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 h igh surface area, smectite-clay minerals. Using X-ray absorption spect roscopy (XAS), it is shown that at low pH and Na ion concentration Co( II) forms outer-sphere, mononuclear surface complexes with permanent-c harge sites in the interlayer region of smectites. XAS analysis indica tes that with increasing pH and Na ion concentration Co(II) is increas ingly excluded from the interlayer permanent-charge sites and forms po lynuclear species with external surface-hydroxyl sites. These studies demonstrate the potential of XAS for distinguishing between the sorpti on of trace metals on interlayer and external sites of smectites as a function of changing solution conditions.