MECHANISM OF MOLYBDENUM REMOVAL FROM THE SEA AND ITS CONCENTRATION INBLACK SHALES - EXAFS EVIDENCE

Molybdenum K-edge EXAFS (extended X-ray absorption fine structure) spe ctra yield new structural information about the chemical environment o f Mo in high-Mo black shales and sediments. Two spectral types are fou nd. The less common one, associated with Mo ores developed in shale in China, is that of a MoS2 phase, possibly X-ray amorphous jordisite. T he other, associated with Cretaceous deep sea sediments and with other black shales, is characterized by short Mo-O distances (1.69-1.71 Ang strom) by Mo-S distances of 2.30-2.38 Angstrom, and in some cases by s econd shell Mo and Fe interactions, which suggests that some Mo reside s in transition metal-rich phases. EXAFS spectra of synthetic amorphou s materials, prepared by scavenging Mo from HS solutions with Fe(II), FeOOH, and humic acid, suggest that the second spectral type arises fr om Mo present chiefly in two forms. One is a compact, Mo-Fe-S ''cubane '' type compound with Mo-S distances of similar to 2.36 Angstrom and M o-Fe distances of similar to 2.66 Angstrom, while the other is probabl y an organic form containing some Mo-O double bonds (similar to 1.69 A ngstrom). Laboratory products, that were prepared by scavenging dissol ved Mo from sulfidic solutions with humic acid, yield spectra quite si milar to the second spectral type observed in shales and sediments, in cluding unexpected indications of Mo-Fe interactions. Molybdenum L-edg e spectra indicate that the mean oxidation state in the sediments and shales lies between IV and VI. This work demonstrates the merit of EXA FS for obtaining structural information on natural materials containin g X-ray amorphous components which defeat conventional mineralogical c haracterization. The implications of these findings regarding Mo scave nging from sulfidic natural waters are considered. We introduce the co ncept of a geochemical switch, in which HS- transforms the marine beha vior of Mo from that of a conservative element to that of a particle r eactive element. The action point of the HS- switch is calculated to b e, a(HS)- = 10(-3.6) - 10(-4.3). When a(HS)- approaches the action poi nt, Mo becomes reactive to particles containing transition metals (e.g ., Fe). We conjecture that thiols, including humic-bound thiol groups, also switch Mo behavior. In contrast to previous ideas, our model for Mo scavenging deemphasizes the role of reduction from Mo(VI) to Mo(V) as the initial step in scavenging; instead, we emphasize the ease wit h which Mo forms covalent bonds to transition metals and organic molec ules via S bridges.