電荷對奈米過濾去除水溶液中全氟羧酸之影響

Abstract

全氟羧酸(perfluorocarboxylic acids, PFCAs)為工業上廣受應用之物質，惟被證實具毒性、持久性與生物累積性，對環境與人體健康均有危害。因此，許多研究致力於尋找成本合理且技術有效之處理方式以去除之。其中，薄膜過濾為一分離程序，未將物質破壞與降解，而能使其純化與濃縮，並回收再利用於製程中。當中，又以奈米過濾較超過濾有較高之截留成效，且其有較佳之通量而較逆滲透節能，故被視為去除與回收PFCAs的前瞻性技術之一。操作條件改變可以影響奈米過濾之成效，其中進流pH及掃留速度能影響奈米薄膜與溶質間電荷互動，進而影響兩者間電荷排斥作用，此排斥作用為主導奈米過濾去除帶電有機溶質之機制。為了優化奈米過濾之成效，探討能影響電荷排斥作用之操作條件為重要之課題。

本研究進行兩種商業化奈米薄膜(NF 90與NF 270)對兩種PFCAs水溶液(PFOA與PFHxA)之掃流式過濾試驗。奈米薄膜之滲透液通量及PFCAs截留率為衡量過濾成效之參數。探討進流pH與操作壓力對過濾成效之影響；掃留速度對NF 270；相同奈米薄膜之PFOA與PFHxA截留率。

研究結果顯示，進流pH提升至7.0及10.0時，NF 90與NF 270之滲透液通量會分別增加，並可以減少薄膜表面附近濃度極化與吸附積垢；PFOA 與PFHxA之截留率亦增加，表示提升進流pH可以強化NF 90與NF 270之截留成效。操作壓力與PFOA、PFHxA截留率及PFOA、PFHxA溶質通量無特定之關係。NF薄膜之滲透液通量隨操作壓力增加而增加，可能非因PFOA與PFHxA溶質通量受操作壓力之影響，而為溶劑通量(即水通量) 隨操作壓力增加而增加所致，惟其增加量較純水過濾時之情況要少。掃流速度由0.28 m/s提升至0.79 m/s時，濃度極化與吸附積垢幾乎不再存在，使得在三種進流pH測試條件下，NF 270對PFOA之截留率皆顯著地減少，推論適度之濃度極化與積垢可能可以改變薄膜表面之電荷特性，進而影響薄膜與溶質間之電荷互動，故對NF 270截流PFOA有幫助。

由過濾試驗之比較結果發現，NF 270與NF 90因薄膜材料特性之不同，而對進流pH之變化具不同之敏感度，造成不同之PFOA與PFHxA截留率。NF 90具明顯較高之PFHxA截留率，實際應用之可能性較高；而NF 270之滲透液通量與NF 270之PFOA截留率對進流pH之敏感度較高，在進流pH 10.0時，NF 270之PFOA截留率已相當接近NF 90者。NF 90與NF 270對PFOA之截留率皆較PFHxA者高，表示分子尺寸可以影響PFCAs 之截留率，且除了電荷排斥作用外，機械篩分亦為重要之截留機制。

Perfluorocarboxylic acids (PFCAs) which were widely used in industries were found to be toxic, persistent and bioacccumulative and had negative effect on environment and human health. Therefore, many research attempted to search the cost-effective techniques which can remove PFCAs. Among those treatments, membrane filtration is a physical separation process in which the materials can be purified, concentrated and recovered back to the industrial process, than a process which destroys and decomposes the target materials. Nanofiltration offers higher flux than reverse osmosis (RO) and better rejection performance than ultrationfiltration (UF). Therefore, nanofiltration is considered as an alternative promising technology for removal and recovery of PFCAs from wastewater. The operation conditions could have an effect on the performance of nanofiltration. The influent pH and crossflow velocity could affect the electrostatic interaction between NF membranes and solutes , and have an effect on the performance of nanofiltration. The electrostatic repulsion is the dominant mechanism which can remove the PFCAs. In order to optimize the performance of nanofiltration, the investigation of the factors which could affect the electrostatic repulsion is the key goal.

In this study, the crossflow filtration tests of two kinds of PFCAs (PFOA and PFHxA) in aqueous solution by two types of commercial NF membrane (NF 270 and NF 90) were conducted. The permeate flux of NF membrane and the rejection rates of PFCAs were the parameters for the filtration performance. The effects of influent pH and transmembrane pressure on the filtration performance were investigated. The effects of crossflow velocity on the filtration performance of PFOA by NF 270 were tested. The filtration performance of the same PFCAs by NF 90, NF 270 and the rejection rates of PFOA, PFHxA by the same NF membrane were compared, respectively.

The experimental results showed that the permeate flux of NF 90 and NF 270 increased, respectively, and the concentration polarization near the membrane surface and the adsorption fouling of membrane were mitigated when the influent pH was adjusted to 7.0 and 10.0. The rejection rates of PFOA and PFHxA by NF membrane also increased, respectively. Thus, increasing influent pH could enhance the rejection performance of NF 90 and NF 270. The rejection rates of PFOA, PFHxA and the PFOA, PFHxA solute flux are independent of transmembrane pressure, respectively. The permeate flux of NF membrane increased with increasing transmembrane pressure could not result from the effect of transmembrane pressure on the PFOA and PFHxA solute, but as a result of the increasing solvent flux (i.e. water fux) with increasing transmembrane pressure. However, the solvent flux increased less than the situation of pure water filtration. The absence of the concentration polarization near the membrane surface and the adsorption fouling of membrane when the crossflow velocity increased from 0.28 m/s to 0.79 m/s caused that the rejection rates of PFOA reduced significantly at three different influent pH conditions tested. Therefore, moderate concentration polarization and membrane fouling could change the charge characteristics of membrane surface and the interaction between membrane and solute, and might be helpful for the rejection of PFCAs by nanofiltration.

The comparison of experimental results obtained from the filtration tests demonstrated that NF 270 and NF 90 showed different response to the change of influent pH because of the different properties of membrane material. Thus, they offered the different rejection rates of PFOA and PFHxA. NF 90 offered higher rejection rate of PFHxA. Therefore, it had higher probability to use in practical engineering. The permeate flux of NF 270 and its PFOA rejection rates were more sensitive to the influent pH. Therefore, the rejection rate of PFOA by NF 270 was comparable to that by NF 90 at influent pH 10.0. The rejection rates of PFOA were higher than those of PFHxA by both NF 270 and NF 90. Thus, the molecular size could affect the rejection of PFCAs, and mechanical sieving was the important removal mechanism besides charge exclusion for PFCAs.