Journal of Nanomaterials

Volume 2013 (2013), Article ID 319637, 14 pages

http://dx.doi.org/10.1155/2013/319637

Review Article

-Based Photocatalytic Process for Purification of Polluted Water: Bridging Fundamentals to Applications

Chuan Wang, Hong Liu, and Yanzhen Qu

Key Laboratory of Reservoir Aquatic Environment, Chinese Academy of Sciences, Chongqing Institute of Green and Intelligent Technology, Chongqing 401122, China

Received 19 June 2013; Revised 6 July 2013; Accepted 7 July 2013

Academic Editor: Jiaguo Yu

Copyright © 2013 Chuan Wang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Recent years have witnessed a rapid accumulation of investigations on TiO₂-based photocatalysis, which poses as a greatly promising advanced oxidation technology for water purification. As the ability of this advanced oxidation process is well demonstrated in lab and pilot scales to decompose numerous recalcitrant organic compounds and microorganism as well in water, further overpass of the hurdles that stand before the real application has become increasingly important. This review focuses on the fundamentals that govern the actual water purification process, including the fabrication of engineered TiO₂-based photocatalysts, process optimization, reactor design, and economic consideration. The state of the art of photocatalyst preparation, strategies for process optimization, and reactor design determines the enhanced separation of photo-excited electron-hole (e-h) pairs on the TiO₂ surface. For the process optimization, the kinetic analysis including the rate-determining steps is in need. For large-scale application of the TiO₂-based photocatalysis, economics is vital to balance the fundamentals and the applied factors. The fundamentals in this review are addressed from the perspective of a bridge to the real applications. This review would bring valuably alternative paradigm to the scientists and engineers for their associated research and development activities with an attempt to push the TiO₂-based photocatalysis towards industrially feasible applications.

1. Introduction

Heterogeneous photocatalysis based on semiconductors has witnessed rapid progress in the last decades [1–3]. The semiconductor photocatalyst, of which the electrons in the valence band can be promoted to the conduction band when being excited by adequate photoenergy, possesses photogenerated electron-hole (e-h) pairs. The e-h pairs enable a series of reductive and oxidative reactions [4–6], and some of them further result in valuable reactions. This method has been initially put forward for the extraction of hydrogen energy from water via the conversion of photoenergy in the 1970s, when the energy crises has emerged [7]. Later in the 1980s, it has been tested as an environmental purification alternative for water [8, 9] and gas [10] as well. This review focuses on the water purification by addressing the fundamentals that serves as a connection with the real-time application.

Many reports have evidenced that numerous organic toxic compounds often present in water can be removed by the photocatalytic method. Chlorinated compounds, alkenes, alkanes, aromatics, dyes, and so forth, have been tested as the model pollutants [2, 3, 11, 12]. The rationale for TiO₂ photocatalytic method is predominantly based on the oxidation of pollutants by means of hydroxyl radicals () that feature the advanced oxidation technologies (AOTs) [13, 14]. The HO∙ production can be expressed as follows:

The h (hole) is highly oxidative, and the oxidation of water by leads to a formation of radicals, which are extremely active and nonselective in attacking the substrates in aqueous solution. Meanwhile, the electron (e) needs to be scavenged by an electron acceptor. Accordingly, the photogenerated e-h pairs are separated, leading to the transformation of pollutants. Otherwise, the photogenerated e-h pairs will be self-combined with an undesired release of thermal energy. As desired, the photogenerated e-h pairs should be separated as far as possible to improve the process performance of photocatalysis.

The TiO₂ photocatalysis has attracted great attention as a promising water treatment technology due to quite a few intrinsic advantages. (i) Powerful ability to decompose the pollutants: the organic pollutants can be decomposed and even mineralized due to the dramatic powerful oxidation ability of HO∙. Consequently, this technique can be utilized widely in the cases once the advanced treatment of water, particularly containing the recalcitrant organic compounds, is demanded. (ii) Ambient operating conditions: the process can be realized under ambient operating conditions, and thus is acknowledged as quite safe. (iii) Low cost: the sunlight can be employed as the energy source, and the oxidant is ambient O₂. Meanwhile, as the TiO₂ photocatalyst can be recycled, the cost can be further cut off to run the process. (iv) Environmentally friendliness: TiO₂ are chemically stable, nearly non-toxic, or relatively safe, even in the harsh conditions. Thus, the TiO₂ photocatalysis is thought as an environmentally-friendly approach, even if recent concerns arise on its environmental risks as it is released into the environment [15].

Many reports available in recent years have evidenced that numerous organic contaminants can be decomposed or even mineralized [16] by photocatalysis, and the mechanism governing the photocatalytic process on the TiO₂ surface has been disclosed in depth [17]. Also, efforts for preparation of photocatalytic materials [7–9, 18–22], which include the modification of the commonly used TiO₂ photocatalyst and the design and preparation of new photocatalysts, are intensively exerted. These efforts inclusively have attempts to accelerate the degradation process of target substrates by enhancing the separation efficiency of e-h pairs or to extend the wavelength of excitation light from ultraviolet (UV) regime to visible regime.

Along with the lab-scale investigation of simulated wastewater, some pilot-scale tests have been performed [23, 24]. It could be understood that, although the results served to demonstrate the technical feasibility for water purification, the TiO₂ photocatalysis is still facing a series of technical challenges for a large-scale installment in wastewater treatment plants (WWTPs). It must be noted that challenges, such as fabrication of active TiO₂ photocatalyst, rapid separation and recycling of TiO₂ after use, and optimization of the overall process, are determined by the fundamentals. In particular, the connection between the fundamentals and the actual scenario should be well understood by both the scientists and engineers as they are performing their own individual tasks.

However, the fundamentals, which have been well established in the literature, are far from being fully understood or sophisticatedly applied in the actual application. Also, the operating parameters optimized in laboratory often suffer a mismatching with the actual scenario. In view of the problem, this review highlights the connection between the two tasks, which is expected to bring valuable information for the academia and industry. Along with this highlight, the economic consideration was covered, which eventually leads to a balance between the various variables working in the TiO₂ photocatalytic process.

2. Water Purification of TiO₂ Photocatalysis

With the increasing demand of the water sources, novel technologies, which serve to purify the polluted water with powerful ability to decompose the pollutants in energy-saving and environmentally-benign manners, are becoming essential. As well known, conventional technologies, such as filtration, coagulation, adsorption, and precipitation, principally serve to transfer the pollutants from one phase to another. The newly emerging membrane separation technology [25] also belongs to such category. Following them, further steps must be taken to treat or dispose the transferred pollutants. In this regard, chemical oxidation, featured by their ability to destroy the molecular structures of pollutants, is an indispensible option. During the processes, the toxicity of the involved organic moieties is likely to extenuate or aggravate, and thus the degree of pollutant degradation should be in control. Moreover, the concept of wastewater will become out of date, since the so-called wastewater is just staying at a stage during the overall recycling of water as a resource. The TiO₂-based photocatalytic process, harnessing the sunlight as an energy source and ambient O₂ as an oxidant for water purification, is capable of exhaustively mineralizing the organic pollutants and eliminating the toxicity. Thus, the TiO₂-based photocatalysis is accepted as an effective tool to purify the polluted water.

As listed in Table 1, the water, which is polluted by textile, pesticides, medicine, and so forth, can be treated by the TiO₂-based photocatalysis. It should be pointed out that the TiO₂-based photocatalysis is just employed as an individual method herein. Actually, however, effluents discharged from industrial processes commonly contain recalcitrant or nonbiodegradable pollutants, and thus an integrated process containing biological and physicochemical units will be more applicable. Basically, in the front of integrated process, the influent can be subjected to a physical unit such as filtration or grilling to separate the solids. Following is a physicochemical unit such as coagulation to remove the colloidal substances. Subsequently, a biological unit including aerobic or inaerobic oxidation serves to degrade most of the biodegradable organic molecules, and thus the water quality is usually improved in terms of the indexes such as chemical oxygen demand (COD) and NH₃-N. Finally, a chemical means, such as the TiO₂-based photocatalytical process, is incorporated to purify the water by finally removing the recalcitrant organic pollutants.

Table 1: Engineered TiO₂, the associated treated targets, reactors and principal operation parameters.

TiO₂
Water
Reactor
Principal operation parameters investigated
References

TiO₂ P25
Pharmaceutical wastewater
Sequencing

batch biological reactor
H₂O₂ dosage, TiO₂ dosage, pH, irradiation, time and hydraulic retention time
[73]

TiO₂/carbon aerogel
High-concentration dye wastewater
Photocatalysis-enhanced electrosorption reactor

Electrochemical adsorption, potential, temperature, initial pH, and wavelength
[74]

TiO₂ P25
Wastewater treatment plant effluents
Lab-scale solar photoreactor
Temperature, pH, and light wavelength
[75]

TiO₂ nanofiber
Hospital wastewater
Batch annular slurry photoreactor (ASP)
Fine bubble aeration, light wavelength, and pH
[76]

Nano TiO₂
Paper mill wastewater
Solar reactor
TiO₂ dosage, pH
[59]

C-TiO₂
Antibiotics wastewater
A computerized Luzchem CCP-4V photochemical reactor
pH, wastewater flow rate
[77]

TiO₂ P25
Antibiotics wastewater
Capacity cylindrical acrylic photoreactor
pH, gas rate, light intensity, and TiO₂ dosage
[78]

TiO₂/SiO₂ beads
Organic compounds-containing wastewater
Batch recirculation type photocatalytic reactor
pH, wastewater flow rate
[79]

TiO₂ P25
Textile wastewater
Pebble bed photocatalytic reactor
TiO₂ pebbles, water treatment after absorption of reflow exhaust gas
[80]

TiO₂ P25
Industrial wastewater
A hermetically sealed photoreactor
Pressure, adsorption time, and temperature
[81]

PFT/TiO₂
Toxic heavy metals wastewater
A column glass photoreactor
Wavelength
[82]

TiO₂ P25
Secondary effluent
A cylindrical glass reactor
Amount of TiO₂ loaded, maximum emission peak
[83]

TiO₂/Ti anode
Textile wastewater
A thin-film photoelectrocatalytic reactor
pH, lamp position, and light intensity
[84, 85]

TiO₂ P25
Pharmaceutical and cosmetic wastewater
Common photoreactor
pH, time, catalysts, and H₂O₂ concentration
[86]

TiO₂ P25
Olive mill wastewater
Common photoreactor
Light wavelength, pH
[87]

TiO₂ P25
Textile dyehouse wastewater
Immersion well batch-type photoreactor
pH, light wavelength, temperature, and H₂O₂ concentration
[22]

TiO₂/SiO₂
DNP and municipal wastewater
Fixed bed circulation type reactor
Water flow rate, residence time
[88]

Ag–TiO₂
Seawater
poly (methyl methacrylate)

reactors

temperature, dosage of bacterial suspension, and O₃
[89]

S–TiO₂
MC-LR in surface water
Borosilicate glass dish
pH, light wavelength
[90]

nano TiO₂
Drinking water
Tank-style reactor
Permeate flux with a membrane,
[91]

TiO₂ P25
Surface water
Photovoltaic reactor
Temperature,
[92]

TiO₂ P25
Benzalkonium chloride
Annular glass reactor
Light wavelength, TiO₂ dosage
[93]

NF-TiO₂-P25 films
Drinking water
Glass

vessel reactor
Light intensity, pH
[94]

TiO₂ microsphere
Organic compounds-containing wastewater
Cylindrical photoreactor
Light wavelength, concentration of TiO₂ suspensions
[66]

TiO₂ films
MC-LR in surface water
Sealed round pyrex reactor
Temperature, water flow rate, pH, TiO₂ coating surface area, and thickness
[95]

TiO₂(P25)-AGS
Toxic heavy metal wastewater
A photocatalytic bath
pH, dosage of catalyst, and temperature
[96]

TiO₂ P25
Drinking water
Cylindrical Pyrex reaction vessel
Catalyst loading, light wavelength
[97]

TiO₂ P25
Organic compounds-containing wastewater
A thermostated reactor
Temperature, dissolved oxygen concentration
[60]

TiO₂-modified nanofiltration membranes
Organic compounds-containing wastewater
Cell reactor
Light intensity, temperatures
[98]

TiO₂ P25
Textile wastewater
Immersion well reactor
Dosage of TiO₂ and H₂O₂, pH
[99]

TiO₂ film supported on a porous nickel net
Pesticides
Reactor with ozone generator
pH, ozone dosage, treatment time and temperature
[100]

TiO₂ nanotubular arrays
Swimming pool water
Cylindrical glass reactor
pH, potential
[101]

From the technical point of view, the TiO₂-based photocatalytical process is applicable as one prerequisite is satisfied, which is the transparency of water. The transparency ensures the transmission of the light. The TiO₂-based photocatalytical process can be installed as a pretreatment unit or an advanced unit. If the effluent is in need of further purification to destroy the nondegradable organic pollutants, the TiO₂-based photocatalytical process follows the biological unit. Alternatively, the TiO₂-based photocatalytical process is put in front of the biochemical unit to improve biodegradability for a better biological oxidization of the organic substrates.

3. Fundamentals

To develop a real TiO₂-based photocatalytical process, optimization of the operating parameters is of importance. However, the fundamentals that have been well established in laboratory are not utilized sophisticatedly or even overlooked in real application. Also, the parameters that are optimized in the laboratory test, which has only one substrate, are not applicable in the full-scale process. The presence of cosubstrates in wastewater may significantly influence the degradation of target substrate. C. Lin and K. S. Lin have observed that the presence of humic acid (HA) retarded the photocatalytic degradation of 4-chlorophenol [26]. Consequently, the fundamentals that closely connect the real scenario should be focused on. Herein, the e-h pair generation and separation, adsorption of organic substrate, and rate determining step (RDS) of the photocatalytic reaction are addressed. These fundaments are analyzed on how to define a maximized efficiency in real application, whereas any successful and competitive real process should get a balance between efficiency and economics.

3.1. Importance of Photogenerated e-h Pair Separation

The TiO₂ photocatalytic process starts with the generation and separation of e-h pairs, which are illustrated in Figure 1. Based on the mechanism of photocatalytic oxidation proposed by Hoffmann et al., [27] the reactions on the TiO₂ can be correspondingly expressed as follows:

It could be understood that upon the photo-excitation, the electrons on the valence band (VB) jump to the conduction band (CB), generating left on the VB. As the electrons and holes initially separate and interact with substrates in the solution and then combine together, an effective conversion of substances is thus resulted in. As recombination occurs, there is no any conversion of substances.

Figure 1: The scheme of TiO₂ photocatalytic process.

Following the generation of holes and electrons, radicals of are generated and considered to work as the principal oxidative species during the photocatalytic process. Xiang et al. have proposed a concept of “OH-index” to quantitatively characterize the production of on different photocatalytic systems [28]. Such index, which can be measured easily by photoluminescence technique, is using coumarin as a probe molecule. It appears that this index is quite adequate to explain the activity difference in different systems. For example, P25 TiO₂, which is a most available commercial photocatalyst, has the highest “OH-index” as illustrated in Figure 2.

Figure 2: Comparison of OH-index of various photocatalysts. Arrows represent OH-index = 0. Am, A, and R denote amorphous, anatase, and rutile of TiO₂, respectively. This figure is taken from the article of Xiang et al. [28].

The photoenergy plays an important role in the e-h separation. The energy that is sufficient to generate the e-h pairs must surpass the energy of TiO₂ band gap. The equation, , designates the relationship between the energy and the associated light wavelength. TiO₂ (-type) has a band gap of 3.2 eV. Accordingly, the light wavelength must be shorter than 380 nm, which falls into the range of UV light. This requirement determines that a full illumination of the photocatalyst is an indispensible factor when designing a real photocatalytic matrix. First, an artificial lamp must be installed. Lamps with 254 nm and 365 nm as the main wavelength are well commercialized. A more powerful illumination of the photocatalyst certainly accelerates the e-h pair generation. High-pressure mercury lamp is also powerful, whereas its illumination is not stable and thus not recommended for real application. Many lamps can be installed together in the photocatalytic reactor to ensure a full illumination, while the heat release should also be taken into account as the illumination time lasts a long time. Second, the target water must be sufficiently transparent for the transmission of the light. Accordingly, following the pretreatment that serves to remove the particles or species that shadow the light, the TiO₂-based photocatalytic process can be well employed. Third, only the UV lamps that are made from quartz glass can be immersed in water. Fourth, even if an immobilization of the TiO₂ photocatalyst serves to recover the TiO₂, it may bring about a decrease in the e-h generation due to the decrease in the illumination intensity caused by the water shadow. As a result, in real application, the suspended TiO₂ instead of the immobilized TiO₂ is more efficient. Li et al. have reported a type of suspended TiO₂ microsphere, which allows both the TiO₂ suspension and recovery [29].

3.2. Measure for e-h Pair Separation

Following the photogeneration of e-h pairs, a fast recombination of charge carries is thermodynamically favorable and thus yields a low quantum yield of the reactive species for organic degradation. Obviously, any effort to facilitate the e-h separation and inhibit its recombination is encouraged. Many measures, including TiO₂ modification, scavenge of electrons, and imposition of external force, are allowed to enhance the e-h separation.

3.2.1. TiO₂ Preparation

The TiO₂ is a cornerstone of the photocatalytic process. In laboratory, it is often prepared by methods such as sol-gel method and hydrothermal method. The physical morphology is controllable through hydrothermal preparation, while the TiO₂ is chemically composed of two crystalline types of anatase and rutile. The rutile type is more stable, and the thermal treatment of anatase type leads to a transformation to rutile. The difference in the distribution of the two types is considered to determine the associated catalytic activity. Basically, pure rutile is found to exhibit a poor activity, while a suitable composition may be more active than the pure anatase type [30]. The preassumed correlation between the composition of crystal phase and the e-h pair separation has been clearly evidenced. Recently, however, Xu and coworkers have found that the anatase and rutile have the same activity in the degradation of 4-chlorophenol, and the difference just lies in their different adsorption ability towards O₂ [31, 32]. Thus, as the TiO₂ with different crystal phases is fabricated, the O₂ adsorption ability on the individual crystal phase needs to be taken into account. The composition of TiO₂ photocatalyst can be adjusted through thermal treatment by means of the increase in temperature.

3.2.2. TiO₂ Doping

To overcome the drawback of low photocatalytic efficiency brought by the e-h self-combination, the bare TiO₂ can be further modified by doping a foreign ion. Table 2 shows that various metal ions, metal oxides, and a few nonmetal elements such as nitrogen, sulfur, fluorine, and silicon can be employed as the dopants.

Table 2: Doping of TiO₂ for the enhanced photocatalytic activity.

Doped TiO₂
Doping method
References

N-doped TiO₂
Sol-gel method; microwave hydrothermal method
[34–36]

Si-doped TiO₂
Hydrolysis of titanium isopropoxide
[102, 103]

Si-doped TiO₂/ZrO₂
Sol-gel method
[37]

S-doped TiO2 nanoparticles
Hydrothermal method
[38]

doped TiO₂ films
Hydrothermal method
[39]

N/S co-doped TiO₂
Manual grinding of thiourea and urea
[104]

S-doped TiO₂/Ti electrode
Anodization method
[105]

S-doped TiO₂-ZrO₂ nanoparticles
Sol-gel method
[40]

C–N-doped TiO₂ nanotube
Chemical vapor deposition

(CVD)
[61]

Se(IV) on Degussa P25
Wet impregnation
[33]

Al(III)-doped TiO₂
Sol-gel method
[106]

V-doped TiO₂
Sol-gel method
[107]

Fe-doped TiO₂
Hydrothermal method
[62]

Polypyrrole/Fe-doped TiO₂
Sol-gel method, emulsion polymerization
[108]

Er³⁺: YAlO₃/Fe- and Co-doped TiO₂
Sol-gel method
[109]

Cu²⁺-doped TiO₂/SiO₂
Sol-gel method
[110]

ZnFe₂O₄ on TiO₂
Sol-gel method
[111]

Ag doped TiO₂
Photoreduction using the sacrificial acid
[112]

Ag/F-TiO₂
Sol-gel method; photoreduction method
[113]

Ag-TiO₂-xNx
Sol-gel method
[114]

Sn-doped TiO₂
Sol-gel method
[115]

Lanthanide ions-doped TiO₂
Sol-gel method
[116]

La³⁺/Zr⁴⁺ co-doped TiO₂
Sol-microwave method, oil-bath condition synthesis
[117]

Ce-doped TiO₂
Sol-gel method
[118]

Ce-doped TiO₂ microspheres
solvothermal method
[67]

Nd/F doped TiO₂
Sol-gel method
[119]

W-doped TiO₂
Solvothermal method
[120]

W-TiO₂ films
Liquid phase deposition (LPD)
[121]

W/N co-doped TiO2 nanoparticles
Sol-hydrothermal method
[122]

Bi-doped TiO₂
Sol-gel method
[123]

C-dope TiO₂ powders
Oxidative annealing of TiC
[41]

Gurkan et al. [33] have reported the density function theory (DFT) calculation of the Se(IV)-doping of TiO₂ sample, showing that the doping does not cause a significant change in the positions of the band edges, whereas produces additional electronic states originating from the Se 3p orbitals in the band-gap. Accordingly, the electrons can be excited from the defect state to the conduction band by a relatively lower energy, and thus visible light can be utilized. Further, the benefit of the doped transition metal lies in the improved trapping of electrons to inhibit e-h recombination during irradiation.

In real application for water purification, special attention should be paid to the potential leakage of the doped ions, which may result in a secondary pollution in the treated water. In this regard, the N-doped TiO₂ appears to be a promising method due to the cleanness brought by the nitrogen and has attracted great attention in recent years. In particular, the photocatalytic activity under visible light illumination has been well evidenced [34–40]. However, the unanimity exists for the photocatalytic mechanism. For example, Irie et al. have stated that the irradiation with UV light excites the electrons in both the VB and the impurity energy levels, but illumination with visible light only excites electrons in the impurity energy level [41]. Ihara et al. have concluded that the oxygen-deficient sites formed in the grain boundaries are importantly attributed to the visible light activity, while the doped nitrogen in part of the oxygen-deficient sites is important as a blocker for reoxidation [42].

Also, for the application of the fabricated TiO₂, the intrinsic advantages brought by the preparation methods should be accurately analyzed, so as to appropriately ascribe the accelerated activities to the e-h separation. For example, Liu et al. [43] have reported that a heat treatment of TiO₂ by hydrogen does not lead to any change in the physical properties including the crystal phase, specific surface area, and particle size. However, results of electron paramagnetic resonance (EPR) detection show that the H₂ treatment leads to formation of oxygen vacancy and Ti³⁺ species on the TiO₂. Moreover, the kinetic constants of phenol degradation increased with the H₂ treatment temperatures lower than 800°C. Therefore, the production of oxygen vacancy and Ti³⁺ species, which are relatively stable as exposure to ambient air, is considered to extend the lifetime of the e-h pairs. The oxygen vacancy and Ti³⁺ species act as holes traps, and oxygen vacancy acts as an electron scavenger. Further, the trapped holes transfer to the organic substrate leading to a degradation reaction and charged defects recover to their original states of oxygen vacancy and Ti³⁺.

3.2.3. Scavenging of the Photogenerated Electrons

To effectively separate the e-h pairs, the electrons need to be scavenged along with the trapping of holes by H₂O to form ∙OH. Otherwise, the electrons will recombine together with the holes without any transformation of pollutants. The most commonly available scavenger is the dissolved oxygen (DO), and thus the TiO₂-based photocatalytic reactions are most often performed in an aerated aqueous system. It has been observed that the reaction rate increases toward a plateau with the partial pressure of O₂ in a gas phase [44]. Actually, before the electron scavenging, an adsorption process of DO occurs, and the adsorption model may be described by a typical Langmuir adsorption.

Besides DO, H₂O₂ is often added as an environmental benign electron scavenger to aid the DO. H₂O₂, being a stronger electron acceptor than oxygen, reacts with the electrons in the valence band of the photocatalyst to generate . Chu et al. have observed that the degradation rate of chlorinated aniline can be improved by adding 0.01 mM H₂O₂ to the reaction solution [45]. However, as the H₂O₂ is overdosed at 100 mM, a retardation of approximately 26% rate is caused. The rate retardation can be accounted for as follows. The H₂O₂ does scavenge the valuable HO∙, while the H₂O₂ in excess consumes the oxidative holes on the TiO₂ catalyst surface (3), where the overall oxidation capabilities of the system are significantly reduced [46]:

Additionally, alternative oxidative species such as Cr(VI) ions can be employed as the electron scavenger in the TiO₂-based photocatalysis [47]. The Cr(VI) ions are also a type of pollutant, and the photocatalysis is sometimes employed to remove the Cr(VI) ions as a reductive process using the electrons [48]. As the Cr(VI) ions coexist with organic pollutant, a special supply of DO as the electron scavenger appears to be unnecessary [49]. Compared to the conventional process of Cr(VI) removal using ferrous reduction, the competition of TiO₂-based photocatalysis may be impeded as the economics is taken into account, because the former appears to be a quite rapid process.

3.2.4. Imposition of Bias to Separate the e-h Pairs

External forces such as electric bias can be employed to accelerate the separation of e-h pairs, which is realized in an electrode system. In this system, the TiO₂ is employed as the anode, where the organic substrate is oxidized by the radicals generated from the interaction between the holes and H₂O. The photogenerated electrons flow through the external circuit and arrive at the cathode where they are scavenged by oxidative species. In this system, an externally imposed anodic bias serves to drive the electrons away from the anode to the cathode, and thus the e-h self-combination of TiO₂ can be efficiently inhibited on the anode. Some reports have shown that a bias of around 0.5 V is efficient to lead to a significant increase in the organic degradation on the anode by means of enhancing the e-h separation [50, 51]. Additional advantage brought by this system leads to the easy recovery of TiO₂ compared to the conventional suspended system.

The lab-scaled prototype of this system, however, may find difficulty in the scale-up for actual water purification, likely because of the utilization of the electrode. As the TiO₂ powder is attached to the anode support, a binder with powerful binding strength is indispensible to anchor the TiO₂ particles on the electrode surface. The binder making of polymer may be degraded after a long-time photocatalytic process, which decreases the lifetime of electrode. Despite this challenge, the photoelectrochemical system can be employed as a powerful platform for the probing of photocatalytic mechanism. In essence, the TiO₂-based photocatalytic process is such one involving electron transfer, which can be exactly investigated by electrochemical means. Liu et al. have employed electrochemical impedance spectroscopy to ascertain the TiO₂-based photocatalytic process, and thus the reaction phenomenon can be accounted for [52]. By means of this powerful tool, the rate-determining steps have been well ascertained.

3.3. Adsorption of Organics onto TiO₂ Surface and Organic Degradation Reaction

Accompanied with the successful e-h separation is the organic degradation reaction. Majority of the reports on the water purification are to investigate the degradation reaction of an individual substrate. The substrate is initially adsorbed onto the TiO₂ surface, and the adsorption ability is often observed to determine the overall degradation efficiency [1]. A large specific surface area of TiO₂ photocatalyst benefits the accumulation of organic substrate on the TiO₂, and thus more active sites are available for the subsequent degradation reaction [53]. Generally, the adsorption behavior can be described by the Langmuir adsorption isotherm, and the kinetic model of organic degradation rate can be described as follows:

In (5), designates the contribution of organic degradation and is the contribution of organic adsorption [54]. Obviously, large values of and are beneficial for the acceleration of reaction rate. As the value is small enough to be neglected, (5) can be considered as the first-order reaction rate as follows:

where equals as the apparent reaction rate constant.

In real application, it is noteworthy that the chemisorption, occurring through a formation of chemical bond, contributes to the acceleration of degradation reaction. In some cases, a great number of the organic substrate is adsorbed, while the degradation reaction of organic substrate does not become more rapid just because of the physisorption instead of chemisorption. It has been disclosed by FT-IR that the organic substrate is chemically adsorbed in the form of complex with the TiO₂ [55]. This complex likely leads to a deviation of the first-order rate model of the degradation reaction, because the large value cannot be neglected in (5). In this scenario, the reaction rate can be described as follows [29, 56, 57]:

Certainly, in real application, the reaction kinetics involving the adsorption should be kept in mind to solve some problems. For example, as the water containing textile as a pollutant is treated, a great number of textile molecules are adsorbed, while physically onto the TiO₂. Obviously, such type of adsorption does not contribute to the degradation. On the other hand, as the chemisorption predominates with a large value, the first-order rate reaction model that is commonly applied to describe the reaction is not applicable, and (6b) is more adequate to fit the degradation data of target pollutants.

3.4. Rate-Determining Steps

Besides the steps of adsorption and degradation reaction, mass transfer and desorption of the reaction products are also the main steps involved in the heterogeneous catalytic reaction. Among these steps, the slowest step determines the overall reaction rate, while which one is the rate-determining step (RDS)? As the aqueous system is commonly fully stirred by means of air bubbling to form a suspension, the step of mass transfer is generally considered as a fast step, and the adsorption and degradation reactions are potentially the RDS.

In real application, the ascertainment of RDS is of importance. Clearly, only manipulation of the RDS leads to an alteration of the overall reaction rate. For example, as the adsorption step is RDS, increase in the specific surface area to improve the adsorption ability enables the acceleration of the overall reaction rate. Otherwise, the associated efforts are in vain. Liu et al. have employed electrochemical impedance spectroscopy (EIS) to ascertain the RDS of TiO₂ photocatalytic process [52]. Their results show that the surface degradation reaction of organic pollutant is commonly an RDS, while the adsorption step is another RDS once the surface degradation reaction becomes quite rapid. In essence, the acceleration of organic degradation reaction can be considered as the acceleration of the e-h separation. To date, the TiO₂ photocatalysis is suffering from the low quantum yield, and TiO₂ that has a sufficiently rapid e-h separation is not available. Accordingly, any effort in the acceleration of the e-h separation is valuable.

4. Balance of the Efficacy and Economics

4.1. Economic Estimation of TiO₂ Photocatalysis for Water Purification

The organic pollutants experience an oxidation pathway during the photocatalytical process, and thus a typical TiO₂-based photocatalytical matrix is composed of four indispensible elements: light source, photocatalyst, oxidant as an electron acceptor, and reactor. The light source supplies energy that excites the electrons on the valence band of TiO₂ to jump to the conduction band, and then holes are left on the conduction band. The TiO₂-based photocatalyst works as a catalyst, which adsorbs organic substrate onto the surface, and then the degradation reaction occurs. As the oxidative holes are consumed to generate ∙OH via the interaction with H₂O, the electrons are left and must be scavenged by an oxidant, mostly, by ambient O₂. The ambient O₂ has an additional function to buoyant the TiO₂ particles to form a suspension, and thus the mass transfer limit can be precluded. The reactor is the place where the photocatalytic process is realized.

To develop an effective TiO₂-based photocatalytic process for water purification, the operating parameters should be well optimized for the full utilization of the light illumination, ability of catalyst, and ambient O₂ in the reactor. When doing this work, the fundamentals that have been established in the laboratory should be used sophisticatedly. In addition, since the solution chemistry varies significantly, the experience in one place, even if quite successful, is not applicable in another place. Particular, the optimized parameters cannot be implemented in some real applications due to the limitation of operating cost.

Basically, the operation cost of photocatalytic reaction matrix includes the cost of photocatalytic materials, electricity, and aging of equipment, while the cost of TiO₂ photocatalyst shares the major part and is discussed herein. The cost of TiO₂ photocatalyst depends on its dosage. For example, 0.1–2 wt% TiO₂, as usually dosed in water treatment, costs too much if it is used one time. It is estimated that the cost of the TiO₂ photocatalyst in treating 1 m³ of water at this dosage level varies from 3 to 60 USD, under the condition that the price of TiO₂ powders is as low as approximately 3000 USD per ton. This cost is undoubtedly unaffordable for most consumers, particularly in the developing countries [58].

4.2. Recovering of the TiO₂ Photocatalyst

This economic estimation implies that the progress obtained in the reaction efficiency must be balanced by the economic consideration. Thus, the stability of TiO₂ photocatalyst may outweigh its temporary activity, even if quite high. Also, the recycling of the TiO₂ powders, which are commonly employed, is also vital. In particular, the TiO₂ particles are often prepared in a scale of nanosize, and it has been well recognized that the nanosized TiO₂ particles exhibit unique photocatalytic ability [3, 13, 59–63]. Obviously, these nanosized TiO₂ powders can be well suspended to form a fine contact between the catalyst particles and the substrate, and thus the illumination of the particles is ensured. Accordingly, the organic substrates can be rapidly destroyed in the photocatalytic matrix with nanosized TiO₂ powders.

However, this nanosized TiO₂ is often synthesized through hydrothermal method that demands high temperature and pressure and thus is quite costly once the preparation is practiced in a large scale. In view of the economic consideration as previously mentioned, these TiO₂ particles will encounter a difficulty in separation from the aqueous phase and recycling after use. An addition of a chemical such as coagulant enables the TiO₂ sedimentation and separation, whereas the reuse of TiO₂ becomes impossible due to the TiO₂ fouling. Mostly, the TiO₂ powders are immobilized onto a support [16] to preclude the postseparation, and the TiO₂ can be reused. Moreover, as the TiO₂ powders are immobilized onto a support such as porous nickel [64] or Ti mesh [65] to form an electrode, a bias can be conveniently applied to the anode TiO₂ to enhance the e-h separation. It is noted that in both cases, however, a significant loss in the contact area between the immobilized photocatalyst and the light source exists, which lowers the global efficiency of photocatalytic degradation of organic substrate. Therefore, the efficacy sometimes must be sacrificed for the sake of cost cutting, and a balance between the efficacy of TiO₂ and the running cost is of significance.

This balance requires a novel concept of design of the photocatalyst matrix, in which the advantage of the suspension system should be kept, while the easy recycling of the photocatalyst should be simultaneously realized. Therefore, a microsphere photocalyst appears to be an ideal option. If the size of microspheres can be designed at a suitable scale in size, they can be well suspended by the air bubbling. The ambient oxygen works as an electron scavenger and thus is inevitably supplied. This way, the suspended TiO₂ can be finely illuminated. Upon the task of photodegradation, the air bubbling stops. Consequently, the microspheres settle at the reactor bottom quickly by the aid of gravity and thus can be readily reused.

Such microsphere photocatalysts have been successfully fabricated [29, 56, 57, 66–70]. For example, as illustrated in Figure 3, a TiO₂ microsphere photocatalyst with a size regime of 30–160 μm and an average size of around 80 m has been prepared by reconstructing the mixture of TiO₂ sol and TiO₂ nanosized powders with a spray drier [29, 57]. After that, the as-prepared microspheres are thermally treated to obtain the anatase-dominant sample TiO₂ microspheres. The experimental results showed that the photoreaction rate using the TiO₂ microspheres was comparable to that using the TiO₂ powder counterparts. Moreover, the TiO₂ microsphere samples could be reused in the photocatalytic oxidation reaction for more than 50 times, without obvious destruction of the physical structure and significant loss in its activity.

(a)

(b)

Figure 3: SEM image of the TiO₂ microspheres samples ((a) has 600 multiple and (b) has 50,000 multiple magnifications). Taken from [57].

4.3. Coupling of Diverse Technologies

Effluents discharged from industrial processes usually contain various pollutants. Accordingly, diverse technologies, including physical, biological, and chemical ones, are needed to be chosen to treat them in light of such properties. Pollutants in the form of colloids or biodegradable solutes are treated by conventional approaches such as coagulation or biological process. The TiO₂ photocatalyst is specially designed to destroy the recalcitrant pollutants that are nonbiodegradable. The ∙OH involved in the TiO₂ photocatalytic process is extremely powerful to destroy the molecular structures and lead to partial or complete mineralization. Generally, the TiO₂ photocatalysis is particularly fitful for the advance treatment of water, with an attempt of final discharge of the effluent or reuse of the purified water. Clearly, it appears infeasible to purify the seriously contaminated water solely by the TiO₂ photocatalytic process. Without surprise, the TiO₂ photocatalysis has its intrinsic drawbacks in treating actual water. For example, the water should be transparent for the light transmission, and the extremely polluted water often shadows the light. Second, the biological processes are cost effective in the decomposition of a great number of biodegradable pollutants at low concentrations. Therefore, the balance between the efficacy of TiO₂ photocalytic process and economics requites a smart coupling of it with other conventional process.

In real application, a successful process for water purification is commonly an integrated one including quite a few technologies for the sake of both technical efficacy and cost effectiveness. Basically, a physical unit such as filtration or grilling unit goes first to separate the solids. Then, a physicochemical process such as coagulation follows to remove the colloidal substances. Further, a biological process is installed to degrade most of the biodegradable organic molecules. Finally, if the effluent requires further purification to destroy the nondegradable organic pollutants, the TiO₂ photocalytic process is desirable. In addition, TiO₂ photocatalytic process is expected to improve the biodegradability of polluted water as a pretreatment unit by decomposing the recalcitrant pollutants. Anyway, any pilot-scale test is usually extremely necessary to verify the technique and economic feasibility [71], and more fundamental research is also in need to understand the enhancing mechanism of the photocatalytic activity [72], while associated reports are very limited.

5. Concluding Remarks

With the rapid accumulation of reports on the TiO₂-based photocatalysis for water purification, its application becomes a task with great importance. In view of the fact that the fundaments of the TiO₂-based photocatalysis have been well established, a link with the actual application in water purification should be followed. Analysis in this review shows that it is of significance to keep in mind that the fundaments, clarified in laboratory, sometimes find it difficult to solve the problems that are encountered in practice. Consequently, some TiO₂-based catalysts, which possess high activity in the degradation of model pollutants in the laboratory, are in great need of further investigation to improve their applicable ability such as separation and recycling. During the further investigation, apart from the fundamentals, economics is believed to play a vital role in actual application, indicating that a balance between the reaction efficiency and the operating cost should also be considered. Moreover, a flexible coupling of TiO₂-based photocatalysis with other technologies is equally important to push it towards real application of water purification.

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (nos. 21067004 and 51208539) and Chongqing Science and Technology Commission (no. cstc2011ggC20013).

References

J. Zhao, T. Wu, K. Wu, K. Oikawa, H. Hidaka, and N. Serpone, “Photoassisted degradation of dye pollutants. 3. Degradation of the cationic dye rhodamine B in aqueous anionic surfactant/TiO₂ dispersions under visible light irradiation: evidence for the need of substrate adsorption on TiO₂ particles,” Environmental Science and Technology, vol. 32, no. 16, pp. 2394–2400, 1998.

J. G. Yu, M. Jaroniec, and G. X. Lu, “TiO₂ photocatalytic materials,” Internation Journal of Photoenergy, vol. 2012, Article ID 206183, 5 pages, 2012.

J. Yu, J. Xiong, B. Cheng, and S. Liu, “Fabrication and characterization of Ag-TiO₂ multiphase nanocomposite thin films with enhanced photocatalytic activity,” Applied Catalysis B, vol. 60, no. 3-4, pp. 211–221, 2005.

S. Wen, J. Zhao, G. Sheng, J. Fu, and P. Peng, “Photocatalytic reactions of phenanthrene at TiO₂/water interfaces,” Chemosphere, vol. 46, no. 6, pp. 871–877, 2002.

H. Chun, W. Yizhong, and T. Hongxiao, “Influence of adsorption on the photodegradation of various dyes using surface bond-conjugated TiO₂/SiO₂ photocatalyst,” Applied Catalysis B, vol. 35, no. 2, pp. 95–105, 2001.

S. Kaneco, Y. Shimizu, K. Ohta, and T. Mizuno, “Photocatalytic reduction of high pressure carbon dioxide using TiO₂ powders with a positive hole scavenger,” Journal of Photochemistry and Photobiology A, vol. 115, no. 3, pp. 223–226, 1998.

F. Moellers, H. J. Tolle, and R. Memming, “On the origin of the photocatalytic deposition of noble metals on TiO₂,” Journal of the Electrochemical Society, vol. 121, no. 9, pp. 1160–1167, 1974.

H. J. Hovel, “TiO₂ antireflection coatings by a low temperature spray process,” Journal of the Electrochemical Society, vol. 125, no. 6, pp. 983–985, 1978.

R. W. Matthews, “Solar-electric water purification using photocatalytic oxidation with TiO₂ as a stationary phase,” Solar Energy, vol. 38, no. 6, pp. 405–413, 1987.

W. Behnke, F. Nolting, and C. Zetzsch, “A smog chamber study on the impact of aerosols on the photodegradation of chemicals in the troposphere,” Journal of Aerosol Science, vol. 18, no. 1, pp. 65–71, 1987.

Y. Tominaga, T. Kubo, and K. Hosoya, “Surface modification of TiO₂ for selective photodegradation of toxic compounds,” Catalysis Communications, vol. 12, no. 9, pp. 785–789, 2011.

C. Hu, J. C. Yu, Z. Hao, and P. K. Wong, “Photocatalytic degradation of triazine-containing azo dyes in aqueous TiO₂ suspensions,” Applied Catalysis B, vol. 42, no. 1, pp. 47–55, 2003.

J. Saien, Z. Ojaghloo, A. R. Soleymani, and M. H. Rasoulifard, “Homogeneous and heterogeneous AOPs for rapid degradation of Triton X-100 in aqueous media via UV light, nano titania hydrogen peroxide and potassium persulfate,” Chemical Engineering Journal, vol. 167, no. 1, pp. 172–182, 2011.

Z. Ai, J. Li, L. Zhang, and S. Lee, “Rapid decolorization of azo dyes in aqueous solution by an ultrasound-assisted electrocatalytic oxidation process,” Ultrasonics Sonochemistry, vol. 17, no. 2, pp. 370–375, 2010.

W. Fan, M. Cui, H. Liu et al., “Nano-TiO₂ enhances the toxicity of copper in natural water to Daphnia magna,” Environmental Pollution, vol. 159, no. 3, pp. 729–734, 2011.

B. Tryba, “Immobilization of TiO₂ and Fe-C-TiO₂ photocatalysts on the cotton material for application in a flow photocatalytic reactor for decomposition of phenol in water,” Journal of Hazardous Materials, vol. 151, no. 2-3, pp. 623–627, 2008.

J. T. Yates Jr., “Photochemistry on TiO₂: mechanisms behind the surface chemistry,” Surface Science, vol. 603, no. 10–12, pp. 1605–1612, 2009.

C. Liang, C. Liu, F. Li, and F. Wu, “The effect of Praseodymium on the adsorption and photocatalytic degradation of azo dye in aqueous Pr³⁺-TiO₂ suspension,” Chemical Engineering Journal, vol. 147, no. 2-3, pp. 219–225, 2009.

Y. Yu, J. Wang, and J. F. Parr, “Preparation and properties of TiO₂/fumed silica composite photocatalytic materials,” Procedia Engineering, vol. 27, pp. 448–456, 2012.

L. F. Qi, J. G. Yu, and M. Jaroniec, “Enhanced and suppressed effects of ionic liquid on the photocatalytic activity of TiO₂,” Adsorption, vol. 19, pp. 557–561, 2013.

H. Zhang, R. Zong, J. Zhao, and Y. Zhu, “Dramatic visible photocatalytic degradation performances due to synergetic effect of TiO₂ with PANI,” Environmental Science and Technology, vol. 42, no. 10, pp. 3803–3807, 2008.

P. A. Pekakis, N. P. Xekoukoulotakis, and D. Mantzavinos, “Treatment of textile dyehouse wastewater by TiO₂ photocatalysis,” Water Research, vol. 40, no. 6, pp. 1276–1286, 2006.

M. N. Chong, B. Jin, C. W. K. Chow, and C. Saint, “Recent developments in photocatalytic water treatment technology: a review,” Water Research, vol. 44, no. 10, pp. 2997–3027, 2010.

H. Xu, Z. Zheng, L. Z. Zhang, H. L. Zhang, and F. Deng, “Recent developments in photocatalytic water treatment technology: a review,” Journal of Solid State Chemistry, vol. 181, no. 9, pp. 2516–2522, 2008.

A. A. Merdaw, A. O. Sharif, and G. A. W. Derwish, “Mass transfer in pressure-driven membrane separation processes, part I,” Chemical Engineering Journal, vol. 168, no. 1, pp. 215–228, 2011.

C. Lin and K. S. Lin, “Photocatalytic oxidation of toxic organohalides with TiO₂/UV: the effects of humic substances and organic mixtures,” Chemosphere, vol. 66, no. 10, pp. 1872–1877, 2007.

M. R. Hoffmann, S. T. Martin, W. Choi, and D. W. Bahnemann, “Environmental applications of semiconductor photocatalysis,” Chemical Reviews, vol. 95, no. 1, pp. 69–96, 1995.

Q. Xiang, J. Yu, and P. K. Wong, “Quantitative characterization of hydroxyl radicals produced by various photocatalysts,” Journal of Colloid and Interface Science, vol. 357, no. 1, pp. 163–167, 2011.

X. Z. Li, H. Liu, L. F. Cheng, and H. J. Tong, “Photocatalytic oxidation using a new catalyst—TiO₂ microsphere—for water and wastewater treatment,” Environmental Science and Technology, vol. 37, no. 17, pp. 3989–3994, 2003.

T. Ohno, K. Tokieda, S. Higashida, and M. Matsumura, “Synergism between rutile and anatase TiO₂ particles in photocatalytic oxidation of naphthalene,” Applied Catalysis A, vol. 244, no. 2, pp. 383–391, 2003.

S. Cong and Y. Xu, “Explaining the high photocatalytic activity of a mixed phase TiO₂: a combined effect of O₂ and crystallinity,” Journal of Physical Chemistry C, vol. 115, no. 43, pp. 21161–21168, 2011.

Q. Sun and Y. Xu, “Evaluating intrinsic photocatalytic activities of anatase and rutile TiO₂ for organic degradation in water,” Journal of Physical Chemistry C, vol. 114, no. 44, pp. 18911–18918, 2010.

Y. Y. Gurkan, E. Kasapbasi, and Z. Cinar, “Enhanced solar photocatalytic activity of TiO₂ by selenium(IV) ion-doping: characterization and DFT modeling of the surface,” Chemical Engineering Journal, vol. 214, pp. 34–44, 2013.

J. Ananpattarachai, P. Kajitvichyanukul, and S. Seraphin, “Visible light absorption ability and photocatalytic oxidation activity of various interstitial N-doped TiO₂ prepared from different nitrogen dopants,” Journal of Hazardous Materials, vol. 168, no. 1, pp. 253–261, 2009.

J. Senthilnathan and L. Philip, “Photodegradation of methyl parathion and dichlorvos from drinking water with N-doped TiO₂ under solar radiation,” Chemical Engineering Journal, vol. 172, no. 2-3, pp. 678–688, 2011.

J. A. Cha, S. H. An, H. D. Jang, C. S. Kim, D. K. Song, and T. Kim, “Synthesis and photocatalytic activity of N-doped TiO₂/ZrO₂ visible-light photocatalysts,” Advanced Powder Technology, vol. 23, no. 6, pp. 717–723, 2012.

X. Xu, D. Yin, S. Wu, J. Wang, and J. Lu, “Preparation of oriented -doped TiO₂ film and degradation of methylene blue under visible light irradiation,” Ceramics International, vol. 36, no. 2, pp. 443–450, 2010.

Y. Liu, J. Liu, Y. Lin, Y. Zhang, and Y. Wei, “Simple fabrication and photocatalytic activity of S-doped TiO₂ under low power LED visible light irradiation,” Ceramics International, vol. 35, no. 8, pp. 3061–3065, 2009.

X. Xu, D. Yin, S. Wu, J. Wang, and J. Lu, “Preparation of oriented -doped TiO₂ film and degradation of methylene blue under visible light irradiation,” Ceramics International, vol. 36, no. 2, pp. 443–450, 2010.

G. Tian, K. Pan, H. Fu, L. Jing, and W. Zhou, “Enhanced photocatalytic activity of S-doped TiO₂-ZrO₂ nanoparticles under visible-light irradiation,” Journal of Hazardous Materials, vol. 166, no. 2-3, pp. 939–944, 2009.

H. Irie, Y. Watanabe, and K. Hashimoto, “Carbon-doped anatase TiO₂ powders as a visible-light sensitive photocatalyst,” Chemistry Letters, vol. 32, no. 8, pp. 772–773, 2003.

T. Ihara, M. Miyoshi, Y. Iriyama, O. Matsumoto, and S. Sugihara, “Visible-light-active titanium oxide photocatalyst realized by an oxygen-deficient structure and by nitrogen doping,” Applied Catalysis B, vol. 42, no. 4, pp. 403–409, 2003.

H. Liu, H. T. Ma, X. Z. Li, W. Z. Li, M. Wu, and X. H. Bao, “The enhancement of TiO₂ photocatalytic activity by hydrogen thermal treatment,” Chemosphere, vol. 50, no. 1, pp. 39–46, 2003.

A. Mills and S. Le Hunte, “An overview of semiconductor photocatalysis,” Journal of Photochemistry and Photobiology A, vol. 108, no. 1, pp. 1–35, 1997.

W. Chu, W. K. Choy, and T. Y. So, “The effect of solution pH and peroxide in the TiO₂-induced photocatalysis of chlorinated aniline,” Journal of Hazardous Materials, vol. 141, no. 1, pp. 86–91, 2007.

J. Marsh and D. Gorse, “A photoelectrochemical and ac impedance study of anodic titanium oxide films,” Electrochimica Acta, vol. 43, no. 7, pp. 659–670, 1997.

H. Yu, S. Chen, X. Quan, H. Zhao, and Y. Zhang, “Fabrication of a TiO₂-BDD heterojunction and its application as a photocatalyst for the simultaneous oxidation of an azo dye and reduction of Cr(VI),” Environmental Science and Technology, vol. 42, no. 10, pp. 3791–3796, 2008.

J. J. Testa, M. A. Grela, and M. I. Litter, “Heterogeneous photocatalytic reduction of chromium(VI) over TiO₂ particles in the presence of oxalate: involvement of Cr(V) species,” Environmental Science and Technology, vol. 38, no. 5, pp. 1589–1594, 2004.

L. Wang, N. Wang, L. Zhu, H. Yu, and H. Tang, “Photocatalytic reduction of Cr(VI) over different TiO₂ photocatalysts and the effects of dissolved organic species,” Journal of Hazardous Materials, vol. 152, no. 1, pp. 93–99, 2008.

Y. S. Sohn, Y. R. Smith, M. Misra, and V. (Ravi) Subramanian, “Electrochemically assisted photocatalytic degradation of methyl orange using anodized titanium dioxide nanotubes,” Applied Catalysis B, vol. 84, no. 3-4, pp. 372–378, 2008.

R. K. Quinn, R. D. Nasby, and R. J. Baughman, “Photoassisted electrolysis of water using single crystal α-Fe₂O₃ anodes,” Materials Research Bulletin, vol. 11, no. 8, pp. 1011–1017, 1976.

H. Liu, X. Z. Li, Y. J. Leng, and W. Z. Li, “An alternative approach to ascertain the rate-determining steps of TiO₂ photoelectrocatalytic reaction by electrochemical impedance spectroscopy,” Journal of Physical Chemistry B, vol. 107, no. 34, pp. 8988–8996, 2003.

H. Tada, A. Hattori, Y. Tokihisa, K. Imai, N. Tohge, and S. Ito, “A patterned-TiO₂/SnO₂ bilayer type photocatalyst,” Journal of Physical Chemistry B, vol. 104, no. 19, pp. 4586–4587, 2000.

C. S. Turchi and D. F. Ollis, “Photocatalytic degradation of organic water contaminants: mechanisms involving hydroxyl radical attack,” Journal of Catalysis, vol. 122, no. 1, pp. 178–192, 1990.

M. Ivanda, S. Musić, S. Popović, and M. Gotić, “XRD, Raman and FT-IR spectroscopic observations of nanosized TiO₂ synthesized by the sol-gel method based on an esterification reaction,” Journal of Molecular Structure, vol. 480-481, pp. 645–649, 1999.

C. Wang, X. Zhang, H. Liu, X. Li, W. Li, and H. Xu, “Reaction kinetics of photocatalytic degradation of sulfosalicylic acid using TiO₂ microspheres,” Journal of Hazardous Materials, vol. 163, no. 2-3, pp. 1101–1106, 2009.

X. Z. Li, H. Liu, L. F. Cheng, and H. J. Tong, “Kinetic behaviour of the adsorption and photocatalytic degradation of salicylic acid in aqueous TiO₂ microsphere suspension,” Journal of Chemical Technology and Biotechnology, vol. 79, no. 7, pp. 774–781, 2004.

H. Liu, C. L. Yu, and C. Wang, “Economics and its implication of photocatalyst recycling for water purification,” in Handbook of Photocatalysts, pp. 527–534, Nova Science Publishers, 2009.

M. Y. Ghaly, T. S. Jamil, I. E. El-Seesy, E. R. Souaya, and R. A. Nasr, “Treatment of highly polluted paper mill wastewater by solar photocatalytic oxidation with synthesized nano TiO₂,” Chemical Engineering Journal, vol. 168, no. 1, pp. 446–454, 2011.

J. G. Yu, Q. Li, S. W. Liu, and M. Jaroniec, “Ionic-liquie-assisted synthesis of uniform fluorinated B/C-codoped TiO₂ nanocrystals and their enhanced visble-light photocatalytic activity,” Chemistry A European Journal, vol. 19, pp. 2433–2441, 2013.

S. Liu, L. Yang, S. Xu, S. Luo, and Q. Cai, “Photocatalytic activities of C-N-doped TiO₂ nanotube array/carbon nanorod composite,” Electrochemistry Communications, vol. 11, no. 9, pp. 1748–1751, 2009.

H. Zhang and H. Zhu, “Preparation of Fe-doped TiO₂ nanoparticles immobilized on polyamide fabric,” Applied Surface Science, vol. 258, no. 24, pp. 10034–10041, 2012.

S. H. Othman, S. A. Rashid, T. I. M. Ghazi, and N. Abdulah, “Dispersion and stabilization of photocatalytic TiO₂ nanoparticles in aqueous suspension for coatings applications,” Journal of Nanomaterials, vol. 2012, Article ID 718214, 10 pages, 2012.

H. Liu, S. Cheng, J. Zhang, C. Cao, and S. Zhang, “Titanium dioxide as photocatalyst on porous nickel: adsorption and the photocatalytic degradation of sulfosalicylic acid,” Chemosphere, vol. 38, no. 2, pp. 283–292, 1999.

X. Z. Li, H. L. Liu, P. T. Yue, and Y. P. Sun, “Photoelectrocatalytic oxidation of rose Bengal in aqueous solution using a Ti/TiO₂ mesh electrode,” Environmental Science and Technology, vol. 34, no. 20, pp. 4401–4406, 2000.

P. F. Lee, X. Zhang, D. D. Sun, J. Du, and J. O. Leckie, “Synthesis of bimodal porous structured TiO₂ microsphere with high photocatalytic activity for water treatment,” Colloids and Surfaces A, vol. 324, no. 1–3, pp. 202–207, 2008.

J. Xie, D. Jiang, M. Chen et al., “Preparation and characterization of monodisperse Ce-doped TiO₂ microspheres with visible light photocatalytic activity,” Colloids and Surfaces A, vol. 372, no. 1–3, pp. 107–114, 2010.

H. Němec, C. Kadlec, F. Kadlec et al., “Resonant magnetic response of TiO₂ microspheres at terahertz frequencies,” Applied Physics Letters, vol. 100, no. 6, Article ID 061117, pp. 891–894, 2012.

F. Cesano, D. Pellerej, D. Scarano, G. Ricchiardi, and A. Zecchina, “Radially organized pillars in TiO₂ and in TiO₂/C microspheres: synthesis, characterization and photocatalytic tests,” Journal of Photochemistry and Photobilogy A, vol. 242, pp. 51–58, 2012.

M. Ye, Y. Yang, Y. Zhang, T. Zhang, and W. Shao, “Hydrothermal synthesis of hydrangea-like F-doped titania microspheres for the photocatalytic degradation of carbamazepine under UV and visible light irradiation,” Journal of Nanomaterials, vol. 2012, Article ID 583417, 8 pages, 2012.

S. Mozia, P. Brożek, J. Przepiórski, B. Tryba, and A. W. Morawski, “Immobilized TiO₂ for phenol degrdation in a pilot-scale photocatalytic reactor,” Journal of Nanomaterials, vol. 2012, Article ID 949764, 10 pages, 2012.

P. Zhou, J. G. Yu, and Y. X. Wang, “The new understanding on photocatalytic mechanism of visible-light response N-S codoped anatase TiO₂ by first-principles,” Applied Catalysis B, vol. 142-143, pp. 45–53, 2013.

E. S. Elmolla and M. Chaudhuri, “The feasibility of using combined TiO₂ photocatalysis-SBR process for antibiotic wastewater treatment,” Desalination, vol. 272, no. 1–3, pp. 218–224, 2011.

Y. Jin, M. Wu, G. Zhao, and M. Li, “Photocatalysis-enhanced electrosorption process for degradation of high-concentration dye wastewater on TiO₂/carbon aerogel,” Chemical Engineering Journal, vol. 168, no. 3, pp. 1248–1255, 2011.

L. Prieto-Rodriguez, S. Miralles-Cuevas, I. Oller, A. Agüera, G. L. Puma, and S. Malato, “Treatment of emerging contaminants in wastewater treatment plants (WWTP) effluents by solar photocatalysis using low TiO₂ concentrations,” Journal of Hazardous Materials, vol. 211-212, pp. 131–137, 2012.

M. N. Chong and B. Jin, “Photocatalytic treatment of high concentration carbamazepine in synthetic hospital wastewater,” Journal of Hazardous Materials, vol. 199-200, pp. 135–142, 2012.

M. Chen and W. Chu, “Degradation of antibiotic norfloxacin in aqueous solution by visible-light-mediated C-TiO₂ photocatalysis,” Journal of Hazardous Materials, vol. 219-220, pp. 183–189, 2012.

D. Nasuhoglu, A. Rodayan, D. Berk, and V. Yargeau, “Removal of the antibiotic levofloxacin (LEVO) in water by ozonation and TiO₂ photocatalysis,” Chemical Engineering Journal, vol. 189-190, pp. 41–48, 2012.

R. Nakano, R. Chand, E. Obuchi, K. Katoh, and K. Nakano, “Performance of TiO₂ photocatalyst supported on silica beads for purification of wastewater after absorption of reflow exhaust gas,” Chemical Engineering Journal, vol. 176-177, pp. 260–264, 2011.

N. N. Rao, V. Chaturvedi, and G. Li Puma, “Novel pebble bed photocatalytic reactor for solar treatment of textile wastewater,” Chemical Engineering Journal, vol. 184, pp. 90–97, 2012.

I. Catanzaro, G. Avellone, G. Marcì et al., “Biological effects and photodegradation by TiO₂ of terpenes present in industrial wastewater,” Journal of Hazardous Materials, vol. 185, no. 2-3, pp. 591–597, 2011.

R. Qiu, D. Zhang, Z. Diao et al., “Visible light induced photocatalytic reduction of Cr(VI) over polymer-sensitized TiO₂ and its synergism with phenol oxidation,” Water Research, vol. 46, no. 7, pp. 2299–2306, 2012.

W. Zhang, Y. Li, Y. Su, K. Mao, and Q. Wang, “Effect of water composition on TiO₂ photocatalytic removal of endocrine disrupting compounds (EDCs) and estrogenic activity from secondary effluent,” Journal of Hazardous Materials, vol. 215-216, pp. 252–258, 2012.

Y. Xu, J. Jia, D. Zhong, and Y. Wang, “Degradation of dye wastewater in a thin-film photoelectrocatalytic (PEC) reactor with slant-placed TiO₂/Ti anode,” Chemical Engineering Journal, vol. 150, no. 2-3, pp. 302–307, 2009.

Y. Yao, K. Li, S. Chen, J. Jia, Y. Wang, and H. Wang, “Decolorization of Rhodamine B in a thin-film photoelectrocatalytic (PEC) reactor with slant-placed TiO₂ nanotubes electrode,” Chemical Engineering Journal, vol. 187, pp. 29–35, 2012.

M. Boroski, A. C. Rodrigues, J. C. Garcia, L. C. Sampaio, J. Nozaki, and N. Hioka, “Combined electrocoagulation and TiO₂ photoassisted treatment applied to wastewater effluents from pharmaceutical and cosmetic industries,” Journal of Hazardous Materials, vol. 162, no. 1, pp. 448–454, 2009.

H. El Hajjouji, F. Barje, E. Pinelli et al., “Photochemical UV/TiO₂ treatment of olive mill wastewater (OMW),” Bioresource Technology, vol. 99, no. 15, pp. 7264–7269, 2008.

K. Nakano, E. Obuchi, S. Takagi et al., “Photocatalytic treatment of water containing dinitrophenol and city water over TiO₂/SiO₂,” Separation and Purification Technology, vol. 34, no. 1–3, pp. 67–72, 2004.

D. Wu, H. You, R. Zhang, C. Chen, and D. J. Lee, “Ballast waters treatment using UV/Ag-TiO₂+O₃ advanced oxidation process with Escherichia coli and Vibrio alginolyticus as indicator microorganisms,” Chemical Engineering Journal, vol. 174, no. 2-3, pp. 714–718, 2011.

C. Han, M. Pelaez, V. Likodimos et al., “Innovative visible light-activated sulfur doped TiO₂ films for water treatment,” Applied Catalysis B, vol. 107, no. 1-2, pp. 77–87, 2011.

R. Bergamasco, F. V. da Silva, F. S. Arakawa et al., “Drinking water treatment in a gravimetric flow system with TiO₂ coated membranes,” Chemical Engineering Journal, vol. 174, no. 1, pp. 102–109, 2011.

T. Ochiai, K. Nakata, T. Murakami et al., “Development of solar-driven electrochemical and photocatalytic water treatment system using a boron-doped diamond electrode and TiO₂ photocatalyst,” Water Research, vol. 44, no. 3, pp. 904–910, 2010.

E. López Loveira, P. S. Fiol, A. Senn, G. Curutchet, R. Candal, and M. I. Litter, “TiO₂-photocatalytic treatment coupled with biological systems for the elimination of benzalkonium chloride in water,” Separation and Purification Technology, vol. 91, pp. 108–116, 2012.

M. Pelaez, P. Falaras, A. G. Kontos et al., “A comparative study on the removal of cylindrospermopsin and microcystins from water with NF-TiO₂-P25 composite films with visible and UV-vis light photocatalytic activity,” Applied Catalysis B, vol. 121, pp. 30–39, 2012.

M. G. Antoniou, P. A. Nicolaou, J. A. Shoemaker, A. A. de la Cruz, and D. D. Dionysiou, “Impact of the morphological properties of thin TiO₂ photocatalytic films on the detoxification of water contaminated with the cyanotoxin, microcystin-LR,” Applied Catalysis B, vol. 91, no. 1-2, pp. 165–173, 2009.

K. Zhang, K. C. Kemp, and V. Chandra, “Homogeneous anchoring of TiO₂ nanoparticles on graphene sheets for waste water treatment,” Materials Letters, vol. 81, pp. 127–130, 2012.

L. Rizzo, “Inactivation and injury of total coliform bacteria after primary disinfection of drinking water by TiO₂ photocatalysis,” Journal of Hazardous Materials, vol. 165, no. 1–3, pp. 48–51, 2009.

G. E. Romanos, C. P. Athanasekou, F. K. Katsaros et al., “Double-side active TiO₂-modified nanofiltration membranes in continuous flow photocatalytic reactors for effective water purification,” Journal of Hazardous Materials, vol. 211-212, pp. 304–316, 2012.

P. A. Pekakis, N. P. Xekoukoulotakis, and D. Mantzavinos, “Treatment of textile dyehouse wastewater by TiO₂ photocatalysis,” Water Research, vol. 40, no. 6, pp. 1276–1286, 2006.

L. Lin, M. N. Xie, Y. M. Liang, Y. Q. He, G. Y. S. Chan, and T. G. Luan, “Degradation of cypermethrin, malathion and dichlorovos in water and on tea leaves with O₃/UV/TiO₂ treatment,” Food Control, vol. 28, no. 2, pp. 374–379, 2012.

J. L. Esbenshade, J. C. Cardoso, and M. V. B. Zanoni, “Removal of sunscreen compounds from swimming pool water using self-organized TiO₂ nanotubular array electrodes,” Journal of Photochemistry and Photobiology A, vol. 214, no. 2-3, pp. 257–263, 2010.

N. Ma, X. Quan, Y. Zhang, S. Chen, and H. Zhao, “Integration of separation and photocatalysis using an inorganic membrane modified with Si-doped TiO₂ for water purification,” Journal of Membrane Science, vol. 335, no. 1-2, pp. 58–67, 2009.

D. N. Bui, S. Z. Kang, X. Li, and J. Mu, “Effect of Si doping on the photocatalytic activity and photoelectrochemical property of TiO₂ nanoparticles,” Catalysis Communications, vol. 13, no. 1, pp. 14–17, 2011.

J. A. Rengifo-Herrera and C. Pulgarin, “Photocatalytic activity of N, S co-doped and N-doped commercial anatase TiO₂ powders towards phenol oxidation and E. coli inactivation under simulated solar light irradiation,” Solar Energy, vol. 84, no. 1, pp. 37–43, 2010.

H. Sun, H. Liu, J. Ma, X. Wang, B. Wang, and L. Han, “Preparation and characterization of sulfur-doped TiO₂/Ti photoelectrodes and their photoelectrocatalytic performance,” Journal of Hazardous Materials, vol. 156, no. 1–3, pp. 552–559, 2008.

D. Zhao, C. Chen, Y. Wang et al., “Enhanced photocatalytic degradation of dye pollutants under visible irradiation on Al(III)-modified TiO₂: structure, interaction, and interfacial electron transfer,” Environmental Science and Technology, vol. 42, no. 1, pp. 308–314, 2008.

S. M. Chang and W. S. Liu, “Surface doping is more beneficial than bulk doping to the photocatalytic activity of vanadium-doped TiO₂,” Applied Catalysis B, vol. 101, no. 3-4, pp. 333–342, 2011.

Q. Li, C. Zhang, and J. Li, “Photocatalytic and microwave absorbing properties of polypyrrole/Fe-doped TiO₂ composite by in situ polymerization method,” Journal of Alloys and Compounds, vol. 509, no. 5, pp. 1953–1957, 2011.

R. Xu, J. Li, J. Wang et al., “Photocatalytic degradation of organic dyes under solar light irradiation combined with Er³⁺:YAlO₃/Fe- and Co-doped TiO₂ coated composites,” Solar Energy Materials and Solar Cells, vol. 94, no. 6, pp. 1157–1165, 2010.

R. F. Chen, C. X. Zhang, J. Deng, and G. Q. Song, “Preparation and photocatalytic activity of Cu²⁺-doped TiO₂/SiO₂,” International Journal of Minerals, Metallurgy and Materials, vol. 16, no. 2, pp. 220–225, 2009.

G. G. Liu, X. Z. Zhang, Y. J. Xu, X. S. Niu, L. Q. Zheng, and X. J. Ding, “Effect of ZnFe₂O₄ doping on the photocatalytic activity of TiO₂,” Chemosphere, vol. 55, no. 9, pp. 1287–1291, 2004.

E. Pipelzadeh, A. A. Babaluo, M. Haghighi, A. Tavakoli, M. V. Derakhshan, and A. K. Behnami, “Silver doping on TiO₂ nanoparticles using a sacrificial acid and its photocatalytic performance under medium pressure mercury UV lamp,” Chemical Engineering Journal, vol. 155, no. 3, pp. 660–665, 2009.

X. Lin, F. Rong, D. Fu, and C. Yuan, “Enhanced photocatalytic activity of fluorine doped TiO₂ by loaded with Ag for degradation of organic pollutants,” Powder Technology, vol. 219, pp. 173–178, 2012.

L. G. Devi, B. Nagaraj, and K. E. Rajashekhar, “Synergistic effect of Ag deposition and nitrogen doping in TiO₂ for the degradation of phenol under solar irradiation in presence of electron acceptor,” Chemical Engineering Journal, vol. 181-182, pp. 259–266, 2012.

X. Li, R. Xiong, and G. Wei, “Preparation and photocatalytic activity of nanoglued Sn-doped TiO₂,” Journal of Hazardous Materials, vol. 164, no. 2-3, pp. 587–591, 2009.

Z. M. El-Bahy, A. A. Ismail, and R. M. Mohamed, “Enhancement of titania by doping rare earth for photodegradation of organic dye (Direct Blue),” Journal of Hazardous Materials, vol. 166, no. 1, pp. 138–143, 2009.

A. F. Shojaie and M. H. Loghmani, “La³⁺ and Zr⁴⁺ co-doped anatase nano TiO₂ by sol-microwave method,” Chemical Engineering Journal, vol. 157, no. 1, pp. 263–269, 2010.

C. Fan, P. Xue, and Y. Sun, “Preparation of Nano-TiO₂ doped with cerium and its photocatalytic activity,” Journal of Rare Earths, vol. 24, no. 3, pp. 309–313, 2006.

L. Yang, P. Liu, X. Li, and S. Li, “The photo-catalytic activities of neodymium and fluorine doped TiO₂ nanoparticles,” Ceramics International, vol. 38, no. 6, pp. 4791–4796, 2012.

W. Sangkhun, L. Laokiat, V. Tanboonchuy, P. Khamdahsag, and N. Grisdanurak, “Photocatalytic degradation of BTEX using W-doped TiO₂ immobilized on fiberglass cloth under visible light,” Superlattices and Microstructures, vol. 52, no. 4, pp. 632–642, 2012.

J. Gong, C. Yang, W. Pu, and J. Zhang, “Liquid phase deposition of tungsten doped TiO₂ films for visible light photoelectrocatalytic degradation of dodecyl-benzenesulfonate,” Chemical Engineering Journal, vol. 167, no. 1, pp. 190–197, 2011.

J. Y. Gong, W. H. Pu, C. Z. Yang, and J. D. Zhang, “Tungsten and nitrogen co-doped TiO₂ electrode sensitized with Fe-chlorophyllin for visible light photoelectrocatalysis,” Chemical Engineering Journal, vol. 209, pp. 94–101, 2012.

Y. Hu, Y. T. Cao, P. X. Wang et al., “A new perspective for effect of Bi on the photocatalytic activity of Bi-doped TiO₂,” Applied Catalysis B, vol. 125, pp. 294–303, 2012.