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A model for the incongruent dissolution of pyrite

University of British Columbia

To understand the incongrucnt dissolution phase of natural pyrite, polished and cleaned samples were subjected to oxidative dissolution at low pH from 0 to 72 hours. All samples were operationally defined as pure based on XRD, XRF, SEM/EDXA and XPS analyses. At 12 hour intervals, one pyrite sample and 10 mLs of solution were extracted from the reaction vessel for analysis. The air-sensitive pyrite samples were introduced into the UHV of an x-ray photoelectron spectrometer. Iron (Fe 2p) and sulfur (S 2p) oxidation states were measured and S/Fe mole ratios were calculated. All samples were compared to a series of controls which indicated the Fe 2p and S 2p binding energies to be 706.7 eV and 162.4 eV respectively. The reference S/Fe ratio was found to be 3.6. This high ratio is believed to be indicative of a natural weathering rind. Curve fitting indicated the presence of elemental sulfur in small amounts (7%), whereas, most sulfur retained its pyritic form. Throughout the dissolution experiment, both photoemission peaks showed no significant signs of oxidation despite the presence of oxygen at the surface. The S/Fe ratio increased from 3.6 to 5.6 (t = 48 h) after which point the ratio decreased rapidly to its original value. The 10 mL aliquot of solution was partitioned into two sub-aliquots - one for the determination of total iron by atomic absorption, the other for total sulfate by nephelometry, a method which proved itself to be superior to the more conventional turbidimetric method. Solution phase data indicated the relatively constant production rate of iron until 48 hours at which point, there is a noticeable increase in this rate. Sulfate analysis indicated a similar trend to that of iron though the increased rate at 48 hours was dramatic. Based on a correlation of the surface data with that of the solution phase, the following mechanism was proposed. Iron diffuses along a S/Fe mole ratio gradient. Eventually, a critical ratio is achieved at which time the surface begins to crumble. Flakes of elemental sulfur and sulfur-rich pyrite detach from the surface. The elemental sulfur is thought to remain as such based on the findings cited in the literature review. Given the significant rise in the production rate of sulfate, the sulfur-rich pyrite flakes are believed lo convert quickly to sulfate while simultaneously releasing iron into solution.

Thesis/Dissertation

application/pdf

2009-02-06

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http://hdl.handle.net/2429/4196

Soil Science

University of British Columbia

UBCV

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