Polyaniline coated membranes for effective separation of oil-in-water emulsions

Abstract

Polyaniline (PANI) decorated commercial filtration membranes, such as stainless steel meshes (SSMs) with 5 μm pore size and polyvinylidene fluoride (PVDF) membranes with 2–0.22 μm pore sizes, were fabricated by a simple one-step dilute polymerization at low temperature. Lots of short PANI nanofibers were firmly and uniformly coated onto the membrane surfaces, forming rough micro- and nanoscale structures and leading to underwater superoleophobicity with low oil-adhesion characteristic. Furthermore, we systematically studied the effect of pore size and pressure difference on oil–water separation ability of the obtained membranes. It was found that the PANI-modified SSMs with 5 μm pore size were suitable for the separation of non-surfactant emulsions with water fluxes of more than 1000 L m⁻² h⁻¹ under gravity only. The PANI-modified PVDF membranes were used for the effective separation of surfactant-stabilized emulsions with water fluxes up to 3000 L m⁻² h⁻¹ for 2 μm pore size under 0.1 bar or 0.22 μm pore size under 0.6 bar. In addition, the superhydrophilic membranes with PANI coatings were demonstrated for high oil rejection, stable underwater superoleophobic properties after ultrasonic treatment and immersing in oils and various harsh conditions, and high and steady water permeation flux after several cycles.

Introduction

Membrane technologies, especially pressure-driven filtration membranes, are still ceaselessly developed for water treatment applications to satisfy the growing demand for clean water. In the past few decades, various kinds of the pressure-driven filtration membranes have been explored according to their characteristic pore size and relative separation objects. Among them, microfiltration (50–500 nm pore sizes) and ultrafiltration (2–50 nm pore sizes) have been successfully applied for emulsion separation [1]. Generally, the polymeric filtration membranes are intrinsically hydrophobic, which requires a transmembrane pressure of up to several bars to selectively filter water, and leads to a low flux and quick decline due to unavoidable fouling issue. Many surface modification methods have been adopted to reduce membrane fouling, usually attempted by an increase in hydrophilicity [2], [3], [4], [5], [6]. However, there are other chemistry parameters rather than hydrophilicity that could affect the membrane fouling [7].

A number of superwetting films have been fabricated to achieve the separation of non-emulsified oil and water mixtures, including superhydrophobic or underwater superoleophobic materials, respectively [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]. These superwetting films are unsuitable for various emulsions with less than 20 μm droplet size, especially surfactant-stabilized emulsions. To realize emulsion separation by integrating the aforementioned approaches, Jin and coworkers transformed hydrophilic poly(acrylic acid)-grafted polyvinylidene fluoride (PVDF) membranes to superhydrophilic and underwater superoleophobic surfaces with a rough micro/nanoscale structure by a salt-induced phase inversion [20]. Consequently, underwater superoleophobic membranes with pore sizes less than the oil droplets have been designed and synthesized to deal with emulsified oily wastewater (Table S1) [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41]. Recently, hydrophilic organic materials have been introduced onto the surface of various commercial porous membranes, providing a strong binding force between hydrophilic coatings and substrates. In order to further enhance the stability and surface roughness of organic coatings, some inorganic materials are widely used to modify the organic coatings. The organic and inorganic materials include for example polydopamine, poly(acrylic acid), poly(sulfobetaine methacrylate), CaCO₃, SiO₂, and TiO₂ [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32]. Notably, the pressure difference on most of underwater superoleophobic membranes can be less than 1 bar compared to commercial hydrophobic membranes, even down to gravity. In general, filtration membranes with a larger pore size require a lower pressure difference. For example, a 500 mesh membrane with Cu(OH)₂ nanowire-haired coatings can separate oil-in-water emulsions under gravity only [33]. However, with this all-inorganic membrane it is hard to achieve separation of surfactant-stabilized emulsions due to large pore size. Therefore, it is desirable to develop a one-step approach to construct superhydrophilic and underwater superoleophobic membranes with different pore sizes, which can maintain high stability under various harsh conditions, and effectively treat different emulsified oily wastewaters with high water fluxes under a low pressure difference.

In our previous work, we prepared underwater superoleophobic membranes by coating polyaniline (PANI) and polypyrrole (PPy) on the surface of stainless steel meshes (SSMs, 300 and 400 mesh size) to separate non-emulsified oil and water mixtures [18]. After in situ polymerization, PANI and PPy can be strongly coated on the walls of the reaction vessels and the surfaces of hydrophobic SSMs. Because of the rigid structures, conductive polymers show the advantages of inorganic materials, forming stable and rough nanocoatings. Furthermore, it has been demonstrated that PANI nanofibers can be well dispersed in water due to their large number of amino groups and the presence of electric double layers around the PANI chains [18], [42], [43], resulting in the hydrophilicity and underwater superoleophobicity. We found that the prepared underwater superoleophobic membranes cannot be used to separate emulsions. In order to realize demulsification, we adopted the same oxidation polymerization method to decorate PANI and PPy onto the membrane surfaces with smaller pore size. However, PANI and PPy cannot be uniformly coated on the membrane surfaces and block the small pore channels, which greatly influences the separation efficiency of oil-in-water emulsions.

Here we slowed down aniline polymerization to make PANI uniformly and firmly coat onto the surface of commercial hydrophobic materials with different pore sizes, including SSMs (2300 mesh size) and PVDF membranes. Note that the method is still unsuitable for PPy. After decoration with PANI coatings, the obtained SSMs and PVDF membranes become superhydrophilic and underwater superoleophobic with low oil-adhesion characteristic and small sliding angles. The PANI-modified SSMs with a pore size of about 5 μm can be used to separate non-surfactant emulsions under gravity only. The separation of various emulsions, stabilized by surfactants such as Tween 80 and sodium dodecyl sulfate (SDS), can be effectively achieved by PANI-decorated PVDF membranes with pore sizes ranging from 2 to 0.22 μm employing different external pressures (less than 1 bar). In addition, these PANI-modified filtration membranes are highly stable after ultrasonic treatment and immersing in oils and various harsh conditions, and maintain underwater superoleophobicity and high flux after reusing.