Impact of Chloride Anions on Proton and Selenium Adsorption by an Aluminum Oxide
2001
Schulthess, C.P. | Hu, Zhiqiang
The quantity of protons adsorbed for each anion adsorbed by a solid surface is an essential component of surface complexation models. This reaction stoichiometry is often assumed, or it is “confirmed” based on the goodness-of-fit of the pH-dependent adsorption models. This ratio can be experimentally measured, but the resulting measurement may be in error if secondary (unaccounted for) reactions are present that are also consuming or releasing protons. Using selenate and selenite adsorption isotherms on an Al oxide, the proton/anion stoichiometries were determined to be ≈2:1 at high pH values (>6), but they decreased rapidly at low pH values. The stoichiometry is ≈2:1 across a broader pH range when corrections are made for the change in Cl adsorption intensity by Al oxide in the presence of Se anions. Generalizing, if a weakly adsorbing anion is affected by another strongly adsorbing anion, then its impact on the balance of protons adsorbed (or balance of surface charge) must also be monitored. Although the Cl anion is very weakly adsorbing, the matrix needs to be considered as a ternary system consisting of H, Cl, and Se ions. A binary approach, one that views only H and Se ions, will misrepresent the H/Se adsorption ratios, particularly at low pH values.
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