Leslie L. Baker

Molecular characterization of copper in soils using X-ray absorption spectroscopy.

Bioavailability of Cu in the soil is a function of its speciation. In this paper we investigated Cu speciation in six soils using X-ray absorption near edge structure (XANES), extended X-ray absorption fine structure (EXAFS), and synchrotron-based micro X-ray fluorescence (mu-XRF). The XANES and EXAFS spectra in all of the soils were the same. mu-XRF results indicated that the majority of the Cu particles in the soils were not associated with calcium carbonates, Fe oxides, or Cu sulfates. Principal component analysis and target transform of the XANES and EXAFS spectra suggested that Cu adsorbed on humic acid (HA) was an acceptable match. Thus it appears that Cu in all of the soils is primarily associated with soil organic matter (SOM). Theoretical fitting of the molecular structure in the soil EXAFS spectra revealed that the Cu in the soils existed as Cu atoms bound in a bidentate complex to O or N functional groups.

Spectral and hydration properties of allophane and imogolite

Allophane and imogolite are common alteration products of volcanic materials. Natural and synthetic allophanes and imogolites were characterized in the present study in order to clarify the short-range order of these materials and to gain an understanding of their spectral properties. Spectral analyses included visible/near-infrared (VNIR), and infrared (IR) reflectance of particulate samples and thermal-infrared (TIR) emissivity spectra of particulate and pressed pellets. Spectral features were similar but not identical for allophane and imogolite. In the near-infrared (NIR) region, allophane spectra exhibited a doublet near 7265 and 7120 cm−1 (1.38 and 1.40 μm) due to OH2v, a broad band near 5220 cm−1 (1.92 μm) due to H2Ov+δ, and a band near 4560 cm−1 (2.19 μm) due to OHv+δ. Reflectance spectra of imogolite in this region included a doublet near 7295 and 7190 cm−1 (1.37 and 1.39 μm) due to OH2v, a broad band near 5200 cm−1 (1.92 μm) due to H2Ov+δ, and a band near 4565 cm−1 (2.19 μm) due to OHv+δ. A strong broad band was also observed near 3200–3700 cm−1 (~2.8–3.1 μm) which is a composite of OHv, H2Ov, and H2O2δ vibrations. Visible/near-infrared spectra were also collected under two relative humidity (RH) conditions. High-RH conditions resulted in increasing band strength for the H2O combination modes near 6900–6930 cm−1 (1.45 μm) and 5170–5180 cm−1 (1.93 μm) in the allophane and imogolite spectra due to increased abundances of adsorbed H2O molecules. Variation in adsorbed H2O content caused an apparent shift in the bands near 1.4 and 1.9 μm. A doublet H2Oδ vibration was observed at 1600–1670 cm−1 (~6.0–6.2 μm) and a band due to OH bending for O3SiOH was observed at ~1350–1485 cm−1 (~6.7–7.4 μm). The Si-O-Al stretching vibrations occurred near 1030 and 940 cm−1 (~9.7 and 10.6 μm) for allophane and near 1010 and 930 cm−1 (~9.9 and 10.7 μm) for imogolite. OH out-of-plane bending modes occurred near 610 cm−1 (16.4 μm) for allophane and at 595 cm−1 (16.8 μm) for imogolite. Features due to Si-O-Al bending vibrations were observed at 545, 420, and 335 cm−1 (~18, 24, and 30 μm) for allophane and at 495, 415, and 335 cm−1 (~20, 24, and 30 μm) for imogolite. The emissivity spectra were obtained from pressed pellets of the samples, which greatly enhanced the spectral contrast of the TIR absorptions. Predicted NIR bands were calculated from the mid-IR fundamental stretching and bending vibrations and compared with the measured NIR values. Controlled-RH X-ray diffraction (XRD) experiments were also performed in order to investigate changes in the mineral structure with changing RH conditions. Both allophane and imogolite exhibited decreasing low-angle XRD intensity with increasing RH, which was probably a result of interactions between H2O molecules and the curved allophane and imogolite structures.

Metal content of charcoal in mining-impacted wetland sediments

Charcoal is well known to accumulate contaminants, but its association with metals and other toxic elements in natural settings has not been well studied. Association of contaminants with charcoal in soil and sediment may affect their mobility, bioavailability, and fate in the environment. In this paper, natural wildfire charcoal samples collected from a wetland site that has been heavily contaminated by mine waste were analyzed for elemental contents and compared to the surrounding soil. Results showed that the charcoal particles were enriched over the host soils by factors of two to 40 times in all contaminant elements analyzed. Principal component analysis was carried out on the data to determine whether element enrichment patterns in the soil profile charcoal are related to those in the soils. The results suggest that manganese and zinc concentrations in charcoal are controlled by geochemical processes in the surrounding soil, whereas the concentrations of arsenic, lead, zinc, iron, phosphorus, and sulfur in charcoal are unrelated to those in the surrounding soil. This study shows evidence that charcoal in soils can have a distinct and important role in controlling contaminant speciation and fate in the environment.

Chemical Extractability of Lead in Field-Contaminated Soils: Implications for Estimating Total Lead

Lead (Pb) is frequently present in urban soils at concentrations of concern for human health. Regulations for Pb are based on total soil concentrations as determined by acid digestion, but a less expensive screening test for Pb would be useful in facilitating more thorough soil testing of urban areas if it could be shown to correlate strongly to total soil Pb. In this study, three extractants (0.1 M citrate, MM, and 1 M nitric acid) were evaluated for their ability to estimate the total Pb in contaminated soils. Nitric acid not only extracted a greater fraction of total soil Pb but also produced the strongest correlation to total Pb and is concluded to be the superior extractant for a soil Pb screening test. As the spatial distribution of Pb was observed in selected soils to be highly heterogeneous on the micron scale, thorough soil homogenization prior to testing is recommended.

XAS STUDY OF Fe MINERALOGY IN A CHRONOSEQUENCE OF SOIL CLAYS FORMED IN BASALTIC CINDERS

The characterization of poorly crystalline minerals formed by weathering is difficult using conventional techniques. The objective of this study was to use cutting-edge spectroscopic techniques to characterize secondary Fe mineralogy in young soils formed in basaltic cinders in a cool, arid environment. The mineralogy of a chronosequence of soils formed on 2, 6, and 15 thousand year old basaltic cinders at Craters of the Moon National Monument (COM) was examined using synchrotron-based X-ray absorption fine structure (XAFS) spectroscopy in combination with selective extractions. Fe K-edge XAFS is useful for determining speciation in poorly crystalline materials such as young weathering products. Over 86% of Fe in the soil clay fractions was contained in poorly crystalline materials, mostly in the form of ferrihydrite, with the remainder in a poorly crystalline Fe-bearing smectite. The XAFS spectra suggest that ferrihydrite in the 15 ka soil clay is more resistant to ammonium oxalate (AOD) extraction than is ferrihydrite in the younger materials. Fe in the poorly crystalline smectite is subject to dissolution during citrate-bicarbonate- dithionite (CBD) extraction. The results indicate that relatively few mineralogical changes occur in these soils within the millennial time frame and under the environmental conditions associated with this study. Although the secondary mineral suite remains similar in the soils of different ages, ferrihydrite crystallinity appears to increase with increasing soil age.

Fe K-edge XAFS spectra of phyllosilicates of varying crystallinity

Fe K-edge X-ray absorption fine structure (XAFS) spectroscopy was used to examine local molecular structure and cation distribution in the natural nanosilicate hisingerite and synthetic nanoaluminosilicate allophane. Fitting procedures for XAFS spectra were established on well-characterized clay mineral standards for application to the poorly crystalline nanosilicate materials. Nanospherical models for these materials were tested against clay-like structures. From XAFS interpretations, the following structural information was concluded: (1) Fe in both hisingerite and allophane is present in well-defined octahedral sites, (2) less long-range structure is observed in hisingerite and allophane than in smectites, and (3) Fe in allophane is present in small clusters in the octahedral sheet. The best fit for hisingerite was obtained using a 1:1 model structure rather than a 2:1 model, supporting a description of it as a ferric halloysite-type structure. Allophane could not be fit using paths based on Fe-substituted proto-imogolite nanospherical models but was successfully fit using a montmorillonite structure. Published models of nanospherical particles suggest backscattering path lengths in allophane and hisingerite should be shortened compared to clay minerals; however, no such shortening was observed, suggesting the nanosphere model does not accurately describe the local atomic structure of these Fe-substituted minerals.

TEMPERATURE EFFECTS ON THE CRYSTALLINITY OF SYNTHETIC NONTRONITE AND IMPLICATIONS FOR NONTRONITE FORMATION IN COLUMBIA RIVER BASALTS

The formation conditions of the ferric smectite nontronite are not fully understood. The present study couples experimental and analytical data with field observations in an attempt to constrain the rate and temperature of formation of naturally occurring nontronites from Columbia River Basalt flows. Synthetic Fe-Al-Si gels were incubated at temperatures ranging from 4 to 150°C for 4 weeks. Samples were analyzed using Fe K-edge X-ray fluorescence spectroscopy (XAFS). Spectra of the synthesized nontronites were compared with spectra of natural samples collected from weathered Columbia River Basalt flows. Cation ordering in the synthetic samples increased with incubation temperature, but the synthetic clays did not approach the degree of crystal ordering of the natural nontronite samples. These observations suggest that highly ordered natural nontronites require longer crystallization times than are typically used in laboratory experiments. The natural samples were found filling open cracks near flow surfaces, indicating that the clays formed at temperatures below the boiling point of water. A comparison of experimental and field timescales with other estimates of nontronite growth rates suggests that natural nontronite crystallization in the region must have occurred at ambient, near-surface temperatures over timescales of up to millions of years.

Biochar Soil Amendment Effects on Arsenic Availability to Mountain Brome (Bromus marginatus)

Biochar is a renewable energy byproduct that shows promise for remediating contaminated mine sites. A common contaminant at mine sites is arsenic (As). In this study, the effects of biochar amendments to a mine-contaminated soil on As concentrations in mountain brome ( Nees ex Steud.) were investigated. In the biochar-amended soil, mountain brome had greater root biomass and decreased root and shoot As concentrations. X-ray absorption near-edge structure spectroscopy results showed that arsenate [As(V)] is the predominant species in both the nonamended and biochar-amended soils. Soil extraction tests that measure phosphate and arsenate availability to plants failed to accurately predict plant tissue As concentrations, suggesting the arsenate bioavailability behavior in the soils is distinct from phosphate. Results from this study indicate that biochar will be a beneficial amendment to As-contaminated mine sites for remediation.

XAFS STUDY OF Fe-SUBSTITUTED ALLOPHANE AND IMOGOLITE

The nano-aluminosilicate mineral allophane is common in soils formed from parent materials containing volcanic ash and often contains Fe. Due to its lack of long-range order, the structure of allophane is still not completely understood. In the present study, Fe K-edge X-ray absorption fine structure (XAFS) was used to examine Fe-containing natural and synthetic allophane and imogolite samples. Results indicated that Fe substitutes for octahedrally coordinated Al in allophane, and that Fe exhibits a clustered distribution within the octahedral sheet. Iron adsorbed on allophane surfaces is characterized by spectral features distinct from those of isomorphically substituted Fe and of ferrihydrite. Fe adsorbed on the allophane surfaces probably exists as small polynuclear complexes exhibiting Fe-Fe edge sharing, similar to poorly crystalline Fe oxyhydroxides. The XAFS spectra of natural allophane and imogolite indicate that the Fe in the minerals is a combination of isomorphically substituted and surface-adsorbed Fe. In the synthetic Fe-substituted allophanes, the Fe XAFS spectra did not vary with the Al:Si ratio. Theoretical fits of the extended XAFS (EXAFS) spectra suggest that local atomic structure around octahedral Fe in allophanes is more similar to Fe in a smectite-like structure than to a published theoretical nanoball structure.

Celadonite in continental flood basalts of the Columbia River Basalt Group

Abstract Celadonite is a common alteration product of basalts in marine environments. It has been argued that marine fluids are necessary for celadonite formation, possibly by providing a source of K and other dissolved cations. Laterally extensive deposits of celadonite occur in basalts of the Grande Ronde Basalt of the Columbia River Basalt Group. The celadonite is found in scoriaceous flow tops of layered basalt flows, where it fills vesicles and replaces the surrounding groundmass. Evolved interstitial glasses are present in the basalts and dissolution of these glasses may provide sufficient K for celadonite formation, whereas dissolution of groundmass augite provides a source of Mg and Fe. These observations show that alteration by seawater or any other external source of dissolved ions is not necessarily required for celadonite formation.