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Article

Agglomeration–Flotation of Microplastics Using Kerosene as Bridging Liquid for Particle Size Enlargement

by
Pongsiri Julapong
1,2,
Jiraphon Ekasin
1,
Pattaranun Katethol
1,
Palot Srichonphaisarn
1,
Onchanok Juntarasakul
1,
Apisit Numprasanthai
1,
Carlito Baltazar Tabelin
3 and
Theerayut Phengsaart
1,*
1
Department of Mining and Petroleum Engineering, Chulalongkorn University, Bangkok 10330, Thailand
2
Department of Mining and Materials Engineering, Prince of Songkla University, Songkhla 90110, Thailand
3
Department of Materials and Resources Engineering Technology, College of Engineering and Technology, Mindanao State University-Iligan Institute of Technology, Iligan City 9200, Philippines
*
Author to whom correspondence should be addressed.
Sustainability 2022, 14(23), 15584; https://doi.org/10.3390/su142315584
Submission received: 30 October 2022 / Revised: 13 November 2022 / Accepted: 17 November 2022 / Published: 23 November 2022
(This article belongs to the Special Issue Marine Environmental Pollution Control: A Sustainable Perspective)
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Abstract

:
Microplastics (MPs), defined as plastics with diameters between 1 and 5000 µm, are problematic pollutants in the environment, but their removal is challenging because of their minute size. One promising approach for their removal is flotation because MPs are inherently hydrophobic. However, the very small particle size of MPs lowers the probability of MPs-bubble collision and attachment that in turn affects the efficiency of the process. To address this challenge, we propose the use of agglomeration-flotation, a technique using kerosene as a bridging liquid to enlarge the particle sizes of MPs and make them amenable to flotation. In this study, the effects of kerosene dosage on particle size enlargement and floatability of six types of MPs with 100–1000 µm size fractions were investigated. The results showed that MPs with lower density compared with water could easily float in water without bubble attachment and particle agglomeration required. So, the effects of agglomeration on removal were negligible. In contrast, agglomeration using kerosene enhanced the floatability of MPs with high-density plastics. Moreover, image analysis was used to determine the agglomerated MPs’ particle size. The results indicate that kerosene could agglomerate the MPs and enhanced the removal of MPs by agglomeration-flotation.

1. Introduction

Plastics are ubiquitous materials in many industries because their properties could be engineered by either using various types of organic molecules or combining plastic resin with additives [1,2]. Recently, plastic wastes have became a serious environmental concern globally, especially during the Coronavirus disease (COVID-19) pandemic. This pandemic altered the lifestyle of everyone and, due to the new normal, online shopping and food deliveries became mainstream, resulting in the generation of huge amounts of single-use plastic wastes from product delivery packaging [3]. Moreover, plastics-based personal protective equipment (i.e., surgical masks, medical gowns, face shields, safety glasses, and gloves) was also dramatically released to the environment, especially facemasks [4].
Among the many types of plastics, polyethylene (PE) and polypropylene (PP) are the most widely used and constitute the bulk of plastic wastes. Other common plastics found in waste streams include polyethylene terephthalate (PET), polyvinylchloride (PVC), polystyrene (PS), and acrylonitrile butadiene styrene (ABS) [5]. Plastic wastes are generally managed by a combination of recycling, incineration, and landfilling [6]. Without proper collection, however, a large portion of plastic wastes is inadvertently discharged into the environment, which is weathered and fragmented, forming microplastics (MPs) [7].
MPs, plastic particles with diameters ranging from 1–5000 µm, have quickly contaminated the environment as implied by their widespread distribution into various ecosystems (i.e., lithosphere, hydrosphere, atmosphere, and biosphere) [8]. MPs generated inland commonly enter water bodies through several pathways, including domestic waste, wastewater treatment plants (WWTPs), industrial effluents, surface runoff, wind currents, and disposal practices [9,10,11,12,13,14,15,16]. MPs easily disperse via fluid (e.g., air and water) due to their low density and small size. They are really difficult to be perceived by organisms including plants, animals, as well as humans [17]. So, they could be transferred into our body by the inhalation (i.e., breathing) and ingestion (eating and drinking) [18].
Water is essential to living things including humans. It is also necessary for sustainable development in various sectors including socio-economic development, energy, food production, ecosystem integrity, and climate change. So, a treatment to prepare clean and safe water with a permissible level of impurities is needed [19,20,21].
Due to their fine particle size and their hydrophobic surface, MPs are well-known carriers of heavy metals (e.g., lead, cadmium, chromium, barium, copper, cobalt, arsenic, aluminum, iron, manganese, and zinc), persistent chemical pollutants (PCPs) such as phathalate esters (i.e., dimethyl phthalate (DMP), diethyl phthalate (DEP), dibutyl phthalate (DnBP), benzylbutyl phthalate (BBZP), di(2-ethylhexyl) phthalate (DEHP), and di-n-octyl phthalate (DnOP)), and organic pollutants (i.e., polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), and orga-nochlorine pesticides (OCPs)), which exacerbate their negative impacts to the environment, biota, and human health [8,9,10,11,12,13,14,15,16,22,23].
Recently, the United Nations (UN) included these issues in their Sustainable Development Goals (SDGs): Goal 6 “Clean water and sanitation” and Goal 14 “Life below water” [24]. To remove MPs from municipal wastewater, many technologies have been applied in WWTPs such as coagulation and flocculation [25], aerated grit chamber [25], filtration [26], membrane bioreactor [27], anaerobic-anoxic-aerobic [28], sequence batch reactor [28], media process [28], and constructed wetlands [29]. Unfortunately, these technologies remain inefficient as noted by the large amounts of MPs released into natural waters reported even after treatment by WWTPs [30]. According to Xu et al. [31], the effluents from WWTPs in Changzhou (China) are contaminated with MPs with D50 (median particle size) of 430 µm at ~9.04 particles/L. To improve the removal rate of MPs from contaminated waters, various studies have been conducted using separation techniques from mineral processing that included coagulation [32,33,34,35], filtration [36,37,38,39], and froth flotation [40].
Among these techniques, flotation—a surface-based separation method—is promising to remove MPs because most of plastics’ surfaces are naturally hydrophobic [41]. Flotation works by collecting hydrophobic materials suspended in solution using air bubbles.
Plastic flotation can be classified into four types (e.g., gamma flotation, adsorption of reagents, surface modification, and physical regulation). Due to the lower density of plastics compared with ores and/or minerals, the capable size of plastic flotation is 2 times larger than that of ore particles. In general, the optimum size range for plastic flotation is 2.0–6.0 mm [42]. Shent et al. [43] proposed that a single bubble is carries several particles in mineral flotation, while aggregates of plastic particles occur in plastic flotation, in which more plastic particles cluster together through bubble attachment. A bubble-particle aggregate will rise to the water surface due to the buoyancy power of the ascending aggregate.
Unfortunately, the efficiency of flotation decreases rapidly when the size of the target materials becomes fine, a phenomenon attributed to the low probability of collision and attachment between fine particles and larger air bubbles [44]. Especially, in the case of MPs, the concentration of MPs in the actual contaminated water in the environment is really low compared with the solid percentage usually used in mineral flotation. This low concentration would also reduce the probability of collision as well as the floatability [45,46].
One promising strategy to address these problems is to increase the size of MPs through oil agglomeration prior to froth flotation, an approach that has been successfully applied to the flotation of fine chalcopyrite particles (<5 µm). According to Hornn et al. [44,47,48,49], improved flotation recovery during agglomeration-flotation could be attributed to size enlargement of particles that increased collision efficiency.
In the case of size enlargement, coagulation and/or flocculation are usually applied in WWTPs to enhance the sedimentation of MPs. However, after that, the sludge with MPs that was generated needs to be disposed of in landfills [25]. In contrast, if kerosene is used, the collected MPs with kerosene could be utilized as the energy [50], supporting the UN-SDGs: Goal 12 “Responsible consumption and production”.
In this study, flotation experiments of six types of MPs—PP, PE, ABS, PS, PET, and PVC—were carried out with and without oil agglomeration. Specifically, the objectives of this study are to (i) investigate the effects of plastic type on the removal rate (floatability) of MPs, (ii) elucidate the importance of kerosene dosage on the removal rate of MPs, (iii) determine the agglomerated sizes of MPs using image analysis, and (iv) identify the mechanisms controlling the removal of MPs using agglomeration-flotation.

2. Materials and Methods

2.1. Sample Preparation

The MPs used in this study are “model samples” of plastics usually found in waste streams including wastewaters [31,51]. PP, PE, ABS, PS, PET, and PVC boards (1000 × 2000 × 2 mm) with specific gravities of 0.92, 0.97, 1.03, 1.06, 1.31, and 1.38, respectively, were first cut using a reciprocating saw and then fed into a cutting mill (Orient mill, VH16, Seishin Enterprise Co., Ltd. Japan) to reduce the particle size. The crushed plastics were then further ground in liquid nitrogen using a cryogenic mill. After this, the ground plastics were screened to obtain a size fraction of 100–1000 µm, which is the most dominant size range found in the environment [26,31].

2.2. Microplastics Flotation Experiments

A Wemco laboratory flotation machine was used in this study. For the flotation tests, 1 g of MP samples were fed into a 110 × 110 × 150 mm flotation cell (Figure 1) with 1500 mL of water. Kerosene (Alfa Aesar, Heysham, UK) was used as the bridging agent or agglomerator. For the conditioning step, the suspension of MPs and water was stirred at an impeller speed of 900 rpm with the addition of 0, 1, 2, and 3 mL of kerosene. After 10 min, the impeller was stopped and a video (30 fps) of MPs particles in water was taken by a digital camera (EOS 6D Mark II, Canon, Canon Inc., Oita, Japan) with a wide angle lens (EF; focal length 17–40 mm; Aperture of f/4-f/22, Canon, Canon Inc., Oita, Japan).
After the video recording, 30 µL of methyl isobutyl carbinol (MIBC; Sigma-Aldrich, Saint-Quentin-Fallavier, Germany) was added into the flotation cell as a frother and the suspension was stirred for 1 min before flotation. Then, the flotation step was carried out for 1 min with an air flow rate of 13 L/min (the average bubble size was 438 µm) [52]. MPs collected in the froth products were labeled as “removed MPs”, while those retained in water were refered to as “remained MPs”. Both products were dried in an oven at 40 °C and weighed to obtain the removal rate under each experimental condition.

2.3. Particle Size Analysis of Microplastics

The particle size enlargement of MPs was investigated using a setup shown in Figure 1. The recorded videos after the conditioning step were extracted into images via VideoPad v11.48. Five images were randomly selected and converted into 8-bit images as shown in Figure 2. Image J software was then used to analyze the particle size of MPs and obtain the D50 of agglomerated MPs.

3. Results and Discussion

3.1. Effects of Plastic Types on the Floatability of Microplastics

The MP flotation experiments without agglomeration (without kerosene addition) were carried out at an impeller speed of 900 rpm. The preliminary experimental results revealed that 900 rpm was the most suitable impeller speed to remove MPs from contaminated water using a mechanical flotation setup. It was also observed that, at slower speeds, the flotability of MPs was low due to the less collision between MPs and bubbles, while at faster speeds, water splashed out of the cell [53].
More than 89% of MPs regardless of plastic type could be removed by flotation without agglomeration as shown in Figure 3a. From visual observations, polyolefin plastics that have SGs less than that of water such as PP and PE were easily floated and removed due to their low densities. More than 96% of PP and PE were removed even without agglomeration, while the other MPs with densities higher than water had lower floatabilities (i.e., removal rate). The floatability of ABS, PS, PET, and PVC were 89, 93, 96, and 89%, respectively. These results indicate that the density of plastics could affect the MPs’ floatability. The plastics lighter than water have high removal rates since they can already float due to their SG, so the bubble and particle attachment in flotation are not required. However, plastics with a higher density will be sunk and need bubble attachment to float them up via flotation.
In addition, the floatability of MPs could also be affected by their surface wettability. The wettability of plastics could be determined by contact angle measurements, but this approach is not applicable to MPs samples because of their small size. As explained in the introduction, the particle size is another important parameter that affects floatability. To improve the removal rate, the application of kerosene as the collector and/or agglomerator to increase the hydrophobicity and enlarge particle size in the agglomeration-flotation were investigated.

3.2. Effects of Kerosene Dosage on the Floatability of Microplastics

In the previous section, PP and PE could be removed from contaminated water at removal rates of more than 96%, while the removal rates of other heavier MPs were lower. To improve the floatability of MPs, agglomeration-flotation experiments using kerosene as a bridging liquid or agglomerator were carried out at dosages of 1, 2, and 3 mL. From our preliminary experiments using mixed MPs, the results showed that these kerosene dosages were suitable for agglomeration-flotation. At higher kerosene dosages, the excess kerosene coated the walls of the flotation cell, which caused the “trapping” of MPs in the cell.
Figure 3a illustrates the removal rate of each type of MP as a function of kerosene dosage. It is interesting to note that only 1 mL of kerosene was enough to improve the floatability of MPs from 89–96% to 96–99%, indicating that kerosene could improve the floatability of MPs.
However, only small improvements in the removal rate at the 2 mL of kerosene dosage for some plastics were observed, while, in others, the removal rate decreased. At 3 mL, the removal rates of most MPs decreased because excess kerosene started to coat on the walls of the flotation cell, causing entrapment of MPs similar to those observed during the preliminary experiments. These results indicate that about 1–2 mL of kerosene is enough to improve the floatability of MPs under the experimental conditions of this study.

3.3. Effects of Kerosene Dosage on the Particle Size Enlargement of Microplastics

In mineral processing, kerosene is not only used as a bridging liquid (agglomerator) to increase the particle size but also as a non-ionic collector to increase the floatability of hydrophobic materials such as coal [54], sulfide minerals [44], and plastics [55]. To confirm that this improvement occurred during the agglomeration of MPs, particle size analysis after agglomeration was investigated.
To investigate the effects of kerosene dosage on particle size enlargement of MPs, image analysis with the setup shown in Figure 1 was carried out. Figure 3b illustrates the relationship of kerosene dosage and the D50 of each type of MPs. The results indicate that the D50 of MPs increased with a higher kerosene dosage, showing strong evidence that agglomeration of MPs occurred. Similar to the effects of kerosene dosage on the floatability of MP, only 1 mL of kerosene could increase the D50 of MPs from 342–504 µm to 727–1166 µm (about two times). The higher relative size of agglomerated MPs improved the probability of particles–bubbles collision, which increased the removal rate [44]. These results indicate that kerosene could agglomerate MPs and agglomeration could enhance the floatability of MPs, especially for the plastics that are heavier than water.

4. Conclusions

From the results of MP flotation experiments without agglomeration, it was found that over 96% of PP and PE with SG < 1 could be removed, while other plastics with SG > 1 had floatabilities of 89–96%.
To improve the floatability of MPs, agglomeration-flotation experiments using kerosene as a bridging liquid (agglomerator) were carried out. The results showed that only 1 mL of kerosene was enough to improve the floatability of MPs to 96–99%. The significant improvements of MP removal rates regardless of the type of plastic in the agglomeration-flotation experiments could be attributed to the bridging effects of kerosene that increased the sizes of particles in suspension. These results indicate that kerosene could agglomerate MPs and enhanced their removal in contaminated waters.

Author Contributions

Conceptualization, O.J. and T.P.; methodology, P.J., O.J. and T.P.; software, P.J., J.E. and P.K.; validation, P.S. and T.P.; formal analysis, P.J., J.E., P.K., O.J. and T.P.; investigation, P.J., J.E. and P.K.; resources, A.N. and T.P.; data curation, P.J.; writing—original draft preparation, P.J., P.S., C.B.T. and T.P.; writing—review and editing, P.J., P.S., C.B.T. and T.P.; visualization, P.J.; supervision, O.J. and T.P.; project administration, T.P.; funding acquisition, A.N. and T.P. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Thailand Science research and Innovation Fund Chulalongkorn University (CU_FRB65_dis(26)_151_21_17).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding authors. The data are not publicly available due to the continuation of a follow-up study by the authors.

Acknowledgments

The authors gratefully acknowledge the Laboratory of Mineral Processing and Resources Recycling, Division of Sustainable Resources Engineering, Faculty of Engineering, Hokkaido University, Japan for sample preparation. The authors also wish to thank the editor and reviewers for their valuable inputs to this paper.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. A schematic illustration of agglomeration-flotation setup used in this study.
Figure 1. A schematic illustration of agglomeration-flotation setup used in this study.
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Figure 2. Examples of (a) 8-bit image and (b) Image J adjusted image of MPs in the flotation cell after the conditioning step.
Figure 2. Examples of (a) 8-bit image and (b) Image J adjusted image of MPs in the flotation cell after the conditioning step.
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Figure 3. The relationships between kerosene dosage and (a) microplastic removal rate and (b) median particle size of microplastics.
Figure 3. The relationships between kerosene dosage and (a) microplastic removal rate and (b) median particle size of microplastics.
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Julapong, P.; Ekasin, J.; Katethol, P.; Srichonphaisarn, P.; Juntarasakul, O.; Numprasanthai, A.; Tabelin, C.B.; Phengsaart, T. Agglomeration–Flotation of Microplastics Using Kerosene as Bridging Liquid for Particle Size Enlargement. Sustainability 2022, 14, 15584. https://doi.org/10.3390/su142315584

AMA Style

Julapong P, Ekasin J, Katethol P, Srichonphaisarn P, Juntarasakul O, Numprasanthai A, Tabelin CB, Phengsaart T. Agglomeration–Flotation of Microplastics Using Kerosene as Bridging Liquid for Particle Size Enlargement. Sustainability. 2022; 14(23):15584. https://doi.org/10.3390/su142315584

Chicago/Turabian Style

Julapong, Pongsiri, Jiraphon Ekasin, Pattaranun Katethol, Palot Srichonphaisarn, Onchanok Juntarasakul, Apisit Numprasanthai, Carlito Baltazar Tabelin, and Theerayut Phengsaart. 2022. "Agglomeration–Flotation of Microplastics Using Kerosene as Bridging Liquid for Particle Size Enlargement" Sustainability 14, no. 23: 15584. https://doi.org/10.3390/su142315584

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