Advances in agronomy vol 75

Advances in agronomy vol 75

P1: FCG/FMZ P2: FUY Advances in Agronomy PS097-FM December 12, 2001 14:19 Stylefile version:April 24, 2000 Contributors Numbers in parentheses indicate the pages on which the authors’ contributions begin. E. BEN-DOR (173), The Remote Sensing and GIS Laboratory, Department of Geography and the Human Environment, Tel-Aviv University, Ramat Aviv, Tel-Aviv 69978, Israel E. LOMBI (1), Agriculture and Environment Division, IACR-Rothamsted, Harpenden, Herts AL5 2JQ, United Kingdom C. LIU (135), Institute of Geographic Sciences and Natural Resources Research, The Chinese Academy of Sciences, Beijing 100101, China S. P. McGRATH (1), Agriculture and Environment Division, IACR-Rothamsted, Harpenden, Herts AL5 2JQ, United Kingdom A. PICCOLO (57), Dipartimento di Scienze Chimico-Agrarie, Universit`a Degli Studi Di Napoli “Federico II,” 80055 Portici, Italy Via Universita 100, Naples, Italy H. WANG (135), State Key Laboratory of Water Environment Simulation, Key Laboratory for Water and Sediment Sciences, Ministry of Education, Beijing Normal University, Beijing 100875, China L. ZHANG (135), CSIRO Land and Water, Canberra Laboratory, P.O. Box 1666, Canberra, ACJ 2601, Australia F. J. ZHAO (1), Agriculture and Environment Divison, IACR-Rothamsted, Harpenden, Herts AL5 2JQ, United Kingdom vii P1: FCG/FMZ P2: FUY Advances in Agronomy PS097-FM December 12, 2001 14:19 Stylefile version:April 24, 2000 Preface Volume 75 contains four outstanding reviews dealing with phytoremediation, issues related to water use in China, humic substances, and remote sensing. Chap- ter 1 is an extensive review on phytoremediation of metals, metalloids, and radionu- clides including discussion on phytoextraction technologies, hyperaccumulator plants, and chemically induced phytoextraction and phytovolatilization. Chapter 2 covers the conservation and use of water in Chinese agriculture including engi- neering, economic, and agronomic aspects and considerations. Chapter 3 presents advances in understanding the structure of humic substances, particularly the con- cept of a supramolecular structure. Analytical and molecular scale evidence for this latter structure are presented as well as discussions on the role of humic su- perstructures in soils. Chapter 4 presented frontiers in quantitative remote sensing of soil properties including principles, methods, mechanisms, and limitations. I thank the authors for their first-rate reviews. D ONALD L. SPARKS ix 12/05/2001 09:33 AM Agronomy-V. 75 PS097A-01.tex PS097A-01.xml APserialsv2(2000/12/19) Textures 2.0 PHYTOREMEDIATION OF METALS, M ETALLOIDS, AND RADIONUCLIDES S. P. McGrath, F. J. Zhao, and E. Lombi Agriculture and the Environment Division IACR-Rothamsted, Harpenden, Herts AL5 2JQ, United Kingdom I. Introduction A. Risks of Metals and Metalloids in Soils B. The Need for Cleanup of Contaminated Soils C. Phytoextraction, Phytomining, and Removal Technologies II. Phytoextraction Using Hyperaccumulator Plants A. Metal Hyperaccumulators B. Phytoextraction Using Hyperaccumulator Plants C. Mechanisms of Metal Hyperaccumulation III. Chemically Enhanced Phytoextraction A. Potential Applications B. Chemically Enhanced Phytoextraction of Lead C. Chemically Enhanced Phytoextraction of Other Heavy Metals D. Chemically Enhanced Phytoextraction of Radionuclides E. Chemically Enhanced Phytomining F. Chemically Enhanced Phytoextraction versus Natural Hyperaccumulation G. Possible Concern Relating to the Use of Chelating Agents IV. Phytovolatilization A. Selenium B. Mercury V. Summary and Future Directions References Phytoremediation is a developing technology that can potentially address the problems of contaminated agricultural land or more intensely polluted areas affected by urban or industrial activities. Three main strategies currently exist to phytoextract inorganic substances from soils using plants: (1) use of natural hy- peraccumulators; (2) enhancement of element uptake of high biomass species by chemical additions to soil and plants; and (3) phytovolatilization of elements, which often involves alteration of their chemical form within the plant prior to volatiliza- tion to the atmosphere. Concentrating on the techniques that potentially remove inorganic pollutants such as Ni, Zn, Cd, Cu, Co, Pb, Hg, As, Se, and radionu- clides, we review the progress in the understanding of the processes involved and 1 Advances in Agronomy, Volume 75 Copyright C 2002 by Academic Press. All rights of reproduction in any form reserved. 0065-2113/02 $35.00 12/05/2001 09:33 AM Agronomy-V. 75 PS097A-01.tex PS097A-01.xml APserialsv2(2000/12/19) Textures 2.0 2 S. P. McGRATH ET AL. the development of the technology. This includes the advances made in the study of the physiology and biochemistry of metal uptake, transport and sequestration by hyperaccumulator plants, as well as the investigation of the processes occurring in soil and plant systems subject to the chemicalenhancement approach. Enoughwork has been carried out inthe last decadeto allow some assessment of the situations and elements in which phytoremediation is likely to be most successful. However, we also identify where there is lack of knowledge. Finally, the likely future directions for research and application are discussed. C 2002 Academic Press. I. INTRODUCTION Phytoremediation can be loosely defined as the use of plants to improve the envi- ronment. Obviously this is an enormous subject and here we will concentrate on the phytoremediation of metals, metalloids, and radionuclides. Phytoremediation of organic compounds in soil and water is a related and rapidly expanding area, which is covered elsewhere (Kruger et al., 1997; Salt et al., 1998; Wenzel et al., 1999). It is very appropriate to review this subject at this time because it was around 1990 that the first field experiments began examining phtyoremediation of metals and Se (reported in Ba˜nuelos et al., 1993; McGrath et al., 1993); and now a decade has passed. We will examine the different strategies that have evolved for phytoremediation and the progress that has been made on the physiology of metal accumulation. On a more practical level, the attempts at field application will be evaluated, and the likely future directions of the science and technology will be discussed. A. RISKS OF METALS AND METALLOIDS IN SOILS Metals and metalloids such as As and Se can pose risks when they build up in soils due to many forms of anthropogenic influences. Some such as Zn, Cu, Mn, Ni, Se, Co, Cr, and Mo are essential for living organisms, and therefore deficiency situations exist either because of very low total amounts of these metals in soil or because of low bioavailability caused by soil chemical conditions. In these cases, when metals are added, there may be positive biological responses in terms of growth and health of organisms. However, these metals and those that are thought to be nonessential such as Pb, As, Hg, and Cd tend to build up in soils; and when their bioavailability becomes high, toxicity can result. These negative effects can occur in soil microbes, soil fauna, higher animals, plants, and humans. A further 12/05/2001 09:33 AM Agronomy-V. 75 PS097A-01.tex PS097A-01.xml APserialsv2(2000/12/19) Textures 2.0 PHYTOREMEDIATION 3 threat is from radionuclides such as those of U, 137 Cs, 90 Sr, and 3 H in soil and water (Negri and Hinchman, 2000). Of course, these elements may occur at elevated concentrations quite naturally in soils and waters. In these cases there may be “effects” on biodiversity and on animal and human health. Examples would be metal-tolerant vegetation that has evolved on metal-mineralized soils (Baker and Proctor, 1990), the effects on human health due to excess Se (Yang et al., 1983), and Cd accumulation in tissues of white-tailed ptarmigan (Lagopus leucurus) in the Colorado Rocky Mountains, resulting in toxicity (Larison et al., 2000). In these cases, it may not be possible or desirable to clean up the soils, but there may be a role for plants in reducing the exposure of biota to these elements, for example, by reduced uptake and exclusion from tissues, or removing elements like Se in geogenically laden water (Ohlendorf et al., 1986; Wu et al., 1995). Indeed, where these natural hot spots occur, there may be specialized fauna and flora, like metallophyte vegetation, which may be in need of preservation (Reeves and Baker, 2000). Metals and metalloids enter soils and waters due to many processes including atmospheric deposition from industrial activities or power generation; disposal of wastes such as sewage sludge, animal manures, ash, domestic and industrial wastes or by-products; irrigation and flood or seepage waters and the utilization of fertilizers, lime, or agrochemicals. Radionuclides may build up in some areas due to deliberate or accidental releases related to their use for energy production or for military purposes. Unlike nitrate or chloride, many of these elements are relatively strongly retained in the surface of soils and do not readily leach, causing the accumulation that may ultimately pose a threat to humans and biota. However, under some conditions, small amounts of these elements do leach and can be an issue in waters, particularly those used for irrigation or drinking. Key examples here would be radionuclides, As, Se, and Cr (Chiou et al., 1995; Kimbrough et al., 1999; Negri and Hinchman, 2000; Ohlendorf et al., 1986). Under these conditions, phytoremediation is an important developing technol- ogy for removal of these elements from either soil or water. It has the potential to be low cost and to be applicable to large areas where other methods may be too expensive and where the concentrations of contaminants are too small for other methods to be effective or economically viable. B. THE NEED FOR CLEANUP OF CONTAMINATED SOILS There isa long history of contaminationaccumulating in soils due tothe practices mentioned earlier. Public and political pressure to reverse this situation and clean up areas only occurs when critical levels are reached. Leaving aside the methods of deriving critical levels for microbes, animals, plants, and humans, once these exist, they provide a direct stimulus for cleanup. 12/05/2001 09:33 AM Agronomy-V. 75 PS097A-01.tex PS097A-01.xml APserialsv2(2000/12/19) Textures 2.0 4 S. P. McGRATH ET AL. Using various ways of defining “contaminated” land, it has been estimated that in the European Union alone, there are potentially 1,400,000 contaminated sites (ETCS, 1998). Not all of these will be contaminated with metals or metalloids, but this gives an indication of the scale of the problem as it may exist worldwide. For example, trace elements are present at high concentrations at 65% of the con- taminated Superfund sites for which the US EPA has signed Records of Decision (US EPA, 1997). Indeed, some areas are not included in these assessments, such as those with low-level contamination due to atmospheric deposition or to the use of chemicals in agriculture. For example, the use of phosphate sources that are contaminated with Cd for agricultural fertilizers may result in crops that contain more than the allowed concentrations of Cd in foodstuffs (Commission of the European Communities, 2001). It is unlikely that these areas are included in the previously described estimates, as they focus more on urban and industrial land. However, for the sustained practice of agriculture with inputs of fertilizers, sewage sludge, and animal manures, there may be a role for plants in removing the small excess amounts of metals such as Cd, Zn, and Cu from soils, perhaps on a long rotational basis. Use of low-Cd phosphate is already taking place, while removal of Cd from phosphate rock is still not considered economically feasible (Oosterhuis et al., 2000). The average concentration in phosphate fertilizers in Europe is still 138 mg Cd kg −1 P (ERM, 1997). In comparison, the background level of cadmium is 0.3 mg Cd kg −1 soil or less in most agricultural soils in Europe. Concentrations in soils are increasing because the inputs are not balanced by the output in terms of removal by crops and leaching out of the ploughing layer (Eriksson et al., 1996; Kofoed and Klausen, 1983). Thus it is likely that phytoremediation will be needed for continued agriculture in the future. C. P HYTOEXTRACTION ,PHYTOMINING, AND REMOVAL TECHNOLOGIES Our focus in this review is on the methods that remove metals and metalloids from soil. This can be achieved by phytoextraction or phytovolatilization, depend- ing on the element considered. A variant of phytoextraction, which applies when the extracted elements are of high value, is phytomining. In the latter case, the aims are to derive a “bio-ore” from the burning of the plant material and to profit from the energy released by combustion of the biomass and the value of the ore itself. The recycling of elements that are bioconcentrated during phytoextraction will not be discussed, and the disposal options for plant biomass will depend on the market for the elements concerned. Related technologies exist or are under development, such as phytostabiliza- tion. This is when the plants are used essentially to stabilize contaminated land or the pollutants present in soil and in so doing prevent or reduce erosion, water 12/05/2001 09:33 AM Agronomy-V. 75 PS097A-01.tex PS097A-01.xml APserialsv2(2000/12/19) Textures 2.0 PHYTOREMEDIATION 5 flow, and flow of pollutants. In this case, metal-tolerant species that do not take up large quantities of metals are often used. In this review, for reasons of space, we chose to focus on remediation of contaminated soils, while preventing con- tamination of groundwater, and not remediation of contaminated water itself. That subject is covered elsewhere (Dushenkov and Kapulnik, 2000; Terry and Ba˜nuelos, 2000). The efficiency of phytoextraction is ultimately the product of a simple equation: biomass × element concentration in biomass. Both factors are important, but it is easy to show that high concentrations in the above-ground material are very important. Harvesting roots or other below-ground organs is difficult and prevents regrowth if the “crop” is a perennial one. The increasing yield from 2 to 20 t ha −1 , which is probably a biological maximum for an annual plant or harvestable from a perennial one, has little influence on the removal rate below about 1000 mg kg −1 of an element in the plant dry matter (Fig. 1). Therefore, maximizing concentra- tions in the plant seems to be the obvious strategy for increasing efficiency, while optimizing yields by agronomic means. However, it must be kept in mind that this thinking relates to very pollutedsoils thatrequire hundreds of kilograms per hectare to be removed. For elements like Cd where relatively small removals (<1kgha −1 ) Figure 1 Modeled removal of an element from soil by crops, showing the dependence on the concentration in the biomass and the effect of yield. All amounts relate to above-ground material. 12/05/2001 09:33 AM Agronomy-V. 75 PS097A-01.tex PS097A-01.xml APserialsv2(2000/12/19) Textures 2.0 6 S. P. McGRATH ET AL. are important, or radionuclides where small quantities are of concern, this may not be so true. But the principle still applies to Cd, i.e., the scope for maximizing Cd concentration is in the magnitude of hundreds (hyperaccumulators, see following), whereas the scope for maximizing biomass is <10). Two strategies exist for obtaining plant biomass with high metal concentrations: (1) use of natural hyperaccumulators and (2) the enhancement of uptake of metals by normally non-accumulating species by applying various chemicals that increase uptake. These are discussed in the following sections. II. PHYTOEXTRACTION USING HYPERACCUMULATOR PLANTS A. M ETAL HYPERACCUMULATORS 1. Definition Based on the relationship between metal concentrations in shoot and in soil, Baker (1981) proposed that plants growing on metalliferous soils can be grouped into three types: (1) excluders, where metal concentrations in the shoot are main- tained at a low level across a wide range of soil concentration, up to a critical soil value above which the mechanism breaks down and relatively unrestricted root-to-shoot transport results; (2) accumulators, where metals are concentrated in above-ground plant parts from low to high soil concentrations; and (3) indicators, where uptake and transport of metals to the shoot are regulated so that internal concentration reflects external levels, at least until toxicity occurs. Exclusion of metals from the shoots is by far the most common strategy em- ployed by many metal-tolerant species. On the other hand, metal accumulation can occur in some plant species that grow mainly on metalliferous soils. Reeves and Baker (2000) traced the earliest qualitative observation, Zn accumulation in Viola calaminaria in the Zn-rich soils in the Aachen area between Germany and Belgium, to A. Braun in 1855. Some 30 years later, Baumann (1885) showed that both Viola calaminaria and Thlaspi calaminare (later called Thlaspi caerulescens) growing over the calamine deposits contained over 1% Zn (10,000 µgg −1 )inthe shoot dry matter. Exceedingly high accumulations of Se in Astragalus plants and of Ni in Alyssum bertolonii were discovered in the 1930s and 1940s, respectively (see Brooks, 1998). Brooks et al. (1977) first introduced the term “hyperaccumulators” to describe plants capable of accumulating more than 1000 µgNig −1 on a dry leaf basis in their natural habitats. The criterion for defining Co, Cu, Pb, and Se hyperaccumulation is also 1000 µgg −1 in shoot dry matter, whereas for Zn and 12/05/2001 09:33 AM Agronomy-V. 75 PS097A-01.tex PS097A-01.xml APserialsv2(2000/12/19) Textures 2.0 PHYTOREMEDIATION 7 Mn the threshold is 10,000 µgg −1 , and for Cd 100 µgg −1 (Baker et al., 2000; Brooks, 1998). Although these criteria are quite arbitrary, in general the concentra- tions of metals in hyperaccumulator plants are about 100- to 1000-fold higher than those in normal plants growing on soils with background metal concentrations, and about 10- to 100-fold higherthan most other plants growing on metal-contaminated soils. In addition to the exceedingly high accumulation of metals in the shoots, hyper- accumulator plants are also characterized by a shoot-to-root metal concentration ratio of >1, whereas non-hyperaccumulator plants generally have higher metal concentrations in roots than in shoots (Baker, 1981; Baker et al., 1994a,b; Brown et al., 1995a; Gabbrielli et al., 1990; Homer et al., 1991a; Kr¨amer et al., 1996; Shen et al., 1997; Zhao et al., 2000). A highly efficient transport of metals from roots to shoots is one of the key features associated with all hyperaccumulator plants. Metal hyperaccumulation is a rare phenomenon in terrestrial higher plants. To date, about 400 plant species have been identified as metal hyperaccumulators, representing <0.2% of all angiosperms (Baker et al., 2000; Brooks, 1998). It is foreseeable that the number of hyperaccumulator plants will increase as more geobotanical surveys are carried out worldwide. On the other hand, some of the hyperaccumulator species reported earlier may not be confirmed as true hyperac- cumulators, but may be incorrectly identified due to contamination or analytical errors (see following). Details of different metal hyperaccumulator species and their geographical distributions have been documented elsewhere (see Baker and Brooks, 1989; Baker et al., 2000; Brooks, 1998; Reeves and Baker, 2000). The following sections give only brief descriptions of the key features of different metal hyperaccumulators. 2. Nickel Hyperaccumulators Ni hyperaccumulatorsare the most numerous among hyperaccumulating species of plants, with a current total number of 318 taxa distributed mainly in the tropical to warm temperature regions of the world (Baker et al., 2000; Reeves and Baker, 2000). The richness of Ni hyperaccumulator species is probably due to the widespread occurrence of Ni-rich ultramafic (serpentine) soils and the long history of geobotanical studies of ultramafic floras. Some of the well- known Ni hyperaccumulators are in the genus Alyssum L. (Brassicaceae), al- though the most remarkable example is perhaps Sebertia acuminata (Sapotaceae), a New Caledonian tree that can grow to a height of about 10 m. Jaffr´e et al. (1976) showed that this plant produces a blue-green latex containing 11.2% Ni on a fresh weight basis (25.7% on a dry weight basis). A mature tree of Sebertia acuminata was estimated to contain 37 kg Ni (Sagner et al., 1998). The other species that has recently attracted attention is Berkheya coddii, which can 12/05/2001 09:33 AM Agronomy-V. 75 PS097A-01.tex PS097A-01.xml APserialsv2(2000/12/19) Textures 2.0 8 S. P. McGRATH ET AL. accumulate Ni to more than 1% and is tall, fast-growing, and productive (Morrey et al., 1989). These are the attributes that are ideal for phytoremediation or phyto- mining (see following). 3. Zinc and Cadmium Hyperaccumulators In comparison to Ni hyperaccumulators, far fewer plant species have been reported that are able to hyperaccumulate Zn and Cd. Baker et al. (2000) listed 11 taxa of Zn hyperaccumulator plants, whereas Reeves and Baker (2000) also con- sidered two otherspecies (Thlaspi ochroleucum and Polycarpaea synandra), which did not reach the criterion of 10,000 µgZng −1 to be hyperaccumulators. The best known examples of the Zn hyperaccumulators are Thlaspi caerulescens (formerly called T. alpestre) and Arabidopsis halleri (formerly named Cardaminopsis halleri), both belonging to the Brassicaceae family. In the case of Thlaspi ochroleucum, Shen et al. (1997) showed that its Zn accumulation and toler- ance are considerably lower than those in T. caerulescens in hydroponic cultures. T. ochroleucum also accumulates more Zn in roots than in shoots, thus behaving in that sense rather like a non-hyperaccumulator. For Cd, T. caerulescens is the only known hyperaccumulator (Ernst, 1974; Reeves and Bakers, 2000), although recent hydroponic experiments showed that A. halleri is capable of accumulat- ing >1000 µgCdg −1 in the shoots without suffering from phytotoxicity (K¨upper et al., 2000), and for this reason may be classified as a Cd hyperaccumulator. Whether A. halleri accumulates over 100 µgCdg −1 in any of its natural habitats (the criterion for defining Cd hyperaccumulation) is probably determined by the Cd concentration in the soil. 4. Copper and Cobalt Hyperaccumulators Twenty-eight and 37 taxa of Co and Cu hyperaccumulator plants, respectively, have been reported (see Brooks, 1998; Reeves and Baker, 2000). These plants are mainly distributed in the Shaban Copper Arc of the Democratic Republic of Congo (formerly Za¨ıre). Some of these plants can hyperaccumulate both metals. However, there have been few experimental studies on the ability of these plants to accumu- late metals, and therefore whether or not they can truly hyperaccumulate Cu and Co remains to be confirmed. A recent study using hydroponic cultures showed that Haumaniastrum katangense and Aeollanthus biformifolius (Lamiaceae), both of which have been described as Cu and Co hyperaccumulators, did not hyperac- cumulate Cu and Co in the shoots, but rather behaved as typical metal excluders (K¨ohl et al., 1997). Contamination of plant samples with dust is a possibility when sampling and analyzing wild plants from their natural habitats. This can cause large errors if the dust happens to be rich in the metals to be analyzed. Reeves and Baker (2000) gave an example in which 0.2 mg of malachite (a secondary [...]... ligands responsible for chelating Cd, Cu, and Co in metal hyperaccumulators In the non-accumulator Arabidopsis thaliana, phytochelatins play an important role in the binding of Cd and the tolerance to Cd (Cobbett, 2000) It is not known whether phytochelatins are also involved in the detoxification of Cd in Cd hyperaccumulators There is no evidence for a role of phytochelatins in Zn or Ni tolerance and hyperaccumulation... xylem loading Translocation of Ni from roots to shoots may involve specific ligands in some hyperaccumulator species Kr¨ mer et al (1996) found that exposing several Ni a hyperaccumulator species of Alyssum to Ni elicited a large and proportional increase in the levels of histidine in the xylem sap Histidine in the xylem sap was shown to be coordinated with Ni, although the concentration of histidine was... histidine, and cell wall, respectively, with the remaining 26% as aqueous free cation The citrate–Zn complex also accounted for 21% of the total Zn in the xylem sap, with the remaining amount apparently present as the free cation In the roots of T caerulescens that contained 1320 µg Zn g−1 dry weight, histidine complexed 70% of the total Zn Malate was found not to be involved in Zn complexation in the... cropping increases For phytoremediation of Ni-contaminated soils, Robinson et al (1997b) estimated that four croppings with B coddii would be required to bring the total soil Ni from 250 µg g−1 to below the EU guideline value of 75 µg g−1 Textures 2.0 12/05/2001 09:33 AM 14 Agronomy- V 75 PS097A-01.tex PS097A-01.xml APserialsv2(2000/12/19) S P McGRATH ET AL 3 Other Metals Phytomining for Tl using Tl... sensing of metal-rich patches remain to be elucidated b Rhizosphere Microbes Mycorrhizal associations increase the area of nutrient exploitation in soil by roots, and this in turn increases acquisition of nutrients like P and Zn under nutrientlimiting conditions (Marschner, 1995) However, in metal-contaminated soils, mycorrhizal associations, particularly those involving ectomycorrhizas, often result in. .. with ligands results in decreased free ion activity and thus decreased toxicity There is evidence for the role of ligands in detoxifying metals in hyperaccumulators For example, histidine has been shown to be involved in the tolerance of Ni by several Alyssum species, possibly through the formation of the Ni–histidine complex in root cells and in xylem saps (Kr¨ mer et al., 1996) a In many Ni hyperaccumulators,... response may not be universal in all Ni hyperaccumulator species Persans et al (1999) did not observe any Ni-inducible responses in terms of histidine concentrations in the roots, shoots, and xylem sap of T goesingense, nor did they find any regulation by Ni of three cDNAs encoding the enzymes involved in the histidine biosynthetic pathway 3 Mechanisms of Metal Tolerance in Hyperaccumulator Plants Thlaspi... phytoextract Zn from a contaminated soil Application of EDTA caused a large increase in Zn solubility and significantly increased the Zn uptake in B juncea This increase was much smaller than that in the study conducted by Blaylock et al (1997) even though the Zn concentration in the soil was much higher These results indicate that real contaminated soils may contain metals in forms more difficult to mobilize... or xylem sap These results suggest that histidine may be important in detoxifying Zn in the root cells; however, organic acids, particularly citrate, may be involved in the root-to-shoot transport and storage of Zn in the leaf vacuoles It is important to point out that although organic acids such as citrate may play a role in metal chelation and storage in vacuoles, they account for neither the metal... increase the mobility of Pb in the soil Several chelating agents have been tested, the most effective having a log of the binding constant higher than 18 Huang et al (1997) tested five chelating agents for their effectiveness in increasing Pb desorption from a Pb-contaminated soil collected from an industrial site EDTA was the most efficient mobilizing agent followed by HEDTA and DTPA They also reported . progress in the understanding of the processes involved and 1 Advances in Agronomy, Volume 75 Copyright C 2002 by Academic Press. All rights of reproduction in. water in Chinese agriculture including engi- neering, economic, and agronomic aspects and considerations. Chapter 3 presents advances in understanding the