Investigation of Ni Sorption on Pyrophyllite: An XAFS Study

Abstract

Sorption reactions at the solid−water interface decrease solute mobility and often control the fate, bioavailability, and transport of metal ions in soils and groundwaters. A thorough understanding of the structural environment of metals at the solid−water interface is therefore of fundamental importance. In this study, X-ray absorption fine structure (XAFS) spectroscopy was used to discern the local atomic structure of Ni(II) sorbed onto pyrophyllite. The first coordination shell consists of 6 O atoms at 2.02−2.04 Å. For the second shell, XAFS data suggest a single Ni−Al/Si distance (2.96−3.03 Å), indicative of edge sharing of Ni and Al octahedra and possibly the presence of mixed nickel−aluminum hydroxides. As Ni surface loading on pyrophyllite increased, the number of Ni second-neighbor atoms at a distance of 2.99−3.00 Å increased from N ≈ 1 to N ≈ 5. The presence of multinuclear surface complexes was depicted at low surface loading and at reaction conditions undersaturated with respect to the formation of Ni(OH)2(s). This observation suggests that the total coverage of surface sites is not necessary for the formation of multinuclear surface complexes and implies that the pyrophyllite surface promotes hydrolysis and multinuclear complex formation.