Uptake of Pb by hydrozincite, Zn5(CO3)2(OH)6--implications for remediation

Hydrozincite, Zn(5)(CO(3))(2)(OH)(6), periodically precipitates from heavy metal contaminated waters of the Rio Naracauli stream, Sardinia, in association with a biological photosynthetic community. The precipitation removes not only zinc from the waters, but also other toxic "heavy metals", such as Cd, Cu, Pb. The phenomenon is therefore of potential interest for "soft" remediation of contaminated waters. Previous cation exchange experiments suggested that binding of Pb to hydrozincite is fairly strong. This suggestion is in agreement with new release tests in deionized water and X-ray absorption spectroscopy (XAS) spectra collected at the Pb L(III) edge for natural hydrozincites from Naracauli, and synthetic Pb-doped hydrozincites. The results suggest that, up to bulk concentration of 1.5 wt.% Pb, uptake of this metal occurs in two distinct ways: (1) as a substitution for Zn in the tetrahedral site of the hydrozincite structure, possibly via formation of a surface mononuclear tridentate inner sphere complex; (2) as an ill-defined, presumably amorphous, phase with a local atomic structure similar to cerussite. These data support the concept that Pb binding to hydrozincite is strong enough to make this mineral a potential sink for the metal.