AN XPS STUDY OF THE ADSORPTION OF CHROMATE ON GOETHITE (ALPHA-FEOOH)

The adsorption behavior of inorganic oxyanions on soil minerals is an important factor in the transport of subsurface environmental pollutan ts. We have studied the adsorption of chromate (CrO42-) from aqueous s olution on the surface of the mineral goethite (alpha-FeOOH) as a func tion of pH and adsorbate concentration in 0.05 M NaNO3 solution. Resul ts obtained from the dried surface by X-ray photoelectron spectroscopy (XPS) are in good agreement with data from spectrophotometric analysi s 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 red ox reaction, but that the large majority of chromium remains in the 6 oxidation state.