Structural and geochemical characterization of Al and Fe minerals with low crystallinityimplication for contaminant mobility

Abstract

The pollution of the aquatic environment by heavy metals and metalloids (from now on referred as ‘metals’) in solution can be potentially associated with acid mine drainage (AMD) or with leachates from acid sulfate soils (ASS). Both AMD and ASS derive from the exposition and oxidation of sulfide minerals contained in mineral ore bodies or in soils formed under reducing conditions,, respectively. In both cases, sulfide oxidation, mainly pyrite, releases protons, sulfate, ferrous iron and other metals; reducing the pH and increasing the ionic load in the solution. This pollution is attenuated by precipitation of nanominerals such as schwertmannite and basaluminite. The schwertmannite structural framework and its affinity by metals have been previously studied in depth, whereas little is known about basaluminite. Thus, the main objective of this doctoral dissertation is to propone a structural model for basaluminite and to study its properties as scavenger of toxic elements by means of non-conventional techniques based on synchrotron radiation (e.g. High Energy X-ray Diffraction and X-ray absorption fine structure). 1) Firstly, neutralization experiments were conducted using a naturally reduced AMD with all aqueous Fe as Fe(II) under atmospheric and anoxic conditions to elucidate, individually, the sorption capacity of trace elements in solution by schwertmannite and basaluminite. Under atmospheric conditions, initial precipitation of schwertmannite led to the total removal of Fe, As, Cr and Pb in solution, masking the possible basaluminite affinity by these elements, as occurs in natural systems. However, under anoxic conditions, neutralization of the reduced solution with all Fe available in solution led first to the basaluminite precipitation, unveiling a heretofore unknown affinity of this phase for As and Cr. 2) Secondly, adsorption isotherm experiments were conducted to quantify the maximum As and Se adsorption capacities on synthetic schwertmannite and basaluminite, as well as the sulfate competitive effect for surface sorption sites. In addition, synchrotron-based techniques were used to determine the local coordination of As and Se complexes in both structural frameworks. The results showed that oxyanion exchange between structural sulfate and As or Se in solution was the main removal mechanism. Maximum As adsorption capacity by basaluminite was twice higher than by schwertmannite and three times higher than Se by both phases. The presence of sulfate had little effect on the sorption As capacity; although it had a strong negative effect on Se removal. Synchrotron results indicated that bidentate binuclear inner-sphere was the most probable ligand for As on both phases and for Se on schwertmannite, whereas Se formed outer-sphere complexes in basaluminite. 3) Finally, synchrotron experiments and reverse Monte Carlo analysis were performed to determine the short-range structure of basaluminite. This nanomineral showed high sulfate contents with different structural coordination between natural (outer-sphere position) and synthetic (inner-sphere position) samples. Both synthetic and natural basaluminites had identical local order with 1.2 nm of coherent domain. On the other hand, A1 in basaluminite showed similar features to Ali3 keggin ions with -1% and 5% of tetrahedral and pentahedral coordination, respectively. According to the structural models, basaluminite revealed similar structural framework to felsobanyaite, which was formed by octahedral A1 layers layers with vacant positions and high angular and longitudinal octahedral distortion, where sulfate and water molecules were emplace in the interlayer space. In conclusion, this doctoral dissertation describes basaluminite as a strategic nanomineral on the metals behavior in areas affected by AMD and ASS, and its study has allowed better understand the natural attenuation processes occurring in systems affected by sulfide oxidation. This information will help also to improve the efficiency of the treatment systems and to develop new strategies focused on metal recovery with economic interest.