Transformation of Pb(II) from Cerrusite to Chloropyromorphite in the Presence of Hydroxyapatite under Varying Conditions of pH Page: 7 of 27
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Ca in apatite may be possible (12). It has been found that formation of pyromorphite from
soluble phosphate and Pb is a rapid process; soluble Pb can be depleted from solution within a
few seconds if a stoichiometric amount of phosphate is provided and the system is well mixed as
observed in our laboratory. Thus the soluble Pb concentration in the aqueous system containing
apatite and Pb-bearing solids will be determined by the dissolution rate of apatite and the overall
Pb release rate (desorption, dissociation and dissolution) of the Pb species. Therefore, knowledge
of reaction kinetics in the Pb-bearing solids and phosphate mineral provides additional
information needed in the evaluation of the technology of immobilization of soil Pb and offers
the basis to improve the technology.
To demonstrate this, a series of kinetic experiments were conducted in systems containing
hydroxyapatite and cerrusite to determine the effects of dissolution rates of apatite and cerrusite
on: 1) the soluble Pb level during the reactions; and 2) formation of pyromorphite. Cerrusite is a
secondary Pb mineral commonly found in soils (20, 21). It is also a weathering product of other
secondary Pb minerals such as anglesite (PbSO4) under appropriate condition of CO2 partial
pressure or activity (22). Cerrusite is not stable and dissolves under acid pH conditions, e.g.,
acidified rain (23). It has been demonstrated that the bioavailability of Pb carbonate was higher
than other soil Pb species such as anglesite and Fe-Mn associated Pb. Further, the Pb content in
the plants grown on the Pb contaminated soils was primarily associated with soil Pb carbonate
content rather than the total soil Pb concentrations (24, 25). Thus, cerrusite can be an important
mineral form of Pb in soil in controlling the soil Pb mobility and reactivity, it also can be a highly
bioavailable form of soil Pb.
Materials and Methods4
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Ryan, J.A. & Zhang, P. Transformation of Pb(II) from Cerrusite to Chloropyromorphite in the Presence of Hydroxyapatite under Varying Conditions of pH, article, October 14, 1998; Albuquerque, New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc675844/m1/7/: accessed July 16, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.