Single Stage Mine Water Treatment with Aeration

[Onur]

Hello everyone,
Previously, I asked how to model a pre-aerated system on PHREEQC. Now, I need to model reactions in a reactor, including mine water, aeration, H2O2 addition and alkali addition. I know the mine water composition pH, alkalinity, reactor's water composition, pH and alkalinity. What would be the best approach to model a single-stage reactor containing lime addition and aeration at the same time?
Mine water pCO2 is calculated as  -1.49 and, reactor water's pCO2 is calculated as -5.30 on PHREEQC. Most probably pCO2 -5.30 (since it is < atmospheric CO2), is related to calcite precipitation.

I prepare a model as follows:

Code: [Select]

EQUILIBRIUM_PHASES 2 
    CO2(g) -5.30
    O2(g) -0.67 # it is assumed that H2O2 addition will oxidize until atmospheric O2 equilibrium
Fix_H+   -9.11 Ca(OH)2 10
An extra question: Which database would better for modeling a circumneutral-wine water with Al (0.8 mg/l), As (2.41 mg/l), Fe (47 mg/l), Mn (6.40 mg/l) and pH 6.26? Minteq.v4 or wateq4f?

[dlparkhurst]

The low pCO2 indicates to me that there is little exchange between the atmosphere and your solution, so I would probably add a specified amount of H2O2 as a reaction and treat CO2 as a closed system (not included in equilibrium phases). The amount of calcite that precipitates could be some amount up to equilibrium with calcite. If you have your final water composition, check the calcite SI to determine how close to equilibrium your reactor approaches equilibrium. The simplest modeling approach is to assume calcite reaches equilibrium (included in EQUILIBRIUM_PHASES with 0 moles initially).

Note that addition of Ca(OH)2 will decrease the pCO2, whereas precipitation of calcite will raise the pCO2.

I'm partial to wateq4f.dat, which is the source of most of phreeqc.dat, but minteq.v4.dat shares some of the same thermodynamic data. I would make comparisons among databases (including llnl.dat).

[Onur]

Hello Sir, many thanks for your answer. I added H2O2 as a reaction and saved it the resultant solution as solution 2. Then, I added an equilibrium phase for fixing pH to 9.11, and the resultant pH was -4.14 (not -5.30 as calculated from the reactor's water). Is this acceptable or should I change something more to make it fit better?
Kind regards

[dlparkhurst]

It is your job to decide when you are satisfied.

Make sure you check the saturation indices to see if there are other minerals that you should consider (calcite, gypsum, and others).