Sorption mechanisms of Sr and Pb on zeolitized tuffs from the Nevada test site as a function of pH and ionic strength

Journal Article · Sat Nov 01 04:00:00 EST 2003 · American Mineralogist

DOI:https://doi.org/10.2138/am-2003-11-1247· OSTI ID:15006236

Um, Wooyong; Papelis, Charalambos

The sorption of divalent strontium, Sr{sup 2+}, and divalent lead, Pb{sup 2+}, on zeolitized tuffs from the Nevada Test Site (NTS) was investigated using macroscopic batch sorption experiments and x-ray absorption spectroscopy (XAS) as a function of geochemical parameters, including pH, ionic strength, and type of background electrolyte. The sorption of Sr{sup 2+} is dependent on the ionic strength of the medium and independent of pH, suggesting that Sr{sup 2+} sorption is controlled by ion exchange at permanent charge sites. At higher ionic strengths, background electrolyte cations compete effectively with Sr{sup 2+} for cation exchange sites and Sr{sup 2+} sorption is suppressed. At the two lower ionic strengths (0.01 and 0.1 M), Pb{sup 2+} sorption is also consistent with adsorption by cation exchange. At the highest ionic strength (1.0 M), however, exclusion of Pb{sup 2+} from cation exchange sites resulted in pH dependent adsorption, consistent with sorption on amphoteric surface hydroxyl sites or formation of surface precipitates. XAS was used to test these hypotheses. Based on XAS data, Sr{sup 2+} formed hydrated surface complexes coordinated with approximately eight oxygen atoms at an average distance of 2.60 ({+-}0.02) {angstrom}, regardless of conditions, consistent with the formation of mononuclear, outer-sphere surface complexes at the Ca2 site in the B channel of clinoptilolite. The coordination environment of sorbed Pb{sup 2+} was more complex and a function of pH and ionic strength. The first shell consisted of two to three oxygen atoms at an average distance of 2.20 ({+-}0.02) {angstrom}. At low pH and ionic strength, XAS data were consistent with Pb{sup 2+} adsorption at the Na1 and Ca2 cation exchange sites in channels A and B of clinoptilolite, respectively. At the highest ionic strength (1.0 M) and low pH, XAS provides evidence for formation of Pb{sup 2+} monodentate, corner-sharing inner-sphere complexes, while at higher pH, XAS analysis is consistent with formation of edge-sharing bidentate inner-sphere complexes. As surface coverage increased, appearance of a second Pb{sup 2+} peak suggests the formation of polynuclear, inner-sphere surface complexes. These results have significant implications for the transport of radionuclides and other contaminants at the NTS and other nuclear test sites and the modeling of these processes.

• Research Organization: Pacific Northwest National Lab., Richland, WA (US)
• Sponsoring Organization: US Department of Energy (US)
• DOE Contract Number: AC06-76RL01830
• OSTI ID: 15006236
• Report Number(s): PNWD-SA-6288
• Journal Information: American Mineralogist, Journal Name: American Mineralogist Journal Issue: 11-12 Pt.2 Vol. 88; ISSN AMMIAY; ISSN 0003-004X
• Country of Publication: United States
• Language: English

Related Subjects

45 MILITARY TECHNOLOGY, WEAPONRY, AND NATIONAL DEFENSE
ABSORPTION SPECTROSCOPY
ADSORPTION
ATOMS
CATIONS
CLINOPTILOLITE
ELECTROLYTES
ION EXCHANGE
NEVADA TEST SITE
NUCLEAR TEST SITES
OXYGEN
RADIOISOTOPES
SIMULATION
SORPTION
STRONTIUM
TRANSPORT
XAS
SR
PB
SORPTION
NTS