Introduction

Gaseous ozone, formed photochemically in the earth's atmosphere by radiation from the sun, is a normal constituent of the earth's atmosphere which is important in shielding the earth from cancer-causing ultraviolet radiation emitted by the sun. Large scale generation of ozone is extremely important commercially, however, due to its strong oxidizing abilities.

O₃(g) + 2H⁺ + 2e^- <====> O₂(g) +H₂O {E^o = 2.075 V}

figure 1: ozone's strong oxidizing ability

The word for ozone comes from the Greek word "ozein" which means "to smell" since ozone was first noticed because of its characteristic pungent odor (1). The odor is detectable in air at levels of about 0.1 parts per million, and exposure to ozone becomes fatal to humans at around levels of 100 ppm for 10,000 minutes or 10,000 ppm for 30 seconds (2). Ozone, O3, is a blue-colored gas at ambient temperatures, but this color is not noticed at the low concentrations at which it is usually generated (2). In the liquid and solid states, ozone is dark blue. Liquid ozone boils at -111.3 °C and solid ozone melts at -192.5 °C (3). Ozone, which is toxic, is an unstable gas and an explosive liquid. The ozone molecule is a bent molecule with an O-O bond length of 1.278 A and a bond angle of 116.8° as shown in the diagram below (3).

figure 2

Second only to fluorine in its oxidizing power, ozone has many uses including but not limited to water purification, bleaching of materials such as paper, synthetic fibers, Teflon, waxes, flour, and other products, treatment of wastes in industry, deodorization and sterilization (3). Previously, chlorine products have been used for these purposes, but recent studies have shown that chlorine products may produce carcinogens such as trihalomethanes and chloramines (4). Ozone is a safe alternative to chlorine products which performs the same functions without the undesirable side effects; it is not harmful to the environment since it is made from oxygen (O2) and decomposes back into O2. Perhaps the most common use of commercially produced ozone is in treatment of water and wastewater. Ozone has been used in water treatment worldwide for more than 100 years (2).

Drinking water, when untreated, often contains undesirable sediments, unwanted colors, and residual tastes and odors which may be successfully removed by treatment with ozone. Treatment of drinking water with ozone disinfects the water by killing bacteria and inactivating viruses present in the water; ozone has been shown to effectively inactivate strains of poliovirus, adenoviruses, rotaviruses, and the viruses which cause vesicular stomatitis and encephalomyocarditis (4). The oxidative properties of ozone are useful in the removal of soluble iron and manganese, the removal of unwanted colors, tastes, and odors, the decomplexing of bound heavy metals, the destruction of inorganic components such as sulfides, cyanides, and nitrites, and the removal of suspended solids (2). One invention even provides a means for maintaining a degree of residual ozone in the water after treatment so that the water will remain pure during storage (5).

Ozone is also extremely useful in the treatment of wastewaters such as sewage, wastewaters associated with pulp and paper mills, and waters polluted with pesticides. The primary application of ozone in sewage treatment is disinfection, but it is also used to control odor, to aid in removal of suspended solids, and to improve the biodegradability of the wastewater (6). An advantage of ozonation of sewage is that it reduces levels of suspended solids in the effluent without the addition of dissolved chemicals, it removes viruses effectively, and it is cost-effective compared to alternate forms of treatment (2).

In the paper production industry, ozone has recently been widely considered as a method of treating pulp and paper wastewaters. These toxic wastewaters typically have high concentrations of organochlorine compounds such as chlorphenolic compounds, chloroacetones, and chloroform and are highly colored; these organochlorine compounds and colored compounds are resistant to conventional methods of treatment (6). Studies have shown that ozone is a viable alternative to chlorine-based bleaching chemicals which is effective in significantly reducing levels of organochlorines, colors, and toxicity levels in pulp and paper wastewaters. Elemental chlorine, which is usually used in pulp bleaching processes, is an effective bleaching agent but is potentially dangerous because the effluents from chlorine-based bleaching processes contain chloride by-products which are corrosive to processing equipment and which are toxic to humans and animals (7).

Industry holds several patents on environmentally improved methods of bleaching pulp with ozone. Among these patents are US Patent #5,164,043, US Patent #5,520,783, US Patent #5,174,861, and US Patent #5,451,296. Ozone is such a powerful oxidizing agent that in pulp bleaching processes, it not only bleaches the lignin portion of the pulp, but also degrades the cellulose in the pulp (7). Wood is made up of two main components, a cellulosic portion and lignin (7). The lignin portion of wood is ideally destroyed in pulp bleaching processes, but the cellulosic portion of wood gives pulp its strength, and therefore should not be attacked by the bleaching agent (8). In order to prevent the problem of ozone attacking the cellulosic portion of pulp, the processes outlined in some patents involve using ozone as a third step in the bleaching treatment under selectively defined and carefully controlled parameters (pH control, use of chelating agents for metal ion control, pulp consistency) so that the ozone minimally degrades the cellulosic portion of the pulp (7).

Although many methods exist for producing ozone, there are three main categories of methods of ozone production, corona discharge methods, electrochemical methods, and methods involving ultraviolet radiation. In the corona discharge method, by far the most common method, oxygen or an oxygen-containing gas, most commonly air, is passed through the space between two electrodes separated by a dielectric material which is usually glass (Figure 2). The electrodes are most often either concentric metallic tubes or flat, plate-like electrodes which are connected to a source of high voltage. When a voltage is supplied to the electrodes, a corona discharge forms between the two electrodes, and the oxygen in the discharge gap is converted to ozone. A corona discharge is a physical phenomenon characterized by a low-current electrical discharge across a gas-containing gap at a voltage gradient which exceeds a certain critical value (9). First, oxygen molecules, O2, are split into oxygen atoms, O, and then the individual oxygen atoms combine with remaining oxygen molecules to form ozone, O3.

The corona discharge generates heat which causes the produced ozone to decompose into oxygen atoms and molecules. In order to prevent this decomposition, ozone generators which utilize the corona discharge method must be equipped with a means of cooling the electrodes. The temperature of the gas inside the discharge chamber must be maintained at a temperature between the temperature necessary for formation of ozone to occur and the temperature at which spontaneous decomposition of ozone occurs. This necessary cooling is usually accomplished by circulating a coolant such as water or air over one surface of the electrodes so that the heat given off by the discharge is absorbed by the coolant. Many variations of this method have been patented because it produces the highest concentrations of ozone per unit of electrical energy used.

Figure 3: Schematic Diagram of A Typical Corona Discharge Ozone Generator

Usually, in the electrochemical method of ozone production, an electrical current is applied between an anode and cathode in an electrolytic solution containing water and a solution of highly electronegative anions. A mixture of oxygen and ozone is produced at the anode. Another common method of ozone generation involves bombarding oxygen with ultraviolet radiation which splits oxygen molecules into oxygen atoms which combine with other oxygen molecules to form ozone. As with the corona discharge method, many modifications of the electrochemical method of ozone production and the ultraviolet radiation method exist. Many other methods of producing ozone and processes for using ozone generators have been patented.

Ozone, like oxygen, chlorine, and hydrogen peroxide, is a strong oxidizing agent. The relatively high (+2.07V) electrochemical potential indicates a very favorable oxidizing agent. Ozone's widespread application is based on its ability to pull electrons from a covalent bond causing it to break, or pulling electrons from an aqueous phase metal (Fe⁺² --> Fe⁺³) causing it to undergo hydrolysis (Fe⁺³ --> Fe(OH)₃(s)) and precipitate from solution.

O2 + 4H3O+ + 4e- --> 6H2 O	Eo = +1.23 V
Cl2 + 2e- --> 2Cl-	Eo = +1.36 V
HClO + H3O+ + e- --> 2H2O + 1/2Cl2 (g)	Eo = +1.63 V
HClO2 + 2H3O+ + 2e- --> HClO + 2H2O	Eo = +1.63 V
H2O2 + 2H3O+ + 2e- --> 4H2O	Eo = +1.77 V
O3(g) + 2H+ + 2e- --> O2(g) + H2O	Eo = +2.07 V
F2 + 2e- --> 2F-	Eo = +2.87 V

The high electrochemical potential shows ozone's favorable thermodynamics. the chart below (from Chemical and Engineering News) illustrates ozone's favorable kinetics as compared to other oxidizing agents. For industrial purposes, whether a reaction takes place in seconds, minutes, or days is of tremendous importance.

COMPOUND	CHLORINE	PERMANGANATE	OZONE
Acetophenone	26 days	43 days	25 minutes
Benzaldehyde	>3.2 days	36 minutes	28 minutes
Camphor	>3.2 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