Lenntech Water treatment & purification

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%.

In what way and in what form does oxygen react with water?

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+
Mn2+ + O2 -> MnO2 + 2 H+
NH4+ + 2 O2 -> NO3- + 6 H+
CH4 + 2 O2 -> CO2 + 4 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.
The oxygen atom is very reactive and forms oxides with virtually all other elements, with the exception of helium, neon, argon and krypton. There are also a large amounts of compounds that react with water.

Solubility of oxygen and oxygen compounds

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.
Oxygen solubility is strongly temperature dependent and decreases at higher temperatures. Oxygen solubility is negatively correlated with the amount of dissolved solids. Consequently, oxygen solubility in freshwater exceeds that in seawater by 1-3 mg/L, depending on temperature.
The saturation constant in rivers and lakes in mountainous areas is usually lower than in lowlands, because it is pressure dependent.

Why is oxygen present in water?

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.

What are the environmental effects of oxygen in water?

Oxygen oxidizes other substances. This occurs for example during fires, but also within organisms, during bacterial destruction and during metal conversion.
All plants and animals require oxygen for breathing. Oxygen is very essential, because it is a part of the DNA and virtually all other compounds that are biologically significant. In the lungs oxygen is bound to iron atoms; central elements of haemoglobin. A total of 200 cm3 of oxygen can dissolve in blood by this mechanism, an amount that clearly exceeds the water soluble amount. Together with energy reserves oxygen causes muscle activity and heat production. This process releases carbon dioxide, which is emitted and subsequently taken up by plants. Plants produce additional oxygen during photosynthesis. Plants contain between 4.1 and 4.4% oxygen (dry mass).
Dissolved oxygen is an important determinant for stability of waters and survival of water organisms. Micro organisms may decompose organic substances in water by means of oxygen. Oxygen application per unit of time is indicated by BOD (Biochemical Oxygen Demand). Organic pollutants may negatively influence water organisms, because they decrease BOD. Thermal pollution causes the same problem, because oxygen solubility is lower in warmer water. This may be a consequence of cooling water discharge on surface waters.
In eutrophic lakes and relatively enclosed sea areas, oxygen concentrations decrease strongly with depth. In some cases conditions may even be anaerobic. Natural examples of influences of temperature on oxygen concentrations in water and environmental impact are seasonal temperature changes in lakes. In winter the water has the same temperature and oxygen concentration everywhere. In summer water in surface layers in warmer than deeper water, resulting in lower oxygen solubility. Algae and plants in the surface layers work oppositely. They produce a high amount of oxygen at high temperatures, causing the water to become oxygen saturated. These plants die off pretty quickly, and are decomposed by micro organisms applying oxygen, which is now abundant in surface layers of the water source. However, organic matter often settles and remains on the bottom of a water body as sediment. This may cause oxygen deficits from decomposition. When an ecological equilibrium is established in lakes, these problems may be solved. However, when discharge, over fertilization, etc. add nutrient that must be decomposed and increase algal blooms, oxygen concentration may decrease to a level where no organism survives. This phenomenon is commonly known as eutrophication (eutrophic = nutrient rich, oligotrophic = nutrient poor). The critical oxygen concentration for fish is achieved at 4 mg O2/ L water.
As pure O2 oxygen is generally not released in amounts that would be hazardous to any aerobic organism. Theoretically, such concentrations are obtainable, and the critical partial pressure differs per species.
Oxygen atoms can be found in a number of toxic organic and inorganic compounds. Toxic compounds are for example hyper oxides and peroxides. Some substances are toxic under low oxygen conditions in water, because breathing of organisms increases and consequently substances are absorbed more rapidly. For obligatory anaerobic organisms, high oxygen concentrations are toxic.
Ozone is an environmental pollutant when it is present in the troposphere. In the stratosphere it functions as a protective layer that reflects solar UV-radiation. Without this ozone layer, life on earth would be impossible. A number of plant species are susceptible to high ozone concentrations in air. This does not show as visible stress symptoms, but rather as growth limitations.
Oxygen has three stable and five instable isotopes.

What are the health effects of oxygen in water?

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.
As was explained earlier for other organisms, humans absorb oxygen through lungs which is than transferred to various organs through the blood. It is delivered by very fine capillaries. The oxygen atom is a part of hydroxyl, carbonyl, and other functional groups. It is transported through blood bound to haemoglobin, and is subsequently stored in muscles in myoglobin. The presence of oxygen in drinking water is favourable, because it assists protective coating formation on the inside of metal water transport pipes. This requires a concentration of 6-8 mg/L.
Oxygen radicals are responsible for derivative diseases such as cancer and cardiovascular illness.
When air contains a lower than 3% oxygen concentration, death by asphyxiation generally follows. At a lower than 7% concentration, one may loose consciousness. Too much oxygen may be lethal. Sports divers that breath pure oxygen often get cramps. Baby's that receive too much oxygen in incubators, generally grow blind.
Oxygen as ozone may damage lungs. Toxic forms of oxygen include hyper oxides, peroxides and hydroxyl radicals.

Which water purification technologies can be applied to remove oxygen from water?

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.
A simpler method that cannot be applied on every occasion is thermal oxygen removal. Gas solubility in water at vaporization temperature equals zero. The principle of thermal degasification is based on this fact. These function as pressure degasifiers under slight over pressure (up to 5 bar), or as vacuum degasifiers under slight under pressure.
On the other hand, water enrichment by oxygen may aid pollutant removal. This may be achieved by artificial aeration, for example by leading water over cascades, by rinsing water through surface aerators, by inserting air through pressure filters, by adding air by increasing the water flow (for example in venturis), or by aeration with pure oxygen. Aeration is applicable in water purification plants, but also in broad rivers.
Oxygen has a cleansing effect because it is essential for micro organisms and because it oxidizes compounds. Therefore, water pollution is indicated by BOD, or COD (Chemical Oxygen Demand).
The often applied BOD5 value indicates the oxygen concentration applied by micro organisms within five days at 20oC in an aerobic environment, to convert organic matter to carbon dioxide, water and new biomass. It is expressed as mg O2 per litre of wastewater. Multiplying this number by the wastewater volume gives the amount of hazardous substances. The BOD5 per unit of time is called BOD load. Hardly decomposable matter is excluded because of the short measuring time.
COD represents the amount of oxygen (mg) that is required for oxidizing all oxidizable matter, per litre of wastewater. This includes not only easily decomposable organic matter, but also hardly decomposable and persistent compounds (for example organic chlorine compounds) and consequently exceeds the BOD5 value.
Ozone can be applied for water purification, for example swimming pool or drinking water disinfection. It is a stronger disinfectant that chlorine gas, but the protection from bacteria lasts only briefly. Ozone is an instable form of oxygen and therefore is quickly converted back to O2, which is favourable, because ozone causes lung damage.

Literature and the other elements and their interaction with water

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