X-ray absorption spectroscopy (XAS) has been applied to numerous problems in soil science, mineralogy, and geochemistry. X-ray absorption spectroscopy was developed in the early 1970s (Sayers et al., 1971) and is widely used at synchrotron radiation facilities. Regardless of the complexity of the sample, the XAS signal comes from all of the atoms of a single element as selected by the X-ray energy. High-quality XAS spectra can be collected on heterogeneous mixtures of gases, liquids, and/or solids with little or no sample pretreatments, making it ideally suited for soils and many other systems. The structural information obtained from XAS is useful for identifying the chemical speciation of an element, including mineral, noncrystalline solid, or adsorbed phases. With the addition of X-ray focusing, samples can be interrogated on length scales comparable to or smaller than some level of natural heterogeneity, thus making it possible to study differences in the atomic environment of an element within or between individual particles and grain boundaries. The acronym XAS covers both X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopies. X-ray absorption near edge structure can be used to determine the valence state and coordination geometry, while EXAFS can be used to determine the local molecular structure of a particular element within a sample. Micrometer-length scale X-ray measurements are designated m-EXAFS and m-XANES. Another variation to the XAS technique utilizes the natural linear polarization of synchrotron X-rays. The use of polarized X-rays allows the atomic environment of the absorber to be probed in the polarization direction and is particularly suited for minerals with layer-type structures such as phyllosilicates and manganese oxides (Manceau et al., 1999). The nature of a complex sample is best revealed by the application of several different experimental techniques, with each individual measurement providing both unique and complementary information. The most common technique for characterizing abundant soil minerals is X-ray diffraction (XRD), which relies on long range ordering of atomic planes to probe crystalline structure at a length scale of approximately 50 Å or more. X-ray absorption spectroscopy probes the immediate environment of the selected element, within about 6 Å, and its theory and interpretation does not rely on any assumption of symmetry or periodicity. While both XRD and XAS can be used to determine distances between atoms, the information is derived from two very different X-ray interactions with the sample. For most systems the application of XRD and XAS is complementary.

Analysis of Soils and Minerals Using X-ray Absorption Spectroscopy

The Hedley sequential-phosphorus (P)-fractionation method has been used in many countries to study the effects of land-use and management systems on soil P. Many data sets have been obtained but collectively never have been considered or to goal topic reviewed. Therefore, the objectives of this review were to compile and systematically evaluate these data. The data generated over many years were grouped into temperate, and subtropical and tropical soils of different land use and duration of soil-management studies. In natural ecosystems, vegetation types and composition and percent of vegetation covers substantially affected all P fractions with pronounced impacts on the labile and moderately labile P. In short-term studies (≤ 10 y), changes in the labile and moderately labile inorganic P (Pi) fractions were detected when more P (e.g., by factor 5) was applied than commonly recommended for agricultural crops. However, without P application the changes in all P fractions were subtle in temperate soils, but declines were significant in labile and moderately labile P in subtropical and tropical soils. In both temperate and tropical climates, medium (10–25 y) and long-term (>25 y) cultivation without P application depleted all P fractions, whereas most of P fractions increased with continuous P application, regardless of the amount and source of P. Synthesis of data resulted in multiple-regression functions which described differences in labile and moderately labile P fractions as function of differences in amount of P application and duration of the experiments. Moreover, the correlation analysis also showed strong association among most of the P fractions. Current limitations in data interpretation of Hedley fractionation can be overcome by the application of 31P nuclear–magnetic resonance (NMR) and X-ray absorption near-edge fine-structure (XANES) spectroscopy.

How does the Hedley sequential phosphorus fractionation reflect impacts of land use and management on soil phosphorus: A review

Phosphorus is an essential nutrient for all life-forms It is a key player in fundamental biochemical reactions involving genetic material (DNA and RNA) and energy transfer (ATP) and in structural support of organisms provided by membranes (phospholipids) and bone (the biomineral hydroxyapatite) Photosynthetic organisms utilize dissolved phosphorus, carbon, and other essential nutrients to build their tissues using energy from the sun Biological productivity is contingent upon the availability of phosphorus to these simple organisms that constitute the base of the food web in both terrestrial and aquatic systems It begins with a brief overview of the various components of the global phosphorus cycle Estimates of the mass of important phosphorus reservoirs, transport rates (fluxes) between reservoirs Following the overview, various aspects of the global phosphorus cycle are examined in more depth, including a discussion of the most pressing research questions currently being posed and research efforts presently underway to address these questions

The Global Phosphorus Cycle

Development of polymer-based nanosized hydrated ferric oxides (HFOs) for enhanced phosphate removal from waste effluents.

Phosphate adsorption on hydroxyl–iron–lanthanum doped activated carbon fiber

Phosphate sorption on Fe- and Al-oxide minerals helps regulate the solubility and mobility of P in the environment. The objective of this study was to characterize phosphate adsorption and precipitation in single and binary systems of Fe- and Al-oxide minerals. Varying concentrations of phosphate were reacted for 42 h in aqueous suspensions containing goethite, ferrihydrite, boehmite, or noncrystalline (non-xl) Al-hydroxide, and in 1:1 (by mass) mixed-mineral suspensions of goethite/boehmite and ferrihydrite/non-xl Al-hydroxide at pH 6 and 22 °C. X-ray absorption near edge structure (XANES) spectroscopy was used to detect precipitated phosphate and distinguish PO4 associated with Fe(III) versus Al(III) in mixed-mineral systems. Changes in the full width at half-maximum height (fwhm) in the white-line peak in P K-XANES spectra provided evidence for precipitation in Al-oxide single-mineral systems, but not in goethite or ferrihydrite systems. Similarly, adsorption isotherms and XANES data showed evidence fo...

XANES Investigation of Phosphate Sorption in Single and Binary Systems of Iron and Aluminum Oxide Minerals

Iron- and Al-(hydr)oxide minerals are important sorbents for retaining PO 4 in soils Our objective was to determine the distribution of adsorbed PO, between ferrihydrite and boehmite in aqueous mixtures of these minerals Phosphate was adsorbed in aqueous suspensions up to maximum concentrations of 1860, 850, and 1420 mmol kg -1 for ferrihydrite, boehmite, and 1:1 (by mass) mixtures of these minerals at pH 6 The solids were analyzed as moist pastes using P K-XANES (X-ray absorption near edge structure) spectroscopy The adsorption isotherm for the mixed-mineral suspensions could essentially be described as a linear combination of Freundlich isotherm models for each single-mineral system, indicating negligible mineral interactive effects on PO 4 adsorption in the mixtures X-ray absorption near edge structure spectra for PO 4 adsorbed on ferrihydrite or in ferrihydrite/ boehmite mixtures showed a pre-edge feature at approximately 2146 eV that was absent in boehmite systems Linear combination fitting of the pre-edge region of XANES spectra for mixtures with average spectra for PO4 adsorbed on boehmite or ferrihydrite alone, indicated that 59 to 97% of the PO 4 was adsorbed on ferrihydrite in the mixtures With increasing concentration of adsorbed PO 4 in the mineral mixtures, the concentration adsorbed on the ferrihydrite component increased linearly Phosphate distribution trends in the mixtures suggested an affinity preference for ferrihydrite at the lowest adsorbed PO 4 concentration (100 mmol kg -1 minerals), no affinity preference for either mineral at intermediate concentrations (200 to 600 mmol PO 4 kg -1 ), and the possibility of a surface precipitate involving Al at the highest concentration (1300 mmol PO 4 kg -1 )

Nidhi Khare

Papers

XANES Investigation of Phosphate Sorption in Single and Binary Systems of Iron and Aluminum Oxide Minerals

XANES Determination of Adsorbed Phosphate Distribution between Ferrihydrite and Boehmite in Mixtures

Phosphate bonding configuration on ferrihydrite based on molecular orbital calculations and XANES fingerprinting