EXAFS Study of Sr sorption to Illite, Goethite, Chlorite, and Mixed Sediment under Hyperalkaline Conditions

Abstract

Strontium is an important contaminant radionuclide at many former nuclear sites. This paper investigates the effect of changing pH and ionic strength on the sorption of Sr to a range of common soil minerals. Specifically it focuses on the sorption of Sr onto illite, chlorite, goethite, and a mixed sediment. The interplay between ionic strength and pH was determined by varying the background ionic strength of the system using both NaCl (for a constant pH) and NaOH (to also vary pH). Under conditions of moderate pH, Sr sorption decreased with increasing ionic strength, due to competition between the Na and Sr atoms for the outer-sphere complexes. However, where increasing ionic strength was accompanied by increasing pH, Sr sorption remained high. This suggested that Sr was sorbed to the minerals without competition from background Na ions. Extended X-ray absorption fine structure (EXAFS) spectra confirmed that at highly alkaline pH (>12.5) Sr was forming inner-sphere complexes on the surfaces of all minerals. This specific adsorption of the Sr (as SrOH(+)) explains why it was still adsorbed to the minerals under very high ionic strength conditions and was not out-competed by Na.

Emerging investigator series: a holistic approach to multicomponent EXAFS: Sr and Cs complexation in clayey soils

“…During both approaches, FEFF6 53 was used to calculate the theoretical (initial) Sr and Cs scattering paths by developing complexation models using the crystallographic information files for anatase, 54 kaolinite 55 and illite, 56 based on previous analytical and molecular dynamics results. 15,20,21,27,31,35,36…”

“…[8][9][10][11][12][13] Transport of fission products like caesium(Cs) and strontium (Sr) through the environment is heavily impacted by the soil mineralogy (including clay minerals and metal oxides in the soils at the Little Forest Legacy Site 3,4,7 ), their surface speciation and complexation with organic matter. 10,11,[14][15][16][17][18][19] Previous studies have identified multiple surface complexes for Sr and Cs complexed with mineral phases present within clayey soils using X-ray absorption spectroscopy and molecular dynamics techniques. Sr has been shown to complex with kaolinite through the formation of hydrated (outer-sphere) surface complexes at elevated concentration and inner-sphere complexes at lower concentrations.…”

“…20,21 A wider range of surface complexes have been identified for Sr complexed with 2:1 clay minerals. This includes outer-sphere 15,[22][23][24] and innersphere surface complexes, 22,25 and incorporation into the interlayer of illite 15 and smectite. 24,26 Sr has been identified to complex with titanium dioxide (common as an accessory mineral in clayey soils 3,4 ) through forming inner-sphere surface complexes with anatase and rutile.…”

“…For example, no spectroscopic evidence exists for Sr complexation with anatase and Cs complexation with either illite or kaolinite. Furthermore, to comprehensively determine the surface complexes in multicomponent systems such as clayey soils (with multiple mineral phases competing for surface complexation) is challenging because spectroscopic techniques only provide an average coordination environment 15,23 and interpreting such spectroscopic information is non-trivial. Here, we utilised a comprehensive/dual approach to fitting the extended X-ray absorption fine structure (EXAFS) spectra on Sr and Cs surface complexation.…”

Kinetic and isothermal adsorption properties of strontium and cesium ions by zeolitic materials synthesized from Jeju volcanic rocks

“…The effect of pH on the Sr and Cs ions adsorption capacities by the Z-S1 was examined in the pH range of 2 -10. The form of Cs and Sr species in aqueous solutions exist in the form of Sr 2+ and Cs + at the pH range of 2-12 [27,28]. As seen in Fig.…”