EDTA and organic acids assisted phytoextraction of Cd and Zn from a smelter contaminated soil by potherb mustard (Brassica juncea, Coss) and evaluation of its bioindicators

Study on the enhancement effect of EDTA and oxalic acid on phytoremediation of Cr(VI) from soil using Datura stramonium L

This study investigated the enhancing effects of soil treatment with ethylene diamine tetraacetic acid (EDTA) and oxalic acid (OA) on the remediation of Cr(VI) contaminated soil by Datura stramonium L. A greenhouse pot experiment was conducted, where Cr(VI) contaminated soil was treated with 100 mg/kg Cr(VI) and varying concentrations of EDTA (5 and 10 mmol/kg) and OA (5 and 10 mmol/kg). The effects of these soil treatments on biomass, chlorophyll content, antioxidant enzyme activities, and Cr(VI) enrichment and translocation efficiency of D. stramonium were evaluated. The results showed that added OA to soil significantly increased the biomass and chlorophyll content of D. stramonium. The addition of 10 mmol/kg of OA to soil increased the plant biomass by 67.16 % and chlorophyll b content by 40.01 %. In addition, OA soil treatment significantly enhanced the activities of superoxide dismutase (SOD) by 6.36 %, peroxidase (POD) by 163.13 %, catalase (CAT) by 36.92 %, and ascorbate peroxidase (APX) by 32.12 %, which effectively alleviated the oxidative stress induced by Cr(VI). In contrast, soil treatment with a high concentration of EDTA (10 mmol/kg) significantly reduced plant biomass and chlorophyll content, although it increased the biological concentration factor (BCF) of the stem and leaf, as well as the translocation factor (TF). In conclusion, appropriate amounts of EDTA and OA added to soil can enhance the phytoremediation efficiency of D. stramonium grown in Cr(VI) contaminated soil, with OA added to soil being more effective than addition of EDTA. This study revealed the potential mechanisms of chelating agents EDTA and OA in enhancing the phytoremediation of Cr(VI) contaminated soil by D. stramonium, providing a scientific basis for further optimization of phytoremediation techniques.

The Role of Low-Molecular-Weight Organic Acids in Metal Homeostasis in Plants

Low-molecular-weight organic acids (LMWOAs) are essential O-containing metal-binding ligands involved in maintaining metal homeostasis, various metabolic processes, and plant responses to biotic and abiotic stress. Malate, citrate, and oxalate play a crucial role in metal detoxification and transport throughout the plant. This review provides a comparative analysis of the accumulation of LMWOAs in excluders, which store metals mainly in roots, and hyperaccumulators, which accumulate metals mainly in shoots. Modern concepts of the mechanisms of LMWOA secretion by the roots of excluders and hyperaccumulators are summarized, and the formation of various metal complexes with LMWOAs in the vacuole and conducting tissues, playing an important role in the mechanisms of metal detoxification and transport, is discussed. Molecular mechanisms of transport of LMWOAs and their complexes with metals across cell membranes are reviewed. It is discussed whether different endogenous levels of LMWOAs in plants determine their metal tolerance. While playing an important role in maintaining metal homeostasis, LMWOAs apparently make a minor contribution to the mechanisms of metal hyperaccumulation, which is associated mainly with root exudates increasing metal bioavailability and enhanced xylem loading of LMWOAs. The studies of metal-binding compounds may also contribute to the development of approaches used in biofortification, phytoremediation, and phytomining.

Extraction of heavy metals from copper tailings by ryegrass (Lolium perenne L.) with the assistance of degradable chelating agents

Heavy metal contamination is an urgent ecological governance problem in mining areas. In order to seek for a green and environmentally friendly reagent with better plant restoration effect to solve the problem of low efficiency in plant restoration in heavy metal pollution soil. In this study, we evaluated the effects of three biodegradable chelating agents, namely citric acid (CA), fulvic acid (FA) and polyaspartic acid (PASP), on the physicochemical properties of copper tailings, growth of ryegrass (Lolium perenne L.) and heavy metal accumulation therein. The results showed that the chelating agent application improved the physicochemical properties of copper tailings, increased the biomass of ryegrass and enriched more Cu and Cd in copper tailings. In the control group, the main existing forms of Cu and Cd were oxidizable state, followed by residual, weak acid soluble and reducible states. After the CA, FA or PASP application, Cu and Cd were converted from the residual and oxidizable states to the reducible and weak acid soluble states, whose bioavailability in copper tailings were thus enhanced. Besides, the chelating agent incorporation improved the Cu and Cd extraction efficiencies of ryegrass from copper tailings, as manifested by increased root and stem contents of Cu and Cd by 30.29-103.42%, 11.43-74.29%, 2.98-110.98% and 11.11-111.11%, respectively, in comparison with the control group. In the presence of multiple heavy metals, CA, FA or PASP showed selectivity regarding the ryegrass extraction of heavy metals from copper tailings. PCA analysis revealed that the CA-4 and PASP-7 treatment had great remediation potentials against Cu and Cd in copper tailings, respectively, as manifested by increases in Cu and Cd contents in ryegrass by 90.98% and 74.29% compared to the CK group.

Impacts of simulated atmospheric cadmium deposition on the physiological response and cadmium accumulation of Sedum plumbizincicola

Atmospheric cadmium (Cd) deposition contributes to the accumulation of Cd in the soil-plant system. Sedum plumbizincicola is a Cd and Zn hyperaccumulator commonly used for the phytoremediation of Cd-contaminated soil. However, studies on the effects of atmospheric Cd deposition on the accumulation of Cd and physiological response in S. plumbizincicola are still limited. A Cd solution spraying pot experiment was conducted with S. plumbizincicola at three atmospheric Cd deposition concentrations (4, 8, and 12 mg/L). Each Cd concentration levels was divided into two groups, non-mulching (foliar-root uptake) and mulching (foliar uptake). The soil type used in the experiment was reddish clayey soil collected from a farmland. The results showed that compared with the non-mulching control, the fresh weight of S. plumbizincicola in non-mulching with high atmospheric Cd deposition (12 mg/L) increased by 11.35%. Compared with those in the control group, the malondialdehyde (MDA) content in the non-mulching and mulching S. plumbizincicola groups increased by 0.88-11.06 nmol/L and 0.96-1.32 nmol/L, respectively. Compared with those in the non-Cd-treated control group, the shoot Cd content in the mulching group significantly increased by 11.09-180.51 mg/kg. Under high Cd depositions, the Cd in S. plumbizincicola mainly originated from the air and was stored in the shoots (39.7-158.5%). These findings highlight that the physiological response and Cd accumulation of S. plumbizincicola were mainly affected by high Cd deposition and suggest that atmospheric Cd could directly be absorbed by S. plumbizincicola. The effect of atmospheric deposition on S. plumbizincicola cannot be ignored.

Effect of ammonium sulfate combined with aqueous bio-chelator on Cd uptake by Cd-hyperaccumulator Solanum nigrum L

The efficacy of using plants to phytoremediate heavy metal (HM) contaminated soils can be improved using soil amendments. These amendments may both increase plant biomasses and HMs uptake. We aimed to determine the composite effect of ammonium sulfate ((NH4)2SO4) combined with the application of an aqueous stem-extracted bio-chelator (Bidens tripartita L) on the plant biomasses and cadmium (Cd) phytoextraction by Solanum nigrum L. The constant (NH4)2SO4 application mode plus bio-chelator additives collectively enhanced the shoot Cd extraction ability owing to the increased plant biomass and shoot Cd concentration by S. nigrum. The shoot Cd extraction and the soil Cd decreased concentration confirmed the optimal Cd phytoextraction pattern in K8 and K9 treatments (co-application of (NH4)2SO4 and twofold/threefold bio-chelators). Accordingly, Cd contamination risk in the soil (2 mg kg-1) could be completely eradicated (<0.2 mg kg-1) after three rounds of phytoremediation by S.nigrum based on K8 and K9 treatments through calculating soil Cd depletion. The microorganism counts and enzyme activities in rhizosphere soils at treatments with the combined soil additives apparently advanced. In general, co-application mode of (NH4)2SO4 and aqueous bio-chelator was likely to be a perfect substitute for conventional scavenger agents on account of its environmental friendliness and cost saving for field Cd contamination phytoremediation by S. nigrum.

Biodegradable chelating agents for enhancing phytoremediation: Mechanisms, market feasibility, and future studies

Heavy metals in soil significantly threaten human health, and their remediation is essential. Among the various techniques used, phytoremediation is one of the safest, most innovative, and effective. In recent years, the use of biodegradable chelators to assist plants in improving their remediation efficiency has gained popularity. These biodegradable chelators aid in the transformation of metal ions or metalloids, thereby facilitating their mobilization and uptake by plants. Developed countries are increasingly adopting biodegradable chelators for phytoremediation, with a growing emphasis on green manufacturing and technological innovation in the chelating agent market. Therefore, it is crucial to gain a comprehensive understanding of the mechanisms and market prospects of biodegradable chelators for phytoremediation. This review focuses on elucidating the uptake, translocation, and detoxification mechanisms of chelators in plants. In this study, we focused on the effects of biodegradable chelators on the growth and environmental development of plants treated with phytoremediation agents. Finally, the potential risks associated with biodegradable chelator-assisted phytoremediation are presented in terms of their availability and application prospects in the market. This study provides a valuable reference for future research in this field.

Lanthanum promotes Solanum nigrum L. growth and phytoremediation of cadmium and lead through endocytosis: Physiological and biochemical response, heavy metal uptake and visualization

Rare earth elements (REEs) are important to enhance agricultural productivity. The utilization of phytoremediation as a green technology for addressing heavy metal (HMs) contamination in soil and wastewater has gained significant attention. In our research, we conducted indoor hydroponic experiments to examine the impacts of lanthanum (La) on the growth and enrichment capacity of Solanum nigrum L. (S. nigrum). S. nigrum was cultivated in 10 mg·L-1 of cadmium (Cd), 25 mg·L-1 of lead (Pb), and a mixture of both (5 mg·L-1 Cd + 15 mg·L-1 Pb). Additionally, S. nigrum were subjected to foliar spray or hydroponic supplementation of La(III). The treatment with La(III) significantly increased total fresh weight by 17.82 % to 42.20 %, compared to the treatment without La(III). Furthermore, La(III) facilitated the endocytosis of roots and enhanced Cd2+ flux ranging from 15.64 % to 75.99 % when compared to the treatment without La(III). Foliar and hydroponic application of La(III) resulted in an increase in the translocation factors (TF) in plants of Cd and Pb compared to treatments without La(III). These findings can offer valuable insights into the potential of La(III) to enhance the phytoremediation of soil or wastewater polluted with compounds.

Exogenous chelating agents influence growth, physiological characteristics and cell ultrastructure of Robinia pseudoacacia seedlings under lead-cadmium stress

Heavy metal pollution of soil, especially by lead (Pb) and cadmium (Cd), is a serious problem worldwide. The application of safe chelating agents, combined with the growing of tolerant trees, constitutes an approach for phytoremediation of heavy-metal-contaminated soil. This study aimed to determine whether the two safe chelators, tetrasodium glutamate diacetate (GLDA) and citric acid (CA), could improve the phytoremediation capacity of black locust (Robinia pseudoacacia L.) in a Pb-Cd-contaminated soil and to find the key factors affecting the biomass accumulation of stressed black locust. In Pb- and Cd-stressed black locust plants, medium- and high-concentration GLDA treatment inhibited the growth, chlorophyll synthesis and maximum photochemical efficiency (Fv/Fm), promoted the absorption of Pb and Cd ions and resulted in the shrinkage of chloroplasts and starch grains when compared with those in Pb- and Cd-stressed plants that were not treated with GLDA. The effects of CA on plant growth, ion absorption, chlorophyll content, chlorophyll fluorescence and organelle size were significantly weaker than those of GLDA. The effect of both agents on Cd absorption was greater than that on Pb absorption in all treatments. The levels of chlorophyll a and plant tissue Cd and rates of starch metabolism were identified as the key factors affecting plant biomass accumulation in GLDA and CA treatments. In the future, GLDA can be combined with functional bacteria and/or growth promoters to promote the growth of Pb- and Cd-stressed plants and to further improve the soil restoration efficiency following pollution by heavy metals. Application of CA combined with the growing of black locust plants has great potential for restoring the Cd-polluted soil. These findings also provide insights into the practical use of GLDA and CA in phytoremediation by R. pseudoacacia and the tolerant mechanisms of R. pseudoacacia to Pb-Cd-contaminated soil.

Effects of magnetic field on cd subcellular distribution and chemical speciation in Noccaea caerulescens

Implementing an external magnetic field of suitable strength has been reported to increase Cd uptake by Noccaea caerulescence. However, only a few mechanisms promoting this efficiency have been reported. A series of culture experiments was conducted to explore how Cd subcellular distribution and speciation vary within the tissue of N. caerulescens when subjected to external magnetic fields of different intensities. Without a magnetic field, over 80% of the Cd was deposited in the cell wall and cytoplasm, indicating that cell wall retention and cytoplasm isolation are significant mechanisms for the detoxification of Cd. An external magnetic field (120 mT) increased the Cd concentrations deposited in the cytoplasm and water-soluble inorganic Cd in the roots, increasing the cell wall-bound Cd and undissolved Cd phosphate in the shoots. Meanwhile, the magnetic field increased carbonic anhydrase activity in plant shoots, except at 400 mT. These results indicated that an external field can elevate the Cd decontamination capacity of N. caerulescens by changing the subcellular compartmentalization and speciation of Cd in different tissues.

Applicability of the sigmoid model to estimate heavy metal uptake in maize and sorghum as affected by organic acids

Although assisted phytoremediation using chemical treatments is a suitable technique for the removal of heavy metals (HMs), the estimation of this process using simple models is also crucial. For this purpose, a greenhouse trial was designed to evaluate the effectiveness of citric, oxalic, and tartaric acid on Cd, Pb, Ni, and Zn phytoremediation by maize and sorghum and to estimate this process using sigmoid HMs uptake model. Results showed that mean values of root and shoot dry weight and metals uptake, translocation factor (TF) of Pb and Zn, and uptake efficiency (UE) of Cd in maize were higher than sorghum but the TF of Cd and the phytoextraction efficiency (PEE) and UE of Pb in sorghum were higher than maize. Citric, oxalic, and tartaric acid significantly increased the UE of Pb by 17.7%, 22.5%, and 32.5%, respectively. Tartaric acid significantly increased the mean values of shoot dry weight, shoot Cd, Pb, and Ni uptake, and PEE of Pb and Ni, but decreased TF of Zn. The R2, NRMSE, and KM values indicated the ability of sigmoid HM uptake model in estimating HMs uptake in maize and sorghum treated with organic acids. Thus, tartaric acid was more effective than citric and oxalic acids to enhance phytoremediation potential. Sigmoid HM uptake model is suitable to estimate the HMs uptake in plants treated with organic acids at different growth stages.

Chelate facilitated phytoextraction of Pb, Cd, and Zn from a lead-zinc mine contaminated soil by three accumulator plants

This study aims to evaluate the enhancement of phytoextraction of heavy metals (Pb, Cd, and Zn) by species Marrubium cuneatum, Stipa arabica, and Verbascum speciosum, through EDTA amendment. Assisted phytoextraction pot experiments were performed at different EDTA dosages (0, 1, 3, and 5 mmol kg-1 soil). The DTPA-extractable metal content increased in the presence of EDTA, followed by their contents in the tissues of all three studied species. Resulting from oxidative stress, the activity of antioxidant enzymes such as glutathione peroxidase (GPX), superoxide dismutase (SOD), and catalase (CAT) increased when the chelating agent was added. EDTA in higher doses partially decreased chlorophyll concentration, and 5 mmol kg-1 of that reduced the biomass of the studied species. The bioconcentration factor (BCF) for Cd was notably high in all studied plants and considerably elevated for Zn and Pb with the addition of EDTA in M. cuneatum and S. arabica (BCF > 1), whilst an accumulation factor greater than one (AF > 1) was found for Cd in all species and for Pb in the case of S. arabica. In general, the results demonstrated that EDTA can be an effective amendment for phytoextraction of Cd, Zn, and Pb by M. cuneatum, V. speciosum and S. arabica in contaminated soils.

Comparative assessment of two biodegradable chelants, S,S-ethylenediamine disuccinic acid and nitrilotriacetic acid, in facilitating Cd remediation by lesser swine cress (Coronopus didymus, Brassicaceae)

Chemically assisted phytoremediation is suggested as an effective approach to amplify the metal-remediating potential of hyperaccumulators. The current study assessed the efficiency of two biodegradable chelants (S,S-ethylenediamine disuccinic acid, EDDS; nitrilotriacetic acid, NTA) in enhancing the remediation of Cd by Coronopus didymus (Brassicaceae). C. didymus growing in Cd-contaminated soil (35-175 mg kg-1 soil) showed increased growth and biomass due to the hormesis effect, and chelant supplementation further increased growth, biomass, and Cd accumulation. A significant interaction with chelants and different Cd concentrations was observed, except for Cd content in roots and Cd content in leaves, which exhibited a non-significant interaction with chelant addition. The effect of the NTA amendment on the root dry biomass and shoot dry biomass was more pronounced than EDDS at all the Cd treatments. Upon addition of EDDS and NTA, bio-concentration factor values were enhanced by ~184-205 and ~ 199-208, respectively. The tolerance index of root and shoot increased over the control upon the addition of chelants, with NTA being better than EDDS. With chelant supplementation, bio-accumulation coefficient values were in the order Cd35 + NTA (~163%) > Cd105 + NTA (~137%) > Cd35 + EDDS (~89%) > Cd175 + NTA (~85%) > Cd105 + EDDS (~62%) > Cd175 + EDDS (~40%). The translocation factor correlated positively (r ≥ 0.8) with tolerance index and Cd accumulation in different plant parts. The study demonstrated that chelant supplementation enhanced Cd-remediation efficiency in C. didymus as depicted by improved plant growth and metal accumulation, and NTA was more effective than EDDS in reclaiming Cd.

Roles and significance of chelating agents for potentially toxic elements (PTEs) phytoremediation in soil: A review

Phytoremediation is a biological remediation technique known for low-cost technology and environmentally friendly approach, which employs plants to extract, stabilise, and transform various compounds, such as potentially toxic elements (PTEs), in the soil or water. Recent developments in utilising chelating agents soil remediation have led to a renewed interest in chelate-induced phytoremediation. This review article summarises the roles of various chelating agents and the mechanisms of chelate-induced phytoremediation. This paper also discusses the recent findings on the impacts of chelating agents on PTEs uptake and plant growth and development in phytoremediation. It was found that the chelating agents have increased the rate of metal absorption and translocation up to 45% from roots to the aboveground plant parts during PTEs phytoremediation. Besides, it was also explored that the plants may experience some phytotoxicity after adding chelating agents to the soil. However, due to the leaching potential of synthetic chelating agents, the use of organic chelants have been explored to be used in PTEs phytoremediation. Finally, this paper also presents comprehensive insights on the significance of using chelating agents through SWOT analysis to discuss the advantages and limitations of chelate-induced phytoremediation.

Effect of citric acid on phytoextraction potential of Cucurbita pepo, Lagenaria siceraria, and Raphanus sativus plants exposed to multi-metal stress

Phytoextraction is a novel technique that involves using plants to remove heavy metals from contaminated soils. An outdoor pot experiment was designed to evaluate the phytoextraction potential of three plant species Cucurbita pepo, Lagenaria siceraria, and Raphanus sativus in soil contaminated with multiple metals (Cd, Co, Cr, Cu, Ni, Pb, and Zn) under the application of citric acid. The results showed that Raphanus sativus, out of all the studied plants, had the highest root and shoot dry weight and the capacity to accumulate all heavy metals at higher concentrations except for Cu. The application of citric acid into the polluted soil significantly increased plant growth, biomass, and heavy metal uptake. High bioconcentration values indicate that Raphanus sativus is a promising plant for absorbing and accumulating Cd and Ni from the soil. The maximum values of bioconcentration were also observed by the application of citric acid. The values of metal translocation from the root to the shoot were varied by plant species and the citric acid application. Regarding the biomass, metal content, as well as removal metal percentage values, it became apparent that the Raphanus sativus plant was the most effective crop in removing heavy metals from multi-metal contaminated Soil. Generally, these findings emphasize that the application of citric acid could be a useful approach to assist Cd and Ni phytoextraction by Raphanus sativus plants. When these plants are growing as vegetable crops, more attention should be given to evaluating the heavy metal content in them, especially when adding citric acid to their soil through fertigation systems to avoid food chain contamination.

Accumulation Potential Cadmium and Lead by Sunflower (Helianthus annuus L.) under Citric and Glutaric Acid-Assisted Phytoextraction

Organic acid assistance is one of the effective methods for phytoremediation of heavy metal contaminated soil. In this experiment, the addition of citric and glutaric acids was selected to improve the accumulation of cadmium and lead by Helianthus annuus L. Results showed that citric and glutaric acids elevated the growth of the plants and stimulated Cd/Pb uptake by plant in single Cd/Pb treatments, but glutaric acid showed inhibitory action on the uptake of metals in complex treatments. Organic acids impacted the translocation of Cd/Pb differently, and citric acids (30 mg·L^-1) enhanced the translocation of Cd to aerial parts of the plants in Cd (5 mg·kg^-1) and Cd (10 mg·kg^-1) plus Pb treatments. Glutaric acid (30 mg·L^-1) could promote the translocation factors in the complex treatments of Cd (5 mg·kg^-1) with Pb (50, 100 mg·kg^-1) added. The application of citric and glutaric acid could be conducive to increase floral growth when proper doses are used, and incorporation of these organic acids can be a useful approach to assist cadmium and lead uptake by sunflower. However, growth, bioaccumulation, and translocation of metals may differ due to the metals' property, kinds, and concentrations of organic acids.

Lead accumulation and biochemical responses in Rhus chinensis Mill to the addition of organic acids in lead contaminated soils

Adding organic acid is an effective approach to assist phytoremediation. The effects of organic acids on phytoremediation efficiency are unknown in Rhus chinensis. This study aimed to evaluate the effect of citric acid (CA) and oxalic acid (OA) on the lead phytoremediation potential of R. chinensis with significantly inhibited growth in Pb-contaminated soil. The experimental pot culture study evaluated the long-term physiological response and metal accumulation patterns of R. chinensis grown in varying Pb-treated soil, and examined the effects of 0.5 and 1.0 mmol L^-1 CA and OA on the growth, oxidative stress, antioxidant system, and Pb subcellular distribution of R. chinensis grown in pots with 1000 mg kg^-1 Pb. Compared with the control, the biomass, leaf area, root morphological parameters, and chlorophyll concentration of R. chinensis decreased, whereas the carotenoid, malondialdehyde, H₂O₂, and O₂˙^- concentrations, and superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) activity increased under Pb stress. A copious amount of Pb was taken up and mainly stored in the cell walls of the roots. The application of CA and OA increased plant growth. The highest shoots and roots biomass increase recorded was 44.4 and 61.2% in 1.0 mmol L^-1 OA and 0.5 mmol L^-1 CA treatment, respectively. The presence of CA and OA increased SOD, POD, and CAT activities and decreased the H₂O₂, O₂˙^- and malondialdehyde content. A concentration of 0.5 mmol L^-1 CA significantly increased the Pb concentration in the organs. The other organic acid treatments changed root Pb concentrations slightly while increasing shoot Pb concentrations. The translocation factor values from organic acid treatments were increased by 38.8-134.1%. Our results confirmed that organic acid could alleviate the toxicity of stunted R. chinensis and improve phytoremediation efficiency.

Citric Acid and Poly-glutamic Acid Promote the Phytoextraction of Cadmium and Lead in Solanum nigrum L. Grown in Compound Cd-Pb Contaminated Soils

Phytoextraction is an efficient strategy for remediating heavy metal-contaminated soil. Chelators can improve the bioavailability of heavy metals and increase phytoextraction efficiency. However, traditional chelators have gradually been replaced due to secondary pollution. In this study, a typical organic acid (citric acid, CA) and a novel biodegradable chelator (poly-glutamic acid, PGA), were investigated using pot experiments to compare the phytoextraction efficiency of Solanum nigrum L. (a Cd (hyper)accumulator) for cadmium (Cd) and lead (Pb) in contaminated soil. The results showed CA and PGA significantly improved plant growth, and total Cd and Pb amounts of S. nigrum, both CA and PGA significantly increased the shoot Cd and Pb concentrations. However, only PGA significantly increased the root Pb concentration. CA and PGA application promoted the bioavailability of Cd and Pb in rhizosphere soils and their translocations from roots to shoots in S. nigrum. Both CA and PGA increased the phytoextraction efficiency of Cd and Pb in S. nigrum plants, and the PGA for Cd and Pb phytoextraction was more effective than CA. Our findings demonstrate that the biodegradable chelator PGA has great potential for enhancing phytoextraction from compound Cd-Pb contaminated soils, suggesting that biodegradable chelator-assisted phytoextraction with (hyper)accumulator is strongly recommended in severely contaminated sites.

Effects of Simultaneous Application of Double Chelating Agents to Pb-Contaminated Soil on the Phytoremediation Efficiency of Indocalamus decorus Q. H. Dai and the Soil Environment

Recent studies have shown that the combined application of ethylenediaminetetraacetic acid (EDTA) and degradable chelating agents can enhance EDTA's affinity for heavy metals and reduce its toxicity, but the effect of this combination on the phytoremediation remains largely unknown. This study evaluated and compared the effects of EDTA, nitrilotriacetic acid (NTA), and glutamic acid-N,N-diacetic acid (GLDA) alone (E, N, G treatment), and in combination (EN and EG treatment), on the growth of dwarf bamboo (Indocalamus decorus Q. H. Dai), their phytoremediation efficiency, and the soil environment in Pb-contaminated soil. The results showed that treatment E significantly reduced the biomass, while treatments N and EN were more conducive to the distribution of aerial plant biomass. Except for treatment E, the total Pb accumulation in all treatments increased significantly, with the highest increase in treatment EN. For double chelating agents, the acid-soluble Pb concentrations in rhizosphere and non-rhizosphere soils of treatments EN and EG were lower than those of treatment E, and the soil water-soluble Pb content after 20 days of treatment EN was significantly lower than that of treatment EG. Furthermore, chelating agents generally increased soil-enzyme activity in rhizosphere soil, indicating that chelating agents may promote plant heavy-metal uptake by changing the rhizosphere environment. In conclusion, treatment EN had the highest phytoremediation efficiency and significantly lower environmental risk than treatments E and EG, highlighting its massive potential for application in phytoremediation of Pb-contaminated soil when combined with I. decorus.

Chelate assisted phytoextraction for effective rehabilitation of heavy metal(loid)s contaminated lands

The contamination of lands and water by heavy toxic metal(loid)s is an environmental issue that needs serious attention as it poses a major threat to public health. The persistence of heavy metals/metalloids in the environment as well as their potentially dangerous effects on organisms underpins the need to restore the areas contaminated by heavy toxic metal(loid)s. Soil restoration can be achieved through a variety of different methods. Being more cost-effective and environmentally sustainable, phytoremediation has recently replaced traditional processes like soil washing and burning. Many plants have been intensively explored to eliminate various heavy metals from polluted soils through phytoextraction, which is a commonly used phytoremediation approach. The ability of chelants to enhance phytoextraction potential has also received wide attention owing to their ability to elevate the efficiency of plants in removing heavy metal(loid)s. Chelants have been found to improve plant growth and the activity of the defense system. Several chelants, either non-biodegradable or biodegradable, have been reported to augment the phytoextraction efficiencies of various plants. The problem of the leaching of heavy metal(loid)s and secondary pollution caused by non-biodegradable chelants can be overcome by the use of biodegradable chelants to an extent. This review is a brief report focusing on recent articles on chelate-assisted phytoextraction of heavy metal (loids) As, Cd, Cu, Cr, Hg, Ni, Pb, U, and Zn.

Cadmium removal potential of hyperaccumulator Solanum nigrum L. under two planting modes in three years continuous phytoremediation

Solanum nigrum L. is a Cd hyperaccumulator, but the potential for continuous remediation, or different planting methods have not been fully characterized. The potential for continuous phytoremediation of Cd-contaminated farmland soil (2.08 mg kg^-1 Cd) by 2 planting methods (flowering harvest twice a year and maturity harvest once a year) was studied in a 3-year pot experiment. The total Cd accumulation (ug plant^-1) of the 3-year flowering stage treatments was 26.3% higher than that of the maturity stage treatments, which was mainly due to that flowering harvest twice a year caused 65.5% increase of shoot biomass. Similarly, the Cd decreased concentration in soil and Cd removal rate in the flowering stage treatments were 29.2% and 27.9% higher than that in the maturity stage treatments, respectively. After 3 years of phytoremediation, the extractable Cd concentration in soil was reduced by 36.4% in the flowering stage treatments and by 27.6% in the maturity stage treatments, which also led to the same decreasing trend of Cd accumulation of S. nigrum. In conclusion, the study results have demonstrated that the planting mode of two harvests a year at the flowering stage seems to be a viable option to apply for continuous phytoremediation of Cd-contaminated farmland soil.

Deep purification of copper from Cu(II)-EDTA acidic wastewater by Fe(III) replacement/diethyldithiocarbamate precipitation

Cu(II)-EDTA is a highly stable typical metal-organic complex in a wide pH range (3.0-12.0) and it is difficult to deeply purify Cu(II) by conventional precipitation methods. In this study, Fe(III) replacement/diethyldithiocarbamate (DDTC) precipitation combined process is proposed as a promising strategy to achieve the deep purification of Cu(II) from Cu(II)-EDTA acidic wastewater. The underlying mechanism has also been systematically elucidated by chemical equilibriums, experiments, and density functional theory (DFT) calculations, laying a foundation for the development and application. Chemical equilibriums show that Fe(III) replacement favors the stoichiometric release of Cu(II) from Cu(II)-EDTA and the formation of Fe(III)-EDTA complex under acidic conditions. Experimentally, Cu(II) is removed (over 99.99%) and deeply purified (under 0.008 mg/L) under the optimal conditions, which is lower than the most stringent discharge standards of copper ions in electroplating effluent (<0.5 mg/L, China). DFT calculations reveal that DDTC could further precipitate the released free copper ions via the carbon disulfide (-C(=S)-S) chelating group while exhibiting a slight effect on the Fe(III) in Fe(III)-EDTA. Considering these results, the electronic structures of Cu(II) and Fe(III), as well as their interaction with EDTA and DDTC ligands, are discussed to understand the mechanism of Fe(III)/DDTC process. By introducing a low dosage of Fe(III), the DDTC could efficiently purify Cu(II) from the Cu(II)-EDTA acid wastewater and realize the near-zero discharge of metal pollutants in metal-organic complex wastewater. It is believed that the main findings may benefit the water pollution reduction and comprehensive recycling of metal resources.

Biodegradable chelant-metal complexes enhance cadmium phytoextraction efficiency of Solanum americanum

Toxic contaminants (metals and metal-containing compounds) are accumulating in the environment at an astonishing rate and jeopardize human health. Remarkable industrial revolution and the spectacular economic growth are the prime causes for the release of such toxic contaminants in the environment. Cadmium (Cd) is ranked the 7th most toxic compound by the Agency for Toxic Substances and Disease Registry (USA), owing to its high carcinogenicity and non-biodegradability even at miniscule concentration. The present study assessed the efficiency of four biodegradable chelants [nitrilotriacetic acid (NTA), ethylenediamine disuccinate (EDDS), ethylene glycol tetraacetic acid (EGTA), and citric acid (CA)] and their dose (5 mM and 10 mM) in enhancing metal accumulation in Solanum americanum Mill. (grown under 24 mg Cd kg^-1 soil) through morpho-physiological and metal extraction parameters. Significant variations were observed for most of the studied parameters in response to chelants and their doses. However, ratio of root and shoot length, and plant height stress tolerance index differed non-significantly. The potential of chelants to enhance Cd removal efficiency was in the order - EGTA (7.44%) > EDDS (6.05%) > NTA (4.12%) > CA (2.75%). EGTA and EDDS exhibited dose-dependent behavior for Cd extraction with 10 mM dose being more efficient than 5 mM dose. Structural equation model (SEM) depicted strong positive interaction of metal extraction parameters with chelants (Z-value = 11.61, p = 0.001). This study provides insights into the importance of selecting appropriate dose of biodegradable chelants for Cd extraction, as high chelant concentration might also result in phytotoxicity. In the future, phytoextraction potential of these chelants needs to be examined through field studies under natural environmental conditions.

Effects of lime and oxalic acid on antioxidant enzymes and active components of Panax notoginseng under cadmium stress

Cadmium (Cd) pollution poses potential safety risks for Panax notoginseng cultivation, a medicinal plant in Yunnan. Under exogenous Cd stress, field experiments were conducted to understand the effects of lime (0, 750, 2250 and 3750 kg hm⁻²) applied and oxalic acid (0, 0.1 and 0.2 mol L⁻¹) leaves sprayed on Cd accumulation, antioxidant system and medicinal components of P. notoginseng. The results showed that Lime and foliar spray of oxalic acid were able to elevate Ca²⁺ and alleviate Cd²⁺ toxicity in P. notoginseng under Cd stress. The addition of lime and oxalic acid increased the activities of antioxidant enzymes and alters osmoregulator metabolism. The most significant increase in CAT activities increased by 2.77 folds. And the highest increase of SOD activities was 1.78 folds under the application of oxalic acid. While MDA content decreased by 58.38%. There were very significant correlation with soluble sugar, free amino acid, proline and soluble protein. Lime and oxalic acid were able to increase calcium ions (Ca²⁺), decrease Cd content and improve the stress resistance of P. notoginseng, while increasing the production of total saponins and flavonoids. Cd content were the lowest, 68.57% lower than controls, and met the standard value (Cd ≤ 0.5 mg kg⁻¹, GB/T 19086-2008). The proportion of SPN was 7.73%, which reached the highest level of all treatments, the flavonoids content increased significantly by 21.74%, which reached the medicinal standard value and optimal yield.

Microwave Irradiation and Glutamic Acid-Assisted Phytotreatment of Tannery and Surgical Industrial Wastewater by Sorghum

We investigated how different doses of microwave irradiation (MR) affect seed germination in Sorghum, including the level of remediation against textile and surgical wastewater (WW) by modulating biochemical and morpho-physiological mechanisms under glutamic acid (GA) application. The experiment was conducted to determine the impact of foliar-applied GA on Sorghum under wastewater conditions. Plants were treated with or without microwave irradiation (30 s, 2.45 GHz), GA (5 and 10 mM), and wastewater (0, 25, 50, and 100). Growth and photosynthetic pigments were significantly decreased in plants only treated with various concentrations of WW. GA significantly improved the plant growth characteristics both in MR-treated and -untreated plants compared with respective controls. HMs stress increased electrolyte leakage (EL), hydrogen peroxide (H2O2), and malondialdehyde (MDA) content; however, the GA chelation significantly improved the antioxidant enzymes activities such as ascorbate oxidase (APX), superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) both in MR-treated and -untreated plants under WW stress compared with respective controls. The results suggested that the MR-treated plants accumulate higher levels of HMs under GA addition in comparison to the WW-only-treated and MR-untreated plants. The maximum increase in Cd accumulation was observed in the range of 14–629% in the roots, 15–2964% in the stems, and 26–4020% in the leaves; the accumulation of Cu was 18–2757% in the roots, 15–4506% in the stems, and 23–4605% in the leaves; and the accumulation of Pb was 13–4122% in the roots, 21–3588% in the stems, and 21–4990% in the leaves under 10 mM GA and MR-treated plants. These findings confirmed that MR-treated sorghum plants had a higher capacity for HMs uptake under GA and could be used as a potential candidate for wastewater treatment.

Efficiency of heterogeneous chelating agents on the phytoremediation potential and growth of Sasa argenteostriata (Regel) E.G. Camus on Pb-contaminated soil

Ethylenediaminetetraacetic acid (EDTA) is one of the most effective chelating agents for enhancing lead (Pb) accumulation in various plant organs. However, it has a higher risk of causing secondary pollution than other chelating agents. To reduce such environmental risks and increase remediation efficiency, EDTA can be combined with degradable chelating agents for use in phytoremediation, but there are few reports on the combination of EDTA and nitrilotriacetic acid (NTA). This study evaluated the effects of combined EDTA and NTA application at different concentrations (900, 1200, or 1500 mg/kg) and with different methods (1 application or 3 applications) on dwarf bamboo (Sasa argenteostriata (Regel) E.G. Camus) growth and phytoremediation efficiency and on the soil environment in pot experiments with Pb-contaminated soil. Applying EDTA and NTA together resulted in lower soil water-soluble Pb concentrations than applying EDTA alone and therefore resulted in lower environmental risk. The increased availability of soil Pb produced a stress response in the dwarf bamboo plants, which increased their biomass significantly. Moreover, under the chelating treatments, the soil Pb availability increased, which promoted Pb translocation in plants. The Pb content in the aerial parts of the dwarf bamboo increased significantly in all treatments (translocation factors increased by 300~1500% compared with that in CK). The Pb content increase in the aerial parts caused high proline accumulation in dwarf bamboo leaves, to alleviate Pb toxicity. Maximum Pb accumulation was observed in the EN1500 treatment, which was significantly higher than that in the other treatments except the EN900 treatment. This study elucidates the choice of remediation techniques and the physiological characteristics of the plants used in such studies. In conclusion, the EN900 treatment resulted in the lowest environmental risk, greatest biomass production, and highest phytoremediation efficiency of all treatments, indicating that it has great potential for application in phytoremediation with dwarf bamboo in Pb-contaminated soil.

Increasing Heavy Metal Tolerance by the Exogenous Application of Organic Acids

Several metals belong to a group of non-biodegradable inorganic constituents that, at low concentrations, play fundamental roles as essential micronutrients for the growth and development of plants. However, in high concentrations they can have toxic and/or mutagenic effects, which can be counteracted by natural chemical compounds called chelators. Chelators have a diversity of chemical structures; many are organic acids, including carboxylic acids and cyclic phenolic acids. The exogenous application of such compounds is a non-genetic approach, which is proving to be a successful strategy to reduce damage caused by heavy metal toxicity. In this review, we will present the latest literature on the exogenous addition of both carboxylic acids, including the Kreb's Cycle intermediates citric and malic acid, as well as oxalic acid, lipoic acid, and phenolic acids (gallic and caffeic acid). The use of two non-traditional organic acids, the phytohormones jasmonic and salicylic acids, is also discussed. We place particular emphasis on physiological and molecular responses, and their impact in increasing heavy metal tolerance, especially in crop species.

Foliar application of flavonoids (rutin) regulates phytoremediation efficiency of Amaranthus hypochondriacus L. by altering the permeability of cell membranes and immobilizing excess Cd in the cell wall

The gap between the current serious soil heavy metal (HM) contamination and the low efficiency of soil remediation threatens human health. The aim of this study was to propose a method to improve the efficiency of phytoremediation by exogenous rutin application and explain the potential mechanism. A series of rutin treatments were designed to evaluate the biomass, cadmium (Cd) accumulation and physiological and biochemical responses of Amaranthus hypochondriacus under different Cd stresses. The results showed a decline in cell membrane damage with rutin application, and more Cd ions were immobilized in the cell wall than in the vacuole, resulting in an increase in Cd tolerance in plants. The addition of rutin caused significant effects on the synthesis of glutathione (GSH), including the advancement of the conversion of GSH to phytochelatins (PCs). Among them, PC₂ and PC₃ in the leaves contributed the most to the high accumulation of Cd. Overall, the phytoremediation efficiency and phytoextraction amount of Amaranthus hypochondriacus with rutin application were improved maximumly by 219.48% and 260.00%, respectively. This study provides a constructive approach for improving the efficiency of phytoremediation by foliar application of flavonoids and contributes to the further development of soil remediation in Cd-contaminated fields.

Phytoremediation of Soils Contaminated with Heavy Metals from Gold Mining Activities Using Clidemia sericea D. Don

Soils contaminated by potentially toxic elements (PTEs) as a result of anthropogenic activities such as mining are a problem due to the adverse effects on human and environmental health, making it necessary to seek sustainable strategies to remediate contaminated areas. The objective of this study was to evaluate the species Clidemia sericea D. Don for the phytoremediation of soils contaminated with PTEs (Hg, Pb, and Cd) from gold mining activities. The study was conducted for three months, with soils from a gold mining area in northern Colombia, and seeds of C. sericea, under a completely randomized experimental design with one factor (concentration of PTEs in soil) and four levels (control (T0), low (T1), medium (T2), and high (T3)), each treatment in triplicate, for a total of twelve experimental units. Phytotoxic effects on plants, bioconcentration (BCF), and translocation (TF) factors were determined. The results obtained for the tissues differed in order of metal accumulation, with the root showing the highest concentration of metals. The highest values of bioconcentration (BCF > 1) were presented for Hg at T3 and Cd in the four treatments; and of translocation (TF > 1) for Hg and Pb at T0 and T1; however, for Pb, the TF indicates that it is transferable, but it is not considered for phytoextraction. Thus, C. sericea demonstrated its potential as a phytostabilizer of Hg and Cd in mining soils, strengthening as a wild species with results of resistance to the stress of the PTEs evaluated, presenting similar behavior and little phytotoxic affectation on the growth and development of each of the plants in the different treatments.

Brassica Species in Phytoextractions: Real Potentials and Challenges

The genus Brassica is recognized for including species with phytoaccumulation potential and a large amount of research has been carried out in this area under a variety of conditions, from laboratory experiments to field trials, with spiked or naturally contaminated soils, using one- or multi-element contaminated soil, generating various and sometimes contradictory results with limited practical applications. To date, the actual field potential of Brassica species and the feasibility of a complete phytoextraction process have not been fully evaluated. Therefore, the aim of this study was to summarize the results of the experiments that have been performed with a view to analyzing real potentials and limitations. The reduced biomass and low metal mobility in the soil have been addressed by the development of chemically or biologically assisted phytoremediation technologies, the use of soil amendments, and the application of crop management strategies. Certain issues, such as the fate of harvested biomass or the performance of species in multi-metal-contaminated soils, remain to be solved by future research. Potential improvements to current experimental settings include testing species grown to full maturity, using a greater amount of soil in experiments, conducting more trials under real field conditions, developing improved crop management systems, and optimizing solutions for harvested biomass disposal.

Effects of plant growth regulator and chelating agent on the phytoextraction of heavy metals by Pfaffia glomerata and on the soil microbial community

Pfaffia glomerata is a candidate for the remediation of heavy metal-contaminated soil, but phytoremediation efficiency requires enhancement. In this study, we evaluated how application of DA-6, EDTA, or CA affected the growth and heavy metal accumulation of P. glomerata and soil microorganisms. We found that P. glomerata removed more Cd and Zn than Pb or Cu from contaminated soil. When compared to the control, application of DA-6, CA, or CA + DA-6 increased plant biomass and increased stem Cd concentration by 1.28-, 1.20-, and 1.31-fold respectively; increased leaf Cd concentration by 1.25-, 1.28-, and 1.20-fold, respectively; and increased the total quantity of Cd extracted by 1.37-, 1.37-, and 1.38-fold, respectively. When compared to the control, application EDTA or EDTA + DA-6 significantly increased the soil available metal and Na concentrations, which harmed plant growth. Application of EDTA or EDTA + DA-6 also significantly decreased the Cd concentration in roots and stems. 16S rRNA high-throughput sequencing analysis revealed that application of EDTA or CA alone to soil significantly reduced the richness and diversity of soil bacteria, while foliar spraying of DA-6 combined with EDTA or CA slightly alleviated this reduction. EDTA or CA addition significantly changed the proportion of Actinobacteria and Proteobacteria. In addition, EDTA or CA addition caused changes in soil properties (e.g. heavy metal availability, K concentration, Na concentration, soil pH, soil CEC, and soil DOC concentration) that were associated with changes in the bacterial community. EDTA addition mainly affected the soil bacterial community by changing soil DOC concentration, the soil available Pb and Na concentration, and CA addition mainly affected the soil bacterial community by changing the soil available Ca concentration.

Effects of S,S-ethylenediamine disuccinic acid on the phytoextraction efficiency of Solanum nigrum L. and soil quality in Cd-contaminated alkaline wheat soil

Degradable chelating agent-assisted phytoextraction is a promising method for the remediation of Cd-contaminated agricultural soil. However, there are limited studies that have examined the effect of chelating agents on soil solutions and soil quality in alkaline soil. In this study, the effects of S,S-ethylenediamine disuccinic acid (EDDS) on the growth and phytoextraction of Solanum nigrum L. (S. nigrum) were studied using pot experiments. The influence of EDDS on the soil solutions, heavy metal contents, and soil enzyme activities was evaluated. EDDS application increased the height of S. nigrum by 7.25-29.25 cm and increased the biomass of stem and leaf by 4.26-14.95 and 1.14-10.78 g/pot, respectively. The Cd concentrations in the leaves and berries of S. nigrum were 1.21-2.17 and 1.7-9.47 times higher than that of the control, respectively, and the Cd extraction amount in the shoots of S. nigrum increased by 22.78-256.16 μg/pot after EDDS application. The chelation of EDDS on heavy metals reached a peak after 7 days of application, decreased gradually with the degradation of EDDS, and disappeared after 30 days of application. Soil pH, available metals, metal speciation, and soil urease were significantly related to the application time of EDDS. Importantly, EDDS application 45 days before S. nigrum harvest treatments decreased the available metal concentrations and improved soil pH and urease activity. However, when EDDS was applied 15 days before S. nigrum harvest, the available Cd and Pb concentrations significantly increased and caused additional Pb pollution. Considering the chelation and degradation effects, the environmental implication, and the cost of EDDS, the results of this study showed that one application of EDDS was better than two applications, a 45-day application before harvest was preferred to a 15-day application, and application of 1-3 mM EDDS 30-45 days before S. nigrum harvest was the most promising application method for the remediation of Cd-contaminated alkaline soil.

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

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

Effects of composited organic mobilizing agents and their application periods on cadmium absorption of Sorghum bicolor L. in a Cd-contaminated soil

Organic mobilizing agents have been advocated for phytoremediation of heavy metals contaminated soils, while the effects of application period of such agents remain unclear. A pot experiment was conducted, with two composited organic agents (oxalic acid or citric acid + dissolved organic fertilizer (OA + DOF and CA + DOF)) and four application periods (seeding, jointing, flag leaf and heading stages) of sorghum (Sorghum bicolor L.), to investigate their impacts on Cd bioavailability in soil. Results indicated that application of the two composited agents increased soil dissolved organic carbon (DOC) and DTPA extractable Cd by 7.31-49.13%, Cd contents in roots and shoots by 21.49-72.10%, bioaccumulation factor (BCF) and translocation factor (TF) of shoots by 4.44-71.99%, while reduced soil pH by 0.25-0.53 units, respectively. Most of these indices increased with the application periods, and largely peaked with their application during the flag leaf to heading stages. Meanwhile, the maximum sorghum biomass (132.84 g pot^-1) and Cd bioaccumulation quantity (BCQ, 0.71 mg pot^-1) in shoots were obtained for the CA + DOF applied at the heading. The DTPA extractable Cd was closely related to soil pH and DOC. Similar close relationships were observed between the Cd contents in shoots and soil DTPA extractable Cd, pH and DOC. The BCQ of Cd was positively related to the shoots biomass rather than their Cd contents. Therefore, the sorghum combined with the CA + DOF may be advocated as an alternative phytoremediation mode in Cd-contaminated soils, and the mobilizing agent should be primarily applied at the heading stage.

The influences of fly ash on stabilization for Cd in contaminated soils

Soil contaminated with potentially toxic metals (PTMs) has being a global environmental issue, which needs to be addressed on the priority basis. Fly ash (FA) is a kind of low-cost alkaline materials, which has been widely used in remediation of soil contaminated by PTMs, while the effects of FA on the stability for PTMs in contaminated farmland soil are still not clearly evaluated. In this study, cadmium (Cd) contaminated soil samples, collected from Shaanxi (SX), Hubei (HB), and Zhejiang (ZJ) province of China, were amended with FA addition (0, 1%, 2.5%, 5%, and 10% dose), and 1-year changes of Cd availability in soil samples were focused on. In addition, biological assessment method through pot culture was carried out to evaluate the reuse potential of Cd contaminated soils amended by FA. The result indicated that FA had a notable impact on decreasing the Cd mobility of SX soil (sand type), with 18.2~52.1% reduction in the DTPA extractable solution, followed by HB soil with 5.9~16.7% reduction, but no obvious effect of FA on ZJ soil (clay type) was observed. Furthermore, the results of pot experiment revealed that FA application could increase the biomass of Chinese cabbage. However, the DTPA extractable Cd in soils after planation and the Cd accumulation of plant increased. The results revealed that FA was not a promising soil stabilizer to immobilize HMs in Cd contaminated soil, and careful consideration should be given to Cd contaminated soils with FA restoration especially in their using for farmland productive due to the remaining risk of Cd bioavailability. These results also contributed to provide references for similar soil pollution remediation.

Performance of Streptomyces pactum-assisted phytoextraction of Cd and Pb: in view of soil properties, element bioavailability, and phytoextraction indices

Microbe-assisted phytoremediation provides an eco-friendly and cost-effective approach to reclaim Cd- and Pb-contaminated soils. In this work, incubation and pot experiments were established to investigate the effect of Streptomyces pactum (Act12) combined with compost on soil physicochemical properties, enzymatic activities, and thereby acted on phytoextraction of Cd and Pb by using potherb mustard (Brassica juncea Coss.). The addition of Act12 and compost increased EC (7.2%), available phosphorus (P) (14.9%), available potassium (K) (17.0 folds), DOC (37.7%), OM (2.8 folds), urease (49.8%), dehydrogenase (2.2 folds), and alkaline phosphatase (23.0 folds) of soil, while reduced pH (7.7%) compared with control. Significant decrease of available Cd and Pb uptake was observed after adding compost and Act12 by 29.1% and 32.2%. Presence of compost and Act12 enhanced the biomass by 3.98 folds and 1.83 folds in shoots and roots of plant. Results showed the assimilation of Cd and Pb in shoots was increased by 103.8% and 48.7% due to the increased of biomass. Meanwhile, the rhizosphere effect of soil microorganisms increased the uptake of Cd (60.4%) and Pb (19.2%) in roots. These findings suggested that Act12 joined with compost-strengthened potherb mustard phytoremediation of Cd- and Pb-polluted soils, which may provide new insights into the clean-up of mining-contaminated soils in field practice.

Performance of the emerging biochar on the stabilization of potentially toxic metals in smelter- and mining-contaminated soils

Soil potentially toxic metals (PTMs) pollution caused by anthropogenic activities has become serious concern with respect to the crop safety production. In this study, an emerging biochar derived from kiwi pruning branches waste was employed as amendment aiming to evaluate its remediation potential on smelter- and mining-contaminated soils. The effect of biochar on the soil physicochemical properties, leachability, and chemical fractions acted on stabilization practice of PTMs in soil was investigated. The results showed that the addition of biochar increased the soil pH, cation exchange capacity, organic matter, and enzymatic activities (dehydrogenase, urease, and sucrase) but reduced the extraction toxicity of PTMs in both smelter (Fengxian, FX) and mining (Tongguan, TG) soils. The fraction analysis showed that the maximum reduction of exchangeable fraction of Cd, Zn, and Pb in the 4% biochar amended soils decreased by 11.1, 13.3, and 24.7% in FX soil and 7.67, 22.8, and 7.89% in TG soil, respectively, in comparison with to control (no biochar added). Additionally, the residual fraction of Cd, Zn, and Pb increased by 55.9, 7.14, and 11.0% in FX soil and 23.7, 5.86, and 10.0% in TG soil, respectively. The further greenhouse experiment showed that the Indian mustard (Brassica juncea) production increased with the increasing application dosages of biochar, while the PTMs uptakes in plant notably decreased after amendments. Conversion of kiwi pruning branches waste into emerging biochar benefits the agricultural waste recycling utilization and enhances PTMs-contaminated soil remediation in practice. Graphical abstract.

Comparison of Myagrum perfoliatum and Sophora alopecuroides in phytoremediation of Cd- and Pb-contaminated soils: A chemical and biological investigation

Phytoremediation is one of the most cost-effective and environmentally friendly ways to reduce adverse effects of cadmium (Cd) and lead (Pb) in the environment. The present study was conducted to investigate the bioaccumulation factor (BF) and translocation factor (TF) of Cd and Pb in muskweed (Myagrum perfoliatum) and foxtail sophora (Sophora alopecuroides). The impact of contamination on some growth responses of plants and soil biological indicators was also evaluated. A non-contaminated soil sample was divided into several subsamples: one subsample was left as control (without contamination) and the others were separately contaminated with three levels of Cd (3, 5, and 10 mg kg^-1) and Pb (100, 300, and 600 mg kg^-1). Pot experiments were performed under greenhouse conditions. The BF values of Cd were greater than 1 at all contamination levels indicating the potential of muskweed and foxtail sophora for the uptake and phytostabilization of Cd. The only TF > 1 was obtained for Cd in muskweed grown at the highest Cd contamination level. The TF values of Pb were much lower than those obtained for Cd indicating that Cd was more translocated from root to aerial parts of muskweed and foxtail sophora compared to Pb. The highest contamination levels of Cd and Pb did not significantly affect growth responses of muskweed and foxtail sophora. Furthermore, the cultivation of muskweed and foxtail sophora reduced the impact of Cd and Pb contamination on biological indicators including carbon mineralization ratio (CMR), substrate-induced respiration (SIR), microbial biomass carbon (MBC), and metabolic quotient (qCO₂).

Effects of low molecular weight organic acids on Cu accumulation by castor bean and soil enzyme activities

Chelating agents have been considered as an important phytoremediation strategy to enhance heavy metal extraction from contaminated soil. A pot experiment was conducted to explore the effects of low molecular weight organic acids (LMWOAs) on the phytoremediation efficiency of copper (Cu) by castor bean, and soil enzyme activities. Results indicated that the addition of all the three kinds of LMWOAs (citric, tartaric, oxalic acids) did not decrease the biomass of castor bean, despite the fact they reduced the concentration of chlorophyll-a in leaves compared to the control. The Cu concentrations in the roots and shoots significantly increased by 6-106% and 5-148%, respectively, in the LMWOAs treatments so that the total accumulation of Cu by whole plants in all the LMWOAs treatments increased by 21-189% in comparison with the control. The values of the translocation factor (TF) and bio-concentration factor (BCF) of Cu in castor bean also rose following the addition of LMWOAs, indicating that the LMWOAs enhanced the uptake and transportation of Cu. Moreover, the application of LMWOAs did not significantly change the soil pH but significantly increased the activity of soil enzymes (urease, catalase, and alkaline phosphatase). The addition of exogenous LMWOAs increased the available Cu significantly in the soil, thus promoted the phytoextraction efficiency of Cu by castor bean. These results will provide some new insights into the practical use of LMWOAs for the phytoremediation of heavy-metal-contaminated soil employing castor bean.

Remediation of iron oxide bound Pb and Pb-contaminated soils using a combination of acid washing agents and l-ascorbic acid

Soil washing is an efficient, rapid, and cost-effective remediation technique to dissolve target pollutants from contaminated soil. Here we studied the effects of leaching agents: hydrochloric acid (HCl), ethylenediamine tetraacetic acid disodium salt (Na₂EDTA) and citric acid (CA), and reductants: hydroxylamine hydrochloride (NH₂OH·HCl) and l-ascorbic acid (VC) on the leaching of Pb from synthetic iron oxide; the changes in mineralogy, morphology, and occurrence of Pb were shown by XRD, SEM, and sequential extraction analyses. Although the washing efficiency of Pb follows the trend HCl (44.24%) > Na₂EDTA (39.04%) > CA (28.85%), the cooperation of the leaching agent with reductant further improves the efficiency. VC is more suitable as a reductant considering the higher washing efficiency by HCl-VC (98.6%) than HCl-NH₂OH·HCl (88.8%). Moreover, increasing the temperature can promote the decomposition and dehydrogenation reaction of VC with more H⁺. Among the mixture agents, Na₂EDTA + VC is the most effective agent to remediate the two kinds of contaminated soils owing to the formation of Fe(ii)-EDTA, a powerful reducing agent so that the efficiencies can reach up to 98.03% and 92.81%, respectively. As a result, these mixture agents have a great prospect to remediate Pb-contaminated soils.

A phytoextraction trial strengthened by Streptomyces pactum and plant nutrients: In view of plant bioindicators and phytoextraction indices

The present work was done to explore the joint effect of Streptomyces pactum (Act12) and plant nutrients on phytoremediation of smelter-contaminated soils. The physiological indicators and phytoextraction indices of potherb mustard (Brassica juncea, Coss) grown in Act12 inoculated soil with or without Hoagland's solution (H), humic acid (HA) and peat (PS) were evaluated. The results indicated that H, HA and PS acted synergistically with Act12, notably increasing chlorophyll and soluble protein contents and thereby promoting plant growth. Soil nutrient treatments reduced the antioxidant activities (PPO, CAT and POD) by 28.2-41.4%, 22.3-90.1% and 15.2-59.4% compared to control, respectively. Act12 and H treatments markedly facilitated plant to accumulate more cadmium (Cd) and zinc (Zn), but it was observed decreases when applied with HA and PS. Metal uptake (MU) values further indicated the differences in phytoextraction efficiency, i.e., H > PS > Control > HA. Taken together, Act12 combined with plant nutrients contributed to alleviating metal toxicity symptoms of plant. Hoagland's solution and peat were highlighted in the present phytoextraction trial, and recommended as soil additives.

Effect of lychee biochar on the remediation of heavy metal-contaminated soil using sunflower: A field experiment

Heavy metal contamination of soils is a serious issue with various consequences in Hunan Province. Here, we aimed to determine the effect and action mechanisms of lychee biochar on the remediation of Pb, Cd, As, and Zn from soil using sunflower (Helianthus annuus). Different amounts of lychee biochar (2.5, 5, and 10%) were added to heavy metal-contaminated soil in the Shuikoushan mining area, Hunan Province. The effects of biochar on the biomass of sunflower plants, and the accumulation and distribution of Pb, Cd, As, and Zn in sunflower plants, and changes in Pb, Cd, As, and Zn concentrations in the rhizosphere soil were studied. The application of biochar stimulated the growth of the sunflower plants, with the maximum biomass recorded in the 5% biochar treatment; however, above this level, biochar inhibited plant growth. Pb, Cd, As, and Zn in sunflower plants were redistributed with biochar addition. The concentration of Pb, Cd, As, and Zn in the leaves and receptacles of sunflower plants increased with biochar application, but their concentration in the roots, stems, and seeds significantly decreased compared with the control. The total amount of accumulated Pb, Cd, and As in sunflower plants increased by 22.9-58.9%, 15.8-42.3%, and 67.9-110%, respectively, compared with that in the control. In the biochar treatments, the total amount of accumulated Zn in sunflowers decreased by 13.8-37.2%, compared with that in the control. The accumulated Pb, Cd, and As in sunflower plants have an antagonistic effect on Zn required by sunflowers. The sunflower plants significantly reduced the concentration of Pb, Cd, As, and Zn in contaminated soil (P < 0.05), which decreased by 12.4, 11.0, 4.35, and 8.17%, respectively, compared with that before planting sunflower. The addition of biochar in heavy metal-contaminated soil significantly enhanced the heavy metal-remediation effect of sunflower. Compared with the control (0% biochar), 10% biochar application decreased the Pb, Cd, As, and Zn concentrations in the rhizosphere of sunflower plants, by 40.6, 31.6, 35.4, and 30.8%, respectively. In conclusion, lychee biochar enhanced the remediation of heavy metals in contaminated soil.

Cadmium, copper, lead and zinc accumulation in wild plant species near a lead smelter

Smelting activities have been shown to increase the likelihood of environmental heavy metal accumulation and bioaccumulation potential within relative proximity to smelter sites. This investigation focused specifically on cadmium (Cd), copper (Cu), lead (Pb) and zinc (Zn) accumulation in 43 wild plant species and corresponding soils near a Pb smelting contaminated area. Soils in the study area had elevated Cd, Cu, Pb and Zn concentrations, with pollution indices ranked as Cd > Pb > Cu > Zn. Aboveground plant heavy metal concentrations ranked Pb > Zn > Cd > Cu, with plants having greater bioconcentration factor (BCF) values for Cd than for Pb, Cu and Zn. Plant Cd BCF averaged 1.42, while Pb, Cu and Zn averaged 0.128, 0.256 and 0.560, respectively. The greatest Cd BCF value was 5.40 for Dendranthema indicum; Cd accumulation for this species has not been reported previously. The greatest shoot Cd concentration (66.5 mg kg^-1) was found in Viola verecunda. Significant correlations existed between plant shoot Cd and Pb concentrations and soil total and DTPA-extractable Cd and Pb. Plant species with greater heavy metal accumulation could potentially be used for phytoextraction in the study area, while those having less accumulation potentially being used to reduce heavy metal flow into the food chain, reducing the health risks associated with elevated heavy metal soil contamination.

Insights into mechanism on organic acids assisted translocation of uranium in Brassica juncea var. foliosa by EXAFS

Citric acid (CA) and Lactic acid (LA) were used as additives to study the mechanism of organic acid promoting the root-to-shoot translocation of uranium (U) in Brassica juncea var. foliosa from molecular and tissue levels. Firstly, the distribution of U in plants under the condition of different organic acids concentrations were studied. The accumulation of U in leafs of 1 mM CA group and 5 mM LA group reached 2225 and 1848 mg/kg respectively, which was about 5 times that of the control group. Secondly, the speciation and distribution of U in plant roots after exposure to different culture solutions were studied by EXAFS and SEM. The result of EXAFS found that the complex of U with organic acids resulted in the U accumulated in the roots was the uranyl carboxylate speciation, while the control group only was the uranyl phosphate speciation. SEM results showed that the lactic acids could enhanced the translocation of U from the cortex to the stele. Thirdly, we further studied the apoplastic pathway and the symplastic pathway of U translocation using transpiration inhibitor and metabolism inhibitor. Compared with the control group, it was likely that the complex of U with organic acids were translocated into the shoot of plants through the apoplastic pathway.

Effects of EDDS on the Cd uptake and growth of Tagetes patula L. and Phytolacca americana L. in Cd-contaminated alkaline soil in northern China

Phytoextraction has been considered an effective and environment-friendly method for removing heavy metals from contaminated soil. However, the efficiency, mechanism, and adaptability of phytoextraction by hyperaccumulators in Cd-polluted weakly alkaline soil have not been investigated in detail. In this study, pot experiments were conducted to evaluate the enhanced effects of S,S-ethylenediamine disuccinic acid (EDDS) on phytoextraction in alkaline soil by measuring the degradation kinetic characteristics of EDDS and Cd absorption dynamics of Tagetes patula L. (T. patula) and Phytolacca americana L. (P. americana) for a period of 55 days. Results showed that the half-life of EDDS varied from 4.20-7.07 days and 3.35-4.36 days for T. patula and P. americana, respectively. EDDS-activated Cd reached saturation at a low dosage (1 mM) and a single application of EDDS was found to be better than double applications. The activation of EDDS on Cd applied before 45 days of harvest was better than that before 15 days of harvest, and disappeared after a 35-day application. Correspondingly, the Cd concentration in P. americana and T. patula leaves increased significantly after 3 days of the EDDS application. However, T. patula had a biomass 2.57 times and Cd absorption capacity 10.06 times higher than P. americana. EDDS showed almost no influence on the stem and leaf biomass of T. patula; however, the root weight decreased by 9.44-71.77%. The Cd concentration in T. patula leaves of all the treatments was 1.00-1.81 times that of the control group. In comparison with other treatments, the EDDS application (3 mM) before 15 days of harvest extracted the highest amount of Cd (601.45 μg/pot) in T. patula shoots, reaching 1.40 times that in the control group. Therefore, T. patula might be a more suitable phytoremediator for Cd-polluted alkaline soil than P. americana; the most effective method was the EDDS application (3 mM) before 15 days of harvest.

Evaluation of phytoremediation potential of five Cd (hyper)accumulators in two Cd contaminated soils

A phytoextraction experiment with five Cd hyperaccumulators (Amaranthus hypochondriacus, Celosia argentea, Solanum nigrum, Phytolacca acinosa and Sedum plumbizincicola) was conducted in two soils with different soil pH (5.93 and 7.43, respectively). Most accumulator plants grew better in the acidic soil, with 19.59-39.63% higher biomass than in the alkaline soil, except for S. plumbizincicola. The potential for a metal-contaminated soil to be cleaned up using phytoremediation is determined by the metal uptake capacity of hyperaccumulator, soil properties, and mutual fitness of plant-soil relationships. In the acidic soil, C. argentea and A. hypochondriacus extracted the highest amount of Cd (1.03 mg pot^-1 and 0.92 mg pot^-1, respectively). In the alkaline soil, S. plumbizincicola performed best, mainly as a result of high Cd accumulation in plant tissue (541.36 mg kg^-1). Most plants achieved leaf Cd bioconcentration factor (BCF) of >10 in the acidic soil, compared to <4 in the alkaline soil. Soil Cd availability was chiefly responsible for such contrasting metal extraction capacity, with 5.02% fraction and 48.50% fraction of total Cd being available in the alkaline and acidic soil, respectively. In the alkaline soil, plants tended to increase rhizosphere soil available Cd mainly through excreting more low molecular weight organic acids, not through changing the soil pH. In the acidic soil, plants slightly decreased soil available Cd. Those species which have high Ca, Zn, Fe uptake capacity extract more Cd from soil, and a positive correlation was found between the concentrations of Cd and Ca, Zn, Fe in leaves. Soil available Ca²⁺, Mg²⁺, SO₄^2-, Cl^- did not play a key role in Cd uptake by plants. In summary, acidic soil was of higher potential to recover from Cd contamination by phytoextraction, while in the alkaline soil, S. plumbizincicola showed potential for Cd phytoextraction.

Organic chelates decrease phytotoxic effects and enhance chromium uptake by regulating chromium-speciation in castor bean (Ricinus communis L.)

There is limited information available on changes in the uptake of essential nutrients and secondary metabolites accumulation in castor bean under Cr toxicity. Besides, the role of organic chelates (EDTA and citric acid) mediated improvement in Cr uptake by castor bean is mostly unknown. Three independent experiments (sand, hydroponics, and soil) were executed to determine the Cr phytoextraction potential of Ricinus communis L. In the sand experiment, optimum doses of organic chelates (EDTA and citric acid) were selected. These optimum doses of chelates were used in the hydroponics and soil experiments. The results of hydroponics and soil experiments manifested a significant decrease in growth characteristics and leaf pigments in response to Cr stress applied as K2Cr2O7 (a source of Cr6+). The application of organic chelates (2.5 and 5 mM) showed a noticeable improvement in oxidative defense and secondary metabolites accumulation that might have decreased oxidative injury reflected as lower hydrogen peroxide (H2O2) and malondialdehyde (MDA) contents. Moreover, chelates improved the uptake of essential nutrients (K+, Ca2+, Mg2+, Fe2+ and P) alongside significant enhancement in total Cr contents of plants. Our results advocated that chelates application resulted in greater endogenous levels of Cr3+ in plants compared with Cr6+ which is more toxic. In nutshell, organic chelates improved growth by regulating Cr species, ion homeostasis and secondary metabolites accumulation in Ricinus communis L.

NTA-enhanced Pb remediation efficiency by the phytostabilizer Athyrium wardii (Hook.) and associated Pb leaching risk

Nitrilotriacetic acid (NTA), a biodegradable chelant, has been promoted to effectively assist Pb phytoextraction, while a few researches available on the phytostabilizer of Athyrium wardii (Hook.). In this study, two incubation experiments and a subsequent column experiment were conducted to investigate the effects of application of NTA on Pb availability in soils and Pb accumulation in A. wardii and associated leaching risk. The application of NTA significantly increased the exchangeable Pb and Pb bound to carbonates along with a decreased pH, leading to enhanced Pb availability in soils. It was more effective in enhancing Pb availability in soils by adding 2 mmol kg^-1 NTA into soils at once for 7 d, thus demonstrating potential for enhancing Pb uptake by A. wardii. After the addition of 2 mmol kg^-1 NTA for 7 d, Pb concentrations in roots of A. wardii was enhanced by 23.8%, along with 10.6% of increase for Pb accumulation in roots. No significant changes were observed for the biomass of A. wardii. Meanwhile, the available Pb and TCLP-extractable Pb in 0-20 cm soils increased by 11.1-23.4% and 7.1-31.2%, thus promoting Pb leaching in 0-20 cm soils. However, there were no changes for Pb leaching risk levels of 20-40 cm soils. No Pb was detected in the leachates from all columns. The application of 2 mmol kg^-1 NTA at once for 7 d is therefore proved to show greater potential in enhancing Pb remediation efficiency by the phytostabilizer of A. wardii without increasing Pb leaching risk into groundwater.

Phytostabilization of Pb-Zn Mine Tailings with Amorpha fruticosa Aided by Organic Amendments and Triple Superphosphate

A greenhouse pot trial was conducted to investigate the effect of organic amendments combined with triple superphosphate on the bioavailability of heavy metals (HMs), Amorpha fruticosa growth and metal uptake from Pb-Zn mine tailings. Cattle manure compost (CMC), spent mushroom compost (SMC) and agricultural field soil (AFS) were applied to tailings at 5%, 10%, 20% and 30% w/w ratio, whereas sewage sludge (SS) and wood biochar (WB) were mixed at 2.5%, 5%, 10% and 20% w/w ratio. Triple superphosphate (TSP) was added to all the treatments at 4:1 (molar ratio). Amendments efficiently decreased DTPA-extracted Pb, Zn, Cd and Cu in treatments. Chlorophyll contents and shoot and root dry biomass significantly (p < 0.05) increased in the treatments of CMC (except T4 for chlorophyll b) and SMC, whereas treatments of SS (except T1 for chlorophyll a and b), WB and AFS (except T4 for chlorophyll a and b) did not show positive effects as compared to CK1. Bioconcentration factor (BCF) and translocation factor (TF) values in plant tissues were below 1 for most treatments. In amended treatments, soluble protein content increased, phenylalanine ammonialyase (PAL) and polyphenol oxidase (PPO) decreased, and catalase (CAT) activity showed varied results as compared to CK1 and CK2. Results suggested that A. fruticosa can be a potential metal phytostabilizer and use of CMC or SMC in combination with TSP are more effective than other combinations for the in situ stabilization of Pb-Zn mine tailings.

Effect of citric acid and vermi-wash on growth and metal accumulation of Sorghum bicolor cultivated in lead and nickel contaminated soil

The aim of the present study is to assess the influence of vermi-wash (VW) and citric acid (CA) on Sorghum bicolor growth and phytoaccumulation of lead (Pb) and nickel (Ni) contaminated soil. The biomass of the S. bicolor has been enhanced by the addition of VW (24 and 26%) and CA (11 and 9%) in Pb and Ni contaminated soil, respectively. The VW treatment showed enhanced shoot and root lengths and chlorophyll concentrations compared to CA. The shoot anatomic structure showed an accumulation of Pb and Ni were positively impacted by the amendment of VW and CA. In addition, VW treatment showed enhanced antioxidant enzymes activity (140, 125 and 152 U/mg of CAT, SOD and POD). Further, the plants grown in Pb contaminated soil treated with VW showed enhanced Rubisco activity of 1.49 U/ml, whereas, CA treatment showed 1.23 U/ml of Rubisco. It has been observed that the VW showed as a potential chelator as well as plant beneficial formulation for the enhanced phyto-remediation of Pb and Ni.