Removal of Cadmium from aqueous solution by synthetic hydroxyapatite

Wastewater containing Cadmium ion could be effectively removed by hydroxyapatite (HAP). Experimental results indicated that Cd removal percentage increased by increasing HAP dosage, solution pH, and reaction time. Cd removal percentage could approach 100%. There existed two stages from kinetic point of view. At the earlier stage, reaction rate was so fast that its kinetic process was intricate. At the latter stage, the rate of reaction became slow and the process of reaction accorded with first-order kinetic equation. The relationship between reaction rate constant k₁ and temperature T accorded to Arrhenius Equation, and the activation energy of sorption (E_a) was 6.075J/mol and frequency factor (A) was 220s^-1. The results of XRD, FT-IR and SEM ruled out the possibility of occurrence of dissolution-precipitation, co-precipitation during the removal of Cadmium ions. It was inferred that ion-exchange governed the process during the removal of Cd by HAP, accompanying with adsorption.