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 δ66Zn 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 δ66Zn compared to Lyon-JMC Zn): laboratory standards (seven samples): − 9.15‰ to + 0.17‰; Zn metal dust purified by thermal distillation (three samples): + 0.09‰ to + 0.19‰; Zn metal shot purified by electrochemically (one sample): + 0.22‰; Special High Grade Zn, as represented in US pennies (six samples): + 0.14‰ to + 0.31‰; galvanized steel (three samples): + 0.12‰ to + 0.58‰; electroplated hardware (three samples): − 0.56‰ to − 0.20‰; and health products (five samples): + 0.09‰ to + 0.24‰. Based on these results, we suggest that the isotopic composition of “common” anthropogenic Zn products ranges from + 0.1‰ to + 0.3‰. All samples studied here had δ66Zn 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 δ66Zn 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 δ66Zn 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 δ66Zn 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.