Zn solubility in low carbonate solutions

While the importance of several metal basic carbonates has been recognized in natural and wastewater systems, the existence of the Zn basic carbonate, hydrozincite, has not been fully appreciated even though solubility data have been available. In the presence of 2 mM total inorganic carbonate, Zn 2+ solutions below pH 8.2 were found to precipitate hydrozincite within 24 h and to contain total dissolved Zn concentrations that were comparable to those predicted from equilibrium with hydrozincite. The identity of the hydrozincite was confirmed by X-ray diffraction and elemental analyses. In the pH range 8.2–10.5, the total dissolved Zn concentrations were less than that expected from equilibrium with hydrozincite by factors of up to 3, while the precipitated solids had C:Zn ratios intermediate between those of hydrozincite and give oxide and exhibited weak hydrozincite X-ray diffraction patterns. At pHs above 10.5, zinc oxides with strong X-ray diffraction patterns were present and total dissolved Zn concentrations approached those expected for equilibrium with zinc oxide. In solutions prepared to exclude carbonate, the total dissolved zinc concentrations in all solutions were similar to those expected for equilibrium with zinc oxide. However, two solids in these “carbonate-free” solutions contained small amounts of inorganic carbonate and exhibited weak hydrozincite X-ray diffraction patterns. The presence of well-defined or poorly-crystalline hydrozincite in all 2 mM inorganic carbon solutions between pH 8–10 and its presence in two solutions prepared to exclude carbonate contamination suggest that hydrozincite is probably a common Zn solid formed in conventional precipitation processes.