One-step reduction and simultaneous decoration on various porous substrates: toward oil filtration from water

Abstract

Effective purification of waste water tainted with either immiscible oil/water mixtures or surfactant-stabilized emulsions is always attractive for long-term water remediation. We report here on a versatile development of polydopamine coated copper surfaces on various porous substrates via a simple one-step immersing method. The incorporation of dopamine into this design not only optimizes the operation of preparing simple substances on copper surfaces on a wide range of substrates, but also endows the as-obtained membranes with superhydrophilicity and underwater superoleophobicity. Such membranes are capable of efficiently separating both immiscible oil/water mixtures and surfactant-stabilized emulsions based on the desired substrates, and they display superior chemical resistance to aggressive reagents, excellent recyclability and robust structural stability, making them promising candidates for a variety of oil filtration applications ranging from the treatment of oil/water mixtures to emulsified oily waste water.

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1. Introduction

Waste water, especially emulsified oil/water mixtures discharged from industrial technologies and daily life, is threatening the environment and human health.¹ Removing oil from waste water is of great significance yet challenging since oily waste water exists in sundry forms including immiscible/emulsified mixtures in state, surfactant-free/stabilized emulsions in components, oil-in-water/water-in-oil emulsions in formulation, and micro/nanometer scale droplets in size, etc.^2–4 Superwetting materials designed by optimal integration of surface chemistry and structure roughness have been explored and widely applied for separation of oil/water mixtures.^5–12 Mesh-based membranes with average pore sizes in the micrometer scale are only capable of separating immiscible mixtures and not capable of processing of emulsions with oil droplet sizes in the nanometer to sub-micrometer range.^13–19 Polymeric filtration membranes have been recently used to solve the deficiency of the mesh-based membranes with acceptable discharge criterion and simple operation.^20–25 However, they also have limitations such as quick decline of the penetration flux and easy fouling owing to severe absorption or plugging of oil droplets and surfactants.^26,27 Hence, it is attractive and challenging to develop a facile method that could realize the process of effective purification of waste water tainted with either immiscible oil/water mixtures or surfactant-stabilized emulsions with improved durability after cyclic water remediation.

Copper, a ubiquitous engineering material, is of great significance and irreplaceable in our society and will remain so in the future. A variety of metallic copper surfaces with special wettability have been fabricated and extensively used over the past few years.²⁸ For instance, superamphiphobic surfaces with CuO/Cu(OH)₂ hierarchical structure,^29,30 superhydrophobic surfaces with CuO microstructure,³¹ the superhydrophobicity of 3D porous copper film,^32,33 superhydrophobic copper tubes,³⁴ underwater superoleophobic surfaces with Cu(OH)₂ hierarchical structure,³⁵ wettability cycling of CuO surfaces between superhydrophobicity and superhydrophilicity,³⁶ underwater superoleophilic to superoleophobic wetting control on nanostructured Cu(OH)₂ substrates,³⁷ etc. have been developed. However, fabrication of special wettable, simple substances on a copper surface is rarely reported. Pitchumani and co-workers developed a two-step electrodeposition process to produce copper coatings on a copper plate.³⁸ Guo et al. realized in situ growth of simple substances of copper nanoparticles on fabrics through reduction of copper sulfate by sodium borohydride.³⁹ Pal et al. fabricated copper nanoparticles on various substrates through the chemical reduction of copper acetate by hydrazine hydrate.⁴⁰ Nevertheless, these methods are either fabricated solely on copper substrates or reduced using noxious and hazardous reductants. Hence, preparing simple substances as copper coatings on various substrates via a milder reduction methodology is of interest.

Herein, we developed copper surfaces on porous substrates through a facile, one-step chemical reduction and decoration simultaneously utilizing dopamine for efficient oil filtration from oil/water mixtures and emulsions. As schematically shown in Scheme 1, the copper precursor is reduced to a simple copper substance and functionalized with a polydopamine (PDA) layer and simultaneously benefits from the benign reduction via dopamine, spontaneous self-polymerization and strong adhesion. The formed polydopamine coated copper (PDA-Cu) surfaces can synchronously decorate on a wide range of porous substrates. Considering the abundant hydrophilic groups, especially catechol and amine groups, the PDA-Cu decorated membranes are endowed with superhydrophilic and underwater superoleophobic wetting behaviors. The resultant membranes are capable of separating both immiscible oil/water mixtures and surfactant-stabilized emulsions with sub-micrometer size oil droplets based on the desired substrates. More importantly, they exhibit superior performance for acidic/alkaline repellency, excellent recyclability and robust structural stability. Thus, the as-presented protocol may offer great potential for developing versatile copper surface materials for oil filtration from oily sewages.

Scheme 1 Schematic of the preparation process for copper surface materials, showing the versatile separation capacity of various oil/water mixtures.

2. Experimental

2.1 Materials

Dopamine-hydrochloride (Sangon, Biotech Co. Ltd., Shanghai, China) was used as purchased. Copper acetate monohydrate and Tween 20 were of analytical grade from Sinopharm Chemical Reagents. All other chemicals were used without further purification.

2.2 Fabrication of PDA-Cu surfaces on porous substrates

The synthesis of copper surfaces on porous commercial substrates, such as stainless steel mesh (SSM, 316 Type), mixed cellulose ester (MCE) filter membrane, copper mesh, nylon mesh and polyurethane (PU) sponge, was carried out through the chemical reduction of copper acetate by dopamine under ambient conditions. Briefly, the SSM used as a substrate was ultrasonically cleaned in acetone to remove the surface dirt. The pretreated mesh was immersed in 100 mL of mixed aqueous solution combining copper acetate monohydrate (5 mmol L⁻¹) and dopamine-hydrochloride (25 mmol L⁻¹). Then, a buffer solution (5 mg of dopamine-hydrochloride per mL of 10 mmol L⁻¹ tris(hydroxymethyl) aminomethane (Tris) buffer, pH = 8.5) was added dropwise. At a weak alkaline pH, dopamine will undergo self-polymerization to produce an adherent PDA coating on the surface of the substrates. The whole reaction was covered with parafilm and kept for 48 h at room temperature. Similar fabrication procedures were adopted for other substrates. To explore the effect of reaction time on the surface morphology, additional pretreated SSM was fabricated according to the same fabrication process and kept for 24 h. Another pretreated SSM was also immersed in the mixed aqueous solution without adding a buffer solution and kept for 48 h to verify the function of the tris(hydroxymethyl) aminomethane component on surface morphology. The as-obtained membranes were removed from the solutions and cleaned with water for further characterization.

2.3 Preparation of oil-in-water emulsions

Surfactant-stabilized, oil-in-water emulsions were prepared by mixing water and oil (toluene, n-hexane, gasoline and diesel) in a 100 : 1 (v/v%) ratio with the addition of 0.5 mg of Tween 20 per mL of mixture with high stirring (at 1200 rpm) for 3 h.

2.4 Oil/water mixtures and emulsions separation tests

The PDA-Cu decorated SSM or MCE filter membrane was fixed between two Teflon fixtures that were attached via a glass tube. The diameter of the glass tube was 30 mm. The oil/water mixtures or freshly obtained emulsions were poured onto the material surface, and the separation was solely driven by gravity. The corresponding fluxes were calculated by measuring the time needed to collect a certain volume of filtrate. The separation efficiency was measured by the oil rejection coefficient (R) according to eqn (1):

R = (1 − C_f/C_o) × 100% (1)

where C_o and C_f are the oil concentration of the original oil/water mixtures and the collected filtrate after one separation, respectively.

2.5 Instrument and characterization

FESEM images were obtained on a field emission scanning electron microscope (SU-8010, Hitachi Limited, Japan). The EDX image was measured using energy-dispersive X-ray analysis (HORIBA, Ltd., Japan). The variation in the Cu²⁺ concentration was tested with a Perkin Elmer Lambda-750 UV spectrometer (United Kingdom). X-ray photoelectron spectroscopy (XPS) measurements were carried out on a Thermo escalab 250Xi spectrometer using an Al Kα X-ray source (1486.6 eV). Contact angles were measured on a contact angle measurement machine (OCA 15 machine, Data-Physics, Germany). Dynamic light scattering (DLS) measurements were performed on a Zeta Plus apparatus (Zeta Plus, Brookhaven Instruments, Holtsville, NY). The oil content in the filtrate was measured by an infrared spectrometer oil content analyzer (Oil480, Beijing Chinainvent Instrument Tech. Co. Ltd., China).

3. Results and discussion

3.1 Morphology investigation

Field emission scanning electron microscope (FESEM) was performed to investigate the morphology variation in various substrates before and after chemical reduction of copper acetate by dopamine. The stainless steel mesh (SSM) knitted by the wires with an average diameter of approximately 60 μm presented an exquisite and ordered structure with a smooth surface (Fig. 1a). After PDA-Cu synthesis, adding Tris buffer and reacting for 24 h, it is clearly seen that the smooth substrate was randomly anchored with many cycloidal lamellar and spindle structures but was not covered throughout the surface (Fig. S1a, ESI†). When the mesh was synthesized by reacting for 48 h, it was covered by porous coatings consisting of numerous irregular micrometer scale pores that were constructed from the nanometer size cross-linked lamellar structures (Fig. 1b). The thickness of these coatings was about 10 μm. In this way, a micro/nano hierarchical porous texture was successfully created. Interestingly, the surface was almost as smooth as the nascent one after reacting for 48 h without adding Tris buffer (Fig. S1b, ESI†), indicating the importance of the Tris component on the fabrication of the PDA-Cu surface decorated samples. The mixed cellulose ester (MCE) filter membrane has an abundant porous structure with a pore size of ∼450 nm and displays similar micro-level structures (Fig. 1c and d) after the simultaneous reduction and decoration, whereby the polymer skeleton was wrapped in a composite coating with a thickness of ∼150 nm. Therefore, a surface with a micro/nano porous structure was achieved on the MCE membrane. The FESEM images of the as-prepared surfaces decorated on other substrates (copper mesh, nylon mesh and PU sponge) were also studied and are displayed in Fig. S2 (ESI†). All of the porous substrates used in this work were either knitted by withy metallic and polymeric wires or a polymer matrix, which ensured a high mechanical strength of the as-prepared membranes. For example, the PDA-Cu surfaces retained adhesion on the copper mesh when the mesh was bent and rinsed with water sequentially (Fig. S3, ESI†). A possible deduction to clarify the growth mechanism of the morphology on different substrates is that the material itself and the intrinsic varied architecture of the substrates are crucial to directly control the facile growth of PDA-Cu surfaces. For SSM and copper mesh, their similar metallic material makes it possible for them to possess the same structures. In regards to the nylon mesh, the polymeric composition and smaller pore sizes lead the lamellar and spherical structures to locate is a disorderly manner on the mesh wires. Compared to the three 2D substrates that were knitted by cylindrical wires, the PU sponge has a 3D interconnected network constructed from the depressed form of the wires, which permit the lamellar structures to grow more densely at the shallow location. For the MCE membrane with pores in the sub-micrometer scale, the PDA-Cu surface has to grow a more refined coating form, taking into account the space hindrance.

Fig. 1 FESEM images of SSM and MCE filter membranes before and after chemical reduction of copper acetate by dopamine. (a) The cross-knitted SSM with a smooth surface, (b) porous coatings consisting of numerous irregular micrometer scale pores decorated on the SSM surface after PDA-Cu synthesis, (c) the MCE filter membrane exhibits an abundant porous structure and (d) the MCE skeleton was wrapped in a composite coating after simultaneous reduction and decoration.

3.2 Chemical state characterization

To test the universal applicability of our approach on sundry porous substrates and to avoid the interference of stainless steel mesh that contains copper, energy dispersive X-ray (EDX) analysis was performed on PDA-Cu decorated nylon mesh. Fig. S4a (ESI†) shows evidence of very strong Cu peaks for the PDA-Cu surfaces, and the peaks of C, O and N should arise from PDA on the resultant surfaces. Additionally, the notable decrease in the UV-Vis spectra of Cu²⁺ solution up to 80% before and after chemical reduction (Fig. S4b, ESI†) indicated that the dopamine successfully reduced Cu²⁺ to some other chemical state and simultaneously decorated the copper surface, which benefited from its mild reduction capacity and high adhesion property at the same time.

To confirm the composition and chemical state of copper for our PDA-Cu surfaces on various substrates (taking the PDA-Cu decorated MCE membrane as an example), X-ray photoelectron spectroscopy (XPS) was performed. The typical survey spectrum of the PDA-Cu surfaces is shown in Fig. 2a. The four prominent peaks of C 1s, N 1s, O 1s and Cu 2p were consistent with that of the above EDX spectra. Fig. 2b displays the Cu 2p_3/2 spectra of the initial Cu²⁺ solution and the fresh sample after reaction (treated with copper reagent and sequentially extracted by CCl₄). The observation of a very broad spectrum at 934.2 eV strongly suggested the presence of Cu²⁺ species in the initial solution. After reduction, the Cu 2p_3/2 spectrum intensified but remained relatively sharp. The satellite peak of the Cu 2p_3/2 band disappeared, and the value of the binding energy shifted to 932.7 eV, which indicated the existence of metallic copper.⁴¹ These results not only provided solid evidence to confirm that the Cu²⁺ species was successfully reduced to the form of a simple substance but indicated that the composite coating on the polymer skeleton had the same composition as the others. Although a few researchers have confirmed the reduction of silver ions by dopamine, the procedures were always tedious and consisted of dopamine spontaneously polymerizing onto substrates, silver nanoparticles forming by reduction and eventually functionalizing by reacting with alkanethiol.⁴² We have reported here the facile fabrication of superwetting copper surfaces by a simple one-step chemical reduction and decoration utilizing dopamine.

Fig. 2 (a) XPS survey spectra of the PDA-Cu surfaces. (b) XPS spectra of Cu 2p_3/2 for the initial Cu²⁺ solution and the fresh sample after reaction (treated with copper reagent and sequentially extracted by CCl₄).

3.3 Wetting behavior and durability

The optimal integration of geometric structure and chemical composition is critical to manipulate the surface wettability of materials. Thus, combining the micro/nano composite structure with the inherent hydrophilicity of PDA can be expected to exhibit the best superwetting behavior.^43–45 Taking the PDA-Cu decorated copper mesh as an example, the wetting behavior of water and oil on the membrane was measured in detail. Once a water droplet contacted the membrane surface, it instantaneously spread within one second, and the water contact angle (CA) was about zero (Fig. S5a, ESI†). As a control, the nascent copper mesh before chemical reduction of copper acetate by dopamine was hydrophobic with a water CA on the surface of about 130 ± 1.3° (Fig. S5b, ESI†), giving indirect proof of the successful decoration of PDA-Cu surfaces on the copper mesh. The optical images in Fig. 3a show oil droplets including toluene, n-hexane, gasoline, diesel and 1,2-dichloroethane deposited on the mesh surface formed almost perfect spheres under water, and the corresponding oil CAs were all greater than 150°. These results demonstrated that the PDA-Cu decorated copper mesh possessed superhydrophilicity as well as underwater superoleophobicity. In addition, the performance of the resultant membranes and the nascent copper meshes for acidic/alkaline repellency was also tested after being soaked in different aqueous solutions with pHs ranging from 1 to 11 and concentrated salt conditions. As shown in Fig. S6 (ESI†), the original copper mesh presented highly oleophobic properties underwater. This was true except when it was soaked in solutions of pH = 1 and 11 and was superoleophobic with oil CAs larger than 150°. However, the inset shows that the color of the copper mesh changed to brown after it was taken out from these two solutions, which demonstrated that the meshes were severely corroded. Whereas, the surface of the as-obtained membrane exhibited stable underwater superoleophobicity for at least 12 h (Fig. 3b). The satisfactory durability makes our membranes favourable for use during oil/water separations.

Fig. 3 (a) Wetting behavior of oil droplets on the PDA-Cu decorated copper mesh in oil/water/solid systems. (b) Acidic/alkaline durability test of the resultant membranes after being soaked in different aqueous solutions with pHs ranging from 1 to 11 and concentrated salt condition.

3.4 Separation test of oil/water mixtures

To evaluate whether the superwetting properties of these membranes could allow them to separate immiscible oil/water mixtures or surfactant-stabilized emulsions with high efficiency and recyclability, diverse separation tests were performed. The chosen membrane pre-wetted by water was fixed in two Teflon fixtures that were attached with glass tubes. Fig. 4a and Video 1 (ESI†) show the diesel/water separation process using the PDA-Cu decorated SSM obtained by reacting for 48 h. It can be seen that the mesh selectively allowed water to quickly pass through, only driven by gravity, whilst the diesel remained on the surface. Significantly, a series of oil/water mixtures including toluene, n-hexane, gasoline and diesel were successfully separated with a permeation flux as high as 6800 L m⁻² h⁻¹ (Fig. S7, labelled by red, ESI†) and efficiencies all in excess of 99.97% (Fig. 4b). To test the effect of reaction time on flux and oil removal efficiency, we also conducted additional separation experiments with gasoline/water mixtures utilizing the original SSM and PDA-Cu decorated SSM obtained by reacting for 24 h. We found that the mixture quickly flowed through the mesh, which indicated that the original SSM lacks a filtration ability (Fig. S8a, ESI†). Notably, the flux of the PDA-Cu decorated SSM obtained by reacting for 24 h was relatively higher than that obtained by reacting for 48 h. However, the corresponding oil removal efficiency was much lower (Fig. S8b, ESI†). Considering the sufficiently high permeation flux as well as the excellent oil removal ability, the reaction time was set at 48 h for the fabrication of sundry PDA-Cu decorated samples in this work.

Fig. 4 (a) Separation apparatus for the immiscible diesel/water mixture with water passing through the mesh whilst the diesel was retained. (b) Separation efficiency of the PDA-Cu decorated SSM for a selection of oil/water mixtures.

3.5 Emulsion separation test

A Tween 20-stabilized toluene-in-water emulsion was fed into the filtration apparatus fitted with the as-prepared MCE membrane (Fig. 5a). The feed was monitored by dynamic light scattering (DLS), and the oil droplet sizes were mainly distributed in the sub-micrometer range (Fig. S9a, ESI†).

Fig. 5 (a) Separation apparatus of the toluene-in-water emulsion with the emulsion being de-emulsified and water penetrating the membrane whilst oil droplets are blocked. (b) Separation efficiency of the PDA-Cu decorated MCE filter membrane for different kinds of oil-in-water emulsions.

The procedure of the filtration separation was solely conducted by gravity (Video 2, ESI†). As the emulsion de-emulsified, the oil droplets were blocked by the membrane, and transparent water penetrated it and flowed into the vial. As a control, the MCE filter membrane used as a substrate was selected to repeat the procedure. The collected filtrate was comparatively opaque (Fig. S10, ESI†), which meant that the initial MCE substrate did not possess the de-emulsifying capacity for sub-microscale emulsions. Additionally, the permeability performances of the as-prepared MCE membrane for different oil-in-water emulsions with all oil droplets in sub-micrometer sizes (Fig. S9b–d, ESI†) are shown in Fig. 5b. The oil concentrations in the corresponding filtrate were all below 25 ppm, indicating the excellent separation capacity of PDA-Cu decorated membranes and their potential application in water remediation. Notably, given that gravity was the sole driving force, the filtration flux of the emulsion separation (Fig. S7, labelled by green, ESI†) was much less in comparison with that of the oil/water mixture separations.

3.6 Recyclability performance

Aside from the separation capabilities, recyclability should also be given attention to attain long-term applicability. The morphologies of the as-prepared membranes with regard to recyclability were tested by conducting repetitive filtrations. After each separation of 100 mL of oil/water mixtures or filtration of 10 mL of emulsion, the membranes were simply washed with deionized water. It can be seen that the architecture of the PDA-Cu decorated SSM and MCE membrane remained almost constant without visible variation up to 20 cycles (Fig. 6). This demonstrated that our membranes have excellent recyclability and robust structural stability for practical applications.

Fig. 6 FESEM images of PDA-Cu decorated SSM (a, b) and MCE membranes (c, d) before and after 20 cycles of filtration of oil/water mixtures and emulsion, respectively.

4. Conclusions

In summary, we reported on the versatile development of PDA-Cu surfaces on various porous substrates via a simple one-step mild chemical reduction and decoration utilizing dopamine. The incorporation of dopamine into this design not only optimizes the operation of preparing simple substance of copper surfaces on a wide range of substrates, but also endows the as-obtained membranes with superhydrophilicity and underwater superoleophobicity. The resultant membranes were verified to be capable of efficiently separating both immiscible oil/water mixtures and surfactant-stabilized oil-in-water emulsions with oil droplet sizes in the sub-micrometer range based on the desired substrates. More importantly, they exhibited superior chemical resistance to aggressive reagents, excellent recyclability and robust structural stability. These results indicate that our PDA-Cu decorated membranes have great potential in water remediation for a variety of oil filtration applications ranging from the treatment of oil/water mixtures to emulsified oily waste water.

Acknowledgements

The authors are grateful for financial support from the National Natural Science Foundation (51173099 and 21134004).

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Footnote

† Electronic supplementary information (ESI) available: Details of FESEM images, EDX and UV-Vis spectra, wetting behavior of water, permeation flux, oil droplet size distribution curves, emulsion filtration test of the initial MCE substrate, and two supporting videos for diesel/water separation and toluene-in-water emulsion separation processes. See DOI: 10.1039/c6ra13698b

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