Debye-Huckel model

[Alexey]

Hi! My question is about using extended Debye-Huckel equation. I guess that for calculation of activities in example 1 (http://wwwbrr.cr.usgs.gov/projects/GWC_coupled/phreeqc/html/final-70.html) this model is applied. But recalculating with hands the provided activities I found a notable divergence with activities calculated by PHREEQC.

I used the following equation:
log10(gamma)=-(ADH*sqrt(I))/(1+BDH*a*sqrt(I))+Bdot*I
at 25°C ADH=0.5114, BDH=0.3288, Bdot=0.041
in this example I=0.6748

For example,
for HCO3- (a=4) this equation provides -0.174 and PHREEQC - -0.170. OK, that is about the same.
for CO3-2 (a=4.5) I calculated -0.731 and PHREEQC - -0.681. The difference is remarkable.
for CO2,aq 0.028 and 0.067. It looks that the Bdot used in PHREEQC for CO2,aq is 0.1.

What is a reason of this?

[dlparkhurst]

I think the equation is the same as used in phreeqc.dat and wateq4f.dat, except there is a z^2 multiplier . The ion-size parameter, a, and the bdot parameter are defined in the database with the -gamma identifier. For phreeqc.dat, HCO3- and CO3-2 have ion-size parameters of 5.4 in these databases. Uncharged species get a bdot value of 0.1 by default; I think based on the work of Drummond (1981).

llnl.dat uses bdot for CO2 that is defined as a function of temperature, also from Drummond. Species defined with
-CO2_llnl_gamma will use this calculated bdot.

[Alexey]

Thanks a lot! Yes, I forgot to write z^2 in the question, but calculated with account of this. Where can I see the ion-size parameters for other species? I thought that 'a' is '-llnl_gamma' in llnl.dat

I just found this in the phreeqc.dat (http://www.phreeplot.org/ppihtml/phreeqc.dat.html)

I understand there are two different parameterizations of Debye-Huckel models in llnl.dat and phreeqc.dat. Can you share some comparison of their robustness at different mineralizations of solution? What is actually better for the case of seawater and solutions with I=1-2?

[dlparkhurst]

I can't say one is better or worse than the other. You can do the calculations and see how solubilities or SIs compare.

Above ionic strength of 1, I would trust the Pitzer calculations (pitzer.dat) more; so, you could compare llnl.dat or phreeqc.dat to pitzer.dat results for the system you are interested in, provided you can use elements that are included in the pitzer.dat database.