title: Effects of sulfate ligand on uranyl carbonato surface species on ferrihydrite surfaces.
abstract: Understanding uranium (U) sorption processes in permeable reactive barriers (PRB) are critical in modeling reactive transport for evaluating PRB performance at the Fry Canyon demonstration site in Utah, USA. To gain insight into the U sequestration mechanism in the amorphous ferric oxyhydroxide (AFO)-coated gravel PRB, U(VI) sorption processes on ferrihydrite surfaces were studied in 0.01 M Na(2)SO(4) solutions to simulate the major chemical composition of U-contaminated groundwater (i.e., [SO(4)(2-)] ~13 mM L(-1)) at the site. Uranyl sorption was greater at pH 7.5 than that at pH 4 in both air- and 2% pCO(2)-equilibrated systems. While there were negligible effects of sulfate ligands on the pH-dependent U(VI) sorption (<24 h) in both systems, X-ray absorption spectroscopy (XAS) analysis showed sulfate ligand associated U(VI) surface species at the ferrihydrite-water interface. In air-equilibrated systems, binary and mono-sulfate U(VI) ternary surface species co-existed at pH 5.43. At pH 6.55-7.83, a mixture of mono-sulfate and bis-carbonato U(VI) ternary surface species became more important. At 2% pCO(2), there was no contribution of sulfate ligands on the U(VI) ternary surface species. Instead, a mixture of bis-carbonato inner-sphere (38%) and tris-carbonato outer-sphere U(VI) ternary surface species (62%) was found at pH 7.62. The study suggests that the competitive ligand (bicarbonate and sulfate) coordination on U(VI) surface species might be important in evaluating the U solid-state speciation in the AFO PRB at the study site where pCO(2) fluctuates between 1 and 2 pCO(2)%.
authors: School of Agricultural, Forest and Environmental Sciences, 270 Poole Agricultural Center, Clemson University, Clemson, SC 29634-0315, USA. yarai@clemson.edu
authors: Yuji
authors: Y
authors: Arai
authors: C C
authors: CC
authors: Fuller
mesh_terms: D005290:Ferric Compounds / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D008024:Ligands; D013431:Sulfates / Q000737:chemistry*; D014501:Uranium / Q000737:chemistry*; D017974:Uranium Compounds / Q000737:chemistry*; D014589:Utah; D014875:Water Pollutants, Radioactive / Q000737:chemistry*; D018508:Water Purification / Q000379:methods*
chemical_list: D005290:Ferric Compounds; D008024:Ligands; D013431:Sulfates; D017974:Uranium Compounds; D014875:Water Pollutants, Radioactive; D014501:Uranium; C092844:ferric oxyhydroxide