Microbial Mediation on As-schwertmannite Formation in Acid Mine Drainage (AMD): A Multiscale Analytical Approach

Acid Mine Drainage (AMD) systems are an increasingly discussed topic within the scientific community due to the severe pollution problems they often cause. Among the potential solutions, investigating secondary minerals as pollutant absorbers, particularly the role of biotic mediation in their formation, has gained significant attention. The San Blasio mine site, located near the village of Caulonia in Southern Italy, where iron (Fe) and arsenic (As) were extracted until the 20th century, represents a valuable location for studying the formation of such biogenic secondary minerals. A whitish/reddish muddy precipitate was identified on the streambed of an extremely acidic creek (pH ~ 3) draining from the mine. The water has a Ca-SO₄ composition and is characterized by high concentrations of As (1150 µg/L), Pb (116 µg/L), and Cd (24 µg/L). The precipitate is primarily composed of amorphous As-schwertmannite (FeO 69%, SO₄ 14%, As₂O₅ 11%; composition by wt%) and, in much smaller quantities, jarosite. To effectively characterize this site, a variety of techniques have been employed, including X-ray diffraction (XRD), optical microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, and Fourier-transform infrared (FTIR) spectroscopy. As-schwertmannite typically forms clusters of subspherical hollow nanospheres ranging from 100 to 300 nm in diameter, with solid mineral walls approximately 30 nm thick. These walls often transition into mineral nanometric needles (ca. 100–300 nm in length and 5–30 nm in width), which radially bulge from the outer portion of the nanospheres. The precipitate is strictly associated with well-preserved to variably degraded microbial cells, possibly actinobacteria, fungi, archaea, or other acidophilic prokaryotes, as well as a high abundance of bacterial outer membrane vesicles (OMVs) and/or spores. All of these organic structures, particularly vesicles and spores, can appear partially or completely mineralized, with < 1 nm nanoparticles of As-schwertmannite replacing the organic material. Since the process of As-schwertmannite biomineralization is not yet well understood, investigating the biochemical mechanisms that mediate precipitation could offer insights into developing sustainable water treatment technologies. These technologies could prevent the dispersion of toxic elements into the environment and facilitate the recovery of As-enriched precipitates. The San Blasio Acid Mine Drainage (AMD) system, located in the South-Eastern sector of Calabria (Southern Italy), is characterized by widespread As-schwertmannite biomineralization developing from the spring and along the whole stream. Below the location image, a brief overview of the used techniques and research significance is provided. X-ray diffraction (XRD) and Raman spectroscopy were used for mineralogical analysis, while optical and electron microscopy (SEM and TEM) enabled micro- and nanoscale structural characterization, with a focus on the identification of organomineral structures. Fourier-transform infrared (FTIR) spectroscopy was also employed to provide molecular insights supporting the hypotheses derived from the morphological and chemical data. The right side of the abstract shows a field picture of the San Blasio AMD system, illustrating key physical and chemical features of the water including: pH, dominant dissolved ions, and toxic metal(loid) concentrations. The schematic representation in the rectangle below the field image, highlights the primary mineral and organomineral structures (with the related SEM and TEM images) involved in the As-schwertmannite biomineralization process. These latter comprise (from left to right): the aggregation of hollow nanospheres associated with organic filaments, often exhibiting needle-like protrusions from the mineral layer; the full mineralization of organic filaments by irregular mineral particles; the formation of an As-schwertmannite mineral layer (~200 nm thick), created through the aggregation of <1 nm particles, which mineralize the cell wall of a degraded cell; and the mineralization of a membranous vesicle (~200 nm in diameter), with mineralization occurring both on the external membrane and within it, witnessing the in-situ growth of the precipitate. As-schwertmannite commonly forms in acid mine drainages and plays a key role in arsenic retention within sediments. A multiscale analytical approach revealed the mineralogy (As-rich schwertmannite) and the mineral-microbe interactions within this precipitate. Hollow nanospheres, generally made by the aggregation of <1 nm particles, represent the main recognized mineral structure. As-schwertmannite precipitation is strictly associated with microbial substrates such as cell walls, vesicles, spores, and EPSs. FTIR corroborated the microbial mediation of As-schwertmannite mineral formation.