Micromorphology and environmental behavior of oxide deposit layers in sulfide-rich tailings in Tongling, Anhui Province, China

Revealing the release and migration mechanism of heavy metals in typical carbonate tailings, East China

Refining the occurrence characteristics of tailings hazardous materials at source is of great importance for pollution management and ecological reclamation. However, the release and transport of heavy metals (HMs) from tailings under rainfall drenching in simulated real-world environments is less well portrayed, particularly highlighting the inherent neutralisation in tailings wastes under superimposed dynamic conditions. In this study, dynamic leaching columns simulating actual conditions were used to observe the release and transport of HMs from tailings under acid rainfall infiltration at spatial and temporal scales. The release rate of trace elements (e.g., As, Cr, Ni, Pb, Cd) is high. Neutralisation in the presence of carbonate rocks in the gangue reduces HMs release intensity from tailings with high heavy metal content, along with the precipitation of iron oxides and chromium-bearing minerals, etc. In addition, the vertical differentiation of HMs is more relevant to physical processes. In the absence of carbonate rocks in gangue, the lowest pH value is reached within 1.2 h after acid rain infiltrates the tailings. At the same time, Cu, Zn and Cd are released significantly from the minerals at the superficial level. The release of As(III) is mainly concentrated in the early and late stages of water-rock contact.

A critical review on the migration and transformation processes of heavy metal contamination in lead-zinc tailings of China

The health risks of lead-zinc (Pb-Zn) tailings from heavy metal (HMs) contamination have been gaining increasing public concern. The dispersal of HMs from tailings poses a substantial threat to ecosystems. Therefore, studying the mechanisms of migration and transformation of HMs in Pb-Zn tailings has significant ecological and environmental significance. Initially, this study encapsulated the distribution and contamination status of Pb-Zn tailings in China. Subsequently, we comprehensively scrutinized the mechanisms governing the migration and transformation of HMs in the Pb-Zn tailings from a geochemical perspective. This examination reveals the intricate interplay between various biotic and abiotic constituents, including environmental factors (EFs), characteristic minerals, organic flotation reagents (OFRs), and microorganisms within Pb-Zn tailings interact through a series of physical, chemical, and biological processes, leading to the formation of complexes, chelates, and aggregates involving HMs and OFRs. These interactions ultimately influence the migration and transformation of HMs. Finally, we provide an overview of contaminant migration prediction and ecological remediation in Pb-Zn tailings. In this systematic review, we identify several forthcoming research imperatives and methodologies. Specifically, understanding the dynamic mechanisms underlying the migration and transformation of HMs is challenging. These challenges encompass an exploration of the weathering processes of characteristic minerals and their interactions with HMs, the complex interplay between HMs and OFRs in Pb-Zn tailings, the effects of microbial community succession during the storage and remediation of Pb-Zn tailings, and the importance of utilizing process-based models in predicting the fate of HMs, and the potential for microbial remediation of tailings.

Geochemical distribution and mineralogy of heavy metals in the gasification residue of coal-waste activated carbon-slurry: Insights into leaching behavior

Here, a novel approach to the detoxification and reuse of waste activated carbon (WAC) through co-gasification with coal-water slurry (CWS) is proposed. To evaluate the harmlessness to the environment of this method, the mineralogical composition, leaching characteristics, and geochemical distribution of heavy metals were investigated, enabling the leaching behavior of heavy metals in gasification residues to be explained. The results showed that the gasification residue of coal-waste activated carbon-slurry (CWACS) contained higher concentrations of Cr, Cu, and Zn, while those of Cd, Pb, As, Hg, and Se were well below 100 μg/g. Further, the spatial distributions of Cr, Cu, and Zn in the mineral phases of the gasification residue of CWACS were relatively uniform overall, and no obvious regional enrichment was observed. The leaching concentrations of various heavy metals in the gasification residues of the two CWACS samples were all lower than the standard limit. Following the co-gasification of WAC with CWS, the stability of the heavy metals in the environment was enhanced. Meanwhile, the gasification residues of the two CWACS samples showed no environmental risk for Cr, low environmental risk for Pb and Hg, and only a moderate environmental risk for Cd, As, and Se.

The geochemical and mineralogical controls on the release characteristics of potentially toxic elements from lead/zinc (Pb/Zn) mine tailings

Large quantities of lead/zinc (Pb/Zn) mine tailings were deposited at tailings impoundments without proper management, which have posed considerable risks to the local ecosystem and residents in mining areas worldwide. Therefore, the geochemical behaviors of potentially toxic elements (PTEs) in tailings were in-depth investigated in this study by a coupled use of batch kinetic tests, statistical analysis and mineralogical characterization. The results indicated that among these studied PTEs, Cd concentration fluctuated within a wide range of 0.83-6.91 mg/kg, and showed the highest spatial heterogeneity. The mean Cd concentrations generally increased with depth. Cd were mainly partitioned in the exchangeable and carbonate fractions. The release potential of PTEs from tailings was ranged as: Cd > Mn > Zn > Pb > As, Cd > Pb > Zn > Mn > As and Cd > Pb > Mn > Zn > As, respectively, under the assumed environmental scenarios, i.e. acid rain, vegetation restoration, human gastrointestinal digestion. The results from mineralogical characterization indicated that quartz, sericite, calcite and pyrite were typical minerals, cumulatively accounting for over 80% of the tailings. Sulfides (arsenopyrite, galena, and sphalerite), carbonates (calcite, dolomite, cerussite and kutnahorite), oxides (limonite) were identified as the most relevant PTEs-bearing phases, which significantly contributed to PTEs release from tailings. A combined result of statistical, geochemical and mineralogical approaches would be provided valuable information for the alteration characteristics and contaminant release of Pb/Zn mine tailings.

A novel method for dearsenization from arsenic-bearing waste slag by selective chlorination and low-temperature volatilization

Because of the widespread presence of arsenic in various smelting waste slags, it not only hinders the recycling and utilization of waste slag, but also causes serious pollution to the ecological environment. In this study, As₂O₃, the main form of arsenic in non-ferrous metal smelting slag, was used as the research object, and FeCl₃ was used as the chlorination agent. As₂O₃ was selectively chlorinated to low-boiling-point AsCl₃ gas which was easy to be volatilized and removed by chlorination roasting. According to the thermodynamic calculation results, the feasibility of FeCl₃ as the chlorination agent for selective chlorination and low-temperature volatilization of dearsenization was analyzed. The TG-DTA diagram was analyzed by thermogravimetric experiment, and the mass change, endothermic and exothermic behaviors of the As₂O₃-FeCl₃ system during the linear heating process were studied. The effects of roasting temperature, roasting time, and molar ratio of reactants on the chlorination-volatilization of the As₂O₃-FeCl₃ system were investigated. The optimal chlorination roasting conditions were determined with a roasting temperature of 290-300 ℃, a roasting time of 50 min, and a reactant FeCl₃/As₂O₃ molar ratio of 4:1. The results of this study provided a novel idea for the removal of arsenic from smelting slag and dust, but the mechanism and process conditions of chlorination still need to be further studied and optimized.

Microbe-mediated transformation of metal sulfides: Mechanisms and environmental significance

Microorganisms play a key role in the natural circulation of various constituent elements of metal sulfides. Some microorganisms (such as Thiobacillus ferrooxidans) can promote the oxidation of metal sulfides to increase the release of heavy metals. However, other microorganisms (such as Desulfovibrio vulgaris) can transform heavy metals into metal sulfides crystals. Therefore, insight into the metal sulfides transformation mediated by microorganisms is of great significance to environmental protection. In this review, first, we discuss the mechanism and influencing factors of microorganisms transforming heavy metals into metal sulfides crystals in different environments. Then, we explore three microbe-mediated transformation forms of heavy metals to metal sulfides and their environmental applications: (1) transformation to metal sulfides precipitation for metal resource recovery; (2) transformation to metal sulfides nanoparticles (NPs) for pollutant treatment; (3) transformation to "metal sulfides-microbe" biohybrid system for clean energy production and pollutant remediation. Finally, we further provide critical views on the application of microbe-mediated metal sulfides transformation in the environmental field and discuss the need for future research.

Study of the micromorphology and health risks of arsenic in copper smelting slag tailings for safe resource utilization

Slag tailings are produced by "cooling-grinding-ball milling-flotation" and other processes of slag, while slag is produced by the flash smelting of the original ore. The utilization and environmental hazards of arsenic in slag tailings have become a focus of attention. This study on slag tailings reveals the presence of arsenic in copper smelting tailings from the mineralogy and leaching perspectives, and the noncarcinogenic and carcinogenic risks of arsenic to the human body were assessed by using the USEPA health risk model. The surface particles of the slag tailings were unevenly dispersed, and the mineral crystals were relatively complete. A small amount of secondary minerals had grown on the mineral surface. Most of the fine particles adhered to the surface of the main mineral to form inclusions. The mineral composition of the slag tailings was dominated by maghemite (Fe₃O₄) and fayalite (Fe₂SiO₄), and the arsenic-bearing minerals were unevenly distributed, where As (Ⅴ) fine particles were embedded in maghemite, amorphous phase and fayalite. There was a large amount of residual arsenic in the slag tailing particles, and the leaching content of arsenic in the toxicity leaching procedure was always lower than the limit of 5 mg/L. The health risk to the exposed population was evaluated by the USEPA health risk model. Since the exposed population in the industrial land is mainly adults, it is determined that the tailings will not cause harm to children's health. In this evaluation, the exposure duration (length of service of the workers) of 30 years, exposure frequency of 314 d/y and body weight of 60 kg (average weight of the workers) were taken as the parameters of three exposure pathways: hand-oral ingestion, respiratory system inhalation and skin contact. Therefore, longer activity time of the workers in the tailing workshop corresponds to a higher HI (hazard index). Although the arsenic in the slag tailings had a certain degree of bioavailability, it was not sufficient to adversely affect human health.

Current situation and forecast of environmental risks of a typical lead-zinc sulfide tailings impoundment based on its geochemical characteristics

The potential environmental implications of a Pb (Lead)-Zn (Zinc) sulfide tailing impoundment were found to be dependent on its geochemical characteristics. One typical lead-zinc sulfide tailing impoundment was studied. Ten boreholes were set with the grid method and 36 tailings were sampled and tested. According to the results of metal content analysis, the tailing samples contained considerably high contents of heavy metals, ranging from 6.99 to 89.0 mg/kg for Cd, 75.3 to 602 mg/kg for Cu, 0.53% to 2.63% for Pb and 0.30% to 2.54% for Zn. Most of the heavy metals in the sample matrix showed a uniform concentration distribution, except Cd. Cd, Pb, Zn, and Mn were associated with each other, and were considered to be the dominant contributors based on hierarchical cluster analysis. XRD, SEM and XPS were employed for evaluation of the tailing weathering characteristics, confirming that the tailings had undergone intensive weathering. The maximum potential acidity of the tailings reached 244 kg H₂SO₄/ton; furthermore, the bioavailability of heavy metals like Pb, Cd, Cr, Cu, and Zn was 37.8%, 12.9%, 12.2%, 5.95%, and 5.46% respectively. These metals would be potentially released into drainage by the weathering process. Analysis of a gastrointestinal model showed that Pb, Cr, Ni and Cu contained in the tailings were high-risk metals. Thus, control of the heavy metals' migration and their environmental risks should be planned from the perspective of geochemistry.