Implement Sorption with defined percentage

[anneimig]

Hi,

I am currently modelling antibiotics fate and transport processes in a column experiment including sorption. From a prior batch experiment, I have obtained a percentage of sorption of the antibiotics to the soil. Now I would like to use this percentage value in the PHREEQC code to implement sorption. If I define a SURFACE_SPECIES or a KINETIC and RATE in the model I am adding new parameters to my model which need to be fitted. I would like to avoid this and only use the percentage as X % are sorbed from a defined SOLUTION_MASTER_SPECIES. Can you suggest a way how to do this in PHREEQC?

Thank you very much
Anne

[dlparkhurst]

I don't see that there is anything you want to model. It seems like you should consider a surface or exchange, but you would need to fit some parameters to use them. If you don't want to fit parameters, then I think you are done.

To model the response of a solution to a surface, you need a surface complexation reaction and log K for the reaction (SURFACE_MASTER_SPECIES and SURFACE_SPECIES). You also need the number of surface sites for the SURFACE definition. If you know the percentage sorbed and the concentration in solution, you can find the equilibrium constant that produces the desired distribution between solution and surface. If you are changing pH or have other species competing for the surface sites, you may need more surface-complexation reactions.

A simpler model is to use an isotherm, although it is tricky to set up in PHREEQC. The isotherm simply uses a K to determine the distribution between solution and sorbed pool. You can look at example 19 in the manual that compares surface complexation with Langmuir and Freundlich isotherms.

Exchange is a bit simpler than surfaces, but you still need similar information about sites, reactions, and equilibrium constants.