Jordan, Guntram; Pokrovsky, Oleg S.; Bahlo, Jens; Guichet, Xavier; Schlueter, Christoph
Sphalerite dissolution kinetics at low hydrothermal conditions.
In: Chemical Geology, Vol. 286, Nr. 3-4: S. 272-279
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A macroscopic (mixed-flow reactor) and microscopic (hydrothermal atomicforce microscopy, HAFM) approach was used to quantify sphalerite (ZnS)dissolution rates and to investigate the stoichiometry of dissolutionand leached layer formation. HAFM observations of reacted (110) surfacesat room temperature revealed surface roughening which was likelygenerated by pit formation, local surface swelling or re-deposition ofsulphur compounds, as also evidenced by a decreased hardness of theroughened surface. At 125 degrees C, appearance of hillocks withthree-sided facets was observed on the (110) surface, most likely of theforms 51116 or 5(1) over bar(1) over bar(1) over bar6. On thefacets, terraces were separated by steps. At these steps, preferentialrelease of material took place during dissolution and caused the stepsto retreat.Macroscopic dissolution rates measured in mixed-flow reactors at pH 2and 125,150 and 200 degrees C are in reasonable agreement withliterature data and yielded an apparent activation energy of Zn-releaseof 96 +/- 4 kJ/mol. Elevated dissolution rates were observed during theinitial stage of the experiment. The time necessary to attainsteady-state ranged from 110 to 250 h. Measured Zn/S concentrationratios in the reactor effluent were at average 1.1 +/- 0.1. These ratiosdo not indicate extensive leached layer formation and agree with HAFMobservations that revealed a morphology transition from 51106 to51116 or 5(1) over bar(1) over bar(1) over bar6. The addition ofFeCl(3) to the solution yielded a strong rate increase with an apparentreaction order with respect to Fe(3+)(aq) ranging from 0.41 to 0.48.