SEPARATION OF COPPER, IRON, AND ZINC FROM COMPLEX AQUEOUS SOLUTIONS FOR ISOTOPIC MEASUREMENT: METAL SOURCES AND PROCESSES OF ACID-ROCK DRAINAGE

The isotopic composition of transition metals in rocks, surficial precipitates, and waters is receiving increased attention because of the development of second-generation multicollector ICP-MS instruments, which allow analysis at the sub-microgram level. Studies of isotopic ratios of Cu, Fe, Zn, etc., in areas with historic mining activity provide a new tool for understanding metal sources and processes in acid rock drainage (ARD). The great sensitivity of the MC-ICP-MS makes it possible to routinely analyze solutions and co-existing solids, allowing direct inferences about fractionating processes. To minimize mass-bias and other polyatomic effects when determining isotopic ratios in complex aqueous solutions such as ARD, it is necessary to first separate the element of interest from other dissolved constituents. Solutions such as ARD typically contain higher concentrations of elements such as transition metals and rare earths than is normally found in natural waters. We have developed a separation scheme that uses a single column of strong-anion exchange resin to separate Cu, Fe, and Zn from each other and from the sample matrix. The ARD solution is first evaporated to dryness and reconstituted with 7N HCl. In this acidic solution, Zn is present as ZnCl₄^2-, Cu as CuCl₂⁰ or CuCl₄^2- and Fe as FeCl₆^3-. This solution is loaded onto a small-volume column containing about 0.5 ml of resin. Fe and Zn are retained on the column as anionic species while Cu is eluted slowly with 7N HCl. Other elements, such as Al, Mn, and the rare earths, remain in cationic form and are rapidly eluted. After complete elution of Cu, the eluent is switched to 1N HCl, which breaks the Fe-chloride complex and rapidly elutes Fe. The eluent is then switched to ultrapure water, breaking the Zn-chloride complex and rapidly eluting Zn. The Cu, Fe, and Zn fractions are collected separately for isotopic analysis. Using this method, sample throughput is approximately 20 samples per day for column elution, followed by one day of instrument time for each of the three elements. Tests performed with natural and synthetic ARD samples indicate the separation scheme is effective in this type of matrix. Our first intended application of this method is to investigate isotopic ratios in areas with historic mining activity.