Study On Synthesis Of Hydroxyapatite And The Adsorption Of Lead Ion

Nano-hydroxyapatite （HAP） is an important inorganic material which is a component ofhuman and animals, bones and teeth, and is also an important environmental functionalmaterial used to treat water and soil pollution. HAP, with high specific surface area, is a kind ofefficient adsorbent to remove heavy metal ions without introducing other impurities ion orcausing secondary pollution to environment. Because of unique structural characteristics inHAP crystal structure, ion exchange reaction between Ca2+and Pb²⁺can take place easilyduring removing heavy metal ions （especially the lead ion）. Consequenly, it will generatehighly stable lead hydroxyapatite （PbHAP）. Therefore, we can use Pb²⁺to activate HAP andthen to adsorb xanthates for creacting a hydrophobic surface, which is important for theflotation separation.As well known, cadmum is a very toxic element for human beings especially itspoisonous effect on human bones. However, to the best of our knowledge, the solubilityproduct of CdHAP at25℃has not been reported so far. The solubility product of HAP andcadmium hydroxyapatite （CdHAP） plays an important role in evaluating of the adsorptionmechanism. Therfore we have determined the solubility product of CdHAP in this study usingsolubility method in combination with Medusa software calculations.In this paper, HAP, PbHAP and CdHAP samples were successfully synthesized by usingthe sol-gel, microwave-assisted liquid precipitation and hydrothermal method, respectively.We analysed HAP through respectively chemical composition, XRD and BET technique. TheBET specific surface area of synthesized HAP sample is51.49m g^-1, and particle size is18.44nm which belongs to the nanoparticles. On the other hand, through the determination ofthe solubility of HAP (Ca₁₀（PO₄）₆（OH）₂) and CdHAP(Cd₁₀（PO₄）₆（OH）₂) samples, thesolubility （log K） at25℃of them are-78.0and-99.2respectively. Furthermore, it is anfundamental criterion in removing heavy metals such as lead or cadmium ions in wastewatertreatment using apatite mineral as adsorbents.The adsorption of Pb²⁺by HAP from aqueous solution was studied. The influence ofdifferent sorption parameters such as contact time, pH of solution and initial concentration of Pb²⁺were investigated and the adsorption mechanism was discussed. Results show that theHAP has good adsorption capability on Pb²⁺, the equilibrium can be achieved after120minutes and kinetic data are well fitted with a pseudo-second-order model and Langmuiradsorption isotherm can be used to describe the experimental data. The maximum capacity ofPb²⁺adsorbed by HAP is515.61mg g^-1. In short, pH value did have the greatest effect duringthe adsorption process, and the adsorption mechanism, which is more complicated can bedescribed as follows: at the pH value between2.92⁴.54, the removal of Pb²⁺is due to thePbHPO₄precipitation formed by HAP dissolution; when the pH value is between4.54⁷.38,it is due to the formation of Pb₃（PO₄）₃precipitation; when the pH value is between7.38⁷.93,PbHAP precipitation is one of the main function of removing Pb²⁺; when the pH valuereaches8.86, however, Pb²⁺removal mechanism may be due to the precipitation of Pb（OH）₂.We studied the xanthates adsorption onto Pb-HAP and PbHAP and their hydrophobicproperties by performing adsorption experiments using ATR/FTIR technique and cantactangle measurements. The absorption equilibrium can be reached at90minutes and150minutes respectively. The adsorption process can fit well the pseudo-first-order andpseudo-second-order model respectively, and the maximum capacity are182.98mg g^-1and596.48mg g^-1respectively. The desorption equilibrium can be achieved at10minutes and180minutes, and the desorption percentage are16.1%and5.3%respectively. It indicated thatthe xanthate has a strong chemisorption character on the surface of the Pb-HAP.The HAP modified by lead ion can adsorb xanthates efficiently and therefore enhance itssurface hydrophobicity. According to the contact angle experiment, we can see that thePb-HAP by absorbing the xanthates has the obvious hydrophobic effect and even canmaintain about3minutes, so it may offer the theoretical basis for developing feasible methodfor the mineral separation by flotation, such as between those of apatite, dolomite or calcite.