**5. Conclusions**

2 2 ( ) ( ) Å - - Å - + + ® -+ + *CB CB TiO EY e TEOA TiO EY e TEOA* (16)

<sup>2</sup>

In dye-sensitized photodegradation under the visible light, the dye molecule is first activated by visible light (λ > 420 nm) and then injects electrons into the conduction band of the semiconductor. Chowdhury et al. [7] described the kinetics of phenol degradation using eosin

> ( ) ( ) <sup>0</sup> 1

*A Ph*

*I*

*dt V KC* (21)

*<sup>V</sup>* (22)

*dt K C* (23)

*r A ph Ph β*

where *W* is the mass of photocatalyst, *CPh* is the phenol concentration at time *t*, *CPh*<sup>0</sup> is the initial phenol concentration, *V* is the volume of the reaction mixture, *KA* is the adsorption equilibrium constant, *kr* is the kinetic rate constant, *I* is the light intensity, and *β* is a constant. The apparent

> = ( ) *app r A <sup>W</sup> K kK*

> > <sup>0</sup> 1

Eq. (23) was used to predict the kinetic parameters of phenol photodegradation at different irradiation intensities (range, 25–100 mW cm−2). Based on a parameter estimation using the experimental data, the values of *Kapp*, *KA*, and *β* were obtained for the degradation of phenol.

*I*

*Ph app Ph β A Ph*


*dC K C*

The values of *Kapp* was 8.02 × 10−6 min−1, *KA* was 0.13 L mg−1, and *β* was 2.15 [7].

Y-sensitized TiO2/Pt with a modified Langmuir-Hinshelwood equation as follows:


*dC W kK C*

**4.3. Dye-sensitized photocatalytic phenol degradation kinetics**

410 Phenolic Compounds - Natural Sources, Importance and Applications

æ ö

Combining Eqs. (21) and (22), they obtained Eq. (23) as follows:

æ ö

kinetic constant as is defined as follows:

è ø +

( ) EY 2 22 2 - ·- - + + ® -+ *CB TiO EY e O TiO O* (17)

·- · ·+ *O H HOO HO* + ® ®® (18)

6 5 <sup>2</sup> · *HO C H OH intermediates CO* +® + (19)

2 2 · *HO intermediates H O CO* <sup>+</sup> ®® + (20)

Phenol and phenolic compounds are considered as priority pollutants by US EPA because of their high toxicity. They impose severe short-term and long-term health problems to human beings. In this review, we discussed different phenol degradation methods such as physical treatments, biological treatments, and advanced oxidation processes (AOPs). AOPs provide much faster degradation rate than conventional treatment methods and undergo complete mineralization instead of transferring the pollutants from one phase to another.

Heterogeneous photocatalysis is such an AOP that limits the use of oxidizing chemicals (e.g., ozone and hydrogen peroxide) and only utilizes light (UV or solar) and photocatalyst to generate hydroxylradicals (HO•). Several photoreactors are used with UV lamp, namely, swirlflow reactor, Taylor vortex reactor, and two-phase monolithic-type reactor for the photocatalytic degradation of phenolic compounds.InUV-light-driven photocatalysis, hydroxylradicals are the active species which react with either phenol or phenolic compounds until complete mineralization. Different parameters such as solution pH, light intensity, initial concentration oftarget compounds,photocatalyst concentration, andelectronacceptorsplay a significantrole on photocatalytic degradation of phenol and phenolic compounds. However, our economic assessment indicates that the use of UV light significantly increases the overall process cost.

Visible-light-active photocatalysts are developed to utilize the most abundant sunlightto make the photocatalysis economically feasible. Compound parabolic collectors (CPCs) are commonlyusedfor solarphotocatalyticdegradationofphenol andphenolic compounds.Photocatalysts are modified via doping, dye sensitization, and coupling method to expand the photoresponse to the visible region. Among these, dye-sensitized photocatalysis is shown to be an efficient method for phenol degradation under the visible solar light. The process involves electron transfer to the conduction band of semiconductor initiated by dye sensitization under the visible solar light. Dye-sensitized photocatalysis processes are shown to be Efficient methods for the degradation of phenol and phenolic compounds under the visible solar light.
