Transient Phytoextraction Agents: Establishing Criteria for the Use of Chelants in Phytoextraction of Recalcitrant Metals

The phytoremediation of recalcitrant metals such as lead and uranium rely on soil amendments to enhance metal availability within the rhizosphere. Because these amendments may persist in soils, agents that not only biodegrade rapidly but also are effective in triggering metal uptake in plants are needed for metals phytoextraction to be considered as an accepted practice. In this study, several biodegradable organic acids and chelating agents were assessed to determine if these amendments can be used in an effective manner, and if their activity and use is consistent with a proposed class of soil amendments for phytoextraction, here termed transient phytoextraction agents (TPAs). A TPA is proposed as an agent that would exhibit both effectiveness in triggering plant accumulation of the targeted metal while minimizing the risk of migration through rapid degradation or inactivation of the soluble complex. Eleven candidate TPAs (acetic acid, ascorbic acid, citric acid, malic acid, oxalic acid, succinic acid, ethylenediaminedisuccinic acid, dicarboxymethylglutamic acid, nitrilotriacetic acid, BayPure® CX 100, and the siderophore desferrioxamine B) were tested in batch studies to evaluate their complexation behavior using contaminated soils, with uranium and lead as the target metals. A growth chamber study was then conducted with Brassica juncea (Indian mustard), Helianthus annuus (sunflower), and Festuca arundinacea (tall fescue) grown in a lead-contaminated soil that was treated with the candidate TPAs to assess phytoextraction effectiveness. For the soils tested, citric acid, oxalic acid, and succinic acid were found to be effective complexing agents for uranium phytoextraction, whereas Baypure® CX 100 and citric acid exhibited effectiveness for lead phytoextraction.     

Applying a solute transfer model to phytoextraction: Zinc acquisition by Thlaspi caerulescens

Phytoextraction is the removal of metals from contaminated soils into harvested plant tissues. The rate of phytoextraction is governed by both soil and plant characteristics. Most effort has focused on identifying appropriate plants for phytoextraction, but the benefits from this effort will be marginal unless the metals are in phytoavailable forms in the rhizosphere. The concentration of a metal in the rhizosphere can be estimated using solute transfer models that incorporate: the metal concentration in the bulk soil solution, the buffer power of the soil, diffusion coefficient for the metal, water movement, root size and morphology, and the rate of entry of metal into the roots. Here a solute transfer model is developed to predict the concentration of Zn in the rhizosphere solution ([Zn]_ext) of Thlaspi caerulescens, a hyperaccumulator species that could be exploited for Zn phytoextraction. The model predicts that Zn accumulation by T. caerulescens is sub-optimal when the Zn concentration in the bulk soil solution is <27 M. Such a high [Zn]_ext is rare in contaminated agricultural soils, but is possible in the metalliferous substrates where T. caerulescens is endemic. Sensitivity analyses indicate that Zn diffusion is more important than transpiration-driven mass flow for Zn delivery to the root, implying that management of soil physical and hydrological properties will improve phytoextraction. Sensitivity analyses also imply that strategies to enhance the Zn absorption power of the root will not necessarily be successful for enhancing phytoextraction per se. Thus, research into enhancing Zn availability and mobility in soil will be as important as understanding and manipulating Zn uptake by plants. In general, such models can be used to identify constraints to efficient phytoextraction (whether plant or soil) and to determine whether commercial phytoextraction is feasible.     

Chemically enhanced phytoextraction of risk elements from a contaminated agricultural soil using Zea mays and Triticum aestivum: performance and metal mobilization over a three year period

Enhanced phytoextraction using EDTA for the remediation of an agricultural soil contaminated with less mobile risk elements Cd and Pb originating from smelting activities in Príbram (Czech Republic) was assessed on the laboratory and the field scale. EDTA was applied to the first years crop Zea mays. Metal mobilization and metal uptake by the plants in the soil were monitored for two additional years when Triticum aestivum was planted. The application ofEDTA effectively increased water-soluble Cd and Pb concentrations in the soil. These concentrations decreased over time. Anyhow, increased concentrations could be still observed in the third experimental year indicating a low possibility of groundwater pollution after the addition of EDTA during and also after the enhanced phytoextraction process under prevailing climatic conditions. EDTA-applications caused phytotoxicity and thereby decreased biomass production and increased Cd and Pb uptake by the plants. Phytoextraction efficiency and phytoextraction potential were too low for Cd and Pb phytoextraction in the field in a reasonable time frame (as less than one-tenth of a percent of total Cd and Pb could be removed). This strongly indicates that EDTA-enhanced phytoextraction as implemented in this study is not a suitable remediation technique for risk metal contaminated soils.     

我国土壤重金属污染植物吸取修复研究进展

我国从上世纪90年代中后期开始土壤重金属（含类金属砷）污染的植物吸取修复研究及技术探索,先后发现了一批具有较高研究价值和应用前景的铜、砷、镉、锰等重金属的积累或超积累植物,并从重金属耐性和超积累生理机制、植物吸取修复的根际过程与机制、吸取修复强化措施和修复植物处置与资源化利用等方面进行了研究,同时开展了植物吸取修复技术的示范与应用,已有一些较成功的植物修复工程应用案例,使我国重金属污染土壤植物修复技术,尤其是植物吸取修复技术在国际上产生了较强的影响力。本文就近年来我国土壤重金属污染植物吸取修复研究进展进行了综述,并对今后的发展趋势进行了展望。     

Phytoextraction: an assessment of biogeochemical and economic viability

Phytoextraction describes the use of plants to remove metals and other contaminants from soils. This low-cost technology has potential for the in situ remediation of large areas of contaminated land. Despite more than 10 years of intensive research on the subject, very few commercial phytoextraction operations have been realised. Here, we investigate the viability of phytoextraction as an effective land-treatment technology. A Decision Support System (DSS) was developed to predict the effect of phytoextraction on soil metal concentration and distribution, as well as the economic feasibility of the process in comparison to either inaction or the best alternative technology. Changes in soil metal concentration are mechanistically predicted on the basis of plant water use, metal concentration in soil solution, soil density, plant root distribution and our so-called root-absorption factor. The root-absorption factor is a `lumped parameter' describing the xylem/soil solution metal concentration quotient. Phytoextraction is considered to be a viable option if it can satisfy environmental regulations and simultaneously be shown to be the most cost-effective technology, either alone, or in combination with other remediation technologies. To date, commercial phytoextraction has been constrained by the expectation that site remediation should be achieved in a time comparable to other clean-up technologies. However, if phytoextraction could be combined with a profit making operation such as forestry, then this time constraint, which has often been considered to be the Achilles heel of phytoextraction, may be less important.     

解磷微生物修复土壤重金属污染研究进展

土壤重金属污染问题日益严重,具有普遍性、隐蔽性、表聚性、不可逆性等特点,已经成为环境污染治理中的热点、难点问题。解磷微生物能够依靠自身的代谢产物或通过与其他生物的协同作用,将土壤中的难溶性磷转化为可供植物吸收利用的磷,具有多重植物促生长功能和重金属解毒能力,可在重金属毒害水平下,促进植物生长、提高植物抗病能力、克服重金属对植物生长的不利影响,从而增强重金属修复植物的生存竞争力。从解磷微生物的研究现状入手,介绍了解磷微生物对土壤重金属污染的修复能力,综述了解磷微生物对土壤重金属污染修复的作用机制,分析了目前解磷微生物在重金属修复过程中存在的问题,并提出了今后研究的方向,为重金属污染土壤的修复提供了新思路。     

Understanding molecular mechanisms for improving phytoremediation of heavy metal-contaminated soils

Heavy metal pollution of soil is a significant environmental problem with a negative potential impact on human health and agriculture. Rhizosphere, as an important interface of soil and plants, plays a significant role in phytoremediation of contaminated soil by heavy metals, in which, microbial populations are known to affect heavy metal mobility and availability to the plant through release of chelating agents, acidification, phosphate solubilization and redox changes, and therefore, have potential to enhance phytoremediation processes. Phytoremediation strategies with appropriate heavy metal-adapted rhizobacteria or mycorrhizas have received more and more attention. In addition, some plants possess a range of potential mechanisms that may be involved in the detoxification of heavy metals, and they manage to survive under metal stresses. High tolerance to heavy metal toxicity could rely either on reduced uptake or increased plant internal sequestration, which is manifested by an interaction between a genotype and its environment.A coordinated network of molecular processes provides plants with multiple metal-detoxifying mechanisms and repair capabilities. The growing application of molecular genetic technologies has led to an increased understanding of mechanisms of heavy metal tolerance/accumulation in plants and, subsequently, many transgenic plants with increased heavy metal resistance, as well as increased uptake of heavy metals, have been developed for the purpose of phytoremediation. This article reviews advantages, possible mechanisms, current status and future direction of phytoremediation for heavy-metal–contaminated soils.     

Competition alters plant species response to nickel and zinc

Phytoextraction can be a cost-efficient method for the remediation of contaminated soils. Using species mixtures instead of monocultures might improve this procedure. In a species mixture, an effect of heavy metals on the species' performance can be modified by the presence of a co-occuring species. We hypothesised that (a) a co-occuring species can change the effect of heavy metals on a target species, and (b) heavy metal application may modifiy the competitive behaviour between the plants. We investigated these mechanisms in a greenhouse experiment using three species to serve as a model system (Carex flava, Centaurea angustifolia and Salix caprea). The species were established in pots of monocultures and mixtures, which were exposed to increasing concentrations of Ni and Zn, ranging from 0 to 2,500 mg/kg. Increased heavy metal application reduced the species' relative growth rate (RGR); the RGR reduction being generally correlated with Ni and Zn concentrations in plant tissue. S. caprea was an exception in that it showed considerable Zn uptake but only moderate growth reduction. In two out of six cases, competitors significantly modified the influence of heavy metals on a target species. The interaction can be explained by an increased uptake of Zn by one species (in this case S. caprea) that reduced the negative heavy metal effect on a target species (C. flava). In two further cases, increasing heavy metal application also altered competitive effects between the species. The mechanisms demonstrated in this experiment could be of relevance for the phytoextraction of heavy metals. The total uptake of metals might be maximised in specific mixtures, making phytoextraction more efficient.     

Phytoremediation of toxic trace elements in soil and water

Toxic heavy metals and metalloids, such as cadmium, lead, mercury, arsenic, and selenium, are constantly released into the environment. There is an urgent need to develop low-cost, effective, and sustainable methods for their removal or detoxification. Plant-based approaches, such as phytoremediation, are relatively inexpensive since they are performed in situ and are solar-driven. In this review, we discuss specific advances in plant-based approaches for the remediation of contaminated water and soil. Dilute concentrations of trace element contaminants can be removed from large volumes of wastewater by constructed wetlands. We discuss the potential of constructed wetlands for use in remediating agricultural drainage water and industrial effluent, as well as concerns over their potential ecotoxicity. In upland ecosystems, plants may be used to accumulate metals/metalloids in their harvestable biomass (phytoextraction). Plants can also convert and release certain metals/metalloids in a volatile form (phytovolatilization). We discuss how genetic engineering has been used to develop plants with enhanced efficiencies for phytoextraction and phytovolatilization. For example, metal-hyperaccumulating plants and microbes with unique abilities to tolerate, accumulate, and detoxify metals and metalloids represent an important reservoir of unique genes that could be transferred to fast-growing plant species for enhanced phytoremediation. There is also a need to develop new strategies to improve the acceptability of using genetically engineered plants for phytoremediation.     

Phytoremediation of toxic trace elements in soil and water

Toxic heavy metals and metalloids, such as cadmium, lead, mercury, arsenic, and selenium, are constantly released into the environment. There is an urgent need to develop low-cost, effective, and sustainable methods for their removal or detoxification. Plant-based approaches, such as phytoremediation, are relatively inexpensive since they are performed in situ and are solar-driven. In this review, we discuss specific advances in plant-based approaches for the remediation of contaminated water and soil. Dilute concentrations of trace element contaminants can be removed from large volumes of wastewater by constructed wetlands. We discuss the potential of constructed wetlands for use in remediating agricultural drainage water and industrial effluent, as well as concerns over their potential ecotoxicity. In upland ecosystems, plants may be used to accumulate metals/metalloids in their harvestable biomass (phytoextraction). Plants can also convert and release certain metals/metalloids in a volatile form (phytovolatilization). We discuss how genetic engineering has been used to develop plants with enhanced efficiencies for phytoextraction and phytovolatilization. For example, metal-hyperaccumulating plants and microbes with unique abilities to tolerate, accumulate, and detoxify metals and metalloids represent an important reservoir of unique genes that could be transferred to fast-growing plant species for enhanced phytoremediation. There is also a need to develop new strategies to improve the acceptability of using genetically engineered plants for phytoremediation.

     

Phytoextraction of cadmium with Thlaspi caerulescens

The in situ phytoextraction of cadmium from soils can only be achieved using plants that are both tolerant to high Cd concentrations and able to extract sufficient amounts of the metal. However, very few plant species are capable of remediating Cd polluted soils in a reasonable time frame. This paper aims to show that the population of the hyperaccumulator Thlaspi caerulescens J. & C. Presl. from Viviez (south of France), which has a high Cd-accumulating capability, is an efficient tool to remove Cd from contaminated soils. Roots of T. caerulescensViviez proliferate in hot spots of metals in soils which is particularly advantageous because of heterogeneity of the distribution of metal in polluted soils. Isotopic techniques showed that plants from this population acquire Cd from the same pools as non-accumulating species, but that it was much more efficient than non-hyperaccumulators at removing the metal from the soil labile pool. This is due: to (i) a specific rooting strategy, and (ii) a high uptake rate resulting from the existence in this population of Cd-specific transport channels or carriers in the root membrane. Growth and overall extraction can be improved with appropriate N fertilisation, supplied either as mineral fertilisers or uncontaminated sewage sludge. Selecting bigger plants is possible from within a suitable Cd-accumulating population to improve the phytoextraction process. Growing the Cd-accumulating populations results in a reduction in the availability of Cd and Zn as shown with field and lysimeter experiments conducted for several years. As a result, on a practical aspect, Cd hyperaccumulating populations of T. caerulescens may be used as a tool to efficiently reduce the availability of Cd in soils, providing appropriate populations are used.     

Phytoextraction: a cost-effective plant-based technology for the removal of metals from the environment

Phytoremediation is an emerging technology that uses plants to clean up pollutants (metals and organics) from the environment. Within this field of phytoremediation, the utilization of plants to transport and concentrate metals from the soil into the harvestable parts of roots and above-ground shoots is usually called phytoextraction. Most traditional remediation methods do not provide acceptable solutions for the removal of metals from soils. By contrast, phytoextraction of metals is a cost-effective approach that uses metal-accumulating plants to clean up these soils. Subsequently, the harvestable parts, rich in accumulated metals, can be easily and safely processed by drying, ashing or composting. Some extracted metals can also be reclaimed from the ash, generating recycling revenues. Phytoextraction appears a very promising technology for the removal of metal pollutants from the environment and may be, at present, approaching commercialization.     

Enhanced phytoextraction: I. Effect of EDTA and citric acid on heavy metal mobility in a calcareous soil

Phytoextraction, the use of plants to extract heavy metals from contaminated soils, could be an interesting alternative to conventional remediation technologies. However, calcareous soils with relatively high total metal contents are difficult to phytoremediate due to low soluble metal concentrations. Soil amendments such as ethylene diaminetetraacetate (EDTA) have been suggested to increase heavy metal bioavailability and uptake in aboveground plant parts. Strong persistence of EDTA and risks of leaching of potentially toxic metals and essential nutrients have led to research on easily biodegradable soil amendments such as citric acid. In our research, EDTA is regarded as a scientific benchmark with which degradable alternatives are compared for enhanced phytoextraction purposes. The effects of increasing doses of EDTA (0.1,1,10 mmol kg(-1) dry soil) and citric acid (0.01, 0.05, 0.25, 0.442, 0.5 mol kg(-1) dry soil) on bioavailable fractions of Cu, Zn, Cd, and Pb were assessed in one part of our study and results are presented in this article. The evolution of labile soil fractions of heavy metals over time was evaluated using water paste saturation extraction (approximately soluble fraction), extraction with 1 M NH4OAc at pH 7 (approximately exchangeable fraction), and extraction with 0.5 M NH4OAc + 05 M HOAc + 0.02 M EDTA at pH 4.65 (approximately potentially bioavailable fraction). Both citric acid and EDTA produced a rapid initial increase in labile heavy metal fractions. Metal mobilization remained constant in time for soils treated with EDTA, but a strong exponential decrease of labile metal fractions was noted for soils treated with citric acid. The half life of heavy metal mobilization by citric acid varied between 1.5 and 5.7 d. In the following article, the effect of heavy metal mobilization on uptake by Helianthus annuus will be presented.     

Using hyperaccumulator plants to phytoextract soil Ni and Cd

Two strategies of phytoextraction have been shown to have promise for practical soil remediation: domestication of natural hyperaccumulators and bioengineering plants with the genes that allow natural hyperaccumulators to achieve useful phytoextraction. Because different elements have different value, some can be phytomined for profit and others can be phytoremediated at lower cost than soil removal and replacement. Ni phytoextraction from contaminated or mineralized soils offers economic return greater than producing most crops, especially when considering the low fertility or phytotoxicity of Ni rich soils. Only soils that require remediation based on risk assessment will comprise the market for phytoremediation. Improved risk assessment has indicated that most Zn + Cd contaminated soils will not require Cd phytoextraction because the Zn limits practical risk from soil Cd. But rice and tobacco, and foods grown on soils with Cd contamination without corresponding 100-fold greater Zn contamination, allow Cd to readily enter food plants and diets. Clear evidence of human renal tubular dysfunction from soil Cd has only been obtained for subsistence rice farm families in Asia. Because of historic metal mining and smelting, Zn + Cd contaminated rice soils have been found in Japan, China, Korea, Vietnam and Thailand. Phytoextraction using southern France populations of Thlaspi caerulescens appears to be the only practical method to alleviate Cd risk without soil removal and replacement. The southern France plants accumulate 10-20-fold higher Cd in shoots than most T. caerulescens populations such as those from Belgium and the UK. Addition of fertilizers to maximize yield does not reduce Cd concentration in shoots; and soil management promotes annual Cd removal. The value of Cd in the plants is low, so the remediation service must pay the costs of Cd phytoextraction plus profits to the parties who conduct phytoextraction. Some other plants have been studied for Cd phytoextraction, but annual removals are much lower than the best T. caerulescens. Improved cultivars with higher yields and retaining this remarkable Cd phytoextraction potential are being bred using normal plant breeding techniques.     

Effects of organic amendments on Cd, Zn and Cu bioavailability in soil with repeated phytoremediation by Sedum plumbizincicola

Organic materials with different functional groups can be used to enhance metal bioavailability. Traditional organic materials (rice straw and clover) and ethylenediamine disuccinic acid (EDDS) were applied to enhance metal uptake from polluted soil by Sedum plumbizincicola after repeated phytoextraction. Changes in pH, dissolved organic carbon (DOC) and metal concentrations were determined in the soil solution after EDDS application. Amendment of the soil with ground rice straw or ground clove resulted in higher concentrations of Cd only (by factors of 1.92 and 1.71 respectively) in S. plumbizincicola compared to control soil. Treatment with 3 mmol kg(-1) EDDS increased all the metals studied by factors of 60.4, 1.67, and 0.27 for Cu, Cd, and Zn, respectively. EDDS significantly increased soil solution DOC and pH and increased soil plant-available metals above the amounts that the plants could take up, resulting in high soil concentrations of soluble metals and high risk of ground water contamination. After repeated phytoremediation of metal contaminated soils the efficiency of metal removal declines as the concentrations of bioavailable metal fractions decline. Traditional organic materials can therefore be much more effective and environmentally friendly amendments than EDDS in enhancing phytoremediation efficiency of Cd contaminated soil     

丛枝菌根在植物修复重金属污染土壤中的作用

丛枝菌根（Arbuscular mycorrhizae,AM）是自然界中分布最广的一类菌根,AM真菌能与陆地上绝大多数的高等植物共生,常见于包括重金属污染土壤在内的各种生境中。在重金属污染条件下,AM真菌可以减轻重金属对植物的毒害,影响植物对重金属的吸收和转运,在重金属污染土壤的植物修复中显示出极大的应用潜力。重点介绍了AM真菌对植物重金属耐性的影响及其在植物提取和植物稳定中的应用等方面的进展,讨论了未来研究所面临的任务和挑战。     

Heavy metals toxicity in plants: An overview on the role of glutathione and phytochelatins in heavy metal stress tolerance of plants

Plants experience oxidative stress upon exposure to heavy metals that leads to cellular damage. In addition, plants accumulate metal ions that disturb cellular ionic homeostasis. To minimize the detrimental effects of heavy metal exposure and their accumulation, plants have evolved detoxification mechanisms. Such mechanisms are mainly based on chelation and subcellular compartmentalization. Chelation of heavy metals is a ubiquitous detoxification strategy described in wide variety of plants. A principal class of heavy metal chelator known in plants is phytochelatins (PCs), a family of Cys-rich peptides. PCs are synthesized non-translationally from reduced glutathione (GSH) in a transpeptidation reaction catalyzed by the enzyme phytochelatin synthase (PCS). Therefore, availability of glutathione is very essential for PCs synthesis in plants at least during their exposure to heavy metals. Here, I reviewed on effect of heavy metals exposure to plants and role of GSH and PCs in heavy metal stress tolerance. Further, genetic manipulations of GSH and PCs levels that help plants to ameliorate toxic effects of heavy metals have been presented.     

Phytoremediation potential of some agricultural plants on heavy metal contaminated mine waste soils,salem district,tamilnadu

The Pot culture experiment performed for phytoextraction potential of selected agricultural plants [millet (Eleusine coracana), mustard (Brassica juncea), jowar (Sorghum bicolor), black gram (Vigna mungo), pumpkin (Telfairia occidentalis)] grown in metal contaminated soils around the Salem region, Tamilnadu, India. Physiochemical characterization of soils, reported as low to medium level of N, P, K was found in test soils. The Cr content higher in mine soils than control and the values are 0.176 mg/L in Dalmia soil and 0.049 mg/L in Burn &; Co soil. The germination rate low in mine soil than control soils (25 to 85%). The content of chlorophyll, carotenoid, carbohydrate and protein decreased in mine soils than control. The morphological parameters and biomass values decreased in experimental plants due to metal accumulation. Proline content increased in test plants and ranged from 0.113 mg g^?1 to 0.858 mg g^?1 which indicate the stress condition due to toxicity of metals. Sorghum and black gram plants reported as metal tolerant capacity. Among the plants, Sorghum produced good results (both biomass and biochemical parameters) which equal to control plant and suggests Sorghum plant is an ideal for remediation of metal contaminated soils.     

用转基因植物修复重金属污染的土壤

将新的性状转入高生物量植物中,以此开发高效的转基因植物修复系统,用于重金属污染的土壤修复是一项具有广阔应用前景的技术。大量实验表明,将细菌、真菌、动物、人类及植物本身与重金属脱毒相关的基因转入高生物量植物,异源表达产物可介导转基因植物耐受和高积累重金属及类似物。综述了这方面的研究进展。     

Large-area experiment on uptake of metals by twelve plants growing in soils contaminated with multiple metals

A site in central Taiwan with an area of 1.3 ha and contaminated with Cr, Cu, Ni, and Zn was selected to examine the feasibility of phytoextraction. Based on the results of a preexperiment at this site, a total of approximately 20,000 plants of 12 species were selected from plants of 33 tested species to be used in a large-area phytoextraction experiment at this site. A comparison with the initial metal concentration of 12 plant species before planting demonstrated that most species accumulated significant amounts of Cr, Cu, Ni, and Zn in their shoots after growing in this contaminated site for 31 d. Among the 12 plant species, the following accumulated higher concentrations of metals in their shoots; Garden canna and Garden verbena (45-60 mg Cr kg(-1)), Chinese ixora and Kalanchoe (30 mg Cu kg(-1)), Rainbow pink and Sunflower (30 mg Ni kg(-1)), French marigold and Sunflower (300-470 mg Zn kg(-1)). The roots of the plants of most of the 12 plant species can accumulate higher concentrations of metals than the shoots and extending the growth period promotes accumulation in the shoots. Large-area experiments demonstrated that phytoextraction is a feasible method to enable metal-contaminated soil in central Taiwan to be reused.