Facile surface treatment and decoration of graphene-based 3D polymeric sponges for high performance separation of heavy oil-in-water emulsions

Highlights

• Superhydrophobic graphene-based sponge (rGO@MF) was engineered through facile surface treatment and hydrothermal steps.

• rGO@MF sponge was developed and utilized for separating various heavy oil-in-water emulsions as well as oil/water mixtures.

• The rGO@MF sponge maintained its high separation performance over ten consecutive adsorption cycles using heavy oil-in-water emulsions.

• General adsorption mechanisms for oil separation using 3D graphene-based sponges were introduced.

Abstract

The separation of oil-in-water emulsions has become an extremely important for both environmentally and industrial related applications. Herein, a graphene-based sponge (rGO@MF) was developed and utilized for separating various heavy oil-in-water emulsions as well as oil/water mixtures. The superhydrophobic sponge was engineered through facile surface-treatment and hydrothermal steps. The surface and structural properties of the rGO@MF sponge and the nanoemulsions were thoroughly characterized by advanced techniques. The high-resolution SEM and EDX mapping confirmed the homogeneous distribution of rGO sheets surrounding the fibers. The developed rGO@MF sponge showed excellent chemical stability and durability. The correlation between the oil type, droplet size and concentration of oil/water mixtures and emulsions, and the rGO@MF adsorption capacity and removal efficiency, were extensively investigated. The developed superhydrophobic rGO@MF sponge showed water contact angle of ~164º and exhibited superior adsorption capacity and removal efficiency of up to 5647 mg/g and 95 ± 3% respectively, for crude oil-in-water emulsions of 30 g/l. In addition, the rGO@MF sponge maintained its high separation performance over ten consecutive adsorption cycles. The adsorption capacity of the rGO@MF sponge maintained up to 92% of its initial values after ten cycles. The calculated activated adsorption energy for crude oil-in-water emulsion on rGO@MF sponge was 16.59 kJ mol⁻¹ indicating a physical adsorption process. The adsorption kinetics and interactions were carefully explored and a general mechanism of separation for both oil-in-water nanoemulsions and oil/water mixtures was introduced.

Introduction

Oily wastewater generated by industries such as crude oil production, oil refining, and metal manufacturing, can cause major environmental issues, and affect human health [1]. In oily wastewater, oils are classified into two types, a mixture of oil/water, and oil-in-water emulsion. The later has stable microdroplets smaller than 20 µm with complex wetting behavior [2]. Therefore, the separation of emulsified oil-in-water possesses a great challenge compared to oil/water mixtures. Various traditional methods have been used to separate oils, including skimming [3], centrifugation [4], precipitation [5], air floatation [6], coacervation [7], membranes [8] and sorption materials [9]. However, most of these techniques showed limitations regarding low processing efficiency; high operation cost and is ineffective for oil-in-water emulsion separation. Therefore, there has been strong demand for novel and highly efficient oil sorbent materials and processes for oil emulsion separation.

To date, tri-dimensional materials (3D) with porous network, high surface area and hydrophobic nature have been utilized to treat oil/water mixtures. For example, Wu et al. used ultra-light carbon nanofiber as an absorbent and showed remarkable absorption capacity up to 310 times its weight for organic solvents and oils removal [10]. Gui et al. developed magnetic carbon nanotube sponge that demonstrated high adsorption capacity for diesel oil [11]. Furthermore, Ren et al. prepared N-doped graphene aerogel, which showed adsorption capacity for organic solvents up to 111.6 g/g [12]. There are few examples in literature in which 3D porous materials have been used for oil-in-water emulsion separation. Tran et al. used PDMS–graphene sponge for separation of organic solvents and emulsified oil–water. The prepared sample showed remarkable adsorption capacity of 800 wt%. In addition, the developed sponge was used to separate the toluene-in-water emulsion with an efficiency of 80% at a concentration of 10 wt% [13]. Huang et al. synthesized graphene aerogel (GA) and exhibited excellent emulsified oil adsorption capacities of 753 mg/g at oil-in-water concentration of 800 mg/l [14]. Swapneel et al. synthesized reduced graphene oxide–silica (rGO-silica) aerogels by a sol-gel method. The rGO-silica aerogel showed a good performance to oil sorption with oil uptake 7–10 times the aerogel mass [15]. Recently, Yang et al. fabricated ultra-light carbon foams with high adsorption capacity up to 1627 mg/g when it was utilized for separation of oil-in-water emulsion of 2020 mg/l [16]. However, in most of these studies, the preparation of the adsorbents was complicated and showed poor mechanical properties which are essential for efficient reuse and recyclability [17].

In recent years, the modification of commercial polymeric sponges such as polyurethane (PU) [18], polyester polyurethane (PESPU) [19], [20], and melamine formaldehyde (MF) [21], has received great attention in the field of oily wastewater separation. These polymeric sponges are characterized by low density, high porosity, and reasonable recyclability. However, it is worth mentioning that polymeric sponge’s surface is normally hydrophilic, accordingly, sponge’s surface modification has been a prerequisite step to turn the surface into a hydrophobic and improve the surface roughness. Several materials such as CNTs [22], PDMS [23], silica nanoparticles [24], MoS₂ [25], [26] and graphene [27] have been utilized for surface modification of the sponges. For instance, Peng et al. developed kaolinite modified graphene oxide-melamine sponge, which exhibited adsorption capacity of 113 g/g and 96 g/g for separating DMF and diesel oil/water mixtures respectively [28]. Zhang et al. fabricated melamine/graphene/carbon black sponge by dip-coating and thermal reduction. The optimized sponge showed adsorption capacity of 120 g/g and 80 g/g for separating chloroform and pump oil/water mixtures respectively [29]. Song et al. reported adsorption capacity for chloroform mixture with water of 112 g/g using MF sponge modified with rGO [30]. Most studies to date are mainly devoted to exploring the optimal adsorption conditions for oil/water mixtures. There have been few reports focusing on improving the adsorption efficiency with detailed mechanism of oil-in-water emulsion separation.

Here, superhydrophobic MF sponge was developed for the separation of high concentration oil-in-water emulsions as well as oil/water mixtures. The commercially available MF sponges were initially surface-treated and subsequently coated with rGO sheets. Five different oils and organic solvents with high concentration (up to 30 g/l) were tested. The correlation between the oil type, droplet size and concentration of oil mixtures and emulsions, and the MF adsorption capacity, were extensively explored. The effects of the reusability on the adsorption capacity and separation efficiency for oil/water mixture and emulsified oil were evaluated for ten successive cycles.