Adsorption of oxalate ion (Ox
2-) in aqueous solutions (30°C, pH 7.5-12.0) to hydroxyapatite (HAP) was confimed to be reversible and of Langmuir type. The amount (x
Ox) of adsorbed Ox
2- from Na
2Ox was larger than that from K
2Ox through the size effect of the counter ion. As the thermodynamic activity (ionic activity product) of adsorbate (Na
2Ox or K
2Ox) increases with an increase in concentration of the counter ion, x
Ox increased with an increase in the amount of added NaCl or KCl. But the amount of adsorption, x
Ox, increased with a decrease in pH because of competitive adsorption with OH
-. It was also suggested that Ox
2- competes with Cl
- for the adsorption sites on HAP. Therefore, the amount of adsorption, x
Ox, increased apparently by the addition of L-lysine HCl or glycine, but it was concluded that the increase was due to the decrease of pH and these substances do not affect x
Ox after pH-correction. On the other hand, x
Ox was decreased by the addition of Na
2HPO
4, NaHCO
3 or sodium L-aspartate even after pH-correction owing to the competitive adsorption between Ox
2- and these anions. It was concluded that Ox
2- is adsorbed to HAP by electrostatic force and that some portion of adsorbed Ox
2- is subjected to the ion-exchange with orthophosphate ion on the HAP surface. Some properties of renal calculus formation was explained on the basis of the results presented here.
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