PNNL

Abstract

The thermal dehydroxylation of dried goethite was studied with combined Fourier Transform Infrared (FTIR)-Temperature Programmed Desorption (TPD) experiments in the presence and absence of adsorbed carbonate and oxalate. The TPD data revealed a dehydroxylation peaks involving the intrinsic dehydroxylation of goethite at 575 K but also a low temperature peak at 475 K which was shown to be associated to the release of non-stoichiometric water in the goethite bulk and surface. The FTIR and the TPD data of goethite in the absence of adsorbed species revealed the presence of adventitious carbonate mostly sequestered in the goethite bulk. The release of carbonate was however not only related to the dehydration of goethite but also to the compaction of the hematite phase at temperatures exceeding 700 K. The relative abundance of surface hydroxyls was shown to change systematically upon goethite dehydroxylation with a preferential stripping of ?-OH sites followed by a dramatic change in the dominance of the different surface hydroxyls upon the crystallization of hematite. The presence of surface-bound carbonate revealed similar FTIR bands as those of bulk-carbonate, suggesting similar bonding structures. The release of surface-bound carbonate was moreover shown to be associated to the low temperature dehydration peak and concomitantly with the preferential stripping of ?–OH sites more than 100 K below the dehydroxylation edge of goethite. The presence of chemisorbed oxalate species produced an important release of CO at 425 K, followed by the presence of a transient carbonyl-bearing surface species (e.g. oxalate of different coordination geometry or formate) which were subsequently decomposed to formic acid and CO2 at higher temperatures. The thermal decomposition of oxalate/formate did not, however, reveal additional adsorbed CO2(g) at the goethite surface.