Geochemical speciation and dynamic of copper in tropical semi-arid soils exposed to metal-bearing mine wastes

Soil contamination around porphyry copper mines: an example from a semi-arid climate

Extraction and processing of disseminated metalliferous ores, porphyry copper in particular, results in significant tonnages of waste and can cause severe disturbances and contamination in natural ecosystems. This is particularly important in semi-arid climates where natural soils are often deprived of organic matter and nutrients. This study was conducted on seven sites around Sungun Copper Mine, northwest Iran. Soil texture, EC, pH, and concentrations of nutrients, organic matter, along with 16 metal and metalloids were measured in 94 soil samples. Results showed a gradient of contamination from low contamination in natural hillsides to high contamination in mine waste depositories, Waste Dump and Oxide Dump, alongside Pakhir and Sungun Rivers. Nutrient deficiency occurred in disturbed sites. The main contaminant point sources were Waste Dump, mine pit drainage, and Oxide Dump. The results of Non-metric multidimensional scaling ordination showed elevated Cd, Zn, Fe, Cu, Pb, As, Mo, Mn, Co, S concentrations, high EC, and higher sand percentage in the sites affected by mine waste and acid mine drainage. Geo-Accumulation and Potential Ecological Risk Indices indicated that Pakhir riverside, Sungun riverside, and Oxide Dump have severe to moderate levels of environmental risks. Positive correlations between certain metal elements suggest common sources and similar reaction pathways, which may contribute to their similar geochemical behaviour in transport, deposition, and interdependence. Overall, the deficiency of organic matter and nutrients along with the soil sandy texture in contaminated sites of Sungun Copper Mine are the main limiting factors in managing metal mobility and soil remediation.

Potential mobilization of cadmium and zinc in soils spiked with smithsonite and sphalerite under different water management regimes

Particulate cadmium (Cd) and zinc (Zn) are ubiquitous in agricultural soils of Pb-Zn mining regions. Water management serves as an important agronomic measure altering the bioavailability of Zn and Cd in soils, but how this affects particulate Cd and Zn and the underlying mechanisms remain largely unknown. Microcosm soil incubation combined with spectroscopic and microscopic characterization was conducted. During a two-year-long incubation period we observed that the concentrations of soil CaCl₂-extractable Zn and Cd increased 3-10 times in sphalerite-spiked soils and 1-2 times in smithsonite-spiked soils under periodic flooding conditions due to the long-term dissolution of sphalerite (SP) and smithsonite (SM). However, the increase in the concentration of CaCl₂-extractable metals (Zn: from 0.607 mg kg^-1 to 1.051 mg kg^-1 and Cd: from 0.047 mg kg^-1 to 0.119 mg kg^-1) was found only in SP-treatment under continuous flooding conditions, indicating the mobilization of metals. Ultrafiltration analysis shows that the nanoparticulate fraction of Zn and Cd in soil pore water increased 5 and 7 times in SP-treatments under continuous flooding conditions, suggesting the increment of metal pools in soil pore water. HRTEM-EDX-SAED further reveals that these nanoparticles were mainly crystalline ZnS and Zn-bearing sulfate nanoparticles in the SP-treatment and amorphous ZnCO₃ and ZnS nanoparticles in the SM-treatment. Therefore, the formation of the stable crystalline Zn-bearing nanoparticles in the SP-treatment may explain the elevation of the concentration of soil CaCl₂-extractable Zn and Cd under continuous flooding. The potential mobility of particulate metals should therefore be expected in scenarios of continuous flooding such as paddy soils and wetland systems.

Potential environmental risk of natural particulate cadmium and zinc in sphalerite- and smithsonite-spiked soils

Cadmium (Cd)-bearing sphalerite and smithsonite ore particles are ubiquitous in soils near metal-mining areas. Previous studies indicate that smithsonite is more readily dissolved in acidic waters and soils than sphalerite but the mobility of Cd and zinc (Zn) derived from these ores in soils is unknown. Using microcosm incubation experiments and microscopic and spectroscopic analysis, we found that the mobility of Cd and Zn derived from smithsonite is higher than from sphalerite. The mobilization rates of Cd (16.6%) and Zn (13.7%) released from smithsonite in soils after 30-day incubation experiments were higher than those from sphalerite (Cd, ~ 1.42%; Zn, ~ 0.75%). Moreover, the percentages of Cd²⁺ and Zn²⁺ in soil pore water showed a dynamic increase in smithsonite-spiked treatments but a decrease in sphalerite-spiked treatments. HRTEM-EDX-SAED analysis further indicates the occurrence of dynamic transformation of amorphous Cd and Zn species in soil pore water to crystalline ZnS and iron oxides in sphalerite-spiked soil but crystalline ZnCO₃ nanoparticles were dynamically transformed to amorphous metal-bearing species in smithsonite-spiked soil. The opposite transformation trends in pore water of Zn ore-spiked soils provide new insights into the Cd environmental risks in soils affected by Zn mining.

Cu Dynamics in the Rhizosphere of Native Tropical Species: Assessing the Potential for Phytostabilization in Mining-Impacted Soils

The use of native plants for reforestation and/or remediation in areas contaminated by mining is a technique with low implantation and maintenance costs. The success of this practice depends on the plant species and geochemical processes at the soil–plant interface (e.g., rhizosphere). This study evaluated the potential of spontaneous species for mobilizing and altering mineral and metal dynamics in the rhizosphere of Cu-rich soils resulting from the abandoned Pedra Verde mine in NE Brazil. Rhizosphere and bulk soil samples were collected from five shrubby/arboreal species. The pH, organic matter content, Cu fractionation, mineralogical characterization, and Cu content in the leaves and roots of all studied species were determined. In addition, the bioaccumulation factor (BCF) and translocation factor (TF) were used to evaluate the potential of these species for Cu hyperaccumulation. The Cu concentration in leaf plant tissues varied from 18 to 34 mg kg−1, and all plants presented TF and BCF < 1, indicating that the species were not Cu hyperaccumulators. However, the root exudates induce mineral dissolution, indicating potential Cu accumulation in the roots (from 36 to 249 mg kg−1). Combretum aff. pisoniodes Taub was the species with the greatest potential for decreasing Cu bioavailability and phytostabilization. Our findings indicate the potential of native Brazilian plants for growth in Cu-contaminated soil. These findings may be used for reforestation programs.

Impact of copper mining wastes in the Amazon: Properties and risks to environment and human health

Improper disposal of copper mining wastes can threaten the ecosystem and human health due to the high levels of potentially toxic elements released into the environment. The objective of this study was to determine the properties of Cu mining wastes generated in the eastern Amazon and their potential risks to environment and human health. Samples of forest soil and artisanal/industrial Cu mining wastes were collected and subjected to characterization of properties and pseudo-total concentrations of Al, As, Ba, Cd, Co, Cr, Cu, Fe, Hg, Mn, Mo, Ni, Pb, and Zn, in addition to chemical fractionation of Cu. The pH ranged from near neutrality to alkaline. Pseudo-total concentrations of Cu were high in all wastes, mainly in the artisanal rock waste, with 19,034 mg kg^-1, of which 61% is concentrated in the most reactive fractions. Pollution indices indicated that the wastes are highly contaminated by Cu and moderately contaminated by Cr and Ni. However, only the artisanal rock waste is associated with environmental risk. Non-carcinogenic and carcinogenic human health risks were detected, especially from exposure to Cr in the artisanal rock waste. Prevention actions and monitoring of the artisanal mining area are necessary to avoid impacts to the local population.

Effect of modified pomace on copper migration via riverbank soil in southwest China

To explore the effects of modified pomace on copper migration via the soil on the banks of the rivers in northern Sichuan and Chongqing, fruit pomace (P) and ethylene diamine tetra-acetic acid (EDTA) modified P (EP) were evenly added (1% mass ratio) to the soil samples of Guanyuan, Nanbu, Jialing, and Hechuan from the Jialing River; Mianyang and Suining from the Fu River; and Guangan and Dazhou from the Qu River. The geochemical characteristics and migration rules of copper in different amended soils were simulated by column experiment. Results showed that the permeation time of copper in each soil column was categorized as EP-amended > P-amended > original soil, and the permeation time of amended soil samples at different locations was Jialing > Suining > Mianyang > Guangan > Dazhou > Nanbu > Guanyuan > Hechuan. Meanwhile, the average flow rate of copper in each soil column showed a reverse trend with the permeation time. Copper in exchangeable, carbonate, and iron–manganese oxide forms decreased with the increase of vertical depth in the soil column, among which the most evident decreases appeared in the carbonate-bonding form. The copper accumulation in different locations presented a trend of Jialing > Suining > Mianyang > Guangan > Dazhou > Nanbu > Guangyuan > Hechuan, and the copper content under the same soil showed EP-amended > P-amended > original soil. The copper proportion of the carbonate form was the highest in each soil sample, followed by the exchangeable form. The proportions of iron-manganese oxide and organic matter forms were relatively small. A significant correlation was observed between the cation exchange capacity and the copper content in exchangeable and carbonate forms. Moreover, total organic carbon and copper contents were negatively correlated.

Copper release from waste rocks in an abandoned mine (NE, Brazil) and its impacts on ecosystem environmental quality

This study aimed to estimate the impact of an abandoned copper (Cu) mine on ecosystem environmental quality, using integrated ecological and biogeochemical analyses. Through a controlled experiment, the amount of Cu released by waste rocks, Cu adsorbed in soils, Cu geochemical behaviour and its leached amount were measured. Furthermore, to investigate the impacts of mine drainage on the adjacent ecosystem, samples of sediments, water and aquatic macroinvertebrates were analysed. We found that waste rocks still have high Cu concentration even after 30 years under weathering, ranging from 7782 to 8717 mg kg^-1, associated mainly with carbonates, amorphous oxides and sulphides. It was estimated that 7.2 tonnes of Cu were released by waste rocks into the environment over last 30 years. The concentration of Cu observed in Ubari stream water was (<dl to 90 μg L^-1), in sediments (28.0-1185 mg kg^-1) and in macroinvertebrates (1.3-28.9 mg kg^-1 d/w). The ecological indexes showed that near mine discharge a significance decrease in the density of aquatic macroinvertebrates and a significance increase of Cu in biological tissues occurs, causing disturbances in biodiversity. The results showed that, even after long periods, the waste rocks from abandoned mines still contain high levels of metal, that are gradually released into the environment through weathering and erosion, representing a potential source of environmental pollution and a clear threat to the environmental quality of adjacent ecosystems.

Role of temperature, wind, and precipitation in heavy metal contamination at copper mines: a review

The increasing demand for minerals pressurizing the mining authorities to extract low-grade ore results in more mining waste and degradation of the environment. The main aim of review was to understand the role of climatic factors (temperature, wind, and precipitation) in dispersal and mobility of heavy metals in soil, water, and vegetation in Cu mining region. The major source of contamination in the mining sector is tailings, overburden rocks, and abandoned mines. The contaminates or fine particles of sulfide-rich mining waste follow two major pathways for the dispersal: aerial and leaching. Sulfides on exposure to oxygen and water generate acid mine drainage which results in leaching of heavy metals. The pit water of abandoned mines is also a cause of concern which contaminates the groundwater resources. Climatic factors such as temperature, precipitation, and wind significantly influence the paths of contaminate dispersal. In arid/semi-arid regions, high temperature forms fine-grained efflorescence salts on tailings or exposed surficial mines which are carried away by strong winds/water and contaminates the surroundings. In wet regions, the leaching of heavy metals from both tailings and overburden rocks sulfides results in environmental contamination. The application of impermeable layers is highly recommended. The climatic factors (temperature, wind, and precipitation) significantly control the dispersal and mobility of heavy metals in Cu mining region. The implementation of waste management policies and pollution control technologies is recommended after considering the climatic factors.

Kinetics and mechanisms of the interaction between the calcite (10.4) surface and Cu2+-bearing solutions

Calcite dissolution, occurring in rocks, soils and sediments, is essential to indicate element cycles and local environments in the lithosphere, biosphere, hydrosphere and atmosphere. Calcite dissolution strongly depends on metal ions in aqueous solutions. Previous studies showed that aquatic Cu²⁺, a typical bio-toxic metal ion, can alter the calcite dissolution behavior. However, wide concentration ranges of Cu²⁺ coexisting with ubiquitous anions in local environments, such as waterways in the oxidation zones of copper deposits and soils near metal processing industry, was overlooked. When a considerable amount of aquatic Cu²⁺ ions are released into the environment, they migrate, diffuse, and hence become an environmental pollutant. Therefore, we focused on the interaction between calcite dissolution and wide concentration ranges of Cu²⁺-bearing solutions with different types of anions (SO₄^2-, Cl^- and NO₃^-). Comprehensive approaches including in situ atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), transmission electron microscope (TEM), and density functional theory (DFT) calculations were employed to investigate kinetics and mechanisms of the interaction between the calcite (10.4) surface and Cu²⁺-bearing solutions. Results demonstrated that both anion types and Cu²⁺ concentrations dramatically affect calcite dissolution. The morphology of etch pits generated in CuSO₄ solutions can be fan-shaped but changed to tear-shaped in Cu(NO₃)₂ or CuCl₂ solutions. Calcite dissolution kinetics is inhibited at c_Cu2+ ≤ 0.1 mM, caused by the coverage of active sites on calcite surfaces. As the Cu²⁺ concentration increases (1 mM ≤ c_Cu2+ ≤ 10 mM), calcite dissolution kinetics is enhanced due to the coupling effect of Cu²⁺-incorporated surface structure and solution chemistry. These results revealed the interactive mechanism between calcite dissolution and the migration of toxic Cu²⁺ in waterways, provided a practical consideration in dealing with the local environment.

Risk assessment and copper geochemistry of an orchard irrigated with mine water: a case study in the semiarid region of Brazil

This study aimed to evaluate mine water reuse, elucidating the potential problems related to trace metal biogeochemistry focusing on Cu dynamics in water, soil, and plants. Water samples were collected from a Cu mine and a reservoir used to store mine water. Additional samples were taken from soils from an uncultivated area and a banana orchard (irrigated with mine water for at least 10 years) and plant from the irrigated area. The following parameters were analyzed: pH, redox potential, dissolved ions in water samples (e.g., Ca²⁺, Mg²⁺, Na⁺, K⁺, Cu²⁺, SO ₄ ^2- , and Cl^-), bioavailable Cu and Cu solid-phase fractionation (in soils and reservoir sediments samples), as well as Cu content in banana plants. Mine water presents high dissolved Cu concentration (mean 2.3 ± 0.0 mg L^-1), limiting its use for irrigation. Water storage at the reservoir increased water quality, reducing dissolved Cu concentration (mean 0.2 ± 0.0 mg L^-1), due to adsorption/precipitation as carbonates (mean 131.8 ± 24.6 mg kg^-1), organic matter (mean 1526.2 ± 4.7 mg kg^-1) and sulfides (mean 158.4 ± 56.9 mg kg^-1). Despite higher water quality at the reservoir, the use of mine water increased the amount of bioavailable Cu in soils, which was primarily associated with organic matter. Increased bioavailable Cu in the soil did not increase the Cu content of banana leaves but resulted in high Cu content of roots and fruit, increasing the risk of toxicity for the population.

The potential of a Technosol and tropical native trees for reclamation of copper-polluted soils

Technosols created to reclaim degraded soils is a promising solution that needs further research. The objectives of the study were: i) to create a Technosol with a very high capacity to immobilize copper from mining, ii) to assess the capacity of the Technosol to immobilize copper after planting two tropical native tree species, and iii) to analyse the capacity of the native trees for extracting copper from polluted soils. Myracrodruon urundeuva (aroeira) and Cedrela fissilis (pink cedar) were planted in pots with Technosol spiked with copper at concentrations of 125, 1525 and 3050 mg Cu kg^-1. Height and stem diameter were measured over 90 days. Biomass and Cu concentration in leaves, stem and roots were determined. Copper was analysed in soils by sequential extraction, as well as in leached water. The Technosol showed a very high capacity to immobilize copper, since 60-80% of the added copper was strongly retained in the soil, mainly by bentonite and carbonates. The Technosol with trees showed the same capacity to immobilize copper as the control, since concentration in shoots was higher than 300 mg Cu kg^-1 and concentration in roots was even higher. These results show that Technosol and both species are useful tools to immobilize copper in polluted soils. Further studies are necessary to determine the total capacity of these trees to immobilize and/or extract copper in the long term and under field conditions.

Screening of native tropical trees for phytoremediation in copper-polluted soils

Due to the limited number of studies on phytoremediation using native tree species in tropical soils, the aim was to identify new phytoremediator species from tropical climate with the purpose of promoting an increase in the diversity of tropical native trees used in phytoremediation projects. Seven native tree species from Brazil were selected: Cedrela fissilis, Handroanthus serratifolius, Copaifera langsdorffii, Hymenaea courbaril, Mimosa caesalpiniifolia, Cecropia sp. and Myracrodruon urundeuva. Seedlings of these species were planted in pots with an unpolluted Arenosol, and then spiked with 60, 100 and 500 mg kg^-1 Cu. Height and stem diameters were measured over 60 days. Biomass and total Cu concentration were determined in leaves, stem and roots. Copper in bulk soils and rhizospheres was analyzed by a sequential extraction method. All species accumulated high concentration of Cu in roots (>300 mg kg^-1), so they could be used as phytostabilizators for this metal. Copper mobilization increased in the rhizospheres, but it was mostly absorbed by roots. Cecropia sp., M. urundeuva and C. langsdorffii are hyperaccumulators of Cu (>300 mg kg^-1 in shoots), so they are potential phytoextractor species. This study evidence the potential of seven tree species native from tropical regions for phytostabilizing copper-polluted soils.

Differential distribution of metals in tree tissues growing on reclaimed coal mine overburden dumps, Jharia coal field (India)

Opencast bituminous coal mining invariably generates huge amount of metal-polluted waste rocks (stored as overburden (OB) dumps) and reclaimed by planting fast growing hardy tree species which accumulate metals in their tissues. In the present study, reclaimed OB dumps located in Jharia coal field (Jharkhand, India) were selected to assess the accumulation of selected metals (Pb, Zn, Mn, Cu and Co) in tissues (leaf, stem bark, stem wood, root bark and root wood) of two commonly planted tree species (Acacia auriculiformis A.Cunn. ex Benth. and Melia azedarach L.). In reclaimed mine soil (RMS), the concentrations of pseudo-total and available metals (DTPA-extractable) were found 182-498 and 196-1877% higher, respectively, than control soil (CS). The positive Spearman's correlation coefficients between pseudo-total concentration of Pb and Cu (r = 0.717; p < 0.05), Pb and Co (r = 0.650; p < 0.05), Zn and Mn (0.359), Cu and Co (r = 0.896; p < 0.01) suggested similar sources for Pb-Cu-Co and Mn-Zn. Among the five tree tissues considered, Pb selectively accumulated in root bark, stem bark and leaves; Zn and Mn in leaves; and Cu in root wood and stem wood. These results suggested metal accumulation to be "tissue-specific". The biological indices (BCF, TF_leaf, TF_{stem bark} and TF_{stem wood}) indicated variation in metal uptake potential of different tree tissues. The study indicated that A. auriculiformis could be employed for Mn phytoextraction (BCF, TF_leaf, TF_{stem bark} and TF_{stem wood} > 1). The applicability of both the trees in Cu phytostabilization (BCF > 1; TF_leaf, TF_{stem bark} and TF_{stem wood} < 1) was suggested. The study enhanced knowledge about the selection of tree species for the phytoremediation of coal mine OB dumps and specific tree tissues for monitoring metal pollution.

Remediation of metalliferous mines, revegetation challenges and emerging prospects in semi-arid and arid conditions

Understanding plant behaviour in polluted soils is critical for the sustainable remediation of metal-polluted sites including abandoned mines. Post-operational and abandoned metal mines particularly in semi-arid and arid zones are one of the major sources of pollution by soil erosion or plant hyperaccumulation bringing ecological impacts. We have selected from the literature 157 species belonging to 50 families to present a global overview of 'plants under action' against heavy metal pollution. Generally, all species of plants that are drought, salt and metal tolerant are candidates of interest to deal with harsh environmental conditions, particularly at semi-arid and arid mine sites. Pioneer metallophytes namely Atriplex nummularia, Atriplex semibaccata, Salsola kali, Phragmites australis and Medicago sativa, representing the taxonomic orders Caryophyllales, Poales and Fabales are evaluated in terms of phytoremediation in this review. Phytoremediation processes, microbial and algal bioremediation, the use and implication of tissue culture and biotechnology are critically examined. Overall, an integration of available remediation plant-based technologies, referred to here as 'integrated remediation technology,' is proposed to be one of the possible ways ahead to effectively address problems of toxic heavy metal pollution. Graphical abstract Integrated remediation technology (IRT) in metal-contaminated semi-arid and arid conditions. The hexagonal red line represents an IRT concept based on remediation decisions by combination of plants and microbial processes.

Assessment of metal toxicity and bioavailability in metallophyte leaf litters and metalliferous soils using Eisenia fetida in a microcosm study

The leaf litters of tree species, Acacia pycnantha (Ap) and Eucalyptus camaldulensis (Ec), predominantly growing at an abandoned copper (Cu) mine and mine soils including controls, were assessed for determining the metal toxicity and bioavailability using earthworm species Eisenia fetida, in a microcosm. Significant reduction in body weight as well as mortality were observed when the worms were introduced into mine soil or its combination with mine Ap litter. Virtually, there were no juveniles when the worms were fed on substratum that contained mine soil or mine leaf litter. The extent of bioaccumulation was dependent on water-soluble fraction of a metal in soil. The accumulation of cadmium, lead and copper in worm tissue was significantly more in treatments that received mine soil with or without mine leaf litter. However, the tissue concentration of zinc did not differ much in earthworms irrespective of its exposure to control or contaminated samples. Mine leaf litter from Ec, a known Cu hyperaccumulator, was more hospitable to earthworm survival and juvenile than that of Ap litter. Validation of the data on bioaccumulation of metals indicated that the mine leaf litter significantly contributed to metal bioavailability. However, it was primarily the metal concentration in mine soil that was responsible for earthworm toxicity and bioavailability. Our data also indicate that detrivores like earthworm is greatly responsible for heavy metal transfer from mines into the ecosystem.