Possible microbial mediated woodwardite precipitation in a circumneutral drainage (preliminary data)

Along the stream bed of a tributary creek of Fiumara Allaro river (Calabria, Italy), which drains ore-rich terrains exploited for mining purposes in the past century, a light blue-greenish fine-grained precipitate occurs. Multiple analytical techniques, including optical microscopy, scanning electron microscopy (SEM), and Raman spectroscopy were employed to characterize the water and the solid phase. The drainage waters are circumneutral (pH 6.8) and characterised by a dominant Ca-SO4 composition, while the dissolved metal load is mainly composed of Zn (15.3 mg/L), Mn (3.1 mg/L), and Cu (1 mg/L). The light blue-greenish precipitate is composed of woodwardite, a secondary hydrated copper-aluminum sulphate hydroxide, appearing with two nanostructures: fine tabular platelet crystals 1 to 5 μm in length consisting of clusters of smaller tabular mineral units, tens of nanometres thick and several micrometres in length, together with irregular aggregates of mineral nanospheres 200 nm in size. Both mineral phases commonly encrust organic structures and possibly substitute organic mucilages, likely produced by microbial bacterial communities. Among these, filamentous structures 2 μm to 10 μm wide and hundreds of micrometres long are particularly abundant and may play an active role in the biomineralisation process.