Strontium (Sr2+) adsorption on barite surfaces is a likely initial step for incorporation of strontium into barite, a potential remediation strategy for long-term storage of radioactive Sr2+. Using high resolution X-ray reflectivity (XR) and density functional theory molecular dynamics (DFT-MD) simulations, we quantified structural changes at the barite (001) – water interface associated with the extent of Sr2+ coverage during adsorption. The XR results indicated that adsorption of Sr2+ disrupted the barite surface structure in the first two monolayers through large rotation of the topmost sulfate ion (by ~ 12°) and relaxation of the top two bariums and sulfates (by up to ~ 0.2 Å). These distortions were larger than that observed in the absence of strontium [1]. Ordering of water beyond 5 Å from the surface was also significantly reduced as compared to the hydration structure in the absence of strontium [1]. The resonant anomalous X-ray reflectivity results showed that Sr2+ adsorbed at [Sr2+] = 25 μM, above which the coverage increased rapidly with increasing [Sr2+] and reached its maximum of ~0.5 Sr2+ per surface unit cell area of barite at [Sr2+]&gt;75 μM. This adsorption behavior follows a Frumkin isotherm with attractive interaction between adsorbed particles, rather than a Langmuir isotherm. At its maximum coverage, adsorbed Sr2+ had a broad distribution centered at ~1 Å from the barite surface. This small average height implies that Sr2+ adsorbed primarily as an inner-sphere surface complex and/or was incorporated in the top monolayer of the barite surface. The DFT simulations predicted favorable exchange of Sr2+ with Ba2+ in the top surface by approximately -35 kJ/mol compared to Sr2+ tightly adsorbed, consistent with the observations. During desorption experiments, only half of adsorbed Sr2+ ions were removed from the surface after exposure to a Sr2+ -free barite-saturated solution. This indicates either Sr2+ adsorption is not fully reversible or Sr2+ desorption is significantly slower than adsorption after exchange.

Strontium adsorption at the barite (001) – water interface

255th National Meeting (2018)

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