Efficient recovery of microplastics from sediments by foam flotation

Efficient recovery of microplastics from sediments by foam flotation | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Efficient recovery of microplastics from sediments by foam flotation Jinghui Wu, Yidi Gao, Guanqiao Li, Xinyuan Li, Yiming Wang, Mingxin Huo, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8909428/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Sediments play a pivotal role in the ecological cycle of microplastics, serving as both a hub for their aggregation and accumulation, and potentially as a source and distributor of these particles. This dual function underscores the importance of sediment analysis in understanding the environmental distribution and origins of microplastics. To comprehensively evaluate microplastic presence and contamination levels in sediments, the development of an efficient and standardized separation method is imperative. Hence, this study introduces an integrated froth flotation approach that has been successfully applied to separate microplastics from various substrates including surface water, wastewater treatment plant sludge, and soil. Compared to conventional froth flotation-based methods, this novel approach yields significantly higher recovery rates of microplastics. Notably, the use of a fully biodegradable surfactant ensures pollution-free separation and achieves recovery rates nearing 100%. Moreover, the study delves into the interaction mechanism between surfactants and plastic particles, particularly focusing on the adsorption process between anionic and cationic surfactants and microplastics. These experimental findings are pivotal in advancing microplastic separation and extraction technologies, guiding future research in this critical area. Microplastics Sediment Integration Foam flotation No contamination Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 1 Introduction Since the inception of plastic products, global plastic production has witnessed exponential growth. Since the 1950s, global plastic production has experienced a spectacular surge—rising from 1.5 million tons back then to an incredible 390.7 million tons by 2021(Shukla, et al., 2024 ). However, a concerning trend has emerged: studies reveal that roughly 80% of plastics ultimately find their way into landfills or the natural environment, many becoming ensnared in various sediment types. Microplastics, characterized by their diminutive size of less than 5 millimeters, are ubiquitously dispersed throughout all sediment varieties. With their minute particle size, expansive specific surface area, and pronounced hydrophobic properties, microplastics exhibit a propensity to adsorb organic pollutants, heavy metals, and potentially pathogenic microorganisms, thereby escalating environmental and ecosystem apprehensions (Liang, et al., 2025 , Priya, et al., 2025, Xiong, et al., 2025 , Jiang, et al., 2025 , Kärkkäinen, et al., 2025 ). Organisms within the sediment and detritus food chain are susceptible to mistakenly ingesting microplastics, thereby amplifying the likelihood of microplastic accumulation within organisms and intensifying the bioconcentration effect within the food chain. This occurrence can potentially induce toxic effects in animals following ingestion, consequently culminating in indirect ramifications for humans (Du, et al., 2025 ). Simultaneously, improper disposal of sludge, coupled with the infiltration of microplastics into soil ecosystems through land use practices, along with the erosion of shoreline microplastics by rivers and oceans, inevitably results in the deposition of these contaminants within sediments. With continued accumulation exacerbated by factors like rainwater infiltration, these contaminants pose a risk of contaminating groundwater reservoirs. These challenges have grown increasingly pronounced and harbor the potential to instigate broader environmental hazards (Sforzi, et al., 2025 , Narayanan, 2023 ). In order to comprehensively and efficiently study microplastics in sediments, it is important to develop a uniform and standardized method for separating microplastics, as there is currently a lack of consistency in the means of separating and extracting microplastics. Therefore, it is crucial to establish an efficient and clean method for separating microplastics in order to analyze the content, type and distribution of microplastics. Currently, methods for separating microplastics in sediments can be broadly categorized into three groups: density separation, oil separation and air flotation (Lechthaler, et al., 2020 , Wu, et al., 2022 , Joshua, et al., 2025 , Yuan, et al., 2025 , Crutchett, et al., 2024). Density separation is usually accomplished by adding saturated brine to the sediment sample, which after thorough mixing is allowed to stand, and then the upper solution is collected. Without considering microplastic attachment, the density of microplastics is usually in the range of 0.8–1.4 g/cm 3 , while the density of sediments is usually around 2.6 g/cm 3 . This method utilizes density differences in order to enable the extraction of microplastics from sediment samples. However, since saturated sodium chloride has a density of 1.19 g/cm³, the separation is relatively ineffective for denser microplastic particles (e.g., PVC, with a density of 1.35–1.39 g/cm 3 ) (Shaw, et al., 2024 , Duong, et al., 2022 ). In addition, some studies have attempted to use saturated sodium iodide, zinc chloride and other salt solutions for density separation of microplastics. However, the insufficiently stable nature of these solutions, along with their high toxicity, has somewhat limited their application. In addition, issues such as the economic costs associated with these methods need to be considered(Crutchett, et al., 2024). The oil separation method mainly utilizes the hydrophobicity and lipophilicity of microplastics by mixing sediment samples with oil and separating the microplastics by oil-water separation. However, this method requires further separation of the oil from the microplastics, which may affect the subsequent characterization of the microplastics and make the separation step relatively complex (Zhang, et al., 2020 , Bellasi, et al., 2021 ). The main principle of air flotation is to use air bubbles to suspend microplastics to the surface of the flotation solution. Microplastics have a special buoyancy due to their small density and hydrophobicity, and when the bubbles are introduced into the water, the hydrophobic interaction between the bubbles and the microplastics causes the microplastics to be carried to the surface of the solution. Subsequently, microplastics floating on the surface of the solution can be collected by overflows. Claessens (Claessens, et al., 2013 ) produced flotation columns to separate microplastics from sediments in 2013, X. Zhu (Zhu, 2015 ) made improvements in the flotation column in 2015, but the results were not as good as expected, especially for the higher density microplastics, the recovery rate only reached about 52% or so. In these experiments, they used flotation columns made of PVC and added a salt solution of surfactant to the flotation solution, but did not reveal more specific details. The use of PVC flotation columns may be a key reason for the low recoveries, as we have found that microplastics are difficult to separate from the flotation column in solution due to hydrophobic interactions when using plastic flotation columns. Imhof (Imhof, et al., 2012 ) also attempted to use froth flotation to separate microplastics from sediments, but the recovery rate was only about 55% or so for plastics 1–5 mm in size. Unlike traditional methods, this study designed an integrated froth flotation method that can accomplish the separation and collection of microplastics at the same time. The flotation column was made of hydrophilic glass material and a hydrophilic glass pipe in the shape of an arch bridge was designed at the top to minimize the adhesion problem between microplastics and the flotation column. In addition, previous studies have often failed to explore in depth the interaction mechanisms between surfactants, bubbles, and microplastics. However, for the first time, we have conducted an in-depth analysis of the flotation mechanism between different types of surfactants, gas bubbles and microplastics, which provides a theoretical foundation and experimental basis for the application of microplastic flotation separation technology. This innovation is expected to bring important progress in the field of microplastics research. 2 Materials and methods 2.1 Experimental materials A The experimental samples were prepared as shown in Table 1 . After the sandy soil was washed repeatedly, 100 microplastics of 500 µm, 3 mm and 5 mm were mixed with 5 g of sandy soil to prepare sediment samples. Table 1 Grain size, color, density and raw materials of self-made microplastics in the recovery rate experiment of different plastic types Polymer Particle size Color Materials Density PET 5 00 µm 3 mm 5 mm Colourless Drinking water bottle 1.36–1.40 g/cm 3 PVC 5 00 µm 3 mm 5 mm Blue PVC sheet 1.35–1.39 g/cm 3 PS 5 00 µm 3 mm 5 mm Pink Spoon 1.14–1.15 g/cm 3 In the previous experiments, we compared the effects of density separation, oil separation and foam flotation on the separation of microplastics in sediments, and the results of the experiments showed that a small amount of separation of microplastics could be achieved by using a high-density solution in the density separation process (Fig. S1 ). Subsequent optimization experiments also indicate that density separation can be applied to extract and separate low-density microplastics in the laboratory, but is not suitable for large-scale separation of microplastics (Fig. S2). The oil separation experiments showed that the separation efficiency was increased by the addition of oil (Fig. S3), but the efficiency was increased to about 99.5% after the introduction of air flotation separation in this system (Fig. S4), so this method can be used to investigate the abundance of microplastics, and it is not suitable for the qualitative study of microplastics in the environment. The separation efficiency of froth flotation for large density microplastics reaches almost 100% (Fig. S5), and the separation step can be completed in less than two minutes. Alkyl polyglucoside (APG), as a kind of green and completely biodegradable surfactant, guarantees the recovery rate in the flotation process without pollution and with high efficiency, which can satisfy the quantitative analysis without affecting the qualitative analysis (Fig. S6). Therefore, the froth flotation separation method shows obvious advantages for the separation of microplastics in sediments. The following is the specific experimental procedure: The prepared sediment samples were placed in the air-float column and then 500 mL of APG solution of different concentrations were added. Air was passed through the air pump for aeration, and with the help of hydrophobic interactions between microplastics and air bubbles, the air bubbles carried the microplastics to float up to the surface of the solution and formed bubbles. Subsequently, the foam was passed through a 0.45 µm pore size filter membrane by means of foam overflow, and then the recovery and uplift rate of microplastics were observed and calculated by the naked eye (Fig. 1 ). 2.2 Experimental methods 2.2.1 Exploratory experiments on the effect of air flow rate on the flotation effect of microplastics The prepared sediment samples were placed in the air flotation column at room temperature, and 500 mL of 0.05 g/L APG 0810 solution was added. The value of the aeration flow rate was changed to study the effect on the flotation effect of microplastics, and the completion of the flotation of microplastics within 5 s was used as the evaluation standard. 2.2.2 Effect of different types of APG concentration on the flotation effect of microplastics In order to better assess the wetting effect between APG and microplastics, the time required for microplastics to rise to the upper layer of the liquid surface in the liquid was used as an evaluation index. When the required time exceeded 5 s, the APG concentration was considered to inhibit microplastic flotation, i.e., wetting. One hundred each of 500 µm, 3 mm, and 5 mm PVC, PET, and PS plastics were added to the separation device, respectively, and aeration was performed by passing 200 mL/min of air. The APG concentration was varied until the foam no longer overflowed. When the number of microplastics present in the liquid was 0, it was recorded as all the microplastics floated, and the time required for the microplastics to float was counted. 2.2.3 Effect of different types of surfactants on the flotation effect of microplastics In order to better assess the wetting effect between APG and microplastics, the time required for microplastics to rise in the liquid to the upper level of the liquid surface was used as an evaluation index. When the time required for uplift exceeded 5 s, the APG concentration was considered to inhibit microplastic flotation as wetting. One hundred each of 5 mm PVC, PET and PS plastics were added to the separation unit and aeration was performed by passing 200 mL/min of air. The concentration of APG0814 was varied until the foam no longer overflowed. When the number of microplastics present in the liquid was 0, it was recorded as all the microplastics floated, and the time taken for the microplastics to float was counted. 3 Results and discussion 3.1 Microplastic Contact Angle Test In order to deeply investigate the interaction mechanism between microplastic particles and surfactants, the contact angle analysis test on the plastic surface was carried out first. The contact angle of various types of microplastic particles was analyzed and tested in detail using a KRUSS DSA255 standard contact angle meter, and the test results are shown in Table 2 and Fig. 2 . Table 2 Contact angle size of different plastic types Polymer PET PVC PS Contact angle/° 99.1 95.1 89.8 3.2 Influence of air flow rate on flotation effect From Fig. 3 , it can be seen that the air flow rate has a certain effect on the flotation effect, with the increase of air flow rate, the uplift of microplastics in 5 s is the first to rise and then fall. The size of air flow affects the volume of bubbles in the flotation column, the distribution density and the degree of plastic tumbling. When the air flow rate is small, the air bubbles in the flotation column are more stable, the distribution density is small, the turbulence degree is small, and the degree of plastic tumbling is small, which will lead to the microplastics not being able to float in time. When the air flow increases, the more bubbles, the greater the distribution density, the greater the degree of turbulence, the greater the degree of plastic tumbling. This will lead to the plastic that has been floated and perhaps washed down by the tumbling bubbles, resulting in a decrease in the floating rate; and in the aeration flow rate of 150–800 mL/min, the floating rate of microplastics is the highest. In order to facilitate the follow-up test, it was decided to choose 200 mL/min ventilation flow rate as the experimental gas flow rate. 3.3 Effect of APG concentration on the flotation effect of microplastics During froth flotation, APG concentration influences the amount of froth and thus the recovery rate of microplastics separation. In order to investigate the minimum concentration of various types of APG surfactants for flotation of microplastics, 200 mL/min air was passed through for aeration. One hundred 5 mm plastics of each type were added to the separation device and aeration was carried out by passing 200 mL/min of air. During this process, the air bubbles attached to microplastics floated up to the upper layer of the solution and overflowed with the foam. With the increasing concentration, the ability of bubbles to carry microplastics gradually decreased. As can be seen from Fig. 4 , before the concentrations of 0.05 g/L, 0.1 g/L and 0.5 g/L for APG1214, APG0814 and APG0810, respectively, the floating process of 5 mm microplastics could be completed within 5 s. In other words, its ability to float 5 mm microplastics could be completed in the same time. That is to say, its inhibition ability of 5 mm microplastic flotation recovery is: APG1214>APG0814>APG0810, and no difference was found between the inhibition points of APG solution on the flotation of three types of microplastics: PVC, PET and PS. However, the time for PS to complete upflotation was lower than that of PET and PVC, which may be due to the lower density of PS. Plastic flotation methods can be roughly categorized into the following three types: chemically regulated flotation, physically regulated flotation and γ flotation. Theoretically, when the flotation system in the liquid surface tension value is located in the two different plastic materials between the wetting critical surface tension value, the flotation effect is optimal. The plastic with large surface tension will be wetted by the liquid or inhibited from floating, while the plastic with small surface tension value will not be wetted or not inhibited from floating, thus realizing plastic flotation separation, which is γ flotation separation. However, in the course of this experiment, no significant difference was observed in the flotation inhibition of different types of microplastics by APG concentration. This may be due to the fact that the contact angles of the selected microplastic particles are similar and have not reached a point where a difference in wetting can be produced. One hundred 500 µm, 3 mm plastics of each type were added to the separation device and aeration was performed by passing 200 mL/min air. During this process, according to the results in Fig. 5 , it can be observed that the flotation effect is similarly suppressed for smaller particle size microplastics as the APG concentration increases. Compared to the 5 mm plastic particles, in all three types of APG solutions, the flotation inhibition for 500 µm and 3 mm plastic particles occurs only after a certain concentration is reached. Specifically, flotation inhibition of 500 µm microplastics occurred only after APG concentrations reached 0.2 g/L, 0.3 g/L, and 0.9 g/L, respectively, whereas inhibition of 3 mm microplastics occurred only after concentrations reached 0.1 g/L, 0.2 g/L, and 0.7 g/L, respectively. This suggests that there is a difference in the effect of microplastic particle size on APG concentration, with smaller particle size microplastics being more susceptible to inhibition by APG solutions with lower concentrations. For the inhibition of microplastic flotation, we suspect that it is related to the critical micelle concentration (CMC) of the surfactant. Typically, conventional surfactants have a special molecular structure in which hydrophilic and hydrophobic portions are interconnected when their concentration reaches a certain level, resulting in the formation of a self-assembled micelle structure. The minimum concentration of this self-assembled structure is known as the critical micelle concentration, i.e., the CMC value of the surfactant (Li, et al., 2025 ). The presence of this micellar structure may lead to complete wetting of the microplastics, thus affecting their flotation performance. The commonly used methods for the determination of CMC are: conductivity method (Silva, et al., 2025) and surface tension method (Jiang, et al., 2025 ). The surface tension method was used in this procedure to test the CMC values of different surfactants. The critical micellar concentration of the surfactant was determined to have been reached when its surface tensi on no longer decreased as the surfactant concentration increased. Subsequently, the surface tension of the three APG surfactants was tested and plotted in Fig. 6 . It can be seen from the figure that the critical micelle concentrations of APG1214, APG0814 and APG0810 were near 0.04 g/L, 0.2 g/L and 1.4 g/L, respectively. In terms of the inhibited concentration of microplastic flotation, the inhibition of flotation of 500 µm microplastics occurred only after the APG concentration reached 0.2 g/L, 0.3 g/L and 0.9 g/L, respectively. Based on these results, we hypothesize that the CMC concentration of surfactant may not be significantly related to the inhibition of microplastic flotation. In order to analyze the mechanism of surfactant flotation for microplastic wetting, the indicators of the three APG surfactants were plotted, as shown in Table 3 . Among them, HLB value represents the hydrophilic-hydrophobic balance of the surfactant, which is used to measure the balance of size and strength between the polar and nonpolar groups of the surfactant, thus determining the degree of lipophilicity or hydrophilicity of the surfactant (Chen, et al., 2024 ). A larger HLB value represents more hydrophilicity, and a smaller HLB value represents more lipophilicity. In order to minimize the experimental error, APG solutions of three anionic surfactants were selected. In the flotation process of microplastics interacting with APG, plastics possessing different groups showed insignificant differences in the wetting process. This may indicate that certain groups between different microplastic surfaces and APG surfactant molecules do not differ much in terms of hydrogen bonding and dispersion forces. On the contrary, the HLB value of APG largely influences the wettability of microplastics. Therefore, we can conclude that the main interaction between microplastics and APG surfactants is the hydrophobic interaction force. Without considering factors such as electrostatic forces, the smaller the HLB value, the stronger the lipophilicity and the stronger the wettability of microplastics. Table 3 Performance indexes of different APG solutions Index APG1214 APG0814 APG0810 Exterior Light yellow paste Light yellow liquid Light yellow liquid pH (50% aqueous solution) 11.5–12.5 11.5–12.5 11.5–12.5 CMC 0.04 0.2 1.4 HLB 10–12 13–15 15–17 structural formula C 18 H 36 O 6 (C 6 H 11 O 5 )nOR C 16 H 32 O 6 However, it was experimentally found that the wettability of APG-type surfactants on large particle size microplastics was significantly higher than that on small particle size microplastics, which might be related to the specific surface area of microplastics. The specific surface area of microplastics increases as the particle size decreases, and a larger specific surface area indicates a stronger adsorption capacity, which also means that microplastics with smaller particle sizes are more difficult to be wetted. In addition, smaller microplastics require less time in the upwelling process, which may be related to the mass of the microplastics. At the same level of turbulence, smaller mass microplastics are more susceptible to turbulence and thus tumble to the upper layers of the liquid to achieve separation. These factors together influence the effectiveness of microplastic flotation, such that microplastics with different particle sizes show different responses to the wetting of APG surfactants (Fig. 7 ). 3.4 Effect of different types of surfactants on the flotation effect of microplastics Having established that the HLB value of the surfactant was inversely related to the wettability of the microplastics, the effect of other types of surfactants on the flotation of 5 mm PVC was further tested. These surfactants included the biodegradable sophorolipid, the cationic surfactant cetyltrimethylammonium bromide (CTAB), and the anionic surfactants sodium dodecyl sulfate (SDS) and sodium dodecyl sulfate (SDBS). The experimental results are shown in Fig. 8 and Table 4 below. From the experimental results, it can be seen that different types of surfactants affected the flotation effect of microplastics to different degrees. This further verifies our previous conclusion that the wettability of surfactants is inversely related to their HLB values. The biodegradable acacia glycolipids showed strong wettability, similar to APG surfactants, which was favorable to the flotation effect of microplastics. On the other hand, the cationic surfactant CTAB and the anionic surfactants SDS and SDBS showed poor wettability and inhibited the flotation effect of microplastics. Table 4 Different surfactants HLB and their inhibitory flotation concentration on 5 mm microplastics Index Sophora glycolipid SDBS APG1214 APG0814 APG0810 CTAB SDS HLB 9–12 10.638 10–12 13–15 15–17 15.8 40 Inhibition concentration (g/L) 0.05 0.8 0.05 0.1 0.5 0.1 1.0 Inhibition concentration (mol/L) 7.26×10 − 5 2.29×10 − 3 7.17×10 − 5 1.43×10 − 4 6.48×10 − 4 2.74×10 − 4 3.67×10 − 3 As can be seen in Table 4 , the HLB values of sophorolipid and APG1214 are similar. However, although the inhibited microplastic flotation concentration of sophorolipid was low (7.26 × 10 − 5 mol/L), the amount of foam generated by sophorolipid was not sufficient to support the upward flotation of microplastics before the inhibited flotation concentration was reached. This suggests that not all surfactants are suitable for the flotation of microplastics, and that the generation of sufficient foam volume and stability to support microplastic flotation also needs to be considered. Therefore, the acacia glycolipid solution cannot be used as a froth flotation agent for microplastics. On the other hand, the cationic surfactant cetyltrimethylammonium bromide (CTAB) had an HLB value of 15.8, and its inhibition of microplastic flotation concentration ranged from 2.74 × 10 − 4 mol/L. This indicates that CTAB has certain wettability in microplastic flotation, but requires a higher inhibition flotation concentration. The anionic surfactant sodium dodecyl sulfate (SDS) has a lower HLB value, but its inhibited flotation concentration is between 2.29 × 10 − 3 mol/L. This suggests that the wettability is not only related to the HLB value, but may also be affected by the chemical and charge properties of the surfactant. CTAB is a cationic surfactant, which exists mainly in the cationic form in solution. Microplastics are usually negatively charged and may be electrostatically charged in solution, enhancing wettability. On the contrary, SDS is an anionic surfactant, which exists mainly in anionic form in solution and may lead to electrostatic repulsion, reducing the wettability. As can be seen from Fig. 9 , the surface of the bubbles is negatively charged, while the surface of the microplastics is also negatively charged. Therefore, adsorption of surfactant occurs on both the bubble and microplastic surfaces. Sun(Sun, et al., 2022 ) Studies have shown that the adsorption effect between cationic surfactants and microplastics is optimal in a certain concentration range. However, these studies mainly focused on the interaction between surfactants and microplastics without considering the interaction between surfactants and air bubbles. In mineral flotation, minerals are usually positively charged, unlike microplastics. We refer to the flotation mechanism in mineral flotation and make an analogy. Combined with our experimental results, we found that when a cationic surfactant is added, the surfactant enhances the adsorption of microplastics and bubbles onto it due to the presence of electrostatic adsorption, leading to a weakening of the hydrophobicity of bubbles and microplastics, and thus the hydrophobic interaction force between them. On the contrary, when anionic surfactant is added, the adsorption of microplastics and bubbles to the surfactant is weakened, which has less effect on the hydrophobicity of bubbles and microplastics. This explains why the wettability of anionic surfactants is lower than that of cationic surfactants. Overall, microplastic flotation still proceeds using hydrophobic interactions with the addition of surfactant, but the adsorption of surfactant affects the hydrophobic interactions between bubbles and microplastics. 4 Conclusion In this study, we devised a froth flotation separation apparatus and meticulously examined the interactions between various surfactants and microplastics. The flotation solution utilizing APG showcased a plethora of attributes such as non-toxicity, harmlessness, non-irritation, complete biodegradability, and cost-effectiveness, rendering it highly conducive for large-scale microplastic separation and extraction. Our findings hold substantial guiding significance for advancing microplastic separation and extraction technologies, leading to the following conclusions: The concentration of APG inversely affects the adherence of bubbles to microplastics. This phenomenon stems from APG's wetting effect on microplastics, transitioning them from hydrophobic to hydrophilic states, consequently diminishing the adherence of air bubbles to plastics and impeding microplastic flotation. Microplastic particle size exerts a discernible influence on froth flotation. Diminutive microplastic particles exhibit augmented specific surface areas and heightened adsorption capacities. However, their reduced wettability poses lesser hindrance in the flotation process. Moreover, microplastic mass influences their behavior in turbulence, consequently impacting flotation outcomes. Not all surfactant types prove suitable for microplastic flotation. It is imperative to ensure that surfactants generate ample foam volume and sustain foam stability to facilitate microplastic flotation without inhibiting the process. The interaction mechanism between surfactants and the solid matrix is intricate, with diverse surfactant types possibly engaging distinct solid particle interaction mechanisms. Excluding other forces, surfactant wettability on plastics correlates with their HLB values, where higher HLB values may yield lower plastic wettability, thereby potentially aiding flotation. Given microplastics' typically negative charge in environmental settings, electrostatic effects influence their interaction with various surfactants. Cationic surfactants may enhance microplastic wettability through electrostatic adsorption, while anionic surfactants may yield contrasting effects. Declarations Credit authorship contribution statement Jinghui Wu : Investigation, Formal analysis, Resources, Writing - Original Draft. Yidi Gao : Investigation, Formal analysis. Guanqiao Li : Investigation, Formal analysis. Xinyuan Li : Investigation. Yiming Wang: Investigation. Mingxin Huo : Resources, Supervision. Xianze Wang : Project administration, Resources, Methodology. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Ethical Approval Not applicable. Availability of data and materials All data generated or analyzed during this study are included in this published article. Funding This work was supported by the National Natural Science Foundation of China (52270037, 52170027 and U20A20322), National Key Research and Development Program of China (2024YFC3907804-3), State Key Laboratory of Water Pollution Control and Green Resource Recycling Foundation (NO. PCRRF25038). References S. Shukla, Y. Pei, W.G. Li, D.S. Pei. (2024). 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Process Safety and Environmental Protection , 164 , 291–298. https://doi.org/10.1016/j.psep.2022.06.014 A.M. Joshua, P.A.D. Aznir, E. Von Lau. (2025). Understanding microplastic flotation through microbubble-microplastic interactions. Surfaces and Interfaces , 64 , 106399. https://doi.org/10.1016/j.surfin.2025.106399 F.Y. Yuan, H. Yuan, C.L. Dai, W. Yu, J.Y. Du, X.J. Yang, D.X. Wang. (2025). Study on the process of microplastic fiber attachment to a rising microbubble-Implications for flotation separation. Journal of Environmental Chemical Engineering , 13 (3), 116819. https://doi.org/10.1016/j.jece.2025.116819 T.W. Crutchett, K.R. Bornt. (2024). A simple overflow density separation method that recovers > 95% of dense microplastics from sediment. MethodsX , 12 , 102638. https://doi.org/10.1016/j.mex.2024.102638 K.R. Shaw, R. Sandquist, C. Fairclough, J. Black, A. Fitzgerald, J.T. Shaw, S. Gallager, J. Lynch. (2024). Separation of microplastics from deep-sea sediment using an affordable, simple to use, and easily accessible density separation device. Microplastics and Nanoplastics , 4 (16). https://doi.org/10.1186/s43591-024-00093-7 T.T. Duong, P.T. Le, T.N.H. Nguyen, T.Q. Hoang, H. Ngo, T.O. Doan, T.P.Q. Le, H.T. Bui, M.H. Bui, V.T. Trinh, T.L. Nguyen, N. Da Le, T.M. Vu, T.K.C. Tran, T.C. Ho, N.N. Phuong, E. Strady. (2022). Selection of a density separation solution to study microplastics in tropical riverine sediment. Environmental Monitoring and Assessment , 194 (2). https://doi.org/10.1007/s10661-021-09664-0 B. Zhang, L. Chen, J.Y. Chao, X. Yang, Q. Wang. (2020). Research Progress of Microplastics in Freshwater Sediments in China. Environmental Science and Pollution Research , 27 , 31046–31060. https://doi.org/10.1007/s11356-020-09473-x A. Bellasi, G. Binda, A. Pozzi, G. Boldrocchi, R. Bettinetti. (2021). The extraction of microplastics from sediments: An overview of existing methods and the proposal of a new and green alternative. Chemosphere , 278 , 130357. https://doi.org/10.1016/j.chemosphere.2021.130357 M. Claessens, L. Van Cauwenberghe, M.B. Vandegehuchte, C.R. Janssen. (2013). New techniques for the detection of microplastics in sediments and field collected organisms. Marine Pollution Bulletin , 70 (1–2), 227–233. https://doi.org/10.1016/j.marpolbul.2013.03.009 X. Zhu. (2015). Optimization of elutriation device for filtration of microplastic particles from sediment. Marine Pollution Bulletin , 92 (1–2), 69–72. https://doi.org/10.1016/j.marpolbul.2014.12.054 H.K. Imhof, J. Schmid, R. Niessner, N.P. Ivleva, C. Laforsch. (2012). A novel, highly efficient method for the separation and quantification of plastic particles in sediments of aquatic environments. Limnology and Oceanography-Methods , 10 (7), 524–537. https://doi.org/10.4319/lom.2012.10.524 H. Li, X.Y. Yu, Z.H. Fu, K. Qiu, J.Y. Wang, S.X. Lu. (2025). Kinetics and dynamics of Gas-liquid separation and bubble generation in surfactant solutions: Role of bulk/interfacial properties and hydrodynamic conditions. Separation and Purification Technology , 355 , 129483. https://doi.org/10.1016/j.seppur.2024.129483 L.M.S. Silva, J.J. Galan-Díaz. (2025). Comparative analysis of critical micelle concentration of cationic surfactants determined by conductivity, sound velocity, and density using weighted orthogonal distance regression. Surfaces and Interfaces , 56 , 105620. https://doi.org/10.1016/j.surfin.2024.105620 B.Y. Jiang, G.Y. Gao, X.H. Wang, B. Ren, S.L. Shi, J. Wang. (2025). The effect of various anionic types of inorganic salts on the wettability of solutions of surfactants. Process Safety and Environmental Protection , 200 , 107367. https://doi.org/10.1016/j.psep.2025.107367 Y. Chen, J.T. Petkov, K. Ma, P. Li, J.R.P. Webster, J. Penfold, R.K. Thomas, J. Allgaier, R. Dalgliesh, G. Smith. (2024). Manipulating the hydrophilic / hydrophobic balance in novel cationic surfactants by ethoxylation: The impact on adsorption and self-assembly. Journal of Colloid and Interface Science , 674 (15), 405–415. https://doi.org/10.1016/j.jcis.2024.06.174 H.M. Sun, S. Zhou, Y.J. Jiang, Z.M. Qin, J. Fei, Y.B. Sun, J. Wang, X.Q. Yin. (2022). Effect of cationic, anionic and non-ionic surfactants on transport of microplastics: Role of adhesion of surfactants on the polyethylene surface. Journal of Hydrology , 612 , 128051. https://doi.org/10.1016/j.jhydrol.2022.128051 Additional Declarations No competing interests reported. Supplementary Files SI.docx Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. 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12:38:51","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-8909428/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-8909428/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":105204709,"identity":"093b4991-a1f1-4dc1-ad48-8d0d014c6894","added_by":"auto","created_at":"2026-03-23 12:30:57","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":122487,"visible":true,"origin":"","legend":"\u003cp\u003eFoam flotation device\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-8909428/v1/0306a72d2e0e9b1538616969.png"},{"id":105564205,"identity":"8b8b4fe1-ef06-4ab2-ba50-0d40f9220c8c","added_by":"auto","created_at":"2026-03-27 12:48:59","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":155746,"visible":true,"origin":"","legend":"\u003cp\u003eContact angle test chart for different types of microplastics\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-8909428/v1/3243678d23dad02dc15771e2.png"},{"id":105563750,"identity":"2d1705dc-8dbe-42ee-ace8-45a7744661d6","added_by":"auto","created_at":"2026-03-27 12:47:42","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":122031,"visible":true,"origin":"","legend":"\u003cp\u003eInfluence of gas flow rate on flotation effect\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-8909428/v1/d282e12e52e49614b3f95628.png"},{"id":105204710,"identity":"259bea75-0d6f-4778-8621-434ac92d48fc","added_by":"auto","created_at":"2026-03-23 12:30:57","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":180808,"visible":true,"origin":"","legend":"\u003cp\u003eRelationship between concentration of different types of APG and recovery rate of microplastics\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-8909428/v1/7c41482c7c2e018fbedf7212.png"},{"id":105204718,"identity":"d87affca-eeb5-47f7-b038-26cc4031d54f","added_by":"auto","created_at":"2026-03-23 12:30:58","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":98702,"visible":true,"origin":"","legend":"\u003cp\u003eRelationship between APG concentration and floating time of 5 mm microplastics\u003c/p\u003e\n\u003cp\u003e(A: 5 mm PVC; B: 5 mm PET; C: 5 mm PS)\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-8909428/v1/ecd9a75299c538bb57b2d669.png"},{"id":105204714,"identity":"5588ac19-bfc8-4c69-ad16-4de9f31c1a50","added_by":"auto","created_at":"2026-03-23 12:30:58","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":165649,"visible":true,"origin":"","legend":"\u003cp\u003eRelationship between APG concentration and floating time of 3 mm and 500 μm microplastics\u003c/p\u003e\n\u003cp\u003e(A\u003csub\u003e1\u003c/sub\u003e: 3 mm PVC; A\u003csub\u003e2\u003c/sub\u003e: 500 μm PVC; B\u003csub\u003e1\u003c/sub\u003e: 3 mm PET; B\u003csub\u003e2\u003c/sub\u003e: 500 μm PET; C\u003csub\u003e1\u003c/sub\u003e: 3 mm PS; C\u003csub\u003e2\u003c/sub\u003e: 500 μm PS)\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-8909428/v1/57e84a0f959ce42366ae2895.png"},{"id":105204715,"identity":"183e891b-1ed9-46ec-a7e6-8089df1adbd2","added_by":"auto","created_at":"2026-03-23 12:30:58","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":136558,"visible":true,"origin":"","legend":"\u003cp\u003eSurface tension coefficient of different APG concentrations\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-8909428/v1/024ab06c33961a1798a4ba32.png"},{"id":105204713,"identity":"73d8577c-69a0-4735-b17c-e194a37f9e67","added_by":"auto","created_at":"2026-03-23 12:30:58","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":244250,"visible":true,"origin":"","legend":"\u003cp\u003eEffect of different surfactant concentrations on the floating time of 5 mm PVC\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-8909428/v1/21d44ad8fb38eb5800cf135e.png"},{"id":105563698,"identity":"a083ef93-6f15-4721-80fa-e641b2fa6b5b","added_by":"auto","created_at":"2026-03-27 12:47:29","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":65806,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic diagram of interaction between microplastics and bubble\u003c/p\u003e","description":"","filename":"9.png","url":"https://assets-eu.researchsquare.com/files/rs-8909428/v1/009defe5c9d9bd0a4aaa1b79.png"},{"id":108644905,"identity":"5726b620-40ab-485a-89cf-72315afb32d1","added_by":"auto","created_at":"2026-05-06 22:09:37","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1490770,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8909428/v1/23ab865c-b62e-4f00-a4c0-7505b47a11b9.pdf"},{"id":105204719,"identity":"c0e886b8-6fff-4728-b9e9-8fcd09e30282","added_by":"auto","created_at":"2026-03-23 12:30:58","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":10871894,"visible":true,"origin":"","legend":"","description":"","filename":"SI.docx","url":"https://assets-eu.researchsquare.com/files/rs-8909428/v1/aa09e2e4f26c3c3feed7e16b.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Efficient recovery of microplastics from sediments by foam flotation","fulltext":[{"header":"1 Introduction","content":"\u003cp\u003eSince the inception of plastic products, global plastic production has witnessed exponential growth. Since the 1950s, global plastic production has experienced a spectacular surge\u0026mdash;rising from 1.5\u0026nbsp;million tons back then to an incredible 390.7\u0026nbsp;million tons by 2021(Shukla, et al., \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). However, a concerning trend has emerged: studies reveal that roughly 80% of plastics ultimately find their way into landfills or the natural environment, many becoming ensnared in various sediment types. Microplastics, characterized by their diminutive size of less than 5 millimeters, are ubiquitously dispersed throughout all sediment varieties. With their minute particle size, expansive specific surface area, and pronounced hydrophobic properties, microplastics exhibit a propensity to adsorb organic pollutants, heavy metals, and potentially pathogenic microorganisms, thereby escalating environmental and ecosystem apprehensions (Liang, et al., \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2025\u003c/span\u003e, Priya, et al., 2025, Xiong, et al., \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2025\u003c/span\u003e, Jiang, et al., \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2025\u003c/span\u003e, K\u0026auml;rkk\u0026auml;inen, et al., \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2025\u003c/span\u003e). Organisms within the sediment and detritus food chain are susceptible to mistakenly ingesting microplastics, thereby amplifying the likelihood of microplastic accumulation within organisms and intensifying the bioconcentration effect within the food chain. This occurrence can potentially induce toxic effects in animals following ingestion, consequently culminating in indirect ramifications for humans (Du, et al., \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2025\u003c/span\u003e). Simultaneously, improper disposal of sludge, coupled with the infiltration of microplastics into soil ecosystems through land use practices, along with the erosion of shoreline microplastics by rivers and oceans, inevitably results in the deposition of these contaminants within sediments. With continued accumulation exacerbated by factors like rainwater infiltration, these contaminants pose a risk of contaminating groundwater reservoirs. These challenges have grown increasingly pronounced and harbor the potential to instigate broader environmental hazards (Sforzi, et al., \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2025\u003c/span\u003e, Narayanan, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2023\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eIn order to comprehensively and efficiently study microplastics in sediments, it is important to develop a uniform and standardized method for separating microplastics, as there is currently a lack of consistency in the means of separating and extracting microplastics. Therefore, it is crucial to establish an efficient and clean method for separating microplastics in order to analyze the content, type and distribution of microplastics. Currently, methods for separating microplastics in sediments can be broadly categorized into three groups: density separation, oil separation and air flotation (Lechthaler, et al., \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2020\u003c/span\u003e, Wu, et al., \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2022\u003c/span\u003e, Joshua, et al., \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2025\u003c/span\u003e, Yuan, et al., \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2025\u003c/span\u003e, Crutchett, et al., 2024). Density separation is usually accomplished by adding saturated brine to the sediment sample, which after thorough mixing is allowed to stand, and then the upper solution is collected. Without considering microplastic attachment, the density of microplastics is usually in the range of 0.8\u0026ndash;1.4 g/cm\u003csup\u003e3\u003c/sup\u003e, while the density of sediments is usually around 2.6 g/cm\u003csup\u003e3\u003c/sup\u003e. This method utilizes density differences in order to enable the extraction of microplastics from sediment samples. However, since saturated sodium chloride has a density of 1.19 g/cm\u0026sup3;, the separation is relatively ineffective for denser microplastic particles (e.g., PVC, with a density of 1.35\u0026ndash;1.39 g/cm\u003csup\u003e3\u003c/sup\u003e) (Shaw, et al., \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2024\u003c/span\u003e, Duong, et al., \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). In addition, some studies have attempted to use saturated sodium iodide, zinc chloride and other salt solutions for density separation of microplastics. However, the insufficiently stable nature of these solutions, along with their high toxicity, has somewhat limited their application. In addition, issues such as the economic costs associated with these methods need to be considered(Crutchett, et al., 2024). The oil separation method mainly utilizes the hydrophobicity and lipophilicity of microplastics by mixing sediment samples with oil and separating the microplastics by oil-water separation. However, this method requires further separation of the oil from the microplastics, which may affect the subsequent characterization of the microplastics and make the separation step relatively complex (Zhang, et al., \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2020\u003c/span\u003e, Bellasi, et al., \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). The main principle of air flotation is to use air bubbles to suspend microplastics to the surface of the flotation solution. Microplastics have a special buoyancy due to their small density and hydrophobicity, and when the bubbles are introduced into the water, the hydrophobic interaction between the bubbles and the microplastics causes the microplastics to be carried to the surface of the solution. Subsequently, microplastics floating on the surface of the solution can be collected by overflows. Claessens (Claessens, et al., \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2013\u003c/span\u003e) produced flotation columns to separate microplastics from sediments in 2013, X. Zhu (Zhu, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2015\u003c/span\u003e) made improvements in the flotation column in 2015, but the results were not as good as expected, especially for the higher density microplastics, the recovery rate only reached about 52% or so. In these experiments, they used flotation columns made of PVC and added a salt solution of surfactant to the flotation solution, but did not reveal more specific details. The use of PVC flotation columns may be a key reason for the low recoveries, as we have found that microplastics are difficult to separate from the flotation column in solution due to hydrophobic interactions when using plastic flotation columns. Imhof (Imhof, et al., \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2012\u003c/span\u003e) also attempted to use froth flotation to separate microplastics from sediments, but the recovery rate was only about 55% or so for plastics 1\u0026ndash;5 mm in size.\u003c/p\u003e \u003cp\u003eUnlike traditional methods, this study designed an integrated froth flotation method that can accomplish the separation and collection of microplastics at the same time. The flotation column was made of hydrophilic glass material and a hydrophilic glass pipe in the shape of an arch bridge was designed at the top to minimize the adhesion problem between microplastics and the flotation column. In addition, previous studies have often failed to explore in depth the interaction mechanisms between surfactants, bubbles, and microplastics. However, for the first time, we have conducted an in-depth analysis of the flotation mechanism between different types of surfactants, gas bubbles and microplastics, which provides a theoretical foundation and experimental basis for the application of microplastic flotation separation technology. This innovation is expected to bring important progress in the field of microplastics research.\u003c/p\u003e"},{"header":"2 Materials and methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\n\u003ch2\u003e2.1 Experimental materials\u003c/h2\u003e\n\u003cp\u003eA The experimental samples were prepared as shown in Table\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e. After the sandy soil was washed repeatedly, 100 microplastics of 500 \u0026micro;m, 3 mm and 5 mm were mixed with 5 g of sandy soil to prepare sediment samples.\u003c/p\u003e\n\u003cdiv class=\"gridtable\"\u003e\n\u003cdiv class=\"colspec\" align=\"left\"\u003e\u0026nbsp;\u003c/div\u003e\n\u003ctable id=\"Tab1\" border=\"1\"\u003e\u003ccaption\u003e\n\u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\n\u003cdiv class=\"CaptionContent\"\u003e\n\u003cp\u003eGrain size, color, density and raw materials of self-made microplastics in the recovery rate experiment of different plastic types\u003c/p\u003e\n\u003c/div\u003e\n\u003c/caption\u003e\n\u003cthead\u003e\n\u003ctr\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003ePolymer\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eParticle size\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eColor\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eMaterials\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eDensity\u003c/p\u003e\n\u003c/th\u003e\n\u003c/tr\u003e\n\u003c/thead\u003e\n\u003ctbody\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003ePET\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003e5\u003c/strong\u003e00 \u0026micro;m 3 mm 5 mm\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eColourless\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eDrinking water bottle\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e1.36\u0026ndash;1.40 g/cm\u003csup\u003e3\u003c/sup\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003ePVC\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003e5\u003c/strong\u003e00 \u0026micro;m 3 mm 5 mm\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eBlue\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003ePVC sheet\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e1.35\u0026ndash;1.39 g/cm\u003csup\u003e3\u003c/sup\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003ePS\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003e5\u003c/strong\u003e00 \u0026micro;m 3 mm 5 mm\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003ePink\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eSpoon\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e1.14\u0026ndash;1.15 g/cm\u003csup\u003e3\u003c/sup\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003c/tbody\u003e\n\u003c/table\u003e\n\u003cp\u003eIn the previous experiments, we compared the effects of density separation, oil separation and foam flotation on the separation of microplastics in sediments, and the results of the experiments showed that a small amount of separation of microplastics could be achieved by using a high-density solution in the density separation process (Fig. \u003cspan class=\"InternalRef\"\u003eS1\u003c/span\u003e). Subsequent optimization experiments also indicate that density separation can be applied to extract and separate low-density microplastics in the laboratory, but is not suitable for large-scale separation of microplastics (Fig. S2). The oil separation experiments showed that the separation efficiency was increased by the addition of oil (Fig. S3), but the efficiency was increased to about 99.5% after the introduction of air flotation separation in this system (Fig. S4), so this method can be used to investigate the abundance of microplastics, and it is not suitable for the qualitative study of microplastics in the environment. The separation efficiency of froth flotation for large density microplastics reaches almost 100% (Fig. S5), and the separation step can be completed in less than two minutes. Alkyl polyglucoside (APG), as a kind of green and completely biodegradable surfactant, guarantees the recovery rate in the flotation process without pollution and with high efficiency, which can satisfy the quantitative analysis without affecting the qualitative analysis (Fig. S6). Therefore, the froth flotation separation method shows obvious advantages for the separation of microplastics in sediments. The following is the specific experimental procedure:\u003c/p\u003e\n\u003c/div\u003e\n\u003cp\u003eThe prepared sediment samples were placed in the air-float column and then 500 mL of APG solution of different concentrations were added. Air was passed through the air pump for aeration, and with the help of hydrophobic interactions between microplastics and air bubbles, the air bubbles carried the microplastics to float up to the surface of the solution and formed bubbles. Subsequently, the foam was passed through a 0.45 \u0026micro;m pore size filter membrane by means of foam overflow, and then the recovery and uplift rate of microplastics were observed and calculated by the naked eye (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e).\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\n\u003ch2\u003e2.2 Experimental methods\u003c/h2\u003e\n\u003cdiv id=\"Sec5\" class=\"Section3\"\u003e\n\u003ch2\u003e2.2.1 Exploratory experiments on the effect of air flow rate on the flotation effect of microplastics\u003c/h2\u003e\n\u003cp\u003eThe prepared sediment samples were placed in the air flotation column at room temperature, and 500 mL of 0.05 g/L APG 0810 solution was added. The value of the aeration flow rate was changed to study the effect on the flotation effect of microplastics, and the completion of the flotation of microplastics within 5 s was used as the evaluation standard.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec6\" class=\"Section3\"\u003e\n\u003ch2\u003e\u003cem\u003e2.2.2 Effect of different types of APG concentration on the flotation effect of microplastics\u003c/em\u003e\u003c/h2\u003e\n\u003cp\u003eIn order to better assess the wetting effect between APG and microplastics, the time required for microplastics to rise to the upper layer of the liquid surface in the liquid was used as an evaluation index. When the required time exceeded 5 s, the APG concentration was considered to inhibit microplastic flotation, i.e., wetting. One hundred each of 500 \u0026micro;m, 3 mm, and 5 mm PVC, PET, and PS plastics were added to the separation device, respectively, and aeration was performed by passing 200 mL/min of air. The APG concentration was varied until the foam no longer overflowed. When the number of microplastics present in the liquid was 0, it was recorded as all the microplastics floated, and the time required for the microplastics to float was counted.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec7\" class=\"Section3\"\u003e\n\u003ch2\u003e2.2.3 Effect of different types of surfactants on the flotation effect of microplastics\u003c/h2\u003e\n\u003cp\u003eIn order to better assess the wetting effect between APG and microplastics, the time required for microplastics to rise in the liquid to the upper level of the liquid surface was used as an evaluation index. When the time required for uplift exceeded 5 s, the APG concentration was considered to inhibit microplastic flotation as wetting. One hundred each of 5 mm PVC, PET and PS plastics were added to the separation unit and aeration was performed by passing 200 mL/min of air. The concentration of APG0814 was varied until the foam no longer overflowed. When the number of microplastics present in the liquid was 0, it was recorded as all the microplastics floated, and the time taken for the microplastics to float was counted.\u003c/p\u003e\n\u003c/div\u003e\n\u003c/div\u003e"},{"header":"3 Results and discussion","content":"\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e3.1 Microplastic Contact Angle Test\u003c/h2\u003e \u003cp\u003eIn order to deeply investigate the interaction mechanism between microplastic particles and surfactants, the contact angle analysis test on the plastic surface was carried out first. The contact angle of various types of microplastic particles was analyzed and tested in detail using a KRUSS DSA255 standard contact angle meter, and the test results are shown in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e and Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e2\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eContact angle size of different plastic types\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePolymer\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003ePET\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003ePVC\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003ePS\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eContact angle/\u0026deg;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e99.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e95.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e89.8\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003e3.2 Influence of air flow rate on flotation effect\u003c/h2\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFrom Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e3\u003c/span\u003e, it can be seen that the air flow rate has a certain effect on the flotation effect, with the increase of air flow rate, the uplift of microplastics in 5 s is the first to rise and then fall. The size of air flow affects the volume of bubbles in the flotation column, the distribution density and the degree of plastic tumbling. When the air flow rate is small, the air bubbles in the flotation column are more stable, the distribution density is small, the turbulence degree is small, and the degree of plastic tumbling is small, which will lead to the microplastics not being able to float in time. When the air flow increases, the more bubbles, the greater the distribution density, the greater the degree of turbulence, the greater the degree of plastic tumbling. This will lead to the plastic that has been floated and perhaps washed down by the tumbling bubbles, resulting in a decrease in the floating rate; and in the aeration flow rate of 150\u0026ndash;800 mL/min, the floating rate of microplastics is the highest. In order to facilitate the follow-up test, it was decided to choose 200 mL/min ventilation flow rate as the experimental gas flow rate.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003e3.3 Effect of APG concentration on the flotation effect of microplastics\u003c/h2\u003e \u003cp\u003eDuring froth flotation, APG concentration influences the amount of froth and thus the recovery rate of microplastics separation. In order to investigate the minimum concentration of various types of APG surfactants for flotation of microplastics, 200 mL/min air was passed through for aeration.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eOne hundred 5 mm plastics of each type were added to the separation device and aeration was carried out by passing 200 mL/min of air. During this process, the air bubbles attached to microplastics floated up to the upper layer of the solution and overflowed with the foam. With the increasing concentration, the ability of bubbles to carry microplastics gradually decreased. As can be seen from Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003e, before the concentrations of 0.05 g/L, 0.1 g/L and 0.5 g/L for APG1214, APG0814 and APG0810, respectively, the floating process of 5 mm microplastics could be completed within 5 s. In other words, its ability to float 5 mm microplastics could be completed in the same time. That is to say, its inhibition ability of 5 mm microplastic flotation recovery is: APG1214\u0026gt;APG0814\u0026gt;APG0810, and no difference was found between the inhibition points of APG solution on the flotation of three types of microplastics: PVC, PET and PS. However, the time for PS to complete upflotation was lower than that of PET and PVC, which may be due to the lower density of PS.\u003c/p\u003e \u003cp\u003ePlastic flotation methods can be roughly categorized into the following three types: chemically regulated flotation, physically regulated flotation and γ flotation. Theoretically, when the flotation system in the liquid surface tension value is located in the two different plastic materials between the wetting critical surface tension value, the flotation effect is optimal. The plastic with large surface tension will be wetted by the liquid or inhibited from floating, while the plastic with small surface tension value will not be wetted or not inhibited from floating, thus realizing plastic flotation separation, which is γ flotation separation. However, in the course of this experiment, no significant difference was observed in the flotation inhibition of different types of microplastics by APG concentration. This may be due to the fact that the contact angles of the selected microplastic particles are similar and have not reached a point where a difference in wetting can be produced.\u003c/p\u003e \u003cp\u003eOne hundred 500 \u0026micro;m, 3 mm plastics of each type were added to the separation device and aeration was performed by passing 200 mL/min air. During this process, according to the results in Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e5\u003c/span\u003e, it can be observed that the flotation effect is similarly suppressed for smaller particle size microplastics as the APG concentration increases. Compared to the 5 mm plastic particles, in all three types of APG solutions, the flotation inhibition for 500 \u0026micro;m and 3 mm plastic particles occurs only after a certain concentration is reached. Specifically, flotation inhibition of 500 \u0026micro;m microplastics occurred only after APG concentrations reached 0.2 g/L, 0.3 g/L, and 0.9 g/L, respectively, whereas inhibition of 3 mm microplastics occurred only after concentrations reached 0.1 g/L, 0.2 g/L, and 0.7 g/L, respectively. This suggests that there is a difference in the effect of microplastic particle size on APG concentration, with smaller particle size microplastics being more susceptible to inhibition by APG solutions with lower concentrations.\u003c/p\u003e \u003cp\u003eFor the inhibition of microplastic flotation, we suspect that it is related to the critical micelle concentration (CMC) of the surfactant. Typically, conventional surfactants have a special molecular structure in which hydrophilic and hydrophobic portions are interconnected when their concentration reaches a certain level, resulting in the formation of a self-assembled micelle structure. The minimum concentration of this self-assembled structure is known as the critical micelle concentration, i.e., the CMC value of the surfactant (Li, et al., \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2025\u003c/span\u003e). The presence of this micellar structure may lead to complete wetting of the microplastics, thus affecting their flotation performance. The commonly used methods for the determination of CMC are: conductivity method (Silva, et al., 2025) and surface tension method (Jiang, et al., \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2025\u003c/span\u003e). The surface tension method was used in this procedure to test the CMC values of different surfactants. The critical micellar concentration of the surfactant was determined to have been reached when its surface tensi on no longer decreased as the surfactant concentration increased. Subsequently, the surface tension of the three APG surfactants was tested and plotted in Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e6\u003c/span\u003e. It can be seen from the figure that the critical micelle concentrations of APG1214, APG0814 and APG0810 were near 0.04 g/L, 0.2 g/L and 1.4 g/L, respectively. In terms of the inhibited concentration of microplastic flotation, the inhibition of flotation of 500 \u0026micro;m microplastics occurred only after the APG concentration reached 0.2 g/L, 0.3 g/L and 0.9 g/L, respectively. Based on these results, we hypothesize that the CMC concentration of surfactant may not be significantly related to the inhibition of microplastic flotation.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eIn order to analyze the mechanism of surfactant flotation for microplastic wetting, the indicators of the three APG surfactants were plotted, as shown in Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. Among them, HLB value represents the hydrophilic-hydrophobic balance of the surfactant, which is used to measure the balance of size and strength between the polar and nonpolar groups of the surfactant, thus determining the degree of lipophilicity or hydrophilicity of the surfactant (Chen, et al., \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). A larger HLB value represents more hydrophilicity, and a smaller HLB value represents more lipophilicity. In order to minimize the experimental error, APG solutions of three anionic surfactants were selected. In the flotation process of microplastics interacting with APG, plastics possessing different groups showed insignificant differences in the wetting process. This may indicate that certain groups between different microplastic surfaces and APG surfactant molecules do not differ much in terms of hydrogen bonding and dispersion forces. On the contrary, the HLB value of APG largely influences the wettability of microplastics. Therefore, we can conclude that the main interaction between microplastics and APG surfactants is the hydrophobic interaction force. Without considering factors such as electrostatic forces, the smaller the HLB value, the stronger the lipophilicity and the stronger the wettability of microplastics.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003ePerformance indexes of different APG solutions\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eIndex\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eAPG1214\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eAPG0814\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eAPG0810\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eExterior\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eLight yellow paste\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eLight yellow liquid\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eLight yellow liquid\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003epH (50% aqueous solution)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e11.5\u0026ndash;12.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e11.5\u0026ndash;12.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e11.5\u0026ndash;12.5\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCMC\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.04\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e1.4\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eHLB\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e10\u0026ndash;12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e13\u0026ndash;15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e15\u0026ndash;17\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003estructural formula\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eC\u003csub\u003e18\u003c/sub\u003eH\u003csub\u003e36\u003c/sub\u003eO\u003csub\u003e6\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e(C\u003csub\u003e6\u003c/sub\u003eH\u003csub\u003e11\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e)nOR\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eC\u003csub\u003e16\u003c/sub\u003eH\u003csub\u003e32\u003c/sub\u003eO\u003csub\u003e6\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eHowever, it was experimentally found that the wettability of APG-type surfactants on large particle size microplastics was significantly higher than that on small particle size microplastics, which might be related to the specific surface area of microplastics. The specific surface area of microplastics increases as the particle size decreases, and a larger specific surface area indicates a stronger adsorption capacity, which also means that microplastics with smaller particle sizes are more difficult to be wetted. In addition, smaller microplastics require less time in the upwelling process, which may be related to the mass of the microplastics. At the same level of turbulence, smaller mass microplastics are more susceptible to turbulence and thus tumble to the upper layers of the liquid to achieve separation. These factors together influence the effectiveness of microplastic flotation, such that microplastics with different particle sizes show different responses to the wetting of APG surfactants (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e7\u003c/span\u003e).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003e3.4 Effect of different types of surfactants on the flotation effect of microplastics\u003c/h2\u003e \u003cp\u003eHaving established that the HLB value of the surfactant was inversely related to the wettability of the microplastics, the effect of other types of surfactants on the flotation of 5 mm PVC was further tested. These surfactants included the biodegradable sophorolipid, the cationic surfactant cetyltrimethylammonium bromide (CTAB), and the anionic surfactants sodium dodecyl sulfate (SDS) and sodium dodecyl sulfate (SDBS).\u003c/p\u003e \u003cp\u003eThe experimental results are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e8\u003c/span\u003e and Table\u0026nbsp;\u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e below. From the experimental results, it can be seen that different types of surfactants affected the flotation effect of microplastics to different degrees. This further verifies our previous conclusion that the wettability of surfactants is inversely related to their HLB values. The biodegradable acacia glycolipids showed strong wettability, similar to APG surfactants, which was favorable to the flotation effect of microplastics. On the other hand, the cationic surfactant CTAB and the anionic surfactants SDS and SDBS showed poor wettability and inhibited the flotation effect of microplastics.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab4\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 4\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eDifferent surfactants HLB and their inhibitory flotation concentration on 5 mm microplastics\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"8\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eIndex\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSophora glycolipid\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eSDBS\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eAPG1214\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eAPG0814\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eAPG0810\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eCTAB\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c8\"\u003e \u003cp\u003eSDS\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eHLB\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e9\u0026ndash;12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e10.638\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e10\u0026ndash;12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e13\u0026ndash;15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e15\u0026ndash;17\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e15.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e40\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eInhibition concentration (g/L)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.05\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.05\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e0.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e1.0\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eInhibition concentration (mol/L)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e7.26\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;5\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e2.29\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e7.17\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;5\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1.43\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e6.48\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e2.74\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e3.67\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eAs can be seen in Table\u0026nbsp;\u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e, the HLB values of sophorolipid and APG1214 are similar. However, although the inhibited microplastic flotation concentration of sophorolipid was low (7.26 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;5\u003c/sup\u003e mol/L), the amount of foam generated by sophorolipid was not sufficient to support the upward flotation of microplastics before the inhibited flotation concentration was reached. This suggests that not all surfactants are suitable for the flotation of microplastics, and that the generation of sufficient foam volume and stability to support microplastic flotation also needs to be considered. Therefore, the acacia glycolipid solution cannot be used as a froth flotation agent for microplastics.\u003c/p\u003e \u003cp\u003eOn the other hand, the cationic surfactant cetyltrimethylammonium bromide (CTAB) had an HLB value of 15.8, and its inhibition of microplastic flotation concentration ranged from 2.74 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e mol/L. This indicates that CTAB has certain wettability in microplastic flotation, but requires a higher inhibition flotation concentration. The anionic surfactant sodium dodecyl sulfate (SDS) has a lower HLB value, but its inhibited flotation concentration is between 2.29 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e mol/L. This suggests that the wettability is not only related to the HLB value, but may also be affected by the chemical and charge properties of the surfactant. CTAB is a cationic surfactant, which exists mainly in the cationic form in solution. Microplastics are usually negatively charged and may be electrostatically charged in solution, enhancing wettability. On the contrary, SDS is an anionic surfactant, which exists mainly in anionic form in solution and may lead to electrostatic repulsion, reducing the wettability.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAs can be seen from Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e9\u003c/span\u003e, the surface of the bubbles is negatively charged, while the surface of the microplastics is also negatively charged. Therefore, adsorption of surfactant occurs on both the bubble and microplastic surfaces. Sun(Sun, et al., \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2022\u003c/span\u003e) Studies have shown that the adsorption effect between cationic surfactants and microplastics is optimal in a certain concentration range. However, these studies mainly focused on the interaction between surfactants and microplastics without considering the interaction between surfactants and air bubbles.\u003c/p\u003e \u003cp\u003eIn mineral flotation, minerals are usually positively charged, unlike microplastics. We refer to the flotation mechanism in mineral flotation and make an analogy. Combined with our experimental results, we found that when a cationic surfactant is added, the surfactant enhances the adsorption of microplastics and bubbles onto it due to the presence of electrostatic adsorption, leading to a weakening of the hydrophobicity of bubbles and microplastics, and thus the hydrophobic interaction force between them. On the contrary, when anionic surfactant is added, the adsorption of microplastics and bubbles to the surfactant is weakened, which has less effect on the hydrophobicity of bubbles and microplastics. This explains why the wettability of anionic surfactants is lower than that of cationic surfactants. Overall, microplastic flotation still proceeds using hydrophobic interactions with the addition of surfactant, but the adsorption of surfactant affects the hydrophobic interactions between bubbles and microplastics.\u003c/p\u003e \u003c/div\u003e"},{"header":"4 Conclusion","content":"\u003cp\u003eIn this study, we devised a froth flotation separation apparatus and meticulously examined the interactions between various surfactants and microplastics. The flotation solution utilizing APG showcased a plethora of attributes such as non-toxicity, harmlessness, non-irritation, complete biodegradability, and cost-effectiveness, rendering it highly conducive for large-scale microplastic separation and extraction. Our findings hold substantial guiding significance for advancing microplastic separation and extraction technologies, leading to the following conclusions:\u003c/p\u003e\n\u003col\u003e\n\u003cli\u003e\n\u003cp\u003eThe concentration of APG inversely affects the adherence of bubbles to microplastics. This phenomenon stems from APG's wetting effect on microplastics, transitioning them from hydrophobic to hydrophilic states, consequently diminishing the adherence of air bubbles to plastics and impeding microplastic flotation.\u003c/p\u003e\n\u003c/li\u003e\n\u003cli\u003e\n\u003cp\u003eMicroplastic particle size exerts a discernible influence on froth flotation. Diminutive microplastic particles exhibit augmented specific surface areas and heightened adsorption capacities. However, their reduced wettability poses lesser hindrance in the flotation process. Moreover, microplastic mass influences their behavior in turbulence, consequently impacting flotation outcomes.\u003c/p\u003e\n\u003c/li\u003e\n\u003cli\u003e\n\u003cp\u003eNot all surfactant types prove suitable for microplastic flotation. It is imperative to ensure that surfactants generate ample foam volume and sustain foam stability to facilitate microplastic flotation without inhibiting the process.\u003c/p\u003e\n\u003c/li\u003e\n\u003cli\u003e\n\u003cp\u003eThe interaction mechanism between surfactants and the solid matrix is intricate, with diverse surfactant types possibly engaging distinct solid particle interaction mechanisms. Excluding other forces, surfactant wettability on plastics correlates with their HLB values, where higher HLB values may yield lower plastic wettability, thereby potentially aiding flotation.\u003c/p\u003e\n\u003c/li\u003e\n\u003cli\u003e\n\u003cp\u003eGiven microplastics' typically negative charge in environmental settings, electrostatic effects influence their interaction with various surfactants. Cationic surfactants may enhance microplastic wettability through electrostatic adsorption, while anionic surfactants may yield contrasting effects.\u003c/p\u003e\n\u003c/li\u003e\n\u003c/ol\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eCredit authorship contribution statement\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eJinghui Wu\u003c/strong\u003e: Investigation, Formal analysis, Resources, Writing - Original Draft. \u003cstrong\u003eYidi Gao\u003c/strong\u003e: Investigation, Formal analysis. \u003cstrong\u003eGuanqiao Li\u003c/strong\u003e: Investigation, Formal analysis. \u003cstrong\u003eXinyuan Li\u003c/strong\u003e: Investigation. \u003cstrong\u003eYiming Wang:\u003c/strong\u003e Investigation. \u003cstrong\u003eMingxin Huo\u003c/strong\u003e: Resources, Supervision. \u003cstrong\u003eXianze Wang\u003c/strong\u003e: Project administration, Resources, Methodology.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eDeclaration of Competing Interest\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEthical Approval\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAvailability of data and materials\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll data generated or analyzed during this study are included in this published article.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e\u0026nbsp;Funding\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis work was supported by the National Natural Science Foundation of China (52270037, 52170027 and U20A20322), National Key Research and Development Program of China (2024YFC3907804-3), State Key Laboratory of Water Pollution Control and Green Resource Recycling Foundation (NO. 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Effect of cationic, anionic and non-ionic surfactants on transport of microplastics: Role of adhesion of surfactants on the polyethylene surface. \u003cem\u003eJournal of Hydrology\u003c/em\u003e, \u003cem\u003e612\u003c/em\u003e, 128051. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.jhydrol.2022.128051\u003c/span\u003e\u003cspan address=\"10.1016/j.jhydrol.2022.128051\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Microplastics, Sediment, Integration, Foam flotation, No contamination","lastPublishedDoi":"10.21203/rs.3.rs-8909428/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8909428/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eSediments play a pivotal role in the ecological cycle of microplastics, serving as both a hub for their aggregation and accumulation, and potentially as a source and distributor of these particles. This dual function underscores the importance of sediment analysis in understanding the environmental distribution and origins of microplastics. To comprehensively evaluate microplastic presence and contamination levels in sediments, the development of an efficient and standardized separation method is imperative. Hence, this study introduces an integrated froth flotation approach that has been successfully applied to separate microplastics from various substrates including surface water, wastewater treatment plant sludge, and soil. Compared to conventional froth flotation-based methods, this novel approach yields significantly higher recovery rates of microplastics. Notably, the use of a fully biodegradable surfactant ensures pollution-free separation and achieves recovery rates nearing 100%. Moreover, the study delves into the interaction mechanism between surfactants and plastic particles, particularly focusing on the adsorption process between anionic and cationic surfactants and microplastics. These experimental findings are pivotal in advancing microplastic separation and extraction technologies, guiding future research in this critical area.\u003c/p\u003e","manuscriptTitle":"Efficient recovery of microplastics from sediments by foam flotation","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-03-23 12:30:50","doi":"10.21203/rs.3.rs-8909428/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"5017ca26-8e7b-4998-9e8c-d5e2f6c77821","owner":[],"postedDate":"March 23rd, 2026","published":true,"recentEditorialEvents":[{"type":"decision","content":"Rejected","date":"2026-05-06T21:52:38+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-04-30T10:42:57+00:00","index":37,"fulltext":""}],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2026-05-06T22:08:35+00:00","versionOfRecord":[],"versionCreatedAt":"2026-03-23 12:30:50","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8909428","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8909428","identity":"rs-8909428","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}