Kinetics of the formation and the dissolution of nickel surface precipitates on pyrophyllite

The kinetics of the formation and the dissolution of Ni surface precipitates on pyrophyllite was studied in order to gain an understanding of the dynamics of heavy metal ion reactions in soils. Ni sorption at pH = 7.5 was initially fast. 25% of the initial Ni was removed within minutes. Thereafter, a gradual decrease in sorption was observed. Based on previous spectroscopic evidence, we attribute the fast reaction stage to adsorption phenomena and the slow reaction stage to nucleation processes on the pyrophyllite surface. The detachment of Ni from surface precipitates at pH = 4 and pH = 6 involves a small amount of Ni (< 10%) being desorbed relatively fast. The desorption of specifically adsorbed, mononuclear bound Ni may account for this rapid Ni release. Thereafter, Ni detachment was extremely slow, and the rate depended strongly on the experimental desorption method. Utilizing a conventional batch technique, further Ni release became negligible. The non-removal of reaction products may have caused the formation of secondary precipitates. Under steady state conditions a constant Ni detachment rate was observed which we attribute to the dissolution of Ni surface precipitates. Compared to the dissolution of crystalline Ni(OH)(2), Ni detachment from pyrophyllite was slow. We hypothesize that the slow Ni detachment is due to the dissolution of mixed Ni-Al-hydroxides which formed prior to the desorption experiment and which have a lower solubility.