Chromate Adsorption on Goethite: Effects of Aluminum Substitution

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.

Contrasting effects of Al substitution on microbial reduction of Fe(III) (hydr)oxides

“…Aluminum co-precipitation in ferrihydrite has been observed in nature and has been synthesized in the lab (Cornell and Schwertmann, 2003) containing up to 20 mol% Al without forming separate Al phases (Masue et al, 2007). Aluminum substitution can impact the properties of Fe(III) (hydr)oxides, including changes in unit-cell edge length and volume of the Fe(III) (hydr)oxide (Cornell and Schwertmann, 2003), average crystallite size, particle morphology, surface area, solubility (Trolard and Tardy, 1987), surface chemistry (Ainsworth et al, 1989), and rates of acid and reductive dissolution (Torrent et al, 1987;Schwertmann, 1991).…”

Fast and Slow Phosphate Sorption by Goethite-Rich Natural Materials

“…The fact that, as for synthetic samples, TEM observations show that natural goethites are also bound by (110) faces (Smith and Eggleton, 1983;Amouric et al, 1986) supports this unifying principle. An alternative hypothesis to explain the constancy of P-sorption capacity in natural crystals (Ainsworth et aL, 1989) is that, when the SA is above a threshold value, the goethite crystals can be described as spheres or cubes rather than needles of various morphologies. Then, all the major crystal faces contribute to adsorption, resulting in an average site density that does not change with further decrease in crystal size.…”

XANES studies at the Al K-edge of aluminium-rich surface phases in the soil environment

“…For example, Al(llI)-rich surface phases on soil particles can dramatically affect the way these particles interact with heavy metals and organic compounds in aqueous solutions relative to the pristine soil particle surface because the Al(Ill)-rich surface phase may have a very different reactivity relative to the substrate. Similarly, the presence of AI(III) substituted for Fe(III) in iron oxyhydroxides can have a major effect on the uptake of aqueous metal ions (Ainsworth et al, 1985;Ainsworth et al, 1989;Cornell and Schwertmann, 1996). In both cases, very little is known at an atomistic level about the reason for these effects.…”