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Newton-Raphson didn't converge after 999 iterations, initial solution is too supersaturated


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I am wondering if anyone can help me successfully model the phosphate species present after Fe2+ addition. I am trying to model how much iron would need to be added to a lake system to precipitate the minerals vivanite and strengite. 

I used values based off of measurements from the USGS on the specific lake system I am trying to model, and I seem to keep getting errors as my model is not showing all 100 steps for sliding the amount of Fe2+ in the system. 

The errors I get when I run the model are: Newton-Raphson didn't converge after 999 iterations, max residual = 1, largest residuals: H+, H2O, SiO2(aq), HPO4, and the initial solution is too supersaturated. 

I have attached my code. Any help would be greatly appreciated!





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After taking a quick look at your input file, I have a couple of suggestions to help you get started. 

Activity is not the same as concentration. If you enter a bulk concentration for Fe++, the program will calculate the distribution of mass amongst dissolved species and precipitated minerals. Do you have a measured value? If you are simply titrating an amount of iron into the system, you can set the initial amount to be very low and then add in a Simple Aqueous reactant for Fe++ in the Reactants pane. Since you're adding a Fe++, you would also want to add an equivalent of anion that balances out the charge (e.g. Cl-). I would not suggest setting a sliding activity activity path. 

I noticed that you are using Cl- as your charge balancing species. In your case, most of your components are anions and therefore, you might consider using a cation as your balancing ion. 

I also noticed that you swapped out O2(aq) for NH3 and swapped in NO2- for NO3-. When this is done, the concentration for the nitrogen redox species is used to set the oxidation state of your system. If you wish to constrain the concentration for NO2- and NH3 separately, you can decouple the redox species (NO2- and NH4+) from its basis species (NO3-). After decoupling, you would be able to add NO2- and NH4+ directly. You can then swap in NH3 for NH4+ if you wish. For fore information redox disequilibrium, please see section 7.3 in the GWB Essentials User Guide.  

Note that the time variable on the Basis pane is used for reactions involving kinetic rates. The program only considers equilibrium reactions unless a kinetic reaction (mineral, aqueous complexation, etc) is specified in the Reactants pane. You can uncheck the time parameter and your model will output the same results. For more information on kinetic reactions, see chapter 4 in the GWB Reaction Model User Guide.

A tip for general troubleshooting is to always start simple and then add complexities. The first step is to check whether the initial chemical constraints for your system are allowing the software to solve the equilibrium state of your system before any reaction takes place. You can do this by selecting the Go initial option under the Run menu. 

Hope this helps,
Jia Wang
Aqueous Solutions LLC

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