Potassium Ferrate - The "Green" Oxidant

K₂FeO₄ is pretty cool stuff. It’s relatively easy to make, has a nice purple color, and it’s a stronger oxidant than permanganate. Oh… and it reacts with water. You can dissolve it in water, but unless the solution is fairly alkaline, it will decompose rapidly, generating O₂ as a product. It is being investigated for use as a cathode material for so-called “super-iron” batteries (using zinc anodes) and for its use as an oxidant for organic reactions. But its biggest claim to fame is its role in the area of water treatment.

A description of the methods used in water treatment facilities is too large to describe here, but one common method is flocculation, a technique used by both the Egyptians and Romans. The addition of alum and/or iron salts to the water to be treated, along with some lime, results in the precipitation of Al(OH)₃, Fe(OH)₂, and Fe(OH)₃. As these precipitates sink, they drag undesirable particulate material along with them, resulting in cleaner water. Chlorine (or chloramines) is then added for disinfection purposes. Chlorine treatments are very successful at removing harmful bacteria, but there has always been a concern that its reaction with organic material still present in the water might form harmful compounds.

This concern has been confirmed by a recently completed 10-year study. Michael Plewa, a geneticist at the University of Illinois, has quantified the toxicity, genotoxicity, and carcinogenicity of these disinfectant by-products (DBPs) using a mammalian cell line specifically developed for this study. He found not only that these DBPs are harmful, but that the degree of toxicity can depend on other factors. For example, it was found that water which contained bromine and iodine (seawater or aquifers associated with ancient sea beds) generated even more toxic DBPs. And DBPs which contained nitrogen were more toxic, genotoxic, and carcinogenic than DBPs which contained no nitrogen.

Plewa is especially concerned with swimming pools and hot tubs, which he refers to as DBP reactors. Organic material from swimmers -- sweat, urine, sunscreen, cosmetics, as well as some disgusting stuff – sits in contact with the chlorinated water for long periods of time, generating DBP levels up to ten times higher than drinking water. This may explain the higher levels of bladder cancer found in people who spend a lot of time swimming in pools.

This is where potassium ferrate comes into play, at least for water treatment applications. If all the organic material could be removed prior to the application of chlorine, then DBPs couldn’t form. So instead of adding iron salts and lime to the water, one could just throw in some K₂FeO₄. Ferrate would chew up the organic material in the water and then decompose to form the Fe(OH)₃ precipitant which removes the particulate matter as usual. And unlike chlorine, you cannot really add too much ferrate – you’ll just end up with more harmless Fe(OH)3. For this reason, ferrate is often referred to as a "green" oxidant. And once the ferrrate has done its work, you can add chlorine without the fear of forming DBPs.

The use of ferrate for this purpose has been investigated for over thirty years. One of the big drawbacks has always been the cost of manufacturing K₂FeO₄, but last year Battelle announced a lower cost method for its production, so it may yet come to pass.