Oxygen (O) and water
Oxygen and water: reaction mechanisms, environmental impact and health effects
| Oxygen is the most abundant element on earth. Oxygen exists as O2 and O3 (ozone), and is present in a number of compounds including water molecules. It can be found dissolved in water as O2 molecules. Consequently, the oxygen content of seawater is 85.7%. |
Gaseous oxygen does not react with water. It is water soluble and functions as an oxidator:
O2 + 2 H2O + 4 e- -> 4 OH-
Oxygen may oxidize organic matter. This is principally a biological process. Each individual compound has a reaction mechanism that can be described by means of an electron balance. Examples are given below (H2O is excluded):
Fe2+ + 0,25 O2 -> Fe(OH)3 + 2,5 H+
These mechanisms show that ammonium and methane apply large amounts of oxygen, and the resulting oxidation reactions form higher or lower amounts of acid. Under normal conditions acid in water reacts with HCO3-, forming CO2.
Water solubility of oxygen at 25oC and pressure = 1 bar is at 40 mg/L water. In air with a normal composition the oxygen partial pressure is 0.2 atm. This results in dissolution of 40 . 0.2 = 8 mg O2/L in water that comes in contact with air.
As was described earlier, oxygen dissolves naturally when water comes in contact with air. Oxygen is also applied commercially. For industrial purposes the element is extracted from air by about 100 million tons annually. Of the total amount, 55% is applied in steel production, 25% is applied in chemical industries, and the remainder is applied in hospitals, for starting missiles, and for slicing metal. In chemical industries a reaction of oxygen and ethylene is applied, and the resulting ethylene oxide is applied as an antifreeze and polyester. Oxygen is highly reactive, and can therefore be applied to break down hazardous substances. It may also be applied as a bleach. Oxygen in ozone compounds is applied for drinking water disinfection. Waters are not contaminated by oxygen when it is applied industrially.
Oxygen oxidizes other substances. This occurs for example during fires, but also within organisms, during bacterial destruction and during metal conversion.
The total oxygen concentration in the human body is about 60% of the total body weight. This value may vary strongly, because it is mainly present in water molecules.
One of the reasons one may want to remove oxygen from water is that is may corrode water pipes. Various physical and chemical processes may solve this problem, for example ion exchange resins. The basic principle of this method is the reaction between hydrogen and oxygen: 2H2 + O2 -> 2H2O. This reaction may de catalysed by various compounds, causing it to end spontaneously. Palladium endowed ion resins may reduce the water oxygen concentration when adequate amounts of hydrogen are present. Hydrazine is another possible reduction compound that may be applied instead of hydrogen: O2 + N2H4 -> N2 + 2 H2O.