Chromate Adsorption on Goethite: Effects of Aluminum Substitution

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Abstract
The adsorption of CrO2‐4 to goethites varying in specific surface area and Al substitution was measured over a range in pH, sorbate and sorbent concentrations and ionic strength. The α‐(Fe, Al)OOH exhibited a lower pH50 (the pH at which 50% adsorption occurs) for the CrO2‐4 at all adsorbate concentrations (6 × 10−6 to 5 × 10−4 M CrO2‐4) than did the two α‐FeOOH samples; this difference could not be attributed to either variations in CrO2‐4 adsorption density or sorbent PZC. The overall proton coefficient (n), calculated from adsorption data for 5 × 10−7 M CrO2‐4, was identical for the two α‐FeOOH samples despite differences in the physical nature and sorbate‐to‐surface site ratio of the two samples. The observed n for the α‐ (Fe, Al)OOH system was larger than the n for the pure goethite systems. These data suggest that differences in the acid‐base properties and interaction with the background electrolyte for the Fe and Al sites affect the overall proton coefficient and CrO2‐4 adsorption on the substituted surface when compared to the pure surface. The surface coordination constants for the Triple Layer Model (TLM) were derived for CrO2‐4 adsorption on the α‐FeOOH solids using the FITEQL computer code. Using these and other TLM constants from the literature, adsorption on all three goethite samples was quantitatively simulated over a range in solute and sorbent concentrations, pH and ionic strength. The successful simulation of CrO2‐4 sorption onto α‐(Fe, Al)OOH required the use of a two‐site model which included the ionization and electrolyte complexation reactions of both Fe and Al sites. Site densities of Fe and Al were taken to be proportional to the mole fraction of each in the α‐(Fe, Al)OOH solid. However, the interaction of the CrO2‐4 species with the Al sites was not required to quantitatively describe the adsorption edge since the log K for the dominant FeOH+2‐HCrO‐4 species is three orders of magnitude greater than the ‐CrO2‐4 or ‐HCrO‐4 on Al sites.