Distinguishing between interlayer and external sorption sites of clay minerals using X-ray absorption spectroscopy

Metals dissolved in natural aquatic environments are often sorbed on high surface area, smectite-clay minerals. Using X-ray absorption spectroscopy (XAS), it is shown that at low pH and Na ion concentration Co(II) forms outer-sphere, mononuclear surface complexes with permanent-charge sites in the interlayer region of smectites. XAS analysis indicates that with increasing pH and Na ion concentration Co(II) is increasingly excluded from the interlayer permanent-charge sites and forms polynuclear species with external surface-hydroxyl sites. These studies demonstrate the potential of XAS for distinguishing between the sorption of trace metals on interlayer and external sites of smectites as a function of changing solution conditions.