skip to main content


Title: Pb Mineral Precipitation in Solutions of Sulfate, Carbonate and Phosphate: Measured and Modeled Pb Solubility and Pb2+ Activity
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.  more » « less
Award ID(s):
1719875
NSF-PAR ID:
10325619
Author(s) / Creator(s):
; ; ;
Date Published:
Journal Name:
Minerals
Volume:
11
Issue:
6
ISSN:
2075-163X
Page Range / eLocation ID:
620
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. null (Ed.)
    This study investigated the reaction kinetics on the oxidative transformation of lead( ii ) minerals by free chlorine (HOCl) and free bromine (HOBr) in drinking water distribution systems. According to chemical equilibrium predictions, lead( ii ) carbonate minerals, cerussite PbCO 3(s) and hydrocerussite Pb 3 (CO 3 ) 2 (OH) 2(s) , and lead( ii ) phosphate mineral, chloropyromorphite Pb 5 (PO 4 ) 3 Cl (s) are formed in drinking water distribution systems in the absence and presence of phosphate, respectively. X-ray absorption near edge spectroscopy (XANES) data showed that at pH 7 and a 10 mM alkalinity, the majority of cerussite and hydrocerussite was oxidized to lead( iv ) mineral PbO 2(s) within 120 minutes of reaction with chlorine (3 : 1 Cl 2  : Pb( ii ) molar ratio). In contrast, very little oxidation of chloropyromorphite occurred. Under similar conditions, oxidation of lead( ii ) carbonate and phosphate minerals by HOBr exhibited a reaction kinetics that was orders of magnitude faster than by HOCl. Their end oxidation products were identified as mainly plattnerite β-PbO 2(s) and trace amounts of scrutinyite α-PbO 2(s) based on X-ray diffraction (XRD) and extended X-ray absorption fine structure (EXAFS) spectroscopic analysis. A kinetic model was established based on the solid-phase experimental data. The model predicted that in real drinking water distribution systems, it takes 0.6–1.2 years to completely oxidize Pb( ii ) minerals in the surface layer of corrosion scales to PbO 2(s) by HOCl without phosphate, but only 0.1–0.2 years in the presence of bromide (Br − ) due the catalytic effects of HOBr generation. The model also predicts that the addition of phosphate will significantly inhibit Pb( ii ) mineral oxidation by HOCl, but only be modestly effective in the presence of Br − . This study provides insightful understanding on the effect of residual disinfectant on the oxidation of lead corrosion scales and strategies to prevent lead release from drinking water distribution systems. 
    more » « less
  2. Monazite is a light rare earth element (REE) phosphate found in REE mineral deposits, such as those formed in (per)alkaline and carbonatite magmatic-hydrothermal systems, where it occurs in association to the development of alteration zones and hydrothermal veins. Although it has been recognized that monazite may undergo replacement by coupled dissolution-precipitation processes, currently there is no model describing the compositional REE variations in monazite resulting from direct interaction with or precipitation from hydrothermal fluids. To develop such a model requires quantification of the thermodynamic properties of the aqueous REE species and the properties of the monazite endmembers and their solid solutions. The thermodynamic properties of monazite endmembers have been determined previously using calorimetric methods and low temperature solubility studies, but only a few solubility studies have been conducted at >100 °C. In this study, the solubility products (logKs0) of LaPO4, PrPO4, NdPO4, and EuPO4 monazite endmembers have been measured at temperatures between 100 and 250 °C and saturated water vapor pressure. The solubility products are reported with an uncertainty of ±0.2 (95% confidence) according to the reaction, REEPO4(s) = REE3+ + PO43−. (see table in manuscript) The REE phosphates display a retrograde solubility, with the measured Ks0 values varying several orders of magnitude over the experimental temperature range. Discrepancies were observed between the experimental solubility products and the calculated values resulting from combining calorimetric data of monazite with the properties of the aqueous REE3+ and PO43− species available in the literature. The differences between the calculated and measured standard Gibbs energy of reaction (ΔrG0) for PrPO4, NdPO4, and EuPO4 increased with higher temperatures (up to 15 kJ mol−1 at 250 °C), whereas for LaPO4 these differences increased at lower temperatures (up to 8 kJ mol−1 at 100 °C). To reconcile these discrepancies, the standard enthalpy of formation (ΔfH0) of monazite was optimized by fitting the experimental solubility data and extrapolating these fits to reference conditions of 25 °C and 1 bar. The optimized thermodynamic data provide the first internally consistent dataset for the solubility of all the monazite endmembers, and can be used to model REE partitioning between monazite and hydrothermal fluids at >100 °C. 
    more » « less
  3. Gilbert C. Walker (Ed.)
    Unequivocally, Pb2+ as a harmful substance damaging children’s brain and nerve systems, thereby causing behavior and learning disabilities, should be detected much lower than the elevated blood lead for children, 240 nM, endorsed by US CDC considering the unknown neurotoxic effects, yet the ultralow detection limit up to sub-ppb level remains a challenge due to the intrinsically insufficient sensitivity in the current analytical techniques. Here, we present nanoemulsion (NE)-integrated single-entity electrochemistry (NI-SEE) toward ultrasensitive sensing of blood lead using Pb-ion-selective ionophores inside a NE, i.e., Pb2+-selective NE. Through the high thermodynamic selectivity between Pb2+ and Pb–ionophore IV, and the extremely large partition coefficient for the Pb2+–Pb–ionophore complex inside NEs, we modulate the selectivity and sensitivity of NI-SEE for Pb2+ sensing up to an unprecedentedly low detection limit, 20 ppt in aqueous solutions, and lower limit of quantitation, 40 ppb in blood serums. This observation is supported by molecular dynamics simulations, which clearly corroborate intermolecular interactions, e.g., H-bonding and π*–n, between the aromatic rings of Pb–ionophore and lone pair electrons of oxygen in dioctyl sebacate (DOS), plasticizers of NEs, subsequently enhancing the current intensity in NI-SEE. Moreover, the highly sensitive sensing of Pb2+ is enabled by the appropriate suppression of hydroxyl radical formation during NI-SEE under a cathodic potential applied to a Pt electrode. Overall, the experimentally demonstrated NI-SEE approach and the results position our new sensing technology as potential sensors for practical environmental and biomedical applications as well as a platform to interrogate the stoichiometry of target ion–ionophore recognition inside a NE as nanoreactors. 
    more » « less
  4. Rare earth elements (REE) are critical elements found in monazite, xenotime, and hydrated REE phosphates which typically form in hydrothermal mineral deposits. Accurate predictions of the solubility of these REE phosphates and the speciation of REE in aqueous fluids are both key to understanding the controls on the transport, fractionation, and deposition of REE in natural systems. Previous monazite and xenotime solubility experiments indicate the presence of large discrepancies between experimentally derived solubility constants versus calculated solubilities by combining different data sources for the thermodynamic properties of minerals and aqueous species at hydrothermal conditions. In this study, these discrepancies were resolved by using the program GEMSFITS to optimize the standard partial molal Gibbs energy of formation (ΔfG°298) of REE aqueous species (REE3+ and REE hydroxyl complexes) at 298.15 K and 1 bar while keeping the thermodynamic properties fixed for the REE phosphates. A comprehensive experimental database was compiled using solubility data available between 25 and 300 °C. The latter permits conducting thermodynamic parameter optimization of ΔfG°298 for REE aqueous species. Optimal matching of the rhabdophane solubility data between 25 and 100 °C requires modifying the ΔfG°298 values of REE3+ by 1–6 kJ/mol, whereas matching of the monazite solubility data between 100 and 300 °C requires modifying the ΔfG°298 values of both REE3+ and REEOH2+ by ∼ 2–10 kJ/mol and ∼ 15–31 kJ/mol, respectively. For xenotime, adjustments of ΔfG°298 values by 1–26 kJ/mol are only necessary for the REE3+ species. The optimizations indicate that the solubility of monazite in acidic solutions is controlled by the light (L)REE3+ species at <150 °C and the LREEOH2+ species at >150 °C, whereas the solubility of xenotime is controlled by the heavy (H)REE3+ species between 25 and 300 °C. Based on the optimization results, we conclude that the revised Helgeson-Kirkham-Flowers equation of state does not reliably predict the thermodynamic properties of REE3+, REEOH2+, and likely other REE hydroxyl species at hydrothermal conditions. We therefore provide an experimental database (ThermoExp_REE) as a basic framework for future updates, extensions with other ligands, and optimizations as new experimental REE data become available. The optimized thermodynamic properties of aqueous species and minerals are available open access to accurately predict the solubility of REE phosphates in fluid-rock systems. 
    more » « less
  5. Critical mineral deposits commonly form in magmatic-hydrothermal systems including carbonatites and/or alkaline syenites, and more evolved peralkaline granites where the rare earth element (REE) undergo a complex array of partitioning, transport and mineralization. Significant alteration and veining zones develop in these deposits and can be used to vector ore zones in the field [1]. The REE ore minerals typically reflect the characteristics of these systems, which are enriched in carbonate, fluoride, and phosphate or a combination thereof. The REE can also be incorporated into vein minerals such as calcite, fluorite and apatite where the REE3+ exchange for Ca2+ on the crystal lattice [2]. These minerals give us clues about the hydrothermal reaction paths of REE in critical mineral deposits. This study aims to: 1) present our recent findings from hydrothermal fluid-mineral REE partitioning experiments, 2) discuss thermodynamic models to simulate REE in critical mineral deposits, and 3) link the thermodynamic simulations to field observations. Hydrothermal fluid-calcite partitioning experiments were conducted between 100 and 200 °C by hydrothermal fluid mixing and precipitation [2] at near neutral to mildly alkaline pH (6 – 9). The REE concentrations in synthetic calcite crystals and aqueous fluids sampled in situ were used to fit the data to the lattice strain model [3] and using the Dual Thermodynamic approach [4]. A second type of experiment consisted of reacting natural fluorite and apatite crystals with acidic to mildly acidic (pH of 2 – 4) aqueous fluids in batch-type reactors to study the behavior of REE and mineral dissolution-precipitation reactions near crystal surfaces. The GEMS code package [5] was used to implement these new data into a thermodynamic model and simulate possible REE reaction paths in hydrothermal fluids. Two REE mineral deposits in New Mexico (Lemitar and Gallinas Mountains) present ideal case studies to illustrate how these models can be linked to field observations from natural systems. [1] Gysi et al. (2016), Econ. Geol. 111, 1241-1276; [2] Perry and Gysi (2020), Geochim. Cosmochim. Acta 286, 177-197; [3] Blundy and Wood (1994) Nature 372, 452-454; [4] Kulik (2006), Chem. Geol. 225, 189-212; [5] Kulik et al. (2013), Computat. Geosci. 17, 1-24. 
    more » « less