Metal precipitations by adding NaOH

[Benkaito]

I am trying to verify lab results on PHREEQCi but the output I am getting, I think, is wrong (the graphs I get for plotting metal concentrations vs pH don't make sense at it). Can someone tell me where I went wrong? I am new to PHREEQCi so the solution I am looking for might be basic.

This is the problem I am trying to solve: "The samples of 500 mL AMD were sequentially neutralized by NaOH under continuous stirring and pH measuring. The resulting precipitate was filtered through a filter funnel with the frit and the filtrate was used for further neutralization. The procedure was repeated at gradually increasing pH values from 4.05 to 11.95 (4.05, 4.26, 4.49, 4.79, 6.11, 7.23, 8.11, 8.97, 9.98, 10.5, 10.99, and 11.95). Afterwards, the precipitate was dissolved in the filter funnel with 10 mL of 10% HCl, the frit was washed with distilled water and the solution was filled up to 200 mL by H2O. Such prepared filtrates were used to determine the concentrations of total Fe, Cu, Mn, Al and Zn with colorimeter DR 890".

This is what I did:

SOLUTION 1
    temp      25
    pH        3.78
    pe        4
    redox     pe
    units     mg/l
    density   1
    Fe        351
    Cu        1.4
    Al        43.2
    Zn        5.5
    Mn        21.8
    -water    1 # kg

EQUILIBRIUM_PHASES 1
    CO2(g)    -3.5 10

SAVE solution 1
END

SOLUTION 2
    temp      25
    pH        7 charge
    pe        4
    redox     pe
    units     mol/l
    density   1
    Na        1.4
    -water    1 # kg

SAVE solution 2
END

SELECTED_OUTPUT
    -file                 selected.output.xls
    -ph                   true
    -totals               Fe  Fe(2)  Fe(3)  Al  Mn  Zn  Cu
MIX 1
    1    0.9999
    2    0.0001
SAVE solution 3
END

MIX 2
    3    0.5
    2    0.015
SAVE solution 4
END

MIX 3
    4    0.9999
    2    0.0001
SAVE solution 5
END
... (mixing goes on until mix 110)
MIX 110
    111    0.9999
    2    0.0001
SAVE solution 112
END

[dlparkhurst]

In MIX 2, you only retain half of the previous solution, so I am not sure about that. Also, you have none of the hydroxide minerals in EQUILIBRIUM_PHASES, so there is no opportunity to precipitate minerals.

As I understand your experiment, here is the way I would approach it. It adds base to the pH that you measured. The phase Fix_H+ is used to set the log activity of H+. The reaction used in the EQUILIBRIUM_PHASES definition is the numberof moles of elements in a liter of 1.4 N NaOH with the assumption that a liter of solution contains a kilogram of water. This could be done more accurately, but considering other uncertainties, it is probably close enough. 1e-3 of the reaction is equal  to a 1-mL addition.

I have assumed gibbsite and goethite could form. There are major uncertainties because it is not clear that mineral equilibria apply in the time frame of your experiment, nor which minerals and corresponding equilibrium constants apply. I leave it up to you to decide how you want to approach these difficulties.

Code: [Select]

PHASES
Fix_H+
H+ = H+
log_k 0
END
SOLUTION 1
    temp      25
    pH        3.78
    pe        4
    redox     pe
    units     mg/l
    density   1
    Fe        351
    Cu        1.4
    Al        43.2
    Zn        5.5
    Mn        21.8
    -water    1 # kg
EQUILIBRIUM_PHASES 1
    CO2(g)    -3.5 10
SAVE solution 1
END
USE solution 1
EQUILIBRIUM_PHASES
    CO2(g)    -3.4 10
    Fix_H+    -4.05  Na1.4(OH)1.4(H2O)55.5 10
    Goethite  0 0
    Gibbsite  0 0
SAVE solution 2
END


[Benkaito]

Thank you so much. Mix 2 was supposed to be at the same ratio too as mix 1, my apologies. I am having issues generating ion concentrations at different pH levels after incorporating your suggestions as follow:

Code: [Select]


PHASES
Fix_H+
    H+ = H+
    log_k     0

END

SOLUTION 1
    temp      25
    pH        3.78
    pe        4
    redox     pe
    units     mg/l
    density   1
    Fe        351
    Cu        1.4
    Al        43.2
    Zn        5.5
    Mn        21.8
    -water    1 # kg
EQUILIBRIUM_PHASES 1
    CO2(g)    -3.4 10
SAVE solution 1

END

USE solution 1

EQUILIBRIUM_PHASES
    CO2(g)    -3.4 10
    Fix_H+    -4.05 Na1.4(OH)1.4(H2O)55.5 10
    Goethite  0 0
    Gibbsite  0 0

SAVE solution 2

END

SELECTED_OUTPUT
    -file                 selected.output.xls
    -ph                   true
    -totals               Fe  Fe(2)  Fe(3)  Al  Mn  Zn  Cu
MIX 1
    1    0.9999
    2    0.0001
SAVE solution 3
END

MIX 2
    3    0.9999
    2    0.0001
SAVE solution 4
END
...

The above code gives me ion concentrations at only 1 pH value (pH of about 3.78) for all simulations. About major uncertainties, I probably have to make some assumptions in relation to them. They investigators did the experiment and verified its results with visual minteq 3 so I thought I could do the same thing with PHREEQCi based on the numbers and conditions outlined in Petrilakova et al (2014).

[dlparkhurst]

I intended the next steps to be similar to the first step. By SAVEing solution 2, you are retaining only the solutes, with some solids sequestered in the equilibrium phases. The next step would use solution 2, have a new EQUILIBRIUM_PHASES definition with the new pH and no initial precipitates.

Code: [Select]

PHASES
Fix_H+
H+ = H+
log_k 0
END
SOLUTION 1
    temp      25
    pH        3.78
    pe        4
    redox     pe
    units     mg/l
    density   1
    Fe        351
    Cu        1.4
    Al        43.2
    Zn        5.5
    Mn        21.8
    -water    1 # kg
EQUILIBRIUM_PHASES 1
    CO2(g)    -3.5 10
SAVE solution 1
END
USE solution 1
EQUILIBRIUM_PHASES 1
    CO2(g)    -3.4 10
    Fix_H+    -4.05  Na1.4(OH)1.4(H2O)55.5 10
    Goethite  0 0
    Gibbsite  0 0
SAVE solution 2
END
USE solution 2
EQUILIBRIUM_PHASES 2
    CO2(g)    -3.4 10
    Fix_H+    -4.26  Na1.4(OH)1.4(H2O)55.5 10
    Goethite  0 0
    Gibbsite  0 0
SAVE solution 3
END
...


[Benkaito]

It has worked. THANK YOU!!!