Pyritic Tailings Research Articles

Enriching acid rock drainage related microbial communities from surface-deposited oil sands tailings.

Little is known about the microbial communities native to surface-deposited pyritic oil sands tailings, an environment where acid rock drainage (ARD) could occur. The goal of this study was to enrich sulfur-oxidizing organisms from these tailings and determine whether different populations exist at pH levels 7, 4.5, and 2.5. Using growth-based methods provides model organisms for use in the future to predict potential activities and limitations of these organisms and to develop possible control methods. Thiosulfate-fed enrichment cultures were monitored for approximately 1 year. The results showed that the enrichments at pH 4.5 and 7 were established quicker than at pH 2.5. Different microbial community structures were found among the 3 pH environments. The sulfur-oxidizing microorganisms identified were most closely related to Halothiobacillus neapolitanus, Achromobacter spp., and Curtobacterium spp. While microorganisms related to Chitinophagaceae and Acidocella spp. were identified as the only possible iron-oxidizing and -reducing microbes. These results contribute to the general knowledge of the relatively understudied microbial communities that exist in pyritic oil sands tailings and indicate these communities may have a potential role in ARD generation, which may have implications for future tailings management.

Mineralogical and geochemical characterization of the Old Tailings Dam, Australia: Evaluating the effectiveness of a water cover for long-term AMD control

Establishing a shallow water cover over tailings deposited in a designated storage facility is one option to limit oxygen diffusion and retard oxidation of sulfides which have the potential to form acid mine drainage (AMD). The Old Tailings Dam (OTD) located at the Savage River mine, western Tasmania contains 38 million tonnes of pyritic tailings deposited from 1967 to 1982, and is actively generating AMD. The OTD was constructed on a natural gradient, resulting in sub-aerial exposure of the southern area, with the northern area under a natural water cover. This physical contrast allowed for the examination of tailings mineralogy and geochemistry as a function of water cover depth across the OTD. Tailings samples (n = 144, depth: ≤ 1.5 m) were collected and subjected to a range of geochemical and mineralogical evaluations. Tailings from the southern and northern extents of the OTD showed similar AMD potential based on geochemical (NAG pH range: 2.1 to 4.2) and bulk mineralogical parameters, particularly at depth. However, sulfide alteration index (SAI) assessments highlighted the microscale contrast in oxidation. In the sub-aerial zone pyrite grains are moderately oxidized to a depth of 0.3 m (maximum SAI of 6/10), under both gravel fill and oxidized covers, with secondary minerals (e.g., ferrihydrite and goethite) developed along rims and fractures. Beneath this, mildly oxidized pyrite is seen in fresh tailings (SAI = 2.9/10 to 5.8/10). In the sub-aqueous zone, the degree of pyrite oxidation demonstrates a direct relationship with cover depth, with unoxidized, potentially reactive tailings identified from 2.5 m, directly beneath an organic-rich sediment layer (SAI = 0 to 1/10). These findings are broadly similar to other tailings storage facilities e.g., Fox Lake, Sherritt-Gordon ZnCu mine, Canada and Stekenjokk mine, Sweden where water covers up to 2 m have successfully reduced AMD. Whilst geotechnical properties of the OTD restrict the extension of the water cover, pyrite is enriched in cobalt (up to 2.6 wt%) indicating reprocessing of tailings as an alternative management option. Through adoption of an integrated mineralogical and geochemical characterization approach for tailings assessment robust management strategies after mine closure can be developed.

Release of Heavy Metals from the Pyrite Tailings of Huangjiagou Pyrite Mine: Batch Experiments

To provide the basic information about the release of heavy metals from the pyrite tailings of Huangjiagou pyrite mine, the pyrite tailings were investigated through a series of batch experiments under different initial pH of extractant, temperature, liquid-solid (LS) ratio, and soaking time conditions. Moreover, calcium carbonate was added in the pyrite tailings to determine the reduction effect on the release of heavy metals. The results show that Fe, Cr, Cu, Mn, Zn, and Ni were the major heavy metals in the pyrite tailings. Low initial pH and high LS ratio significantly promoted Fe, Cu, Mn, Ni, and Zn release, and high temperature significantly promoted Fe, Cu, Mn, and Ni release. Only small amounts of Cr were detected at low LS ratios. With the increase of soaking time, the released amount of Fe, Cu, Mn, Ni, and Zn increased to the maximum value within 48 h, respectively. After adding calcium carbonate, the released amounts of Fe, Cu, and Zn reduced at least 70.80% within 48 h soaking time. The results indicate that summer and the early soaking stage are the main phases for the release of heavy metals from the pyrite tailings. In the pyrite tailings, Cr is difficult to release. Adding calcium carbonate can effectively reduce the release of Fe, Cu, and Zn.

Biogeotechnology Application in the Recovery of Metals from the Wastes of Processing Plants

In order to recovery base and precious metals from processing plants tailings of Southern Ural and the Murmansk region, test work on heap biooxidation using these products was carried out. These tests involved the chemical and mineralogical analyses of the samples, the bacteria adaptation for the products tested, heap biooxidation in percolation columns, base metals (copper, nickel, zinc) precipitation and gold cyanidation. The recoveries of base metals to the solution from the products of magnetic separation of the wastes were 75% Ni and 50% Cu. This work is aimed at studying sustainability of magnetic separation products and pyrite tailings biotechnological processing. Also, gold and base metals recovery to the solution and the options of base metals removal from the solutions were studied.

Evaluation of the genotoxic potential of soil contaminated with mineral coal tailings on snail Helix aspersa

Coal remains an important source of energy, although the fuel is a greater environmental pollutant. Coal is a mixture of several chemicals, especially inorganic elements and polycyclic aromatic hydrocarbons (PAH). Many of these compounds have mutagenic and carcinogenic effects on organisms exposed to this mineral. In the town of Charqueadas (Brazil), the tailings from mining were used for landfill in the lower areas of the town, and the consequence is the formation of large deposits of this material. The purpose of this study was to evaluate the genotoxic potential of soil samples contaminated by coal waste in different sites at Charqueadas, using the land snail Helix aspersa as a biomonitor organism. Thirty terrestrial snails were exposed to different treatments: 20 were exposed to the soil from two different sites in Charqueadas (site 1 and 2; 10 in each group) and 10 non-exposed (control group). Hemolymph cells were collected after 24h, 5days and 7days of exposure and comet assay, micronucleus test, oxidative stress tests were performed. Furthermore, this study quantified the inorganic elements present in soil samples by the PIXE technique and polycyclic aromatic hydrocarbons (PAH) by HPLC. This evaluation shows that, in general, soils from sites in Charqueadas, demonstrated a genotoxic effect associated with increased oxidative stress, inorganic and PAH content. These results demonstrate that the coal pyrite tailings from Charqueadas are potentially genotoxic and that H. aspersa is confirmed to be a sensitive instrument for risk assessment of environmental pollution.

Pyritic Tailings as a Source of Plant Micronutrients in Calcareous Soils

Pyrite (FeS2) is usually a waste from complex sulfide ores. Yet, it may be a remediation additive for calcareous soils deficient in iron (Fe) and other micronutrients such as copper (Cu), zinc (Zn), and manganese (Mn). In this study, leaching experiments were conducted under laboratory conditions and a 30-day pot trial (with wheat) to evaluate the effect of applying different amounts of pyritic tailings on micronutrient and heavy-metal concentrations in a calcareous soil and on crop growth (dry-matter production). The application of pyritic tailings to the calcareous soils improved the levels of Fe, Cu, Zn, and Mn, and dry-matter biomass of wheat also significantly increased. The heavy-metal contents in soil and plant were well below the permissible limit for soil and plants.

Mycorrhizal Fungal Community of Poplars Growing on Pyrite Tailings Contaminated Site near the River Timok

Background and Purpose: Mycorrhizal fungi are of high importance for functioning of forest ecosystems and they could be used as indicators of environmental stress. The aim of this research was to analyze ectomycorrhizal community structure and to determine root colonization rate with ectomycorrhizal, arbuscular mycorrhizal and endophytic fungi of poplars growing on pyrite tailings contaminated site near the river Timok (Eastern Serbia). Materials and Methods: Identification of ectomycorrhizal types was performed by combining morphological and anatomical characterization of ectomycorrhizae with molecular identification approach, based on sequencing of the nuclear ITS rRNA region. Also, colonization of poplar roots with ectomycorrhizal, arbuscular mycorrhizal and dark septated endophytic fungi were analysed with intersection method. Results and Conclusions: Physico-chemical analyses of soil from studied site showed unfavourable water properties of soil, relatively low pH and high content of heavy metals (copper and zinc). In investigated samples only four different ectomycorrhizal fungi were found. To the species level were identified Thelephora terrestris and Tomentella ellisi, while two types remained unidentified. Type Thelephora terrestris made up 89% of all ectomycorrhizal roots on studied site. Consequently total values of Species richness index and Shannon-Weaver diversity index were 0.80 and 0.43, respectively. No structures of arbuscular mycorrhizal fungi were recorded. Unfavourable environmental conditions prevailing on investigated site caused decrease of ectomycorrhizal types diversity. Our findings point out that mycorrhyzal fungal community could be used as an appropriate indicator of environmental changes.

Laboratory Study of Highly Pyritic Tailings Submerged Beneath a Water Cover Under Various Hydrodynamic Conditions

Many studies have examined the efficiency of water covers for control of acid mine drainage, but few have addressed underwater deposition of highly pyritic tailings with low neutralizing capacity. This study simulated the effects of various hydrodynamic conditions on water quality when tailings containing 80 % pyrite were deposited beneath a water cover. The tailings were placed at the bottom of laboratory columns and covered with deionized water. The applied hydrodynamic conditions were: stagnant, stagnant with downward intermittent infiltration, continuously stirred with low tailings resuspension (130–180 mg/L), and continuously stirred with high tailings resuspension (2,910–3,100 mg/L). A continuously stirred water cover with a sand layer placed over the tailings was also studied. Water cover and pore water samples were analyzed monthly to determine variations in the geochemical parameters over monitoring periods ranging from 338 to 840 days. With stagnant water covers, the pH remained near neutral and concentrations of dissolved metals were generally low compared to columns with tailings resuspension. The addition of a layer of inert material significantly prevented pyrite oxidation. Zn was the dissolved metal with the highest concentration (except for Fe) in all of the column leachates, although the tailings contained only 0.38 % Zn.

EVALUATION OF PYRITIC TAILINGS FROM A COPPER CONCENTRATION PLANT FOR CALCAREOUS SODIC SOIL RECLAMATION

This study was aimed to investigate the feasibility to use waste pyrite and sulphuric acid produced from waste pyrite for the reclamation of calcareous sodic soils. The final aim is to displace Na from an exchange complex and replace it with Ca, leading to a decrease in pH and exchangeable sodium percentage (ESP) and an improvement of soil structure. A fertility of the soil may also be enhanced by this technique since waste pyrite can be rich in several micronutrients. An additional advantage to this strategy is that waste pyrite is inexpensive and readily available in large quantities, by contrast to gypsum. In this study, column-leaching tests were carried out to evaluate a change in soil properties upon addition of gypsum and pyritic tailing amendments from a copper concentration plant. An availability of essential micronutrients for a plant growth (Fe, Cu, Zn and Mn) and hazardous potential of pyritic tailings in terms of heavy metal contamination were taken into account. Gypsum, powder waste pyrite and sulfuric acid produced from waste pyrite were applied to the soil with reference to the gypsum requirement (GR) of the soils. The results showed that application of waste pyrite with a dose of 44.74 megagrams per ha was superior to gypsum of 55.20 Mg/ha dose in terms of exchangeable sodium percentage (ESP) in 42 weeks. In addition, application of all the concentrations of sulfuric acid produced from waste pyrite also decreased the ESP values. The best values with sulfuric acid were obtained with a dose of 35.31 Mg/ha in 26 weeks. It was observed that the content of all micronutrients in the soil increased significantly (P<0.01) with waste pyrite and sulfuric acid applications. The levels of the micronutrients after treatments can be classified sufficient for the plant growth except for the iron level with the minimum dose of sulfuric acid application 17 Mg/ha. The heavy metal content in the soil after treatment with the pyritic tailings was found to be lower than the legal limit values. Thus, waste pyrite can be effectively used since rapid amelioration of calcareous sodic soils is possible with no deleterious heavy metal contamination.

Bioprocessing of Mining and Metallurgical Wastes Containing Non-Ferrous and Precious Metals

Mining and metallurgical treatments of sulphide ores are characterised by present significant losses of non-ferrous and precious metals as different types of waste. These elements are accumulated in heaps due to the lack of efficient technology for the recovery of the metals from metallurgical waste. The treatment of two types of industrial metallurgical waste (copper converter slag and old flotation pyrite tailings) containing non-ferrous and precious metals were examined in the laboratory. Leaching of the slag containing 2.74% Cu (as digenite, bornite, and free metal) and 2.49% Zn (as a ferrite ZnFe2O4 and silicate) by an Fe3+-containing solution was studied. The effect of various experimental parameters on the leaching dynamics of copper, zinc, and iron under batch conditions was investigated. The following experimental parameters were recommended: a pH of 1.5, a pulp density of 10% (w/v), a temperature of 70 °C, and an initial Fe3+ concentration of 15 g/L. Leaching under these conditions resulted in the solubilisation of 89.4% copper and 35.3% zinc within 2.5 hours. Percolation leaching of the pyrite tailings containing 0.29% Cu (as chalcopyrite), 0.26% Zn (as sphalerite), 0.00007% gold, and 0.00108% silver was also studied. Acidic percolation leaching and the resulting biooxidation lasting 134 days resulted in the solubilisation of 73.4% zinc and 50.8% copper. The recovery rates of gold and silver from the bioleaching residues by cyanidation were 57.2% and 50.7%, respectively. The data obtained in the present work may be used to estimate the operating parameters for the industrial-scale processing of non-ferrous and precious metals from mining and metallurgical waste.

In Situ Calibration Checking Technique for Permanently Installed Oxygen Probes in the Vadose Zone

Oxygen probes used for long-term in situ monitoring in the vadose zone generally cannot be retrieved for calibration purposes. Therefore, a method is needed for confirming the calibration of permanently installed O2 probes. We developed a novel in situ calibration checking technique for use with permanently installed probes. The technique was tested at a pyritic tailings disposal site where O2 probes were installed below a bentonite–polypropylene composite cover. The technique involved (i) opening a gas sampling tube to allow atmospheric O2 ingress to the probe's location via barometric pumping and (ii) analyzing the resultant probe data collected during periods of stable, maximum probe response. Probe calibration checking was done six times during a 12-yr period, and results showed that response data were within 5% of the original calibration. This confirmed the long-term reliability of the probes and identified that the probes' useable life span extended to at least 12 yr.

Heavy Metals Leaching Experiment from the Pyritic Tailings by the T.F and T.T Bacterias

The produced and extraction process of pyritic would generate a large amount of pyritic mineral processing solid waste. This kind of waste can lead to land occupation, and contaminate the local water, air and soil. Therefore, in this paper, Thiobacillus ferrooxidans and Thiobacillus thiooxidans are used to deal with the pyritic mineral processing solid waste. The particles, catalysts and bioleaching products between microorganism and pyritic mineral processing solid wast were studied. It is indicated that the microorganism growth and the heavy metals leaching efficiency involve biological and chemical interactions. The experimental results suggested that the T.t was best to bioleach the pyritic mineral processing solid waste, and the efficiency of T.f was worse than that of T.t. The leaching effect of mixed bacterias of T.t and T.f fell in between, the leaching rate would decrease when the pulp density increased. The micro-morphology of bacteria was observed by SEM detector, the single cellular dimension of bioleaching bactreia was about (1~2) μm × (0.2~0.5) μm.

Geochemistry and mineralogy of surface pyritic tailings impoundments at two mining sites of the Iberian Pyrite Belt (SW Spain)

Two pyritic tailings impoundments located in two mining areas of the Iberian Pyrite Belt (Cueva de la Mora and Minas de Ríotinto-Zarandas) were selected to asses their potential environmental impact. Mineralogical (XRD diffraction study), physico-chemical characterization (colour, particle size, pH, acid–base account, total Fe, As, Cu, Pb and Zn) as well as a speciation study (by means of a seven-step sequential extraction procedure) were performed in superficial (0–20 cm) tailings samples. Arsenic and metal contents in soils around the tailings impoundments were also studied. Zarandas dam, a reclaimed impoundment, which has been limed, partially topsoiled and planted, has supported and allowed the growth of pine trees and other plants for many years. The surface of this impoundment can be considered very acid but nonacid forming. Although total As and metal concentrations were relatively high, it is not possible to conclude that the Zarandas tailings have polluted the surrounding soils. Tailings in Cueva de la Mora showed high total and easily mobilizable concentration of toxic elements. The net neutralization potential was strongly negative as a consequence of the acid generation caused by the sulphide oxidation, the presence of secondary acid-generating minerals and the absence of neutralizing materials. Coquimbite and rhomboclase efflorescences formed during the Mediterranean dry summers on the surface of this impoundment contained very high levels of soluble As, Cu and Zn that were easily dissolved and released to the running water in the first rains of autumn.

Study on Autoclaved Performance of Pyrite Tailing

Autoclaved characteristics of pyrite tailing are studied mixed different materials by different curing condition. The results show that compressive strength of pyrite tailing autoclaved sample is 0. Pyrite tailing autoclaved sample acquires a higher compressive strength mixed with lime. But effect of gypsum is few on compressive strength of pyrite tailing lime autoclaved system. Mixed pyrite tailing is disadvantage for compressive strength of steel slag autoclaved samples. But a higher compressive strength of steel slag pyrite tailing autoclaved sample is acquired mixed gypsum, and compressive strength of steel slag lime autoclaved sample increases mixed pyrite tailing.

Bioprocessing of pyrite concentrate from coal tailings for the production of the coagulant ferric sulphate

The aim of this work was to produce a high concentration ferric sulphate solution from coal pyrite tailings that could be used as coagulant for water and wastewater treatment. At laboratory scale it was performed the oxidation of pyrite in an aqueous medium in a packed bed leaching column in an oxidizing environment with the presence of acidophilic bacteria ( Acidithiobacillus ferrooxidans) as well as nutrients for bacterial growth. It was indicated that an aqueous solution rich in ferric sulphate can be produced which was found suitable for application in water treatment plants.

Phosphorus deficiency is the major limiting factor for wheat on alluvium polluted by the copper mine pyrite tailings: a black box approach

Failures of tailings dams have degraded large areas of agricultural alluvial soils worldwide, and concomitant soil pollution studies are abundant. Yet, the data on the actual effects of thereby imposed stresses on major crops are scarce. This work analyses the effect of pyrite tailings from a copper mine, deposited over crop fields by long-term flooding, on wheat (Triticum aestivum L.) under field conditions. The major previously reported polluting agents were Cu, As, Zn, Pb and acidity generated by sulphide oxidation. Flexible systematic sampling, based on visual symptoms in wheat, included transects through partially damaged fields (from calcareous to acid soils). Multivariate analysis of soil properties, leaf mineral composition and growth parameters revealed a consistent underlying soil gradient of decreasing available P and increasing Stot. Phosphorus was shown to have the highest unique contribution to predicting wheat yield, consistent correlation with growth and visual symptoms, and concentrations in the range of severe deficiency. In P deficient plants N deficiency, decrease of available micronutrients and increase of As occur irrespectively of their soil concentrations, and the competition with superior “pyrite” weeds increases. Different sorption of P and possible rhizotoxic effects of other pollutants imply that fertilization can hardly be a solution.

Mobility of iridium in terrestrial environments: Implications for the interpretation of impact-related mass-extinctions

Traditionally, iridium has been considered an element of low mobility, but its behavior is still debated. Ir concentration in a soil affected by a catastrophic mining spill in 1998 that covered the soil with a layer of tailings offers the opportunity to analyse an exceptional Ir-bearing horizon 10 years after deposition. This has enabled comparisons with the values of past Ir-bearing horizons associated to impact-related mass-extinction events. Iridium concentration in the tailings (0.349 ppm) was 5-fold higher than the anomaly in the K–Pg at The Moody Creek Mine section (the highest values obtained from terrestrial sections). The oxidative weathering of the tailings caused the release of Ir and infiltration into the soil. Iridium distribution in depth indicates redistribution throughout the profile in relation to the change in the physico-chemical properties of the soil. With regard to the background concentration in the soil (0.056 ppm), anomalous values of Ir (0.129 ppm) can be detected to 11 cm below the layer of tailings. The correlation analysis between the Ir concentration and the main properties and constituents of the soils indicated a significant correlation with sulfur, iron, clay content, and pH. Selective extractions were made to study the forms in which Ir can be mobilized in the soil. The residual/insoluble fraction was >90% of the total Ir concentration in soil. Soluble-in-water concentration of Ir (1.5% of total) was detected in the uppermost 2–3 cm of the soil, which were directly affected by the leaching of acidic waters coming from the oxidation of the pyrite tailings. Iridium retention in the affected part of the soil reached 9% of the total Ir concentration; this retention could be related to the amorphous iron forms dissolved by the oxalic-oxalate extraction. However, according to our research, original Ir abundance could be secondarily modified, and then a direct analysis of the iridium values recorded in sediments could induce misinterpretations. The comparison between the actual example and the fossil record belonging to terrestrial settings, can be considered as a valuable approach, especially when Iridium data were used by researchers to interpret the impact-related mass-extinction events in the past.

Use of liming in the remediation of soils polluted by sulphide oxidation: A leaching-column study

Pure CaCO 3 in ascending quantities was added to a soil to study the effect of liming after contamination by an acidic solution from the oxidation of pyrite tailings. The samples were placed in percolation columns, and soils and leachates were monitored. In the soil samples, the mineralogy, pH, CaCO 3, iron, and total content in Cu, Zn, As, Cd, and Pb were determined. The presence of CaCO 3 in the soils considerably limited their acidification, favouring the precipitation of Cu, Zn, and Cd, and promoting precipitation of iron and SO 4 2− ions in the form of iron hydroxysulphates and gypsum. The iron hydroxysulphates tended to retain the less mobile elements (As, and Pb) near the top of the soil. The more mobile elements (Zn, Cd and Cu) precipitated in deeper layers, directly related mainly to the CaCO 3 added and to pH. The CaCO 3 clearly did not reduce Zn and Cd toxicity effectively enough, given that the concentrations of both elements were above the toxic level in all leachates. The amounts of liming needs to be properly controlled, as excessively high pH limits As fixing, at the same time as the effectiveness of CaCO 3 is limited by coating precipitation, reducing its capacity to react with the acidic solution.

Evaluation of the Genotoxic Potential of the Mineral Coal Tailings Through the Helix aspersa (Müller, 1774)

Coal mining is an activity with a high potential for environmental pollution. Coal has been described as the most significant pollutant of all the fossil fuels, containing a heterogeneous mixture. Many elements present in coal byproducts as well as coal tailings are rich in potentially toxic and genotoxic metals, which ultimately lead to profound changes in cells, tissues, populations, and ecosystems. The purpose of this study was to assess the genotoxic potential of the mineral coal tailings using the land snail Helix aspersa. Animals were divided in three groups, clustered in plexiglass cages: control (animals fed with organic lettuce), coal tailings (animals living in a layer of pyrite tailings and fed with organic lettuce), and mine lettuce (animals fed with lettuce grown in an area located in a deposit of coal tailings). The hemolymph was collected at different exposure times (24 h, 48 h, 72 h, 96 h, 1 week, 2 weeks, 3 weeks, and 1 month) for comet assay analyses. Results showed that the animals of the coal tailings and mine lettuce groups presented higher levels of DNA damage in relation to the control group at all exposure times, but with a peak of DNA damage in 48 h and 96 h. These results demonstrate that the coal pyrite tailings are potentially genotoxic and that H. aspersa has proven to be a sensitive instrument for a better risk assessment of environmental pollution.

Factor analysis and sequential extraction unveil geochemical processes relevant for trace metal distributions in fluvial sediments of a pyrite mining area, China

Four fluvial sediment cores were geochemically analysed for their major elements and for their trace metal contents and represent a sensitive environmental record for heavy metal contamination in a pyrite mining area, Pearl River Basin, South China. While an identification of depositional and post-depositional processes is not possible by means of the vertical profiles of the trace metal contents alone, factor analysis uncovers four main factors that control trace metal distributions in the sediment cores. After analysing the geochemical fractions of heavy metals by a sequential extraction procedure, these four factors could be explained as (i) complexation with organic matter in the sediment (As, Cu, Ni and Zn), (ii) weathering processes by iron reduction and oxidation (Pb, Mo and Cr), (iii) weathering by Mn reduction and oxidation (Tl and Co) and (iv) binding effects of sulphur in the sediment or physical transport of pyrite tailings (Zn). The environmental evaluation by geoaccumulation indices and enrichment factors reveals that the studied sediment cores are significantly contaminated and enriched with As, Tl, Pb and Zn. The risk assessment code additionally suggests low to medium risk of these studied heavy metals on the whole.