Michel Mench

Phytoremediation of contaminated soils and groundwater: lessons from the field.

Background, aim, and scopeThe use of plants and associated microorganisms to remove, contain, inactivate, or degrade harmful environmental contaminants (generally termed phytoremediation) and to revitalize contaminated sites is gaining more and more attention. In this review, prerequisites for a successful remediation will be discussed. The performance of phytoremediation as an environmental remediation technology indeed depends on several factors including the extent of soil contamination, the availability and accessibility of contaminants for rhizosphere microorganisms and uptake into roots (bioavailability), and the ability of the plant and its associated microorganisms to intercept, absorb, accumulate, and/or degrade the contaminants. The main aim is to provide an overview of existing field experience in Europe concerning the use of plants and their associated microorganisms whether or not combined with amendments for the revitalization or remediation of contaminated soils and undeep groundwater. Contaminations with trace elements (except radionuclides) and organics will be considered. Because remediation with transgenic organisms is largely untested in the field, this topic is not covered in this review. Brief attention will be paid to the economical aspects, use, and processing of the biomass.Conclusions and perspectivesIt is clear that in spite of a growing public and commercial interest and the success of several pilot studies and field scale applications more fundamental research still is needed to better exploit the metabolic diversity of the plants themselves, but also to better understand the complex interactions between contaminants, soil, plant roots, and microorganisms (bacteria and mycorrhiza) in the rhizosphere. Further, more data are still needed to quantify the underlying economics, as a support for public acceptance and last but not least to convince policy makers and stakeholders (who are not very familiar with such techniques).

Cadmium toxicity effects on growth, mineral and chlorophyll contents, and activities of stress related enzymes in young maize plants (Zea mays L.)

Plants were cultivated in a nutrient solution containing increasing cadmium concentrations (i.e. 0.001–25 µM), under strictly controlled growth conditions. Changes in both growth parameters and enzyme activities, directly or indirectly related to the cellular free radical scavenging systems, were studied in roots and leaves of 14-day-old maize plants (Zea mays L., cv. Volga) as a result of Cd uptake. A decrease in both shoot length and leaf dry biomass was found to be significant only when growing on 25 µM Cd, whereas concentrations of chlorophyll pigments in the 4th leaf decreased from 1.7 µM Cd on. Changes in enzyme activities occurred at lower Cd concentrations in solution leading to lower threshold values for Cd contents in plants than those observed for growth parameters. Peroxidase (POD; E.C. 1.11.1.7) activity increased in the 3rd and 4th leaf, but not in roots. In contrast, glucose-6-phosphate dehydrogenase (G6PDH; E.C. 1.1.1.49), isocitrate dehydrogenase (ICDH; E.C. 1.1.1.42) and malic enzyme (ME; E.C. 1.1.1.40) activities decreased in the 3rd leaf. According to the relationship between the POD activity and the Cd content, a toxic critical value was set at 3 mg Cd per kg dry matter in the 3rd leaf and 5 mg Cd per kg dry matter in the 4th. Anionic POD were determined both in root and leaf protein extracts; however, no changes in the isoperoxidase pattern were detected in case of Cd toxicity. Results show that in contrast with growth parameters, the measurement of enzyme activities may be included as early biomarkers in a plant bioassay to assess the phytotoxicity of Cd-contaminated soils on maize plants.

Progress in remediation and revegetation of the barren Jales gold mine spoil after in situ treatments

A series of single extractions and short-term plant tests were performed in order to test a variety of inexpensive mineral amendments for the in situ inactivation of trace elements on the fine-grained spoil of the former gold mine of Jales, Portugal. Based on the results of these tests, mesocosms (lysimeters) were constructed and a small-scale semi-field trial was carried out since 1998. The long-term effect of steelshots (SS, iron grit), beringite (B), and municipal compost (C) as spoil amendments was investigated. Vegetation establishment on the treated spoils was successful with Holcus lanatus L. in year 1 and Pinus pinaster Ait. in year 2. Therefore, a detailed monitoring program was implemented for determining the sustainability of trace elements in situ inactivation by C (5%), CB (5% compost combined with 5% beringite), CSS (5% compost combined with 1% steelshots), and CBSS (5% compost combined with 5% beringite and 1% steelshots) treatments (all amendments are expressed by soil dry weight) and of the revegetation. After 3 years, revegetation was excellent in the CSS treatment, and successful for the CBSS. Volunteer plant species became established in treated spoils during year 2. In contrast, the trees planted on the C treated spoil declined from year 2 and some died. In year 3, the trees on the CB treatment started to decline. Arsenic and zinc exposure are suggested to explain the negative effects on pine growth. In agreement with results of single extractions, compost addition in the spoil increased long-term arsenic percolation. Lead leaching was also enhanced. The CBSS and CSS treatments were the most effective for limiting water-soluble As and decreasing long-term metal leaching.

Copper toxicity in young maize (Zea mays L.) plants: effects on growth, mineral and chlorophyll contents, and enzyme activities

Changes in the growth parameters and in enzyme activities were studied in roots and leaves of 14-days old maize grown in a nutrient solution containing various copper concentrations (i.e. 0.01 to 10 μM). A significant decrease in root and leaf biomass was only found at 10 μM Cu. In contrast, changes in several enzyme activities occured at lower copper concentrations in the solution, corresponding to different threshold values which are lower than those observed for growth parameters. Peroxidase (POD) activity significantly increased in all investigated plant organs (i.e. 3rd-leaf, 4th-leaf and roots) in relation to their copper content. Additionally, glucose-6-phosphate dehydrogenase (G-6-PDH), and isocitrate dehydrogenase (ICDH) activities decreased in the leaves, especially in the 4th-leaf. However, the activity of malic enzyme (ME), G-6-PDH, glutamate dehydrogenase (GDH) and ICDH increased with the copper content in roots. According to the relationship between POD activity and copper content, the toxic critical value was set at 26 mg Cu per kg dry matter (DM) in roots and 21 mg Cu per kg DM in the 3rd-leaf. In roots, a new isoenzyme of peroxidase appeared for copper content above 12.6 mg Cu kg DM−1. Measurement of enzyme activity, especially that of POD and Cu-specific changes in the (iso)peroxidase pattern, might be used as biomarkers to assess the phytotoxicity for maize grown on copper-contaminated substrata.

Assessment of successful experiments and limitations of phytotechnologies: contaminant uptake, detoxification and sequestration, and consequences for food safety.

PurposeThe term “phytotechnologies” refers to the application of science and engineering to provide solutions involving plants, including phytoremediation options using plants and associated microbes to remediate environmental compartments contaminated by trace elements (TE) and organic xenobiotics (OX). An extended knowledge of the uptake, translocation, storage, and detoxification mechanisms in plants, of the interactions with microorganisms, and of the use of “omic” technologies (functional genomics, proteomics, and metabolomics), combined with genetic analysis and plant improvement, is essential to understand the fate of contaminants in plants and food, nonfood and technical crops. The integration of physicochemical and biological understanding allows the optimization of these properties of plants, making phytotechnologies more economically and socially attractive, decreasing the level and transfer of contaminants along the food chain and augmenting the content of essential minerals in food crops. This review will disseminate experience gained between 2004 and 2009 by three working groups of COST Action 859 on the uptake, detoxification, and sequestration of pollutants by plants and consequences for food safety. Gaps between scientific approaches and lack of understanding are examined to suggest further research and to clarify the current state-of-the-art for potential end-users of such green options.Conclusion and perspectivesPhytotechnologies potentially offer efficient and environmentally friendly solutions for cleanup of contaminated soil and water, improvement of food safety, carbon sequestration, and development of renewable energy sources, all of which contribute to sustainable land use management. Information has been gained at more realistic exposure levels mainly on Cd, Zn, Ni, As, polycyclic aromatic hydrocarbons, and herbicides with less on other contaminants. A main goal is a better understanding, at the physiological, biochemical, and molecular levels, of mechanisms and their regulation related to uptake–exclusion, apoplastic barriers, xylem loading, efflux–influx of contaminants, root-to-shoot transfer, concentration and chemical speciation in xylem/phloem, storage, detoxification, and stress tolerance for plants and associated microbes exposed to contaminants (TE and OX). All remain insufficiently understood especially in the case of multiple-element and mixed-mode pollution. Research must extend from model species to plants of economic importance and include interactions between plants and microorganisms. It remains a major challenge to create, develop, and scale up phytotechnologies to market level and to successfully deploy these to ameliorate the environment and human health.

Evaluation of metal mobility, plant availability and immobilization by chemical agents in a limed-silty soil.

Metal-contaminated soils in the vicinity of industrial sites become of ever-increasing concern. Diagnostic criteria and ecological technologies for soil remediation should be calibrated for various soil conditions; actually, our knowledge of calcareous soil is poor. Silty soils near smelters at Evin (Pas de Calais, France) have been contaminated by non-ferrous metal fallout and regularly limed using foams. Therefore, the mobility, bioavailability, and potential phytotoxicity of Cd, Pb and Zn, were investigated using single soil extractions (i.e. water, 0.1 n Ca(NO(3))(2), and EDTA pH 7), and vegetation experiments, in parallel with a biological test based on (iso)-enzymes in leaves and roots, before and following soil treatment with chemical agents, i.e. Thomas basic slags (TBS), hydrous manganese oxide (HMO), steel shots (ST) and beringite. No visible toxicity symptoms developed on the above-ground parts of ryegrass, tobacco and bean plants grown in potted soil under controlled environmental conditions. Cd, Zn and Pb uptake resulted in high concentrations in the above-ground plant parts, but the enzyme capacities in leaves and roots, and the peroxidase pattern indicated that these metal concentrations were not phytotoxic for beans as test plants. The addition of chemical agents to the soil did not increase biomass production, but treatment with either HMO, ST or beringite markedly decreased the mobility of Cd, Zn and Pb. These agents were proven to be effective in mitigating the Cd uptake by plants. HMO and ST decreased either Pb or Zn uptake by ryegrass. TBS was effective in lowering Pb uptake by the same species. Beringite decreased Cd uptake by beans. If fallout could be restricted, the metal content of food crops in this area should be lowered by soil treatment. However, the differences in Cd uptake between plant species were not suppressed, regardless of the type of agents applied to the soil.

Selected bioavailability assays to test the efficacy of amendment-induced immobilization of lead in soils

Lead immobilization in 10 soils contaminated with Pb from different origin was examined using lime (CaCO3), a mix of cyclonic ash and steelshots (CA+ST), and a North Carolina phosphate rock. The immobilization efficacy of the three amendments was evaluated using single (CaCl2solution) and sequential (BCR method) chemical extractions in tandem with a microbiological Pb biosensor (BIOMET), a Pb phytotoxicity test, Pb plant uptake, and a Physiologically Based Extraction Test (PBET) mimicking Pb bioavailability in the human gastro-intestinal tract. The results demonstrated the necessity of using a diverse suite of bioavailability methodology when in situ metal immobilization is assessed. Sequential (BCR) extractions and PBET analysis indicated that PR was the most effective amendment. PR however, proved ineffective in totally preventing Pb phytotoxicity and Pb uptake on all soils tested. On the contrary, CA+ST and lime decreased BIOMET Pb, Pb phytotoxicity, and Pb uptake to a far greater extent than did PR. BIOMET detectable Pb and Pb uptake, however, were strongly related to Pb in soluble or exchangeable soil fractions (i.e., CaCl2 extractable). By combining these fractions with the acid-extractable Pb, accomplished by using acetic acid extractant, the recently developed BCR sequential extraction scheme appeared to have lost some valuable information on judging Pb bioavailability. The data show that different amendments do not behave consistently across different soils with different sources of contamination. Different indices for measuring Pb bioavailability are also not necessarily consistent within particular soil and amendment combinations.

Immobilization of trace metals and arsenic by different soil additives: Evaluation by means of chemical extractions

INRA, Bordeaux Aquitaine Res Ctr, Agron Unit, F-33883 Villenave Dornon, France. Limburgs Univ Ctr, Dept SBG, B-3590 Diepenbeek, Belgium. Univ Kuopio, Dept Ecol & Environm Sci, FIN-70211 Kuopio, Finland. Univ Tras os Montes & Alto Douro, P-5001 Villa Real, Portugal.Boisson, J, INRA, Bordeaux Aquitaine Res Ctr, Agron Unit, BP 81, F-33883 Villenave Dornon, France.

Thallium in French agrosystems—I. Thallium contents in arable soils

The thallium (Tl) content of the upper horizons of 244 French soils was determined as the first step towards the creation of a reference data bank for total Tl content of arable soils. Forty soil samples were collected in the vicinity of potential anthropogenic sources of Tl, but the remainder came from rural areas. The distribution of Tl concentrations in soils was characterized by a median value of 0.29 mg Tl kg(-1) and a 90th percentile value of 1.54 mg Tl kg(-1). Very high pedogeochemical contents were found (up to 55 mg Tl kg(-1)) but none could be attributed to obvious anthropogenic pollution. Areas of very high Tl concentration belong to an epihercynian transgression zone with a contact between a sedimentary basin and a crystalline massif. This contact is associated with stratified mineralizations (Zn, Pb, F, Sb, Ba, Tl and pyrites). High Tl concentrations were common in limestone, marl or granite derived soils, and the Tl in limestones or marls is probably concentrated in the sulfides contained in these rocks because Tl has a high affinity to S. In granites, Tl may be in the micas and feldspars because Tl+ can replace K+ in these minerals. Silty or clay-silty soils showed the highest concentrations. These granulometric fractions contain the majority of the minerals, which are supposed to be the major hosts of Tl in soils, i.e. clay minerals, oxides and micas. Tl in the soils was positively correlated with Ba, V, Pb, Fe, Ni, Cd, Zn, Co, As and especially Mn. A significant proportion of Tl may be in the Mn oxides: in oxidizing conditions, Tl(III) could enter the Mn oxides by sorption, or Tl(I) could replace K(I) in the oxide.

A microbial biosensor to predict bioavailable nickel in soil and its transfer to plants

Ralstonia eutropha strain AE2515 was constructed and optimised to serve as a whole-cell biosensor for the detection of bioavailable concentrations of Ni2+ and Co2+ in soil samples. Strain AE2515 is a Ralstonia eutropha CH34 derivative containing pMOL1550, in which the cnrYXH regulatory genes are transcriptionally fused to the bioluminescent luxCDABE reporter system. Strain AE2515 was standardised for its specific responses to Co2+ and Ni2+. The detection limits for AE2515 were 0.1 microM Ni2+ and 9 microM Co2+, respectively. The signal to noise (S/N) bioluminescence response and the metal cation concentration could be linearly correlated: for Ni2+ this was applicable within the range 0.1-60 microM, and between 9 and 400 microM for Co2+. The AE2515 biosensor strain was found to be highly selective for nickel and cobalt: no induction was observed with Zn(II), Cd(II), Mn(II), Cu(III) and Cr(VI). In mixed metal solutions, the bioluminescent response always corresponded to the nickel concentrations. Only in the presence of high concentrations of Co2+ (2 mM), the sensitivity to nickel was reduced due to metal toxicity. AE2515 was used to quantify the metal bioavailability in various nickel-enriched soils, which had been treated with additives for in situ metal immobilisation. The data obtained with strain AE2515 confirmed that the bioavailability of nickel was greatly reduced following the treatment of the soils with the additives beringite and steel shots. Furthermore, the data were found to correlate linearly with those on the biological accumulation of Ni2+ in specific parts of important agricultural crops, such as maize and potato. Therefore, the test can be used to assess the potential transfer of nickel to organisms of higher trophic levels, in this case maize and potato plants grown on nickel-enriched soils, and the potential risk of transfer of these elements to the food chain.