| Summary: | 碩士 === 國立臺北科技大學 === 環境工程與管理研究所 === 105 === Forward osmosis (FO) is an emerging membrane separation process of water driven by the osmotic pressure difference across a semipermeable membrane without the aid of external energy. FO is considered a promising technology due to the advantages of low energy consumption and high solute rejection. In Taiwan, thin film transistor liquid crystal displays (TFT-LCD) based industries are growing exponentially. In the optoelectronic industry, iodide is an important raw material for the process so that optoelectronic wastewater contains high concentrations of iodide. This study aims at concentrating iodide by utilizing FO system, then reused for TFT-LCD production. Cellulose triacetate (CTA-ES) membranes were used for all these FO experiments. Potassium iodide solution was used as the feed solution and EDTA-2Na at pH8 was utilized as the draw solution which produced a lower reverse salt flux 0.63 g/m2h. The optimum flow rate was maintained at 7.7 L/m2h when EDTA-2Na concentration was 0.6 M and the cross flow rate was 700 cm/min. In this experiment, the rejection efficiency was not affected by the change of the feed concentration, the draw concentration and the cross flow rate. The rejection efficiency was found to be 93%. The overall results showed that with potassium iodide solution between pH 3 and 8, the I- species was found to be more than 99%, whereas I2 was measured to be less than 1%. Therefore, the oxidation process was applied to change the iodide type in the solution. By adding an oxidizing agent (NaClO) into the FS side, part of the iodide were oxidized to iodine and could be combined to form a large ion radius of the triodide, resulting in an improvement of the iodide rejection efficiency. In particular, iodide type (I-, I2 and I3-) and the molar ratio were analyzed at different concentrations of oxidizing agent and different iodide solution pH values. The best efficiency of iodide oxidation was obtained when the sodium hypochlorite concentration was 0.0144 M and the solution pH value was 3. The highest conversion efficiency is 44.77% (iodide is converted into iodine) so that the rejection efficiency from the original 93% increases to 98.49%. Eventually, the iodide concentration decreased from 430 ppm to 92 ppm in the permeate stream. The concentration of iodide increased from the initial 6,120 mg/L to 15,665 mg/L along with the concentration efficiency at 2.6 times after 48 hours of long-term operation. Finally, the membrane distillation process was utilized in order to recover EDTA-2Na. The recovery rate was 70% and the rejection rate was 99.99% after 7 hours of operation. The hybrid FO-MD system was effectively applied to concentrate and purify the draw solution that made it possible to reuse iodide.
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