Kinetics and mechanisms of the formation and dissolution of nickel(II) surface precipitates on clay minerals and metal oxides using macroscopic, spectroscopic, microscopic, and thermogravimetric techniques

Retention of heavy metals at the mineral-water interface is an important process in maintaining environmental health and quality. Knowledge of the kinetics and mechanisms of heavy metal sorption and desorption on clay mineral and oxide surfaces is necessary in order to accurately predict the fate and mobility of these environmental contaminants. This study examines the kinetics and mechanisms of the formation and dissolution of Ni(II) surface precipitates on clay minerals and oxides using macroscopic, spectroscopic (XAFS and DRS), microscopic (AFM and HRTEM), and thermogravimetric (HRTGA) techniques.; Macroscopic batch studies demonstrated that Ni sorption reactions were initially rapid and the rate then slowed until relatively no Ni was in solution. Initial product formation of Ni-sorbent reactions, confirmed through spectroscopic and microscopic methods, resulted in mixed Ni-Al layered double hydroxides (LDH) phases on pyrophyllite and gibbsite, and alpha-Ni(OH)2 precipitates on talc, amorphous silica, and the gibbsite/silica mixture. As aging time progressed, experimental evidence showed that the precipitates on the Si-containing sorbents transformed from a nitrate-containing interlayer to a Si-containing interlayer which underwent further conversion to a Ni-phyllosilicate precursor. Additionally, temperature sorption studies suggest that these mineral-like precipitate reactions are surface-controlled and an associative mechanism that consume energy.; Formation of these precipitate phases drastically reduces metal concentration in soil and sediment solutions, and effectively competes with adsorption onto soil minerals. However, an important question that has been left unanswered is the stability of the precipitates and the potential long-term release of the metal back into the environment. Proton- and ligand-promoted dissolution via a batch replenishment technique of Ni-Al LDH and alpha-Ni(OH)2 precipitates aged from one hour to two years showed that as aging time increased, the amount of Ni removed from the precipitate phase decreased, indicating an increase in stability with residence time. The aging effect may be attributed partly to the solid-state transformation of the precipitate phases (silication of Ni-Al LDH and alpha-Ni(OH)2), and partly to crystal growth due to Ostwald ripening. Spectroscopic, microscopic, and thermogravimetric analyses show that during dissolution, the chemical environment of the precipitate was not changed, however, quantitative changes were evident as observed in the macroscopic studies.