Guillaume Echevarria

Phytoextraction of cadmium with Thlaspi caerulescens

The in situ phytoextraction of cadmium from soils can only be achieved using plants that are both tolerant to high Cd concentrations and able to extract sufficient amounts of the metal. However, very few plant species are capable of remediating Cd polluted soils in a reasonable time frame. This paper aims to show that the population of the hyperaccumulator Thlaspi caerulescens J. & C. Presl. from Viviez (south of France), which has a high Cd-accumulating capability, is an efficient tool to remove Cd from contaminated soils. Roots of T. caerulescensViviez proliferate in hot spots of metals in soils which is particularly advantageous because of heterogeneity of the distribution of metal in polluted soils. Isotopic techniques showed that plants from this population acquire Cd from the same pools as non-accumulating species, but that it was much more efficient than non-hyperaccumulators at removing the metal from the soil labile pool. This is due: to (i) a specific rooting strategy, and (ii) a high uptake rate resulting from the existence in this population of Cd-specific transport channels or carriers in the root membrane. Growth and overall extraction can be improved with appropriate N fertilisation, supplied either as mineral fertilisers or uncontaminated sewage sludge. Selecting bigger plants is possible from within a suitable Cd-accumulating population to improve the phytoextraction process. Growing the Cd-accumulating populations results in a reduction in the availability of Cd and Zn as shown with field and lysimeter experiments conducted for several years. As a result, on a practical aspect, Cd hyperaccumulating populations of T. caerulescens may be used as a tool to efficiently reduce the availability of Cd in soils, providing appropriate populations are used.

Effect of pH on the sorption of uranium in soils

This work was undertaken to study the influence of soil type and chemical composition on uranium sorption ratios (SR in 1 kg-1) in order to reduce the uncertainty associated with this parameter in risk assessment models. Thirteen soil samples were collected from three different locations in France under different geological conditions. Clay content varied from 7.0 to 50.0%, pH ranged from 5.5 to 8.8 and organic matter content from 1.0 to 4.6%. Soils were incubated at room temperature in polyethylene packets for 28 days in the presence of 1 mg U kg-1 soil. Sorption ratio values varied from 0.9 to 3198 for all soils with no significant effect of soil texture or of organic matter. However, soil pH was highly linearly correlated with (log SR) as a probable consequence of the existence of different uranium complexes as a function of soil pH. The sorption behaviour differences between UO2(2+) and UO2(2+)-carbonate complexes are so great that any other effect of soil properties on U sorption is hidden. Thus, soil pH should be the focus variable for reduction of the uncertainty associated with the soil Kd value used in environmental risk assessments, even for reducing the uncertainty in site-specific Kd values.

In-situ phytoextraction of Ni by a native population of Alyssum murale on an ultramafic site (Albania)

Ultramafic outcrops are widespread in Albania and host several Ni hyperaccumulators (e.g., Alyssum murale Waldst. & Kit.). A field experiment was conducted in Pojske (Eastern Albania), a large ultramafic area in which native A. murale was cultivated. The experiment consisted in testing the phytoextraction potential of already installed natural vegetation (including A. murale) on crop fields with or without suitable fertilisation. The area was divided into six 36-m2 plots, three of which were fertilised in April 2005 with (NPKxa0+xa0S). The soil (Magnesic Hypereutric Vertisol) was fully described as well as the mineralogy of horizons and the localisation of Ni bearing phases (TEM-EDX and XRD). Ni availability was also characterised by Isotopic Exchange Kinetics (IEK). The flora was fully described on both fertilised and unfertilised plots and the plant composition (major and trace elements) and biomass (shoots) harvested individually were recorded.The soil had mainly two Ni-bearing phases: high-Mg smectite (1.3% Ni) and serpentine (0.7% Ni), the first one being the source of available Ni. Ni availability was extremely high according to IEK and confirmed by Ni contents in Trifolium nigriscens Viv. reaching 1,442xa0mgxa0kg−1 (A new hyperaccumulator?). Total biomass yields were 6.3xa0txa0ha−1 in fertilised plots and 3.2xa0txa0ha−1 in unfertilised plots with a highly significant effect: fertilisation increased dramatically the proportion of A. murale in the plots (2.6xa0txa0ha−1 vs. 0.2xa0txa0ha−1). Ni content in the shoots of A. murale reached 9,129xa0mgxa0kg−1 but metal concentration was not significantly affected by fertilisation. Phytoextracted Ni in total harvest reached 25xa0kg Nixa0ha−1 on the fertilised plots. It was significantly lower in unfertilised plots (3xa0kg Nixa0ha−1). Extensive phytomining on such sites could be promising in the Albanian context by domesticating already installed natural populations with fertilisation.

Agromining: farming for metals in the future?

Phytomining technology employs hyperaccumulator plants to take up metal in harvestable plant biomass. Harvesting, drying and incineration of the biomass generates a high-grade bio-ore. We propose that agromining (a variant of phytomining) could provide local communities with an alternative type of agriculture on degraded lands; farming not for food crops, but for metals such as nickel (Ni). However, two decades after its inception and numerous successful experiments, commercial phytomining has not yet become a reality. To build the case for the minerals industry, a large-scale demonstration is needed to identify operational risks and provide real-life evidence for profitability.

Identification of nickel chelators in three hyperaccumulating plants: An X-ray spectroscopic study

We have investigated the accumulation of nickel in a hyperaccumulating plant from the Brassicacae family Leptoplax emarginata (Boiss.) O.E. Schulz. Two supplementary hyperaccumulating plants, which have been the subject of a high number of publications, Alyssum murale Waldst. & Kit and Thlaspi caerulescens J.&C. Presl, and a nonaccumulating species Aurinia saxatilis were also studied for reference. The plants were grown during 4 months in specific rhizoboxes with Ni-bearing minerals as a source of nickel. Nickel speciation was analyzed through X-ray absorption spectroscopy at Ni K-edge (X-ray absorption near edge spectroscopy and extended X-ray absorption fine structure spectroscopy) in the different parts of the plants (leaves, stems and roots) and compared with aqueous solutions containing different organo-Ni(II) complexes. Carboxylic acids (citrate, malate) appeared as the main ligands responsible of nickel transfer within those plants. Citrate was found as the predominant ligand for Ni in stems of Leptoplax and Alyssum, whereas in leaves of the three plants, malate appeared as the chelating organic acid of accumulated metal. Histidine could not be detected either in leaves, stems nor roots of any studied plant sample.

Assessment and control of the bioavailability of nickel in soils

Nickel, a potentially toxic metal, is present in all soils with an average concentration of 20 to 30 mg/kg, sometimes exceeding 10,000 mg/kg (e.g., ultramafic soils). The ecotoxicological risk of Ni in soils to organisms is controlled by its availability. It is therefore essential to identify an efficient and reliable method for the evaluation of this risk. This paper presents a complete study of the effect of Ni origin, localization, and soil properties on its availability as assessed with the isotopic exchange kinetics (IEK) method and compares plant response to isotopically exchangeable properties of Ni in soils. We performed IEK on 100 soil samples representing a worldwide range of Ni fate, and concentrations showed that pH was the main influencing parameter and that labile Ni (i.e., isotopically exchangeable Ni, Et) could be reasonably well assessed by a single diethylene triamine pentaacetic acid extraction. The identification of the soil mineral phases that bear Ni (bearing phases) in 16 Ni-rich samples selected among the 100 soils showed a strong effect of the mineralogy of the bearing phases on Ni availability (IEK). Plants with different Ni accumulation strategies all took up Ni from the same labile pool of Ni in four contrasting soils, and the amount taken up by hyperaccumulator plants could be anticipated with the IEK parameters, thus confirming the usefulness of isotopic dilution methods for risk assessment.

Nickel bioavailability in an ultramafic toposequence in the Vosges Mountains (France)

A serpentinised harzburgite outcrop located in the Vosges Mountains hosts a population of the Ni-hyperaccumulator Thlaspi caerulescens J. & C. Presl. A complete study was undertaken to relate the variability of Ni availability along the ultramafic toposequence to pedogenesis, soil mineralogy and functioning with X-Ray Diffraction, Transmission Electron Microscope observations coupled with Isotopic Exchange Kinetics and diethylenetriamine pentaacetic acid extraction of Ni. The soil profiles ranging from Dystric Cambisol to Hypermagnesic Hypereutric Cambisol were distributed unevenly along the toposequence probably due to geochemical variability of the bedrock and also complex quaternary erosion features. The richest soils were characterised by slight mineral weathering leading to Ni, Cr and Fe accumulation in the B horizons whereas the lowest saturated soils had very low-metal contents. Most soil minerals were inherited from the parent materials and there were only few traces of formation of secondary minerals. Primary minerals (e.g. serpentine, chlorite) contained low Ni concentrations (0.2%) whereas neoformed goethite, mainly in the B horizons of the richest soils, contained up to 4.3% Ni. Ni was probably sorbed onto amorphous Fe oxy−hydroxide particles (oxalate extraction) rather than incorporated within the crystal lattice of goethite. Ni availability in the B horizon of Hypereutric Cambisols was extremely high and so was the oxalate extractable Fe. At the toposequence level, there was a high level of Ni availability in the least weathered soils and a very low-availability level in the more intensively weathered soils (strongly acidic pH). Ni availability was unexpectedly positively correlated to pH and was controlled by soil mineralogy and Ni-bearing mineral phases. Ni hyperaccumulation (above 1,000xa0mgxa0kg−1) by native T. caerulescens was only reached in the Ni-rich soils as a consequence of the local edaphic factors. Ni uptake by T. caerulescens is strongly regulated by Ni availability in soils and therefore related to pedogenesis.

The flora and biogeochemistry of the ultramafic soils of Goiás state, Brazil

Major collections of the ultramafic flora of Goiás, central Brazil, were made by Brooks and co-workers in 1988 and 1990. At the time of reports on this material in 1990–1992 much of it had been identified only tentatively and incompletely, but the area was clearly interesting for taxonomic and biogeochemical reasons. Further progress has been made but still only two-thirds of the specimens are identified at the species level. Following a third collection in early 2005, we now have 800 specimens from this area, with chemical analyses of all the plants and of more than 120 representative soil samples. New species have been found, e.g., in Paspalum (Poaceae) and Pterolepis (Melastomataceae). There is a need for more taxonomic work in genera such as Cnidoscolus (Euphorbiaceae), Lippia (Verbenaceae), Turnera and Piriqueta (Turneraceae), and Vellozia (Velloziaceae). Ni hyperaccumulation (>1,000xa0mg/kg in dry plant matter) has now been found in a total of 79 specimens, representing more than 30 different species. Notable Ni hyperaccumulators include Pfaffia sarcophylla (Amaranthaceae), species of Justicia, Lophostachys and Ruellia (Acanthaceae), Porophyllum (Asteraceae), several species of Lippia (Verbenaceae), Turnera and Piriqueta (Turneraceae), and a possibly new Cnidoscolus (Euphorbiaceae). Ni hyperaccumulation has not been found in plants of the outcrops of Morro Feio or Crominia-Mairipotaba; it seems to be confined to the extensive layered ultramafics of Barro Alto and the Macedo-Niquelândia areas. The distribution of Ni-values in the Brazilian plant collection is different from that found in the Mediterranean and California, where there is a clear distinction between accumulator and non-accumulator plants: in Brazil the distribution is more continuous, and median Ni concentrations are much greater. An ultramafic hill just north of Niquelândia deserves to be protected because of the presence there of many of the hyperaccumulators and species probably endemic to the Goiás ultramafics.

Designing Cropping Systems for Metal-Contaminated Sites: A Review

Considering that even contaminated soils are a potential resource for agricultural production, it is essential to develop a set of cropping systems to allow a safe and sustainable agriculture on contaminated lands while avoiding any transfer of toxic trace elements to the food chain. In this review, three main strategies, i.e., phytoexclusion, phytostabilization, and phytoextraction, are proposed to establish cropping systems for production of edible and non-edible plants, and for extraction of elements for industrial use. For safe production of food crops, the selection of low-accumulating plants/cultivars and the application of soil amendments are of vital importance. Phytostabilization using non-food energy and fiber plants can provide additional renewable energy sources and economic benefit with minimum cost of agricultural measures. Phytoextracting trace elements (e.g., As, Cd, Ni, and Zn) using hyperaccumulator species is more suitable for slightly and moderately polluted sites, and phytomining of Ni from serpentine soils has shown a great potential to extract Ni-containing bio-ores of economic interests. We conclude that appropriate combinations of soil types, plant species/cultivars, and agronomic practices can restrict trace metal transfer to the food chain and/or extract energy and metals of industrial use and allow safe agricultural activities.

Nickel and Zinc Isotope Fractionation in Hyperaccumulating and Nonaccumulating Plants

Until now, there has been little data on the isotope fractionation of nickel (Ni) in higher plants and how this can be affected by plant Ni and zinc (Zn) homeostasis. A hydroponic cultivation was conducted to investigate the isotope fractionation of Ni and Zn during plant uptake and translocation processes. The nonaccumulator Thlaspi arvense, the Ni hyperaccumulator Alyssum murale and the Ni and Zn hyperaccumulator Noccaea caerulescens were grown in low (2 μM) and high (50 μM) Ni and Zn solutions. Results showed that plants were inclined to absorb light Ni isotopes, presumably due to the functioning of low-affinity transport systems across root cell membrane. The Ni isotope fractionation between plant and solution was greater in the hyperaccumulators grown in low Zn treatments (Δ(60)Ni(plant-solution) = -0.90 to -0.63‰) than that in the nonaccumulator T. arvense (Δ(60)Ni(plant-solution) = -0.21‰), thus indicating a greater permeability of the low-affinity transport system in hyperaccumulators. Light isotope enrichment of Zn was observed in most of the plants (Δ(66)Zn(plant-solution) = -0.23 to -0.10‰), but to a lesser extent than for Ni. The rapid uptake of Zn on the root surfaces caused concentration gradients, which induced ion diffusion in the rhizosphere and could result in light Zn isotope enrichment in the hyperaccumulator N. caerulescens. In high Zn treatment, Zn could compete with Ni during the uptake process, which reduced Ni concentration in plants and decreased the extent of Ni isotope fractionation (Δ(60)Ni(plant-solution) = -0.11 to -0.07‰), indicating that plants might take up Ni through a low-affinity transport system of Zn. We propose that isotope composition analysis for transition elements could become an empirical tool to study plant physiological processes.