Artifacts associated with the chemical leaching of sediments for rare-earth elements

Abstract

Using radioactive Eu as a tracer for rare-earth elements (REE), simple experiments were carried out to determine if an re-adsorption artifact occurs during the chemical leaching of marine sediments. Fourteen commonly used chemical leaching solutions were used with and without (controls) the presence of Buzzards Bay (Massachusetts, U.S.A.) sediment. All controls show 100% recovery of soluble 152Eu, indicating no adsorption or precipitation during handling. In contrast, almost all chemical leaching solutions show poor recoveries even at low (2–4) pH. Even for HCl and HNO 3, recoveries ranged from 25% to 85% at ≤ 0.2 M. Hydroxylamine hydrochloride in acetic acid, for example, shows only 33% and 57% recoveries for 0.02 M and 0.04 M concentrations, respectively. The results show that large-scale re-adsorption of REE onto sediments occurs even at low pH associated with mineral acids, organic chelators and reductive solutions. Given their large hydrolysis constants, a case is made that one would expect that REE, like Pb and Th, to exhibit significant re-adsorption onto sediments following their dissolution from specific phases. This result reinforces concerns in the literature that chemical leaching techniques have major artifacts associated with non-selectivity and re-adsorption of trace metals. Interpretations of REE diagenesis and authigenesis, based on chemical leaching data, should be viewed with caution.