Oil clean up using Electrospun Fibers

Contact angle photos of (a) water and (b) diesel oil droplet on polyacrylonitrile/polystyrene (1:1) electrospun membrane showing contact angle of 144° and 0° respectively. [Qiao et al IOP Conf. Series: Materials Science and Engineering 2014; 62: 012032. This work is licensed under a Creative Commons Attribution 3.0 Unported License.]

High surface area to volume ratio of electrospun fibers makes it a potential candidate for oil spill clean-up application. A good oil sorbent material shall have the following characteristics,

• High oil sorption capacity
• High rate of sorption
• Good buoyancy
• High oil/water selectivity

Electrospun fibers especially those in the nanoscale or with porous surface will contribute to oil sorption capacity and rate of sorption. High surface area to low mass also makes electrospun membrane light weight and this contributes to good buoyancy. However, above all, the material must demonstrate high oil/water selectivity. Generally, a non-polar material is able to adsorb oil on its surface while rejecting water. Flexibility of electrospinning allows a variety of non-polar materials to be selected for this purpose.

Small diameter of electrospun fibers meant that it has a large surface area to mass as compared to many other commercially available oil-sorption materials. Electrospun polyacrylonitrile/polystyrene (1:1) fibers membrane has an oil-sorption capacity of nearly 20 times that of commercial polypropylene nonwoven fabric with diameter of about 1 µm for the electrospun fiber and about 10 µm for the commercial fabric fibers [Qiao et al 2014]. The surface area of electrospun fibers may be further enhanced by modifying its physical architecture to exhibit surface or intra-fiber pores. Electrospun polystyrene in particular has been shown to exhibit such structures by varying its electrospinning environment. Lin et al (2012) has shown that with polystyrene the presence of intra-fiber pores were able to adsorb almost twice the amount of motor oil compared to solid core fibers. However, due to the poor mechanical strength of polystyrene, polyurethane was blended to form a stronger hybrid scaffold although the oil-sorption capacity was reduced. To improve the mechanical strength of the fiber while maximizing the surface exposure of polystyrene for oil sorption, a core-shell structure may be constructed with polystyrene as the fiber shell and polyurethane at the core [Lin et al 2013]. With this setup, the oil sorption is more than twice that from polystyrene/polyurethane blend. Another material that has been tested for this purpose is Ethylene-propylenediene which is known to be a low density artificial rubber with good resistance against degradation by heat, water, light etc. Sorption kinetics of cyclohexane on the electrospun fibers was found to follow the first-order kinetic model with smaller diameter fibers giving greater sorption [Liu et al 2010]. Poly(lactic acid) (PLA) is a biodegradable and renewable polymer which is mostly used in medical implants. Liu et al (2018) electrospun porous PLA fibers for the purpose of oil-water separation. Surface pores on the electrospun PLA fibers were generated by increasing the relative humidity of the electrospinning environment. The increased surface roughness due to the pores enhanced the hydrophobicity of the membrane. Rising the environmental relative humidity from 40% to 80% increases the membrane porosity from 81% to 92%. Flux of the three model oils, n-hexane, olive oil and lubricant oil showed higher rate with higher membrane porosity. Separation efficiency of the electrospun membrane for all three oils were greater than 99.98%. A demonstration of improved oil absorbency through the introduction of pores in electrospun PLA fibers were shown by Liang et al (2019). In their study, the pore size on the fiber was controlled by varying the electrospinning nozzle diameter. They found that when the nozzle diameter is smaller, the narrower electrospinning jet and its corresponding higher surface area encourages faster solvent evaporation leading to smaller fiber diameter and larger pore size. The larger pore volume from smaller diameter fibers increases the oil absorption capacity. For the largest inner diameter nozzle, the fiber diameter was 1.5 µm and vacuum pump oil absorption was 23.44 g/g. For the smallest inner diameter nozzle, the fiber diameter was 1.27 µm and vacuum pump oil absorption was 42.38g/g. A study by Mikaeili et al (2018) suggested that hydrophobicity of conventionally hydrophilic material may increased significantly due to changes in its molecular arrangement during electrospinning. This will increase the selection of material available for oil sorbent mats. Cellulose acetate (CA) thin film from spin-coating has a water contact angle of 63.67° which is hydrophilic. However, after electrospinning, the CA nanofibrous membrane with nanofiber diameter 350-400?nm, has a contact angle of 154.3°. Their calculation showed that Cassie's model alone is not able to explain such a high water contact angle. Investigation into its molecular structure suggested that concentration of the hydroxyl groups on the electrospun fiber is much lower than spin coated CA thin film. The reduction in hydroxyl groups lowers the surface energy and thus increases its hydrophobicity. The electrospun CA membrane was able to remove 30 times its own weight of motor oil.

Table 1: Electrospun materials and oil-sorption capacity unless otherwise stated.

Material	oil	Oil-adsorption capacity	Reference
Commercial polypropylene nonwoven fabric (fiber dia. 10 - 15 µm)	Pump oil	10 g/g	Qiao et al 2014
Polyacrylonitrile/polystyrene (1:1) electrospun fibers (fiber dia. about 1.2 to 2.7 µm)	Pump oil	195 g/g	Qiao et al 2014
Polyvinyl chloride/polystyrene fiber	Motor oil	146 g/g	Zhu et al 2011
Polystyrene shell and polyurethane core fiber	Motor oil	64.4 g/g	Lin et al 2013
Polystyrene	Motor oil	113.87 g/g	Lin et al 2012
Polystyrene/polyurethane blend (4:1)	Motor oil	27.75 g/g	Lin et al 2012
Polysytrene porous fiber	Motor oil	112.3 g/g	Wu et al 2012
Polysytrene porous fiber	Silicon oil	81.4 g/g	Wu et al 2012
Polyacrylonitrile/polystyrene (1:1) electrospun fibers (fiber dia. about 1.2 to 2.7 µm)	Gasoline	43 g/g	Qiao et al 2014
Polyacrylonitrile/polystyrene (1:1) electrospun fibers (fiber dia. about 1.2 to 2.7 µm)	Diesel	67 g/g	Qiao et al 2014
Polyvinyl chloride/polystyrene fiber	Diesel	38 g/g	Zhu et al 2011
Polysytrene porous fiber	Diesel	7.13 g/g	Wu et al 2012
Polycaprolactone/beeswax (PCL/BW), 25 wt% beeswax	Diesel	16.95 g/g	Reshmi et al 2017
Polyacrylonitrile/polystyrene (1:1) electrospun fibers (fiber dia. about 1.2 to 2.7 µm)	Peanut oil	132 g/g	Qiao et al 2014
Polyvinyl chloride/polystyrene fiber	Peanut oil	119 g/g	Zhu et al 2011
Polysytrene porous fiber	Peanut oil	112.3 g/g	Wu et al 2012
Polystyrene shell and polyurethane core fiber	Sunflower seed oil	47.48 g/g	Lin et al 2013
Polycaprolactone/beeswax (PCL/BW), 25 wt% beeswax	Sunflower seed oil	31.05 g/g	Reshmi et al 2017
Polyvinyl chloride/polystyrene	Ethylene glycol	81 g/g	Zhu et al 2011
Polycaprolactone/beeswax (PCL/BW), 25 wt% beeswax	gingelly	25.17 g/g	Reshmi et al 2017
Polycaprolactone/beeswax (PCL/BW), 25 wt% beeswax	Kerosene	20.72 g/g	Reshmi et al 2017

Most electrospun membrane for oil clean up uses straight fibers, utilizing high porosity of the membrane to trap oil. Zhao et al (2017) suggested that membrane made of helical fibers will perform better in trapping oil within the coils. In their study, electrospun helical fibers were made from polyvinylidene fluoride (PVDF) which has a good ductility as the core and stiffer, porous polystyrene (PS) as the shell. To construct straight fibers of the same material, a rotating drum was used instead of a stationary collector. Their study demonstrated significantly better oil sorption in the helical fibers compared to straight fibers.

Sorption of crude oil by the helical porous fibers. a) Photographs showing the crude oil sorption process of the helical porous fibers from oil/water system. b) Photographs showing the crude oil desorption process of the helical porous fibers from pure oil system. c) Sorption efficiency of the various helical fibers (B, C, D, E, F) and their corresponding straight fibers (b, c, d, e, f) for crude oil. d) Schematic showing crude oil sorption mechanism of the helical fibers, compared to the straight fibers [Zhao et al 2018].

Beyond selecting a material that is a good absorber of oil, the physical characteristics of the absorbent may also influence its oil absorption performance. Xu et al (2016) compared the oil absorption capacities of the fibrous membranes of polystyrene (PS) and poly(styrene-co-butyl acrylate) (PS-BA) with different ratios of butyl acrylate. Looking at the morphology of the electrospun membrane, PS-BA electrospun membrane was more compact compared to PS. The loose arrangement of PS fibers demonstrated better absorbency for high viscosity oil. However, for low viscosity oil, the absorbency is similar to PS-BA electrospun membrane. For high viscosity oil, absorbency of the oil may be dependent on the ease of oil penetration into the pores of the membrane. This would favor the loose PS fiber membrane. For lower viscosity oil, absorbency may depend on the total surface area of the nanofibers and not the pore size. This may account for the similarity of oil absorbency for both membranes.

The availability of channels and pores to improve passage and storage of oil will certainly improve the membrane's adsorption and storage capacity. Zhang et al (2022) constructed a PVDF/polydimethylsiloxane (DP8)/SiO₂ composite membrane with a distinct hierarchical structure made from microspheres forming obvious honeycomb structures on a base electrospun fiber membrane layer. The composite membrane was made by first electrospinning a layer of polyvinylidene fluoride followed by electrospraying of DP8/SiO₂ microspheres. The DP8/SiO₂ microspheres formed honeycomb-like through hole structures on the electrospun membrane probably due to electrostatic repulsion. The lower surface energy of DP8/SiO₂ microspheres increases the surface roughness and hydrophobicity of the composite membrane. With 1.5 wt% SiO₂, the membrane exhibited an ultra-high hydrophobic angle 162.1°. The flux and separation efficiency of oil-water after 10 cycles was greater than 4500 Lmh^-1 and 99.4% respectively.

SEM of pure PVDF nanofiber membrane DP8 composite membrane and electrosprayed DP8-X with different SiO2 content. (a-a2) PVDF, (b-b2) DP8, (c-c2) DP8-0.5% SiO2, (d-d2) DP8-1.0% SiO2 [Zhang et al 2022].

For actual application as oil/water separation, other characteristics such as separation efficiency, environmental durability and reusability needs to be investigated. Reshmi et al (2017) tested the suitability of electrospun polycaprolactone/beeswax (PCL/BW) electrospun membranes for gravity driven oil/water separation. With PCL/BW electrospun fiber membrane with 25 wt% beewax, oil sorption capacity across different oils range from 17 to 31 g/g. Gravity driven oil/water separation efficiency was 98.1% and this was maintained for 15 cycles. The membrane was also found to maintain its superhydrophobicity when exposed to UV and across a range of pH.

Another consideration in the selection of material for constructing oil clean up membrane is its environmental impact. The high surface area of electrospun nanofibrous membrane has made its membrane an attractive candidate for soaking up oil spill. However most of the materials used in studies require the use of organic solvents to prepare the solution for electrospinning. Ge et al (2021) chose polyvinyl alcohol (PVA) which is a water soluble polymer with lipophilic properties for electrospinning into nanofibrous membrane for oil adsorption. Engine oil adsorption at optimum fiber diameter was 12 g/g. Several factors were found to influence oil adsorption. A smaller pore size between the interconnected fibers is preferred to trap oil droplets. Smaller fiber diameter also increases the surface area available for oil adsorption. Interestingly, electrospun PVA membrane with 9 wt% solution has a larger pore size compared to 10 wt% PVA solution. Therefore, membrane electrospun with 10 wt% PVA solution was found to have the best engine oil adsorption performance. The interconnected fibers also facilitate oil adhesion by spreading the oil droplet through capillary action while high porosity and interconnected pores allows oil penetration and storage within the pores.

Adsorption of oil by membrane goes beyond removal of oil spill. It may also be used in the removal of contaminants in biodiesel, in this case free fatty acids (FFA). For practical use of biodiesel, the level of FFA in it must be below 1 wt%. However, the produced biodiesel often contains 2 to 3 wt% FFA which needs to be removed before the biodiesel can be used. Wang et al (2022) investigated the use of polyethersulfone/magnesium silicate membranes in the removal of FFA in biodiesel. A practical adsorbent material should have a high capacity to contain the contaminant and high surface area for fast adsorption. Magnesium silicate (MS) has been developed for the purpose of non-aqueous adsorbent and has surface area and pore volume greater than PES cast film. By electrospinning a blend of PES and MS, the resultant PES/MS nanofibrous membrane has a higher surface area and pore volume than MS. Pure MS probably has a lower surface area and pore volume compared to PES/MS electrospun membrane due to agglomeration. The PES/MS electrospun composite membrane showed the highest adsorption capacity of 670 mg g^-1 and the most efficient removal rate over 90% for FFA. This is higher than pure MS powder. The adsorption membrane may be recovered by washing with ethanol to remove the FFA. The electrospun membrane retained an adsorption capacity over 94% even after 8 cycles of adsorption-desorption cycle while the cast membrane showed significant drop in adsorption after 3 cycles of washing. Hence the PES/MS electrospun composite membrane has the potential for purifying biodiesel for greater utilization.

[+]

[-] comments powered by Disqus

Content [Environment Care]