Oxidation and Reduction

Oxidation and  reduction are terms heard often in potable water treatment.  We talk about organics or iron being oxidized by chlorine or potassium permanganate.  What does that really mean?  We are going to look at these specific reactions today to try to determine what is happening when something is oxidized. 

 

First of all, it is important to realize that whenever oxidation takes place, reduction also takes place.  When something is oxidized there is something else being reduced.  Oxidation originally meant the addition of oxygen to a compound.  It has now come to mean an increase in positive valence or decrease in negative valence.  Conversely, reduction is the increase in negative valence or decrease in positive valence.  Another way to say it is that oxidation is the removal of electons and reduction is the addition of electrons.  Electons are negatively charged.  The following equation illustrates this.

 

Cl2 + 2 FE++ = 2 FE+++ + 2Cl-

 

In this reaction chlorine is being reduced to chloride ion and iron is being oxidized from the ferrous state (+2) to the ferric state (+3).  Chlorine is an oxidizing agent so in the process it becomes reduced.  Ferrous iron becomes ferric iron so it is being oxidized.  It is termed the reducing agent.  Chlorine is receiving electrons while ferrous iron is giving up electrons.  This is the reaction that takes place in well water when it is treated with chlorine .  Iron present in well water is nearly always in the ferrous state.  As it comes to the surface it contacts air, chlorine, and sometimes potassium permanganate all of which convert the ferrous to ferric.  Simple aeration is all that is needed somtimes and towers are constructed for this purpose.  In aeration the ferrous iron is being oxidized by the oxygen gas in the air.  The key to the usefulness of this reaction is that ferrous iron is soluble and ferric iron is not.  Ferric can be settled out with a flocculent or filtered out. 

Potassium permanganate reacts the following way when removing iron:

MnO4- + Fe++ = MnO2 + Fe+++

The Manganese goes from a +7 state to a +4 state. Iron goes from +2 (ferrous) to +3 (ferric).  Manganese is being reduced and iron is being oxidized. 

Similarly, permanganate oxidizes Mn+2 to MnO2 (Mn is +4 here)

Mn+2 is the form found naturally in water and which causes the black stains.

We all know permanganate is used to remove manganese which seems impossible at first.  MnO4 is used to remove Mn+2 from water by converting it to MnO2 (Mn is +4 here).  MnO2 is insoluble in water and can be settled and filtered out.  Both Mn+2 and MnO4 are soluble in water. 

So, you are actually dealing with manganese in three forms in water treatment, +2, +4, and +7. 

 

Organics are another substance encountered in water treatment and they can cause taste and odor problems. Chlorine can be used to oxidize them but in the process can cause formation of chlorinated organics such as THMs, HAAs, and other undesirables.  Permanganate can be used to oxidize organics but it forms MnO2 which needs to be filtered out.  Ozone is an excellent oxidizer for organics because it breaks down into oxygen.  What happens when an organic gets oxidized?  It breaks up into smaller components and hence loses its ability to cause taste and odor.  Organics are long chain carbon compounds which become several smaller chains when oxidized.  Some are converted all the way to CO2 and H2O which are completely harmless.  Carbon is also used, of course, to remove organincs.  This is through adsorption which is a subject for another day. 

This has been a basic discussion of oxidation and reduction.  We hope it has been useful to you.  If you have any questions please do not hesitate to call or email me at the following addresses:

Bill Rolph

warolph@walterlouis.com

636-233-2681

Thank you for your time.