5. POTASSIUM PERMANGANATE
Potassium permanganate (KMnO4) is used primarily to control taste and odors, remove color, control biological growth in treatment plants, and remove iron and manganese. In a secondary role, potassium permanganate may be useful in controlling the formation of THMs and other DBPs by oxidizing precursors and reducing the demand for other disinfectants (Hazen and Sawyer, 1992). The mechanism of reduced DBPs may be as simple as moving the point of chlorine application further downstream in the treatment train using potassium permanganate to control taste and odors, color, algae, etc. instead of chlorine. Although potassium permanganate has many potential uses as an oxidant, it is a poor disinfectant.
5.1 Potassium Permanganate Chemistry
5.1.1 Oxidation Potential
Potassium permanganate is highly reactive under conditions found in the water industry. It will oxidize a wide variety of inorganic and organic substances. Potassium permanganate (Mn 7+) is reduced to manganese dioxide (MnO2) (Mn 4+) which precipitates out of solution (Hazen and Sawyer, 1992). All reactions are exothermic. Under acidic conditions the oxidation half-reactions are (CRC, 1990): MnO4- + 4H+ + 3e- à MnO2 + 2H2O MnO4- + 8H+ + 5e-à Mn2+ + 4H2O Under alkaline conditions, the half-reaction is (CRC, 1990): MnO4- + 2H2O + 3e- à MnO2 + 4OHEo = 0.60V Eo = 1.68V Eo = 1.51V
Reaction rates for the oxidation of constituents found in natural waters are relatively fast and depend on temperature, pH, and dosage.
5.1.2 Ability To Form a Residual
It is not desirable to maintain a residual of KMnO4 because of its tendency to give water a pink color.
Potassium permanganate is only supplied in dry form. A concentrated KMnO4 solution (typically 1 to 4 percent) is generated on-site for water treatment applications; the solution is pink or purple in color. KMnO4 has a bulk density of approximately 100 lb/ft3 and its solubility in water is 6.4 g/mL at 20ºC.
EPA Guidance Manual Alternative Disinfectants and Oxidants
5. POTASSIUM PERMANGANATE
Depending on the amount of permanganate required, these solutions can be made up in batch modes, using dissolver/storage tanks with mixers and a metering pump for small feed systems. Larger systems will include a dry chemical feeder, storage hopper and dust collector configured to automatically supply permanganate to the solution dissolver/storage tank. KMnO4 solution is made up of dry crystalline permanganate solids added to make-up water and then stirred to obtain the desired permanganate concentration. The cost of KMnO4 ranges from $1.50 to $2.00 per pound (1997 costs), depending on the quantity ordered. Shipment containers are typically buckets or drums. Potassium permanganate is supplied in various grades. Pure KMnO4 is nonhygroscopic but technical grades will absorb some moisture and will have a tendency to cake together. For systems using dry chemical feeders, a free-flowing grade is available that contains anticaking additives (Hazen and Sawyer, 1992). Potassium permanganate is a strong oxidizer and should be carefully handled when preparing the feed solution. No byproducts are generated from making the solution. However, this dark purple/black crystalline solid can cause serious eye injury, is a skin and inhalation irritant, and can be fatal if swallowed. As such, special handling procedures include the use of safety goggles and a face shield, an MSA™/NIOSH approved dust mask, and wearing impervious gloves, coveralls, and boots to minimize skin contact.
5.3 Primary Uses and Points of Application
Although potassium permanganate can inactivate various bacteria and viruses, it is not used as a primary or secondary disinfectant when applied at commonly used treatment levels. Potassium permanganate levels that may be required to obtain primary or secondary disinfection could be cost prohibitive. However, potassium permanganate is used in drinking...
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