An XPS study of the adsorption of lead on goethite (α-FeOOH)

The adsorption behavior of divalent metal ions on amphoteric surface hydroxyl groups of metal oxides is an important factor in the transport of subsurface environmental pollutants. We have studied the adsorption of lead ions from aqueous solution on the surface of the mineral goethite (α-FeOOH) as a function of pH and adsorbate concentration in 0.1 M NaNO3 solution. Results obtained from the dried surface by X-ray photoelectron spectroscopy (XPS) are in good agreement with data from spectrophotometric analysis of lead ions remaining in the supernatant liquid. Lead adsorption as a function of pH shows an adsorption edge near pH 6 and maximizes by pH 7.5. The surface atomic Pb/Fe ratio at saturation as measured by XPS is 0.93 leading to an apparent high adsorption site density of 14.2 sites/nm2. A characteristic low binding energy (BE) XPS feature is present in the Pb 4f XPS signal in the pH range 6–8. Comparison with literature predictions of the speciation of lead at a goethite surface suggests that this XPS feature is perhaps associated with a FeOPb+ complex. At high Pb(II) concentrations some polynuclear complexes probably form. They may be responsible for the high site density value which, while crystallographically possible, is much higher than values reported by others working at lower Pb concentrations.