Plasma-induced PAA-ZnO coated PVDF membrane for oily wastewater treatment: Preparation, optimization, and characterization through Taguchi OA design and synchrotron-based X-ray analysis

Highlights

• A super-hydrophilic PVDF-PAA-ZnO membrane was obtained by plasma-induced polymerization and nano-ZnO assembly.

• The permeation flux for oily wastewater treatment was increased more than 10 times.

• The experimental parameters were optimized using Taguchi OA design method.

• The mechanisms of nano-ZnO self-assembly were revealed through synchrotron-based X-ray analyses.

Abstract

A novel membrane surface modification approach was proposed to successfully obtain a poly(vinylidene fluoride)-poly(acrylic acid)-ZnO (PVDF-PAA-ZnO) membrane with super-high water permeability and great oil rejection through cold plasma-induced PAA graft-polymerization followed by simple nano-ZnO self-assembly. The experimental parameters of modification were optimized and their optimal combination was identified using Taguchi orthogonal array (OA) design method. The PVDF-PAA-ZnO membrane was comprehensively characterized and the mechanism of nano-ZnO self-assembly was explored by contact angle measurement, scanning electron microscope (SEM) images, elemental analysis, tension test, Attenuated Total Reflection-Fourier Transform Infrared Spectroscopy (ATR-FTIR) and synchrotron-based X-ray analyses. It was revealed that ZnO nanoparticles were immobilized onto membrane surface through the adsorption of PAA layer to form a PAA-ZnO coating without valence change. The carboxyl groups of PAA layer provided complexing ligands to coordinate with Zn²⁺ and form bidentate species on the nano-ZnO surface. The firm PAA-ZnO coating on PVDF membrane surface converted its hydrophobic nature to hydrophilic, bringing about the dramatically improvement of membrane performance both in water permeation flux and oil rejection rate. The permeation flux of the PVDF-PAA-ZnO membrane was more than 10 times as great as that of the pristine PVDF membrane.

Introduction

Increasing oily wastewater pollution caused by petrochemical activities such as onshore/offshore oil recovery and marine transportation have brought about serious health risks and the destruction of ecosystems, becoming an urgent global environmental problem [1,2]. The development of effective methods for oily wastewater treatment is desired and full of challenges, particularly for the separation of oil/water emulsions [3,4]. Most of the traditional techniques, such as gravity separation, flocculation and flotation, suffer from low efficiency, high cost, and secondary pollution [[5], [6], [7], [8], [9]]. Polymer filtration membranes have been frequently applied in practical applications of oily wastewater treatment owing to their lower cost and ready availability [10,11]. Among them, polyvinylidene fluoride (PVDF) membranes are extensively used due to its outstanding mechanical properties, thermal stability, and chemical resistance [12]. Nevertheless, they still encounter two main limitations which affect their separation efficiency and operating cost. Firstly, both water and oil are adsorbed on membrane surfaces during the treatment process because of their poor separation selectivity. Secondly, serious pore clogging and surface fouling is caused by oil or grease, leading to a recessive reduction in permeate flux.

To address these two limitations, a number of advanced PVDF membranes have been developed by various modification techniques with the incorporation of nanoparticles (NPs) to improve the membrane hydrophilicity and antifouling ability. Many types of NPs have been utilized in membrane modification, such as iron (Fe⁰, Fe₂O₃, Fe₃O₄), silica dioxide (SiO₂), alumina (Al₂O₃), titanium dioxide (TiO₂), zirconium dioxide (ZrO₂), carbon nanotubes, graphene oxide, etc. [[13], [14], [15], [16], [17]]. For example, the incorporation of TiO₂ NPs in membrane modification as an additive in the polymer matrix or immobilized on the membrane surface has been widely studied [18,19]. Alternatively, zinc oxide (ZnO) NPs have been applied as a replacement for TiO₂ NPs since they have similar properties but the crystal form of nano-ZnO is easier to control and the price is slightly lower [[20], [21], [22]]. Hence, ZnO NPs have attracted an increasing amount of interest in membrane modification to improve the performances of PVDF membranes. Hong and He (2012) reported a composite PVDF-ZnO membrane that exhibited improved mechanical properties and BSA (blood serum albumin) rejection by adding 0.1% ZnO NPs into the casting solution. The highest pure water flux of the composite PVDF membrane was achieved when the supplemental nano-ZnO content was increased to 1.5%, and it was nearly five times higher than that of the pristine PVDF membrane [23]. Liu et al. (2016) prepared electrospun PVDF membranes with controllable structures and tunable wettability for oil/water separation by adding ZnO NPs into the polymer matrix [24]. Liang et al. (2012) modified PVDF membranes for synthetic municipal wastewater treatment by blending ZnO NPs in its cast solution to improve the anti-irreversible fouling properties. The water permeability was almost doubled when the dosage (6.7% nano-ZnO) was added [20].

However, most of previous research on ZnO NPs for PVDF membrane modification was limited to blending ZnO NPs into the casting solution. The improvements of membrane properties such as hydrophilicity and fouling resistance were restricted by doing so. The modification efficiency was affected because the ZnO NPs agglomerated in the casting solution, causing them to be entirely enfolded by the polymer matrix. In comparison, immobilizing ZnO NPs on the membrane surfaces using techniques of coating or chemical grafting to form a stable functional layer could be a more effective modification approach. Most of the ZnO NPs can disperse on PVDF membrane surfaces to maximally improve their performances. The challenge of this approach is how to stably immobilize ZnO NPs on the membrane surface, since ZnO NPs cannot self-assemble onto PVDF membrane surface without bonding with suitable functional groups.

Herein, a novel membrane surface modification approach was proposed to obtain a PVDF-PAA-ZnO membrane which was modified from PVDF membrane through cold plasma-induced poly(acrylic acid) (PAA) graft-polymerization followed by simple nano-ZnO self-assembly. The technique of cold plasma surface treatment was applied to induce PAA polymerization by introducing chemical initiators on PVDF membrane surface. An ultrathin and uniform PAA layer can thus be formed on the membrane surface to realize nano-ZnO self-assembly without compromising the bulk structure. To maximize the improvement of membrane hydrophilicity, the Taguchi orthogonal array (OA) design was applied to optimize the experimental parameters and identify their optimal combination. The obtained PVDF-PAA-ZnO membrane was comprehensively characterized and the mechanism of nano-ZnO self-assembly was explored by contact angle measurement, scanning electron microscope (SEM) images, elemental analysis, tension test, attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR), and synchrotron-based X-ray analyses. The PVDF-PAA-ZnO membrane was subjected to physical and chemical stresses to evaluate the binding performance of the PAA-ZnO coating. The improvement in membrane performance was further assessed for the application of oily wastewater treatment.