**1. Introduction**

Considering the nature of contaminants, chlorinated hydrocarbons and other groups of organic compounds belong to the group of chemicals that have widely been used in the past due to their environmental persistence and toxicity their use has been prohibited and became greatly controlled [1–7]. They can be decomposed by many chemical oxidation processes among which Advanced oxidation processes (AOPs) prevail. AOPs represent a group of methods of chemical oxidation in liquid phase which are employed to destroy organic compounds. AOPs were developed in order to oxidize organic compounds that can be resistant or which are able to deactivate traditionally used biological stage at sewage disposal plants (these compounds are non-biodegradable) including also pharmaceutical residues [8, 9]. If the final results of chemical oxidation are just inorganic products, such as CO2, H2O and other

harmless inorganic compounds, we talk about complete mineralization or total oxidation. AOPs are employed to replace standard oxidation technologies, such as oxidation with KMnO4, K2Cr2O7 and Na2S2O8 because they can oxidize many organic compounds only partially [10]. Those oxidants can also serve as secondary source of pollution, e.g. hexavalent chromium ions are environmentally non-friendly. Some of the AOPs can also be based on sulphate chemistry combined with UV irradiation or photochemical processes combined with electrochemical processes [11, 12]. The effectiveness of oxidation agents is given by their standard oxidation potentials that were listed in **Table 1** in Section 2.1.3. AOPs comprise several common features that can be briefly described as follows: [13–17].


The main disadvantages of AOPs are relatively high treatment costs and special safety requirements because of the use of very reactive chemicals (ozone, hydrogen peroxide), etc. and high-energy sources (UV lamps, electron beams, etc.). Attention is also paid to low energy sources, such as UV LED [18]. Among AOPs the following


#### **Table 1.**

*Standard redox potentials of some typical oxidative species [13].*

**Figure 1.**

*Suitability of water treatment technologies according to COD contents [12].*

processes can be categorized: Fenton oxidation (Fe2+/H2O2); Fenton-like oxidation (Fe3+/H2O2); photo assisted Fenton (Fe2+/3+/H2O2/UV); photocatalysis (TiO2/hv/O2); ozone systems (O3/H2O2, O3/UV), UV photolysis (UV/H2O2). It is favorable to treat wastewaters with maximum content of COD = 10 to 15 g/L (chemical oxygen demand) [19]. For higher values of COD, other oxidation methods are usually applied as can be seen in **Figure 1**.

Significance of the AOPs' usage in water treatment is supported by existing registered trademarks like ULTROX®, RAYOX®, UVOX®, ECOCLEAR®. Trademarks ULTROX®, RAYOX®, UVOX® are *ex-situ* water remediation technologies utilizing ultraviolet irradiation and ozone used by company Ultrox International in California, US [20]. ECOCLEAR® is a heterogeneous catalytic ozonation process [21].

A brief summary of oxidative species used for chemical oxidation is briefly given in **Table 1**. The oxidative species are arranged according to their standard redox potentials. Standard redox potential describes capability of certain oxidizing agents for oxidation reaction. The higher the redox potential is revealed, the stronger the oxidizing agent is.

### **2. Fenton oxidation**

Fenton oxidation is the most traditional method of AOPs. It was invented by Henry John Horstman Fenton in 1890 [22]. He discovered oxidation with reagent containing Fe2+ and hydrogen peroxide. The OH• production occurs by means of H2O2 addition to the solution containing Fe2+ salts:

$$\mathrm{Fe}^{2+} + \mathrm{H}\_{2}\mathrm{O}\_{2} \rightarrow \mathrm{Fe}^{3+} + \mathrm{OH} + \mathrm{OH}\bullet \tag{1}$$

This is a very simple way of producing OH• requiring neither special reactants nor special reaction apparatus. Iron is naturally very abundant and non-toxic element to the environment. Hydrogen peroxide is also environment friendly chemical and easy

to store and handle. It was pointed recently that at low values of pH = 2,5–3 Fe3+ salts are reduced to Fe2+ and reaction becomes Fenton-like [23]. It is described by Eqs. (2) and (3):

$$\mathrm{Fe^{3+} + H^{2}O^{2} \rightleftharpoons H^{+} + FeOOH^{2+}} \tag{2}$$

$$\text{FeOOH}^{2+} \rightarrow \text{HO}\_2\text{\bullet} + \text{Fe}^{2+} \tag{3}$$

Fenton oxidation also exists in several modifications. One of them is the photo assisted Fenton reaction. It is the classical Fenton reaction enhanced by presence of UV–VIS irradiation [24, 25]. It utilizes a product yielding from reaction (2) and upon irradiation it yields Fe2+ ions and OH• as described by reaction (4).

$$\text{Fe}(\text{OH})^{2+} + \text{h}\nu \rightarrow \text{Fe}^{2+} + \text{OH}\bullet \tag{4}$$

There are also other modifications of Fenton-like reactions, e.g. electro-Fenton, nano-Fenton utilizing graphene oxide wrapped nanoparticels of Fe3O4 [26, 27] and various modifications.
