Water Problems — Manganese
Manganese Contamination in Aquatic Sources
The element manganese is inherently found within geological structures such as rocks and soil, and its presence in water can also originate from underground pollutants. It is rare to detect manganese in water sources alone; it is more commonly found in conjunction with iron, though less frequently. Chemically, manganese resembles iron, forming similar compounds. Its presence in water can be as troublesome as iron, or even more so. Even at minimal concentrations, it can discolor surfaces it comes into contact with. It causes sediment buildup in pipes and can create black particles and cloudiness in tap water due to manganese precipitation. Washing with water high in manganese results in dark stains due to the oxidation of manganese. The U.S. EPA recommends a secondary drinking water standard of 0.05 mg/l for manganese to avoid staining. Industrial uses might necessitate even stricter limits of 0.01 to 0.02 mg/l, which can still be considered too high in certain situations. Concentrations above 0.05 mg/l may alter the water’s color, odor, or taste, but health concerns are only significant at roughly ten times these levels. If manganese contamination is unacceptable, treatment techniques such as cation exchange, distillation, filtration, or reverse osmosis can be utilized to treat the water.
Health Risks Associated with Manganese
Ingestion of manganese through diet and drinking water is frequent. Manganese in water does not increase exposure during bathing or showering, as it does not penetrate the skin or become airborne. Elevated manganese exposure has been connected to nervous system toxicity, with symptoms similar to Parkinson’s disease. It is not usually linked to other toxic effects, such as cancer or reproductive damage. Younger individuals tend to absorb more manganese and eliminate less, emphasizing the necessity of clean water for pregnant women and children.
Challenges in Manganese Removal and Its Properties
Manganese removal from water is generally more difficult than iron removal due to its slower oxidation rate. Pure manganese metal has a pinkish-gray color, is brittle, and harder than iron. It does not occur in its pure form in nature but is present in various compounds, with deposits located throughout the US and worldwide. In aqueous solutions, manganese is commonly found as the manganous ion (Mn++). Manganese salts tend to be more soluble in acidic conditions than in alkaline, similar to iron. Manganous bicarbonate dissolves, typically introducing the ion into water, while organic or colloidal manganese compounds may be found in surface waters and shallow wells. Manganese bacteria can cause issues comparable to those caused by iron bacteria, including blockages and discoloration.
The formation of the insoluble manganic hydroxide, known as "black water," is less common due to the high pH required for precipitation compared to ferric hydroxide. Manganese bicarbonate in solution is colorless, which is why well waters containing manganous ions initially appear clear but turn black upon aeration, forming manganese dioxide. This conversion can cause issues. When manganous ions are oxidized, the reaction is as follows:
2Mn++ + O2 + 2H2O → 2MnO2 + 4H+
To decrease manganese concentrations, a water softener can be effective for lower levels, whereas higher concentrations might necessitate oxidizing filters. For very high concentrations or those combined with organic matter, chemical oxidation followed by filtration is essential. Chlorine is not adequate for complete oxidation of manganese unless the pH is above 9.5; potassium permanganate is effective at pH levels above 7.5 and is usually the preferred oxidizing agent.
*Colloidal refers to particles that are insoluble and suspended in a liquid without settling, which do not affect the liquid’s physical properties such as freezing or boiling points.