In-Situ U Stabilization by Microbial Metabolites; Sequestration of U by Melanin and its Sorption to Minerals.
Anna S. Knox and Charles Turick. Savannah River National Laboratory, Savannah River Site, Aiken, SC 29808
This study is based on pyomelanin-producing bacteria isolated from soils at Savannah River Site (SRS), Aiken, SC. Pyomelanin is a microbial pigment and a humic type compound in the class of melanin pigments. Pyomelanin has electron shuttling capabilities and thus accelerates the bacterial reduction of soluble and insoluble metals oxides. Pyomelanin is produced outside the cell and either diffuses away or attaches to the cell surface. In either case, the reduced pyomelanin is capable of transferring electrons to metals. Hence, soil amendments that stimulate pyomelanin production are expected to contribute to metal reduction and/or sequestration in-situ. We have demonstrated pyomelanin production in soil from the Tims Branch area of SRS as a result of tyrosine amendments. These results were documented in laboratory soil column studies and field deployed lysimeters. The amended soils demonstrated increased redox behavior and sequestration capacity of U following pyomelanin production. Treatments with tyrosine and lactate demonstrated the highest levels of pyomelanin production. Decreased soil permeability was associated with lactate additions and may be a result of colloid formation. The presented study quantifies melanin sequestration of uranium and the sorption behavior of melanin-uranium complexes to predominant soil minerals. Sequestration of U by melanin produced by model dissimilatory metal reducing bacteria was evaluated as a function of pH and melanin mass. Total U concentrations was determined with ICP-MS and U(VI) by pulsed-laser phosphorimetry. The U concentration data was used to calculate distribution coefficients (Kd values), defined as the ratio of the concentration of solute sorbed to the solid divided by its concentration in solution. Non-electrostatic (Kd, Freundlich or Langmuir) models were evaluated and a simple model was identified to fully describe the data. The main intent of conducting desorption experiments of sorbed U by melanin or a mixture of melanin and minerals was to determine how strongly U is bound to the melanin or melanin mixed with minerals. Uranium immobilization evaluated by various short term and long term geochemical approaches provided a critical indication of the effectiveness of this method. Effective short-term measures of performance included measurements of U in pore water as a time function (3 to 8 weeks) and sorption/desorption experiments. Geochemical measurements (sequential extractions, mineralogical analysis, etc) were performed to support long-term performance evaluation. Long term performance was assessed by determining precipitated or co-precipitated mineral phases and identifying, where possible, natural analogs.