STABILITY OF LAYERED NI HYDROXIDE SURFACE PRECIPITATES-A DISSOLUTION KINETICS STUDY

Abstract

In recent years, studies have shown that sorption of metals onto natural materials results in the formation of new mineral-like precipitate phases. However, the stability of the precipitates and the potential long-term release of the metal back into the soil solution are poorly understood. Therefore, we investigated the influence of residence time and dissolution agent on the release of nickel from three sorbents, pyrophyllite, talc, and gibbsite, complementing the macroscopic observations with X-ray absorption fine structure (XAFS) and diffuse reflectance spectroscopies (DRS), and high-resolution thermogravimetric analysis (HRTGA). Dissolution of the surface precipitates was compared to dissolution of reference Ni compounds.In the sorption experiments conducted at pH 7.5, Ni-Al layered double hydroxide (LDH) formed in the presence of pyrophyllite and gibbsite, and α-Ni hydroxide formed with talc, in line with former studies. The stability of the phases decreased from Ni-Al LDH on pyrophyllite to α-Ni hydroxide on talc to Ni-Al LDH on gibbsite. This sequence could be explained by the greater stability of precipitates with Al-for-Ni substituted hydroxide layers compared to pure Ni hydroxide layers, and by the greater stability of precipitates with silicate-for-nitrate exchanged interlayer. With increasing residence time, all precipitate phases drastically increased in stability, as was documented by decreasing amounts of Ni released by nitric acid (HNO3) and ethylenediaminetetraacetic acid (EDTA) treatments. This aging effect may be partly explained by the silicate-for-nitrate exchange during the first days of reaction, and subsequently by silicate polymerization and partial grafting onto the hydroxide layers (Ford et al., 1999). However, even Si-free, Ni-reacted gibbsite showed a substantial aging effect, suggesting that factors other than interlayer silication may be equally important. Such a factor may be crystal growth due to Ostwald ripening. The Ni precipitates which remained at the end of the dissolution experiments were structurally similar to the precipitates at the beginning of the dissolution, indicating that no preferential dissolution of a less stable phase occurred. Therefore, the precipitate phase within each sorbent system was apparently homogeneous in structure.