From adsorption to precipitation: Sorption of Mn 2+ on FeCO 3 (s)

Details From adsorption to precipitation: Sorption of Mn 2+ on FeCO 3 (s) From adsorption to precipitation: Sorption of Mn 2+ on FeCO 3 (s)

Nov 1989

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1989gecoa..53.2787w&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 53, Issue 11, pp.2787-2796

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Sorption of Mn 2+ on siderite was investigated as a function of Mn concentration and time at constant pH, pco 2 , p , and ionic strength. The log [Mn 2+ ]versus log Mn plot revealed an S-shaped isotherm which was modeled on an extended version of the adsorption/precipitation model previously formulated for metal oxides. At low concentrations, Mn was adsorbed up to the inflection point of the isotherm, where nearly all surface carbonate sites are covered by Mn 2+ . At higher concentrations, ESR spectroscopy gave evidence for a co-precipitation phenomenon by the change in the line width of the ESR signal, ruling out the other suggested mechanisms (e.g., adsorption on sites with lower reactivity). In the MnCO 3 --FeCO 3 solid solution that was formed, and within the range of isomorphous substitution considered, the activity coefficient of MnCO 3 (s) remained remarkably constant. The total solubility product was about 500 times larger than that of the end members. The present version of the model explicitly includes the solid phase activity coefficients, thus allowing the use of "thermodynamic" and not "adjustable" solubility constants. The kinetics of Mn 2+ sorption at high Mn concentration exhibited a three step mechanism, identified by ESR. A rapid initial adsorption was followed by an intermediate surface complexation process before the co-precipitation reaction itself occurred, giving rise to a solid solution. The observed increased solubility of such a solid solution relative to that of the end members is a common feature in reducing tropical environments.

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