Beryllium desorption from minerals and organic ligands over time

Beryllium isotopes sorbed to sediments have provided useful tools in the field of geochronology and geomorphology over the last few decades. The use of beryllium isotopes relies on the premise that beryllium sorbed to sediments is unaltered over large timescales. Changes in the environmental chemistry, either in-situ or en route from soil to fluvial system, to the ocean, can cause beryllium desorption and may preclude some beryllium isotopic applications. Four mechanisms were tested to determine the relative desorption potential of beryllium including a reduction in pH, an increase in ionic strength (NaCl) and complexation by soluble organic (malonic acid) and inorganic species (NaF). To assess the relative effect of each mechanism on beryllium desorption from both organic and mineral fractions, we prepared separate solutions of beryllium bound to minerals and organic compounds and measured beryllium concentrations in solution before and after each chemical perturbation. We conclude a reduction in pH resulted in the greatest amount of desorption among the four treatments, removing 97% and 75% of sorbed beryllium from illite and montmorillonite, respectively, and none from the organic ligands tested. The addition of malonic acid and increasing the ionic strength also resulted in desorption from montmorillonite. Although increasing the ionic strength did remove 32% and 8.4% of beryllium from montmorillonite and sulfonate, respectively, the presence of sodium significantly enhanced sorption to illite. The addition of NaF did not result in any beryllium desorption. Our results demonstrate that various chemical processes can promote the exchange of beryllium between solid and dissolved phases, the extent to which depends on the composition of the system. We also related differences in beryllium desorption behavior to complexation mechanisms driving retention among organic and mineral species. We estimate inner sphere complexation is the predominant sorption mechanism among the organic ligands tested due to the minimal amounts of desorption and the large stability constants previously reported in the literature. Additionally, we found that different complexation processes are involved in beryllium sorption to illite versus montmorillonite. Because beryllium desorbed from montmorillonite due to changes in pH, ionic strength and organic acid complexation, we hypothesize that a portion of beryllium-montmorillonite associations involve outer sphere processes, driven by weaker electrostatic attractions. However, beryllium exhibited a unique relationship with illite in that sorption not only involves inner sphere processes but also physical inclusion within collapsed interlayer spaces. (C) 2016 Elsevier B.V. All rights reserved.