*2.1.3. Kinetic mechanism of WAO*

WAO of organic pollutants is generally described by a free-radical chain reaction mechanism in which the induction period to generate a minimum radical concentration is of great significance [6-9]. In the most detailed studied reaction in WAO, in the oxidation of phenol water solution during the induction period, practically no change was observed in phenol concentration. Once the critical concentration of free radical is reached, fast reaction takes place (propagation step) when almost all phenol is oxidized. It has been found that the induction period length depends on oxygen concentration, temperature, type of organic compound and if applied on the catalyst concentration [2, 10-13]. The pH also has influence on the induction period that actually is shorter for pH values of about 4, and is increasing with the increase in pH [14].

In wet oxidation, the reaction chains are thermally initiated [15]:

$$\text{RH} \rightarrow \text{R}^\* + \text{H}^\* \tag{1}$$

$$\text{RH} + \text{O}\_2 \rightarrow \text{R'} + \text{HO}\_2^- \tag{2}$$

$$\rm{R'+O\_2} \rightarrow \rm{ROO'}\tag{3}$$

$$\text{ROO}^\* + \text{RH} \to \text{ROOH} + \text{R}^\* \tag{4}$$

$$\text{RH} + \text{(OH', HO\_2', RCO')} \rightarrow \text{R'+} \left(\text{H}\_2\text{O,H}\_2\text{O}\_{2'}, \text{ROOH}\right) \tag{5}$$

$$\text{H}\_2\text{O}\_2 \rightarrow 2\text{ HO}^\cdot\tag{6}$$

In the mechanism, R is any organic molecule present in the reaction mixture. In addition to the organic radicals several inorganic radicals participate in the degradation such as H, HO, and HO2/O2 - . Among these radicals HO is the most reactive, it reacts with aromatic molecules (Ph=phenol) in radical addition reaction with practically diffusion limited rate coefficient at all temperatures up to the supercritical value [16].

presence of iron that usually forms insoluble scale, which has to be removed by chemical and/ or mechanical treatment. Chloride is dangerous because of corrosion, in spite of the use of

A special process is the so-called VerTech oxidation of sludges. This applies an underground installation, consisting of concentric tubes as heat exchangers going down to 1200 m depth to an oxidation vessel. Due to the depth of the vessel, the bottom pressure in the reactor is above 100 bar at 275°C without the need of high-pressure pumps at the surface. The building of the installation required drilling and casing technology developed by the gas and oil production industry. The operation of the system required a frequent descaling operation with nitric acid in order to preserve the efficiency of heat exchange in the concentric tubes. The output of this

WAO of organic pollutants is generally described by a free-radical chain reaction mechanism in which the induction period to generate a minimum radical concentration is of great significance [6-9]. In the most detailed studied reaction in WAO, in the oxidation of phenol water solution during the induction period, practically no change was observed in phenol concentration. Once the critical concentration of free radical is reached, fast reaction takes place (propagation step) when almost all phenol is oxidized. It has been found that the induction period length depends on oxygen concentration, temperature, type of organic compound and if applied on the catalyst concentration [2, 10-13]. The pH also has influence on the induction period that actually is shorter for pH values of about 4, and is increasing with the increase in

R + O ROO g g

RH R + H ® g g (1)

<sup>2</sup> ® (3)

RH + O R + HO 2 2 ® g g (2)

ROO + RH ROOH + R g g ® (4)

H O 2 HO 2 2 ® <sup>g</sup> (6)

RH + OH , HO , ROO R + H O, H O , ROOH ( <sup>2</sup> ) ® ( 2 22 ) <sup>g</sup> <sup>g</sup> gg (5)

In wet oxidation, the reaction chains are thermally initiated [15]:

special alloys in the WAO equipments.

158 Wastewater Treatment Engineering

plant was 80 tons of sludge per day [5].

*2.1.3. Kinetic mechanism of WAO*

pH [14].

$$\text{Ph} + \text{HO}^{\cdot} \rightarrow \text{PhHO}^{\cdot} \tag{7}$$

The hydroxycyclohexadienyl radical formed in Reaction (7) from phenol is also highly sensitive to oxygen.

In all real PWW and in the reaction mixtures of wet oxidation in stainless steel autoclaves, Fe ions are present with measurable concentration [17-19]. These ions and other transition metal ions present accelerate the decomposition of peroxide to HO radicals in Fenton-type processes. The steady state (propagation step) is then followed by the third step (termination step) characterized by a slow oxidation rate.

The first step is the chain initiation, in which free radicals (R, OH, HO2) are produced by dissociation (1) and the bimolecular reaction of dissolved oxygen with the organic compound (2), which is found to be very slow at low temperatures. When the free radical R is formed, it can readily react with molecular oxygen to give peroxoradical (ROO) (3). The other reaction is the formation of hydrogen peroxide, which decomposes with metal catalysis to OH radicals (6). Finally the peroxo radical gives with the parent compound a free radical and hydroper‐ oxide (4). The OH radicals oxidize the parent compound into a free radical again (5).

In the mechanism, the organic parent compound (RH) can react thus with molecular oxygen, the organic peroxyl (ROO ) (5), hydroxyl (OH) (6) and hydroperoxyl (HO2 ) (3) radicals [20, 21].

Intermediates formation is of great importance in WAO and has been reviewed by Devlin and Harris for the oxidation of aqueous phenol with dissolved oxygen [22]. The conclusion was that, at elevated temperatures, oxygen is capable of three different oxidation reactions with the organic: (i) introducing an oxygen atom into an aromatic ring to form a dihydric phenol or quinone; (ii) attacking carbon to carbon double bonds to form carbonyl compounds; and (iii) oxidizing alcohols and carbonyl groups to form carboxylic acids. The ring compound intermediates (dihydric phenols and quinones) were formed under conditions near the stoichiometric ratio of phenol and oxygen, increasing in quantity when oxygen was in deficiency. The unsaturated acids, namely maleic and acrylic and saturated ones, namely formic, acetic and oxalic appear independently of phenol to oxygen ratio used. Malonic, propionic and succinic acids were identified only in case of deficit of oxygen. Malonic acid undergoes decarboxylation to produce acetic acid and carbon dioxide.

Another interesting research was done about the kinetics of oxidation of phenol, and nine substituted phenols were investigated [10]. The process was studied in a 1 L stainless steel autoclave at temperatures in the range of 150°C-180°C and the initial phenol concentration was 200 mg/L. The oxidation reaction found to be the first order for oxygen and also first order with respect to phenolic substrates in both cases. The overall oxidation reaction rate was found to be kinetically controlled when the temperature was less than 195°C and the phenol con‐ centration was less than 200 mg/L. At higher temperatures (>240°C) and higher phenol concentration (>20000 mg/L) the overall oxidation reaction rate became mass transfer control‐ led.

**Figure 2.** Proposed reaction pathway for phenol oxidation by molecular oxygen.
