KINETICS OF THE FORMATION AND THE DISSOLUTION OF NICKEL SURFACE PRECIPITATES ON PYROPHYLLITE

The kinetics of the formation and the dissolution of Ni surface precip itates on pyrophyllite was studied in order to gain an understanding o f the dynamics of heavy metal ion reactions in soils. Ni sorption at p H = 7.5 was initially fast. 25% of the initial Ni was removed within m inutes. Thereafter, a gradual decrease in sorption was observed. Based on previous spectroscopic evidence, we attribute the fast reaction st age to adsorption phenomena and the slow reaction stage to nucleation processes on the pyrophyllite surface. The detachment of Ni from surfa ce precipitates at pH = 4 and pH = 6 involves a small amount of Ni (< 10%) being desorbed relatively fast. The desorption of specifically ad sorbed, mononuclear bound Ni may account for this rapid Ni release. Th ereafter, Ni detachment was extremely slow, and the rate depended stro ngly on the experimental desorption method. Utilizing a conventional b atch technique, further Ni release became negligible. The non-removal of reaction products may have caused the formation of secondary precip itates. Under steady state conditions a constant Ni detachment rate wa s observed which we attribute to the dissolution of Ni surface precipi tates. Compared to the dissolution of crystalline Ni(OH)(2), Ni detach ment from pyrophyllite was slow. We hypothesize that the slow Ni detac hment is due to the dissolution of mixed Ni-Al-hydroxides which formed prior to the desorption experiment and which have a lower solubility.