利用靜電紡絲技術固定微藻於殼聚醣纖維膜去除水中重金屬

Abstract

近年來利用生物復育重金屬污染的研究日漸熱門，其中使用微藻做為吸附劑的研究也顯示自營生長的微藻具有去除水中重金屬之潛力，而使用材料固定生物體作為吸附劑的方式也蔚為風潮，雖生物體因固定於材料內部或表面，易降低其與污染物接觸頻率以及減少吸附位，造成固定化生物體去除污染物能力常低於懸浮態生物體，但固定化後生物體仍可保持活性、吸附能力並藉此降低系統空間需求、提高重複使用潛力、提高的環境耐受性、提高污染物回收潛力與更簡易的操作與維護等優點。 本研究主題為利用靜電紡絲技術製作具有固定化微藻功能之纖維膜，並藉此去除水中重金屬之污染。靜電紡絲纖維膜具有高比表面積、易改質、材料多樣等特性，因此優選出固定微藻的最佳方法有其重要性，經過三種固定微藻方法比較（夾層法、浸塗法、共紡法），並發現浸塗法較適合批次且無水壓下使用；夾層法需要給水壓力提供必要流速以維持微藻活性；共紡法因微藻需投入酸性靜電紡絲溶液中，因此不適合非嗜酸性的小球藻。 此外，本研究進一步評估出在浸塗法下的最佳化靜電紡絲溶液比例（殼聚醣:聚乙烯醇 = 60:40）與其操作參數的最佳條件；聚乙烯醇因具有水中溶解特性，因此採用浸泡法、噴覆法、乾燥皿法等三種戊二醛交聯方式進行比較，結果發現噴覆法無明顯交聯效果，浸泡法與乾燥皿法皆能有效交聯以降低水溶速度，但因為浸泡法需要大量使用到有毒戊二醛，故選用乾燥皿法做為交聯方法。殼聚醣因具有功能性氨基與羥基可做為固定微藻之吸附位，但會因為不同溶液酸鹼值而產生帶電性差異，本研究得出，溶液在pH=3、4、5、6、7下，面積12.56 cm2之纖維膜固定微藻之生物量分別為-2.6、9.3、29、19.3、9 mg，以pH = 5有最佳固定微藻效果，而重金屬吸附效果以pH=6較佳，並確認纖維膜在酸性溶液條件下，有較高的有機物溶解並可能與微藻表面吸附位結合而降低吸附效果，因此反而在pH = 6下固定微藻纖維膜具有最佳的重金屬去除效果，而如預期固定化微藻確實會導致吸附能力降低，在pH = 6條件下，單位微藻重量之重金屬吸附量從懸浮態之9.08 mg/g - Ni下降到固定化微藻之5.8 mg/g - Ni以及14.09 mg/g -Cu下降到10.54 mg/g -Cu，而因為交聯纖維膜吸附位可以提供負電物質良好去除效果，單位微藻重量之鉻(CrO72-)吸附量反而從懸浮態之4.26 mg/g提升到59.7 mg/g。 本研究成功利用靜電紡絲技術生產以殼聚醣為主要材料的纖維膜，其具有固定微藻效果，並能保持微藻活性以及去除重金屬能力。

In recent years, bioremediation of heavy metals has attracted more and more attention, and studies using microalgae as an adsorbent have shown potential to remove heavy metals in contaminated water by autotrophic microalgae. Immobilization as one of important approaches to contain large amount of microalgae in smaller space, can not only maintain microalgal activities and adsorption capacity on heavy metals, but also reduce space requirements, increase reuse potential, improve environmental tolerance, and simplify operation and maintenance. In this study, we successfully fabricated a fibrous membrane with immobilized microalgae on it by utilizing electrospinning technique to remove heavy metal in contaminated water. Fibrous membrane based on electrospinning technique has the characteristics of high specific surface area, ease of modification, various materials to choose, etc. Hence evaluating methods for immobilizing microalgae, such as interlayer method, dip-coating method and blending method and finding out the best one is necessary. Results indicated the interlayer method needs to provide flow rate to maintain microalgae activity, whereas the blending method is only suitable for acidophilic microalgae as the microalgae needs to be added to the acidic electrospinning solutions. The dip-coating method is more suitable for batch application. In addition, optimal ratio of the electrospinning solution （chitosan : polyvinyl alcohol = 60:40） has been observed, and optimal electrospinning of polyvinyl alcohol and chitosan have been evaluated, respectively. For polyvinyl alcohol, the soluble properties in water was addressed by three glutaraldehyde crosslinking methods, including immersion method, spraying method, and dry dish method. Results demonstrated the spraying method has no cross-linking effect, and both the immersion method and the dry-dish method can effectively cross-link to reduce water solubility. However, because of the large amount of toxic glutaraldehyde used in the immersion method, the dry-dish method was chosen instead as the cross-linking method. The amino and hydroxyl groups of chitosan can be used as adsorption sites for microalgae, but differences in surface potential depend on the pH of the solution. Therefore, the effect of pH on the immobilization of microalgae was tested respectively at solution pH = 3, 4, 5, 6, 7, and results showed the biomass of microalgae, which was successfully adsorbed on the 12.56 cm2 membrane was -2.6, 9.3, 29, 19.3, 9 mg, respectively, suggesting pH = 5 was the optimal condition for immobilization of microalgae. However, it is speculated that the acidic condition of the solution caused a slight dissolution of chitosan. The surface adsorption site of the microalgae may bind to the chitosan to reduce the adsorption effect. Therefore, instead of pH = 5, pH = 6 was observed to provide the best heavy metal removal effect. However, the adsorption capacity of microalgae to remove heavy metals after immobilization is lower than that of suspended microalgae. After the immobilization of microalgae at pH = 6, the adsorption capacity of microalgae for Ni and Cu decreased from 9.08 and 14.09 to 5.8 and 10.54 mg/g, separately. Although the immobilization of microalgae causes to lower adsorption capacity of microalgae, the membrane immobilized by microalgae provides adsorption sites for negatively charged ions due to the positively charged functional groups of the chitosan. The adsorption capacity of microalgae for Cr (CrO72-) significantly increased from 4.26 to 59.7 mg/g. In this study, we first successfully constructed the microalgae/chitosan-based electrospun membrane by utilizing electrospinning technique to remove heavy metals, which can immobilize microalgae, maintain microalgal activities and remove heavy metals simultaneously. Nonetheless, further research will still be needed to improve the capacity of this microalgae/chitosan-based electrospun membrane on the removal of heavy metals.