Removal of Co2+ from aqueous solutions by hydroxyapatite.

A study on the removal of cobalt ions from aqueous solutions by synthetic hydroxyapatite was conducted in batch conditions. The influence of different sorption parameters, such as: initial metal concentration, equilibration time, solution pH and presence of EDTA on the amount of Co(2+) sorbed was studied and discussed. The sorption process followed pseudo-second-order kinetics with necessary time of 24 h to reach equilibrium. Cobalt uptake was quantitatively evaluated using the Langmuir and Dubinin-Kaganer-Radushkevich (DKR) model. The Langmuir adsorption isotherm constant corresponding to adsorption capacity, Xm, was found to be 20.92 mg/g. Sorption of Co(2+) is constant in the initial pH range 4-8, because HAP surface buffers these solutions to the constant final pH value of 5.1. In the presence of EDTA, sorption of Co(2+) decreases due to formation of complex with lower sorption affinities. Cobalt desorption depends on the composition of the extracting solution. The desorbed amount of cobalt decreased continuously with increasing pH, and increased with increasing Ca(2+) concentration in leaching solution.