Cd Phytoremediation Research Articles

Effect of farmyard manure, elemental sulphur and EDTA on growth and phytoextraction of cadmium by spider plants (Chlorophytum comosum L.) under Cd stress

Cadmium (Cd) contamination is considered as a widespread concern at global scale which is serious threats to human health. Phytoremediation is an eco-friendly approach which can remove or immobilize Cd from the soil. Different organic and inorganic amendments can potentially enhance Cd phytoremediation efficiency but the comparison of farmyard manure (FM), elemental sulphur (S) and ethylenediaminetetraacetic acid (EDTA) for Cd phytoremediation through spider plants (Chlorophytum comosum L.) remained unanswered. The present study evaluated the efficiency of S (0.1 and 0.2%), EDTA (0.1 and 0.2%, represented as EDTA-0.1 and EDTA-0.2) and FM (0.5 and 1%, represented as FM-0.5 and FM-1) for remediation of Cd contaminated soils (50 and 100 mg kg-1, represented as Cd-50 and Cd-100) through spider plants. Results depicted that the highest shoots and roots dry biomass was found in FM treated plants followed by S, EDTA and control except in EDTA-0.2 treatment in which the lowest values of these parameters were observed. Application of FM-1 significantly increased the shoot dry weight (120%), root dry weight (99%), as well as photosynthetic attributes in Cd-50 as compared to control. Application of EDTA-0.2 increased the bioavailable fraction of Cd than control and the maximum increase was observed in Cd-100. The highest Cd concentrations in shoot and roots were found in EDTA treated plants followed by S, control and FM irrespective of Cd and amendment levels. Maximum Cd in roots (109%) and shoots (156%) was recorded in plants grown in Cd-100 with EDTA-0.2 than control. The maximum bioaccumulation factor, translocation index, harvest index and root to shoot translocation were observed with EDTA than control and other treatments. EDTA along with spider plants may enhance the uptake of Cd but lower biomass production in the highest dose of EDTA may questioned the efficiency of EDTA.

Salicylic acid improves Nasturtium officinale phytoremediation capability for cadmium-contaminated paddy soils.

Cadmium (Cd) contamination is a severe problem in paddy soils that has affected crops' safety. The present study aimed at remediating Cd-contaminated paddy soil by improving the phytoremediation capability of aquatic accumulator plants. We conducted an experiment to investigate the effects of salicylic acid (SA) on the growth and Cd phytoremediation capability of the aquatic accumulator plant Nasturtium officinale. SA with the concentrations of 100, 150, and 200 mg/L increased the root and shoot biomass of N. officinale, while only 150 mg/L increased the chlorophyll a and b contents. SA increased the activities of peroxidase and catalase of N. officinale to a great extent, but decreased the superoxide dismutase activity and soluble protein content. SA also increased the root Cd content, shoot Cd content, root Cd extraction, and shoot Cd extraction to a large extent. At concentrations of 100, 150, and 200 mg/L, SA increased the shoot Cd extraction by 17.59%, 47.16%, and 43.27%, respectively, compared with the control. Moreover, SA concentration had a quadratic polynomial regression relationship with the root Cd extraction and shoot Cd extraction. The correlation and grey relational analyses revealed that root Cd extraction, shoot biomass, and root biomass were closely associated with shoot Cd extraction of N. officinale. Thus, our results suggest that SA promoted the growth and improved the phytoremediation (extraction) capability of N. officinale, and 150 mg/L SA was the most suitable concentration.

The application of exogenous Se improved the remediation efficiency of Lolium multiflorum Lam grown in nonylphenol-Cd Co-contaminated soil

Nonylphenol (NP) and cadmium (Cd), very toxic to microorganisms, plants, and animals, are often detected simultaneously in soil and water. Selenium (Se) is important to microorganisms and plants. A pot experiment was carried out to explore the effect of Se on the efficiency of ryegrass in the remediation of NP-Cd co-contaminated soil. The removal rate of soil NP and Cd increased by 6.2 % and 30.8 %, respectively, with the application of exogenous Se in NP-contaminated soil (50 mg·kg−1). The selenium significantly increased the catalase activity (11.5–24.5%), but decreased the activities of acid phosphatase (5.5–9.0 %), invertase (19.8–22.5 %), and urease (11.1–18.4 %) in NP contaminated soil (50 mg·kg−1) (p＜0.05). The selenium also increased the relative abundance of beneficial bacteria, including Massilia, Burkholderiaceae, Micrococcales, and Sphingomonas in the NP-Cd co-contaminated soil. These bacteria are closely related to Cd and NP pollution remediation. In summary, applying Se to the contaminated soil effectively improved the efficiency of the phytoremediation of NP and Cd, and also increased the relative abundance and diversity of microorganisms in the NP-Cd co-contaminated soil. These findings can attract practical interest in phytoremediation measures of organic and heavy metals co-contaminated soil.

Dual tolerance of ageratum (Ageratum conyzoides L.) to combined pollution of acid and cadmium: Field survey and pot experiment

A field survey and pot experiment were carried out to screen tolerant plants growing in cadmium (Cd)-polluted mining areas which were co-polluted with acid in soil, and the related physiological and biochemical mechanisms were also analyzed. Thirty-seven species of wild plants and their corresponding soil were collected from a farmland around the mining areas. Ageratum (Ageratum conyzoides L.) with strong Cd-accumulative ability was selected, and its tolerance experiment for acid and Cd with different levels were carried out separately or orthogonally, respectively. Furthermore, the activities of superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT), and the contents of malondialdehyde (MDA), photosynthetic pigments, soluble sugar and proline in its leaves were determined. The results showed that the Cd accumulation in ageratum and sticktight (Bidens pilosa L.) was relatively high, but the latter has been well documented, so we focused on ageratum in the present work. In pot experiment, ageratum grew normally at 100 mg kg−1 Cd in soil, and the Cd concentrations in its roots, stems and leaves were 75.37 ± 7.37, 31.01 ± 3.76 and 53.92 ± 10.05 mg kg−1, respectively. In the case of acid tolerance experiment, all plant individuals of ageratum grew normally when soil pH was over 3.5. In the orthogonal experiment, the Cd accumulation in this plant increased with the decrease of soil pH under the same Cd treatment. Under strong acid conditions, the activity of SOD in leaves of ageratum was increased significantly. When the Cd concentration was 10 mg kg−1 and the soil pH was 5.5 or 3.5, the activities of POD and CAT were significantly increased. In addition, based on stepwise regression analysis, the leaf Cd concentration was significantly positive correlated with the activities of SOD and POD in leaves of ageratum. Therefore, ageratum not only had a strong tolerance for Cd and acid pollution in soil, but also had a strong ability to accumulate Cd. As a common plant in the mining area, it has a great potential for the phytoremediation of Cd and acid co-contaminated soil.

Lactic acid bacteria promoted soil quality and enhanced phytoextraction of Cd and Zn by mustard: A trial for bioengineering of toxic metal contaminated mining soils

Microbial-assisted phytoremediation provides a green approach for remediation of metal contaminated soils. However, the impacts of mono and co-applications of lactic acid bacteria (LAB) on soil biochemical properties and phytoavailability of toxic metals in contaminated mining soils have not yet been sufficiently examined. Consequently, here we studied the effects of Lactobacillus plantarum (P), Lactobacillus acidophilus (A), and Lactobacillus rhamnosus (R) applications alone and in combination on soil enzyme activities and bioavailability and uptake of Cd and Zn by mustard (Brassica juncea) in a smelter-contaminated soil under greenhouse conditions. Among the studied bacteria, P was the most tolerant to Cd-and-Zn contamination. As compared to control, R increased the fresh and dry weight of mustard plants by 53.5% and 63.2%, respectively. Co-application of P + A increased the chlorophyll content by 28.6%, as compared to control. Addition of LAB to soil increased the activity of soil urease, alkaline phosphatase and β-D glucosidase increased by 1.86-fold (P + R), 1.80-fold (R) and 55.16% (P + R), respectively. Application of P + A + R enhanced catalase activity (19.3%) and superoxide dismutase activity (51.2%), while addition of A alone increased peroxidase activity (POD: 15.7%). Addition of P alone and together with A (P + A) enhanced Cd and Zn phytoextraction by mustard shoots up to 51.5% and 52.5%, respectively. We conclude that the single and/or co-application of LAB decreased soil pH, promoted plant growth, antioxidant and enzyme activities, and enhanced the phytoavailability of Cd and Zn in the studied contaminated soil. These findings might be an aid for enhancing the phytoremediation of Cd and Zn using LAB and mustard as a bioenergy crop, which may offer new ideas for field treatment of toxic metals contaminated soils.

Subcellular localization and chemical speciation of Cd in Arabidopsis halleri ssp. gemmifera to reveal its hyperaccumulating and detoxification strategies

The study involving low-level Cd using Arabidopsis halleri ssp. gemmifera is likely suitable for realistic Cd phytoremediation since the available Cd in the contaminated soil is considered low. However, the comprehensive evidence on how this plant undergoes hyperaccumulating-detoxification strategies under low Cd levels in its root, stem, and leaves remained questionable. Therefore, Cd uptake and distribution, intracellular localization, Cd speciation, and metal chelation compounds, glutathione (GSH) and phytochelatins (PCs), were assessed in a time-course analysis to portray their involvement in the Cd hyperaccumulating mechanisms. The results showed that under low Cd stress, 0.3 µmol/L of Cd was completely absorbed by this plant during 336 h, indicating a high Cd uptake system. At the same time, more Cd content accumulated in the stem and leaves compared to Cd content in the root. Excessive Cd being transported to the upper part was supported by the availability of more than 80% of Cd speciation categorized as mobile fractions (ethanol-and H 2 O-extractable forms). The highest Cd soluble fraction found in the root during initial and middle exposure promoted Cd intracellular translocation to stem and leaves. At the end of exposure (336 h), we predicted Cd was compartmentalized in the less metabolite sites, cell wall and vacuole due to less toxicity effect, which was also found in high Cd level (30 µmol/L). Furthermore, GSH and PC as metal complexation compounds owned a minor role even under high Cd exposure level. Therefore, regardless of the Cd supply level, elevating of metal transport and sequestration in the shoots were preferred over thiol complexed forms as common hyperaccumulating strategies in A. halleri ssp. gemmifera. • At low Cd supply, Arabidopsis halleri ssp. gemmifera does not accumulate in the root. • Cd is finally sequestrated in the leaves and existed mainly as soluble fraction. • More than 80 % of Cd chemical form in the tissues was mobile forms. • Glutathione and phytochelatins play a minor role in Cd chelating mechanisms.

Combined effect of humic acid and vetiver grass on remediation of cadmium-polluted water.

Effective treatment of water pollution is an economic and social requirement globally. Humic acid (HA) is a popular mitigator for such waters. However, the combined effect of HA and restorative plants on cadmium (Cd) remediation is not well understood. Therefore, we experimented on Cd remediation using HA along with vetiver grass and HA-vetiver grass. We observed that vetiver grass effectively removed Cd at 15～30mg/L. The accumulation capacity of the root was significantly higher than the shoots (P<0.05), and Cd distribution followed the trend: cell wall >organelle >soluble substance (F1> F2> F3). The plant's accumulation capacity against 25mg/L Cd was higher than for other treatments. The root accumulation capacity was much higher (702.3mg/L) than those without added HA. However, upon adding 200 and 250mg/L HA, the phytoremediation of Cd in the root and shoot significantly reduced (P<0.05). Conversely, HA improved the Cd removal efficiency of the plants, notably at a lower HA concentration (150mg/L). In addition, HA (especially at 150mg/L) influences Cd distribution in vetiver cells (P<0.05) and can significantly increase the proportion of Cd in the root cytoplasm. Consequently, a low HA concentration can significantly improve Cd accumulation in the vetiver, shorten the metal's bioremediation cycle, and improve the biological absorption efficiency.

Cadmium phytoremediation potential of Brassica genotypes grown in Cd spiked Loamy sand soils: Accumulation and tolerance.

Phytoremediation acts as an efficient methodology for management of toxic elements spiked soils. The accumulation and tolerance potential of hyper-accumulator plants for toxic elements act as an index for in-situ removal of toxic elements. Extraction of cadmium (Cd) through its accumulation in harvestable parts of plants has attracted attention as the economic and environment friendly technique. Brassica genotypes have greater potential to accumulate Cd when grown in Cd spiked soils. Therefore, for evaluation of comparative efficiency of three Brassica genotypes (B. juncea, B. campestris and B. napus) in phytoremediation of Cd spiked soils, a pot study was carried out in Cd contaminated soil with 6 levels as 0, 5, 10, 20, 40, and 80mgkg-1 soil. Results indicated that dry biomass production of Brassica genotypes declined with the enhanced Cd contamination in soil. The reduction in grain and shoot yield varied from 2.87 to 1.85 and 11.85 to 8.00g pot-1 with increased Cd contamination from 5 to 80mgkg-1 soil. Similarly, increased levels of Cd contamination resulted in enhanced concentration and accumulation in grains as well as shoots of all Brassica genotypes. Among Brassica genotypes, B. juncea recorded the highest production of dry biomass (12.8g pot-1), Cd accumulation (736.0μg pot-1). Also, the bioaccumulation coefficient and tolerance index indicated that B. juncea is the most tolerant genotype to Cd contamination in soil. Therefore, B. juncea could act as the most potential genotypes for decontamination of Cd spiked soils by preventing its entry into food chain.

Sulfur enhances cadmium bioaccumulation in Cichorium intybus by altering soil properties, heavy metal availability and microbial community in contaminated alkaline soil.

Cadmium (Cd) contamination seriously threatens the soil health and food safety. Combination of amendment and accumulator plant is a green and effective technique to improve phytoremediation of Cd-contaminated alkaline soil. In this study, a potting experiment was conducted to investigate the effect of sulfur on Cd phytoextraction by Cichorium intybus (chicory). Soil chemical and microbial properties were determined to reveal the mechanism of sulfur-assisting Cd phytoremediation by chicory. Soil pH decreased from 7.77 to the lowest 7.30 with sulfur addition (0.6, 0.9 and 1.2 g kg-1, LS, MS and HS treatment); Electric conductivity, sulfate anion and available cadmium concentration increased gradually with increasing sulfur doses. Cd concentration of shoot and root significantly increased from 1.47 to 4.43 mg kg-1, 6.15 to 20.16 mg kg-1 by sulfur treatment relative to CK, which were attributed to increased available Cd concentration induced by decreased pH. Sulfur treatments significantly increased the Cd bioconcentration factor by 64.1%, 118.6%, 201.0% for shoot, 76.3%, 145.6% and 227.7% for root under LS, MS and HS relative to CK treatment, respectively (P < 0.05). However, only MS treatment significantly improved the Cd removal efficiency by 82.9% in comparison of CK treatment (P < 0.05). Microbial community diversity measured by 16SrRNA showed that Thiobacillus and Actinobacteria were the key and dominant strains of soil microbial communities after sulfur addition, which played a pivotal role in the process of sulfur oxidation involved in decrease of soil pH and the transformation of Cd forms. Correlation analysis and path analysis by structural equation model indicated that soil sulfate anion and Thiobacillus directly affected Cd removal efficiency by chicory in Cd-contaminated alkaline soil. This suggests that combination of sulfur and chicory may provide a way to promote Cd bioaccumulation for phytoremediation of Cd-contaminated alkaline soil.

Influence of magnetized water irrigation on characteristics of antioxidant enzyme, ferritin, and Cd excretion in Festuca arundinacea during phytoextraction

The magnetic field can alter the hydrogen-bond structure and polarity characteristics of water; therefore, we hypothesize that magnetized water can affect plant physiological functions, including metal detoxification and excretion. In this study, the amount of Cd excreted on the leaves of Festuca arundinacea was estimated using magnetized water and normal water irrigation patterns. Irrigation with magnetized water improved the shoot dry weight and Cd content in F. arundinacea by 13.6% and 52.8%, respectively, compared to the control. Magnetized water irrigation also increased antioxidant enzyme activities in plant leaves, thereby alleviating the oxidative damage. The concentration of ferritin was 0.91 folds higher than that of the control, increasing the Fe sequestration and detoxification capacity of F. arundinacea. The amount of Cd excreted was significantly higher under magnetized water irrigation, thereby increasing the annual Cd removal by 109.7% from soil by leaf washing compared with that of the control. In contrast, F. arundinacea irrigated with magnetized water excreted 38.1% less Fe owing to the increase in ferritin levels, compared with that of the control. This study suggests a novel pathway of Cd phytoremediation by rinsing excreted Cd from the leaf surface without harvesting and replanting F. arundinacea.

Physiological and rhizospheric response characteristics to cadmium of a newly identified cadmium accumulator Coreopsis grandiflora Hogg. (Asteraceae)

Screening for superior cadmium (Cd) phytoremediation resources and uncovering the mechanisms of plant response to Cd are important for effective phytoremediation of Cd-polluted soils. In this study, the characteristics of Coreopsis grandiflora related to Cd tolerance and accumulation were analyzed to evaluate its Cd phytoremediation potential. The results revealed that C. grandiflora can tolerate up to 20 mg kg−1 of Cd in the soil. This species showed relatively high shoot bioconcentration factors (1.09–1.85) and translocation factors (0.46–0.97) when grown in soils spiked with 5–45 mg kg–1 Cd, suggesting that C. grandiflora is a Cd accumulator and can potentially be used for Cd phytoextraction. Physiological analysis indicated that antioxidant enzymes (i.e., superoxide dismutase, peroxidase, and catalase) and various free amino acids (e.g., proline, histidine, and methionine) participate in Cd detoxification in C. grandiflora grown in soil spiked with 20 mg kg–1 of Cd (Cd20). The overall microbial richness and diversity remained similar between the control (Cd0) and Cd20 soils. However, the abundance of multiple rhizospheric microbial taxa was altered in the Cd20 soil compared with that in the Cd0 soil. Interestingly, many plant growth-promoting microorganisms (e.g., Nocardioides, Flavisolibacter, Rhizobium, Achromobacter, and Penicillium) enriched in the Cd20 soil likely contributed to the growth and vitality of C. grandiflora under Cd stress. Among these, some microorganisms (e.g., Rhizobium, Achromobacter, and Penicillium) likely affected Cd uptake by C. grandiflora. These abundant plant growth-promoting microorganisms potentially interacted with soil pH and the concentrations of Cd and AK in soil. Notably, potassium-solubilizing microbes (e.g., Rhizobium and Penicillium) may effectively solubilize potassium to assist Cd uptake by C. grandiflora. This study provides a new plant resource for Cd phytoextraction and improves our understanding of rhizosphere-associated mechanisms of plant adaptation to Cd-contaminated soil.

Tagetes lemmonii (Atapethiya) as a Potential Candidate for Phytoremediation of Cadmium in Soils

Phytoremediation is one of the eco-friendly strategies used to remove heavy metals such as Cadmium (Cd) from the environment. Tagetes erecta is one such plant used in the phytoremediation of Cd polluted soils. T. lemmonii which belongs to the same genus was used in the current study to investigate its tolerance to cadmium to determine its suitability in phytoremediation of Cd under greenhouse conditions. One-month-old nursery plants were uprooted and transferred into soil containing pots and allowed to establish under greenhouse conditions for one week. Then, soil in pots was treated with 20 mgkg-1, 50 mgkg-1, 100 mgkg-1, and 200 mgkg-1 Cd2+ concentration series with four replicates for each. The control treatments were four pots of plants without Cd2+ added and four pots of soil treated with Cd2+ in each concentration without plants. At two-day intervals, morphological changes such as the height of the plant, and the number of leaves were observed and after 14 days, plants were uprooted and the number of leaves, leaf area, number of rootlets, length of the root, length of the shoot, and total length of the plants were compared among treatments and controls. Cadmium-treated plants, in comparison to the control plants, showed a significant increase in leaf area (p=0.000), fresh weight (p=0.000), dry weight (p=0.000) and a significant decrease in root length (p=0.005 and the number of rootlets (p=0.000) except at 100 and 50 mgkg-1respectively. The increase in plant height (p=0.950), the increase in shoot length (p=0.914), and the decrease in the number of leaves (p=0.322) were not significant. The results suggest that T. lemmonii has a significant survival ability under Cd2+ in soil. Significant total height increments of T. lemmonii plants under Cd2+ treatments suggest that this plant is a hyperaccumulator of Cd2+. Future investigations on anatomical parameters, accumulation, and reduction of Cd2+ in plant tissues and soil respectively will reveal its remediation potential of Cd2+. The results of this study will be utilized to develop phytoremediation strategies for Cd2+ polluted soils using T. lemmonii.&#x0D; Keywords: Heavy metals, Ornamental plants, Eco-friendly, Hyperaccumulation

Enhanced Cd Phytoextraction by Solanum nigrum L. from Contaminated Soils Combined with the Application of N Fertilizers and Double Harvests.

It is very important to increase phytoremediation efficiency in practice in suitable climatic conditions for plant growth through multiple harvests. Solanum nigrum L. is a Cd hyperaccumulator. In the present experiment, after applying different types of N fertilizers (NH4HCO3, NH4Cl, (NH4)2SO4, CH4N2O), root and shoot biomasses and Cd phytoextraction efficiency of S. nigrum effectively improved (p < 0.05). Shoot biomasses of S. nigrum harvested at the first florescence stage plus the amounts at the second florescence stage were higher than those harvested at the maturation stage, which indicates that S. nigrum Cd phytoaccumulation efficiency was higher in the former compared to the latter as there was no clear change in Cd concentration (p < 0.05). The pH value and extractable Cd contents showed no changes, regardless of whether N fertilizer was added or not at different growth stages. In addition, after N fertilizer was applied, H2O2 and malondialdehyde (MDA) contents in S. nigrum in vivo were lower compared to those that had not received N addition (CK); similarly, the concentration of proline was decreased as well (p < 0.05). The activity of the antioxidant enzyme catalase (CAT), harvested at different growth periods after four types of N fertilizer applications, obviously decreased in S. nigrum shoots, while peroxidase (POD) and superoxide dismutase) (SOD) activities increased (p < 0.05). Our study demonstrated that (NH4)2SO4 treatment exerted the most positive effect and CH4N2O the second most positive effect on S. nigrum Cd phytoremediation efficiency in double harvests at florescence stages, and the growth conditions were better than others.

Plant growth-promoting actinobacterial inoculant assisted phytoremediation increases cadmium uptake in Sorghum bicolor under drought and heat stresses

In this study, two proficient Cadmium (Cd) resistant and plant growth-promoting actinobacterial strains were isolated from metal-polluted soils and identified as Streptomyces sp. strain RA04 and Nocardiopsis sp. strain RA07. Multiple abiotic stress tolerances were found in these two actinobacterial strains, including Cd stress (CdS), drought stress (DS) and high-temperature stress (HTS). Both actinobacterial strains exhibited multifarious plant growth-promoting (PGP) traits such as phosphate solubilization, and production of indole-3-acetic acid, siderophores and 1-aminocyclopropane-1-carboxylate deaminase under CdS, DS and HTS conditions. The inoculation of strains RA04 and RA07 significantly increased Sorghum bicolor growth and photosynthetic pigments under CdS, DS, HTS, CdS + DS and CdS + HTS conditions as compared to their respective uninoculated plants. The actinobacterial inoculants reduced malondialdehyde concentration and enhanced antioxidant enzymes in plants cultivated under various abiotic stress conditions, indicating that actinobacterial inoculants reduced oxidative damage. Furthermore, strains RA04 and RA07 enhanced the accumulation of Cd in plant tissues and the translocation of Cd from root to shoot under CdS, CdS + DS and CdS + HTS treatments as compared to their respective uninoculated plants. These findings suggest that RA04 and RA07 strains could be effective bio-inoculants to accelerate phytoremediation of Cd polluted soil even in DS and HTS conditions.

Risk assessment of groundwater pollution during GLDA-assisted phytoremediation of Cd- and Pb-contaminated soil

Using biodegradable chelate agents during phytoremediation of heavy metal contaminated soil is a growing consensus. However, the potential risk of groundwater pollution and the corresponding quantification during biodegradable chelate-assisted phytoremediation is currently ignored. This study provides a risk-assessment framework containing the practical boundaries and the transport model of chelate-metal complexes and metals at the subsoil, to quantify and predict the leaching risk of biodegradable N,N-bis(carboxymethyl)glutamic acid (GLDA)-induced phytoremediation during heavy rainfall. The case of ethylenediaminetetraacetic acid (EDTA)-assisted phytoremediation was also considered. Here, adsorption experiments and percolation experiments were conducted based on the subsoil to obtain adsorption parameters and hydrodynamic parameters. Optimal parameters derived from inverse fitting and the boundary condition originated from pot leaching experiments were used for the framework establishment and Hydrus-1D simulation. Results showed that fewer GLDA-metal complexes were adsorbed on the soil particles compared with metals, inferring their great mobility in the subsoil. However, the chemical and physical non-equilibrium process indicated by the asymmetric breakthrough curves of the percolation experiments would also affect the transport behavior of contaminants in the sub-vadose zone. In addition, adsorption parameters and optimal hydrodynamic parameters aptly described breakthrough curves of the chelate-metal complexes, and could better predict their transport behavior at the subsoil. Opposite to the previous findings, prediction results showed that heavy rainfall during GLDA-assisted phytoextraction resulted in Cd and Pb pollution in shallow groundwater, where the pollution degree was related to properties of the soil horizon, depth of the water table, and types of contaminants. The degradation feature of GLDA-metal complexes would fail to keep groundwater from heavy metal contamination at the sub-vadose zone with high permeability. Thus, we recommend our risk-assessment framework to optimize the operating condition before utilizing the biodegradable chelate-assisted phytoremediation.

Organic acids in conjunction with various oilseed sunflower cultivars promote Cd phytoextraction through regulating micro-environment in root zone

Exogenous chelates in conjunction with phytoextraction have been demonstrated as a prominent potential for remediating Cd-contaminated soils. Among bath chelates, low molecular-weight organic acids have the advantages of smaller impact on plant growth and lower negative effects on soil environment. In this research, oilseed sunflowers were selected as the main extraction plants, and the Cd phytoextraction effects of different cultivars, varies organic acid types and concentrations combinations were systematically examined. Results showed that adding exogenous organic acids improved biomass of all issues, but the increase varied with the cultivar. For both cultivars, their biomass increase with acid application concavely, with the acid application rate at which the biomass peaked differed between the two cultivars. Adding exogenous acids boosted Cd concentration in roots and shoots in general, but it reduced the Cd content in seeds as well as the rhizosphere and bulk soil. In addition to its impact on Cd, application of organic acids also reduced nutrient and salinity in the rhizosphere and promoted root uptake of nutrients as a result. Cd content plant an soil was closely related to nutrients in soil and plant. The organic acids reduced soil Cd and nutrients in soil proximal to the roots. It was also found that adding acids reduced Cd content in the top 0–10 cm soil but increased the Cd content in the 10–30 cm soil, alluding a potential leaching. Therefore, adding organic acids to improve phytoremediation of Cd needs to proceed with care.

Endophyte colonization enhanced cadmium phytoremediation by improving endosphere and rhizosphere microecology characteristics

This study investigated the phytoremediation efficiency of Cd-contaminated soils by hyperaccumulator P. acinosa and its endophyte B. cereus, and evaluated the variation of rhizosphere/endosphere microecology characteristics. The result showed that endophyte PE31, which could successfully colonize on P. acinosa root, increased plant Cd uptake by 42.90% and 28.85% in low and high Cd contaminated soils by promotion of plant biomass and Cd concentration in plant tissues. The improved phytoremediation may attribute to the endophyte inoculation, which significantly improved the bioavailable heavy metal (HM) percentage, nutrient cycling related enzyme activities and nutrient contents including available potassium, phosphorus and organic matter. Additionally, the relative abundance beneficial bacteria Bacillus (significantly increased by 81.23% and 34.03% in the endosphere, and by 4.86% and 8.54% in rhizosphere in low and high Cd contaminated soils) and Lysobacter, showed positive and close correlation with plant growth and HM accumulation. These results indicated that endophyte inoculation could reshape rhizosphere and endosphere microecology characteristics, which enhanced the potential for phytoremediation of Cd contaminated soils.

Large-scale screening and characterization of Cd accumulation and ultrastructural deformation in duckweed

Cadmium (Cd) pollution in soil, rivers and lakes is a serious problem due to the current industrialization and urbanization in China. Duckweeds are recognized as promising species for Cd phytoremediation. However, intraspecific variations in Cd accumulation in duckweeds remain largely unknown. In this study, 16 accessions selected from 39 geographically isolated duckweed strains were chosen to investigate their Cd remediation abilities. The optimal accession Landoltia punctata named 07SGZP01 (L. punctata 0701) was identified and shown to accumulate maximal Cd in the body while maintaining the highest biomass. The dominant variety treated with different Cd concentrations showed that the biomass of L. punctata 0701 was significantly lower than that of the control group (CK). Cd contents in L. punctata 0701 were substantially increased from 2511.1 to 30,641.01 mg kg−1 with an increase in Cd treatment levels from 0.3 to 20 mg L−1. The transport coefficient (TF) increased as Cd levels increased from 0.3 to 2 mg L−1. In addition, the Cd content in leaves was greater than that in roots (TF > 1) within this Cd concentration range, whereas the Cd content in roots was greater than that in leaves (TF < 1) when the concentration of the Cd treatment was greater than 5 mg L−1. The bioaccumulation factor (BCF) decreased significantly with increasing Cd levels (P < 0.05). The rate of Cd removal in the solution gradually decreased with increasing Cd concentrations, and the removal rate achieved the highest value (75%) when the Cd concentration was 0.5 mg L−1. In addition, Cd treatment (2 mg L−1) not only damaged the ultrastructure of L. punctata 0701, as characterized by chloroplast deformation and cell vacuolation but also caused most of the stomata to close, and the leaf epidermal cells were damaged and ruptured.

Cytological and physiological tolerance of transgenic tobacco to Cd stress is enhanced by the ectopic expression of SmZIP8

Zrt and Irt-like proteins (ZIPs) are responsible for transporting various divalent metal cations. However, information about the characteristics of the cellular and physiological tolerance of plant ZIPs to Cd stress is still limited. The expression levels of SmZIP8 in Salix matsudana Koidz were upregulated by Cd stress. The complete length of SmZIP8 from S. matsudana was cloned, and transgenic tobacco was obtained by Agrobacterium-mediated transformation. Then, the tolerance to Cd stress of wild-type (WT) and transgenic tobacco seedlings was analyzed and compared by studying the cytotoxicity of the root tip cells, photosynthetic parameters, histochemical staining of O2− and H2O2, the activities of antioxidant enzymes, and malondialdehyde content under Cd stress. In comparison with WT tobacco, the ectopic expression of SmZIP8 in tobacco promoted the cytological tolerance of the transgenic tobacco to Cd stress by reducing cell damage, raising the mitotic indexes, and reducing the rate of chromosome aberration of the root cells. Meanwhile, the results of increased photosynthetic capacity, decreased oxidative damage, and activated antioxidant enzymes showed that the physiological tolerance of transgenic tobacco to Cd was enhanced. The principal component analysis for the above physiological parameters explained 96.08% of the total variance (PC1, 77.77%; PC2, 18.31%), indicating a significant difference in Cd tolerance abilities between the tobacco expressing SmZIP8 and WT tobacco. Therefore, SmZIP8 may be considered as an important genetic resource for the phytoremediation of Cd or other heavy metal pollution via the use of transgenic plants obtained through genetic transformation.

Effects of Pb-, Cd-resistant bacterium Pantoea sp. on growth, heavy metal uptake and bacterial communities in oligotrophic growth substrates of Lolium multiflorum Lam.

Phosphate-solubilizing bacteria (PSB) can accelerate phytoremediation, especially in those fertilized soils. However, PSB function in oligotrophic growth substrates remains poorly studied. In this study, we isolated lead (Pb)- and cadmium (Cd)-resistant PSB from contaminated sandy soil at an abandoned lubricant plant. The isolated Pantoea sp. PP4 (PP4 hereafter) can produce organic acid and IAA (Indole-3-acetic acid) and dissolve up to 238mg/L of inorganic phosphate Ca2(PO4)3, exhibiting biosorption capability for Pb and bioprecipitation for Pb and Cd. In the sand pot experiment, inoculation of PP4 increased the accumulation of Pb and Cd in Lolium multiflorum Lam. by 28.9% and 95.5%, respectively, and increased the available phosphorous in oligotrophic river sand by 30.8% (P < 0.05). Meanwhile, the growth of Lolium multiflorum Lam. was also stimulated, resulting in 89.2%, 57.1%, 184.6%, and 28.5% increase in fresh weight, dry weight, root length, and shoot length, respectively. NMDS analysis showed that the bacterial communities in river sand were more clustered after inoculation with PP4. These results indicated that the application of Pantoea sp. PP4 can facilitate the phytoremediation of Pb and Cd in oligotrophic growth substrates, forming a convergent bacterial community. Our findings highlighted the importance of identifying ideal PSB to improve phytoremediation efficiency in oligotrophic environments.