**2. Degradation methodologies of phenol and phenolic compounds**

It is of utmost importance to treat wastewater containing phenols before disposal to the environment in order to save the aquatic life. Physical/chemical treatment methods such as activated carbon adsorption, ion exchange, liquid-liquid extraction, chlorine oxidation, chlorine dioxide oxidation, and hydrogen peroxide oxidation are mostly applied for the removal of phenols. However, these methods are expensive and produce hazardous byproducts. On the other hand, biological treatment methods for phenol degradation are superior in the above aspects, but these are applicable only for low concentration of phenols [3]. AOPs normally produce hydroxyl radicals (HO•) as active species which have very low selectivity and can drive phenol degradation through complete mineralization [27]. A review of the conventional and advanced degradation methods of phenol and phenolic compounds is presented in **Table 2**.


#### Degradation of Phenolic Compounds Through UV and Visible-Light-Driven Photocatalysis: Technical... http://dx.doi.org/10.5772/66134 399

Finally, we demonstrate a dye-sensitized method to improve the photocatalytic activity and visible-light response of TiO2-based photocatalyst to perform visible-light-driven phenol

It is of utmost importance to treat wastewater containing phenols before disposal to the environment in order to save the aquatic life. Physical/chemical treatment methods such as activated carbon adsorption, ion exchange, liquid-liquid extraction, chlorine oxidation, chlorine dioxide oxidation, and hydrogen peroxide oxidation are mostly applied for the removal of phenols. However, these methods are expensive and produce hazardous byproducts. On the other hand, biological treatment methods for phenol degradation are superior in the above aspects, but these are applicable only for low concentration of phenols [3]. AOPs normally produce hydroxyl radicals (HO•) as active species which have very low selectivity and can drive phenol degradation through complete mineralization [27]. A review of the conventional and advanced degradation methods of phenol and phenolic compounds is

**Experimental results References**

[32]

[33]

[34]

[35]

(i) Phenol oxidation follows a second-order reaction

(ii) at Fe(VI)/phenol of 10:1, phenol degrades 100%, TOC decreases 57%, and COD decreases 82%; (iii) oxidation reaction follows a radical pathway to undergo ring opening forming intermediates such as

(i) Oxidation follows a second-order reaction kinetics; (ii) oxidation of both phenol and BPA improve under

(iii) oxidation of phenol delays at pH range 7.5–8.5; (iv) manganese intermediates such as Mn(V) and

(i) BPA oxidation with aqueous O3 follows a second

(ii) O3 conc. and solution pH show a significant

(i) Reach adsorption equilibrium in 150 min; (ii) adsorption follows a Freundlich isotherm; (iii) iron oxide impregnation improves BPA removal

phenoxyphenol and benzoquinone

mildly acidic conditions (pH 4–6);

Mn(VI) form during the reactions


effect on BPA removal

**2. Degradation methodologies of phenol and phenolic compounds**

degradation.

presented in **Table 2**.

**compound** 

398 Phenolic Compounds - Natural Sources, Importance and Applications

Phenol and bisphenol A (BPA)

4. Adsorption BPA Adsorbent:

**Process details and significant factors**

9, stirring for 1 h, Fe(VI)O4 2− to phenol molar ratios (1:1, 10:1, and 15:1), phenol initial conc. 30 ppm

Solution pH (4–8.5), initial conc. of BPA 0–16.8 μM, phenol initial conc. 10 μM, permanganate conc. 181 μM

BPA O3 concentration, solution pH, bicarbonate concentration, initial conc. of BPA 35 μM

> powdered activated

kinetics;

Phenol Solution pH

**No. Technology Target**

1. Chemical oxidation with sulfatoferrate (VI)

2. Chemical oxidation with potassium permanganate

3. Chemical oxidation with ozone (O3)



Degradation of Phenolic Compounds Through UV and Visible-Light-Driven Photocatalysis: Technical... http://dx.doi.org/10.5772/66134 401


**No. Technology Target**

9. Enzymatic process

10. Advanced oxidation with UV, O3, and TiO2

11. Advanced oxidation by Fenton process

12. Photocatalysis with UV-TiO2

13. Photocatalysis -visible-light BiVO4

 4- Nitrophenol (4-NP)

**compound** 

400 Phenolic Compounds - Natural Sources, Importance and Applications

Phenol (in refinery effluent)

**Process details and significant factors**

Packed bed bioreactor, biocatalyst weight 135 g, effluent pH 7, temperature (20–32°C), flow rate( 3–6 ml min −1), H2O2 conc. (1–9 mM), phenol initial conc. 100 ppm

Phenol A low-pressure

Phenol Concentration

of iron (II) sulfate 0.001 mol L−1, flow rate of H2O2 (0.075, 0.15, and 0.3 mol per 30 min), pH 3, temperature 30°C, phenol initial conc. 0.012 mol L−1

Initial concentration of 4-NP, light intensity, partial pressure of oxygen, photocatalyst concentration, pH, chloride ion, and temperature

Phenol Chelating agents

(ascorbic acid or citric acid), solvent volumetric ratio, electron scavenger

mercury lamp (λ, 184.9 and 253.7 nm), circulation flow rate for phenol 1000 ml min−1, phenol initial conc. 50–200 ppm

kinetics;

of phenol within 2 h;

of Fenton process;

(ii) Cl−

acid are the reaction intermediate

**Experimental results References**

[40]

[41]

[42]

[14]

[43]

(i) 97% phenol degradation is attained;

(ii) H2O2 concentration, temperature, and flow rate have a positive effect on phenol degradation; (iii) immobilized enzyme shows better stability at broad pH range and at high temperature

(i) Phenol degradation follows a pseudo-first-order

(ii) O3−UV-TiO2 process achieves complete degradation

(iii) formic acid, acetic acid, propionic acid, and fumaric

(i) About 94% organic degradation possible in 2 h; (ii) excess iron(II) is responsible for the lower efficiency

(iii) higher H2O2 flow rate provides best results

(i) Degradation rate of 4-NP follows pseudo-first-order

(i) Three-dimension ordered macroporous (3D-OM) bismuth vanadates successfully remove phenol (94% removal) from wastewater under visible light; (ii)Bi(+V)/chelating agent optimum molar ratio is 2:1

ion shows a negative effect on the degradation

kinetics with respect to its concentration;

**Table 2.** Different treatment methods for removal/degradation of phenol and phenolic compounds.

There are two major constraints that need to be considered for industrial applications: (i) technical feasibility and (ii) economic feasibility. The overall costs of the processes are calculated by summing up the capital cost, operating cost, and maintenance cost [28]. In the following section, we compare the costs associated with different advanced oxidation methodologies. The treatment costs of the AOPs are ranked on a 0–5 scale, 0 being the most expensive and 5 being the least. In between 0 and 5, the ranking is evaluated based on Eq. (1) [29]:

$$\text{Rank}\_{\text{cost}\_i, i} = \left(\frac{\text{Cost}\_{\text{max}} - \text{Cost}\_i}{\text{Cost}\_{\text{max}} - \text{Cost}\_{\text{min}}}\right) \times 5\tag{1}$$

where *Rankcost*, *<sup>i</sup>* is the cost rank of AOP, *i*. *Cost*max is the most expensive AOP, and *Cost*min is the least expensive AOP.

The different process costs are compared by few authors. Saritha et al. [30] compare UV, UV/TiO2, UV/H2O2, UV/Fenton, Fenton, and H2O2-based AOPs for the degradation of 4 chloro-2 nitrophenol (4C-2-NP). Based on the overall costs, we find that AOP carried out using H2O2 and Fenton are least expensive having ranks of ~5, while UV is the most expensive, assigned a rank of 0. **Figure 1** shows the cost ranking of the different AOPs for the degradation of 4C-2-NP. Esplugas et al. [31] compare UV, O3/H2O2, O3/UV, O3/UV/H2O2, UV/H2O2, and O3 processes for phenol degradation. Again, based on the overall costs, the different O3-based AOPs are least costly, while UV is the most expensive, as evident from **Figure 2**. We can infer from the cost comparison that incorporating a photocatalyst such as TiO2 with UV lowers the overall cost by one-third [30]. In the future, using sunlight in place of UV could make AOPs economically more efficient.

**Figure 1.** Cost comparison on AOPs for the degradation of 4C-2-NP.

**Figure 2.** Cost comparison on AOPs for the degradation of phenol.
