The safe disposal of 60Co, 63Ni, and 59Ni has required considerable information on the interactions of Co2+ and Ni2+ with clay minerals in the geosphere. X-ray photoelectron spectroscopy (XPS) has been used to probe the sorption sites for Co2+ and Ni2+ on hectorite and montmorillonite. The spectra were measured for Co-hectorite, Ni-hectorite, and Ni-montmorillonite immediately following ion exchange and after washing the clay two and five times with distilled water. The spectra, recorded following etching of the surface with an argon ion beam, differentiate two sorption sites; a labile (to washing) fraction sorbed as ion pairs, and a non-labile fraction sorbed by ion exchange at broken bond and interlamellar sites. The data were consistent with the sorption of metal ions (Co2+, Ni2+) in a common “MO6” ligand environment.

Co2+ had a greater affinity for exchange on hectorite than did Ni2+; but Ni2+ had a greater affinity for the surface of montmorillonite than for hectorite. The argon ion etching of Ni-montmorillonite gave rise to a new photopeak of 853 eV, which was probably due to elemental Ni formed consequent to the chemical violation of the surface by ion etching.

The structure and composition of short-range ordered aluminosilicates (SROAS) may control their affinity for organic acids with potential effects on soil organic matter stabilization. Adsorption mechanisms of model organic acids were studied to resolve the effect of Si incorporation. Adsorption of oxalic, salicylic, and octanoic acid on Al-rich (Al:Si = 3.7) and Si-rich (Al:Si = 1.4) SROAS was quantified by analyses of dissolved organic carbon using catalytic high-temperature combustion. The initial pH of 5 and 6.5 increased to 6.3–8.2 during adsorption of oxalic and salicylic acid, demonstrating hydroxyl release by ligand exchange. Minor changes in pH indicated weak interactions of octanoic acid with both SROAS. Adsorbates were characterized by Fourier-transform infrared spectroscopy. Asymmetric stretching of carboxylate groups at 1720 and 1700 cm–1, and symmetric stretching at 1430 cm–1 evinced the formation of chelate complexes for oxalic acid. An absorption band centered at 1545 cm–1 indicated partial inner-sphere binding of salicylic acid on both SROAS. Silicon-rich SROAS adsorbed 80–90% less than Al-rich SROAS, suggesting that adsorption of oxalic and salicylic acid was controlled by surface aluminol groups. Fast kinetics of oxalate adsorption on Al sites was studied by a conductivity-based stopped-flow technique. Ligand exchange proceeded at a rate constant of 3.5 s–1 (25°C), similar to solute Al complexation, with an activation energy of up to 34.1 kJ mol–1. A slow process with a rate constant of 0.13 s–1 (25°C) was attributed to diffusion of oxalate at the surface or into SROAS particles. As supported by structural characterization of Si-rich SROAS, the much lower susceptibility of Si-rich SROAS to ligand exchange relates to Al speciation. The formation of tetrahedral Al precludes its complexation by carboxyl groups.