MARCASITE OXIDATION IN LOW-TEMPERATURE ACIDIC (PH-3.0) SOLUTIONS - MECHANISM AND RATE LAWS

Marcasite surfaces were analyzed using X-ray photoelectron spectroscop y (XPS) and auger electron spectroscopy (AES). XPS data of a pristine marcasite surface are used as a template to examine the characteristic s of a marcasite surface after exposure to vigorous cleaning procedure s and after reaction in oxygenated and mildly acidic (pH 3.0) solution . Minor changes are observed to the Fe(2p(3/2)) spectrum after cleanin g the surface with concentrated HCl. A new species is observed at appr oximately 709 eV, representing 10-15% of the total Fe spectrum. Chlori de was detected by XPS broadscans and OH- was observed in the O(1s) sp ectrum. The new Fe species at 709 eV may be associated with either OH- or Cl-. XPS sulfur spectrum of the surface exposed to oxygenated, HCl solution (pH = 3.0) indicates that polysulfide increases at the expen se of disulfide. The Fe species observed at 709 eV is also present and represents 10-15% total Fe. XPS broadscan analyses indicate trace amo unts of chloride. Oxide O2- is absent from the O(1s) spectrum but OH- is present. AES depth profiles reveal no compositionally distinct zone s after reaction. Leach rates for the aqueous oxidation of marcasite w ere determined at 25 degrees C in O-saturated chloride solution at pH 3.0. Two rate experiments were performed on crushed and sieved size fr actions of marcasite: one sample was vigorously cleaned to investigate fundamental aspects of marcasite leaching and the other was untreated to simulate conditions found in natural environments. The oxidative l each rate of Fe(aq) from pristine marcasite is 4.25 x 10(-5) mmol/(m(2 ).s). Analyses of aqueous S speciation reveal fluctuations in S conten t of oxidation state lower than SO42-. The XPS results suggest that th e fluctuation may result from periodic release of polysulfide to solut ion, after accumulation on the reactive marcasite surface.