SURFACE and KINETICS Keywords

[Rozeran]

Hello everyone,

I am trying to model Cr(VI) reduction to Cr(III) with SURFACE ferrihydrite. The following kinetic reaction and rate expression will be used:

3/2 Fe(0) (s) + CrO4-2 + 5H+ (aq) -> 3/2 Fe2+ (aq) + Cr(OH)3 (s) + H2O(l)

dC/dt = -k1*C (first order kinetics)

So Fe(0) dissolves as Fe(3) and precipitates as ferrihydrite i.e. Hfo.  I want to use SURFACE "Hfo" as a kinetic reactant. In the minteq database there exists surface species for Hfo_s and Hfo_w with CrO4-2. Thats why I thought I dont have to define SURFACE_SPECIES or SURFACE MASTER SPECIES...

We can associate surfaces with kinetic reactants i.e. KINETICS, acc. to manual. But there is no example input for such action. At this point I am stuck. Do you happen to know an example input ?

[dlparkhurst]

You could kinetically generate Hfo, but here is an example that is a little more complete; hopefully it gives you something to start from.

Initially, there is 1 millimole of zero-valent iron that reacts by 1st order kinetics. As it reacts, Fe(0) is dissolved in solution that is in equilibrium with atmospheric oxygen. The iron is oxidized to Fe(3) and precipitates as goethite. Hfo_w and Hfo_s are related to the amount of goethite present. As goethite precipitates surface sites are created, and the surface sorbs Cr.  I am using Minteq.v4.dat for the database.

Code: [Select]

RATES
Fe(0)
-start
10 k = 0.1  # 1/s
20 rate = k * M
30 save rate * TIME
END
SOLUTION 1
pH 7
Cr(6) 1
Na 1 charge
END
KINETICS 1
Fe(0)
-m 1e-3
-formula Fe 1
-step 10 in 10
END
EQUILIBRIUM_PHASES 1
Goethite 0 0
O2(g) -0.7 10
END
SURFACE 1
Hfo_w Goethite equilibrium_phase 0.1 1e5
Hfo_s Goethite equilibrium_phase 0.01
END
USE solution 1
USE kinetics 1
USE equilibrium_phases 1
USE surface 1
USER_GRAPH 1
    -headings               time KIN_Fe(0) Sorbed_Cr
    -axis_titles            "Seconds" "Moles" "Moles"
    -initial_solutions      false
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
10 GRAPH_X TOTAL_TIME
20 GRAPH_Y KIN("Fe(0)")
30 GRAPH_SY SURF("Cr", "Hfo")
  -end
    -active                 true
END