Date of Graduation
Statler College of Engineering and Mineral Resources
Civil and Environmental Engineering
This study examined the adsorptive removal of selenate using iron coated granular activated carbon (Fe-GAC) and further investigated its complexation mechanisms and coordination structures using spectroscopic techniques. Adsorption kinetics and equilibrium experiments with initial selenium concentration of 1 mg/L were conducted under three different ionic strengths to study selenate adsorption behavior. Selenate adsorption reached equilibrium within 48 hours with more than 85% of the equilibrium capacities being obtained within the first 6 hours. High removal efficiency (i.e., > 75%) was achieved for pH range 2 - 5. Pseudo-second-order kinetic model characterized selenate adsorption kinetics well (R2 = 0.999) and the rate constant decreased with ionic strength. Adsorption capacity decreased significantly with increasing ionic strength, which was not observed in selenite adsorption with the same adsorbent. Competitive adsorption with other four oxy-anions (SiO 32-, SO42-, PO4 3- and CO32-) showed that selenate removal efficiency was reduced to various degrees in the presence of each individual anion. Competitive adsorption of binary adsorbates (selenite and selenate) showed that the Fe-GAC had higher adsorption affinity for selenite over selenate and the adsorption preference increased with ionic strength. Sheindorf-Rebuhn-Sheintuch (SRS) multi-adsorbate competitive adsorption model was applied to quantify the binary competitive adsorption between selenate and selenite.;Acid-base titration experiments were carried out under three ionic strengths (i.e., 0.001, 0.01 and 0.1M) and showed point of zero charge (pHpzc) at pH 7.8 for the tested Fe-GAC. It indicates a preferred positively charged surface for anions under acidic conditions. Raman spectroscope was used to determine the type of surface complex and the coordination state of adsorbed selenate at different pH with 0.1 M ionic strength. The results showed that inner-sphere and outer-sphere complexes were formed at pH 5 and 8, respectively. Additionally, mixture of mono-dentate and bi-dentate binding was observed at pH 5 with selenate. In comparison, selenite was only observed to form bi-dentate inner sphere bindings with surface sites.;Three electrolytes (LiCl, NaCl and KCl) were used as background electrolytes in this study to investigate their effects on selenate adsorption. Adsorption isotherms carried out under three ionic strengths (0.01, 0.05 and 0.1 M) were all well fitted with the Freundlich isotherm (R2 > 0.97). The results suggested that as ionic strength increased, selenate adsorption decreased to various degrees in the presence of all three electrolytes, which followed the order: KCl < NaCl < LiCl. Additionally, under the same ionic strength controlled by three background electrolytes, Kruskal - Wallis Statistical Test showed increasing difference among selenate adsorption as ionic strengths increased from 0.01 to 0.1 M. This result can be attributed to the variations of cation radius.;X-ray photoelectron spectroscope (XPS) was used to investigate the chemical state and composition of coated iron on GAC surface. The multiplet peak fitting results for high resolution XPS spectra for O1s and Fe2p3/2 indicated that Fe(III) was formed after coated with 0.1 M ferrous chloride, and alpha-FeOOH was the possible chemical composition. Furthermore, the suspension of Fe-GAC in NaCl electrolytes at studied pH range (3 - 8) did not cause appreciable changes of iron chemical states and composition. Raman spectra for selenate loaded Fe-GAC indicated the formation of bidentate inner sphere complex at pH lower than 7 and a mixture of outer sphere and monodentate inner sphere complex at pH 8. Theses results explained the low selenate adsorption under alkaline conditions. Two modification methods of Fe-GAC preparation were proposed based on the XPS and Raman results: (1) binary coating with other metal oxides in order to increase the point of zero charge (pHzpc); and (2) ferrous iron coating to reduce selenate first followed by high adsorption removal of selenite.
Zhang, Ning, "Study of Chemical States, Composition and Adsorption Behavior of Iron Coated Granular Activated Carbon (Fe-GAC) for Selenate Removal and Feasible Adsorbent Modifications" (2012). Graduate Theses, Dissertations, and Problem Reports. 481.