| Nanofiltration (NF) membrane plays an important role in the field of membrane filtration technology. Based on the special configuration structure, thin-film composite NF membrane has becom the predominat NF membranes for its advantages such as higher permeability, better compact resistance and super separation selectivity when compared with the primitive asymmetric NF membrane. Nevertheless, with the industry developing, the waste fluids become more complicated and their treatment becomes more difficulty. Membranes adopted to treat these waste effluents not only need nicer separation performance but also should possess good physical and chemical stability in order to assure the long-term separation performance. Nowadays, the common polyamide (PA) and cellulose acetate (CA) membranes are all failed to treat the strong acidic fluids due to their insufficient acid resistance. While other acid stable polymer materials like polypropylene (PP) and polysulfone (PSf) show bad separation performance. One of the key means to slove the above problem is to develop acid-resistance NF membrane through the selection of membrane material with good acid stability, the optimization of membrane fabrication technique, as well as membrane modification.The objective of this study is to develop the acid-resistant nanofiltration membrane. To achieve this goal, the chemical-stable material polysulfonamide (PSA) was selected as the membrane material to prepare nanofiltration membrane possessing both good separation performace and superior acid resistance. Firstly, the PSA selective layer was formed on the PSf support membrane through the interfacial polymerization of piperazine (PIP) in aqueous phase and1,3,6-naphthalenesulfonate chloride (NTSC) in organic phase. The surface chemical property and morphology of the resultant composite membrane were investigated. The membrane preparation parameters such as the monomer concentration, additives, reaction and curing conditions were studied systematically to obtain the optimal membrane preparation conditions. Secondly, the separation performance of the PSA composite membrane to aqueous solutions containing different salts or dyes were evaluated through cross flow permeation tests employing a circulation filtration testing apparatus under different feed concentrations and the operation pressures.Thirdly, the acid resistance of the PSA membrane was investigated in terms of the changes of the membrane surface chemical property and the separation performance through static acid soaking test and running test under acidic condition, respectively. For comparasion polyamide (PA) composite nanofiltration memebrane prepared from trimesoyl chloride (TMC) and PIP was also studied. Additionally, the performace of the resultant PSA membrane in treating copper/acid wastewater was also evaluated.Finally, membrane modification was performed by incorporating the second monomer TMC into the organic phase, through which modified membranes with the selective layer of PSA-co-PA and PSA/PA (or PA/PSA) could be obtained. The effects of the NTSC and TMC concentrations in organic phase on the performance of the PSA-co-PA membrane were investigated, and the separation performance, the hydrophilicity as well as the acid stability of the composite membranes before and after modification were compared. The conclusions were as follows:(1) The results of attenuated total reflectance fourier transform infrared spectroscopy (ATR) and the zeta potential measurements revealed that the partially cross-linked PSA reactive layer were successfully formed on the PSf support. The isoelectric point of the membrane surface was at pH3.07, the composite membrane was negatively charged at neutral pH due to the existence of large amount of the residual sulfonic groups, which were favarable to the membrane performance. According to the morphological studies, the surface layer of the resulting membrane was relative loose and showed an un-conspicuous ridge and valley structure with a root mean square roughness (Rms) of20.9nm, and the thickness of the thin layer was less than0.3μm. The loose structure of the thin surface layer was possibly due to the relatively low reactivity of monomer NTSC. (2) Preparaing parametric studies indicated that, the effects of the monomer concentrations, the reaction and curing conditions on the membrane performance were remarkable. With the monomer concentration increasing, the rejection rate of the PSA membrane to Na2SO4increased firstly and then leveled off. The increase of the reaction temperature was useful to the improvement of the rejection, while the prolongation of reaction time led to the serious flux reduction. Membranes with maximal rejection rate could be obtained by optimizing the reaction and curing temperatures. The optimal conditions for the preparation of the PSA composite membrane were as follows:PIP=1.32w/v%, triethylamine=4w/v%, lauryl sodium sulfate=0.01w/v%, NTSC=0.0316w/v%, ethylene glycol monomethyl ether=1.0w/v%, reaction time=10min, curing temperature=100℃and curing time=5min.(3) The study of membrane performance showed that the desired membrane has a molecular weight cut off of around5150Da and a calculated pore size in radius of about1.95nm, suggesting that the composite membrane was a kind of relatively loose NF membrane. The rejection rate and permeability of the PSA composite membrane for an aqueous solution containing0.5g/1Na2SO4at0.5MPa and25℃was71.0%and4.31/m2h bar, respectively. The rejection rate of the resultant membrane to different salts followed the order of Na2SO4> Mg2SO4> CuSO4> NaCl> MgCl2at neutral pH, implying that the membrane possessed nicer separation selectivity based on the Donnan effect. The separation performance for dyes was largely affected by the property of the dyes (e.g. molecular size, structure, and charge) and the interaction between dye molecules and membrane surface, the membrane exhibited higher rejection rate and lower fouling to the anionic dye aqueous solution than the cationic dye, for instance, the rejection rate of Congo red (MW=696.70g/mol) was more than97.8%. Both operation pressure and feed concentration affected the separation performance, the flux could be significantly improved by and increasing the pressure.(4) Acid soaking tests demonstrated that, after soaking in H2SO4aqueous solution with the concentration of20w/v%for8days, the PSA membrane showed no change in the surface morphological structure, while large amount of crack or big holes were occurred with the comparative PA membrane. In addition, the acid soaked PSA membrane showed no evident flux reduction under given trans-membrane pressure, but serious flux decline was occurred with the PA membrane. The PSA membrane could maintain the chemical property and separation performance during the60days’acid treatment process, while under the same condition, chemical degradation was took place with the PA membrane, which lost the separation effectiveness gradually. During the100days of acid soaking process, there was no obvious change of contact angle for the PSA membrane surface, while the contact angle for the PA membrane decreased firstly and then increased and finally reached to a value that equal to that of the surface of PSf substrate. The rejection of the PSA membrane to PEG4000increased as the feed changed from neutral to acidic, and could be recovered to its normal value when the feed pH returned to neutral, which suggesting that the polymer matrix has undergone reversible compaction. During the130hours’running test with aqueous solution containing4.9w/v%H2SO4and0.5g/1Na2SO4, the PSA membrane exhibited slight performance change, while the rejection of the PA membrane to Na2SO4decreased firstly and then increased and finally decreased. Moreover, the copper rejection and the acid permeate rate of the resultant PSA composite NF membrane were more than60.0%and89.0%, respectively, and the performance could be maintained during130hours’running test.(5) Membrane modification study suggested that thin-film composite membranes with hybrid active layer of PSA-co-PA and dual active layers of PSA/PA (or PA/PSA) could be prepared by incorporating the second organic phase monomer TMC. The PSA-co-PA composite membrane prepared using0.01w/v%TMC and0.028w/v%NTSC processed better hydrophilicity and outstanding separation selectivity, the flux and rejection of the copolymerized membrane for Na2SO4were55.61/m2h and74.2%, respectively, which were higher than those of the PSA membrane.The formation of dual active layers resulted in a slight improvement in both of the rejection and flux, and a significantly enhancement in both of the surface hydrophilicity and permselectivity of the resulting composite membrane. Furthermore, the modified composite membrane with the dual active layers of PSA/PA still processed outstanding acid stability. |
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