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Abstract
Heavy metal contamination of waters and soils is particularly dangerous to the living
organisms. Different studies have demonstrated that hydroxyapatite has a high removal
capacity for divalent heavy metal ions in contaminated waters and soils. The removal
of Cd from aqueous solutions by hydroxyapatite was investigated in batch conditions
at 25+/-2 degrees C. Cadmium was applied both as single- or multi-metal (Cd + Pb +
Zn + Cu) systems with initial concentrations from 0 to 8 mmol L(-1). The adsorption
capacity of hydroxyapatite in single-metal system ranged from 0.058 to 1.681 mmol
of Cd/g of hydroxyapatite. In the multi-metal system competitive metal sorption reduced
the removal capacity by 63-83% compared to the single-metal system. The sorption of
Cd by hydroxyapatite follows the Langmuir model. Cadmium immobilization occurs through
a two-step mechanism: rapid surface complexation followed by partial dissolution of
hydroxyapatite and ion exchange with Ca resulting in the formation of a cadmium-containing
hydroxyapatite.