| Summary: | 碩士 === 國立成功大學 === 環境工程學系碩博士班 === 91 === Arsenic is commonly present in the groundwater of southwestern and northeastern parts of Taiwan. Although the arsenic concentration in the groundwater may be as high as a few hundred □g/L, some of the groundwater is still used as drinking water sources. To comply with national drinking water standard of Taiwan for arsenic, which is 10 □g/L, advanced treatment techniques, such as granular ferric hydroxide (GEH), are needed. Granular ferric hydroxide is a new adsorbent that is designed for the removal of arsenic from water. Therefore, the kinetic and equilibrium adsorption of arsenic onto GEH is still not clear. In this study, transport and equilibrium of arsenate and arsenite onto GEH is examined.
A batch reactor with temperature control was used to determine the adsorption kinetics and adsorption capacity for arsenate and arsenite onto GEH. The kinetic experiments showed that equilibrium is established for both arsenate and arsenite after about 250 hours. Equilibrium experiment revealed that the uptake of arsenate decreased as pH increased, while the uptake of arsenite increased until pH 7.5, and then dropped as pH increased. Both Freundlich and Langmuir isotherm equations can successfully describe the equilibrium data.
The competition of phosphate and sulfate with arsenate for the adsorption on GFH was also studied. The presence of sulfate did not affect the uptake of arsenate for most pH condition. As pH is less than 4, arsenic uptake is suppressed by sulfate. For the effect of phosphate, the arsenate uptake reduced significantly due to the presence of phosphate, especially under low pH condition.
A pore diffusion model, combined with isotherm parameters, was used to simulate the adsorption kinetic data. In fitting the models to the experimental data, only one parameter, pore diffusivity (Dp), is adjusted. The models conform closely to the experimental data, and the extracted pore diffusion coefficient was 7.0×10-11m2/sec for arsenate and 4.0×10-11 to 8.0×10-11m2/sec for arsenite. These diffusivities are very close to those for arsenate and arsenite in activated alumina grains reported in the literature.
A Triple Layer Model (TLM) was employed to simulate the equilibrium data of arsenate onto GEH and another metal oxide, activated alumina. In the model, three complexation reactions were needed to simulate the adsorption data for GEH, while only one reaction is needed for activated alumina. The models fit the equilibrium uptake data for both oxides fairly well. The extracted equilibrium constants from the models were then used to predict the observed data under different conditions. The model predictions conform closely to the data, substantiating the appropriateness of the model.
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