An ATR-FTIR and EXAFS Study of Mixed Fe-Al (oxy) Hydroxides.
Katya A. Bazilevskaya, Douglas Archibald, and Carmen E. Martínez. Department of Crop and Soil Sciences, The Pennsylvania State University, 404 ASI Bld, University Park, PA 16802
In well-developed soils, iron and aluminum are abundant and can co-precipitate forming metastable colloids which eventually crystallize into more ordered Fe and Al oxides. Knowledge of the structure of these intermediate phases is important with respect to their role as adsorbents of trace elements. In this study we combine two spectroscopic approaches, attenuated total reflectance Fourier transform infrared (ATR-FTIR) and extended x-ray absorption fine structure (EXAFS) spectroscopy, and follow changes in the structure and phase composition of mixed Fe-Al (oxy)hydroxides with time. Mixed Fe-Al colloids were prepared by raising the pH (to 5) of solutions containing iron ([FeIII]=0.01 M) and aluminum at specific concentrations in order to obtain the desired Fe:Al molar ratios. Once the pH reached 5, the colloidal suspensions were dialyzed using 1000 MWCO (~1 nm pore size) membranes. Nanocolloids (particle size 100-300 nm) were found to be stable for up to 6 months. Crystalline Fe and Al minerals (goethite, Al-substituted goethite and gibbsite) were synthesized and characterized for use as reference materials. ATR-FTIR measurements were performed using a Bruke Tenon 27 FTIR instrument with a diamond-Zn-Se ATR sensor. Two µL of the oxide suspension were placed on the detector and dried to form a thin oxide coating before data collection. For EXAFS analyses, suspensions were placed in 3-µm thick 3516 MYLAR film bags and the spectra collected in fluorescence mode at the Fe K-edge using a 13-element Ge-detector. EXAFS analyses were performed on Beamline X18-B at the National Synchrotron Light Source, Brookhaven National Laboratory. Our results show that, in the absence of Al, poorly crystalline ferrihydrite was formed after 6 months, while the formation of gibbsite took place in suspensions containing an Fe:Al molar ratio of 1. A more detailed structural analysis of the Fe-Al mixtures is in progress. Preliminary results show that the spectra of the coatings on the ATR element (for poorly crystalline Fe-Al (oxy)hydroxides, for example) have much higher peak resolution than the spectra obtained for the centrifuged powders using Diffuse Reflectance (DR-FTIR).