Kinetic approach for cadmium sorption using microwave synthesized nano-hydroxyapatite

Nano-crystalline hydroxyapatite adsorbent that was prepared by microwave processing was utilized for cadmium removal from aqueous solutions using the batch technique. Cadmium sorption on the prepared adsorbent was studied as a function of initial cadmium concentration in the aqueous solution, adsorbent dosage, agitation speed, hydrogen ion concentration of the aqueous solution (pH), and the solution temperature. The highest Cd2+ sorption was achieved at agitation rate of 500 rpm. The sorption process was relatively fast and equilibrium was achieved after about 240 min of contact. The optimum sorption of cadmium occurred at pH range 4–7. The kinetic process of Cd2+ sorption onto the synthesized nano-hydroxyapatite was tested by applying the pseudo-first order, the pseudo-second order, the simple Elovich and intraparticle diffusion rate models. The sorption process follows a pseudo second-order kinetics with a contribution of intraparticle diffusion. Thermodynamic parameters ΔH, ΔS and ΔG have been calculated. Positive value of ΔH and negative value of ΔG show endothermic and spontaneous nature of sorption respectively. The relatively small value of the activation energy that equal to 8.61 kJ/mol confirms that cadmium sorption process is diffusion controlled. The main mechanism for cadmium ions removal using the synthesized nano-hydroxyapatite was suggested to be ion-exchange and diffusion controlled.

Research Highlights
► Microwave synthesized hydroxyapatite produced in nano-scale with particle diameter ranged between 40 and 70nm with crystalline properties.
► Nano-hydroxyapatite has rapid cadmium sorption rate and good sorption capacity.
► Sorption kinetics system follows pseudo-second order kinetics and governed mainly by intraparticle diffusion.
► The ion exchange mechanism plays a significant role in cadmium sorption systems.
► The cadmium sorption process is thermodynamically favourable, spontaneous and endothermic in nature.