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.
Highlights
The widespread use of nuclear power over roughly the last 50 years has left a large volume of radioactive nuclear waste
In all mineral systems increasing ionic strength of NaCl caused a decrease in percent sorption of strontium onto the surface
The minor decrease in Sr sorption in the 1 mol L−1 systems may reflect the saturation of the minerals reactive sites, as the total ionic strength (IS) of the system is above the reported cation exchange capacity (CEC)
Summary
The widespread use of nuclear power over roughly the last 50 years has left a large volume of radioactive nuclear waste. Historic leaks and authorized discharge from nuclear facilities have released 90Sr (and other radionuclides) into the geosphere This poses a potential risk to the environment and to human health. Sorption of Sr2+ to common mineral phases in soils under moderate environmental conditions (low ionic strength, pH 4−10) is well understood. Under these conditions, Sr primarily sorbs to those minerals with large surface area and cation exchange capacity, clays and iron oxides.[1−4] Sorption is known to occur at circumneutral pH though charge compensating cation exchange with the Sr2+
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