*3.4.4 UV/TiO2 photo catalysis (UVPC)*

*Environmental Issues and Sustainable Development*

is alkyl free radical.

due to precipitation of iron, scavenging of HO•

radicals (Eq. 5) [40].

PhACs can be inhibited by PO4

form a less reactive complex [39].

*3.4.2 Photo-Fenton processes (PFP)*

the illumination of UV light [41].

*3.4.3 UV/H2O2 photolysis (UVP)*

bond of H2O2 forming HO•

below (Eqs. 6–11) [42]:

where, R•

tional HO•

•3 2 R Fe R Fe ++ + + →+ (4)

The major parameters like solution pH, amount of ferrous ion, concentration of H2O2, initial concentration of pollutants/ PhACs and presence of other background ions [36] that are affecting FP. The optimum pH for FP generally ranges from 2 to 4. At pH > 4, Fe2+ ions are unstable, and they are easily transformed to Fe3+ forming complexes with hydroxyl ion. Moreover, under alkaline conditions H2O2 loses its oxidative power as it breakdowns to water [17]. An effluent pH was Adjusted usually before addition of Fenton reagent. Increase of Fe2+ ions and H2O2 concentration boosts up the degradation rate [37]. The use of excess amount of H2O2 can deteriorate the overall degradation efficiency of FP coupled with biological treatment due to toxic nature of H2O2 to microorganisms [38]. Fenton oxidation of organics/

3−, SO4

Photo-Fenton process (H2O2/Fe2+/UV) involves formation of HO•

2−, F−

through photolysis of hydrogen peroxide (H2O2/UV) by UV-irradiation along with the Fenton reaction (H2O2/Fe2+). In presence of UV irradiation, ferric ions (Fe3+) are also photo-catalytically converted to ferrous ions (Fe2+) with formation of addi-

( ) 2 •

Likewise, PFP gives faster rates and higher degree of mineralization compared to conventional FP [39]. The reaction can be driven by low energy photons and it also can be achieved using solar irradiation [39]. The employment of solar light significantly reduces the operational cost. Another important advantage of PFP is that iron-organic complexes formed during Fenton oxidation can be broken under

UVP includes H2O2 injection with continuous mixing in a reactor equipped with UV irradiation system (wavelength 200 to 280 nm). UV light is used to cleave O-O

, Br−

and Cl−

<sup>2</sup> Fe OH h Fe HO <sup>+</sup> <sup>+</sup> + ν→ + (5)

radicals. The reactions describing UVP are presented

• H O h 2HO 2 2 + ν→ (6)

• 2HO H O → 2 2 (9)

• 2HO H O O 2 22 2 → + (10)

• • H O HO H O 2 2 + → ΗΟ + 2 2 (7)

• • H O HO H O H 22 2 + ΗΟ → + + 2 2 (8)

ions. The inhibition may be

radicals

radicals or coordination with Fe3+ to

**360**

Photocatalysis is the acceleration of a photoreaction using a catalyst in presence of light/photon. It is a well-recognized approach where light energy is employed to excite the semiconductor material producing electron (e− cb)/hole (h+ vb) pair (Eq. 12) which eventually involves in the detoxification of pollutants (in water or air). e− cb from the valence band (VB) is promoted to the conduction band (CB) of the semiconductor and a h+ vb is created in the VB. The photo generated e− migrates to the surface without recombination can reduce and oxidize the contaminants adsorbed on the surface of the semiconductor [44]. e− cb react with surface adsorbed molecular oxygen to yield superoxide radical anions (Eq. 13), while h+ vb react with water to form HO• ad radicals on the surface of the catalyst (Eq. 14) [45].

$$\text{TiO}\_2 + \text{hv} \rightarrow \text{e}^-\_{\text{cb}} + \text{h}^+\_{\text{vb}} \tag{12}$$

$$\text{e}^-\_{\text{cb}} + \text{O}\_2 \rightarrow \text{O}\_2^{\cdot -} \tag{13}$$

$$\text{pH}^{\*}\_{\text{vb}} + \text{H}\_{2}\text{O} \rightarrow \text{H}^{\*} + \text{HO}^{\*}\_{\text{ad}} \tag{14}$$

TiO2 is widely used as a photocatalyst due to high photo-catalytic activity, low cost, low toxicity, high oxidation power, easy availability and chemical stability under UV light (λ˂380 nm) [46]. TiO2 has two common crystal structures i.e., rutile and antase. TiO2 Degussa 25 consisting of 20% rutile and 80% anatase is considered as a standard photocatalyst. Organic compounds can undergo oxidative degradation through reactions with h<sup>+</sup> vb, HO• ad, and O2 −• radicals as well as through reductive cleavage by e<sup>−</sup> cb. The key advantages of UVPC are treatment at ambient conditions, lower mass transfer limitations using nanoparticles and possibility of use of solar irradiation. UVPC is capable for destruction of a wide range of organic chemicals into harmless compounds such as CO2 and H2O [47]. The major factors affecting UVPC are initial pollutant load, amount of catalyst, reactor design, irradiation time, temperature, solution pH, light intensity and presence of ionic species. The use of excess catalyst may reduce the amount of photon transfer into the medium due to opacity offered by the catalyst particles [36]. The design of reactor should assure uniform irradiation of the catalyst [48].

#### **3.5 Advantages and limitations of AOPs**

AOPs using H2O2 and Fe2+ suffer from the requirement of acidic conditions, interference by inorganic ions, iron-organic complexation and formation of iron sludge. Some of the above limitations can be overcome when heterogeneous photocatalytic treatments like UVPC is used. However, uniform illumination of UV light and separation of catalyst particles could limit the application. Application of artificial UV light increases the cost of treatment and also poses health hazard to the working personnel.

The typical advantages of iron based AOPs are:

