以薄膜外加電場應用於含全氟辛酸廢水之研究

Abstract

全氟碳化物(perfluorinated compounds, PFCs)因在環境中不易被分解且具有生物累積等特性，近年來受到各領域之關注，目前由於半導體產業仍未實質找到其替代物質，仍為環境中一主要汙染源。全氟辛酸(perfluorooctanoic acid, PFOA)為PFCs中最具代表物質之一，比起其他PFCs更具毒性、更難以分解。由於實場廢水成分複雜，故本研究以濃度0%、0.05%、0.1%二氧化矽(SiO2)及0 mg/L、20 mg/L、40 mg/L腐質酸(HA)，分別做為廢水中無機及有機物之代表，配置成九種不同背景基質之水樣，並討論外加電場在薄膜過濾程序中對通量及PFOA去除率之影響。 通量部分，二氧化矽及腐質酸皆會對薄膜表面造成積垢現象，施加於臨界電場95 V/cm時，最終過濾之正規化通量比值J/J0從4.2%提升到75% (SiO2=0.05%)，腐質酸則是從56%提升到100% (HA=20 mg/L, E=142.5 V/cm)。在阻力分析上，可觀察到總積垢阻力(Rf)隨著電場強度增加而減小，但在大於臨界電場時，阻力又會以不可逆積垢(Ri)增加。 去除率部分，在無施加電場下，薄膜程序無法直接經由篩濾作用提升PFOA去除率；但當背景基質僅有0.1%二氧化矽時，PFOA去除率提高至31%，此因為薄膜表面二氧化矽積垢使其帶電性增加，利用靜電斥力避免PFOA靠近膜面；當背景基質僅有腐質酸時，去除率則不因薄膜表面積垢而有大幅度之改變。施加電場下，PFOA可直接經由電泳作用力分離，而去除率隨著電場強度升高而提升(SiO2=0%, HA=0 mg/L)，於142.5 V/cm電場中去除率可達49%。若背景基質存在二氧化矽於臨界電場95 V/cm下，其去除率可從41% (SiO2=0%)提升到79% (SiO2=0.1%)，此亦為薄膜表面積垢提高靜電斥力使然；但在背景基質含腐質酸之情況下則對去除率無影響。此外背景基質同時存在二氧化矽及腐質酸時，於47.5 V/cm電場強度下，PFOA去除率會隨腐質酸濃度增加而下降，其因腐質酸積垢會使薄膜表面電位降低。綜合以上結果，以本程序應用於含全氟辛酸之模擬水樣，在兼顧水質水量之考量下，最適操作之電場強度應以臨界電場附近最佳。 由實場廢水與模擬水樣實驗比較，顯示實場廢水亦因外加電場之施加，可有效地提升PFOA之去除率，顯示外加電場具有實務應用之可行性，惟實場廢水之混凝加藥之前處理，會導致水中導電度大幅增加，造成通量及去除率下降。故若能應用此模組於混凝處理前之廢水，對於實務應用之競爭性將能有所提升。

Recently, it has been concerned by every field that PFCs have the characteristics of persistence and bioaccumulation. Because semiconductor industry cannot find the substitute of PFCs, it is still one of the major pollution sources in our environment. Compared to other PFCs, PFOA has more toxicity and is hard to decompose. Therefore, PFOA is the most representative material of PFCs. Because of the complexity of wastewater, this study used 0%, 0.05%, 0.1% SiO2 and 0 mg/L, 20 mg/L, 40 mg/L humic acid (HA) as the representatives of inorganic and organic matters of wastewater to dispense nine various background matrix samples. In addition, the study investigated the strength of electric field that affects the quantity and quality of filtrate. For the flux part, both of SiO2 and humic acid cause the fouling of membrane surface. Under 95 V/cm electric field, normalized flux ratio (J/J0) raised from 4.2% to 75% (SiO2=0.05%) and 56% to 100% (HA=20 mg/L, E=142.5 V/cm), respectively. In resistance analysis, it was observed that total resistance (Rf) would decrease along with the strength of electric field. However, resistance would increase due to irreversible resistance (Ri) when electric field was above the critical electric field strength. For the rejection part, membrane process could not separate PFOA via screening mechanism without electric field. The rejection of PFOA elevated to 31% when background matrix contained 0.1% SiO2, because membrane surface charge is changed due to SiO2 fouling. However, PFOA rejection would not change when membrane fouling by humic acid. PFOA can be directly separated by electrophoretic force, and its rejection would increase with application of electric field strength. Under background matrix contained 0.1% SiO2, PFOA rejection increased from 41% (0% SiO2) to 79% (0.1% SiO2) with 95 V/cm of critical electric field strength, which also revealed the effect of electrostatic repulsion. On the other hand, under background matrix contained HA, PFOA rejection had no significant change with electric field application. Moreover, PFOA rejection would decrease with the increase of humic acid concentration when background matrix contained SiO2 (47.5 V/cm). This result could attribute to the humic acid fouling which decreased membrane surface charge. In conclusion, the above outcomes showed that the optimal operating electric field is close to the critical electric field. According to the comparison between wastewater and simulation sample, the data demonstrated that PFOA rejection could be improved by application of electric field. However, wastewater treatment plants with the coagulation/flocculation process that could greatly increase conductivity, the flux and rejection would be reduced. As the result, if electro-membrane filtration could be applied in front of coagulation/flocculation process, it would improve its rejection efficiency.