Formation of Mixed Complexes of Type Phosphate-Ca-Dye

Abstract

Many of the complex biological properties of hydroxylapatite HA are related to the multiple exchange capacities inside the HA structure, including formation of vacancies to accommodate ions with other valencies, in particular carbonate ions and alkali and other metal ions of biological importance. We have recently illustrated the possibilities of using HA to remove various textile dyes from industrial waste waters by co-precipitation with HA. Most dyes can be treated efficiently and the HA adsorbent can be regenerated for reuse following thermal treatment in air to burn of the organic materials adsorbed. However, important facts point to a situation more complex than simple adsorption. To elucidate the mode of action of HA on the removal of dyes or organic matter in general, we investigated the interactions of alizarinsulfonate ARS with calcium and phosphate ions in solution as a function of pH and dye/mineral fraction. Alizarin or its salt is thus regarded as a model compound for more complex dyes, which led us to investigate the way calcium ions interact simultaneously with phosphate and ARS. We report here the observation of the formation of mixed complexes of the type phosphate-Ca-dye which demonstrate the competition for calcium ions between phosphate and organic ligands.