**1. Introduction**

During recent years, large discharge quantities of oily wastewater has attracted the attention of the public, and it also has drawn researchers' interests in the field of fabricating novel materials having higher oil/water separation e fficiency. There are two primary ways to generate oily wastewater: the

first is by releasing the oil-contaminated industrial wastewater; these industries often constitute of petrochemical industries, printing industries, metallurgical-production industries, food-processing industries, and so on [1–6]. Such wastewater has become the most common contaminant all over the world and it has seriously threatened our habitat. The other reason for oily wastewater generation is the large numbers of oil spill incidents and the recent estimates show that nearly two million tons of oil spills into the ocean annually [5–7]. So, a significant quantity of waste oil being disposed into the oceans has not only caused substantial energy loss, but also seriously threatened aquatic life because the spilled oil decreases the oxygen present in the water [8–13]. In addition, the fouling of surfaces is a significant problem that affects not only people' daily lives, but also does harm to industrial production systems, and it could be mitigated via a method by combining particular designed interfaces and further chemical treatments. This is a good technique with broad application prospects.

If the membranes with low surface adhesion to contaminants on the surface could be prepared, it will greatly benefit the lifetime and separation efficiency of the membranes in practical applications [14]. At present, several traditional methods including ultrasonic separation, skimming, centrifugation, combustion, etc., are being widely applied for the oily wastewater separation [9,15–18]. Nevertheless, the application range of these techniques is often hampered by some limitations such as the production of secondary pollutants, low efficiency, and the complex operation [3,15,19–21]. Considering the issues above, there's an urgen<sup>t</sup> requirement for novel technologies having characteristics, higher separation efficiency, low operational cost and simple operation for separation of oil/water mixture.

Inspired by the naturally occurring phenomenon of superhydrophobicity at the surface of lotus leaf and goose feathers, scholars have researched and fabricated new membrane-based materials with high separation efficiency of oil/water mixtures. These membrane materials with unique wetting properties have gradually become a hot research topic in the past decade [6,22–24]. Wettability exists as an inherited characteristic of solid surfaces, and it affects the wetting phenomenon as the droplets touch the surface of the solid [25–28]. According to the different wetting phenomena when oil droplets and water droplets touch the solid surface, it can be summarized into four wetting properties, namely: oleophilicity, oleohobicity, hydrophilicity and hydrophobicity. Researchers found that the hierarchical micro/nanostructures could increase the roughness of the solid surface and then enhanced the four-fundamental wetting properties into superoleophilicity, superoleohobicity, superhydrophilicity and superhydrophobicity [29–34] and some organics with lower surface energy could decrease the wettability of liquid and solid surfaces. By employing physical refining as well as chemical treatment approaches such as corrosion, alteration of some substances with lower energy, electric deposition etc., to formulate the micro and nanocomposite structure membrane with relatively lower energy surfaces [20,35]. On the basis of above analyses the membrane-based separation material driven solely by gravity can be divided into two categories: "water-removing" and "oil-removing". The first type material called "water-removing" membrane means the membrane shows "water-loving" and "oil-hating" characteristics, and water easily passes across the membrane; however, oil is blocked over the membrane simultaneously. On the contrary, the membrane of the "oil-removing" material shows "oil-loving" and simultaneously "water-hating" characteristics, and water stops above the membrane while oil can permeate the membrane smoothly [36–39].

Researchers first prepared the superhydrophobic/superoleophilic surface, which could selectively let the oil pass through but stopped the water and the material successfully separated the heavier oil/water mixture [9,24]. However, there are two main problems associated with the practical application of the superhydrophobic/superoleophilic membranes: on the one hand, this material is suitable to separate the heavier oils/water, like 1,2-dichloroethane, whose density is heavier than the density of water, but oils having a density lower than water; it becomes complex to practically separate the mixture because the oil cannot touch the membrane even though it could easily pass across the membrane, and is occluded by the lower water layer. In such a situation, placing the separation device obliquely is necessary to separate the mixture smoothly [38–40]. While, on the other hand, as a result of higher viscosity, the oil phase quickly chokes the membrane

during the separation. Thus, the separation rate can be much lower, and that limits the practical application in the field of oil/water separation [41–43]. Considering the problems above, scholars have investigated the superhydrophilicity/superoleophobicity phenomenon compared with the superhydrophobicity/superoleophilicity of the oil-removing-type material.

After continuous exploration and attempts by many scholars, this material was successfully fabricated [44]. The "water-removing" membrane could ideally avoid the two problems mentioned above to separate oil with a lower density than the water, as water smoothly flows across the superhydrophilic membrane and oil stays above the superoleophobic membrane. Even if it perfectly avoids these two problems, there still remain a few disadvantages of the superhydrophilic/superoleohobic materials. Many "water-removing" materials need to have characteristics with lower surface energy to further modify its solid surface, but actually those materials with lower surface energy are usually fluorine-containing organic matter [23,31,45,46]. In that case, the formation process is usually complex and costly, but also the chemical stability of the adapted surface coating is not very effective. Furthermore, the literature shows that the current research hitherto primarily focused on separating the single oil and water mixture. However, it should be noted that the composition of wastewater containing oil varies and was often complex due to presence of different oil components; this certainly yields a high demand for membrane adaptability to the practical applications of oil/water separation [8,15,36].

The current study reports two facile and one-step chemical reaction methods on the copper mesh to fabricate the "water-removing" membrane with superhydrophilicity and underwater-superoleophobicity. The prepared membranes possess a nanoneedle Cu(OH)2 structure and a micro/nanocomposite Ag structure. With these two easily operable methods, we successfully fabricated the water-removing membranes and experimentally studied the separation efficiency, permeation pressure and fluid flux. In addition, to authenticate the more widespread adaptability of the membranes, the mixture with a single component of oil and water were successfully separated, and the multi-constituents of the oil/water mixture were experimentally investigated.
