Abstract:Adsorption mechanisms of sulfonated organic compounds by hydroxylapatite (HAP) were studied. Alkylsulfonates such as sodium dodecyl sulfonate were easily adsorbed after ion exchange (i.e.,isomorphous substitution) between their terminal sulfonate groups and phosphate ions on the surface of HAP, because the size of the sulfonate group is quite similar to that of phosphate ion. Azo dyes such as methyl orange (MO) and Ponceau R(PR) were adsorbed, because these compounds also have sulfonate groups in their chemical structure. The adsorption amount of PR was larger than that of MO. This fact is suggesting that the amount increases with the number of sulfonate groups in an adsorbate molecule. The adsorption amount of each of these compounds was, however, smaller than that of a long chain alkylsulfonate. The role of hydrophobic group in an alkyl sulfonate was emphasized. Hydrophobic interaction on the surface of HAP resulted in formation of hemimicelle or admicelle, which accelerated the adsorption. The adsorption amounts of MO and PR were increased in the presence of bovine serum albumin (BSA), because these dyes formed an intermolecular complex with BSA on the surface of HAP which promotes their adsorption.

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Methyl orange (MO) was not adsorbed directly to hydroxyapatite (HAP) when the concentration of MO was low, while bovine serum albumin (BSA) was adsorbed by HAP in the presence or absence of MO. Formation of the intermolecular complex between MO and BSA both in an aqueous phase and on the surface of HAP was discussed in the present paper. Apparent adsorption amount of MO to HAP increased with a concentration of BSA due to the adsorption of BSA and formation of the complex of MO-BSA on the surface when the [BSA] was low. The adsorption amount of MO, however, decreased with a concentration of BSA when the [BSA] was high. This fact was explained in terms of an increase in the concentration of BSA free from HAP, i.e., formation of the complex of MO-BSA in the mother solution.