Acid mine Drainage mixed with NaOH and CaO

[mleal]

Hello,

a treatment has been proposed for an acid mine drainage (AMD), which is going to be treated in a water treatment plant. We have to verify if the water after the proposed treatment will meet the water quality criteria.

Firstly, 30 mg/L of NaOH are going to be added to neutralize the water. After reaching a pH of around 5.5, 100 mg/L CaO (95% purity) is going to be added to precipitate some heavy metals.

After reading this discussion (https://wwwbrr.cr.usgs.gov/projects/GWC_coupled/phreeqc/mail/msg00481.html) I have decided to use the MIX option. Attach you can find the input file. I have use the minteq.v4.dat database.

I have run some tests changing the amount of added CaO but at the end I allways get the same final heavy metal concentrations.

I suppose I am doing something wrong, but I can't find it.

Please, could anyone help me with this? I apologize in advance, because it is surely a very basic question.

Thank you very much,

María

[dlparkhurst]

From your description, it may be better to use REACTION instead of MIX. With REACTION you can simply add NaOH, CaO, or both, without adding the water that would be associated with MIX. The units of REACTION are moles. The following would add 30 mg of NaOH to solution.

Code: [Select]

REACTION
NaOH 1
7.5e-4   # 30 mg / gram formula wt of NaOH

You have not defined any sources or sinks for your metals, so the metals are conservative through your mix reactions. You will have to consider which metal phases are likely to precipitate and include them in an EQUILIBRIUM_PHASES data block. Minerals with positive saturation indices are candidates, although some are not likely to form under your conditions.

You will need to be careful with interaction with the atmosphere. Loss or gain of CO2 from the atmosphere will affect your reactions. Also, the redox conditions will be important. The simplest approach is to assume equilibrium between your solutions and the atmosphere by adding CO2(g) (atmospheric log P(CO2) is -3.4) and O2(g) (atmospheric log P(O2) is -0.7), but equilibrium is not instantaneous.

[mleal]

Hello Parkhurst,

Thank you very much for your repply and for the information. I am going to write again the input file with your suggestions.

Have a nice day,