X-Ray Absorption Spectroscopy of Strontium(II) Coordination

Area of Science:

• Geochemistry
• Environmental Science
• Materials Science

Background:

• Strontium sorption on mineral surfaces is crucial for understanding its environmental fate.
• The molecular-level interactions governing strontium sorption are not fully elucidated.
• Factors like pH, concentration, and co-contaminants influence sorption behavior.

Purpose of the Study:

• To investigate the sorption mechanisms of dissolved strontium on kaolinite, amorphous silica, and goethite.
• To determine the local molecular coordination of strontium under various experimental conditions.
• To assess the impact of pH, strontium concentration, CO2, phosphate, and aging on strontium sorption.

Main Methods:

• Batch sorption experiments were conducted varying pH, strontium concentration, and the presence of CO2 or phosphate.
• Synchrotron X-ray absorption spectroscopy (XAS), specifically extended X-ray absorption fine structure (EXAFS), was used to analyze strontium coordination.
• Samples were analyzed at both low (13-23 K) and room temperatures.

Main Results:

• Sorption on kaolinite and amorphous silica, and most goethite samples, resulted in hydrated surface complexes with 9-10 oxygen atoms at ~2.61 Å.
• EXAFS spectra remained unchanged after 57 days of aging, indicating stable sorption complexes.
• On goethite at pH ~8.5, strontium precipitated as strontianite (SrCO3), particularly in the presence of atmospheric CO2.
• Strontium uptake on goethite increased with atmospheric CO2, suggesting carbonate nucleation of SrCO3 precipitation.

Conclusions:

• Strontium primarily forms hydrated surface complexes on kaolinite and silica.
• Goethite can facilitate strontium precipitation as strontianite (SrCO3) under specific pH and CO2 conditions.
• The presence of CO2 is a key factor in strontium precipitation on goethite, influencing its environmental mobility.