*3.4.3 UV/H2O2 photolysis (UVP)*

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 bond of H2O2 forming HO• radicals. The reactions describing UVP are presented below (Eqs. 6–11) [42]:

$$\text{H}\_2\text{O}\_2 + \text{hv} \rightarrow 2\text{HO}^\*\tag{6}$$

$$\text{H}\_2\text{O}\_2 + \text{HO}^\* \rightarrow \text{HO}\_2^\* + \text{H}\_2\text{O} \tag{7}$$

$$\rm H\_2O\_2 + HO\_2^\* \rightarrow HO^\* + H\_2O + H\_2 \tag{8}$$

$$\text{2HO}^\* \rightarrow \text{H}\_2\text{O}\_2 \tag{9}$$

**361**

*Fate and Occurrences of Pharmaceuticals and Their Remediation from Aquatic Environment*

Reaction 6 is the rate limiting because the rates of other reactions are much

[42]. A disadvantage of UVP is that it cannot utilize solar light as the source of UV illumination. The required UV irradiation for the photolysis of H2O2 is not available in the solar spectrum [43]. H2O2 has poor UV absorption characteristics and input

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

valence band (VB) is promoted to the conduction band (CB) of the semiconductor

recombination can reduce and oxidize the contaminants adsorbed on the surface of

TiO h e h <sup>2</sup> cb vb

• h H O H HO vb 2 ad

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

ad, and O2

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

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

concentration (Eq. 6), which decomposes the target compounds. However, an optimal H2O2 concentration exists because overdosing of H2O2 leads to reaction with

higher. In UVP, a higher initial H2O2 concentration produces higher HO•

irradiation to the reactor is wasted if the water matrix absorbs UV light.

eventually involves in the detoxification of pollutants (in water or air). e−

vb is created in the VB. The photo generated e−

vb, HO•

assure uniform irradiation of the catalyst [48].

**3.5 Advantages and limitations of AOPs**

•

• • HO H O O + ΗΟ → + 2 22 (11)

(Eq. 7). UVP is quite efficient in mineralizing PhACs

cb)/hole (h+

cb react with surface adsorbed molecular oxygen to yield

•

cb. The key advantages of UVPC are treatment at ambient conditions,

vb react with water to form HO•

− + + ν→ + (12)

cb 2 2 e OO − − + → (13)

−• radicals as well as through reductive

+ + + →+ (14)

radical

vb) pair (Eq. 12) which

migrates to the surface without

cb from the

ad radicals

*DOI: http://dx.doi.org/10.5772/intechopen.94984*

radicals leaving off HO2

*3.4.4 UV/TiO2 photo catalysis (UVPC)*

the semiconductor material producing electron (e−

superoxide radical anions (Eq. 13), while h+

on the surface of the catalyst (Eq. 14) [45].

HO•

and a h+

the semiconductor [44]. e−

through reactions with h<sup>+</sup>

cleavage by e<sup>−</sup>

$$\text{2HO}\_2\text{"} \rightarrow \text{H}\_2\text{O}\_2 + \text{O}\_2\tag{10}$$

*Fate and Occurrences of Pharmaceuticals and Their Remediation from Aquatic Environment DOI: http://dx.doi.org/10.5772/intechopen.94984*

$$\rm HO^{\*} + HO\_{2}^{\*} \rightarrow \rm H\_{2}O + O\_{2} \tag{11}$$

Reaction 6 is the rate limiting because the rates of other reactions are much higher. In UVP, a higher initial H2O2 concentration produces higher HO• radical concentration (Eq. 6), which decomposes the target compounds. However, an optimal H2O2 concentration exists because overdosing of H2O2 leads to reaction with HO• radicals leaving off HO2 • (Eq. 7). UVP is quite efficient in mineralizing PhACs [42]. A disadvantage of UVP is that it cannot utilize solar light as the source of UV illumination. The required UV irradiation for the photolysis of H2O2 is not available in the solar spectrum [43]. H2O2 has poor UV absorption characteristics and input irradiation to the reactor is wasted if the water matrix absorbs UV light.
