薄膜外加電場處理程序：水中天然有機物及奈米碳管之影響研究

Abstract

本實驗使用無機陶瓷薄膜進行薄膜外加電場掃流過濾程序，以5mg/L DOC濃度之腐植酸（HA）模擬水中天然有機物作為進流液，並加入40 mg/L之單壁奈米碳管（SWCNT）與多壁奈米碳管（MWCNT）配置成三種水樣（HA、HA+SWCNT、HA+MWCNT）， 探討不同配比水樣在改變操作壓力、施加電場強度等操作因子，對於濾液通量衰減及水中有機物去除率之影響。 觀察薄膜過濾通量，施加有效電場強度以接近理論臨界電場值可以達到較佳情形，而當所施加之電場強度達到臨界電場值，UV254吸光度及DOC去除率便難再有顯著提升。因此就成本效益加以評估，外加電場輔以奈米碳管進行薄膜程序時，施加之電場強度以接近理論臨界電場為宜。加入奈米碳管之效益，在98kPa操作壓力下較為顯著，含有奈米碳管之配比，在不同電場強度下均比只含腐植酸之溶液有較佳的通量表現。而在49kPa下加入單壁奈米碳管之水樣，當電場強度由43.42 V/cm提高至72.36 V/cm，水樣之DOC去除率僅增加3.61％，相較於電場強度由19.30 V/cm提高至接近理論臨界電場43.42 V/cm時去除率增加18.11%要小很多。 在外加電場作用下，加入單壁奈米碳管之水樣對於UV254吸光度及DOC去除率有較顯著的提升，以操作壓力為49kPa時，其平均去除率較單純腐植酸溶液之去除率可提升11%，添加多壁奈米碳管之配比則與單純腐植酸溶液差異不大。顯示應用外加電場薄膜處理程序去除水中粒子時，欲達到目標去除物種之最適去除效率，不同種類之奈米碳管有其選擇性。 薄膜過濾之總阻力（Rtot）隨外加電場強度提高而顯著下降。外加電場產生之電泳作用使薄膜積垢與濃度極化阻力（Rs）減小，而電滲透作用亦使薄膜本身阻力（Rm）隨著施加電場強度的增加而下降。 奈米碳管加入腐質酸溶液後，僅持續攪拌但不經薄膜程序，則就其（UV254, DOC）平均去除率而言，單壁奈米碳管為（5.72%, 5.55%）、多壁奈米碳管為（1.30%, 1.26%），相較於經過薄膜外加電場處理程序，整體之去除率無論是UV254或DOC至少在50%以上有很大差異，且以單壁奈米碳管更為顯著。可見對於腐植酸有較佳吸附能力、碳結構較為完整之單壁奈米碳管，加上外加電場產生之電泳、電滲透現象，均是本系統可使過濾通量及污染物去除率提升之重要因素。

An electro-membrane cross-flow filtration was conducted with the inorganic ceramic tubular membrane in this study. Three various mixed solutions were prepared from 5mg/L DOC humic acid (HA) which was used to simulate nature organic matter in water, and 40 mg/L single-wall carbon nanotubes (SWCNT) or multi-wall carbon nanotubes (MWCNT). Both the effects of decrement in flux and removal of organic matters in water on different solutions under various pressures and electric field strength were investigated. The best performance of permeate flux and rejection of UV254 as well as DOC was resulted when applied electric field strength was near theoretical critical electric field value. With respect to the cost-effectiveness evaluation, it is advisable to apply the strength near critical electric field when an electro-membrane filtration process with carbon nanotube addition was implemented. The benefit of carbon nanotube addition to electro-membrane filtration process is more obvious under 98kPa pressure. The permeate flux of solutions with carbon nanotubes was better than pure humic acid (HA) solution under various electric field strength. When HA+SWCNT solution under 49kPa pressure, the DOC rejection only increased 3.61% with electric field strength rose from 43.42 V/cm to 72.36 V/cm. It was much less than 18.11% increase of DOC rejection when electric field strength rose from 19.30 V/cm to 43.42 V/cm which was close to the critical electric field strength. When electric field was applied, HA+SWCNT solution has dramatic improvement on rejection of UV254 and DOC. For example, the average rejection of HA+SWCNT solution was 11% more than that of HA solution under 49kPa pressure but there’s little difference between HA+MWCNT solutions and HA solutions. It showed the selectivity of different type of carbon nanotubes to the rejection of target substances in the electro-membrane filtration process. The total resistance (Rtot) of filtration process could be greatly reduced due to electrokinetic phenomenon induced by electric field application. The effect of electrophoresis caused the decline of the resistance of fouling and concentration polarization layer (Rs), as well as the membrane intrinsic resistance (Rm) under the influence of electro-osmosis while electric field strength increased. Average rejection of humic acid after adding carbon nanotubes without membrane process, (UV254, DOC) removal were (5.72%, 5.55%) of SWCNT and (1.30%, 1.26%) of MWCNT, respectively. It was much different compared with more than 50% rejection under electro-membrane filtration process, and seemed to be more obvious of adding SWCNT. It could be suggested that SWCNT with more complete carbon structure has better ability of adsorption to humic acid. This phenomenon could attribute to the electric field application like electrophoresis and electro-osmosis, both play important role of the system to enhance permeate flux and rejection of pollutant.