How do I input precipitation reactions?

[olivertheseira]

Hi,

According to the manual, PHASES data block is only used for dissolution reactions. May I know how I can input precipitation reactions in PHREEQC, specifically calcite precipitation from albite. This reaction is clearly not in the database. How should I go about this with respect to the PHASES, RATES and KINETICS data blocks? This is for the purpose of reactive transport modeling.

Thanks in advance,
Oliver Theseira

[dlparkhurst]

PHASES requires dissolution reactions only to be able to identify the mineral, which is the first reactant on the left hand side. PHASES is just a list of potential reactants, and a set of minerals for which saturation indices will be calculated. PHASES does not produce any mole transfers during calculations.

Now phreeqc.dat clearly contains a mineral Albite, defined in the PHASES definitions. If you put Albite in an EQUILIBRIUM_PHASES definition with some moles, for example

EQUILIBRIUM_PHASES
Albite 0.0  1
Calcite 0.0 1

then there is one mole of Albite that may react, which is more than enough for the solution to come to equilibrium (the 0.0 following Albite). If you include calcite as shown, the the solution will equilibrate with both calcite and albite.

There is also a RATES definition for Albite. If you define a KINETICS data block including Albite, the albite will react kinetically, depending on the rate parameters and the amount of albite that is defined in KINETICS.

I suggest that you work with batch reaction modeling with PHREEQC to understand the interactions of KINETICS and EQUILIBRIUM_PHASES before you start reactive-transport modeling.