The nature of Zn precipitates formed in the presence of pyrophyllite

The partitioning of Zn to the pyrophyllite surface was studied as a function of surface loading for periods up to 4 months. Examination of the reaction products using X-ray absorption fine structure spectroscopy (XAFS) indicated the formation of a Zn precipitate at each surface loading. Comparison of the local structure of the surface precipitates to the structure of various hydroxide- and carbonate-bearing phases indicates the formation of a Zn-Al layered double hydroxide (LDH). The solubility of Zn following aging in pyrophyllite systems indicated that the initial Zn-Al LDH precipitates transformed to a more stable form. Increased Zn stability in these experimental systems may be attributed to an increase in LDH crystallinity (Ostwald ripening) or incorporation of Si within the LDH interlayer leading to transformation to a phyllosilicate-like phase. Our results support formation of an LDH precipitate as a precursor to Zn fixation in soils abundant in aluminosilicate minerals. These results augment recent findings that transition metals may form layered hydroxide and phyllosilicate-like precipitates during sorption to clay minerals. Acknowledgment of this process as a potential metal sequestration mechanism in certain soil types is important to assessment of contaminant attenuation. Development of a more comprehensive database of solubilities for these surface precipitates will facilitate more reliable estimates.