Polysulfone/sulfonated polysulfone alloy membranes with an improved performance in processing mariculture wastewater
Introduction
With increasing demand for seafood and the continuous reduction of its annual yield, mariculture industry consistently develop in a more sustainable and more efficient way. Generation of large quantities of solid waste and wastewater remains a challenge of the commercial mariculture industry [1]. Because feed and additive chemicals are added into seawater as a nutrition and disinfectant for the growth of aquaculture during the cultivation process, wastewater contains mainly organic contaminants in soluble, colloidal and particulate forms, which has to be treated properly before discharge to avoid the deterioration of marine ecological environment such as eutrophication and water pollution in the coastal areas. The purified wastewater can also be reused as recirculated seawater in a large-scale marine aquaculture plant to achieve zero-liquid-discharge and can efficiently prevent the breakout of infectious disease.
Nowadays, the wastewater treatment for mariculture industries has attracted many interests whether from academic or practical points of view [2], [3]. The existing major obstacles involve the complexity of process and operation management, large footprint, low retention efficiency and high operating cost. In particular, biochemical methods such as conventional activated sludge (CAS), which is considered to be an efficient way for the wastewater treatment, often fails to treat mariculture wastewater due to the poor growth of activated sludge at a high salinity (seawater), leading to poor removal efficiencies of COD and NH3-N [4], [5], [6].
To overcome these challenges, membrane separation has been proposed as a useful method for the mariculture wastewater treatment to avoid the negative influence of the salinity effect of seawater [7]. A mineral tubular ultrafiltration (UF) membrane with molecular weight cut-off (MWCO) of 15 kDa exhibited a flux of 97.7 L/m2h and a protein rejection of 26.0% at 0.4 MPa in processing seafood wastewater [8]. Mameri et al. [9] also investigated the UF performances of a ZrO2 membrane and a polysulfone (PSf) membrane in treating seafood wastewater, exhibiting the high protein rejections more than 70.2%, respectively. However, the steady-fluxes decreased with the operating time, even lowered to 58 L/m2h after operation of 2 h [9]. It indicated that the membranes suffered a severe membrane fouling due to the contaminant accumulation on the membrane surface in processing mariculture wastewater, resulting in a significant decline in the separation performance. Therefore, the membranes with an improved separation performance and anti-fouling property are urgently required.
Polysulfone (PSf) with high thermal, mechanical and chemical stabilities [10], [11] is now one of the commonly commercial ultrafiltration (UF) membrane materials. However, the hydrophobic nature of PSf makes membrane surface easily suffer from an undesirable membrane fouling, especially organic fouling and bio-fouling [12]. Consequently, a great deal of studies has focused on the hydrophilic modification of membrane surface to improve the anti-fouling property and separation performance, including mainly surface grafting [13], surface coating [14] and blending [15]. Compared to other techniques, blending method was advantageous on the process simplification, easy industrialization and no damage impact on the substrate.
Recently, membranes with high performance have been prepared by incorporating a certain amount of hydrophilic sulfonated polymers into the membrane matrix via blending-phase inversion, involving PSf /sulfonated polysulfone (SPSf) [16], polyethersulfone (PES)/ SPSf [17], [18], PSf/sulfonated poly(ether ketone) (SPEK) [19] and PSf/sulfonated polyphenylenesulfone (sPPSU) [20]. These alloyed sulfonated polymers were proved that they not only adjust the surface hydrophilicity and membrane morphology, but also significantly enhance the separation performance [21], [22] as well as anti-fouling property [23]. Among the various sulfonated polymers, SPSf is one of the most important hydrophilic sulfonated aromatic polymers, which exhibits the unique chemical and physical properties involving high thermal and oxidation stabilities, adjustable hydrophilicity and excellent mechanical property [24], [25], which was usually blended into PSf or PES casting solution to fabricate NF membranes [16], [18] and serve as substrate of thin film composite membranes [26]. Water transport and salt rejection can be enhanced by the introduction of the hydrophilic SPSf into the hydrophobic PSf substrates [16], [26]. More important, on a basis of similar dissolve mutually theory, SPSf is characteristic of high compatibility with PSf due to the quite similar molecular chains between them, which would facilitate the long-term stability and durability of membranes. Therefore, we believe that the incorporation of SPSf in the PSf membrane can adjust the membrane structure, enhance the membrane hydrophilicity, and thus improve the UF performance as well as the membrane durability in a real application such as mariculture wastewater treatment.
In this work, a homemade water-soluble SPSf was synthesized via direct sulfonation method using chlorosulfonic acid (CSA) as sulfonating agent and used as part of membrane materials to fabricate the hydrophilic PSf/SPSf UF membranes via phase inversion method. The objectives of this study are (1) to investigate the effects of SPSf content on the membrane formation, structure and separation performance of the resulting PSf/SPSf membranes; (2) to evaluate the separation performance in treating a model organic-foulant solution as well as a real mariculture wastewater, which holds promise for the treatment of mariculture wastewater through an ultrafiltration process. As a benchmark, a homemade non-sulfonated PSf membrane and a commercial PSf membrane were also tested for comparison.
Section snippets
Materials
Polysulfone (PSf, Udel P-1700, USA) was dried at 100 °C for 24 h prior to use. Chlorosulfonic acid (HSO3Cl, CSA, 99%) from Xiya Reagent Research Center and chloroform (CHCl3) from the Iron Pagoda Reagent were employed to synthesize SPS powders. N-methyl-2-pyrrolidone (NMP) and polyethylene glycol 400 (PEG-400) purchased from Sinopharm Chemical Reagent Co. Ltd were used as the solvent and pore-former in the casting solution, respectively. HA purchased from Sigma (St. Louis, USA) was applied as the
Characterization of the synthesized SPSf
As shown in Fig. 1, the sulfonic groups introduced in the sulfonation are likely localized at the ortho position relative to the ether oxygen atom on an aromatic ring of the main chain, which is attributed to the activation of ortho positions by the electron-donating oxygen atoms via electron resonance [22]. According to the Eqs. (1), (2), DS and IEC of the synthesized SPSf equal to approximately 30% and 0.60 meq/g, respectively, indicating that ten repeating units of PSf polymer chain yield
Conclusion
In summary, we fabricated a hydrophilic and negatively charged polysulfone/sulfonated polysulfone (PSf/SPSf) blended membrane for the mariculture wastewater treatment. As demonstrated, the resulting PSf/SPSf membranes exhibit good performance including high flux and excellent rejection in the UF process. For example, a high pure water permeability of 2521.6 L/m2h MPa, excellent steady-state permeation flux of 118.0 L/m2h and HA rejection of 86.8% were achieved when filtrating 10 mg/L humic acid
Acknowledgment
This work was supported by Shandong Province Key Project (2014GHY115033), NSFC (No. 21306178), Qingdao applied basic research project (No. 15-9-1-21-jch) and Fundamental Research Funds for the Central Universities (No. 201564015).
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