X-ray absorption spectroscopy of cobalt(II) multinuclear surface complexes and surface precipitates on kaolinite

Abstract We have examined the local atomic structure of multinuclear complexes and surface precipitates of Co(II) sorbed to kaolinite (Al2Si2O5(OH)4), a common clay mineral, using extended X-ray absorption fine structure (EXAFS) spectroscopy. Structural information from quantitative analysis of EXAFS spectra is used to examine differences, on a molecular level, among Co species sorbed on kaolinite at different coverages from solutions under- and over-saturated with respect to solid Co(OH)2 phases, and between sorbed species and Co hydroxide solids precipitated from solutions in the absence of kaolinite. The EXAFS results indicate formation of oxide- or hydroxide-bridged multinuclear surface complexes on kaolinite at surface coverages well below a statistical monolayer from undersaturated solutions. As Co surface loading on kaolinite is increased, the number of Co second-neighbor atoms at 3.10-3.12 A increases from ≈2 to 6, the number of second-neighbor Co backscatterers in crystalline Co (OH)2(s), and interatomic distances determined by EXAFS remain constant. These observations suggest that hydroxide-like multinuclear complexes form at low surface coverages in addition to monomeric surface complexes, perhaps as precursors to surface precipitation. At high surface coverages and apparent solubilities (i.e., solution ion activity products) both below and above saturation with respect to stable Co(OH)2(s), the spectra of the sorption products differ from the spectra of Co solids precipitated in the absence of kaolinite, including dry, crystalline Co(OH)2(s) (pink form) and wet, freshly precipitated Co(OH)2(s) (blue and pink forms). Compared to the precipitates, sorption samples have Co-Co distances shorter by 0.050.07 (±0.02) A, lack small "beat" patterns in normalized spectra, and have reduced backscattering amplitudes from multiple scattering among Co atoms at ≈ 3 and 6 A. Long-term equilibration of oversaturated sorption samples (21 or 45 days) resulted in a decrease in backscattering amplitudes for both single- and multiple-scattering paths among neighboring Co atoms relative to short-term (24-36 h) equilibration. These differences are attributed to a decrease in the average number of Co atoms in a hydroxide-like complex and/or to an increase in disorder in the local atomic structure. This is not expected in typical aging of a freshly precipitated solid phase, where average particle size and degree of long-range order normally increase with time, suggesting that kaolinite influences the sorption product. The contraction in interatomic distances and disruption of long-range structure indicated by EXAFS for sorption samples can be accounted for by small particle sizes of the new surface phase, structural misregistry on an atomic scale between a Co hydroxide phase and kaolinite, or dissolution of the kaolinite substrate and substitution of minor impurities into a Co hydroxide phase. The apparent reduced solubilities of the sorption products relative to bulk solubilities of Co(OH)2(s) phases may be related to Ostwald-step processes, the formation of an impure Co hydroxide phase, or a lower near-surface solvent dielectric constant.