Ozone (O3), a strong oxidizing agent made from stable molecular oxygen (O2), is replacing chlorinated compounds in a variety of applications including wastewater treatment, air treatment, and as a disinfectant. Ozone is unstable and decomposes slowly (minutes) at ambient temperatures and rapidly (< 1 sec) at higher temperatures. Because of this instability, it must be manufactured on-site for industrial applications. Current technology in ozone production is carried out by several techniques. One approach utilizes electrochemical techniques. These systems suffer from high electrical /consumption and the cells produce chemicals that can be toxic or difficult to dispose. The second general area involves the use of high energy methods such as UV light, beta rays, and lasers to convert the oxygen to ozone but these means of converting oxygen to ozone have found no large scale commercial application.The third general approach utilizes electrical discharges or plasmas. In this approach, pure oxygen or air in the gas phase is passed through an electric field generated by putting a high voltage across an anode and a cathode. These systems typically suffer from high electrical consumption and a relatively low conversion efficiency (1-15%) of oxygen to ozone. Yokomi et al. demonstrated the efficiency for the inert gas (Ar, Ne, He)-oxygen mixtures in their generator are less then a nitrogen-oxygen mix where as the efficiencies measured here for the inert gas-oxygen mixtures are greater than the nitrogen-oxygen mixture. Like fluorine, chlorine, and hydrogen peroxide, ozone is a strong oxidizing agent.
O2 + 4H3O+ + 4e- ® 6H2O Eo = +1.23 V [1]
Cl2 + 2e- ® 2Cl- Eo = +1.36 V [2]
HClO + H3O+ + e- ® 2H2O + 1/2Cl2 (g) Eo = + 1.63 V [3]
HClO2 + 2H3O+ + 2e- ® HClO + 2H2O Eo = + 1.63 V [4]
H2O2 + 2H3O+ + 2e- 4H2O Eo = +1.77 V [5]
O3(g) + 2H+ + 2e- ® O2(g) + H2O Eo = +2.07 V [6]
F2 + 2e- ® 2F- Eo = +2.87 V [7]
Ozone has some well-known advantages over other strong oxidizing agents. For example, HClO2 and HClO leave behind a chlorine-based residue and fluorine gas (F2) is highly corrosive. The kinetics of ozone reacting with a variety of organic compounds have been measured (see table 1) and shown to be quite favorably compared to other strong oxidizing agents.
Table 1. Data that shows the kinetics of reaction of ozone on certain organic compounds compared to other oxidizing agents .
COMPOUND
CHLORINE
PERMANGANATE
OZONE
Acetophenone
26 days
43 days
25 minutes
Benzaldehyde
>3.2 days
36 minutes
28 minutes
Camphor
>3 days
>5.8 days
12 minutes
p-nitrophenol
2.1 hours
1.1 days
2 minutes
Borneol
1.4 days
7 days
53 minutes
Methyl-m-toluate
>20 days
22 days
5.5 minutes
Diacetone-L-sorbose
100 days
14 days
2.8 minutes
The following thermodynamic considerations are considered for optimum design in the efficient and economic production of ozone from oxygen. First, the production of ozone from oxygen is an endothermic reaction:
3O2(g) Û 2O3(g) D H = +286 kJ/mol [9]
The discharge provides the energy needed to convert oxygen to ozone. Second, the decomposition of ozone to oxygen is thermodynamically favored:
2O3(g) Û 3O2(g) D G = -326 kJ/mol [10]
Lowering the temperature of the system can slow this decomposition. The goal of this process is to provide enough energy to convert the oxygen to ozone and to cool the ozone molecules as rapidly as possible. The inert gases rapidly dissipate energy gained in collisions by emitting electromagnetic radiation. If the discharge energy is to low [Eq. 9], the formation of ozone from oxygen, will not take place. If the discharge energy is not dissipated rapidly [Eq. 10] the decomposition of ozone will be accelerated.