Sorption mechanisms and transport behavior of strontium(II) and lead(II) on zeolitized tuffs from the Nevada Test Site

The sorption mechanisms and transport behavior of strontium (Sr) and lead (Pb) on zeolitized tuffs from the Nevada Test Site (NTS) were investigated using macroscopic sorption/desorption, diffusion, and column experiments, as well as X-ray absorption spectroscopic (XAS) studies.; Sr(II) sorption was primarily dependent on the ionic strength of the background electrolyte solution and independent of pH. Sr(II) sorption was controlled by ion-exchange at permanent charge sorption sites. The mobility of Sr(II) in columns increased as the ionic strength of the solution increased. Due to weak binding, sorbed Sr(II) was easily desorbed from the zeolitized tuffs by an increase in solution ionic strength or changes in solution composition. Pb(II) showed stronger sorption affinity for zeolitized tuffs compared to Sr(II). Because Pb(II) reacted with surface hydroxyl sorption sites at high pH, sorbed Pb(II) was not easily released from the adsorbents by an increased background electrolyte concentration.; XAS results revealed that Sr(II) formed hydrated surface complexes coordinated with approximately 8.0 oxygen atoms at an average distance of 2.60 (±0.02) Å. Because of the single frequency spectra for sorption and solution samples in the EXAFS, sorbed Sr(II) complexes were consistent with forming of mononuclear, outer-sphere surface complexes. The coordination environment of Pb(II) consisted of two or three oxygen atoms at an average distance of 2.20 (±0.02) Å. Additional peaks at high pH and high ionic strengths were consistent with formation of strong polynuclear, inner-sphere surface complexes.; Diffusion was shown to be a significant process in solute transport retardation. For reactive solutes, the apparent diffusivities were five to seven orders of magnitude lower compared to molecular diffusivities. These low diffusivities of reactive metals resulted from strong sorption affinities of Sr(II) and Pb(II) for zeolitized tuffs.; Due to the presence of mobile colloids, the concentration of eluted Pb(II) increased to approximately 10–30% of the initial concentration depending on geochemical conditions, even though Pb(II) bound very strongly on zeolitized tuffs. In the absence of colloids, no breakthrough was observed under similar conditions.