modeling transport at nanometric scale?

[mapi]

Hi,
I was wondering if it is possible to simulate diffusion at nanometric scale with PHREEQC.
I am trying to dissolve ACC until it reaches the equilibrium, only at the beginning, and then, have diffusion of Ca and CO3 and precipitation of calcite in the last cell. I could do it when the cell size was bigger but when I try to simulate with cells of nanometers I can't. Is there a way to do this?
Thank you!
Maria


PHASES
ACC
    CaCO3:1.2H2O = CO3-2 + Ca+2 + 1.2H2O
    log_k     -6.3


SOLUTION 0
    temp      25
    pH        7
    pe        4
    redox     pe
    units     mmol/kgw
    density   1
    -water    1 # kg

SOLUTION 1-200
    temp      25
    pH        7
    pe        4
    redox     pe
    units     mmol/kgw
    density   1
    -water    1 # kg

EQUILIBRIUM_PHASES 1
    ACC       0 10


RATES 200
Calcite #precipitation
-start
10 si_cc = SI("Calcite")
15 if (si_cc <= 0) then goto 200
20 rate = 10^-7*(si_cc-1)
30 moles = rate * TIME
200 SAVE moles
-end

KINETICS 200
Calcite
-formula Ca -1  CO3 -1
-m        1
-m0       1
-tol 1.0e-8

INCREMENTAL_REACTIONS true

TRANSPORT 2-200
    -cells                 200
    -shifts                100
    -time_step             43200
    -flow_direction        diffusion_only
    -lengths               200*1e-8 
    -correct_disp          true
    -diffusion_coefficient 1.3e-09
    -print_cells           5-200
    -print_frequency       5
    -punch_cells           5-200
    -punch_frequency       5

END

[dlparkhurst]

Thought I had answered this before, but here it is again.

No, PHREEQC will not be able to run with that set of cell sizes and time step. The method of PHREEQC uses an explicit mixing method to determine a number of steps to accurately solve the diffusion problem. Your set of parameters requires 1e12 mixing calculations an simply will not run in a reasonable amount of time.

However, I think there are simpler simulations that would be more instructive. At that length scale, diffusion is very fast. Below is a modified version of your file (hopefully correct) that runs diffusion for 4e-3 seconds. With closed boundary conditions, the concentration in the column is virtually uniform. The rate of calcite precipitation is slow relative to this time scale. So, I think a batch simulation with equilibrium with ACC and kinetic precipitation of calcite would give the same information and the simulation would run much more quickly.

Code: [Select]

PHASES
ACC
    CaCO3:1.2H2O = CO3-2 + Ca+2 + 1.2H2O
    log_k     -6.3

RATES
Calcite #precipitation
-start
10 si_cc = SI("Calcite")
15 if (si_cc <= 0) then goto 200
20 rate = 10^-7*(si_cc-1)
30 moles = rate * TIME
200 SAVE moles
-end
END

SOLUTION 1-10
    temp      25
    pH        7
    pe        4
    redox     pe
    units     mmol/kgw
    density   1
    -water    1 # kg
END
EQUILIBRIUM_PHASES 1
    ACC       0 10

KINETICS 10
Calcite
-formula Ca -1  CO3 -1
-m        1
-m0       1
-tol 1.0e-8

INCREMENTAL_REACTIONS true

TRANSPORT
    -cells                 10
    -shifts                1
    -time_step             4.320e-3
    -flow_direction        diffusion_only
    -lengths               0.2e-6 #200*1e-8 
    -correct_disp          true
    -diffusion_coefficient 1.3e-09
USER_GRAPH 1
    -headings               distance ACC Calcite
    -axis_titles            "Distance" "Saturation Index" ""
    -initial_solutions      false
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
10 GRAPH_X dist
20 GRAPH_Y SI("ACC"), SI("Calcite")
  -end
    -active                 true
END