Interaction of Phosphate with Lithium Cobalt Oxide Nanoparticles: A Combined Spectroscopic and Calorimetric Study.

Adsorption of small ions such as phosphates to the surfaces of metal oxides can significantly alter the behavior of these materials, especially when present in the nanoscale form. Lithium cobalt oxide is a good model system for understanding small-molecule interactions with emerging nanomaterials because of its widespread use in lithium ion batteries and its known activity as a water oxidation catalyst. Here, we present a thermodynamic analysis of phosphate adsorption to LiCoO2 and corroborate the results with additional in situ techniques, including zeta potential measurements and attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy, at pH values relevant to potential environmental release scenarios. Flow microcalorimetry measurements of phosphate interaction with LiCoO2 at pH 7.4 show that there are two distinct exothermic processes taking place. Time-sequence in situ ATR-FTIR with two-dimensional correlation analysis reveals the spectroscopic signatures of these processes. We interpret the data as an interaction of phosphate with LiCoO2 that occurs through the release of two water molecules and is therefore, best described as a condensation process rather than a simple adsorption, consistent with prior studies, demonstrating that phosphate interaction with LiCoO2 is highly irreversible. Additional measurements for over longer times of 5 months show that phosphate adsorption terminates with one surface layer and that continued transformation over longer periods of time arises from H+/Li+ exchange and slow transformation to a cobalt hydroxide, with phosphate adsorbed to the surface only. To the best of our knowledge, this is the first time that flow microcalorimetry and two-dimensional correlation spectroscopy have been applied in tandem to clarify the specific chemical reactions that occur at the interface of solids and adsorbates.

Toxicity of manufactured zinc oxide nanoparticles in the nematode Caenorhabditis elegans.

Information describing the possible impacts of manufactured nanoparticles on human health and ecological receptors is limited. The objective of the present study was to evaluate the potential toxicological effects of manufactured zinc oxide nanoparticles (ZnO-NPs; 1.5 nm) compared to aqueous zinc chloride (ZnCl2) in the free-living nematode Caenorhabditis elegans. Toxicity of both types of Zn was investigated using the ecologically relevant endpoints of lethality, behavior, reproduction, and transgene expression in a mtl-2::GFP (gene encoding green fluorescence protein fused onto the metallothionein-2 gene promoter) transgenic strain of C. elegans. Zinc oxide nanoparticles showed no significant difference from ZnCl2 regarding either lethality or reproduction in C. elegans, as indicated by their median lethal concentrations (LC50s; p = 0.29, n=3) and median effective concentrations (EC50s; Z = 0.835, p = 0.797). Also, no significant difference was found in EC50s for behavioral change between ZnO-NPs (635 mg Zn/L; 95% confidence interval [CI], 477-844 mg Zn/L) and ZnCl2 (546 mg Zn/L; 95% CI, 447-666 mg Zn/L) (Z = 0.907, p = 0.834). Zinc oxide nanoparticles induced transgene expression in the mtl-2::GFP transgenic C. elegans in a manner similar to that of ZnCl2, suggesting that intracellular biotransformation of the nanoparticles might have occurred or the nanoparticles have dissolved to Zn2+ to enact toxicity. These findings demonstrate that manufactured ZnO-NPs have toxicity to the nematode C. elegans similar to that of aqueous ZnCl2.

Energetics of arsenate sorption on amorphous aluminum hydroxides studied using flow adsorption calorimetry.

Flow adsorption calorimetry was used to investigate the energetics of arsenate sorption on amorphous aluminum hydroxide (AHO) and its effect on surface charge and ion exchange. Arsenate sorption at pH 5.7 was exothermic and the molar heats of adsorption were quite variable, ranging from -3.0 to -66 kJ/mol. Repetitive exposure of the same sample to arsenate in the calorimeter showed that the AHO was able to regenerate a considerable amount of reactive surface over time periods as short as 15 to 20 min. The large variability in heats of arsenate adsorption and the ability to regenerate reactive surface is believed to result from the amorphous nature of the AHO used. Heats of Cl/NO3 exchange were much smaller and more consistent, ranging from about 3.0 to 6.0 kJ/mol. The molar ratio of exchangeable Cl:Al was about 6:1 for the AHO, indicating a highly porous material. At pH 5.7, arsenate sorption neutralized surface positive charge as measured by Cl/NO3 exchange. Only at the two highest loadings (>60,000 mg/kg) did arsenate sorption result in any negative surface charge as measured by Na/K exchange. These results showed that most of the arsenate was adsorbed by a mechanism that involved no increase in surface negative charge. The PZNC of the AHO decreased by about 1 pH unit when exposed to arsenate in the flow calorimeter. Exposure to arsenate in a batch system decreased the PZNC about 4 pH units. This difference in behavior between batch and flow systems was related to differences in the amount of arsenate adsorbed.