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Iron Removal from borehole water

By mass, iron is the most abundant element in (or on) the earth.  It is therefore not surprising that iron is so frequently found in ground water.  The origins of elemental iron root back to when a sufficiently large star would collapse under it’s own gravitational mass.  As the star collapses, under conditions of tremendous heat and pressure, new elements are formed.  Initially hydrogen, then helium etc.  At the point where iron is being formed, the collapsing process stops and the star explodes.  The resultant super-nova scatters the make up elements of the star into the cosmos.  A few quantum steps later, planets agglomerate from the scattered elements, our earth being an example of such.

Iron Removal borehole water

Iron Removal borehole water

In a mysterious way, un-oxidized iron is soluble in water.  It is counter intuitive that a metal can be dissolved in water the way sugar is dissolved in coffee.  If you have a look at the electron shell diagram on the left, it can be seen that the outer electron orbit has only two electrons.  On closer examination, the outer orbit but one is also not full.  This means, inter-alia, that in some way, there is a tension of sorts.  For complete stability, an electron orbit likes to be full.  To achieve orbital equilibrium, the atomic iron would have to either gain whole bunch of electrons or give away one or two.  A full 4th orbit is home to 32 electrons, whilst orbit 3 can house a maximum of 18 electrons.  For iron, the 3rd orbit has 4 vacancies and in the outer orbit there are 30!

This peculiar property of iron is shared with a number of other elements in the periodic table.  In broad terms, these are the transition elements and they tend to have the well known properties of metals such as malleability, good conductivity and magnetic propensity.

Another feature of the transition metals is the relatively large scope of compound formation through oxidation.  Old school chemistry says that Loss of Electrons equals Oxidation.  When iron in it’s un-oxidized state comes into contact with oxygen, an oxidation reaction will spontaneously occur.  The resultant product will be some type of iron oxide, rust by another name.  The oxidation reaction also takes place with chlorine and flourine.

In water, the oxidation of soluble iron results in the formation of a insoluble iron oxide.  In the solid state, iron is easily removed from water via a combination of sedimentation and filtration.  In the real world, the text book chemistry is sometimes hampered by extraneous factors.  If the pH of the water is on the low side, there will be an excess of positive hydrogen ions.  These too would like to be oxidized and because of their size, are always at the front of the queue.  Dissolved organic material is a well known oxidation reaction inhibitor.

Iron removal from borehole water

In summary.  Removing dissolved iron from ground water requires a change of state.  The dissolved iron needs to become particulate.  The treatment process addresses this challenge by incorporating various unit processes that include;

  1. pH adjustment
  2. Oxidation via either
    1. Atmospheric pressurized aeration
    2. Chemical oxidation using
      1. chlorine (Cl)
      2. potassium permanganate (KMnO4)
      3. ozone (O3) or
      4. hydrogen peroxide (H2O2)
  3. Sedimentation and filtration

Alternatively the stepped iron removal process can be substituted for a single pass through either

  1. Ion exchange using a resin or
  2. Catalytic oxidation filtration using a manganese dioxide (MnO2) based media

Each process requires contact and reaction time.  The stepped process has a low operating cost whilst the single pass will require regular replacement of chemicals.  As with so many applications in water treatment, the selected technology is determined by site and practical considerations.