Processes > Inverse modelling
Inverse modeling of Zn release in pure water
dlparkhurst:
OK, you made me look more carefully.
First, your units are mg/L, so is phosphorus reported as P or as PO4. Make sure you have the correct units.
Let's step back and look at your second analysis. The highest concentrations are Fe, P, and Al. It does not seem like apatite would be the source of the phosphorus, or else you would have large Ca concentrations. Is it possible that you have dissolved P-rich iron oxyhydroxides?
You will have to consider the source of the Al. Is it primary aluminosilicates, or amorphous aluminum oxides?
RadiantRocks23:
Thanks for checking. I confirm that the units are all in mg/L except N(5). I also confirm that is P concentration and not PO4 concentration.
I made a mistake for Ca concentration. It is way higher than what I initially wrote. It is associated with the high concentration of P. I do not have the F content yet but I estimated it from the formula of fluorapatite as Ca and P mostly result from the dissolution of apatite. It does not come from aluminosilicates given the lower concentration of Na, K, Al and Si. Al is indeed hosted in aluminosilicates; there are not amorphous aluminum oxides in my samples.
I added chrysotile and montmorillonite for an additional source of Mg and Fe (I wanted a Fe-rich amphibole, but I guess the combination of the two will be similar). I also added Illite because I have charge balance issue otherwise.
With the following code, I managed to obtain a solution with only Montmorillonite and Illite precipitating. It's better than the previous solution but I don't expect precipitation of such minerals. I have many elements in my input (12 elements) so I think I need at least as many phases in my model. This is why I added Illite but I don't really expect it, or in a very minor proportion only. I tried to play with the composition of solution 2 by adding much more Mg and Al for instance and check if Montmorillonite could dissolve, but it still precipicates.
I will try to play with the code but I don't have a solution so far.
--- Code: ---SOLUTION 1
temp 25
pH 0.5 charge
pe 4
redox pe
units mg/l
density 1 calc
Al 0
Mg 0
Ca 0
F 0
Fe 0
P 0
S 0
K 0
Na 0
Si 0
Zn 0
N(5) 0.31 mol/l
-water 0.01 # kg
SOLUTION 2
temp 25
pH 0.72 charge
pe 4
redox pe
units mg/l
density 1 calc
-water 0.01 # kg
Al 270
Mg 250
Ca 16000
F 2800
Fe 1100
P 8600
S 61
Zn 7
K 200
Na 160
Si 150
N(5) 0.2 mol/l
INVERSE_MODELING 1
-solutions 1 2
-uncertainty 0.025
-phases
Fluorapatite diss
Sphalerite diss
O2(g) diss
N2(g) precipitation
Pyrite diss
Albite diss
Anorthite diss
Quartz diss
Muscovite diss
Annite diss
Kaolinite diss
Chrysotile diss
Montmorillonite
Illite
-mineral_water true
-tolerance 1e-10
PHASES
Fluorapatite
Ca4.999Zn0.001P3O12F1 + 3H+ = 4.999Ca+2 + F- + 3HPO4-2 + 0.001Zn+2
-analytical_expression 29064.06 8.907085 -1192683 -11346.08 53046470 -0.003060213
-Vm 157.56 cm3/mol
Muscovite
KAl3Si3O10(OH)F + 9H+ + H2O = K+ + 3Al+3 + 3H4SiO4 + F-
log_k 12.99
delta_h -59.34 kcal
Annite
KFe3AlSi3O10(OH)2 + 10H+ = K+ + 3Fe+2 + Al+3 + 3H4SiO4
log_k 23.29
delta_h -65.72 kcal
Montmorillonite
Mg0.485Fe.22Al1.71Si3.81O10(OH)2 + 6.76H+ + 3.24H2O = 3.81H4SiO4 + 0.485Mg+2 + 0.22Fe+3 + 1.71Al+3
--- End code ---
dlparkhurst:
This is not worth doing without accurate analyses. If you don't know the F concentration, you can't do the exercise. If you charge balance you solution with pH, you get a pH of 7, so I am skeptical of your new Ca concentration.
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