The use of maize and poplar in chelant-enhanced phytoextraction of lead from contaminated agricultural soils

Chelant-enhanced phytoextraction of heavy metals is an emerging technological approach for a non-destructive remediation of contaminated soils. The main objectives of this study were (i) to assess the extraction efficiency of two different synthetic chelating agents (ethylenediaminetetraacetic acid (EDTA) and ethylenediaminedisuccinic acid (EDDS)) for desorbing Pb from two contaminated agricultural soils originating from a mining and smelting district and (ii) to assess the phytoextraction efficiency of maize (Zea mays) and poplar (Populus sp.) after EDTA application. EDTA was more efficient than EDDS in desorbing and complexing Pb from both soils, removing as much as 60% of Pb. Maize exhibited better results than poplar when extracting Pb from the more acidic (pH approximately 4) and more contaminated (up to 1360 mg Pb kg(-1)) agricultural soil originating from the smelting area. On the other hand, poplars proved to be more efficient when grown on the near-neutral (pH approximately 6) and less contaminated (up to 200 mg Pb kg(-1)) agricultural soil originating from the mining area. Furthermore, the addition of EDTA led to a significant increase of Pb content especially in poplar leaves, proving a strong translocation rate within the poplar plants.     

Enhanced phytoextraction of Cu, Pb, Zn and Cd with EDTA and EDDS

Chemically enhanced phytoextraction has been proposed as an effective approach to removing heavy metals from contaminated soil through the use of high biomass plants. Using pot experiments, the effects of the application of EDTA, EDDS and citric acid on the uptake of Cu, Pb, Zn and Cd by corn (Zea mays L. cv. Nongda 108) and bean (Phaseolus vulgaris L. white bean) plants were studied. The results showed that EDDS was more effective than EDTA at increasing the concentration of Cu in corn and beans. The application of 5 mmol kg-1 soil EDDS to soil significantly increased concentrations of Cu in shoots, with maximum levels of 2060 and 5130 mg kg-1 DW in corn and beans, respectively, which were 45- and 135-fold higher than that in the corresponding control plants to which chelate had not been applied. Concentrations of Zn in shoots were also higher in the plants treated with EDDS than in those treated with EDTA. For Pb and Cd, EDDS was less effective than EDTA. The maximum Cu phytoextraction was found with the EDDS treatment. The application of EDTA and EDDS also significantly increased the shoot-to-root ratios of the concentrations of Cu, Pb, Zn and Cd in both plant species. The results of metal extraction with chelates showed that EDDS was more efficient at solubilizing Cu and Zn than EDTA, and that EDTA was better at solubilizing Pb and Cd than EDDS.     

Chelate-induced phytoextraction of metal polluted soils with Brachiaria decumbens

Chelate-induced phytoextraction with high biomass plant species has been proposed for the clean-up of heavy metal polluted soils. In the current work, the effect of the application of two different chelating agents, i.e. EDTA and EDDS, on the metal phytoextraction capacity of Brachiaria decumbens was studied. Although EDTA was, in general, more effective in soil metal solubilization, EDDS, a chelate less harmful to the environment, was more efficient inducing metal accumulation in B. decumbens shoots than EDTA. Indeed, in a moderately heavy metal polluted soil, EDDS caused a 2.54, 2.74 and 4.30-fold increase in Cd, Zn, and Pb shoot metal concentration, respectively, as compared to control plants. In this same soil, EDTA caused a 1.77, 1.11 and 1.87-fold increase in Cd, Zn, and Pb shoot metal concentration, respectively, as compared to control plants. EDDS was also more effective than EDTA in stimulating the translocation of metals from roots to shoots. B. decumbens plants were able to grow in the metal polluted soils showing no visible symptoms of phytotoxicity, which suggests their metal tolerance. Finally, B. decumbens, a fast-growing, high biomass, aluminum tolerant plant species, that has a well-established agronomic system, fulfills most of the requirements for chemically-induced phytoextraction.     

EDDS and EDTA-enhanced phytoextraction of metals from artificially contaminated soil and residual effects of chelant compounds

The potential of 18 different plants to be used in the chemically enhanced phytoextraction of Cu, Pb, Zn and Cd was assessed using pot experiments. Chrysanthemum coronarium L. was the species most sensitive to the application of EDTA, and had the highest enhancement of Cu and Pb concentrations in its shoots. Compared with EDTA, EDDS was more effective in enhancing the concentration of Cu in the shoots of Chrysanthemum coronarium L. and Zea mays L. grown on multi-metal contaminated soils. The EDTA-treated soil still had a significant ability to enhance the concentrations of Cu and Pb in the shoots of Zea mays L. six months after the chelant treatment. However, the EDDS-treated soil did not have any effect in enhancing the concentrations of metals in the shoots of Zea mays L. in the second crop test. The results may indicate that EDDS biodegrades more rapidly than EDTA in soil and is better in limiting potential metal leaching.     

The EDTA effect on phytoextraction of single and combined metals-contaminated soils using rainbow pink (Dianthus chinensis)

Rainbow pink (Dianthus chinensis), a potential phytoextraction plant, can accumulate high concentrations of Cd from metal-contaminated soils. The soils used in this study were artificially added with different metals including (1) CK: original soil, (2) Cd-treated soil: 10 mg Cd kg(-1), (3) Zn-treated soil: 100 mg Zn kg(-1), (4) Pb-treated soil: 1000 mg Pb kg(-1), (5) Cd-Zn-treated soil: 10 mg Cd kg(-1) and 100 mg Zn kg(-1), (6) Cd-Pb-treated soil: 10 mg Cd kg(-1) and 1000 mg Pb kg(-1), (7) Zn-Pb-treated soil: 100 mg Zn kg(-1) and 1000 mg Pb kg(-1), and (8) Cd-Zn-Pb-treated soil: 10 mg Cd kg(-1), 100 mg Zn kg(-1), and 1000 mg Pb kg(-1). Three concentrations of 2Na-EDTA solutions (0 (control), 2, and 5 mmol kg(-1) soil) were added to the different metals-treated soils to study the influence of applied EDTA on single and combined metals-contaminated soils phytoextraction using rainbow pink. The results showed that the Cd, Zn, Pb, Fe, or Mn concentrations in different metals-treated soil solutions significantly increased after applying 5 mmol EDTA kg(-1) (p<0.05). The metal concentrations in different metals-treated soils extracted by deionized water also significantly increased after applying 5 mmol EDTA kg(-1) (p<0.05). Because of the high extraction capacity of both 0.005 M DTPA (pH 5.3) and 0.05 M EDTA (pH 7.0), applying EDTA did not significantly increase the Cd, Zn, or Pb concentration in both extracts for most of the treatments. Applying EDTA solutions can significantly increase the Cd and Pb concentrations in the shoots of rainbow pink (p<0.05). However, this was not statistically significant for Zn because of the low Zn concentration added into the contaminated soils. The results from this study indicate that applying 5 mmol EDTA kg(-1) can significantly increase the Cd, Zn, or Pb concentrations both in the soil solution or extracted using deionized water in single or combined metals-contaminated soils, thus increasing the accumulated metals concentrations in rainbow pink shoots. The proposed method worked especially well for Pb (p<0.05). The application of 2 mmol EDTA kg(-1) might too low to enhance the phytoextraction effect when used in silty clay soils.     

Effects of EDTA on solubility of cadmium, zinc, and lead and their uptake by rainbow pink and vetiver grass

Rainbow pink (Dianthus chinensis), a potential phytoextraction plant, can accumulate high concentrations of Cd from contaminated soils. Vetiver grass (Vetiver zizanioides) has strong and long root tissues and is a potential phytostabilization plant since it can tolerate and grow well in soils contaminated with multiple heavy metals. Soil was moderately artificially contaminated by cadmium (20 mg/kg), zinc (500 mg/kg), and lead (1000 mg/kg) in pot experiments. Three concentrations of Na2-EDTA solution (0, 5, and 10 mmol/kg soil) were added to the contaminated soils to study the influence of EDTA solution on phytoextraction by rainbow pink or phytostabilization by vetiver grass. The results showed that the concentrations of Cd, Zn, and Pb in a soil solution of rainbow pink significantly increased following the addition of EDTA (p < 0.05). The concentrations of Cd and Pb in the shoots of rainbow pink also significantly increased after EDTA solution was applied (p < 0.05), but the increase for Zn was insignificant. EDTA treatment significantly increased the total uptake of Pb in the shoot, over that obtained with the control treatment (p < 0.001), but it did not significantly increase the total uptake of Cd and Zn. The concentrations of Zn and Pb in the shoots of rainbow pink are significantly correlated with those in the soil solution, but no relationship exists with concentrations in vetiver grass. The toxicity of highly contaminating metals did not affect the growth of vetiver grass, which was found to grow very well in this study. Results of this study indicate that rainbow pink can be considered to be a potential phytoextraction plant for removing Cd or Zn from metal-contaminated soils, and that vetiver grass can be regarded as a potential phytostabilization plant that can be grown in a site contaminated with multiple heavy metals.     

Enhanced phytoextraction of Pb and other metals from artificially contaminated soils through the combined application of EDTA and EDDS

Chemically enhanced phytoextraction is achieved by the application of chelates to soils. Using pot experiments, the effect of the combined application of EDTA and EDDS on the uptake of Cu, Pb, Zn and Cd by Zea mays L. was studied. Among the tested application ratios of 1:1, 1:2, and 2:1 (EDTA/EDDS), 2:1 of EDTA:EDDS was the most efficient ratio for increasing the concentrations of Cu, Pb, Zn and Cd in the shoots. The combined application of 3.33 mmol kg(-1) soil of EDTA+1.67 mmol kg(-1) soil of EDDS produced 650 mg kg(-1) of Pb in the shoots, which was 2.4 and 5.9 times the concentration of Pb in the shoots treated with 5 mmol kg(-1) of EDTA and EDDS alone, respectively. The total phytoextraction of Pb reached 1710 microg kg(-1) soil, which was 2.1 and 6.1 times the total Pb from 5 mmol kg(-1) EDTA and EDDS alone, respectively. The combined application of EDTA and EDDS also significantly increased the translocation of Pb from the roots to the shoots. The mechanism of enhancing the phytoextraction of Pb by the combined application of EDTA+EDDS did not involve a change in the pH of the soil. The increase in the phytoextraction of Pb by the shoots of Z. mays L. was more pronounced than the increase of Pb in the soil solution with the combined application of EDTA and EDDS. It was thought that the major role of EDDS might be to increase the uptake and translocation of Pb from the roots to the shoots of plants.     

The influence of EDDS and EDTA on the uptake of heavy metals of Cd and Cu from soil with tobacco Nicotiana tabacum

Phytoextraction, the use of plants to extract contaminants from soils and groundwater, is a promising approach for cleaning up soils contaminated with heavy metals. In order to enhance phytoextraction the use of chelating agents has been proposed. This study aims to assess whether ethylene diamine disuccinate (EDDS), a biodegradable chelator, can be used for enhanced phytoextraction purposed, as an alternative to ethylene diamine tetraacetate (EDTA). EDDS revealed a higher toxicity to tobacco (Nicotiana tabacum) in comparison to EDTA, but no toxicity to microorganisms. The uptake of Cu was increased by the addition of EDTA and EDDS, while no increase was observed in the uptake of Cd. Both chelating agents showed a very low root to shoot translocation capability and the translocation factor was lower than the one of the control. Heavy metals where significantly more phytoavailable than in the control, even after harvesting, resulting in a high heavy metal leaching possibility, probably owing to a low biodegradation rate of EDDS. New seedlings which were transplanted into the EDDS treated pots 7d after the phytoextraction experiment, showed signs of necrosis and chlorosis, which resulted in a significantly lower biomass in comparison to the control. The seedlings on the EDTA treated pots showed no toxicity signs. Contrary to previous opinions the results of this study revealed the chelating agents EDTA and EDDS as unsuitable for enhanced phytoextraction using tobacco.     

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.     

Role of microbial inoculation and chitosan in phytoextraction of Cu, Zn, Pb and Cd by Elsholtzia splendens--a field case

A field experiment was carried out to study the effect of microbial inoculation on heavy metal phytoextraction by Elsholtzia splendens and whether chitosan could have a synergistic effect with the microbial inocula. The microbial inocula consisted of a consortium of arbuscular mycorrhizal fungi and two Penicillium fungi. Three treatments were included: the control, inoculation with microbial inocula, and the inoculation combined with chitosan. Microbial inoculation increased plant biomass especially shoot dry weight, enhanced shoot Cu, Zn and Pb concentrations but did not affect Cd, leading to higher shoot Cu, Zn, Pb and Cd uptake. Compared with microbial inoculation alone, chitosan application did not affect plant growth but increased shoot Zn, Pb and Cd concentrations except Cu, which led to higher phytoextraction efficiencies and partitioning to shoots of Zn, Pb and Cd. These results indicated synergistic effects between microbial inocula and chitosan on Zn, Pb and Cd phytoextraction.     

Leaching of heavy metals from contaminated soils using EDTA

Ethylenediaminetetraacetic acid (EDTA) extraction of Zn, Cd, Cu and Pb from four contaminated soils was studied using batch and column leaching experiments. In the batch experiment, the heavy metals extracted were virtually all as 1:1 metal-EDTA complexes. The ratios of Zn, Cd, Cu and Pb of the extracted were similar to those in the soils, suggesting that EDTA extracted the four heavy metals with similar efficiency. In contrast, different elution patterns were obtained for Zn, Cd, Cu and Pb in the column leaching experiment using 0.01 M EDTA. Cu was either the most mobile or among the most mobile of the four heavy metals, and its peak concentration corresponded with the arrival of full strength EDTA in the leachate. The mobility of Zn and Cd was usually slightly lower than that of Cu. Pb was the least mobile, and its elution increased after the peaks of Cu and Zn. Sequential fractionations of leached and un-leached soils showed that heavy metals in various operationally defined fractions contributed to the removal by EDTA. Considerable mobilisation of Fe occurred in two of the four soils during EDTA leaching. Decreases in the Fe and Mn oxide fraction of heavy metals after EDTA leaching occurred in both soils, as well as in a third soil that showed little Fe mobilisation. The results suggest that the lability of metals in soil, the kinetics of metal desorption/dissolution and the mode of EDTA addition were the main factors controlling the behaviour of metal leaching with EDTA.     

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 (NH₄)₂SO₄, (2) to examine the effectiveness of EDTA and (NH₄)₂SO₄ 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 (NH₄)₂SO₄, and 100 mmol L^?1?+?1 g?L^?1 (NH₄)₂SO₄) 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 (NH₄)₂SO₄. 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 (NH₄)₂SO₄ significantly enhanced the efficiency of mulberry in removing Cd from the compost medium. Adding (NH₄)₂SO₄ 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.     

Metal associations in soils before and after EDTA extractive decontamination: implications for the effectiveness of further clean-up procedures

The distribution of Pb, Ni and Zn in two contaminated soils was determined before and after treating the soils with an EDTA solution. After the EDTA extraction, the proportion of Pb accumulated in the acid-extractable fraction considerably increased, which was related to the greater degree of metal extraction from the other fractions. EDTA was also able to extract certain amounts of Pb, Zn and Ni from the silicate matrix, which implied that these extractable amounts were not so strongly fixed to the residual fraction as previously supposed. As a consequence, after EDTA application, metal content (especially Pb) remained more weakly adsorbed to soil components (more easily leachable), potentially favouring the application of phytoremediation technologies. The extraction recoveries (for only one application) were generally low for the three metals (33-37% for Pb, 5-11% for Ni and 14-19% for Zn), although this fact is an advantage as plants would not be able to assimilate very high mobilised contents of metals.     

Comparison of EDTA and EDDS as potential soil amendments for enhanced phytoextraction of heavy metals

Phytoextraction has been proposed as an alternative remediation technology for soils polluted with heavy metals or radionuclides, but is generally conceived as too slow working. Enhancing the accumulation of trace pollutants in harvestable plant tissues is a prerequisite for the technology to be practically applicable. The chelating aminopolycarboxylic acid, ethylene diamine tetraacetate (EDTA), has been found to enhance shoot accumulation of heavy metals. However, the use of EDTA in phytoextraction may not be suitable due to its high environmental persistence, which may lead to groundwater contamination. This paper aims to assess whether ethylene diamine disuccinate (EDDS), a biodegradable chelator, can be used for enhanced phytoextraction purposes. A laboratory experiment was conducted to examine mobilisation of Cd, Cu, Cr, Ni, Pb and Zn into the soil solution upon application of EDTA or EDDS. The longevity of the induced mobilisation was monitored for a period of 40 days after application. Estimated effect half lives ranged between 3.8 and 7.5 days for EDDS, depending on the applied dose. The minimum observed effect half life of EDTA was 36 days, while for the highest applied dose no decrease was observed throughout the 40 day period of the mobilisation experiment. Performance of EDTA and EDDS for phytoextraction was evaluated by application to Helianthus annuus. Two other potential chelators, known for their biodegradability in comparison to EDTA, were tested in the plant experiment: nitrilo acetic acid (NTA) and citric acid. Uptake of heavy metals was higher in EDDS-treated pots than in EDTA-treated pots. The effects were still considered insufficiently high to consider efficient remediation. This may be partly due to the choice of timing for application of the soil amendment. Fixing the time of application at an earlier point before harvest may yield better results. NTA and citric acid induced no significant effects on heavy metal uptake.     

Leaching and uptake of heavy metals by ten different species of plants during an EDTA-assisted phytoextraction process

In a pot experiment, the potential use of 10 plant species, including six dicotyledon species and four monocotyledon species, was investigated for the EDTA-enhanced phytoextraction of Pb from contaminated soil. Mung bean and buckwheat had a higher sensitivity to the EDTA treatment in soils. In the 2.5 and 5.0 mmol kg(-1) EDTA treatments, the Pb concentrations in the shoots of the six dicotyledon species ranged from 1,000 to 3,000 mg kg(-1) of dry matter, which were higher than those of the monocotyledon species. The highest amount of phytoextracted Pb (2.9 mg Pb pot(-1)) was achieved in sunflowers, due to the high concentration of Pb in their shoots and large biomass, followed by corns (1.8 mg Pb pot(-1)) and peas (1.1 mg Pb pot(-1)). The leaching behavior of heavy metals as a result of applying EDTA to the surface of the soil was also investigated using short soil-leaching columns (9.0-cm diameter, 20-cm height) by the percolation of artificial rainfall. About 3.5%, 15.8%, 13.7% and 20.6% of soil Pb, Cu, Zn and Cd, respectively, were leached from the soil columns after the application of 5.0 mmol kg(-1) of EDTA. The growth of sunflowers in the soil columns had little effect on the amount of metals that were leached out. This was probably due to the shallowness of the layer of soil, the short time-span of the uptake of metals by the plant and the plant's simple root systems.     

Mobility of Zn, Cd and Pb in soils as affected by poultry litter extract--I. Leaching in soil columns

Application of poultry litter to cropland may increase metal mobility, because the soluble organic ligands in poultry litter can form water-soluble complexes with metals. In this study, one uncontaminated soil and two metal-contaminated soils were sampled. A portion of the uncontaminated soil was amended with Zn, Pb, and Cd at rates of 400, 200, and 8 mg kg(-1), respectively. Packed soil columns were leached with H2O, EDTA, CaCl2, and poultry litter extract (PLE) solutions separately. No leaching of Zn, Cd, and Pb with the PLE was found in the uncontaminated soil. The retention of PLE-borne Zn indicated the potential for Zn accumulation in the soil. A large portion of the metals was leached from the metal-amended soil, and EDTA solubilized more Zn, Cd, and Pb than CaCl2 and PLE. In the metal-contaminated soils, the leaching of Zn and Cd with PLE was consistently larger than that for CaCl2, indicating that these metals were mobilized by organic ligands. The PLE did not mobilize Pb in these soils. The utilization of poultry litter in metal-contaminated soils might accelerate the movement of Zn and Cd in soil profiles.     

Leaching variations of heavy metals in chelator-assisted phytoextraction by Zea mays L. exposed to acid rainfall

Chelant-enhanced phytoextraction method has been put forward as an effective soil remediation method, whereas the heavy metal leaching could not be ignored. In this study, a cropping-leaching experiment, using soil columns, was applied to study the metal leaching variations during assisted phytoextraction of Cd- and Pb-polluted soils, using seedlings of Zea mays, applying three different chelators (EDTA, EDDS, and rhamnolipid), and artificial rainfall (acid rainfall or normal rainfall). It showed that artificial rainfall, especially artificial acid rain, after chelator application led to the increase of heavy metals in the leaching solution. EDTA increased both Cd and Pb concentrations in the leaching solution, obviously, whereas EDDS and rhamnolipid increased Cd concentration but not Pb. The amount of Cd and Pb decreased as the leaching solution increased, the patterns as well matched LRMs (linear regression models), with R-square (R ²) higher than 90 and 82% for Cd and Pb, respectively. The maximum cumulative Cd and Pb in the leaching solutions were 18.44 and 16.68%, respectively, which was amended by EDTA and acid rainwater (pH 4.5), and followed by EDDS (pH 4.5), EDDS (pH 6.5), rhamnolipid (0.5 g kg⁻¹ soil, pH 4.5), and rhamnolipid (pH 6.5).

     

The effects of exogenous plant growth regulators in the phytoextraction of heavy metals

The term "assisted phytoextraction" usually refers to the process of applying a chemical additive to contaminated soil in order to increase the metal uptake by crop plants. In this study three commercially available plant growth regulators (PGRs) based on cytokinins (CKs) were used to boost the assisted phytoextraction of Pb and Zn in contaminated soil collected from a former manufactured gas-plant site. The effects of EDTA treatment in soil and PGR treatment in leaves of Helianthus annuus were investigated in terms of dry weight biomass, Pb and Zn accumulation in the upper parts of the plants, Pb and Zn phytoextraction efficiency and transpiration rate. Metal solubility in soil and its subsequent accumulation in shoots were markedly enhanced by EDTA. The combined effects of EDTA and cytokine resulted in an increase in the Pb and Zn phytoextraction efficiency (up to 890% and 330%, respectively, compared to untreated plants) and up to a 50% increase in foliar transpiration. Our results indicate that exogenous PGRs based on CKs can positively assist the phytoextraction increasing the biomass production, the metal accumulation in shoots and the plant transpiration. The observed increase in biomass could be related to its action in stimulation of cell division and shoot initiation. On the other hand, the increase in metal accumulation in upper parts of plant could be related to both the role of PGRs in the enhancement of plant resistance to stress (as toxic metals) and the increase in transpiration rate, i.e. flux of water-soluble soil components and contaminants by the regulation of stomatal opening.     

Recycling EDTA solutions used to remediate metal-polluted soils

The objective of this research was to investigate the recycling of ethylenediamine-tetraacetic acid (EDTA) used for the removal of trace metals from contaminated soils. We successfully used Na2S combined with Ca(OH)2 to precipitate the trace metals allowing us to recycle the EDTA. The results of batch and column leaching experiments show that both Ca-EDTA and Na-EDTA are powerful chelating agents with a similar soil remediation potential. The major advantage of Ca-EDTA is the preservation of soil organic matter. We found that Na2S was capable of separating the metals Cd, Cu and Pb from EDTA; however, the precipitation of Zn required the addition of Ca(OH)2. After reusing the reclaimed EDTA seven times, over a 14-day period, EDTA reagent losses ranged from 19.5% to 23.5%. Successive washing cycles enhanced the removal of trace metals from contaminated soils. The metal sulfide precipitates contain high concentrations of metals and could potentially be recycled.     

The use of NTA and EDDS for enhanced phytoextraction of metals from a multiply contaminated soil by Brassica carinata

The potential of nine different species to grow in the presence of metals (As, Cd, Cu, Pb and Zn) and to accumulate them in the shoots was assessed for each metal separately by germination and root length tests, and successively by hydroponic experiments. Of the nine species tested, Brassica carinata was the species that accumulated the highest amounts of metals in shoots without suffering a significant biomass reduction. To further evaluate the potential of B. carinata for chelant-enhanced phytoextraction of a natural, multiply metal-polluted soil (As, Cd, Cu, Pb and Zn), both hydroponic and pot experiments were carried out with nitrilotriacetic acid (NTA) or (S,S)-ethylenediamine disuccinic acid (EDDS) as complexing agents. The hydroponic study with solutions containing the five metals together showed that accumulation of Cd, Cu, Pb and Zn in shoots was higher following EDDS addition compared to NTA. EDDS was more effective than NTA in desorbing Cu, Pb and Zn from the soil, whereas As and Cd were poorly extracted. B. carinata plants were grown for 4 weeks in the multiply metal-contaminated soil and then the soil was amended with 5 mmol kg(-1) NTA or EDDS. All plants were harvested 1 week after amendment. In comparison to NTA, EDDS was more effective in enhancing the concentrations of Cu, Pb and Zn in B. carinata shoots (2- to 4-fold increase compared to the control). One week after chelant addition, the DTPA-extractable metal concentrations in the polluted soil were lower in the EDDS treatment in comparison with the NTA amendment. Even though B. carinata showed a reduced growth and a relatively low metal uptake, it demonstrated the ability to survive and tolerate the presence of more metals simultaneously.