Variations in physico-chemical properties of water associated with bio-precipitation of hydrozincite [Zn-5(CO3)(2)(OH)(6)] in the waters of Rio Naracauli, Sardinia (Italy)

摘要

A photosynthetic microbial community promotes the bio-precipitation of hydrozincite [Zn-5(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 HCO3- 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.According to theoretical calculations, the main chemical species of Zn, Cd and Pb are Zn, Cd and Pb in the upper reaches of Naracauli Creek, while further downstream ZnCO30, CdCO30, and PbCO30 complexes prevail. The sat uration index with respect to hydrozincite shows an oversaturation of up to 30 times the equilibrium value.Bulk partition coefficients between solution and hydrozincite show that K-D(Pb/Zn) > K-D(Cu/Zn) > K-D(Ba/Zn) > K-D(Ni/Zn) > K-D(Cd/Zn) > K-D(Sr/Zn) K-D(Me/Zn) = (Me/Zn)(s)/(Me2+/Zn2+)(aq) where (Me/Zn)(s) represents the ratio between trace metal Me to Zn in hydrozincite, (Me2+/Zn2+)(aq) represents the ratio in associated water, and Me stands for Cd2+, Pb2+, Ni2+, Cu2+, Sr2+ and Ba2+). Heavy metals appear to be strongly bound in hydrozincite, because only small quantities of these are removed by cation exchange.These results may have implications for the development of remediation methods for waters contaminated with heavy metals. (C) 2004 Elsevier Ltd. All rights reserved.