ABSTRACT
We report an extensive study of the optical and structural properties of NiWO4 combining experiments and density functional theory calculations. We have obtained accurate information on the pressure effect on the crystal structure determining the equation of state and compressibility tensor. We have also determined the pressure dependence of the band gap finding that it decreases under compression because of the contribution of Ni 3d states to the top of the valence band. We report on the sub-band-gap optical spectrum of NiWO4 showing that the five bands observed at 0.95, 1.48, 1.70, 2.40, and 2.70 eV correspond to crystal-field transitions within the 3d8 (t2g6eg2) configuration of Ni2+. Their assignment, which remained controversial until now, has been resolved mainly by their pressure shifts. In addition to the transition energies, their pressure derivatives are different in each band, allowing a clear band assignment. To conclude, we report resistivity and Hall-effect measurements showing that NiWO4 is a p-type semiconductor with a resistivity that decreases as pressure increases.
ABSTRACT
Hydroxyapatite (HAp) was successfully synthesized from egg shells, a low cost and easily available biodegradable waste, by the precipitation method and characterized by X-ray diffraction (XRD), scanning electron microscopy, Fourier transform infrared, and Brunauer-Emmett-Teller (BET) surface area analysis. The surface area of HAp was found to be 144 m2/g with a crystalline size of 9-99 nm from the BET and XRD data. The maximum fluoride removal efficiency within 1 h using 0.3 g of the synthesized adsorbent at pH 6 was 95%. The adsorption of fluoride followed second-order kinetics, indicating that chemisorptions are the rate-limiting step. The experimental data were well fitted with Langmuir and Freundlich isotherms, validating both monolayer and multilayer sorption during the fluoride adsorption onto the porous HAp. The positive adsorption of F- ions at the HAp interface can be attributed to ion exchange/ion pairing and H-bonding below the pHpzc of HAp (pHpzc = 8), and the negative adsorption can be attributed to the electrostatic repulsion between O- and F- ions at alkaline pH. Both physical and chemical adsorption phenomena were also evidenced from the molecular parking area data. The results of a batch experiment show that the HAp synthesized from egg shells can be used as an effective, low-cost adsorbent for fluoride removal from a contaminated aqueous solution as well as groundwater compared to other adsorbents.
