Use of soil amendments such as synthetics (ammonium thiocyanate) and natural zeolites has yielded promising results [62-66]. EDTA, NTA, citrate, oxalate, malate, succinate, tartrate, phthalate, salicylate, acetate, etc. have been used as chelators for rapid mobility and uptake of metals from contaminated soils by plants. Use of synthetic chelators significantly increased Pb and Cd uptake and translocation from roots to shoots, facilitating phytoextraction of the metals from low-grade ores. Synthetic cross-linked polyacrylates, hydrogels have protected plant roots from heavy metal toxicity and prevented the entry of toxic metals into roots. Application of low-cost synthetics and natural zeolites on a large scale are applied to the soil through irrigation at specific stages of plant growth; this might be beneficial to accelerate metal accumulation .
A major factor influencing the efficiency of phytoextraction is the ability of plants to absorb large quantities of metal in a short period of time. Hyperaccumulators accumulate appreciable quantities of metal in their tissue regardless of the concentration of metal in the soil , as long as the metal in question is present. This property is unlike moderate accumulators now used for phytoextraction in which the quantity of absorbed metal is a reflection of the concentration in the soil. Although the total soil metal content may be high, it is the fraction that is readily available in the soil solution that determines the efficiency of metal absorption by plant roots. To enhance the speed and quantity of metal removal by plants, some researchers advocate the use of various chemicals for increasing the quantity of available metal for plant uptake.
Chemicals suggested for this purpose include various acidifying agents [6,20,69], fertilizer salts [71,72], and chelating materials [67,73,74]. These chemicals increase the amount of bioavailable metal in the soil solution by liberating or displacing metal from the solid phase of the soil or by making precipitated metal species more soluble. Research in this area has been moderately successful, but the wisdom of liberating large quantities of toxic metal into soil water is questionable.
Soil pH is a major factor influencing the availability of elements in the soil for plant uptake . Under acidic conditions, H+ ions displace metal cations from the cation exchange complex (CEC) of soil components and cause metals to be released from sesquioxides and variably charged clays to which they have been chemisorbed (i.e., specific adsorption) . The retention of metals to soil organic matter is also weaker at low pH, resulting in more available metal in the soil solution for root absorption. Many metal cations are more soluble and available in the soil solution at low pH (below 5.5), including Cd, Cu, Hg, Ni, Pb, and Zn [2,76]. It is suggested that the phytoextraction process is enhanced when metal availability to plant roots is facilitated through the addition of acidifying agents to the soil [2,24,70].
Possible amendments for acidification include NH4-containing fertilizers, organic and inorganic acids, and elemental S. Trelease and Trelease  indicated that plant roots acidify hydroponic solutions in response to NH4 nutrition and cause solutions to become more alkaline in response to NO3 nutrition. Metal availability in the soil can be manipulated by the proper ratio of NO3 to NH4 used for plant fertilization by the effect of these N sources on soil pH, but no phytoremediation research has been conducted on this topic to date.
The acidification of soil with elemental S is a common agronomic practice that can be used to mobilize metal cations in soil. Brown et al.  acidified a Cd- and Zn-contaminated soil with elemental S and observed that accumulation of these metals by plants was greater than when the amendment was not used. Acidifying agents are also used to increase the availability of radioactive elements in the soil for plant uptake. Huang et al.  reported that the addition of citric acid increases U accumulation in Indian mustard (B. juncea) tissues more than nitric or sulfuric acid, although all acids decrease soil pH by the same amount. These authors speculated that citric acid chelates the soil U, thereby enhancing its solubility and availability in the soil solution. The addition of citric acid causes a 1000-fold increase of U in the shoots of B. juncea compared to accumulation m
FIGURE 25.11 Citric acid is a naturally occurring chelating agent. The chelation process is water activated. EDTA, NTA, citrate, oxalate, malate, succinate, tartrate, phthalate, salicylate, acetate, etc. have been used for chelate-induced hyperaccumulation. Synthetic soil amendments such as ammonium thiocyanate and natural zeolites have yielded promising results in inducing hyperaccumulation of metals.
in the control (no citric acid addition) (Figure 25.11). Despite the promise of some acidifying agents for use in phytoextraction, little research is reported on this subject.
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