Lead And Cadmium Cations Removal By Hydroxyapatite In Cotton Morphology

With the development of industry, environment problems faced by human have been becoming more and more serious. Water pollution was one of the critical environmental problems, it was particularly critical caused by wastewater containing heavy metal ions discharged from industry. Heavy metals were of great concern because of their bad effects on human health and ecological environment. How to effectively and deeply remove of heavy metals from water systems was a very important but still challenging task for environmental engineering.Hydroxyapatite (HAP) in cotton morphology was prepared for the first time and used for adsorption of Pb2+and Cd2+in water. The crystal phase, micro-structure, particles and surface area of the prepared HAP powder were characterized by XRD, SEM, BET. Effect of several parameters, include pH value, time, temperature and metal ion concentration on the absorption process was investigated. Pseudo-second-order kinetics equation, Langumir adsorption isotherm model, Freundlich adsorption isotherm model and thermodynamic equation were used to fit and analyze the experiment results. The adsorption mechanism was studied by anlyzing the change of pH value in the adsorption process and comparing the original HAP and the adsorption product. The paper included two parts:(1) Cotton was used as biotemplate to fabricate HAP with controllable microscopic morphology for the first time. The effects of sintering temperature on morphology of HAP were studied. In the range of sintering temperature between400℃and700℃, HAP particles could fabricate into cotton morphology and hollow structure, while when the temperature was up to800℃and900℃, cotton morphology and hollow structure disappeared, bulk structure formed. Meanwhile, shape and size of HAP particles changed along the increasing sintering temperature. In the range of400-500℃, rod-like HAP particles were achieved. When the sintering temperature was in the range of600-700℃, layer structure was observed. At800℃, the product was HAP pieces in elliptical shapes, which intended to stick together to form bundles. At900℃, sphere-like structure was observed. (2) At the sintering temperature of500℃, cotton morphology and hollow structure was fabricated by the rod-like HAP particles. BET surface area was increased effectively with the mesoporous formed by the accumulation of HAP particles. So this kind of HAP was used to remove Pb2+and Cd2+from water, the best adsorption conditions were optimized and possible mechanisms were proposed.Solution pH had great effect on the adsorption efficiencies of Pb2+and Cd2+on HAP with cotton morphology and hollow structure and the optimum experimental pH were2.5and6.5, respectively. Under the temperature range from5℃to55℃, the adsorption efficiencies of the two metal ions increased with the increasing of temperature, this indicated that the adsorption processes were endothermic.It was found that all the adsorption of Pb2+and Cd2+on HAP with cotton morphology and hollow structure was a fast adsorption process that could be reached equilibrium within50and120minustes, respectively. The adsorption processes could be described by the pseudo-second-order equation well (R2=0.9998, R2=0.9998)By analyzing the adsorption isotherms, the adsorption of Pb2+and Cd2+on HAP with cotton morphology and hollow structure fitted both Langmuir and Freundlich model, but fitted with Langmuir better than Freundlich. The maximum adsorption capacity was1690.2mg/g and97.08mg/g, respectively.Based on the analysis of the changes of X-ray powder diffraction (XRD) spectrum, scanning electron microscopy (SEM) images, transmission electron microscopy (TEM) images and BET surface areas between original HAP and adsorption products, the mechanisms of the adsorption of Pb2+and Cd2+were proposed.The mechanism of the Pb2+removed was varied depending on the pH of aqueous solution. At the low pH of2.5, the mechanism of dissolution/precipitation was the dominated one. The mechanism of ion exchange dominated at pH4.5-6.5. At pH of3.5, the adsorption capacity was the worst since the two mechanisms was competing. At pH higher than7.5, the precipitate of hydroxyl complex formed.Ion exchange was the main mechanism of Cd2+removed from water accompanied by the surface complexation and surface electrostatic adsorption.