Characterization of lead on mineral surfaces in soils contaminated by mining and smelting

Abstract

Mine tailings and soils impacted by mining, milling and smelting of Cu-Pb-Zn-Ag-Au ores in a small mining town in central Colorado were analyzed by X-ray photoelectron spectroscopy (XPS), and surface analysis by laser ionization (SALI) utilizing stimulated desorption (pulsed Ar +) coupled with nonresonant photoionization (using 266 nm light) and time-of-flight mass spectrometry. These surface-sensitive techniques revealed that Pb is present on mineral grain surfaces in some soild and mine tailings. The presence of surface-bound Pb correlates with soil pH; carbonate-sulfide tailing with pH≈7 have Pb-bearing iron hydroxide (and to a lesser extent iron sulfate) coatings that are several μm thick, whereas sulfide tailings with pH≈2 have little or no detectable surface-bound Pb. Grains from residential soils (pH≈7) have thin (a few nm thick) Pb-bearing surface layers. Soils impacted by smelter aerosols and fly ash (pH = 4.5) also have thin (a few nm thick) Pb-bearing surfaces layers. Measured Pb4f 7/2 binding energies (corrected for charge shifting by referencing the Si2s peak to 153.0 eV) ranged from 137.4 to 138.2 eV and suggest that the surface-bound Pb is present as Pb-O or Pb-OH species.