Efficient oil/water separation by a durable underwater superoleophobic mesh membrane with TiO2 coating via biomineralization
Graphical abstract
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.
Section snippets
Materials
Titanium(IV) bis(ammonium lactato)dihydroxide solution (Ti-BALDH, 50 wt% in H2O) was purchased from Sigma-Aldrich and branched polyethylenimine (PEI, MW = 1800) was purchased from Polysciences Inc. Stainless steel mesh (SSM) composed by 25 μm wires using the Twill Dutch weave style with an opening diameter of 5 μm was obtained from Utah Biodiesel Supply. Nonwoven stainless steel mesh (400 × 400) was obtained from McMaster-Carr Supply Company. Filter paper (grade 413, diameter: 5.5 cm, particle
Morphology and surface chemical compositions
The surface compositions and chemical states of SSM and four-cycle-coated TSSM were investigated by XPS. The survey spectra (Fig. 1a) prove the existence of Fe in SSM and Ti in TSSM. The Ti 2p spectra of TSSM (Fig. 1b) are well resolved into two spin orbit components at binding energies at 458.4 eV and 464.1 eV that are attributed to Ti 2p3/2 and Ti 2p1/2, respectively. XRD testing shows that SSM and TSSM have almost identical spectra and no characteristic peaks of crystalline TiO2 were
Conclusions
In conclusion, TiO2-coated stainless steel meshes were facilely fabricated via the biomineralization approach in an environmentally benign manner. The uniform, smooth, thin and conformal coating of TiO2 endowed the mesh with superhydrophilicity and underwater superoleophobicity. It could selectively separate water from various oil/water mixtures driven by gravity with a high separation flux (∼3 × 104 L m−2 h−1), high oil rejection efficiency (∼99.999%), and excellent resistance to oil fouling.
Notes
The authors declare no competing financial interest.
Acknowledgements
This study was supported by Texas A&M Energy Institute. The use of the Texas A&M University Materials Characterization Facility is also acknowledged.
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