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Geosciences 2018, 8, 56 3 of 18 HAP traits include: higher metal content in the leaves of the plant than non-HAP’s, high metal tolerance, low growth rates and low biomass yields [31,32]. Some of these traits like slow growth and low biomass make it somewhat impractical to use these plants for agro-mining; for this reason, more recent research has focused on high biomass crop species and the technology of induced metal accumulation in plants. Induced metal accumulation uses non-HAPs with a large biomass to accumulate significant amounts of metals. One form of induced metal accumulation in plants uses chelating agents. These chelating agents are applied to the soil where they form water-soluble metal organic complexes through dissolution of precipitated compounds and desorption of sorbed elements, making metals more available for plant uptake [33,34]. These chelating agents are added to the soil near the end of the plant-growth phase, the plants are then harvested within several days or a week [9]. The solubilized metals are taken into the plant via the apoplast pathway rather than the symplast pathway [35]. There are numerous studies discussing the efficacy of using chelating agents to induce metal accumulation in plants [35,36]. Probably the best known and most successful chelating agent is ethylenediaminetetraacetic acid (EDTA), first synthesized in 1935 by I.G. Farbenindustrie [37]. EDTA is one of the cheapest and most suitable complexing agents for many technical purposes and has the best cost/performance ratio of all chelates [38]. EDTA is not readily biodegradable, although it experiences some photodegradation at a very slow rate in the environment. Its biodegradation has been demonstrated using specialized bacterial cultures [37–39]. The use of EDTA in phytoremediation/phytoextraction has been banned in most countries because of the dangers associated with complexed metals been leached into the environment. The problem with EDTA is that it is persistent in the environment and can easily leach into and accumulate in natural waters, its environmental toxicity has been discussed [40,41]. Levels of 2.2 mg/L EDTA or greater in natural waters can cause problems, but this level is rarely observed [35]. Thousands of tons of EDTA are used every year in industries such as detergent and paper production [37]. EDTA has a low toxicity profile for humans and is commonly used in cosmetics and pharmaceuticals. Its environmental toxicity is also low and limited to point source emissions to natural waters. Environmental risk levels for EDTA in the environment are available [42]. Ethylenediamine-N, N’-disuccinic acid (EDDS) is a biodegradable alternative of EDTA and is its closest performing counterpart. EDDS is readily degraded and one of the more widely studied biodegradable chelating agents, it has seen some commercial application in the detergent industry as a replacement for EDTA [43–45]. EDDS has three stereoisomers [SS], [RR], and [SR]/[RS] of which only the [SS]-isomer is 100% biodegradable [46,47]. All works discussed in this paper consider only the [SS]-isomer when referring to EDDS. The biodegradability of several chelating agents in activated sludge has been studied [48]. The authors of this study found that the EDTA molecule remained intact for up to 100 days, whereas EDDS was biodegraded rapidly in the activated sludge. The biodegradation of EDDS has been shown to be effective even in polluted soils [49]. Some metal complexes of EDTA and EDDS are susceptible to photodegradation [40,47,50]. In general, chelating agents which form complexes with relatively low stability constants are readily degradable whereas those forming stronger complexes (i.e., higher stability constants) are more resistant to biodegradation [45]. Crown ethers (Monocyclic polyethers) such as 12-crown-4 have a marked selectivity for alkali metals and are normally used to complex alkali cations like Li. These compounds and similar compounds like cryptands and lariat ethers which have equivalent properties to crown ethers are generally very expensive. The complexation constants for Li are very weak compared to other metals but multidentate ligands do form complexes with Lithium. The Li-EDTA complex has a stability constant of 2.79 while the Li-EDDS complex has a very small stability constant. The stability of an EDDS metal complex is relatively low when compared to an EDTA complex for example, a Ca-EDTA complex has a stability constant of 10.65 while the Ca-EDDS complex is around 4.6. This is especially true for Pb, because of the high stability constant of the Pb-EDTA complex at 18.0 versus the Pb-EDDS complex at 12.7 [28]. Several studies show that chelating agents such as EDTA and EDDS can be used to increase metal mobility in soils making them more available to plants [25,28,34,45,51,52].PDF Image | Induced Plant Accumulation of Lithium
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