Reactions
The reaction mechanism illustrated in Figure 2 translates into the mass balance equations for aldicarb, aldicarb sulfoxide, aldicarb sulfone, aldicarb oxime, aldicarb sulfoxide oxime, and aldicarb sulfone oxime. Solving this set of coupled ODEs provides information on the time scales of the degradation processes.
First treat the aldicarb decomposition kinetics in the water pond as a perfectly mixed system. Start by importing a set of global parameters defining the rate constants kj expressed in 1/day, and the reactions for the chemical species given by Equation 2 through Equation 7.
Note: The location of the files used in this exercise varies based on the installation. For example, if the installation is on your hard drive, the file path might be similar to C:\Program Files\COMSOL\COMSOL64\Multiphysics\applications\.
Parameters
1
In the Model Builder window, under Global Definitions click Parameters 1.
2
In the Settings window for Parameters click the Load from File button .
3
Browse to the file pesticide_transport_parameters_1.txt in the Application Libraries folder on your computer, Subsurface_Flow_Module\Solute_Transport. Double-click to add or click Open.
The rate constants are added to the table.
Reactions
1
In the Model Builder, under Global ODEs and DAEs, click Global Equations .
2
In the Settings window for Global Equations click the Load from File button .
3
Browse to the file pesticide_transport_reactions.txt in the Application Libraries folder on your computer, Subsurface_Flow_Module\Solute_Transport. Double-click to add or click Open.
The reactions are added to the table.
To obtain consistent units for the equations, locate the Units section and click the Select dependent variable quantity button . In the Physical Quantity dialog type concentration in the text field. Choose Concentration (mol/m^3) and click OK. Repeat the same procedure for the Source term quantity section, and choose Reaction rate (mol/(m^3*s)) as unit.
The chemical reactions (given by Equation 2 through Equation 7) are slightly modified. All the terms are moved to the left-hand side. Also note that the initial values are zero for all the concentrations, except for aldicarb.