Adsorption of 177Lu from Water by Using Synthetic Hydroxyapatite

Adsorption behavior of lutetium by hydroxyapatite (HAP) was investigated by considering pH, adsorbent dose, contact time, initial concentration of lutetium, and temperature. The adsorbent was synthesized and characterized by X-ray diffraction (XRD; JCPDS file 01-04-3708), and the point of zero charge was 7.22. When the initial pH (pH(i)) was 3, the final pH (pH(f)) of the system was approximately 6, and in these conditions, approximately 98% of lutetium is present as Lu-.(3+) The equilibrium of the adsorption system was reached in 5 min, and the HAP retained 99.2 +/- 0.3% of lutetium. The experimental kinetic and isotherm data were adjusted to the pseudo-second-order and Freundlich models, respectively, indicating a chemisorption mechanism on a heterogeneous surface. Experimental and literature data revealed that Freundlich parameters depend on the cation size of the lanthanide elements; K-f (Freundlich constant) values linearly decrease with the ionic radii of the elements. The adsorption process was exothermic and spontaneous, as indicated by the negative values of enthalpy and Gibbs free energy values. The entropy value is small and negative, which may indicate that the affinity between Lu3+ and HPA is not strong.