Precipitation rates

[Ami]

Dear Colleagues,

I am interested in modelling the precipitation rate of calcium carbonates in seawater using phreeqc. I have found an example of calcite kinetics in the phreeqc user guide (version 2) by Parkhurst and Appelo (1999). I have added the routine below, but changing the phase to aragonite.

I would be grateful if you could advise me as to whether this program can be used to determine the precipitation rate of calcium carbonates in seawater.
If this is not the appropriate data block, could you kindly direct me to a valid expression for the simulation of calcium carbonate precipitation using phreeqc?
Thanks so much for your time and help
Ami

Code: [Select]

KINETICS 1
Aragonite
    -formula  CaCO3  1
    -m        0.003
    -m0       0.003
    -parms    4.43 0.6
    -tol      1e-08
-steps       172800 in 10 steps # seconds
-step_divide 1
-runge_kutta 3
-bad_step_max 500

INCREMENTAL_REACTIONS True

 RATES
Aragonite
-start
rem M = current number of moles of aragonite
rem M0 = number of moles of aragonite initially present
rem PARM(1) = A/V, cm^2/L
rem PARM(2) = exponent for M/M0
10 si_cc = SI("Aragonite")
20 if (M <= 0 and si_cc < 0) then goto 200
30 k1 = 10^(0.198 - 444.0 / TK )
40 k2 = 10^(2.84 - 2177.0 / TK)
50 if TC <= 25 then k3 = 10^(-5.86 - 317.0 / TK )
60 if TC > 25 then k3 = 10^(-1.1 - 1737.0 / TK )
70 t = 1
80 if M0 > 0 then t = M/M0
90 if t = 0 then t = 1
100 area = PARM(1) * (t)^PARM(2)
110 rf = k1*ACT("H+")+k2*ACT("CO2")+k3*ACT("H2O")
120 rem 1e-3 converts mmol to mol
130 rate = area * 1e-3 * rf * (1 - 10^(2/3*si_cc))
140 moles = rate * TIME
200 SAVE moles
-end


[dlparkhurst]

You must make your own decision. I think it is a reasonable place to start.

[Ami]

Thank you, Dr. Parkhurst

I know it's a complex matter. I just wanted to be sure that the mathematical expression is valid for precipitation. Any recommendations to address this issue?

Best regards,
Ami

[dlparkhurst]

The "rem" statements need line numbers too.

Read the Plummer and other papers on which the rates are based.

[Ami]

Thank so much!