The Troublesome Trio & Water Purity: Part 2
The Troublesome Trio & Water Purity: Part 2
Part 2: Manganese & Hydrogen Sulfide
Three undesirable water characteristics-iron, manganese, and hydrogen sulfide–can he called the “Bermuda Triangle” of the water processing business. Singularly or in combination, this trio can present a very complex challenge to the water treatment specialist. The same processing method used for removal of one species can often be applied to all three, but because of the many variations in the nature of these substances, determining corrective treatment calls for a complete and reliable water analysis; thorough equipment know-how; and proper application of the “art” of water conditioning.
Both physical and water chemistry factors determine success or failure in removing these substances to improve the aesthetic quality of water. Important physical factors include (1) the pump and plumbing system, (2) available flow rates, and (3) water temperature. Water chemistry factors include (1) pH values, (2) concentration of the species, and (3) oxygen levels. Geographically, iron, manganese, and hydrogen sulfide often are objectionable characteristics in the same water supply. Along the Eastern Seaboard, the upper Great Lakes region, and the Great Plains states are areas where unacceptable levels of these three species are common in well waters and municipal water supplies. Below, and in the next installament, we’ll look at the background and chemical behavior of the troublesome trio in water sources.
The presence of manganese in groundwaters, like iron, is generally attributed to the solution of rocks and minerals, chiefly oxides, sulfates, carbonates, and silicates, that contain some degree of manganese. Manganese-bearing minerals are less abundant than iron-bearing minerals; consequently, manganese is found less often than iron in water sources. The hydroxides and carbonates of manganese are, on the other hand, more soluble than the corresponding iron complexes. Despite these differences, concentrations of manganese seldom run higher than 2.0 ppm-mg/L. Comparative test data of many public water supplies in Illinois show that the concentration iron encountered is roughly 10 times the manganese concentration in the same source water. the solution of manganese-bearing minerals into a water source is often attributed to the action of carbon dioxide in groundwater — the same general theory as iron. Often, groundwater carbon dioxide is presumably generated by the bacterial decomposition of organic matter leached from soils. The solution of manganese and iron may take place under anaerobic conditions and in the presence of reducing agents (organic substances, hydrogen sulfide) that are capable of reducing the higher oxides of manganese and iron into the manganous manganese (Mn²⁺) and ferrous iron Fe²⁺ states.
Manganese is a vital micronutrient for both plants and animals. When manganese is not present in sufficient quantities, plants exhibit a yellowing of leaves (chlorosis). While the average daily intake of manganese for humans is 10mg, large ingested amounts are reported to have caused some liver damage.
Manganese will be found in water in the same forms as iron: dissolved (clear water) chiefly, as manganese bicarbonate, precipitated (oxidized) manganese, and organic manganese.
Manganese is rarely found alone in a water source; it is generally found in conjunction with iron. Concentrations of 0.1 ppm-mg/L are considered troublesome. The EPA has listed the maximum level of manganese at 0.05 ppm-mg/L in the secondary drinking water regulations. Both iron and manganese are stain-causing substances. While oxidized manganese is dark brown, accumulations on a surface will appear as black. This is why manganese-bearing water may be referred to as “black water”. Laundries, textile mills, and paper mills are among the industries that are most critically affected by manganese in the process water supply. Food process water must also be extremely low in manganese concentration.
Organic (bacterial) manganese behavior is similar to that of iron bacteria. There is an old saying in the water treatment industry that “manganese holds up iron”. It doesn’t, but organic matter can sometimes hold both metals in a water solution.
The third link in the trio is the offensive odor-producing substance of hydrogen sulfide — often called sulfur water in the water treatmet trade. Unlike manganese and iron, which are inorganic dissolved metals, hydrogen sulfide is a gas that dissolves readily in water. When released, such as at a water faucet, it’s disagreeable “rotten egg” odor is very noticeable, even at low concentrations. This gas is also flammable and, in higher atmospheric concentrations, poisonous to humans. In addition H₂S is corrosive to most metals and can tarnish silverware. At the level of only 0.25 ppm-mg/L, hydrogen sulfide is detectable by most persons.
Despite its unpleasant and corrosive characteristics, hydrogen sulfide has played a role in spas and hot sulfur springs over the years. Hot Springs National Park in Arkansas, Warm Springs in Georgia, and Hot Sulfur Springs in Colorado were known for their potential therapeutic cures.
Occasionally, a hydrogen sulfide odor may be detectied in a softened or filtered water supply that showed no H₂S in the raw water sample analysis. This condition generally occurs in the morning when the first water is drawn at a hot water faucet. The cause is the presence of a harmless sulfur bacteria, usually in the hot water heater tank.
Because of the changing nature of gases dissolved in water, hydrogen sulfide is sensitive to water temperature and pH value. Raising the water temperature from, say, 50°F to 70°F will result in 25 percent less H₂S in the water. Hydrogen sulfide is most readily removed at a pH value of 5.5. Ninety-eight percent of the sulfide in water at pH 5.5 value exists as H₂S and only two percent exists as the bisulfite ion. On the other hand, at 9.0 pH value, just a little over 0.05 percent would be present as hydrogen sulfide.