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Re: biogenic decalcification

Well, Larry asked for it...

> Here's a question for a chemist or bio-chemist:
> I am a little confused about the mechanism of biogenic decalsification.

In short, plants force the pH up when they take CO2 or bicarbonate out of 
the water.  Decalcification happens as an incidental side effect of the 
increased pH.

> 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).

[cut and paste]

> I   CO2 + H2O <--> H2CO3
>               (Carbonic acid)
> II  H2CO3      +       CaCO3       <-->     Ca(HCO3)2
>    (C. Acid)      (Calcium carbonate)   (Calcium bicarbonate)

> 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?)

For starters, "H2CO3" (at least in geochemical literature) is regarded as
just H20 + CO2.  H2CO3 has never been isolated as a pure component.  It
might be conceptually helpful, but it complicates the formulas.  I've 
seen other data that seems to dispute this, but the common thermochemical 
tables are pretty clear.

Second, the species "Ca(HCO3)2" exists as an ion association in sea water 
and other concentrated solutions, but isn't significant in freshwater.  
Ca(HCO3)2 should be replaced with Ca+2 + 2(HCO3-).

So, replace equation I with

I'	CO2 + H2O <--> H+ + HCO3-

The reaction is forced to the right when CO2 concentrations increase and
pH drops.  The reaction can be forced to the left by plants consuming CO2
and in that case the pH increases.  I think plants will force this
reaction only while CO2 is available at "practical" levels - at levels
where the plant can actually use it.  So plant uptake of CO2 can increase 
pH, but only to moderately high levels, not to the 9+ levels that 
sometimes occur.

If calcium carbonate is present, then equation II might go on as:

II'	H+ + CaCO3 <--> Ca+2 + HCO3-

When when we force reaction I' to the right by adding CO2 the increased H+
from the reaction also forces reaction II' to the right.  That tends to
use up the increased H+ and cancel the pH drop created by adding CO2.  It
also increases general hardness and alkalinity.  But it only works if
CaCO3 is present.  If CaCO3 isn't present then the reaction can't go to
the right.  It can be forced to the left (precipitating calcium carbonate)
if something causes an increase in pH. 

We can also force this reaction to the right with ion-exchange softening,
which removes Ca+2 and replaces it with 2Na+.  Calcium carbonate 
dissolves and cancels out the work done to soften the water.

Snails force the reaction to the left when they build their shells.  That
is a form of biogenic decalcification and I don't know how they do it. 
Because of snails, this system seems to procede to the left in my tanks,
even with added CO2. 

These reactions could also be written with magnesium in the place of calcium.

Equations I' and II' probably do not operate on the same time scales.  
I' occurs entirely in solution and is a relatively fast reaction.  II' 
occurs only at the surface of solid CaCO3 and is a relatively slow reaction.
You will see changes caused by I' over the scale of minutes, but changes 
caused by II' over the course of hours or days.

> 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.

[cut and paste]

> 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)

These are really just the first 2 equations, combined and written 
in reverse order.

As I understand the process, some aquatic plants are capable of breaking 
down bicarbonate ion to obtain CO2.  The reaction might happen on the 
surface of the plant and might be written as:

2(HCO3-) --> CO3-2 + CO2 + H2O.

CO2 is imported by the plant, and the CO3-2 remains in the water.  (This
same reaction occurs when bicarbonates are heated.  Throw sodium bicarb
on a grease fire and it breaks down to sodium carbonate and the released
CO2 smothers the fire.)

You could also look at this as two reactions:

HCO3- --> OH- + CO2
OH- + HCO3- --> CO3-2 + H2O

The reaction might also be done inside the plant, which would import 
HCO3- in one area of the plant surface and export CO3-2 at another area.  
This is mechanistically a little more difficult.

Either way, the result is uptake of CO2 and production of carbonate 
ion at the plant's surface.  The carbonate ion reacts with any available 
H+ to form bicarbonate ion and that increases the pH:

III'	CO3-2 + H+ <--> HCO3-

Plant production of CO3-2 can force pH over 9.

The carbonate can also react with calcium ion to precipitate calcium 

IV'	CO3-2 + Ca+2 <--> CaCO3
Reaction IV' goes to the right (thus precipitating calcium carbonate) at
very high pH levels that are created at the plant's surface.  If the pH
gets high enough farther away from the leaf then reaction II' will also 
precipitate calcium carbonate.

Equation IV' reacting to the right is probably what is usually called
biogenic decalcification when we're talking about plants.

> 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?

Equations I' and III' express equilibrium conditions between dissolved 
inorganic carbon species.  It's this equilibrium that normally 
determines the pH of water.  When plants remove CO2 they increase the 
pH.  At higher pH they remove HCO3- and that forces the pH even higher.

Equations II' and IV' are the decalcification reactions.  In both
reactions, precipitation of calcium carbonate (decalcification) can be
induced by the increase in pH caused by plant consumption of dissolved
inorganic carbon.

Enough already!

Roger Miller