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. © 2025 Elsevier B.V., All rights reserved.

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