A diatomite coated mesh with switchable wettability for on-demand oil/water separation and methylene blue adsorption

Chitosan-polyvinyl alcohol-diatomite hydrogel removes methylene blue from water

Dye pollution in the aquatic environment can harm ecosystems and human health. Here, we developed a new green adsorbent by applying an improved drying process. Diatomite was embedded in a network structure formed between chitosan and polyvinyl alcohol without using any crosslinking agent to prepare chitosan-polyvinyl alcohol-diatomite hydrogel beads through alkali solidification. The beads were tested for removing a cationic dye (methylene blue (MB)) from water. The structure of the adsorbent beads was analysed using scanning electron microscopy, energy-dispersive spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and Fourier-transform infrared spectroscopy. The adsorption capacity was investigated, and the results indicated excellent MB adsorption properties. The adsorbents had a rough surface and high swelling capacity of 66.9 g/g. The maximum MB adsorption capacity was 414.70 mg/g, and the adsorption followed the Freundlich isothermal and quasi-second-order kinetic models. The adsorption was an endothermic spontaneous process governed by both intra-particle and external diffusion processes. The proposed adsorption mechanisms involved hydrogen bonding and electrostatic interactions. These adsorbent beads have considerable application potentials owing to their high adsorption capacity, green composition, and non-polluting nature.

Selective and Continuous Oil Removal from Oil-Water Mixtures Using a Superhydrophobic and Superoleophilic Stainless Steel Mesh Tube

The development of continuous oil-water separation processes has applications in the treatment of industrial oily wastewater and effective management of oil spills. In this research, the performance of a superhydrophobic-superoleophilic (SHSO) membrane in oil-water separation is investigated through dynamic tests. We investigate the effects of the total flow rate and oil concentration on the separation efficiency using an as-fabricated SHSO mesh tube. To construct the SHSO membrane, a tubular stainless steel mesh is dip-coated into a solution, containing a long-chain alkyl silane (Dynasylan F8261) and functionalized silica nanoparticles (AEROSIL R812). The as-prepared SHSO mesh tube illustrates a water contact angle of 164° and an oil contact angle of zero for hexane. A maximum oil separation efficiency (SE) of 97% is obtained when the inlet oil-water mixture has the lowest flow rate (5 mL/min) with an oil concentration of 10 vol %, while the minimum oil SE (86%) is achieved for the scenario with the highest total flow rate (e.g., 15 mL/min) and the highest oil concentration (e.g., 50 vol %). The water SE of about 100% in the tests indicates that the water separation is not affected by the total flow rate and oil concentration, due to the superhydrophobic state of the fabricated mesh. The clear color of water and oil output streams also reveals the high SE of both phases in dynamic tests. The outlet oil flux increases from 314 to 790 (L/m²·h) by increasing the oil permeate flow rate from 0.5 to 7.5 (mL/min). The linear behavior of the cumulative amounts of collected oil and water with time demonstrates the high separation performance of a single SHSO mesh, implying no pore blocking during dynamic tests. The significant oil SE (97%) of the fabricated SHSO membrane with robust chemical stability shows its promising potential for industrial-scale oil-water separation applications.

Special Superwetting Materials from Bioinspired to Intelligent Surface for On-Demand Oil/Water Separation: A Comprehensive Review

Since superwetting surfaces have emerged, on-demand oil/water separation materials serve as a new direction for meeting practical needs. This new separation mode uses a single porous material to allow oil-removing and water-removing to be achieved alternately. In this review, the fundamentals of wettability are systematically summarized in oil/water separation. Most importantly, the two states, bioinspired surface and intelligent surface, are summarized for on-demand oil/water separation. Specifically, bioinspired surfaces include micro/nanostructures, bioinspired chemistry, Janus-featured surfaces, and dual-superlyophobic surfaces that these superwetting materials can possess asymmetric wettability in one structure system or opposite underliquid wettability by prewetting. Furthermore, an intelligent surface can be adopted by various triggers such as pH, thermal and photo stimuli, etc., to control wettability for switchable oil/water separation reversibly, expressing a thought beyond nature to realize innovative oil/water separation by external stimuli. Remarkably, this review also discusses the advantages of all the materials mentioned above, expanding the separation scope from the on-demand oil/water mixtures to the multiphase immiscible liquid-liquid mixtures. Finally, the prospects of on-demand oil/water separation materials are also concluded.

The separation of oily water using low-cost natural materials: Review and development

This paper reviewed the recent progress on a number of important natural/biomass materials (fibers, leaves, woods, nutshells, algae, and sands, etc.) for the separation of oily water mixture/emulsions. Experiments were conducted with desert pristine sand samples, which were collected, sieved, characterized and tested for the separation of two surfactant-stabilized oil in water (O/W) emulsions in a simple cross-flow sand bed filter under the simulated natural gravity. The wettability of the sand samples was checked through contact angle measurements. The morphology of sands was characterized with SEM/EDX and FTIR. Experimental results revealed that natural sand filtration is a promising technology with the characteristics of: 1) a surface of superhydrophilicity and underwater superoleophobicity; 2) sands bed achieved separation efficiency and flux comparable or higher than commercial microfiltration membranes under natural gravity conditions, and 3) the separation efficiency and flux of the sand bed are relatively stable with respect to the operation parameters. This technology is technically feasible, low-cost, and environmental-benign and can play an important role in the practical applications.

Mussel-inspired superhydrophilic membrane constructed on a hydrophilic polymer network for highly efficient oil/water separation

HYPOTHESIS

Superhydrophilic/underwater superoleophobic membrane constructed by hydrophilic polymers possesses great advantage in the separation of oily waste water, due to its intrinsic oil-repellent property. The formation of hydration layer to repel and block oil is considered as the mechanism of underwater superoleophobicity and subsequent oil/water separation. Constructing a stable hydrophilic polymer network on the substrate surface would significantly improve the robustness of hydration layer.

EXPERIMENTS

In this work, a feasible and universal mussel-inspired dip-coating method was developed for constructing stable hydrophilic polymer network onto target substrate surface, via successively immersing substrate membranes into aqueous solutions of polydopamine (PDA) and catechol-functionalized hydrophilic polymer (CFHP). After pre-wetting with water, the polymer network would swell with water to form a thin and stable water film layer, serving as a barrier against oil penetration.

FINDINGS

The as-prepared CFHP/PDA modified membranes exhibit outstanding performance in separating various oil/water mixtures and oil-in-water emulsions stabilized by surfactants, with separation flux up to 5641.1 L·m^-2·h^-1 and separation efficiency achieving 99.98%. The surface modification method developed in this work can be easily extended to various materials and membrane systems, for achieving a variety of practical applications such as industrial wastewater treatment.

Adsorption performance of basic fuchsin on alkali-activated diatomite

The natural diatomite was treated with NaOH to obtain alkali-activated diatomite. The materials were systematically characterized by X-ray powder diffraction, X-ray fluorescence, Fourier transform infrared spectroscopic, scanning electron microscopy, and N2 adsorption–desorption. Meanwhile, the potential use of alkali-activated diatomite as adsorbent for the removal of basic fuchsin from aqueous solution was assessed by batch experiment. Results indicated that the structure and textural properties of diatomite were obviously changed via alkali activation, and then affecting its adsorption performance. The adsorption capacity of alkali-activated diatomite for basic fuchsin was higher than that of natural diatomite. In the case of alkali-activated diatomite, its adsorption capacity was increased with increasing the activation temperature, and the diatomite activated at 115°C (alkali-activated diatomite-115) exhibited the maximum adsorption capacity. The pseudo-first-order kinetics and the Sips isotherm model were preferable to describe the adsorption process of basic fuchsin on alkali-activated diatomite-115 and the thermodynamic parameters indicated that the adsorption process was endothermic and spontaneous.

Superhydrophobic Polypropylene Functionalized with Nanoparticles for Efficient Fast Static and Dynamic Separation of Spilled Oil from Water

Frequent oil spills not only threaten the ecosystem, but they are also a waste of a valuable source of energy. There is an urgent need to develop materials that can readily remove spilled oil from water bodies and also have the capacity to collect it for energy applications. Herein, a superhydrophobic fiber of functionalized polypropylene is engineered with the help of palmitic acid interaction with incorporated copper oxide nanoparticles. The successful development of functionalized polypropylene is confirmed by Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, and energy-dispersive X-ray spectroscopy. The scanning electron microscopy images reveal that the surface roughness of the polypropylene is enhanced after functionalization. The optimized functionalized polypropylene displays an ultrahydrophobic surface with a water contact angle of 162.42°. The functionalized polyprolyene displays good absorption capacity. It has the capacity to take 30 to 40 times its own weight in oils and nonpolar organic solvents, which makes it useful for small spills. With a flux of 11 204 Lm-2 h-1, functionalized polypropylene is as an ideal material for the dynamic separation of oil spills from water. It also has excellent selectivity towards oil, water rejection, and oil absorption capacity.

High-Flux and Robust Co3O4 Mesh for Efficient Oil/Water Separation in Harsh Environment

Material with special wettability for oil/water separation has drawn more and more attention, since the oil spill accidents and industrial processing are growing in frequency and in volume. A superhydrophilic and underwater superoleophobic mesh was prepared by introducing Co₃O₄ on a stainless steel mesh, through a simple hydrothermal process and subsequent calcination. The as-prepared Co₃O₄ mesh can not only separate various oil/water mixtures with high efficiency and high flux, but also work effectively in harsh environment such as highly acidic, alkaline, and salty solutions. Moreover, the Co₃O₄ mesh can still retain good separation performance after 40 abrasion cycles with sandpaper. The outstanding anticorrosion and antiabrasion behaviors make the Co₃O₄ mesh promising for oil/water separation even in harsh environment.

Electrophoretic Deposition of Graphene Oxide on Laser-Ablated Copper Mesh for Enhanced Oil/Water Separation

The fabrication of bionic surfaces resembling hydrophobic plants through micro manufacturing, which creates abundant multi-level micro/nanostructures and elemental variations, has been widely employed to change the surface wettability of metallic materials. Based on the mechanisms for selective permeation of various liquids, it could achieve the function of oil/water separation. Herein, a separation copper membrane fabricated with pulsed laser ablation and modified with graphene oxide (GO) deposition showed a synergetic effect on tunable surface wettability. Micro/nanostructures were generated on the copper substrate membrane through concentric circular scanning, which was followed by hole drilling. Afterwards, charged GO nanosheets were deposited via electrophoresis. The spacing of circular lines, the diameter of the holes and the abundant high-surface-energy hydrophilic oxygen contained in deposited GO amounts could be regulated in the laser processing and deposition, resulting in oleophobicity and hydrophilicity at the same time. The highest contact angle of oil in water of the prepared mesh could reach above 165° with a hole size of 200 µm and a circular line spacing of 100 µm after the laser processing. Water flux and oil-holding capacity, which represent the separation capability of the mesh, were also evaluated. The as-prepared separation mesh also showed great stability under harsh environments.