> We're using PHREEQCI for speciation modelling. When we give the program an input solution and then perform a batch reaction the program automatically adjusts the pe in the resultant solution. Presumably this is to maintain charge balance. Is there a way to over-ride this in order for the resultant solution to have a pe which we specify even if there will be a charge imbalance? PHREEQC does not easily allow fixed activities of individual species. In part, this is by design because maintaining a fixed pe in your case, while allowing pH to vary can move the solution out of the stability field of water. You can also implicitly be requiring many moles of mass transfer to maintain a fixed activity. Usually, the change in pe from reaction is telling you something. However, although you can not simply specify a constant pe, you can define a phase that reacts until you get the pe that you want. Thus, you have to explicitly add something to solution to adjust the pe. The following is a bit artificial, but organic matter reacts with water, the reaction would normally produce methane, CO2, and a very low pe. The phase "Fix_pe" is defined in PHASES and required to equilibrate with the solution with EQUILIBRIUM_PHASES. However, the definition in EQUILIBRIUM_PHASES specifies that oxygen (O2) be added or removed from the solution until the saturation index of -7 is reached for the mineral. This combination of definitions causes oxygen to be added until the pe is 7.0. The oxygen allows all the carbon to be CO2 rather than half as methane. David SOLUTION 1 pe 7 REACTION CH2O 1 1 mmol EQUILIBRIUM_PHASES Fix_pe -7 O2 PHASES Fix_pe e- = e- log_k 0.0 END 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
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