Synthesis of a novel composite nanofiltration membrane incorporated SiO₂ nanoparticles for oily wastewater desalination

Abstract

A novel composite nanofiltration membrane incorporated with silica nanoparticles was prepared by interfacial polymerization on polysulfone (PSF) ultrafiltration membrane. The silica nanoparticles (∼15 nm) were homogeneously dispersed in poly(amidoamine) (PAMAM) dendrimer aqueous monomers and incorporated in situ into the polyamide films. Series of tests including FTIR–ATR, AFM, XPS, etc. were performed to characterize the polyamide (PA)-SiO₂ membrane's morphologies, structure, physical and chemical characteristics, separation properties and permeability, etc. The results showed that silica loading into the PA membrane resulted in roughness and hydrophilicity promotion. The permeation performance for PA-SiO₂ membrane increased nearly 50% without loss of salt rejection rate by adding 1.0% (wt.) nano-SiO₂ nanoparticles in aqueous solution. The order of rejection to inorganic salts is Na₂SO₄ > MgSO₄ > MgCl₂ > NaCl revealed both PA and PA-SiO₂ membrane were nagatively charged. The zeta potentials testing results indicated addition of SiO₂ will increase the negative charge quantities on the surface of PA-SiO₂ membrane for negatively charged hydroxyl groups and silanol-covered nano-SiO₂ surface. The value of molecular weight cutoff (MWCO) for PA-SiO₂ membrane was about 1000 g/mol and the additive of SiO₂ nanoparticles to PA membrane enlarges the pore size slightly. The PA-SiO₂ membrane had a higher stable flux and could remove nearly 50% salts when treated with oily wastewater in one-cycle filtration. Desalination of oily wastewater using the nacomposite PA-SiO₂ membrane is feasible.

Introduction

Compared with reverse osmosis (RO) and ultrafiltration (UF), nanofiltration (NF) offers several advantages such as low operation pressure, high flux, high rejection of multivalent anion salts and organic moleculars above 300, relatively low investment and low operation and maintenance costs. The NF is increasingly gaining attention in many separation and treatment processes such as water softening, color removal, chemical oxygen demand (COD) reduction and separation of medicines, inorganic ions, and regulated and unregulated organic compounds such as endocrine disrupting compounds (EDCs), pharmaceutically active compounds (PhACs), pesticides, and other organic micropollutants [1], [2], [3], [4], [5]. RO and NF are the dominant membrane processes for desalination [6]. At the same time, the NF membranes are increasingly recognized as one of the best processes for water treatment and production of portable water.

Among all the preparation techniques of thin film composite NF membranes, interfacial polymerization (IP) is the most commonly followed technique because the selective layer and the porous support layer can be optimized separately [7], [8]. The selective layer is the key factor for determining the total properties of nanofiltration and it should be an important promising work for membrane manufacturing and technicists to develop a novel NF membrane with special properties using high-performance materials [9].

Poly (amidoamine) (PAMAM) dendrimer has a global shape, high surface areas, hollow microspheres, tidy and fine highly branched structure. In recent years, they have been applied in many fields, such as nanoscale catalysts micelle mimics, drug delivery, sustained-release and controlled-release agents, chemical sensors, high-performance polymers and adhesives, etc [9], [10]. Now, the applications of PAMAM in the membrane separation field primary include improvement of separation performance of UF [11] and preparation of CO₂ separation membrane [12], [13], [14], [15], [16]. The structure of PAMAM dendrimer is shown in Fig. 1. G0 and G1 in the Fig. 1 are generations. Because the higher densities of terminal –NH₂ groups on the surface, it can provide a large number of reactive sites for preparing NF membrane. The number of amino groups grows exponentially with generation of PAMAM, It can also provide more primary amine and secondary amine group than normal macromolecules, and the hydrophilicity of the resulted NF membrane would be better than those prepared with small or ordinary macromolecules. Now few researchers prepared NF membranes with dendrimers, especially with PAMAM dendrimer. Li et al. [9] prepared NF membrane with PAMAM dendrimer and the resulted membrane displayed good performance. In addition to unique chemical structure, PAMAM also has unique inner cavity properties which can be used as a carrier, which has been widely used in the field of medicine. Further studies are still necessary to utilize PAMAM special space structure properties to prepare high-quality NF membrane.

The NF membranes include organic polymeric membranes and inorganic ceramic membranes according to material properties. Recently many new types of organic/inorganic hybrid materials have the potential to combine the desired properties of inorganic and organic materials [17], including improving thermal and mechanical properties, electrochemical and anti-swelling, anti-fouling performance, hydrophilicity, flexibility and ductility, etc [18], [19]. These nanoparticles include TiO₂, Al₂O_3, ZrO₂, Al₂O₃ and TiO₂ mixtures [20] and so on. Among all preparation methods, the way of doping inorganic oxide particles to polymer to prepare organic–inorganic composite membranes is intensively studied for its simple operating process and preparation technology [21]. Most of the nanoparticles in the market were modified with hydrophily or hydrophobicity properties, it is convenient to add various ceramic fillers into polymer to adjust the properties of resultant nanocomposite polymer membrane, some researches have been done like this, such as polyvinyl alcohol (PVA)/TiO₂, polyamide (PA)/TiO₂, PVA/SiO₂,polyether sulfone (PES)/SiO₂, etc [18], [22], [23]. The nanocomposite membranes containing smaller sizes (1–20 nm) inorganic particles will be even better performing membranes than those containing particles sizes of 50 nm or above as the smaller particles essentially permit the extensive organic inorganic interactions and the smaller particles show larger surface area. In order to overcome the nanoparticles aggregation and increase its dispersibility in the casting solution, dispersing agent, e.g. sodiumdodecyl sulfate (SDS) commonly was added to the solution.

The SiO₂ nanoparticles have wide resource, low price and many excellent characteristics. Some of the latest research [21] focused on preparing nano-silica modified PSF ultrafiltration membrane to treat with wastewater containing oil. In nanofiltration preparation area, Jadav and Singh [24] synthesized the silica-polyamide nanocomposite membrane and studied the influence of silica nanoparticles on membrane structure and performances. Winberg et al. [25] prepared membrane modified by nano-silica and the gas diffusivity and solubility of the membrane were improved.

In the present work, we attempted to prepare composite PA-SiO₂ nanofiltration membrane by interfacial polymerization of PAMAM and trimesoyl chloride (TMC) on the substrate of thermally stable PSF ultrafiltration membrane. The composite membrane was characterized with FTIR–ATR, AFM, XPS, TGA, mechanical performance, etc to characterize the PA-SiO₂ membrane's properties. The effects of SiO₂ nanoparticles on nanofiltration membrane performances including permeation and salt rejection performance were discussed. The pore-size variation with the generation of PAMAM and small organics rejection properties of PA-SiO₂ were also investigated. Finally the PA and PA-SiO₂ nanofiltration membrane were used to treat with oily wastewater to evaluate their separation performance.