Magnetically responsive Janus nanoparticles synthesized using cellulosic materials for enhanced phase separation in oily wastewaters and water-in-crude oil emulsions

Highlights

• Biwettable Janus nanoparticles are facily synthesized via sequential adsorption of cellulosic materials.

• Biodegradable and environmentally friendly cellulosic materials used for the synthesis.

• The M−Janus nanoparticles enhance phase separation of oily wastewaters and water-in-crude oil emulsions.

• The M−Janus nanoparticles exhibit excellent interfacial activities.

Abstract

A new class of magnetically responsive and interfacially active Janus (M−Janus) nanoparticles was designed and synthesized by sequential adsorption of cellulosic materials: hydrophobic ethyl cellulose (EC) and hydrophilic carboxymethyl cellulose (CMC) on the opposite sides of magnetite (Fe₃O₄) nanoparticles. The adsorption study using quartz crystal microbalance with dissipation (QCM-D) proved the concept of proposed synthesis of M−Janus nanoparticles. The adsorption of EC and CMC on magnetite nanoparticles was confirmed by zeta-potential measurements, thermogravimetric analysis (TGA), characterization using Fourier transform infrared spectroscopy (FTIR) and TEM. The surface wettabilities of the opposite sides on the M−Janus nanoparticles were investigated by measuring contact angles of nanoparticle surfaces deposited from the oil-water interfaces using the Langmuir-Blodgett method. SEM images revealed an excellent dispersion of M−Janus nanoparticles in both aqueous and organic phases. The results from the coalescence time and crumpling ratio measurement of particles-stabilized oil droplets along with the interfacial pressure-area isotherms demonstrated stronger interfacial activities of M−Janus nanoparticles and a stiffer interface with adsorbed M−Janus nanoparticles as compared with the interfaces stabilized by conventional interfacially-active nanoparticles. The microscopy images confirmed the deposition of M−Janus nanoparticles at the emulsion droplet surface during the phase separation process. The M−Janus nanoparticles not only exhibited excellent capability and high efficiency in separating emulsified water from water-in-crude oil emulsions and the oil from oily wastewaters under an external magnetic field, but also retained high interfacial activity and hence desirable separation efficiency after five-cycle applications. Because of the environmentally friendly and biodegradable cellulosic materials used in the synthesis, the M−Janus nanoparticle can achieve effective oil/water phase separation without causing further pollution to the continuous phase.

Introduction

Formation of emulsions is inevitable in various situations such as the water-in-crude oil (W/O) emulsions (water content <5%) in the petroleum-related industry and the oily wastewaters (O/W) in daily life [1], [2]. Typically, those emulsions are undesirable and even detrimental. For instance, the emulsified water phase in the crude oil emulsions can cause problems and damage the equipment of crude oil production because the dissolved salt in the aqueous phase can poison refinery catalyst and cause severe corrosions of the process equipment; while stable oil droplets in oily wastewaters can easily cause pollution on water resource [3], [4], [5], [6], [7]. Therefore, the oil-water phase separation of such emulsions is necessary to remove undesirable phase in advance of the further process. Unfortunately, the oil-water phase separation of these emulsions is a well-known challenging issue due to the inherent high stability of the emulsions, resulting from the adsorption/assembly at the oil-water interface of interfacially active components such as asphaltenes in the crude oil and/or surfactants in the oily wastewaters.

Recently, various techniques have been reported for treating oily wastewaters, including oil sorption [8], [9], [10], filtration [4], [5], [11], [12], [13], electrocoagulation [6], [14], [15], solvent extraction [16] and coalescers [17], [18]. However, each of these techniques bears inherent drawbacks, such as high cost of raw materials, low efficiency, and high energy cost in the treatment, and complicated procedures for equipment set-up [19], [20], [21]. As for dewatering of the water-in-crude oil emulsions, the key point is to disturb and destroy the rigid asphaltene film, which often forms at the oil-water interface and prevents the water droplets from coalescing. In general, heating and chemical treatment are used to destabilize the water-in-crude oil emulsions. However, these methods consume too much energy, and the residual chemicals are hardly recyclable and often an environmental liability [22]. Up to date, studies regarding destabilization of water-in-crude oil emulsions have been focused mainly on breaking the elastic interfacial films using chemical demulsifiers to accelerate the coalescence of the water droplets [23], [24]. Zhang et al. reported a commercial copolymer: ethylene oxide (EO)/propylene oxide (PO) demulsifier to soften the rigid asphaltene film [25]. Later on, an interfacially active polymer ethyl cellulose (EC) was reported by Feng et al. to effectively demulsify the water-in-diluted bitumen emulsions by disrupting the asphaltene film and reducing asphaltene aggregation [26]. Although asphaltene film can be destroyed by such polymeric demulsifiers, following oil-water phase separation still relied on gravitational settling, which was inefficient due to the small difference in density between water and oil phase as well as the high viscosity of the bitumen. Furthermore, conventional demulsifiers were not able to be recycled after demulsification, leading to the high cost of their applications. It is therefore highly desirable to develop a new class of demulsifiers that is not only efficient, but also reusable for oil-water phase separation in the treatment of oily wastewaters and water-in-crude oil emulsions.

Recently, the magnetically responsive and interfacially active particles have attracted considerable attentions [27], [28], [29], [30], [31], [32]. With the firm adsorption of such particles at the oil-water interface, undesirable phases such as the waste oil droplets in the oily wastewaters or the emulsified aqueous phase in the water-in-crude oil emulsions can be attracted, gathered and eventually removed under the external magnetic field, leaving a clean and purified phase for the further processing. For example, Mirshahghassemi et al. designed polymer-coated iron oxide nanoparticles to effectively collect and remove waste oil from water systems [33]. Xu et al. reported a kind of magnetic nanoparticles grafted with expanded perlite to absorb oil spills from the oily wastewaters [21]. Specifically, to dewater the water-in-crude oil emulsions, functional materials such as EO/PO copolymer, ethyl cellulose, polyelectrolyte are necessarily required to effectively disrupt or disturb the asphaltene film [2], [34], [35]. The particles modified with those materials can tag the water droplets and such tagged droplets can be effectively removed under the external magnetic field. Peng et al. firstly reported a novel magnetic iron oxide nanoparticle grafted with interfacially active polymer ethyl cellulose (EC) using esterification reaction [36], [37]. The magnetic iron oxide nanoparticles grafted with EC showed excellent performance in destroying aged asphaltene film and dewatering the water-in-crude oil emulsions. Later on, Pensini et al. reported the adsorption of hydrophilic carboxymethyl cellulose (CMC) on iron oxide substrate, which revealed the potential of CMC to be used as a connective material [38]. Recently, we investigated the strong interactions of carboxyl methyl cellulose (CMC) with bare iron oxide nanoparticles and CMC with EC, which led to magnetic iron oxide nanoparticles fully covered with EC (M−CMC−EC) using convenient procedures [39]. In the procedures, the CMC was used as a bridge to directly link iron oxide particles with EC, leading to the formation of M−CMC−EC nanoparticles with EC wholly coated on the nanoparticle surfaces [39]. According to the discussions above, these magnetic nanoparticles are interfacially active, induced by homogeneous surface coatings of nanoparticles. However, such particles are sometimes found inefficient and ineffective due to the possible desorption of the particles from the interface under the influence of external magnetic force, leading to insufficient efficiency of demulsification [40], [41]. Therefore, magnetic particles as demulsifiers with stronger interfacial activity are more desirable in phase separating the oily wastewaters and the water-in-crude oil emulsions under an external magnetic field.

Janus-type particles are individual particles with two opposite surface properties in one single particle such as hydrophilicity or hydrophobicity [42], [43], negatively charged surface or positively charged surface [44]. For example, a spherical particle can have one side with hydrophilicity while another is hydrophobic. Compared with homogeneously surface-modified particles, biwettable Janus particles have better interfacial activities with stronger pinning to stabilize emulsions. They are also more difficult to desorb from the oil-water interface [45], [46]. Various of Janus particles have been applied for phase separating either oily wastewaters or water-in-crude oil emulsions [46], [47], [48]. However, such Janus particles applied to the oily wastewaters are not very effective to deal with the water-in-crude oil emulsions due to the lack of functional materials for effectively disturbing or destroying asphaltene film [46]. Also, the particles workable for dewatering the water-in-crude oil emulsions are usually hard to effectively adsorbed to the oil-water interface of oily wastewaters due to the high hydrophilicity of the particles [47], [48]. Based on our previous work, EC and CMC can both attach onto iron oxide surface while having opposite wettability and EC can effectively disturb the asphaltene film. The biwettable Janus nanoparticles can be synthesized by coating hydrophilic CMC and hydrophobic EC on the opposite sides of the nanoparticle surfaces. With their unique biwettability, such Janus nanoparticles are expected to have stronger interfacial properties, leading to more efficient removal of emulsified oil droplets under the external magnetic fields, as compared with M−CMC−EC nanoparticles of homogeneous wettability. Also, due to the coating of functional material EC, the interfacially active Janus nanoparticles can be applied to effectively phase separate not only the oily wastewaters, but also the water-in-crude oil emulsions under an external magnetic field. Furthermore, due to the biodegradability and environmental friendliness of the cellulosic materials, such Janus nanoparticles would not cause pollution to the demulsified emulsion systems.

In this study, we report a novel magnetically responsive and interfacially active Janus (M−Janus) nanoparticle with hydrophobic EC and hydrophilic CMC coated on the opposite sides of the nanoparticle surface. Compared with the previously reported EC wholly-coated magnetic nanoparticles [39], better interfacial properties of the M−Janus nanoparticles were investigated by measuring coalescence time using induction timer, interfacial pressure-area isotherms (π-A) using Langmuir trough and dynamic interfacial tension change of the M−Janus nanoparticles adsorbed oil-water interface. M−Janus nanoparticles were applied to effectively phase separate both of the water-in-crude oil emulsions and the oily wastewaters under an external magnetic field. The quick and efficient phase separation of such emulsions suggests potential applications of M−Janus nanoparticles to dewatering of water-in-crude oil emulsions in the heavy oil industry and removal of undesirable oil phases from the oily wastewaters. Furthermore, owing to the biocompatibility and biodegradability of the cellulosic materials used in synthesizing M−Janus nanoparticles, the M−Janus nanoparticles can effectively deal with the oily wastewaters and the water-in-crude oil emulsions without polluting the continues phase.