Characterization Of Cadmium By Selective Adsorption On Nanoparticles In Environmental Samples

In this paper, Titanium dioxide nanoparticles were employed for the sorption of cadmium ions from aqueous solution. The adsorptive capability and mechanism of cadmium on nano-TiO₂ at different sorption time, temperature, pH value, and concentration were studied in thesis. The research contents and experimental results as follows:1. The conditions of determining trace cadmium by acetylene-oxygen flame atomic absorption spectrometry was optimized, the optimum conditions was obtained: wavelength 228.8 nm, slit width 0.7 H, the filament current of Cd hollow cathode lamp 4.0 mA, acetylene flow rate 1.4, air flow 16.0 L·min^-1. The detection limits of determining Cd（Ⅱ） ions was 0.0016mg·L^-1, and the relative standard deviations was 0.66%.2. The effect of different sorbents on the adsorption behaviour for cadmium in aqueous was studied, and the nano-TiO₂ （rutile） was the best sorbent. Both adsorption and desorption of nano-TiO₂ to cadmium ions depended on the initial pH of the solution. The sorption has been optimized with respect to the pH of the solution. A removal percentage （ > 97%） was found for Cd（Ⅱ） in the pH range of 4.0 7.0.The best conditions of static adsorption method are as follows: The adsorption after ultrasonically dispersed for 3 min and static adsorption for 10 min, and the adsorption percentage was up to 97%. 7.0 mL 0.1 mol·L^-1 HCl was used for the desorption of Cd（Ⅱ） on nano-TiO₂, and the recovery was 95%.Dynamic adsorption:the addition of 80 mg nano-TiO₂ and 5 g porous carrier can remove 90% Cd（Ⅱ） in the water sample.It has reached the requirement of potable wanter quality.3. The adsorption of Cd（Ⅱ） ions on nano-TiO₂ properly correlate with the second-order rate equation. The rate constant of second-order adsorption of nano-TiO₂ was 1.89 g·mg^-1·min^-1,and the activation energy of nano-TiO₂ was 3.16 kJ·mol^-1. In the overall rate process, the mechanism of the adsorption of cadmium ions on nano-TiO₂ is predominantly intraparticle diffusion, but not the only rate-controlling step, the adsorption rate is also controlled by the diffusion process outside the particles.4. The Langmuir model could describe the adsorption of nano-TiO₂ to Cd（Ⅱ） ions successfully over the whole range of concentration studied. At room temperature, maximum adsorption capacity was 6.34 mg·g^-1. The value of standard enthalpy and standard entropy （ΔH⁰,ΔS⁰ > 0） may be interpreted as the endoergic adsorption process. The negative value ofΔG⁰ showed the adsorptions of cadmium by nano-TiO₂ were spontaneous process. The adsorption energy of nano-TiO₂ was 11.54 kJ·mol^-1, which indicate that the process belongs to ion-exchange.5. The effect of the most common heavy metal ions on absorption of cadmium by nano-TiO₂ in industrial water is in this order: Zn²⁺>Cu²⁺>Pd²⁺ ,the common anions on Cd（Ⅱ） adsorption is in the order of PO₄^3->NO₃^->SO₄^2->CO₃^2->Cl^->F^-. Forthermore, other coexisting ions almost didn’t have effects and the allowable volume for these species were large. With the increase of ionic strength, Cd（Ⅱ） adsorption decreases, and the higher Cd（Ⅱ） ion concentration, the greater impact. With Se（IV） and Cr（VI） exists, the adsorption of cadmium were significantly reduced, and Cr（VI） has greater impact.