Potassium permanganate
(endorsed 2005)
Potassium permanganate is mainly used for the oxidation and removal of iron and manganese; it can also be used as a disinfectant, or to control tastes and odours.
General description
Potassium permanganate, , is a dark purple crystal with a blue metallic sheen. It has a sweetish, astringent taste, is odourless and is an oxidant. The chemical is commercially available in crystalline form. Potassium permanganate is highly soluble in water, but heating is usually needed to prepare solutions with concentrations of more than 2.5%.
Appropriate handling materials include iron, steel, stainless steel, fibreglass-reinforced plastic, polyethylene and polyvinyl chloride.
Chemistry
Potassium permanganate is produced by fusing manganese dioxide with potassium hydroxide to form potassium manganate; a solution of the manganate is then electrolysed at about 60Ā°C using iron electrodes.
Under most treatment applications, permanganate () is reduced to insoluble manganese dioxide ().
Divalent manganese () is removed from water by the oxidation to insoluble manganese dioxide (). As the oxidant, the permanganate ion is itself reduced to manganese dioxide. The reaction proceeds as follows:
The stoichiometric ratio of to soluble is 1.92:1; however, reactions with organics usually require significantly higher ratios. The alkalinity consumed is 1.2 mg of per milligram of , and the sludge produced (based on as the precipitate) is 2.6 kg/kg .
Potassium permanganate can also be used to oxidise iron and organics. The stoichiometric ratio of to soluble is 0.94:1; however, reactions with organics usually require higher ratios. The alkalinity consumed is 1.5 mg per mg of and the sludge produced (based on as the precipitate) is 2.4 kg/kg .
Manganese dioxide resulting from permanganate reduction is an effective adsorbent for ferrous iron (), manganous manganese (), radium () and other trace inorganic cationic species. These contaminants can be removed by permanganate treatment.
Manganese dioxide also adsorbs natural organic materials that serve as precursors for disinfection by-products. This characteristic of manganese dioxide is particularly pronounced in hard waters, presumably because of the bridging action of calcium and manganese.
Typical use in Australian drinking water treatment
In drinking water treatment, potassium permanganate can be fed into solution directly using a dry chemical feeder or as a liquid bulk supply using dosing pumps. Alternatively, a concentrated solution can be prepared on site, from which the desired concentration is added to the water.
Permanganate is often added at the head of the treatment plant, as close to the intake as possible. This allows sufficient time for the permanganate to perform its oxidative function and to be reduced completely to solid manganese dioxide before filtration. In some cases, an alkali (usually lime) is added before, or soon after, addition of potassium permanganate, to assist in the oxidation process. Potassium permanganate can be effective over a range of pH values, but is most effective at pH 8.5 or higher.
The adsorptive property of is the principle underlying the historical manganese greensand process, in which the filter medium is coated with manganese dioxide, which subsequently serves as an adsorbent for , and other metals in the filter influent. Filter media can be coated with manganese oxide by applying a potassium permanganate solution to the filter bed and oxidising it (through chlorination or aeration). Low doses of chlorine or permanganate are applied to the filter influent to catalyse the oxidation of the adsorbed metals, thereby creating additional adsorption sites. Alternatively, the filter backwash water may be treated with chlorine or permanganate.
Concentrations of potassium permanganate used in drinking water treatment depend on the concentrations of metals and organics, but are usually 0.3ā5 mg/L. Overdosing of the chemical should be avoided because of the pink colour of unreacted permanganate. The potassium permanganate levels required should be determined by jar testing.
Advantages of potassium permanganate include its ease of use and the fact that it is effective for the oxidation of both iron and manganese and for certain types of taste and odour. As a disinfectant, it produces no halogenated disinfection by-products, but has only a limited disinfection capability.
Contaminants
The purity of chemicals used in Australia for the treatment of drinking water varies, depending on the manufacturing process. The following chemical contaminants may be present in this product (NRC 1982):
cadmium
chloride
chromium
mercury
sulfate
Residual and by-product formation in drinking water
When employed in drinking water treatment, potassium permanganate should be used in such a way that any contaminant or by-product formed by the use of the chemical does not exceed guideline values in the Australian Drinking Water Guidelines.
The manganese dioxide produced is a black precipitate that can be removed by a conventional clarification or filtration process. If manganese dioxide is not properly removed, the precipitates will create particulate deposits in the distribution system and on household plumbing fixtures. At manganese concentrations above 0.02 mg/L, an increase in consumer complaints is common.
Status
Potassium permanganate was endorsed by the NHMRC for use as a drinking water treatment chemical in 1983. The revision undertaken in 2003 did not change the status of this chemical for the treatment of drinking water.
References
ANSI (American National Standards Institute)/AWWA (American Water and Wastewater Association) Standard no B603-98. AWWA CD-ROM (April 2003). Available at <www.awwa.org>
Clesceri LS, Greenberg AE and Eaton AD (eds) (1998). Standard Methods for the Examination of Water and Wastewater, 20th edition. American Public Health Association, Washington, DC.
NRC (National Research Council) (1982). Water Chemicals Codex. Committee on Water Treatment Chemicals, Food and Nutrition Board, Assembly of Life Sciences, NRC, Washington, DC.
White GC (1992). Handbook of chlorination and alternative disinfectants, 3rd edition. Van Nostrand Reinhold, New York.
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