kinetic rate

[Selene Eliana Patani]

Hi everybody,
I'm dealing with this kind of kinetic for the dissolution or precipitation of a mineral rate=constant*(1-SR) where SR is the saturation ratio of the mineral considered. I'm using this kinetic for several types of minerals but I'm having problems with pyrite in certain conditions. Infact even if I obtain a SR very close to zero(so the mineral is dissolving) I have the message that Mn moles in solution are negative: instead I expect that them are increasing because the mineral is dissolving. Can anyone help me plese? Thank you very much!! I attach the code.
Bye!!

[dlparkhurst]

The message about negative moles is not fatal. It is a warning that the program may be having trouble with the integration, but it continues with a smaller time step.

The fatal warning is too many Runge Kutta time substeps, which means it has tried 500 times (by default) to attempt to integrate one of the time intervals. This error does indicate that the RK method is having difficulty and has indeed failed (with the specified maximum time steps). Usually, increasing the number of time steps allowed is not the solution. The problem is usually that the set of rates equations is "stiff", and the explicit Runge Kutta method is not going to work. The solution is to use an implicit method. With PHREEQC, that means including -cvode in the KINETICS definitions. CVODE is an implicit method that works better, although more slowly, than the RK method for stiff sets of equations.

[Selene Eliana Patani]

Thank you very much!! It works now :))