Sliding pH is sensitive to starting pH?

[CShort]

I'm attempting to evaluate sorption of several parameters over a range of pH. I attempted to use the "slide" feature in react to do this: the initial solution has a pH of 4, and I slide it to 10. Precipitation is disabled. In this case, at pH 7.3, U is sorbed 99.48%. Great.

 

if I run this same model, instead over the range of 7-10, U is sorbed 12.8% at pH 7.3. I would expect to get similar sorbed fractions at each pH, regardless of the start and end point.

 

 

I'm expecting that these models should have the same results at every pH. I'm afraid I'm missing something fundamental about how the "slide" function works - what am I missing?

 

Thanks!

Clark

20170616_U_Sorption_2005_pH4.rea

20170616_U_Sorption_2005_pH7.rea

FeOH_minteq_plusU.sdat

[Brian Farrell]

Hi Clark,

I think the issue here is how you’re constraining the amount of the sorbing species. By default, the concentrations you specify on the Basis pane refer to the amount in the fluid. React figures how much additional mass is present on the surface. This option is useful when you know the composition of the fluid, such as when you sample water from a well.

An alternative method is to set the sum of dissolved and sorbed mass on the Basis pane using the “sorbate include” option. In a lab experiment, for example, you might know the total mass of a species that you add to a reactor with a sorbing mineral, but not how much is in solution or how much gets sorbed.

If the amount of sorption is pH-dependent, you can set different initial pH values only if you’re using the “sorbate include” option. The constraints you specify on the Basis pane in this case must correspond to measurements of the sum of dissolved and sorbed mass, of course. Otherwise, with React’s default setting (“sorbate exclude”) you have to start with the known values in solution at the particular pH.

For more information, please see the “sorbate” command in the GWB Reference Manual.

By the way, I noticed that Mg++ also sorbs in your model. It’s probably best to pick a non-sorbing species for the charge balancing ion.

Regards,

Brian Farrell

Aqueous Solutions LLC