Speciation model at varying pH fails to converge

[mbcomins]

Hello,

I am working on modeling Neodymium complexes with citrate at varied pH. My current model fails to converge to a numerical solution for my first solution, which is at pH 0. I have tried both charge balancing and not charge balancing. Hopefully someone can provide some insight as to how I might remedy this.

My database and input file are attached.

[dlparkhurst]

I think the main issue is that both Nd(OH)3(s) and Nd2O3(s) are pretty soluble in acid solutions. Further, Nd2O3(s) is many orders of magnitude more soluble than Nd(OH)3(s), which seems unlikely to me. Here is a simulation that attempts to equilibrate with the more stable of the two phases at pH 3, 7, and 10. Solubility is greater than 3 molal at pH 3. Also look at the saturation index of Nd2O3(s) in each case.

Code: [Select]

PHASES
Fix_H+
H+ = H+
log_k 0
END
SOLUTION 1
    temp      25
    pH        7
    pe        4
    redox     pe
    units     mol/l
    density   1
    Cit       0.01
    Cl        1  charge
    Na        1
    -water    1 # kg
END
USE solution 1
EQUILIBRIUM_PHASES
Fix_H+ -3 HCl 10
Nd2O3(s) 0 1
Nd(OH)3(s) 0 1
END
USE solution 1
EQUILIBRIUM_PHASES
Fix_H+ -7 HCl 10
Nd2O3(s) 0 1
Nd(OH)3(s) 0 1
END
USE solution 1
EQUILIBRIUM_PHASES
Fix_H+ -10 NaOH 10
Nd2O3(s) 0 1
Nd(OH)3(s) 0 1
END

[mbcomins]

I apologize, but I'm not sure I follow. I decided to try to use only the Nd(OH)3(s) phase, since Nd2O3 is not stable in aqueous solution. I also tried playing with a few KNOBS parameters, including tolerance, step size, etc. Even so, I keep getting the convergence error, with residual Nd of about 3.17e+01 and I'm not sure how to interpret this.

[dlparkhurst]

PHREEQC will fail at high concentrations. The solubility of Nd(OH)3 (given your log K) is several molal at pH < 6.5, and PHREEQC does not converge. In the attached file, the solubility of Nd(OH)3 is calculated as HCl is added to lower pH. The amount of Nd(OH)3 is limited to 3 moles to avoid the convergence problems that you find, but you can see that the solubility increases exponentially below pH 7. At low pH, the solubility of Nd(OH)3(s) is practically infinite, so solid Nd(OH)3(s) will not be stable under acid conditions.

Code: [Select]

SOLUTION 1
    temp      25
    pH        12
    pe        4
    redox     pe
    units     mol/l
    density   1
    Cit       0.01
    Cl        1  charge
    Na        1
    -water    1 # kg
END
USE solution 1
EQUILIBRIUM_PHASES
Nd(OH)3(s) 0 3
REACTION
HCl 1
10 in 200
USER_GRAPH 1
    -axis_titles            "pH" "Nd, mol/kg water" ""
    -initial_solutions      false
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
10 GRAPH_X -LA("H+")
20 GRAPH_Y TOT("Nd")
  -end
    -active                 true
END