Boron substituted MFI-type zeolite-coated mesh for oil-water separation

Abstract

A boron substituted zeolite-coated mesh was synthesized by a secondary growth method. The inclusion of boron ions significantly increased the hydrophilicity of the resultant mesh. Using gravity as the driving force and upon exposure to a water/oil mixture, water permeated into and through the zeolite mesh, while oil was rejected. With increasing boron content in the zeolite-coated mesh, the hydrophilicity of the mesh was enhanced, while the oil droplet contact angle increased from 136.8° to 162.0°. For a B/Si ratio of 0.04, the zeolite-coated mesh showed super-hydrophilicity. At this ratio, the oil rejection rate of the zeolite-coated mesh reached >99% for a water flux of >14,000 L m⁻² h⁻¹. The zeolite-coated mesh showed high chemical stability and the aforementioned oil rejection rate whether treated with acidic, basic or hot media. The mesh were successfully reproduced by a simple re-calcination method due to their thermal stability. Slight degradation of the oil rejection performance less than 1% was observed after the third re-calcination. Various organic solvents, such as n-hexane, cyclohexane, mineral oil, and vegetable oil were also separated via boron substituted zeolite-coated mesh, in conjunction with oil rejection rates of >96.5%.

Introduction

Because of the ever-increasing demand for clean water, combined with the increasing generation of industrial oily water discharge, there is a worldwide water crisis. The presence of oil in water is immediately detrimental to the environment, but also has the long term effect of damaging the self-cleaning circulatory processes of the ecological system [1]. Thus, considerable efforts are underway to obtain energy-efficient and environmentally-friendly methods for the remediation of oily contaminants. Over the past few decades, polymeric membranes have dominated the R&D landscape because of their good performance and oil rejection rates of more than 95% [[2], [3], [4], [5], [6], [7], [8], [9], [10]]. However, due to their extremely small pore sizes, large pressure gradients across these membranes are required, which translates into power consumption and a rising cost of operation. In addition, at these pressures, polymeric membranes are prone to fouling [[4], [5], [6], [7], [8], [9], [10], [11]], where the flux through the membrane rapidly declines [12,13]. Despite efforts to develop novel self-cleaning polymeric membranes, the ratio of oil droplets adsorbed onto the surfaces of such membrane is 70–90% and the ratio of oil droplets cleaned away from membrane surface is as low as 30–40% [[9], [10], [11]].

In an effort to resolve the fouling problem, super-hydrophilic polymeric materials have been coated on stainless-steel wire mesh, which allow water to pass through the mesh and reject oil [[14], [15], [16], [17], [18]]. These super-hydrophilic membranes exploit the differences in density between water and oil and the formation of a water layer on the membrane surface that acts as barrier to oil [19]. Xue et al. [14] successfully fabricated hydrogel-coated mesh to realize a 99% oil rejection rate within only a few seconds of operation, while Xiao et al. [17] synthesized super-hydrophilic polymeric materials and with oil rejection rates of 99% or better. Chen et al. [18] prepared inorganic-organic thiol-ene coated mesh with almost 100% oil rejection. The downside of polymeric coated membranes is their intolerance to corrosive solutions, such as acids and bases, organic solvents and high temperatures [12,20,21]. They are also susceptible to swelling, which detrimentally impacts molecular diffusion and the convection of water in the pores [22]. This can produce a swelling ratio in hydrogels upwards of 2.0–3.7 [23,24], thereby limiting their use in challenging environments.

Recently, researches have turned their attention to ceramic materials as an alternative to polymers. Li et al. [23] coated SiO₂ on stainless steel mesh, which separated 98% kerosene from corrosive and hot water. Li’s group utilized a spray coating process for the development of ZnO-coated mesh and TiO₂-coated mesh to obtain kerosene/water separation rates of 97.3% and 97.5%, respectively, over 40–50 separation cycles [25,26]. More recently, the separation efficiencies of a variety of materials coated on stainless steel mesh coating, such as zeolite [19,27], ZnO [25,28,29], TiO₂ [26], Cu₂S [30], CuC₂O₄ [31], and SiO₂ [23,32] have been examined. Among these, pure-silica or high-silica zeolite are the most promising due in part to their corrosion-resistance and their chemical, mechanical, and thermal stability [[33], [34], [35]], which enables their operation under severe conditions. The IIIA ion (e.g. B³⁺, Al³⁺) can be introduced into zeolite structure in order to make use of their empty orbitals so that more hydroxyl groups can be connected, which will form more hydrogen bonds and thereby increase the hydrophilicity [36].

Herein, we demonstrate for the first time the novel fabrication of boron substituted zeolite-coated meshes that resist fouling, highly recyclable, and efficiently separate water from oil. With the substitution of B³⁺ ions, the zeolite mesh becomes more hydrophilic and a higher water flux. In order to evaluate the stability of the zeolite-coated mesh when operating in corrosive aqueous solutions, oil/water separation was conducted under acidic, basic, and hot-media conditions, respectively. The rejuvenation and reusability of the zeolite-coated mesh by re-calcination was also investigated. We also report on the separation efficiency as a function of oil type.