The removal of microplastics (MPs) from water using oil has shown early promise; however, incorporation of this technique into a feasible in situ method has yet to be developed. Here, a simple yet effective method of MP capture from water using vegetable oil with bubbles is demonstrated to achieve high removal efficiencies of > 98%. Comparisons are made with other methods of agitation, and higher removal efficiencies are observed when bubbles are used. Due to the low agitation provided by the bubbles, the oil layer remains unbroken, meaning that no oil is released into the bulk water phase. In this way, secondary contamination is avoided—unlike membrane filtration, another effective removal method, in which polymer-based membranes can break down due to chemical backwashing and ageing. It is demonstrated that variation in MP size within the micrometer range (50-170 μm) has minor impact on the removal efficiency; however, 100% removal is achieved for larger, millimeter-sized MPs (500-5000 μm). Similarly, a high removal efficiency of greater than 99% is achieved in the capture of microfibers. Other factors such as oil volume and water salinity are also investigated and discussed. Based on these results, the proposed method can be introduced into multiple setting types as a passive and continuous method of MP capture.
Joshua Saczek, Xiaoxue Yao, Vladimir Zivkovic, Mohamed Mamlouk, Steven Wang, Stevin S. Pramana.
Utilization of Bubbles and Oil for Microplastic Capture from Water.
Engineering, 2024, 41(10): 74-87 DOI:10.1016/j.eng.2023.01.021
Although plastics are one of the most useful and versatile materials of recent times, they are often discarded without consideration of their environmental impact. One endpoint is water, with around 8.75 million metric tons of plastic entering the ocean every year and breaking down into microplastics (MPs; < 5 mm) [1], [2]. These small particles can now be found throughout the world’s water systems, from the Mariana Trench and the Great Barrier Reef to potable drinking water [3], [4], [5], [6], [7]. MPs have become a focus of global attention, with specific interest on their effects on human health, impacts on the environment, and methods of removal [8], [9], [10], [11], [12], [13], [14], [15], [16]. This concern has been exacerbated by a lack of legislative limits on the concentration of MPs permissible in various consumable liquids or a focused method for their removal [17], [18], [19], [20]. Studies by Triebskorn et al. [21] and Lu et al. [22] have shown that the concentration of MPs can be as high as 37-40 g∙L-1. As the proposed procedure applies the same principles regardless of its locale, it can be used to address the plastic contamination of water within domestic households, industry drinking water, and passive ocean settings.
As reported by Poerio et al. [23] and Ma et al. [24], traditional filtration technologies exhibit low effectiveness against MPs, particularly in examples where sedimentation (with and without coagulant) and ultrafiltration (with coagulants) were used, which achieved 2%, 13.6%, and < 15% removal, respectively (polyethylene, diameter (d) < 0.5 mm) [24], [25]. On the other hand, within conventional waste water treatment, filtration can achieve a minimal MP removal of > 88% [6], [10], [16], [23], [26], [27], [28]. In fact, membrane technologies—especially membrane bioreactors—are capable of MP removal approaching 100%. Nevertheless, the filtration of large volumes of MPs and/or nanoplastics (NPs) may lead to filter pore clogging and a reduction in flux, with smaller particle sizes being more difficult, expensive, and energy intensive to separate [16], [29].
A recent assessment by Pizzichetti et al. [30] on the use of membranes for MP filtration showed good removal of approximately 94%; however, some MPs (20-300 μm polystyrene (PS)) larger than the nominal pore size (5 μm) were found to pass through. The most likely explanation is the existence of membrane abrasion, which becomes more apparent with membranes of low hardness and particles with sharp corners. Membrane abrasion coupled with the mechanical stress induced by the transmembrane pressure can cause the polymer membrane to break down [30]. Moreover, when the capture of microfibers is attempted using a membrane, the fibers often escape, passing longitudinally through the membrane’s pores [31], [32].
Dissolved air filtration is another physical method of MP removal with improved cost effectiveness. This process relies on the hydrophobic interactions of MPs with bubbles and coagulants, as well as general floatation principles [33]. High removal efficiencies are attainable, but only for a specific particle size under specific conditions, rather than for a wide range of MP types and sizes [34]. In general, when plastic floatation (i.e., adapted mineral floatation) is discussed, only larger MPs with sizes of 1-5 mm are considered [35], [36], [37], [38]. In the proposed method, bubbles produced by an aerator transfer MPs from the bulk water to the water-oil interface at the surface of the water. By utilizing the MPs’ oleophilic and hydrophobic behaviors via oil capture and mineral particle floatation principles, a highly efficient and lasting method of MP removal from water is achieved [35], [39], [40], [41], [42], [43], [44], [45], [46], [47].
Within this study, the capture of six of the most prevalent types of MPs from water is conducted and examined by varying key parameters. This study targets the removal of six plastic types: low-density polyethylene (LDPE), polypropylene (PP), PS, nylon (PA66), poly(ethylene terephthalate) (PET), and polyvinyl chloride (PVC). These plastics are subject to high-volume production and, as such, have the greatest probability of entering water systems [6], [12], [21], [48], [49], [50], [51]. Here, we report a simple, swift, and highly effective method for MP removal from water based on the canola/castor oil capture procedure and mineral microparticle floatation. This method takes advantage of polymer properties, which aid in both their interaction with the surface of the bubbles and their retention within the oil layer (Figs. 1(a) and (b)).
2. Materials and methods
Vegetable oil was purchased from KTC Edibles (UK), and plastic fibers (PP, polyethylene (PE), PA66) were purchased from Goonvean Fibers (UK). PP, LDPE, PS, PA66, PVC, and PET MPs were supplied by City University of Hong Kong (China).
2.1. Microplastic dyeing
To visually identify and track the movement of the MPs, dyes were applied. Dyeing was achieved using an adapted version of a previously reported procedure [52]. Typically, 100 mg of MPs were added to 50 mL of dye solution. Blue MPs were selected due to their contrast; the dying was achieved using a 2:1 ratio of deionized water and Kentucky Sky Rit DyeMore Synthetic liquid dye (Disperse Blue 3 anthraquinones dye; Nakoma Products, USA). This dying was carried out for each type of MP. The MP dye solution was then heated in an oven for 2 h at 70 °C, below the melting points of the MPs, allowing for the diffusion of the dye molecules into the matrix of each polymer [52], [53]. The samples were then left for 72 h at room temperature under dark conditions to ensure that the dye molecules remained within. The dyed MPs were rinsed and vacuum filtered multiple times, ensuring no leeching. This MP labeling approach was successfully applied to each of the six MP types (Fig. S1 in Appendix A) [52].
2.2. Microplastic characteristics
2.2.1. Particle size distribution
A real-world scenario of plastic capture encompasses fragments ranging from the nanometer to millimeter scale and larger; therefore, an overly tight control of particle size within this work was not considered necessary, as the MPs in application will never be just one specific size. Instead, a more general focus was on the removal of plastic fragments in their respective order of magnitude, for NPs with d < 1 μm, small MPs with 1 μm ≤ d < 1 mm, and larger MPs with 1 mm ≤ d < 5 mm [9], [16], [54], where d refers to the diameter or particle size. The MPs used within this study were characterized using a VHX-970F Keyence Digital Microscope (Keyence Corporation, USA). Subsequent images were analyzed via ImageJ using a particle counter feature [55]. The size distribution of the MPs studied ranged between 50 and 200 μm. To obtain three clear size distributions corresponding to the smallest, intermediate, and largest particles, granular convection was employed. This process is otherwise known as the Brazil nut effect, in which a mixture of granular material is shaken, and the largest particles move to the surface [56], [57], [58]. A 50 mL beaker was filled with an MP type and vibrated on a vortex mixer for 1 min. Following this, the top surface layer, 10 mm in thickness, was removed from the beaker. The process was then repeated until all the powder had been removed from the beaker. Fig. S2 in Appendix A shows the distribution of particles acquired in each layer. Layers 1, 3, and 5 represent the largest ((160.0 ± 3.6) μm), intermediate ((110.0 ± 8.0) μm), and smallest ((70.0 ± 13.4) μm) MPs, respectively.
2.2.2. Microplastic contact angle
The contact angles of each type of MP in their dyed and undyed states were measured using the sessile drop method (Fig. S3 in Appendix A) [37], [59], [60], [61]. Pellets were formed via uniaxial compression using a hydraulic hand press (SPX Power Team SPM256C; SPX Power Team Corporation, USA) at 2500 psi (1 psi = 6.895 kPa) for 10 min. Following compression, a sessile droplet of about 10 μL of deionized water was placed on the surface of each MP pellet. An image of the side profile of the droplet on the powder bed was taken (RS Pro Polarized USB Microscope with 200× magnification; RS Americas, Inc., USA). The contact angles were calculated using the DropSnake plugin [62] on ImageJ; this process was repeated five times for each type of MP.
2.3. Microplastic capture via the oil encapsulation procedure
A constant and dense stream of fine air bubbles was created using a porous stone, and 1 L∙min-1 of air was supplied using a pump (Charles Austen Linear Air Pump, 40 L∙min-1; Charles Austen Pumps, UK), controlled via a rotameter (FR200 Variable Area Flow Meter, 0.1-1.0 L∙min-1; Brooks Instruments, USA) (Figs. 1(a) and (b); Fig. S4 in Appendix A). The aerator was placed at the base of a 400 mL cylinder and filled with 200 mL of deionized water containing 20 mg (0.1 mg∙mL-1) of MPs. We attempted to ensure that a concentration comparable to other oil-capture methods was used; however, it should be noted that the concentration range found in fresh water spans from 7 × 10-10 to 39 mg∙mL-1 [22], [30], [63]. A 10 mL droplet of oil was then added onto the surface of the water. The aerator produced bubbles constantly for 10 min to allow for adequate capture time, which was followed by another 5 min for settling. The oil containing the MPs was then removed from the water surface using the outlet at the top of the cylinder (Fig. 1(a)). The remaining water in the column was vacuum filtered, and the mass of MPs remaining in the water was weighed. This process was repeated six times for each type of MP under each set of experimental conditions—five times for undyed MPs and once for dyed MPs. The dyed MPs allowed visual identification for further verification. The MPs that were captured within the oil were not released into effluent water; all oil used within the study was stored and subsequently filtered to remove the captured MPs. Any residual oil remaining on the MP following filtration was removed using a reagent alcohol protocol employed by Crichton et al. [40], allowing for reuse.
A previously reported method by Mani et al. [39] for MP separation using oil and manual agitation was followed to function as a comparison with the bubble separation. The same MP concentrations, water and oil volumes, and conditions as the bubble separation were employed. In the method by Mani et al. [39], agitation occurs when a pear-shaped separation funnel is manually shaken. This is followed by venting the valve at the bottom of the funnel, allowing the water to be vacuum filtered.
2.4. Microplastic properties
The MPs used in this study (65%) were between 90 and 130 μm in diameter (range: 50-200 μm), and all had similar morphologies (Fig. S1). The largest, intermediate, and smallest MPs were analyzed using a digital microscope and ImageJ, yielding the respective size distributions of (160.0 ± 3.6), (110.0 ± 8.0), and (70.0 ± 13.4) μm (Fig. S5 in Appendix A).
The degree of wetting of each MP type is provided in Table 1 [64], [65], [66]. As can be seen for both the dyed and undyed MPs, the contact angle is greater than what is observed in the literature for a solid surface. This is due to the more porous structure of the MP pellets in comparison with a solid surface; pockets of air are present between the MPs, which instigates a Cassie-Baxter wetting regime, increasing the hydrophobic behavior [67], [68], [69], [70]. Moreover, the contact angles remain similar between the dyed and undyed powder. However, as a hydrophobic anthraquinone dye was used to dye the MPs, slight differences in contact angle exist between the dyed and undyed MPs, due to the chemical structures on the dyed MPs’ surface. Although this may have impacted the packing and wetting in general, it had negligible effect on the removal efficiencies, as the results for the dyed MPs were within the standard deviation of those for the undyed MPs [64], [65].
3. Results
3.1. Bubble characteristics
Throughout the experiments, the air flow rate and, therefore, the bubble volume and density were kept constant. This was justifiable, as the system used in these experiments was a batch system, so the MPs were subjected to continual mixing. Upon integration into a continuous system, the air flow rate and subsequent bubbles will be a key parameter and, as such, will be the focus of further investigation. It was also paramount to select an air flow rate that did not disrupt the oil layer, to ensure it remained as a layer at the surface of the water only, thereby avoiding oil contamination.
The bubble diameter ((880 ± 93) μm), volume (0.41 mm3), velocity (0.509 m∙s–1), and density (3342 ± 116 per 200 mL water) were determined through image sequences captured via a high-speed camera (Photron Fastcam Viewer SA3, 2000 frames per second (fps); Photron Ltd., Japan), with subsequent analysis using ImageJ and its manual tracking plugin (Fig. 2) [55]. The pump supplied was capable of an air flow rate of 1 L∙min–1. As the time for a bubble to rise to the surface is known (0.100 s), the volume of air present in the system for any still image and the subsequent volumetric flow rate could be found, 1666.7 mm3 and 16 666.7 mm3∙s–1, respectively. Using the average bubble volume, the theoretical maximum number of bubbles could be found (i.e., 3727). This was compared with the number of bubbles observable through ImageJ (3342 ± 116); in this way, the flow rate was determined to be (14 370.6 ± 499.0) mm3∙s–1 (Fig. S6 in Appendix A). The flow rate obtained through ImageJ is approximately 14% less than the calculated value; the discrepancy was accounted for by the overlapping projected images of the bubbles. It can therefore be assumed that the flow rate provided by the pump is observable through the number of bubbles formed. The rise velocity for spherical bubbles was then verified using Eqs. (1), (2). Confirmation of the bubble shape was achieved via bubble Reynolds, Bond, and Morton dimensionless numbers (Fig. S7 in Appendix A) [71], [72], [73], [74], [75], [76]. The bubble rising velocity for a spherical regime could now be calculated and was found to be 0.539 m∙s–1. When compared with the 0.509 m∙s–1 observed through ImageJ sequences, there is an acceptable difference of about 5%. where Ut is the terminal velocity of the bubble, Δρ is the density difference between continuous medium and the dispersed fluid, g is gravitational acceleration, db is the bubble diameter, k1 is the viscosity ratio, and ηg and ηl are the gas and liquid viscosity, respectively.
3.2. Microplastic capture results
3.2.1. Bubble oil system basis for microplastic removal
When using vegetable oil, the proposed bubble and oil system achieved minimal removal efficiencies of 98% (Fig. 3). For MPs less dense than water (i.e., LDPE and PP), 100% removal was achieved; however, MPs with a density greater than water had a lower removal, with the removal decreasing as the density increased. This result occurred because the MPs denser than water sank, making the mixing of these MPs with the oil layer more difficult in comparison with that of their less dense equivalents.
To assess the impact of factors that can aid in MP removal, control experiments were set up. The method of agitation and capture medium were compared with ① manually shaking with oil, ② using a magnetic stirrer at 250 r∙min−1 with oil, ③ using a magnetic stirrer at 500 r∙min−1 with oil, and ④ using bubbles with no oil. As shown in Fig. 3, the removal efficiency decreased with an increase in MP density for all removal methods. This finding is most apparent in the comparison of the bubble removal methods with and without oil capture. When no oil was employed to capture MPs, the removal efficiencies of the three densest MPs (i.e., PA66, PET, and PVC) did not exceed 2.5%; however, when oil was employed, approximately 98% removal was achieved. In both instances, the bubbles forced the MPs to the surface but, when oil was absent in the control experiment and the aerator was turned off, the MPs sank back to the base of the column—meaning that capture from the surface could not occur.
The remaining three control experiments examined how the method of agitation impacts capture. The manually shaken method was based upon work by Mani et al. [39]; it generated very vigorous mixing, resulting in an emulsion. The formation of an emulsion was disadvantageous in this work, due to the long settling time required for the oil layer to reform and the presence of oil on the sides of the vessel following agitation. The degree of mixing was varied using a magnetic stirrer with stirring speed of 250 and 500 r∙min−1. These two speeds were selected because they did not break the oil down into small droplets, avoiding the formation of an emulsion (Figs. S8 and S9 in Appendix A). The 250 r∙min−1 mixing speed maintained a uniform oil layer, whereas 500 r∙min−1 generated some larger oil droplets that broke off from the main oil structure. The removal efficiencies of these control methods were much less than that of the bubble and oil method (Fig. 3).
To indicate the significance of the difference between these methods, statistical analysis was undertaken using Prism software utilizing two-way analysis of variance with multiple comparisons [77]. Variation in the removal method had little significance for MPs less dense than water (i.e., LDPE and PP); it was only when the least vigorous type of mixing—that is, the 250 r∙min−1 magnetic stirrer—was employed that significant variance was observed (Table S1). As the density of the MPs increased, the type of mixing generated P values of < 0.0001. The improved removal efficiencies for all MPs when using the bubble and oil method are due to the constant upward force transporting the MPs to the surface of the water, where they mix with the oil layer. With manual shaking and 500 r∙min−1 mixing, instead of the single large oil layer in the proposed method, numerous small oil droplets formed due to the vigorous type of mixing; as a result, the MPs had less of an affinity to move into the oil phase. This issue was not crucial with the lower density MPs, as these MPs remained at the surface of the water, which was in constant contact with the oil layer. However, the type of mixing employed is important for the denser MPs, with the mixing induced from bubbles proving to be superior.
The application of bubbles also allows for rapid removal and capture. In all instances, it was only necessary to run the system for 30 s before the majority (> 97%) of the plastic contaminants were captured within the oil layer (Table S2 in Appendix A). Millimeter-sized MPs, both small and large, required additional time to move to the surface, due to their size and reliance on more successful bubble collisions. Moreover, in all instances, the MP settling velocity was less than the MP-bubble rise velocity (Table 2); in the case of the PA66 fibers, there was a magnitude ×1000 difference.
3.2.2. Impact of particle morphology
Within the current literature, several studies have investigated how oil can be used as a method to isolate MPs from certain mediums, including water, sand, or slurry. However, these studies focused on MPs in the millimetric range of 0.20-6.17 mm [39], [40], [41], [42], [78], whereas this study considers MPs of that range and smaller, at 50-200 μm. Within the micrometer range, the removal efficiency was not affected by size, and the three specified size distributions were all within the standard deviation of the mixed sample (99.1% ± 0.6%; Fig. S10 in Appendix A). In comparison, Mani et al. [39] found that, with larger (500-1000 μm) and smaller MPs (300-500 μm), removal efficiencies of 100% ± 2% and 98% ± 4% were achieved, respectively [39]. To effectively compare the proposed method with manually shaken strategies, millimeter-range MPs were also examined, separated into small (0.5-2.0 mm) and large (3-5 mm) size fractions [39], [40]. When these two size fractions were considered, complete removal of MPs was achieved, aligning with the results reported in the literature. As shown in Fig. 4, the oil layer proved effective in the capture of MPs of various densities and sizes. This process was similar to froth floatation, in which bubbles float the MPs to the surface. However, instead of froth being present at the surface, formed by a surfactant or similar chemical, an oil layer is utilized, which acts as a more effective medium for capture. When bubbles were introduced into the oil layer at low turbulence, the oil layer remained intact and thus did not leave the system in the treated water. Unlike froth floatation, where particle characteristics play a crucial role in froth stability and the froth often collapses, this consideration is not necessary for oil layer stability.
One issue encountered with conventional membrane filtration is that microfibers can pass through more easily than particles [31], [32]. The effectiveness of the bubble and oil procedure in the capture of fibers was assessed using three different materials: PP, PA66, and PET. As shown in Table 3, each of the three types was captured with a high rate of removal (∼99.4%), due to the fibers’ highly floatable nature, with most of the capture occurring within the initial moments of removal.
3.2.3. Impact of liquid conditions
The volume of the oil was varied to better understand how the surface coverage of the water with oil impacted MP capture. Within these experiments, the oil formed either a “lens” that did not cover the whole surface of the water or an oil layer that completely covered the surface (Table 4) [61]. As the volume of oil increased, a greater percentage of the two MPs denser than water—PA66 and PVC—was captured (Table 4). This was largely because the available contact area for mixing increased with the increase in oil volume. The increase leveled off once the diameter of the oil lens (doil) was larger than the aerator’s diameter (daerator). Therefore, it is crucial for the oil layer “lens” diameter to be at least equivalent to the size of the area producing the bubbles, as this increases the chance that any MPs forced to the surface will encounter the oil. The thickness of the oil layer had no impact on the removal percentages of the MPs, with both 25 and 50 mL of oil achieving 99.4% removal.
Fresh water [79], [80], [81], [82], sea water [83], [84], [85], [86], waste water [6], [27], [87], [88], [89], [90], [91], and potable water [6], [7], [88], [92], [93] all suffer from MP contaminants present in detectable numbers. Surface salt water conditions were simulated to assess what impact—if any—salt water would have on the removal of MPs via the aforementioned method, using the ocean’s salt water density of approximately 1025.0 kg∙m-3 [94], [95], [96]. As shown in Fig. 5, there was no discernable difference between the removal efficiencies of MPs in pure water versus artificial sea water, suggesting the applicability of this technique to either setting.
3.3. Microplastic retention
One of the benefits of utilizing oil as the capture medium is that, once an MP is taken into the oil phase, it will not move back into the water phase, even if the oil and water remain in contact. To highlight the degree to which this occurs, the retention in either the oil layer or at the water surface (with no oil present) following bubble mixing was observed. After three weeks, the oil layer retained 99.96% of all MPs; in contrast, when oil was absent, only 57.9% of the MPs were retained (Fig. 6; Tables S3 and S4 in Appendix A). The latter decreased to 21.8% when only the MPs denser than water were considered and reached 0% for PA66, PET, and PVC well before three weeks. This finding highlights the pivotal role played by the oil layer in the proposed method—not only in the initial capture of the MPs from the bubbles but also in ensuring that any captured MPs are not returned to the water (Fig. 4) [39], [40], [41], [78].
4. Discussion
4.1. Interactions of microplastics with bubbles
MPs are forced to the water/oil interface via two main routes: either by directly attaching to the air bubble or by being pushed up by the upward flow generated by the bubbles. If the former scenario occurs, as is observed within mineral floatation, the MPs move very quickly to the surface due to the buoyance of the bubble, exhibiting almost exactly the same rise velocity as a bubble without an attached MP (Figs. 1(a), 1(b), and 7(a)-(c)) [97], [98], [99], [100], [101], [102]. The MPs transported without direct attachment reach the surface more slowly, with a time to surface of (0.312 ± 0.038) s. These two routes allow for all the MPs—not just those attached to bubbles—to interact with the oil (Figs. 1(b) and Figs. 7(a)).
The first method of movement is associated with general floatation mechanisms, consisting of three stages: collision, attachment, and detachment (Figs. 1(b), 7(a), and 7(b)). By examining each of these subprocesses, the total probability of collection, Pcoll, can be calculated, where Pc, Pa, and Pd are the probabilities of collision, attachment, and detachment, respectively (Eq. (3)). In this study, two additional processes are proposed: uptake into and collapse within the oil layer (Figs. 7(a)–(c)). In general, when plastic floatation is considered, the literature tends to discuss larger MPs, in a size range of 1–5 mm [32], [33], [34], [35]. In such instances, unlike in mineral floatation, the particles tend to be larger than the bubbles; thus, a different mechanism for floatation is experienced in which multiple bubbles attach to the particle surface (Figs. 7(d) and (e)) [35], [36], [44], [103], [104], [105]. In scenarios where the particle is much larger than 5 mm, changes to the system may be required, with larger bubbles being introduced. On the other hand, mineral floatation tends to focus on particle sizes in the range of 1–1000 μm, which are classified into fine (< 100 μm) and coarse (> 200 μm) particles [43], [44], [106], [107], [108], [109], [110].
A high Pcoll is linked with Pc, which largely depends on the hydrodynamics of the system and is strongly impacted by the sizes of the particles and bubbles [37], [111]. Traditionally, fine particles have lower floatation due to the reduced Pc [35], [42], [111]. However, as the MPs have a lower density than mineral microparticles, attachment to a bubble is not necessarily a major factor in forcing the MPs to move to the surface of the water (the average density of MPs is 1120.8 kg∙m-3, the density of quartz is 2650 kg∙m-3, and the density of zircon is 4560 kg∙m-3) [35], [112]. Therefore, an increase in the volume and number of bubbles present allows for quicker removal from the system, rather than dictating the overall removal efficiency.
Unlike Pc, Pa is dependent on particle characteristics rather than the surrounding fluid [105], [113], [114], [115], [116], [117]. Pa increases with an increase in contact angle and a decrease in both particle size and density, resulting in the high removal efficiencies observed throughout this study for MPs [105], [113], [117]. The floatability of fine particles is fundamentally governed by the contact angle, as fine particles with larger contact angles and lower density have not only greater Pa but also greater floatability [36], [37], [38]. Minerals such as quartz and zircon are much more hydrophilic than the studied MPs, with contact angles of 20°-50° in comparison with the average contact angle of the MPs at 86.2° [118], [119], [120]. The larger contact angles observed within this study allowed the MPs to attach more easily to the bubbles, resulting in higher Pa, Pcoll, and floatation recovery [37], [121]. The relatively high hydrophobicity of the MPs results in not only a higher likelihood of attachment but also a reduced probability of detachment, Pd, due to the submerged particles’ repulsion to water [37], [43]. Like Pa, Pd is dependent on the properties of the particles rather than on the system’s hydrodynamics [117]. Detachment can often be ignored in the floatation of fine particles, as its impact is negligible—unlike coarse particles, where particle detachment is one of the main causes of poor floatation [43], [105], [117].
For the MPs within this study, detachment from the bubble rarely occurred, even with the millimetric polymer pieces, due to the MPs’ high contact angles and lower densities. However, if the density and size of the MP particles were much higher, detachment would play an increasingly important role [111]. The upper size limit of the plastics removed within this system was determined by the definition of MPs, which refers to fragments < 5 mm in size. Plastic larger than this would potentially require the attachment of multiple bubbles or the introduction of larger bubbles to successfully float [43], [111]. Pieces much larger than MPs, at > 20 mm, could potentially still be removed by this system; however, the method of removal would more likely occur via the second route (driven by bubble force; Fig. 1(b)). A system that considers both MPs and larger fragments would therefore require a much denser stream of fine bubbles in order to ensure that MPs can still be removed via the first route, while the second route can be employed for the larger fragments by providing greater bubble force. Further work will be directed toward investigating the applicability of the theoretical framework of Pcoll used within mineral floatation for MPs and testing critical parameters to accurately describe the removal of lower density and higher contact angle particles of different sizes.
The proposed system may also be applicable for NP removal. Within froth floatation, nanoparticle capture often proves challenging due to the low probability of collision, resulting in mediocre removal rates [122]. Particles at this scale rely on the instigation of bubble collisions by means of Brownian diffusion and colloidal forces, as well as interception, with particles often lacking the required energy to disrupt the bubble’s film [97], [123]. To overcome this issue, particle size can be increased to improve the probability of collision, often via hydrophobic aggregation and oil-assisted floatation, both of which rely on the “hydrophobic force” [124]. As this situation readily occurs with non-polymer particles, the more hydrophobic plastic pieces may be more strongly attracted to one another and develop aggregates on their own. At this particle size, it may also be beneficial for mixing to take place for a longer duration, to allow for the formation of these NP aggregates. Conversely, it has been suggested that the additional movement caused by Brownian diffusion may aid in the probability of collision, as silica nanoparticles (259 nm) exhibited improved collection efficiency with a decrease in particle size and an increase in hydrophobicity [125]. Thus, it could be argued that the properties such as relatively high hydrophobicity that aid in the high removal of micro and millimetric polymer pieces may also be beneficial for NPs.
4.2. Interactions of bubbles and microplastics with oil
A common issue with floatation is that an optimum parameter must be sought; for example, a larger particle size will improve the probability of collision but will also increase the chance of detachment [43], [111]. This is not the case with the proposed method; because the froth is replaced by an oil layer, there is no need to consider the froth stability. In froth floatation in general, the structural stability of the froth decreases when particles with an intermediate level of hydrophobicity are present (contact angle > 65°) [98], [126], [127], [128]. This is one of the main drawbacks of froth floatation, as over-encumbered bubbles will collapse in the froth layer before the particles are removed from the system. On the other hand, an oil layer is not negatively impacted by bubble collapse. In fact, when bubbles burst within the oil layer, this permits the transfer of the MP from the bubble interface into the bulk oil layer. A greater number of bubbles collapsing within the oil layer would allow for faster rates of MP uptake. Using the oil layer as a capture medium also means that particle size—which would ordinarily impact a froth floatation complex—is not an important factor, as the MPs are strongly attracted to the oil phase [129]. This oleophilic interaction occurs between the non-polar components of the long-chain fatty acid molecules within the oil and the non-polar parts of the polymer backbone [39]. This interaction is strong enough to hold even the densest polymers within the layer, preventing captured pieces from returning to the water [40]. The use of a viscous oil, such as the one used in this study, ensures that the plastic pieces have a lower sinking velocity within the oil layer, holding them in place. Replacing the froth layer with an oil layer makes it possible to exploit the benefits of the bubbles used in floatation to capture MPs, as MPs have the most appropriate characteristics for such a system: ① The MPs have low density, so floatation of the full range of MP sizes can be achieved; ② the MPs have a high contact angle and are hydrophobic, making them more likely to attach to a bubble and less likely to detach; ③ the MPs are oleophilic, with a natural affinity to move from the water phase to the oil phase.
It is postulated that millimeter-sized particles are more readily taken into the oil phase, while smaller particles resist transference from the water phase. As observed with the liquid marble (LM) formation, although a particle is inherently philic to a specific liquid, it can demonstrate phobic behavior, where the particles do not move into the liquid immediately. Instead, the particles decorate the outside of the liquid droplet, causing an LM or Pickering emulsion to form. LMs are considered to be non-wetting soft solids; they can be formed when a water droplet is rolled through a bed of particles (often hydrophobic MPs with contact angle > 90°). These particles coat the exterior of the droplet, fully encapsulating the liquid within [130], [131], [132]. However, hydrophilic powders (contact angle < 90°) can also form stable LMs, as either stable aggregates exhibiting Cassie-Baxter wetting or single particles in which the particle remains in a metastable state [62], [99], [132], [133], [134], [135]. This phenomenon could be used to explain the slightly reduced removal efficiencies of the micrometer MPs when compared with their millimeter counterparts. As all the MPs used within this study were oleophilic, the majority were taken into the oil, with a small percentage remaining in the water nearby or at the oil/water interface. This result can be explained via the oleophobic behavior exhibited by some of the MPs at the oil boundary, like the formation of a hydrophilic LM. This issue can be remedied by further prolonged mixing, which is achieved by leaving the aerator running for longer than 10 min, thereby overcoming the initial resistance to the wetting of some of the MPs by the oil.
As has been mentioned throughout this work, oil layer stability is of key importance and, as such, is the main limitation of the proposed method. Once the bubble-particle structure has formed within the water, it is transported into the oil layer where bubble collapse occurs, resulting in MP retention in the oil. It is important that the air flow rate used ① is adequate in producing a stream of fine bubbles for successful collision and attachment, and ② does not move oil from the water surface. When air flow rates higher than this are used, the oil layer breaks down into globules, which reduces the efficiency of the mixing of the bubble-particle structure with the oil; in addition, the oil drops move down to the base of the column, contaminating the surfaces. This could be overcome with the addition of a coating to the surface to help prevent oil contaminants from affecting the system [46], [47].
4.3. Comparisons with existing removal methods
Many other methods are used for separating plastics from water, such as froth floatation, density separation, and filtration; however, these methods were not specifically designed for MP removal [23], [100], [101], [102], [136], [137], [138], [139]. The separation of MPs by filtration is commonplace, often conducted using membrane bioreactors, ultrafiltration, microfiltration, nanofiltration, or reverse osmosis [140], [141]. Filtration is a simple form of removal, and numerous papers have demonstrated that excellent removal (up to 100%) can be achieved with filtration, especially when membrane bioreactors are employed [28], [31], [32], [140], [141], [142], [143], [144], [145]. However, for membranes to perform at peak efficiency, strict fouling controls must be in place, such as high-pressure backwashing and chemical treatment. These fouling controls, along with general aging from the long-term operation of the membrane, contribute to its breakdown. In the case of polymer-based membranes, the membrane then becomes a secondary source of pollution; for membranes of other materials, more and larger MPs are able to pass through a breaking-down membrane uncaptured [32], [102], [138], [145], [146], [147]. A recent study by Pizzichetti et al. [30] perfectly highlights these points. Polycarbonate, cellulose acetate, and polytetrafluoroethylene membranes achieved high removal efficiencies of > 94%. During operation, however, fragmentation of the membrane and of the MPs themselves into smaller particles was observed. These smaller MPs and NPs are also part of wider issues with membrane filtration, as the small pore sizes of the membrane quickly become fouled, resulting in high pressure drop and subsequent incurred cost [137], [139], [147], [148]. In addition, it has been shown that membranes do not capture plastic fibers well, as such fibers can pass longitudinally through the filter’s pores—especially under high operating pressures of 100-400 kPa [31], [32]. In contrast, the method proposed in this study does not release polymer fragments as a secondary source and has exceptionally high rates of microfiber entrapment.
Filtration, coagulation, and sedimentation are removal methods associated with drinking water treatment plants. It has been shown that specific coagulants can improve MP removal. A document published by World Health Organization [149] demonstrated this with a polyacrylamide coagulant; although high removal efficiencies were achieved (∼90%, using 15 mg∙L-1 of the coagulant), the quantity of coagulant was much higher than the maximum authorized dose of 1 mg∙L-1. Depending on the treatment plant and the methods employed, MP removal efficiencies vary in the range of 70%-99%. Given the high volume of water being treated, many MPs—in some instances, billions—are missed each day [28]. Therefore, the addition of a cost-effective method that specifically targets MPs, such as the one proposed herein, would prevent these missed MPs from entering potable water or the environment [6], [23], [24], [25], [150].
Froth floatation requires the addition of chemical agents, usually surfactants, to the water to induce the formation of a bubble layer at the surface of the liquid, where the particles removed via floatation are captured. Varying removal efficiencies have been observed, with low removal efficiencies of 0, 0, and 8% for polymethylmethacrylate, PVC, and PS (d = 4.18 mm), respectively [35]. In other plastic floatation studies, it has been shown that, while significantly higher removal efficiencies (∼97.5%) could be achieved, these were for a very specific size range, with great fluctuations dependent on particle size [37]. Wang et al. [36] highlighted this issue using MPs of four different sizes (25 mg∙L-1 tannic acid and 5 mg∙L-1 terpineol; 15 min conditioning time and 15 min floatation time). They observed a decrease in MPs of all particle sizes within the liquid phase except the intermediate size (2-4 mm), indicating poor capture of MPs outside of the range [36]. The poor floatation behaviors within this study could be attributed to incompatible bubble and particle sizes, with coarse particles having poor adhesion to the bubbles and fine particles relating very closely to their wetting characteristics; the behaviors could also be related to poor froth stability. One of the shortcomings of froth floatation is that the layers depend on the size and hydrophobicity of the particles within the froth to ensure stability. This often means that the entrapment of a very specific size range is necessary to prevent a loss of particles back to the bulk liquid, with slight changes in the system characteristics changing the particles from foam stabilizers to destabilizers [43]. This is less of a problem when an oil layer is employed; as the froth layer is replaced with oil, there is no need to consider froth stability. Using an oil layer as a capture medium also means that particle size is less of a key parameter, as the MPs—regardless of size—are naturally oleophilic and attracted to the oil phase, which would ordinarily impact the froth floatation complex. As plastic fragments are both hydrophobic and oleophilic, they easily interact with the oil layer rather than the bulk water [46], [47].
As shown in Fig. 4, once MPs are within the oil layer, they do not return to the water. When the oil layer containing the captured polymer is removed from the system, the MPs can be removed from the majority of the oil. In this study, the used MPs coated with slight oil residue were added to virgin polylactic acid pellets and extruded for use as filament in three-dimensional (3D) printing projects. When the MPs were reused in additional capture experiments, the remaining oil residue was removed from the particles using reagent alcohol [39], [40]. Both protocols ensured that the MPs used within the investigation were not lost to the environment. Moreover, studies by Fernandes et al. [151], Wu et al. [152], and Cui et al. [153] have shown that waste vegetable oils can be converted into resins for 3D printing. Such a method could be modified to allow for printing with the recycled oil resins containing the captured MPs, thereby forming a waste-capturing-waste system. In general, regardless of its final use as a resin or not, waste oils can be used within the proposed type of MP-capture system.
5. Conclusions
Bubbles and oil can be utilized as a simple and effective method for MP capture and removal. This mechanism can remove > 99.4% of six of the most prevalent MP types in differing environments and for various MP particle sizes. The removal efficiencies have been shown to be exceptionally competitive, exceeding those of other MP removal methods such as filtration and floatation. In addition, factors that reduce removal efficiencies in traditional methods, such as low density and high contact angle of the particles, are beneficial for oil capture in the proposed system, due to the oleophilic nature of the MPs. Furthermore, the proposed system does not suffer from the drawbacks of traditional methods, such as fouling and the need for particle-specific system design. This work has demonstrated that the recent interest in using oil to capture MPs can be incorporated into a feasible method that can be used beyond manually shaking a beaker. The naturally hydrophobic and oleophilic behaviors of the MPs allow for easy encapsulation within an oil layer, where they remain entrained. This paper elaborated the principles of this separation, which can be applied to capture MPs from not only seas and rivers but also potable water and consumables.
Acknowledgments
Stevin S. Pramana acknowledges the start-up financial support from the School of Engineering, Newcastle University. Joshua Saczek thanks Engineering and Physical Sciences Research Council (EPSRC) for his Doctoral Training Partnership (DTP) studentship.
Compliance with ethics guidelines
Joshua Saczek, Xiaoxue Yao, Vladimir Zivkovic, Mohamed Mamlouk, Steven Wang, and Stevin S. Pramana declare that they have no conflicts of interest or financial conflicts to disclose.
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