Biomimetic Construction Of Hollowand Porous Octacalcium Phosphate Superstructuresand Their Adsorption Properties Of Heavy Metals And Tetracycline Hydrochloride

Heavy metals and antibiotics are ubiquitous in the biosphere,which are particularly difficult to clean up and may cause great harm to the ecological environment and human health.In the current study,the biomimetic mineralization was proposed to prepare the hollow and porous OCP superstructures,where the polyelectrolyte PSS was applied as the modifier in a gas diffusion process.The microstructure and growth mechanism were researched by using modern analytical techniques.The OCP superstructures composed of ultrathin nanosheets show excellent adsorption properties for heavy metal pollutants and TC.We investigated the effects of pH,concentrations of adsorbents and temperature on the adsorption capacity of pollutants.The adsorption isotherms,adsorption kinetics and adsorption thermodynamics of heavy metals Pb2+,Cu2+,Cd2+and TC by OCP/PSS were tracked.The adsorption mechanism was further explored by the XPS analysis.Some results are list as follows:(1)Under the regulation of PSS,the synthesized OCP/PSS superstructures are hollow and porous,composed of ultrathin nanosheets.In contrast,pure OCP presents a solid spherical structure,which is mainly made up of long nanoribbons.OCP/PSS has a larger surface area and a richer microporous structure than pure OCP.(2)At 25℃,the adsorption of heavy metals like Pb2+by OCP/PSS conforms to the Langmuir model,indicating that the adsorption is chemisorption of single molecular layer determined with ion exchange between metal ions and Ca2+ on the surface of OCP.The adsorption of TC conforms to the Freundlich mode,which proves that the adsorption is the physical adsorption of a single molecular layer,where the adsorption is completed by the complex between the active site Ca2+ and TC molecular groups on the surface of the OCP.In the adsorption kinetics,the adsorption of heavy metals and TC all conforms to the quasi-secondary adsorption dynamic mode.(3)Meanwhile,XPS analysis showed that the excellent adsorption of Pb2+derived from the minimal change of binding energy before and after ion exchange.That meant smaller change in electronic rearrangement around active site PO43-after adsorption,which might result in the easiest ion exchange between Ca(Ⅱ)and Pb(Ⅱ).Simultaneously,this process would cause the least mismatching in crystal lattice.Thus,the adsorption differences among Cu(Ⅱ),Cd(Ⅱ)and Pb(Ⅱ)were mainly caused by the different degree of matching in crystal lattice among metal bearing precipitate and OCP.Traditionally,the ion radius and the hydration ion radius of metal ions have a great influence on the adsorption capacity of metals,but in the current case,the adsorption data indicate that they have little influence on the adsorption properties.Further analysis showed that the higher adsorption of OCP/PSS than other OCP materials relates closely with the ultrathin nanosheets,the amorphous nanoregions and structural defects in microstructures to a certain degree.