Study On The Adsorption Properties Of Different Types Of HEDP/HAP Composites For Heavy Metal Ions In Water

The presence of heavy metals in water has caused serious harm to the natural ecological environment and human health,and the pollution of heavy metals in wastewater has also caused more and more attention.Hydroxyapatite（HAP）is an important inorganic mineral component in human and animal bones and teeth.Its unique chemical composition and open lattice structure make it have a strong ion exchange capacity.It is a new type of environmental functional material.Hydroxyethylidene diphosphonic acid（HEDP）is a bisphosphonate containing a P-C-P bond.It has a strong binding ability with HAP and has a strong chelating ability for a variety of metal ions.In this paper,the adsorption properties of metal ions on hybrid HEDP/HAP systems,surface grafted HEDP/HAP and bulk-doped HEDP/HAP composites were studied.The main research contents are as follows:（1）The adsorption properties of HEDP-HAP mixed system on copper and lead ions were studied.The effects of the mixing ratio of HEDP and HAP,the pH of the solution,the initial concentration of metal ions,the reaction time,and the reaction temperature were investigated.The results showed that when the mixing ratio of HEDP and HAP was 0.5:1 and1:1,respectively,it has the best adsorption performance for copper ions and lead ions,and the maximum adsorption amounts reached 171.8 mg/g and 1584 mg/g,respectively.For the single commodity HAP,the maximum adsorption capacities for copper ions and lead ions were 40.32 mg/g and 152.2 mg/g,respectively.（2）HEDP-HAP composites were prepared by surface grafting method.FTIR characterization showed that HEDP was successfully grafted to the surface of HAP.XRD results showed that new crystal phases were formed after surface grafting.The grafting amounts of HEDP were 50%and 100%,respectively,the composites had the best adsorption performance for copper ions and lead ions,with the maximum adsorption amounts of 168.9mg/g and 1521 mg/g,respectively.Moreover,molecular dynamics simulations have been applied to study the mechanism of HEDP and HAP.Molecular dynamics studies have shown that the positively charged Ca²⁺of HAP and the phosphonic acid group negatively charged O atoms of HEDP formed ionic bond,and the hydrogen bond between the PO₄^3-group of HAP with the OH group of HEDP enhanced the affinity.At the same time,taking copper ions as an example,the Multifwn program based on density functional theory was used to theoretically study the adsorption mechanism of metal ions on HEDP/HAP composites.The calculation results showed that the minimum points of the electrostatic potential（ESP）and the average localized ionization energy（ALIE）were basically concentrated on the phosphonic acid groups of HEDP,indicating that the oxygen atoms of the phosphonic acid group were more likely to supply lone electrons to form ionic bond with metal ions,and meanwhile,the phosphate groups of HAP and the copper ions also have a strong interaction to form HAP-HEDP-Cu ternary surface complex resulting in improving the adsorption of copper ions.The adsorption mechanism of copper ions may include ion exchange and ternary surface complex formation.（3）HEDP-HAP hybrid composites were prepared by the bulk doping method.XRD characterization showed that the crystallinity of the HEDP-HAP composites was greatly reduced,and became amorphous HAP.The composites had the best adsorption effect on the four ions of lead,copper,zinc,and cadmium when the doping amount of HEDP was 20%.The maximum adsorption capacities were 1893 mg/g,239.3 mg/g,254 mg/g,and 277 mg/g,respectively.The maximum adsorption capacity of the synthesized HAP for the four ions was998.5 mg/g,97.1 mg/g,105.3 mg/g,and 133.5 mg/g,respectively.XRD results showed that the adsorption of lead ions on HAP and HEDP/HAP included dissolution-precipitation and the formation of Ca_10-xPb_x（PO₄）₆（OH）₂.The other three heavy metal ions have not formed a new crystal phase after adsorption.Adsorption mechanisms include ion exchange and surface complexation.