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Confusion about ion exchange water softener

  1. Feb 26, 2018 #1

    sophiecentaur

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    I am about to have a water softener installed and I thought I should learn a bit about the Chemistry involved. There is loads of advertising blurb on the Web but very little precise Chemistry. I am sure someone here can help me with this.
    The basic softening process is fair enough with sodium +ions replacing the Calcium and Magnesium +ions. Why the Na+ ions in brine would displace the Ca+ ions on the beads is not clear. Something about ionisation energy, no doubt.
    Also, the Na+ ions in the softened water would be left behind on surfaces if the water evaporates but what would they be combined with and what solid would they form? Sodium Carbonate? Obvs, it would wash off easily and would never be deposited in pipes etc..
     
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  3. Feb 26, 2018 #2

    Borek

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    It is an equilibrium process - add high concentration of Na+ and they will displace the Ca2+ just because there is plenty of them (that's why brine is used, to have as large concentration of Na+ as possible). Exact numbers depend on the resin, but even if you assume identical cation affinity it works this way - ratio of cations adsorbed by the resin more or less follows the ratio of concentrations of ions in the solution.

    Counteranions in the water after softening will be exactly the same ones that were present before, solution is electrically neutral from the very beginning, the thing that changes is that one kind of cation is displace by another.
     
  4. Feb 26, 2018 #3
    Assuming that calcium is in form of Ca(HCO3)2, of course in dissociated, ionic form as solid compound of this formua does not exist, then you will be left with solution of NaHCO3, eg solution of baking soda, once cationite have done its deeds.
    Cationite takes a form of resin with strongly acidic sulphonic acid functionality.
    Initial form could be described as Res-SO3H where Res is just organic group derived from polymer. Task of polymer is to make cationite insoluble.
    Sodium form is initially obtained as follows:
    Res-SO3H + Na+ ---> Res-SO3Na + H+ (this initial step is usually done in factory).
    In your water system it goes as follows:
    2 Res-SO3Na + Ca++ ---> (Res-SO3)2Ca + 2Na+ (this is how during normal use calcium is replaced by sodium, calcium is binded to insoluble resin, hence taken out of water which is now softer)
    However reaction above is reversible, particularly if large excess of sodium ions is present, say in concentrated brine, hence regeneration of active, sodium form of resin is possible:
    (Res-SO3)2Ca + 2Na+ ---> 2 Res-SO3Na + Ca++ (effluent after regeneration, containing calcium chloride mixed with some unreacted sodium chloride will be discarded and regenerated resin is ready to work with next batch of calcium contaminated water and so on.
     
  5. Feb 26, 2018 #4

    sophiecentaur

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    So it's all a matter of concentration. That makes sense. It's analogous to Haemoglobin picking up Oxygen molecules from air and losing them in the capillaries where there is low concentration. Something you may not have heard before but Foetal Haemoglobin is a different formula from adult Haemoglobin because it has to work between lower Oxygen concentration gradient across the placenta compared with in the lungs. Some damn clever engineering at work there. That beats my water softener.
    Thanks chaps. I am happy now. :smile:
     
  6. Feb 26, 2018 #5

    Tom.G

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    Just a heads-up. Do not drain the regeneration effluent into a septic tank or anyplace you want plants, trees, etc to grow. The effluent is concentrated enough to kill off almost anything. (I haven't tried salt grass though)
     
  7. Feb 27, 2018 #6

    sophiecentaur

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    Yep, I got that one! Also, the sodium in softened water upsets the water balance in plants so it's not good to use it regularly on the garden. Even as I type, the man is fitting a hard water tap outside for the sake of the plants.
     
  8. Feb 27, 2018 #7
    Most of plants require considerable amounts of calcium to grow. Exceptions are Ericaceae which still require calcium but in small quantities only.
    So unless you are certain that your soil is calcium rich it is not a good idea to use decalcified water.
    Watering garden with not treated water is good idea, but if you have anyone of Ericaceae, decalcified version is beneficial under these particular plants.
     
  9. Feb 27, 2018 #8

    sophiecentaur

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    But I understand that high sodium levels are bad. I guess that would apply even to Ericacious plants. I will read further on gardening links but everyone seems to advise against it.
     
  10. Feb 27, 2018 #9
    There must be a compromise, I guess.
    These particular plants like rain water most, if you are using one with calcium, you also need to acidify soil with sulphur powder from time to time.
    I have in my garden V. corymbosum. Very nice berries. These plants really hate too much of calcium and I have to maintain high acidity of soil by occassional application of sulphur powder. This is oxidized by soil bacteria to sulphuric acid which is removing excess of calcium in form of poorly soluble calcium sulphate (gypsum).
    Any excess of calcium is signalled by this plant as yellowing leaves.
     
  11. Feb 27, 2018 #10

    jim mcnamara

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    Plants in the Ericaceae family have mycorrhizal fungi that do most of the "uptake" work for nutrients and create nitrate - these fungi require fairly low pH soils to do their job.

    FWIW:

    https://www.actahort.org/books/525/525_11.htm

    Short blurb from a good book. You can inoculate blueberries with varying strains of the fungi - Amazon sells several different ones. Adding these beasties into the reasonable pH soil, if needed, it really improves growth and productivity. Search term - blueberry inoculant
     
  12. Feb 27, 2018 #11
    I actually did this innoculation but still try to keep pH in range of 4-5.
    Manufacturer of innoculant is claiming that it can keep plant happy up to pH 6-6.5, but I don't believe in it. Decided to keep it within safe range anyway and results are very good.
     
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