skip to main content
OSTI.GOV title logo U.S. Department of Energy
Office of Scientific and Technical Information

Title: Sequestration of Sr(II) By Calcium Oxalate - a Batch Uptake Study And EXAFS Analysis of Model Compounds And Reaction Products

Abstract

Calcium oxalate monohydrate (CaC{sub 2}O{sub 4}{center_dot}H{sub 2}O -- abbreviated as CaOx) is produced by two-thirds of all plant families, comprising up to 80 wt.% of the plant tissue and found in many surface environments. It is unclear, however, how CaOx in plants and soils interacts with metal ions and possibly sequesters them. This study examines the speciation of Sr(II){sub aq} following its reaction with CaOx. Batch uptake experiments were conducted over the pH range 4--10, with initial Sr solution concentrations, [Sr]{sub aq}, ranging from 1 x 10{sup -4} to 1 x 10{sup -3} M and ionic strengths ranging of 0.001--0.1 M, using NaCl as the background electrolyte. Experimental results indicate that Sr uptake is independent of pH and ionic strength over these ranges. After exposure of CaOx to Sr{sub aq} for two days, the solution Ca concentration, [Ca]{sup aq}, increased for all samples relative to the control CaOx suspension (with no Sr added). The amount of Sr{sub aq} removed from solution was nearly equal to the total [Ca]{sup aq} after exposure of CaOx to Sr. These results suggest that nearly 90% of the Sr is removed from solution to a solid phase as Ca is released into solution. We suggestmore » that the other 10% is sequestered through surface adsorption on a solid phase, although we have no direct evidence for this. Extended X-ray absorption fine structure (EXAFS) spectroscopy was used to determine the molecular-level speciation of Sr in the reaction products. Deconvolutions of the Sr K-edge EXAFS spectra were performed to identify multi-electron excitation (MEE) features. MEE effects were found to give rise to low-frequency peaks in the Fourier transform before the first shell of oxygen atoms and do not affect EXAFS fitting results. Because of potential problems caused by asymmetric distributions of Sr-O distances when fitting Sr K-edge EXAFS data using the standard harmonic model, we also employed a cumulant expansion model and an asymmetric analytical model to account for anharmonic effects in the EXAFS data. For Sr-bearing phases with low to moderate first-shell (Sr-O pair correlation) anharmonicity, the cumulant expansion model is sufficient for EXAFS fitting; however, for higher degrees of anharmonicity, an analytical model is required. Based on batch uptake results and EXAFS analyses of reaction products, we conclude that Sr is dominantly sequestered by a solid phase at the CaOx surface, likely the result of a dissolution-reprecipitation mechanism, to form SrC{sub 2}O{sub 4} of mixed hydration state (i.e. SrO{sub x}{center_dot}nH{sub 2}O, where n = 0, 1, or 2). Surprisingly, no spectroscopic or XRD evidence was found for a (Sr,Ca)Ox solid solution or for a separate SrCO3 phase. In addition, we found no evidence for Sr(II) inner-sphere sorption complexes on CaOx surfaces based on lack of Sr-Ca second-neighbor pair correlations in the EXAFS spectra, although some type of Sr(II) surface complex (perhaps a type B Sr-oxalate ternary complex or an outer-sphere Sr(II) complex) or some as yet undetected Sr-bearing solid phases are needed to account for approximately 10% of Sr uptake by CaOx. The formation of a hydrated SrOx phase in environments under conditions similar to those of our experiments should retard Sr mobility and could be a significant factor in the biogeochemical cycling of Sr in soils and sediments or in plants and plant litter where CaOx is present.« less

Authors:
; ; ; ; ;
Publication Date:
Research Org.:
SLAC National Accelerator Lab., Menlo Park, CA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
953566
Report Number(s):
SLAC-REPRINT-2009-317
Journal ID: ISSN 0016-7037; GCACAK; TRN: US201002%%1394
DOE Contract Number:  
AC02-76SF00515
Resource Type:
Journal Article
Journal Name:
Geochim. Cosmochim. Acta 72:5055,2008
Additional Journal Information:
Journal Volume: 72; Journal Issue: 20; Journal ID: ISSN 0016-7037
Country of Publication:
United States
Language:
English
Subject:
04 OIL SHALES AND TAR SANDS; ABSORPTION; ADSORPTION; ATOMS; CALCIUM; COMPLEXES; CONTROL; CORRELATIONS; DATA; EXCITATION; EXPANSION; FINE STRUCTURE; HARMONICS; HYDRATION; IONS; METALS; MOBILITY; OXALATES; OXYGEN; PEAKS; PLANT TISSUES; PLANTS; POTENTIALS; RANGE; RISE; SEDIMENTS; SHELLS; SOILS; SOLID SOLUTIONS; SOLIDS; SOLUTIONS; SORPTION; SPECTRA; SPECTROSCOPY; STANDARDS; SURFACES; SUSPENSIONS; UPTAKE; X-RAY DIFFRACTION; Other,OTHER, EARTH, CHEM

Citation Formats

Singer, D M, Johnson, S B, Catalano, J G, Farges, F, Brown, G E, and Jr,. Sequestration of Sr(II) By Calcium Oxalate - a Batch Uptake Study And EXAFS Analysis of Model Compounds And Reaction Products. United States: N. p., 2009. Web. doi:10.1016/j.gca.2009.07.002.
Singer, D M, Johnson, S B, Catalano, J G, Farges, F, Brown, G E, & Jr,. Sequestration of Sr(II) By Calcium Oxalate - a Batch Uptake Study And EXAFS Analysis of Model Compounds And Reaction Products. United States. https://doi.org/10.1016/j.gca.2009.07.002
Singer, D M, Johnson, S B, Catalano, J G, Farges, F, Brown, G E, and Jr,. 2009. "Sequestration of Sr(II) By Calcium Oxalate - a Batch Uptake Study And EXAFS Analysis of Model Compounds And Reaction Products". United States. https://doi.org/10.1016/j.gca.2009.07.002.
@article{osti_953566,
title = {Sequestration of Sr(II) By Calcium Oxalate - a Batch Uptake Study And EXAFS Analysis of Model Compounds And Reaction Products},
author = {Singer, D M and Johnson, S B and Catalano, J G and Farges, F and Brown, G E and Jr,},
abstractNote = {Calcium oxalate monohydrate (CaC{sub 2}O{sub 4}{center_dot}H{sub 2}O -- abbreviated as CaOx) is produced by two-thirds of all plant families, comprising up to 80 wt.% of the plant tissue and found in many surface environments. It is unclear, however, how CaOx in plants and soils interacts with metal ions and possibly sequesters them. This study examines the speciation of Sr(II){sub aq} following its reaction with CaOx. Batch uptake experiments were conducted over the pH range 4--10, with initial Sr solution concentrations, [Sr]{sub aq}, ranging from 1 x 10{sup -4} to 1 x 10{sup -3} M and ionic strengths ranging of 0.001--0.1 M, using NaCl as the background electrolyte. Experimental results indicate that Sr uptake is independent of pH and ionic strength over these ranges. After exposure of CaOx to Sr{sub aq} for two days, the solution Ca concentration, [Ca]{sup aq}, increased for all samples relative to the control CaOx suspension (with no Sr added). The amount of Sr{sub aq} removed from solution was nearly equal to the total [Ca]{sup aq} after exposure of CaOx to Sr. These results suggest that nearly 90% of the Sr is removed from solution to a solid phase as Ca is released into solution. We suggest that the other 10% is sequestered through surface adsorption on a solid phase, although we have no direct evidence for this. Extended X-ray absorption fine structure (EXAFS) spectroscopy was used to determine the molecular-level speciation of Sr in the reaction products. Deconvolutions of the Sr K-edge EXAFS spectra were performed to identify multi-electron excitation (MEE) features. MEE effects were found to give rise to low-frequency peaks in the Fourier transform before the first shell of oxygen atoms and do not affect EXAFS fitting results. Because of potential problems caused by asymmetric distributions of Sr-O distances when fitting Sr K-edge EXAFS data using the standard harmonic model, we also employed a cumulant expansion model and an asymmetric analytical model to account for anharmonic effects in the EXAFS data. For Sr-bearing phases with low to moderate first-shell (Sr-O pair correlation) anharmonicity, the cumulant expansion model is sufficient for EXAFS fitting; however, for higher degrees of anharmonicity, an analytical model is required. Based on batch uptake results and EXAFS analyses of reaction products, we conclude that Sr is dominantly sequestered by a solid phase at the CaOx surface, likely the result of a dissolution-reprecipitation mechanism, to form SrC{sub 2}O{sub 4} of mixed hydration state (i.e. SrO{sub x}{center_dot}nH{sub 2}O, where n = 0, 1, or 2). Surprisingly, no spectroscopic or XRD evidence was found for a (Sr,Ca)Ox solid solution or for a separate SrCO3 phase. In addition, we found no evidence for Sr(II) inner-sphere sorption complexes on CaOx surfaces based on lack of Sr-Ca second-neighbor pair correlations in the EXAFS spectra, although some type of Sr(II) surface complex (perhaps a type B Sr-oxalate ternary complex or an outer-sphere Sr(II) complex) or some as yet undetected Sr-bearing solid phases are needed to account for approximately 10% of Sr uptake by CaOx. The formation of a hydrated SrOx phase in environments under conditions similar to those of our experiments should retard Sr mobility and could be a significant factor in the biogeochemical cycling of Sr in soils and sediments or in plants and plant litter where CaOx is present.},
doi = {10.1016/j.gca.2009.07.002},
url = {https://www.osti.gov/biblio/953566}, journal = {Geochim. Cosmochim. Acta 72:5055,2008},
issn = {0016-7037},
number = 20,
volume = 72,
place = {United States},
year = {Tue May 26 00:00:00 EDT 2009},
month = {Tue May 26 00:00:00 EDT 2009}
}