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Sorption Preference

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My question is regarding the quantification of sorption preferences of metals onto a given surface. If I have both arsenic and molybdenum in groundwater sorbing onto HFO, is there a way to tell which (As or Mo) is thermodynamically more stable after sorption? In other words, would some metals have the potential to 'kick off' others that have already sorbed (or out-compete other ions in the groundwater), and if so how can I tell what would remove what? I could probably test this qualitatively by adding and removing metals to see what happens, but am wondering if there is a way to calculate this a bit more quantitatively. I read the section in Bethke, perhaps comparing the products of the equilibrium constant and the boltzman factor of various surface complexes would tell me what I need?




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Hi Scott,

I apologize for the delay. This is a slightly complicated question. While the stability constant (log K) and boltzman factor certainly plays a role on the sorbed species, there are a few other things to consider.

Comparing the product between the log K and boltzmann factor can work relatively well if the conditions for your types of surface species are similar. For example, if we take a look at the 3 metals example, all the surface species have the same charges and consists of similar reactions with bivalent cations, similar types of weak and strong reactions, similar reaction stoichiometry and etc. Things can get more complicated if the charge of the binding metal differs from each other or if the pH shifts, as many complexation reactions can be pH-dependent. If the site types for certain ions bind only to weak and the other only to strong sites, the log K and boltzmann factor alone won't capture that. The degree to which other aqueous complexes form when your system will also affect the ions available for binding. Without a full speciation calculation, you won't be able to account for quite a few of these things. Additionally, unless your calculation involves a non-electrostatic, the boltzmann factor can vary with your system when comparing between different charges of surface species.

Hope this helps,
Jia Wang
Aqueous Solutions LLC

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