Research articleStudy of a tropical soil in order to use it to retain aluminum, iron, manganese and fluoride from acid mine drainage
Introduction
Acid mine drainage (AMD) is a persistent environmental problem that directly affects the mining industry. Many researchers have studied environmental problems arising from AMD in Brazil and in the world (Singh and Rawat, 1985, Fernandes et al., 1998, Diz et al., 1999, Ladeira and Gonçalves, 2007, Motsi et al., 2009, Vazquez et al., 2010, Campos et al., 2011, Kwon et al., 2016, Rakotonimaro et al., 2017, Godoi et al., 2017). AMD is very acidic (pH < 3.0) and contains iron, aluminum, sulfate and heavy metals such as lead, mercury, cadmium and arsenic (Kwon et al., 2016).
The oxidation of sulfide minerals is the main cause of acid drainage from mining tailings. In the case of the uranium mine of Caldas Mineral Treatment Unit (UTM-Caldas), pyrite (FeS2) is the sulfide responsible for the occurrence of acid drainage (Franklin, 2007). UTM-Caldas was inaugurated in 1982, being the first Brazilian uranium mine. In 1996 it finished its operations and is currently in decommissioning stage. UTM-Caldas is located in the municipality of Caldas, in the plateau of Poços de Caldas, Southwest region of the State of Minas Gerais, Brazil. The average annual rainfall is 1750 mm and the average annual temperature is 17 °C.
According to Sicupira et al. (2015), the AMD generated in the mining region of Poços de Caldas contains radionuclides (U, Th, among others) as well as species of Mn, Zn, Fe, and F− ions at concentration levels above those permitted by Brazilian law regarding their discharge into the environment.
The solid waste from the mine (waste rock) was defined as material with less than 170 mg kg−1 of uranium in the ore. This material was disposed in the so-called Waste Rock Piles (WRP). The waste rock pile number 4 (WRP-4) is the main type in the UTM-Caldas, occupies an area of 56.9 ha and contains 12.4 × 103 m3 of waste rocks, slope height of 90 m and slope of 70°. The waste rocks present a tinguatic texture, constantly impregnated by pyrite, fluorite, uranium, molybdenum and zirconium minerals (Cipriani, 2002). WRP-4 was formed by the disposal of waste rocks over the valley and bed of the Consulta Creek, without any compaction control, allowing waste rocks of higher particle size formed drainage channels in the base.
Currently, the acid drainage of the WRP-4 is discharged in the Nestor Figueiredo retention pond (NFP), with an approximate area of 2170.5 m2, which was constructed to capture almost all the water that percolates through WRP-4, besides receiving part of the run-off from the same pile. The pH of WRP-4 effluent water ranges from 3.0 to 4.0 and has high concentration of chemical elements and ions such as aluminum, manganese, iron, fluoride among others, and radioactive elements such as uranium, radium and thorium. The surplus water from the NFP flows through a channel located on the left bank of the Consulta Creek, which discharges the water downstream from its dam. However, leaking acid water by the NFP dam foundation has been observed frequently.
As there are no reports of the construction of a liner system at the NFP bottom, this research aimed to investigate a typical tropical soil, located in the UTM-Caldas area, in order to use it as a mineral liner for the NFP. It is intended to minimize the leakage of acid waters by the dam foundation and to retain the predominant elements in those waters (aluminum, iron, manganese and fluoride). In this study, we present the geotechnical and chemical-mineralogical characterization of the tropical soil and the performance and analysis of a column test using this soil and the acid water from the WRP-4 as a percolating solution.
Section snippets
Contaminant transport
The transport of contaminants refers to the transport of mass in porous medium in which the mass moves with the water between the voids of the soil in unsaturated and saturated zones. The main mechanisms of contaminant transport are advection, diffusion and hydrodynamic dispersion.
Advection is a physical process whereby the contaminant is carried by the water in a flow generated in the medium, where the rate of transport is directly proportional to the velocity, as a consequence of a gradient
Material and methods
The sampling site was the ground adjacent to the Nestor Figueiredo pond (NFP) at a level corresponding to the pond bottom. The geographical coordinates of the collection point were: Latitude (S) 21°56′24.72″; Longitude (W) 46°29′24.13″. The UTM coordinates were: N (m): 7573066.488 and E (m): 346123.741.
An undisturbed soil sample was collected by carefully crimping a beveled cylindrical polyethylene mold with an internal diameter of approximately 20 cm and a height of about 30 cm. Part of the
Characterization tests
The particle size curves of the soil sample with the use of deflocculant (WD) and without deflocculant (ND) is shown in Fig. 1. The results of the geotechnical and chemical characterization tests of the soil sample are presented in Table 1, Table 2.
The soil sample was classified as silty clay with the use of the deflocculant, and as sand-clayey silt, without it, and the result was the micro aggregation of particles, typical of tropical soils (Gidigasu, 1976, Committee on Tropical Soils of
Conclusion
The tropical soil located in the area of the Caldas Mineral Treatment Unit (UTM-Caldas) – characterized to be used as a mineral liner base for the NFB pond (NFP) that receives the acid drainage from the WRP-4 – was not suitable for retaining the main chemical species present. The soil has micro aggregation, acid pH, low values of organic matter content and cation exchange capacity, typical characteristics of highly weathered tropical soils. The estimated point of zero charge was 3.5, below the
Acknowledgments
The authors would like to thank the National Nuclear Energy Commission (CNEN) and the Nuclear Industries of Brazil SA (INB) for the support given to the research, and the National Council for Scientific and Technological Development (CNPq) for granting master's scholarship (process number 300789/2011-4).
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