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I Extractable Ni □ Plant Ni

FIGURE 6.3 (A) The effect of chelating agents added at 2 g/kg soil, on shoot Ni concentration (dry matter) by the hyperaccumulator Berkheya coddii. (B) The effect of increasing EDTA addition on the shoot Ni concentration (dry matter) of Arrhenatherum elatius. Extractable Ni was determined using a 1 M NH4OAc extractant. * denotes plant death.

I Extractable Ni □ Plant Ni

FIGURE 6.3 (A) The effect of chelating agents added at 2 g/kg soil, on shoot Ni concentration (dry matter) by the hyperaccumulator Berkheya coddii. (B) The effect of increasing EDTA addition on the shoot Ni concentration (dry matter) of Arrhenatherum elatius. Extractable Ni was determined using a 1 M NH4OAc extractant. * denotes plant death.

tially be used in combination with organic matter and Fe-rich material, as soil amendments to reduce plant availability of Cd, Zn, and Pb.

Phosphate amendments such as hydroxyapatite are effective in reducing the solubility of Pb, Cd, Zn, Al, Ba, Co, Mn, Ni, and U. However, phosphate has been shown to promote the solubility of As and Cr [45], possibly through reduced sorption of the oxyanions due to an increase in pH and competition from PO43-.

A variety of inorganic and organic amendments have been used to reduce Cr(VI) to the less mobile, less toxic Cr(III) species. Fe(II)-bearing minerals form effective reductants. Surface-bound organic matter does this also. The latter is catalyzed by soil mineral surfaces, and Cr(III) binds tightly to surface species or is precipitated as Cr(OH)3 [46]. Similarly, Bolan et al. [47] have shown that organic amendments, such as animal and poultry manures rich in dissolved organic carbon, are very effective in reducing Cr(VI) to Cr(III).

Lombi et al. [48] demonstrated that the 2% addition of bauxite residue, "red mud," to contaminated soils reduced the solubility of Cd, Pb, Ni, and Zn, but not Cu. The remedial action of this material was attributed to a rise in soil pH and adsorption of the metals onto oxides of Fe and Mn.

Liming has been demonstrated to be effective in reducing the mobility of trace element cations in variable-charge soils by increasing the negative charge on oxides, clays, and organic matter. The effectiveness of raising the pH on metal immobilization also depends on the liming agent. Bolan et al. [49] found that Ca(OH)2 was less effective than KOH in immobilizing Cd2+ due to competition between Ca2+ and Cd2+ for adsorption sites.

Establishing vegetation on a contaminated site can reduce the solubility and mobility of trace elements. Plant growth reduces trace element mobility in the substrate through the addition of organic matter, creation of an aerobic environment, root uptake, and returning rainfall to the atmosphere by evapotranspiration [50]. Romkens et al. [51] showed that Cu solubility was lowered significantly in the root zone of Agrostis capillaries (var Parys Mountain). Turpeinen et al. [52] demonstrated that Pb solubility was reduced by up to 93% by pine seedlings. The use of vegetation for the remediation of contaminated sites, phytoremediation, has some advantage over other in situ immobilization techniques. Once established, the physicochemical change induced by vegetation is permanent (Figure 6.4).

FIGURE 6.4 Vegetation established on the Tui mine tailings, Te Aroha, New Zealand. The vegetation has reduced metal mobility by adding organic matter to the substrate and returning some rainfall to the atmosphere via evapotranspiration.

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