Other
Scientific paper
Mar 1981
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1981gecoa..45..363d&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 45, Issue 3, pp.363-379
Other
2
Scientific paper
The `in vitro' metal fixation ability of a recent sedimentary organic matter of algal origin has been investigated with seven divalent cations (UO 2+ 2 , Ni 2+ , Mn 2+ , Zn 2+ , Co 2+ , Pb 2+ , Cu 2+ ) and three anionic species (VO - 3 , MoO 2- 4 , GeO 2- 3 ). In the case of the cations, the great selectivity of the fixation, the independence of the amounts of metal fixed with regard to the cation exchange capacity of the organic matter and the existence of a `pH effect', suggest the fixation mechanism to be a complexation-chelation process. The similarities between the cation fixation capacity vs pH curves [in the 2 to (4-7) pH range]emphasize the existence of a common and determining step in the chelation mechanism for all the cationic species studied. We assess this phenomenon to be an interaction between the cationic species and anionic carboxylate groupings. This first, non-selective, interaction is supposed to be followed by the formation of selective linkages between the metal and various organic functional groups. Moreover, the thermodynamic expression of these hypotheses allows to take into account, for the cations, both the pH dependence of their fixation capacity and the possibility to apply a Langmuir adsorption model to their fixation on the organic matter. Some competition studies between two cationic species have also been performed. They indicate a good fixation of Cu 2+ vs other metals, but show that the best one seems to be that of UO 2+ 2 . As to the anions, the fixation is weak and the anion exchange capacity of the algal organic matter is small. Taking into account the experimental results concerning U, V and Mo and the correlation often observed in caustobioliths between these three elements, we propose a metallogenic scheme for the accumulation of these three metals in such sediments, starting with fixation of metals under oxycationic (or neutral) forms leading to organometallic linkages.
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