MECHANISMS OF NICKEL SORPTION ON PYROPHYLLITE - MACROSCOPIC AND MICROSCOPIC APPROACHES

Retention of heavy metal ions on soil mineral surfaces is a crucial pr ocess for maintaining environmental quality. A thorough understanding of the sorption mechanisms of heavy metals on soil mineral surfaces is therefore of fundamental importance. This study examined Ni(II) sorpt ion mechanisms on pyrophyllite. The removal of Ni from solution was st udied as a function of pH (pH = 5-8.5), initial Ni concentration (1 x 10(-4) to 1 x 10(-2) M), and ionic strength (0.01-1 M). The data sugge st that Ni sorption behavior can be divided into two distinct pH regio ns. In the lower pH region (i.e., pH <7), relative Ni sorption increas ed with decreasing ionic strength and initial Ni concentration. The ad sorption maximum at pH = 6 was significantly higher than the cation-ex change capacity (CEC) at the same pH. Based on these findings, we prop ose that both specific and nonspecific adsorption are involved. In the higher pH region (pH >7), nickel sorption becomes slow and does not s eem to be affected by the ionic strength and the initial Ni concentrat ion. We employed high-resolution transmission electron microscopy (HRT EM) to ascertain whether any alteration in the surface structure of py rophyllite could be detected after reaction with Ni at pH >7. Surface deposits, not present on untreated samples, were found. At low Ni sorp tion densities, surface precipitation seems to occur preferentially al ong the edges of the particles. Based on the HRTEM findings, and on re sults from a previous x-ray absorption fine structure spectroscopy (XA FS) study, we hypothesize that the formation of a mixed Ni-Al hydroxid e phase on the pyrophyllite surface is responsible for the sorption be havior above pH 7.