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Some heavy trace elements (Cu, Zn, etc.) are micronutrients essential for plant growth and therefore are beneficial to the crops; however, in excessive amounts they can reduce growth or can be toxic to plants [25]. Other heavy trace elements present in sewage sludges (As, Cd, Hg, Pb, and Se) are not essential for plants, but are toxic above certain defined levels [26-28]. Similarly, animals will have different tolerance levels for heavy trace elements, e.g., cattle are more susceptible to Se poisoning than sheep are [4]. Many trace elements present in sewage sludge form stable complexes, when applied to soil, with biomolecules and their presence in even small amounts can be detrimental to plants and animals [29,30].

In the soil environment, a heavy metal ion can undergo a number of processes and will be distributed among its different chemical forms and physical phases (water-soluble mobile phase, organic matter bound, oxide bound, etc.). It was found that trace elements that form stable complexes with ligands tend to be more toxic and remain in the soil for much smaller periods of time [15,31,32]; this can result in phytotoxicity and increased movement of trace elements into the food chain. Figure 31.1 illustrates the speciation of heavy trace elements in a soil-water system [33,34].

The processes that determine the speciation of trace elements in a soil environment are [33-35]:

• Precipitation and dissolution

• Sorption and desorption

• Complexation with organic compounds

• Complexation with inorganic compounds

Which of these processes occurs depends on the chemical characteristics of the trace elements and properties of the soil environment.

Sorbed to

- Organic matter

- Clay particles

- Other soil constituents

mobile fraction immobile fraction

Complexed with inorganic soluble ligands such as:

- hydroxides

- carbonates

- sulfates

- chloride

- phosphates

- nitrate

Desorption

Desorption

Comp]

Comp]

Metal ion in soil liquid

Precipitation

Precipitation

Dissolution

Dissolution

Complexation

Precipitated with - Sulfates

Complexed with organic soluble molecules

- Hydroxides

- Carbonates

- Hydroxides

- Phosphates

FIGURE 31.1 Fractionation of trace elements in soil-water system.

Depending on their nature, heavy trace elements are associated in a variable manner with different phases making up the sludge. The distribution of heavy trace elements in the different forms and phases in which they occur in soil and sludge can be determined using sequential extraction procedures [36,37]. These procedures provide information about the differentiation of the relative binding strength of the metal on various solid phases and about their potential reactivity under different physicochemical environmental conditions.

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