Biochar-assisted phytoextraction of Cd and Zn by Noccaea caerulescens on a contaminated soil: A four-year lysimeter study

Amendments of biochar, the residual solid of biomass pyrolysis, have been shown to enhance metal phytoextraction from contaminated soils with hyperaccumulating plants in specific situations. In order to investigate this phenomenon over successive harvests in field conditions, two identical undisturbed soil cylinders (1-m² section × 1.85-m height) were excavated from a contaminated agricultural plot and monitored with instrumented lysimeters. Wood-derived biochar was added at a rate of 5% (w/w) in the first 30 cm of one of the two lysimeters. The Cd/Zn-hyperaccumulator Noccaea caerulescens was then grown for the next four years on both lysimeters. Our results showed that the hyperaccumulating plant was able to remove about 2 g m^-2 of Cd and 12-16 g m^-2 of Zn within four years, representing about 40% and 4% of the initial Cd and Zn soil contamination, respectively. Biochar amendment improved plant germination and survival and increased root surface density. However, no significant effect of biochar on shoot metal content of N. caerulescens was observed. Mass balances suggested that up to 10% the metal contamination moved from the disturbed Ap horizon to the deeper horizons, particularly in the biochar-amended soil profile. Furthermore, shoot Cd and Zn concentration generally decreased over the successive harvests, together with soil metal availability. Depending on the way to account for this progressive decrease in efficiency, our estimations of the time necessary to remove the excess of metals in the topsoil in these conditions ranged from 11 to 111 years for Cd and from 97 years to an infinite time for Zn. In conclusion, the simultaneous use of N. caerulescens and biochar amendment can lead to a significant removal of specific metallic elements from the topsoil, but the risk of metal movement down the soil profile and the observed decrease in phytoextraction efficiency over time deserve further investigations.

Geochemical Referencing of Natural Forest Contamination in Poland

Various studies have established possible threats posed due to pollution using ecological risk indices, but most have focused strictly on anthropogenic areas, so the data from these studies are less comparable with those obtained from natural forest sites, which was the focus of this current study. The main reason for this focus could be attributed to the commonly agreed reference provided by natural forest parks, which are assumed to be uncontaminated. The aim of this research was to determine if the Kampinoski National Park (Poland) could be considered a geochemical referencing ecosystem for Pb, Cd, and Ni levels. The specific purpose was to conduct a soil-background-based evaluation of metal contamination with a focus on geochemical indices as normative tools for assessing similar forest ecosystems at local and international levels. The toxicity response factors indicated some specific metal features that seemed highly magnified for Cd compared with Pb and Ni. The use of geochemical indices when assessing the contamination status of various ecosystems, either natural or strongly anthropogenic, is recommended to enable worldwide comparison, rather than only assessing metal contents. This approach considers the background metal concentrations for local on-site targets as well as pre-industrial reference levels for international referencing.

Water as a mild extractant of metals and metalloids from the samples of the selected certified reference materials and subsequent multi-elemental quantification by ICP-AES

An assessment of mobility and bioavailability of trace elements present in the soil requires the determination of these elements in soil samples by an appropriate methodology. In such a context, the use of mild extraction reagents-such as water-is considered to be appropriate. On the other hand, performing an analysis of a reference material together with real samples is recommended in order to control the quality of analytical procedure. The quantification of 27 analytes in aqueous extracts of the soil CRMs samples is described. The methodology consisted of single-step extraction of analytes by deionized water (m/v = 1/10) with their subsequent direct determination by inductively coupled plasma-atomic emission spectrometry (ICP-OES). Three certified reference materials (CRM) for soils have been selected as model samples: NCS DC 77302 (alias GBW 07410), Metranal-31, and Metranal-33. Although the recoveries of the selected elements obtained by water extractions are very low (i.e., the values usually do not exceed 1%), the results obtained in this study reveal the elements that by means of ICP-OES can be quantified in the water extracts of unpolluted soils are as follows: Al, Ba, Ca, Cr, Cu, Fe, K, Li, Mg, Mn, Mo, Na, Ni, P, S, Sr, Ti, and V. However, ICP-OES is not sensitive enough to quantify the elements As, Be, Cd, Co, Pb, Sb, Se, Tl, and Zn that are present in the water extracts of clean soil samples in too low mass fractions. The results obtained in this paper are useful for future uses of the three tested CRMs, in the cases of the extraction of the analytes by deionized water at room temperature.

Trace metal availability in soil horizons amended with various urban waste composts during 17 years - Monitoring and modelling

Recycling organic residues in agrosystems presents several benefits but faces the question of contaminants, among them a few trace metals which eventually accumulate in soils following regular applications of organic waste products (OWP) and represent an ecological risk. The increase of total trace metal contents in amended topsoils can be predicted by a mass balance approach, but the evolution of their available fractions is a more intricate issue. We aimed at modelling this evolution by using the dataset of a long-term field experiment of OWP applications (manure and three urban waste composts). Two operationally-defined fractions of 6 trace metals have been quantified in the OWP and amended topsoils between 2002 and 2015: the soluble and potentially available metals, extracted in 0.01 M CaCl₂ and 0.05 M EDTA solutions, respectively. The potentially available metals have progressively increased in amended topsoils, at rates depending on elements and types of OWP. For Zn, these increases corresponded in average to inputs of potentially available Zn from OWP. But the soil stocks of potentially available Cu increased faster than from the inputs of EDTA-extractable Cu, showing linear regression slopes between 1.4 and 2.5, depending on OWP type. The influence of OWP has been provisionally interpreted in the light of their efficiency to increase soil organic matter and their inputs of reactive oxides. Soluble copper has increased with repeated amendments. But soluble cadmium, nickel and zinc have generally decreased, as they are influenced by changing soil variables such as pH and organic matter. Statistic models were used to unravel the relationships between soluble and EDTA-extractable metals and other soil variables. For Cu, the most satisfactory models just relate soluble and potentially available Cu. Developing such models could contribute to predict the long-term effects of a precise scenario of agricultural OWP recycling upon available trace metals in soils.

Phosphoric acid purification sludge: Potential in heavy metals and rare earth elements

The present study was carried out to show the potential in heavy metals (HM) and the rare earth elements (REE) which presents the residues of phosphoric-acid(PA) purification. Three different cadmiferous solid residues (according to the nature of the purification process of the PA: BG, BC and BS) were collected from an industrial site located in the south of Tunisia. The mineralogical study showed the predominance of anhydrite, accompanied by quartz, malladrite; calcium sulfate hemihydrate and fluorapophyllite. The microanalysis showed (i) the association of cadmium and zinc, (ii) as well as the presence of associated REEs. The chemical analysis showed that (i) the calcium sulfate concentrations are majority in samples BS, BG and BC (44, 34 and 44%, respectively), (ii) significant concentrations of phosphoric acid (28, 18 and 21% P₂O₅, respectively), (iii) the HM: Cd, Zn, Cr, Ni, V, Cu, Pb, Co, Mo, Mn and U have proportion in the order of 0.1%. The concentrations of Cd, Zn and Cr are respectively in the order of: 230, 149 and 189 mg/kg for BS, 346, 243 and 153 mg/kg for BG and 183, 129 and 440 mg/kg for BC and (iv) the REEs: La, Ce, Nd, Eu, Y et Yb present considerable mass percentages able to reach 0.2%. A series of extraction tests was led on the cadmiferous sludges to evaluate the rates of HM (Cd, Zn) and REE dissolution, using two solvents (deionized water (DW) and aqueous sodium based alkaline metal solution). The results showed that the dissolution rates of Cd and Zn are respectively in the order of (12-29% and 41-45% for DW; 67-86% and 83-93% for Na₂SO₄ solution). The extractability of HM and REE is strongly influenced by pH, solvent nature and mineral load in the cadmiferous sludges. The water-soluble metals represent a significant mobile fraction, making the toxic elements more sensitive to mobilization processes, such as leaching and erosion. Whereas, the metals extractable by the Na₂SO₄ solution represent a very important exchangeable and "co-crystallization" fraction, which reflects the bioavailability of these metals.

Predictive statistical modelling of cadmium content in durum wheat grain based on soil parameters

Regulatory limits on cadmium (Cd) content in food products are tending to become stricter, especially in cereals, which are a major contributor to dietary intake of Cd by humans. This is of particular importance for durum wheat, which accumulates more Cd than bread wheat. The contamination of durum wheat grain by Cd depends not only on the genotype but also to a large extent on soil Cd availability. Assessing the phytoavailability of Cd for durum wheat is thus crucial, and appropriate methods are required. For this purpose, we propose a statistical model to predict Cd accumulation in durum wheat grain based on soil geochemical properties related to Cd availability in French agricultural soils with low Cd contents and neutral to alkaline pH (soils commonly used to grow durum wheat). The best model is based on the concentration of total Cd in the soil solution, the pH of a soil CaCl₂ extract, the cation exchange capacity (CEC), and the content of manganese oxides (Tamm's extraction) in the soil. The model variables suggest a major influence of cadmium buffering power of the soil and of Cd speciation in solution. The model successfully explains 88% of Cd variability in grains with, generally, below 0.02 mg Cd kg^-1 prediction error in wheat grain. Monte Carlo cross-validation indicated that model accuracy will suffice for the European Community project to reduce the regulatory limit from 0.2 to 0.15 mg Cd kg^-1 grain, but not for the intermediate step at 0.175 mg Cd kg^-1. The model will help farmers assess the risk that the Cd content of their durum wheat grain will exceed regulatory limits, and help food safety authorities test different regulatory thresholds to find a trade-off between food safety and the negative impact a too strict regulation could have on farmers.

Bioaccumulation of heavy metals and ecophysiological responses to heavy metal stress in selected populations of Vaccinium myrtillus L. and Vaccinium vitis-idaea L

The aim of this study was to determine the concentrations of heavy metals (Cd, Pb, Zn, Fe, and Mn) in soil, and their bioavailability and bioaccumulation in Vaccinium myrtillus L. and Vaccinium vitis-idaea L. organs. Analysis also concerned the physiological responses of these plants from three polluted sites (immediate vicinity of a zinc smelter in Miasteczko Śląskie, ArcelorMittal Poland S.A. iron smelter in Dąbrowa Górnicza-Łosień, and Jaworzno III power plant in Jaworzno) and one pseudo-control site (Pazurek nature reserve in Jaroszowiec Olkuski). All of the sites are situated in the southern parts of Poland in the Śląskie or Małopolskie provinces. The contents of proline, non-protein thiols, glutathione, ascorbic acid, and the activity of superoxide dismutase and guaiacol peroxidase in the leaves of Vaccinium myrtillus L. and Vaccinium vitis-idaea L. were measured. In soil, the highest levels of Cd, Pb, and Zn (HNO3 extracted and CaCl2 extracted) were detected at the Miasteczko Śląskie site. At all sites a several times lower concentration of the examined metals was determined in the fraction of soil extracted with CaCl2. Much higher Cd, Pb, Zn and Fe concentrations were found in V. myrtillus and V. vitis-idaea grown at the most polluted site (located near the zinc smelter) in comparison with cleaner areas; definitely higher bioaccumulation of these metals was found in lingonberry organs. Additionally, we observed a large capability of bilberry to accumulate Mn. Antioxidant response to heavy metal stress also differed between V. myrtillus and V. vitis-idaea. In V. myrtillus we found a positive correlation between the level of non-protein thiols and Cd and Zn concentrations, and also between proline and these metals. In V. vitis-idaea leaves an upward trend in ascorbic acid content and superoxide dismutase activity accompanied an increase in Cd, Pb, and Zn concentrations. At the same time, the increased levels of all tested metals in the leaves of V. vitis-idaea were accompanied by a decreased activity of guaiacol peroxidase. In both species increased Mn accumulation caused a decrease in antioxidant response.

Zinc solubility and fractionation in cultivated calcareous soils irrigated with wastewater

The solubility, lability and fractionation of zinc in a range of calcareous soils from Peshawar, Pakistan were studied (18 topsoils and 18 subsoils). The lability (E-value) of Zn was assessed as the fraction isotopically exchangeable with (70)Zn(2+); comparative extractions included 0.005 M DTPA, 0.43 M HNO3 and a Tessier-style sequential extraction procedure (SEP). Because of the extremely low concentration of labile Zn the E-value was determined in soils suspended in 0.0001 M Na2-EDTA which provided reliable analytical conditions in which approximately 20% of the labile Zn was dissolved. On average, only 2.4% of soil Zn was isotopically exchangeable. This corresponded closely to Zn solubilised by extraction with 0.005 DTPA and by the carbonate extraction step (F1+F2) of the Tessier-style SEP. Crucially, although the majority of the soil CaCO3 was dissolved in F2 of the SEP, the DTPA dissolved only a very small proportion of the soil CaCO3. This suggests a superficial carbonate-bound form of labile Zn, accessible to extraction with DTPA and to isotopic exchange. Zinc solubility from soil suspended in 0.01 M Ca(NO3)2 (PCO2 controlled at 0.03) was measured over three days. Following solution speciation using WHAM(VII) two simple solubility models were parameterised: a pH dependent 'adsorption' model based on the labile (isotopically exchangeable) Zn distribution coefficient (Kd) and an apparent solubility product (Ks) for ZnCO3. The distribution coefficient showed no pH-dependence and the solubility model provided the best fit to the free ion activity (Zn(2+)) data, although the apparent value of log10 Ks (5.1) was 2.8 log units lower than that of the mineral smithsonite (ZnCO3).

Assessment of bioavailability of heavy metal pollutants using soil isolates of Chlorella sp

Biotests conducted with plants are presently used to estimate metal bioavailability in contaminated soils. But when plants are grown in soils, especially the plants with fine roots, root collection is easily biased and tedious. Indeed, at harvest, small amounts of soil can adhere to roots, resulting in overestimation of root metal content, and the finest roots are often discarded from the analysis because of their difficult and almost impossible recovery. This report presents a novel method for assessing the bioavailability of heavy metals in soils using microalgae. Two species of green unicellular microalgae were isolated from two highly contaminated soils and identified by phylogenetic and molecular evolutionary analyses as Chlorella sp. RBM and Chlorella sp. RHM. These two cultures were used to determine the metal uptake from metal-contaminated soils of South Australia as a novel, cost-effective, simple and rapid method for assessing the bioavailability of heavy metals in soils. The suggested method is an attempt to achieve a realistic estimate of bioavailability which overcomes the inherent drawback of root metal contamination in the bioavailability indices so far reported.

Potentially toxic element phytoavailability assessment in Technosols from former smelting and mining areas

This study reports the chemical bioavailability of several potentially toxic elements (Zn, Pb, Cd, As, and Sb) in contaminated Technosols from two former smelting and mining areas. Though these elements have long been recognized as potentially harmful elements, understanding of their toxicity and environmental behavior in Technosols developed on former mining and smelting sites are more limited, particularly for As and Sb. Surface soils were sampled from metallophyte grassland contaminated with Zn, Pb, and Cd located at Mortagne-du-Nord (North France) and from a former mining settling basin contaminated with As, Pb, and Sb located at la Petite Faye (Limoges, France). Various selective single extraction procedures (CaCl2, NaNO3, NH4NO3, DTPA, and EDTA) were used together with germination tests with dwarf beans whose shoots were analyzed for their potentially toxic element concentrations after 21 days of growth. The extraction capacity of the potentially toxic elements followed the order EDTA > DTPA > NH4NO3 > CaCl2 > NaNO3 for both studied areas. Pearson's correlation coefficient analysis between the concentrations of potentially toxic elements accumulated in bean primary leaves or their mineral mass with their extractable concentrations showed a positive significant correlation with dilute CaCl2 and nitrate solutions extraction procedures. In contrast, for all studied elements, except Pb, the complexing and chelating extractants (EDTA and DTPA) exhibited poor correlation with the dwarf bean leaves concentrations. Moreover, results showed that the 0.01 M CaCl2 extraction procedure was the most suitable and provided the most useful indications of metal phytoavailability for studied elements.

Remediation of sediment and water contaminated by copper in small-scaled constructed wetlands: effect of bioaugmentation and phytoextraction

The use of plants and microorganisms to mitigate sediment contaminated by copper was studied in microcosms that mimic the functioning of a stormwater basin (SWB) connected to vineyard watershed. The impact of phytoremediation and bioaugmentation with siderophore-producing bacteria on the fate of Cu was studied in two contrasted (batch vs. semi-continuous) hydraulic regimes. The fate of copper was characterised following its discharge at the outlet of the microcosms, its pore water concentration in the sediment, the assessment of its bioaccessible fraction in the rhizosphere and the measurement of its content in plant tissues. Physico-chemical (pH, redox potential) and biological parameters (total heterotrophic bacteria) were also monitored. As expected, the results showed a clear impact of the hydraulic regime on the redox potential and thus on the pore water concentration of Cu. Copper in pore water was also dependent on the frequency of Cu-polluted water discharges. Repeated bioaugmentation increased the total heterotrophic microflora as well as the Cu bioaccessibility in the rhizosphere and increased the amount of Cu extracted by Phragmites australis by a factor of ~2. Sugar beet pulp, used as a filter to avoid copper flushing, retained 20% of outcoming Cu and led to an overall retention of Cu higher than 94% when arranged at the outlet of microcosms. Bioaugmentation clearly improved the phytoextraction rate of Cu in a small-scaled SWB designed to mimic the functioning of a full-size SWB connected to vineyard watershed. Highlights: Cu phytoextraction in constructed wetlands much depends on the hydraulic regime and on the frequency of Cu-polluted water discharges. Cu phytoextraction increases with time and plant density. Cu bioaccessibility can be increased by bioaugmentation with siderophore-producing bacteria.

Phytotoxicity of trace metals in spiked and field-contaminated soils: Linking soil-extractable metals with toxicity

Soil tests have been widely developed to predict trace metal uptake by plants. The prediction of metal toxicity, however, has rarely been tested. The present study was set up to compare 8 established soil tests for diagnosing phytotoxicity in contaminated soils. Nine soils contaminated with Zn or Cu by metal mining, smelting, or processing were collected. Uncontaminated reference soils with similar soil properties were sampled, and series of increasing contamination were created by mixing each with the corresponding soil. In addition, each reference soil was spiked with either ZnCl2 or CuCl2 at several concentrations. Total metal toxicity to barley seedling growth in the field-contaminated soils was up to 30 times lower than that in corresponding spiked soils. Total metal (aqua regia-soluble) toxicity thresholds of 50% effective concentrations (EC50) varied by factors up to 260 (Zn) or 6 (Cu) among soils. For Zn, variations in EC50 thresholds decreased as aqua regia > 0.43 M HNO3  > 0.05 M ethylenediamine tetraacetic acid (EDTA) > 1 M NH4 NO3  > cobaltihexamine > diffusive gradients in thin films (DGT) > 0.001 M CaCl2 , suggesting that the last extraction is the most robust phytotoxicity index for Zn. The EDTA extraction was the most robust for Cu-contaminated soils. The isotopically exchangeable fraction of the total soil metal in the field-contaminated soils markedly explained the lower toxicity compared with spiked soils. The isotope exchange method can be used to translate soil metal limits derived from soils spiked with metal salts to site-specific soil metal limits.

Arsenic (As), antimony (Sb), and lead (Pb) availability from Au-mine Technosols: a case study of transfer to natural vegetation cover in temperate climates

Soils from old Au-mine tailings (La Petite Faye, France) were investigated in relation to the natural vegetation cover to evaluate the risk of metals and metalloids (Pb, As, Sb) mobilizing and their potential transfer to native plants (Graminea, Betula pendula, Pteridium aquilinum, Equisetum telmateia). The soils are classified as Technosols with high contamination levels of As, Pb, and Sb. The single selective extractions tested to evaluate available fraction (CaCl2, acetic acid, A-Rhizo, and DTPA) showed low labile fractions (<5 % of bulk soil contents), but still significant levels were observed (up to 342.6 and 391.9 mg/kg for As and Pb, respectively) due to the high contamination levels of soils. Even at high soil contaminations (considered as phytotoxic levels for plants), translocation factors for native plants studied are very low resulting in low concentrations of As, Sb, and Pb in their aerial part tissues. This study demonstrates the important role of (1) native plant cover in terms of "stabilization" of these contaminants, and (2) the poor effectiveness of extraction procedures used for this type of soil assemblages, i.e., rich in specific mineral phases.

Phytoavailability and phytovariety codetermine the bioaccumulation risk of heavy metal from soils, focusing on Cd-contaminated vegetable farms around the Pearl River Delta, China

Five random vegetable farms were selected to investigate the bioaccumulation risk of heavy metals (HMs) by different type of vegetables around the Pearl River Delta (PRD), China. The concentration order of four major HMs in the surface soil samples was Cd<Cu<Pb<Zn, with only Cd concentrations (1.4-1.8mgkg(-1)) significantly higher than the permissible limit (≤0.3mgkg(-1)) for agricultural soils. Soil DTPA-extractable (phytoavailable) Cd concentrations differed markedly amongst the five farms, and varied within 0.017-0.17mgkg(-1). Meanwhile, 28.0% of vegetable samples collected from these five farms were contaminated with Cd according to the permissible limit (≤0.05mgkg(-1)), and 71.4% of these polluted samples belonged to stem/leaf vegetables. The average bioaccumulation factors of Cd from cultivated soil to stem/leaf vegetables and melon/fruit/bean vegetables varied within 0.021-0.050 and 0.005-0.020 (soil total Cd basis), and 0.50-2.01 and 0.13-0.53 (soil DTPA-extractable Cd basis), respectively. Redundancy analysis (RDA) showed that DTPA-extractable Cd, which negatively but significantly correlated (P<0.05) to soil pH, was the key factor in influencing vegetable Cd accumulation, notably stem/leaf vegetables. The results show that Cd was the primary metal of risk in vegetable farms around the PRD region, and stem/leaf vegetables posed about 2.2 times higher health risks associated with exposure to Cd than melon/fruit/bean vegetables.

Using data mining to predict soil quality after application of biosolids in agriculture

The amount of biosolids recycled in agriculture has steadily increased during the last decades. However, few models are available to predict the accompanying risks, mainly due to the presence of trace element and organic contaminants, and benefits for soil fertility of their application. This paper deals with using data mining to assess the benefits and risks of biosolids application in agriculture. The analyzed data come from a 10-yr field experiment in northeast France focusing on the effects of biosolid application and mineral fertilization on soil fertility and contamination. Biosolids were applied at agriculturally recommended rates. Biosolids had a significant effect on soil fertility, causing in particular a persistent increase in plant-available phosphorus (P) relative to plots receiving mineral fertilizer. However, soil fertility at seeding and crop management method had greater effects than biosolid application on soil fertility at harvest, especially soil nitrogen (N) content. Levels of trace elements and organic contaminants in soils remained below legal threshold values. Levels of extractable metals correlated more strongly than total metal levels with other factors. Levels of organic contaminants, particularly polycyclic aromatic hydrocarbons, were linked to total metal levels in biosolids and treated soil. This study confirmed that biosolid application at rates recommended for agriculture is a safe option for increasing soil fertility. However, the quality of the biosolids selected has to be taken into account. The results also indicate the power of data mining in examining links between parameters in complex data sets.

Mobilité et spéciation du chrome dans un système à phragmites australis de traitement des eaux usées de tanneries

L’objectif de ce travail est l’étude de la répartition, la migration, la spéciation et la biodisponibilité du Cr dans les différents horizons du sol au niveau d’un pilote à Phragmites australis (Cav.) Trin ex Steudel de traitement du rejet du tannage au chrome. Les résultats obtenus sont comparés à ceux d’un pilote non planté. La spéciation chimique du Cr total a été réalisée par cinq extractions séquentielles. L’extraction du chrome biodisponible est effectuée par CaCl2 (0,1 M). Les résultats de la répartition du Cr montrent que l’horizon superficiel renferme des teneurs élevées en Cr 80 % et que cette accumulation est statistiquement différente entre les deux pilotes (p<0,05) : 48 ± 4 g·kg-1 pour le système planté et 39 ± 7 g·kg-1 pour le système non planté. Les résultats de la spéciation chimique totale du Cr indiquent qu’il est très lié à la phase oxydable : 36 ± 10 % pour le système planté (PP) et 54 ± 9 % pour le système non planté (NPP). La teneur en Cr retenue dans la fraction résiduelle présente 26 ± 3 % et 25 ± 2 % respectivement pour le système PP et NPP. La proportion du Cr retenue dans la fraction carbonate est de 19,5 ± 5 % pour le système PP et 10,7 ± 5 % pour le système NPP. La part du Cr associée à la phase réductible présente 18 ± 6 % et 10 ± 4 % respectivement pour le système PP et NPP. Pour les deux pilotes le Cr total est peu échangeable et sa teneur biodisponible est faible. En outre, les résultats montrent que la présence de Phragmites australis a provoqué la libération partielle du Cr de la fraction organique mobile vers les formes carbonate et réductible relativement stables dans le système planté. Les résultats trouvés montrent que Phragmites australis présente une accumulation assez importante en Cr au niveau des racines : 1690 ± 124 mg·kg-1 en matière sèche. Le calcul du bilan global de l’épuration révèle que le Cr est surtout retenu par le sol (94 %) et que 5 % du Cr est prélevé par la plante. Cependant, la présence de Phragmites australis assure une porosité suffisante pour la percolation des eaux en traitement et permet le traitement d’un volume d’eau usée plus grand.

Use of the modified BCR three-step sequential extraction procedure for the study of trace element dynamics in contaminated soils

The modified BCR three-step sequential extraction procedure was used to examine the temporal dynamics of trace elements in soils contaminated by an accidental spill from an opencast mine in south-west Spain. Soils were mainly contaminated with pyritic sludge and acidic wastewater, whereas some soils were affected only by acidic wastewater. The distributions obtained for both some major (Ca, Fe and Mn) and trace elements (As, Cd, Cu, Pb and Zn) in the sludge and soil samples taken at different times after the accident, 1-3 months and 21 months, were compared. Sequential extractions were useful in identifying different sources of contamination, and in obtaining additional information on the solubility of secondary minerals formed by pyrite oxidation. Thus, the effectiveness of the BCR procedure has proved to be a useful tool for predicting short- and long-term mobility of trace elements, even in complex environmental scenarios.

Evaluation of extractants for estimation of the phytoavailable trace metals in soils

Despite its environmental (and financial) importance, there is no agreement in the literature as to which extractant most accurately estimates the phytoavailability of trace metals in soils. A large data set was taken from the literature, and the effectiveness of various extractants to predict the phytoavailability of Cd, Zn, Ni, Cu, and Pb examined across a range of soil types and contamination levels. The data suggest that generally, the total soil trace metal content, and trace metal concentrations determined by complexing agents (such as the widely used DTPA and EDTA extractants) or acid extractants (such as 0.1M HCl and the Mehlich 1 extractant) are only poorly correlated to plant phytoavailability. Whilst there is no consensus, it would appear that neutral salt extractants (such as 0.01 M CaCl(2) and 0.1 M NaNO(3)) provide the most useful indication of metal phytoavailability across a range of metals of interest, although further research is required.

Soil microbial diversity as affected by the rhizosphere of the hyperaccumulator Thlaspi caerulescens under natural conditions

It is hypothesized that metal hyperaccumulator plants have specific rhizosphere conditions, potentially modifying the bioavailability of soil metals. This article aims to further the knowledge about the rhizosphere of the hyperaccumulator Thlaspi caerulescens, focusing on its microflora isolated from metalliferous soils collected in situ where the plants grow naturally. We characterized the cultivable microbial communities isolated from the rhizosphere of one population of this Ni hyperaccumulator species grown on a serpentine soil. The rhizosphere soil harbored a wide variety of microorganisms, predominantly bacteria, confirming the stimulatory effect of the T. caerulescens rhizosphere on microbial growth and proliferation. We tested the hypothesis that the rhizosphere of T. caerulescens influences (1) the metabolic diversity of the bacterial community and (2) the bacterial resistance to metals. The principal component analysis of the Biolog plate's data confirmed a structural effect of the rhizosphere of T. caerulescens on bacterial communities. The percentage of Ni-resistant bacteria was higher in the rhizosphere than in the bulk soil, suggesting a direct effect of the rhizosphere on Ni tolerance, reflecting a greater bacterial tolerance to Ni in the rhizosphere.

Performance of a Mixed Binding Layer for Measuring Anions and Cations in a Single Assay Using the Diffusive Gradients in Thin Films Technique

The performance of a mixed binding layer (MBL) for use in diffusive gradients in thin films (DGT) was investigated. The MBL consisted of ferrihydrite and Chelex-100 cation-exchange resin combined together in a binding gel in an attempt to allow measurement of anions and cations in a single assay. Results from the MBL were compared to experiments performed using individual Chelex gels and ferrihydrite gels that have been shown to work successfully for DGT methodology. To facilitate combined analysis of P and cations by ICP-MS, HCl (1 M) was used for gel elution to minimize interferences from 14N16OH or 15N16O on 31P. All elements tested (Cd, Cu, Mn, Mo, P, and Zn) were bound successfully to the MBL. An elution efficiency of 0.92 was obtained for all elements, apart from Mo (0.79). This is higher than the elution efficiencies obtained previously for pure Chelex or ferrihydrite gels using HNO3 (1 M) as the eluent. Uptake of cations by DGT using the MBL was consistent across the pH range 5-9, which compares well with results using pure Chelex. Below pH 5, accumulated masses were lower for Mn, Cu, and Zn. Uptake of P and Mo was unaffected by pH in the range 3-8, and the amount absorbed compared well with results obtained previously for pure ferrihydrite gels. Performance of the MBL at different ionic strengths (0.001, 0.01 M) was comparable to performance using the pure Chelex gel. DGT measurements obtained using the MBL on agricultural soils correlated well (r2 = 0.95) with separate measurements obtained using either pure Chelex or ferrihydrite binding layers. This suggests that the MBL could be used for simultaneous measurement of cationic and anionic element availability in soils.

Desorption kinetics of Cd, Zn, and Ni measured in soils by DGT

DGT (diffusive gradients in thin films) was used to measure the distribution and rates of exchange of Zn, Cd, and Ni between solid phase and solution in five different soils. Soil texture ranged from sandy loam to clay, pH ranged from 4.9 to 7.1, and organic carbon content ranged from 0.8% to 5.8%. DGT devices continuously remove metal to a Chelex gel layer after passage through a well-defined diffusion layer. The magnitude of the induced remobilization flux from the solid phase is related to the pool size of labile metal and the exchange kinetics between dissolved and sorbed metal. DGT devices were deployed over a series of times (4 h to 3 weeks), and the DIFS model (DGT induced fluxes in soils) was used to derive distribution coefficients for labile metal (Kdl) and the rate at which the soil system can supply metal from solid phase to solution, expressed as a response time. Response times for Zn and Cd were short generally (<8 min). They were so short in some soils (<1 min) that no distinction could be made between supply of metal being controlled by diffusion or the rate of release. Generally longer response times for Ni (5-20 min) were consistent with its slow desorption. The major factor influencing Kdl for Zn and Cd was pH, but association with humic substances in the solid phase also appeared to be important. The systematic decline, with increasing pH, in both the pool size of Ni available to the DGT device and the rate constant for its release is consistent with a part of the soil Ni pool being unavailable within a time scale of 1-20 min. This kinetic limitation is likely to limit the availability of Ni to plants.

Use of sorption and extraction tests to predict the dynamics of the interaction of trace elements in agricultural soils contaminated by a mine tailing accident

Over 2000 ha of agricultural soils were contaminated by a pyritic sludge and acidic waste waters coming from a spill from a mining exploitation. The affected soils were acidic with sandy-loam texture (SL), loamy with neutral pH (L), and calcareous, saline, with clay texture (Cs). The Cs soils were contaminated only with acidic waste waters. Sorption and extraction tests were applied to examine the medium-term dynamics of the interaction of trace elements (As, Cd, Cu, Pb and Zn) in the soils. The solid-soil solution distribution coefficient (KD) was determined in soil samples taken 3 months (initial stage, 3M samples) and then nearly 2 years (final stage, 21M samples) after the accident. Distribution coefficient values ranged from the lowest values in the SL samples (from 0.2 l kg(-1) for Cd and Zn to 25 l kg(-1) for As) to higher values in the L and Cs soils. Lead and As had the highest KD in all soils (over 10(5) l kg(-1) in the L soils). No clear dynamics pattern could be derived from these data because of the low heavy metal concentrations in the soil solution. As a complementary approach, four single extractions (0.01 and 1 mol l(-1) CaCl2; 0.05 mol l(-1) EDTA; 0.43 mol l(-1) CH3COOH) were applied to soil and sludge samples. Samples derived from submitting field 3M samples to drying-wetting (DW) cycles were included to define a complete laboratory approach to predict field dynamics. Results from extraction tests indicated that changes of the trace element interaction over time depended on the soil pH and on the source of contamination. For those soils affected only by the acidic waste waters, where an increase in fixation occurred, natural processes such as diffusion controlled dynamics. For those soils contaminated by a mixed source, the dynamics of the interaction was the resultant process of the combination of the natural attenuation and the oxidation of the pyritic sludge. This latter process led to an increase in the remobilization for those elements associated with soluble secondary minerals (Cd and Zn) and to an increase in the fixation for those elements coprecipitating with insoluble secondary minerals (Pb and As). Drying-wetting cycles were useful to predict the dynamics in the field at month-year scale. The CH3COOH was recommended as the best test among those studied here to derive conclusions about dynamics pattern because it provided significant desorption yields in most scenarios. Acidic soils with sludge contamination represented the scenarios with the highest risk, while calcareous soils better attenuated contamination. In a longer time scale, the depletion of calcareous components needs to be controlled.

Is metal extraction by Arabidopsis halleri related to exchangeable metal rates in soils amended with different metal-bearing solids?

Metals are associated to various constituents in polluted soils, and their availability is closely related to their chemical speciation. Studies on relations between metal extraction efficiency by hyperaccumulators and location of metals with respect to soil constituents are scarce. In this study. we investigate the relationship between metal extraction by Arabidopsis halleri and the exchangeable metals from substrates amended with various metal-bearing solids collected in the vicinity of a Zn smelter complex. These consisted of fresh and decomposing organic matter, the soil clay fraction, and two types of waste slags. ZnSO4 was also used as metal-bearing solid. Each was mixed with an unpolluted soil to produce two types of substrate, one moderately polluted and the other highly polluted. Total Zinc, Cd, Cu, and Pb were measured in substrates and in roots and shoots of A. halleri. Analysis of 0.01 M CaCl2 exchangeable metals in each substrate was performed before and after plant growth. The results showed different concentrations of exchangeable metals after plant growth, depending on the nature of the metal-bearing solids. In the ZnSO4 soil substrate, the proportion of exchangeable Zn decreased after plant growth, whilst it increased significantly on substrates amended with the two waste slags. For the other substrates, exchangeable Zn was not significantly different before and after plant growth. The same trend was observed for Cd. In the case of Cu, exchangeable rates increased in all substrates. The results were discussed according to the characteristics of the metal-bearing solids and to the metal-uptake strategy of A. halleri.

Evaluation of heavy metal lability in polluted soils by a cation exchange batch procedure

A new extraction procedure with cationic exchange resins is proposed for the assessment of heavy metal lability in polluted soils. This method classifies soluble metal compounds in three levels according to their lability: very labile, moderately labile and non-labile. The concept of lability is based on the use of resins with different ability to interact with metals. The proposed procedure was applied to five samples of polluted soils. In the five soils, soluble Cd was found to be very labile, soluble Ni and Pb were mostly non-labile, Mn and Zn lability depended on the soil, while Cu had a fraction of moderately labile forms in four of the five soils. The resin extraction procedure was compared with the DTPA and CaCl2 extractions.