NATURAL AND MAN-INDUCED PHYTOREMEDIATION IN SEVERAL MINE RESIDUES CONTAINING HIGH CONCENTRATION OF CD - Management of Hazardous Residues Containing Cd - page 160

on the soil cover or on a mixture of slag and soil, and the root system develops
into the cover and avoids growing down inside the slag. Some of the plant
trees such as Casuarina equisetifolia or Eucalyptus sp. produce litter that is
accumulate on the slag surface forming a layer where the root system grows
(Figure 2a). The main disadvantage of these options for tailing management is
that these trees can easily fall down, because the root does not penetrate in the
slag layer (Figure 2b).
Residues range in color from dark brown or dark gray to yellow
depending of the oxidation condition (Table 2). Mineral composition
determines the kinetics of oxidation. In the same tailing heap, different
oxidation conditions can be found. Oxidation occurs depending on the mineral
composition and water input on the heap. Reaction in the tailing wastes is
commonly alkaline, but as the oxidation occurs, it becomes acidic until the pH
is lower than 3.7. In some places, pH can deplete even more. This acidic
environment causes a corrosive effect on minerals and organism tissues,
increasing the amount of bioavailable metals. Oxidation of sulphide to
sulphate causes the reduction of the pH values. The organic matter content is
usually low, but the use of xanthates during flotation process (O'connor and
Dunne, 1994) increases their content. Metals content and the oxidation of
sulphide can cause interferences during the wet oxidation of organic carbon
and the determination of carbon could be overestimated.
Total cadmium concentration ranged from 21 to 814 mg kg -1 and the
bioavailable concentration (DTPA-extracted) was less than 5% of the total
(Table 2). Based in the total Cd content, some of these residues may be
considered as highly toxic substrates for growing plants, but different factors
affect the availability of Cd. The concentration in each tailing heap was very
variable; for instance, in the tailing heap from Zacatecas town total contents
varied from 12 to 47 mg kg -1 (Figure 3a). In the wastes from Sombrerete,
concentration ranged from 18 to 123 mg kg -1 (Figure 3b) and from Fresnillo it
varied from 10 to 73 mg kg -1 (Figure 3c). The reduction of the Cd and other
metals concentration by the addition of soil or another material containing low
concentration of Cd, could improve revegetation. Other tailing heaps
containing Cd have been reported in Pachuca, Hidalgo 0.13 to 20.07 mg kg -1
(Jonathan et al., 2010); Guanajuato with 2.5 mg kg -1 (Mendoza-Amezquita et
al., 2006); in Guerrero with 1-780 mg kg -1 (Talavera Mendoza et al., 2008,
Romero et al., 2010); in Chihuahua with 10-239 mg kg -1 (Martin y Gutierrez,
2008) and in San Luis Potosi with 33-1716 mg kg -1 (Luna et al., 2008, Mendez
Ortiz et al., 2007); and 16 ppm (Castro-Larragoitia et al., 1997).