 |
| Titel |
Experimental model of cerussite PbCO3 transformation into phosphate phases at pH 3-11 |
| VerfasserIn |
Monika Kwaśniak-Kominek, Maciej Manecki |
| Konferenz |
EGU General Assembly 2014
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250086990
|
| Publikation (Nr.) |
 EGU/EGU2014-945.pdf |
|
|
|
|
|
| Zusammenfassung |
| Bioavailability of lead depends strongly on mineral speciation. Mobility of this toxic element
is strongly reduced when lead in the form of relatively soluble phases (carbonates) is
transformed into insoluble forms like phosphates. Despite the fact, that in-situ immobilization
of Pb by phosphate amendment (phosphate-induced lead immobilization) is widely applied in
remediation of polluted soils and waters, the mechanism of transformation of lead carbonate
into lead phosphates is only recently under investigation [1]. Our understanding of
this mechanism might in future allow for optimization of the methods applied in
environmental engineering. Although the transformation of carbonate minerals into
phosphate phases has been studied before, little is known of this system with Pb.
There is no systematic study of the effect of pH on the mechanism and the reaction
product.
The mechanism of cerussite reaction with phosphate solution at pH 3 – 11 was studied
using fragments of natural gem-quality crystals from Morocco. The mineral was reacted with
0.1M PO43- or a mixture of 0.1M PO43- and 0.03M Cl- either at 140 °C in the autoclave
for 24 hours or at 8 °C in the cooler (for around 6 weeks). Reaction products were identified
ex-situ by X-Ray diffraction (XRD) and micro Raman spectroscopy. Scanning electron
microscopy and microprobe analysis (EMP) were applied to cut and polished crystals in
epoxy.
The products of the reaction depend on pH. This dependency is the most pronounced
in Cl—free environment. At low pH (below 6) phosphoshultenite PbHPO4 and
hydroxylpyromorphite Pb5(PO4)3OH (HPY) are formed while at alkaline range HPY is the
only product. Pyromorphite Pb5(PO4)3Cl (CPY) forms in the wider pH range. The
replacement reaction proceeds through coupled dissolution-precipitation mechanism. The
lead phosphate coating does not armor the surface resulting in complete replacement of
cerussite grain by polycrystalline porous reaction products. There is evidence for formation
of precursors at the reaction front, which later undergo transformation into final products.
This is probably combine effect of carbonate dissolution associated with relatively slow
diffusion of ions though porous coating, affecting both pH and saturation indices. High
concentrations of carbonate ions result in CO32- substitution in the structure of precipitating
HPY. This is less pronounced or non-existing in the case of CPY precipitating in the presence
of Cl-.
Monika Kwaśniak-Kominek was partially financed from AGH-UST grant No.
15.11.140.329. Maciej Manecki was financed from research grant No. NCN
2011/01/M/ST10/06999.
[1] Wang L., Putnis CV, Ruiz-Agudo E., King HE, Putnis A. Coupled Dissolution and
Precipitation at the Cerussite-Phosphate Solutions Interface: Implications for Immobilization
of Lead in Soils, Environmental Science & Technology, 2013. |
|
|
|
|
|
|