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Arbuscular mycorrhizal (AM) fungi are known to have beneficial effects on host plants growing on contaminated soils. The present study aimed at investigating the influence of two different AM fungi (Rhizophagus intraradices and Funneliformis mosseae) on the growth of plants and Cu uptake by Mimosa pudica L. grown in polluted soils containing various levels of Cu (Control, 400, 500, or 600 mg kg

Arbuscular mycorrhizal fungi (AMF) and earthworms have potential uses in the bioremediation of contaminated soils. In recent years, heavy metal-contaminated sites have been remediated by adding plants and AMF or earthworms to the soil. However, there are few studies on remediation using combinations of plants, animals, and microbes, especially for the remediation of Cu-contaminated soil. The present study investigated the separate and combined effects of AMF and earthworms on Cu-contaminated soil in which Tagetes patula L. was grown. The results show that the combined application of AMF and earthworms markedly increased the biomass of plant shoots and roots by more than 100%. It also increased Cu extraction by T. patula by 270%. The combined treatment was effective in increasing the CEC, contents of OM, and available Cu, P and K, but reduced the soil pH. Furthermore, the combined treatment significantly increased the abundance and diversity of the soil microbial community. In particular, the abundances of the bacteria Bacteroides, Proteobacteria, and Actinobacteria were increased, with the genera Flavobacterium, Pedobacter, Algoriphagus, Gaetbulibacter, Pseudomonas, Luteimonas, and Arthrobacter dominating. Meanwhile, the abundance of the fungus Zygomycota was increased, with Mortierella dominating. Moreover, inoculation with earthworms greatly improved the structure of the soil microbial community.

Metallothioneins (MTs) are small, cysteine-rich, heavy metal-binding proteins involved in metal homeostasis and detoxification. The increasing numbers of available genomic sequences of ectomycorrhizal (ECM) fungi enable deeper insights into the characteristics of MT genes in these fungi that form the most important symbiosis with the host trees in forest ecosystems. The aim of this study was to establish a comprehensive, genome-wide inventory of MT genes from the ECM fungus Laccaria bicolor. Eight MT genes in L. bicolor were cloned, and the expression patterns of their transcripts at various developmental stages based on expressed sequence tag (EST) counts were analyzed. The expression levels of four MTs were significantly increased during symbiosis stages. Quantitative real-time PCR (qRT-PCR) analysis revealed that transcripts of LbMT1 were dominant in free-living mycelia and strongly induced by excessive copper (Cu), cadmium (Cd), and hydrogen peroxide (H2O2). To determine whether these eight MTs functioned as metal chelators, we expressed them in the Cu- and Cd-sensitive yeast mutants, cup1∆ and yap1∆, respectively. All LbMT proteins provided similar levels of Cu(II) or Cd(II) tolerance, but did not affect by H2O2. Our findings provide novel data on the evolution and diversification of fungal MT gene duplicates, a valuable resource for understanding the vast array of biological processes in which these proteins are involved.

Sedum alfredii and Sedum plumbizincicola typically have high heavy metal (such as Zn and Cd) accumulation capacities with fast growth rates and relatively high Pb tolerance in contaminated soils. We compared the accumulation characteristics of heavy metals in Sedum species through meta-analysis. Furthermore, we analyzed the effects of soil organic matter (SOM) and soil pH on Cd, Pb and Zn accumulation by S. alfredii and S. plumbizincicola and the correlation between various metals. Results showed that the accumulations of Cd and Zn in shoots were higher than that of roots, but Pb accumulated in roots more than shoots. Moreover, there is a significant positive correlation between the accumulation of Zn and Cd in shoots. We found that the heavy metal accumulation rate in shoots was higher with lower soil pH. Sedum species had the highest Cd adsorption capacity in 20–30 g/kg SOM and the highest Zn adsorption capacity in SOM less than 20 g/kg. The accumulation rate of Cd in shoots of S. plumbizincicola was increased with exposure time, while the accumulation rate of Zn was slightly decreased. S. alfredii and S. plumbizincicola are two common Cd and Zn hyperaccumulators. We systematically compared the accumulation characteristics of heavy metals in Sedum plants and the effects of soil organic matter and pH on the accumulation of Cd, Pb and Zn in S. alfredii and S. plumbizincicola through meta-analysis. This provides certain theoretical knowledge for the application of sedum plants to the phytoremediation of heavy metal contaminated soils.

Microplastic (MP) pollution problem is severe in China. As the main component of mulch film, whether polyethylene (PE) poses a threat to the safe production of sweet potato is unknown. In this study, micron-sized original or weathered PE was simulated as the field film particles, and pot, hydroponic experiment were conducted to explore the effects of original and weathered MPs on physicochemical properties in soil, growth and phosphorus (P), potassium (K) absorption in sweet potato; P and K adsorption in liquid environment, and also the distribution of original MPs in sweet potato tissues respectively. The results showed that 5 μm original PE MPs significantly reduced pH (5.6-7.9%) and increased EC (6.0-12.1%) of soil compared to weathered PE MPs. In addition, original PE MPs can also significantly improved the biomass growth rates (5.9-19.0%) of sweet potato compared with weathered PE MPs by adsorbing more Olsen-P and Olsen-K in soil, and increasing K concentration in stems compared with control (17.1-55.4%). Although there was no significant difference between original and weathered PE MPs on the degree of membrane lipid peroxidation in sweet potato leaves, the original PE MPs made sweet potato exhibit the stronger oxidative stress. The tissue distribution of PE MPs-fluorescent spheres were only observed in cortical tissues of roots and stems. Results from our study suggest that sweet potato were not significantly affected by a short term exposure to single PE MPs.

Cadmium (Cd) in agricultural soils can be absorbed by wheat and transferred into the grains, risking human health. In order to find the optimal foliar treatment method to reduce Cd accumulation in wheat grain, nineteen single-factor foliar treatments and multi-factor combination treatments were used to study the effects of different foliar sprays on Cd accumulation of wheat grain. The results showed that the foliar application of ethylenediaminetetraacetate (EDTA), selenium (Se), and sodium nitroprusside (SNP) can significantly reduce Cd concentration in wheat grains by 49.2%, 29.6%, and 28.8%, respectively, in the field. Foliar application of EDTA, Se, zinc (Zn), ascorbic acid (ASC), silicon (Si), and molybdenum (Mo) can significantly reduce Cd concentration of wheat grains by 32.3%, 32.0%, 27.7%, 27.7%, 26.3%, and 25.9%, respectively, in pot experiment. Foliar application of 2 mM EDTA and 2 mM Se exerted excellent effects on controlling the Cd accumulation of wheat grains both in pot and field experiment. Foliar application with 0.1 mM Se or 2 mM EDTA significantly reduced Cd concentrations in grains both in grain filling stage and heading + grain filling stage. Spraying at the filling stage has a better effect on reducing Cd concentration in grains than spraying at the heading stage. In addition, the relationship between Cd concentration in grains and husks was significantly positive, while the Cd concentration in grains and flag leaves was significantly negative. Our research proves that foliar spraying of Se and EDTA is feasible to reduce the Cd concentration in wheat grains, which provides technical guidance for the safe production of wheat in low-Cd-contaminated soils.

Pisolithus sp. 1 (P sp. 1) is an ectomycorrhizal fungus (EMF) with a strong Cr(VI) tolerance and reduction ability. The noninvasive microttest technique (NMT), real-time quantitative PCR (qRT-PCR), and the three-dimensional excitation-emission matrix (3D-EEM) were used to deeply explore the physiological mechanism of the P sp. 1 response to Cr(VI) and investigate the relationship between Cr(VI) reduction and denitrification in P sp. Cr(VI) induced the strongest elevations in nitrate reductase (NR) activity and NO production in the mycelia after treatment with Cr(VI) for 48 h under aerobic conditions. The NR inhibitor tungstate significantly inhibited Cr(VI) reduction, proton efflux and the expression of the NR gene (niaD) and NiR gene (niiA). In addition, NO was generated via NR-regulated denitrification. Combined treatments with Cr(VI) and the NO scavenger carboxy-PTIO (cPTIO) significantly increased O

Currently, the research and application of biochar in the remediation of heavy metal contaminated soil has become a hotspot, especially regarding the remediation of agricultural land. Biochar has been proved to be effective in reducing the content of available heavy metals in the soil as well as the heavy metals in plants. However, the long-term effectiveness of biochar immobilization has not been widely studied. In this review, retrospective search was carried out on the published literature results concerning remediation effects of biochar on different areas of heavy metal contaminated soil in the recent years, its application in field remediation (several years), and some potential abiotic and biotic factors that may weaken the immobilization effects of biochar. This results indicate that: (1) biochar is widely used in the remediation of heavy metal contaminated soil in different areas and has excellent immobilization effect. (2) Most of the research demonstrate that the immobilization effect of biochar is effective for 2-3 years or according to few results even for 5 years. However, there have been various reports claiming that the immobilization effect of biochar decreases with time. (3) Abiotic factors such as acid rain, flooded environment, changes in soil condition (pH, redox and dissolved organic matter) and changes in biochar (Cl- and alkali leaching) can significantly weaken the immobilization effect of biochar. (4) Biotic factors such as plant roots, earthworms and soil microorganisms can also significantly reduce the immobilization effect of biochar. Therefore, field experiments having longer time span with biochar need to be further carried out, and the developmental research of modified biochar with a more stable immobilization effect also needs further attention.

Conventional chemical soil amendments and novel material biochars have been widely reported for the immobilisation of cadmium (Cd) and lead (Pb) in polluted soil. However, information regarding their comparative effectiveness is poor. In the present study, rice husk biochar (RHB) was compared with two chemical soil amendments including hydroxyapatite (HAP) and hydrated lime (HDL) for their effectiveness to enhance plant growth and the reduction of Cd uptake and translocation by Triticum aestivum L. grown in heavy-metal-polluted soil. Compared with control and two chemical soil amendments, RHB rapidly improved wheat growth. The HAP, HDL, and RHB treated plants retained Cd and Pb in roots and restricted their translocation. The RHB treatment had the best effect on growth, yield promotion and the reduction of Cd and Pb in wheat grain. Furthermore, the soils treated with RHB and HAP showed lower DTPA-extracted Cd concentrations, and the maximum reduction was observed in HAP-amended soil. However, the DTPA-extracted Pb concentration was not significantly decreased after the application of two chemical soil amendments for 40 days; the maximum reduction was found in soil treated with RHB for 80 days. In all treatments, Cd in post-harvest soil was mainly present in exchangeable, carbonate bound, and Fe-Mn oxide Cd, while the dominant chemical form of Pb was Fe-Mn oxide Pb. Three soil amendments application decreased exchangeable and organic bound- Cd and Pb levels. HAP and RHB displayed significantly immobilisation for soil Cd and reduced translocation of heavy metal as well as its availability in soil, but the HAP had significant inhibition on growth of wheat in contaminated soil. Therefore, RHB shows a promising potential for the reduction of Cd and Pb bioaccumulation in grains from wheat grown on heavy-metal-polluted soils.

Cadmium is a non-essential element for plants and that inhibits plant growth and development. The Zhangye Mawan (ZM) variety of Vicia sativa is more sensitive to Cd toxicity than that Lanjian 3# (L3) variety, but the underlying mechanism is not fully understood. Here, we demonstrated that ZM showed higher Cd accumulation than L3 based on root Cd content and Cd fluorescence intensity in root protoplasts. VsRIT1, a member of the ZIP (ZRT/IRT-like protein) family, showed expression levels in ZM roots 8-fold higher than those in L3 roots under Cd exposure. VsRIT1 expression increased Cd transport and accumulation in Arabidopsis and yeast. These suggests that VsRIT1 participates in Cd uptake by V. sativa roots. Furthermore, ZM root tips have a higher capacity for transient Cd influx than L3 roots when exposed to Cd alone or Cd and iron (Fe) together, owing to the higher VsRIT1 expression in ZM. Our findings also imply that Cd may compete with Fe or/and zinc (Zn) for uptake via VsRIT1 in V. sativa or yeast.

The effect of earthworms on the immobilization of heavy metals by biochar was investigated using 4-week soil pot experiments. Our results showed that both sludge and rice husk biochars applied to Cd, Pb and Zn contaminated soils significantly reduced the bioavailability of those heavy metals in soils, and their concentrations in soil pore water. The Cd and Pb concentrations in pakchoi shoots were decreased by 10.8%–11.7% and 21.5%–26.5%, respectively, in biochar treatments alone. Biochar and earthworm treatments, alone or in combination, can significantly increase pakchoi growth. However, compared with biochar-treated alone, applying earthworms into pots with biochar treatments significantly reduced soil pH by 0.1–0.19 units, increased the Cd, Pb and Zn concentrations in soil pore water, their bioavailability and total uptake in shoots. Additionally, earthworms weaken the immobilization effect of biochar on heavy metals. The results of principal component analysis and correlation analysis showed that earthworm treatment was the main cause of soil pH reduction, and soil pH was significantly negatively correlated with the bioavailable Cd, Pb and Zn concentrations in the soil. Furthermore, the effect of the earthworm digestive system (casts) on bioavailable Cd, Pb and Zn concentrations could not explain their changes in the soil. In addition, the application of biochar had no significant effect on the survival and heavy metal enrichment of earthworm. Therefore, the effect of earthworms must be considered in the process of Cd, Pb and Zn contaminated farmland soil remediation using biochar.

The results of Cd (cadmium) concentration, Cd2+ fluorescent staining, NMT (non-invasive micro-test technology) analysis of Cd absorption revealed the remarkably positive role of HRW in reducing Cd uptake by root of pak choi seedlings. BcIRT1 (iron-regulated transporter 1) and BcZIP2 (zinc-regulated transporter protein 2) are the main Cd transporters in pak choi, but their roles in the process of HRW-reduced Cd uptake is still far from being answered. In this study, we specifically verified the function of IRT1 and ZIP2 in HRW-reduced Cd absorption in pak choi and Arabidopsis thaliana. Heterologous and homologous expression in Arabidopsis thaliana displayed that Cd concentrations in wild-type (Col-0) and transgenic A. thaliana of IRT1 and ZIP2 were significantly reduced by HRW, except for irt1- and zip2-mutant. NMT detection showed that HRW not only decreased Cd2+ influx in root of WT and transgenic lines, but also enhanced the competition between Zn and Cd. Taken together, the HRW-induced reduction of Cd accumulation in plants may be result from depressing the expression of BcIRT1 and BcZIP2 and affecting the preference of BcIRT1 and BcZIP2 in ion uptake.

Soil provides a critical environment for microbial community development. However, microorganisms may be sensitive to substances such as heavy metals (HMs), which are common soil contaminants. This study investigated bacterial communities using 16S ribosomal RNA (rRNA) gene fragment sequencing in geographic regions with and without HM pollution to elucidate the effects of soil properties and HMs on bacterial communities. No obvious changes in the richness or diversity of bacterial communities were observed between samples from mining and control areas. Significant differences in bacterial richness and diversity were detected between samples from different geographic regions, indicating that the basic soil characteristics were the most important factors affecting bacterial communities other than HMs. However, the abundances of several phyla and genera differed significantly between mining and control samples, suggesting that Zn and Pb pollution may impact the soil bacterial community composition. Moreover, regression analyses showed that the relative abundances of these phyla and genera were correlated significantly with the soil-available Zn and Pb contents. Redundancy analysis indicated that the soil K, ammoniacal nitrogen (NH4+-N), total Cu, and available Zn and Cu contents were the most important factors. Our results not only suggested that the soil bacteria were sensitive to HM stresses but also indicated that other soil properties may affect soil microorganisms to a greater extent.

We used Pisolithus tinctorius and Cenococcum geophilum to determine the copper (Cu) resistance of ectomycorrhizal (ECM) fungi and their potential for improving phytoremediation of Cu-contaminated soil by Chinese red pine (Pinus tabulaeformis). The results showed that nutrient accumulation in C. geophilum mycelium was significantly lower under higher Cu concentrations in the soil, which was not observed in P. tinctorius. Meanwhile, P. tinctorius exhibited greater Cu tolerance than C. geophilum. Inoculation with ECM fungi significantly improved the growth of pine shoots planted in polluted soil in pot experiments (p < 0.01). The total accumulated Cu in pine seedlings planted in Cu-contaminated soil increased by 72.8% and 113.3% when inoculated with P. tinctorius and C. geophilum, respectively, indicating that ECM fungi may help their host to phytoextract heavy metals. Furthermore, the majority of the total absorbed metals remained in the roots, confirming the ability of ECM fungi to promote heavy metal phytostabilization. There were no differences between the effects of the two fungi in helping the host stabilize and absorb Cu, even though they have different Cu tolerances. Inoculation with ECM fungi can benefit plant establishment in polluted environments and assist plants with phytoremediating heavy-metal-contaminated soils.

Within the domain of phytoremediation research, the proper disposal of harvestable plant parts, that remove pollutants from contaminated soil, has been attracted extensive attention. Here, the bioenergy generation capability of trace metals (Cu, Pb, Zn, Cd, Mn, and As) polluted plants was assessed. The biogas production potential of accumulators or hyperaccumulator plants, Elsholtzia haichowensis, Sedum alfredii, Solanum nigrum, Phytolacca americana and Pteris vittata were 259.2 ± 1.9, 238.7 ± 4.2, 135.9 ± 0.9, 129.5 ± 2.9 and 106.8 ± 2.1 mL/g, respectively. The presence of Cu (at approximately 1000 mg/kg) increased the cumulative biogas production, the daily methane production and the methane yield of E. haichowensis. For S. alfredii, the presence of Zn (≥ 500 mg/kg) showed a significant negative impact on the methane content in biogas, and the daily methane production, which decreased the biogas and methane yield. The biogas production potential increased when the content of Mn was at 5 000–10,000 mg/kg, subsequently, decreased when the value of Mn at 20,000 mg/kg. However, Cd (1–200 mg/kg), Pb (125–2000 mg/kg) and As (1250–10,000 mg/kg) showed no distinctive change in the cumulative biogas production of S. nigrum, S. alfredii and P. vittata, respectively. The methane yield showed a strong positive correlation (R2 = 0.9704) with cumulative biogas production, and the energy potential of the plant residues were at 415–985 kW h/ton. Thus, the anaerobic digestion has bright potential for the disposal of trace metal contaminated plants, and has promising prospects for the use in energy production.

To explore the potential of ectomycorrhizal (ECM) Pinus sylvestris (P. sylvestris) utilizing in the phytoremediation of a combined heavy metal contaminated tailings pond, Pisolithus sp.1(P1)-. Pisolithus sp.2 (P2)-. Cenococcum geophilum (Cg)-. Laccaria sp. (L1)- ECM, and non-ectomycorrhizal (NM) P. sylvestris were planted separately in lead (Pb)-zinc-(Zn)-cadmium-(Cd)-combined polluted soil, collected from a tailings pond. After four months, growth, photosynthetic parameters, nutrient and heavy metal levels of the plants were evaluated. The physical and chemical properties and enzyme activities of soil before and after ECM plants planting were also investigated. The results showed that inoculation with ECM fungi improved the survival rates of host plants by increasing the biomass, photosynthesis (photosynthetic rate (Pn), stomatal conductance (Gs), transpiration rate (Tr), intercellular CO2 concentration (Ci)), and mineral nutrients (phosphorus (Pi), magnesium (Mg), iron (Fe), calcium (Ca)), while it decreased the transfer factors of Cd, Pb, and Zn. In addition, ECM P. sylvestris significantly accumulated much more Cd, Pb, and Zn than NM seedlings, while it reduced pH and the availability of heavy metals (DTPA-Cd, DTPA-Pb, DTPA-Zn) in soil and increased activity of soil enzymes (acid phosphatase, alkaline phosphatase, urease). Therefore, the ECM symbionts have the great potential for phytoremediation of abandoned tailings pond, and this study provides a theoretical basis and application premise for the phytoremediation of abandoned tailings pond.

The proper disposal of copper (Cu) polluted plant residues after phytoremediation has attracted extensive attention. In this study, the Cu-polluted biogas residue produced through anaerobic digestion was applied directly. Wheat, soybean and pakchoi were grown in pots for four seasons over two years. The application dosage of Cu-polluted biogas residue was evaluated by measuring growth conditions of crops, Cu content in edible parts, and amelioration of saline-alkali soil. The results showed that the biomass of the crops, the content of soil organic matter, total N and available P and microbial diversity can be improved, and the Cu concentration of the edible parts was all lower than limit standard. Amendment with 2% biogas residue enhanced the growth of beneficial bacteria and fungi, and decreased the relative abundances of potentially pathogenic fungi in the saline-alkali soil. The results of this study provide a basis for the safe utilisation of copper-polluted plant residues.

Ectomycorrhizal (ECM) fungi can improve the growth of pine trees and enhance their tolerance to heavy metal stress, and may also be useful during the afforestation and phytoremediation of polluted regions with pine trees. Hebeloma vinosophyllum (Cr(VI)-sensitive strain) and Pisolithus sp1 ((Cr(VI)-tolerant strain) were selected through liquid culture experiment, and were used in symbiosis with Japanese black pine (Pinus thunbergii) in pot experiments, to determine their potential for improving phytoremediation of Cr(VI)-contaminated soils. Our results indicated that Pisolithus sp1 also had a significantly higher accumulation of Cr than H. vinosophyllum in mycelium under the same Cr(VI) treatments in liquid culture experiment. The tolerance index of Pisolithus sp1 ECM seedlings’ shoots and roots to Cr(VI) were significantly higher than that of H. vinosophyllum ECM and non-ectomycorrhizal (NM) seedlings while the total accumulated Cr per seedling in Pisolithus sp1 ECM seedlings were 1.50–1.96 and 2.83–27.75 fold higher that of H. vinosophyllum ECM and NM seedlings, respectively, within 0–800 mg kg−1 Cr(VI) treatments in pot experiments. In addition, the significant differences ratios of photosynthetic rate, stomatal conductance, transpiration rate and intercellular CO2 concentration between Pisolithus sp1 ECM and NM seedlings were significantly higher than those between H. vinosophyllum ECM and NM seedlings under 400 and 800 mg kg−1 Cr(VI) treatments. Compared with the control (no plant), and planting NM or H. vinosophyllum ECM seedlings, the planting of Pisolithus sp1 ECM seedlings significantly reduced the percentage content of exchangeable Cr in the soil.

Whether the huge external hyphal system of ectomycorrhizae that promotes host plants' acquisition of water and nutrients can selectively inhibit their transport of heavy metals at the same time remains unclear. In this experiment, we designed and conducted two types of soil-pot test to clarify the effects of EMF on the absorption and transport of copper (Cu) and cadmium (Cd) by host Pinus thunbergii seedlings. In the root-bag test, external hyphae took the initiative into the Cu/Cd-contaminated bulk soil, absorb and transport Cu and Cd to the rhizosphere soils and further transport it to the shoots of the host plants. Inoculation with EMF also promoted the uptake of nutrients by host plants, thereby increasing their biomass and improving Cu/Cd tolerance compared with non-inoculated plants. Inoculation with EMF species with higher Cu or Cd tolerance generated more phytostabilization and phytoextraction of Cu or Cd by host plants. In a short-term exposure test, inoculation with EMF accelerated the absorption of Cu and Cd by P. thunbergii within 12 h of Cu or Cd irrigation. Therefore, we concluded that EMF do not act as a barrier inhibiting the absorption of heavy metals by host plants, but rather promote this absorption. Improving the plant's nutritional status and promoting growth, diluting heavy metal concentrations, thereby reducing the toxic effects of heavy metals on host plants. These results provide the theoretical basis for the application of EMF in plant–microbial combinations for the phytostabilization and phytoextraction of heavy metal–contaminated soils.

We examined the effects of Cd on growth, lipid peroxidation, reactive oxygen species (ROS) accumulation, antioxidant enzymatic activity, and lignin content in the roots of two varieties of Vicia sativa. Treatment with Cd decreased plant growth and increased ROS and lipid peroxidation levels to a greater extent in the Cd-sensitive variety ZM than in the Cd-tolerant variety L3. Most hydrogen peroxide (H2O2) and superoxide anion (O2(•-)) were accumulated in the cell walls and extracellular spaces in response to Cd treatments. Chemical assays and experiments using inhibitors showed that larger increases in H2O2 and O2(•-) production in ZM than in L3 were probably attributed to elevated Cd-induced nicotinamide adenine dinucleotide-peroxidase (NADH-POD) activity. Cd treatment increased the accumulation of lignin and the guaiacol peroxidase (GPOD) activities in the apoplast more significantly in ZM root than in L3. Howerver, root laccase activity was higher in L3 than in ZM. Thus Cd toxicity induced significant lignification in the roots of V. sativa, and increases in H2O2 accumulation and apoplastic GPOD activity were likely responsible for this effect.

Metallothioneins (MTs) are low-molecular-weight, cysteine-rich metal-binding proteins found in numerous genera and species, but their functions in abiotic stress tolerance remain unclear. Here, a MT gene from Oryza sativa, OsMT2c, was isolated and characterized, encoding a type 2 MT, and observed expression in the roots, leaf sheathes, and leaves, but only weak expression in seeds. OsMT2c was upregulated by copper (Cu) and hydrogen peroxide (H2O2) treatments. Excessive Cu elicited a rapid and sustained production and release of H2O2 in rice, and exogenous H2O2 scavengers N,N'-dimethylthiourea (DMTU) and ascorbic acid (Asc) decreased H2O2 production and OsMT2c expression. Furthermore, the expression of OsMT2c increased in the osapx2 mutant in which the H2O2 levels were higher than in wild-type (WT) plants. These results showed that Cu increased MT2c expression through the production and accumulation of Cu-induced H2O2 in O. sativa. In addition, the transgenic OsMT2c-overexpressing Arabidopsis displayed improved tolerance to Cu stress and exhibited increased reactive oxygen species (ROS) scavenging ability compared to WT and empty-vector (Ev) seedlings.

In this study, the effectiveness of six soil amendments (ferrihydrite, manganese dioxide, gibbsite, calcium carbonate, biochar, and organic fertilizer) was investigated to assess the feasibility of minimizing possible environmental contaminant leaching during S,S-ethylenediaminedisuccinic acid (EDDS)-enhanced phytoextraction process based on 0.01-M CaCl2 extraction. Results showed that the application of EDDS could significantly increase Cu concentrations in the leaching solution. Compared with control, incorporation of six amendments (excluding organic fertilizer) significantly decreased CaCl2-extractable Cu concentrations in both soils. When EDDS-containing solutions leached from the soil columns (mimicking the upper soil layers) were added to soils with different amendments (mimicking the subsoil), CaCl2-extractable Cu in the soils amended with ferrihydrite, manganese dioxide, gibbsite, and calcium carbonate was significantly lower than that in the control soil (no amendments) and remained relatively constant during the first 14 days. Incorporation of biochar or organic fertilizer had no positive effect on the immobilization of Cu in EDDS leachates in soils. After 14 days, CaCl2-extractable Cu concentration decreased rapidly in soils incorporated with various amendments. Integrating soil washing with biodegradable chelating agents or chelant-enhanced phytoextraction and immobilization of heavy metals in subsoil could be used to rapidly reduce the concentration of bioavailable metal fractions in the upper soil layers and minimize environmental risks of secondary pollution.

The present study investigated the effects of the biodegradable chelant S , S -ethylenediaminedisuccinic acid (EDDS) and the plant-growth-promoting bacterium DGS6 on pollutant uptake by corn from e-waste–contaminated soils. The highest concentration and total uptake of Cu and Zn in corn shoots were observed in the presence of EDDS and DGS6 + EDDS, respectively. The ΣPCB concentrations in shoots ranged from 0.53 to 0.72 ng g −1 , and the highest PCB concentration was observed in the presence of EDDS. This could be ascribed to the enhanced dissolved organic carbon, increased dissolution and efficient translocation of PCBs from roots to shoots, as well as potential root damage due to increased soluble metal levels in soil solution. In contrast, the highest total uptake of PCBs in shoots was observed in the presence of DGS6, likely due to enhanced shoot biomass and high levels of air deposition.

The present manuscript reviews the research progresses on the phytoextraction of metals using biodegradable chelants in our research group. It mainly focuses on the theoretical considerations including metal solubilization, root uptake and translocation upward to the shoots, the effects of chelant application on metal leaching and soil microbes, and the optimization of the chelant-assisted phytoextraction. In addition, the potential of phytoextraction of heavy metals and persistent organic pollutants (POPs) with biodegradable chelant in the heavy metal-POPs co-contaminated is reviewed as well.

Phytoremediation as an alternative strategy has been a widespread attention. The screening of enriched plants and hyperaccumulators is the key of the strategy. So this study examined the status of heavy metal pollution in molybdenum (Mo) mine soils, metal accumulation in plants growing on mine, and their tolerance strategies. The analysis of 14 soils and 27 plant samples in mining area showed that Mo, zinc (Zn), and cadmium (Cd) concentrations exceeded soil safety standards and their levels varied in 27 plant samples. Mo was the heavy pollution with an average total content of 256.1 mg/kg in soils. As Mo-enriched plants, Mo concentrations of Macleaya cordata (Willd.) R. Br. and Morus australis Poir. were 704.4 and 772.4 mg/kg, respectively. M. cordata was selected as the research material, due to its high biomass. Molybdenum significantly decreased the biomass and photosynthesis of M. cordata at high concentration (> 200 μmol/L), but its biomass and photosynthesis reached the maximum after 50 μmol/L Mo treatment, respectively. Analysis of the subcellular distribution and chemical speciation showed that Mo was distributed a certain way in the extracts and that this suggested that it may be present in cell wall and soluble fraction of roots (51.9–63.9%; 26.1–44.7%) or shoots (30.0–44.4%; 47.3–56.0%) and complexed to organic acid, pectate, oxalate, and protein. This might be responsible for the adaptation of M. cordata to Mo stress. Therefore, M. cordata could serve as a potential plant to utilize for the phytoremediation of Mo-contaminated soil.

This study investigated the anaerobic digestion capability of five plants and the effects of copper (Cu) and S,S'-ethylenediaminedisuccinic acid (EDDS, a chelator widely used in chelant-assisted phytoremediation) on biogas production to determine a feasible disposal method for plants used in remediation. The results showed that in addition to Phytolacca americana L., plants such as Zea mays L., Brassica napus L., Elsholtzia splendens Nakai ex F. Maekawa, and Oenothera biennis L. performed well in biogas production. Among these, O. biennis required the shortest period to finish anaerobic digestion. Compared to normal plants with low Cu content, the plants used in remediation with increased Cu levels (100 mg kg(-1)) not only promoted anaerobic digestion and required a shorter anaerobic digestion time, but also increased the methane content in biogas. When the Cu content in plants increased to 500, 1000, and 5000 mg kg(-1), the cumulative biogas production decreased by 12.3%, 14.6%, and 41.2%, respectively. Studies also found that EDDS conspicuously restrained biogas production from anaerobic digestion. The results suggest that anaerobic digestion has great potential for the disposal of contaminated plants and may provide a solution for the resource utilization of plants used in remediation.

Traditional metal chelators, such as ethylenediaminetetraacetic acid (EDTA), have been gradually replaced due to their poor biodegradability in soil and high risk of heavy metal leaching into groundwater, which pose high environmental risks to the health of humans and animals. In this study, a liquid amino acid fertilizer (LAAF, waste proteins from hydrolysates of animal carcasses) and polyaspartate (PASP) were used as additives to enhance the phytoextraction of cadmium (Cd) and lead (Pb) from contaminated soil. We conducted pot experiments to investigate the phytoextraction capacity of Solanum nigrum, a Cd accumulator, grown on soil highly contaminated with Cd and Pb in the absence (as controls) or presence of PASP and LAAF. Both PASP and LAAF significantly improved plant growth, Cd accumulation, and total Cd and Pb content in S. nigrum shoots and roots. PASP and LAAF application promoted Cd translocation from roots to shoots in S. nigrum and Cd bio-accessibility in rhizosphere soils, but this was not the case for Pb. Both PASP and LAAF increased Cd and Pb phytoextraction by S. nigrum plants, and Cd phytoextraction was more effective in LAAF-assisted S. nigrum than in PASP-assisted S. nigrum. These findings demonstrate that the low cost and ecofriendly features of recycled waste proteins make them good candidates for chelant-enhanced phytoextraction from heavy metal-contaminated soils.

The present study evaluated the effects of a biodegradable chelant, S,S-ethylenediaminedisuccinic acid (EDDS), on enzyme activities and microbial community composition in copper (Cu)-contaminated soils, planted with either corn or beans. Results showed that the application of EDDS did not affect urease and acid phosphatase activities in the soil, but greatly reduced catalase and saccharase activities, and increased β-glucosidase activity on the seventh day after EDDS application. On the 28th day, no significant difference was observed in the enzyme activities (except for β-glucosidase) of EDDS-treated soils compared to the controls. Analysis of phospholipid fatty acids (PLFAs) showed that the application of 3 mmol kg−1 EDDS did not cause significant stress to soil microbial communities. However, PCR-denaturing gradient gel electrophoresis (PCR-DGGE) fingerprint revealed that EDDS influenced the bacterial communities in the soils, and the effects on bean soils were more significant than that with corn. In general, the enzyme activities and bacterial communities were influenced by the application of EDDS, but the impact became weaker or even disappeared with the biodegradation of EDDS.

In this study, we investigated the role of rhizospheric bacteria in solubilizing soil copper (Cu) and promoting plant growth. The Cu-resistant bacterium DGS6 was isolated from a natural Cu-contaminated soil and was identified as Pseudomonas sp. DGS6. This isolate solubilized Cu in Cu-contaminated soil and stimulated root elongation of maize and sunflower. Maize was more sensitive to inoculation with DGS6 than was sunflower and exhibited greater root elongation. In pot experiment, inoculation with DGS6 increased the shoot dry weight of maize by 49% and sunflower by 34%, and increased the root dry weight of maize by 85% and sunflower by 45%. Although the concentrations of Cu in inoculated and non-inoculated seedlings did not differ significantly, the total accumulation of Cu in the plants increased after inoculation. DGS6 showed a high ability to solubilize P and produce iron-chelating siderophores, as well as significantly improved the accumulation of P and Fe in both maize and sunflower shoots. In addition, DGS6 produced indole-3-acetic acid (IAA) and ACC deaminase, which suggests that it may modulate ethylene levels in plants. The bacterial strain DGS6 could be a good candidate for re-vegetation of Cu-contaminated sites. Supplemental materials are available for this article. Go to the publisher's online edition of International Journal of Phytoremediation to view the supplemental file.

A field investigation, field experiment, and hydroponic experiment were conducted to evaluate feasibility of using Oenothera glazioviana for phytostabilization of copper-contaminated soil. In semiarid mine tailings in Tongling, Anhui, China, O. glazioviana, a copper excluder, was a dominant species in the community, with a low bioaccumulation factor, the lowest copper translocation factor, and the lowest copper content in seed (8 mg kg(-1)). When O. glazioviana was planted in copper-polluted farmland soil in Nanjing, Jiangsu, China, its growth and development improved and the level of γ-linolenic acid in seeds reached 17.1%, compared with 8.73% in mine tailings. A hydroponic study showed that O. glazioviana had high tolerance to copper, low upward transportation capacity of copper, and a high γ-linolenic acid content. Therefore, it has great potential for the phytostabilization of copper-contaminated soils and a high commercial value without risk to human health.

One concern about the chelant-enhanced phytoextraction is the potential metal leaching associated with chelant application. A field study was carried out and the metal leaching along the 60-cm depth soil profiles were evaluated within 36 days after the biodegradable chelant EDDS was applied. Results showed EDDS significantly increased soluble Cu in the top 5 cm soil layer 1 day after the application, and the increase of soluble metals was generally limited in the top 20 cm soil. Metal speciation analysis indicated all Cu and Zn were in forms of Cu-EDDS and Zn-EDDS complexes in soil solution, and Ca was the major competitor with trace metals to EDDS. The soluble metals decreased quickly with time, and no significant difference was observed in the extractable Cu between EDDS treatments and the controls 22 days after the EDDS addition. The potential leaching associated with biodegradable EDDS addition may be controlled under field conditions.

Copper contamination in soils and vegetables in the vicinity of an abandoned copper mine in China was investigated. The Cu concentrations of 93 soil samples ranged from 30.4 to 3,191 mg kg−1 soil for a mean of 816.8 mg kg−1 soil. Among 15 samples from a 0 to 20-cm soil layer used for the toxicity characteristic leaching procedure (TCLP) test, the highest value of Cu-TCLP was 133.8 mg kg−1 soil and the TCLP values were positively correlated with the total Cu content of the soils. The sequential extraction of soils in the 0–20-, 20–40-, and 40–60-cm soil layers showed that Cu existed mainly in the Fe–Mn oxide fraction, sulfide/organic fraction, and residual fraction. The copper contamination of 21 species of vegetables from in situ sampling was also examined. Cu concentrations in the edible portions of Brassica chinensis and Solanum melongena were higher than the FAO/WHO standard (40 mg kg−1 DW). The health risk of copper for local inhabitants from consuming these vegetables was assessed on the basis of the target hazard quotient. Enriched concentrations of copper were also found in situ in eight cultivars of B. chinensis planted in the fields, with two levels of Cu concentration. The results showed that there is severe copper contamination in this mine area, and the pollutant in soils show a high risk of leaching into the groundwater and diffusing through the food chain.

The effects of excess copper (Cu) on the production of reactive oxygen species (ROS) and antioxidant enzyme activities in leaves of the Cu-accumulator Elsholtzia haichowensis Sun were investigated. The addition of 100 microM of copper significantly increased the accumulation of hydrogen peroxide (H(2)O(2)) and the activities of superoxide dismutase (SOD) and other antioxidant enzymes. The increase in SOD activity was attributable to an increase in apoplastic and symplastic copper-zinc superoxide dismutase (CuZn-SOD) activity. Induction of CuZn-SOD proteins was demonstrated by immunoblot analysis. This study also provides the first cytochemical evidence of an accumulation of superoxide anion in the chloroplasts of mesophyll cells, and H(2)O(2) in the mesophyll cell walls and extracellular space, as a consequence of Cu treatment. Experiments with diphenyleneiodonium as an inhibitor of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase and N-N-diethyldithiocarbamate as an inhibitor of SOD showed that the source of H(2)O(2) in the cell wall could be, in part, NADPH oxidase. Apoplastic guaiacol peroxidase (POD) and symplastic ascorbate peroxidase (APX) activities were induced in leaves of E. haichowensis with 100 microM Cu, suggesting that apoplastic POD and symplastic APX may be important in avoiding the buildup of toxic H(2)O(2) concentrations.

The concentrations of heavy metals in Phytolacca americana L. and corresponding soil samples from three contaminated sites and an uncontaminated site were studied. Hydroponic experiments were also conducted to investigate the Cd uptake ability and mechanism of P. americana. The field results showed that the average Cd concentration was 42 mg kg(-1) in P. americana leaves, with the highest concentration of 402 mg kg(-1) found at Datianwan. A significant relationship was observed between the concentrations of Cd in leaves and those of corresponding soils on a logarithmic scale. Under laboratory hydroponic conditions, the maximum Cd concentration in aerial tissues of P. americana was 637 mg kg(-1), under treatment with 100 microM Cd. The population from the uncontaminated site (Zijinshan) also had a remarkable ability to accumulate Cd in shoots to concentrations well in excess of 100 microM in the hydroponic experiment, similar to the population from contaminated site, suggesting that Cd accumulation is a constitutive trait of P. americana. In the presence of 100 microM Cd, the addition of polyethylene glycol decreased leaf transpiration, the shoot Cd concentration, and the shoot/root Cd concentration ratio. There was a significantly positive relationship between the shoot Cd concentration and the leaf transpiration of P. americana. A similar significant positive correlation was also obtained between the shoot/root Cd concentration and leaf transpiration. Moreover, pretreatment with 5 microM abscisic acid or 5 microM HgCl(2) significantly decreased the Cd concentration in P. americana shoots. These results suggest that transpiration has an important role in Cd accumulation in shoots of P. americana.

Using both histochemical and cytochemical methods, we investigated the effects of copper (Cu) on the production of hydrogen peroxide (H2O2) and superoxide anion (O2(.-)) in the leaves of the moss Plagiomnium cuspidatum. Cu treatment significantly increased the contents of total thiobarbituric acid-reactive substances and H2O2, as well as the activity of guaiacol peroxidase and superoxide dismutase (SOD). Native PAGE detected all three forms of SOD (Mn-SOD, Fe-SOD and CuZn-SOD) in P. cuspidatum, and the increase in the total SOD activity appeared to be mainly caused by an increase in CuZn-SOD activity. According to cytochemical results, H2O2-dependent CeCl3 precipitates were primarily localized in the plasma membranes and cell walls, and O2(.-) was chiefly localized on the inner side of the plasma membrane and in the cytoplasm surrounding the chloroplasts. Experiments using imidazole as an inhibitor of NADPH oxidase, N-N-diethyldithiocarbamate as an inhibitor of CuZn-SOD, and 1,2-dihydroxybenzene-3,5-disulphonic acid as an O2(.-) scavenger indicated that a partial source of H2O2 in the cell walls may be NADPH oxidase. The results also showed that peroxidase (POD) is involved in the detoxification of H2O2. Increased POD activity induced by Cu may remove excess H2O2 caused by Cu.

In the present study, the accumulation of Mn and other metals by Phytolacca Americana L. from contaminated soils in Hunan Province, South China, was investigated. Results showed that the average concentrations of Mn in the leaves and roots reached 2198 and 80.4 mg kg(-1) (dry weight), respectively, with a maximum 13,400 mg kg(-1) in the leaves. A significant correlation was found between Mn concentrations in the plant leaves and those in the corresponding soils. Hydroponic experiments were also conducted to study the Cd uptake ability and interactions between Mn and Cd in the plant. It was found that P. americana hyperaccumulated not only Mn, but also Cd in the leaves. In the presence of Cd, adding Mn to the solution significantly improved the plant growth and reduced the concentrations of Cd in all organs of the plant.

The concentrations of heavy metals in the leaves of two aquatic plants Potamogeton pectinatus L. and Potamogeton malaianus Miq., and the corresponding water and sediment samples from the Donghe River in Jishou City of Hunan Province, China were studied to investigate metal contamination from the intensive industrial activities in the surrounding area. Results showed that the concentrations of heavy metals in the sediments, especially Cd, Mn and Pb, were much higher than the eco-toxic threshold values developed by the U. S. Environmental Protection Agency. Between the two plant species, P. pectinatus showed the higher capacity in metal accumulation. The highest concentrations of Cd, Pb, Cu, Zn and Mn were found in the leaves of P. pectinatus, reaching 596, 318, 62.4, 6590 and 16,000 mg kg(-1) (DW), respectively. Significantly positive relationships were observed among the concentrations of Zn, Cu and Mn in the leaves of both aquatic plants and those in water, indicating the potential use of the two plants for pollution monitoring of these metals. In addition, a laboratory experiment was conducted to investigate the ability of P. pectinatus and P. malaianus to remove heavy metals from contaminated river water. The average removal efficiencies by P. pectinatus and P. malaianus for Cd, Pb, Mn, Zn and Cu from the spiked Donghe River water were 92%, 79%, 86%, 67% and 70%, respectively. The results indicated that P. pectinatus and P. malaianus had high capabilities to remove heavy metals directly from the contaminated water. The potential use of these plants in wastewater treatment is worth further exploration.

To increase the phytoextraction efficiency of heavy metals and to reduce the potential negative effects of mobilized metals on the surrounding environment are the two major objectives in a chemically enhanced phytoextraction process. In the present study, a biodegradable chelating agent, NTA, was added in a hot solution at 90°C to soil in which beans (Phaseolus vulgaris L., white bean) were growing. The concentrations of Cu, Zn and Cd, and the total phytoextraction of metals by the shoots of the plant from a 1 mmol kg−1 hot NTA application exceeded those in the shoots of plants treated with 5 mmol kg−1 normal NTA and EDTA solutions (without heating treatment). A significant correlation was found between the concentrations of metals in the shoots of beans and the relative electrolyte leakage rate of root cells, indicating that the root damage resulting from the application of a hot solution might play an important role in the process of chelate-enhanced metal uptake in plants. The application of hot NTA solutions did not significantly increase metal solubilization in soil in comparison with a normal application of solution of the same dosage. Therefore, the application of a hot NTA solution may provide a more efficient alternative in chemical-enhanced phytoextraction, although further studies of techniques of application in fields are sill required.

Aquatic macrophytes can absorb heavy metals either from sediments via the root system, from the water phase by leaves, or from both sources. In this study, cadmium accumulation and distribution in the aquatic plant Hydrilla verticillata were investigated, with a focus on the role of roots. Results showed that leaves of H. verticillata had a higher Cd concentration than roots when intact plants were grown in sediments and solutions containing Cd. Cadmium can significantly decrease the leaf chlorophyll content, and the leaves of intact plants with roots had lower chlorophyll contents than the leaves of detached ones without roots due to the transfer effect of roots. The majority of the Cd accumulated in leaves of H. verticillata was bound to the cell walls. When roots were submerged in a solution containing Cd, with shoots in a control solution without Cd, the Cd concentrations in leaves were considerably lower than in roots. In contrast, Cd was almost undetectable in roots when the shoots were submerged in a solution containing Cd, with roots in the control solution. Compared to the leaves and stems of detached shoots without roots, the concentrations of Cd were much higher in the leaves and stems of intact plants with roots. It is suggested that the roots of intact plants absorb Cd and transfer it to leaves and that more Cd is removed from the solution by intact plants.

Soil that is co-contaminated with heavy metals (HMs) and polycyclic aromatic hydrocarbons (PAHs) is difficult to bioremediate due to the ability of toxic metals to inhibit PAH degradation by bacteria. We demonstrated the resistance mechanisms to Cu(II) and Cd(II) of two newly isolated strains of Sphingobium sp. PHE-SPH and Ochrobactrum sp. PHE-OCH and further tested their potential application in the bioremediation of HM-phenanthrene (PhA) co-contaminated sites. The PHE-SPH and PHE-OCH strains tolerated 4.63 and 4.34 mM Cu(II) and also showed tolerance to 0.48 and 1.52 mM Cd(II), respectively. Diverse resistance patterns were detected between the two strains. In PHE-OCH cells, the maximum accumulation of Cu(II) occurred in the cell wall, while the maximum accumulation was in the cytoplasm of PHE-SPH cells. This resulted in a sudden suppression of growth in PHE-OCH and a gradual inhibition in PHE-SPH as the concentration of Cu(II) increased. Organic acid production was markedly higher in PHE-OCH than in PHE-SPH, which may also have a role in the resistance mechanisms, and contributes to the higher Cd(II) tolerance of PHE-OCH. The factors involved in the absorption of Cu(II) or Cd(II) in PHE-SPH and PHE-OCH were identified as proteins and carbohydrates by Fourier transform infrared (FT-IR) spectroscopy. Furthermore, both strains showed the ability to efficiently degrade PhA and maintained this high degradation efficiency under HM stress. The high tolerance to HMs and the PhA degradation capacity make Sphingobium sp. PHE-SPH and Ochrobactrum sp. PHE-OCH excellent candidate organisms for the bioremediation of HM-PhA co-contaminated sites.

The non-selective apoplastic passage of Cu and Cu-citrate complexes into the root stele of monocotyledonous corn and dicotyledonous soybean was investigated using an inorganic-salt-precipitation technique. Either Cu ions or Cu-citrate complexes were drawn into root through the apoplast from the root growth medium, and K4[Fe(CN)6] was subsequently perfused through xylem vessels or the entire root cross section. Based on microscopic identification of the reddish-brown precipitates of copper ferrocyanide in the cell walls of the xylem of corn and soybean roots, Cu(2+) passed through the endodermal barrier into the xylem of both species. When the solution containing 200 μM CuSO4 and 400 μM sodium citrate (containing 199.98 μM Cu-citrate, 0.02 μM Cu(2+)) was drawn via differential pressure gradients into the root xylem while being perfused with K4[Fe(CN)6] through the entire root cross-section, reddish-brown precipitates were observed in the walls of the stele of soybean, but not corn root. However, when a CuSO4 solution containing 0.02 or 0.2 μM free Cu(2+) was used, no reddish-brown precipitates were detected in the stele of either of the two plants. Results indicated that endodermis was permeable to Cu-citrate complexes in primary roots of soybean, but not corn. The permeability of the endodermal barrier to the Cu-citrate complex may vary between dicotyledonous and monocotyledonous plants, which has considerable implications for chelant-enhanced phytoextraction.

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.

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 μg 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 plant species composition and their ability to accumulate heavy metals were investigated at three contaminated sites in Xiangxi area, Southern China. The concentrations of Cd, Pb, Zn, and Cu in more than 363 samples of 125 plant species were analyzed in the present study. The average concentrations of Cd, Pb, Zn, and Cu in the plants were 19, 81, 637, and 8 mg kg(-1), respectively. The highest concentration of Cd in above-ground plant tissues was found to be 287 mg kg(-1) in the leaves of Lobelia chinensis Lour, at the Datianwan site, followed by Solamim nigrum L. with 99 mg kg(-1) Cd in the leaves. They might be potential Cd hyperaccumulators. At the three contaminated sites, some dominant and relative dominant species with high accumulation potential of metals, such as Kalimeris indice (L.) Sch.-Bip. and Solanum nigrum L., might be suitable for use in the phytoextraction of contaminated soils. The dominant and relative dominant species with low accumulation of metals and dense fibrous root systems, such as Imperata cylindrical (L.) Beauv. var. major C. E. and Miscanthus floridulus (Labill.) Warb., might be suitable for stabilizing such metal contaminated sites.

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.

Recent research has shown that phytoextraction approaches often require soil amendments, such as the application of EDTA, to increase the bioavailability of heavy metals in soils. However, EDTA and EDTA–heavy metal complexes can be toxic to plants and soil microorganisms and may leach into groundwater, causing further environmental pollution. In the present study, vetiver grass (Vetiveria zizanioides) was studied for its potential use in the phytoremediation of soils contaminated with heavy metals. In the pot experiment, the uptake and transport of Pb by vetiver from Pb-contaminated soils under EDTA application was investigated. The results showed that vetiver had the capacity to tolerate high Pb concentrations in soils. With the application of EDTA, the translocation ratio of Pb from vetiver roots to shoots was significantly increased. On the 14th day after 5.0 mmol EDTA kg−1 of soil application, the shoot Pb concentration reached 42, 160, 243 mg kg−1 DW and the root Pb concentrations were 266, 951, and 2280 mg kg−1 DW in the 500, 2500 and 5000 mg Pb kg−1 soils, respectively. In the short soil leaching column (9.0-cm diameter, 20-cm height) experiment, about 3.7%, 15.6%, 14.3% and 22.2% of the soil Pb, Cu, Zn and Cd were leached from the artificially contaminated soil profile after 5.0 mmol EDTA kg−1 of soil application and nearly 126 mm of rainfall irrigation. In the long soil leaching experiment, soil columns (9.0-cm diameter, 60-cm height) were packed with uncontaminated soils (mimicking the subsoil under contaminated upper layers) and planted with vetiver. Heavy metal leachate from the short column experiment was applied to the surface of the long soil column, the artificial rainwater was percolated, and the final leachate was collected at the bottom of the soil columns. The results showed that soil matrix with planted vetiver, could re-adsorb 98%, 54%, 41%, and 88% of the initially applied Pb, Cu, Zn, and Cd, respectively, which may reduce the risk of heavy metals flowing downwards and entering the groundwater.

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.