2. Photocatalytic reduction over TiO2 to detoxify metal ions

The reducible hazardous heavy metal ions that are widely found in the wastewater are silver Ag(I), copper Cu(II), hexavalent chromium Cr(VI), mercury Hg(II), and uranium U(VI). Silver pollutant can be found with high concentration in the wastewater of radiophotography activity that is usually disposed from hospitals [41, 42], silver electroplating, and electronic fabrication [43]. Wastewaters containing copper in high concentration are disposed from electroplating and electrical stuffs [43, 44]. Hexavalent chromium is mostly emitted by metal plating activity and paint industry [45, 46]. Mercury is usually contained in wastewater originated from gold recovery and incinerator in hospitals [46, 47]. The environmental contamination by uranium (VI) ion may originate from uranium purification or extraction from its respected mineral. A leak of nuclear reactor releasing uranium into the water may also contribute to the uranium contamination [48, 49].

resulted by excitation of some electrons from valence band (evb) to the conduction one (ecb) with the formation of positive holes (hvb+) [3, 4]. The reactions of electron–hole generation from

e� þ O2 ! O2

The electrons, in water media and in the presence of dissolve oxygen, can react with the oxygen to form super oxide, as presented by reaction (4) [4]. This is a reduction path. The use of the electrons from TiO2 for some reducible metal ions such as Ag(I) [5–13], Au(III) [13–17], Cr(VI) [18–23], Cu (II) [24–26], Hg(II) [27, 28], and U(VI) [29, 30] has also been developed.

Meanwhile the hole generates the free OH radical after contact with water and TiO2 surface, as illustrated by Eq. (2) and (3). The OH radical acts as a strong oxidizing agent with high oxidation potential (E0 = 2.8 V) that can degrade organic compounds into smaller CO2 and H2O molecules [3, 4]. This is called as oxidation path. Due to strong ability in organic degradation, OH radicals from TiO2 have been intensively applied for cyanide oxidation [31], treating hazardous phenol [32, 33], di-nitrophenol [34, 35], various organic dyes [36–39], and surfactant of detergent [40].

The applications of the photodegradation process catalyzed by TiO2 have been frequently published through journals and books. Meanwhile, less intensive photoreduction methods are explored. In fact, the photoreduction process over TiO2 has some advantages compared to the other reduction reactions, with respect to simplicity, cost-effectiveness, efficiency, less

It is interesting therefore to address the applications of the reduction over TiO2 photocatalyst for various purposes. The applications of the photocatalytic reduction in the presence of TiO2 for detoxifying the toxic metal ions, doping by transition and noble metals, and converting

The reducible hazardous heavy metal ions that are widely found in the wastewater are silver Ag(I), copper Cu(II), hexavalent chromium Cr(VI), mercury Hg(II), and uranium U(VI). Silver pollutant can be found with high concentration in the wastewater of radiophotography activity that is usually disposed from hospitals [41, 42], silver electroplating, and electronic fabrication [43].

greenhouse gas CO2 into more valuable chemicals are described in more detail.

2. Photocatalytic reduction over TiO2 to detoxify metal ions

Notes: hv represents photon energy of UV light (E = hv), O2

chemical usage, and green chemistry principles.

TiO2 <sup>þ</sup> <sup>h</sup><sup>ν</sup> ! TiO2 <sup>e</sup>� <sup>þ</sup> <sup>h</sup><sup>þ</sup> (1)

h<sup>þ</sup> þ H2O ! �OH þ H<sup>þ</sup> (2)

h<sup>þ</sup> þ TiOH ! Ti OH (3)

TiO2 <sup>þ</sup> <sup>e</sup>� <sup>þ</sup> <sup>h</sup><sup>þ</sup> ! TiO2 <sup>þ</sup> heat (5)

� (4)

\_ is called as superoxide, and �OH

TiO2 are presented as Eqs. (1):

130 Photocatalysts - Applications and Attributes

indicates hydroxide radical.

Contamination of the hazardous metals can create environmental and human health problems. Silver contamination can induce argyria syndrome [50]. For human, the excessive copper intake can disturb the gastrointestinal (GI) system [44]. Hexavalent chromium is carcinogenic [45, 51], and mercury Hg(II) can cause neurological dysfunction [47]. The uranium ion, as a radioactive element, must be very dangerous, both for environment and human [44]. Therefore, a method that can detoxify the hazardous heavy metals is urgently.

The products of the reduction of Ag(I), copper Cu(II), hexavalent chromium Cr(VI), mercury Hg(II), and uranium (VI) are Ag(0), Cu(0), Cr(III), Hg(0), and U(IV), respectively, that are less toxic [44]. Hence, these facts motivate many researchers for detoxification of the hazardous heavy metal ions by reduction method, especially by photoreduction catalyzed by TiO2.

Photoreduction of Ag(I) ions in the aqueous solution by electron provided by TiO2 takes place effectively, due to its high standard reduction potential (E0 ) = 0.79 V [52]. The reaction is presented by Eq. (6) that produces undissolved solid silver that is less toxic and easier to be handled. It is clear that by photoreduction process, the silver contaminant is detoxified:

$$\text{Ag}^+ + \text{e}^- \rightarrow \text{Ag}^0 \qquad \qquad \text{E}^0 = 0.79 \text{ V} \tag{6}$$

Hexavalent chromium in the aqueous solution existed as chromate (CrO4 = ) and bichromat (Cr2O7 = ) ions, depending on the pH that has high standard reduction potential (E<sup>0</sup> ) = 1.33 V in acid condition [52]. This allows them to be easily reduced into Cr(III) as Cr3+ ions as presented by reaction (7). The chromate (Cr(VI)) is highly toxic, while Cr3+ is less toxic or is even needed by feeding women [45]. Hence, detoxification of the toxic Cr(VI) can be carried out by reduction method. Photoreduction of Cr(VI) over TiO2 photocatalyst has been frequently reported [18–23] with satisfaction result. Further, the Cr3+ ions can precipitate into undissolved Cr(OH)3 in basic condition that is remediable by solidification/stabilization technique:

$$\text{Cr}\_2\text{O}\_7^- + 14\text{H}^+ + \text{e}^- \rightarrow 2\text{Cr}^{3+} + 7\text{H}\_2\text{O} \qquad \quad \text{E}^0 = 1.52\text{ V} \tag{7}$$

Copper ions in the solution formed as Cu2+ can be reduced into dissolved Cu (I) and/or undissolved Cu(0), with respective E<sup>0</sup> = 0.153 V and 0.34 V [52]. Photoreduction method in the presence of TiO2 has been subjected to detoxify the toxic Cu2+ that can prominently form the undissolved toxic Cu(0), with very small Cu(I) ion [24–26]. The reactions of the Cu(II) reduction are shown by Eqs. (8) and (9). The photoreduction of Cu(II) is found to be less effective compared to Ag(I) photoreduction that may be caused by the low standard reduction potential, E<sup>0</sup> = 0.34 V [52]. To improve the effectiveness, a reducing agent, such as oxalic acid, can be added in the Cu(II) photoreduction [26]:

$$\text{Cu}^{2+} + \text{e} \rightarrow \text{Cu}^{+} \qquad \qquad \text{E}^{0} = 0.153 \text{ V} \tag{8}$$

shift the absorption of TiO2 to visible light region [5–11]. The latter is supposed to give some advantages, as the photocatalytic process under metal-doped TiO2 can take place under sun light that must be low-cost and safer and so greener than that of by UV light irradiation.

Photoreduction Processes over TiO2 Photocatalyst http://dx.doi.org/10.5772/intechopen.80914 133

The transition metals that have been examined as dopants on TiO2 are Cu(II) [53, 54], Fe [55, 56], Co [56, 57], Ni [57], Mn [56], and Cr [58]. Moreover, Ag(I) [5–13], Au(III) [13, 14], Pd(II) [14], and Pt(IV) [14] are the noble metals that are frequently doped into TiO2 structure. All the metals doped on TiO2 are reported to improve the photocatalytic activity of TiO2 under UV irradiation as well as to shift the visible absorption with various effects, from very low, shown by Cr(III) up

A doping process basically involves the conversion of the metal ions in the solution to be deposited solid metal on TiO2 powder that is frequently carried out by sol–gel method [56].

From the above methods, the regard salt solution is usually used as the dopant source, and high temperature is required that makes the method costly due to high energy consumption. In addition, large metal particle is usually resulted from the process that retards the metal insertion into gap of valence and conduction gaps. As a consequence, the small absorption shift is resulted that yields less significant improvement of the photocatalyst activity under UV

In addition to the four doping methods, photoreduction has also been examined. The photoreduction method becomes a great of interest, because the process takes place at room temperature, no need of chemicals, except UV light, and has resulted small cluster of metal dopant particles. The small particles are well inserted into the gap between valence and conduction band of TiO2. Such insertion has considerably shortened the gap that enhances the activity under UV light and pronounces shift of the light absorption into wide visible region. However, the photoreduction method is limited only for dopants that are reducible metal ions, including Cu(II) representing transition metal and Ag(I), Au(III), Pd(II), and Pt(IV) for noble metal ions. In general, the doping process by photoreduction method is carried out by UV light irradiation toward the regard metal salt solutions in a certain period of time. Then M/TiO2 (M = metal

Photoreduction of Ag (I) in the solution over TiO2 for doping purpose principally follows the same procedure as in the detoxification, as described earlier. The starting salt for Ag doping

and the small Ag particle resulted can enter into the gap between the conduction and the valence. The present of the small particle dopant in the gap shortens the bandgap. This allows the metal-doped TiO2 to be active under visible light and to work better with UV irradiation,

In the doping Au on TiO2 through photoreduction method, the salt frequently introduced as

, Au<sup>+</sup>

whether for degradation of the organic pollutants or for bacterial combating.

following reactions (14)–(16) with their own standard reduction potentials [52]:

), the photoreduction of Ag+ takes place efficiently,

[13–16]. The doping follows reaction (13).

, and Au3+ that are also reducible by the

However, hydrothermal [57] and chemical vapor [58] methods are also introduced.

to the very significant effect, observed on Ag(I).

light or the slight visible light responsiveness.

usually used is AgNO3 [5–13].

As its high standard reduction potential (E<sup>0</sup>

gold source is KAuCl4 that dissolves to form AuCl4

The other gold ions may form as AuCl2

dopant) resulted is dried at about 110C to remove the water.

$$\text{Cu}^{2+} + 2\text{ e } \rightarrow \text{Cu}^{0} \qquad \qquad \text{E}^{0} = 0.34\text{ V} \tag{9}$$

Photoreduction over TiO2 has been also used to detoxify mercury (Hg2+) ion in the aqueous solution, by converting it to be undissolved Hg<sup>0</sup> [27, 28]. Based on the standard reduction potential as seen in the reaction (10) [52], the reduction should proceed effectively. To handle the elemental or solid mercury may be easier than that of the dissolved ions. As presented by previous authors [47], the order of the toxicity level of mercury forms, from the most toxic, is methyl mercury (CH3Hg), Hg(0) vapor, Hg2+ dissolved ion, and Hg(0) element:

$$\text{Hg}^{2+} + 2\text{e}^- \rightarrow \text{Hg}^0 \qquad \qquad \text{E}^0 = 0.85 \text{ V} \tag{10}$$

The photoreduction catalyzed by TiO2 suspension has also been studied for removal of the radioactive uranium (VI) [48, 49] that exists as UO2 2+ anionic in the solution. The anion is the most stable form and so the one that is found in the solution. The photoreduction of the anionic has produced the less radioactive uranium (V), Eq. (11) [52]:

$$\text{U}\text{O}\_2^{2+} + \text{e}^- \rightarrow \text{U}\text{O}\_2^{+} \qquad \qquad \text{E}^0 = 0.163 \text{ V} \tag{11}$$

Detoxification of the hazardous (toxic and radioactive) heavy metals by photoreduction pathway offers a simple, practical, economic, and green procedure that meets with the future requirement method in solving the environmental pollution problem.
