ARTICLE pubs.acs.org/est
Mineralogy and Geochemistry of Zn-Rich Mine-Drainage Precipitates From an MgO Passive Treatment System by Synchrotron-Based X-ray Analysis Rafael Perez-Lopez,†,‡,* Francisco Macías,‡ Manuel A. Caraballo,‡ Jose Miguel Nieto,‡ Gabriela Roman-Ross,§ Remi Tucoulou,|| and Carlos Ayora† †
Institute of Environmental Assessment and Water Research, IDÆA - CSIC, Jordi Girona 18, 08034, Barcelona, Spain Department of Geology, University of Huelva, Campus ‘El Carmen’, 21071, Huelva, Spain § AMPHOS 21 Consulting S.L., Passeig de Garcia I Faria, 49-51, 1-1, 08019 Barcelona, Spain European Synchrotron Radiation Facility, 6 rue Jules Horowitz, Grenoble, France
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‡
ABSTRACT: Synchrotron radiation-induced micro-X-ray analysis were applied to characterize the newly formed phases that precipitate in a passive treatment system using magnesium oxide to remove high concentrations of zinc (ca. 440 mg/L) and other minor metals from neutral pretreated waters in the Iberian Pyrite Belt (SW Iberian Peninsula). MicroX-ray fluorescence (μ-XRF) maps of polished samples were used to find spatial correlations among metals, pinpointing zones of interest where micro-X-ray diffraction (μ-XRD) data were exploited to identify the mineral phases responsible for metal retention. This coupled technique identified hydrozincite (Zn5(CO3)2(OH)6) and minor loseyite ((Mn, Zn)7(CO3)2(OH)10) as the mineral sinks for Zn and also other potentially toxic elements such as Co and Ni. Although hydrozincite retains traces of Mn, this metal is mainly retained by precipitation of loseyite. The precipitation of zinc hydroxy-carbonates and their ability to uptake other metals (Mn, Co, and Ni) is hence of potential interest not only for the treatment of contaminated waters but also for the generation of a solid waste that could be exploited as a new Zn economic resource.
1. INTRODUCTION Acid mine drainage (AMD) generated by oxidation of sulfidebearing wastes is a main source of acidity and metal pollution of watersheds, and becomes a serious environmental problem due to the low biodegradability and high ability of contaminants to accumulate in the biota.1,2 Mine waters are treated in plants where chemicals are added in controlled doses to precipitate the metals. This treatment is only feasible while the industry or mine is active, but it may be impracticable at abandoned sites located in remote areas or where no current owner can be held responsible for treatment costs. Passive treatment systems may be a costeffective alternative.3 Most of the AMD passive remediation systems add alkalinity via calcite dissolution, inducing the precipitation of hydroxides of trivalent metals (Fe, Al), which in turn are able to retain some trace elements by adsorption and/or coprecipitation processes.4,5 These systems, however, are deficient to remediate high concentrations of divalent metals such as Zn, Mn, Cu, Pb, Ni, Cd, and Co (tens to hundreds of mg/L, commonly found at metal mining and smelting sites), because in Ca-rich solutions calcite dissolution only raises pH to values around 7, which are insufficient to precipitate these metals. Caustic magnesia (MgO) dissolution buffers solution pH between 8.5 and 10 where r 2011 American Chemical Society
solubilities of many divalent metal compounds are low. Cortina et al.6 and R€otting et al.7,8 conducted laboratory column experiments which demonstrated that caustic magnesia, an inexpensive MgO-rich product obtained from magnesium carbonate calcination, can be used to deplete concentrations of 75 mg/L of Zn, Mn, Cu, Pb, Ni, Cd, and Co to concentrations below 0.5 100 μg/L depending on the metal. In passive remediation systems, metals pass from the soluble forms to an insoluble solid waste. For a proper management of these wastes, the mineralogical characterization of the secondary phases hosting the metals is crucial. Note that the release of metals from these solid wastes could be another source of contamination and again pose a risk to the environment. The rapid nucleation and precipitation of secondary phases in treatment systems produces a very small size of the newly formed crystallites and poor crystallinity that limit the use of conventional X-ray techniques for their mineralogical characterization. X-ray diffraction (XRD) studies of these mineral phases commonly Received: May 16, 2011 Accepted: August 5, 2011 Revised: July 6, 2011 Published: August 05, 2011 7826
dx.doi.org/10.1021/es201667n | Environ. Sci. Technol. 2011, 45, 7826–7833
Environmental Science & Technology
7827
A basic statistic (mean and standard deviation for n = 5 sampling campaigns) is shown. nd: not detected;
