Mechanisms of nickel sorption on pyrophyllite: Macroscopic and microscopic approaches

Retention of heavy metal ions on soil mineral surfaces is a crucial process 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) sorption mechanisms on pyrophyllite. The removal of Ni from solution was studied 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 suggest that Ni sorption behavior can be divided into two distinct pH regions. In the lower pH region (i.e., pH <7), relative Ni sorption increased with decreasing ionic strength and initial Ni concentration. The adsorption maximum at pH = 6 was significantly higher than the cation-exchange capacity (CEC) at the same pH. Based on these findings, we propose that both specific and nonspecific adsorption are involved. In the higher pH region (pH >7), nickel sorption becomes slow and does not seem to be affected by the ionic strength and the initial Ni concentration. We employed high-resolution transmission electron microscopy (HRTEM) to ascertain whether any alteration in the surface structure of pyrophyllite could be detected after reaction with Ni at pH >7. Surface deposits, not present on untreated samples, were found. At low Ni sorption densities, surface precipitation seems to occur preferentially along the edges of the particles. Based on the HRTEM findings, and on results from a previous x-ray absorption fine structure spectroscopy (XAFS) study, we hypothesize that the formation of a mixed Ni-Al hydroxide phase on the pyrophyllite surface is responsible for the sorption behavior above pH 7.