Hydrophilic Nanoparticle Embedded Nanofiltration Membrane by Electrospray Interfacial Polymerization for Dye Recovery from Saline Wastewater

Abstract

Dyes and pigments are commonly applied in textiles, printing, rubber, paper, plastics, and cosmetics industries. During the dyeing process, inorganic salts (sodium chloride and sodium sulfate) are added to increase the dye absorption of the fabric. However, improperly treated effluents containing dyes and salts deteriorate the environment. Furthermore, dyes are toxic to humans, and owing to their persistence, they bioaccumulate in living organisms. Therefore, it is urgent to thoroughly treat dye wastewater. Coagulation/flocculation, ion exchange, adsorption, and membrane treatment systems are typically used in dye wastewater treatment processes. Among them, the membrane treatment system has several advantages such as high removal efficiency, ease of fabrication, operation, scale-up, and control, etc. However, it has a few weak points, for example, selectivity-permeability trade-off and fouling effect. Therefore, the purpose of this doctoral research is to develop a membrane suitable for dye wastewater treatment. Due to the molecular weight cut-off (MWCO) of nanofiltration (NF) membranes (100-1000 Da), dye wastewater treatment systems are mainly using NF membranes. However, since the NF membrane removes inorganic salts and dye molecules together, valuable resources cannot be reused or recovered. A new concept of membrane called loose NF membrane is spotlighted. It shows high water permeability due to a relatively larger MWCO (500-2500 Da) than the NF membrane. In addition, it has the characteristic of being able to permeate most salt ions while showing high dye rejection. The electrospray interfacial polymerization (EIP) method was applied to fabricate the active layer of the loose NF membrane. It can produce continuous fine microdroplets in submicron to nanoliter scale range by the electric force. The active layer can be synthesized simply and easily, the fabricated membrane shows a smooth surface, and the thickness of the active layer can be controlled in nanometers. In addition, using the EIP method, nanoparticles can be evenly distributed on the membrane surface, and the amount of nanoparticles used can be calculated. Therefore, the EIP method is suitable for fabricating a nanoparticles embedded membrane. The thin-film nanocomposite (TFN) membrane was fabricated to improve the performance and antifouling effect of the membrane by containing nanoparticles in the active layer, and a thin film composite (TFC) membrane with nanocomposite substrate was used to give the dye adsorption property. In this case, functionalized graphene oxide (fGO) was used as the nanoparticles to improve the properties of GO. The sulfonated graphene oxide (SGO) was prepared by substituting the oxygen functional groups with sulfonic acid groups; therefore, the negative charge was strengthened, which eventually improved the Donnan exclusion effect of the membrane. Also, magnetite (iron oxide, Fe3O4) decorated SGO (MGO) was prepared and dispersed in the matrix of the substrate to give the dye adsorption property to the membrane. To begin with, Chapter 1 introduces the thesis scope and outlines, and Chapter 2 extensively reviews the general background and previous studies. In Chapter 3, the loose NF membrane was fabricated by using the EIP method. It is a novel and facile interfacial polymerization method, which controls the thickness of poly(piperazine-amide) (PPA) layer in nanometers (1 nm/min) and changes cross-linking degree of PPA layer and pore size by varying the electrospray time; consequently, water permeance and molecule and ion selectivity can be handled. The fabricated EIP membrane with an optimized fabrication condition (M30, electrospray time was 30 min) possessed excellent pure water permeance (PWP, 20.2 LMH bar−1), high dye rejection (e.g. 99.6% for Congo red (CR)), and low salt rejection (e.g. 6.3% for NaCl). As shown in Chapter 3, the loose NF membrane showed the best performance when the PPA thickness was 31 nm; therefore, I fixed the thickness at around 31 nm. Thus, in Chapter 4, SGO composite loose NF membranes with high water permeance were fabricated by the EIP method to treat saline dye wastewater. SGO incorporated EIP membrane showed enhanced characteristics and performances even with a small amount of nanoparticle (0.04 g per membrane area (m2)). The SGO nanoparticles were uniformly dispersed on the membrane surface by virtue of electrospray that ejects nano-sized droplets. The fabricated loose NF membrane with 2000 ppm SGO has the highest water permeance of 16.7 ± 2.2 LMH bar−1, which increased by 53% than that of a pristine membrane (11.0 ± 0.8 LMH bar−1), and showed superior specific selectivity with high rejection for EB (99.6%) and CR (99.8%) with high NaCl permeation (92.3%). The surface roughness of the membrane decreased and the hydrophilicity and negative charge were enhanced with increasing concentration of SGO nanoparticles. These properties improved the antifouling property of the membrane as the FRR of the SGO embedded membrane remained at 97.9% after the reusability test. This work provides a facile approach to preparing highly efficient loose nanocomposite NF membranes for molecule/ion separation. For further improvement of functionality, a TFC loose NF membrane comprising a PPA active layer was successfully fabricated by the EIP method. SGO and magnetite-decorated SGO (MGO) were synthesized and individually embedded in the polymer matrix of the substrate via a phase inversion method. The hydrophilicity, surface roughness, porosity, and pure water permeance of the prepared substrates, in which the additive existed, were found to be improved in comparison with those of the pristine substrate. The MGO nanoparticle-incorporated TFC membrane (M-MGO) exhibited high permeation of salt (96.1% for NaCl) and water permeance (44.4 ± 4.0 LMH bar‒1) owing to the loose structure of the active layer. The M-MGO also exhibited the highest dye rejection (CR, 99.0%) owing to the presence of MGO nanoparticles with an adsorption capacity. When permeating the dye and salt mixture, M-MGO achieved the highest dye/salt selectivity in comparison with those of the other membranes. Moreover, the M-MGO achieved better antifouling properties with a high FRR of 99.0%, whereas the pristine membrane exhibited a low FRR of 97.6% or less. This method presents a high potential for the fabrication of novel saline wastewater treatment membranes. This study provides the successful synthesis of SGO and MGO nanoparticles and performance evaluation of novel loose NF membranes fabricated by the EIP method for dye and removal. It presents an in-depth study of the effect of the nanoparticles incorporated into the membranes in terms of performance-enhancing mechanism. However, so far experiments have only been performed under very limited conditions in laboratory-scale systems. In that sense, current research has limitations to apply in real conditions. Therefore, additional studies such as the evaluation of membrane performance under the actual conditions of using actual dye wastewater containing high concentrations of salts, several types of dyes, and surfactants and the development of a large-scale EIP membrane manufacturing method should be performed.