TY - JOUR
T1 - A self-consistent model describing the thermodynamics of Eu(III) adsorption onto hematite
AU - Estes, Shanna L.
AU - Arai, Yuji
AU - Becker, Udo
AU - Fernando, Sandra
AU - Yuan, Ke
AU - Ewing, Rodney C.
AU - Zhang, Jiaming
AU - Shibata, Tomohiro
AU - Powell, Brian A.
N1 - Funding Information:
The authors thank two anonymous reviewers and GCA associate editor Dan Giammar for their valuable comments which helped advance the quality of this manuscript. The authors also thank L. Rao, G. Tian, and coworkers for TRLFS analyses and helpful conversations. This work was supported by the Subsurface Biogeochemical Research Program of the U.S. Department of Energy’s Office of Biological and Environmental Research under project number DE-SC0004883. MRCAT operations are supported by the US Department of Energy and the MRCAT member institutions.
PY - 2013/12/1
Y1 - 2013/12/1
N2 - The environmental fate of actinides is greatly influenced by interfacial reactions, including adsorption onto solid surfaces where the adsorption of trivalent and tetravalent actinides is generally a very strong and potentially irreversible reaction. Changes in the primary hydration sphere of the actinide during inner-sphere adsorption could greatly influence the thermodynamics of these reactions. However, few researchers have studied actinide adsorption thermodynamics. Therefore, using Eu(III) as an analog for trivalent actinides, we examined the thermodynamics of Eu(III) adsorption onto hematite, with particular emphasis on changes in the Eu(III) coordination number and the influence of temperature upon sorption. Our working hypothesis was that a decrease in hydration number upon adsorption, as indicated by a decrease in coordination number and an increase in adsorption with increasing temperature, results in energetically favorable sorption reactions, which are driven by a large, positive entropy term. To perform these studies, we applied the diffuse layer model to describe Eu(III) adsorption onto hematite at pH values ranging from ~3 to 7 and at 15, 25, 35, and 50°C. Additionally, we characterized the Eu(III)-hematite surface complex and changes in the Eu(III) primary hydration sphere using extended X-ray absorption fine structure spectroscopy (EXAFS) and computational modeling. High-resolution transmission electron microscopy (HRTEM) was used to identify possible europium surface precipitates or morphological changes in the hematite. The data indicate that the adsorption reaction (1) is endothermic, (2) proceeds with a decrease in the Eu(III) coordination number, and (3) results in the formation of a bidentate mononuclear surface complex, (FeO)2Eu+. The enthalpy and entropy values for the formation of this surface complex, which were estimated using a van't Hoff plot, were 131±8 kJmol-1 and 439±26JK-1mol-1, respectively, indicating that adsorption of Eu(III) onto hematite is entropically driven. Additionally, we suggest that the decrease in Eu(III) coordination number and the large entropy term are due to the loss of coordinating water molecules from the Eu(III) hydration sphere.
AB - The environmental fate of actinides is greatly influenced by interfacial reactions, including adsorption onto solid surfaces where the adsorption of trivalent and tetravalent actinides is generally a very strong and potentially irreversible reaction. Changes in the primary hydration sphere of the actinide during inner-sphere adsorption could greatly influence the thermodynamics of these reactions. However, few researchers have studied actinide adsorption thermodynamics. Therefore, using Eu(III) as an analog for trivalent actinides, we examined the thermodynamics of Eu(III) adsorption onto hematite, with particular emphasis on changes in the Eu(III) coordination number and the influence of temperature upon sorption. Our working hypothesis was that a decrease in hydration number upon adsorption, as indicated by a decrease in coordination number and an increase in adsorption with increasing temperature, results in energetically favorable sorption reactions, which are driven by a large, positive entropy term. To perform these studies, we applied the diffuse layer model to describe Eu(III) adsorption onto hematite at pH values ranging from ~3 to 7 and at 15, 25, 35, and 50°C. Additionally, we characterized the Eu(III)-hematite surface complex and changes in the Eu(III) primary hydration sphere using extended X-ray absorption fine structure spectroscopy (EXAFS) and computational modeling. High-resolution transmission electron microscopy (HRTEM) was used to identify possible europium surface precipitates or morphological changes in the hematite. The data indicate that the adsorption reaction (1) is endothermic, (2) proceeds with a decrease in the Eu(III) coordination number, and (3) results in the formation of a bidentate mononuclear surface complex, (FeO)2Eu+. The enthalpy and entropy values for the formation of this surface complex, which were estimated using a van't Hoff plot, were 131±8 kJmol-1 and 439±26JK-1mol-1, respectively, indicating that adsorption of Eu(III) onto hematite is entropically driven. Additionally, we suggest that the decrease in Eu(III) coordination number and the large entropy term are due to the loss of coordinating water molecules from the Eu(III) hydration sphere.
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U2 - 10.1016/j.gca.2013.08.023
DO - 10.1016/j.gca.2013.08.023
M3 - Article
AN - SCOPUS:84884801412
SN - 0016-7037
VL - 122
SP - 430
EP - 447
JO - Geochimica et Cosmochimica Acta
JF - Geochimica et Cosmochimica Acta
ER -