(Na, Pb)-Jarosite nucleation and growth on anglesite: Implications for inhibition of Pb releasing

The mobility and bioavailability of Pb can be significantly reduced by Pb-bearing minerals encapsulation in jarosite-group minerals, especially in sulfate-rich environments. However, the kinetic pathways and mechanisms of jarosite-group minerals formations on Pb-bearing mineral surfaces are not well understood. Here, time-resolved heterogeneous (Na, Pb)-jarosite nucleation and growth on anglesite were explored to gain insights into the encapsulation mechanisms. The initial dissolution of anglesite were clearly distinguished, and for the first time, the facet-specific heterogeneous nucleation of (Na, Pb)-jarosite on anglesite was demonstrated. Density functional theory calculations revealed higher adsorption energies and electronic interactions of FeSO4+ complex on anglesite (020), (140), (110) facets, attributed to the preferential nucleation of (Na, Pb)-jarosite on these facets, which resulted in effective passivation of the facets resistant to dissolution. An interpretation was proposed where (Na, Pb)-jarosite grew via a particle-attachment pathway involving the formation of amorphous intermediate, and subsequently, it transformed to the crystalline phase by solid-state conversion. These observations might improve the mechanistic understanding of interface interactions between slightly soluble Pb-bearing minerals and iron minerals, with implications for Pb immobilization in sulfate-rich environments.

A fast one-pot synthesize of crystalline anglesite by hydrothermal synthesis for environmental assessment on pure phase

Anglesite (PbSO₄) is a lead sulfate that belongs to the barite group and is naturally ubiquitous in the environment. This work describes a simple way to synthesize crystalline lead sulfate by using a straightforward hydrothermal procedure. Typically, Pb(NO₃)₂ and Fe₂(SO₄)₃ precursors were mixed and heated at 94 °C for 24 h. The synthesized samples have been characterized by coupling X-Ray diffraction (XRD) to spectroscopic methods (FTIR and micro-Raman), X-ray absorption spectroscopy (XAS), and electronic microscopy (SEM and TEM). In fine, the results about this new well crystalline synthetic anglesite confirm the efficiency and the importance of this cheap protocol and the synthesized phases obtained. Moreover, the environmental stability and bioaccessibility of anglesite have been done to evaluate environmental stability of anglesite under various physico-chemical conditions and sanitary risks. Finally, the paper allows to obtain precise data on a pure phase in order to be able to more easily evaluate and understand the role of anglesite in as-polluted sites and soils.

Induced biotransformation of lead (II) by Enterobacter sp. in SO4-PO4-Cl solution

Pb is a toxic heavy metal in contaminated soil and water, resulted from industrial activities, mine exploration, etc. Phosphate solubilizing bacteria are able to secrete organic acids and further to enhance the solubility of phosphates. Enterobacter. sp and geological fluorapatite (FAp) were applied to investigate the biotransformation of Pb²⁺ in solution with SO₄^2-, PO₄^3-, and Cl^- species by ICP-OES, ATR-IR, XRD, and SEM. Enterobacter. sp can lower pH of the medium to ∼4. Meanwhile, >90% mobile Pb (declining from 1000 to 30 ppm) was immobilized via the combination of Enterobacter. sp and FAp. With the addition of FAp and Pb, pyromorphite was precipitated, but with relatively low content. In contrast, abundant anglesite mineral was formed in such weakly acidic system. These anglesite crystals can even absorb phosphates particles onto their surface. Additionally, geochemical modeling confirms the formation of anglesite and cerussite under weekly acidic and alkalic condition respectively, especially when H₂PO₄^- concentration <10^-8 mM. Furthermore, the presence of Cl^- in solution leads to the formation of chloropyromorphite when H₂PO₄^- concentration >10^-12 mM, especially under neutral environment. This study explored the biotransformation of Pb in SO₄-PO₄-Cl aqueous system and hence provided guidance on bioremediation of Pb by bacteria and FAp.

Assessing the influence of humic acids on the weathering of galena and its environmental implications

Galena weathering often occurs in nature and releases metal ions during the process. Humic acid (HA), a critical particle of natural organic matter, binds metal ions, thus affecting metal transfer and transformation. In this work, an electrochemical method combined with spectroscopic techniques was adopted to investigate the interfacial processes involved in galena weathering under acidic and alkaline conditions, as well as in the presence of HA. The results show that the initial step of galena weathering involved the transformation Pb²⁺ and S°, regardless of whether the solution was acidic or alkaline. Under acidic conditions, S° and Pb²⁺ further transform into anglesite, and HA adsorbs on the galena surface, inhibiting the transformation of sulfur. HA and Pb (II) ions form bridging complexes. Under alkaline conditions without HA, the sulfur produced undergoes no transformation, whereas Pb²⁺ will transform into PbO. The presence of HA changes the galena weathering mechanism via ionization effect, and Pb²⁺ is ultimately converted into anglesite. Higher acidity in acidic conditions or higher alkalinity in alkaline conditions causes galena corrosion when the electrolyte does not contain HA. Conversely, higher pH always accelerates galena corrosion when the electrolyte contains HA, whether the electrolyte is acidic or alkaline. At the same acidity/alkalinity, increasing the concentration of HA inhibits galena weathering. Galena will release 134.7 g m^-2·y^-1 Pb²⁺ to solution at pH 2.5, and the amount decreases to 28.09 g m^-2·y^-1 in the presence of 1000 mg/L HA. This study provides an in situ electrochemical method for the assessment of galena weathering.

Cd isotope fractionation during sulfide mineral weathering in the Fule Zn-Pb-Cd deposit, Yunnan Province, Southwest China

Zinc (Zn)-Lead (Pb) deposits are generally rich in cadmium (Cd), and the weathering of sulfide minerals in such deposits results in large releases of Cd into the environment. From an environmental and public health standpoint, understanding Cd sources and cycling is critical to identifying potential hazards to humans. In this study, the Cd isotope compositions (expressed as δ^114/110Cd) of secondary minerals such as anglesite (-0.57±0.03‰; 2S.D.), granular smithsonite (0.04±0.14‰; 2S.D.), layered smithsonite (0.15±0.40‰; 2S.D.), hydrozincite (0.26±0.01‰; 2S.D.) and clay minerals (-0.01±0.06‰; 2S.D.) from the Fule Zn-Pb-Cd deposit, Southwest China, are investigated to better understand the Cd sources and cycling in this area. Combined with our previous study (Zhu et al., 2017), the work herein elucidates the patterns of Cd isotopic fractionation during the formation processes of such secondary minerals and traces the weathering of these minerals into the ecosystem. The δ^114/110Cd values of secondary minerals exhibit the following decreasing trend: hydrozincite>large granular smithsonite>small granular smithsonite>anglesite. Although different amounts of Cd were lost during the formation of equally sized samples, no or minor variations in Cd isotopic composition were observed. However, significant isotopic differences were observed between different size fractions. These results demonstrate that the particle size of secondary minerals and weathering products of sulfide significantly influence Cd isotope composition and fractionation during natural weathering. This systematic fractionation provides an initial foundation for the use of Cd isotopes as environmental tracers in ecosystems and in the global Cd isotope budget.

Alterations of lead speciation by sulfate from addition of flue gas desulfurization gypsum (FGDG) in two contaminated soils

This is the first study to evaluate the potential application of FGDG as an in situ Pb stabilizer in contaminated soils with two different compositions and to explain the underlying mechanisms. A smelter Pb contaminated soil (SM-soil), rich in ferrihydrite bound Pb (FH-Pb), cerussite and litharge with a total Pb content of 65,123mg/kg and an organic matter rich orchard soil (BO-soil), rich in FH-Pb and humic acid bound Pb with a total Pb content of 1532mg/kg were amended with 5% FGDG (w/w). We subjected the two soils to three leaching tests; toxicity characteristic leaching protocol (TCLP), synthetic precipitation leaching protocol (SPLP), kinetic batch leaching test (KBLT) and in-vitro bioaccessibility assay (IVBA) in order to evaluate the FGDG amendment on Pb stabilization. Solid residues of original and FGDG amended soil were analyzed using X-ray absorption spectroscopy (XAS) to identify changes in Pb speciation after each leaching test. The leachate Pb concentrations of FGDG amended soil were lower compared to those of in non-amended soil. The linear combination fitting analysis of XAS confirmed the formation of anglesite and leadhillite in FGDG amended soil. FGDG reduced the Pb desorption from ferrihydrite (FH), by forming FH-Pb-SO4 ternary complexes. FGDG decreased the Pb adsorption onto humic acid (HA) possibly due to the release of divalent cations such as Ca and Mg, which can compete with Pb to get adsorbed onto HA. The FGDG can successfully be used to remediate Pb contaminated soil. The efficiency of the treatment highly depends on the soil composition.

Recovery of lead from smelting fly ash of waste lead-acid battery by leaching and electrowinning

Fly ash that was enriched with lead (Pb), formed as an intermediate in waste lead-acid battery (WLAB) smelting, was recycled by the hydro-electrometallurgy. Characterization of fly ash thereof indicated that the Pb was in the forms of PbSO4 (anglesite) and Pb2OSO4 (lanarkite). Nitric acid and sodium hydroxide were firstly used to study the leaching of the fly ash sample, which was affected by leachant dosage and solid-to-liquid ratio (S/L). At an S/L of 60gL(-1), the leachability of Pb was 43% and 67% in 2M acidic and basic solutions, respectively, based on an average 70wt% of Pb in the original fly ash. Anglesite was completely soluble in NaOH and lanarkite was mildly soluble in HNO3. Pb was recovered from the pregnant leach solution within an electrolytic cell constructed with graphite or RuO2/IrO2-coated titanium (Ti-DSA) anodes and a stainless steel cathode. Properties of anodes deposited with lead dioxides were analyzed by cyclic voltammetry. The optimized parameters of electrowinning were 2M NaOH leachant, a current density of 0.75Adm(-2) and an electrolytic process duration of 120min, which yielded a Pb removal of higher than 99% and a specific energy consumption of 0.57Whg(-1). This process constitutes an eco-friendly and economic alternative to the presently utilized secondary pyrometallurgy for treating lead-containing fly ash.