Grass cover density and metal speciation in profiles of a tailings-pile from a mining zones in Zacatecas,North-Central Mexico |
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Authors: | F Figueroa J Castro-Larragoitia A Aragón J V García-Meza |
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Institution: | 1.Facultad de Ingeniería, Instituto de Metalurgia,UASLP,San Luis Potosí,Mexico;2.Facultad de Ingeniería,UASLP,San Luis Potosí,Mexico |
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Abstract: | The 7-year-old mine tailings pile P2 from Concepción del Oro has been revegetated spontaneously by xerophyte grasses, covering
about the 30% of its surface. To elucidate the effect of the grass cover in the geochemical behavior of the sulfide minerals
(SM) and metals, the strata of four selected profiles (P2-I, P2-II, P2-III and P2-IV, with high, middle, low, and null grass
cover, respectively) were analyzed mineralogically and chemically, using scanner electron microscopy, X-ray diffractometer
and performing a six-step sequential extraction method. An older (50-year-old) and uncovered (without grass cover) profile
of the tailings pile P3 was also analyzed. In all the profiles from P2, the SM oxidation is not yet an extended process; however,
the samples from the uncovered profile P2-IV showed evidences of SM (pyrite and chalcocite) oxidation, as well as the presence
of gypsum and Fe oxides, as the major secondary phases resulted from the SM oxidation. Additionally, the carbonate content
and pH values in P2-IV were lower than in the covered profiles from P2. The oldest and uncovered P3-I profile showed an extensive
oxidation of sulfurs which resulted in the depletion of carbonates and a pH 2. Another distinctive characteristics of the
covered profiles was that Pb, Cu, and Zn were mainly associated with the reducible fractions (carbonates and/or amorphous
oxides); meanwhile, in the uncovered (P2-IV and P3-I) such metals were mainly associated with the oxidizable fraction (crystalline
oxides). The results suggest that the mineralogical transformations control metal stability in plant-covered impoundments
(phytostabilization): the occurrence of a thick grass cover, with an efficient water usage and retention, seems to shun both
the acidic dissolution of carbonates and the reductive dissociation of the formerly present oxyhydroxides, which is desired
for remediation tasks. This is the first report about the effects of grass cover for a carbonaceous and unsaturated mine tailings
from a semiarid region, that can help in a better understanding of the scope of phytoremediation in such conditions. |
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