Trace elements mobility during the early diagenesis of iron precipitates in acid drainage systems

Abstract

Both mining and industrial activities are the main pollution sources for the environment. However, many of these processes have a natural origin, as in the case of the acid rock drainage (ARD). The ARD results from the exposure of metal sulfide minerals to atmospheric conditions. When interacting with meteoric water, oxidative dissolution of sulfides releases protons, metals and sulfates to solution and provokes the acidification of the environment. Acid mine drainage (AMD) is a leaching process derived from the mining activity increasing this effect provoked by ARD. The main metallic sulfide mineral associated with AMD environments is pyrite, yet it is commonly associated with other sulfides as arsenopyrite, chalcopyrite, galena, or sphalerite. Many of the metal(loid)s (from now on metals) hosted by sulfides are thought to be toxins for the environment, making its control something important. One of the main metallogenic provicndes of massive sulfide deposits in the world is located at the SW of the Iberian Peninsula, in the Iberian Pyrite Belt (IPB), where the mining activity dates back to Pre-Roman times. Even so, the huge discharge of contaminants to the water causes the formation of a series of low-crystallinity mineral phases that are able to retain metal traces. Schwertmannite is the nano-crystalline Fe oxy-hydrosulfate that spontaneously precipitates, creating sedimentary terraces in waters affected by AMD and buffering the pH between 2 and 4. Its poor crystallinity leads to two characteristics (1) it can capture a high quantity of trace metals from solution and (2) as it is a metastable phase, it is easily and rapidly transformed into goethite and jarosite, and at long-term into hematite. The behavior of the trace metals previously retained by schwertmannite during its transformation is a focus for discussion in the scientific community; it is here where the present thesis is framed. Several samples at different maturation stages were collected and chemically and mineralogically characterized by micro-Raman spectroscopy, reflected optic microscope, X-ray diffraction (XRD), scanning electron microscopy (SEM), electronic probe micro-analyzer (EPMA) and total acid digestion. In AMD environments, newly-formed terraces were observed to be composed by schwertmannite in the early stage, which was formed on the riverbed, trapping As from the solution. It was also noticed that as going deeper in a terrace profile goethite and, eventually, jarosite appear, being finally goethite the predominant phase at deeper levels. After diagenetic maturation, fossil terraces were composed by highly crystalline goethite and hematite. It is shown in this study that ^-Raman is a useful tool for the mineralogical characterization of Fe(III) phases, for oxides, hydroxides, and oxy-hydroxysulfates. However, it was not possible to detect the As incorporation to the structure, due to the low As concentration in the natural precipitates. In a second study, representative samples of the extreme diagenetic stages were used; a sample of a present-day terrace and an old terrace. The samples were mineralogically characterized by ^-Raman. Moreover, acid digestions of the samples were done, as well as microprobe analysis and, most importantly, X-ray fluorescence maps with synchrotron radiation (^-XRF) focalized on those ranges of major trace metals; As, Cu, Zn. The high number of data obtained was analyzed using statistic techniques such as principal component analysis (PCA). A progressive loss of metal affinity of the trace metals was detected with PCA in favor of the main element, Fe, as the phases were more crystalline (Fe R2 schwertmannite > goethite > crystalline goethite > hematite). Experiments with synthetic samples were performed in order to simulate the diagenetic transformation processes of schwertmannite. Schwertmannite was precipitated at different temperatures (40, 60 and 85 2C), during different exposure times and with different initial As(V) concentrations, [As]o. The supernatant water was analyzed and the precipitates characterized by high-energy X-ray diffraction (HEXD) with synchrotron radiation. During the synthesis process, schwertmannite was the only phase of the initial precipitates, except for those cases with [As]0 > 0.5 mM, where a amorphous Fe- and As-rich phase was formed. As a result, a delay in the schwertmannite precipitation and its transformation was observed in those experiments with a higher As added concentration. During the experiment a release of As was detected at long-term aging. Also, the introduction of As to the structure can generate local defects that have been elucidated with Pair distribution function (PDF) analysis. It mainly affects to Fe-octahedra, disappearing in part from the structure.