Kinetics and mechanisms of Zn complexation on metal oxides using EXAFS spectroscopy.

Zn(II) sorption onto Al and Si oxides was studied as a function of pH (5.1-7.52), sorption density, and ionic strength. This study was carried out to determine the role of the various reaction conditions and sorbent phases in Zn complexation at oxide surfaces. Extended X-ray absorption fine structure (EXAFS) spectroscopy was used to probe the Zn atomic environment at the metal oxide/aqueous interface. For both amorphous silica and high-surface-area gibbsite, Zn sorption kinetics were rapid and reached completion within 24 h. In contrast, Zn sorption on low-surface-area-gibbsite was much slower, taking nearly 800 h for a sorption plateau to be reached. In the case of silica, EXAFS revealed that Zn was in octahedral coordination with first-shell oxygen atoms up to a surface loading of approximately 1 micro molm(-2), changing to tetrahedral coordination as surface loading and pH increased. For the high-surface-area gibbsite system, the Znz.sbnd;O first-shell distance was intermediate between values for tetrahedral and octahedral coordination over all loading levels. Zn formed inner-sphere adsorption complexes on both silica and high-surface-area gibbsite over all reaction conditions. For Zn sorption on low-surface-area gibbsite, formation of Znz.sbnd;Al layered double hydroxide (LDH) occurred and was the cause for the observed slow Zn sorption kinetics. The highest pH sample (7.51) in the Zn-amorphous silica system resulted in the formation of an amorphous Zn(OH)(2) precipitate with tetrahedral coordination between Zn and O. Aging the reaction samples did not alter the Zn complex in any of the systems. The results of this study indicate the variability of Zn complexation at surfaces prevalent in soil and aquatic systems and the importance of combining macroscopic observations with methods capable of determining metal complex formation mechanisms.

Zinc speciation in a smelter-contaminated soil profile using bulk and microspectroscopic techniques.

A soil profile contaminated as a result of Zn smelting operations from the historic Palmerton, PA smelting facility was characterized using X-ray absorption fine structure spectroscopy (XAFS) and X-ray diffraction (XRD) as bulk techniques, coupled with electron microprobe (EM), and microfocused XAFS as microscopic techniques to determine the chemical forms of Zn and elucidate its geochemical fate. The black, organic matter-rich topsoil contained 6200 mg/ kg Zn and was strongly acidic (pH 3.2). Bulk XAFS revealed that about 2/3 of Zn was bound in franklinite and 1/3 bound in sphalerite. Both minerals may have been aerially deposited from the smelter operation. Microspectroscopy detected also minor amounts of Zn adsorbed to Fe and Mn (hydr)oxides as inner-sphere sorption complexes, which may have formed after weathering of the Zn minerals. About 10% of the total Zn in this sample could be easily leached. In contrast, the yellowish, loamy subsoil contained less Zn (890 mg/kg) and had a higher pH of 3.9. XAFS revealed that Zn was mostly bound to Al-groups and to a lesser extent to Fe and Mn (hydr)oxides. Minor amounts of outer-sphere complexes or organic matter-bound Zn species could also be detected. About 70% of the total Zn content could be easily leached, indicating that outersphere sorption complexes have been underestimated and/ or inner-sphere sorption complexes are weak due to the low pH. The Zn forms in the subsoil most likely derive from weathering of the Zn minerals in the topsoil. Due to the lack of minerals incorporating Zn and due to the low pH, the availability of Zn in the subsoil is as high as in the topsoil, while the total concentration is almost 1 order of magnitude smaller.

Combining selective sequential extractions, X-ray absorption spectroscopy, and principal component analysis for quantitative zinc speciation in soil.

Selective sequential extractions (SSE) and, more recently, X-ray absorption fine-structure IXAFS) spectroscopy have been used to characterize the speciation of metal contaminants in soils and sediments. However, both methods have specific limitations when multiple metal species coexist in soils and sediments. In this study, we tested a combined approach, in which XAFS spectra were collected after each of 6 SSE steps, and then analyzed by multishell fitting, principal component analysis (PCA) and linear combination fits (LCF), to determine the Zn speciation in a smelter-contaminated, strongly acidic soil. In the topsoil, Zn was predominately found in the smelter-emitted minerals franklinite (60%) and sphalerite (30%) and as aqueous or outer-sphere Zn2+ (10%). In the subsoil, aqueous or outer-sphere Zn2+ prevailed (55%), but 45% of Zn was incorporated by hydroxy-Al interlayers of phyllosilicates. Formation of such Zn-bearing hydroxy-interlayers, which has been observed here for the first time, may be an important mechanism to reduce the solubility of Zn in those soils, which are too acidic to retain Zn by formation of inner-sphere sorption complexes, layered double hydroxides or phyllosilicates. The stepwise removal of Zn fractions by SSE significantly improved the identification of species by XAFS and PCA and their subsequent quantification by LCF. While SSE alone provided excellent estimates of the amount of mobile Zn species, it failed to identify and quantify Zn associated with mineral phases because of nonspecific dissolution and the precipitation of Zn oxalate. The systematic combination of chemical extraction, spectroscopy, and advanced statistical analysis allowed us to identify and quantify both mobile and recalcitrant species with high reliability and precision.