Assessing the ecotoxicological effects of long-term contaminated mine soils on plants and earthworms: relevance of soil (total and available) and body concentrations

Are the Brazilian prevention values for copper and zinc in soils suitable for protecting earthworms against metal toxicity?

The current Brazilian copper (Cu) and zinc (Zn) prevention values (PV) for soil quality do not take into account the ecotoxicological impacts on soil organisms, which suggests these guiding values may not be protective of soil ecological trophic levels. This study assessed the acute (mortality) and chronic toxicity (reproduction), as well as the cumulative (bioaccumulation) potential of Cu and Zn (pseudo-total and available fractions) for earthworms Eisenia andrei in a Tropical Artificial Soil (TAS) and two tropical field soils (Oxisol and Alfisol). Toxicity data based on pseudo-total fractions were compared to PV. The Lowest Observed Effect Concentrations (LOEC) for the mortality endpoint were found at Cu and Zn concentrations higher than their PV (60 and 300 mg kg^-1, respectively), regardless of the soil type. However, concentrations lower than PV reduced the reproduction of E. andrei by 20% (compared to the controls) for Cu in all tested soils (EC₂₀s from 31.7 to 51.2 mg kg^-1) and by 50% for Zn in Oxisol and Alfisol (EC₅₀s = 225 and 283 mg kg^-1, respectively). In TAS, only the EC₂₀ (273 mg kg^-1) for Zn was lower than PV. Increases of Cu in earthworm tissues occurred at concentrations higher than PV in all tested soils (LOEC values from 70 to 107 mg kg^-1). The same was observed for Zn in TAS (LOEC = 497 mg kg^-1), while in the field soils, the increases of Zn in earthworm tissues were lower than PV (LOEC = 131 and 259 mg kg^-1 in Alfisol and Oxisol, respectively). We suggest the following: (1) The current Brazilian PV for Cu and Zn are not protective for earthworms (E. andrei) in the field soils tested; (2) PV derived from ecotoxicological assays in artificial soil cannot be representative for Brazilian field soils; (3) Using PV based on the pseudo-total fraction, without a soil-type normalizing factor, may limit the representativeness of this threshold for different soil types.

Ecotoxicological risk assessment in soils contaminated by Pb and As 20 years after a mining spill

This study evaluates the potential toxicity of the soils of the Guadiamar Green Corridor (GGC) affected by the Aznalcóllar mine spill (Andalusia, Spain), one of the most important mining accidents in Europe in recent decades. Twenty years after the accident, although the area is considered to be recovered, residual contamination in soils persists, and the bioavailability of some contaminants, such as As, is showing trends of increasing. Therefore, the potential residual toxicity in 84 soil samples was evaluated by bioassays with lettuce (Latuca sativa L.), earthworms (Eisenia andrei) and determining the microbial activity by basal respiration and metabolic quotient. The selected soils sampled along the GGC were divided into 4 types according to their physicochemical properties. In the closest part of the mine two soil types appear (SS1 and SS2), originally decarbonated and loamy, with a reduction in lettuce root elongation of 57% and 34% compared to the control, as well as a the highest metabolic quotient (23.9 and 18.1 ng Cc_O2 μg Cmicrob^-1 h^-1, respectively) with the highest risk of Pb and As toxicity. While, located in the middle and final part of the affected area of the spill (SS3 and SS4), soils presented alkaline pH, finer textures and the lowest metabolic quotient (<9.5 ng Cc_O2 μg Cmicrob^-1 h^-1). In addition, due to Pb and As exceeded the Guideline values established in the studied area, the human toxicity risk was determined according to US-EPA methodology. Although the total contents were higher than the Guidelines established, the obtained hazard quotients for both contaminants were less than one, so the risk for human health was discarded. However, monitoring over time of the toxicity risks of the GGC soils would be advisable, especially due to the existence of areas where residual contamination persist, and soil hazard quotient obtained for As in children was higher and close to unity.

Role of potassium transporter KUP8 in plant responses to heavy metals

Heavy metal concentrations, which have been increasing over the last 200 years, affect soil quality and crop yields. These elements are difficult to eliminate from soils and may constitute a human health hazard by entering the food chain. Recently, we obtained a selection of mutants with different degrees of tolerance to a mixture of heavy metals (HMmix) in order to gain a deeper insight into the underlying mechanism regulating plant responses to these elements. In this study, we characterized the mutant obtained Atkup8 (in this work, Atkup8-2), which showed one of the most resistant phenotypes, as determined by seedling root length. Atkup8-2 is affected in the potassium transporter KUP8, a member of the high-affinity K⁺ uptake family KUP/HAK/KT. Atkup8-2 mutants, which are less affected as measured by seedling root length under HMmix conditions, showed a resistant phenotype with respect to WT seedlings which, despite their delayed growth, are able to develop true leaves at levels similar to those under control conditions. Adult Atkup8-2 plants had a higher fresh weight than WT plants, a resistant phenotype under HMmix stress conditions and lower levels of oxidative damage. KUP8 did not appear to be involved in heavy metal or macro- and micro-nutrient uptake and translocation from roots to leaves, as total concentrations of these elements were similar in both Atkup8-2 and WT plants. However, alterations in cellular K⁺ homeostasis in this mutant cannot be ruled out.

Assessment of biopiles treatment on polluted soils by the use of Eisenia andrei bioassay

A long-term case of residual pollution is studied after 20 years since the largest mining accident in Spain (the Aznalcóllar spill) happened. This pollution is manifested through a surface zoning consisting of bare soils (B0), sparsely vegetated soils (B1), and densely vegetated and recovered soils (B2). A biopiles treatment with a mixture of contaminated soils (B0 and B1) with recovered soils (B2) at 50% (w/w), and vermicompost addition (50 tons ha-1) was evaluated. To assess the effectiveness of treatments, total, water-soluble, and bioavailable fractions of the most polluting elements in the zone (Cu, Zn, As, Pb, Cd, and Sb) was analyzed. To evaluate the potential risk of contamination for the ecosystem, a bioassay with earthworm Eisenia andrei was carried out. Twenty years after the accident, there are still soils where total As and Pb exceed the regulatory levels and water-soluble Zn and As exceed the toxicity guidelines. According to toxicity bioassay, weight variation and juvenile production of earthworms showed an improvement after biopiles treatment, with values that trend to be similar to those of recovered soils. The only bioaccumulated element in earthworms was Cd (BAF>1), both in polluted as in treated soils, which indicates the possible existence of exclusion mechanisms of the other pollutants by earthworms. The comparison between biopiles and polluted soils showed no significant differences for the bioaccumulation factor of trace elements, with the exception of Zn and Cu, which slightly increased after treatment. According to our results, biopiles treatment combined with vermicompost addition is a good technique for the recovery of residual contaminated areas, by the improvement of soil properties and the reduction of the potential toxicity; anyway, monitoring of soils and organisms is needed to prevent the increase of bioavailability of some potentially pollutant elements over time.

Ecological criteria for zinc in Chinese soil as affected by soil properties

The increasing accumulation of zinc (Zn) in agricultural soils has led to the need to assess the potential risk of this element for terrestrial organisms. However, the soil ecological criteria in agricultural soil as a function of soil properties have been sparsely reported. In the present study, we derived the ecological criteria (expressed as predicted no effect concentration (PNEC)) for Zn in soils, based on ecotoxicity data for 19 terrestrial species in Chinese soils, the effect of soil properties on Zn ecotoxicity, differences in species sensitivity, and differences between laboratory and realistic field conditions. First, all ecotoxicity data of Zn for terrestrial organisms in Chinese soils were collected and filtered with given criteria to obtain reliable database. Second, the ecotoxicity data were normalized using Zn ecotoxicity predictive models to eliminate the effect of soil properties on Zn ecotoxicity, and corrected with leaching and aging factors to minimize the differences in Zn ecotoxicity under laboratory and field conditions. Then, species sensitivity distribution (SSD) curves were generated with a Burr Ⅲ function based on corrected ecotoxicity data. The concentration of Zn in soil that provides ecological safety for (100 - x)% of species (HC_x), was calculated from the SSD curve and HC₅ was used for estimation of PNEC. Finally, we developed the predictive models for HC_x by quantifying the relationship between the Zn HC_x and soil properties. Results showed that soil pH was the most crucial factor affecting Zn HC_x values, with HC₅ values varying from 38.3 mg/kg in an acidic soil to 263.3 mg/kg in an alkaline calcareous soil. Both the two-factor (soil pH and OC) and the three-factor (soil pH, OC and CEC) models predicted HC_x values well, with determination coefficients (R²) of 0.941-0.959 and 0.978-0.982, respectively. This study provides a scientific and reliable basis for the improvement of ecological risk assessment and the establishment of soil environmental quality standards.

Assessment of phytotoxicity of leachates from landfilled waste and dust from foundry

The study assesses the contamination, classification and phytotoxicity of foundry waste. The presented results are a part of the research on the agrotechnical use of foundry waste. Landfilled foundry waste (LFW) and dust samples were taken from one of the Polish foundries. An analysis of the waste and its leachate composition was conducted. Phytotoxicity tests were carried out using Lepidium sativum. The aim of the phytotoxicity study was to evaluate germination and root growth after 72 h and the accumulation of heavy metals after 7 days. LFW was least contaminated with heavy metals and metalloids compared to dust. The composition of the foundry dusts depended on the unit of the foundry, from which it was collected. It was found that electric arc furnace dust (EAFD) was the most polluted by heavy metals among the dust samples. According to the requirements of Polish regulations most of tested waste were classified as non-hazardous, and EAFD as hazardous waste due to high Pb concentration in leachate. Phytotoxicity tests have shown a low phytotoxicity of the leachate from most of the tested waste. The results of the accumulation test showed that an excess of metal and metalloids in leachate was not directly related to its accumulation in plants. A negative correlation between EC, Cu, Co, Fe, Pb, Cr, K, Na, sulfate, fluoride, ammonia, phenol and formaldehyde concentration in leachate and GI was found. It was stated that the Fe, Mn, As and Se in plants was significantly correlated with concentrations in leachate.

Interactive effects of earthworm Eisenia fetida and bean plant Phaseolus vulgaris L on the fate of soil selenium

Selenium (Se) is an essential micronutrient for animals with a narrow margin between essentiality and toxicity. Se toxicity is largely related to inorganic forms of Se in soil, i.e., selenite and selenate that enter food chains through plant uptake, threatening higher trophic level organisms. This experiment investigated effects of earthworm activity on Se bioavailability in soil and the subsequent plant uptake, using earthworm Eisenia fetida and bean plant Phaseolus vulgaris L, both exposed to either selenite or selenate at 1 or 4 mg Se kg^-1 for 16 weeks. Plants took up selenate (up to 221-fold) faster than selenite, with up to 84% of the Se rapidly transported to shoots. In the presence of earthworms, Se accumulation obviously increased for selenate-supplied plants, leading to an up to 4% increase in Se translocation factor for all treatments except for 1 mg kg^-1 selenite treatment. Earthworms also concentrated Se faster in tissues (up to 274 mg kg^-1 DW) at exposure to selenate. For Se toxicity, Se speciation analysis was conducted on the plants and earthworms using XAS. Compared to worm-free treatments, the percentage of organo-Se, i.e., SeMet and CysSeSeCys, increased in beans (up to 34%) in the presence of earthworms for selenate, while the elemental Se portion was significantly reduced or absent, opposite to the results for selenite. Surprisingly, elemental Se (up to 65%) dominated earthworms, regardless of the form of Se supplied. In conclusion, earthworms clearly enhanced Se uptake and translocation in plants, leading to elevated Se levels in shoots. To prevent resulting hazards to humans and other animals, caution should be taken while consuming the shoots, particularly beans, harvested from the Se contaminated soil where earthworm activity is high. Finally, the significant reduction in soil Se suggests phytoextraction of Se from the soil could be improved using earthworms as an aid to plants.

Bioavailable arsenic and amorphous iron oxides provide reliable predictions for arsenic transfer in soil-wheat system

The application of current soil quality standards based on total arsenic (As) fails to assess the ecological risks of soil arsenic or to ensure the safety of crops and foods. In this study, bioavailable arsenic instead of total arsenic was applied to improve predictive models for arsenic transfer from soil to wheat (Triticum turgidum L.). The stepwise multiple-linear regression analysis showed that bioavailable arsenic and amorphous iron oxides (Fe_OX) were the two most important factors contributing to arsenic accumulation in wheat grain, with the explained percentage of variation being up to 82%. Compared with the bioavailable arsenic extracted by NH₄H₂PO₄, bioavailable arsenic extracted by HNO₃ from soils generated better predictions of the amount of arsenic in grain. The best reliable model was log[As_grain] = 0.917 log[HNO₃-As] - 0.452 log[Fe_OX] - 1.507 (R² = 0.82, P <  0.001). Consistently, bioavailable arsenic and Fe_OX were also the key factors to predict arsenic accumulation in wheat straw, leaves and spikes. Our prediction models was successfully verified for three independent soils. Our results highlight the role of soil bioavailable heavy metals in predicting their transfer in soil-plant systems and can be used to improve existing Chinese soil quality standards.

Study of Zn availability, uptake, and effects on earthworms of zinc oxide nanoparticle versus bulk applied to two agricultural soils: Acidic and calcareous

The increasing use of zinc oxide nanoparticles (ZnO NPs) in agriculture renders it necessary to evaluate their impact on soil non-target organisms. This work studies Zn availability to earthworms from the ZnO (NP and bulk) applied to two agricultural soils with a different pH at 20, 225, 500, and 1000 mg Zn kg^-1. Zn uptakes and the effects on Eisenia andrei, grown under controlled conditions, were determined. Effects were assessed at three levels: organisms, mortality, growth and reproduction; biochemical, catalase and glutathione S-transferase activities, malondialdehyde (MDA), and protein content; cellular in coelomocytes, reactive oxygen species (ROS) generation, lysosomal membrane alterations (RN) and mitochondrial dysfunction (MTT). Available Zn was 100-fold higher in acidic than in calcareous soil and did not differ among ZnO (NP or bulk). Zn in worms was auto-regulated regardless of the soil Zn concentration, pH and ZnO size. Effects on mortality and weight were observed only in the acidic soil at the highest concentration, ZnO NPs reduced survival and body weight, while ZnO bulk reduced body weight. Reproduction parameters in acidic soil were: EC50 (fecundity) 277 and 256 mg Zn kg-1 and EC50 (fertility) 177 and 179 mg Zn kg-1 for ZnO NPs and bulk, respectively, with no found NP-specific effects. No responses of enzymatic activities, MDA and MTT were detected. ROS and RN were altered in the coelomocyte cells of earthworms in the two soils, but effects depended on ZnO size suggesting nanospecific effects. Soil pH governs toxicity more than ZnO size regardless of body Zn concentration.

Effects of diclofenac and salicylic acid exposure on Lemna minor: Is time a factor?

The global occurrence of pharmaceuticals in the aquatic environment has been considered a particularly concerning problem with unknown consequences. Non-steroidal anti-inflammatory drugs (NSAIDs) including diclofenac (DCF) and salicylic acid (SA), are among the most frequently prescribed drugs in the world, being consequently commonly found in the aquatic environment. Prolonged experiments (with duration of exposure that surpass those recommended by already established testing guidelines) are important to obtain ecologically relevant data to address the issue of NSAIDs ecotoxicity, because by being more realistically (namely in terms of levels and durations of exposure), such tests may indicate realistic challenges posed to aquatic organisms. Among the most common test species that are used for assessing environmental quality, plants play a leading role. Lemna species are among the most important plants used for ecotoxicity testing. Therefore, the aim of this study was to evaluate the temporal effect of a prolonged exposure of DCF and SA on Lemna minor. To attain this purpose, L. minor plants were chronically exposed to 0, 4, 20, and 100 μg/L of both pharmaceuticals, and samplings were performed at 6, 10 and 14 days of exposure. The analyzed endpoints were: levels of chlorophyll a, b and total, carotenoids; and enzymatic biomarkers, such as catalase, ascorbate peroxidase and glutathione-S-transferases. Diclofenac was responsible for alterations in all analyzed parameters in different intervals of exposure. Salicylic acid exposure was not capable of causing alterations on pigment contents of L. minor, however, enzymatic biomarkers were altered at all sampling intervals. Thus, it is possible to conclude that both pharmaceuticals can cause damage on the tested macrophyte species, biochemical parameters being more sensitive than physiological ones. Additional prolonged experiments are required to understand the chronic effects of different pharmaceuticals in the aquatic environment, especially in plants.

Assessment of the environmental impact of an abandoned mine using an integrative approach: A case-study of the "Las Musas" mine (Extremadura, Spain)

The mine abandonment is generally associated with the release of potentially toxic metals into the environment, which may depend on metals speciation, soil properties and climate conditions. The goal of the present work was to assess the environmental impact of the abandoned Pb-Zn mine "Las Musas" (Spain) using an integrative approach. The impact on soils and surface waters was performed using: chemical parameters, quantification of potentially toxic metals (Cd, Cu, Pb and Zn), and ecotoxicological responses using lethal and sub-lethal bioassays with organisms' representative of different trophic level ((soil: Eisenia fetida (mortality and reproduction test); Latuca sativa and Lollium perenne (seedling emergence); and water: Vibrio fischeri (luminescence inhibition), Daphnia magna (immobility and reproduction test), Thamnocephalus platyurus (mortality), Pseudokirchneriella subcapitata (growth inhibition)). The results showed soils with neutral to slight alkaline pH (7.64-8.18), low electric conductivity (125-953 μS/cm) and low organic matter levels (0.20-1.85%). For most of the soil samples, Pb was the only metal which surpassed the limit proposed by the Canadian soil quality guidelines, with values ranging from 42.2 to 181.4 mg/kg. The ecotoxicological results showed that the soils with the highest levels of Pb induced a decrease on E. fetida reproduction and on L. sativa germination, indicating negative impacts on the habitat function. The analysis of the surface waters showed levels of Zn surpassing the legal limit adopted from the Water Framework Directive (37.0 to 69.0 μg/L). The ecotoxicological results highlight the importance of bioassays that evaluate the behavior of species, when assessing the risk of mining areas with non-acid soils and waters with high nutrients/organic matter concentrations and low concentrations of potentially toxic metals. The results indicated a moderate environmental risk from potentially toxic metals, at the areas analyzed around the Azuaga mine.

A new conceptual framework for plant responses to soil metals based on metal transporter kinetic parameters

Based on a review of the literature, we have developed a functional conceptual framework of plant metal uptake in relation to plant available metal concentration in the soil. This framework applies to all plant parts and plant available metal levels in soils, and was validated using independent datasets from field surveys and the literature. This is the first framework based on metal transporter kinetic parameters and combining Michaelis-Menten (hyperbolic) kinetics facilitated by the High Affinity Transport System (HATS) for soil concentrations below the transition concentration between transport systems, and linear metal uptake facilitated by the Low Affinity Transport System (LATS) for higher soil available metal concentrations. We propose a new terminology for metal tolerant plants, i.e. metal tolerators, based on this framework. Depending on the plant available metal levels in the soil, tolerator responses to metals can be described best by either V_max and K_m for soil concentrations below the transition concentration between metal transport systems (HATS), or by the slope for greater soil concentrations (LATS). This conceptual framework may be a useful tool for selecting suitable metal tolerators for specific phytoremediation purposes, and may be also applied to non-metal elements or ions.

Assessment of the stabilization of heavy metal contaminants in soils using chemical leaching and an earthworm bioassay

Soil stabilization is a remedial technique that reduces the exposure of the soil environment to soil contaminants. Its efficacy can be assessed by determining whether the environmental availability of a contaminant decreases following treatment. We evaluated several chemical leaching treatments by assessing both contaminant leachability and bioaccumulation in the earthworm Eisenia fetida, and determined the most effective treatment for achieving soil stabilization. Soil samples contaminated with As, Cd, Cu, Pb, and/or Zn were collected from abandoned mine areas and stabilized by adding limestone and steel slag (5% and 2% w/w, respectively). All leaching and earthworm tests were conducted using both contaminated and stabilized soils. In addition to bioaccumulation in earthworms, several toxicity parameters (number of cocoons, growth changes, and survival rates) were also assessed to determine the effects of the treatments on the earthworms. The study showed that treatment of soil with EDTA-NH₄OAc resulted in a significant decrease in contaminant leachability following soil stabilization. There was an increase in survival and growth of earthworms exposed to the stabilized soil compared with those exposed to the non-stabilized soil. Bioaccumulation in earthworms was found to be statistically correlated with the leachability of As by EDTA-NH₄OAc. We conclude that limestone and steel slag effectively decreased the availability of heavy metals in the soil and reduced the toxicity to earthworms. Leaching with EDTA-NH₄OAc has the potential to be predictive in estimating the bioavailability of As in soils, but further studies are needed if it is to be proposed as a standard method.

Influence of metal-contamination on distribution in subcellular fractions of the earthworm (Metaphire californica) from Hunan Province, China

Earthworms have the ability to accumulate of heavy metals, however, there was few studies that addressed the metals in earthworm at subcellular levels in fields. The distributions of metals (Cd, Cu, Zn, and Pb) in subcellular fractions (cytosol, debris, and granules) of earthworm Metaphire californica were investigated. The relationship between soil metals and earthworms were analyzed to explain its high plasticity to inhabit in situ contaminated soil of Hunan Province, south China. The concentration of Cd in subcellular compartments showed the same pattern as Cu in the order of cytosol > debris > granules. The distribution of Zn and Pb in earthworms indicated a similar propensity for different subcellular fractions that ranked as granules > debris > cytosol for Zn, and granules > cytosol > debris for Pb. The internal metal concentrations in earthworms increased with the soil metals (p<0.05). Significant positive correlations were found between soil Cd and Cd concentrations in cytosol and debris (p<0.01). Moreover, the soil Pb concentration significantly influenced the Pb concentrations in cytosol and debris (p<0.01), similar to that of Cd. The soil Cu concentrations was only associated with the Cu in granules (p<0.05). Soil Zn concentrations correlated with the Zn concentrations in each subcellular fraction (p<0.05). Our results provide insights into the variations of metals partitioning in earthworms at subcellular levels and the relationships of soil metals, which could be one of the detoxification strategies to adapt the long-term contaminated environment.

Arsenic characteristics in the terrestrial environment in the vicinity of the Shimen realgar mine, China

In this study, multiple types of samples, including soils, plants, litter and soil invertebrates, were collected from a former arsenic (As) mine in China. The total As concentrations in the soils, earthworms, litter and the aboveground portions of grass from the contaminated area followed the decreasing order of 83-2224 mg/kg, 31-430 mg/kg, 1-62 mg/kg and 2-23 mg/kg, respectively. X-ray absorption near-edge structure (XANES) analysis revealed that the predominant form of As in the soils was arsenate (As(V)), while no arsenite (As(III)) was detected. In the grass and litter of the native plant community, inorganic As species (As(V) and As(III)) were the main species, while minor amounts of DMA, MMA, AsC, and an unknown As species were also detected in the extracts analyzed with high-performance liquid chromatography inductively coupled plasma mass spectrometry (HPLC-ICP-MS). The As speciation and As concentrations varied with the plant species, and very high As levels (197-584 mg/kg) and proportions of inorganic As (>99%) were found in two As-hyperaccumulating ferns, Pteris vittata and Pteris cretica. The major As species extracted from earthworms were inorganic, with proportions of 51-53% As(III) and 38-48% As(V). AsB was the only organic species present in the earthworm samples, although at low proportions (<8.99%). The internal bioconversion of other As species is hypothesized to contribute greatly to the formation and accumulation of AsB in earthworms, although the direct external absorption of organic As from soils might be another source. This study sheds light on the potential sources of complex organoarsenicals, such as AsB, in terrestrial organisms.

Ecotoxicological impact of arsenic on earthworms and collembolans as affected by attributes of a highly weathered tropical soil

High levels of heavy metals in soils may impose serious impacts on terrestrial organisms. In Brazil, the prevention values for evaluating the ecological risk of these elements are based only on soil chemical analyses and/or on data from ecotoxicological assays performed in soils of temperate regions. However, the attributes of the Brazilian highly-weathered tropical soils can influence the availability of heavy metals for soil fauna, resulting in different toxic values. To provide more accurate ecotoxicological risk values for arsenic (As) in tropical soils, we assessed the impacts of sodium arsenate (Na₂HAsO₄·7H₂O) on the reproduction of earthworms (Eisenia andrei) and collembolans (Folsomia candida), as well as on As bioaccumulation and growth (weight loss) of E. andrei in a tropical artificial soil (TAS) and in an Oxisol. In TAS, As doses reduced the reproduction of the species and promoted weight loss of earthworms. On the other hand, the reproductions of the species as well as the earthworm growth were not altered by As in the Oxisol. The effective concentrations that reduce the reproduction of E. andrei and F. candida by 50 % (EC₅₀) obtained in TAS (22.7 and 26.1 mg of As kg^-1 of dry soil, respectively) were lower than those in the Oxisol (>135 mg kg^-1, for both species). Although there was As bioaccumulation in earthworms in both soils, the internal concentrations in the earthworms were much higher in the oligochaetes exposed to arsenic in TAS. All these differences were attributed to the higher availability of As in the TAS, compared to the Oxisol, which increased the exposure of the species to the metal. The lower availability in the Oxisol was related to higher contents of type 1:1 silicate minerals and Fe and Al oxides and hydroxides, which strongly bind to As. These results highlight the importance of using tropical soils of humid regions to derive the Brazilian ecological risk prevention values for heavy metals, since the toxicity values are specific for these soils.

Screening Arabidopsis mutants in genes useful for phytoremediation

Emissions of heavy metals have risen over the past 200 years and significantly exceed those from natural sources. Phytoremediation strategies may be able to recover soil productivity in self-sustaining ecosystems; however, our knowledge of the molecular mechanisms involved in plant heavy-metal perception and signalling is scarce. The aim of this study was to assemble a "molecular tool box" of genes useful for phytoremediation. To identify mutants with different heavy-metal-tolerance, we first selected a medium from mixtures containing three metals based on their presence in two Spanish mining areas and then screened about 7000 lines of Arabidopsis T-DNA mutants and found 74 lines more resistant and 56 more susceptible than the wild type (WT). Classification of the genes showed that they were mainly linked to transport, protein modification and signalling, with RNA metabolism being the most representative category in the resistant phenotypes and protein metabolism in the sensitive ones. We have characterized one resistant mutant, Athpp9 and one sensitive, Atala4. These mutants showed differences in growth and metal translocation. Additionally, we found that these mutants keep their phenotype in amended former soils, suggesting that these genes may be useful for phytoremediation and the recovery of contaminated soils.

Evaluation of remediation techniques in soils affected by residual contamination with heavy metals and arsenic

Residual soil pollution from the Aznalcóllar mine spill is still a problem in some parts of the affected area, today converted in the Guadiamar Green Corridor. Dispersed spots of polluted soils, identified by the absence of vegetation, are characterized by soil acid pH and high concentrations of As, Pb, Cu and Zn. Ex situ remediation techniques were performed with unrecovered soil samples. Landfarming, Composting and Biopiles techniques were tested in order to immobilize pollutants, to improve soil properties and to promote vegetation recovery. The effectiveness of these techniques was assessed by toxicity bioassays: Lactuca sativa L. root elongation test, Vibrio fischeri bioluminescence reduction test, soil induced respiration test, and Eisenia andrei survival and metal bioaccumulation tests. Landfarming and Composting were not effective techniques, mainly due to the poor improvement of soil properties which maintained high soluble concentrations of Zn and Cu after treatments. Biopile technique, using adjacent recovered soils in the area, was the most effective action in the reduction of soil toxicity; the improvement of soil properties and the reduction in pollutants solubility were key to improve the response of the tested organisms. Therefore, the mixture of recovered soils with polluted soils in the areas affected by residual contamination is considered a more suitable technique to reduce the residual pollution and to promote the complete soil recovery in the Guadiamar Green Corridor.

Long-term toxicity assessment of soils in a recovered area affected by a mining spill

Residual pollution in the Guadiamar Green Corridor still remains after Aználcollar mine spill in 1998. The polluted areas are identified by the absence of vegetation, soil acidic pH and high concentrations of As, Pb, Zn and Cu. Soil toxicity was assessed by lettuce root elongation and induced soil respiration bioassays. In bare soils, total As and Pb concentrations and water-extractable levels for As, Zn and Cu exceeded the toxicity guidelines. Pollutants responsible for toxicity were different depending on the tested organism, with arsenic being most toxic for lettuce and the metal mixture to soil respiration. Soil properties, such as pH or organic carbon content, are key factors to control metal availability and toxicity in the area. According to our results, there is a risk of pollution to living organisms and the soil quality criteria established in the area should be revised to reduce the risk of toxicity.

Thresholds of arsenic toxicity to Eisenia fetida in field-collected agricultural soils exposed to copper mining activities in Chile

Several previous studies highlighted the importance of using field-collected soils-and not artificially-contaminated soils-for ecotoxicity tests. However, the use of field-collected soils presents several difficulties for interpretation of results, due to the presence of various contaminants and unavoidable differences in the physicochemical properties of the tested soils. The objective of this study was to estimate thresholds of metal toxicity in topsoils of 24 agricultural areas historically contaminated by mining activities in Chile. We performed standardized earthworm reproduction tests (OECD 222 and ISO 11268-2) with Eisenia fetida. Total soil concentrations of Cu, As, Zn, and Pb were in the ranges of 82-1295 mg kg(-1), 7-41 mg kg(-1), 86-345 mg kg(-1), and 25-97 mg kg(-1), respectively. In order to differentiate between the effects of different metals, we used regression analysis between soil metal concentrations and earthworm responses, as well as between metal concentrations in earthworm tissues and earthworm responses. Based on regression analysis, we concluded that As was a metal of prime concern for Eisenia fetida in soils affected by Cu mining activities, while Cu exhibited a secondary effect. In contrast, the effects of Zn and Pb were not significant. Soil electrical conductivity was another significant contributor to reproduction toxicity in the studied soils, forcing its integration in the interpretation of the results. By using soils with electrical conductivity ≤ 0.29 dS m(-1) (which corresponds to EC50 of salt toxicity to Eisenia fetida), it was possible to isolate the effect of soil salinity on earthworm reproduction. Despite the confounding effects of Cu, it was possible to determine EC10, EC25 and EC50 values for total soil As at 8 mg kg(-1), 14 mg kg(-1) and 22 mg kg(-1), respectively, for the response of the cocoon production. However, it was not possible to determine these threshold values for juvenile production. Likewise, we were able to determine EC10, EC25 and EC50 of earthworm tissue As of 38 mg kg(-1), 47 mg kg(-1), and 57 mg kg(-1), respectively, for the response of the cocoon production. Finally, we determined the no-observed effect concentration of tissue As in E. fetida of 24 mg kg(-1). Thus, earthworm reproduction test is applicable for assessment of metal toxicity in field-collected soils with low electrical conductivity, while it might have a limited applicability in soils with high electrical conductivity because the salinity-induced toxicity will hinder the interpretation of the results.

Integrating ecotoxicity and chemical approaches to compare the effects of ZnO nanoparticles, ZnO bulk, and ZnCl2 on plants and microorganisms in a natural soil

This work compared the toxicity of ZnO nanoparticles (ZnO-NPs), ZnO bulk, and ZnCl2 on microbial activity (C and N transformations and dehydrogenase and phosphatase activities) and their uptake and toxic effects (emergence, root elongation, and shoot growth) on three plant species namely wheat, radish, and vetch in a natural soil at 1000 mg Zn kg(-1). Additionally, plants were also tested at 250 mg Zn kg(-1). The effects of the chemical species on Zn extractability in soil were studied by performing single and sequential extractions. ZnCl2-1000 presented the highest toxicity for both taxonomic groups. For microorganisms, ZnO-NPs demonstrated adverse effects on all measured parameters, except on N transformations. The effects of both ZnO forms were similar. For plants, ZnO-NPs affected the growth of more plant species than ZnO bulk, although the effects were small in all cases. Regarding accumulation, the total Zn amounts were higher in plants exposed to ZnO-NP than those exposed to ZnO bulk, except for vetch shoots. The soil sequential extraction revealed that the Zn concentration in the most labile forms (water soluble (WS) and exchangeable (EX)) was similar in soil treated with ZnO (NP and bulk) and lower than that of ZnCl2-treated soil, indicating the higher availability of the ionic forms. The strong correlations obtained between WS-Zn fraction and the Zn concentrations in the roots, shoots, and the effects on shoot weight show the suitability of this soil extraction method for predicting bioavailable Zn soil for the three plant species when it was added as ZnO-NPs, ZnO bulk, or ZnCl2. In this work, the hazard associated with the ZnO-NPs was similar to ZnO bulk in most cases.

Influence of soil properties on the bioaccumulation and effects of arsenic in the earthworm Eisenia andrei

This study aimed at assessing the influence of soil properties on the uptake and toxicity effects of arsenic in the earthworm Eisenia andrei exposed for 4 weeks to seven natural soils spiked with different arsenic concentrations. Water-soluble soil concentrations (AsW) and internal As concentrations in the earthworms (AsE) were greatly different between soils. These two variables were highly correlated and were key factors in earthworm toxicity response. AsW was explained by some soil properties, such as the pH, calcium carbonate content, ionic strength, texture or oxide forms. Toxicity showed a clear variation between soils, in some cases without achieving 50 % adverse effect at the highest As concentration added (600 mg kg(-1)). Nevertheless, soil properties did not show, in general, a high relation with studied toxicity endpoints, although the high correlation with AsW could greatly reduce indirectly As bioavailability and toxicity risk for earthworms. Obtained results suggest that soil properties should be part of the criteria to establishing thresholds for contaminated soils because they will be key in controlling As availability and thus result in different degrees of toxicity.

Modeling the transfer of arsenic from soil to carrot (Daucus carota L.)--a greenhouse and field-based study

Reliable empirical models describing arsenic (As) transfer in soil-plant systems are needed to estimate the human As burden from dietary intake. A greenhouse experiment was conducted in parallel with a field trial located at three sites through China to develop and validate soil-plant transfer models to predict As concentrations in carrot (Daucus carota L.). Stepwise multiple linear regression relationships were based on soil properties and the pseudo total (aqua regia) or available (0.5 M NaHCO3) soil As fractions. Carrot As contents were best predicted by the pseudo total soil As concentrations in combination with soil pH and Fe oxide, with the percentage of variation explained being up to 70 %. The constructed prediction model was further validated and improved to avoid overprotection using data from the field trial. The final obtained model is of great practical relevance to the prediction of As uptake under field conditions.

Interaction between nickel and cobalt toxicity in Enchytraeus crypticus is due to competitive uptake

Uptake and toxicity of Ni-Co mixtures in Enchytraeus crypticus were determined after 4 d, 7 d, 10 d, and 14 d exposure. Generally, body concentrations of Ni and Co increased with increasing exposure concentrations. Ni body concentration was significantly reduced in the presence of Co, whereas Ni only marginally affected Co uptake. When expressed as free ion activities, individual toxicity of Ni and Co increased with time, with median lethal concentrations (LC50) decreasing from 78.3 μM and 511 μM at 4 d to 40.4 μM and 393 μM at 14 d, respectively. When expressed as body concentrations, LC50BodyNi remained constant with time whereas LC50BodyCo increased during the first 7 d but remained stable afterwards. As identified by the MIXTOX model, interactions between Ni and Co were mainly antagonistic when based on free ion activities, however, no interaction was observed when based on body concentrations. A process-based model, incorporating exposure time to analyze the mechanisms underlying the dynamic mixture toxicity confirmed the differences in toxicokinetics of the 2 metals. The author's findings suggest that body concentrations, which incorporate bioaccumulation processes, are time-independent and can act as a more constant indicator of metal toxicity. The observed antagonism was mainly caused by competition between Co and Ni for binding sites and subsequent inhibition of Ni uptake. This competitive interaction occurred at the uptake level (toxicokinetics), but not at the target level (toxicodynamics).