Mechanism of molybdenum removal from the sea and its concentration in black shales: EXAFS evidence

摘要: Molybdenum K-edge EXAFS (extended X-ray absorption fine structure) spectra yield new structural information about the chemical environment of Mo in high-Mo black shales and sediments. Two spectral types are found. The less common one, associated with ores developed shale China, is that a MoS2 phase, possibly amorphous jordisite. other, Cretaceous deep sea sediments other shales, characterized by short Mo-O distances (1.69-1.71 Angstrom) Mo-S 2.30-2.38 Angstrom, some cases second shell Fe interactions, which suggests resides transition metal-rich phases. synthetic materials, prepared scavenging from HS solutions Fe(II), FeOOH, humic acid, suggest type arises present chiefly two forms. One compact, Mo-Fe-S ''cubane'' compound similar to 2.36 Angstrom Mo-Fe 2.66 while probably an organic form containing double bonds (similar 1.69 Angstrom). Laboratory products, were dissolved sulfidic quite observed sediments, including unexpected indications interactions. L-edge indicate mean oxidation state lies between IV VI. This work demonstrates merit for obtaining on natural materials components defeat conventional mineralogical characterization. implications these findings regarding waters considered. We introduce concept geochemical switch, HS- transforms marine behavior conservative element particle reactive element. action point switch calculated be, a(HS)- = 10(-3.6) - 10(-4.3). When approaches point, becomes particles metals (e.g., Fe). conjecture thiols, humic-bound thiol groups, also behavior. In contrast previous ideas, our model deemphasizes role reduction Mo(VI) Mo(V) as initial step scavenging; instead, we emphasize ease forms covalent molecules via S bridges.