Almost all natural waters contain some carbon dioxide which they gain in several ways. Carbon dioxide gas (CO,) is present in the air to the extent of 0.03 percent by volume and 0.05 percent by weight. As rain falls through the air, it absorbs some of this gas.
Free carbon dioxide. Refers to carbon dioxide gas dissolved in water. The term is used to distinguish a solution of the gas from the combined carbon dioxide present in bicarbonate and carbonate ions.
On reaching the earth, the rainwater now slightly acid will absorb additional amounts of carbon dioxide if it flows through decaying vegetation. At the same time, the carbon dioxide becomes carbonic acid. If the water now passes through limestone formations, its carbonic acid content will react with the limestone to form soluble calcium bicarbonate. In this process the carbonic acid is partially neutralized.
Limestone. A sedimentary rock formation wholly, or in large degree, composed of calcium carbonate. There are many important varieties of limestone, such as chalk, travertine and marble.
On the other hand, if water passes through rock formations, such as granite, no such reaction occurs. The carbonic acid is not neutralized. It continues as carbonic acid until drawn to the surface where it can then cause corrosion if not neutralized.
Granite. A type of rock that consists primarily of quartz, alkali feldspar and mica. The quartz and feldspar are always present in granite. Other minerals are sometimes present as well. These are all silicates.
If nature or chemical agents do not neutralize carbonic acid, it will cause corrosion of both copper and galvanized plumbing systems . In those parts of the country where the problem is prevalent it is serious, for it can lead to serious damaging of plumbing equipment. Carbon dioxide, together with carbonic acid, is primarily a problem in water containing relatively low concentrations of minerals. In such water there are not sufficient alkaline salts to buffer the effect of the carbonic acid.
The simplest method for removal of carbonic acid is to pass the water through a tank containing limestone chips. A neutralizing filter of this type affects the carbonic acid just as does the underground limestone formation. The limestone in the filter reacts with the carbonic acid to produce calcium bicarbonate. In the same way, lesser amounts of magnesium bicarbonate are formed. Note: Not all forms of limestone are suitable for this purpose. Excessively soft material may break down to form a solid mass and block the filter. The best types are hard, strong granules which retain their physical structure, even as they are dissolved.
Another type of material used in this neutralizing process is magnesium oxide. Although this procedure does add hardness and alkaline salts to the water, it effectively neutralizes a considerable amount of carbonic acid at a relatively low cost.
Where high carbon dioxide concentrations are encountered, a solution of soda ash sodium carbonate (Na2C03) may be fed into the water. The carbonic acid and the sodium carbonate react directly to form sodium bicarbonate. This method of treatment offers the advantage of not adding hardness to the water. Also, it is especially effective where it is necessary to remove carbonic acid from large volumes of water. This method, as we have seen, has the disadvantage of requiring more attention in the preparation and maintenance of proper feeds.
Jan
16
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