Tracing anthropogenic mercury in soils from Fe-Hg mining/smelting area: Isotopic and speciation insights.

Mercury (Hg) stable isotope ratios supplemented by Hg solid speciation data were determined in soils in a former Fe-Hg mining/smelting area (Jedová hora, Czech Republic, Central Europe). The dominant Hg phase in the studied soils was found to be cinnabar (HgS). A secondary form of soil Hg(II) was represented by Hg weakly and strongly bound to mineral (micro)particles, as revealed by thermo-desorption analysis. These Hg species probably play a key role in local soil Hg processes and biogeochemical cycling. The Hg isotopic data generally showed small differences between HgS (-1.1 to -0.8‰; δ202Hg) and the soil samples (-1.4 to -0.9‰; δ202Hg), as well as limited isotopic variability within the two studied soil profiles. On the other hand, the detected negative δ202Hg shift (∼0.4‰) in organic horizons compared to mineral soils in the highly contaminated profile suggests the presence of secondary post-depositional Hg processes, such as sorption or redox changes. For the less contaminated profile, the observed Hg isotopic variation (∼0.3‰; δ202Hg) in the subsurface mineral soil compared to both overlying and underlying horizons is likely due to cyclic redox reactions associated with Hg isotopic fractionation. We assume that the adsorption of Hg(II) to secondary Fe(III)/Mn(III,IV)-oxides could be of major importance in such cases.

Bioaccessible metals in dust materials from non-sulfide Zn deposit and related hydrometallurgical operation.

Mining and processing of ores in arid (desert) areas generates high amounts of dust, which might be enriched in potentially harmful elements. We studied dust fractions of ores, soils, and technological materials from mining and related hydrometallurgical operation at former Skorpion Zinc non-sulfide Zn deposit in southern Namibia (closed and placed under maintenance in 2020). Chemical and mineralogical investigation was combined with oral bioaccessibility testing of fine dust fractions (<48 µm and <10 µm) in simulated gastric fluid (SGF) to assess potential risk of intake of metallic contaminants (Cd, Cu, Pb, Zn) for staff operating in the area. The bulk metals concentrations were largely variable and ranked as follows: soils < tailings â‰ª Skorpion ores < imported ores and dross used for feed ore blending. Maximum contaminant concentrations in the original granular materials were 927 mg Cd/kg, 9150 mg Cu/kg, 50 g Pb/kg and 706 g Zn/kg, respectively, and generally increased as a function of decreasing grain size. The highest bioaccessible concentrations of Cd and Pb yielded imported ores from Taiwan and Turkey and, together with the milled dross, these samples also exhibited the highest Zn bioaccessibilities. The exposure estimates calculated for a worker (weighing 70 kg) in this mining/ore processing operation at a dust ingestion rate of 100 mg/day indicated that most dust samples (soils, tailings, Skorpion ores) exhibited metals intake values far below tolerable daily intake limits. The overall health risk was limited in all mining and ore processing areas except for the ore blending area, where imported ores and recycled dross enriched in bioaccessible Cd, Pb and/or Zn were used for the ore blending. Safety measures required by the mine operator (wearing of masks by the operating staff) helped to prevent the staff's exposure to potentially contaminated dust even in this blending ore area.

Contaminant Binding and Bioaccessibility in the Dust From the Ni-Cu Mining/Smelting District of Selebi-Phikwe (Botswana).

We studied the dust fractions of the smelting slag, mine tailings, and soil from the former Ni-Cu mining and processing district in Selebi-Phikwe (eastern Botswana). Multi-method chemical and mineralogical investigations were combined with oral bioaccessibility testing of the fine dust fractions (<48  and <10 µm) in a simulated gastric fluid to assess the potential risk of the intake of metal(loid)s contaminants. The total concentrations of the major contaminants varied significantly (Cu: 301-9,600 mg/kg, Ni: 850-7,000 mg/kg, Co: 48-791 mg/kg) but were generally higher in the finer dust fractions. The highest bioaccessible concentrations of Co, Cu, and Ni were found in the slag and mine tailing dusts, where these metals were mostly bound in sulfides (pentlandite, pyrrhotite, chalcopyrite). On the contrary, the soil dusts exhibited substantially lower bioaccessible fractions of these metals due to their binding in less soluble spinel-group oxides. The results indicate that slag dusts are assumed to be risk materials, especially when children are considered as a target group. Still, this exposure scenario seems unrealistic due to (a) the fencing of the former mine area and its inaccessibility to the local community and (b) the low proportion of the fine particles in the granulated slag dump and improbability of their transport by wind. The human health risk related to the incidental ingestion of the soil dust, the most accessible to the local population, seems to be quite limited in the Selebi-Phikwe area, even when a higher dust ingestion rate (280 mg/d) is considered.

Thallium and lead variations in a contaminated peatland: A combined isotopic study from a mining/smelting area.

Vertical profiles of Tl, Pb and Zn concentrations and Tl and Pb isotopic ratios in a contaminated peatland/fen (Wolbrom, Poland) were studied to address questions regarding (i) potential long-term immobility of Tl in a peat profile, and (ii) a possible link in Tl isotopic signatures between a Tl source and a peat sample. Both prerequisites are required for using peatlands as archives of atmospheric Tl deposition and Tl isotopic ratios as a source proxy. We demonstrate that Tl is an immobile element in peat with a conservative pattern synonymous to that of Pb, and in contrast to Zn. However, the peat Tl record was more affected by geogenic source(s), as inferred from the calculated element enrichments. The finding further implies that Tl was largely absent from the pre-industrial emissions (>~250 years BP). The measured variations in Tl isotopic ratios in respective peat samples suggest a consistency with anthropogenic Tl (ε205Tl between ~ -3 and -4), as well as with background Tl isotopic values in the study area (ε205Tl between ~0 and -1), in line with detected 206Pb/207Pb ratios (1.16-1.19). Therefore, we propose that peatlands can be used for monitoring trends in Tl deposition and that Tl isotopic ratios can serve to distinguish its origin(s). However, given that the studied fen has a particularly complicated geochemistry (attributed to significant environmental changes in its history), it seems that ombrotrophic peatlands could be better suited for this type of Tl research.

Slag dusts from Kabwe (Zambia): Contaminant mineralogy and oral bioaccessibility.

The former Pb-Zn mining town of Kabwe in central Zambia is ranked amongst the worst polluted areas both in Africa and in the world. The fine dust particles from the ISF and Waelz slags deposited in Kabwe represent a health risk for the local population. Here, we combined a detailed multi-method mineralogical investigation with oral bioaccessibility testing in simulated gastric fluid (SGF; 0.4 M glycine, pH 1.5, L/S ratio of 100, 1 h, 37 °C) to evaluate the risk related to the incidental dust ingestion. The slag dust fractions contain up to 2610 mg/kg V, 6.3 wt% Pb and 19 wt% Zn. The metals are mainly bound in a slag glass and secondary phases, which formed during the slag weathering or were windblown from nearby tailing stockpiles (carbonates, Fe and Mn oxides, phosphates, vanadates). The bioaccessible fractions (BAFs) are rather high for all the main contaminants, with the BAF values generally higher for the ISF slags than for the Waelz slags: Pb (24-96%), V (21-100%) and Zn (54-81%). The results clearly indicate the potential risks related to the incidental slag dust ingestion. Even when a conservative value of the dust daily intake (100 mg/day) is considered, the daily contaminant intake significantly exceeds the tolerable daily intake limits, especially for Pb â‰« V > Zn. At higher ingestion rates, other minor contaminants (As, Cd) also become a health risk, especially for children. The slag heaps in Kabwe should be fenced to prevent local people entering and should be covered to limit the dust dispersion.

Thallium isotopic fractionation in soil: the key controls.

We studied the key geochemical and mineralogical factors that could affect the fractionation of stable thallium (Tl) isotopes in soil. A set of grassland soil samples enriched in geogenic Tl in combination with selected Tl-containing mineral materials from the Czech Republic (Kluky) were investigated for this purpose. The results demonstrate significant incorporation of Tl in pedogenic (specific) Mn-oxide, which led to a large accumulation of the heavy 205Tl isotope (∼+14 ε205Tl units), presumably resulting from oxidative Tl sorption. Consequently, we concluded that the Mn-oxide-controlled Tl uptake is the primary cause of the observed 205Tl enrichment in the middle profile zone, at the A/B soil horizon interface, with up to +4 of ε205Tl. Furthermore, our results displayed a clear relationship between the Tl isotopic fractionation degree and the Mn-oxide soil concentration (R2 = 0.6), as derived from the oxalate-extractable data. A combination of soil and mineralogical considerations suggests that 205Tl enrichment in respective soil samples is also partly due to the Tl present in micaceous clay minerals, mainly illite, which is the predominant pedogenic Tl host phase. In line with our previous results, this Tl behavior can be inferred from systematic Mn-oxide degradation and the associated Tl (enriched in 205Tl) cycling in the studied soils and thus, presumably in the redoximorphic soils in general.

The influence of land-use on tropical soil chemical characteristics with emphasis on aluminium.

Composition of soil vegetation cover and land management directly influences the cycling of chemical elements and is a key factor for soil biogeochemistry and also Al behaviour. Moreover, Al is an important factor limiting the growth of cultural plants. Our results are based on long-term observations of soils translocated from selected small areas of eight 1 ha plots of different land-use gradient, with identical geological, climatic and geographical conditions, located in the North of Congo Basin (near Mbalmayo, Cameroon). The plots are established in primary and secondary forests, cocoa agroforestry systems and a maize field (two plots per habitat). All soil plots were exchanged between each other in two layers; A. 0-5 cm, and B. 5-20 cm of depths. The soil was sampled at the times 0, +3, +6 months, and soil chemical parameters were determined. The most important differences between the particular habitats comprise of vegetation cover as a consequence of the land management. Particular plots differed mainly in their pH, organic C, exchangeable Al and contents of base cations. The most marked trends comprise of significant decrease of pH, increase of Al and decrease of the Ca/Al ratio in A layer after translocation to the agricultural plots. All translocations resulted into rapid loss of organic C and release of Al, which was more obvious when the forest-to-agriculture translocation took place.

Thallium stable isotope fractionation in white mustard: Implications for metal transfers and incorporation in plants.

We studied thallium (Tl) isotope fractionation in white mustard grown hydroponically at different Tl doses. Thallium isotope signatures in plants indicated preferential incorporation of the light 203Tl isotope during Tl uptake from the nutrient solution. Negative isotope fractionation was even more pronounced in dependence on how much the available Tl pool decreased. This finding corresponds to the concept of isotope overprinting related to a high contamination level in the growing media (solution or soil). Regarding Tl translocation in plants, we observed a large Tl isotope shift with an enrichment in the heavy 205Tl isotope in the shoots relative to the roots in treatments with low/moderate solution Tl concentrations (0.01/0.05 mg Tl/L), with the corresponding α205/203Tl fractionation factors of ˜1.007 and 1.003, respectively. This finding is probably a consequence of specific (plant) reactions of Tl replacing K in its cycle. The formation of the S-coordinated Tl(I) complexes, potentially affecting both Tl accumulation and Tl isotope fractionation in plants, however, was not proven in our plants, since we did not have indication for that on the basis of X-ray absorption spectroscopy, suggesting that Tl was mainly present as free/hydrated Tl+ ion or chemically bound to O-containing functional groups.

Oral bioaccessibility of metal(loid)s in dust materials from mining areas of northern Namibia.

Ore mining and processing in semi-arid areas is responsible for the generation of metal(loid)-containing dust, which is easily transported by wind to the surrounding environment. To assess the human exposure to dust-derived metal(loid)s (As, Cd, Cu, Pb, Sb, Zn), as well as the potential risks related to incidental dust ingestion, we studied mine tailing dust (n = 8), slag dust (n = 5) and smelter dust (n = 4) from old mining and smelting sites in northern Namibia (Kombat, Berg Aukas, Tsumeb). In vitro bioaccessibility testing using extraction in simulated gastric fluid (SGF) was combined with determination of grain-size distributions, chemical and mineralogical characterizations and leaching tests conducted on original dust samples and separated PM10 fractions. The bulk and bioaccessible concentrations of the metal(loid)s were ranked as follows: mine tailing dusts < slag dusts ≪ smelter dusts. Extremely high As and Pb bioaccessibilities in the smelter dusts were caused by the presence of highly soluble phases such as arsenolite (As2O3) and various metal-arsenates unstable under the acidic conditions of SGF. The exposure estimates calculated for an adult person of 70 kg at a dust ingestion rate of 50 mg/day indicated that As, Pb (and also Cd to a lesser extent) grossly exceeded tolerable daily intake limits for these contaminants in the case of slag and smelter dusts. The high risk for smelter dusts has been acknowledged, and the safety measures currently adopted by the smelter operator in Tsumeb are necessary to reduce the staff's exposure to contaminated dust. The exposure risk for the local population is only important at the unfenced disposal sites at Berg Aukas, where the PM10 exhibited high levels of bioaccessible Pb.

The impact of wetland on neutral mine drainage from mining wastes at Luanshya in the Zambian Copperbelt in the framework of climate change.

The impact of a natural wetland ("dambo" in Zambia) on neutral mine drainage at Luanshya in the Zambian Copperbelt has been investigated during an intermediate discharge period (July) using a multi-method characterization of solid phase samples, sequential extraction analysis, X-ray diffraction, Mössbauer spectroscopy, and scanning electron microscopy combined with water analyses, isotopic analyses, and geochemical modeling. In the wetland, the principal identified solid phases in sediments were carbonates, gypsum, and ferric oxyhydroxides. A significant portion of the ochres was present as insoluble hematite. Mine drainage pH values decrease, and log [Formula: see text] values increase after inflow of water into the wetland; dissolved and suspended concentrations of Fe, Mn, Cu, and Co also decrease. Based on speciation calculations, there is no precipitation of secondary Cu and Co minerals in the period of sampling, but it can occur later in dry period when the flow rate is reduced. Concentrations of sulfate decrease, and values of δ34S(SO4) in the wetland increase in parallel, suggesting sulfate reduction is occurring. In more advanced dry period, the discharge in mine drainage stream is probably much lower and water can reach supersaturation with respect to minerals such as gypsum, which has been found in sediments. Wetlands have a positive impact on mine drainage water quality due to the removal of metals by adsorption, co-precipitation, and filtration of colloids. However, there can also be a rebound of contamination by seepage inflow downstream from the wetland. Ongoing climate change with extreme hydrologic events may enhance differences between dry and rainy seasons with resulting faster mobilization of contaminants.

Thallium contamination of desert soil in Namibia: Chemical, mineralogical and isotopic insights.

We studied arid desert soils from Namibia (Rosh Pinah) that were contaminated with up to 7 mg kg-1 of thallium (Tl) via dust emitted from a local flotation tailing dam. Chemical extractions of waste and soil materials indicated that most of the Tl is strongly bound, in accordance with X-ray diffraction and X-ray absorption spectroscopy data that point to the predominant association of Tl with metal sulfides and phyllosilicates. The isotope fractionation factor ε205Tl of the soil samples (from -0.4 to +3.8) shows a positive linear relationship (R2 = 0.62) with 1/Tl, indicative for the mixing of two major Tl pools, presumably anthropogenic Tl and geogenic Tl. The ε205Tl value for the topmost soil samples (∼+3) closely matches the ε205Tl value for post-flotation waste particles with a diameter of <0.05 mm, whereas the bulk flotation waste exhibits a significantly larger ε205Tl value (∼+6). These variations are in accordance with predominant atmospheric transfer of Tl from the tailings to the adjacent soils via fine (dust) particles. The identified minimal Tl alteration in soils indicates that only a small part of the Tl could be potentially released and passively enter the vegetation, local population and/or food chain in the long term. From this viewpoint, Tl does not represent such an important environmental concern as other (abundant) contaminants at the locality. Furthermore, there could be a relevance for other alkaline desert soils, including those where Tl pollution plays a major role.

Transformation of arsenic-rich copper smelter flue dust in contrasting soils: A 2-year field experiment.

Dust emissions from copper smelters processing arsenic-bearing ores represent a risk to soil environments due to the high levels of As and other inorganic contaminants. Using an in situ experiment in four different forest and grassland soils (pH 3.2-8.0) we studied the transformation of As-rich (>50 wt% As) copper smelter dust over 24 months. Double polyamide bags with 1 g of flue dust were buried at different depths in soil pits and in 6-month intervals; then those bags, surrounding soil columns, and soil pore waters were collected and analysed. Dust dissolution was relatively fast during the first 6 months (5-34%), and mass losses attained 52% after 24 months. The key driving forces affecting dust dissolution were not only pH, but also the water percolation/retention in individual soils. Primary arsenolite (As2O3) dissolution was responsible for high As release from the dust (to 72%) and substantial increase of As in the soil (to a 56 × increase; to 1500 mg kg-1). Despite high arsenolite solubility, this phase persisted in the dust after 2 years of exposure. Mineralogical investigation indicated that mimetite [Pb5(AsO4)3(Cl,OH)], unidentified complex Ca-Pb-Fe-Zn arsenates, and Fe oxyhydroxides partly controlled the mobility of As and other metal(loid)s. Compared to As, other less abundant contaminants (Bi, Cu, Pb, Sb, Zn) were released into the soil to a lesser extent (8-40% of total). The relatively high mobility of As in the soil can be seen from decreases of bulk As concentrations after spring snowmelt, high water-extractable fractions with up to ∼50% of As(III) in extracts, and high As concentrations in soil pore waters. Results indicate that efficient controls of emissions from copper smelters and flue dust disposal sites are needed to prevent extensive contamination of nearby soils by persistent As.

Copper isotopic record in soils and tree rings near a copper smelter, Copperbelt, Zambia.

The copper (Cu) content and isotopic composition were studied in soils and in pine tree rings at locations close to and far from the Cu smelter, located at Kitwe, Zambia. The soil in the remote area contained 25-75mgkg-1 Cu, whereas the soil close to the smelter contained 207-44,000mgkg-1 Cu. The δ65Cu at the remote area and close to the smelter varied in the range -0.40 to -0.11‰, and -0.44 to 0.01‰ respectively. The δ65Cu of the surface soil at both profiles (-0.44 to -0.40‰) is similar to the isotopic composition of the concentrates processed in the smelter (-0.75 to -0.45‰), i.e. both locations are affected by Cu ore dust. The increase in the δ65Cu in the direction towards the centre of the profile is caused by the oxidative dissolution of Cu(I) from ore minerals, during which heavier Cu is released. In deeper parts of the profile, there is a slight decrease in δ65Cu because of easier mobilisation of the lighter isotope. The tree rings at the two locations differ in the total contents and isotopic composition. At the less contaminated site, the Cu contents equal 0.4 to 1.1mgkg-1 while, at the polluted site, the Cu contents vary in the range 3 to 47mgkg-1. Whereas, at the less contaminated location, the tree rings are substantially enriched in lighter Cu (δ65Cu=-0.76 to -2.2‰), at locations close to the smelter the tree rings have an isotopic composition (-0.31 to -0.88‰) similar to that of the contaminated soil or processed ore. The isotopic compositions of the tree rings close to the smelter are affected particularly by interception of dust containing Cu ore. The δ13C in tree rings demonstrate the interconnection of acidification and Cu mobility.

Thallium isotopes in metallurgical wastes/contaminated soils: A novel tool to trace metal source and behavior.

Thallium (Tl) concentration and isotope data have been recorded for contaminated soils and a set of industrial wastes that were produced within different stages of Zn ore mining and metallurgical processing of Zn-rich materials. Despite large differences in Tl levels of the waste materials (1-500mgkg-1), generally small changes in ε205Tl values have been observed. However, isotopically lighter Tl was recorded in fly ash (ε205Tl∼-4.1) than in slag (ε205Tl∼-3.3), implying partial isotope fractionation during material processing. Thallium isotope compositions in the studied soils reflected the Tl contamination (ε205Tl∼-3.8), despite the fact that the major pollution period ended more than 30 years ago. Therefore, we assume that former industrial Tl inputs into soils, if significant, can potentially be traced using the isotope tracing method. We also suggest that the isotope redistributions occurred in some soil (subsurface) horizons, with Tl being isotopically heavier than the pollution source, due to specific sorption and/or precipitation processes, which complicates the discrimination of primary Tl. Thallium isotope analysis proved to be a promising tool to aid our understanding of Tl behavior within the smelting process, as well as its post-depositional dynamics in the environmental systems (soils).

Mobility of Pb, Zn, Cu and As in disturbed forest soils affected by acid rain.

Early efforts at remediation of contaminated soils involve overturn or removal of the uppermost soil horizons. We find that such disruption is counterproductive, as it actually increases the mobility of the heavy metals involved. In our study, we sought to replicate in a controlled manner this commonly used remediation strategy and measure Pb, Zn, Cu and As concentrations in all soil horizons-both prior to and 1 year after disruption by trenching. BCR analyses (sequential leaching) indicate that Pb is affected to the greatest degree and is most highly mobile; however, Zn and As remain insoluble, thus partially ameliorating the detrimental effect. Differences in vegetation cover (i.e. spruce vs. beech forest) have little influence on overall element mobility patterns. The Krusné hory (Ore Mts., Czech Republic) study area is one of the more heavily contaminated areas in Central Europe, and thus the results reported here are applicable to areas affected by brown-coal-burning power plants.

Influence of Elevation Data Resolution on Spatial Prediction of Colluvial Soils in a Luvisol Region.

The development of a soil cover is a dynamic process. Soil cover can be altered within a few decades, which requires updating of the legacy soil maps. Soil erosion is one of the most important processes quickly altering soil cover on agriculture land. Colluvial soils develop in concave parts of the landscape as a consequence of sedimentation of eroded material. Colluvial soils are recognised as important soil units because they are a vast sink of soil organic carbon. Terrain derivatives became an important tool in digital soil mapping and are among the most popular auxiliary data used for quantitative spatial prediction. Prediction success rates are often directly dependent on raster resolution. In our study, we tested how raster resolution (1, 2, 3, 5, 10, 20 and 30 meters) influences spatial prediction of colluvial soils. Terrain derivatives (altitude, slope, plane curvature, topographic position index, LS factor and convergence index) were calculated for the given raster resolutions. Four models were applied (boosted tree, neural network, random forest and Classification/Regression Tree) to spatially predict the soil cover over a 77 ha large study plot. Models training and validation was based on 111 soil profiles surveyed on a regular sampling grid. Moreover, the predicted real extent and shape of the colluvial soil area was examined. In general, no clear trend in the accuracy prediction was found without the given raster resolution range. Higher maximum prediction accuracy for colluvial soil, compared to prediction accuracy of total soil cover of the study plot, can be explained by the choice of terrain derivatives that were best for Colluvial soils differentiation from other soil units. Regarding the character of the predicted Colluvial soils area, maps of 2 to 10 m resolution provided reasonable delineation of the colluvial soil as part of the cover over the study area.

Isotopic Tracing of Thallium Contamination in Soils Affected by Emissions from Coal-Fired Power Plants.

Here, for the first time, we report the thallium (Tl) isotope record in moderately contaminated soils with contrasting land management (forest and meadow soils), which have been affected by emissions from coal-fired power plants. Our findings clearly demonstrate that Tl of anthropogenic (high-temperature) origin with light isotope composition was deposited onto the studied soils, where heavier Tl (ε(205)Tl ∼ -1) naturally occurs. The results show a positive linear relationship (R(2) = 0.71) between 1/Tl and the isotope record, as determined for all the soils and bedrocks, also indicative of binary Tl mixing between two dominant reservoirs. We also identified significant Tl isotope variations within the products from coal combustion and thermo-desorption experiments with local Tl-rich coal pyrite. Bottom ash exhibited the heaviest Tl isotope composition (ε(205)Tl ∼ 0), followed by fly ash (ε(205)Tl between -2.5 and -2.8) and volatile Tl fractions (ε(205)Tl between -6.2 and -10.3), suggesting partial Tl isotope fractionations. Despite the evident role of soil processes in the isotope redistributions, we demonstrate that Tl contamination can be traced in soils and propose that the isotope data represent a possible tool to aid our understanding of postdepositional Tl dynamics in surface environments for the future.

Composition and fate of mine- and smelter-derived particles in soils of humid subtropical and hot semi-arid areas.

We studied the heavy mineral fraction, separated from mining- and smelter-affected topsoils, from both a humid subtropical area (Mufulira, Zambian Copperbelt) and a hot semi-arid area (Tsumeb, Namibia). High concentrations of metal(loid)s were detected in the studied soils: up to 1450mgAskg(-1), 8980mgCukg(-1), 4640mgPbkg(-1), 2620mgZnkg(-1). A combination of X-ray diffraction analysis (XRD), scanning electron microscopy (SEM/EDS), and electron probe microanalysis (EPMA) helped to identify the phases forming individual metal(loid)-bearing particles. Whereas spherical particles originate from the smelting and flue gas cleaning processes, angular particles have either geogenic origins or they are windblown from the mining operations and mine waste disposal sites. Sulphides from ores and mine tailings often exhibit weathering rims in contrast to smelter-derived high-temperature sulphides (chalcocite [Cu2S], digenite [Cu9S5], covellite [CuS], non-stoichiometric quenched Cu-Fe-S phases). Soils from humid subtropical areas exhibit higher available concentrations of metal(loids), and higher frequencies of weathering features (especially for copper-bearing oxides such as delafossite [Cu(1+)Fe(3+)O2]) are observed. In contrast, metal(loid)s are efficiently retained in semi-arid soils, where a high proportion of non-weathered smelter slag particles and low-solubility Ca-Cu-Pb arsenates occur. Our results indicate that compared to semi-arid areas (where inorganic contaminants were rather immobile in soils despite their high concentrations) a higher potential risk exists for agriculture in mine- and smelter-affected humid subtropical areas (where metal(loid) contaminants can be highly available for the uptake by crops).

The influence of sample drying procedures on mercury concentrations analyzed in soils.

Methods commonly used for soil sample preparation may be unsuitable for measuring Hg concentrations due to the possible loss of volatile Hg species when drying at higher temperatures. Here, the effects of freeze-drying, air drying at 25°C and oven-drying at 105°C on Hg concentrations in two soil types and three standard reference materials were tested. Two soils with different levels of Hg contamination and three reference materials were examined. A systematic decrease of Hg concentrations was observed in air-dried (24 %) and oven-dried (3 %) contaminated upland soils in comparison to freeze-dried control samples. The 105°C oven drying also led to loss of Hg from reference materials (5 %-8 % in comparison with the certified Hg concentration). Different results from the drying of sterilized reference materials and natural soils were probably related to the extent of microbiological activity, demonstrating the importance of this parameter in sample preparation for Hg analysis.

Thallium contamination of soils/vegetation as affected by sphalerite weathering: a model rhizospheric experiment.

The environmental stability of Tl-rich sphalerite in two contrasting soils was studied. Rhizospheric conditions were simulated to assess the risk associated with sulfide microparticles entering agricultural (top)soils. The data presented here clearly demonstrate a significant effect of 500 µM citric acid, a model rhizospheric solution, on ZnS alteration followed by enhanced Tl and Zn release. The relative ZnS mass loss after 28 days of citrate incubation reached 0.05 and 0.03 wt.% in Cambisol and Leptosol samples respectively, and was up to 4 times higher, compared to H2O treatments. Incongruent (i.e., substantially increased) mobilization of Tl from ZnS was observed during the incubation time. Generally higher (long-term) stability of ZnS with lower Tl release is predicted for soils enriched in carbonates. Furthermore, the important role of silicates (mainly illite) in the stabilization of mobilized Tl, linked with structural (inter)layer Tl-K exchange, is suggested. Thallium was highly bioavailable, as indicated by its uptake by white mustard; maximum Tl amounts were detected in biomass grown on the acidic Cambisol. Despite the fact that sulfides are thought as relatively stable phases in soil environments, enhanced sulfide dissolution and Tl/trace element release (and bioaccumulation) can be assumed in rhizosphere systems.