Strategies For Enhanced Uptake Of Trace Elements To Facilitate Phytoextraction

27.6.1 Chelate-Assisted or Chemically Induced Phytoextraction

This strategy of phytoextraction is based on the fact that the application of metal chelates to the soil significantly enhances metal accumulation by plants. The literature to date reports a number of chelates that have been used for chelate-induced hyperaccumulation. These include EDTA, CDTA (irans-1,2-diaminocyclohexane-N,N,N ',N'-tetraacetic acid), DTPA (diethylene triaminepentaacetic acid), EGTA [ethyleneglycol-bis(P-aminoethyl ether),N,N,N',N-tetraacetic acid], EDDHA [ethyl-enediaminedi (o-hydroxyphenylacetic acid)], HEDTA (N-hydroxyethyl enediaminetriacetic acid), HEIDA [N-(2-hydroxyethyl)iminodiacetic acid], and NTA (nitrilo-triacetic acid) [101].

For chemically enhanced phytoextraction, establishment of a high biomass crop is required before chelate application. Brassica sp. are to be directly related to the affinity of the applied chelate for the metal [105]. Therefore, it can be concluded that, for efficient phytoextraction to occur, synthetic chelates having a high affinity for the metal of interest should be used: EDTA for lead, EGTA for cadmium [104], possibly citrate for uranium [106], etc. Also, adding ammonium thio-cyanate to the substrate [49] showed that Brassica juncea can be induced to accumulate up to 57 mg/kg gold. The mechanisms involved in metal-chelate induced plant uptake and translocation of metals are not well understood. Chemically induced phytoextraction has been described as a two-step process in which plants first accumulate metals in their roots and then, by application of an inducing agent, enhanced transfer of the metals to the shoots occurs [105,107]. This transfer is due to disrupting the plant metabolism that regulates the transport of metal to the shoots. Lombi et al. [84] reported that the application of EDTA alone increases metal mobility in soil and accumulation in roots, but does not substantially increase the transfer of metals to shoots.

Apart from the addition of synthetic chelates, plants secrete to the rhizosphere natural metal-chelating molecules to mobilize soil-bound metals. Thus far, only phytosiderophores, iron-chelating compounds, have been studied in detail. Some of these phytosiderophores include mugeneic and deoxymugeneic acids from barley and corn, and avenic acid from oats [108]. It is also possible that metal-chelating proteins, perhaps related to metalothioneins or phytochelatins, may act as phytosiderophores [81].

Chelate-assisted phytoextraction in field conditions is likely to increase the risk of adverse environmental effects such as ground water pollution due to leaching of metal-laden seepage during extended periods after chelate application. Wenzel et al. [109] hypothesize that free protonated EDTA enters the roots, subsequently forming metal complexes that enhance metal transport to shoots. However, the study of Vassil et al. [110] was conducted in hydroponic conditions, whereas EDTA in soil is expected to form complexes with Ca and other metals. Greman et al. [111] reported ethylenediaminedisuccinate (EDDS) as a promising new chelate for enhanced, environmentally safe phytoextraction of Pb-contaminated soils. It caused only minor leaching of Pb and was significantly less toxic to plants and soil microbes. To avoid possible chelate-metal movement into ground water, the amount, time, and method of chelate application should be carefully controlled.

Due to the severe limitations of chelate-assisted phytoextraction, further efforts should focus on natural, continuous technologies using high biomass perennial plants such as willows or poplar.

27.6.2 Rhizosphere-Assisted Processes for Metal Accumulation and Exclusion

27.6.2.1 Bioavailability of Metals in Soils

Heavy metal accumulation in soils is highly dependant on the availability of metals for plant uptake. Soils consist of a heterogeneous mixture of different minerals (primary minerals, clay minerals, and hydrous oxides of Al, Fe, and Mn); organic and organo-mineral substances, and other solid components. The binding mechanisms for heavy metals are therefore complex and vary with the composition of the soil, soil acidity, and redox conditions. Heavy metal behavior (e.g., mobility, bioavailability) depends upon several factors (Table 27.2), which can be classified as [31]:

• Geochemical characteristics of a metal

• Plant capacity to take up a metal

• Soil chemical equilibria

• Climatic and other environmental variables

• Agricultural or remedial soil management

Generally, the solubility of metal fractions is in the order [112]: exchangeable > carbonate specifically adsorbed > Fe-Mn oxide > organic-sulfide > residual

Furthermore, only a fraction of soil metal is readily available (bioavailable) for plant uptake. The bulk of soil metal is commonly found as insoluble compounds unavailable for transport into roots. With the exception of Hg, metal uptake into roots occurs from the aqueous phase. In soil, easily mobile metals such as Zn and Cd occur primarily as soluble or exchangeable, readily bioavailable forms. Cu and Mo predominate inorganically bound and exchangeable fractions. Slightly mobile metals such as Ni and Cr are mainly bound in silicates (residual fraction). Soluble, exchangeable, and chelated species of trace elements are the most mobile in soils and govern their migration and phytoavailability [31]. Others, such as Pb, occur as insoluble precipitates (phosphates, carbonates, and hydroxyoxides), which are largely unavailable for plant uptake [113]. Binding and immobilization within the soil matrix can significantly restrict the potential for metal phytoextrac-tion.

Despite the adverse effect on metal root uptake, soil inactivation with chemical amendments has been proposed as a temporary solution for the remediation of metal-contaminated soils, especially for Pb. Also, the effect of soil amendment on bioavailability is metal specific. Increased mobility of metals can be stimulated by plant roots; this includes changes in pH, reducing capacity, the amount and composition of exudates [114], and use of chelating agents. Soil amendments can increase or decrease biological availability of the contaminant for plant uptake. Bioavailability and metal uptake can often be increased by lowering soil pH, adding chelating agents, using appropriate fertilizers (containing ammonium), altering soil ion composition, soil microorganisms, phytosid-erophores, and root exudates [2].

• Soil pH. The lower soil pH increases concentration of heavy metals in solution via decreasing their adsorption. Soil pH was adjusted using HNO3 and CaCo3 to provide a range of pH before planting. Acidified treatments were leached to remove excess nitrate before fertilizers were added. Chaney et al. [97] pointed out that, because soil pH is known to affect plant uptake of most heavy metals from soils, studies needed to be conducted to evaluate the independent effect of soil pH and soil metal concentration on hyperaccumulator yield and metal uptake.

• Chelate amendments. Chelate additions (EDTA, HEDTA, DTPA, EGTA, EDDHA, NTA, citrate, and hydroxylamine) are commonly used in soil washing technologies because they cause metal desorption from clay minerals and dissolution of certain precipitates such as Fe and Mn oxides. Artificial chelates, such as EDTA, have been tested to enhance metal phytoavailability and subsequent uptake and translocation in shoots. Two strategies have been proposed regarding the mode of chelate application. The chelates may be added at once a few days before harvest [103] or gradually during the growth period [115]. The type of chelate and its time of application are important considerations. Pierzynski and Schwab [116] investigated the effect of chemical amendments on the potential for phytoextraction of several toxic metals including Cd, Pb, and Zn. They showed that addition of limestone, cattle manure, and poultry litter to soil significantly reduced Zn bioavailability. Experiments indicate that biosurfactants have the potential to enhance metal bioavailability in contaminated soil and sediments [117]. EDTA-amended soils increased Pb availability. Therefore, chelator application might pose a risk to the environment [118]. If the metal availability could be locally improved by increasing reductase activity or the amount of chelating agents — e.g., phytosiderophores [119] — without harmful effects on the environment, hyperaccumulators might be used safely for phytoremediation.

• Soil fertilizers. Fertilization with N, P, and K more than doubled annual biomass production without reducing the shoot Ni concentration. This suggested that soil fertility management will be important for commercial phytoextraction [120].

• Competition for sorption sites. Using the competition of metal ions in solution for sorption sites may also be a useful tool. For example, addition of phosphate to soil may help to extract Cr, Se, and As on exchange sites by binding to the sites, thereby increasing bioavailability.

• Soil microorganisms. The soil microbes have been documented to catalyse redox reactions leading to changes in metal mobility in soils and propensity for uptake into roots. For example, chemolithotrophic bacteria have been shown to enhance environmental mobility of metal contaminants via soil acidification or, in contrast, to decrease their solubility due to precipitation as sulfides [121]. Several strains of Bacillus and Pseudomonas increased the total amount of Cd accumulated by Brassica juncea seedlings [122]. Furthermore, soil microorganisms have been shown to exude organic compounds, which stimulate bioavailability and facilitate root absorption of a variety of metal ions, including Fe2+ [123], Mn2+ [124], and possibly Cd2+ [122]. The microbial activity is stimulated by adding carbon substrates such as agricultural wastes [125], water, and nutrients. Growth of crops also provides these materials to the soil microbiota due to standard farming practices and the process of C loss from roots, called "rhizodeposition." It is interesting that rhizodeposition increases after clipping plants [126]; this could partly explain the enhanced Se removal after cutting treatments, plus the enhanced biomass production.

• Phytosiderophores. Plants possess highly specialized mechanisms to stimulate metal bioavailability in the rhizosphere and to enhance uptake into roots [127]. Thus, graminaceous species (grass sp.) have been documented to exude a class of organic compounds termed siderophores (mugineic and avenic acids) capable of enhancing the availability of soil Fe for uptake into roots [128].

• Root exudates. It is well established that roots of many plant species release specific metal-chelating or reducing compounds into the rhizosphere to mobilize Fe and, possibly, Zn [85]. For Zn/Cd/Cu/Pb hyperaccumulators, there are no studies on the role of root exudates in metal accumulation to date.

27.6.2.2 Exclusion of Trace Elements to Foster Phytostabilization

The bioavailability of metal ions depends on their solubility in the soil solution, i.e., their general solubility and the stage of equilibrium between the metal cation in its bound form and the free soluble cation. Because the concentration of heavy metal cations in forest soils is usually so low that the solubility behavior of the metal salts will govern their concentration in the soil solution, the dominating factor for the bioavailability of heavy metal cations in soils is their adsorption to soil structures.

The capacity of a given soil to bind given heavy metals depends on the amount and nature of binding sites in the soil structures and the pH of the soil solution. Generally, it can be stated that the lower the pH value is, the more soluble are the metal cations, and the more binding sites that are available in a given soil, the lower will be the solubility of the heavy metals. In the case of cadmium ions, the increase in solubility with decreasing pH values starts at a pH of 6.5. In the case of lead and mercury ions, it starts at a pH value of 4; ions of arsenic, chrome, nickel, and copper start to dissolve at pH values between these two extremes [129]. Thus, the pH value of the soil solution in principle is one of the main factors governing the solubility of heavy metal cations in the soil solution; its influence on plants under heavy metal stress is well established [130-138]. Unfortunately, the acid deposition prevalent all over Europe during recent decades has increased the mobility of heavy metals considerably [139,140].

Thus, increasing the pH could be a measure to reduce the bioavailability of heavy metals. This has been shown by Walendzik [141] for spruce in the Western Sudety Mountains. Liming, however, may not always be a good solution because it may increase the rate of nitrogen mineralization and thus aggravate the NO3 load in the groundwater [142-144]. The approach of using waste materials such as fuel ash [145] or sewage sludge [146] to improve the growth of trees on mine spoils may not always be successful, as the authors cited previously have shown. Better results using municipal sewage sludge for establishing sagebrush vegetation on copper mine spoils were reported by Sabey et al. [147]. The simple addition of inorganic fertilizer may not work at all [146].

Another way to decrease the bioavailability of heavy metals is to increase the binding sites for heavy metal ions in the soil, e.g., by amendment with humic substances or zeolites [148] or expanded clay and porous ceramic material [149]. When organic substances are added to the soil, it is very important to work with water-insoluble material, which is not available for rapid degradation by microorganisms [150]. The authors found that an addition of hay to a soil contaminated with heavy metals increased the solubility of Cu, Cd, and Zn, but this effect was not observed for Pb. Amendment of the soil with peat had the opposite effect.

Huttermann and coworkers applied cross-linked polyacrylates, hydrogels, to metal-contaminated soils. When such a compound (Stockosorb K400) was applied to hydrocultures of Scots pine (Pinus sylvestris), which contained 1 |M of Pb, two effects were observed: (1) the hydrogel increased the nutrient efficiency of the plants; and (2) the detrimental effect of the heavy metal was completely remediated. Determination of the heavy metal content of the roots revealed that the uptake of the lead was greatly inhibited by the hydrogel. Analysis of the fine roots of 3-year-old spruce grown for one vegetation period in lead-contaminated soil, with and without amendments with the hydrogel, showed that the amendment of the soil with the cross-linked acrylate did indeed prevent the uptake of the lead into the stele of the fine roots. The hydrogel acts as a protective gel that inhibits the entrance of the heavy metal into the plant root [151].

27.6.2.3 Metal Exclusion by Organic Acids

Organic acids are natural products of root exudates, microbial secretions, and plant and animal residue decomposition in soils [152] (Figure 27.3 and Figure 27.4). These biomolecules have been implicated for altering the bioavailabilities and phytoremediation efficiencies of heavy metals in soils. Some researchers showed that amendment of contaminated soils with organic acids reduced

Apoplast -Exudation

FIGURE 27.3 A comprehensive model to explain the availability of substrates required for the biosynthesis and exudation of organics acids, namely, citrate and malate. These organic acids are present in continuous exchange between mitochondria and the cytosol. Organic acids can be accumulated in the vacuole or excreted into the apoplast by specific carrier proteins, transported towards phloem, and directed to roots for exudation. Plants that exclude toxic trace metals would be the best for photostabilization.

Apoplast -Exudation

FIGURE 27.3 A comprehensive model to explain the availability of substrates required for the biosynthesis and exudation of organics acids, namely, citrate and malate. These organic acids are present in continuous exchange between mitochondria and the cytosol. Organic acids can be accumulated in the vacuole or excreted into the apoplast by specific carrier proteins, transported towards phloem, and directed to roots for exudation. Plants that exclude toxic trace metals would be the best for photostabilization.

the bioavailability of heavy metals [153]. In contrast, Huang et al. [103] investigated the effect of organic acids amendment of uranium-contaminated soils and found that citric acid significantly increased metal availability and enhanced uranium accumulation many folds in the shoots of selected plants. The contradictory results may be tightly related to the concentration of heavy metal in soil solution and may sequentially be the results of desorption behavior of heavy metal from this soil.

In plants, organic acids may be implicated in detoxification, transport, and compartmentalization of heavy metals. Organic acids are low molecular weight compounds containing carbon, hydrogen, and oxygen and are characterized by one or more carboxylic groups. The number and the dissociation properties of the carboxylic groups determine the negative charges carried by the molecules: the number of metal cations that can be bound in solution or the number of anions that can be displaced from the soil matrix [39]. The most stable ligand-metal complexes have the highest number of carboxyl groups available for binding metal cations. Metal complexes with citrate (tricarboxylate) are more stable than those with malate2-, oxalate2-, or malonate2- (dicarboxylate) and acetate (monocarboxylate) [154].

In several plant species, organic acids participate in the metal exclusion mechanism as metal chelators excreted by the root apex outside the plant and in metal hyperaccumulation as metal chelators inside the plant, with various degrees of metal retention within root and shoot [155,156]. The total concentration of organic acids in the root is generally about 10 to 20 mM, but may vary depending on the degree of cation-anion imbalance because organic acids often provide the negative charges that balance excess cations [157]. Within the plant cell, organic acids are mainly synthesized in mitochondria through the tricarboxylic acid cycle, but the site of preferential storage is the vacuole. Usually, root vacuoles contain two- to tenfold higher concentrations of malate and citrate than cytosol (5 mM) [157] and organometallic chelates can be found in the cell wall, cytoplasm,

Al-citrate Ca-citrate

FIGURE 27.4 The ubiquity of organic acids mediating the response of plants to soil stress. Organic acids are strong cation chelators, which act in important adaptive processes in the rhizosphere, such as P and Fe acquisition, Al tolerance, NH4 uptake and reduction, and microbial attraction.

Al-citrate Ca-citrate

FIGURE 27.4 The ubiquity of organic acids mediating the response of plants to soil stress. Organic acids are strong cation chelators, which act in important adaptive processes in the rhizosphere, such as P and Fe acquisition, Al tolerance, NH4 uptake and reduction, and microbial attraction.

and vacuoles. The composition of root exudates varies greatly, depending on environment, plant species, and age [157-159].

Researchers have found that plant roots exude a variety of organic compounds. Root exudates contain components that play important roles in nutrient solubilization (e.g., organic acids, phyto-siderophores, and phenolics), restricting the passage of toxic metals across the root (e.g., citrate, malate, small peptides) and attracting beneficial microorganisms (e.g., phenolics, organic acids and sugars). Often, the excretion of these organic molecules increases in response to soil stress.

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