Kinetics of Ni(II) sorption and desorption on kaolinite: Residence time effects

Recent studies have shown that aging or increased residence time can reduce the availability of trace element cations sorbed to common soil minerals. Numerous explanations have been given to explain the observed residence time effect. However, most of these studies begin only with sorbed species and not surface precipitates. The formation of Ni2+ surface precipitates on common soil minerals has been observed in the laboratory by a number of researchers. Accordingly, the influence residence time on the sorption/desorption kinetics of Ni2+ on kaolinite was examined. Nickel sorption kinetics were conducted at three aqueous concentrations (0.10, 0.50, 0.75 mM) of Ni2+ in the presence of 25 g L-1 kaolinite at pH 7.5. More than 99% of the Ni2+ was sorbed to the kaolinite surface at the end of 14 days for all aqueous concentrations of Ni2+ Adsorption was characterized by an initial fast reaction followed by a slower reaction. Both reactions followed first order kinetics. Based on previous spectroscopic studies, the fast reaction was attributed to chemisorption, whereas the slow reaction was attributed to nucleation and surface precipitation of a Ni-AL layered double-hydroxide (LDH). Desorption experiments were conducted on kaolinite samples after 14 days (short-term) and 20 weeks (long-term) in the presence of 1 mM oxalate at pH 6.0. Similar to adsorption kinetics, desorption kinetics were characterized by an initial rapid reaction followed by a slower reaction, both of which followed first order kinetics. For all surface coverages the total quantity of Ni2+ desorbed and the desorption rate coefficients (k(1) and k(2)) were greater for the short-term than for the long-term experiments. It is suggested that the residence time effect observed for the slow desorption/dissolution reaction was caused by an increase in crystallinity of the LDH surface precipitate and, to a lesser extent, phase transformation into a Ni-Al phyllosilciate. In contrast, several processes may be responsible for the residence time effect observed for the fast desorption/dissolution reaction, including movement of weakly bound Ni2+ to a more strongly bound phase (eg, change in the type of surface complex), diffusion into micropores or intraparticle spaces, or an increase in crystallinity (eg, Ostwald ripening) of weakly precipitated Ni2+ The above results demonstrate and suggest potential mechanisms for the long-term natural attenuation of trace metal cations such as Ni2+ adsorbed to mineral surfaces.