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

The adsorption behavior of inorganic oxyanions on soil minerals is an important factor in the transport of subsurface environmental pollutants. We have studied the adsorption of chromate (CrO42−) from aqueous solution on the surface of the mineral goethite (α-FeOOH) as a function of pH and adsorbate concentration in 0.05 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 chromate remaining in the supernatant liquid. Chromate adsorption increases with decreasing pH of the solution and eventually reaches a maximum at pH 6.5. The chromium XPS signal indicates that initially a small amount of chromium adsorbs in the +3 oxidation state via a redox reaction, but that the large majority of chromium remains in the +6 oxidation state.