Enhanced phytoextraction: II. Effect of EDTA and citric acid on heavy metal uptake by Helianthus annuus from a calcareous soil

Citric acid-assisted phytoextraction of trace elements in composted municipal sludge by garden plants

Sludge landscaping after compost stabilization is a popular recycling process; however, until trace elements (TEs) are extracted by plants and reduced to safe concentrations, they present a potential exposure risk. Three garden plants, Liriope platyphylla Wang et Tang (L. platyphylla), Iris tectorum Maxim (I. tectorum), and Photinia x fraseri Dress (P. x fraseri), were selected for field experiments, and their ability to phytoremediate TEs and the promotion effect of citric acid (CA) were studied over 3 months of observation. Among the three kinds of plants, L. platyphylla had the highest biomass per unit soil area, and the CA treatment further increased the biomass of this plant per unit soil area as well as the uptake of TEs. When treated with 3 mmol kg^-1 CA, L. platyphylla showed increases in the bioconcentration factors of Cu, Zn, Pb, and Cd by 24%, 63%, 27%, and 123%, respectively. Because of the large biomass and high concentrations of TEs, L. platyphylla had high phytoremediation indexes for Zn, Cu, Pb, Ni, and Cd, which reached 18.5, 3.7, 3.2, 2.2, and 0.4 mg m^-2, respectively, and were further improved by 60%-187% by the CA treatment. These advantages indicate the potential usefulness of L. platyphylla for phytoremediation. The results provide basic data and technical support for the use of sludge-based compost and phytoremediation by garden plants.

Assessment of biodegradable chelating agents in the phytoextraction of heavy metals from multi-metal contaminated soil

A pot incubation experiment under natural conditions was designed to investigate the effects of three biodegradable chelating agents, namely; the [S,S]-isomer of ethylenediamine disuccinate (EDDS), citric acid (CA), and tetrasodium N,N-Bis(carboxymethyl)-L-glutamate acid (GLDA), on two plant species (Brassica juncea and Brassica rapa) in terms of plant foliar growth, dry matter yield, and heavy metal (HM) accumulation. Both plant species exhibited diminished growth and symptoms of phytotoxicity under HM stress. The application of EDDS and CA affected plant foliar growth, biomass production, and led to the development of chlorotic lesions on leaves. EDDS and CA also decreased the shoot length by 38.5% and 45.2% in B. juncea, and 60.1% and 100% in B. rapa, respectively. In contrast, GLDA relieved HM stress by significantly increasing plant growth (P > 0.05) and was shown to be well tolerated (tolerance index [TI]; B. juncea = 99% and B. rapa = 123%). Among both plants, B. juncea displayed the ability to accumulate a wider range of HMs at higher concentrations. Amongst the three chelators, EDDS induced the highest bioconcentration (BCF) of Pb (2.45), Zn (2.68), and Cd (3.36) while CA achieved better results for Ni (4.01) and Cr (1.45). However, the current results showed that even with the application of chelating agents, HMs were predominantly accumulated in roots and translocation factor was generally <1. The findings of this investigation emphasize that chelate-assisted phytoextraction with Brassica spp. is highly limited in multi-metal settings, making it an unsuitable option for severely contaminated sites.

Cadmium uptake and transfer by Sedum plumbizincicola using EDTA, tea saponin, and citric acid as activators

Sedum plumbizincicola (S. plumbizincicola) is known as a sufficient plant for phytoremediation of cadmium (Cd) polluted soils. This study aimed to investigate the effects of ethylene diamine tetraacetic acid (EDTA), tea saponin (TS), and citric acid (CA) on Cd uptake and translocation by S. plumbizincicola. To do so, using a pot experiment, we set four concentration levels of activators (1, 3, 5, and 10 mmol L^-1) and a control (CK). Results showed that none of the applied activators had significant impact on soil pH. Except for CA-10, the concentration of available Cd in Cd polluted soils increased by 65.8-72.9% compared with CK. The EDTA-1, CA-1, and TS-5 treatments caused significant increases of 52.3, 67.2, and 38.4%, respectively, in the biomass of aerial parts of S. plumbizincicola (p < 0.05) compared with CK. Except for CA-3, activators increased Cd accumulation in the aerial parts of plants by 47-124% compared with CK. Of all activators, EDTA-3 caused the highest Cd accumulation of 6.64 g pot^-1 in the aerial plant tissues followed by CA-10 (6.25 g pot^-1) and TS-1 (5.48 g pot^-1). Finally, our results suggested that the application of S. plumbizincicola together with different activators sufficiently reduced soil total Cd by 4.64-48.4% compared with CK. These findings suggest that appropriate application of EDTA, TS, and CA can promote phytoremediation of Cd contaminated soils by hyper-accumulators. In particular, the combined use of EDTA and S. plumbizincicola is an affordable and promising strategy for remediation of Cd contaminated soil.Novelty statement: Sedum plumbizincicola (S. plumbizincicola) is a well-known hyper-accumulator plant for remediation of cadmium (Cd) and zinc (Zn) contaminated soils. In addition, low molecular rganic acids and macromolecular chelating agents can improve the solubility and leaching of soil heavy metals. In the present work, we examined the combined effects of three activators (EDTA, tea saponin, and citric acid) with S. plumbizincicola to remediate a Cd contaminated soil in Anhui Province, East China. Our results indicated the effectiveness of these activators to increase soil available Cd, as well as improving the biomass of S. plumbizincicola and its Cd uptake. We believe that this study provides an efficient approach to increase the uptake of Cd by S. plumbizincicola, restoring Cd contaminated soils. Nevertheless, excessive activators may have adverse effects on soil aggregates and soil microorganisms. Therefore, it is necessary to control the amount of chelating agents and subsequently the deterioration of soil quality.

Efficacy of Zea mays L. for the management of marble effluent contaminated soil under citric acid amendment; morpho-physiological and biochemical response

The adverse industrial activities discharged contaminated wastewater directly into the water bodies that contain toxic substances such as heavy metals. The contours use of marble industrial effluents may affect the fertility of soil and crop growth. The present study was conducted to investigate the toxic effects of marble industrial effluents (M.E) on Zea mays L under the exogenous application of citric acid (CA) with different combinations such as marble industrial effluent (0, 30%, 60%, 100%) diluted with distilled water and CA (10 mM). The results showed significant decrease in the growth of Zea mays with increasing concentration of marble industrial effluent. The maximum reduction in plant height, root length, number of leaves, leaf area and fresh and dry biomass was observed at the application of 100% M.E as compared to control. Similar to growth conditions the photosynthetic machinery and the activities of antioxidant enzymes (Superoxide dismutase (SOD), Peroxidases (POD), Catalases (CAT), Ascorbate peroxidase (APX)) was also decreased with increasing concentration of M.E. The application of CA significantly alleviated the M.E induced toxic effect on Zea mays and ameliorated the growth, biomass, photosynthesis and antioxidant enzymes activities by reducing the production of reactive oxygen species. The C.A application also enhanced the heavy metal content such as chromium (Cr), cadmium (Cd), Zinc (Zn) in different parts of Zea mays. The results concluded that the Zea mays tolerant varieties can be a potential candidate for the M.E irrigated soil and might be suitable for the phyto-extraction of Cr, Cd and Zn.

Effects of biodegradable chelator combination on potentially toxic metals leaching efficiency in agricultural soils

Soil washing with chelators, a viable method for treating soils contaminated with potentially toxic metals, has drawn increasing attentions. The objective of this study was to determine a new generation of mixed degradable chelating agents from N, N-bis (carboxymethyl) glutamic acid (GLDA), [S, S]-stereoisomer of ethyleneiaminedisucc--inic acid (EDDS), nitrilotriacetic acid (NTA), and citric acid (CA), and to evaluate its effectiveness and feasibility to reduce toxic metals contamination in two different agricultural soils. A comparative leaching test conducted on the four individual degradable chelating agents showed that the capacity of single chelator in mobilizing copper (Cu), zinc (Zn), cadmium (Cd), and lead (Pb) varied significantly. Using a combination of GLDA and NTA was more advantageous than using a single chelating agent in extracting potentially toxic metals. The removal efficiencies of Cu, Zn, Cd, and Pb reached 38.2, 9.8, 71.4, and 19.5% for soil 1, and 25.0, 5.2, 59.7, and 18.5% for soil 2, respectively, at mixed chelator (MC) concentrations of 3 mmol/L (GLDA) and 2 mmol/L (NTA), pH of 6.0, and a contact time of 4.0 h. The effects of washing conditions, chelator concentration, pH values, and contact time on the removal efficiencies of target toxic metals were investigated. The results showed that the combined chelating agent has a lower pH dependence, making it feasible for a wider range of applications. The effects of the chelating agents on the morphological distribution of potentially toxic metals and the soil enzyme activity before and after the treatments were also studied. After washing, the content of the water-soluble, acid-soluble, reducible, and oxidizable target metals showed a certain degree of decrease. Although the activities of catalase, urease, and invertase appeared to be inhibited during a short period of time, their activities were stimulated and later promoted with the degradation of the chelating agent. In general, the chelating agent combination has a great potential for toxic metals leaching.

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.

Citric acid as soil amendment in cadmium removal by Salix viminalis L., alterations on biometric attributes and photosynthesis

During the past decade, the target in cleaning polluted sites is an application of chelating agents, such as citric acid (CA), which is proposed as a good candidate in the promotion of phytoremediation. Among heavy metals, cadmium (Cd) is one of the most common and dangerous elements, which strongly disturbs morphophysiological properties in plants. A pot experiment was assessed to evaluate the influence of CA in Cd phytoremediation in alkaline soil by Salix viminalis (clone SV068). The effects of CA on Cd bioavailability, mobility, and distribution in plants, various morphometric measurements, and physiological performances as photosynthesis, transpiration, water use efficiency, and pigment content were tested. The highest Cd accumulation was evident after 60 days of growing, in plants subjected to combined treatment of CA with a higher dose of Cd. Application of CA showed a beneficial effect in maintaining the photosynthetic rate as well as gas exchange capacity in willows grown in Cd-contaminated soil. Furthermore, CA slightly increased plant growth and biomass production, depending on applied Cd dose and harvest period. A chelating agent like CA showed strong influence in plant response to combat Cd toxicity.

Chelate-assisted phytoextraction of lead using Fagopyrum esculentum: laboratory vs. field experiments

The development of more sustainable remediation techniques has been receiving greater attention, as an alternative to soil excavation plan in urban gardens. An in situ phytoextraction experiment with buckwheat (Fagopyrum esculentum) was performed with a 5 mmol kg^-1 citric acid (CA) application. Joint experiments under laboratory conditions were conducted using various cultivars of F. esculentum in two soils with a Pb contamination of either geogenic or anthropogenic origin and various chelate concentrations. Results show that a minimum dose of 50 mmol kg^-1 of CA is required to lower soil pH and raise the concentration of mobile Pb-CaCl₂ for both soils. Consequently, Pb shoot uptake is increased from 6.3 to 8.9 times depending on soil type. Phytoextraction efficiency is found to be 1.3 to 2.0 times higher in the anthropogenic contaminated soil than in the soil with geogenic Pb. A scale effect has also been identified since Pb root accumulation under laboratory conditions was 2.4 times higher than in the field experiment. Despite an increase in the Pb extraction rate with CA, buckwheat appears to lack the efficiency needed to remove Pb in moderately contaminated soils. The calculated remediation period would last 166 years to remove the mobile Pb fraction.

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 application of plant growth regulators to improve phytoremediation of contaminated soils: A review

Soil contamination is one of the most important environmental problems around the world. The transfer of organic contaminants and heavy metals to the food chain is a major threat to human health. Purging these contaminants often involves a lot of energy and complex engineering processes. Phytoremediation technology can be used in various environments, such as water, soil, and air, to reduce or eliminate different contaminants. The major mechanisms involved in phytoremediation include plant extraction, rhizofiltration, plant evaporation, plant stabilization, plant decomposition, and rhizosphere degradation. The efficiency of phytoremediation can be increased through using chelating and acidifying agents, applying electric current in the soil, using organic chemicals and fertilizers, planting transgenic plants, using bacteria, and applying plant growth regulators. Recently, the use of plant growth regulators has been investigated as a suitable method for improving the efficacy of phytoremediation. Effective plant growth regulators to improve phytoremediation include auxins, gibberellins, cytokinins, and salicylic acid. The activity of these materials depends on their concentration, environmental factors that affect their absorption, and the physiological state of the plant. Using these materials increases the biomass of the plant and reduces the negative effects of the presence of contaminants in the plant. The present study aimed to review the latest studies performed on the improvement of phytoremediation using plant growth regulators and their mechanisms.

Low-Cd tomato cultivars (Solanum lycopersicum L.) screened in non-saline soils also accumulated low Cd, Zn, and Cu in heavy metal-polluted saline soils

Many reclaimed tidal flat soils feature high salinity and heavy metal (HM) accumulation. Consumption of vegetables cultivated in this type of cropland may cause health risks. Low-Cd tomato cultivars (Solanum lycopersicum L.) were identified in non-saline soil in our previous studies (Tan et al. 2014). However, further research should determine whether these low-Cd cultivars will maintain in the repeatability and stability in saline soil and whether they have low accumulation abilities for accompanying metals (such as Zn and Cu). A soil-pot trial was implemented to measure Cd, Zn, and Cu concentrations in low- and high-Cd cultivars of both common and cherry-type tomatoes grown on HM-polluted reclaimed tidal flat saline soil. Then, cultivar differences in dissolution of Cd, Zn, and Cu in soil and their uptake and redistribution in plants were analyzed. Results showed that the cherry type accumulated more Cd, Zn, and Cu than the common type. Low-Cd cultivars of both types in saline soil accumulated low concentrations of Cd, Zn, and Cu in fruits. Low HM accumulation in fruits is partly attributed to a low root/shoot (R/S) biomass ratio. Low amounts of soil HMs were dissolved because of the low level of rhizosphere organic compounds, which possibly decreased HM uptake by the roots. Low-Cd cultivars of both tomato types had a higher ability to retain HMs in the roots than their high-Cd cultivars. These findings may provide a scientific guidance for the safe cultivation of HM-polluted saline soils.

Phytoextraction of 55-year-old wastewater-irrigated soil in a Zn-Pb mine district: effect of plant species and chelators

Untreated water from mining sites spreads heavy metal contamination. The present study assessed the phytoextraction performance of heavy metal-accumulating plants and the effects of chemical chelators on cadmium (Cd), lead (Pb), zinc (Zn), and copper (Cu) removal from paddy fields that have been continuously irrigated with mining wastewater from mines for 55 years. Outdoor pot experiments showed that the total Pb, Zn, and Cd content was lower in the rhizosphere soil of Amaranthus hypochondriacus than in that of Sedum alfredii, Solanum nigrum, and Sorghum bicolor. The aboveground biomass (dry weight) and relative growth rate of A. hypochondriacus were significantly higher than that of the other three species (P < .05). However, the total metal accumulation was significantly higher in the A. hypochondriacus system than in the other plants' system (P < .05). The increase in shoot biomass of A. hypochondriacus depended mostly on the chelator type [ethylenediaminetetraacetic acid (EDTA), malate, oxalate, and citrate] and their application frequency. Single application of EDTA significantly increased the shoot biomass of A. hypochondriacus and total metal removal loading from soil (P < .05). In conclusion, A. hypochondriacus may be effective for in situ phytoremediation of heavy metal-contaminated farmland soil and EDTA can accelerate the phytoextraction effect.

Citric acid assisted phytoextraction of chromium by sunflower; morpho-physiological and biochemical alterations in plants

Soil and water contamination from heavy metals and metalloids is one of the most discussed and burning global issues due to its potential to cause the scarcity of healthy food and safe water. The scientific community is proposing a range of lab and field based physical, chemical and biological solutions to remedy metals and metalloids contaminated soils and water. The present study finds out a possibility of Chromium (Cr) extraction by sunflower from spiked soil under chelating role of citric acid (CA). The sunflower plants were grown under different concentrations of Cr (0, 5, 10 & 20mgkg^-1) and CA (0, 2.5 & 5mM). Growth, biomass, gas exchange, photosynthesis, electrolyte leakage (EL), reactive oxygen species (ROS; malondialdehyde (MDA), hydrogen peroxide (H₂O₂) and the activities of antioxidant enzymes such as, superoxide dismutase (SOD), guaiacole values peroxidase (POD), ascorbate peroxidase (APX), catalase (CAT) were measured. The results depicted a clear decline in plant height, root length, leaf area, number of leaves and flowers per plant along with fresh and dry biomass of all parts of plant with increasing concentration of Cr in soil. Similar reduction was observed in chlorophyll a and b, total chlorophyll, carotenoids, soluble protein, gas exchange attributes and SPAD. The increasing concentration of Cr also enhanced the Cr uptake and accumulation in plant roots, stem and leaves along with the production of ROS and EL. The activities of antioxidant enzymes increased with increasing Cr concentration from 0 to 10mg, but decreased at 20mgkg^-1 soil. The CA application significantly alleviated Cr-induced inhibition of plant growth, biomass, photosynthesis, gas exchange, soluble proteins and SPAD value. Presence of CA also enhanced the activities of all antioxidant enzymes and reduced the production of ROS and EL. The chelating potential of CA increased the concentration and accumulation of Cr in plant roots, stem and leaves. It is concluded that the sunflower can be a potential candidate for the remediation of Cr under CA treatment, while the possibility may vary with genotype, Cr level and CA concentration.

Impact of chelator-induced phytoextraction of cadmium on yield and ionic uptake of maize

Enhanced phytoextraction uses soil chelators to increase the bioavailability of heavy metals. This study tested the effectiveness of ethylenediaminetetraacetic acid (EDTA) and citric acid in enhancing cadmium (Cd) phytoextraction and their effects on the growth, yield, and ionic uptake of maize (Zea mays). Maize seeds of two cultivars were sown in pots treated with 15 (Cd₁₅) or 30 mg Cd kg^-1 soil (Cd₃₀). EDTA and citric acid at 0.5 g kg^-1 each were applied 2 weeks after germination. Results demonstrated that the growth, yield per plant, and total grain weight were reduced by exposure to Cd. EDTA increased the uptake of Cd in shoots, roots, and grains of both maize varieties. Citric acid did not enhance the uptake of Cd, rather it ameliorated the toxicity of Cd, as shown by increased shoot and root length and biomass. Cadmium toxicity reduced the number of grains, rather than the grain size. The maize cultivar Sahiwal-2002 extracted 1.6% and 3.6% of Cd from soil in both Cd+ EDTA treatments. Hence, our study implies that maize can be used to successfully phytoremediate Cd from soil using EDTA, without reducing plant biomass or yield.

Citric acid enhances the phytoextraction of chromium, plant growth, and photosynthesis by alleviating the oxidative damages in Brassica napus L

Chromium (Cr) toxicity is widespread in crops grown on Cr-contaminated soils and has become a serious environmental issue which requires affordable strategies for the remediation of such soils. This study was performed to assess the performance of citric acid (CA) through growing Brassica napus in the phytoextraction of Cr from contaminated soil. Different Cr (0, 100, and 500 μM) and citric acid (0, 2.5, and 5.0 mM) treatments were applied alone and in combinations to 4-week-old seedlings of B. napus plants in soil under wire house condition. Plants were harvested after 12 weeks of sowing, and the data was recorded regarding growth characteristics, biomass, photosynthetic pigments, malondialdehyde (MDA), electrolytic leakage (EL), antioxidant enzymes, and Cr uptake and accumulation. The results showed that the plant growth, biomass, chlorophyll contents, and carotenoid as well as soluble protein concentrations significantly decreased under Cr stress alone while these adverse effects were alleviated by application of CA. Cr concentration in roots, stem, and leaves of CA-supplied plant was significantly reduced while total uptake of Cr increased in all plant parts with CA application. Furthermore, in comparison with Cr treatments alone, CA supply reduced the MDA and EL values in both shoots and roots. Moreover, the activity of superoxide dismutase (SOD), guaiacol peroxidase (POD), catalase (CAT), and ascorbate peroxidase (APX) in shoots and roots markedly increased by 100 μM Cr exposure, while decreased at 500 μM Cr stress. CA application enhanced the activities of antioxidant enzymes compared to the same Cr treatment alone. Thus, the data indicate that exogenous CA application can increase Cr uptake and can minimize Cr stress in plants and may be beneficial in accelerating the phytoextraction of Cr through hyper-accumulating plants such as B. napus.

Effect of citric acid on metals mobility in pruning wastes and biosolids compost and metals uptake in Atriplex halimus and Rosmarinus officinalis

To assess metal mobility in pruning waste and biosolids compost (pH 6.9 and total concentration of metals in milligram per kilogram of Cd 1.9, Cu 132, Fe 8,513, Mn 192, Pb 81, and Zn 313), shrubs species Atriplex halimus and Rosmarinus officinalis were transplanted in this substrate and irrigated with citric acid (4 g L(-1), pH 2.9) and nutrient solution daily for 60 days. Citric acid significantly increased the concentrations of soluble Mn and Fe in the nutrient substrate solution measured by suction probes, while other metals did not vary in concentration (Cu and Zn) or were not observed at detectable levels (Cd and Pb). In plants, citric acid significantly increased the concentrations of Cu (2.7 ± 0.1-3.3 ± 0.1 mg kg(-1)), Fe (49.2 ± 5.2-76.8 ± 6.8 mg kg(-1)), and Mn (7.2 ± 1.1-11.4 ± 0.7 mg kg(-1)) in leaves of R. officinalis, whereas the concentration of only Mn (25.4 ± 0.3-42.2 ± 2.9 mg kg(-1)) was increased in A. halimus. Increasing Fe and Mn solubility by citric acid addition indicates the possibility of using it to improve plant nutrition. The mobility of metals in this substrate was influenced for the concentration of the metal, the degree of humification of organic matter and its high Fe content.

Enhancing radium solubilization in soils by citrate, EDTA, and EDDS chelating amendments

The effect of three chelating agents (citrate, EDTA, and EDDS) on the solubilization of radium from a granitic soil was studied systematically, considering different soil pH values, chelating agent concentrations, and leaching times. For all the chelating agents tested, the amount of radium leached proved to be strongly dependent on the pH of the substrate: only for acidic conditions did the amount of radium released increase significantly relative to the controls. Under the best conditions, the radium released from the amended soil was greater by factors of 20 in the case of citrate, 18 for EDTA, and 14 for EDDS. The greatest improvement in the release of radium was obtained for the citrate amendment at the highest concentration tested (50 mmol kg(-1)). A slightly lower amount of radium was leached with EDTA at 5 mmol kg(-1) soil, but the solubilization over time was very different from that observed with citrate or EDDS. With EDTA, a maximum in radium leaching was reached on the first day after amendment, while with citrate, the maximum was attained on the fourth day. With EDDS, radium leaching increased slightly but steadily with time (until the sixth day), but the net effect for the period tested was the lowest of the three reagents.

Optimization of chelators to enhance uranium uptake from tailings for phytoremediation

A greenhouse experiment was set up to investigate the ability of citric acid (CA), oxalic acid (OA), nitrilotriacetic acid (NTA) and EDTA for phytoremediation of uranium tailings by Indian mustard [Brassica juncea (L.) Czern. et Coss]. Uranium tailings were collected from Umra mining region and mixed with 75% of garden soil which yielded a 25:75 mixture. Prepared pots were divided into four sets and treated with following different concentrations - 0.1, 0.5, 2.5 and 12.5 mmol kg(-1) soil additions for each of the four chelators. Control pots which were not treated with chelators. Experiments were conducted in completely randomized block design with triplicates. The optimum concentrations of these chelators were found on the basis of biomass production, tolerance and accumulation potential. The data collected were expressed statistically. EDTA produced maximum growth depression whereas, minimum occurred in the case of NTA. Maximum U uptake (3.5-fold) in the roots occurred at 2.5 mmol of CA, while NTA proved to be the weakest for the same purpose. Severe toxicity in the form of reduced growth and plant death was recorded at 12.5 mmol of each chelator. Minimum growth inhibition produced by chelators occurred in NTA which was followed by OA, moderate in CA and maximum was traced in EDTA applications. Chelator strengthened U uptake in the present study follows the order: CA>EDTA>OA>NTA.

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.

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.

Enhancing uranium solubilization in soils by citrate, EDTA, and EDDS chelating amendments

A systematic study was made of the effects of three soil amendments on the solubilization of uranium from a granitic soil. The aim was to optimize solubilization so as to enhance bioavailability for the purposes of remediation. The three amendments tested were with citrate, EDTA, and EDDS as chelating agents. The effects of pH, chelator concentration, and leaching time were studied. The most important factor in uranium solubilization was found to be the pH. In the absence of chelating agents, the greatest solubilization was obtained for alkaline conditions, with values representing about 15% of the total uranium activity in the bulk soil. There were major differences in uranium solubilization between the different amendments. The citrate treatment was the most efficient at acidic pH, particularly with the greatest concentration of citrate tested (50 mmol kg(-1)) after 6 days of treatment. Under these conditions, the uranium concentration in solution was greater by a factor of 356 than in the control suspension, and represented some 63% of the uranium concentration in the bulk soil. Under alkaline conditions, the EDTA and EDDS treatments gave the greatest uranium activity concentrations in solution, but these concentrations were much lower than those with the citrate amendment, and were not very different from the control results. The uranium extraction yield with EDDS amendment was greater than with EDTA.

The impact of EDTA on the rate of accumulation and root/shoot partitioning of cadmium in mature dwarf sunflowers

In addition to increasing the mobility of metal ions in the soil solution, chelating agents such as EDTA have been reported to alter both the total metal accumulated by plants and its distribution within the plant structures. Here, mature Mini-Sun Hybrid dwarf sunflowers exposed to 300 μM Cd(2+) in hydroponic solution had initial translocation rates of at least 0.12 mmol kg(-1)h(-1) and reached leaf saturation levels within a day when a 3-fold molar excess of EDTA was used. EDTA also promoted cadmium transfer from roots to the shoots. A threefold excess of EDTA increased the translocation factor (TF) 100-fold, resulting in cadmium levels in the leaves of 580 μg g(-1) and extracting 1400 μg plant(-1). When plants were exposed to dissolved cadmium without EDTA, the vast majority of the metal remained bound to the exterior of the root. The initial accumulation could be successfully modeled with a standard biosorption pseudo second-order kinetic equation. Initial accumulation rates ranged from 0.0359 to 0.262 mg g(-1)min(-1). The cadmium binding could be cycled, and did not show evidence of saturation under the experimental conditions employed, suggesting it might be a viable biosorbant for aqueous cadmium.

Comparison of organic and inorganic amendments for enhancing soil lead phytoextraction by wheat (Triticum aestivum L.)

Phytoextraction has received increasing attention as a promising, cost-effective alternative to conventional engineering-based remediation methods for metal contaminated soils. In order to enhance the phytoremediative ability of green plants chelating agents are commonly used. Our study aims to evaluate whether, citric acid (CA) or elemental sulfur (S) should be used as an alternative to the ethylene diamine tetraacetic acid (EDTA)for chemically enhanced phytoextraction. Results showed that EDTA was more efficient than CA and S in solubilizing lead (Pb) from the soil. The application of EDTA and S increased the shoot biomass of wheat. However, application of CA at higher rates (30 mmol kg(-1)) resulted in significantly lower wheat biomass. Photosynthesis and transpiration rates increased with EDTA and S application, whereas these parameters were decreased with the application of CA. Elemental sulfur was ineffective for enhancing the concentration of Pb in wheat shoots. Although CA did not increase the Pb solubility measured at the end of experiment, however, it was more effective than EDTA in enhancing the concentration of Pb in the shoots of Triticum aestivum L. It was assumed that increase in Mn concentration to toxic levels in soil with CA addition might have resulted in unusual Pb concentration in wheat plants. The results of the present study suggest that under the conditions used in this experiment, CA at the highest dose was the best amendment for enhanced phytoextraction of Pb using wheat compared to either EDTA or S.

Chemically enhanced phytoextraction of Pb by wheat in texturally different soils

A pot study was used to examine the effects of amendments such as EDTA and elemental sulfur on the growth potential, gas exchange features, uptake and mobilization of Pb by wheat (Triticum aestivum L.) in two texturally different contaminated soils at three levels of EDTA (2, 4, 8 mmol kg(-1) dry soil) and two levels of elemental sulfur (100, 200 mmol kg(-1) dry soil). EDTA resulted in more solubilization of Pb than elemental sulfur in both soils. Application of EDTA and elemental sulfur increased shoot dry matter in the loamy sand soil, whereas in the sandy clay loam soil EDTA treated plants produced lower shoot dry matter compared to that observed with elemental sulfur. Application of EDTA 10d prior to harvest increased the amount of Pb accumulated into wheat shoots with more Pb accumulated by plants from the loamy sand than from the sandy clay loam soil. However, evaluation of the relative extraction efficiency expressed as the percentage of solubilized Pb that is subsequently also effectively accumulated by the plant shoots reveals that the relatively low efficiency does not warrant the massive mobilization induced by the environmentally persistent EDTA chelator. More modest mobilization of Pb induced by elemental sulfur and the higher relative extraction of mobilized Pb therefore deserves further attention in future research. In particular, attention needs to be paid to determining soil types in which elemental sulfur can induce significant impact on soil pH and metal mobility after application of a practically realistic dosage.

EDTA-assisted Pb phytoextraction

Pb is one of the most widespread and metal pollutants in soil. It is generally concentrated in surface layers with only a minor portion of the total metal found in soil solution. Phytoextraction has been proposed as an inexpensive, sustainable, in situ plant-based technology that makes use of natural hyperaccumulators as well as high biomass producing crops to help rehabilitate soils contaminated with heavy metals without destructive effects on soil properties. The success of phytoextraction is determined by the amount of biomass, concentration of heavy metals in plant, and bioavailable fraction of heavy metals in the rooting medium. In general, metal hyperaccumulators are low biomass, slow growing plant species that are highly metal specific. For some metals such as Pb, there are no hyperaccumulator plant species known to date. Although high biomass-yielding non-hyperaccumulator plants lack an inherent ability to accumulate unusual concentrations of Pb, soil application of chelating agents such as EDTA has been proposed to enhance the metal concentration in above-ground harvestable plant parts through enhancing the metal solubility and translocation from roots to shoots. Leaching of metals due to enhanced mobility during EDTA-assisted phytoextraction has been demonstrated as one of the potential hazards associated with this technology. Due to environmental persistence of EDTA in combination with its strong chelating abilities, the scientific community is moving away from the use of EDTA in phytoextraction and is turning to less aggressive alternative strategies such as the use of organic acids or more degradable APCAs (aminopolycarboxylic acids). We have therefore arrived at a point in phytoremediation research history in which we need to distance ourselves from EDTA as a proposed soil amendment within the context of phytoextraction. However, valuable lessons are to be learned from over a decade of EDTA-assisted phytoremediation research when considering the implementation of more degradable alternatives in assisted phytoextraction practices.

Enhanced phytoextraction of uranium and selected heavy metals by Indian mustard and ryegrass using biodegradable soil amendments

The applicability of biodegradable amendments in phytoremediation to increase the uptake of uranium (U), cadmium (Cd), chromium (Cr), copper (Cu), lead (Pb) and zinc (Zn) by Indian mustard (Brassica juncea) and ryegrass (Lolium perenne) was tested in a greenhouse experiment. Plants were cultivated during one month on two soils with naturally or industrially increased contaminant levels of U. Treatments with citric acid, NH4-citrate/citric acid, oxalic acid, S,S-ethylenediamine disuccinic acid (EDDS) or nitrilotriacetic acid (NTA) at a rate of 5 mmol kg(-1) dry soil caused increases in soil solution concentrations that were up to 18 times higher for U and up to 1570 times higher for other heavy metals, compared to the controls. Shoot concentrations increased to a much smaller extent. With EDDS, 19-, 34-, and 37-fold increases were achieved in shoots of Indian mustard for U, Pb and Cu, respectively. The increases in plant uptake of Cd, Cr and Zn were limited to a factor of four at most. Ryegrass generally extracted less U and metals than Indian mustard. Despite a marked increase of U and metal concentrations in shoots after addition of amendments, the estimated time required to obtain an acceptable reduction in soil contaminant concentrations was impractically long. Only for Cu and Zn in one of the studied soils, could the Flemish standards for clean soil theoretically be attained in less than 100 years.

The ability of Helianthus annuus L. and Brassica juncea to uptake and translocate natural uranium and 226Ra under different milieu conditions

Seedlings of Helianthus annuus L. (HA) and Brassica juncea (BJ) were used to test the effect of the pH, the presence of phosphates, and the addition of ethylene-diamine-tetraacetic acid (EDTA) or citrate on the uptake and the translocation of uranium isotopes ((238)U, (235)U, and (234)U) and (226)Ra. The results indicated that the presence of phosphates generally reduces the uptake and transfer of uranium from the roots to the shoots of HA. In the case of BJ, while phosphate enhanced the retention of uranium by roots, the translocation was poorer. Likewise, for (226)Ra, the best translocation was in the absence of phosphates for both species. The addition of citrate increased the translocation of uranium for both species, but had no clear effect on the transfer of (226)Ra. The effect of EDTA was much more moderate both for uranium and for (226)Ra, and for both plant species. Only noticeable was a slightly better uptake of (226)Ra by BJ at neutral pH, although the translocation was lower.

Chemically assisted phytoextraction: a review of potential soil amendments for increasing plant uptake of heavy metals

The contamination of soils by trace metals has been an unfortunate sideeffect of industrialization. Some of these contaminants can interfere with vulnerable enduses of soil, such as agriculture or nature, already at relatively low levels of contamination. Reversely, conventional civil-technical soil-remediation techniques are too expensive to remediate extended areas of moderately contaminated soil. Phytoextraction has been proposed as a more economic complementary approach to deal with this specific niche of soil contamination. However, phytoextraction has been shown to be a slow-working process due to the low amounts of metals that can be annually removed from the soil under normal agronomic conditions. Therefore, extensive research has been conducted on process optimization by means of chemically improving plant availability and the uptake of heavy metals. A wide range of potential amendments has been proposed in the literature, with considerable attention being spent on aminopolycarboxylic acids such as ethylenediaminetetraacetic acid (EDTA). However, these compounds have received increasing criticism due to their environmental persistence and associated risks for leaching. This review presents an overview of potential soil amendments that can be employed for enhancing metal uptake by phytoextraction crops, with a distinct focus on more degradable alternatives to persistent compounds such as EDTA.

Potential of Brassic rapa, Cannabis sativa, Helianthus annuus and Zea mays for phytoextraction of heavy metals from calcareous dredged sediment derived soils

Remediation of soil pollution is one of the many current environmental challenges. Anthropogenic activity has resulted in the contamination of extended areas of land, the remediation of which is both invasive and expensive by conventional means. Phytoextraction of heavy metals from contaminated soils has the prospect of being a more economic in situ alternative. In addition, phytoextraction targets ecotoxicologically the most relevant soil fraction of these metals, i.e. the bioavailable fraction. Greenhouse experiments were carried out to evaluate the potential of four high biomass crop species in their potential for phytoextraction of heavy metals, with or without with the use of soil amendments (EDTA or EDDS). A calcareous dredged sediment derived surface soil, with high organic matter and clay content and moderate levels of heavy metal pollution, was used in the experiments. No growth depression was observed in EDTA or EDDS treated pots in comparison to untreated controls. Metal accumulation was considered to be low for phytoextraction purposes, despite the use of chelating agents. The low observed shoot concentrations of heavy metals were attributed to the low phytoavailability of heavy metals in this particular soil substrate. The mobilising effects induced by EDTA in the soil were found to be too long-lived for application as a soil amendment in phytoextraction. Although EDDS was found to be more biodegradable, higher effect half lives were observed than reported in literature or observed in previous experiments. These findings caution against the use of any amendment, biodegradable or otherwise, without proper investigation of its effects and the longevity thereof.