Fractionation of chemical elements including the REEs and ²²⁶Ra in stream contaminated with coal-mine effluent

Water draining from abandoned open-pit coal mines in southeastern Ohio typically has a low pH and high concentrations of Fe, Al and Mn, as well as of trace metals (Pb, Cu, Zn, Ni, Co, etc.) and of the rare earth elements (REEs). The cations of different elements are sorbed selectively by Fe and Al hydroxide precipitates which form with increasing pH. As a result, the trace elements are separated from each other when the hydroxide precipitates are deposited in the channel of a flowing stream. Therefore, the low-energy environment of a stream contaminated by mine effluent is a favorable site for the chemical fractionation of the REEs and of other groups of elements with similar chemical properties. The interpretation of chemical analyses of water collected along a 30-km-stretch of Rush Creek near the town of New Lexington, Perry County, Ohio, indicates that the abundances of the REEs in the water appear to change downstream when they are normalized to the REE concentrations of the mine effluent. In addition, the Ce/La ratios (and those of all REEs) in the water decrease consistently downstream. The evidence indicates that the REEs which remain in solution are enriched La and Ce because the other REEs are sorbed more efficiently. The solid Fe(OH)₃ precipitates in the channel of Rush Creek upstream of New Lexington also contain radioactive ²²⁶Ra that was sorbed from the water. This isotope of Ra is a decay product of ²³⁸U which occurs in the Middle Pennsylvanian (Upper Carboniferous) coal and in the associated shale of southeastern Ohio. The activity of ²²⁶Ra of the Fe(OH)₃ precipitates increases with rising pH, but then declines farther downstream as the concentration of Ra remaining in the water decreases.