Chemical speciation in waters influenced by lead–zinc metallurgical industry

Abstract

The Lead–Zinc Company region, Kardjali city, Bulgaria, is known to be highly polluted with heavy metals from its pyrometallurgical activities. The polluted levels and the chemical speciation in surface natural waters in the region as well as in the wastewaters of the factory were investigated in January 2008 by application of monitoring studies, thermodynamic modeling, and interpretation in terms of the “softness–hardness” factor. It was found that the levels of trace metals pollution of surface waters were lower than the legislation limits for the regions with Pb and Zn production. The wastewater treatment facilities of the company were found to operate properly, and the quality of the cleaned waters in station Kar4 was comparable to the other surface waters studied (e.g., station Kar5). The trace metals were divided into three groups: (1) Fe^3 + and Al^3 +, being “hard” acids, existed in all the studied waters as hydroxy species Fe(OH)$_{2}^{+}$, AlOH^2 +, and Al(OH)$_{2}^{+}$, followed by the phosphate species AlPO$_{4}^{0}$ and Al₂(OH)₂PO$_{4}^{+}$; (2) Mn^2 +, Zn^2 +, and Cd^2 + being “soft” acids with crystal field stabilization energy (CFSE) = 0 were present in natural waters mainly as free Me^2 + ions. Small concentrations of their MeSO$_{4}^{0}$, MeCO$_{3}^{0}$ species, and of MeCl$_{2}^{0}$ (Me = Zn, Cd) species were also calculated. In the wastewaters, two more species [Me(SO$_{4})_{2}^{2-}$ and Me(SO$_{4})_{3}^{4-}$] of the softer Zn and Cd metals were also calculated; (3) Cu^2 + and Pb^2 +, as “soft” acids with CFSE $\ne$ 0 preferentially coordinated with softer CO$_{3}^{2-}$ ions and in natural waters existed mainly as MeCO$_{3}^{0}$ and PbHCO$_{3}^{+}$, followed by free Me^2 +ions and MeOH⁺. In the wastewaters, MeSO$_{4}^{0}$ and Pb(SO$_{4})_{2}^{2-}$ species increased at the expense of the free Me^2 + ions. The highest self-cleaning capability of natural waters was found with respect to Al and Fe, followed by Mn and Cd. The lowest corresponded to Pb, Cu, and Zn.