Soil physical characteristics after EDTA washing and amendment with inorganic and organic additives

Core-shell design of UiO66-Fe3O4 configured with EDTA-assisted washing for rapid adsorption and simple recovery of heavy metal pollutants from soil

The coupling of washing with adsorption process can be adopted for the treatment of soils contaminated with heavy metals pollution. However, the complex environment of soil and the competitive behavior of leaching chemicals considerably restrain adsorption capacity of adsorbent material during washing process, which demands a higher resistance of the adsorbents to interference. In this study, we synthesized strongly magnetic, high specific surface area (573.49 m2/g) UiO66 composites (i.e., UiO66-Fe3O4) using hydrothermal process. The UiO66-Fe3O4 was applied as an adsorbent during the ethylene diamine tetraacetic acid (EDTA)-assisted washing process of contaminated soil. The incorporation of UiO66-Fe3O4 results in rapid heavy metal removal and recovery from the soil under low concentrations of washing agent (0.001 mol/L) with reduced residual heavy metal mobility of soil after remediation. Furthermore, UiO66-Fe3O4 can quickly recollect by an external magnet, which offers a simple and inexpensive recovery method for heavy metals from contaminated soil. Overall, UiO66-Fe3O4 configuration with EDTA-assisted washing process showed opportunities for heavy metals contaminated sites.

How exogenous ligand enhances the efficiency of cadmium phytoextraction from soils?

Many experiments showed that exogenous ligands could enhance cadmium (Cd) phytoextraction efficiency in soils. Previous studies suggested that the dissociation and the apoplastic uptake of Cd complex could not fully explain the increase of root Cd uptake. Two hypotheses are evaluated to explain enhanced Cd uptake in the presence of ligand: i) enhanced apoplastic uptake of complex due to reduced apoplastic resistance and ii) complex internalization by membrane transporters. RESULTS: show that the ligand affinity for Cd is a key characteristic determining the potential mechanism for enhanced Cd uptake. When low molecular weight organic acids are applied, the complex dissociation could generally be fast (> 10-3.3 s-1) and result in the increased Cd uptake. When hydrophilic aminopolycarboxylic acids (APCAs) are applied in experiments without water or temperature stresses to the plant, the root water uptake flux could very likely be high (> 10-7.8 dm s-1), and the strong apoplastic complex uptake could enhance the root Cd uptake. When lipophilic APCAs are applied, the strong internalization of the complex by membrane transporters could result in the increased Cd uptake if the maximum internalization rate is high (> 10-12 mol dm-2 s-1). However, the complex internalization by membrane transporters must be experimentally confirmed.

Preparation of landscape gardening soil using undersized fraction from aged MSW by EDTA or citric acid coupled with humic acid: Effect assessment, properties, and optimization

Undersized fraction from aged municipal solid waste (UFAMSW), as a kind of soil-like material, has been proved effective in providing a large amount of organic matter and nutrients for soil and plants. The characteristics and effectiveness of heavy metal pollution removal in UFAMSW attracted tremendous research interest from scientists recently. In this study, the heavy metal removal efficiencies and bioavailability of washing on contaminated UFAMSW were evaluated with three washing reagents including ethylene diamine tetra acetic acid (EDTA), citric acid (CA), and humic acid (HA). The effects of chelating agent concentration, pH, and washing time on metal removal were investigated and response surface methodology (RSM) was employed to optimize the washing conditions. The results indicated that the removal efficiencies of Cu, Zn, and Mn could be 53.68%, 52.12%, and 30.63% by EDTA/HA washing and 42.36%, 39.67% and 28.49% by CA/HA washing, respectively. The European Community Bureau of Reference (BCR) sequential extraction was applied to analyze the fraction change of heavy metals in UFAMSW before and after washing, and it was found that chelating agent combined with HA could contribute to the removal of the exchangeable fraction. Physical and chemical properties of UFAMSW were improved to some extent after washing with mixed HA and chelating agent and could achieve the quality standard of landscape gardening soil. Accordingly, the mixture of HA and other chelating agents could be a promising washing process for preparation of landscape gardening soil using UFAMSW.Implications: Our manuscript studies the removal of heavy metals from the contaminated undersized fraction from aged municipal solid waste (UFAMSW). UFAMSW, as a kind of soil-like material, has been proved effective in providing a large amount of organic matter and nutrients for soil and plants however often limited by heavy metal pollution. The UFAMSW used in this experiment was collected after the excavation and screening-sorting of aged refuse from Changshankou Domestic Waste Sanitary Landfill in Wuhan City, Hubei Province, Southern China. This study investigated the effects of EDTA, CA, HA, mixed EDTA/HA, and mixed CA/HA washing on heavy metal removal (Cu, Zn, and Mn), bioavailability of residual heavy metal and properties. The effects of chelating agent concentration, pH, and washing time on metal removal were investigated and then response surface methodology was employed to optimize the washing conditions. The results showed that washing by CA/HA and EDTA/HA, had a higher removal efficiency of heavy metals (Cu, Zn, and Mn) in UFAMSW compared to single HA. Meanwhile, HA has a higher removal for exchangeable fraction of heavy metals, the exchangeable concentration of Cu, Zn, and Mn in CA/HA and EDTA/HA washed UFAMSW were lower compared with UFAMSW washed by single CA and EDTA. Thus, mixing HA with EDTA or CA makes a less risk to environmental and the removal efficiency is acceptable. Additionally, CA/HA and EDTA/HA washing tend to improve soil physicochemical properties and soil fertility. Thus, mixing HA with different washing agent are potential methods for preparation of landscape gardening soil using UFAMSW.

Best management practices for minimizing undesired effects of thermal remediation and soil washing on soil properties. A review

The use of remediated soils as end-of-life materials raises some challenges including policy and regulation, permits and specifications, technological limitations, knowledge and information, costs, as well as quality and performance associated with using them. Therefore, a set of procedures must be followed to preserve the quality and fundamental properties of soil during a remediation process. This study presented a comprehensive review regarding the fundamental impacts of thermal desorption (TD) and soil washing (SW) on soil characteristics. The effects of main operating parameters of TD and SW on the physical, chemical, and biological properties of soil were systematically reviewed. In TD, temperature has a more remarkable effect on physic-chemical and biological characteristics of soil than heating time. Therefore, decrease in temperature within a suitable range prevents unreversible changes on soil properties. In SW, more attention should be paid to extraction process of contaminants from soil particles. Using the right dosage and type of chelating agents, surfactants, solvents, and other additives can help to avoid problems with recovery or treatment using conventional methods. In addition, this review introduced a framework for implementing sustainable remediation approaches based on a holistic approach to best management practices (BMPs), which, besides reducing the risks associated with different pollutants, might provide new horizons for decreasing the unfavourable impacts of TD and SW on soil.

Field experiments to assess the remediation efficiency of metal-contaminated soil by flushing with ferric chloride followed by applying amendments

The Cd, Cu, Pb, and Zn removal efficiencies achieved by flushing with FeCl₃ were determined in a field experiment using soil contaminated with multiple metals. Soil was first flushed with FeCl₃ and then with FeCl₃ or a mixture of chelators. Flushed soil was amended with lime and organic matter to revitalize the soil, then the soil was used to grow Zea mays and Brassica juncea. The heavy metal concentrations in groundwater were determined to assess the risks of leaching caused by soil flushing. The Cd, Cu, Pb, and Zn removal efficiencies were 70%, 40%, 33%, and 17%, respectively, when FeCl₃ (25 mmol (kg topsoil)^-1) was applied. The second washing generally did not significantly decrease the heavy metal contents of the soil but the second FeCl₃ washing did decrease the Pb content. Pb leached from topsoil was partly retained by the subsoil 20-40 cm deep. The Zea mays yields were significantly lower but the Brassica juncea yields were significantly higher after the combined soil flushing and amendment treatment than after only the amendment treatment. This indicated that soil flushing only negatively affected growth of deep-rooted Z. mays. The Cd, Cu, Pb, and Zn concentrations in Z. mays grains and the edible parts of B. juncea grown in remediated soil were below the Chinese tolerable limits for contaminants in food. Washing with FeCl₃ did not increase groundwater contamination during the study. The results indicated that flushing soil with FeCl₃ and subsequent amendments is a technically feasible method for remediating agricultural soil contaminated with Cd.

Leaching and characterization studies of heavy metals in contaminated soil using sequenced reagents of oxalic acid, citric acid, and a copolymer of maleic and acrylic acid instead of ethylenediaminetetraacetic acid

In this work, the removal performance of three environmentally friendly reagents, oxalic acid (OA), citric acid (CA), and a copolymer of maleic and acrylic acid (PMAA), on heavy metals in polluted soil was studied at the optimum conditions and compared their sequenced performance. The results showed that the consecutive washing with the individual acids significantly improved the removal percentage of heavy metals in the soil compared to that of EDTA (10.2%, 71.3%, 29.8%, 61.6%, and 52.4% removal for As, Cd, Cu, Pb, and Zn, respectively). The removal of As, Cd, Cu, Pb, and Zn in the sequence of CA-OA was 65.6%, 79%, 59.1%, 64.6%, and 63.5%, respectively. In addition, the organic acids had little influence on the soil physicochemical properties after washing with slight reductions of acidity (pH) and soil organic matter (SOM), which are the major determinants of the usability of washed soils for plant growth. The germination rate of Sorghum bicolor in CA-OA-washed soils reached over 70% on the 7th day. CA-OA-washed soils collectively stand out in using washed soils for plant growth with the following advantages: simultaneous removal of cationic and anionic metals, less harmful impact on soil properties, and successful support for the germination of crops. Based on the findings, we recommend the CA-OA sequence as the best alternative to EDTA with higher metal removal efficiency and germination success.

Effect of multiple washing operations on the removal of potentially toxic metals from an alkaline farmland soil and the strategy for agricultural reuse

Few studies have carried out soil washing experiments using pot experiments to simulate in situ soil washing operations, particularly for alkaline soils. This study explored the effects of multiple washing operations using pot experiments on the removal efficiencies of potentially toxic metals (PTM) from alkaline farmland soil and the reuse strategy of washed soil for safe agricultural production. The results showed that the removal efficiencies of Cd, Pb, Cu, and Zn after seven washings with a mixed chelator (EDTA, GLDA, and citric acid) were 41.1%, 47.1%, 14.7%, and 26.5%, respectively, which was close to the results of the EDTA treatment. For the alkaline soil studied, the second washing with the mixed chelators most effectively removed PTM owing to the activation of them after the first washing operation. The mixed chelator more effectively increased the proportion of stable fraction of PTM and maintained soil nutrients (e.g., nitrogen content) than EDTA, indicating little disturbance of alkaline soil quality after washing with the mixed chelator. After the amendment of the washed soil, there was no visible difference in the biomass weight of crops from the soils washed with different agents, indicating that the inhibitory effect of both washing agents on plant growth was effectively alleviated. The Cd and Pb contents in Z. mays were below the threshold of Hygienical Standard for Feeds of China (GB 13078-2017) (1 and 30 mg·kg^-1). Moreover, after three cropping operations, the available concentrations of PTM in the soil washed with the mixed chelator were lower than those in the soil washed with EDTA, indicating the value and potential of agricultural reuse of alkaline farmland soil washed with the mixed chelator.

Remediating Garden Soils: EDTA-Soil Washing and Safe Vegetable Production in Raised Bed Gardens

Soil remediation is an important practice in the restoration of heavy metal-contaminated soils and reduce the heavy metal exposure of the local population. Here, we investigated the effect of an ex-situ soil washing technique, based on ethylenediaminetetraacetic acid (EDTA) as a chelating agent, on a contaminated Cambisol. Lead, Cd and Zn were investigated in different soil fractions, drainage water and four vegetables from August 2019 to March 2021. Three treatments consisting of (C) contaminated soil, (W) washed soil and (WA) washed soil amended with vermicompost and biochar were investigated in an outdoor raised bed set up. Our results showed that the total and bioavailable metal fractions were significantly reduced but failed to meet Austrian national guideline values. Initial concentrations in the soil leachate increased significantly, especially for Cd. Vegetables grown on the remediated soil took up significantly lower amounts of all heavy metals and were further reduced by the organic amendment, attaining acceptable values within EU guideline values for food safety. Only spinach exceeded the thresholds in all soil treatments. The increase in soil pH and nutrient availability led to significantly higher vegetable yields.

Co-high-efficiency washing agents for simultaneous removal of Cd, Pb and As from smelting soil with risk assessment

Soil washing is considered a highly efficient technology due to its higher removal rate of multiple heavy metals from contaminated soil. However, previous studies on Cd, Pb and As washing agents for soils with complex contaminations did not consider the differences in As and Cd/Pb properties, resulting in the lack of effective washing compounds and washing conditions for soils with complex contaminations. Moreover, most traditional washing agents can cause secondary pollution. In this study, HEDTA and lactic acid (LA) treatments resulted in a higher Cd and Pb removal, while 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP) was more effective in As removal. Most importantly, a new washing strategy was proposed with a new combined high-efficiency washing agents consisting of HEDP + LA + FeCl₃ with a ratio of 6:3:1. Considering washing efficiency and consumption under optimal washing conditions, i.e. the soil/liquid (S/L) ratio of 1:20 and washing time of 48 h, the rates of Cd, Pb and As removal were 79.93%, 69.84% and 61.55%, respectively. In addition, washing process could influence the speciation of heavy metals, especially oxidizable and residual Cd and Pb fractions, as well as reducible As fraction. The washing process using the new washing agent can significantly reduce the pollution level and health risk of Cd, Pb and As contamination. The results of this study can provide an efficient washing agent for the remediation of heavy metal-contaminated soils at smelting sites, which will help protect human health.

Changes in organic matter composition caused by EDTA washing of two soils contaminated with toxic metals

Two soils contaminated with potentially toxic metals (PTMs) contrasting in pH and mineralogy were remediated with CaEDTA, and changes in soil organic matter (SOM) composition were investigated. Previous studies showed no significant loss of SOM from CaEDTA-treated soils, but the results of our study reflected significant decreases (from 46 to 49%) in the free fraction of humic acids (HAs). Remediation affected the composition of the free HA fraction via disturbance of intermolecular bonds - an increase in phenolic and aromatic groups with a simultaneous decrease in carbohydrates - which was confirmed by FTIR spectroscopy in both soils. Because non-radical molecules such as carbohydrates were selectively removed, the concentration of free radicals in the free HA fraction increased in acidic soil. The bound fraction of HAs and fulvic acids (FAs) in SOM, which are important due to their stability and the permanent effects they have on the soil's physical properties, remained unchanged in both remediated soils. The effect of soil recultivation was observed only in the excitation emission matrix (EEM) fluorescence spectra of HAs. In terms of SOM, CaEDTA soil washing can be considered moderately conservative; however, the restoration of free humic fractions is likely to be a long-term process.

Application of Soil Washing and Thermal Desorption for Sustainable Remediation and Reuse of Remediated Soil

Global governance of soil resources as well as revitalizations and remediation of degraded areas seem to be necessary actions for sustainable development. A great deal of effort has gone into developing remediation technologies to remove or reduce the impact of these contaminants in the environment. However, contaminated soil remediations in stringent conditions deteriorate soil properties and functions and create the need for efficient soil revitalization measures. Soil washing (SW) and thermal desorption (TD) are commonly used to remediate contaminated soil and can significantly reduce the contaminant, sometimes to safe levels where reuse can be considered; however, the effects of treatment on soil quality must be understood in order to support redevelopment after remediation. In this review, we discussed the effects of SW and TD on soil properties, including subsequent soil quality and health. Furthermore, the importance of these techniques for remediation and reclamation strategies was discussed. Some restoration strategies were also proposed for the recovery of soil quality. In addition, remediated and revitalized soil can be reused for various purposes, which can be accepted as an implementation of sustainable remediation. This review concludes with an outlook of future research efforts that will further shift SW and TD toward sustainable remediation.

Demonstration gardens with EDTA-washed soil. Part III: Plant growth, soil physical properties and production of safe vegetables

In previous reports large-scale EDTA-based soil washing using ReSoil® technology was demonstrated. In the current study, we established a vegetable garden with nine raised beds (4 × 1 × 0.5 m), three with original (contaminated) soil, three with remediated soil, and three with remediated soil vitalized by addition of vermicompost, earthworms, and rhizosphere inoculum. The garden was managed in 6 rotations between July 2018 and November 2019. Buckwheat was sown first as a green manure followed by spinach, lamb's lettuce, chicory, garlic, onion, leek, lettuce, carrot, kohlrabi and spinach again. Buckwheat growth on the remediated soil was reduced by half. Throughout the gardening process there were no remarkable differences in bulk density, hydraulic conductivity, available water capacity, and aggregate stability of the original and remediated soil. Biomass yield and plant performance, as measured by NDVI, also remained similar regardless of soil treatment. Remediation reduced Pb concentration in edible parts of vegetables from 76 (garlic) to 95% (kohlrabi), Zn concentration from 14 (lettuce) to 76% (first cutting of chicory), and Cd concentration from 33% (carrot) to 91% (leek and second cutting of chicory). The transfer of metals from soil to root and from root to shoot occurred in the order: Pb < Zn < Cd. The bioconcentration of toxic metals in edible plant parts was generally lower in the remediated soils. Application of ReSoil® technology and growing vegetables that exclude metals, especially Cd, has potential for safe food production on remediated soils. Vitalization had little effect on the properties of the remediated soil.

Demonstrational gardens with EDTA-washed soil. Part II: Soil quality assessment using biological indicators

In this study, we evaluated the impact of washing of Pb, Zn and Cd contaminated soil using EDTA-based technology (ReSoil®) on soil biological properties by measuring some of the most commonly used/sensitive biological indicators of soil perturbation. We estimated the temporal dynamics of the soil respiration, the activities of soil enzymes (dehydrogenase, β-glucosidase, urease, acid and alkaline phosphatase), and the effect of the remediation process on arbuscular mycorrhizal (AM) fungi in original (Orig), remediated (Rem) and remediated vitalized (Rem+V) soils during a more than one-year garden experiment. ReSoil® technology initially affected the activity level of soil microbial respiration and all enzyme activities except urease and reduced AM fungal potential in the soil. However, after one year of vegetable cultivation and standard gardening practices, soil microbial respiration, acid and alkaline phosphatase in the Rem and Rem+V reached similar activities as in the Orig. Only the activities of dehydrogenase and β-glucosidase remained lower in the remediated soil compared to the Orig. The frequency of arbuscular mycorrhiza in the root system, arbuscular density in the colonized root fragment, and the intensity of mycorrhizal colonization in the colonized root fragments in the remediated treatments increased with time; at the end of the experiment, no consistent differences in these parameters of mycorrhizal colonization were found among the treatments. Our results suggest a restored biological functioning of the remediated soil after one year of vegetable cultivation. In general, no differences were found between the Rem and Rem+V treatments, indicating that simple common garden practices are sufficient to restore soil functioning after remediation.

Effect of Chelant-Based Soil Washing and Post-Treatment on Pb, Cd, and Zn Bioavailability and Plant Uptake

The remediation of Pb, Cd, and Zn contaminated soil by ex situ EDTA washing was investigated in two pot experiments. We tested the influence of (i) 0, 0.5, 1.0, and 1.5%wt zero-valent iron (ZVI) and (ii) a combination of 5%wt vermicompost, 2%wt biochar, and 1%wt ZVI on the metal availability in EDTA-washed soil using different soil extracts (Aqua regia, NH₄NO₃) and plant concentrations. We found that EDTA soil washing significantly reduced the total concentration of Pb, Cd, and Zn and significantly reduced the Cd and Zn plant uptake. Residual EDTA was detected in water extracts causing the formation of highly available Pb-EDTA complexes. While organic amendments had no significant effect on Pb behavior in washed soils, an amendment of ≥ 1%wt ZVI successfully reduced EDTA concentrations, Pb bioavailability, and plant uptake. Our results suggest that Pb-EDTA complexes adsorb to a Fe oxyhydroxide layer, quickly developing on the ZVI surface. The increase in ZVI application strongly decreases Zn concentrations in plant tissue, whereas the uptake of Cd was not reduced, but even slightly increased. Soil washing did not affect plant productivity and organic amendments improved biomass production.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s11270-021-05356-0.

Quality of heavy metal-contaminated soil before and after column flushing with washing agents derived from municipal sewage sludge

Removal of heavy metals (HMs) from soil is a priority in soil washing/soil flushing. However, for further management of remediated soil, it should be characterized in detail. This study presents, for the first time, an evaluation of soil quality after column flushing with new-generation washing agents (WAs) recovered from municipal sewage sludge (dissolved organic matter, DOM; soluble humic-like substances, HLS; soluble humic substances, SHS) and Na₂EDTA as a standard benchmark. Sandy loam soil was spiked with industrial levels of Cu, Pb and Zn, then flushed in a column reactor at two WA flow rates (0.5 and 1.0 ml/min). Soil quality was assessed by determining both physico-chemical (pH, total HMs and their mobility, soil organic matter, OM, humic substances, HS and their fractions, macroelements) and biological indicators (dehydrogenase activity, DHA; germination rate, GR; and inhibition factors for roots and shoots of Triticum aestivum). Total residual HMs contents and HMs contents in the mobile fraction were significantly lower in soil flushed at 1.0 ml/min than in soil flushed at 0.5 ml/min. With all WAs, the decrease in Cu content was larger than that of the other HMs, however this HM most effectively was removed with DOM. In contrast, Pb most effectively was removed by HLS and Na₂EDTA, and DOM should not be used to remediate Pb-contaminated soil, due to its very low effectiveness. Flow rate did not appear to affect the fertilizing properties of the soil, DHA activity or soil toxicity indicators. Soil flushing with all SS_WAs increased OM, HS, and exchangeable P, K and Na content in remediated soils, but decreased exchangeable Ca content, and in most cases, exchangeable Mg content. Soil flushing substantially improved DHA activity and GR, but only slightly improved the shoot and root inhibition factors.

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.

Soil washing with biodegradable chelating agents and EDTA: Effect on soil properties and plant growth

Soils contaminated with Pb, Zn and Cd are hazardous. Persistent EDTA and biodegradable GLDA, EDDS and IDS have been used as chelators in the ReSoil soil washing technology, which recycles chelator and curbs toxic emissions. The washed soils supported similar growth of buckwheat (F. esculentum) and better growth of Chinese cabbage (B. rapa) compared with the original (not-remediated) soil. The growth of buckwheat on EDDS-washed soil was an exception and was 67% suppressed. The activities of enzymes of the plant antioxidant preventive system were assessed in roots and leaves of Chinese cabbage on all soils. Similar activities were measured, confirming that washed soils are not harmful to the plants. Plant uptake of potentially toxic elements was reduced from all washed soils, i.e. buckwheat grown on GLDA-washed soils accumulated up to 27 and 83 times less Pb and Cd than in the original soil. The initial Pb emissions in leachate from GLDA and IDS washed soils were up to 89 and 92% higher than those of the original soil, respectively. The latter emissions ceased to the levels measured in original, EDTA and EDDS washed soils. Soil physical properties (water holding capacity, aggregate stability) and soil functionality, assessed as soil respiration and activity of enzymes indicative for soil C, N and P cycle, were similar in all soils after 10 weeks of plant growth experiment. The overall results indicate a low impact of the remediation on soil quality. Soils washed with EDTA performed slightly better compared to GLDA-, EDDS- and IDS-washed soils.

On-line microcolumn-based dynamic leaching method for investigation of lead bioaccessibility in shooting range soils

In this work, a miniaturized flow-through leaching test is presented for rapid screening of potential chemical extractants to explore the bioaccessibility of lead (Pb) in contaminated shooting range soils in Valkeala, Finland. The method combines the versatility of microcolumn-based extraction methods with on-line inductively coupled plasma optical emission spectrometry (ICP OES) analysis for expedient assessment of the magnitude of the bioaccessible pools and the leaching kinetics of lead from polluted soils under variable physicochemical scenarios. Acids and salt solutions were studied as potential extractants. The efficiency of the extractants relative to the initial total amount of lead in the soil sample (509 ± 21 mg/kg) were found to increase in the following order: 0.11 M acetic acid (55%) < 1 M MgCl₂ (58%) < 0.1 M NH₂OH·HCl (61%) < 0.1 M citric acid (93%) < 0.1 M HCl (96%). The proposed on-line microcolumn-based method was further explored for implementation of the modified BCR (now termed Standards, Measurements and Testing Programme, SM&T) sequential extraction procedure to avail the information about different fractions available in the solid sample, and validated by mass balance calculations. The equivalent sequential procedure in a batch format was then studied and compared against the on-line microcolumn extraction method. The on-line dynamic extraction system presented in this work accepts a substantial amount of sample (2.5 g) as compared to previous flow-through mini-column setups (generally accommodating < 0.25 g of sample), thus maintaining sample representativeness and fostering comprehension of the extraction patterns for non-homogenous soil materials. The use of cotton buds and Teflon membranes and holders in the microcolumn setup facilitates the repeatable flow-through leaching of trace elements and restrict formation of preferential channels. Monitoring of the leachable trace elements in real time delivers detailed insight into the ongoing extraction process and provides a time-saving assessment of potential chemical extractants.

Recovery of the biological function of ethylenediaminetetraacetic acid-washed soils: Roles of environmental variations and microbes

To understand the recovery of the biological functions of washed soil, we studied changes in the microbial communities of soils washed with 10 or 60 mmol kg^-1 ethylenediaminetetraacetic acid (EDTA) for 90 d of incubation. The relative abundance of tolerant or degrading species decreased, while that of microorganisms with chemical autotrophic ability increased as the incubation time increased. The changes in the enzyme activity followed different trends. As an intracellular enzyme, dehydrogenase was initially most severely damaged by the washing process but could recover over time, while the activity of urease increased after washing with EDTA, which may be related to the use of N as a nutrient source by microorganisms. Phosphatase did not significantly change over time. The redundancy discriminant analyses indicated that there were distinct factors driving such changes in the soils washed with different EDTA dosages. For the soil washed with 10 mmol kg^-1 EDTA, bacteria with tolerance or degradation capacity of toxic pollutants, such as Nocardioidaceae, played a more important role in the recovery of soil functions; therefore, the EDTA stress indicator was the main driving factor. However, in the soil washed with 60 mmol kg^-1 EDTA, chemolithoautotrophic bacteria, such as Nitrososphaeraceae, exerted a greater influence on the recovery of biological functions due to the higher loss of nutrients and EDTA residue; therefore, the main driving factor was the nutrients supply.

Soil washing remediation of heavy metal from contaminated soil with EDTMP and PAA: Properties, optimization, and risk assessment

Soil washing is a rapid and efficient remediation technique for soil contaminated by heavy metals. In this study, Cd, Pb, and Zn were removed from contaminated soil by ethylenediamine tetra (methylene phosphonic acid) (EDTMP) and polyacrylic acid (PAA). We then investigated the effect of varying the concentration, pH and duration of the washing processes. Single-factor experiments suggest that the PAA washing process may be dominated by electrostatic adsorption, and is suitable for remediation under weak acid and neutral conditions. Meanwhile, EDTMP remediation might be dominated by chelation, which is favorable in strong acid and alkaline environments. In a quadratic saturation D-optimization design (QSDD), we optimized the washing parameters and further explored the washing mechanism including primary factor, principal effect, interaction effect, and the optimal washing conditions, with simultaneously changing multiple influencing factors. The optimum efficiencies of Cd, Pb, and Zn removal were 92.74%, 96.14%, and 50.76% respectively in EDTMP remediation, and 84.62, 79.24, and 41.66% respectively in PAA remediation. The washing processes effectively reduced the availability of Cd, Pb, and Zn in contaminated soil, without noticeably affecting soil chemical properties. Therefore, the washing incurred little ecological risk. EDTMP and PAA are suitable remediation agents of soil contaminated by heavy metals.

A Soluble Humic Substance for the Simultaneous Removal of Cadmium and Arsenic from Contaminated Soils

With abundant oxygen-containing functional groups, a humic substance (HS) has a high potential to remediate soils contaminated by heavy metals. Here, HS was first extracted from a leonardite and analyzed for its chemical compositions and spectroscopic characteristics. Then it was assessed for its ability as a washing agent to remove Cd and As from three types of soils (red soil, black soil, and fluvo-aquic soil) that were spiked with those contaminants (Cd: 40.5–49.1 mg/kg; As: 451–584 mg/kg). The operational washing conditions, including the pH and concentration of the HS, washing time and cycles, and liquid–soil ratio, were assessed for Cd and As removal efficiency. At pH 7, with an HS concentration (3672 mg C/L) higher than its critical micelle concentration and a liquid–soil ratio of 30, a single washing for 6–12 h removed 41.9 mg Cd/kg and 199.3 mg As/kg from red soil, 33.5 mg Cd/kg and 291.5 mg As/kg from black soil, and 30.4 mg Cd/kg and 325.5 mg As/kg from fluvo-aquic soil. The removal of Cd and As from the contaminated soils involved the complexation of Cd and As with the carboxyl and phenolic groups of HS. Outcomes from this research could be used to develop a tailor-made HS washing agent for the remediation of Cd- and As-contaminated soils with different properties.

Chemical and microbiological responses of heavy metal contaminated sediment subject to washing using humic substances

Washing of contaminated soils or sediments using humic substances (HS) extracted either from source-rich materials or compost has been tested effective to remove various heavy metals. Nevertheless, the remaining chemical fractionation of metals and post-washing biological responses were not discussed in previous research. In this study, we used a HS extracted from green waste compost to wash off Cd, As, and Ni from a contaminated sediment, and evaluated the washing effect on sediment microbes by measuring a series of indexes with regard to microbial biomass and enzyme activities. Results showed that HS washing was more effective in removing the cationic metals Cd and Ni than the anionic metal As. The highest HS dose of 2000 mg L^-1 resulted in 24.5-, 33.1-, and 12-fold increases of removal for Cd, Ni, and As, respectively. The remaining Cd and As were found to migrate to less stable fractions, whereas the remaining Ni was dominantly found in the residual fraction. Increases of metal removal efficiency, microbial biomass, and dehydrogenase activity were found to correlate with the increase of HS concentrations. Increasing doses of HS slightly altered sediment pH to the lower range but did not cause any significant effect on microbial activities. The study proves that HS washing is indeed a more environmental-friendly strategy than many existing washing agents which have exerted various side effects on soil properties.

Multi-step column leaching using low-molecular-weight organic acids for remediating vanadium- and chromium-contaminated soil

In soil, vanadium (V) contamination is commonly concomitant with chromium (Cr) contamination, which poses potential risks to humans, animals, and plants due to the transfer of toxic metals and the increase in their concentrations via the food chain or through direct exposure. This study applied a multi-step column leaching process using low-molecular-weight organic acids (LMWOAs) to treat V-contaminated soil from a smelter site that contains 2015.1 mg V kg^-1 and 1060.3 mg Cr kg^-1. After leaching three times with an equivalent solution/soil ratio of 0.3 mL/g using 1.0 M oxalic acid solution, the total removal rates reached 77.2% and 7.2% for V and Cr, respectively, while the removal rates of the extractable fractions reached 118.6% and 99.2% due to the reduction in residual fraction (F₄) of toxic metals. Simultaneously, the distribution and redistribution of geochemical fractions of V and Cr were determined with a sequential extraction technique, and the greater proportion of potential mobile fractions of V (65.1%) may increase its leaching from soil relative to Cr (7.1%). In addition, a lower pH of the leaching agent increased the efficiency of the leaching process to an extent. Compared with batch extraction with a typical solution to soil ratio of 10 mL/g, multi-step column leaching used less agent and hence produced less wastewater. This strategy could reduce the mobilization and bioavailability of toxic metals, and potentially enhance in situ soil flushing for the remediation of V- and Cr- contaminated soil.

Remediation of cadmium, lead and zinc in contaminated soil with CETSA and MA/AA

Soil washing, which is used to remove heavy metals from soil, is dependent on suitable washing agents. However, there is still a lack of economical, environmentally friendly washing agents with high removal efficiency. In this study, three washing agents, carboxyalkylthiosuccinic acid (CETSA), copolymer of maleic and acrylic acid (MA/AA) and ethylenediamine tetra acetic acid (EDTA), were used to remove heavy metals from contaminated soil. The influence of washing solution concentration, pH and washing time on heavy metals removal was also investigated. The cadmium (Cd), lead (Pb), and zinc (Zn) removal efficiencies increased as washing solution concentrations increased from 0 to 60 g L^-1, while they declined as pH increased from 3 to 8. Despite fluctuations between 90 and 120 min, heavy metal removal efficiencies increased continuously from 10 to 90 min. The three agents also effectively reduced the potential risks of Cd, Pb, and Zn in contaminated soil, but only CETSA and MA/AA produced no significant changes in chemical properties. Fourier transform infrared spectroscopy revealed that the hydroxyl, carboxyl, carbonyl, methoxyl, and sulfur groups were related to the heavy metal ions from the soil colloids. Thus, CETSA and MA/AA were suitable washing agents for remediation of heavy metals contaminated soil.

Vanadium and chromium-contaminated soil remediation using VFAs derived from food waste as soil washing agents: A case study

Food waste (FW) is environmentally unfriendly and decays easily under ambient conditions. Vanadium (V) and chromium (Cr) contamination in soils has become an increasing concern due to risks to human health and environmental conservation. Volatile fatty acids (VFAs) derived from FW was applied as soil washing agent to treat V and Cr-contaminated soil collected from a former V smelter site in this work. The Community Bureau of Reference (BCR) three-step sequential extraction procedure was used to identify geochemical fractions of V and Cr influencing their mobility and biological toxicity. Optimal parameters of a single washing procedure were determined to be a 4 h contact time, liquid-solid ratio of 10:1, VFAs concentration of 30 g/L, and reaction temperature of 25 °C, considering for typical soil remediation projects and complete anaerobic fermentation of FW. Under the optimal conditions, butyric acid fermentation VFAs attained removal rates of 57.09 and 23.55% for extractable fractions of V and Cr, respectively. Simultaneously, a multi-washing process under a constant liquid-solid ratio using fresh and recycled VFAs was conducted, which led to an improvement on the total removal efficiency of toxic metals. The washing procedure could reach the pollution thresholds for several plants, such as of S. viridis, K. scoparia, M. sativa, and E. indica. This strategy enhances the utilization of VFAs derived from food waste, has a positive effect on V and Cr-contaminated soil remediation, wastewater control of soil washing and FW disposal.

The use of zero-valent Fe for curbing toxic emissions after EDTA-based washing of Pb, Zn and Cd contaminated calcareous and acidic soil

The use of EDTA-based soil washing is prevented by chelant environmental persistence and the hazard of toxic post-remedial emissions. Calcareous and acidic soils with 828 and 673 mg Pb kg^-1, respectively, and co-contaminated with Zn and Cd, were washed with 90 and 60 mM EDTA, respectively, to remove 67 and 80% of Pb. Washed soils were rinsed until 6.5 and 5.1 mM EDTA, respectively, was measured in the final rinsing solutions. Emissions of residual EDTA and chelated metals from remediated soils were mitigated by adsorption on zero-valent Fe (ZVI), which was added (0.5-1.5%, w/w) to the slurry of washed soil immediately before rinsing. ZVI addition prevented the initial post-remedial surge of toxic metals leachability and minimised toxic emissions from calcareous and acidic soil as soon as 6 and 7 days after remediation, respectively. The extractability/leachability of EDTA and toxic metals from remediated and ZVI amended soils diminished to close to emissions from the original soils, frequently below the limit of quantification by flame-AAS, and was not affected by the pH of the leaching solutions. Efficient curbing of toxic post-remediation emissions as demonstrated herein is of paramount importance for recognition of EDTA-based remediation as environmentally safe.

Effect of mixed chelators of EDTA, GLDA, and citric acid on bioavailability of residual heavy metals in soils and soil properties

Soil washing is an effective technology for the remediation of multi-metal contaminated soils. However, bioavailability of residual heavy metals in soils and soil properties could be changed during washing processes. This study investigated the effects of EDTA, FeCl₃ and mixed chelators (MC) on bioavailability of residual heavy metals in soils and soil biological properties after soil washing. The results showed that soil washing by chelators successfully decreased the total concentration of heavy metals in soils, while it did not effectively decrease the exchangeable fraction of heavy metals, especially for calcareous contaminated soil. The toxic effects of the washed soils seemed to exhibit higher correlations with the changes in the soil properties such as soil pH and nutrient concentrations. As compared with FeCl₃ and EDTA, MC tended to moderately change soil properties (e.g., pH, total N, available N, available P, and exchangeable K, Ca, and Mg). Additionally, MC-washed soil had the least influence on the soil enzymes activities, and had the highest germination and growth of Chinese cabbage. Accordingly, MC is a moderate washing solution in the removal of heavy metals from multi-metal contaminated soils, and had minimal negative effects on soil qualities.

Remediation of multiple heavy metal-contaminated soil through the combination of soil washing and in situ immobilization

The remediation of heavy metal-contaminated soils is a great challenge for global environmental sciences and engineering. To control the ecological risks of heavy metal-contaminated soil more effectively, the present study focused on the combination of soil washing (with FeCl₃) and in situ immobilization (with lime, biochar, and black carbon). The results showed that the removal rate of Cd, Pb, Zn, and Cu was 62.9%, 52.1%, 30.0%, and 16.7%, respectively, when washed with FeCl₃. After the combined remediation (immobilization with 1% (w/w) lime), the contaminated soils showed 36.5%, 73.6%, 70.9%, and 53.4% reductions in the bioavailability of Cd, Cu, Pb, and Zn (extracted with 0.11M acetic acid), respectively, than those of the soils washed with FeCl₃ only. However, the immobilization with 1% (w/w) biochar or 1% (w/w) carbon black after washing exhibited low effects on stabilizing the metals. The differences in effects between the immobilization with lime, biochar, and carbon black indicated that the soil pH had a significant influence on the lability of heavy metals during the combined remediation process. The activity of the soil enzymes (urease, sucrase, and catalase) showed that the addition of all the materials, including lime, biochar, and carbon black, exhibited positive effects on microbial remediation after soil washing. Furthermore, lime was the most effective material, indicating that low soil pH and high acid-soluble metal concentrations might restrain the activity of soil enzymes. Soil pH and nutrition were the major considerations for microbial remediation during the combined remediation. These findings suggest that the combination of soil washing and in situ immobilization is an effective method to amend the soils contaminated with multiple heavy metals.

Combined application of EDDS and EDTA for removal of potentially toxic elements under multiple soil washing schemes

Chelant-enhanced soil washing, such as EDTA (ethylenediaminetetraacetic acid) and biodegradable EDDS ([S,S]-ethylene-diamine-disuccinic acid), has been widely studied, however, EDTA is persistent under natural conditions while EDDS has a low efficiency for Pb extraction. Therefore, we investigated the efficacy of mixed chelants (EDDS and EDTA mixture at 1:1 M ratio) for the removal of Cu, Zn, and Pb from a field-contaminated soil using various washing schemes (multi-pulse, step-gradient chelant, and continuous washing schemes). Speciation modelling of the target metals, mineral elements, and EDDS/EDTA was performed, while the leachability and bioaccessibility of residual metals in the treated soils were also assessed. Our results suggested that the combined use of EDDS and EDTA reached equivalent extraction efficiency of the target metals as EDTA, i.e., 50% reduction in the dosage of EDTA was made possible. This was accomplished by selective extraction of Cu by EDDS and Pb by EDTA, which was supported by the results of speciation calculation. Multi-pulse washing scheme with intermittent water rinsing steps removed entrapped metal-chelant complexes and free chelants, therefore reducing the leachability and bioaccessibility of residual metals in the treated soils. Step-gradient chelant washing with the maximum dosage of chelants in the first washing step only achieved marginal improvement but undesirably promoted Pb bioaccessibility. Continuous washing for 24 h enhanced metal extraction but promoted mineral dissolution, together with a large amount of uncomplexed chelants and increase in Cu leachability. Thus the combined use of EDDS and EDTA in multi-pulse washing is recommended for further studies.

Feasibility of four wastes to remove heavy metals from contaminated soils

Soil washing is one of the permanent techniques to remove heavy metals, and washing agent is a key influence factor for this technique, but there is still lack of high-efficiency, eco-friendly, and inexpensive agents. In this study, four wastes including pineapple peel (PP), soybean straw (SS), broad bean straw (BBS) and tea residue (TR) were employed to remove cadmium (Cd), lead (Pb) and zinc (Zn) in contaminated soils. The Fourier transform infrared spectroscopy (FTIR) analysis indicated that hydroxyl, carboxyl, amine, carbonyl and amide groups were involved in the interaction with metal ions by complexation or ion exchange. We then investigated the influences of various conditions including washing solution concentration, pH, and washing time. The metal removal efficiencies with these agents increased as the concentration augmented from 5 to 80 g L^-1, decreased or presented an asymmetric V-shaped curve with increasing pH from 2.5 to 7.5, and fit intraparticle diffusion or Elovich model with washing time increasing. PP has the highest removals for Cd (90.1%), Pb (18.6%), and Zn (15.2%) in soil A, and 85.8, 24.8, and 69.4% in soil B, respectively. The relatively high metal removal was mainly attributed to effective removal of the exchangeable and acid soluble fractions. Moreover, single washing not only lowered the potential ecological risk of the heavy metals, but moderated the effects on soil chemical properties. Therefore, PP was a feasible washing agent to remediate soils contaminated by heavy metals.

Enhancing the soil heavy metals removal efficiency by adding HPMA and PBTCA along with plant washing agents

Plant washing agents-water-extracted from Coriaria nepalensis (CN), Clematis brevicaudata (CB), Pistacia weinmannifolia (PW) and Ricinus communis (RC)-are feasible and eco-friendly for soil heavy metal removal, but their single application has limited removal efficiency. To improve their metal removal efficiencies, two biodegradable assistant agents, hydrolytic polymaleic anhydride (HPMA) and 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTCA), were investigated in combination with plant washing agents through batch soil washing experiments. Results showed that the addition of HPMA or PBTCA with plant agents greatly enhanced the removal efficiencies of soil heavy metals (p<0.05). Under acidic conditions, the maximum improvements in soil heavy metal removal reached 18.69% and 18.00% for soil Cd and Zn by PW+HPMA, respectively, and 12.89% for soil Pb by CN+HPMA. Under neutral or alkaline conditions, the largest improvements in soil Cd, Pb and Zn were 24.18%, 54.38% and 25.47% by PW+PBTCA, respectively. When compared with EDTA, the loss rates of soil nitrogen, phosphorus and potassium significantly decreased (p<0.05) and the soil organic carbon significantly increased (p<0.05) after washing with the combinations. Hence, the addition of HPMA or PBTCA with the plant agents could improve the removal of soil heavy metals.

Evaluation methods for assessing effectiveness of in situ remediation of soil and sediment contaminated with organic pollutants and heavy metals

Soil and sediment contamination has become a critical issue worldwide due to its great harm to the ecological environment and public health. In recent years, many remediation technologies including physical, chemical, biological, and combined methods have been proposed and adopted for the purpose of solving the problems of soil and sediment contamination. However, current research on evaluation methods for assessing these remediation technologies is scattered and lacks valid and integrated evaluation methods for assessing the remediation effectiveness. This paper provides a comprehensive review with an environmental perspective on the evaluation methods for assessing the effectiveness of in situ remediation of soil and sediment contaminated with organic pollutants and heavy metals. The review systematically summarizes recent exploration and attempts of the remediation effectiveness assessment based on the content of pollutants, soil and sediment characteristics, and ecological risks. Moreover, limitations and future research needs of the practical assessment are discussed. These limitations are not conducive to the implementation of the abatement and control programs for soil and sediment contamination. Therefore, more attention should be paid to the evaluation methods for assessing the remediation effectiveness while developing new in situ remediation technologies in future research.

Removal of Pb, Zn, and Cd from contaminated soil by new washing agent from plant material

Soil washing is an effective approach to remove soil heavy metals, and the washing agent is generally regarded as one of the primary factors in the process, but there is still a lack of efficient and eco-friendly agents for this technique. Here, we showed that four plant washing agents-from water extracts of Coriaria nepalensis (CN), Clematis brevicaudata (CB), Pistacia weinmannifolia (PW), and Ricinus communis (RC)-could be feasible agents for the removal of soil lead (Pb), zinc (Zn), and cadmium (Cd). The metal removal efficiencies of the agents increased with their concentrations from 20 to 80 g L^-1, decreased with the increasing solution pH, and presented different trends with the reaction time increasing. CN among the four agents had the highest removal efficiencies of soil Pb (62.02%) and Zn (29.18%) but owned the relatively low Cd removal efficiencies (21.59%). The Fourier transform infrared spectroscopy showed that the abilities of plant washing agents for the removal of soil heavy metals may result from bioactive substances with specific functional groups such as -COOH, -NH₂, and -OH. Our study provided CN as the best washing agents for the remediation of contaminated soil by heavy metals.

EDTA retention and emissions from remediated soil

EDTA-based remediation is reaching maturity but little information is available on the state of chelant in remediated soil. EDTA soil retention was examined after extracting 20 soil samples from Pb contaminated areas in Slovenia, Austria, Czech Republic and USA with 120 mM kg(-1) Na2H2EDTA, CaNa2EDTA and H4EDTA for 2 and 24 h. On average, 73% of Pb was removed from acidic and 71% from calcareous soils (24 h extractions). On average, 15% and up to 64% of applied EDTA was after remediation retained in acidic soils. Much less; in average 1% and up to the 22% of EDTA was retained in calcareous soils. The secondary emissions of EDTA retained in selected remediated soil increased with the acidity of the media: the TCLP (Toxicity Characteristic Leaching Procedure) solution (average pH end point 3.6) released up to 36% of EDTA applied in the soil (28.1 mmol kg(-1)). Extraction with deionised water (pH > 6.0) did not produce measurable EDTA emissions. Exposing soil to model abiotic (thawing/freezing cycles) and biotic (ingestion by earthworms Lumbricus rubellus) ageing factors did not induce additional secondary emissions of EDTA retained in remediated soil.

Effect of soil washing with only chelators or combining with ferric chloride on soil heavy metal removal and phytoavailability: Field experiments

In a field experiment on multi-metal contaminated soil, we investigated the efficiency of Cd, Pb, Zn, and Cu removal by only mixture of chelators (MC) or combining with FeCl3. After washing treatment, a co-cropping system was performed for heavy metals to be extracted by Sedum alfredii and to produce safe food from Zea mays. We analyzed the concentration of heavy metals in groundwater to evaluate the leashing risk of soil washing with FeCl3 and MC. Results showed that addition of FeCl3 was favorable to the removal of heavy metals in the topsoil. Metal leaching occurred mainly in rain season during the first co-cropping. The removal rates of Cd, Zn, Pb, and Cu in topsoil were 28%, 53%, 41%, and 21% with washing by FeCl3+MC after first harvest. The application of FeCl3 reduced the yield of S. alfredii and increased the metals concentration of Z. mays in first harvest. However, after amending soil, the metals concentration of Z. mays in FeCl3+MC treatment were similar to that only washing by MC. The grains and shoots of Z. mays were safe for use in feed production. Soil washing did not worsen groundwater contamination during the study period. But the concentration of Cd in groundwater was higher than the limit value of Standard concentrations for Groundwater IV. This study suggests that soil washing using FeCl3 and MC for the remediation of multi-metal contaminated soil is potential feasibility. However, the subsequent measure to improve the washed soil environment for planting crop is considered.

Remediation and reclamation of soils heavily contaminated with toxic metals as a substrate for greening with ornamental plants and grasses

Soils highly contaminated with toxic metals are currently treated as waste despite their potential inherent fertility. We applied EDTA washing technology featuring chelant and process water recovery for remediation of soil with 4037, 2527, and 26 mg kg(-1) of Pb, Zn and Cd, respectively in a pilot scale. A high EDTA dose (120 mmol kg(-1) of soil) removed 70%, 15%, and 58% of Pb, Zn, and Cd, respectively, and reduced human oral bioaccessibility of Pb below the limit of quantification and that of Zn and Cd 3.4 and 3.2 times. In a lysimeters experiment, the contaminated and remediated soils were laid into two garden beds (4×1×0.15 m) equipped with lysimeters, and subjected to cultivation of ornamental plants: Impatiens walleriana, Tagetes erecta, Pelargonium×peltatum, and Verbena×hybrida and grasses: Dactylis glomerata, Lolium multiflorum, and Festuca pratensis. Plants grown on remediated soil demonstrated the same or greater biomass yield and reduced the uptake of Pb, Zn and Cd up to 10, 2.5 and 9.5 times, respectively, compared to plants cultivated on the original soil. The results suggest that EDTA remediation produced soil suitable for greening.

Efficiency of nanoscale zero-valent iron on the enhanced low molecular weight organic acid removal Pb from contaminated soil

The Pb removal efficiencies from contaminated soils by low molecular weight organic acid (LMWOA) and nanoscale zero-valent iron (nZVI) were investigated through batch soil washing experiments. Results showed that significant promotion on Pb-removal with the mixed solutions of LMWOA and nZVI (p < 0.05). The Pb removal efficiencies reached 64% and 83% for mine and farmland soil by addition of 0.2 M citric acid and 2.0 g L−1 nZVI, respectively. They decreased with increasing pH from 3 to 9. The mixed solutions of LMWOA and nZVI induced Pb(II) releases processes including a rapid desorption within 4 h and a slow desorption in the following hours. The second-order model was the most appropriate for describing the kinetic processes of Pb(II) desorption. The main fractions of Pb removal were exchangeable and reducible. Compared with LMWOA, the loss rates of nitrogen, phosphorus and potassium decreased after washing with the mixed solutions. Our study suggests that combining of LMWOA and nZVI would be a promising alternative approach for remediation Pb-contaminated soils.

Assessment of EDTA heap leaching of an agricultural soil highly contaminated with heavy metals

The efficiency of heavy metal removal from soil by EDTA leaching was assessed in a column leaching experiment at the laboratory scale and field heap leaching at the pilot scale using a sandy loam sierozem agricultural soil contaminated with Cd, Cu, Pb, and Zn. Soil amendment and aging were conducted to recover leaching soils. The percentages of Cd, Cu, Pb, and Zn removed by column leaching were 90%, 88%, 90%, and 67%, respectively, when 3.9 bed volumes of 50mM EDTA were used. At the pilot scale, on-site metal removal efficiencies using the selected leaching procedure were 80%, 69%, 73% and 62% for Cd, Cu, Pb and Zn, respectively. EDTA leaching decreased soil CEC, total P, total K and available K concentrations but increased organic matter and total Kjeldahl N concentrations. The subsequent amendment and soil aging further reduced the DTPA-extractable heavy metals in the leached soils. Growth of the first crop of pak choi in the leached soil was inhibited but the second crop grew well after the soil was aged for one year and the concentrations of Cd and Pb in the edible parts were below the Chinese statutory limits. The results demonstrate the potential feasibility of the field leaching technique using EDTA combined with subsequent amendment and soil aging for the remediation of heavy metal-contaminated agricultural soils.