Effect of chelating agents in phytoremediation of heavy metals

Chelate‐assisted metal uptake by plants has only recently been discovered in the remediation industry. The simultaneous accumulation of lead, arsenic, copper, and cadmium in plants after application of chelating agents to soil is a promising technology enhancement for phytoremediation. One of the most powerful and commonly used chelating agents is ethylene diamine tetra acetic acid (EDTA), which forms complexes with many of the metal contaminants within the natural environment. This study was conducted to determine the efficiency of an emergent wetland plant species Typha sp. and floating wetland macrophytes such as Pistia sp., Azolla sp., Lemna sp., Salvinia sp., and Eichhornia sp. in phytoremediation of various heavy metals with addition of a chelating agent such as EDTA. EDTA addition to the treatment systems increased the uptake of heavy metals by plants, which was much pronounced with lead and copper. However, the pattern of uptake by plants was similar as that of heavy metals without EDTA amendments. © 2012 Wiley Periodicals, Inc.     

Remediation of metal-contaminated sites using plants

Heavy metal contamination of soil resulting from anthropogenic sources poses a significant challenge in many industrialized societies. The current technologies employed for removal of heavy metals often involve expensive ex-situ processes requiring sophisticated equipment and removal, transportation, and purification of the soil. Generally, in-situ remedial technologies are favored to ex-situ methods for detoxification, neutralization, degradation, or immobilization of contaminants. In-situ bioremediation is increasingly favored because of its effectiveness and low cost. A new type of bioremediation, known as vegetative remediation or “phytoremediation,” uses metal-tolerant hyperaccumulator plants to take up metal ions from soils and store them in their aboveground parts. To select the appropriate phytoremediation technology, one must understand the technical feasibility, cost effectiveness, and availability of the suitable plant species. Equally important is determining whether the site's soil conditions are optimal to enhance or restore the soil biological activity. Before phytoremediation can be exploited on a contaminated site, greenhouse-scale confirmatory testing is necessary to measure plant uptake and correlate shoot metal concentrations to available soil metals. These tests also validate that the harvesting and subsequent disposal of metal-containing plant tissues are environmentally safe and manageable.     

Field trial experiment: Phytoremediation with Salix sp. on a dredged sediment disposal site in Flanders,Belgium

Remediation of heavy metal contamination in soil is a widespread environmental issue. Conventional remediation techniques are invasive and often too expensive, particularly if large areas of soil are contaminated. Phytoremediation is the use of plants to remediate soil and groundwater. Phytoremediation of inorganic comtaminants such as metals can be further catagorized into phytostabilization and phytoextraction. These techniques have gained an increasing amount of attention and research over the last ten years. Phytoextraction of heavy metals and periodical removal of harvestable plant parts results in a gradual decrease of pollutant levels in the top soil. Woody species such as Salix sp. (willow) do not represent the fastest phytoextraction procedure compared to uptake by herbaceous species; however, they offer the added advantage of possible reuse of the produced biomass (wood) for the production of renewable energy. Here we present the results of a field experiment conducted to evaluate the use of Salix to remediate soil contaminated with cadmium and zinc at a dredged sediment disposal site in Flanders, Belgium. © 2003 Wiley Periodicals, Inc.     

Hype or Hope: The Potential of Phytoremediation as an Emerging Green Technology

Phytoremediation is defined as the use of green plants and their associated microorganisms, soil amendments, and agronomic techniques to remove, contain, or render harmless environmental pollutants. At the present time, phytoremediation is an emerging technology and there is still a significant need to pursue both fundamental and applied research to fully exploit the metabolic and growth habits of higher plants. It is precisely the purpose of the European COST Action 837 to stimulate the development and evaluate the potential of plant biotechnology for the removal of organic pollutants and toxic metals from wastewater and contaminated sites. However, green plants grow under nonsterile conditions and thus strongly interact with many microorganisms, like bacteria and mycorrhizal fungi. In this context, an Inter‐COST Workshop on bioremediation was recently organized to address the significance of soil microorganisms for plants, and the importance of their interactions, with regard to their potential for phytoremediation. Based on the outcomes of this workshop, the potential use of phytoremediation is presented in this article. © 2001 John Wiley & Sons, Inc.     

Phytoremediation of lead-contaminated soil at a New Jersey Brownfield site

Phytoremediation is a new technology that uses specially selected metal-accumulating plants as an attractive and economical method to clean up soils contaminated with heavy metals and radionuclides. The integration of specially selected metal-accumulating crop plants (Brassica juncea (L) Czern.) with innovative soil amendments allows plants to achieve high biomass and metal accumulation rates. In a recent study conducted at a lead-contaminated site in Trenton, New Jersey, the soil was treated with phytoremediation using successive crops of B. juncea combined with soil amendments. Through phytoremediation, the average surface soil lead concentration was reduced by 13 percent. In addition, the target soil concentration of 400 mg/kg was achieved in approximately 72 percent of the treated area in one cropping season.     

Enhanced phytoremediation: A study of mycorrhizoremediation of heavy metal–contaminated soil

Arbuscular mycorrhizal fungi (AMF) are microscopic fungi that occur naturally in soil and form a symbiosis with plant roots. By colonizing the roots, the fungus increases plant growth by making soil essential elements like zinc and phosphorus more accessible. AMF can play a role in the phytoremediation of heavy metal–contaminated soil (mycorrhizoremediation). Two research experiments were conducted to evaluate the impact of AMF on the extraction of different heavy metals (arsenic, cadmium, lead, selenium, and zinc) in contaminated soil. A grass mixture composed of Festuca rubra, Festuca eliator, Agropyron repens, and Trifolium repens was used in the experiments, and four different types of AMF were investigated: Glomus intraradices, Glomus mosseae, Glomus etunicatum, and Gigaspora gigantea. The results of the study showed that heavy metal extraction by Glomus intraradices colonized plants was the highest of all four AMF tested and was generally higher than nonmycorrhizal plants, depending on the heavy metal concentration in soil and whether it interacted with other metals in soil. However, metal extraction by AMF colonized grasses reached a plateau after an approximately two‐month period showing no further phytoaccumulation. © 2006 Wiley Periodicals, Inc.     

Arbuscular mycorrhizal fungi involvement in zinc and cadmium speciation change and phytoaccumulation

Over the past few years, there has been a greater study and understanding of the application of phytoremediation to remediate contaminated soil. The enhancement of phytoaccumulation of heavy metals—zinc (Zn), cadmium (Cd), arsenic (As), and selenium (Se)—in plants has been shown by inoculation of roots using arbuscular mycorrhizal fungi (AMF). This article presents the results of in vitro lab experiments conducted to verify the effects of AMF ( Glomus intraradices) hyphae on speciation of essential Zn and nonessential Cd heavy metals in order to change these metals from a water‐ insoluble carbonate to a soluble and phytoavailable form. Results show that in the presence of heavy metals in a nonavailable form to plants, endomycorrhizal hyphae can change the metal from carbonate to a water‐ soluble species. This phenomenon is more apparent with a nonessential (Cd) than with an essential metal (Zn). Zn saturation is reached in the G. intraradices colonized roots at around 400 ppm, independently of initial ZnCO₃ concentrations. Cd saturation is not reached; in the lower Cd treatment, the plant/media metal ratio is 3:1, and in the higher treatment, the ratio is 1:1. © 2005 Wiley Periodicals, Inc.     

Zinc,lead, and cadmium tolerance and accumulation in Cistus libanotis,Cistus albidus,and Cistus salviifolius: Perspectives on phytoremediation

Heavy metal contamination is of particular concern for human health and the environment. Phytoremediation is an emerging cost‐effective strategy to remediate heavy metal contaminated soil. However, this technique is limited by the small number of plants that are tolerant to heavy metals and are also accumulators. This study assayed zinc, lead, and cadmium tolerance and accumulation in Cistus libanotis, Cistus albidus, and Cistus salviifolius. The plants were cultivated in hydroponic conditions and exposed to different concentrations of Pb(NO₃)₂ (100 and 200 µM), ZnSO₄ (100 and 200 µM), or CdCl₂ (10 and 20 µM) for 3 weeks. Plant biomass and metal accumulation in roots and aboveground parts varied greatly among the species. All three species appeared to be sensitive to Zn. However, C. albidus displayed strong tolerance to Pb and accumulated large quantities of Pb and Cd in its roots. C. libanotis accumulated large quantities of Pb and Cd in its aboveground parts. C. libanotis can thus be classified as a Pb and Cd accumulator species. The study results show that C. albidus is suitable for phytostabilization of Pb‐contaminated soils, while C. libanotis can be used for phytoextraction of both Pb and Cd.     

Potential benefits and risks of land application of sewage sludge

Sewage sludge, also referred as biosolids, is a byproduct of sewage treatment processes. Land application of sewage sludge is one of the important disposal alternatives. Characteristics of sewage sludge depend upon the quality of sewage and type of treatment processes followed. Being rich in organic and inorganic plant nutrients, sewage sludge may substitute for fertilizer, but availability of potential toxic metals often restricts its uses. Sludge amendment to the soil modifies its physico-chemical and biological properties. Crop yield in adequately sludge-amended soil is generally more than that of well-fertilized controls. Bioavailability of metals increases in sludge amended soil at excessive rates of application for many years. Plants differ in their abilities to absorb sludge-derived metals from the soil. The purpose of this paper is to review the available information on various aspects of sewage sludge application on soil fertility and consequent effects on plant production to explore the possibility of exploiting this byproduct for agronomy and horticulture.     

Phytoremediation of arsenic‐contaminated soil as affected by the chelating agent CDTA and different levels of soil pH

Elevated levels of arsenic can pose a major threat to both human health and the environment. The phytoremediation of heavy metals from soil is emerging as a cost‐effective technology for the remediation of contaminated soils. The present greenhouse study was undertaken to identify plants capable of tolerating and accumulating high concentrations of arsenic. Asparagus fern and rye grass were found to tolerate and accumulate more than 1,100 ppm of arsenic in plant tissue. Arsenic uptake as affected by different levels of the chelating agent trans‐1, 2‐ cyclohexylenedinitrilotetraacetic acid (CDTA) and soil pH were also studied. The application of 5 mmol kg^?1 of CDTA to arsenic‐contaminated medium loam field soil enhanced the accumulation of arsenic by the test plants. Under these conditions, plants accumulated up to 1,400 ppm of arsenic as compared to 950 ppm by the plants grown in soil containing 1,200 ppm of arsenic but without any amendment of the chelating agent. Plants grown in field soil of pH 5 containing 300 ppm of arsenic absorbed higher concentrations of arsenic than at other tested pH levels. Corresponding reductions in arsenic content of soil after plant harvests were observed. © 2001 John Wiley & Sons, Inc.     

Electroremediation part II: A study of the movement of heavy metals during an electroremediation process in Andisol soil

In the Summer 2004 issue of Remediation, the authors presented a study of the influence of buffering behavior in contaminated Andisol soil. This article, Part II, expands on this research by presenting the results of laboratory tests conducted to study the movement of heavy metals in contaminated Andisol soil during the first stage of an electrochemical remediation process. The analysis was performed on the soil after treatment and also on the washing solutions collected during the first four hours. In order to analyze the effectiveness of fast and simple techniques for monitoring the electroremediation process, computer‐aided modeling of speciation in the soil solution was performed in connection with the remediation treatment. The results show that the metals moved mainly as positive species in the soil and later occurred as insoluble forms relative to the pH value in the washing solution from the cathode chamber. © 2005 Wiley Periodicals, Inc.     

硫酸锰渣污染土壤中重金属的形态分布及生物活性

测定了硫酸锰渣污染土壤中Cu,Zn,Cd,Pb,Mn的总量和各形态含量.结果表明重金属总量远超过环境背景值和土壤环境二级标准.重金属各形态分布特征:Cu,有机态>残渣态>铁锰氧化态>碳酸盐态>可交换态;Zn,残渣态>铁锰氧化态>有机态>碳酸盐态>可交换态;Cd,铁锰氧化态>可交换态>碳酸盐态>残渣态>有机态;Pb,铁锰氧化态>残渣态>有机态>碳酸盐态>可交换态;Mn,铁锰氧化态>残渣态>可交换态>有机态>碳酸盐态.重金属的生物可利用性系数和迁移系数均为Cd>Mn>Pb>Cu>Zn.     

Health risk assessment of heavy metals (Cr, Ni, Cu, Zn, Cd, Pb) in circumjacent soil of a factory for recycling waste electrical and electronic equipment

The aim of the study was to determine the potential environmental contamination in a typical factory for recycling waste electrical and electronic equipment in Shanghai. Heavy metals (Cr, Ni, Cu, Zn, Cd, Pb) in the soil around the factory have been evaluated in this paper. Compared with the background value, the concentrations of six metals detected in all the samples were higher, which showed that toxic metals were released into soil around the factory. Compared with the Environmental Quality Standards for Soils, China grade III, all the six metals are under soil guidelines. The non-cancer risk in different directions from the factory was in the order of: the north > the west > the south > the east. For inhalation and ingestion, the non-cancer risk in the soil west of the factory was biggest. Nevertheless, the non-cancer risk in the soil north of the factory was the biggest for dermal contact. The trend of cancer risk was the west > the south > the north > the east. The non-cancer risk and the carcinogenic risk for Cr, Ni, and Cd were all below the limiting value. This study might provide a reference for the risk assessment involved in electronic waste management and recycling activities.     

Metal fractionation in soil profiles at automobile mechanic waste dumps.

Soil profiles at five automobile mechanic waste dumps in Port-Harcourt, Nigeria were investigated to assess the spatial distribution, chemical speciation, and likely mobility of Cd, Cu, Pb, Zn, Cr and Ni in the soil as a function of the soil properties. A sequential fractionation protocol was used that generated six different fractions into which soil metal could partition. Cadmium was associated with non-residual fractions at surface horizons, but at lower depths it was in the residual fractions. Copper and Cr partitioned into organic and residual fractions, while Pb was associated with an Fe-Mn oxide fraction and the residual fractions. Zinc in surface horizons partitioned into an Fe-Mn oxide fraction and a fraction that captured carbonate-bound species, but in subsurface horizons, it was mainly in the residual fractions. Ni was predominantly found in the residual fractions. Mobility factors were calculated, and their values tended to decrease with increasing profile depth, indicating that these metals are relatively mobile in the surface horizons compared the subsurface except for chromium in the 15-30 cm depths. The mobility factors for the heavy metals follow the order: Cd > Zn > Pb > Cu > Cr > Ni. The results suggest that there is serious contamination hazard with Cd, Pb, and Zn in the soil profiles.     

Application of MSWI fly ash on acid soil and its effect on the environment

This study evaluated the feasibility of using municipal solid waste incinerator (MSWI) fly ash as acid soil amendment. In particular, changes in soil physicochemical properties and the potential environmental problems caused by the application of MSWI fly ash were investigated. The results showed that application of MSWI fly ash to the acid soil could raise the soil pH. The contents of rapidly available P and K, and slowly available K in the amended soil had a linear relationship with the addition ratio of MSWI fly ash. An addition of less than 20% of MSWI could raise the soil respiratory intensity after incubation for 3-5 days. Application of MSWI fly ash to the soil increased its content of water soluble salts and heavy metals, which could cause phytotoxicity in the plants. Therefore, the addition of MSWI fly ash to the soil should not be excessive, and less than 5-10% is an advisable addition level depending on the acidity of the soil and the plants growing on it.     

Brownfield Sites Remediation Technology Overview,Trends, and Opportunities in China

Over decades of economic development, China's industrialization has led to significant environmental issues due to unregulated discharges into air, water, and soil. As cities continue to expand (i.e., urbanization trend) and awareness/concerns about environmental pollution rises, many industrial facilities along the edge of or within the city boundaries have been relocated or closed. This urbanization trend leaves behind idled and abandoned land that is contaminated from the former industrial activities and unregulated discharges. China released its first nationwide soil quality survey in April 2014, and the survey suggests that soil conditions in China represent a significant challenge. China has encouraged local engineering firms to demonstrate soil treatment technologies through pilot‐scale studies, but the outcomes of many demonstrations have not been promising due to the lack of remediation experience and underdeveloped technical guidelines that are needed to guide the remediation processes. During the past decade, some local soil remediation experience has been established, but it is limited for certain technologies that address their primary contaminants of concern: heavy metals and persistent organic pollutants. In 2014, national technical guidelines were published regarding environmental investigation, risk assessment, monitoring, and remediation; however, regulations and funding systems are still underdeveloped. Thus, the remediation processes that should maximize economic and environmental benefits are not streamlined. This article provides an overview of the latest regulatory developments, remediation technologies applied, technology trends, and market opportunities in China. The provided information aims to allow international remediation practitioners to better understand and appreciate this unique and emerging remediation market, which is growing fast, and to highlight the importance of developing a sustainable model that not only provides for cleanup of the environment but also supports economic development. ©2015 Wiley Periodicals, Inc.     

Changes in Extractability of Cr and Pb in a Polycontaminated Soil After Bioaugmentation With Microbial Producers of Biosurfactants, Organic Acids and Siderophores

Partly because of the low bioavailability of metals, the soil cleaning-up using phytoremediation is usually time-consuming. In order to enhance the amount of metals at the plant's disposal, the soil bioaugmentation coupled together with phytoextraction is an emerging technology. In this preliminary work, two agricultural soils which mainly differed in their Cr, Hg and Pb contents (LC, low-contaminated soil; HC, high-contaminated soil) were bioaugmented in laboratory conditions by either bacterial (Bacillus subtilis, Pseudomonas aeruginosa, Pseudomonas fluorescens or Ralstonia metallidurans) or fungal inocula (Aspergillus niger or Penicillium simplicissimum) and incubated during three weeks. The LC soil pots bioaugmented with A. niger and P. aeruginosa contained higher concentrations of Cr (0.08 and 0.25 mg.kg⁻¹ dw soil) and Pb (0.25 and 0.3 mg.kg⁻¹ dw soil) in the exchangeable fraction F1 (extraction with MgCl₂) by comparison with the non-bioaugmented soil where neither Cr nor Pb was detected. Conversely, immobilization of Cr and Pb in the soil were observed with the other microorganisms. The soil bioaugmentation not only modified the metal speciation for the most easily extractable fractions but also modified the distribution of metals in the other fractions, to a lesser extent nevertheless. The difference in microbial concentrations between the bioaugmented or not HC soils reached up to 1.8 log units. Thus the microorganisms that we chose for the soil bioaugmentation were competitive towards the indigenous microflora. The PCA analysis showed close positive relationships between the microorganisms which potentially produced siderophores in the soil and the amount of Cr and Pb in the fraction F1.     

Plant selection for a pilot phytoremediation study at a former skeet range

A pilot phytoremediation project was conducted at the Mukluk site in Sprague, Connecticut, formerly a private skeet shooting range. A series of experiments was conducted to investigate if any plants can be effective lead phytoextractors for this site that has very high soil lead concentrations and low soil pH. Greenhouse screening of plants for lead resistance and accumulation using field soil was implemented as the initial step before the field installation. Herbaceous plant species with known lead phytoextraction capabilities included Indian mustard and blue fescue; a few willow clones with purported heavy metal resistance were also tested. Based on the results of the greenhouse experiments, blue fescue appeared to be sensitive to high lead concentration in soil, and only willows and Indian mustard along with various soil amendments were selected for the field installation. Indian mustard grew poorly in most of the treatments at the site except in the compost and lime treatment. Lead accumulation by this species was low in all treatments. In contrast, willows showed tolerance to very high lead concentrations present in the soil and were able to uptake and translocate lead into aboveground tissues. However, lead content in aerial tissues was low, and no change in soil lead concentration at the site was recorded post‐harvest after one growing season. It appeared that highly unfavorable soil characteristics at the Mukluk site complicated the species selection, and no effective phytoextractors have been found for this location. These suggest that the feasibility of phytostabilization and possible production of biofuel from willow biomass on these types of sites should be further investigated. © 2010 Wiley Periodicals, Inc.     

Characteristics of environmental factors and their effects on CH4 and CO2 emissions from a closed landfill: An ecological case study of Shanghai

To elucidate the influence of landfill gas (LFG) emission on environmental factors, an ecological investigation that was primarily concerned with the characteristics of vegetation, cover soil, and solid waste in the landfill was carried out. Temporal and spatial variations in vegetation diversity and coverage and their effects on reducing the emission of methane in the landfill were investigated. The results showed that both vegetation coverage and diversity increased with elapsed landfill closure time. The transition trend of the vegetation species was from perennial plant (Phragmites australis) to annual plants. Perennial vegetation was the dominant type of vegetation during the early closure period, and annual vegetation coverage increased with closure time. Vegetation preferentially appeared in areas of comparatively high depth of cover soil, which was characterized by high moisture retentiveness that enabled vegetation growth. The concentrations of methane and carbon dioxide in the cover soil significantly decreased with increasing closure time. The concentrations of methane and carbon dioxide from bare cover soil were higher than those from vegetated cover soil whereas the CO₂ flux of bare cover soil was less than that of vegetated cover soil.     

Cyclodextrin‐enhanced remediation of organic and metal contaminants in porous media and groundwater

Heavy metals and toxic organic contaminants are found at numerous industrial and military sites. The generally poor performance of conventional pump‐and‐treat schemes has made the development of improved methods for contaminated site remediation a significant environmental priority. One such innovative method is cyclodextrin‐enhanced flushing of the contaminated porous media and groundwater. Cyclodextrin is a glucose‐based molecule that is produced on industrial scales by microorganisms. Over the last years, several cyclodextrin derivatives have received extensive research interest. It was shown that cyclodextrins can significantly enhance the solubility of toxic organics, and in some cases, heavy metals and radioactive isotopes. As a sugar, cyclodextrin is considered relatively non‐toxic to humans, plants, and soil microbes. Thus, there are minimal health‐related concerns associated with the injection of cyclodextrin into the subsurface, which is an inherent advantage for use of cyclodextrins as a remediation agent. This paper provides a review of the available literature concerning use of cyclodextrin for remediation of groundwater and soil.