Oxidation processes with Ozone

Ozone

Ozone (O3) is an unstable gas made of three oxygen atoms; it may be produced from oxygen gas or air by exposing it to an electric discharge, UV radiation and sometimes chemical reactions. At ambient conditions ozone is a colorless gas with a pungent smell. Man can already perceive ozone at concentrations in the 0.02 - 0.05 ppm range; this concentration is 1/20 of what is considered safe for a 15' exposure and 1/4 of what is accepted in a working environment. While it is forming ozone has a purple color, is highly unstable and tends to decompose back to oxygen with increasing temperature. For this reasons, ozone must be produced where it will be used, as it cannot be stored for long periods.

Ozone is a very strong oxidizer and may react with many organic substances that have double or triple bonds; this characteristic is very useful for water and air treatment applications. Ozone effect on bacteria, yeast and viruses has been known for a long time (Sonntag, 1890). The first potable water ozone installations have been realized in 1906 (Nice). Today many cities (Amsterdam, Moscow, Paris, Turin, Florence, Bologna, Ferrara) use ozone for surface water treatment in their potable water production plants. Ozone is definitely a stronger sterilizer compared to chlorine based products, both on bacteria and viruses; moreover, ozone does not leave taste on treated water (Viebahn, 1977) and generates less dangerous sterilization by-products (see alogenated hydrocarbons).

As of June 26th 2001, FDA regulations accept the use of ozone in food industry processes; this is another demonstration of ozone compatibility with human activity. Ozone has been extensively used for bottled water production since 1982; all Olympic pools must include ozone treatment, effective since 1984.

In July 1996, in protocol nr 24482 the Ministry of Health recognized ozone as a "NATURAL MEANS OF PROTECTION FOR THE STERILIZATION OF ENVIRONMENTS"

Chemical-Physical properties of ozone

Molecular formula	O3
Principal characteristics	Oxidising gas
Molecular weight	48,0
Concentrations in gaseous state	up to18% w/w
Boiling point	-111,9 ºC
Melting point	-192,7 ºC
Critical temperature	-12,1ºC
Critical pressure	54,6 atm.
Density	2,14 kg O3/m3 @ 0ºC 1013mbar
Relative density (on air)	1,7
Solubility in water	3 ppm at 20 ºC
DH of formation	144.7 kJ/mole
Bond angle	116º
Electrochemical potential	-2,07 V
Flammability	non-flammable at standard working concentrations and pressures
Hazardous products of decomposition	none

The solubility of ozone in water is higher than that of oxygen and depends on temperature and on the concentration of ozone.

Water temperature (ºC)	Ozone concentration in gaseous state (% by weight)
	0.1 %	1.0 %	1.5 %	2.0 %	3.0 %
	Ozono solubility (mg/l)
5	0.74	7.39	11.09	14.79	22.18
10			9.75	13.00	19.5
15			8.40	11.19	16.79
20			6.43	8.57	12.86
25	0.35	3.53	5.29	7.05	10.58
30	0.27	2.70	4.04	5.39	8.09

Ozone removing time

Ozone is not stable and it is therefore important to know its decomposition time. For ozone in water half-time (time needed to reduce its concentration by 1/2) is dependent on many parameters. pH relations are as follows (T=21ºC, P=1 bar):

• pH=6.0, about 20'
• pH=7.0, about 15'
• pH=8.0, about 5'

Temperature is also relevant. At pH=7:

• T=15ºC, about 30'
• T=20ºC, about 20'
• T=25ºC, about 15'
• T=30ºC, about 12'

Decomposition is much slower in the gaseous state:

• T=-50ºC, about 90 days
• T=-35ºC, about 18 days
• T=-25ºC, about 8 days
• T=20ºC, about 3 days
• T=120ºC, about 1,5 hours
• T=250ºC, about 1,5 seconds

In reality decomposition is usually much faster due to the influence of additional parameters such as humidity, presence of organic substances and decomposition catalysts.

Exposure times at different concentrations and effects on human health

Ozone's strong oxidising power makes it harmful to human health, especially affecting respiratory pathways.

Maximum allowable concentration for working environment, with an exposure of 8 h/day - 5 d/week, is 0,1 ppm. A concentration of 1 ppm may be tolerated for short periods (e.g. 15'). Concentrations of 100-1000 ppm may kill an adult even in very short time. Anyway, ozone tolerance is really dependant on the specific individual and is also influenced by ambient condition, above all temperature. It is highly recommended to be very cautious when room temperature is high. 

The following graph displays the effects of ozone on human health at different concentrations and different exposure times:

Cutaneous exposure to ozone in solution is associated with a smaller risk, also due to the fact that in such cases concentrations are ususally very low. The main worry for people exposed to ozone-containing waters (such as swimming pools) remains the possibility of accumulation of the gas on the area overhead, as a result of degassing phenomena. 

Sanitizing properties of ozone in water

Due to its high oxidizing potential, ozone can attack cell walls and break up large molecules that are essential for bacteria, yeasts, molds and viruses life cycle. This powerful sanitizing potential is widely used for potable water and waste water disinfection. 

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