Adsorption and Desorption Behaviors of Sr on Montmorillonite: A Triple Sr Isotope Perspective

Abstract

Montmorillonite is a widespread mineral, which affects the migration of Sr in the critical zone. Hydrochemical factors influencing Sr adsorption have been evaluated in the literature through laboratory batch experiments. Since those experiments were conducted using high-concentration and high-purity standard solutions, the influences of chemical matrices or element relative concentrations in water remain unclear. Here, we reassessed the adsorption and desorption behaviors of Sr on montmorillonite using natural waters with various chemical matrices and ionic strengths. Triple Sr isotopes ( 87 Sr/ 86 Sr and δ 88/86 Sr) were used to trace the exchange processes between the minerals and the waters. Adsorption and desorption occurred in all processed samples, but to varying degrees. These phenomena (especially desorption) were principally controlled by the ionic competition of divalent ions (i.e., Ca and Mg), rather than by ionic strength. Sr isotope fractionation occurred during adsorption. Isotopically lighter Sr was preferentially adsorbed: the mean Δ 88/86 Sr mineral−water (a value independent from the clay type) was −0.17‰. Consequently, water δ 88/86 Sr depended on both the mixing of mineral-bound and initial water Sr and on the degree of Sr adsorption; moreover, the exchange process was a reversible reaction and reached its chemical and isotopic equilibrium within a short time.

Equilibrium Fractionation of Stable Strontium Isotopes During Adsorption to Mn Oxides

“…Lastly, the observation that adsorption ratios are lower at high IS conditions compared to low IS conditions results from the competition for finite adsorption sites by other ions present in the solution, such as Ca 2+ and Mg 2+ , which interfere with Sr 2+ adsorption (Figure 1). This phenomenon is similar to what has been observed in the adsorption behaviors of other ions on different minerals, such as Ca 2+ and Mg 2+ on Sr 2+ adsorption by montmorillonite (Liu et al, 2022), and Ca 2+ and Na + on Tl + adsorption by illite (Martin et al, 2018).…”

Fractionation Mechanism and Flux Estimation of Strontium Isotopes During Basalt Weathering

“…Clay minerals in soils at different locations can have differing sorption capacities and behave differently due to variability in chemical composition. Similar to the isotope fractionation of other elements such as Mg, Li, K, and Rb, stable Sr isotope fractionation can occur through adsorption by clay minerals (H.-C. Liu, You, & Tu, 2012;H.-C. Liu et al, 2022;Su et al, 2022). In this regolith, the clay minerals include smectite and kaolinite.…”

“…Sr can be adsorbed on the smectite surface or incorporated into its interlayer sites. In the absence of ionic competition, mineral-bound Sr is stable in smectite (H.-C. Liu et al, 2022). However, the regolith is an integrated system rich in multiple ions and the presence of competing cations (e.g., Ca 2+ and Mg 2+ ) can lead to the saturation of smectite interlayer sites, altering the adsorption of Sr (Ahmad, 1995;Sugiura et al, 2021).…”

“…In this regolith, Sr is adsorbed onto the smectite surface to form outer-sphere complexes (Atun & Kaplan, 1996;Fuller et al, 2016;Wallace et al, 2012;Yamaguchi et al, 2018). The interatomic distance between Sr and O on the smectite surface is slightly longer than that between hydrated Sr and O (H.-C. Liu et al, 2022). Therefore, the isotopically light Sr tends to adsorb onto the smectite surface, forming slightly shorter Sr-O bonds.…”

“…In addition, the mechanism of stable Sr isotope fractionation during secondary mineral formation is not well comprehended. The formation of secondary minerals during weathering promotes either incorporation of 86 Sr into their lattice or adsorption onto their surface (H.-C. Liu et al, 2022;Stevenson et al, 2016;Su et al, 2022). However, Bouchez and von Blanckenburg (2021) documented negligible fractionation when Sr was adsorbed onto the exchangeable sites of clay mineral surfaces.…”