Remediation of aqueous chromium(VI), lead(II), and technetium(VII) by supported zero-valent iron nanoparticles and ion exchange of cesium from complex aqueous streams

Cr(VI), Pb(II) and Tc(VII) were separated and immobilized from aqueous streams by nanoparticles of zero-valent iron mounted on a support ("Ferragels"). Ferragel were synthesized by the borohydride reduction of an aqueous iron salt in the presence of a support. A variety of support materials can be used.; The remediation of aqueous metal ions occurred by redox reaction between the iron and the metal ions. In batch tests, the greater specific surface area for iron provided by dispersion of the nanoparticles across the support surface created rates of reduction 3--5 times greater than iron filings and 12--20 times the total moles of metal ions remediated. The redox reaction between the zero-valent iron and the contaminant metal ions resulted in insoluble mixed oxides deposited on the spent Ferragel. Ferragels were also shown to reduce Tc(VII) from solution to a reduced, insoluble solid. Re-containing Ferragel (where Re was used as a surrogate for Tc) was encapsulated in a borosilicate frit, as a test of the spent Ferragel as a vehicle for final disposal of the recovered Tc. In contrast to the active remediation process, which was apparently controlled by the surface area and amount of iron nanoparticles, passive oxidation of Ferragel appeared to be controlled by products of the borohydride reduction. Passive corrosion was anodically limited, with significant passivation of the surface.; Continuous-flow processes for the separation of aqueous Cs from complex, aqueous mixtures at high pH were proposed. The processes were modeled on the Savannah River Site operation, which used tetraphenylborate ions (TPB) to precipitate 137CS from solution. The proposed process entailed counter-current injection of NaTPB into a bulk stream, followed by immediate cross-current filtration. The recovered slurry was mixed with propylene carbonate (PC) and tripropylamine (TPA) to form a liquid-liquid-liquid extraction. The Cs was recovered as CsNO3(aq), while the tetraphenylborate was stabilized in the PC phase. The two drivers for the extraction of Cs were: the increase in entropy from the disordering of the Na+ hydration sphere when Na+(aq) was exchanged for Cs +(PC); and the base hydrolysis of PC, which formed HCO 3-(aq) and H-TPA +TPB -(PC).