Which is the major form of alkalinity and how it is formed?
Answers
Bicarbonate is the major form of alkalinity. The alkalinity of a sample is the measure of its capacity to neutralize acids. Knowledge of the kind of alkalinity present in water and their magnitude is important. The alkalinity of water refers to its acid-neutralizing capacity. It is the sum of all titratable bases. Highly alkaline waters, above pH 7.0, can create a bitter taste and a slippery feel and also cause drying of the skin.
Origin of Alkalinity
Alkalinity in freshwater systems is derived from several sources: weathering of rocks and soil, exchange reactions in soils, biological uptake and reduction of strong acid anions, evaporation and precipitation of minerals, and atmospheric deposition of dust. Precipitation has little if any alkalinity, and in most systems, weathering is typically the dominant source of alkalinity for inland waters. In relatively unpolluted regions, precipitation is only slightly acidic because the carbon dioxide in the atmosphere in equilibrium with pure water results in a weak carbonic acid solution with zero alkalinity but a pH of about 5.6. In industrialized areas, the sulfates and nitrates from emissions may result in precipitation with a pH of about 4.3 and an alkalinity of about negative 50 μeq l−1. In some arid regions, dry deposition of alkaline dusts may result in atmospheric alkalinity inputs to the inland waters.
Weathering
Most of the large variations in the alkalinity of inland waters can be attributed to the amount of weathering of bedrock material and soils derived from the bedrock. Rock types can be divided into three broad categories: slow weathering noncarbonate rock (igneous granite, gneiss) with resulting thin soils, low cation exchange capacity, and low pH; moderate weathering of rocks with low carbonate content, but deeper soils; and rapid weathering carbonate bedrock (e.g., sedimentary limestone), deep soils with high cation exchange and high soil pH.
Cation Exchange
Cation exchange can increase alkalinity whenever hydrogen ions in solution exchange on surfaces for base cations. The effect is generally reversible, and thus the process may not contribute to long-term increases in alkalinity once the cation exchange sites are depleted. In fact, cation exchange can act in reverse if base cations are added from sea spray or road salt to a soil solution, causing temporary acidification and loss of alkalinity. Nevertheless, soils with large cation exchange capacities can act as a large buffering reserve against relatively short-term acidification events.
Assimilatory Uptake
The uptake of inorganic carbon and other cations and anions during plant growth are generally balanced with little net effect on alkalinity with the general exception of nitrogen uptake. The uptake of nitrate during plant growth tends to raise alkalinity of the solution. Nitrate is assimilated by plants and reduced to amine groups R-NH2 with the net result being the removal of nitric acid. A general equation for the reaction is
[6]106CO2+16H++16NO3−+H3PO4+122H2O→(CH2O)106(NH3)16H3PO4+138O2
Forests, for example, may produce net gains in alkalinity in stream water by the uptake of nitrates found in acid precipitation. The opposite may occur in some systems, where ammonium [NH4+] is the dominant form of nitrogen uptake by plants or algae and results in loss of alkalinity as a hydroxide [OH−] is also taken up to balance the charges.
Dissimilatory Redox Reactions
A similar reaction may occur with dissimilatory uptake of acid anions. In this case anaerobic bacteria do not use the ions to build cellular material, but instead use the nitrate and sulfate ions as electron acceptors during anaerobic decomposition of organic matter. A simplified equation for sulfate reduction is
[7]SO42−+2(CH2O)→H2S+2HCO3−
This process strips the oxygen from the acid anions to produce CO2 and in effect removes the strong acids from solution. The reduced end products from the processes of denitrification and sulfate reduction are nitrogen gas and hydrogen sulfide, respectively. As long as the reduced end product is not reoxidized, the alkalinity gain is permanent. Nitrogen gas is stable and will not readily oxidize, and similarly, hydrogen sulfide gas may be lost to the atmosphere as a gas release or permanently stored in the sediments (e.g., as iron sulfide), thus preventing reoxidation.
Evaporation and Precipitation
The amount of alkali produced by chemical precipitation reactions depends on the cations and anions involved. As noted earlier, precipitation of calcium carbonate as calcite removes two equivalents of alkalinity from solution because the base cation calcium is removed by a weak acid anion carbonate. However, the precipitation of gypsum, calcium sulfate CaSO4, has no effect because equal equivalents of base cation and strong acid anion are removed together.