José-Miguel Nieto

Recovery of Rare Earth Elements and Yttrium from Passive-Remediation Systems of Acid Mine Drainage

Rare earth elements and yttrium (REY) are raw materials of increasing importance for modern technologies, and finding new sources has become a pressing need. Acid mine drainage (AMD) is commonly considered an environmental pollution issue. However, REY concentrations in AMD can be several orders of magnitude higher than in naturally occurring water bodies. With respect to shale standards, the REY distribution pattern in AMD is enriched in intermediate and valuable REY, such as Tb and Dy. The objective of the present work is to study the behavior of REY in AMD passive-remediation systems. Traditional AMD passive remediation systems are based on the reaction of AMD with calcite-based permeable substrates followed by decantation ponds. Experiments with two columns simulating AMD treatment demonstrate that schwertmannite does not accumulate REY, which, instead, are retained in the basaluminite residue. The same observation is made in two field-scale treatments from the Iberian Pyrite Belt (IPB, southwest Spain). On the basis of the amplitude of this process and on the extent of the IPB, our findings suggest that the proposed AMD remediation process can represent a modest but suitable REY source. In this sense, the IPB could function as a giant heap-leaching process of regional scale in which rain and oxygen act as natural driving forces with no energy investment. In addition to having environmental benefits of its treatment, AMD is expected to last for hundreds of years, and therefore, the total reserves are practically unlimited.

Acid mine drainage in the Iberian Pyrite Belt: 2. Lessons learned from recent passive remediation experiences

The Iberian Pyrite Belt (IPB), SW Spain and Portugal, contains about 100 abandoned mine wastes and galleries that release acid mine drainages (AMD) to the Tinto and Odiel rivers. In situ passive remediation technologies are especially suitable to remediate the drainages of these orphan sites. However, traditional remediation systems, designed for coal mines, have been demonstrated inefficient to treat the IPB mine waters. Due to their high acidity and metal loads, large amount of solids precipitate and fast clogging of porosity or passivation (coating) of the reactive grains occurs. To overcome these problems, the dispersed alkaline substrate (DAS) a mixture of fine-grained limestone sand and a coarse inert matrix (e.g., wood shavings) was developed. The small grains provide a large reactive surface and dissolve almost completely before the growing layer of precipitates passivates the substrate. The high porosity retards clogging. However, calcite dissolution only raises pH to values around 6.5, at which the hydroxides of trivalent metals (Al and Fe) precipitate, but it is not high enough to remove divalent metals. Caustic magnesia (MgO) buffers the solution pH between 8.5 and 10. A DAS system replacing limestone with caustic magnesia has been tested to be very efficient to remove divalent metals (Zn, Cd, Mn, Cu, Co, Ni, and Pb) from the water previously treated with calcite.

Silicate and oxide exsolution in pseudo-spinifex olivine from metaultramafic rocks of the Betic Ophiolitic Association: A TEM study

Abstract Transmission electron microscopy (TEM) has been used to study submicroscopic particles in spinifex- like textured olivine from secondary harzburgites collected from the Cerro del Almirez locality in the Mulhacén Complex (Betic Cordillera, SE Spain). Three main types of submicroscopic oxides have been identified: (1) equidimensional Fe-rich spinel (magnetite), with average grain size in the order of 1-2 μm; (2) elongated Cr-bearing spinels (Fe-chromite to Cr-magnetite) with sizes ranging from 0.2 to 1 μm long and 0.01 to 0.1 μm thick; and (3) equidimensional Ti-rich particles from <0.01 to 0.1 μm and compositions ranging from ilmenite to Fe-Cr-Ti oxide. Chromite and ilmenite particles form parallel, chromite-rich and ilmenite-rich bands, extending along the a-axis of the host olivine. Both phases show a fixed orientation relationship with olivine, with the approximately hexagonal close-packed oxygen planes being parallel in both structures. These textural relationships indicate that both chromite and ilmenite were formed during a common exsolution process. Magnetite particles also are orientated preferentially relative to the olivine, but these particles are homogeneously distributed within the olivine, suggesting either a primary origin or an exsolution process that was not contemporaneous with formation of chromite and ilmenite. Chromite particles commonly are accompanied by lamellae of talc and/or enstatite, both showing a consistent orientation relationship with olivine. Talc lamellae are twice as thick as the associated chromite crystals, whereas enstatite lamellae show a greater thickness and, moreover, form single enstatite particles, which consist of clino- and orthoenstatite intergrowths. Talc formation may be explained by exsolution, together with spinel, from olivine containing OH-groups, probably related to incomplete dehydration of serpentine during olivine formation. On the basis of these results and previously reported petrological data, we have concluded that exsolution of chromite-silicate and ilmenite occurred during the retrograde stage that followed the climax of the eo-Alpine metamorphic event.

Assessment of metal contamination, bioavailability, toxicity and bioaccumulation in extreme metallic environments (Iberian Pyrite Belt) using Corbicula fluminea.

The Iberian Pyrite Belt (SW Iberian Peninsula) has intense mining activity. Currently, its fluvial networks receive extremely acid lixiviate residue discharges that are rich in sulphates and metals in solution (acid mine drainage, AMD) from abandoned mines. In the current study, the sediment and water quality were analysed in three different areas of the Odiel River to assess the risk associated with the metal content and its speciation and bioavailability. Furthermore, sediment contact bioassays were performed using the freshwater clam Corbicula fluminea to determine its adequacy as a biomonitoring tool in relation to theoretical risk indexes and regulatory thresholds. Reburial activity and mortality were used as the toxic responses of clams when exposed to contaminated sediment. The results showed coherence between the water and sediment chemical contamination for most of the metals. The reburial activity was correlated with the metal toxicity, but no clam mortality was registered. The bioaccumulation of the studied metals in the clam did not have a significant correlation with the bioavailable fraction of the metal content in the environment, which could be related to a potential different speciation in this singular environment. The bioaccumulation responses were negative for As, Cd and Zn in highly contaminated environments and were characterized as severe, considerable and low potential environmental risks, respectively. The results show that C. fluminea is a good biomonitor of Cu and Pb.

Water acidification trends in a reservoir of the Iberian Pyrite Belt (SW Spain).

Scarcity of waters is the main limiting factor of economic development in most arid and semi-arid regions worldwide. The construction of reservoirs may be an optimal solution to assure water availability if the drainage area shows low disturbances. This is the quandary of mining areas where economic development relies on water accessibility. Water acidification trends were investigated in the Sancho Reservoir (SW Spain) in the last 20 years. The acidity (pH3-5) and high dissolved metal concentrations (e.g., 4.4 mg/L of Al, 2.1mg/L of Mn, 1.9 mg/L of Zn) observed in the Sancho, together with the large volume stored (between 37 and 55 Mm(3)), makes this reservoir an extreme case of surface water pollution worldwide. A progressive acidification has been observed since 2003, as evidenced by decreasing pH values and increasing dissolved metal concentrations, especially noticeable after 2007. The increase in the net acidity in the reservoir originates from the higher input of metals and acidity due to the rebound effect after the mining closure in 2001. This trend was not detected in the river feeding the reservoir due to its great hydrological and hydrochemical variability, typical of the Mediterranean climate. Chemical analysis and absolute dating of sediments identified a progressive enrichment in S and metals (i.e., Fe, Zn Cu, Ni, Co and Cd) in the upper 20 cm, which reinforce the year 2002/03 as the onset of the acidification of the reservoir. The decrease of pH values from 4-5 to 3-4 occurred later than the increase in sulfate and metals due to pH-buffering by Al. The acid mine drainage (AMD) pressure has caused an increment of dissolved Fe and other metals, as well as a change in the pH buffering role, exerted now by Fe. These processes were simulated by PHREEQC, which confirms that the acidification trend will continue, causing pH values to reach 2.5 if AMD pressure persists.

The nanocrystalline structure of basaluminite, an aluminum hydroxide sulfate from acid mine drainage

Abstract Basaluminite is a poorly crystalline aluminum hydroxysulfate that precipitates in waters affected by acid mine drainage (AMD) and in acid sulfate soils (ASS). Its ability to sequester potentially toxic elements, such as Cu and As, makes it an important component of these systems, with strong environmental implications. Although it was initially described as a mineral, basaluminite is now considered a nanoscale variety of felsöbányaite, a rare mineral. In the present study, chemical analyses of natural and synthetic basaluminites are combined with data from advanced nanoscale characterization techniques such as high-energy X-ray diffraction (HEXD) and their corresponding pair distribution function (PDF) analyses, extended X-ray absorption fine structure (EXAFS), and solid-state nuclear magnetic resonance (ssNMR) spectroscopy. X-ray scattering data are analyzed with reverse Monte Carlo (RMC) modeling to obtain an atomistic representation of the disorder presents in this nanomineral. Sulfur K-edge EXAFS results show that sulfate is coordinated to the aluminum-octahedral framework of basaluminite mainly through outer-sphere ligands, though the existence of inner-sphere ligands seems to be significant in synthetic samples. PDF analyses show that both synthetic and natural basaluminites have identical short-range order, with ~1.2 nm coherent domain size, and share structural characteristics with felsöbányaite. Interestingly, 27Al ssNMR reveals the presence of, respectively, ~1 and 5% of tetrahedral and pentahedral coordinations. RMC models of basaluminite highlight the presence of structural point defects. The understanding of this nanocrystalline character has important implications in terms of the reactivity of this nanomineral in AMD and ASS. The lack of correlation between the spatial and temporal occurrence of basaluminite and felsöbányaite suggests that the similarities between both mineral structures could be fortuitous, and highlights the need for a re-evaluation of the status of basaluminite as a nanomineral.