Preparation Of Hydroxyapatite / Chitosan Three - Dimensional Porous Materials And Study On Removal Of Lead Ion In Wastewater

Heavy metals ions are highly harmful to environment and human health because of their acute or chronic toxicity. Notably, lead ions not only interfere with the metabolism of human bodies, but also indirectly damage the central nervous system, inhibit DNA repair, and cause cognitive deficits. Environmental pollution of heavy metals caused widespread concern around the world. Various techniques have been developed to remove heavy metal ions from aqueous solutions, adsorptive separation is considered as an effective and economic method to remove heavy metals. Ideal adsorbent material must meet the following requirements: first, the porous materials can effectively remove the heavy metal ions from wastewater; second, materials have a three-dimensional macroporous structure through a high specific surface area, pore volume and they should be easily separated from the water; third, materials are environmentally friendly materials that do not produce toxic effects to human beings; fourth, suitable mechanical properties and good processability can make the adsorbent materials to be widely used in industrial wastewater. In this study, combined with freeze-drying technique, we prepared hydroxyapatite/chitosan three-dimensional porous materials(HCPMs、THCMs and MHCMs) by mixing, in situ transformation and mineralization, respectively. The morphology, phase structure and mechanical properties of porous materials before and after adsorption of lead ions were studied by means of FESEM, TEM, EDS, XRD, FTIR, BET, PMI and electronic universal testing machine. This work calculate the adsorption properties of three materials for lead and study their Pb(II) adsorption properties.Hydroxyapatite/chitosan porous materials fabricated by mixing(HCPMs) possessed three-dimensional interconnected porous structures with pore sizes of 100~400 nm. For the HCPMs, many HA nanoparticles existed on and within the films. HA crystals with low degree of crystallinity mainly grow along the c-axis. The total porosity of the porous material were estimated by liquid replaced method. The total porosity and maximum stress of HCPMs were 90.89% and 0.90±0.05 MPa, respectively. Many particles with particle sizes of approximately 10 mm were observed on the surface of CS porous material after the adsorption of Pb(II) ions. These particles may be CS-Pb complexes, as confirmed by the element distribution image and XRD pattern. After adsorbing Pb(II)ions, many nanorods lead apatite(PbHA) deposited on the surface of HCPMs. These nanorods had different particle sizes at different locations. The flow rate was controlled at 0.8 mL/min. The concentration and pH of the Pb(II) aqueous solutions were 400 mg/L and 5.5, respectively. The HCPMs possessed a maximum adsorption amount of 264.62 mg/g.Hydroxyapatite/chitosan porous materials fabricated by in situ transformation(THCMs) possessed three-dimensional interconnected porous structures with pore sizes of 50~150μm. The as-obtained carbonated HA nanoplates with thickness of 50~150 nm were perpendicular to the particle surfaces, and coalesced to form a flower-like structure with an aperture size of 0.2~1.0 μm. The total porosity and maximum stress of THCMs were 90.89% and 1.02±0.02 MPa, respectively. After the adsorption of Pb(II) ions, the HA plates were converted into PbHA nanorods with diameters of ~50 nm. Different initial concentrations of lead ions also have an impact on the adsorption properties of the materials. The corresponding maximum adsorption amount arrived at387.91 mg/g, 559.29 mg/g and 848.30 mg/g when the concentration the Pb(II) aqueous solutions were 200 mg/L, 400 mg/L and 800 mg/L, respectively.Hydroxyapatite/chitosan porous materials fabricated by mineralization(MHCMs) possessed three-dimensional interconnected porous structures. The pore size of the MHCMs is mainly distributed around 100~300μm, many HA nanorods with diameters of ~20 nm and lengths of ~500 nm existed on the films of the MHCMs. The total porosity of MHCMs estimated by using liquid displacement method was 94.4% and the porosities order of HCPMs, THCMs and MHCMs was MHCMs>HCPMs>THCMs. The MHCMs were texted by electronic universal testing machine and have the good compression strength of 0.93±0.03 MPa, and the mechanical properties order of the materials was THCMs>MHCMs>HCPMs. After adsorbing Pb2+ ions on the MHCMs, lots of PbHA rods with a length of 1~3 mm and a diameter of 50~200 nm are detected. And many nanoplates surrounded by nanorods are detected in the MHCMs. With the decrease of pH values, the maximum adsorption amounts of Pb2+ ions increase. Under the different lead solutions with pH values of 2.5, 4.0, 5.5 and 7.0, the experimental equilibrium adsorption amounts on the MHCMs are 543.9 mg/g, 337.7 mg/g, 281.7 mg/g and 208.0 mg/g, respectively.Under the same experimental conditions, the adsorption properties order of the materials was THCMs > MHCMs > HCPMs, and they depend on content and topographies of HA. The content of HA in THCMs is the most among the three materials and the specific surface area is the largest due to the HA nanoplates. The great changes of the HCPMs、THCMs and MHCMs in phases and morphologies between before and after adsorbing Pb2+ ions suggest that the conversion mechanism of nano-HA into PbHA is a dissolution–precipitation reaction. The released of PO43-ions and OH- ions increases the local ions concentration around the nano-HAP crystals in HCPMs. As the ionic activity product exceeds the thermodynamic solubility product of PbHAP crystals, they in situ deposit on the surfaces of the porous materials by using the HAP and CS as active sites. The sorption behaviors of Pb2+ ions on HCPMs, THCMs and MHCMs are verified using pseudo first order and pseudo second order kinetic models. The kinetic processes for the adsorption of Pb2+ ions on the HCPMs, THCMs and MHCMs powders follow pseudo first order and pseudo second order kinetic models. The correlation coefficient(R2) of the pseudo second order kinetic model is greater than the pseudo first order kinetic model, suggesting that the adsorption process of Pb2+ ion is more suitable for pseudo second order kinetic model.