Efficient oil/water separation by a durable underwater superoleophobic mesh membrane with TiO₂ coating via biomineralization

Highlights

• Uniform TiO₂ was coated on mesh membranes via a facile biomineralization approach.

• The coated-mesh showed superhydrophilicity and underwater superoleophobicity.

• The coated mesh separated oil/water mixtures efficiently with anti-fouling ability.

• The coated mesh had great chemical, thermal and mechanical durability.

Abstract

In this work, a novel and facile method was used to coat a uniform and conformal TiO₂ film on a stainless steel mesh via polyethylenimine-catalyzed biomineralization of titanium bis(ammonium lactato)dihydroxide at neutral pH, ambient temperature and pressure. The fabricated mesh showed excellent superhydrophilicity and underwater superoleophobicity. It separated immiscible oil/water mixtures by gravity at a high flux (∼3 × 10⁴ L m⁻² h⁻¹), high separation efficiency (∼99.999%), and exhibited excellent anti-oil-fouling ability. The mesh also demonstrated great durability: (1) it had outstanding stability in 1 M NaCl, 0.1 M HCl, 0.1 M NaOH solutions, and absolute ethanol; (2) it maintained underwater superoleophobicity in solutions of pH from 1 to 13 and saturated NaCl solutions; and (3) the separation efficiency was retained after high temperature treatment and mechanical wear tests (sonication, abrasion and crumpling). The biomineralization coating method was also applied on other substrates (nonwoven mesh, filter paper and PVDF membrane), which also showed good capability for oil/water separation. Therefore, this new environmentally benign approach of preparing superwetting surfaces provides a durable and sustainable solution to the treatment of oily wastewater.

Introduction

Oil/water separation is important in treating industrial oily wastewater and rapidly responding to oil spills. Membrane technology is attractive for oil/water separation because it works without chemicals addition, with undemanding operations, and it can achieve high separation efficiency and speed if the surface wettability of the membranes is well designed [1]. Typically, surface wettability of membranes for oil/water separation can be categorized to two types: hydrophilic/oleophobic surfaces that are permeable to water but impermeable to oils, and oleophilic/hydrophobic surfaces that are permeable to oils but impermeable to water. By achieving an appropriate combination of surface energy and surface roughness, desirable surface wettability can be procured [2]. Therefore, surface modification of membranes is deemed a fundamental but powerful approach to engineer surface wettability [3].

Applying coatings on membranes is widely used to modify membrane surface wettability. Stainless steel mesh (SSM) is a type of low-cost and chemically stable porous membrane. It has better thermal durability than polymer membranes and superior mechanical properties comparing to ceramic membranes, but pristine SSM cannot separate oil/water mixtures because its surface is both hydrophobic and underwater oleophobic. Therefore, suitable surface coatings are necessary to prepare SSM for oil/water separation. A wide range of materials have been studied as coating materials on SSM to engineer surface wettability. Polymers have been extensively researched as coating materials on SSM as polymers have good chemical stability and are easy to construct microstructures [4], [5], [6], [7], [8], [9], [10], [11], [12]. Inorganic materials, with good thermal and mechanical properties, are deemed promising candidate coating materials. Stainless steel meshes coated with boron nitride nanotubes [13], ZnO nanowires [14], [15], silica nanoparticles [16], [17], zeolite films [18], [19], [20], Cu microflakes [21], and graphene oxide [22] have been developed for oil/water separation. TiO₂ has also attracted specific research interest as a coating material to equip substrates with hydrophilicity and oleophobicity, because it is inexpensive, nontoxic, and has good thermal, chemical and mechanical stability. Various TiO₂ nanostructures, such as nanoparticles, nanotubes, nanofibers, and nanowires, have been used to construct coatings on meshes and membranes for oil/water separation purpose and the factors governing the separation process have been investigated [23], [24], [25], [26], [27]. However, these reported surface coatings require complex coating procedures or precursor preparation processes, such as long-time hydrothermal process and spray/dip-coating followed by calcination, that are often chemically, energetically and operationally intensive. For example, reported coating methods for TiO₂ are limited to spray or dip coating using sols made from either TiO₂ nanostructures or titanium precursors. In these methods, preparation of the stable sols is usually time-consuming and post-annealing at high temperatures is required. Moreover, some of these coatings are not chemically and mechanically durable. For example, ZnO is naturally not stable in acidic or basic environment (pH less than 5 or greater than 11) [28]; Cu is readily oxidized in air; and resistance to mechanical wear, such as abrasion, of these prepared meshes and membranes was rarely studied. Therefore, research on coating materials and methods is still in great need to facilely prepare durable superwetting membranes.

Biomineralization is a biomimetic synthesis process that has emerged as a promising method to synthesize metal oxides such as TiO₂ as it generally attains a high-yield mineral product in a cost-effective, energy-efficient, and environmentally benign manner [29], [30]. This process makes use of the hydrolysis and condensation of water-soluble titanium precursor to produce TiO₂, catalyzed by amino group-rich chemicals [29]. The process involves only two precursors reacting in aqueous solutions at environmentally benign conditions: neutral pH, ambient temperature and pressure, and no hazardous materials, like concentrated acids and explosive chemicals, are involved. These features make the approach a facile and scalable synthesis approach. A large number of TiO₂ and TiO₂-based materials have been synthesized by biomineralization and applied for photocatalytic [29], [30], [31], [32], [33], electrochemical [34], [35], [36], [37], and sensing applications [38], [39]. However, no study has been reported on applying this biomineralization approach in preparing TiO₂ coatings on meshes or membranes for engineering surface wettability and for oily wastewater treatment.

Because of these eminent advantages, biomineralization may be a suitable process for coating TiO₂ on SSM to endow the surface with hydrophilicity and oleophobicity. In this work, we prepared TiO₂-coated SSM (TSSM) via a biomineralization approach and studied its surface wettability, separation capacity (flux and efficiency), anti-fouling capability, and durability under chemically, thermally and mechanically harsh conditions. We also extended the developed coating route to apply TiO₂ coatings on other substrates, including nonwoven SSM, cellulose filter paper, and PVDF membrane for oil/water separation. The proof-of concept studies in this work suggest the potential applicability of the biomineralization-enabled TiO₂-coating method and the facilely fabricated durable TSSM for oil/water separation and oil-spill cleanup.