Research Conducted by the University of Delaware Environmental Soil Chemistry Group

Workshop on Shell Fitting

This workshop is designed as an introduction to EXAFS shell fitting for environmental samples. It was put on by post doctoral scholar Matt Siebecker. The talk is geared towards people new to shell fitting of EXAFS data.

Part 1 slides
Part 1 video
Part 2 slides
Part 2 video
Data files

Group Research Foci

Metal(loid) Reactivity and Speciation in Soils and Natural Systems

Phosphorus Biogeochemistry in Soils

Sea Level Rise

Mineral Complexation and Metal Redox Coupling Impacts on Soil Carbon Sequestration

Metal(loid) Reactivity and Speciation in Soils and Natural Systems

The Environmental Soil Chemistry Group conducts research on metal associations and reactions with soil constituents, such as minerals and organic materials. Investigations of these relationships help to determine not only in what forms or species the metal is present, but also the chemical process of phase formation and the length of the reaction time for phase development. Soils that contain the metals of interest, either occurring naturally or by contamination, are examined as well as model systems in which metals are reacted with a limited number of soil constituents. This research is important in understanding, modeling, and predicting the fate of metals in environmental systems and is essential for the development of remediation strategies in situations where contaminants are present.

Figure 1

Figure 1: a) Elemental distributions by µ-XRF mapping of Cd, Zn, Ca, and Fe in a contaminated soil under different conditions; and b) Cd—K-edge XANES spectra of Cd standards and of the contaminated soil (from Khaokaew et al. Environ. Sci. Technol. 2011, 45, 4249–4255). From the µ-XRF maps, spatial relationships between the various metals in the soil can be observed, and the XANES spectra will determine the species of Cd present.

Phosphorus Biogeochemistry in Soils

P biogeochemistryPhosphorus (P) is an essential plant nutrient that is also a common pollutant in aquatic systems such as the Chesapeake Bay and the Delaware Inland Bays’ watershed. Eutrophication caused by increased levels of phosphorus can lead to decreased dissolved oxygen levels and excessive growth of Pfiesteria, which has been linked to fish kills as and human toxicity. The dense poultry industry of southern Delaware produces approximately 300,000 ton of broiler litter annually. Water sources in proximity to southern Delaware’s poultry industry are particularly susceptible to phosphorus pollution due to application of broiler litter as fertilizer to agricultural soils.
A fundamental understanding of soil phosphorus speciation is vital in order to improve forecasting for phosphorus retention and transport in soils. Although the chemistry of phosphorus has been extensively studied, specific mechanisms for phosphorus adsorption to soils remain largely unknown. Research in the Environmental Soil Chemistry group at University of Delaware includes 1) use of spectroscopic techniques (FTIR, 31P NMR and XAS) to elucidate mechanisms of phosphorus bonding during phosphate sorption on soil minerals, 2) in-situ speciation of P in soils and agricultural bio-solids, and 3) biogeochemical cycling among P, Si, and Fe in Delaware farmland soils. A recent project in our group characterized the P speciation of Chesapeake Bay sediments to understand the contribution of farmland released phosphorous to bay eutrophication. We are also involved in supporting the development of a novel synchrotron tender X-ray microprobe at the updated National Synchrotron Light Source (NSLS-II) to evaluate the speciation and reactivity of phosphorus in soils. Understanding how phosphorus is retained by soils will facilitate the identification of management practices to mitigate further loss of phosphorus to the environment.

P biocheochemisrty

Sea Level Rise

The United States Geological Survey has determined the Mid-Atlantic region of the country is experiencing the fastest rate of sea level rise in the world. Delaware offers a unique opportunity to study the impacts of sea level rise on contaminated coastal sites, because of its large coastline and industrial legacy. The majority of these contaminated areas are clustered near areas of dense population due people's natural tendency to live near the water. The large impact on the environment and human health associated with this issue make it critical to understand the fate of trace metals in soil systems vulnerable to salt water intrusion.
Although recent studies have raised alarm, there has been little research to investigate the threat of seawater induced contaminant mobility. The groups' research will provide fundamental knowledge of these processes in order to better understand the chemistry behind this threat. We theorize salinization and rising water tables are the two primary processes that could lead to release and migration of sediment-bound contaminants in response to sea level rise. For this study, soils heavily contaminated with arsenic (As) and chromium (Cr) from waterfront sites have been selected. Arsenic and chromium are two common soil contaminants that will be examined because of their redox sensitivity and their toxicity's dependence on redox state. The toxicity of redox sensitive elements, such as As and Cr, depends upon oxidation states and associations with sorbed and mineral phases. Determining these associations in heterogeneous sediments at the solid-phase molecular scale will be achieved with micro-X-ray Absorption Spectroscopy (μ-XAS) and μ-XRD. Utilizing these synchrotron-based methods in addition to aqueous phase chemical methods, will allow us to determine how a changing climate will impact existing contaminants in soils of areas susceptible to sea level rise.
Furthermore, once the contaminants are released they may be taken up by nearby wetland flora. For this reason, the interaction between contaminant mobility and marsh plants is being investigated. The native and densely rooted Spartina alterniflora is being compared to invasive exotic and thick rooted Phragmites australis. The roots of these plants may play a critical role in capturing heavy metals released by influxes of seawater through direct absorption into root tissue or by sorption of the metals to iron plaques formed around the roots. Potential contaminant mobility is being determined with μ-XAS, μ-XRD, and Inductively Coupled Mass Spectroscopy (ICP-MS). The findings of these studies will elucidate how the shift in plant communities from native Spartina to non-native Phragmites will impact a marsh ecosystem’s ability to sequester heavy metal contaminants during storm surge events and with increasing sea level rise.

Mineral Complexation and Metal Redox Coupling Impacts on Soil Carbon Sequestration

Carbon (C) sequestration in soil systems has been recognized as one of the potential measures through which greenhouse gas emissions can be mitigated. Mineral complexation and metal redox coupling are important geochemical processes impacting carbon cycling in terrestrial systems. Microbe-mediated biological processes also greatly influence the molecular structure of soil organic matter (SOM) and complexation with mineral surfaces. Our understanding of the chemical and biological processes for SOM sequestration by soil minerals is still largely unknown. Our research group combines both field and lab studies to investigate the complexation mechanisms of SOM by minerals, and the impacts of metal redox coupling and microbial processes, using a multi-scale approach. We use synchrotron-based C K-edge NEXAFS (XANES) spectroscopy and nano-spatially resolved STXM techniques. One of the goals of our research is to provide a theoretical basis for better understanding SOM stability and carbon sequestration in soils.
Figure 1
Fig. 1 Submicron and nano-scale characterization of different C species in organo-mineral microaggregates using STXM coupled with C K-edge NEXAFS spectroscopy and PCA-Cluster analysis (Spatial resolution, 30 nm; STXM data collected at the Canadian Light Source)

Figure 2

Fig. 2 Submicron and nano-scale correlation of elements (C, Ca, Fe, Al and Si) in organo-mineral aggregates using STXM coupled with C K-edge NEXAFS spectroscopy
 
               

Home | Members | News | Links | Research | Photos | Awards | Alumni | Publications