Pb Mineral Precipitation in Solutions of Sulfate, Carbonate and Phosphate: Measured and Modeled Pb Solubility and Pb2+ Activity

Abstract

Lead (Pb) solubility is commonly limited by dissolution–precipitation reactions of secondary mineral phases in contaminated soils and water. In the research described here, Pb solubility and free Pb2+ ion activities were measured following the precipitation of Pb minerals from aqueous solutions containing sulfate or carbonate in a 1:5 mole ratio in the absence and presence of phosphate over the pH range 4.0–9.0. Using X-ray diffraction and Fourier-transform infrared spectroscopic analysis, we identified anglesite formed in sulfate-containing solutions at low pH. At higher pH, Pb carbonate and carbonate-sulfate minerals, hydrocerussite and leadhillite, were formed in preference to anglesite. Precipitates formed in the Pb-carbonate systems over the pH range of 6 to 9 were composed of cerussite and hydrocerussite, with the latter favored only at the highest pH investigated. The addition of phosphate into the Pb-sulfate and Pb-carbonate systems resulted in the precipitation of Pb3(PO4)2 and structurally related pyromorphite minerals and prevented Pb sulfate and carbonate mineral formation. Phosphate increased the efficiency of Pb removal from solution and decreased free Pb2+ ion activity, causing over 99.9% of Pb to be precipitated. Free Pb2+ ion activities measured using the ion-selective electrode revealed lower values than predicted from thermodynamic constants, indicating that the precipitated minerals may have lower KSP values than generally reported in thermodynamic databases. Conversely, dissolved Pb was frequently greater than predicted based on a speciation model using accepted thermodynamic constants for Pb ion-pair formation in solution. The tendency of the thermodynamic models to underestimate Pb solubility while overestimating free Pb2+ activity in these systems, at least in the higher pH range, indicates that soluble Pb ion-pair formation constants and KSP values need correction in the models.

Recycling of Lead Pastes from Spent Lead–Acid Batteries: Thermodynamic Constraints for Desulphurization

“…Cerussite (PbCO 3 ) is the dominant lead phase in carbonate-bearing solutions [14,42,43]. Therefore, lead pastes can be desulphurized in carbonate-bearing solutions by taking advantage of the stability of cerussite in such solutions [44].…”

Worth a Closer Look: Raman Spectra of Lead-Pipe Scale

“…The chemistries of natural drinking-water sources differ (e.g., pH, alkalinity, and concentrations of Ca, Al, and dissolved inorganic carbon) and, thus, may require site-specific optimization of the approaches to their corrosion-control treatment. For instance, with respect to orthophosphate additives, it is important to understand how chemical differences in the precipitation solutions will affect the identity, chemical-structural properties, and thus, solubility of the phosphate phases that form [25]. In order to explore some of the compositions of phosphohedyphane-like phases identified in studies of harvested LSLs [12,17,18], our group initiated synthesis experiments (experimental details in SI Section S6) using phos-phate solutions with different molar ratios of Ca/Pb and (Ca+Pb)/P in the presence or absence of NaCl.…”

“…In this case, pyromorphites of various OH-Cl-F chemistries demonstrated the ability to accommodate within their structures some of the carbonate dissolved in the water, thereby precluding formation of a separate lead carbonate phase (cf. [25]). The differences in anion content among the pyromorphites is spectroscopically detectable (especially in the OH-region of the spectrum; see inset in Figure S3, panel A), and some cation substitution also probably accounts for the visible shifts in the ν 1 peak positions (Figure 8).…”

“…In principle, one can calculate which phase will be stable under specific compositional and pH conditions in the drinking/synthetically produced water. However, bulk XRD analyses of scale material in some cases show deviations from the predicted scale minerals [2,4,11,12], which means that the predicted lead concentrations in the water will be in error [25]. These deviations should be investigated.…”

Characterization of Mine Waste from a Former Pb–Zn Mining Site: Reactivity of Minerals During Sequential Extractions

“…The primary Pb-and Zn-bearing minerals, related to the ores that were mined, identified are: sphalerite (ZnS) (PL_2, PL_4) and galena (PbS) (all samples), as well as secondary minerals, such as willemite (Zn2SiO4) (PL_1, PL_4), cerussite (PbCO3)(all samples), anglesite (PbSO4) (PL_2), plumbojarosite (PbFe6(SO4)4(OH)12) and beudantite (PbFe3(AsO4) SO4)(OH)6) (PL_4). During progressive weathering of galena, cerussite or anglesite forms, depending mainly on the pH value [23,24]. The primary PbS is often coated with these alteration products [25].…”