The use of poplar during a two-year induced phytoextraction of metals from contaminated agricultural soils

Evaluation for phosphorus accumulation and removal capability of nine species in the Polygonaceae to excavate amphibious superstars used for phosphorus-phytoextraction

Reducing excessive phosphorus (P) from both soils and eutrophic waters is attractive to achieve environmental P balance, and P-phytoextraction by amphibious plants with great biomass and P uptake is an amazing method, as already reported for P-accumulating plant, Polygonum hydropiper. However, it is still unknown how widespread high P tolerance and great P accumulation is among species in the Polygonaceae, and if there are new amphibious superstars used for P-phytoextraction. We used six Polygonum species and three non-Polygonum species to compare P accumulation and removal capability in hydroponics and soils with different P treatments. In high P hydroponics, all species showed superiority in growth and P accumulation without P toxicity, except for F. multiflora. In high P soils, all species showed much better growth performance with green leaves at 8 weeks, with shoot biomass being 3.60-29.49 g plant^-1. At 8 weeks, Polygonum species displayed obviously higher shoot P accumulation (31.32-152.37 mg plant^-1), P extraction ratio (3.16%-15.36%), maximum potential P removal (13.89-67.59 kg ha^-1), and much lower plant effective number (7-32) than non-Polygonum species under high P soils. Besides, P. lapathifolium, P. divaricatum and P. orientale ranked the top three in growth with P concentration more than 10 mg g^-1 dry weight in hydroponics and showed dominant advantage in P accumulation and P removal from high P soils. Through the cluster analysis, P. lapathifolium was always separated into a class, and P. divaricatum and P. orientale more likely clustered together. It is therefore that P. lapathifolium, P. divaricatum and P. orientale are tolerant to high P and attractive in P accumulation and P removal from high P waters and soils, and thus can be used as new amphibious superstars for P-phytoextraction, particularly P. lapathifolium.

Is the harvest of Salix and Populus clones in the growing season truly advantageous for the phytoextraction of metals from a long-term perspective?

The phytoextraction potential of short-rotation field coppice plantations was investigated on soil historically contaminated with risk elements (REs), especially Cd, Pb and Zn. The main objective of the study was to assess the effect of biomass harvest time (summer harvest versus traditional winter one) on biomass yield, REs accumulation and removal in the long-term study. The precise field experiment with two Salix clones and two Populus clones was established in April 2009. Shoots of all clones were harvested in February 2012 for the first time, and then identical branches and leaves were harvested every two years in September (summer harvest = SH) and the branches every two years in February (winter harvest = WH). The first summer harvest seemed to be more promising compared to the winter one, but the yields in the second and third harvests were inconsistent. The total phytoextraction efficiency of the SH declined in second and third harvests due to a decrease of leaves/aboveground biomass ratio, and the RE concentrations in leaves. Clonal Salix smithiana was the most productive one in the SH, with a dry matter yield of 15.1 t ha^-1 year^-1 and showed promising extraction ability for Cd (11.65 %) and Zn (4.59 %) over a 6-year field experiment. A lower portion of Cd (6.97 %) and Zn (2.38 %) was removed by this clone in the WH (calculated from the total soil content of REs). SH was more reasonable for phytoextraction compared to WH. Higher RE concentrations were found in leaves of Salix compared to Populus. Populus accumulated the highest Pb content in the branches; unfortunately, the Pb extraction was low, due to extremely high soil Pb contamination. Locally bred willows and poplars performed substantially better than internationally recognised ones, indicating the importance of phytoremediation optimisation, including clone selection, for suitable climatic conditions.

Removal of cadmium, lead, and zinc from multi-metal-contaminated soil using chelate-assisted Sedum alfredii Hance

Biodegradable chelator-assisted phytoextraction is an effective method to enhance remediation efficiency of heavy metals. A greenhouse experiment was conducted to investigate the effects of S,S-ethylenediamine disuccinic acid (EDDS), citric acid (CA), and oxalic acid (OA) application before planting on the biomass and physiological characteristics of hyperaccumulator Sedum alfredii Hance, and its cadmium (Cd), lead (Pb), and zinc (Zn) uptake. The results showed that EDDS and CA slightly inhibited the plant growth, while the 1.0 mmol kg^-1 (OA-1) and 2.5 mmol kg^-1 OA (OA-2.5) addition produced 55.3% and 35.2% greater shoot biomass compared with the control, which may be related to that OA can produce higher leaf chlorophyll and soluble protein contents, as well as lower concentrations of malondialdehyde. At the same time, the concentrations of Pb and Zn in leaf after OA-2.5 treatment significantly increased by 127% and 28.4%, and the Cd, Pb, and Zn uptake by shoot was obviously enhanced by 21.5%, 117%, and 44.9% for OA-1 addition and by 39.1%, 80.0%, and 58.3% for OA-2.5 addition, respectively, in comparison with the control (P < 0.05). The reductions in available contents of Cd, Pb, and Zn in soil were observed after phytoextraction by Sedum alfredii Hance when OA was treated. These findings imply that OA was suitable for facilitating Sedum alfredii Hance to remove Cd, Pb, and Zn in co-contaminated soil.

Enhanced Phytoextraction for Co-contaminated Soil with Cd and Pb by Ryegrass (Lolium perenne L.)

A pot experiment was conducted to investigate the effect of three additives - citric acid (CA), polyaspartic acid (PASP), and FeCl₃ - on the phytoextraction efficiency of cadmium (Cd) and lead (Pb) by ryegrass (Lolium perenneL.) from artificially contaminated soils with different heavy metal concentrations. The results showed that as the concentration of pollutants increased, the TI (tolerance index) and BCF (bio-concentration factor) of ryegrass gradually increased only when FeCl₃ was applied. FeCl₃ also exhibited the most significant biomass enhancement and heavy metal accumulation of ryegrass, as well as the highest phytoextraction efficiency in heavily-polluted soils. The overall orders of the optimal phytoextraction efficiency for the three additives in terms of their MER (metal extraction ratio) were: FeCl₃ > PASP > CA. Therefore, FeCl₃ can be used to improve the Cd and Pb phytoextraction efficiency of ryegrass in heavily-polluted soils.

Chelator-assisted phytoextraction of arsenic, cadmium and lead by Pteris vittata L. and soil microbial community structure response

Using biodegradable chelators to assist in phytoextraction may be an effective approach to enhance the heavy-metal remediation efficiencies of plants. A pot experiment was conducted to investigate the effects of ethylenediamine disuccinic acid (EDDS), citric acid (CA), and oxalic acid (OA) on the growth of the arsenic (As) hyperaccumulator Pteris vittata L., its arsenic (As), cadmium (Cd), and lead (Pb) uptake and accumulation, and soil microbial responses in multi-metal(loid)-contaminated soil. The addition of 2.5-mmol kg^-1 OA (OA-2.5) produced 26.7 and 14.9% more rhizoid and shoot biomass, respectively compared with the control, while EDDS and CA treatments significantly inhibited plant growth. The As accumulation in plants after the OA-2.5 treatment increased by 44.2% and the Cd and Pb accumulation in plants after a 1-mmol kg^-1 EDDS treatment increased by 24.5 and 19.6%, respectively. Soil urease enzyme activities in OA-2.5 treatment were significantly greater than those in the control and other chelator treatments (p < 0.05). A PCR-denatured gradient gel electrophoresis analysis revealed that with the addition of EDDS, CA and OA enhanced soil microbial diversity. It was concluded that the addition of OA-2.5 was suitable for facilitating phytoremediation of soil As and did not have negative effects on the microbial community.

The suitability of growing mulberry (Morus alba L.) on soils consisting of urban sludge composted with garden waste: a new method for urban sludge disposal

Efficient disposal of urban sewage sludge, material that typically contains high concentrations of heavy metals, has become a significant concern worldwide. The empirical purpose of the current study is to investigate physical and chemical parameters of composted sludge and garden waste at different ratios. Results reveal that nutrient content has significantly increased after the application of composts as compared to the controlled sample. Composting garden waste with sewage sludge at a 1:1 ratio promoted plant growth and gradually showed superiority in the later period. The maximum plant height, total biomass, and crown width of mulberry trees increased by 12.1, 33.5, and 45.7%, respectively, compared with the control treatment. The bound to organic matter of Hg, Cr, and Pb in the sewage sludge increased after composting with garden waste, and the mulberry exhibited a high ability to accumulate Ni and Cd from the soil. Conclusively, compared to using the two soil mediums separately, composting garden waste and sewage sludge together is beneficial for soil improvement and vegetation growth.

Effects of summer and winter harvesting on element phytoextraction efficiency of Salix and Populus clones planted on contaminated soil

The clones of fast-growing trees (FGTs) were investigated for phytoextraction of soil contaminated with risk elements (REs), especially Cd, Pb, and Zn. As a main experimental factor, the potential effect of biomass harvesting time was assessed. The field experiment with two Salix clones (S1 - (Salix schwerinii × Salix viminalis) × S. viminalis, S2 - S. × smithiana) and two Populus clones (P1 - Populus maximowiczii × Populus nigra, P2 - P. nigra) was established in April 2009. Shoots of all clones were first harvested in February 2012. After two further growing seasons, the first half of the trees was harvested in September 2013 before leaf fall (summer harvest) and the second half in February 2014 (winter harvest). Remediation factors (RFs) for all clones and all REs (except Pb for clone S1) were higher in the summer harvest. The highest annual RFs for Cd and for Zn (1.34 and 0.67%, respectively) were found for clone S2 and were significantly higher than other clones. Although no increased mortality of trees harvested in the summer was detected in the following season, the effect of summer harvesting on the phytoextraction potential of FGTs clones should be investigated in long-term studies.

Relationships between soil parameters and physiological status of Miscanthus x giganteus cultivated on soil contaminated with trace elements under NPK fertilisation vs. microbial inoculation

Crop growth and development can be influenced by a range of parameters, soil health, cultivation and nutrient status all play a major role. Nutrient status of plants can be enhanced both through chemical fertiliser additions (e.g. N, P, K supplementation) or microbial fixation and mobilisation of naturally occurring nutrients. With current EU priorities discouraging the production of biomass on high quality soils there is a need to investigate the potential of more marginal soils to produce these feedstocks and the impacts of soil amendments on crop yields within them. This study investigated the potential for Miscanthus x giganteus to be grown in trace element (TE)-contaminated soils, ideally offering a mechanism to (phyto)manage these contaminated lands. Comprehensive surveys are needed to understand plant-soil interactions under these conditions. Here we studied the impacts of two fertiliser treatments on soil physico-chemical properties under Miscanthus x giganteus cultivated on Pb, Cd and Zn contaminated arable land. Results covered a range of parameters, including soil rhizosphere activity, arbuscular mycorrhization (AM), as well as plant physiological parameters associated with photosynthesis, TE leaf concentrations and growth performance. Fertilization increased growth and gas exchange capacity, enhanced rhizosphere microbial activity and increased Zn, Mg and N leaf concentration. Fertilization reduced root colonisation by AMF and caused higher chlorophyll concentration in plant leaves. Microbial inoculation seems to be a promising alternative for chemical fertilizers, especially due to an insignificant influence on the mobility of toxic trace elements (particularly Cd and Zn).

Effect of soil washing with only chelators or combining with ferric chloride on soil heavy metal removal and phytoavailability: Field experiments

In a field experiment on multi-metal contaminated soil, we investigated the efficiency of Cd, Pb, Zn, and Cu removal by only mixture of chelators (MC) or combining with FeCl3. After washing treatment, a co-cropping system was performed for heavy metals to be extracted by Sedum alfredii and to produce safe food from Zea mays. We analyzed the concentration of heavy metals in groundwater to evaluate the leashing risk of soil washing with FeCl3 and MC. Results showed that addition of FeCl3 was favorable to the removal of heavy metals in the topsoil. Metal leaching occurred mainly in rain season during the first co-cropping. The removal rates of Cd, Zn, Pb, and Cu in topsoil were 28%, 53%, 41%, and 21% with washing by FeCl3+MC after first harvest. The application of FeCl3 reduced the yield of S. alfredii and increased the metals concentration of Z. mays in first harvest. However, after amending soil, the metals concentration of Z. mays in FeCl3+MC treatment were similar to that only washing by MC. The grains and shoots of Z. mays were safe for use in feed production. Soil washing did not worsen groundwater contamination during the study period. But the concentration of Cd in groundwater was higher than the limit value of Standard concentrations for Groundwater IV. This study suggests that soil washing using FeCl3 and MC for the remediation of multi-metal contaminated soil is potential feasibility. However, the subsequent measure to improve the washed soil environment for planting crop is considered.

Plant responses to a phytomanaged urban technosol contaminated by trace elements and polycyclic aromatic hydrocarbons

Medicago sativa was cultivated at a former harbor facility near Bordeaux (France) to phytomanage a soil contaminated by trace elements (TE) and polycyclic aromatic hydrocarbons (PAH). In parallel, a biotest with Phaseolus vulgaris was carried out on potted soils from 18 sub-sites to assess their phytotoxicity. Total soil TE and PAH concentrations, TE concentrations in the soil pore water, the foliar ionome of M. sativa (at the end of the first growth season) and of Populus nigra growing in situ, the root and shoot biomass and the foliar ionome of P. vulgaris were determined. Despite high total soil TE, soluble TE concentrations were generally low, mainly due to alkaline soil pH (7.8-8.6). Shoot dry weight (DW) yield and foliar ionome of P. vulgaris did not reflect the soil contamination, but its root DW yield decreased at highest soil TE and/or PAH concentrations. Foliar ionomes of M. sativa and P. nigra growing in situ were generally similar to the ones at uncontaminated sites. M. sativa contributed to bioavailable TE stripping by shoot removal (in g ha(-1) harvest(-1)): As 0.9, Cd 0.3, Cr 0.4, Cu 16.1, Ni 2.6, Pb 4, and Zn 134. After 1 year, 72 plant species were identified in the plant community across three subsets: (I) plant community developed on bare soil sowed with M. sativa; (II) plant community developed in unharvested plots dominated by grasses; and (III) plant community developed on unsowed bare soil. The shoot DW yield (in mg ha(-1) harvest(-1)) varied from 1.1 (subset I) to 6.9 (subset II). For subset III, the specific richness was the lowest in plots with the highest phytotoxicity for P. vulgaris.

In-Situ Remediation Approaches for the Management of Contaminated Sites: A Comprehensive Overview

Though several in-situ treatment methods exist to remediate polluted sites, selecting an appropriate site-specific remediation technology is challenging and is critical for successful clean up of polluted sites. Hence, a comprehensive overview of all the available remediation technologies to date is necessary to choose the right technology for an anticipated pollutant. This review has critically evaluated the (i) technological profile of existing in-situ remediation approaches for priority and emerging pollutants, (ii) recent innovative technologies for on-site pollutant remediation, and (iii) current challenges as well as future prospects for developing innovative approaches to enhance the efficacy of remediation at contaminated sites.

Enhanced accumulation of Cd in castor (Ricinus communis L) by soil-applied chelators

Phytoextraction has been identified as one of the most propitious methods of phytoremediation. This pot experiment were treated with varying amounts of (ethylenediamine triacetic acid) EDTA 3-15, (Nitriloacetic acid) NTA 3-10, (Ammonium citrate) NH4 citrate 10 - 25 mmol and one mg kg(-1)Cd, filled with 5 kg soil. The addition of chelators significantly increased Cd concentration in soil and plant. The results showed that maximum Cd uptake was noted under root, shoot and leaf of castor plant tissue (2.26, 1.54, and 0.72 mg kg(-1)) under EDTA 15, NTA 10, and NH4 citrate 25 mmol treatments respectively, and in soil 1.08, 1.06 and 0.52 mg kg(-1) pot(-1) under NH4 citrate 25, NTA 10 and EDTA 15 mmol treatments respectively, as against to control (p < 0.05). Additions of chelators reduction biomass under the EDTA 15 mmol as compared to other treatments, However, Bioconcentration factor (BCF), translocation factor (TF) and remediation factor (RF) were significantly increased under EDTA 15 and NH4 citrate 25 mmol as against control. Our results demonstrated that castor plant proved satisfactory for phytoextraction on contaminated soil, and EDTA 15 and NH4 citrate 25 mmol had the affirmative effect on the Cd uptake in the artificial Cd-contaminated soil.

Distribution of P, K, Ca, Mg, Cd, Cu, Fe, Mn, Pb and Zn in wood and bark age classes of willows and poplars used for phytoextraction on soils contaminated by risk elements

Fast-growing clones of Salix and Populus have been studied for remediation of soils contaminated by risk elements (RE) using short-rotation coppice plantations. Our aim was to assess biomass yield and distributions of elements in wood and bark of highly productive willow (S1--[Salix schwerinii × Salix viminalis] × S. viminalis, S2--Salix × smithiana clone S-218) and poplar (P1--Populus maximowiczii × Populus nigra, P2--P. nigra) clones with respect to aging. The field experiment was established in April 2008 on moderately Cd-, Pb- and Zn- contaminated soil. Shoots were harvested after four seasons (February 2012) and separated into annual classes of wood and bark. All tested clones grew on contaminated soils, with highest biomass production and lowest mortality exhibited by P1 and S2. Concentrations of elements, with exception of Ca and Pb, decreased with age and were higher in bark than in wood. The Salix clones were characterised by higher removal of Cd, Mn and Zn compared to the Populus clones. Despite generally higher RE content in young shoots, partly due to lower wood/bark ratios and higher RE concentrations in bark, the overall removal of RE was higher in older wood classes due to higher biomass yield. Thus, longer rotations seem to be more effective when phytoextraction strategy is considered. Of the four selected clones, S1 exhibited the best removal of Cd and Zn and is a good candidate for phytoextraction.

Agronomic Practices for Improving Gentle Remediation of Trace Element-Contaminated Soils

The last few decades have seen the rise of Gentle soil Remediation Options (GRO), which notably include in situ contaminant stabilization ("inactivation") and plant-based (generally termed "phytoremediation") options. For trace element (TE)-contaminated sites, GRO aim to either decrease their labile pool and/or total content in the soil, thereby reducing related pollutant linkages. Much research has been dedicated to the screening and selection of TE-tolerant plant species and genotypes for application in GRO. However, the number of field trials demonstrating successful GRO remains well below the number of studies carried out at a greenhouse level. The move from greenhouse to field conditions requires incorporating agronomical knowledge into the remediation process and the ecological restoration of ecosystem services. This review summarizes agronomic practices against their demonstrated or potential positive effect on GRO performance, including plant selection, soil management practices, crop rotation, short rotation coppice, intercropping/row cropping, planting methods and plant densities, harvest and fertilization management, pest and weed control and irrigation management. Potentially negative effects of GRO, e.g., the introduction of potentially invasive species, are also discussed. Lessons learnt from long-term European field case sites are given for aiding the choice of appropriate management practices and plant species.

Problems inherent to a meta-analysis of proteomics data: a case study on the plants' response to Cd in different cultivation conditions

This meta-analysis focuses on plant-proteome responses to cadmium (Cd) stress. Initially, some general topics related to a proteomics meta-analysis are discussed: (1) obstacles encountered during data analysis, (2) a consensus in proteomic research, (3) validation and good reporting practices for protein identification and (4) guidelines for statistical analysis of differentially abundant proteins. In a second part, the Cd responses in leaves and roots obtained from a proteomics meta-analysis are discussed in (1) a time comparison (short versus long term exposure), and (2) a culture comparison (hydroponics versus soil cultivation). Data of the meta-analysis confirmed the existence of an initial alarm phase upon Cd exposure. Whereas no metabolic equilibrium is established in hydroponically exposed plants, an equilibrium seems to be manifested in roots of plants grown in Cd-contaminated soil after long term exposure. In leaves, the carbohydrate metabolism is primarily affected independent of the exposure time and the cultivation method. In addition, a metabolic shift from CO2-fixation towards respiration is manifested, independent of the cultivation system. Finally, some ideas for the improvement of proteomics setups and for comparisons between studies are discussed.

BIOLOGICAL SIGNIFICANCE

This meta-analysis focuses on the plant responses to Cd stress in leaves and roots at the proteome level. This meta-analysis points out the encountered obstacles when performing a proteomics meta-analysis related to inherent technologies, but also related to experimental setups. Furthermore, the question is addressed whether an extrapolation of results obtained in hydroponic cultivation towards soil-grown plants is possible.

Adaptive response of poplar (Populus nigra L.) after prolonged Cd exposure period

An outdoor pot experiment was designed to study the changes of growth parameters, accumulation, and distribution of Cd in poplar (Populus nigra L.) during a prolonged exposure period (growing period of 17 months including three harvest points), allowing the consideration of time effects and prolonged adaptation to Cd stress. Simultaneously, changes to the antioxidant system in roots and leaves were monitored. It was demonstrated that poplar could adapt to the Cd-contaminated soils after prolonged exposure. Total Cd accumulation in the aerial parts of poplar, due to high biomass production and acceptable Cd accumulation parameters, implies that the tested poplar species could be a good candidate for Cd phytoextraction application as well as could be used as phytostabilizer of Cd in heavily polluted soil. Furthermore, the activity of the antioxidant machinery displays both a tissue- and exposure-specific response pattern to different Cd treatments, indicating that strict regulation of the antioxidant defense system is required for the adaptive response of poplar. In addition, this report highlights the importance of prolonged exposure studies of physiological responses of plants, especially for long-life-cycle woody species under heavy metal stress, since some misleading conclusions could be reached after shorter time periods.

Phytoextraction potential of poplar (Populus alba L. var. pyramidalis Bunge) from calcareous agricultural soils contaminated by cadmium

To investigate the phytoextraction potential of Populus alba L. var. pyramidalis Bunge for cadmium (Cd) contaminated calcareous soils, a concentration gradient experiment and a field sampling experiment (involving poplars of different ages) were conducted. The translocation factors for all experiments and treatments were greater than 1. The bioconcentration factor decreased from 2.37 to 0.25 with increasing soil Cd concentration in the concentration gradient experiment and generally decreased with stand age under field conditions. The Cd concentrations in P. pyramidalis organs decreased in the order of leaves > stems > roots. The shoot biomass production in the concentration gradient experiment was not significantly reduced with soil Cd concentrations up to or slightly over 50 mg kg(-1). The results show that the phytoextraction efficiency of P. pyramidalis depends on both the soil Cd concentration and the tree age. Populus pyramidalis is most suitable for remediation of slightly Cd contaminated calcareous soils through the combined harvest of stems and leaves under actual field conditions.

Polyaspartate, a biodegradable chelant that improves the phytoremediation potential of poplar in a highly metal-contaminated agricultural soil

Phytoremediation is a cost-effective and environment friendly in situ technique for the reclamation of heavy metal-polluted soils. The efficacy of this technique, which relies on tolerant plant species, can be improved by the use of chelating agents. A pot experiment was carried out to evaluate the phytoextraction and phytostabilisation capacities of a white poplar (Populus alba L.) clone named AL35 previously selected for its marked tolerance to copper (Cu) and zinc (Zn). Cuttings were grown on agricultural soil highly contaminated with Cu and Zn, in the presence or not (controls) of a chelant mixture (EDTA/EDDS) known to enhance metal bioavailability and, hence, uptake by plant roots, or the not yet investigated synthetic, highly biodegradable polyaspartic acid (PASP). Both chelant treatments improved the phytostabilisation of Cu and Zn in AL35 plants, whilst the phytoextraction capacity was enhanced only in the case of Cu. Considering that the effectiveness of PASP as phytostabilizer was comparable or better than that of EDTA/EDDS, the low cost of its large-scale chemical synthesis and its biodegradability makes it a good candidate for chelant-enhanced metal phytoextraction from soil while avoiding the toxic side-effects previously described for both EDTA and EDDS.

Toxicity of synthetic chelators and metal availability in poultry manure amended Cd, Pb and As contaminated agricultural soil

Chelating agents added to contaminated soils may increase solubility and phytoextraction efficiency of soil metals. However, they can create negative effects on soil biological quality. A 90-day incubation experiment was conducted to evaluate mixed effects of chelating agents and poultry manure on changes in available Cd, Pb and As, CO2-C efflux, microbial biomass C, dissolved organic C (DOC), and N mineralization in metal-polluted agricultural soil. Application of poultry manure resulted in a considerable increase in soil pH, DOC, CO2-C efflux, net N mineralization, net N nitrification, and microbial biomass C compared to those in unmanured soil. Availability of arsenic increased twice in manure amended soil due to changes in pH and DOC. However, adding poultry manure did not affect the concentrations of available Pb and Cd compared to those in control soil. Chelating agents increased CO2-C efflux, DOC, and metal availability but decreased microbial biomass C and net N mineralization. Maximum decrease in microbial biomass C, net N mineralization, and net N nitrification, was observed in EDTA applied soil possibly due to high metal availability to soil microorganisms. Overall results revealed that the application of synthetic chelators in combination with poultry manure enhances available As and demonstrates better environment for soil biota.

Heavy metal accumulation by poplar in calcareous soil with various degrees of multi-metal contamination: implications for phytoextraction and phytostabilization

The object of this study was to assess the capacity of Populus alba L. var. pyramidalis Bunge for phytoremediation of heavy metals on calcareous soils contaminated with multiple metals. In a pot culture experiment, a multi-metal-contaminated calcareous soil was mixed at different ratios with an uncontaminated, but otherwise similar soil, to establish a gradient of soil metal contamination levels. In a field experiment, poplars with different stand ages (3, 5, and 7 years) were sampled randomly in a wastewater-irrigated field. The concentrations of cadmium (Cd), Cu, lead (Pb), and zinc (Zn) in the poplar tissues and soil were determined. The accumulation of Cd and Zn was greatest in the leaves of P. pyramidalis, while Cu and Pb mainly accumulated in the roots. In the pot experiment, the highest tissue concentrations of Cd (40.76 mg kg(-1)), Cu (8.21 mg kg(-1)), Pb (41.62 mg kg(-1)), and Zn (696 mg kg(-1)) were all noted in the multi-metal-contaminated soil. Although extremely high levels of Cd and Zn accumulated in the leaves, phytoextraction using P. pyramidalis may take at least 24 and 16 years for Cd and Zn, respectively. The foliar concentrations of Cu and Pb were always within the normal ranges and were never higher than 8 and 5 mg kg(-1), respectively. The field experiment also revealed that the concentrations of all four metals in the bark were significantly higher than that in the wood. In addition, the tissue metal concentrations, together with the NH4NO3-extractable concentrations of metals in the root zone, decreased as the stand age increased. P. pyramidalis is suitable for phytostabilization of calcareous soils contaminated with multiple metals, but collection of the litter fall would be necessary due to the relatively high foliar concentrations of Cd and Zn.

Role of chelating agents on release kinetics of metals and their uptake by maize from chromated copper arsenate-contaminated soil

Kinetic aspects of metal release were investigated to understand the effects of synthetic chelating agents (EDTA, EDDS and NTA) and low molecular weight (LMW) organic acids (oxalic and citric acids) on the release kinetics of Cr, Cu and As in chromated copper arsenate (CCA)-contaminated soil, as well as their uptake by maize (Zea mays L.). The results showed that the release of metals from soil was better described by parabolic diffusion, power function or simple Elovich model than by the first- and second-order models, indicating a heterogeneous diffusion of metals. Synthetic chelating agents afforded a higher release of Cu than that of LMW organic acids, whereas citric acid was the most effective chelating agent for Cr and As release. The most effective treatments for stimulating metal uptake in plant shoots were EDDS for Cu, EDTA for Cr, and citric acid for As, as indicated by the removal efficiencies of 0.046%, 0.036%, and 0.004%, respectively. However, Zea mays is not an attractive species for chelate-enhanced phytoremediation of CCA-contaminated soils due to its low phytoextraction rate, even in the presence of chelating agents and ligands.

The use of a biodegradable chelator for enhanced phytoextraction of heavy metals by Festuca arundinacea from municipal solid waste compost and associated heavy metal leaching

In a column experiment with horizontal permeable barriers, the effects of a biodegradable chelator-nitrilotriacetic acid (NTA) on the uptake of heavy metals from municipal solid waste (MSW) compost by Festuca arundinacea and metal leaching were investigated. The use of NTA was effective in increasing Cu, Pb, and Zn uptakes in shoots of two crops of F. arundinacea. In columns with barriers and treated with 20 mmol NTA per kg MSW compost, metal uptakes by the first and second crop of F. arundinacea were, respectively, 3.8 and 4.0 times for Pb, and 1.8 and 1.7 times for Zn greater with the added NTA than without it. Though NTA application mobilized metals, it caused only slight leaching of metals from MSW compost. Permeable barriers positioned between compost and soil effectively reduced metal leaching. NTA-assisted phytoextraction by turfgrass with permeable barriers to cleanup heavy metal contaminated MSW compost should be environmentally safe.

Accumulation and spatial distribution of Cd, Cr, and Pb in mulberry from municipal solid waste compost following application of EDTA and (NH4)2SO4

Municipal solid waste compost can be used to cropland as soil amendment to supply nutrients and improve soil physical properties. But long-term application of municipal solid waste (MSW) compost may result in accumulation of toxic metals in amended soil. Phytoremediation, especially phytoextraction, is a novel, cost-effective, and environmentally friendly approach that uses metal-accumulating plants to concentrate and remove metals from contaminated soils. Ethylenediaminetetraacetate (EDTA) was applied to metal-contaminated soil to increase the mobility and phytoavailability of metals in soil, thereby increasing the amount of toxic metals accumulated in the upper parts of phytoextracting plants. The objectives of this study were (1) to investigate the accumulation and spatial distribution of toxic metals (Cd, Cr, and Pb) in mulberry from MSW compost with the application of EDTA and (NH(4))(2)SO(4), (2) to examine the effectiveness of EDTA and (NH(4))(2)SO(4) applied together on toxic metals (Cd, Cr, and Pb) removal by mulberry under field conditions, and (3) to evaluate the potential of mulberry for phytoextraction of toxic metals from MSW compost. The tested plant-mulberry had been grown in MSW compost field for 4 years. EDTA solution at five rates (0, 50, 100, 50 mmol L(-1) + 1 g L(-1) (NH(4))(2)SO(4), and 100 mmol L(-1) + 1 g L(-1) (NH(4))(2)SO(4)) was added into mulberry root medium in September 2009. Twenty days later, the plants were harvested and separated into six parts according to plant height. Cd, Cr, and Pb contents in plant samples and MSW compost were analyzed using an atomic absorption spectrophotometer. In the same treatment, Cd, Cr, and Pb concentrations in mulberry shoot were all higher than those in root, and Cd and Pb concentrations in shoot increased from lower to upper parts, reaching the highest in leaves. Significant increases were found in toxic metal concentration in different parts of mulberry with increasing EDTA concentration, especially when combined with (NH(4))(2)SO(4). Mulberry exhibited high ability to accumulate Cd with bioconcentration factors (BCFs) higher than 1. EDTA application also significantly increased Cd BCFs. More than 30 % of metal uptake was concentrated in mulberry branches (stem of above 100 cm height) and leaves. Results presented here show that mulberry is a woody plant that has the potential of Cd phytoextraction from MSW compost by removing leaves and cutting branches. The application of EDTA combined with (NH(4))(2)SO(4) significantly enhanced the efficiency of mulberry in removing Cd from the compost medium. Adding (NH(4))(2)SO(4) into the compost will lower the risk of the exposure of environment to excessive non-biodegradable EDTA in a large-scale EDTA-assisted phytoextraction by reducing the dosage of EDTA. In China, the need for sod is increasing day by day. Sod is often produced on arable soil and sold together with soils. This would lead to the soil being infertile and the soil layer thin. After several times' production, the soil can no longer be used for cultivating crops and be destroyed. In order to fully utilize MSW compost resources and save valuable soil resources, MSW compost can be used to replace arable soil to produce sod after extraction of toxic metals in it.

Uptake of Heavy Metals by Trees: Prospects for Phytoremediation

It is known that heavy metals are taken up and translocated by plants to different degrees. Phytoremediation, the use of plants to decontaminate soil by taking up heavy metals, shows considerable promise as a low-cost technique and has received much attention in recent years. However, its application is still very limited due to low biomass of hyperaccumulators, unavailability of the suitable plant species and long growing seasons required. Therefore, to maximize phytoextraction efficiency, it is important to select a fast-growing and high-biomass plant with high uptake of heavy metals, which is also compatible with mechanized cultivation techniques and local weather conditions. Trees in particular have a number of attributes (e.g. high biomass, economic value), which make them attractive plants for such a use. This paper reviews the potential for the phytoremediation of heavy metal-contaminated land by trees. In summary, we present the research progress of phytoremediation by trees and suggest ways in which this concept can be applied and improved.

Phytoextraction of Cd-Contaminated Soils: Current Status and Future Challenges

Cadmium (Cd) is one of the most toxic and widely distributed pollutants in the environment. Cadmium contamination of soils has posed a serious threat to safe food production in many parts of the world. The authors present a comprehensive review of present status of phytoextraction technology for cleaning up Cd-contaminated soils, based primarily on the data resulting from both laboratory and field-scale studies that have been conducted to assess or improve the Cd phytoextraction potential of various plant species in the past decade. The encouraging results of field-scale studies have provided a fundamental basis to usher phytoextraction technology into practical use to remediate slightly to moderately Cd-contaminated soils in Europe and Asia, although this technology is not yet ready for widespread application. Chelators and microorganisms tested so far seem not to contribute to the applicability of Cd phytoextraction. The major challenges for the large-scale application of Cd phytoextraction are (a) how to further improve the efficiency of Cd phytoextraction, (b) how to cut the overall costs of Cd phytoextraction, and (c) how to get greater stakeholders' acceptance of Cd phytoextraction as a reliable option.

Chemically enhanced phytoextraction of risk elements from a contaminated agricultural soil using Zea mays and Triticum aestivum: performance and metal mobilization over a three year period

Enhanced phytoextraction using EDTA for the remediation of an agricultural soil contaminated with less mobile risk elements Cd and Pb originating from smelting activities in Príbram (Czech Republic) was assessed on the laboratory and the field scale. EDTA was applied to the first years crop Zea mays. Metal mobilization and metal uptake by the plants in the soil were monitored for two additional years when Triticum aestivum was planted. The application ofEDTA effectively increased water-soluble Cd and Pb concentrations in the soil. These concentrations decreased over time. Anyhow, increased concentrations could be still observed in the third experimental year indicating a low possibility of groundwater pollution after the addition of EDTA during and also after the enhanced phytoextraction process under prevailing climatic conditions. EDTA-applications caused phytotoxicity and thereby decreased biomass production and increased Cd and Pb uptake by the plants. Phytoextraction efficiency and phytoextraction potential were too low for Cd and Pb phytoextraction in the field in a reasonable time frame (as less than one-tenth of a percent of total Cd and Pb could be removed). This strongly indicates that EDTA-enhanced phytoextraction as implemented in this study is not a suitable remediation technique for risk metal contaminated soils.

Impact of two iron(III) chelators on the iron, cadmium, lead and nickel accumulation in poplar grown under heavy metal stress in hydroponics

Poplar (Populus jacquemontiana var. glauca cv. Kopeczkii) was grown in hydroponics containing 10 μM Cd(II), Ni(II) or Pb(II), and Fe as Fe(III) EDTA or Fe(III) citrate in identical concentrations. The present study was designed to compare the accumulation and distribution of Fe, Cd, Ni and Pb within the different plant compartments. Generally, Fe and heavy-metal accumulation were higher by factor 2-7 and 1.6-3.3, respectively, when Fe(III) citrate was used. Iron transport towards the shoot depended on the Fe(III) chelate and, generally, on the heavy metal used. Lead was accumulated only in the root. The amounts of Fe and heavy metals accumulated by poplar were very similar to those of cucumber grown in an identical way, indicating strong Fe uptake regulation of these two Strategy I plants: a cultivar and a woody plant. The Strategy I Fe uptake mechanism (i.e. reducing Fe(III) followed by Fe(II) uptake), together with the Fe(III) chelate form in the nutrient solution had significant effects on Fe and heavy metal uptake. Poplar appears to show phytoremediation potential for Cd and Ni, as their transport towards the shoot was characterized by 51-54% and 26-48% depending on the Fe(III) supply in the nutrient solution.

Increase of Cd accumulation in five poplar (Populus L.) with different supply levels of Cd

The present investigation reports the results of the Cd accumulated by five Populus at six Cd supply levels (0, 0.5, 2, 6, 25, 60 mg/kg) in soils. For all tested Populus species, Cd accumulated by the leaves, stems, and roots linearly increased with increasing Cd supply levels, the higher concentration Cd treatments significantly promoted the Cd accumulation. P. hopeiensis, and P. tomentosa always performed the stronger ability of Cd accumulation than other three Populus species under different Cd supply treatments, and P. nira var. thevestina and P. leucoides had the poorer accumulation ability. Cd in soil was more intensively absorbed in the leaves and stems for all 5 Populus species, was not retained in roots and was transferred to aboveground plant tissues.

Evolution of bioavailable copper and major soil cations in contaminated soils treated with ethylenediaminedisuccinate: a two-year experiment

This study evaluates the long-term behavior of metals in soils treated with ethylenediaminedisuccinate for remediation purposes. The addition of the chelant led to a significant increase of water-extractable copper, iron, aluminum contents and their uptake by poplar. Increased concentrations of the metals were present in the soil solution even after the 2 years of the experiment (up to a 30-, 170- and 270-fold increase for copper, iron and aluminum, respectively). Therefore, soils treated with chelants must be monitored not only for the targeted metal concentrations but also for major soil cations originating from chelant-induced dissolution of soil oxides.

Influence of the application renewal of glutamate and tartrate on Cd, Cu, Pb and Zn distribution between contaminated soil and Paulownia tomentosa in a pilot-scale assisted phytoremediation study

The influence of repeated applications of tartrate (TAR) and glutamate (GLU) at 50-mmol kg(-1) of soil on Cd, Cu, Pb, and Zn distribution between a contaminated soil and Paulownia tomentosa was investigated. TAR and GLU were applied by a single or a double dosage, the latter carried out with an interval between the two applications of thirty days. The comparison of the differences in mean amounts of metals accumulated in the whole plant at the end of single and double TAR and GLU application experiments indicated the positive effect of repeated GLU applications on the accumulation of Cu, Pb, and Zn by Paulownia tomentosa as compared to untreated controls. A similar effect was not observed for the TAR treatments. When soil treated with either TAR or GLU was compared with untreated controls, no significant effect on heavy-metal concentrations in the soil solution was observed 30 days after treatment, suggesting the absence of an increase of the long-term leaching risk of heavy metals in aquifers and surface waters due to the ligand application. A cost analysis of the treatment is also reported.

Ethyl lactate-EDTA composite system enhances the remediation of the cadmium-contaminated soil by autochthonous willow (Salix x aureo-pendula CL 'J1011') in the lower reaches of the Yangtze River

In order to explore a practical approach to the remediation of the cadmium (Cd)-contaminated soil in the lower reaches of the Yangtze River, we evaluated the effects of a local willow (Salix x aureo-pendula CL 'J1011') of absorbing, accumulating, and translocating Cd; and assessed the potential of chelator ethylenediaminetetraacetic acid (EDTA) in combination with ethyl lactate for enhancing the efficiency of the willow in removing Cd in two water-culture growth chamber trials and a field one. The willow showed a high tolerance to Cd in growth chamber trial 1 where the Cd concentration in the medium reached up to 25 mg L(-1) medium, and the bioaccumulation factors (BAFs) of the shoots for Cd rose from 3.8 to 7.4 as the Cd concentration in the medium was elevated from 5 to 25 mg L(-1) medium. In growth chamber trial 2, the average Cd removal rates in two treatments with EDTA and ethyl lactate (molar ratios of EDTA to ethyl lactate=68/39 and 53.5/53.5, respectively) reached 0.71 mg d(-1) pot(-1) for the duration of Day 5-8 and 0.59 mg d(-1) pot(-1) for that of Day 8-11, which were 5- and 4-fold of their counterparts in the control, respectively. In the field trial, for the remediational duration of 45 days, three treatments-willow alone, willow combined with EDTA, and willow combined with EDTA and ethyl lactate-led to decreases in the Cd concentration in soil by 5%, 20%, and 29%, respectively; increases in that in the leaves by 14.6%, 56.7%, and 146.5%, respectively; and increases in that in the stems by 15.6%, 41.2%, and 87.4%, respectively, compared to their counterparts on Day 0. These results indicate that EDTA combined with ethyl lactate significantly enhanced the efficiency of willow in removing Cd from the soil. Therefore, a phytoextration system consisting of the autochthonous willow, EDTA, and ethyl lactate has high potential for the remediation of the Cd-polluted soil in the lower reaches of the Yangtze River.

Potential and drawbacks of EDDS-enhanced phytoextraction of copper from contaminated soils

Incubation and pot experiments using poplar (Populus nigra L. cv. Wolterson) were performed in order to evaluate the questionable efficiency of EDDS-enhanced phytoextraction of Cu from contaminated soils. Despite the promising conditions of the experiment (low contamination of soils with a single metal with a high affinity for EDDS, metal tolerant poplar species capable of producing high biomass yields, root colonization by mycorrhizal fungi), the phytoextraction efficiency was not sufficient. The EDDS concentrations used in this study (3 and 6 mmol kg(-1)) enhanced the mobility (up to a 100-fold increase) and plant uptake of Cu (up to a 65-fold increase). However, despite EDDS degradation and the competition of Fe and Al for the chelant, Cu leaching cannot be omitted during the process. Due to the low efficiency, further research should be focused on other environment-friendly methods of soil remediation.

Interactions of EDDS with Fe- and Al-(hydr)oxides

The efficiency of EDDS ([S,S]-ethylenediaminedisuccinate) in metal (phyto) extraction has been discussed in many recent papers. This study demonstrated that the presence of Fe- and Al-(hydr)oxides in soils influences the speciation of EDDS and thus can decrease the extraction of the targeted metallic contaminants (e.g., Pb, Cu, Zn). Above all, amorphous and poorly crystalline oxides (e.g., ferrihydrite) seem to significantly control dissolved Fe and Al concentrations in soils in the presence of metal-EDDS complexes and especially uncomplexed EDDS. Metals released from these minerals compete for the chelating agent and the extraction efficiency of the targeted metals is lowered. The formation of stable Cu-EDDS complexes, which are preferentially formed in soils with high Cu concentrations, results into a lower dissolution of ferrihydrite and goethite compared to free EDDS and Al-EDDS. Information about the contents of amorphous and poorly crystalline oxides in the treated soils would thus be beneficial for choosing efficient EDDS dosages.

Interactions of Trametes versicolor, Coriolopsis rigida and the arbuscular mycorrhizal fungus Glomus deserticola on the copper tolerance of Eucalyptus globulus

The presence of high levels of Cu in soil decreases the shoot and root dry weights of Eucalyptus globulus. However, higher plant tolerance of Cu has been observed in the presence of the arbuscular mycorrhizal (AM) fungus Glomus deserticola. The hyphal length of G. deserticola was sensitive to low Cu concentrations, and the percentage of AM root colonisation and the metabolic activity of the AM fungus were also decreased by Cu. Therefore, a direct effect of Cu on the development of the AM fungus inside and outside the root cannot be ruled out. E. globulus colonised by G. deserticola had higher metal concentrations in the roots and shoots than do non-mycorrhizal plants; however, the absence of a higher root to shoot metal ratio in the mycorrhizal plants (1.70+/-0.11) indicated that G. deserticola did not play a filtering/sequestering role against Cu. The saprobe fungi Coriolopsis rigida and Trametes versicolor were able to remove Cu ions from the asparagine-glucose growth media. However, plants inoculated with C. rigida and T. versicolor did not accumulate more Cu than non-inoculated controls, and the growth of the plant was not increased in the presence of these fungi. However, C. rigida increased the shoot dry weight, AM root length colonisation, and metabolic mycelial activity of plants colonised with G. deserticola in the presence of Cu; only this saprobe-AM fungus combination increased the tolerance of E. globulus to Cu. Inoculation with G. deserticola and C. rigida increased the E. globulus Cu uptake to levels reached by hyperaccumulative plants.

Phytoextraction of Cd-contaminated soil by carambola (Averrhoa carambola) in field trials

Use of metal-accumulating woody species to extract metals from heavy metal contaminated soil has received more attention. While considerable studies have focused on the phytoextraction potential of willow (Salix spp.) and poplar (Populus spp.), similar information is rare for other woody species. Carambola (Averrhoa carambola) is a high-biomass tree and has been identified as a new Cd-accumulating species. The present study aimed to evaluate the Cd phytoextraction potential of carambola under field condition. After growing in a slightly Cd-contaminated site for about 170 d, the carambola stand initiated by seed-seedling with high planting density (encoded with "HD-1yr") attained a high shoot biomass yield of 18.6 t ha(-1) and extracted 213 g Cdha(-1), resulting in a 1.6-fold higher Cd removal efficiency than that of a contrasting stand established by grafted-seedling with low planting density (5.3% vs. 2%). That is, "HD-1yr" would remove 50% of the total soil Cd with 13yr, assuming that the Cd removal efficiency would not change over time. Further, one crop of "HD-1yr" significantly decreased (63-69%) the Cd uptake by subsequent vegetables. Among the four carambola stands established using grafted-seedling, the 2-yr-old stand exhibited the highest annual Cd removal efficiency (3.7%), which was yet lower than that of "HD-1yr". These results suggested that phytoextraction of Cd by carambola (especially for "HD-1yr" stand) presented a feasible option to clean up agricultural soils slightly contaminated by Cd.

The potential of willow for remediation of heavy metal polluted calcareous urban soils

Growth performance and heavy metal uptake by willow (Salix viminalis) from strongly and moderately polluted calcareous soils were investigated in field and growth chamber trials to assess the suitability of willow for phytoremediation. Field uptakes were 2-10 times higher than growth chamber uptakes. Despite high concentrations of cadmium (>/=80 mg/kg) and zinc (>/=3000 mg/kg) in leaves of willow grown on strongly polluted soil with up to 18 mgCd/kg, 1400 mgCu/kg, 500 mgPb/kg and 3300 mgZn/kg, it is unsuited on strongly polluted soils because of poor growth. However, willow proved promising on moderately polluted soils (2.5 mgCd/kg and 400 mgZn/kg), where it extracted 0.13% of total Cd and 0.29% of the total Zn per year probably representing the most mobile fraction. Cu and Pb are strongly fixed in calcareous soils.

PHYTOREMEDIATION OF INORGANICS: REALISM AND SYNERGIES

There are very few practical demonstrations of the phytoextraction of metals and metalloids from soils and sediments beyond small-scale and short-term trials. The two approaches used have been based on using 1) hyperaccumulator species, such as Thlaspi caerulescens (Pb, Zn, Cd, Ni), Alyssum spp. (Ni, Co), and Pteris vittata (As) or 2) fast-growing plants, such as Salix and Populus spp. that accumulate above-average concentrations of only a smaller number of the more mobile trace elements (Cd, Zn, B). Until we have advanced much more along the pathway of genetic isolation and transfer of hyperaccumulator traits into productive plants, there is a high risk in marketing either approach as a technology or stand-alone solution to clean up contaminated land. There are particular uncertainties over the longer-term effectiveness of phytoextraction and associated environmental issues. Marginally contaminated agricultural soils provide the most likely land use where phytoextraction can be used as a polishing technology. An alternative and more useful practical approach in many situations currently would be to give more attention to crops selected for phytoexclusion: selecting crops that do not translocate high concentrations of metals to edible parts. Soils of brownfield, urban, and industrial areas provide a large-scale opportunity to use phytoremediation, but the focus here should be on the more realistic possibilities of risk-managed phytostabilization and monitored natural attenuation. We argue that the wider practical applications of phytoremediation are too often overlooked. There is huge scope for cross-cutting other environmental agenda, with synergies that involve the recovery and provision of services from degraded landscapes and contaminated soils. An additional focus on biomass energy, improved biodiversity, watershed management, soil protection, carbon sequestration, and improved soil health is required for the justification and advancement of phytotechnologies.

Effect of ozonation on polychlorinated biphenyl degradation and on soil physico-chemical properties

The objectives of this study were to investigate the effectiveness of ozone treatment on degradation of polychlorinated biphenyl (PCB) contaminated soils and to observe the subsequent changes in soil physico-chemical properties. Furthermore, the ability of plants to grow on the ozone-treated soils was evaluated. Soils with different physico-chemical characteristics spiked with seven PCB congeners in two different time periods were chosen. Ozonation was more efficient for PCB degradation in freshly spiked soils and the removal efficiency increased with increasing ozonation time. The highest decrease was found in the soil with a lower soil organic matter (SOM) content and a coarser soil structure indicating the substantial effect of soil characteristics on the efficiency of ozonation. The composition of individual PCB congeners changed in all treatments in terms of higher accumulation rate of highly chlorinated biphenyls with a higher ozonation time. Increased mobility of several elements, changes in SOM content and in soil pH were detected after ozonation. Vulnerability of plants to these modifications was documented on rape seedlings. No inhibition in growth during any treatment and predominantly higher concentration of PCB in non-ozonated treatments were observed. Results suggest that this method can present a promising environmental friendly remediation technology for PCB contaminated soils.