Ni(II) sorption mechanism at the vermiculite-water interface: Effects of interlayer

Abstract

In this study, the effects of interlayer on Ni(II) sorption at the vermiculite-water interface was investigated by combining batch technique, surface complexation model (SCM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and extended X-ray absorption fine structure (EXAFS) analyses. The results showed that alkali cations can enter the interlayer of vermiculite, and cause the collapse of vermiculite interlayer to different extent mainly due to the different ionic radii and hydrated energies. XRD patterns confirmed the order of collapse extent following the trend of Cs+ ≈ Rb+ > K+ > Na+. Subsequently, the sorption edges of Ni(II) on vermiculite were exhibiting the same trends as the interlayer collapse extents in different electrolytes. SCM confirmed that the sorption was mainly controlled by ion exchange (IE) in low pH region, while the inner-sphere complexes (ISCs) and surface-induced precipitates became the dominant species in high pH region. Both XPS and EXAFS analyses have confirmed that the formations of Ni-precipitates and NiAl layer double hydroxide (NiAl LDH) were indeed enhanced after the interlayer collapsed. Therefore, the sorption mechanisms of Ni(II) could be regulated by the interlayer of vermiculite. These findings are of great importance to understand the sorption mechanisms of heavy metals at the expandable clay-water interfaces and their environmental behaviors.