Water Depollution Using Ferrites Photocatalysts

Abstract

The presence of organic pollutants and pathogenic microorganisms in water has become an increasing concern throughout the world. Heterogeneous photocatalytic technologies have been applied to control the organic pollutants and microorganisms in water. Development of narrow band-gap photocatalysts which function in the visible light remains a challenge in the wastewater treatment processes. Spinel ferrites has attracted a remarkable attention because of a relatively narrow band gap of about 2.0 eV, which has considerable photo-response in the visible light region. This chapter reviews recent advances in ferrites and the application of visible light photocatalysts to the remediation of contaminants such as H₂S, phenols, and dyes in water. Recent development in synthesis and characterization of ferrite and hybrid ferrites with other semiconductors is reviewed. The applications of ferrites in photocatalytic conversion of visible solar energy to generate e^–/h⁺, which in turn produce reactive oxygen species through redox processes for the degradation of the pollutants in water, are demonstrated. We discuss the enhanced visible photocatalytic activity of ferrites by doping with metals and combing with other photocatalysts. Moreover, the addition of H₂O₂ to ferrite either in dark or visible light irradiation indicates the enhanced degradation efficiency for organic pollutants.

4.1 Introduction

Freshwater is a precious resource on earth and is critical to sustain life. In many regions of the world, daily need of water is not met. According to World Health Organization (WHO), more than 880 million people in the world do not have access to potable water (WHO 2010). Water contamination caused death to 1.8 million children from diarrhea every year (WHO 2010). Water scarcity can also affect ecosystem as numerous species might not be able to cope with decrease in availability of freshwater. One of the greatest challenges in this century is to provide access to clean water. The development of nanotechnology in the past decade offers prospects of meeting challenges of safe and sustainable water demand (Di Paola et al. 2012; Qu et al. 2012). Furthermore, a combination of nanotechnology and solar energy may lead to innovative water purification technologies.

One of the well studied technologies to clean water is the photocatalytic remediation (Di Paola et al. 2012; Rajeshwar 2011; Rajeshwar et al. 2012). The photocatalytic processes on bare semiconductor titanium dioxide (TiO₂) have been studied for several decades, but TiO₂ is only active under UV irradiation (λ < 400 nm) (Abe 2011; Chen et al. 2011; Hoffmann et al. 1995; Li and Liu 2011; Serpone et al. 2012; Tao et al. 2011). The solar spectrum has very small fraction of incoming light in the UV region (ca. 4 %). A use of all the UV light would result in only 2 % solar conversion efficiency (Abe 2011). Comparatively, the portion of visible light (400 < λ < 800 nm) in the solar spectrum is much more abundant (ca. 46 %). A use of visible light up to 600 nm increases the efficiency to 16 % and further improvement to 32 % is achievable if visible light use is extended to 800 nm (Kubacka et al. 2012; Linic et al. 2011; Paracchino et al. 2011; Serpone and Emeline 2012).

Currently, the efficiency for degradation of pollutants under visible-light irradiation is still low due to the fast charge recombination and backward reactions of photocatalysts (Chen et al. 2011). The efficient visible-light-driven photocatalysts requires closing the band gaps to harvest visible light in the long-wavelength regions as well as to improve the separation of photogenerated electrons (e⁻) and holes (h⁺) efficiently. Dopants into TiO₂ have been added to narrow the band gap in order for TiO₂ to be appropriate for absorbing visible light (Kamat 2012; Kubacka et al. 2012; Serpone et al. 2009). However, doped TiO₂ materials under visible light have lower chemical activity of surface active centers and decreased photoactivities compared to those formed under UV light irradiation (Serpone and Emeline 2012). Moreover, commonly used dopants are rare, expensive and/or toxic metals, which may not fulfill the principle of sustainable chemistry. Identification of suitable candidates capable of degrading pollutants under visible light is essential to provide clean remediation processes. The objective of this article is to provide information on the progress made in remediation of water using environmentally benign visible-light active iron-based oxides, ferrites.

Ferrites have a molecular formula M-Fe₂O₄ in which Fe₂O₃ is combined with a metal oxide (M-O). Ferrites may provide an alternative to TiO₂ because they have shown to be efficient in the visible light region (Han et al. 2007). Metals used in ferrites include Ca²⁺, Zn²⁺, Mg²⁺, Ni²⁺, Co²⁺, and Mn²⁺ (Benko and Koffyberg 1986; Borse et al. 2008; Dom et al. 2011; Han et al. 2007; Tamaura et al. 1999). These ferrites, when prepared with a spinel structure, show promising photocatalytic activity (Kim et al. 2009). Spinel structures have the general formula AB₂X₄, where, in the case of ferrites, A is a metal, B is Fe³⁺ and X is oxygen (Burdett et al. 1982). As shown in Fig. 4.1 for example for ZnFe₂O₄, the spinel structure usually consists of both tetrahedral and octahedral sites, where the metal atoms occupy one-eighth of the tetrahedral sites and Fe³⁺ occupies one-half of the octahedral sites (Degueldre et al. 2009). The lattice parameter a and the cation-oxide distance R_ZnO and R_FeO in the octahedral and tetrahedral substructures were related by Eq. 4.1 (Degueldre et al. 2009)

Fig. 4.1

Schematic description of the crystallographic structure for bulk ZnFe₂O₄. Occupied tetrahedral site in spinel sub-cell, Zn is gray and O is in red. Occupied octahedral site in spinel sub-cell, Fe is yellow, and O is in red. The arrangement in one unit cell with 3D succession of octahedral and tetrahedral sub-cells (Adapted from (Degueldre et al. 2009) with permission from Elsevier Ltd.)

$$ \mathrm{A} = 2{(2)^{1/2 }}[{{\mathrm{R}}_{\mathrm{FeO}}} \cos (\psi /2) + {{\mathrm{R}}_{\mathrm{ZnO}}} \sin (\varphi /2)] $$

(4.1)

The tetrahedron internal angle (φ) and the octahedron equatorial angle (ψ) were identified as 109.471° and 95.375° for O-Zn-O and Fe-O-Fe, respectively.

The ferrites of Zn²⁺, Ca²⁺, and Mg²⁺ seem to be favorable as they are more relatively environmentally friendly. The energy diagram of these ferrites is shown in Fig. 4.2 (Dom et al. 2011). The band gaps of ∼2.0 eV show the potential of ferrites to absorb visible light absorption and also the possibility for degrading pollutants in water (Casbeer et al. 2012; Hou et al. 2010; Ida et al. 2010; Li et al. 2011; Su et al. 2012).

Fig. 4.2

Schematic diagram of MFe₂O₄ (M: Mg, Ca, Zn) showing the feasibility of materials as visible light photocatalysts. The thickness of the band edge (CB-conduction/VB valence band) indicates the possible variation in the value depending on various physico–chemical parameters of electrolyte and environmental conditions viz. pH, temperature, concentration etc. and there possible effects on the Fermi energy (Adapted from (Dom et al. 2011) with the permission of Elsevier Ltd.)

The present chapter first describes briefly the synthesis and characterization of ferrites, followed by application of ferrites to degrade contaminants. Most of studies in literature have been performed on ZnFe₂O₄ and hence examples are presented using this form of ferrite.

4.2 Synthesis of Ferrites

4.2.1 Spinel Ferrites

A number of methods used in synthesis of spinel ferrites are shown in Fig. 4.3. These method include thermal, sol-gel and citrate, co-precipitation, and solid-state reactions (Casbeer et al. 2012; Dom et al. 2011; Hou et al. 2011; Jadhav et al. 2012; Li et al. 2011; Pardeshi and Pawar 2011; Pradeep et al. 2011; Salunkhe et al. 2012; Su et al. 2012; Zhang et al. 2010a). In synthesis of ferrites, salts of Fe(III) and M(II) were used as precursors. Synthesis methods have recently been summarized (Casbeer et al. 2012). Highly visible-light active ZnFe₂O₄ nanotube array has also been prepared using sol-gel methods (Li et al. 2011). Microwave sintering process may be advantageous due to its shorter synthesis time compared to conventional methods (Dom et al. 2011).

Fig. 4.3

Preparation methods of ferrites using Fe(III) and M(II) salts as precursors (Adapted from (Casbeer et al. 2012) with the permission of Elsevier Ltd)

4.2.2 Composite Ferrites

In composite ferrites, the emphasis has been on the coupling of titania and zinc ferrite through different approaches. Ferrite is sensitive to visible light while titania has high photoactivity (Chen et al. 2010; Cheng et al. 2004) and composites showed increased photocatalytic activity (Tan et al. 2012). Sol-gel and hydrothermal methods were successfully used to prepare either titania doped ferrite or ferrite doped titnia (Cheng et al. 2004; Liu et al. 2004; Moreira et al. 2012). More recently, TiO₂-ZnFe₂O₄ with an intermediate layer of alumina has been prepared by a multistep wet chemical method (Hankare et al. 2011). Other methods include the liquid catalytic phase transformation at low temperature and the two-step electrochemical processes (Hou et al. 2010; Shihong et al. 2009). Nanocomposites of ZnFe₂O₄ nanoplates and Ag nanoparticles and ZnFe₂O₄/multi-walled carbon nanotubes (MWCNTs) have also been prepared (Cao et al. 2011; Chen et al. 2010).

4.3 Characterization

Figure 4.4 shows the optical properties of ferrites, analyzed by UV–vis absorption spectroscopy. Ferrites have strong absorption in the visible range of 400–700 nm (Dom et al. 2011; Liu et al. 2009; Subramanian et al. 2004; Valenzuela et al. 2002). This is significant because of performing photocatalytic activity under visible light. Comparatively, TiO₂ does not show any significant absorption in the visible region. The electron excitation from the O-2p level into the Fe 3d level for ferrites may be causing absorption in the visible region (Lv et al. 2010). The two persistent absorption bands of spinel structures of nano ZnFe₂O₄ at ₁ = 545 and ₂ = 292 cm⁻¹ in the Fourier Transform Infrared (FTIR) spectrum were observed (Pradeep et al. 2011). These bands could be related to the oxygen-metal cation complex presence in the tetrahedral and octahedral sites of the ferrite. Other surface techniques have also been applied to characterize ferrite, which include X-ray diffraction (XRD), X-ray spectroscopy (XAS), transmission electron microscopy (TEM), scanning electron microscopy (SEM), neutron diffraction analysis, and Brunauer-Emmett-Teller (BET) surface area (Blanco-Gutierrez et al. 2011; Hankare et al. 2011; Hou et al. 2011; Li et al. 2011; Moreira et al. 2012; Nilsen et al. 2007).

Fig. 4.4

Visible light spectra of metal ferrites (Adapted from (Dom et al. 2011; Liu et al. 2009; Subramanian et al. 2004; Valenzuela et al. 2002))

Mossbauer spectroscopy has also been used to characterize ZnFe₂O₄ samples (Blanco-Gutiérrez et al. 2011). The zero-field Mossbauer spectra of samples, prepared differently are presented in Fig. 4.5 (Blanco-Gutiérrez et al. 2011). Samples Z3 and Z19 were synthesized using solvothermal method while Z11 was prepared by means of sol-gel method. All samples had different treatment time while temperature was ranged from 160 °C to 200 °C. Treatment times were 2 and 288 h for Z3 and Z19 samples, respectively. Sample Z11 was treated for 24 h. The mean particle sizes of samples were 3, 11, and 19 nm for Z3, Z11, and Z19, respectively. Spectra of samples at 1.4 and 4.2 K had two broadened sextets in which suggested the presence of Fe³⁺. The non-Lorentzian shape of lines gave mean hypefine parameters as 47.2, 50.2, and 49.9 T for Z3, Z11, and Z19, respectively. The characteristic doublets appeared with increase in temperature (Fig. 4.5). This is indicative of superparamagnetic behavior of particles of such samples.

Fig. 4.5

Mössbauer spectra obtained at different temperatures for Z3 (a), Z11 (b), and Z19 (c) samples (Adapted from (Blanco-Gutiérrez et al. 2011) with the permission of the American Chemical Society)

4.4 Oxidation by Ferrites

The absorption of light by semiconductor photocatalysis results in several processes (Fig. 4.6) (Teoh et al. 2012). In the first step, formation of electron–hole (e-h⁺) occurs through bandgap excitation. A number of reactions can take place in absence and presence of contaminants in water (Fig. 4.6). Both electron and hole can diffuse to the surfaces of semiconductors and reduce and oxidize the adsorbed contaminants, respectively (Fig. 4.6). Other possibility is the recombination of electron and hole, which decreases the efficiency of the photocatalysts to react with contaminants in water. Hydroxyl radicals (^•OH) have been suggested to be dominated species to oxidize contaminants. Hydroxyl radicals are efficient in abstracting hydrogen atom and attaching to electron-rich moieties (Park and Choi 2005; Turchi and Ollis 1990). Superoxide species (O₂ ^•−) are generated by the reaction of O₂ with electron (Gerischer and Heller 1991; Schwitzgebel et al. 1995). Other reactive oxygen species such as singlet oxygen (¹O₂), hydrogen peroxide (H₂O₂), and hydroperoxyl radicals (HO₂ ^•) may also be produced and be involved in the photocatalytic oxidation reactions. In the next section, applications of ferrites (e.g., ZnFe₂O₄) as a photocatlysts in degrading contaminants are presented.

Fig. 4.6

Possible reaction pathways arising from the excitation of photocatalyst (Adapted from (Teoh et al. 2012) with the permission of American Chemical Society)

4.4.1 Degradation of Contaminants

Ferrites have been useful in decontamination of inorganic compounds and disinfection (Li et al. 2008; Liu et al. 1996; Rana et al. 2005; Rawat et al. 2007; Zhang et al. 2010b; Zhao et al. 2010). Examples include photodehydrogenation of H₂S using ZnFe₂O₄ and inactivation of Escherichia coli by composite ferrites, Ag/MgFe₂O₄, Ag/Ni₂Fe₂O₄, Ag/Zn₂FeO₄, and Ag/CoFe₂O₄. Ferrites have also shown effectiveness in degrading aliphatic compounds such as methanol, ethanol, isopropanol, acetaldehyde, oxalic acid, and butenes (Gibson and Hightower 1976; Manova et al. 2004, 2011; Shchukin et al. 2004; Tsoncheva et al. 2010). Most of studies on the remediation by ferrites are on the oxidation of phenols and dyes and are discussed below.

4.4.1.1 Phenols

A few studies on the oxidation of phenols by ferrites have been performed and are summarized in Table 4.1 (Aziz et al. 2012b; Hou et al. 2010; Li et al. 2012, 2011; Valenzuela et al. 2002; Xu et al. 2007). The concentrations of phenols in these studies were generally from 10 to 50 mg l⁻¹. These studies have demonstrated that phenols could be removed successfully using ferrite alone and composite ferrites. The current focus is on the composite ferrites to enhance the photocatalytic oxidation of phenols through synergistic effects. The presence of ferrites in the TiO₂-ferrite composite not only increases absorbance of visible light, but also increases absorbance in the UV light in order to assist photooxidation of phenol (Li et al. 2012, 2011). Furthermore, composites could be magnetically separated (Aziz et al. 2012a, b; Li et al. 2012). The photocatalytic oxidation of phenols using a magnetically recyclable photocatalyst, multi-walled carbon nanotubes (MWNT) supported NiFe₂O₄ is shown in Fig. 4.7 (Xiong et al. 2012). Figure 4.7a shows the degradation of phenol using NiFe₂O₄/MWNT nanocomposites having different MWNT content. The degradation of phenols increased with increasing amount of MWNT in the nanocomposites, however, the degradation rate had no such relationship. The degradation of phenol under UV radiation followed pseudo-first-order kinetics and obtained rate constant (k) as a function of content of MWNT are shown in Fig 4.7b. NiFe₂O₄/MWNT nanocomposite with 30 wt % MWNT exhibited the best photocatalytic activity. Degradation of picric acid, o-nitrophenol, and p-nitrophenol are shown in Fig. 4.7c. Most of the phenol degraded significantly under UV light in 400 min. This study also determined the total organic carbon (TOC) for the degradation of phenol and p-nitrophenol (Fig. 4.7d). Removals of TOC were ∼80 % and ∼40 % for phenol and p-nitrophenol, respectively. Incomplete removal of TOC suggests that the oxidation of phenol and p-nitrophenol resulted in simple organic compounds in the NiFe₂O₄/MWNT/UV radiation (Xiong et al. 2012).

Table 4.1 Degradation of phenols by ferrites

Fig. 4.7

(a) Photocatalytic degradation of phenol by NiFe₂O₄/multi-walled carbon nanotubes (MWNT) photocatalysts with differing MWNT contents under UV light irradiation; (b) The pseudo-first-order rate constant k as a function of MWNT content; (c) Photocatalytic degradation of nitrophenols (o-nitrophenol, p-nitrophenol, picric acid) by NiFe₂O₄/MWNT(0.30) nanocomposite photocatalysts under UV light irradiation; (d) Evolution of TOC removal of phenol and p-nitrophenol with irradiation time (Adapted from (Xiong et al. 2012) with the permission of Elsevier Ltd.)

4.4.1.2 Dyes

Numerous studies on the oxidation of degradation of dyes by ferrites alone and composite ferrites have been carried out (Baldrian et al. 2006; Casbeer et al. 2012; Li et al. 2011; Moreira et al. 2012). Dyes studied include methyl orange (MO), methylene blue (MB), rhodamine B (RhB), bromophenol blue, Chicago sky blue, eosine yellow, evans blue, naphthol blue black, phenol red, poly B-411, and reactive orange 16. The concentration levels of dyes were usually in the range of 50–500 mg L⁻¹. In a recent work, results of the studies have been summarized (Casbeer et al. 2012). Ferrites as photocatalyst alone were effective in degrading dyes, but the efficiency of degradation was enhanced when ferrites were used as composite photocatalysts. This is shown in Fig. 4.8. The degradation of RhB by ferrites was examined by UV light while visible light was applied in irradiating the MO dye. The temperature of ferrite synthesis influenced the degradation of RhB. Increase in the amount of TiO₂ in the composite ferrites increased the degradation efficiency of dyes (Fig. 4.8). Decrease in crystal size of ferrites also increased the degradation efficiency of dyes (Fan et al. 2009).

Fig. 4.8

Degradation of methyl orange (MO) and rhodamine B (RhB) dyes by TiO₂/ZnFe₂O₄ composite photocatalysts. All listed reactions contain the composite unless otherwise noted. MO solutions irradiated with visible light, while RhB solutions are irradiated with UV light (Adapted from (Casbeer et al. 2012) with the permission of Elsevier Ltd.)

In a recent work, the photocatalytic degradation of acid orange II (AOII) was studied by combining ZnFe₂O₄ with H₂O₂ under visible light (λ > 400 nm) (Su et al. 2012). Significantly direct photolysis of AOII by visible light was difficult, but oxidation of AOII was possible due to generation of ^•OH radicals in the system (Fig. 4.9). The Fenton-type reaction can produce ^•OH radicals (Path A). The formation of ^•OH radicals is also due to oxidation of water by holes on the surface of the ferrite (Path B). The capture of electron by H₂O₂ gives ^•OH radicals (Path C). The Path C is advantageous because it decreases the recombination of electron and hole and hence enhance the formation of ^•OH radicals. The mechanism of the reactions was examined by adding scavengers of reactive species, oxalate, iso-propoanol, Cr(VI), and KI, into the system. Degradation rate did not show any influence by adding oxalate. This suggests that oxidation of AOII by hole was less likely to be involved because hole is highly reactive with oxalate. Cr(VI) can react rapidly with electron and the degradation rate decreases in the presence of Cr(VI), which suggest that electron formed at the surface of the ferrite is involved. Both iso-propoanol and KI are good scavengers of ^•OH radicals and degradation of AOI decreased very significantly. This confirmed the dominant role of the ^•OH radicals in degradation of AOII. Basically, Paths, A, B, and C enhanced the degradation of AOII (Su et al. 2012). Degradation of AOII in the ZnFe₂O₄/H₂O₂/visible light was almost complete within 2 h. The amount of ZnFe₂O₄ and the concentration of H₂O₂ influenced the degradation of AOII.

Fig. 4.9

A hypothetical scheme for the generation of ^●OH radical in H₂O₂-ZnFe₂O₄-visible light system (Adapted from (Su et al. 2012) with the permission of Elsevier Ltd.)

4.5 Conclusion

ZnFe₂O₄-mediated photocatalytic oxidation of organic contaminations is a promising alternative technology for remediation of contaminants in water treatment. Pioneering works indicate that ZnFe₂O₄ possesses a relatively low visible-light-driven activity for organic pollutants, however, the technology is a potentially economical and benign process due to its capability to absorb the visible light of solar energy, stability against photo- and chemical- corrosion, low cost, and non-toxicity. In this case, it is still a challenge to explore the highly-efficient modified ferrites for remediation of contaminants under visible light. The size and morphology control of ferrites is one of the most important approaches that can enhance the photocatalytic activity. Compared with a traditional synthesized method, i.e. co-precipitation and sol-gel techniques, resulting in large particles and a broad size distribution, some innovative methods should be developed which can synthesize smaller size ZnFe₂O₄. An alternative promising approach is to modify ferrites with an appropriate matching-band-gap-energy semiconductor in a way that the photogenerated e⁻/h⁺ can be transferred between ferrites and matching-band-gap-energy semiconductor. This effective charge suppresses the combination of the photoinduced electrons and holes, which is beneficial to improve the photocatalytic efficiency. Further improvement in the modification of ferrites and ferrite composite for photocatalytic degradation of organic pollutants is still needed.