Studies on the Mechanisms of Lead Immobilization by Hydroxyapatite

Abstract

The sorption of lead by synthetic hydroxyapatite (HA) from solutions containing Pb2+ initial concentrations up to 1770 mg L(-1) was studied. X-ray diffractometry (XRD) associated with Rietveld methodology for refining the spectra pattern was used in order to characterize the mechanisms of lead uptake. It is shown that the dissolution of hydroxyapatite is followed by the formation of a solid solution, Pb(10-x)Ca(x)(PO4)6(OH)2, with Pb ions mostly occupying Ca(II) sites. The Ca/Pb molar ratio of this solid solution decreases continuously until it reaches the structure of a pure hydroxypyromorphite. The cell parameters and the crystallite mean size behavior of both mineral phases reinforce the hypothesis that hydroxypyromorphite, PbHA, formation is the end of a process in which Pb(10-x)Ca(x)(PO4)6(OH)2 crystallites are continuously dissolved and recrystallized producing crystals with lower calcium content. Combination of Inductively Coupled Plasma spectrometry (ICP), chemical analysis, and XRD results permitted the conclusion that lead ions are not completely immobilized by precipitating Pb(10-x)Ca(x)(PO4)6(OH)2. Additional surface mechanisms also contribute to Pb2+ uptake. During Pb2+ sorption process, pH variations of the solution phase showed a more complex pattern than previously reported. Contribution of surface mechanisms, in addition to the hydroxyapatite dissolution, could explain this behavior.

Mechanisms of lead, copper, and zinc retention by phosphate rock

“…For initial Pb concentrations of up to 0.4 mM, the equilibrium Pb concentrations were nearly at the detection limit of the analytical method employed. The obtained sorption patterns were similar to those observed elsewhere (Mavropoulos et al, 2002;Singh et al, 2001). According to Echeverria et al (1998), isotherms that have similar equilibrium concentrations for different amounts of metal added reflect a precipitation mechanism.…”

“…5). Similar results were observed in previous studies (Mavropoulos et al, 2002;Singh et al, 2001;Xu et al, 1994). In this study, metal retention induced pH reduction from 6.83 to 5.38e5.56, which was less than the PZC of PR, i.e.…”

“…Competitive metal retention by PR suggested romorphite precipitation at greater Pb concentration Table 1 Langmuir parameters for the sorption of mono-metal and multi-metal onto phosphate rock may result from the competition of Cu and Zn for phosto form solid when aqueous Zn reacted with apatite. phate ions (Chen et al, 1997;Mavropoulos et al, 2002).…”

“…Inconsistent Pb desorption responding to solution pH may indicate that not all the Pb was chemisorbed by precipitating as fluoropyromor phite, and a fraction of Pb was weakly adsorbed or complexed on the surface of PR. Comparing the total amount of the immobilized Pb with the Pb incorporated into the hydroxypyromorphite phase, Mavropoulos et al (2002) concluded that up to 30% of Pb was immobilized by other surface mechanisms such as ad sorption or complexation besides apatite dissolution and hydroxypyromorphite crystallization. So the high acid ity of pH ¼ 2:93 is likely to induce dissolution of both weakly-bound Pb and PR, facilitating formation of fluoropyromorphite and leading to reduction of Pb desorption (Basta et al, 2001 Flow calorimetry enables the direct measurement of the heat associated with a reaction Steinberg, 1981).…”

Interactions between calcium phosphate and heavy metal ions in aqueous solution

“…The use of mineral materials such as apatite showed big efficiency for decontamination of industrial waste [1][2][3][4][5][6]. Apatite is the name given to a group of crystals of the general chemical formula M 10 (RO 4 )X 2 , where R is most generally phosphorus, M could be one of the several metals, although it is usually calcium, and X is frequently hydroxide or a halogen such as fluorine or chlorine [7].…”