Surface and mass transfer kinetic and equilibrium modeling of Pb(II) ions adsorption on hydroxyapatite scaffold: Batch and fixed-bed column studies

This paper presents the results of a study on removal of Pb2+ ions by using hydroxyapatite (HAP) scaffold prepared by replication method. To this aim, the effects of suspension solid content and polymeric foam density on the properties of ceramic foam were investigated. The mean total porosity and apparent porosity of the optimized scaffolds were 92.67 % and 92.04 %, respectively. The Taguchi method was performed for designing batch experiments by considering three controllable variables, namely: (1) initial pH of the solution, (2) initial concentration of Pb2+ ions (C0), and (3) circulating rate of the solution (Ǭ), on the removal efficiency of Pb2+ ions as the response. Based on the obtained results from the ANOVA analysis, the removal efficiency of Pb2+ ions was mainly affected by the initial pH of the solution (90.88 %). The highest removal efficiency (99.1 %) of Pb2+ ions was obtained by optimizing the process under operating conditions of pH = 3.5, C0 = 50 mg.L−1, and Ǭ= 0.5 L.min−1. The Langmuir isotherm model with good accuracy was fitted to the equilibrium results, and the maximum adsorption capacity of 106.38 mg.g−1 was predicted for the HAP scaffold adsorbent. Kinetic studies showed that the mass transfer diffusion steps in the external film and inside the pores are fast, being the adsorption on the adsorbent sites the rate-limiting step of the kinetics, what is plausible given the high porosity of the material. Also, the primary mechanism for Pb2+ adsorption using HAP scaffold adsorbents was found to be dissolution-precipitation. The fixed-bed column adsorption experiments performed under the predicted optimum conditions of batch experiments. In comparison to the powder adsorbents tested in fixed-bed column, this adsorbent presents several advantages. These include the elimination of the requirement for mechanical separation of the shaped adsorbent from the solution after adsorption, prevention of bed clogging, and its high surface area derived from interconnected open pores.