Theoretical and experimental study of organic fouling of loose nanofiltration membrane
Graphical abstract
Introduction
Water scarcity and contamination severely hamper the sustainable development of the modern global society, and are the critical challenges of the 21st century [1], [2]. A sustainable access to clean water is a strategic solution to these challenges. Nanofiltration (NF) emerges as an advanced separation technology for the removal of organic micro-pollutants (i.e., pharmaceuticals, endocrine disrupting compounds, and pesticides) from groundwater, surface water and seawater, based on the synergistic effect of size exclusion and electrostatic repulsion [3], [4], [5], [6]. In particular, loose NF membranes retaining compounds in the range of 500–1000 Da are of interest as a state-of-the-art technology, which allows for sufficient fractionation of organics/salt mixtures with an acceptably high permeation flux, due to the loose surface structure, which facilitates the salt transmission and reduces concentration polarization effect [7], [8], [9], [10], [11], [12], [13].
Loose NF membranes show a vast potential for wastewater treatment, i.e., dye or antibiotics removal, in view of resource extraction and water reclamation [14], [15], [16], [17], [18]. For instance, Da et al. developed an yttria-stabilized-zirconia loose NF membrane through the sol–gel process, presenting 99% dye retention and ∼98% NaCl removal [15]. The sulfonated thin-film composite nanofiltration membrane through interfacial polymerization of 2,2′-benzidinedisulfonic acid and trimesoyl chloride displayed over 99% rejection for Congo red, but extremely low retention (<1.8%) for NaCl, which tremendously outperforms commercial NF 270 membrane for dye desalination [19]. The novel loose NF membrane designed by bio-inspired co-deposition of polydopamine and copper nanoparticles yielded an extremely high rejection for dye molecules and fast transport of NaCl, with strong antimicrobial property [10]. Furthermore, Cheng et al. developed a loose NF membrane through bio-inspired coating of gallic acid and branched polyethyleneimine, yielding a 96.7% rejection to Azithromycin [17]. Unfortunately, fouling caused by organic matters is a ubiquitous problem across all applications of NF technology, which substantially deteriorates the membrane flux and permeate quality.
In the past decades, comprehensive investigations on organic fouling of commercially available (tight) NF membranes, i.e., NF 90 from Dow-Filmtec (MWCO of 200 Da), NE 70 from Saehan (MWCO of ∼350 Da), and NP030 from Microdyn-Nadir (MWCO of 400 Da), that retain compounds in the range of 100–500 Da were conducted, revealing the critical fouling-related factors, i.e., solution chemistry, hydrodynamic conditions, and membrane properties [20], [21], [22], [23], [24], [25], [26], [27], [28]. However, the fouling behavior of loose NF membranes is not well documented, failing to serve as guideline for their industrial application. Therefore, a systematical investigation for understanding organic fouling of loose NF membranes is of great significance in broadening their applications in drinking water production, brackish water and seawater desalination, and wastewater treatment.
This study aims to understand the effects of the solution chemistry and hydraulic conditions on organic fouling and solute rejection of a loose NF membrane (i.e., Sepro NF 6 with MWCO of 862 Da, Ultura). Specifically, the dependence of flux behavior and solute rejection on humic acid concentration, pH, ionic strength, calcium concentration and magnesium concentration was investigated. Furthermore, the interactions between the membrane surface, organic foulants and inorganic solutes at different hydraulic conditions were validated by COMSOL simulation and scanning electronic microscopy (SEM) observation.
Section snippets
Materials
The model foulants including humic acid, sodium alginate and bovine serum albumin (BSA) were obtained in powder form from Sigma Aldrich (Belgium). Specifically, 2 g L−1 humic acid, sodium alginate and BSA solutions were prepared as storage solution. The pH and ionic compositions of the feed solution were adjusted by the addition of hydrochloric acid, sodium hydroxide, sodium chloride (NaCl) or calcium chloride (CaCl2). MilliQ water (electrical resistance of 18.2 MΩ cm, Millipore, USA) was used
COMSOL simulation
Generally, membrane fouling remains an inevitable challenge associated with membrane processes in large industrial modules as well as in small bench-scale cells [31]. Specifically, the flow behavior in the membrane system is substantially related to the filtration performance of nanofiltration membranes [32], [33], [34]. Therefore, the implementation of computational fluid dynamics (CFD) is an asset to understand the flow behavior in the membrane module or on the membrane surface.
In this work,
Effect of foulant concentration
The filtration of a humic acid solution with different concentrations using Sepro NF 6 in is shown in Fig. 3.
As indicated in Fig. 3, humic acid has no significant effect on the flux of a loose NF membrane. Although frequent collisions of humic acid molecules onto the membrane surface occur with increasing humic acid concentrations [21], a slight flux decline for Sepro NF 6 was observed without sacrificing the humic acid rejection (>99.9%). This is mainly due to the strong hydrophilicity of the
Conclusion
In this work, the flux behavior of a loose NF membrane fouled by humic acid at variable solution compositions (i.e., humic acid concentration, pH, ionic strength, and calcium concentration) was systematically investigated in view of its potential practical application in fractionation of organic matters and salts for resource recovery from wastewater. The Sepro NF 6 membrane showed outstanding antifouling properties against humic acid, BSA, and alginate acid, due to its strong hydrophilicity.
Acknowledgment
This work was supported by the National Natural Science Foundation of China (Grant Nos. 21706035 and 21707018), the Natural Science Foundation of Fujian Province (Grant No. 2017J01413), the Fujian Agriculture and Forestry University Program for Distinguished Young Scholar (Grant No. xjq201704) and Fuzhou University (Grant Nos. XRC-1622 and 2017T017) for this work. Ultura, USA, is greatly thanked for providing the nanofiltration membrane samples.
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