chicken and Egg?-biogenic decalsification

[larry at creative_net (Larry Frank)]

Here's a question for a chemist or bio-chemist:

I am a little confused about the mechanism of biogenic decalsification. I
have read that CO2 causes calcium hardness in water by forming Carbonic
acid which dissolves Calcium carbonate. The Carbonic acid is also
responsible for the lowering of Ph with CO2 injection.( Equations I and
II).

Now biogenic decalsification occurs when the calcium bicarbonate comes out
of solution and forms calcium carbonate. My confusion is over the
mechanism. Does this occur by plants pulling CO2 out of solution causing
Equation I to reverse, and in turn causing equation II to reverse. ( The
rise in PH from reversing equation I would cause a reverse in equation II?)


Or as suggested in some literature the plants force equation III ( or in
extreme cases Equation IV--Which must require more energy because it is
claimed that plants which use IV raise the PH higher than from using III)
In this case equation III would force a reverse in equation II which in
turn would reverse equation I , thus lowering the PH.

So this is sort of a chicken and egg question. Does the rise in PH due to
CO2 being pulled from solution cause biogenic decalsification. Or does
biogenic decalsification cause a rise in PH?



I   CO2 + H2O <--> H2CO3
              (Carbonic acid)

II  H2CO3      +       CaCO3       <-->     Ca(HCO3)2
   (C. Acid)      (Calcium carbonate)   (Calcium bicarbonate)

__________________________________________________________________________

III CA(HCO3)2   ---> CaCO3    + H2O   + CO2
                                       (Taken directly from Calcium bicarbonates
                                        by plants)

IV H2CO3 ---> CO2 + H20                 (Taken directly from the carbonate
ion
                                        by plants)

Thanks,

Larry

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