[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: quick question re: PHREEQ




> I'm a postdoc with Patricia Dove at Virginia Tech.  I'm studying growth
of
calcite at different temperatures in the AFM.  I worked out my recipes for
supersaturated solutions with Geochemist's Workbench, but plugged in
solubility constants from Plummer and Busenberg.

> Niel Plummer suggested running the recipes in PHREEQ to compare results,
and
I've found that the main difference is that GWB predicts equilibrium
concentrations of CaCl+ that are on the order of the concentration of Ca++,

while PHREEQ does not include CaCl+ as a species at all.  This makes a
large
difference in supersaturation log(aCa*aCO3/Ksp).  Niel thinks the CaCl+
association is very weak.  He suggested looking at some other versions of
PHREEQ that include Pitzer parameters, but I am unable to find these on the

web.  I don't think I should need Pitzer equations, though, because my
solutions have ionic strength of ~0.1.

It wouldn't hurt to use the Pitzer approach for a third opinion. I believe
GWB allows calculation with the Pitzer equations. PHRQPITZ is also a Pitzer
model, although it is rather old and hard to use. It can be found at the
web address given below. I think the Pitzer calculations should be
consistent with little or no CaCl+ pairing in an ion association model, but
it is worth a check.

> Do you know where I might get good data for the CaCl+ complex?  I have no
idea
where the data come from in GWB, but it seems to think CaCl+ has even
higher
activities than Ca++.

I looked in a version of the EQ3/6 database, which has the same numbers as
used in llnl.dat, the PHREEQC database that was translated from the GWB
database. The EQ3/6 database gives the following reference for the aqueous
species CaCl+ and CaCl2. It also indicates that further extrapolation was
done using the techniques of SUPCRT, which I assume is for temperature
dependence.

94sve Sverjensky, D.A., 1994, Thermodynamic properties of aqueous inorganic
metal
complexes:  Geochim. Cosmochim. Acta (in prep.), .

David


David Parkhurst (dlpark@xxxxxxxx)
U.S. Geological Survey
Box 25046, MS 413
Denver Federal Center
Denver, CO 80225

Project web page: https://wwwbrr.cr.usgs.gov/projects/GWC_coupled




Project Home Page
Complete Water Resources Division Software
USGS Home Page
Water Resources Division Home Page
NRP Home Page
Help Page
USGS Privacy Statement       

Please note that some U.S. Geological Survey (USGS) information accessed through this page may be preliminary in nature and presented prior to final review and approval by the Director of the USGS. This information is provided with the understanding that it is not guaranteed to be correct or complete and conclusions drawn from such information are the sole responsibility of the user.

Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government. 


The URL of this page is: https://wwwbrr.cr.usgs.gov/projects/GWC_coupled/phreeqc/mail/msg00447.html
Email:dlpark@usgs.gov
Last modified: $Date: 2005-09-13 21:04:21 -0600 (Tue, 13 Sep 2005) $
Visitor number 1934 since Jan 22, 1998.