SOLUBILIZATION OF NATURAL HYDROXYAPATITE BY LACTIC ACID PRODUCING RHIZOPUS ARRHIZUS IMMOBILIZED IN POLYURETHANE FOAM

Biotechnological solubilization of insoluble inorganic phosphate-bearing materials is now accepted as one of the most viable alternative to the traditional chemical methods. Many microorganisms are capable of solubilizing phosphates although this feature is mainly attributed to metabolites such organic acids. During the recent years, we have developed various biotechnological schemes of P-solubilization based on citric acid (Aspergillus niger, Yarrowia lipolytica), gluconic acid (Penicillium variabile), and oxalic acid (white-rot fungi) producers. In this work, P-solubilization was carried out employing the filamentous fungus Rhizopus arrhizus in polyurethane-foam-immobilized form. Experiments were performed in shake flasks under the conditions of five repeated-batch fermentation cycles. Hydroxyapatite (HA) was supplemented to the fermentation medium at a concentration of 1.0 and 2.0 g/l and particle size of 40 and 60 mesh. Results showed that lactic acid produced by Rhizopus arrhizus was able to solubilize the inorganic phosphate by its hydroxyl and carboxyl groups. The process of solubilization depended on the size of the P-bearing particles (the lower the size the higher the concentration of soluble P in the medium). The highest solubilization level (percentage soluble P/total P) of 47% was reached at the 3d batch cycle using the lower size particles applied at concentration of 1.0 g/l. Higher concentration of soluble phosphate was found when using 2.0 g/l of HA but the solubilization efficiency was lower. HA was found to stimulate lactic acid production as it served as a pH corrector during the fermentation. Similarly, the slow release of phosphate due to phosphate solubilization enhances lactic acid production in the P-deficient production medium.