How to prevent a mineral to precipitate?

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I could not find that exact string, but it looks like the way that PHREEQC sequentially updates the master variables to arrive at a solution to the non-linear equations. You definitely don't want to change reset and that basic part of the numerical method. If the program does not converge, that is one thing (and I will lood at it), but if the program does converge, then the results should be the correct solution to the non-linear equations. If the solution is incorrect, please let me know.

HSO4- is orders of magnitude smaller than SO4-2 in the printout that you showed, so its presence should not affect the result significantly.

If you want to reduce HSO4- even further, you can decrease the log K for the reaction. Including the following in your input file will effectively eliminate the effect of HSO4-:

--- Code: ---SO4-2 + H+ = HSO4-
#log_k    1.979
        log_k -20
#delta_h 4.91    kcal
#-analytic   -5.3585   0.0183412   557.2461
-dw 1.33e-9
-Vm 8.2 9.2590   2.1108   -3.1618 1.1748  0 -0.3 15 0 1 # ref. 1

--- End code ---

If you are trying to reproduce results from your code, I don't think the problem is in HSO4-.  I think it is more likely that the Pitzer coefficients are different, or the calculation of non-symmetric mixing factors or some other calculations are different. To debug that, I would probably start with a pure Na2SO4 solution, setting all the PITZER parameters involving SO4-2 in that solution to zero in a PITZER data block in the input file. I would do the same for your code. You can then add parameters to your code and PHREEQC to see where the calculations diverge.


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