The isotopic composition of some common forms of anthropogenic zinc

Anthropogenic sources account for much of the zinc (Zn) in the environment. Constraining the isotopic composition of anthropogenic Zn is therefore essential to understanding the environmental biogeochemical cycling of Zn isotopes. This study examines the isotopic variability in several different categories of anthropogenic Zn. Pure Zn metal and Zn dust are the raw materials used in many Zn-containing products. We have measured delta Zn-66 for Zn dust purified by thermal distillation, electrochemically purified Zn metal, and US pennies which are made from the most common grade of Zn metal (Special High Grade). Zn in galvanized steel and electroplated hardware was studied because this is a common use of Zn, and Zn in these products may be easily released into the environment through corrosion. Vitamins were studied because they are more highly purified than typical Zn metals and are made from processed chemical compounds such as Zn oxide or Zn gluconate. The isotopic composition of samples measured in this study are as follows (reported as delta Zn-66 compared to Lyon-JMC Zn): laboratory standards (seven samples): -9.15 parts per thousand to +0.17 parts per thousand; Zn metal dust purified by thermal distillation (three samples): +0.09 parts per thousand to +0.19 parts per thousand; Zn metal shot purified by electrochemically (one sample): +0.22 parts per thousand; Special High Grade Zn, as represented in US pennies (six samples): +0.14 parts per thousand to +0.31 parts per thousand; galvanized steel (three samples): +0.12 parts per thousand to +0.58 parts per thousand; electroplated hardware (three samples): -0.56 parts per thousand to -0.20 parts per thousand; and health products (five samples): +0.09 parts per thousand to +0.24 parts per thousand. Based on these results, we suggest that the isotopic composition of common anthropogenic Zn products ranges from +0.1 parts per thousand to +0.3 parts per thousand. All samples studied here had delta Zn-66 values within this range except four laboratory standards, all electroplated hardware samples, and a single galvanized steel sample. The isotopic range for common anthropogenic Zn is much smaller than the total delta Zn-66 range found in Zn ore-field hydrothermal samples, demonstrating the effects of Zn isotope homogenization during ore processing and purification. Laboratory standards may have anomalously light delta Zn-66 values because they undergo extra purification steps and Zn recovery during these steps is not quantitative. Electroplated hardware was also isotopically light, consistent with previous studies showing that lighter isotopes are electroplated more quickly than heavy isotopes. Our results suggest that isotopically heavy Zn may be incorporated into the waste stream during the electroplating process. By defining the range of delta Zn-66 values that is typical for some common Zn products, and discovering some kinds of products that fall outside this range, we will be better able to use Zn isotopes to trace different anthropogenic Zn sources. (C) 2007 Elsevier B.V. All rights reserved.