Phenol is a simple aromatic compound that has many important uses. It is a common component in the production of resins, pharmaceuticals, adhesives, surface finishes and many other chemical products. Phenol products are also present in the wastewater of many industries such as oil refining, petrochemicals, coking operations, plastics, and wood products. However, phenols are toxic compounds in parts-per-million levels and if discharged to surface waters without treatment they may adversely impact aquatic systems and human health.
The U.S. EPA lists phenol as a Priority Pollutant. Discharge to surface water is regulated under the National Pollutant Discharge Elimination System (NPDES) program and discharge limits are set based on steam flow and background concentrations.
CAIROX® potassium permanganate and CARUSOL® liquid permanganate oxidation chemistries are a recognized method for breaking down and eliminating phenols. The reaction is fast and permanganate can achieve very low residual phenol levels in many waters.
Complete Oxidation Reaction
The permanganate reaction is capable of fully mineralizing the phenol molecule to carbon dioxide and water, according to the following equation:
3 C6H5OH + 28 KMnO4 + 5 H2O ⇒ 18 CO2 + 28 KOH + 28 MnO2
This reaction proceeds quickly at neutral pH and in 60 minutes or less the phenol concentration can be reduced by 99%. If necessary, the reaction rate can be improved by raising the solution pH with the addition of a base, such as sodium hydroxide or by acidifying the solution with, for example, sulfuric acid. To achieve complete oxidation, 15.7 weight parts of KMnO4 are required for every 1 part of phenol.
In this equation, the permanganate ion was supplied in the form of potassium permanganate (KMnO4). It is also available in the form of sodium permanganate (NaMnO4) which is just as effective. The sodium permanganate reaction is:
3 C6H5OH + 28 NaMnO4 + 5 H2O ⇒ 18 CO2 + 28 NaOH + 28 MnO2
The weight ratio for this reaction is 14.1 parts sodium permanganate to 1 part phenol.
Step-wise Breakdown of Phenol
Although complete mineralization is possible, in actual treatment applications permanganate breaks down the phenol molecule in a series of small steps. The oxidation starts when the aromatic ring structure is split open, resulting in a six-carbon, unsaturated organic acid. This reaction continues further, very quickly, as permanganate attacks the double bonds in the chain. Oxalic acid, C2H2O4, has been identified as a common end-point. The permanganate reaction equation to an oxalic acid end-point is:
3 C6H5OH + 22 KMnO4 + 11 H2O ⇒ 9 C2H2O4 + 22 KOH + 22 MnO2
The weight ratio for this reaction is 12.3 parts potassium permanganate to 1 part phenol.
Because there is often a mixture of end-products that are formed, a good starting dosage for permanganate tests is often a 10:1 ratio of potassium permanganate to phenol. This assumes that phenol is the only compound in the solution that will be oxidized.
The Reaction Conditions
As with most chemical reactions, the oxidation of phenol by permanganate is influenced by other system factors. The speed and completeness of the reaction is affected by (1) the temperature of the system, (2) the pH of the solution, (3) the concentration of the phenol, and (4) the other reactive components of the waste that will compete for the permanganate.
Permanganate Jar Testing
The actual amount of permanganate that will be needed at any site should be determined by running a simple jar test to measure the permanganate consumption rate and the disappearance of the phenol. You can experiment with different reaction conditions and assess the benefits. Because CAIROX potassium permanganate and CARUSOL liquid permanganate are stable, standard laboratory solutions, typically 10 gram/Liter, can be prepared and stored for multiple screening tests. Test procedures and guidelines have been published explaining how to run a bench scale test with permanganate.
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