| Summary: | 碩士 === 國立臺灣大學 === 環境工程學研究所 === 106 === 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.
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