Concentration of Aluminum in water after coagulation using Alum

[DAVID21]

Hello everyone

I'm trying to model coagulation.
Adding 1mmol of alum per kgw and I looking to find the concentration of Al in the water after the process, in a certain pH range and in mg/L units.

I wrote this code but the results come out very high.
I would appreciate any help.

Code: [Select]

PHASES
     Fix_pH
    H+ = H+
    log_k     0

SOLUTION 1   
units mg/l
pH       6.84
Temp 22.3
O(0) 5.03
pe      2.015
Na 110
K 3.7
Ca 155
Mg 47
Si 24  as SiO2
Cl 320
N(5) 36
S(6) 41
Alkalinity 322 as HCO3
B 0.127
Sr 1.12
SAVE SOLUTION 1
END

USE SOLUTION  1
REACTION 1
Al2(SO4)3 0.001
SAVE SOLUTION 2
END

USER_GRAPH 1
-headings  pH Al
-connect_simulations true
-initial_solutions   false
-chart_title  "Al concentration in water after adding 1 mmol/kgw of Alum"
-axis_titles  "pH"  "Al [mg/L]"
-axis_scale x_axis 2 11 0.25
#-axis_scale y_axis  0 3 0.1
#-axis_scale sy_axis  min  max  unit
-start
10 GRAPH_X -LA("H+") 
20 GRAPH_Y TOT("Al")*GFW("Al")*1000
USE SOLUTION 2   # The reagent (Ca(OH)2 or HCl)
EQUILIBRIUM_PHASES 1
    Fix_pH    -11 Ca(OH)2 10
Calcite  0  0
Al(OH)3(a) 0 0     
END
USE SOLUTION 2
EQUILIBRIUM_PHASES 2
    Fix_pH    -10.75 Ca(OH)2 10
    Calcite  0  0
Al(OH)3(a) 0 0
END

[dlparkhurst]

Just a couple of comments. There are a series of aluminum oxyhydroxide phases of varying stability. Al(OH)3 is on the soluble end (it is undersaturated at pH 11). You might pick gibbsite as a more stable phase and assume the precipitating phase is somewhere in between depending on the reaction conditions and length of time.

The script below shows the pH dependence of Al(OH)3 solubility. The gibbsite curve will be similar, except a log unit or two lower. As you can see, solubilities are higher at low pH and high pH with a minimum at neutral pH. Avoid high pH if you want lower concentrations of Al.

[DAVID21]

Problem solved.

Thank you very much for the help!