Significance of Anions and Cations in Natural Water

 

[12/26, 2:25 PM] Dr. AMAR NATH GIRI: Significance of Anions and Cations in Natural Water

The principal constituents of ionic species and their distribution in natural waters

vary greatly depending on the geographical formations and soil type. Important

ionic species (Cation & Anion) in all natural waters that influence water quality

and represent the principal chemical constituents, which are listed below:

Cation Anions

Calcium (Ca2+)

Bicarbonate (HCO3-) and

Magnesium (Mg2+)

Carbonate (CO3 2-)

Sodium (Na +)

Chloride (Cl-)

Potassium (K+)

Sulfate (SO42-)

Iron(Fe2+)

Nitrate (NO3-)

Manganese (Mn2+)

Phosphate (PO43-)

Fluoride (F-)

Calcium: It is derived mostly from rocks, and maximum concentrations come

from lime stone, dolomite, gypsum, and gypsiferrous shale. Calcium is the

second major constituent, after bicarbonate, present in most natural waters, with

a concentration range between 10 and100 mg/l. Calcium is a primary constituent

of water hardness and calcium level between 40 and 100 mg/l are generally

considered as hard to very hard.

[12/26, 2:26 PM] Dr. AMAR NATH GIRI: Magnesium: Source of magnesium includes ferromagnesium minerals in

igneous and metamorphic rocks and magnesium carbonate in limestone and

dolomite. Magnesium salts are more soluble than calcium, but they are less

abundant in geological formations. At high concentration in drinking water,

magnesium salts may have laxative effects. They may also cause unpleasant

taste at concentrations above 500 mg/l. For irrigation purposes, magnesium is a

necessary plant nutrient as well as a necessary soil conditioner. Magnesium is

associated with hardness of water, and is undesirable, in several industrial

processes.

Sodium: The major source of sodium in natural waters is from weathering of

feldspars, evaporates, and clay. Sodium salts are very soluble and remain in

solution. Typical sodium concentrations in natural waters range between 5 and

50 mg/l. Excessive sodium intake is linked to hypertension in humans. A

deficiency may result in hyponatremia and muscle fatigue. The recommended

USEPA limit of sodium in drinking water supply is 20 mg/l.

Potassium: Potassium is less abundant than sodium in natural waters. Its

concentration rarely exceeds 10 mg/l in natural waters. In highly cultivated areas,

runoff may contribute to temporarily high concentrations as plants take up

potassium and release it on decay. From the point of view of domestic water

supply, potassium is of little importance and creates no adverse effects. There is

presently no recommended limit in drinking water supply.

Iron: Iron is present in soils and rocks as ferric oxides (Fe2O3) and ferric

hydroxides [Fe(OH)3]. In natural waters, iron may be present as ferrous

bicarbonate [Fe(HCO3)2], ferrous hydroxide, ferrous sulfate (FeSO4), and organic

(chelated) iron. The USEPA secondary drinking water regulations limit for iron is

0.3 mg/l, for reasons of aesthetics and taste.

Manganese: Manganese is present in rocks and soils. In natural waters, it

appears with iron. Common manganese compounds in natural waters are

manganous bicarbonate [Mn(HCO3)2], manganous chloride (MnCl2), and

manganous sulfate (MnSO4). The toxicity of Mn may include neurobehavioral

[12/26, 2:27 PM] Dr. AMAR NATH GIRI: changes. The USEPA secondary standard for aesthetic reasons for Mn is 0.05mg/l.

Bicarbonate – Carbonate: Bicarbonate is the major constituent of natural water.

It comes from the action of water containing carbon dioxide on limestone, marble,

chalk, calcite, dolomite, and other minerals containing calcium and magnesium

carbonate. The carbonate-bicarbonate system in natural waters controls the pH

and the natural buffer system. The typical concentration of bicarbonate in surface

waters is less than 200 mg/l as HCO3. In groundwater, the bicarbonate

concentration is significantly higher.

Chloride: Chloride in natural waters is derived from chloride-rich sedimentary

rock. In typical surface waters, the chloride concentration is less than 10 mg/l.

Drinking water standards have been formulated and updated time to time, as

more and more knowledge about effect of various parameters in drinking water is

acquired. Drinking water standards formulated by Bureau of Indian Standards

(BIS) and also guidelines of Central Public Health and Environmental

Engineering Organization (CPHEEO), as recommended by the World Health Organization (WHO)