**2.1. Photo-catalytic oxidation**

 Photocatalytic method [8, 9] is an advanced oxidation process (AOP). In common, semiconductor photocatalysts were needed to the oxidation reaction. Using higher than the semiconductor energy band gap light. For example, anatase which band gap is about 3.2 eV. Thus only ultraviolet light (wavelength <388nm) can be effectively applied for electricity to be inspired to the conduction band from electronic valence band to generating a pair of holes and electrons in **Figure 3**. The hole–electron pair may be not stable enough. If they are not to combine together immediately, the two species (hole–electron pair) respectively may react with oxygen or water to generate oxidizing species, such as hydroxyl radicals, hydrogen oxygen free radical, superoxide free radicals and so on, hydroxyl radicals directly attack the phenol. The


**Table 1.** The classification of dechlorination reaction.-

**Figure 3.** Theory of photocatalytic method.

proposed mechanism for the photocatalytic degradation of 4-chlorophenol by photocatalysts (GR-Eu2 O3 /TiO2 ) was described by Mallanaicker Myilsamy in **Figure 4** [10]:-

**Figure 4.** Proposed mechanism for the photocatalytic degradation of 4-chlorophenol [10].

There are several influence factors of this AOP.-

**The first: Photocatalysis**: catalyst which determines the type of the light and speed of the reaction is a key factor we need to consider. Due to its high catalytic activity, low cost, chemical stability, non-toxicity, TiO2 was usually applied as the most efficient photocatalyst in this AOP [11, 12]. Based on the photocatalytic mechanism of electron–hole, some metal nanoparticles were also used to prevent the combination of hole and electron pair [13, 14].

Other factors such as the amount of catalyst added, illumination angle, solvent, reaction temperature and so on will also affect the whole reaction efficiency [9].

Recent research directions focused on the development and synthesis of improved photocatalytic materials (such as core-shell nanoparticles, bimetals catalyst and so on), with two major lines aiming at (i) better charge separation and better charge migration (ii) visible-light driven photocatalysis. Claude Descorme has summed up most of the catalyst types in his review [9].

## **2.2. Sonocatalysis (ultrasonic reaction method)**

Ultrasonic reaction (sonocatalysis) is also considered as an advanced oxidation process (AOP). It is well known that ultrasonic wave can greatly promote heat movement of molecules in the reaction, accelerating the reaction process, thus lots of scientists like to choose ultrasonic reaction method to the low reaction activities reagents, the entire process reaction temperature is not too high. The chemical processes of the sonocatalysis may involve two parts (I) free radical process and (II) pyrolysis process under ultrasonic wave. Recent research directions focused on the development of the more efficient reactor, for example (**Figure 5**), and catalyst which can be applied to decrease the frequencies [15, 16].

### **2.3. Fenton method**

Fenton method [17] is also a AOP method which uses ferrous or cuprous salts (Fe2+ +Cu2+) and hydrogen peroxide (H2 O2 ) for generating powerful oxidant, i.e., hydroxyl radicals produced in situ **Figure 6**.

 For the Fenton method to the dechlorination of Chlorophenols, the important parameter is probably the operating pH, In the presence of a homogeneous catalyst, an optimum is generally observed around pH 3. In fact, when pH < 2.5, the Fe(II) (H2 O)2 + will generate while the pH higher than 4, free iron and reduced concentration of hydroxyl precipitation of iron oxide. The pH can also affect the stability of the heterogeneous catalyst, now most research has focused on- the development of more stable and regeneration or be used in a wide pH range catalyst [18].

#### **2.4. O3 oxidation**

Ozone oxidation is also a kind of VOP, both ozone oxidation and Fenton oxidation also produce free radicals such as •OH to degradation of organochlorine, especially for the organochlorine

**Figure 5.** Scheme of the self-excited ultrasonic irradiation device.

**Figure 6.** The mechanism of Fenton method.

pollution which contain multiple bonds such as C〓N, C〓C has a better effect; while some limitations still exist in terms of the O3 oxidation, chlorophenol concentrations, catalyst costs and its recovery and so on.
