Abstract
Isotherms were obtained for the adsorption of benzenehexacarboxylic acid (HexA) and benzene1,3,5-tricarboxylic acid (TriA), as well as for the corresponding sodium salts (HexNa and TriNa), onto synthetic hydroxyapatite crystals (HA) or enamel powder (EP). The studies were also performed as a function of pH and in the presence of various amounts of PO 4 −3 ions. Different results were obtained when studying adsorption of benzenepolycarboxylic acids or their corresponding salts and when treating EP or HA. The fact that the amounts of HexA or HexNa taken up by either HA or EP were 7 to 10 times greater than those of TriA is particularly important. Taking into account specific surface areas and the maximum molecular surfaces of HexA and TriA, an adsorption shell was formed in the case of EP which is 8 times thicker than that on HA. Adsorption versus pH appears to be maximum at pH 5, whereas the presence of phosphate ions increased uptake of HexNa and TriNa and decreased adsorption of HexA and TriA. A Langmuir-type adsorption model fitted the experimental results for low concentrations. The facts that affinities for HexA were higher than those for HexNa and that N parameters for HexA were four times greater for EP than for HA were also significant.
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