An Investigation into Selenium Geochemistry in Phosphate Mine Soils

In the western United States, elevated selenium (Se) levels in soil have resulted in documented cases of ruminant fatalities. This is due to the ingestion of Se-hyperaccumulating vegetation growing on previously reclaimed phosphate mine soils. A field-scale analysis was first conducted to examine Se bioavailability to plants. Soil and plant samples were collected from transects from five study locations in Soda Springs, Idaho. Soils were analyzed for Se speciation and geochemical phases using a sequential extraction procedure (SEP). Additionally, speciation, SEP results, and Se bioavailability in the hyperaccumulator, western aster (Symphyotrichum ascendens (Lindl.)), were related using simple linear regression. Soil speciation and the validity of this SEP were then evaluated using synchrotron-sourced X-ray absorption fine structure (XAFS) spectroscopy for both whole and a sequence of extracted soils. Lastly, competitive adsorption of Se with two dissolved organic carbon (DOC) species, citric and salicylic acid, was examined on an amorphous iron oxide mineral surface. A strong relationship was identified for western aster Se and the first two combined SEP fractions, water-soluble and PO43--extractable Se (R2 = 0.85; P = <0.0001). Results also indicated a strong relationship between selenate and water-soluble Se (R2 = 0.83; P = 0.0002). This suggests that water extracts could be useful Se bioavailability assessment tools in highly contaminated systems. XAFS analyses indicated that elemental and organic Se were the most predominant phases overall in whole soils. The dominant oxidized species present was selenite sorbed onto iron oxides and calcite. Critical SEP evaluations using XAFS also indicate that oxidized Se species were underestimated by the SEP and elemental Se was overestimated. In extracted soils, XAFS results indicated partial recovery of carbonate, iron oxide and organic Se occurred. Therefore, it is suggested that researchers exert caution when employing SEPs. Additionally, sorption analyses demonstrated the highly competitive behavior of citric acid with both selenite (pH 5-8) and selenate (pH 5-6). Little competition was observed in the presence of salicylic acid for both Se species. Competition and subsequent desorption of both sorbed species in the presence of citric acid suggest a possible mechanism for Se solubilization and bioavailability in seleniferous environments.

Name thesis:degree_name

Ph. D.

Level thesis:degree_level

doctoral

Discipline thesis:degree_discipline

Crop and Soil Environmental Sciences

Department dc:contributor.department

Crop and Soil Environmental Sciences

Grantor dc:publisher

Virginia Tech

Year dc:date.issued

2017

Author dc:creator

• Favorito, Jessica Elizabeth

Chairs dc:contributor.committeechair

• Eick, Matthew J.
• Grossl, Paul R.

Committee members dc:contributor.committeemember

• Daniels, W. Lee
• Xia, Kang

• Adsorption
• Bioavailability
• Dissolved Organic Carbon
• Phosphate Mine Soils
• Selenium
• Sequential Extraction Procedure
• XAFS

Statement dc:rights

• In Copyright

Licence dc:rights.uri

• http://rightsstatements.org/vocab/InC/1.0/

Dc Identifier Other

vt_gsexam:11039

OAI identifier oai:identifier

oai:vtechworks.lib.vt.edu:10919/77945