Soil-Plant Transfer Coefficients (TC) of Heavy Metals
Element TC estimations3 Pot experimentsb Field experiments0
a Obtained from Alloway, B.J., in Heavy Metals in Soils, Alloway, B.J., Ed., Blackie Academic and Professional, London, 1995, 38-57.
bData from Antoniadis, V. and Alloway, B.J., Water Air Soil Pollut., 132, 201-214, 2001; Antoniadis, V. and Alloway, B.J., Environ. Pollut., 117, 515-521, 2002; Tsadilas, C.D. et al. Commun. Soil Sci. Plant Anal., 26, 2603-2619, 1995; Hooda, P.S. and Alloway, B.J., J. Soil Sci., 44, 97-110, 1993; and Jackson, A.P. and Alloway, B.J., Plant Soil, 132, 179-186, 1991.
c Data from Antoniadis, V. et al., in Proc. 8th Conf. Hellenic Soil Sci. Soc., Kavala, 2000, 459-469; Berti, W.R. and Jacobs, L.W., J. Environ. Qual., 25, 1025-1032, 1996; Bergkvist, P. et al., Agri. Ecosyst. Environ., 97, 167-179, 2003; Brown, S.L. et al., J. Environ. Qual., 127, 1071-1078, 1998; Chang et al., J. Environ. Qual., 12, 391-397, 1983; and Schaecke, W., Tanneberg, H., and Schilling, G., J. Plant Nutr. Soil Sci., 165, 609-617, 2002.
concerns is that after sewage sludge has been applied to soils, heavy metals borne in it may accumulate in plants and, subsequently, enter the human food chain or have toxic effects on plants and/or animals grazing on them. Heavy metal concentrations in plants depend on the concentrations (total and available) of the metals in the soil in which the plants are grown. The transfer coefficient (TC) between soil and plant gives a measure of metal mobility. Table 3.6 gives the TC of the main heavy metals as obtained by Alloway  and values of the same coefficient from several other works.
Although TCs are not supposed to be precise in predicting metal uptake by plants (because only orders of magnitude are indicated), it is evident that Cd, Ni, and Zn are among the most mobile elements and that the ones strongly sorbed onto humic substances (including Cr and Pb) are less available. From Table 3.6, it is evident that heavy metals exhibit behavior inside a greenhouse in pot studies different from that in field experiments; the latter usually gives smaller TC values. An explanation of this trend was given by De Vries and Tiller , who attributed the higher metal concentrations in pot experiments to the more favorable conditions (temperature, humidity, and light distribution) usually found inside a greenhouse.
Although McBride  agreed with that, he added that differences in plant rooting patterns are more critical. Most of the crops' rooting systems in the field go well beyond the sludge incorporation zone and thus heavy metal uptake is more conservative. In pot experiments, plant roots are "forced" to remain where sludge is thoroughly incorporated with the soil. Some researchers, however, have found no relationship between metal concentration in soil and in the plant. Kuo  and Del Castilho and Chardon  found no apparent relationship between DTPA-extractable Cd (probably the most widely used availability index) and plant Cd in five soils with a pH of 5 to 6. The same was also found by Barbarick and Workman  and O'Connor .
Although Cd and Pb are nonessential elements with no biological function within the plant, Ni and Zn are essential micronutrients for plant growth. However, Ni and Zn can cause toxicities when they are present in plants in excess. Zinc can be relatively easily translocated from roots to shoots in a plant and, in high concentrations, tends to accumulate in mature leaves although heavy metals accumulate mostly in roots. This is also shown in Figure 3.4. Other heavy metals, however,
stay in the roots and move slowly upwards . Kabata-Pendias and Pendias have summarized the several ways by which plants can tolerate high heavy metal concentrations :
• Selective uptake of ions
• Decreased permeability through cell membranes
• Immobilization of ions in roots, foliage, or seeds
• Removal of ions from metabolism by deposition in insoluble forms
• Alteration in metabolic patterns: increased enzyme system that is inhibited, or increased antagonistic metabolite, or reduced metabolic pathway by-passing an inhibited site
• Adaptation to toxic metal replacement of a physiological metal in an enzyme
• Release of ions to plants by leaching from foliage and excretion from roots
However, it is evident that different plant species and different cultivars in the same species have different capabilities in tolerating heavy metals . That is, different plants, grown in the same soil contaminated with heavy metals, may accumulate variable quantities of these metals. The response of some plant species to heavy metal exposure is shown in Table 3.7.
3.3.3 Concerns of Heavy Metal Leaching Out of Soil and into Groundwater
The movement of heavy metals down the soil profile is of a great importance because it involves the risk of groundwater contamination and deterioration of drinking water quality. The literature provides some extreme examples of cases of massive heavy metal movement up to 3 m depth . After heavy metals have been introduced with sewage sludge application to land, downward movement will be a possibility when some, or any combination, of the following is evident:
• High sludge application rates
• Enhanced heavy metal load in sludge
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