Microstructural characteristics of naturally formed hardpan capping sulfidic copper-lead-zinc tailings (original) (raw)
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Environmental Pollution, 2019
Hardpans are massively indurated layers formed at the top layer of sulfidic tailings dams, which develop cementation structures and result in heavy metal immobilization. However, the micro-structural and complex forms of the cementing materials are not fully understood, as well as the mechanisms by which Zn and Pb are stabilized in the hardpans. The present study deployed synchrotron-based X-ray fluorescence microscopy (XFM) to have characterized the cementing structures, examined the distribution of Fe, Zn and Pb, and obtained laterally-resolved speciation of Zn within the hardpans using fluorescence X-ray absorption near-edge structure (XANES) imaging. The XFM analyses revealed that the Fe-rich cement layers consisted of Fe (oxyhydr)oxides coupled with amorphous Si materials, immobilizing Zn and Pb. Through laterally-resolved XANES imaging analyses, Znferrihydrite-like precipitates were predicted to account for > 76% of the total Zn within the Fe-rich cement layers. In contrast, outside of the cement layers, 9-63% of the Zn was estimated as labile ZnSO 4. 7H 2 O, with the remainder in the form of Zn-sulfide. These findings demonstrated that the Fe-rich cement layers were critical in immobilizing Zn and Pb within hardpans via mineral passivation and encapsulation, as the basis for long-term geochemical stability in the hardpan layer of sulfidic mine tailings.
Journal of Hazardous Materials, 2019
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2020
Extensive mineral weathering and formation of large amounts of Fe-rich secondary mineral gels have been identified as precursors critical to forming massive hardpan caps in the surface layers of sulfidic tailings. However, how to initiate and accelerate these precursor processes remains to be established before developing this hardpan-based novel method to rehabilitate sulfidic tailings landscapes. In a 5-month microcosm experiment, the present study has demonstrated the concept of bio-engineering sulfidic tailings by inoculating Fe/S-oxidizing bacterial consortium to accelerate the weathering of sulfides and other Si-rich minerals for mineral gels formation. Synchrotron-based X-ray absorption fine structure spectroscopy (XAFS) demonstrated that the weathering of pyrite and biotite-like minerals was rapidly accelerated by the presence of Fe/S-oxidizing bacterial consortium. The microbial process and associated mineral transformation led to the formation of critical precursor mineral...
Environmental Geology, 2003
Column leaching experiments were used to determine the effects of an iron-rich hardpan layer, on the rate of tailings oxidation and the composition of leachate waters, from the Renison Bell tailings dams in western Tasmania, Australia. One-meter-long PVC columns, filled with tailings, cover material (Cassiterite Flotation Tailings) and hardpan samples from the tailings dams, were leached over a period of 14 weeks. Under dry cover conditions, when hardpan was present, the solute loads peaked at 21-49 days (Fe at 2,294 ppm and SO 4
The role of climate in hardpan formation in mine tailings and its environmental effect
2020
Mine tailings discharge is one of the most important environmental aspects to be taken into consideration for most mines before, during and after operations. To prevent environmental problems resulting from the weathering of sulfide minerals within the tailings impoundments and the subsequent generation of acid mine waters rich in SO − 2 4 , Fe 2+ and some toxic metals, or through erosion and spreading out to e.g agricultural areas, a detailed investigation of a number of aspects including the mineralogical, geochemical and sedimentological status quo are required. In this study, mine tailings located in completely different climatic regions (Germany, sulfide mine tailings and Egypt, gold mine tailings) were studied for their mineralogical, geochemical and physical properties, in addition to, the tailings disposal techniques used, in order to understand the processes affecting hardpan formation. This was combined with column experiments for monitoring the transport/precipitation processes in small scale. Mineral Liberation Analysis (MLA) was applied, especially for the samples from Germany in order to quantify the textural, mineralogical and chemical changes in a µm-and mm scale for better understanding mineral alteration, and precipitation of secondary phases and gels, and last but not least the processes leading to the formation of hardpans. From the following study it was concluded, that thickened tailings disposal in Egypt, did not lead to grain size and mineral fractionations, and therefore the critical amount of reactive phases cannot be achieved within the lamina required for hardpan formation. This is in sharp contrast to the tailings disposal in Germany where deposition of the slurry by spigot points caused mineral/grain size segregations and therefore different lamina with contrasting physical, mineralogical and chemical pattern enhancing hardpan formation. The amounts of acid producing sulfides and acid buffering compounds such as carbonates and e.g plagioclase, biotite in these lamina are critical for the very local acid/neutralization potentials. The higher amount of carbonates compared to reactive sulfides in a homogeneous environment (Egypt case, Barramiya site) doesn't favour hardpan formation compared to slightly higher sulfides in a highly heterogeneous sequence (Germany case). The availability of water, oxygen, and temperature, besides the basic premises such as microbiology, mineralogy, grain size etc., drive reactions within the tailings. During the rarely rain fall events in Egypt, water mainly runs off, little is soaked by the material and driven out quickly by capillary ABSTRACT ii transport generating scattered efflorescence, since reaction times are minimized. Alteration of the few sulfides generates rims of Fe-sulfates around the sulfide grains and disseminated gypsum spots close to the upper surface and in the immediate vicinity of desiccation cracks. The tailings in Germany are characterized by a comparable humid climate which favours periods of raining events (mobilization directions downward) and periods of drying (mobilization upward under capillary forces) which are very important for oxidation, transport and precipitation processes within and between the lamina. The increased electrical/fluid conductivities especially in the critical zone above the water table depend mainly on the fast rising capillary head fluids which accumulate all the available ions around the particles and moved upward. Efflorescent sulfate crusts were recognized as first alteration products in the course of element mobilization combined with evaporation in column experiments, especially with high temperatures. Within these crusts toxic elements like As and Pb can be fixed through co-precipitation processes. However, their attenuation depends on the stability of the hosting secondary phases in the new environment over time even under climatic changes. The development of hard crusts capable of inhibiting rain and oxygen infiltrations is highly enhanced in areas with high temperature and water contents but their formation requires definitely an elevated potential of reactive compounds in the accessible zone of alteration and a multiple repetition of drying and wetting cycles, and eventually long reaction time, dependent on the primary mineral and grain size distribution.
Applied Geochemistry, 2007
The roles of mineral dissolution, precipitation, transformation and mass transport processes related to formation of characteristic cemented layer-hardpan sequences were studied in low sulfide and low carbonate Freiberg polymetallic mine tailings. Using high resolution profiling, combined geochemical, geomicrobiological and geophysical methods allowed description of the process of weathering of reactive mineral phases and the position of the oxidation front in detail, as well as revealing the mechanisms of cementation of tailings predominantly by the formation of gels/poorly crystalline phases. Autochthonous and allochthonous gels reduced the porosity of cemented layers to values 61%, whereas secondary crystalline phases were less efficient in filling the pore space. Electron microprobe analysis of cemented tailings showed that common jarosite-group minerals contained up to about 8 wt.% PbO and 0.2-1.9 wt.% As 2 O 5. Iron-As-Si gels reached contents of up to $44 wt.% As 2 O 5 in gel-rich cemented layers. Zinc was below the detection limit in the studied secondary phases. Sequential extraction of cemented and related oxidized brown silt layers confirmed that the bulk of As was bound to amorphous/poorly crystalline hydrous oxides of Fe, whereas Pb was often bound to jarosite. Zinc was found preferentially in the water-soluble and the exchangeable fractions. In the grey silt and the sand directly underlying the oxidized layers, As, Pb and Zn occurred as sulfide minerals. The main effects of the cemented layer-hardpan sequences at the studied site are (1) a temporary natural attenuation of the toxic compounds, (2) a restriction of the downward movement of the oxidation front, and (3) a reduction of the extent of the erosion of the surface of the tailings impoundment by wind and water. The potential of a heap to form cemented layers and hardpans is greatly increased by a heterogeneous distribution of grain sizes and reactive materials in the topmost zone, as well as by the occurrence of sulfide-rich tailings on top of layers with low permeability.
Lead and zinc in the structure of organic and mineral soil components
Revista Brasileira de Ciência do Solo, 2013
In addition to the more reactive forms, metals can occur in the structure of minerals, and the sum of all these forms defines their total contents in different soil fractions. The isomorphic substitution of heavy metals for example alters the dimensions of the unit cell and mineral size. This study proposed a method of chemical fractionation of heavy metals, using more powerful extraction methods, to remove the organic and different mineral phases completely. Soil samples were taken from eight soil profiles (0-10, 10-20 and 20-40 cm) in a Pb mining and metallurgy area in Adrianópolis, Paraná, Brazil. The Pb and Zn concentrations were determined in the following fractions (complete phase removal in each sequential extraction): exchangeable; carbonates; organic matter; amorphous and crystalline Fe oxides; Al oxide, amorphous aluminosilicates and kaolinite; and residual fractions. The complete removal of organic matter and mineral phases in sequential extractions resulted in low participation of residual forms of Pb and Zn in the total concentrations of these metals in the soils: there was lower association of metals with primary and 2:1 minerals and refractory oxides. The powerful methods used here allow an identification of the complete metal-mineral associations, such as the occurrence of Pb and Zn in the structure of the minerals. The higher incidence of Zn than Pb in the structure of Fe oxides, due to isomorphic substitution, was attributed to a smaller difference between the ionic radius of Zn 2+ and Fe 3+ .
Applied Geochemistry, 2009
Desert mine tailings may accumulate toxic metals in the near surface centimeters because of low water through-flux rates. Along with other constraints, metal toxicity precludes natural plant colonization even over decadal time scales. Since unconsolidated particles can be subjected to transport by wind and water erosion, potentially resulting in direct human and ecosystem exposure, there is a need to know how the lability and form of metals change in the tailings weathering environment. A combination of chemical extractions, X-ray diffraction, micro-X-ray fluorescence spectroscopy, and micro-Raman spectroscopy were employed to study Pb and Zn contamination in surficial arid mine tailings from the Arizona Klondyke State Superfund Site. Initial site characterization indicated a wide range in pH (2.5-8.0) in the surficial tailings pile. Ligand-promoted (DTPA) extractions, used to assess plant-available metal pools, showed decreasing available Zn and Mn with progressive tailings acidification. Aluminum shows the inverse trend, and Pb and Fe show more complex pH dependence. Since the tailings derive from a common source and parent mineralogy, it is presumed that variations in pH and ''bio-available" metal concentrations result from associated variation in particle-scale geochemistry. Four sub-samples, ranging in pH from 2.6 to 5.4, were subjected to further characterization to elucidate micro-scale controls on metal mobility. With acidification, total Pb (ranging from 5 to 13 g kg À1 ) was increasingly associated with Fe and S in plumbojarosite aggregates. For Zn, both total (0.4-6 g kg À1 ) and labile fractions decreased with decreasing pH. Zinc was found to be primarily associated with the secondary Mn phases manjiroite and chalcophanite. The results suggest that progressive tailings acidification diminishes the overall lability of the total Pb and Zn pools.
Polish Journal of Environmental Studies, 2005
The aim of our paper was to determine selected morphometric indices of the main fabric units of humus layers in soils with different vegetation cover and contaminated by heavy metals. The study sampled soil from three sites located about 4 km NW from a zinc plant. In all humus layers decomposed organic matter occurred mainly in form of fine excrements, originated from small soil arthropods, while macrofauna excrements were the absent. The dominance of soil mesofauna over macrofauna was probably attributed to elevated heavy metal concentrations. The high concentration of heavy metals affected the major soil components, but the kind of vegetation and the degree of cover of the soil surface were as important for the development of the humus layers as heavy metal contamination.
Interaction of Humic Acid with Cu/Pb-Zn Tailings of Different Degrees of Weathering
Soil Science Society of America Journal, 2017
Aggregation of tailings is instrumental for developing a physical environment permitting water infiltration, aeration, and root penetration. Previous research found that microaggregation of Cu-Pb-Zn tailings was stimulated by amendment of composted sugarcane residues. The present study aimed to identify organic functional groups in humic acids (HA) involved in the aggregation of Cu-Pb-Zn tailings, which were considered to be formed from long-term organic matter decomposition in the engineered tailing-soil. The adsorption of HA by the weathered (WT) and freshly deposited (FT) tailings was evaluated at a range of pH conditions in tailing-HA suspension. Humic acid adsorption decreased with increasing pH from 5.0 to 9.0. The FT was found to contain more oxy-(hydr)oxides, particularly poorly crystalline Fe and Al oxy-(hydr)oxides than the WT, which was one of the factors attributing to FT's higher HA affinity. The Fourier transform infrared spectroscopy analysis of tailing particles before and after HA adsorption indicated that carboxylic and phenolic groups of HA preferentially interacted with tailing mineral particles, probably through ligand exchange, polyvalent cation bridge and electrostatic attraction. The pH-dependent HA adsorption indicated the potential role of functional groups of organic matters in microaggregation of Cu/Pb-Zn tailings. Moreover, the presence of abundant Al/Fe oxyhydroxides promoted organo-mineral interactions. The present findings will contribute toward developing field application to engineer functional Technosols from the tailings by selecting organic matter rich in functional groups. Abbreviations: HA, humic acid; FT, freshly deposited tailings; OM, organic matter; WT, weathered tailings. L arge volumes of soil and cover materials are required to reconstruct root zones and soil profile for tailings rehabilitation at metal mines, leading to huge financial costs associated with the closure and rehabilitation of tailings storage facilities. Developing Technosols from mine tailings by ecological engineering inputs and practices have been advocated as a cost-effective and ecologically sustainable technology for rehabilitating large areas of tailings landforms at mine sites (Huang et al., 2014). While many studies in literature focused on physical development in natural soils, much less is known about aggregation in novel and engineered parent materials like tailings. Rehabilitating physical properties (e.g., aggregates and aggregate stability, development of macropores) in tailings of silty texture is one of the critical challenges when engineering soil formation from mine tailings toward functional technosols with the primary goal of supporting sustainable plant communities (Uzarowicz and Skiba, 2011; Huang et al., 2014). The formation of microaggregates (<250 mm) is a process of solid-phase reaction among organic matter (OM), polyvalent cations and mineral particles of different sizes (Edwards and Bremner, 1967), mainly through organo-mineral interactions (Tisdall and Oades, 1982). These interactions are the foundation of soil