KINETIC DISSOLUTION OF QUARTZ

The major breakthrough of PHREEQC-2 in hydrogeochemistry is its facility to model kinetic reactions of any type very easily.
Kinetic reaction rates can be defined in the form of BASIC programs which are included in the input file or read from the database. BASIC programs consist of numbered lines with statements that resemble the formula in a spreadsheet cell. Special functions give access to the chemical variables of PHREEQC. These functions are described in the PHREEQC manual under keyword RATES.
A simple example is the kinetic dissolution of quartz, cf. PHREEQC file kin_qu.txt.

Dissolution of quartz,
     SiO2 + 2 H2O ↔ H4SiO4
proceeds kinetically:
     d Si / dt = (A0 / V) (m / m0)0.67 k (1 - SR)
where Si is the concentration of H4SiO4 (mol/L); t is time (s); A0 is the initial surface of quartz (m2); V is the solution volume (L); m is the remaining mass of quartz (moles); m0 is the initial mass of quartz (moles); k is the rate constant = 10-13.7 mol/m2/s (25 C); SR is the Saturation Ratio for quartz.

The rate is programmed in BASIC as: 


       RATES
        Quartz
        -start
         1 A0 = parm(1)
         2 V = parm(2)
         10 rate = (A0 / V) * (m/m0)^0.67 * 10^-13.7 * (1 -  SR("Quartz"))
         20 save rate * time
        -end

Keyword KINETICS calls the rate. Under this keyword, A0 and V are defined with the identifier -parms.
The graph shows the increase of the Si concentration in time for typical conditions in a soil, A0 = 23 m2 and V = 0.16 L.

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