Remediation of Soils and Wastes Contaminated with Uranium and Toxic Metals

The presence of radionuclides and toxic metals in soils and wastes due to nuclear related activities is a major environmental concern. To restore the contaminated sites, both the radionuclides and toxic metals must be removed. We have developed a comprehensive method to remediate contaminated soils and wastes with the removal and recovery of uranium and toxic metals. The overall process consist of three steps: extraction, biodegradation, and photodegradation. Radionuclides and toxic metals are removed from contaminated soils and wastes by extracting them with citric acid, a natural multidentate complexing agent. Citric acid forms different types of complexes with the transition metals and actinides that may involve the formation of bidentate, tridentate, binuclear, or polynuclear complex species. Several metal−citrate complexes were readily biodegraded by Pseudomonas fluorescens, resulting in the bioprecipitation and recovery of the metals, whereas uranyl citrate, which is recalcitrant to biodegradation, upon exposure to light was photodegraded with the precipitation of uranium as UO3·xH2O. For example, uranium was removed from contaminated soils and sludge with >85% efficiency. Biodegradation followed by photodegradation of the citric acid extract resulted in >99% recovery of the extracted uranium. In addition, significant amounts of Al, Ca, Co, Cr, Cu, Mg, Mn, Ni, Pb, Sr, Th, and Zn were also removed during biodegradation of the extract. In this process, the toxic metals and uranium are recovered in separate fractions in concentrated form for recycling or disposal.