Effects of Low Molecular Organic Acids on Immobilazition of Lead Ions by Nanosized Hydroxyapatite
|School||Nanjing Agricultural College|
|Keywords||Nanosized hydroxyapatite organic acids immobilization of lead neutralization reaction method|
Nanosized hydroxyapatite (nHAP) exhibits greater surface area and more active sites on the surface than bulk materials, which lead to an increase in physical adsorption capacity, accompanied by a rise in ion exchange and complexation ability. Thus, the immobilization of heavy metals by nHAP would be more effective than bulk materials. In this work, we studied the immobilization of lead ions by nHAP synthesized through a sol-gel method and a neutralization reaction method, meanwhile, the adsorption of low molecular weight (LMW) organic acids on nHAP, as well as the effect of LMW organic acids on the removal of lead ions by nHAP were investigated. The main results were shown as follows:(1) Two types of nanosized hydroxyapatite (nHAP(A) and (nHAP(B)) with different crystallinity were prepared by a sol-gel method and a neutralization reaction method, respectively. And the crystallinity of nHAP(A) was smaller than nHAP(B). The nHAP(A) and nHAP(B) were used to remove lead ions from the aqueous solution, in order to study the effects of adsorption times and pH. The mechanism was also studied through the variety of pH and the analysis of XRD and TEM. Results showed that the removal ability of two materials decreased with the increasing pH and both have the largest removal amount of lead ions in pH=2～4. When the concentration of lead ions was 500mg/L, the removal capacity of lead ions by nHAP(A) was about 1.5 times higher than nHAP(B). The nHAP(A) which has stronger immobilization ability of lead ions was used to study the kinetics and the variety of pH values with different concentration lead ions in solution. It was found that the addition of nHAP to the solution containing lead ions caused a significant decrease of pH initially. It could be concluded that besides dissolution-precipitation mechanism, surface complexation mechanism should be also contributing to the lead uptake, and lead ions coordinated with≡POH on the suface of nHAP. XRD datas showed the formation of Pb(10-x)Cax(P04)6(OH)2(PbCaHAP) as an intermediate phase in the dissolution-precipitation procedure, and lead ions mainly occupied in Ca(Ⅱ) of nHAP. TEM micrographs showed that the final product was PbHAP with a low bioavailability Nanosized hydroxyapatite was proved to be effective for lead immobilization. Furthermore, poorly crystallized nHAP(A) prepared by neutralization method showed a higher efficiency in lead immobilization, and this method was relatively simple and could be used for mass production. Therefore, nHAP synthesized by neutralization method was expected to be an alternative in the remediation of heavy metal in soil and water.(2) The adsorption behavior of LMW organic acids on nHAP was studied, and five kinds of LMW organic acids were used to sudy their influnces on the removal of lead ions by nHAP and PR, including acetic, malic, oxalic, citric acid and EDTA. The results showed that the poorly crystallized nHAP(A) adsorbed greater amounts of oxalic, citric, or malic acid than the well crystallized nHAP(B). It was found that organic acids such as malic, oxalic and citric acid (<5 mmol/L) could enhance the adsorption of lead ions by nHAP(B) and PR, and having the largest removal amount of lead ions when the concentration was 1 mmol/L. Acetic, malic, citric acid and EDTA (>5 mmol/L) could decrease the removal of lead ions by nHAP(B) and PR, and their inhibitory capacities were in the order: EDTA>citric acid>malic acid>acetic acid. Oxalic acid could enhance the removal of lead ions by nHAP(B) and PR significantly, and XRD datas showed that oxalic acid enhance the removal of lead ions through the formation of lead oxalate. However, organic acids had little effect on the removal of lead ions by nHAP(A). The smaller particle sized nHAP(A) can achieve the best adsorption effect no matter what the changes of organic acid concentrations and species, it could transform the harmful lead ions to insoluble hydroxypyromorphite completely and should be the best adsorbent for the removal of lead ions from aqueous solution.Based on the above results, it could conclude that, in the process of immobilization of lead ions by nHAP in aqueous solution, not only dissolution-precipitation mechanism, but also surface complexation mechanism should be contributing to the lead uptake. In addition, the poorly crystallized nHAP(A) prepared by a neutralization reaction method was proved to be efficient for the remediation of lead-polluted water and soil. Moreover, adding low concerntration of LMW organic acids could enhance the immobilization of lead ions by nHAP and decrease the bioavailability of Pb through the formation of hydroxypyromorphite.