Tom Meuzelaar Posted May 13, 2008 Share Posted May 13, 2008 [admin notice: the below is from the former GWB users group email distribution list. This message was originally posted 12/12/2005] Posted by: Sherry Samson My question concerns carbonate in high pH solutions. The example solution is from a flow-through mineral dissolution experiment and model input consists of the pH measured at 25C, the measured cations, and the estimated concentrations of F (from the mineral stoichiometry) and Na (estimated from titration of the experiment inlet solution: approximately 1 mL of 10 N NaOH in 1 L of DI water). The only available anion other than F is OH. My objectives are to calculate pH at the experimental temperature (70C) and determine saturation indices. I am using GWB version 4.0.3 of React with Windows XP Professional and the thermo.dat database. In the initial run (scripts for each scenario attached), I fixed pH at 25C and attempted to charge balance on Na (the species with the largest concentration and uncertainty). The system will not converge (“Residuals too large”). The same result is obtained when attempting to balance on any other species. I then ran a script with the charge balance off. Convergence was achieved with a final pH at 70C of 10.741, but there was a charge imbalance of -2.52E-05 faradays. I then tried balancing on H. The system converged with charge balanced and similar results for pH (11.949 at 25C and 10.740 at 70C). The problem arises in attempting to introduce CO2 into the system. The carbonate content of the samples was not analyzed, but the solutions are in equilibrium with the atmosphere so CO2 should be included. I ran two scripts with CO2(g) added: one with the initial log fugacity at -3.5, and another with the log fugacity fixed at -3.5. The only constraint on pH is I know the measured pH at 25C is 11.95 and here in each case the 25C pH was computed as 9.193 so we know this isn’t correct (final pH was 8.943 or 9.404, respectively). If my only objective was to calculate pH, it would appear that simply balancing on H (and omitting CO2) gives reasonable results, but CO2 also affects the calculation of the saturation indices. In this particular example, all the secondary phases of most interest (amorphous Si, gibbsite, Fe(OH)3, and brucite) are undersaturated in all scenarios, but the saturation index varies from 1 to 3 orders of magnitude so this may not hold true in other experiments. Although the initial solutions when prepared are in equilibrium with the atmosphere at 25C, both the inlet solutions and the water bath containing the dissolution reactors are heated to 70C so the scenario fixing fugacity was not appropriate. Just setting the initial fugacity, however, also doesn’t give logical results as the measured 25C pH cannot be duplicated and I am quite confident in that measurement. The samples are allowed to cool (in capped test tubes) to room temperature before pH is measured and some CO2(g) is absorbed by the solutions during the course of the pH measurement as evidenced by a continual downward drift in pH for samples from experiments at lower pH (e.g., < 9), but no drift is observed with samples this high in pH. The fact that this experiment was done at 70C introduces some complications, but the same problems are encountered in attempting to model experiments that were done at 25C (example of pH 12 at 25C attached), so high pH is the common factor, not the temperature. Any suggestions anyone may have about handling this CO2 problem will be appreciated. I’m also wondering why charge balancing on Na was unsuccessful. Sherry D. Samson Posted by: Craig Bethke Hi Sherry, If your fluid is of pH 12, you can be sure it is not in equilibrium with CO2 in the atmosphere. Whatever CO2 has dissolved into the fluid has reacted to form CO3--. The reason you can't change balance on Na+ is that you have complete information about the cations in solution, but not the anions. To attempt a charge balance on the fluid, which is overbalanced with positive ions, the program drove the Na+ concentration negative, preventing it from converging. Theoretically, you can solve this problem by constraining the concentration of CO3-- by charge balance. In doing so, however, you should make sure you the charge balancing reflects reality and not simply the sum of errors in analyzing for the other components. Hope this helps, Craig Posted by: Sherry Samson Dear Craig: Thank you for your reply. Although it’s the carbonate anion we lack information on, because of speciation at this pH the charge imbalance is -2.21E-5 faradays which means there was actually an excess of negative charge, correct? If so, why would the system reduce the Na concentration to a negative value? Wouldn’t it increase it? And since the imbalance represents only 0.2% of the Na concentration, wouldn’t this adjustment be within the tolerances of the system (i.e., not grossly unbalanced)? I couldn’t get the system to charge balance on carbonate. Were you able to achieve this with the script that I sent? I tried another approach which appears to give reasonable results. I used just a pure water system and specified the CO2(g) log fugacity as -3.5 and temp as 70C. This gave me a total carbonate concentration of 6.4E-6 molal. I ran my original script with the initial CO3 concentration set to this value and balanced on Na. The system converged and the charge balance was successful (Na concentration adjusted from 0.01 to 0.01004). The pH went from 11.95 at 25C to 10.741 at 70C. Initially I had tried setting the CO3 concentration to the equilibrium value at 25C, but because it’s treated as a closed system, CO2 isn’t allowed to exsolve and the total carbonate concentration remained the same at 70C. Is there another way to handle this that allows for exsolution? Thank you for your help. Sherry Posted by: Craig Bethke Hi Sherry, I looked at you input files a little more and you are correct that your run is not overbalanced with respect to cations. Sorry for the false lead. Your run is unusual in that it contains no significant anions in the basis (OH- is in the system, but represented in the basis by H+). This makes it hard for the program to converge. There are two ways to help it along, both of which solve the problem: (1) Swap OH- into the basis in the place of H+, and instead of setting pH to 12, set the log activity of OH- to -2. This helps convergence because H+, a weak basis entry due to its very low concentration (~10^-12 molal), is replaced by OH-, the dominant anion and hence a strong entry. (2) Constrain water in terms of free kg, rather than total kg. This eliminates an unknown value from the numerical solution and lets the program converge. There is a flaw we just noticed in Releases 4.0 to 6.0 of the routine that saves the model configuration: the program saves the water constraint in total kg, even if it has not been set and should default to free kg. This issue is pretty much a formality, since the two values are nearly identical. The calculation results aren't affected, but in your case it did make it a little harder for the program to converge; the routine will be corrected in the next maintenance release of the software. Hope this helps, Craig Quote Link to comment Share on other sites More sharing options...
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