Proton Consumption with Phosphate Adsorption on Amorphous Fe(III) Hydroxide

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Proton consumption with phosphate adsorption on amorphous Fe (III) hydroxide (am-Fe(OH)_3) was compared between two different pH-controlled conditions in a 0.1 mol dm^<-3> NaCIO_4 solution at initial pH values of 5.50 and 4.50, at 298±0.005 K. The number of protons caused by phosphate dissociation was subtracted from the total number of protons consumed, then the amount of surface OH groups released by the ligand exchange reaction were determined. When a sequential acid titration by a pH-stat maintained pH values of the systems at initial pH values, the percentage of OH groups released during the ligand exchange reaction was almost constant, 29-37%. When the pH values of the systems increased with phosphate adsorption, the percentage of OH groups released by the ligand exchange reaction varied from -4.3 to 33%. The difference in the proton migration between the two pH-controlled conditions not only depended on the phosphate dissociation, but on the difference in the adsorption mechanism, i.e. the ratio of ligand exchange with OH groups to total phosphate adsorption.
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