reaction rate as function of pH

[Hydroman]

Hello everybody,

at the moment I'm dealing with the reaction rate as function of pH (Figure 8.16 on page 396 in Geochemistry, Groundwater and Pollution). Therefore, I use the examplecode of kinetic from database phreeqc.dat modified with the right coefficients of the mineralphase anorthite. For decreasing and increasing pH I used the keyword "reaction". So, my first question is:
1) Is it necessary to define the "time" in the kinetic part if I use the keyword reaction to decrease/ increase pH? Is there a combination between the steps from reaction-bloc and kinetics-blok (Reaction: 5000 steps and time 40: steps). Is it necessary that the steps have the same value (here: 5000)?

2) The second question is about the formula of the reaction rate.
a) Example 8.4 (page 398 in Geochemistry, Groundwater and Pollution) uses the following formula to calculate the reaction rate:
70 rate = rate * A0 / V * (m/m0)^0.67 * (1 - SR("K-feldspar"))
with 60 rate = rate_H + rate_w + rate_OH + rate_CO2

b) The example in the database uses the following formula to calculate the reaction rate:
220 area = PARM(1) * M0 *(M/M0)^0.67
230 rate = PARM(2) * area * rate * (1-SR("Anorthite"))
with Parm(1) = A0.

I don’t understand why the formulas are different. In b) there is no volume V defined and why is there defined M0 twice? Maybe someone can give me a little hint. That would be amazing.

BR Hydroman

[dlparkhurst]

Yes, you need to define -time in KINETICS. Default will be 1 second. (If you use RUN_CELLS , TRANSPORT, or ADVECTION, the time can be overridden).

If you have a different number of steps in REACTION (5000) than KINETICS (40), I think the last time step from KINETICS (1.5 y/40) will be used for the remaining 4960 steps. It would be clearer if you define the total time that you want to run in 5000 steps. Then again, I am not sure what you are modeling with this combination of REACTION and KINETICS. The KINETIC reaction also affects pH.

As for rate expressions, you are free to choose any expression you want provided the final result is moles. As my high school chemistry teacher taught me, follow the units. M0 * (M/M0)^.67 is a factor to account for smaller particles as M decreases relative to M0.

[Hydroman]

Thanks for your advisements.

So maybe it's easier to understand if I explain what I want to model. At the end I want to model a column experiment, with

- rock sample material that includes the silicate mineral phases (anorthite, k-feldspar, k-mica, albite) and quartz,
- destilled water at the beginning of the experiment,
- circulation of the water through the column until an equilibrium was reached.

One result of the experiment is that the measured electric conductivity as function time shows the course of a sigmoid function. So at the beginning I have got a less increasing of the values of EC with time. After a few days the measured EC started to increase rapidly.
My thesis is that the result of the experiment shows the effect of pH on the dissolution rate. At the beginning the water has a pH of nearly 7. The increasing of EC is slowly. Because of water-rock interaction there is a variation of pH that leads to a variation on log dissolution rate. That explains the course of a sigmoid function of EC.

And therefore, I’m interested to model the dissolution rate as function of pH with PhrreqC. In my understanding I use REACTION to increase or decrease pH (HCl and NaOH). For investigation the influence on log_r I can use RATES and KINTETICS. By using KINTEICS I can influence A0 and M0 of the different mineral phase (different GfW and different result of XRD-analysis).
So at the end I want to produce diagrams like figure 8.16 (in geochemistry, groundwater and pollution p.396) for different mineralphases.

Yes, you are right the kinetics also influences pH. But with the combination of REACTION (increasing/decreasing pH) and KINETICS I can explain the sigmoid course of the function of EC.

And because of that I asked myself if I have to use the same steps in REACTION and KINETICS.

BR Hydroman

[dlparkhurst]

It seems to me that all of the reaction is between the circulating water and the minerals. There is no external forcing of pH with HCl, so I would not use a REACTION that adds Cl.

If you have analyses of the circulating water that are sufficiently complete and accurate, you could use INVERSE_MODELING to evaluate the reactions that have occurred , including secondary mineral formation. Over short periods of time, surface reactions may be as important as mineral dissolution. Ultimately, you would try to include all of the pertinent reactions and estimate their rates or equilibrium conditions. I'm not sure you will have enough information to do that.

[Hydroman]

Dear Mr. Parkhurst,

thank you for your advice. I followed the units. Now it's clear. I think my describtion was not clear enough. Thank you very much for your advices.

BR

Hydroman