Search

Pseudomorphs of barite (BaSO4) and Cd-rich ZnS after whewellite (CaC2O4·H2O) occur within remnants of Scots pine bark tissues in the peat layer of a poor fen located near a zinc smelter in south Poland. A two-step formation of the pseudomorphs is postulated based on SEM observations: (1) complete dissolution of whewellite, possibly caused by oxalotrophic bacteria, and (2) subsequent bacterially induced precipitation of barite and spheroidal aggregates of ZnS together with galena (PbS) in voids left by the dissolved whewellite crystals. Local increase in pH due to microbial degradation of whewellite, elevated concentrations of Zn(II) and Ba(II) in pore water due to the decomposition of atmospheric particles of sphalerite and barite in the acidic (pH 3.5-3.8) environment, oxidation of S species during drying and rewetting of the peat layer, and subsequent partial reduction of sulfate anions by sulfur-reducing bacteria were all factors likely involved in the crystallization of ZnS and barite in the microenvironment of the post-whewellite voids.

Human activities have led to a considerable increase in trace metal cycling in recent times. The mobilized elements get subjected to a variety of processes leading eventually to their re-concentration. The polluted sites, transformed by Earth's surface processes, become similar to metal accumulations known from geological records. This study examines authigenic metal sulfide mineralization in two peatlands polluted by atmospheric deposition from a nearby Pb-Zn smelter. We use the polluted peatlands as a small-scale model of Zn-Cd-Pb sulfide deposit to determine the role of organic matter in ore genesis and the textural and structural organization of biogenic precipitates. The study shows that the air-derived metal enrichment (up to 2.3 g Zn kg−1, 1.1 g Pb kg−1, and 62 mg Cd kg−1) is retained in a thin layer (∼30 cm) around 10–15 cm below the peat surface. A combination of focused ion beam (FIB) technology and scanning (SEM) and transmission (TEM) electron microscopy reveals that micrometric spheroids are most characteristic for ZnS and (Zn,Cd)S, although the sulfides readily form pseudomorphs after different plant tissues resulting in much larger aggregates. The aggregates have a complex polycrystalline sphalerite structure much more advanced than typically obtained during low-temperature synthesis or observed in other modern occurrences. Platy highly-disordered radially-aggregated submicrometre crystals develop within the time constraints of several decades in the cold (∼15 °C) and acid (pH 3.4–4.4) peat. The less abundant Pb sulfides occur as small cube-like crystals (<1µm) between ZnS or as flat irregular or square patches on plant root macrofossils. All PbS are crystalline and defect-free. Pb ion complexation with dissolved and solid organic matter is probably responsible for the low number and equilibrium shape of PbS crystals. Iron is absent in the authigenic sulfide mineralization and occurs entirely as organically bound ferric iron (Fe3+), as revealed by Mössbauer spectroscopy. The different affinity of metals to organic matter enhances the precipitation of Zn and Cd as sulfides over Pb and Fe. Our findings demonstrate that human activities lead to the formation of near-surface stratiform metal sulfide accumulations in peat, and the polluted sites can be of use to understand and reconstruct ancient ore deposits' genesis and mechanisms of formation. [ABSTRACT FROM AUTHOR]

Soil constitutes a direct sink for elements mobilized due to mining and smelting activities. One of the desired pathways for reducing the bioavailability and toxicity of the contaminants is their transformation into sparingly soluble solid phases. Here, we report the formation of secondary mineral phases in extremely contaminated (up to 210 g Zn kg−1,102 g Pb kg−1, 5.7 g Cd kg−1, and 6.4 g As kg−1, respectively) organic-rich soil. Combining mineralogical techniques (SEM, XRD), a nonspecific sequential extraction (CISED) with Mössbauer and X-ray photoelectron (XPS) spectroscopies evidenced two poorly crystalline goethite components differing in crystallite size and As, Zn, Pb, Ca, Al, P substitutions and minor magnetite associated with plant roots (mainly Deschampsia caespitosa , Equisetum palustre, and Carex rostrate) directly below a layer of smelter-derived particles deposited into the soil. SEM was the only method that unambiguously documented the occurrence of ZnCd sulfide microsize aggregates incrusting plant roots and located in between the (hydrous) Fe oxides. Sequential extraction confirmed a complete As immobilization by goethite, while Cd forms a solid solution with ZnS and is lacking in the Fe hydroxides. The partitioning of Zn and Pb between the goethite and sulfide phases depends on soil water saturation. It is proposed that the coexistence of hydrous Fe oxides and nonferrous metal sulfides in the soil subsurface is possible because of redox heterogeneity of the rhizosphere and the decoupling of sulfur and iron cycles. Low mobility of biogenic sulfide ions and the protecting role of organic matter limits goethite sulfidation. The system remains active, adapting to the seasonally changeable plant roots ecology and fluctuations in water saturation. The obtained results are of value in remediation and managing strategies for contaminated soils and in reconstructing processes related to the formation and/or transformation of low-temperature sulfide deposits. [Display omitted] • Smelter-impacted organic-rich soils differing in water saturation were studied. • Chemical, mineralogical, and spectroscopic methods reveal metal(oid) forms in soil. • As, Pb, Zn, and Cd are sequestered in poorly crystalline secondary mineral phases. • Arsenic is immobilized in goethite, while Cd is entirely encapsulated in sulfides. • Rhizosphere is crucial for mineral phase formation and retention of metal(oid)s. [ABSTRACT FROM AUTHOR]

The supply of Cd, Cu, Fe, Pb, Zn, and Tl into a wetland in the industrial area of Upper Silesia, southern Poland via atmospheric precipitation and dust deposition has been counterbalanced by the biogenic metal sulfide crystallization in microsites of the thin (<30 cm) peat layer, despite the overall oxidative conditions in the wetland. Disequilibrium of the redox reactions in the peat pore water (pH 5.4-6.2) caused by sulfate-reducing microorganisms has resulted in the localized decrease in Eh and subsequent precipitation of micron- and submicron-sized framboidal pyrite, spheroidal ZnS and (Zn,Cd)S, and galena as revealed by high-resolution scanning electron microscopy (SEM)/energy dispersive spectrometer (EDS). Saturation index for each sulfide is at a maximum within the calculated Eh range of −80 and −146 mV. Lead was also immobilized in galena deposited in fungal filaments, possibly at a higher Eh. Thallium (up to 3 mg kg) in the peat strongly correlates with Zn, whereas Cu (up to 55 mg kg) co-precipitated with Pb. The metal sulfides occur within microbial exudates, which protect them from oxidation and mechanical displacement. Vertical distribution of toxic metals in the peat layer reflects differences in pollution loads from atmospheric deposition, which has been much reduced recently. [ABSTRACT FROM AUTHOR]