Dodecyl Mercaptan Functionalized Copper Mesh for Water Repellence and Oil-water Separation

Reckless discharge of industrial wastewater and domestic sewage as well as frequent leakage of crude oil have caused serious environmental problems and posed severe threat to human survival. Various nature inspired superhy-drophobic surfaces have been successfully applied in oily water remediation. However, further improvements are still urgently needed for practical application in terms of facile synthesis process and long-term durability towards harsh environment. Herein, we propose a simple one-step dodecyl mercaptan functionalization method to fabricate Super-hydrophobic-Superoleophilic Copper Mesh (SSCM). The prepared SSCM possesses excellent water repellence and oil affinity, enabling it to successfully separate various oil-water mixtures with high separation efficiency (e.g., > 99% for hexadecane-water mixture). The SSCM retains high separating ability when hot water and strong corrosive aqueous solutions are used to simulate oil-water mixtures, indicating remarkable chemical durability of the dodecyl mercaptan functionalized copper mesh. Additionally, the efficiency can be well maintained during 50 cycles of separation, and the water repellence is even stable after storage in air for 120 days, demonstrating the reusability and long-term stability of the SSCM. Furthermore, the functionalized mesh also shows good mechanical robustness towards abrasion by sandpaper, and oil-water separation efficiency of > 96% can be obtained after 10 cycles of abrasion. The reported one-step dodecyl mercaptan functionalization could be a simple method for increasing the water repellence of copper mesh, and thereby be a great candidate for treating large-scale oily wastewater in harsh environments.


Introduction
Frequent occurrence of oil spills, and massive discharge of industrial and domestic oily wastewater make the problem of oil and water pollution increasingly serious, causing great harm to the ecological environment [1][2][3][4] . How to efficiently treat the oil-water mixture and thus obtain reusable pure water and oil is one of the most attractive scientific problems. Traditional methods for treating water and oil pollution can be broadly divided into three categories: biodegradation [5] , chemical treatment [6,7] and physical separation by skimmer and oil absorption felt, etc. [8,9] . However, these methods are generally characterized by low separation efficiency, poor oil-water selectivity, low reusability and high cost.
Many plants and animals in nature have special wettability on their surfaces. For example, the self-cleaning function of lotus leaves originate from the superhydrophobicity of their upper surface [10] ; fish swims in oily water without being contaminated by oil because their scales are superhydrophilic and underwater superoleophobic [11] ; desert beetles can survive in extreme dry environment due to the superhydrophilic-superhydrophobic patterned surface on their backs facilitates the collection of fog [12] . Therefore, in recent years, biomimetic interfaces with special wettability have attracted great attention from both scientific researchers and engineers [13][14][15][16][17][18][19] . Because of their outstanding liquid repellence or affinity, high separation efficiency and flux, functional surfaces with super wettability have been widely developed for oil-water separation, and these surfaces can be classified as superhydrophobic-superoleophilic surfaces, superhydrophilic-underwater superoleophobic surfaces and con-888 vertible superwetting surfaces [20][21][22][23][24][25][26][27] . Typically, oil-water separation can be realized via filtering or adsorption by these surfaces with super wettability. The adsorbent materials are mainly 3D porous foams [28] , sponges [29,30] , aerogels [31,32] , powders or particles [33] , which are suitable for the adsorption and transfer of oil spills on water surface. In contrast, filtering is more desired for the separation of collectible oil-water mixtures, and porous metal mesh [34][35][36] , membranes [37,38], fiber fabrics [39,40] and filter papers [41] are always selected as the substrates to create surfaces with super wettability for filtering type separation. Among them, metal meshes (e.g., Copper Mesh (CM)) are characterized with good mechanical strength, cheapness and easy availability, etc. More importantly, due to the high activity of metal meshes, chemical reactions and modifications can be easily performed on their surfaces to construct super wettability. For example, Cheng et al. prepared spherical structures of Cu(OH) 2 on CM and modified them with thiols to obtain superhydrophobic surfaces with controlled adhesion [42] . Liu et al. prepared rough Cu(OH) 2 nanopins on CM with tunable wettability to separate mixed-phase solutions [43] . Ansah et al. prepared various morphologies of Cu(OH) 2 and CuO on CM, such as needle-shaped, hair-shaped and arch-shaped structures, which were further modified with fluorosilanes to obtain superhydrophobic surfaces [44] . However, those universal synthesis methods are relatively complex and environmentally-unfriendly, often requires two steps consisting of building micro-nanostructures and low surface energy material modification, and the direct construction of superhydrophobic surfaces by a one-step method with good durability towards harsh environments (e.g., corrosive aqueous solutions) remains a great challenge [45] . Therefore, it is of great relevance and urgency to develop a robust superhydrophobic interface with simple preparation process and high environmental adaptability.
Herein, we report a simple and eco-friendly one-step immersion strategy to fabricate Super-hydrophobic-Superoleophilic Copper Mesh (SSCM). The self-assembly reaction of copper and dodecyl mercaptan could generate a layer of micro-nanostructures on the surface of the CM and reduce its surface energy, imparting the mesh with superhydrophobicity and superoleophilicity. The whole process can be completed in 2 h at room temperature without using any harmful fluorinated chemicals. The effect of different immersion time on the wettability of CM was investigated via water contact angle measurement. Since the excellent superhydrophobic and superoleophilic properties, the separation efficiency of oil-water mixtures was measured to be > 97%. It was found that the as-prepared SSCM showed excellent separation ability even under hot water, acidic, alkaline and salty conditions. The separation efficiency could be well maintained after 50 separation cycles.

Materials
CMs with different size were purchased from Shanghai composite sieve works. The distilled water was provided by a UCP-III water purification system. Sodium chloride (NaCl), sodium hydroxide (NaOH), hydrochloric acid (HCl) and dodecyl mercaptan [CH 3 (CH 2 ) 10 CH 2 SH] were purchased from Shanghai Aladdin Biochemical Technology Co. Hexadecane, peanut oil, hexane, octane for oil-water separation were purchased from Tianjin Yuanli Chemical Co, and dichloromethane was purchased from Tianjin Kosmeo Chemical Reagent Co. All chemicals were AR level and used without further purification.

Sample preparation
Before preparation, the raw CM was cut into circle samples (D = 5 cm) firstly. The oxide on the CM surface was removed by immersing 0.1 mol·L −1 hydrochloric acid solution for 1 min. Then the CMs were ultrasonically washed with ethanol and distilled water in turn, and dried in the air. Finally, the samples were immersed in 0.1 mol·L −1 dodecyl mercaptan ethanol solution for 2 h at room temperature. The modified CMs were flushed with anhydrous ethanol and dried in air.

Sample characterization
Static contact angle and sliding angle of water and oil on the prepared samples were measured by an optical contact angle meter (ASTVCM Optima, USA) at room temperature, and the used volume of the droplets were 7 μL. The contact angle at 5 different positions of the surface were measured and the average value was calculated and reported in this paper. Scanning Electron Microscope (SEM, SUPRA 55 SAPPHIRE, Germany) was used to observe the surface morphology of the samples, and the chemical composition of the surface was analyzed by Energy-Dispersive X-ray spectroscopy (EDS, SUPRA 55 SAPPHIRE, Germany) and X-ray Photoelectron Spectroscopy (XPS, Thermo ESCALAB 250Xi, USA).

Oil-water separation
The prepared SSCM was fixed between two identical glass tubes with a clamping device, and the glass tubes were fixed at an inclination angle of 45˚. The prepared oil-water mixture was slowly poured into the upper tube and the whole separation process was driven by gravity. The oil-water separation efficiency could be calculated by η where V represented the volume of the oil in the mixture, S was the effective cross-sectional area of the SSCM used for oil-water separation, and t was the time required for the oil in the mixture to completely pass through the SSCM. The water was dyed blue for better visual effect. The oils used in the experimental process included hexadecane, peanut oil, hexane, octane and dichloromethane, and their basic parameters are presented in Table S1.

Formation of superhydrophobicity and wettability
The preparation process of the SSCM is shown in Fig. 1. When the CM was immersed into dodecyl mercaptan ethanol solution, a direct adsorption chemical reaction occurred on the surface, which was accompanied by oxidative addition to the surface bonds of the CM and the reduction of hydrogen elimination [46] : As a result of the adsorption reaction, self-assembled monolayers were formed on the surface.
The micro/nano structures and the low surface energy of the self-assembled monolayers enabled the CM to be superhydrophobic by this one-step adsorption reaction. Fig. 2a shows the digital image of droplets (water, 1 mol·L −1 HCl, 1 mol·L −1 NaOH and 20 wt% NaCl) deposited on the as-prepared superhydrophobic CM, which demonstrated that all the droplets retained as spherical shape, and the corresponding Water Contact Angle (WCA) of the functionalized mesh is about 153˚ (Fig. 2b). By contrast, a slowly released hexadecane droplet was observed to spread on the mesh  rapidly, and the Oil Contact Angle (OCA) is nearly 0˚ (Fig. 2c), indicating that the one-step dodecyl mercaptan functionalization created a SSCM. When a water jet was sprayed onto the surface of the SSCM, the jet bounced and detached from the surface without any residual on the SSCM (Figs. 2d -2f). Furthermore, as depicted in Fig. 2g, when a water droplet adhered on a needle was moved downward and compressed to make contact with the SSCM, it could easily detach from the surface by pulling the needle upwards. These results demonstrated that the as-prepared SSCM was characterized by excellent water repellence, low water adhesion and high oil affinity.

Surface morphological analysis
To explore the mechanism of superhydrophobicity and superoleopholicity induced by dodecyl mercaptan functionalization, surface morphology and chemistry of the meshes were characterized and analyzed. Firstly, the surface morphologies of the pristine CM and the SSCM were recorded by SEM and shown in Figs. 3a -3f. The original CM was composed of pores with size of 70 μm and copper fibers with diameter of 50 μm (Fig. 3a). The pristine fibers were relatively smooth and only some scratch-like structures that probably formed during manufacturing of the mesh existed (Figs. 3b and 3c). After being functionalized by dodecyl mercaptan, the average size of the fibers and pores changed little, but some irregular submicron and nano scale trenches and humps appeared on the fibers (Figs. 3d -3f). EDS elemental mapping analysis shown in Fig. 3g indicated that C and S originated from the dodecyl mercaptan molecules ( Fig. 1) were uniformly distributed on the fibers of the SSCM, demonstrating successful self-assembly of the dodecyl mercaptan molecules via Eq. (1).

XPS analysis
Additionally, the detailed chemical composition of the pristine CM and SSCM were recorded by XPS and depicted in Fig. 4. Compared with the original CM, dodecyl mercaptan functionalization resulted in a decrease of the relative element content of Cu (from 63.57 at.% to 13.43 at.%) and an increase of C (from 15.35 at.% to 72.89 at.%). More importantly, a S 2p peak could be clearly observed on the XPS spectrum of the SSCM (Fig. 4a), and the relative element content of S 2p was 3.44 at.%. Fig. 4b shows the high-resolution S 2p peak fitting results of the SSCM surface. The S 2p was deconvoluted into four components: the peaks centered at 162.3 eV and 163.0 eV were respectively assigned to S−C (2p3/2) bonds and S−C (2p1/2), and the components at 163.7 eV and 164.7 eV were attributed to S−Cu (2p3/2) and S−Cu (2p1/2) groups, respectively [47] . Moreover, the high-resolution C 1s peaks of the original CM and SSCM were also fitted, and the results were shown in Figs. 4c and 4d. The high-resolution C 1s peak of original CM can be fitted to three components at  [48] (Fig. 4c). By contrast, in addition to these three peaks, a new C−S peak could be found at 287.8 eV in the fitted C 1s peak of SSCM (Fig. 4d). These results all demonstrated the formation of a self-assembled molecular layer on the surface of the mesh during the immersion in dodecyl mercaptan, which lowered the surface energy of the mesh due to the long C−C/C−H chains of dodecyl mercaptan molecules. The roughened surface morphology and low surface energy met the requirements of lotus-leaf inspired superhydrophobic surfaces, and thereby enabled the functionalized mesh to be super water repellent. Fig. 5a shows the influence of mesh number on wettability of the functionalized CM. When the mesh number was 100, the WCA was only 135.2˚, while the WCAs became larger than 150˚ when the mesh number was 200, 300 and 400. However, previous works demonstrated that when the mesh number increased, the flux of oil-water separation decreased [49,50] . Therefore, the CM with mesh number of 200 was used to conduct oil-water separation test in this paper.

Effect of the processing time on wettability
Then the influence of immersing time on wettability of the functionalized CM was examined, and the WCAs and Water Sliding Angles (WSAs) under different immersing times were illustrated in Fig. 5b. When the original CM was immersed in the solution for 30 min, the WCA increased to 108.9˚, while the water droplet remained sticky on the mesh, which was probably due to the slight change of the mesh morphology (Fig. 5c). As the immersing time increased to 90 min, irregular micro-nano structures formed on the CM surface (Fig. 5d), making the WCA increase to 142.3˚ and the WSA decrease to 78.5˚. When the immersing time was extended to 120 min, the surface of the CM was further roughened (Fig. 5e), and the CM obtained super water repellence with a WCA of 153.7˚ and a WSA of 21.8˚. Further increase of the immersing time had little effect on the WCA and WSA of the mesh. Notably, the CM showed superoleophilicity for all the dodecyl mercaptan functionalized CM, therefore the SSCM immersed for 120 min was employed for subsequent oil-water separation.

Oil-water separation
Firstly, we examined the intrusion pressure of the oil-wetted SSCM, which represented the maximum height of water column that could be supported biny the SSCM. The intrusion pressure can be calculated using the equation Δp = ρgh max , where Δp was the intrusion pressure, ρ was the density of water, g was the gravitational acceleration, h max was the maximum height of water column. The maximum heights of water column for oil-prewetted SSCM were measured to be > 11.7 cm, corresponding to the intrusion pressures of > 1.15 kPa (see Fig. S1). This indicated that the SSCM had a good water supporting capacity when it was wetted by oils. Gravity-driven oil-water separation is shown in Fig. 6 and video S1. The SSCM was fixed between two glass tubes by clamps, and then mounted on an iron support. The glass tube was tilted at an angle of 45˚ so that it could separate mixtures containing both light oil whose density was smaller than water and heavy oil whose density was larger than water (Fig. 6a). For heavy oils like dichloromethane, when the oil-water mixture was poured into the upper glass tube, water was blocked due to the superhydrophobicity of the SSCM, while dichloromethane could sink to the bottom of the water layer and contact with the SSCM, and then oil passed through the SSCM easily due to its superoleophilicity, leading to the separation of heavy oil-water mixture, and no residual water was observed in the separated oil (Fig.   6b). When the mixture of water and light oil (e.g., hexadecane, hexane, octane and peanut oil) was poured into the tube, water was stopped by the inclined SSCM and formed a water layer that covered part of the mesh. The oil floated on water surface could contact with the uncovered region of the mesh and flow through the SSCM (Fig. 6a), and finally the oil-water mixture was separated. Figs. 6c and 6d show the separation of water-peanut oil mixture and water-hexane mixture, respectively, demonstrating effective separation of the light oil-water mixtures.
Separation efficiency is an important index to evaluate the oil-water separation capacity of the prepared SSCM, which was measured and shown in Fig. 7a. It could be seen that the separation efficiency of the SSCM was up to 99.2% for dichloromethane-water mixture, and above 97.1% for other testing oil-water mixture, and the peanut oil-water mixture was found to be separated with the lowest separation efficiency due to the relatively high viscosity of peanut oil which resulted in some residue on the glass tube wall. Oil flux is also important for the evaluation of the oil-water separation property of the SSCM, which was calculated and shown in Fig. 7b. Peanut oil-water mixture showed an oil flux of 3800 L·m −2 ·h −1 due to its high viscosity, while other oil-water mixtures had oil flux larger than 14000 L·m −2 ·h −1 .
In actual separation of oily wastewater, the aqueous phases are always complex, such as high temperature and corrosive solutions, therefore it is important to test the oil-water separation property of the prepared SSCM  under complex conditions. Firstly, we tested the stability of water repellence of the mesh after being immersed in complex solutions for 12 h, and the employed solutions included hot water (80 ˚C), HCl (1 mol·L −1 ), NaOH (1 mol·L −1 ), NaCl (20 wt%), and organic solvent (acetone). As shown in Fig. 7c, the water repellence of the SSCM immersed in these solutions deteriorated little, and the WCAs were all around 150˚, and the WSAs remained about 21˚, showing durable superhydrophobicity in these complex environments. Then the oil-water separation property of the solution-immersed SSCM was conducted (see Fig. S2 and video S2), and the separation efficiencies of dichloromethane-based mixtures are shown in Fig. 7d. It could be easily ob-served that the dichloromethane passed through the SSCM quickly while the water was blocked, and the separation efficiency of these oil-water mixtures were all above 98%. This indicated that the dodecyl mercaptan functionalized SSCM possessed excellent heat and corrosion resistance and could be used for oil-water separation in harsh environments.
Reusability of the functionalized mesh used for oil-water separation was another important issue for practical application. To investigate the reusability of the SSCM in oil-water separation process, the WCA and separation efficiency of the mesh under different separation cycles were investigated by using hexadecane-water mixture as an example, and the results were depicted in Fig. 8a. During 50 cycles of the oil-water separation, the separation efficiency maintained above 98% and the WCA retained at about 150˚, demonstrating excellent reusability of the SSCM in terms of multiple oil-water separation. In addition, the SEM images of the SSCM after 50 cycles of the oil-water separation were shown in Figs. 8b and 8c. It could be seen that the surface morphology of the SSCM was not obviously damaged during repetitive oil-water separations, which enabled the ability to retain its water repellence and separation capacity. Moreover, Fig. 8d shows the stability of water repellence of the SSCM in terms of storage in open air. It could be found that the WCA remained above 150˚ even after being stored in air for 120 days, indicating long-term stable superhydrophobicity of the prepared SSCM in air condition.

Mechanical durability
Mechanical robustness of superhydrophobic sur-faces play a very important role in their practical applications. Here we employed sandpaper abrasion testing to evaluate the mechanical durability of the SSCM. As shown in the illustration in Fig. 9a, the SSCM was bonded to a glass slide and placed them face-down to a 1000 grit SiC sandpaper, making direct contact between the SSCM and sandpaper. Then a 50 g weight was placed on the slide, and the sample was moved for 10 cm along the horizontal direction and then moved back in the opposite direction, which was defined as one abrasion cycle. The WCA of the SSCM was measured after each abrasion cycle, and the results were shown in Fig. 9a. It could be seen that the WCA gradually decreased as the abrasion cycle increased, when the sample was abraded for 4 cycles the WCA tended to be 150˚, and after 10 abrasion cycles the WCA decreased to about 140˚. SEM images (Fig. 9b) showed that after 10 cycles abrasion, some micro/nano structures of the mesh fibers were damaged. Subsequently, these sandpaper abraded SSCMs were employed to separate dichloromethane-water mixture, and the separation efficiency was calculated and presented in Fig. 9c. It showed that the separation efficiency changed little after 4 cycles abrasions, and though the water repellence decreased slightly after 10 cycles of abrasion, the separation efficiency retained higher than 96%.

Conclusion
In summary, SSCM was successfully fabricated using a simple one-step dodecyl mercaptan functionalization. The SSCM showed excellent water repellence and oil affinity (the WCA and OCA were 153.7˚ and 0˚, respectively), which imparted the SSCM with remarkable oil-water separation capacity. The separation efficiency and oil flux for hexadecane-water mixture were > 99% and 16000 L·m −2 ·h −1 , respectively. The SSCM could be easily reused and the separation efficiency retained > 98% even after 50 separation cycles. The functionalized SSCM could maintain excellent water repellence even after being immersed in hot water (80 ˚C), HCl (1 mol·L −1 ), NaOH (1 mol·L −1 ), NaCl (20 wt%) and organic solvent for 12 h, enabling realizable oil-water separation under these harsh conditions. Additionally, the SSCM could maintain its water repellence and oil-water separation ability even after 10 cycles sandpaper abrasions. This one-step dodecyl mercaptan immersion could be a simple method to functionalize CM with robust water repellence and oily wastewater treatment ability. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made.
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