help writing a simple reaction model for transport

[Mark Himenz]

Can I write the reaction block in such a way that it removes a percentage of a solution species for each shift and cell in a transport simulation? From what I understood it can only remove a specified molar amount of a species but not a fraction. Or is this something that can only be modeled using the kinetics functionality?

I have a column experiment with a slug injection of uranium, where we only get a ~60% recovery. We have evidence that this is due to both co-precipitation with calcite as well as some U6 reduction and precipitation, both of which are kinetically controlled in our experiment. However we want to keep our modeling efforts relatively simple for now, and see if we can model uranium removal as an unspecified precipitate as a function of influent uranyl.

[dlparkhurst]

You can use kinetics. The only thing you need to be careful of is the formula by which you remove uranium with KINETICS. If you use

-formula U

it is equivalent to removing uranium metal, and the uranium will be reduced by reaction with water. Alternatively, you can use

-formula UO2CO3

which would not cause the redox reactions. Just pointing out that the choice of the reaction may be important.

SOLUTION
Ca 1
Alkalinity 2
U(6) 1e-3
RATES
U_precipitation
10 k = 1 / (3600*24)  # 1/day
20 rate = k * TOT("U")
30 moles = rate * TIME
40 SAVE MOLES
KINETICS
U_precipitation
-formula UO2CO3 -1
-step 86400 in 10
USER_GRAPH 1
    -headings               Days U
    -axis_titles            "Time, days" "Micromoles per kilogram water" ""
    -initial_solutions      true
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
10 GRAPH_X TOTAL_TIME / (3600*24)
20 GRAPH_Y TOT("U") * 1e6
  -end
    -active                 true
END

[bannings]

Hi, I'm a newbie in the RATE module. I have some questions for you. 1. What is the meaning of rate in "20 rate = k * TOT("U")"? Does rate, like K in the previous code, represent a simple function? 2. What is the meaning of "30 moles = rate * TIME"? I noticed that almost all code examples in the RATE module used it. I tried to read the explanation in the manual, but still can't understand the meaning of this code. Could you please use a simple example to explain?

[dlparkhurst]

The rate you cite is about as simple as it gets. The units of k are 1/s, TOT("U") is the total dissolved concentration of uranium (mol/kgw), so the product is mol/kgw/s. The rate produces an exponential decay of uranium.

Code: [Select]

20 rate = k * TOT("U")"

TIME is an internally generated sub-time step. rate*time is mol/kgw and is the extent of reaction over the given TIME interval. Multiple time intervals are evaluated to integrate the reaction over the time intervals specified in KINETICS or TRANSPORT. The integration method has an error estimate, so that the integration is restarted from a point in time with smaller sub-time steps if the error estimate is too large.

(Note that to be entirely accurate, the rates should be multiplied by the mass of water to produce mol/s, but generally, the mass of water is about 1 kg.)

[bannings]

I get it, the unit of "30 moles = rate * TIME" is mol/kgw. I have another question, if I want to express dc/dt whose unit is mol/kgw/s, how should I write it? Here is my guess.

Code: [Select]

20 rate = TOT("U")
30 moles = rate / TIME


[dlparkhurst]

TOT("U") is mol/kgw. K is 1/s. K*TOT("U") is mol/kgw/s.