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.     

Field evaluation of Cd and Zn phytoextraction potential by the hyperaccumulators Thlaspi caerulescens and Arabidopsis halleri

Field trials were undertaken to investigate the effect of the application of metal mobilizing agents, different sowing strategies and length of growing season on the extraction of Cd and Zn from soils by Thlaspi caerulescens and Arabidopsis halleri. None of the mobilizing agents used enhanced metal accumulation by T. caerulescens. Between 1998 and 2000, on average across plots where Cd or Zn exceeded allowable limits, T. caerulescens removed 1.3 and 0.3% of the total soil Cd and Zn. In one season when T. caerulescens was grown for 14 months, 21.7 and 4.4% of the total soil Cd and Zn was removed. This was larger than values found when T. caerulescens was grown for 4 months. A. halleri accumulated similar concentrations of Zn, but lower Cd concentrations than T. caerulescens. The results indicate that metal phytoextraction using T. caerulescens can be used to clean up soils moderately contaminated by Cd.     

PCBs and DDT in the serum of juvenile California sea lions: associations with vitamins A and E and thyroid hormones

In order to better understand the fate of metals during the biodegradation of organic matter in soils, an in vitro incubation experiment was conducted with metal-rich and metal-free leaves of Arabidopsis halleri introduced in a non-contaminated soil. During incubation of these microcosms, we followed the partitioning of Zn and Cd between the solution and their solid components, by determining the metal contents of six soil fractions and dissolved metals after granulo-densimetric separations at selected times. Microbial biomass and exchangeable metals in K(2)SO(4) solutions were also determined at the same times, and two main stages were identified. The first one takes place after a fast abiotic transfer of Zn and Cd from readily soluble plant tissues onto fine soil constituents, keeping metals away from the liquid phase: during about 14 days, microbial biomass increased as well as metal contents of some soil fractions, particularly those rich in particulate organic matter. During the second stage, between 14 and 60 days and for the metal-rich microcosms, Zn and Cd contents in solution increased, while microbial biomass decreased instead of staying constant as in control. A change of Zn and Cd speciation is assumed, from non-toxic adsorbed forms to more toxic species in solution. Remaining metal-rich plant residues seem to create a stable organic C compartment in the soil.     

Enhanced multi-metal extraction with EDDS of deficient and excess dosages under the influence of dissolved and soil organic matter

This study investigated the influence of dissolved and soil organic matter on metal extraction from an artificially contaminated soil. With high concentration of DOM, the extraction of Cu, Zn and Pb was enhanced by forming additional metal-EDDS complexes under EDDS deficiency. However, the enhancement of metal extraction under EDDS excess was probably due to the soil structure being disrupted owing to humic acid enhanced Al and Fe dissolution, which induced more metals dissolving from the soils. Fulvic acid was found to enhance metal extraction to a greater extent compared with humic acid because of its high content of the carboxylic functional group. Cu extraction from the soil with high organic matter content using EDDS was the lowest due to the high binding affinity of Cu to SOM, whereas Zn extraction became the highest because of a preference for EDDS to extract Zn due to the high stability constant of ZnEDDS.     

Zinc tolerance and uptake by <Emphasis Type="Italic">Arabidopsis halleri ssp. gemmifera</Emphasis> grown in nutrient solution

Background, aim, and scope  

Zinc is an essential micronutrient element but its concentrations found in contaminated soils frequently exceed those required by the plant and soil organisms, and thus create danger to animal and human health. Phytoremediation is a technique, often employed in remediation of contaminated soils, which aims to remove heavy metals or other contaminants from soils or waters using plants. Arabidopsis (A.) halleri ssp. gemmifera is a plant recently found to be grown vigorously in heavy metal contaminated areas of Japan and it contained remarkably high amount of heavy metals in its shoots. However, the magnitude of Zn accumulation and tolerance in A. halleri ssp. gemmifera need to be investigated for its use as a phytoremediation plant.     

Strategies of heavy metal uptake by three plant species growing near a metal smelter

Some higher plant species have developed heavy metal tolerance strategies which enable them to survive and reproduce in highly metal-contaminated soils. We have investigated such heavy metal uptake and accumulation strategies of two absolute metallophyte species (Armeria maritima ssp. halleri and Cardaminopsis halleri) and one pseudometallophyte (Agrostis tenuis) growing near a former metal smelter. Samples of plant parts and soil were analysed for Zn, Cd, Pb, and Cu. In soil, there were two dominant types of metal concentration gradients with depth. Under the absolute metallophytes, extremely high metal contents were measured in the surficial Ah horizon, followed by a strong decrease in the underlying soil horizons (L(11) and L(12)). Under the pseudometallophyte, metal concentrations in the Ah horizon were much lower and fewer differences were observed in metal concentrations among the Ah, L(11), and L(12) horizons. The concentrations of Zn, Cd, Pb, and Cu in Agrostis tenuis roots were greater than concentrations in leaves, indicating significant metal immobilisation by the roots. For C. halleri, Zn and Cd concentrations in leaves were >20,000 and >100 mg kg(-1), respectively, indicating hyperaccumulation of these elements. Armeria maritima ssp. halleri exhibited root concentrations of Pb and Cu that were 20 and 88 times greater, respectively, than those in green leaves, suggesting an exclusion strategy by metal immobilisation in roots. However, Zn, Cd, Pb, and Cu concentrations in brown leaves of Armeria maritima ssp. halleri were 3-8 times greater than in green leaves, suggesting a second strategy, i.e. detoxification mechanism by leaf fall.     

Impact of sewage sludge spreading on heavy metal speciation in tropical soils (Réunion, Indian Ocean)

This study was designed to evaluate the impact of sewage sludge spreading on tropical soilborne heavy metal speciation. Sludgeborne heavy metal speciation was also assessed, and the potential mobility of the elements was classified as follows: Zn>Ni>Cu approximately Cr. Two sequential extraction procedures were applied to study Cr, Cu, Ni and Zn speciation in control soils and sludge-amended soils. We demonstrated that sewage sludge spreading over a 2-year period did not have an impact on soilborne heavy metal concentrations but affected speciation of the most mobile fractions of Ni and Zn. Both protocols were consistent for the organic matter fractions, with an increase in Cu, Zn and Cr concentrations in the amended soil as compared to the control soil. In addition, we highlighted that the two protocols characterized different pools of organic matter and that organic compounds remained in the solid matrix after extraction. With respect to the reducible fraction, completely opposite results were obtained with the two protocols and the solid residue study revealed that the two schemes were ineffective in characterizing iron and manganese fractions.     

Effects of compost, pig slurry and lime on trace element solubility and toxicity in two soils differently affected by mining activities

The use of organic wastes as amendments in heavy metal-polluted soils is an ecological integrated option for their recycling. The potential use of alperujo (solid olive-mill waste) compost and pig slurry in phytoremediation strategies has been studied, evaluating their short-term effects on soil health. An aerobic incubation experiment was carried out using an acid mine spoil based soil and a low OM soil from the mining area of La Unión (Murcia, Spain). Arsenic and heavy metal solubility in amended and non-amended soils, and microbial parameters were evaluated and related to a phytotoxicity test. The organic amendments provoked an enlargement of the microbial community (compost increased biomass-C from non detected values to 35 μg g⁻¹ in the mine spoil soil, and doubled control values in the low OM soil) and an intensification of its activity (including a twofold increase in nitrification), and significantly enhanced seed germination (increased cress germination by 25% in the mine spoil soil). Organic amendments increased Zn and Pb EDTA-extractable concentrations, and raised As solubility due to the influence of factors such as pH changes, phosphate concentration, and the nature of the organic matter of the amendments. Compost, thanks to the greater persistence of its organic matter in soil, could be recommended for its use in (phyto)stabilisation strategies. However, pig slurry boosted inorganic N content and did not significantly enhance As extractability in soil, so its use could be specifically recommended in As polluted soils.     

Influence of organic acids on the transport of heavy metals in soil

Vegetation historically has been an important part of reclamation of sites contaminated with metals, whether the objective was to stabilize the metals or remove them through phytoremediation. Understanding the impact of organic acids typically found in the rhizosphere would contribute to our knowledge of the impact of plants in contaminated environments. Heavy metal transport in soils in the presence of simple organic acids was assessed in two laboratory studies. In the first study, thin layer chromatography (TLC) was used to investigate Zn, Cd, and Pb movement in a sandy loam soil as affected by soluble organic acids in the rhizosphere. Many of these organic acids enhanced heavy metal movement. For organic acid concentrations of 10mM, citric acid had the highest R(f) values (frontal distance moved by metal divided by frontal distance moved by the solution) for Zn, followed by malic, tartaric, fumaric, and glutaric acids. Citric acid also has the highest R(f) value for Cd movement followed by fumaric acid. Citric acid and tartaric acid enhanced Pb transport to the greatest degree. For most organic acids studied, R(f) values followed the trend Zn>Cd>Pb. Citric acid (10mM) increased R(f) values of Zn and Cd by approximately three times relative to water. In the second study, small soil columns were used to test the impact of simple organic acids on Zn, Cd, and Pb leaching in soils. Citric acid greatly enhanced Zn and Cd movement in soils but had little influence on Pb movement. The Zn and Cd in the effluents from columns treated with 10mM citric acid attained influent metal concentrations by the end of the experiment, but effluent metal concentrations were much less than influent concentrations for citrate <10mM. Exchangeable Zn in the soil columns was about 40% of total Zn, and approximately 80% total Cd was in exchangeable form. Nearly all of the Pb retained by the soil columns was exchangeable.     

Potential availability of heavy metals to phytoextraction from contaminated soils induced by exogenous humic substances

Effective phytoremediation of soils contaminated by heavy metals depends on their availability to plant uptake that, in turn, may be influenced by either the existing soil humus or an exogenous humic matter. We amended an organic and a mineral soil with an exogenous humic acid (HA) in order to enhance the soil organic carbon (SOC) content by 1% and 2%. The treated soils were further enriched with heavy metals (Cu, Pb, Cd, Zn, Ni) to a concentration of 0, 10, 20, and 40 microg/g for each metal and allowed to age at room temperature for 1 and 2 months. After each period, they were extracted for readily soluble and exchangeable (2.5% acetic acid), plant-available (DTPA, Diethylentriaminepentaacetic acid), and occluded (1 N HNO(3)) metal species. Addition of HA generally reduced the extractability of the soluble and exchangeable forms of metals. This effect was directly related to the amount of added HA and increased with ageing time. Conversely, the potentially plant-available metals extracted with DTPA were generally larger with increasing additions of exogenous HA solutions. This was attributed to the formation of metal-humic complexes, which ensured a temporary bioavailability of metals and prevented their rapid transformation into insoluble species. Extractions with 1 N HNO(3) further indicated that the added metals were present in complexes with HA. The observed effects appeared to also depend on the amount of native SOC and its structural changes with ageing. The results suggest that soil amendments with exogenous humic matter may accelerate the phytoremediation of heavy metals from contaminated soil, while concomitantly prevent their environmental mobility.     

The influence of pH and organic matter content in paddy soil on heavy metal availability and their uptake by rice plants

The experiments were done to investigate the effect of soil pH and organic matter content on EDTA-extractable heavy metal contents in soils and heavy metal concentrations in rice straw and grains. EDTA-extractable Cr contents in soils and concentrations in rice tissues were negatively correlated with soil pH, but positively correlated with organic matter content. The combination of soil pH and organic matter content would produce the more precise regression models for estimation of EDTA-Cu, Pb and Zn contents in soils, demonstrating the distinct effect of the two factors on the availability of these heavy metals in soils. Soil pH greatly affected heavy metal concentrations in rice plants. Furthermore, inclusion of other soil properties in the stepwise regression analysis improved the regression models for predicting straw Fe and grain Zn concentrations, indicating that other soil properties should be taken into consideration for precise predicting of heavy metal concentrations in rice plants.     

Metal extractability patterns to evaluate (potentially) mobile fractions in periurban calcareous agricultural soils in the Mediterranean area—analytical and mineralogical approaches

A set of periurban calcareous agricultural Mediterranean soils was spiked with a mixture of Cd, Cu, Pb and Zn at two levels within the limit values proposed by current European legislation, incubated for up to 12 months, and subjected to various one-step extraction procedures to estimate mobile (neutral salts) and potentially mobile metal fractions (complexing and acidic extraction methods). The results obtained were used to study metal extractability patterns according to the soil characteristics. The analytical data were coupled with mineralogical investigations and speciation modelling using the Visual Minteq model. The formation of soluble metal-complexes in the complexing extracts (predicted by the Visual Minteq calculations) led to the highest extraction efficiency with complexing extractants. Metal extractability patterns were related to both content and composition of carbonate, organic matter, Fe oxide and clay fractions. Potentially mobile metal fractions were mainly affected by the finest soil fractions (recalcitrant organic matter, active lime and clay minerals). In the case of Pb, scarce correlations between extractable Pb and soil constituents were obtained which was attributed to high Pb retention due to the formation of 4PbCO₃·3PbO (corroborated by X-ray diffraction). In summary, the high metal proportion extracted with complexing agents highlighted the high but finite capacity to store potentially mobilizable metals and the possible vulnerability of these soils against environmental impact from metal accumulation.     

Cu and Zn mobilization in soil columns percolated by different irrigation solutions

We investigated the effects of different concentrations of nitrate and ammonium in irrigation water on the mobilization of Zn and Cu in repacked soil columns with a metal-polluted topsoil and unpolluted subsoils over two and a half years. Soil solution samples were collected by suction cups installed at vertical distances of a few centimeters and analyzed for dissolved organic carbon (DOC), Cu, and Zn (total and labile). During high N treatments the pH decreased and the presence of exchangeable cations resulted in Zn mobilization from the surface soil. The nitrogen input stimulated the biological activity, which affected both concentration and characteristics of DOC and consequently Cu speciation. Metal leaching through the boundary between the polluted topsoil and the unpolluted subsoils increased soil-bound and dissolved metals within the uppermost 2 cm in the subsoils. Our study shows that agricultural activities involving nitrogen fertilization can have a strong influence on metal leaching and speciation.     

Sequential extraction of heavy metals during composting of sewage sludge

The major limitation of soil application of sewage sludge compost is the total heavy metal contents and their bioavailability to the soil-plant system. This study was conducted to determine the heavy metal speciation and the influence of changing the physico-chemical properties of the medium in the course of composting on the concentrations, bioavailability or chemical forms of Cu, Zn, Pb and Ni in sewage sludge. Principal physical and chemical properties and FTIR spectroscopical characterization of sludge compost during treatment show the stability and maturity of end product. The total metal contents in the final compost were much lower than the limit values of composts to be used as good soil fertilizer. Furthermore, it was observed by using a sequential extraction procedure in sludge compost at different steps of treatment, that a large proportion of the heavy metals were associated to the residual fraction (70-80%) and more resistant fractions to extraction X-NaOH, X-EDTA, X-HNO3 (12-29%). Less than 2% of metals bound to bioavailable fractions X-(KNO3+H2O). Heavy metal distribution and bioavailability show some changes during composting depending on the metal itself and the physico-chemical properties of the medium. Bioavailable fractions of all elements tend to decrease except Ni-H2O. Zn and mainly Cu present more affinity to organic and carbonate fractions. In contrast, Pb is usually preferentially bound to sulfide forms X-HNO3. Nickel shows a significant decrease of organic form. Significant degrees of correlation were found between heavy metal fractions and changes of some selected variables (e.g. pH, ash, organic matter, humic substance) during the course of composting. Mobile fractions of metals are poorly predictable from the total content. The R2 value was significantly increased by the inclusion of other variables such as the amount of organic matter (OM) and pH.     

A remediation strategy based on active phytoremediation followed by natural attenuation in a soil contaminated by pyrite waste

Phytoremediation of metal-polluted soils can be promoted by the proper use of soil amendments and agricultural practices. A 4-year phytoremediation programme was applied to a site affected by the toxic spill of pyrite residue at Aznalcóllar (Spain) in 1998, contaminated with heavy metals (Zn, Cu, Pb, Cd) and arsenic. This consisted of active phytoremediation, using organic amendments (cow manure and compost) and lime and growing two successive crops of Brassica juncea (L.) Czern., followed by natural attenuation without further intervention. Changes in soil pH, extractable metal and As concentrations, organic carbon content and microbial biomass was evaluated. The initial oxidation of metal sulphides from pyrite residues released soluble metals and reduced soil pH to extremely acidic values (mean 4.1, range 2.0-7.0). The addition of lime (up to 64 t ha(-1)) increased soil pH to adequate values for plant growth, resulting in a significant decrease in DTPA-extractable metal concentrations in all plots. The natural attenuation phase showed also a decrease in extractable metals. Organic treatments increased the soil total organic carbon, which led to higher values of microbial biomass (11.6, 15.2 and 14.9 g kg(-1) TOC and 123, 170 and 275 microg g(-1) biomass-C in control, compost and manure plots, respectively). Active phytoremediation followed by natural attenuation, was effective for remediation of this pyrite-polluted soil.     

A comparison of phytoremediation capability of selected plant species for given trace elements

In our experiment, As, Cd, Pb, and Zn remediation possibilities on medium contaminated soil were investigated. Seven plant species with a different trace element accumulation capacity and remediation potential were compared. We found good accumulation capabilities and remediation effectiveness of Salix dasyclados similar to studied hyperaccumulators (Arabidopsis halleri and Thlaspi caerulescens). We have noticed better remediation capability in willow compared to poplar for most of the elements considered in this experiment. On the contrary, poplar species were able to remove a larger portion of Pb as opposed to other species. Nevertheless, the removed volume was very small. The elements found in plant biomass depend substantially on the availability of these elements in the soil. Different element concentrations were determined in natural soil solution and by inorganic salt solution extraction (0.01 molL(-1) CaCl(2)). Extracted content almost exceeded the element concentration in the soil solution. Element concentrations in soil solution were not significantly affected by sampling time.     

Dynamic chemical characteristics of soil solution after pig manure application: a column study

When manures from intensive livestock operations are applied to agricultural or vegetable fields at a high rate, large amounts of salts and metals will be introduced into soils. Using a column leaching experiment, this study assessed the leaching potential of the downward movement of Cu and Zn as well as some salt ions after an intensive farm pig manure at rates of 0%, 5% and 10% (w/w) were applied to the top 20 cm of two different textured soils (G soil -sandy loam soil; H soil-silty clay loam soil), and investigated the growth of amaranth and Cu and Zn transfer from soil to amaranth (Amaranthus tricolor). Soil solutions were obtained at 20, 40 and 60 cm depth of the packed column and analyzed for pH, electrical conductivity (EC), dissolved organic matter (DOC) and Cu and Zn concentrations. The results indicated that application of pig manure containing Cu and Zn to sandy loam soil might cause higher leaching and uptake risk than silty clay loam soil, especially at high application rates. And manure amendment at 5% and 10% significantly decreased the biomass of amaranth, in which the salt impact rather than Cu and Zn toxicity from manures played more important role in amaranth growth. Thus the farmer should avoid application the high rate of pig manure containing metal and salt to soil at a time, especially in sandy soil.     

Comparison of a rhizosphere-based method with other one-step extraction methods for assessing the bioavailability of soil metals to wheat

There is no method recognized as a universal approach for evaluation of bioavailability of heavy metals in soil. Based on the simulation of the rhizosphere soil conditions and integration of the combined effects of root-soil interactions as a whole, a rhizosphere-based method has been proposed. Wet fresh rhizosphere soil was extracted by low-molecular-weight organic acids (LMWOAs) to fractionate metal fractions of soil pools, which were then correlated with the metal contents of wheat roots and shoots. The rhizosphere-based method was compared with other one-step extraction methods using DTPA, EDTA, CaCl2, and NaNO3 as extractants and the first step of the Community Bureau of Reference (BCR) method. Simple correlation and stepwise multiple regression analysis were used for the comparison. Simple correlation indicated that the extractable Cu, Zn, Cr, and Cd of soils by the rhizosphere-based method were significantly correlated with the metal contents of wheat roots. For DTPA, BCR1 and EDTA methods there was a relatively poor correlation between the extractable Cu, Zn and Cd of soil and metal contents of wheat roots. Stepwise multiple regression analysis revealed that the equation of the rhizosphere-based method was the simplest one, and no soil properties variables needed to be added. In contrast, the equations of other one-step extraction methods were more complicated, and soil properties variables needed to be entered. The most distinct feature of the rhizosphere-based method was that the recommended method was suitable for acidic, neutral and near alkaline soils. However, the DTPA and EDTA extraction methods were suitable for calcareous soils only-or-only for acidic soils. The CaCl2, and NaNO3 extraction methods were only suitable for exchangeable metals. In short, the rhizosphere-based method was the most robust approach for evaluation of bioavailability of heavy metals in soils to wheat.     

Plant accumulation of potentially toxic elements in sewage sludge as affected by soil organic matter level and mycorrhizal fungi.

Leek (Allium ameloprasum) was grown in pot trials in two clay loams of contrasting organic contents, with and without indigenous mycorrhizal propagules. Sewage sludges containing varying levels of Cd, Cu and Zn were added. Extractable soil metals, plant growth, major nutrient content and accumulation of metals, and soil microbial indices were investigated. The aim was to establish whether soil organic content and mycorrhizal status affected plant and microbial exposure to these metals. Extractable metals were higher and responses to inputs more pronounced in the arable, lower organic matter soil, although only Cd showed a soil difference in the CaCl2 fraction. There were no metal toxic effects on plants and some evidence to suggest that they promoted growth. Uptake of each metal was higher in the larger plants of the grassland, higher organic matter soil. Inoculation with arbuscular mycorrhizal fungi increased root Cd and Zn concentrations. With the exception of Cd (roots) and Zn (shoots), higher inputs of sludge metals did not increase plant metals. Zn and Cu, but not Cd, concentrations were higher in roots than in shoots.     

Simulating the long-term chemistry of an upland UK catchment: heavy metals

CHUM-AM was used to investigate the behaviours of atmospherically-deposited heavy metals (Ni, Cu, Zn, Cd and Pb) in three moorland sub-catchments in Cumbria UK. The principal processes controlling cationic metals are competitive partitioning to soil organic matter, chemical interactions in solution, and chemical weathering. Metal deposition histories were generated by combining measured data for the last 30 years with local lake sediment records. For Ni, Cu, Zn and Cd, default parameters for the interactions with organic matter provided reasonable agreement between simulated and observed present-day soil metal pools and average streamwater concentrations. However, for Pb, the soil binding affinity in the model had to be increased to match the observations. Simulations suggest that weakly-sorbing metals (Ni, Zn, Cd) will respond on timescales of decades to centuries to changes in metal inputs or acidification status. More strongly-sorbing metals (Cu, Pb) will respond over centuries to millennia.