I synthesized thiol modified adsorbent to adsorb Au(III). I investigated it in several pH. Surprisingly, at a pH of 4 to 7, the adsorption capacity tended to reduce due to the weak Au-S bonding (based on literature?) What the nature of this phenomenon?
Did you get better adsorption beyond pH app. 9.5? If yes, then reason may be linked to pKa value of your thiol modified adsorbent.
well, I tested at pH 2-6, then best one is of pH 3. I try to search in literature, found that its bond tend to weak due to the increasing of pH system.
The answer to this question needs a little of back ground as:
The formation of Au-thiolate bond starting from alkane thiol and dialkyl di sulphide may be written as:
RS - H + (n+1)Au^0 =RS -Au +n Au^0 + 1/2H2
2RS –Au = RS – SR+2Au
RS - SR + (n+1)Au^0 = RS - Au + nAu^0
The origin of the S-Au(111) bond and the binding site preference are discussed from the viewpoint of orbital interaction. The site preference is in the order of three-fold hollow (fcc and hcp) > bridge > on-top. The second layer Au atoms have little influence on the S-Au(III) bonding. Adsorptions to the fcc and hcp sites are almost identical with respect to energy and S-Au bonding nature. Although ó-type S-Au orbital interactions dominate the S-Au(III) bonding in the on-top model, ð-type S-Au orbital interactions play an important role in the bridge, fcc, and hcp models.
The bonding modes change with the change of pH and are in weak bridging mode at 4-7 pH.
Will you please spare some time to read in between the lines of the following paper?
Who knows I might have drawn a different conclusion?
J. Phys. Chem. B 2002, 106, 12727-12736.
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