**4. Modifications of photocatalysis and photo-Fenton processes**

#### **4.1 Photocatalysis method**

Photocatalytic degradation using TiO2 has recently received considerable attention for removal of the persistent organic pollutants (POPs) due to its cost-effective technology, non-toxicity, fast oxidation rate, and chemical stability [24–28]. However, the wide band gap of TiO2, that is 3.2 eV for anatase, allows it only to be excited by photons with wavelengths shorter than 385 nm or UV region that limits its application under visible light [14, 22]. Therefore, an effort has been focused to overcome this deficiency, such as by doping TiO2 structure with either non-metal, and metals elements.

Doping Ag metal on TiO2 to increase the activity on the degradation of LAS in the laundry wastewater under visible light has been studied [14]. The results are seen as **Figure 14**. The increase of the TiO2-Ag activity is promoted by the smaller Eg allowing TiO2-Ag to be activated by visible light to provide OH radicals in adequate number. In contrast, TiO2 with higher Eg (3.0–3.2 eV) is difficult to be excited by the visible light, that can only form fewer number of OH radicals. Moreover, the process with TiO2-Ag under visible light takes place faster than TiO2-Ag under UV

#### **Figure 14.**

*The effectiveness of the LAS degradation with conditions of: (1) TiO2/UV light, (2) TiO2/visible light, (3) TiO2-Ag/UV light, and (4) TiO2-Ag/visible light [14].*

*Photo-Processes as Effective and Low-Cost Methods for Laundry Wastewater Treatment DOI: http://dx.doi.org/10.5772/intechopen.94336*

light. In this case, the metal dopant can act as a separation center, where electron transfer from the TiO2 conduction band to Ag particles at the interface is thermodynamically possible because the Fermi level of TiO2 is higher than that of Ag metal [14, 22]. This doping resulted in the formation of a Schottky barrier at metal semiconductor contact region and improved the photocatalytic activity of TiO2. Hence doping Ag atoms essentially reduced the band gap of TiO2 for the photo-excitation or red shift, and simultaneously reduced the recombination rate of photogenerated electron–hole pairs [14, 22].

#### **4.2 Photo-Fenton modification**

The photo-Fenton process appears as an attractive alternative for removing emerging contaminants. Photo-Fenton processes are reported to be effective in removing several classes of contaminants, such as phenols [29], amoxicillin [30], and dyes [31]. On the other hand, the use of photo-Fenton process is restricted to acidic pH values, with associate high operating costs for industrial scale applications. To overcome these drawbacks, photo-Fenton processes modified by adding selected chelating agents such as polycarboxylates and amino polycarboxylates compounds, can be successfully performed at neutral pH. The chelating agent acting as a ligand is able to form strong complexes with Fe3+ that can prevent the precipitation of Fe(OH)3 [32–33].

As pointed out in Eq. (17) and (18), such ligand (L) should be able to form stable complexes with Fe3+ which significantly absorb UV–vis light and then undergo photochemical reductions leading to Fe2+ ions [33].

$$\text{Fe}^{3+} + \text{L} \rightarrow \left\{ \text{FeL} \right\}^{3\text{\textquotedblleft}} \tag{17}$$

$$\left(\left\{\text{FeL}\right\}^{3\text{\textdegree}} + \text{h}\,\text{v}\right)\xrightarrow{\text{\textdegree}}\left(\left\{\text{FeL}\right\}^{3\text{\textdegree}}\bullet\Rightarrow\text{Fe}^{2\text{\textdegree}} + \text{L}\,\text{\textdegree}\tag{18}$$

A study [32] reported that by addition of ethylenediamine-N,N′-disuccinic acid (EDDS), photo-Fenton process was more effective at neutral pH compared to the process at acidic condition. Other study as referred by Clarizia, *et al.* [33] also examined the effect of the adding humic acid to an aqueous solution containing benzene compound in the pH range of 5.0–7.0. The result exhibited that the rate for the oxidation of benzene were as high as those measured at pH 3.0 in absence of humic acid. However, so far, the use of chelating agents in the photo-Fenton for degradation of LAS in wastewater has not been explored. Therefore, there is a great challenge to realize experimentally the use of chelating compounds in the photo-Fenton for laundry wastewater treatment through LAS degradation.

In addition, the other drawback appearing in photo-Fenton is the use of UV light, that is more expensive and hazard for people health and ecosystem [14]. This limits in the large scale application of the photo-Fenton process [14, 21, 34]. Finding solutions of such weakness is obviously essential. An example solution of the weakness is by exploring synthetic or real solar light. The synthetic solar light is represented by wolfram or tungsten lamp [14] emitting visible light, that is low price and environmentally benign.

The results of the nitro-phenols degradation under solar light photo-Fenton, as well as under UV photo-Fenton [33] exhibit that the use of solar light can result in the degradation as high as resulted by UV photo-Fenton process. It is implied that the amount of OH radicals produced by decomposition of H2O2 induced by visible

light is equal to that of by UV light. In fact, the power of UV light (λ < 350 nm) is higher than the visible one (λ > 350 nm), that should give more OH radicals, as seen in Eq. 8. This fact suggests that OH radicals provided by Fenton's reagent is much more prominent compared to that of by light. With the promising results, the possibility of employing solar energy in photo-Fenton processes helps improving their economic and environmental sustainability.
