Extent and isotopic composition of Fe and Mo release from two Pennsylvania shales in the presence of organic ligands and bacteria

Isotopic fractionation of Fe and Mo during weathering could contribute toward the isotopic signatures of river and ocean waters. To investigate weathering processes, batch experiments were carried out at pH 6 under oxygenated conditions to investigate the influence of a nitrogen-fixing soil bacterium and organic ligands on the extent and isotopic signature of Fe and Mo release from two Pennsylvania shales with different mineralogies: an olive to grey shale (Rose Hill) and a black shale (Marcellus). Results of these studies showed that Fe and Mo were mainly released from illite, chlorite/vermiculite, and Fe oxides in the RHS and from pyrite in the MS. Dissolution rates of the clays estimated from our batch experiments are broadly consistent with published estimates. Release of Fe from both shales was only enhanced by the hydroxamate siderophore desferrioxamine B (DFAM). In contrast, none of the treatments enhanced release of Mo. Furthermore, release of this metal was measurable only for the black shale. For both shales, Fe that was released in the presence of DFAM was depleted in ⁵⁶Fe as compared to the bulk rock. In contrast, leachate solutions were enriched in the heavier Mo isotope as compared to the bulk Marcellus in all experiments with and without organic ligands and the bacterium. The isotopic fractionations in Fe and Mo are broadly consistent with previously reported fractionation effects during sorption onto precipitating Fe,Mn oxides. The observed Mo isotope fractionation is also consistent with all published studies of rivers, i.e., riverine Mo is enriched in heavy Mo isotopes. The observed Fe isotope fractionation is likewise consistent with published studies for some rivers, i.e., riverine Fe is sometimes depleted in ⁵⁶Fe. Our experimental results are consistent with sorption of Fe and Mo onto Fe (oxyhydr)oxides during weathering as a possible explanation for the isotopic signatures of these metals in many rivers.