No abstract

The molecular structure of ions retained on mineral surfaces is needed to accurately model their sorption process and to determine their stability. Extended X-ray absorption fine structure (EXAFS) spectroscopy was used in this study to deduce the local coordination environment of two environmental contaminants, arsenate and chromate, on the mineral goethite (R-FeOOH). Based on the oxyanion-Fe distances, it was concluded that three different surface complexes exist on goethite for both oxyanions: a monodentate complex, a bidentate-binuclear complex, and a bidentate-mononuclear complex. At low surface coverages, the monodentate complex was favored while at higher coverages the bidentate complexes were more prevalentsthe bidentate-binuclear complex appears to be in the greatest proportion at these highest surface coverages. Therefore, modeling efforts for chromate or arsenate retention on goethite need to consider a monodentate complex at very low coverages, both the monodentate and bidentate complexes at intermediate coverages, and predominantly the bidentate complexes at very high coverages.

Minerals are more complex than previously thought because of the discovery that their chemical properties vary as a function of particle size when smaller, in at least one dimension, than a few nanometers, to perhaps as much as several tens of nanometers. These variations are most likely due, at least in part, to differences in surface and near-surface atomic structure, as well as crystal shape and surface topography as a function of size in this smallest of size regimes. It has now been established that these variations may make a difference in important geochemical and biogeochemical reactions and kinetics. This recognition is broadening and enriching our view of how minerals influence the hydrosphere, pedosphere, biosphere, and atmosphere.

No abstract

Global soil resources under stress The future of humanity is intertwined with the future of Earth's soil resources. Soil provides for agriculture, improves water quality, and buffers greenhouse gases in the atmosphere. Yet human activities, including agricultural soil erosion, are rapidly degrading soil faster than it is naturally replenished. At this rate, human security over the next century will be severely threatened by unsustainable soil management practices. Amundson et al. review recent advances in understanding global soil resources, including how carbon stored in soil responds to anthropogenic warming. Translating this knowledge into practice is the biggest challenge remaining. Science , this issue 10.1126/science.1261071

The association of organic matter (OM) with minerals is recognized as the most important stabilization mechanism for soil organic matter. This study compared the properties of Fe-OM complexes formed from adsorption (reaction of OM to postsynthesis ferrihydrite) versus coprecipitation (formation of Fe solids in the presence of OM). Coprecipitates and adsorption complexes were synthesized using dissolved organic matter (DOM) extracts from a forest little layer at varying molar C/Fe ratios of 0.3-25.0. Sample properties were studied by N2 gas adsorption, XRD, FTIR, Fe EXAFS, and STXM-NEXAFS techniques. Coprecipitation resulted in much higher maximum C contents (∼130 mg g(-1) C difference) in the solid products than adsorption, which may be related to the formation of precipitated insoluble Fe(III)-organic complexes at high C/Fe ratios in the coprecipitates as revealed by Fe EXAFS analysis. Coprecipitation led to a complete blockage of mineral surface sites and pores with ≥177 mg g(-1) C and molar C/Fe ratios ≥2.8 in the solid products. FTIR and STXM-NEXAFS showed that the coprecipitated OM was similar in composition to the adsorbed OM. An enrichment of aromatic C was observed at low C/Fe ratios. Association of carboxyl functional groups with Fe was shown with FTIR and STXM-NEXAFS analysis. STXM-NEXAFS analysis showed a continuous C distribution on minerals. Desorption of the coprecipitated OM was less than that of the adsorbed OM at comparable C/Fe ratios. These results are helpful to understand C and Fe cycling in the natural environments with periodically fluctuating redox conditions, where coprecipitation can occur.

No abstract

No abstract