Cadmium (Cd) contamination of farmland soils is a growing concern because of its highly toxic impact on ecosystems and human health. Chelator-assisted washing and chemical immobilization are ...effective remediation strategies for Cd-contaminated soils. Ethylenediaminetetraacetic acid (EDTA) has traditionally been used for soil washing, but its persistence in the environment and subsequent toxicity have raised significant ecological concerns. Consequently, biodegradable chelators have gained increasing attention as eco-friendly alternatives to the persistent chelator, EDTA. Therefore, this study evaluated the performance and efficacy of three biodegradable chelators: L-glutamate-N,N'-diacetic acid (GLDA), methylglycine-diacetic acid (MGDA), and 3-hydroxy-2,2'-iminodisuccinic acid (HIDS) in comparison to EDTA for remediating a real Cd-contaminated agricultural soil. The influence of treatment parameters, including chelator variants, washing time, chelator concentration, solution pH, and liquid-to-soil ratio (L/S) on Cd extraction was studied and optimized to attain the maximum removal rate. Following chelator-assisted washing, the efficacy of a stabilization preference combining FeCl3 and CaO in reducing the leaching potential of residual Cd in chelator-washed soil residues was also investigated. GLDA demonstrated comparable Cd extraction efficiency to EDTA, and the Cd extraction efficiency was found to be positively correlated with the soil washing parameters. However, under the optimized conditions (chelator concentration: 10 mmol L-1; washing time: 3 h; solution pH: 3; L/S ratio: 10:1), GLDA exhibited a higher Cd extraction rate than EDTA or the other chelators. Furthermore, a post-treatment process incorporating FeCl3 and CaO substantially diminished the water-leachable Cd content in the resultant soil residues. The proposed remediation strategy, which combines chemically assisted washing and stabilization, could be a practical option for extracting bulk Cd from soil and reducing the leaching potential of residual Cd.
In washing soils contaminated with toxic metals, the replacement of recalcitrant EDTA with biodegradable chelators has gained high expectations. Herein we investigated the feasibility of using EDTA ...and biodegradable GLDA, EDDS and IDS under conditions pertinent to operational remediation technology, in a pilot-scale experiment. GLDA and IDS did not precipitate from process solutions, which lessened their recyclability. In other process parameters, chelator supplement, Na-saturation of process solutions and processing time, EDTA outperformed biodegradable chelators. Treatment with EDTA was also the most effective in total Pb and Zn removal and least impacted soil properties. GLDA was slightly better in Cd removal. EDDS and IDS were inefficient. All chelators effectively removed easily-available Pb, Zn and Cd from the exchangeable soil fraction. EDTA was the most efficient chelator in reducing the bioaccessibility of Pb and GLDA in reducing the bioaccessibility of Cd from simulated human gastrointestinal tract. Treatment with GLDA had an edge in reducing plant bioaccessibility of toxic metals, but induced worrying leachability of Pb. This was 8.3-times higher than with the process with EDTA and 3.4-times higher than in original soil. In general, our results demonstrate the advantage of EDTA over tested biodegradable chelators in process and remediation efficiency and environmental safety.
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•In soil washing biodegradable chelators are often preferred over persistent EDTA.•Current knowledge is based on data from small-scale soil extraction experiments.•EDTA and biodegradable chelators were tested in operational soil-washing process.•Recyclability, efficiency, soil impact and safety of chelators were assessed.•In most aspects EDTA outperformed biodegradable chelators GLDA, EDDS and IDS.
In this study, we investigated the level of contamination of agricultural soil near an old recycling lead smelter in Vietnam and proposed an effective treatment for the remediation of the soil. The ...analysis of soil samples using an ICP-MS method revealed that the soil in the area was heavily contaminated by heavy metals, especially lead (Pb) with concentrations in surface soil of >3000 μg g−1. High concentrations of metals, including Pb, copper (Cu) and zinc (Zn), were found in whole soil profile. The FE-EPMA and Laser-Raman spectrometer results suggested that iron minerals and carbon materials in the soil are the important hosts of the toxic metals. Subsequently, a series of washing experiment were performed on the soil using biodegradable chelators, including N, N-dicarboxymethyl glutamic acid tetrasodium salt (GLDA), ascorbic acid and citric acid. The results showed that the mixture of GLDA-ascorbic (100 mM: 100 mM) can be considered as a potential candidate for Pb and Zn removal, which removes approximately 90% of Pb and 70% of Zn. Meanwhile, a mixture of GLDA-citric would be preferred for Cu removal based on its greater extraction efficiency compared to other mixtures.
•The investigated soil was heavily contaminated with Pb conc. > 3000 μg g−1.•Pb mainly adsorbs on surface of black-carbon particles and minerals in soil.•∼90% of Pb and 70% of Zn removed from soil using GLDA & ascorbic acid mixture.•Ascorbic acid seems to inhibit the removal of Cu from the contaminated soil.•GLDA is highly effective in removing Pb, Zn and Cu in wide pH range.
In chelator-enhanced Zn phytoremediation studies, it is crucial to understand how the degradable chelators and the competition from other ions influence the concentration of Zn in soil solutions. ...This study investigated the biodegradability of two chelators (EDDS: Ethylenediamine-N,N′-disuccinic acid, and NTA: Nitrilotriacetic acid) and their effects on the Zn concentration in the soil solution during the growth of alfalfa (Medicago Sativa L.). The chelators were added at four doses (0, 0.5, 2 and 5 mmol kg−1) in soils with varying Zn levels (189, 265 and 1496 mg kg−1). The results showed that the lag phase before EDDS and NTA biodegradation varied from 0 to 7 days in the three soils. EDDS and NTA were completely decomposed within the assessed 57 days regardless of the applied dosage, with a half-life of 1.3–3.0 days in highly Zn-contaminated soil and 4.2–10.8 days in the two other soils. In soil solutions, the change in solubilized Zn was in line with EDDS and NTA degradation kinetics. Cu, Al, Fe and Mn were the main metal ions that competed against Zn for chelation. Besides, Ni competed with Zn in the whole process. Ca did not compete effectively in the three soils, while Mg was a competitor only at the initial stage. Our results show the importance of considering both the biodegradation rate and the competition between the target cation and other elements present in the soil when using chelators to enhance phytoremediation. A 30-day explorative incubation experiment is recommended to evaluate the appropriate application time of chelators and the target Zn exposure time for plants during phytoremediation.
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•The lag phase for the degradation of EDDS and NTA varied from 0 to 7 days.•EDDS biodegradation rates varied for soils but were independent of applied doses.•Zn in soil solutions changed in line with the chelator degradation kinetics.•Ca did not compete effectively for chelation in soils with varying Zn levels.•Ni competed effectively with Zn for complexation in the whole procedure.
Lead (Pb) contamination in shooting range soils is an increasing concern due to high Pb content, threatening the ecosystems and human health. Chelator-assisted washing and chemical immobilization are ...the most viable treatment options for Pb-contaminated soil remediation. Ethylenediaminetetraacetic acid (EDTA) has been the most widely used chelators in washing remediation. However, the prolonged environmental persistence and subsequent toxicity of EDTA have governed eco-concerns. Alternatively, biodegradable chelators have received substantial focus as a promising substitute for EDTA. Herein, the performance and efficiency of S,S-ethylenediaminedisuccinic acid (EDDS), N,N-dicarboxymethyl glutamic acid (GLDA), and 3-hydroxy-2,2′ -iminodisuccinic acid (HIDS) were evaluated as eco-compliant alternatives to EDTA for the remediation of Pb-contaminated shooting range soil. Additionally, a post-treatment preference for immobilizing residual Pb in treated soils using FeCl3 and CaO was highlighted. Among the tested biodegradable chelators, EDDS exhibited the highest Pb extraction and displayed comparable efficiency to EDTA. The Pb removal efficiency of chelators was positively correlated with soil washing parameters. In contrast, the post-treatment using FeCl3 and CaO significantly reduced the leachable Pb-content in chelator-washed soils. The proposed remediation strategy coupled with washing and fixation could be a practical treatment option for extracting bulk Pb from soil and suppressing the leaching potential of residual Pb.
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•Biodegradable chelators evaluated to remediate Pb-contaminated shooting range soil.•EDDS is effective in extracting Pb and featured comparable efficiency to EDTA.•Chelator-induced Pb-extraction significantly correlated with washing variables.•The FeCl3 and CaO based post-treatment suppressed the mobility of residual Pb.•An efficient and eco-friendly process is designed combining washing and fixation.
Purpose
The aim of this study was to evaluate the effectiveness of nitrilotriacetic acid (NTA) on cadmium (Cd) fractions and microbial biomass in a calcareous soil spiked with Cd under cultivated (
...Zea
mays
L.) and uncultivated regime subject to soil leaching condition. Expanding investigations related to soil–plant interactions on metal-contaminated soils with insights on microbial activity and associated soil toxicity perspective provides novel perspectives on using metal-chelating agents for soil remediation.
Methods
The experimental factors were three levels of Cd contamination (0, 25, and 50 mg kg
−1
soil) and three levels of NTA (0, 15, and 30 mmol L
−1
) in loamy soil under maize-cultured and non-cultured conditions. During the experiment, the adding NTA and leaching processes were performed three times.
Results
The results showed that the amount of leached Cd decreased in cultivated soil compared to uncultivated soil due to partial uptake of soluble Cd by plant roots and changes in Cd fractions in soil, so that Cd leached in Cd
50
NTA
30
was 9.2 and 6.1 mg L
−1
, respectively, in uncultivated and cultivated soils. Also, Cd leached in Cd
25
NTA
30
was 5.7 and 3.1 mg L
−1
respectively, in uncultivated and cultivated soils. The best treatment in terms of chemical and microbial characteristics of the soil with the high percentage of Cd removed from the soil was Cd
25
NTA
30
in cultivated soil. In Cd
25
NTA
30
compared to Cd
25
NTA
0
in cultivated soil, pH (0.25 unit), microbial biomass carbon (MBC, 65.0 mg kg
−1
), and soil respiration (27.5 mg C-CO
2
kg
−1
24 h
−1
) decreased, while metabolic quotient (qCO
2
, 0.05) and dissolved organic carbon (DOC, 20.0 mg L
−1
) increased. Moreover, the changes of Cd fractions in Cd
25
NTA
30
in cultivated soil compared to uncultivated soil were as follows; the exchangeable Cd (F
1
, 0.27 mg kg
−1
) and Fe/Mn-oxide-bounded Cd (F
4
, 0.15 mg kg
−1
) fractions increased, in contrast, carbonate-Cd (F
2
, 2.67 mg kg
−1
) and, organically bounded Cd (F
3
, 0.06 mg kg
−1
) fractions decreased. NTA had no significant effect on the residual fraction (F
5
).
Conclusion
The use of NTA, especially in calcareous soils, where most of the Cd is bound to calcium carbonate, was able to successfully convert insoluble fractions of Cd into soluble forms and increase the removal efficiency of Cd in the phytoremediation method. NTA is a non-toxic chelating agent to improve the accumulation of Cd in maize.
In this study, we discuss the effects of biodegradable chelators on the bioavailability of iron and the growth of radish sprouts. In alkaline soils, iron exists as hardly soluble states of hydrated ...oxide. In experiments at pH 10, iron moved into the upper layer (without iron) from the bottom layer (with iron) of the medium, and the mobility depended on the chelators. The content of iron in the upper layer of the medium was 7.0 % of the total iron in the medium without chelators, and 17.3 % in the medium with HIDS. These results suggest that chelators can dissolve the hardly soluble iron hydroxide species in alkaline conditions, increase the mobility of iron, and increase plant growth.
Biodegradable chelators (BCs) are promising substitutes for conventional washing agents in the remediation of heavy metal contaminated soil with strong complexing ability and less cost. However, ...great challenges for the applications of BC–assisted washing still exist, such as the assessment of the factor affecting the efficiency of metal removal and the unclear of the metal removal mechanism. Batch washing was therefore explored to evaluate the potential for four BCs for removing Cd, Pb, and Zn from polluted soils. The soil spectroscopic characteristics before and after washing were also investigated. The results demonstrated that iminodisuccinic acid (ISA) and glutamate–N, N–diacetic acid (GLDA) were an appealing alternative to commonly used non–biodegradable ethylenediaminetetraacetic acid, but glucomonocarbonic acid (GCA) and polyaspartic acid (PASP) were less efficient. Optimal parameters of BCs were determined to be a concentration of 50 mmol L−1, a pH of 5.0, a contact time of 120 min, and a solid/liquid ratio of 1:5, considering metal removal efficiencies and the suitable cost. A single removal washing could be up to 52.39% of Cd, 71.79% of Pb, and 34.13% of Zn from mine soil, and 98.28% of Cd, 91.10% of Pb, and 90.91% of Zn from polluted farmland soil. After washing, the intensity of heavy metal binding to soil colloids increased while the metal mobility reduced because of weakly bound fractions removed by BCs. The BCs–induced soil washing revealed that the possible mechanisms of metal removal included the acid dissolution, ion exchange, and surface complexation. Our findings highlight the potential application of especially ISA and GLDA as efficient washing agents to remove potentially toxic elements from contaminated soils.
•Metal removal efficiency with different biodegradable chelator washing was studied.•The mobility of residual metals in chelator-washed soil was lower than unwashed soil.•The ISA and GLDA more effectively remove Cd, Pb, and Zn than that of GCA and PASP.•Metal removal mechanism dominated by dissolution, ion exchange, and complexation”.
Soil washing with chelators is a promising and efficient method of remediating metals-contaminated soils. However, the toxicity of residual metals and the effects on soil microbial properties have ...remained largely unknown after washing. In this study, we employed four biodegradable chelators for removal of metals from contaminated soils: iminodisuccinic acid (ISA), glutamate-N,N-diacetic acid (GLDA), glucomonocarbonic acid (GCA), and polyaspartic acid (PASP). The maximum removal efficiencies for Cd, Pb, and Zn of 85, 55, and 64% and 45, 53, and 32% were achieved from farmland soil and mine soil using biodegradable chelators, respectively. It was found that the capacity of ISA and GLDA to reduce the labile fraction of Cd, Pb, and Zn was similar to that of the conventional non-biodegradable chelator ethylenediaminetetraacetic acid (EDTA). The leachability, mobility, and bioaccessibility of residual metals after washing decreased notably in comparison to the original soils, thus mitigating the estimated environmental and human health risks. Soil β-glucosidase activity, urease activity, acid phosphatase activity, microbial biomass nitrogen, and microbial biomass phosphorus decreased in the treated soils. However, compared with EDTA treatment, soil enzyme activities distinctly increased by 5–94% and overall microbial biomass slightly improved in the remediated soils, which would facilitate reuse of the washed soils. Based on soil toxicity tests that employed wheat seed germination as the endpoint of assessment, the washed soils exhibited only slight effects especially after ISA and GLDA treatments, following high-efficiency metal removal. Hence, ISA and GLDA appear to possess the greatest potential to rehabilitate polluted soils with limited toxicity remaining.
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•GLDA and ISA efficiently removed 25–85% of Cd, Pb, and Zn from polluted soils.•Leachability and bioaccessibility of metals reduced by 24–92% in GLDA and ISA washing.•Biodegradable chelates allow higher soil enzyme activity than that of EDTA treatment.•Wheat seed germination bioassay was used to evaluate the phytotoxicity of washed soil.
In the study, the dynamics of Sr2+ and geochemically correlated elements (Ca2+, Ba2+, and Y3+) in soil with chelators in the mix (soil to chelator ratio, 1:10; matrix, H2O) were assessed to ...understand chemical-induced washing remediation of radiogenic waste solids. Specifically, EDTA (2,2′,2″,2‴-(ethane-1,2-diyldinitrilo)tetraacetic acid), EDDS (2-2-(1,2-dicarboxyethylamino)ethylaminobutanedioic acid), GLDA (2-bis(carboxymethyl)aminopentanedioic acid), and HIDS (2-(1,2-dicarboxyethylamino)-3-hydroxy-butanedioic acid) are chelators that are used as extractants. The effect of solution pH on chelator-induced extractions of the target elements (t-Es: Sr2+, Ca2+, Ba2+, or Y3+) from soil and stability constants of the t-Es complexes with chelators were used to explain the trends and magnitudes in interactions. Pre- and post-extractive solid-phase speciation was used to define the extent of the competence of each chelator in persuading dissolution of t-Es in the soil. The effects of ultrasonic energy, admixtures of biodegradable chelators, and excess chelators in solution (1:20) were also analyzed on the extractive removal of t-Es from soil. The results indicate that the Sr2+ removal with biodegradable chelators significantly exceeded (approximately 70%) when compared to that of environmentally-persistent EDTA at lower solution pHs and a higher soil to chelator ratio (GLDA > HIDS > EDDS ≈ EDTA). However, the extraction of the geochemically related element was significantly lower.
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•Sr2+ extracted at a higher ratio than its geochemical analogs with chelators.•Sr2+-extractability of biodegradable chelators was higher than that of EDTA.•Impact of sonication and mixed biodegradable chelator application was evaluated.•Sonication followed by mechanical mixing persuaded a higher rate of Sr2+ release.•Addition of excess chelator in solution can induce a similar impact to sonication.
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