan et al., 2016). Anatase possesses a band gap of 3.2 eV, while rutile and brookite have a 3.0 and 3.2 eV band gap, respectively. Although anatase and rutile showed similar physical properties, anatase has better photocatalytic activity than rutile due to its prolonged lifetime of charge carriers and spatial charge separation (Kaplan et al., 2016). Since the discovery of the phenomenon of photocatalytic splitting water on titanium dioxide (TiO2) under UV light, TiO2 has been widely used as a photocatalyst and adsorbent in water and wastewater treatment systems (Mahlambi et al., 2015, Das et al., 2017). Many works have demonstrated that TiO2 can be used for the adsorption of both metal and organic pollutants (Ali, 2012). Wagle and Shipley (2016) observed that TiO2 nanoparticles have a greater adsorption capacity for As(V) when normalized by mass, and TiO2 bulk has a greater adsorption capacity when normalized by surface area. However, the adsorption rate for TiO2 nanoparticles was 87 times greater than that of TiO2 bulk. Interestingly, Jegadeesan et al. (2010) showed that amorphous TiO2 nanoparticles may be a better arsenic adsorbent than crystalline TiO2 nanoparticles due to a higher concentration of Ti31 on the surface. Thus is important to notice that the sorption behavior of TiO2 is largely dependent on crystallinity, particle size, and surface energy.

Figure 5.16 Crystalline structure of (A) anatase, (B) rutile, and (C) brookite. Reproduced with permission from Pelaez, M., Nolan, N.T., Pillai, S.C., Seery, M.K., Falaras, P., Kontos, A.G., et al., 2012. A review on the visible light active titanium dioxide photocatalysts for environmental applications. Appl. Catal. B Environ. 125, 331349.

212 Chapter 5 Actually, TiO2 is considered a photocatalyst with great potential due to its chemical stability, nontoxicity, and high reactivity (Jing et al., 2013). However, it is necessary to expose TiO2 to UV photons with a 240400 nm (3.104.43 eV) wavelength range, in order to be adsorbed and for the excitation of an electron to the conduction band (e2 ðCBÞ ) to occur, 1 generating a positve hole in the valence band (hðVBÞ ) (Pelaez et al., 2012), that is: 2 TiO2 1 hν-h1 ðVBÞ 1 eðCBÞ

(5.16)

The generated charges on TiO2 may undergo a recombination process causing energy liberation or migration to the catalyst surface, thus, implying redox reactions (Pelaez et al., 2012). It is interesting to note that the oxidation potential of the photoinduced holes is up to 3.0 eV, which is much higher than that of hydrogen (1.36 eV) and ozone (2.07 eV) (Lan et al., 2013). Therefore the generation of electronhole pairs on the catalytic surface is an essential step for the formation of ROS, which is the main factor responsible for organic pollutant degradation (Pelaez et al., 2012). Eqs. (5.17)(5.23) summarize the possible ROS formation and pollutant degradation reactions. 1 H2 O 1 h1 ðVBÞ -  OH 1 H

(5.17)

U2 O2 1 e2 ðCBÞ -O2

(5.18)

OH 1 pollutant---H2 O 1 CO2

(5.19)

1 OU2 2 1 H -  OOH

(5.20)

OOH 1  OOH---H2 O2 1 O2

(5.21)

OU2 2 1 pollutant---H2 O 1 CO2

(5.22)

OOH 1 pollutant---H2 O 1 CO2

(5.23)

In general, all water and wastewater treatment devices that use TiO2 as a photocatalyst are based on the steps showed in Eqs. (5.16)(5.23) and on surface adsorption for pollutant degradation. For example, Serna-Galvis et al. (2017) applied TiO2 as a photocatalyst for antibiotic cloxacillin degradation. They see that the adsorption of cloxacillin is the most key step in the process and that degradation occurs on the catalyst surface due to photogenerated holes and adsorbed hydroxyl radicals. It has been well reported that TiO2 and its derivatives have a good photocatalytic activity when expose to UV light. However, photocatalysis has attracted much attention due to a significant hope of using sunlight as an energy source for pollutant degradation (Bora and Mewada 2017). Thus much effort has been made to develop strategies that improve the photocatalytic activity of TiO2 under sunlight. This topic has been well reviewed by Pelaez et al. (2012), Schneider et al. (2014), and Rehman et al. (2009).

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5.3.2.3 Zinc Oxide Zinc oxide (ZnO) is an environment-friendly material that can be used without any risk to human health or any environmental impact (Lee et al., 2016). ZnO is crystalline (wurtzite phase) and thermodynamically stable at room temperature. It has a hexagonal structure and an ionicity at the borderline between covalent and ionic semiconductor (Samadi et al., 2016). Besides these characteristics, ZnO is an n-type semiconductor with a broadband gap at 3.37 eV that has a large excitation binding energy of 60 meV and a deep violet/borderline ultraviolet adsorption at room temperature (Choi et al., 2012). Thus due to its excellent optical, electrical, and mechanical properties, similar to TiO2, ZnO has emerged as a promising material for water and wastewater treatment (Gharoy Ahangar et al., 2015). Many hierarchical ZnO nanostructures with controlled morphology and size have been applied in heavy metal adsorption and advanced oxidative processes (AOP) (Ong et al., 2018). As an adsorbent, ZnO is mostly applied to eliminate H2S (Wang et al., 2010b). However, in the past few years, it has been used for heavy metal adsorption (Hua et al., 2012). Wang et al. (2010b) synthesized porous ZnO nanoplates with a yield of more than 94% and a high SSA (147 m2 g21), and assessed its capacity and selectivity to cationic contaminants. Interestingly, they observed that porous ZnO nanoplates have an unsaturated adsorption capacity of more than 1600 mg g21 for Cu(II) ions. This sorption capacity of ZnO can also be employed to synthesize other environmental materials (Ma et al., 2010). Nonetheless, it is important to mentioned that, in general, the amount adsorbed is smaller than that observed, as pointed out by Wang et al. (2010b). Among the diverse AOPs, photocatalysis has gained attention due to the possibility of using solar energy for organic pollutant degradation. From this perspective, several types of photocatalysts, such as TiO2, ZnO, Fe2O3, ZrO2, V2O5, and WO3, have been applied for water and wastewater treatment (Lee et al., 2016). ZnO has been suggested as a good photocatalyst due to its unique characteristics, such as direct and wide band gap near the UV spectral region, strong oxidation ability, and large free-exciton binding energy (Ong et al., 2018). Fig. 5.17 shows the overall heterogeneous photocatalysis pathway process. When ZnO is photoinduced by light radiation with energy (hυ) equal to or higher than the excitation energy (Eg), electrons (e2) are excited and transferred from the valence band 1 (VB) to the conductance band e2 ðCBÞ , creating a hole hðVBÞ in the valence band. The electronhole pairs can migrate to the ZnO surface and be involved in redox reactions to produce hydroxyl radicals (∙OH) and superoxide anion radicals O2: 2 (Lee et al., 2016). However, the efficiency to generate highly reactive radicals can decrease mainly due to the 1 recombination process of e2 ðCBÞ and hðVBÞ , leading to a reduction of the quantum yield for the photocatalytic process (Ong et al., 2018).

214 Chapter 5

Figure 5.17 Schematic illustration of the formation of the e2 and h1 pair and photocatalytic degradation of pollutants on the ZnO nanostructured surface. Reproduced with permission from Samadi, M., Zirak, M., Naseri, A., Khorashadizade, E., Moshfegh, A.Z., 2016. Recent progress on doped ZnO nanostructures for visible-light photocatalysis. Thin Solid Films 605, 219.

Since the recombination process is mainly influenced by parameters that are correlated to ZnO nanostructure, a strategy for improving the photocatalytic activity is changing the morphology and size of the ZnO nanoparticles and/or doping with different organic or inorganic materials (Rehman et al., 2009). Ma et al. (2011) demonstrated that ZnO nanorods and nanoflowers have superior photocatalytic performance compared to commercial ZnO particles, which have a spherical morphology. Pirsaheb et al. (2017), using a doping strategy, developed chromium-doped zinc oxide nanoparticles immobilized on sandblasted glass for the photocatalytic degradation of aniline in a continuous reactor. They observed that, in optimal conditions and under sunlight illumination, 93% of aniline was removed after 6 hours. 5.3.2.4 Manganese Oxide Manganese (Mn) oxide is considered the strongest natural oxidant agent and can be found in a wide range of natural environments and oxidation states: Mn(II), Mn(III), and Mn(IV) (Remucal and Ginder-Vogel, 2014). In general, the reactivity of Mn oxides is related to their oxidation state, crystalline phase, and surface area (Najafpour et al., 2016). Besides these, another important factor for assessing the reactivity of Mn oxides in water and wastewater treatment is the chemical characteristic of the pollutants present, which significantly influence the process (Nidheesh, 2015). For example, the relative oxidation rate of substituted anilines and phenols decrease when the redox potential or Hammett coefficients (σ) increase (Stone, 1987, Li et al., 2000). Several studies involving phenols and anilines have been utilized to show the reactivity of Mn oxides and some of them were summarized and discussed by Remucal and GinderVogel (2014). The mechanisms of phenol and aniline oxidation by Mn oxide are similar and can be summarized in three steps: (1) The diffusion of the aromatic compound into the boundary layer; (2) the formation of a surface complex; and (3) the oxidation of the

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compound via a one-electron transfer within the surface complex. Ulrich and Stone (1989) studied the chlorophenol compounds on the surface of Mn oxide and found that the formation of a complex is a rate-limiting step, while for aromatic amine oxidation the first electron transfer is a rate-limiting step (Salter-Blanc et al., 2016). An important advantage of a system containing Mn oxide compared with traditional advanced oxidation processes, is its high reactivity, which allows for the removal of traces of organic contaminants with low energy costs and no addition of hazardous chemicals. However, Mn oxides present the same disadvantage in the degradation of selected organic pollutants; the rate of contaminant oxidation is pH-dependent and the leaching of Mn(II) to the treated effluent make the use of a Mn oxide system a challenge (Forrez et al., 2009; Forrez, Carballa et al., 2011). 5.3.2.5 Metal Oxide Supports The use of NMOs for organic pollutant degradation and the adsorption of heavy metals have increased in works involving water and wastewater treatment (Srikanth et al., 2017). However, drawbacks have often been observed due to their size, such as activity loss due to agglomeration, difficult separation, and excessive pressure drops when applied in a flowthrough system (Hua et al., 2012). In order to overcome these technical bottlenecks, a capping agent and supports based on metallic oxide were developed (Granger et al., 2018; Vojoudi et al., 2018). Silica is the main metal oxide used as a capping agent and support due to its facile process of synthesis and its high SSA (Mauˇcec et al., 2017; Vojoudi et al., 2018). However, another commonly used support is alumina, which has a strongly alkaline surface, wide range of SSAs, and ion exchange capacity due to the presence of Al31 in its framework. For the degradation of organic pollutants, researches are often found that explore the use of silica and alumina as supports for iron and iron oxides (Nidheesh, 2015). Jusoh et al. (2015) synthesized two types of Fe species supported on mesostructured silica nanoparticles and used them as heterogeneous catalysts for the degradation of 2-chlorophenol under fluorescent light irradiation. From the results, the authors suggested the existence of a synergistic effect between the dual type of Fe species (SiOFe and ISFeOOH colloid) and mesostructured silica nanoparticles, which play an important role in enhancing the degradation of 2-chlorophenol. Nogueira et al. (2014) developed a heterogeneous catalyst based on magnetite-MCM-41, and observed the complete removal of methylene blue with a reaction time of 180 minutes. In addition, they also showed the existence of a synergistic effect between MCM-41 and magnetite. Ferric iron supported in alumina has been investigated by Ghosh et al. (2012). They compared the degradation efficiency of a heterogeneous catalyst and homogeneous ferric iron in a Fenton process and observed that in the optimal conditions the degradation efficiency was 98% and 92.5%, respectively.

216 Chapter 5 Bautista et al. (2011) synthesized an Fe/γ-Al2O3 catalyst by incipient wetness impregnation of γ-Al2O3 with an aqueous solution of Fe(NO3)3∙9H2O, followed by calcination at 300 C for 4 hours. It had an efficiency of almost 80% mineralization of the phenol, as a heterogeneous catalyst in optimal conditions. It is important to note that, in general, works that investigated the effects of supports in water and/or wastewater treatment focused mainly on heterogeneous catalysts.

5.4 Conclusions This chapter provided a revision on water treatment devices based on metal oxide nanomaterials. Size and morphology affect the reactivity of a material, its quantum confinement, and different optical and electric behaviors. For instance, nanostructures of metal oxide are used as electrochemical supercapacitors, sensors, magnetic controlling systems, and adsorbents and catalysts to remove organic compounds from water through adsorption. Another important application is related to the usage of these materials in membrane separation processes in water treatment with nanoparticles of iron oxide, silica, alumina, silver, zeolites, carbon nanotubes, enhanced titanate oxide membranes, as well as for the functionality of membranes with integrated nanoparticles in water treatment.

Acknowledgment The authors acknowledge the financial support of the following Brazilian agencies for scientific and technological development: CNPq (408790/2016-4, 433168/2016-1), CAPES and Funcap (PNE-011200048.01.00/16); BNB-FUNDECI (2017.0002), FUNCAP (P13-0086-00057.01.00/13), MCTI/CNPq/CT-Biotec (402835/2013-1).

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