Metal organic mobilisation

Tyre-derived rubber materials should not be used in areas with high pH (e.g., pH 8 or above) or very low pH (e.g., pH 5 or below) as there is a greater potential for metal/organic mobilisation. 

Speciation in solution is considered a major factor in the mobilisation and leaching of metal cations (DeKoninck, 1980 Bloomfield, 1981 Stevenson and Fitch, 1986). Complexation increases the total soluble concentration of a metal and hence increases its potential to be leached. Organic ligands (e.g. humate, ful-vate, citrate, polyphenols) are the major complexers involved in this mechanism, but they are effective only if the soluble organic complex does not become saturated and precipitate (DeKoninck, 1980). 

In assessing environmental risks, however, the lack of specificity can be not so crucial. It might be more interesting to get information on the possibility of mobilisation or on bioavailability of PTMs than to identify the exact chemical species of metals in soil. It could be not so important to know whether the metals come from sulphides or organic matter, but could be more useful to know that they can be released under reducing or oxidising conditions . 

Fraction 1 is likely to define the actual concentration of metals in soil solution. Fractions 2-4 might be mobilised in the short and medium term by changes in soil chemistry. Fraction 2 is supposed to be easily mobilised from the surface of soil constituents (soil sorptive complex) by changes in pH. Fractions 3 and 5, which represent metals bound to Mn oxides and amorphous Fe oxides, should be sensitive to changes in redox potential. Fraction 4, reflecting the organically bound metals, will probably be mobilised by the decomposition of soil organic matter. Fractions 6 and 7 are expected to be relatively stable, particularly in well-aerated soils, as reaction kinetics of iron oxides and silicates are slow. 

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