Fix pH during transport

[llanduyt]

Dear,

In the framework of my master thesis, I am simulating the transport of Sb in river systems, taking into account it's pH dependent sorption behaviour. I would like to simulate some general scenarios to demonstrate the influence of pH.

To fix the pH value, I have been using the PHASE Fix_H+, for which I then defined a saturation ratio using the EQUILIBRIUM_PHASES data block. This is working fine for some of my scenarios, but when I for example model a pulse input of Sb at pH 7 I am still observing quite some pH changes (with values ranges between 6.1 and 8.7 when I fix the pH to 7). Since the sorption behaviour I am modelling is very pH-sensitive, these are quite significant changes. Thus I was wondering if I am making mistakes in the syntax (e.g. with the SAVE statements) or if this is just the numerical limit of the system, which is very sensitive to pH-changes at pH 7 when only a background electrolyte of 0.01 M is present.

Thank you in advance for any help or clarifications.

Most sincerely,

Lisa

The sorption reactions I implemented:
 Hfo_sOH + Sb(OH)6- + H+ = Hfo_sSbO(OH)4 + 2H2O; log_k 4.4779
 Hfo_wOH + Sb(OH)6- + H+ = Hfo_wSbO(OH)4 + 2H2O; log_k 4.4779
 Hfo_sOH + Sb(OH)6- = Hfo_sOHSbO(OH)4- + H2O; log_k 3.7350
 Hfo_wOH + Sb(OH)6- = Hfo_wOHSbO(OH)4- + H2O; log_k 3.7350
 Hfo_sOH + Sb(OH)3 = Hfo_sSbO(OH)2 + H2O; log_k   4.837
 Hfo_wOH + Sb(OH)3 = Hfo_wSbO(OH)2 + H2O; log_k   4.837

My script:
database C:\phreeqc\database\adapted_minteq.dat

PHASES
Fix_H+
  H+ = H+
  log_k 0.

KNOBS
-tol 1e-20

SOLUTION 0
pH 7
Na 10
Cl 10
Sb 1E-01 # 10^-3 mmol = 1 µmol
EQUILIBRIUM_PHASES
Fix_H+ -7 NaOH 
   -force_equality true
O2(g)   -1.736386
   -force_equality true
SAVE SOLUTION 0
END

SOLUTION 1-40
Na 10
Cl 10
pH 7
N(5) 1E-12 charge
EQUILIBRIUM_PHASES
Fix_H+ -7 NaOH 
   -force_equality true
O2(g)   -1.736386
   -force_equality true
SURFACE 1-40
Hfo_s      5.9533E-05   600 1.057901145
Hfo_w      0.002381319
SAVE SOLUTION 1-40
SAVE SURFACE 1-40
END

SELECTED_OUTPUT
 -file advection_hfo_pH7_O2-2_pulse_chargebalance.sel
 -reset false
 -step
 -high_precision true

USER_PUNCH
 -headings timing Sb_diss Sb_sorb Sb_tot pe pH
 -start
10 microSb=tot("Sb")*1e+06
20 sorbedSb=SURF("Sb", "Hfo")*1e+06
30 totSb=SYS("Sb")*1e+06
40 PUNCH total_time/3600
50 PUNCH microSb
60 PUNCH sorbedSb
70 PUNCH totSb
90 PUNCH -la("e-")
100 PUNCH -la("H+")
-end

TRANSPORT
 -cells       40
 -shifts       550
 -time_step      312.5
 -flow_direction     forward
 -boundary_conditions    flux # constant flux
 -lengths       250
 -initial_time       0
 -dispersivities    0
 -diffusion_coefficient 0
 -stagnant       0
 -print_cells      1-40
 -print_frequency   1
 -punch_cells      1-40
 -punch_frequency   1
 END
 
SOLUTION 0
pH 7
Na 10
Cl 10
N(5) 1E-12 charge
EQUILIBRIUM_PHASES
Fix_H+ -7 NaOH 
   -force_equality true
O2(g)   -1.736386
   -force_equality true
SAVE SOLUTION 0
END

TRANSPORT
 -cells       40
 -shifts       4000
 -time_step      312.5
 -flow_direction     forward
 -boundary_conditions    flux # constant flux
 -lengths       250
 -dispersivities    0
 -diffusion_coefficient 0
 -stagnant       0 #
 -print_cells      1-40
 -print_frequency   1
 -punch_cells      1-40
 -punch_frequency   1
 END

[dlparkhurst]

You have defined only EQUILIBRIUM_PHASES 1 (default when no number is given), so the pH is required to be fixed only in cell 1. Use EQUILIBRIUM_PHASES 1-40 to define Fix_pH in all 40 cells.

(Perhaps fixing the pH is appropriate for your calculations, but I prefer to let the reactions determine the pH.)

[llanduyt]

A stupid negligence, thank you so much for the quick help! (& for all the other tips you have given, they are very much appreciated.)

I am aware that what I am doing is quite artificial, but in my case I am lacking the necessary data to make the calculations more natural.

Lisa