Ambient-temperature synthesis, evolution, and characterization of cobalt-aluminum hydrotalcite-like solids

Double hydroxide solids precipitated homogeneously from three laboratory-sy nthesized aqueous solutions that simulated mildly contaminated surface or g roundwater. Over a limited pH range, precipitates formed rapidly from disso lved ions, and more slowly by incorporating ions dissolving from other soli ds, including highly soluble aluminous solids. The precipitates were charac terized by size and shape via transmission electron microscopy (TEM), by co mposition via inductively coupled plasma-mass spectrometry (ICP-MS) of moth er solutions and analytical electron microscopy (AEM) of precipitates, and by structure via powder X-ray diffraction (XRD), TEM, and extended X-ray ab sorption fine structure (EXAFS) spectroscopy. They were identified as nanoc rystalline cobalt hydrotalcite (CoHT) of the form [Co(II)(1-x)Al(III)(x)(OH )(2)](x+)(A(x/n)(n-)). mH(2)O, with x = 0.17-0.25, A = CO32-, NO3-, or H3Si O4-, n = anion charge and m undetermined. Complete solid solution may exist at the macroscopic level for the range of stoichiometries reported, but cl ustering of Co atoms within hydroxide layers indicates a degree of immiscib ility at the molecular scale. Composition evolved toward the Go-rich endmem ber with time for at least one precipitate. The small layer charge in the x = 0.17 precipitate caused anionic interlayers to be incomplete, producing interstratification of hydrotalcite and brucite-like layers. Solubility pro ducts estimated from solution measurements for the observed final CoHT stoi chiometries suggest that CoHT is less soluble than the inactive forms of Co (OH)(2) and CoCO3 near neutral pH. Low solubility and rapid formation sugge st that CoHT solids may be important sinks for Co in contact with near neut ral pH waters. Because hydrotalcite can incorporate a range of transition m etals, precipitation of hydrotalcite may be similarly effective for removin g other trace metals from natural waters.