Variations in physico-chemical properties of water associated with bio-precipitation of hydrozincite [Zn5(CO3)2(OH)6] in the waters of Rio Naracauli, Sardinia (Italy)

A photosynthetic microbial community promotes the bio-precipitation of hydrozincite [Zn5(CO3)2(OH)6], containing tens to thousands of mg kg−1 of Pb, Cd, Cu, and Ni, in the Rio Naracauli, a streamlet polluted by mine tailings. Major variations in chemical and physical parameters occur in the waters as a consequence of hydrozincite precipitation induced by an initial environmental change (including a pH increase) produced by microbial activity. Specifically, the pH increases from 6.4 to 7.7 during hydrozincite precipitation, while at the same time the image concentration decreases. High concentrations of heavy metals (Zn 350 mg l−1, Cd 3.3 mg l−1, Pb 1.1 mg l−1 and Ni 0.17 mg l−1) were found upstream from the region of bio-precipitation in the outflow of tailings ponds. All metal concentrations decrease significantly immediately after the bio-precipitation of hydrozincite and continue to decrease further downstream.