Sequential kinetic reactions

[peterwadeuk]

Greetings good people.

I am attempting to reproduce experimental data on a silica system.

Silicic acid in solution (H4SiO4) converts at a rate proportional to the fourth power of its concentration, to a nanocolloidal form, and then the nanocolloidal form converts to a precipitated form. All have the same stoichiometry of silica to oxygen = 1:2.

I am failing to obtain a concentration for the precipitated form of silica.

I request guidance.

I reproduce the input file below.

All the best, Peter

DATABASE C:\Program Files\USGS\Phreeqc Interactive 2.18.5570\database\wateq4f.dat
TITLE Kinetic formation and dissolution of silica nanocolloid cf Conrad et al. 71 (2007) 531-542.
#     CSTR

PHASES
SiO2_nano
    H4SiO4 = H4SiO4
    log_k     -25
SiO2_ppt
    H4SiO4 = H4SiO4
    log_k     -25


SOLUTION 1
    temp      25
    pH        6
    pe        4
    redox     pe
    units     mmol/kgw
    density   1
    -water    1 # kg
    Si  12.5
    Na 10
    Cl 10 charge

RATES
    SiO2_nano
-start
30 k1 = PARM(1)
40 SiO2_mono = MOL("H4SiO4")
200 rate = k1 * SiO2_mono^4
210 moles = -rate*TIME
220 SAVE moles
-end
    SiO2_ppt
-start
10 k2 = PARM(1)
20 sio2nano = KIN("SiO2_nano")
40 rate = k2 * sio2nano
50 moles = rate*TIME
60 SAVE moles
-end

KINETICS 1
SiO2_nano
    -formula  SiO2  1
    -m        0
    -m0       0
    -parms    35.8
    -tol      1e-008
SiO2_ppt
    -formula  SiO2  1
    -m        0
    -m0       0
    -parms    1.15e-002
    -tol      1e-008
-steps       330000 in 3 steps # seconds
#-steps       11000000 in 100 steps # seconds
-step_divide 1
-runge_kutta 3
-bad_step_max 500

SELECTED_OUTPUT
    -file                 170403_2221_nanocolloid_ppt_diss.tsv
    -totals               Si
    -molalities           H4SiO4
    -kinetic_reactants    SiO2_nano  SiO2_ppt

[dlparkhurst]

If you want nano to convert to ppt, then you need to decrease the amount of nano precipitate at a given rate. Here is a file that precipitates SiO2_nano, but the rate also includes a rate of dissolution. That rate of dissolution is matched by the rate of SiO2_ppt formation.