calculation of k_qu

[Hydroman]

Hello community,

I'm interested in the kinetic reaction of quartz dissolution in general and especially in the temperature dependency of the reaction. I already read the chapter "Kinetics of geochemical processes (4.6)" in "Geochemistry, Groundwater an Pollution, 2nd Edition" and calculated the given example 4.15.

1. Problem: My first problem is the value for k_qu. In the example the value of k_qu is 10^-13.7 mol/m²/s. I've tried to recalculate the value with the formula published in the paper from Rimstidt and Barnes (1980) [log k=1.174 - 2.028*0.001*298.15 - 4158/298.15]. The result of this calculation is logk=-13.38 and this is probobly wrong if I compare that with the value of logk_qu=10^-13.7.

2. Problem: My second problem is the calculation of the temperature dependency. In my opinion it should be possible to calculate logk_qu, for an other temperature, let's say 50°C with the formula of Rimstidt and Barnes. Or is ist better to calculate it with the Arrhenius equastion (Formula 4.62 in Geochemistry, Groundwater an Pollution, 2nd Edition)? In that case I have no idea how to to calculate the pre-exponential factor A and the activation energy Ea.

My third problem is more a question. Does anbody knows whether it's possible to calculate the kinetic effects in combination with the keyword "reaction_pressure"? If yes, is it necessary to calculate k_qu with an other formula?

Mybe some of you had the same problems with the kinetic reactions. I would be happy if I could unterstand the calculation.

Thanks and many greetings from germany.

[dlparkhurst]

1. I agree that Rimstidt and Barnes' formula should give -13.3766. Perhaps it is simply a typo in A&P, but the Ea (90) is also different than 67-76 given in their paper. You could look in Palandri and Kharaka, who synthesize rate expressions for many minerals. They give an activation energy of 90.9.

2. You could use the Rimstidt and Barnes formula for temperature dependence. Palandri and Kharakha give a "pre-exponential" factor A for Aexp(-E/RT), the log K (-13.4), and the activation energy (90.9).

If you want to consider pressure, then you can get the pressure corrected log K by using the Basic function LK_PHASE (phreeqc.dat, Amm.dat, and pitzer.dat consider pressure). You would also have to consider how you want to adjust the rate constant for the rate expression that you use. You may be able to find something in the literature, but I can't give you any direction on how pressure would be incorporated into the parameters of the rate expression.

[Hydroman]

 thank you for the rapid and efficient response.