Open AccessArticle

Enhancing Pb Adsorption on Crushed Microplastics: Insights into the Environmental Remediation

Sen Li

¹,

Lu Cao

^1,*

Qiyuan Liu

²,

Shuting Sui

¹,

Jiayin Bian

²,

Xizeng Zhao

^1,3

and

Yun Gao

School of Marine Engineering Equipment, Zhejiang Ocean University, Zhoushan 316022, China

Oceanic Environmental Monitoring and Forecasting Center of Zhoushan City, Zhoushan 316022, China

Ocean College, Zhejiang University, Zhoushan 316021, China

Author to whom correspondence should be addressed.

Water 2024, 16(23), 3541; https://doi.org/10.3390/w16233541

Submission received: 29 October 2024 / Revised: 2 December 2024 / Accepted: 6 December 2024 / Published: 9 December 2024

(This article belongs to the Topic Microplastics Pollution)

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Figure 1
(a) SEM image of primary MPs (magnification: 10.00KX); (b) SEM image and corresponding elemental composition (EDS) bar chart of crushed MPs; (magnification: 10.00K X) (c) SEM image and corresponding elemental composition (EDS) bar chart of Pb-adsorbed MPs (magnification: 10.00K X). "> Figure 2
The adsorption of Pb on primary MPs and crushed MPs with time. "> Figure 3
Adsorption curves of crushed MPs with different particle sizes in Pb solution. Experimental conditions: [adsorbents] = 1 g/L; [Pb] = 30 mg/L. "> Figure 4
Adsorption characteristics of crushed MPs in Pb solutions: adsorption curves (a), adsorption kinetics (b), and intra-particle diffusion model (c). Experimental conditions: [adsorbents] = 1 g/L; [MP size] = 0.2 mm. "> Figure 5
Adsorption isotherm of heavy metal Pb solution on MPs. Experimental conditions: [adsorbent] = (0.1 g/L, 0.2 g/L, 0.3 g/L); [MP size] = 0.2 mm. "> Figure 6
Effect of the pH (a) and salinity (b) on the adsorption of Pb on crushed PVC. Experimental conditions: [adsorbents] = 0.3 g/L; [MP size] = 0.2 mm. "> Figure 7
Adsorption curves of crushed MPs with different concentrations in Pb solution. Experimental conditions: [MP size] = 0.2 mm; [Pb] = 30 mg/L. "> Figure 8
Biofilm formation on the surface of MPs (magnification: 350X). "> Figure 9
Effect of biofilm-coated MPs and non-coated MPs on Pb adsorption in crushed PVC. Experimental conditions: [adsorbent] = 1 g/L; [Pb] = 30 mg/L. "> Figure 10
The effect of Fe3O4 mass on the recovery efficiency of MPs. "> Figure 11
The effect of adsorption capacity of MPs on their recovery efficiency by Fe3O4 nanoparticles in Pb-contaminated wastewater. ">

Versions Notes

Abstract

This study investigates the pollution characteristics and environmental risks of crushed microplastics (MPs) generated during plastic recycling, emphasizing their adsorption capacity for heavy metals, particularly lead (Pb). SEM-EDS analysis revealed that crushed MPs exhibit significantly higher adsorption capacity than primary MPs due to their larger surface area and more available adsorption sites, including oxygen-containing functional groups. The adsorption behavior of MPs was influenced by key factors such as MP size, MP quantity, pH, salinity, and biofilm formation. Smaller MPs demonstrated higher adsorption efficiency, while elevated pH enhanced Pb adsorption. Conversely, increased salinity reduced adsorption due to competition for adsorption sites. Increasing MP concentrations improved Pb removal efficiency, but higher MP quantities led to a decrease in maximum adsorption capacity, demonstrating a trade-off between removal efficiency and adsorption capacity. Isothermal adsorption experiments revealed that Pb adsorption on MPs follows a multi-layer mechanism, best characterized by the Freundlich model. The adsorption capacity increased nonlinearly with Pb concentration and stabilized as surface sites became saturated. The formation of biofilms on MPs further enhanced their adsorption capacity by providing additional functional groups and facilitating multi-layer adsorption, increasing ecological risks. Adsorption kinetics were best described by pseudo-second-order and intra-particle diffusion models, indicating chemical adsorption and boundary layer diffusion as dominant mechanisms. Magnetic Fe₃O₄ nanoparticles demonstrated a high recovery efficiency of 99.3% for MPs, highlighting their potential for environmental remediation. However, the presence of adsorbed Pb slightly reduced recovery performance, emphasizing the need to optimize recovery conditions for maximum efficiency. These findings underscore the dual threat posed by crushed MPs: their capacity to adsorb and concentrate harmful substances, increasing ecological toxicity, and the challenges associated with their recovery. This research provides critical insights into mitigating MP pollution and developing effective recovery strategies under realistic environmental conditions.

Keywords:

crushed microplastics; Pb; adsorption capacity; ecological threat; environmental remediation

1. Introduction

Microplastics (MPs), plastic fragments smaller than 5 mm in diameter [1,2,3], have been reported to be widely distributed across diverse ecosystems, including oceans, land, freshwater, and even polar regions [4,5,6,7]. MPs are normally generated by the breaking down of plastics under various environmental factors, such as ultraviolet radiation, thermal degradation, biodegradation, and oxidative weathering. However, plastic recycling processes are an overlooked way of MP generation but have been reported recently, and research has shown that the plastic recycling industry and wastewater treatment plants could be major sources of MP pollution entering the environment [8,9,10]. In the recycling industry, MPs are mainly generated during the mechanical crushing of plastic waste, and they are subsequently released into the environment through washing processes [10]. Due to their smaller sizes, larger surface areas, and higher stability, MPs are easily prone to absorb persistent organic pollutants (POPs) and heavy metals, which exacerbates their ecological toxicity and presents significant challenges to environmental health [11,12,13,14]. Studies have detected trace metals such as cadmium (Cd), chromium (Cr), copper (Cu), and lead (Pb) in plastic particles collected from beaches on the island of Vis, Croatia [15]. Additionally, harmful metals, including Pb, have been detected in MP samples collected from the Musi River [16]. So, MPs have the potential to passivate heavy metals in water. Compared to other MPs in soils or oceans, crushed MPs are easier to collect. However, the adsorption capacity of crushed MPs remains unclear and requires further research into its various influencing factors, such as MP type, MP size, the number of MPs, pH, salinity, and the presence of biofilms on MPs.

Furthermore, due to their large surface areas and strong hydrophobicity, MPs are highly susceptible to microbial colonization in aquatic environments, leading to the formation of biofilms on their surfaces. These biofilms alter the physical and chemical properties of the MPs, significantly affecting their ability to adsorb heavy metals. As the biofilm matures, the concentration of heavy metals on the MPs increases [17,18]. This is because the presence of biofilms enhances electrostatic interactions, coordination, complexation, and surface precipitation between MPs and heavy metals [18]. During the adsorption process, MPs may form complex pollutants that pose more severe threats to aquatic ecosystems and are more likely to be ingested by aquatic organisms, introducing new ecological risks. Therefore, investigating the effects of biofilm attachment on MPs’ adsorption of heavy metals is of great environmental significance.

To address MP pollution, researchers have explored various physical, chemical, and biological methods. In recent years, magnetic nanomaterials, especially Fe₃O₄ nanoparticles, have emerged as a promising environmental remediation technology due to their excellent adsorption performance and magnetic recovery capabilities [19]. Studies have shown that magnetic Fe₃O₄ nanoparticles can effectively adsorb various types of MPs, such as polyethylene (PE), polypropylene (PP), and polystyrene (PS), and can be rapidly separated from water by applying a magnetic field [20]. By optimizing the density and contact time of Fe₃O₄ nanoparticles, experiments have achieved over 80% removal efficiency, with high recovery rates observed in environmental waters such as rivers and seawater [19]. This magnetic separation-based MP recovery technology, while effective, faces challenges in dealing with the diverse types, shapes, and sizes of MPs in the environment, which can lead to variations in removal efficiency. Therefore, improving the production process of magnetic nanomaterials and enhancing recovery efficiency, particularly under real-world environmental conditions, remain pressing issues.

In this study, we systematically investigate the adsorption capacity of crushed MPs for Pb under various environmental conditions, including MP type, MP size, the number of MPs, pH, salinity, and the presence of biofilms on MPs. By analyzing both kinetic and isothermal adsorption models, this work provides a comprehensive understanding of the adsorption mechanisms and highlights the factors that exacerbate the toxicity of MPs and their environmental impact. Additionally, this study addresses the effects of biofilm formation, a naturally occurring process in aquatic environments, on the adsorption properties of MPs. Biofilm attachment significantly alters the physical and chemical characteristics of MPs, enhancing their adsorption of heavy metals and thereby increasing ecological risks. Furthermore, this study explores the application of magnetic Fe₃O₄ nanoparticles specifically for the recovery of crushed MPs after adsorption. The recovery efficiency was evaluated based on the amount of Fe₃O₄ nanoparticles added, the adsorption capacity of MPs, and their interaction during the recovery process. This method demonstrates its practicality by providing insights into optimizing Fe₃O₄ nanoparticle usage and improving recovery efficiency under controlled experimental conditions. These findings highlight the potential of Fe₃O₄ nanoparticles as a practical solution for addressing MP pollution, contributing to the development of sustainable remediation strategies.

2. Methods and Materials

2.1. Materials and Chemicals

The PVC MP particles used in the experiments included primary MP particles and crushed MP particles. The primary PVC particles were purchased from Guangdong Guangyuan Plastic Raw Material Co., Ltd. (Guangzhou, China) and the crushed PVC particles were derived from PVC-U pipes produced by Ningbo Yongcai Plastic Co., Ltd. (Ningbo, China); they were crushed into MPs with a diameter smaller than 5 mm, followed by sieving using screens with diameters of 1 mm, 0.5 mm, and 0.2 mm. Before the experiments, the MP particles underwent a pretreatment process, including washing with 95% ethanol, 2% nitric acid, and deionized water (three times for each solution), followed by air drying in a well-ventilated area. Reagents such as lead chloride (PbCl₂), nitric acid (HNO₃), ethanol (CH₃CH₂OH), hydrochloric acid (HCl), sodium hydroxide (NaOH), sodium chloride (NaCl), iron trichloride hexahydrate (FeCl₃·6H₂O), sodium acetate (CH₃COONa), L-Lysine (C₆H₁₄N₂O₂), and dopamine hydrochloride (C₈H₁₂ClNO₂) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).

2.2. Characterization

In this study, the morphological changes of MPs were observed both before and after the experiment, utilizing a scanning electron microscope (SEM, Sigma500, Zeiss, Oberkochen, Germany). Furthermore, the element composition of the MPs after the adsorption process was analyzed through energy-dispersive X-ray spectroscopy (EDS, Oxford Instruments, Abingdon, Britain).

2.3. Adsorption Experiments

Before conducting the adsorption experiments, conducting preliminary experiments using MPs (PVC, PP, PET, PE) obtained from a recycling process in a certain company under the same experimental conditions; MP size of 0.5 mm, initial Pb concentration of 30 mg/L, and MPs concentration of 1 g/L. The aim was to determine the type of MPs to be used in the subsequent experiments, and detailed results are provided in Table S1. The research findings indicated that PVC exhibited the best adsorption efficiency for Pb, and thus, PVC was predominantly used in the subsequent experiments.

The research goal is to explore the possibility of MPs carrying a significant amount of heavy metal Pb in highly polluted environments. Considering the detected Pb concentrations in North Africa (14.19 ± 7.5 mg/L) and in the Ganges river of Rishikesh–Allahabad (ranging from 2.4 to 26.9 mg/L) [21,22], the Pb solution concentrations were set at 30 mg/L for the majority of the experiments.

To investigate the influence of various factors on the adsorption performance of Pb onto crushed PVC MPs, multiple experimental conditions were designed, including MP type, MP size, pH, salinity, the number of MPs, and the presence of biofilms on MPs. The specific experimental procedures are described as follows. A predetermined amount of MPs was weighed and placed into 300 mL glass conical flasks containing 250 mL of Pb solution. The flasks were then placed on a shaker operating at 120 rpm and kept at room temperature. Sampling intervals were set at 4, 8, 16, 24, 48, 96, 144, 192, 240, 288, and 336 h. At each sampling point, 10 mL of the solution was extracted using a pipette, filtered through a 0.45 μm membrane to separate the MPs from the solution, and subsequently analyzed to determine the Pb concentration in the filtrate. A UV–visible spectrophotometer (UV2365, Unico, Shanghai, China) was used for Pb concentration analysis. The adsorption capacity of MPs was calculated based on the change in Pb concentration over time. To ensure reproducibility, three parallel samples were prepared for each experimental condition. The average of the three parallel results was used as the experimental data. Prior to use, all glass conical flasks were soaked in 2% HNO₃ for two hours and rinsed thoroughly with Milli-Q water to eliminate potential contamination. Detailed experimental setups for each factor are summarized in Table 1. For each factor, the experimental procedures followed the same general protocol as described above.

To further understand the adsorption behavior of Pb in crushed PVC MPs, adsorption kinetics and isothermal adsorption experiments were conducted. For the adsorption kinetics experiments, the initial Pb concentration was set as the variable, with concentrations of 10, 30, and 50 mg/L. A constant MP concentration of 1 g/L and a particle size of 0.2 mm were used. The experimental setup and sampling intervals followed the same protocol as described above. The adsorption kinetics were analyzed by fitting the experimental data to pseudo-first-order and pseudo-second-order kinetic models to determine the adsorption rate and underlying mechanisms. For the isothermal adsorption experiments, MPs with a particle size of 0.2 mm were used at concentrations of 0.1, 0.2, and 0.3 g/L, while the initial Pb concentrations ranged from 30 to 55 mg/L (30, 35, 40, 45, 50, and 55 mg/L). The experiments were carried out under the same shaking conditions (120 rpm) and room temperature. The samples were allowed to reach equilibrium after 192 h, after which the equilibrium Pb concentrations in the solutions were measured. The adsorption isotherms were evaluated using Langmuir and Freundlich models to determine the adsorption capacity, adsorption mechanisms, and the nature of the adsorption process. All adsorption models and parameters are listed in Table S2.

This experiment utilized a UV–Vis spectrophotometer (UV2365, Unico, China) to monitor the changes in the concentration of Pb solution before and after the experiment. The method to detect Pb was the improved xylene orange spectrophotometric method, described as follows: (1) Two drops of 0.1% thymol blue solution were added into a 5 mL sample. The solution was then neutralized to a slightly red color with hydrochloric acid and further neutralized to yellow with ammonium hydroxide. (2) Six drops of 1% ascorbic acid, 1 mL of 0.1% potassium ferrocyanide, and 3 mL of acetic acid-sodium acetate buffer solution (pH 5.1) were added to the solution, which was then diluted to 22 mL with water and thoroughly mixed. (3) An amount of 1 mL of 0.5% ammonium fluoride solution and 1 mL of 0.2% xylene orange solution were added to the solution. The resulting solution was further diluted to 25 mL and shaken for 10 min. (4) A portion of the solution was transferred to a 3 cm colorimetric dish (or a 2 cm colorimetric dish for high concentrations) with water as the reference liquid, and the absorbance was measured at a wavelength of 580 nm after subtracting the blank absorbance obtained from the control sample.

2.4. Approaches to Microplastic Recovery

To investigate the effectiveness of magnetic Fe₃O₄ nanoparticles in removing MPs, the following steps were conducted for the synthesis of magnetic Fe₃O₄: The synthesis of magnetic Fe₃O₄ nanoparticles: 3.40 g of FeCl₃·6H₂O and 6 g of CH₃COONa were added to 100 mL of ethylene glycol. The mixture was stirred at 50 Hz for 90 min and then placed in an oven and heated to 190 °C for 10 h. The product was washed with ethanol five times and dried in an oven at 65 °C for 3 h to obtain magnetic Fe₃O₄ nanoparticles [23].

Dopamine coating of magnetic particles: 100 mg of Fe₃O₄ and 2 mg of L-lysine were added to 20 mL of a water/ethanol solution (1:1) and the mixture was sonicated for thorough dispersion. The mixture was stirred at 100 Hz for 4 h, during which 5 mL of dopamine hydrochloride solution was added. After stirring, the mixture was dried at 90 °C in an oven for 3 h, followed by carbonization in a tube furnace at 700 °C for 3 h.

To ensure consistency with previous studies, the concentration of MPs was maintained at 1 g/L using PVC particles with an average diameter of 0.2 mm. In the initial adsorption experiments, PVC particles were placed in 40 mL of water, with three parallel samples prepared for each group. After 3 days, magnetic Fe₃O₄ nanoparticles were added to the solution containing the MPs for adsorption experiments. Upon the addition of the magnetic Fe₃O₄ nanoparticles, the mixture was stirred at 100 Hz for 10 min to allow the Fe₃O₄ to adsorb onto the MPs. The magnetized MPs were then removed using a magnet, separated, and washed with pure water in a beaker. After filtration and drying, the removal efficiency was calculated. The removal efficiency of MPs under different conditions was compared to identify the factors influencing MP adsorption.

3. Results and Discussion

3.1. Adsorption Capacity Between Primary MPs and Crushed MPs

Scanning electron microscopy (SEM) images reveal distinct differences in the surface morphology of MPs. As depicted in Figure 1a, the surface of primary MPs exhibits a smooth and flat characteristic with only subtle depressions. In contrast, Figure 1b illustrates the rough and multi-layered surface of crushed MPs, featuring prominent protrusions and depressions. This morphology may provide additional adsorption sites for heavy metals, leading to a significantly enhanced adsorption capacity of MPs for heavy metals [24,25]. Figure 1c highlights the presence of columnar structures absorbed on the surface of MPs, consistent with the Pb ions confirmed in the EDS spectrum in Figure 1c. EDS scans of MPs used in the heavy metal adsorption experiment clearly show a significant accumulation of Pb ions on the surface in Figure 1b,c. Analysis of the EDS spectrum indicates the presence of elements such as Ca (18.04%) on the surface of crushed MPs, potentially influencing the adsorption capacity of MPs for Pb. This aligns with previous research findings [26].

Figure 2 shows the comparison of the adsorption capacities between primary and crushed MPs. It is worth noting that the adsorption performance of primary MPs is relatively poor, with little change in Pb concentration, resulting in an adsorption capacity of 0.169 mg/g. In contrast, for crushed MPs, a significant trend is observed. Over the next 96 h, the Pb concentration reached an adsorption equilibrium state. As previously reported, primary MPs indeed exhibit limitations in the adsorption of heavy metals, with a slow adsorption process and low adsorption capacity [27,28]. The significant disparity in adsorption capacities among MPs of different morphologies suggests that untreated primary MPs have a relatively lower adsorption capacity for heavy metals compared to other types of MPs. This also implies that crushed MPs have a stronger adsorption capacity for heavy metal substances in the environment, posing greater environmental toxicity. The adsorption capacity of crushed PVC in the final experiment was 28.272 mg/g. The crushing treatment significantly enhanced the MPs’ adsorption capacity for pollutants, thereby increasing environmental risk.

3.1.1. Effect of MP Size on Adsorption

The impact of MP size on Pb adsorption was evaluated, as illustrated in Figure 3. In the process of Pb adsorption in crushed MPs, the equilibrium was reached at approximately 48 h for MPs with diameters of 0.2 mm and 0.5 mm, whereas MPs with a diameter of 1 mm required up to 192 h to achieve adsorption equilibrium. This delay in reaching adsorption equilibrium for larger MPs is attributed to the saturation of adsorption sites as the adsorption progresses, limiting the interaction between MPs and Pb ions [29]. The adsorption capacity of MPs for heavy metals is influenced by their specific surface area, which is related to different particle sizes [30,31]. As evidenced by Figure 3, it is apparent that MPs with diameters of 0.2 mm and 0.5 mm reach adsorption equilibrium faster than those with a diameter of 1 mm, exhibiting higher adsorption capacities. This phenomenon is attributed to the increased complexity of surface morphology as the MP size decreases, resulting in smaller MPs having larger surface areas and more unoccupied adsorption sites [32,33]. This finding is consistent with previous research results. For instance, Wang et al. [31] compared three different-sized polyethylene MPs and found that smaller-sized MPs exhibited the highest adsorption capacity for Cd. Similarly, Gao et al. [34] compared four different-sized polypropylene MP particles and observed a decrease in the adsorption capacity of MPs for Pb, copper, and cadmium with increasing particle size. This experimental study confirms the high sensitivity of crushed MPs to Pb ions and their characteristic of rapidly reaching adsorption saturation, thereby increasing their toxicity in a shorter duration.

3.1.2. Adsorption Kinetics

From Figure 4a, it can be observed that under the same number of MPs, the higher the Pb concentration in the solution, the smaller the adsorption rate and the larger the adsorption capacity. The adsorption process of MPs for Pb exhibits distinct stages. The initial stage occurs from 0 to 24 h, demonstrating rapid adsorption, reaching 80.39% to 97.45% of the maximum adsorption capacity within the first 24 h. Subsequently, a slow adsorption stage occurs from 24 to 48 h, during which 93% to 99.81% of the maximum adsorption capacity is attained. After 48 h, the adsorption rate gradually approaches zero, indicating the achievement of adsorption equilibrium. The maximum adsorption capacity of MPs in this experiment is calculated to be 49.3 mg/g. When the initial concentration of Pb is 10 mg/L, the Pb concentration in the solution decreases to 0. However, for initial concentrations of 30 mg/L and 50 mg/L, the concentrations decrease to 0.1045 mg/L and 0.8943 mg/L, respectively. These results confirm the effective removal capacity of crushed PVC for Pb ions from the solution. In a study by Liu et al. (2022) on Pb adsorption isotherms, a significant positive correlation was observed between the increased concentration of Pb and the adsorption capacity of MPs [26]. Therefore, varying the quantity of crushed PVC MPs at different Pb concentrations may reduce the Pb concentration in the solution to 0. Crushed MPs demonstrate significant potential for removing heavy metal pollutants from water environments. The strong adsorption capacity can be utilized for environmental applications, and over time, the adsorption of heavy metal pollutants by MPs increases, enhancing their toxicity and causing irreversible harm to organisms [13].

To gain a deeper understanding of the adsorption process of Pb, we explored pseudo-first-order and pseudo-second-order kinetic models. The fitting results of the experimental data are shown in Figure 4b, and the corresponding adsorption parameters are listed in Table S3. The adsorption rate of MPs with initially high concentrations is significantly lower than those at low concentrations, and the adsorption capacity is larger [35]. This may be attributed to the increase in Pb concentration, leading to an elevated collision probability between particles [36]. By comparing the fitting results of the two kinetic models in Table S3, it can be inferred that the pseudo-second-order kinetic model with correlation coefficients (R² values ranging from 0.986 to 0.997) outperforms the pseudo-first-order kinetic model (values from 0.970 to 0.997). This suggests that chemical adsorption may occur during the adsorption process, consistent with previous research. The adsorption process of crushed PVC for Pb is evidently complex and likely involves chemical adsorption mechanisms [26,37].

Further investigation of the mass transfer process of Pb adsorption was conducted using the intra-particle diffusion model, as shown in Figure 4c, and the corresponding adsorption parameters are listed in Table S4. The fitted line of the model does not pass through the origin, indicating that internal diffusion is not the sole limiting step but is also influenced by the surface water film on MPs [38]. The adsorption process is divided into three stages, with the diffusion rates continually decreasing (k_t₁ > k_t₂ > k_t₃). This could be attributed to the decreasing Pb solution concentration and the reduction in adsorption sites over time [29]. Firstly, the first stage involves surface diffusion, where adsorption sites are unoccupied, leading to a fast adsorption rate. In the second stage, internal pore diffusion occurs where metal ions slowly diffuse into the particles. As adsorption sites decrease, the adsorption rate begins to decline. The third stage involves micropore diffusion, reaching adsorption equilibrium, and the adsorption rate returns to zero [35,39]. In this process, the C value continually increases, indicating a growing boundary layer effect.

3.1.3. Adsorption Isotherms

To further investigate the adsorption behavior of Pb on MPs, an isothermal adsorption experiment was conducted to complement the kinetic studies. Building on the findings that the adsorption of Pb on MPs involves chemical adsorption and is influenced by initial concentrations, the isothermal adsorption experiment aimed to explore the adsorption capacity and its relationship with Pb concentrations under equilibrium conditions.

In this experiment, 0.2 mm MPs at concentrations of 0.1, 0.2, and 0.3 g/L were placed in Pb solutions with initial concentrations ranging from 30 to 55 mg/L (30, 35, 40, 45, 50, and 55 mg/L). The adsorption process was monitored over 192 h to ensure equilibrium, and the maximum adsorption capacity was determined. The results revealed that the maximum adsorption capacity of MPs increased with higher Pb concentrations, exhibiting a nonlinear growth pattern. At lower Pb concentrations (30–40 mg/L), the adsorption rate was highest, suggesting that abundant adsorption sites on the surface of MPs facilitated efficient adsorption. However, as the Pb concentration increased, the adsorption sites on the surface of MPs gradually became saturated, limiting further adsorption [26].

Figure 5 and Table S5 present the corresponding results and parameters. As shown in Figure 5, the maximum adsorption capacity increased nonlinearly with Pb concentration, eventually stabilizing as the surface adsorption sites became saturated at an MP concentration of 0.3 g/L. The data in Table S5 further indicate that the Freundlich model provides a better fit for the adsorption behavior of Pb on MPs, with correlation coefficients (R² values ranging from 0.9604 to 0.9910) surpassing those of the Langmuir model (R² values ranging from 0.9412 to 0.9936). This suggests that the adsorption mechanism involves multi-layer adsorption, as characterized by the Freundlich model. The value of 1/n being less than 0.500 indicates an uneven distribution of adsorption sites on the surface of MPs, which was further confirmed through SEM. The SEM images revealed a heterogeneous surface morphology of MPs, consistent with the observation of non-uniform surface multi-layer adsorption [40]. These results align with the kinetic findings and highlight the complex and heterogeneous nature of adsorption sites on MPs during the Pb adsorption process.

3.1.4. Effect of pH on Adsorption

The pH of the water body highly influences the surface charge of the adsorbent and the metal ions. Figure 6a depicts the dependence of Pb adsorption on MPs within the pH range of 4.0 to 8.0 under experimental conditions. The experiments were conducted with a Pb solution concentration of 30 mg/L at room temperature, and samples were analyzed after 192 h to ensure equilibrium. In the deionized water system, as the solution pH increases, the adsorption of Pb on MPs also increases. Initially, there is the competitive adsorption of H⁺ ions, and as the pH reaches or exceeds 7, Pb ions in the solution mainly exist in the forms of Pb²⁺, Pb(OH)⁺, and Pb(OH)₂, with the precipitation observed in the solution when adjusting the pH to 8.0, indicating the occurrence of precipitation. This phenomenon is attributed to precipitation, enhanced electrostatic forces, and the reduced competitive effect of H⁺ ions in the solution. Conversely, as the solution pH increases, the adsorption capacity for anionic pollutants gradually decreases [41]. The highest adsorption at pH 8.0 was observed, consistent with similar findings reported by Liu et al. [26].

3.1.5. Effect of Salinity on Adsorption

Using NaCl to simulate seawater, we investigated the impact of salinity on the adsorption capacity of MPs for Pb. Given that the average salinity of the world’s oceans is 3.5%, with the Baltic Sea having the lowest at 1%, we selected 1% and 3.5% as contrasting experimental conditions. As shown in Figure 6b, salinity significantly affects the ability of MPs to adsorb Pb, with a decrease in Pb adsorption as salinity increases. This aligns with our previous research findings [26,42]. Due to the negative charge present in MPs, Na ions compete with Pb ions for adsorption. With the increasing concentration of Na ions, limited adsorption sites on MPs lead to a reduction in the specific surface area for Pb ion accumulation, hindering the adsorption of Pb by MPs. Therefore, the inhibitory effect of seawater is limited, and crushed MPs with higher adsorption capacities still pose considerable ecological toxicity [13].

3.1.6. Effect of MPs Quantity on Adsorption

To evaluate the effect of MP concentration on Pb adsorption, varying amounts of 0.2 mm-sized MPs (0.1 g/L, 0.5 g/L, 1 g/L, 1.5 g/L, and 2 g/L) were added to a 30 mg/L Pb solution. The results demonstrate a clear trend of increased Pb adsorption capacity with increasing MP concentrations (Figure 7). Notably, when the MP concentration was 0.1 g/L, the Pb concentration only decreased by 29.7%. However, as the MP concentration increased to 0.5 g/L, the Pb concentration dropped significantly by 97.1%, indicating a substantial reduction in Pb in the solution. Further increasing the MP concentration to 1.5 g/L and 2 g/L resulted in the complete removal of Pb from the solution, with the Pb concentration dropping to 0 mg/L. Across all experimental groups, a rapid initial adsorption process was observed during the first 48 h. At the highest MP concentration of 2 g/L, Pb removal efficiency reached 99% within this period. Additionally, for MP concentrations exceeding 1 g/L, the adsorption process approached equilibrium within 48 h. Interestingly, despite the increased Pb removal efficiency with higher MP concentrations, the maximum adsorption capacity of MPs decreased as the MP concentration increased. Specifically, the maximum adsorption capacity decreased from 86.2 mg/g at 0.1 g/L to 15 mg/g at 2 g/L.

3.2. Effect of Surface Biofilm on Pb Adsorption by MPs

To assess the impact of biofilms on Pb adsorption, crushed MP samples of varying sizes (0.2, 0.5, and 1 mm) were exposed to seawater for two weeks, allowing for natural biofilm formation (Figure 8). Following biofilm development, both biofilm-coated and non-coated MPs were subjected to adsorption experiments using a Pb solution with a concentration of 30 mg/L. The experimental conditions included an adsorbent concentration of 1 g/L, and the adsorption was monitored for 48 h to track the equilibrium point.

As shown in Figure 9, biofilm-coated MPs exhibited a significantly higher Pb adsorption capacity compared to non-coated MPs, quickly reaching the adsorption equilibrium within 48 h. The extracellular polymeric substances (EPS) within the biofilm matrix are believed to play a crucial role in enhancing Pb adsorption by providing additional binding sites [17]. Overall, biofilm-coated MPs demonstrated a markedly higher adsorption capacity for Pb ions compared to their non-coated counterparts. This result aligns with previous studies, suggesting that biofilm presence enhances pollutant adsorption by increasing the available surface area and providing additional functional groups for metal binding. The EPS in the biofilm, which include polysaccharides, proteins, and lipids, play a crucial role in facilitating this enhanced adsorption [18].

The results highlight the influence of biofilm formation in enhancing the adsorption of heavy metals, such as Pb, onto MPs. The biofilm matrix acts as a secondary adsorbent, contributing to the overall adsorption capacity of the MPs and accelerating the adsorption speed of MPs for Pb. This finding suggests that in natural environments, where biofilms are likely to form on MPs over time, the ecological risks posed by heavy metal-laden MPs could be greater than previously anticipated. On the contrary, this characteristic of biofilm-coated MPs can have a high potential to control Pb pollution by passivating it.

3.3. MP Recovery

Magnetic nanoparticles, particularly Fe₃O₄, have shown significant potential for the recovery of MPs. However, it remains unclear whether this method is equally effective for MPs with Pb on their surface. In this study, Fe₃O₄ nanoparticles were introduced into a system containing Pb-contaminated wastewater and crushed MPs to evaluate their recovery efficiency.

3.3.1. The Effect of Different Masses of Fe₃O₄ on Microplastic Adsorption

To assess the feasibility of the recovery process, initial experiments were conducted using deionized water and crushed MPs. A total of 40 mg of crushed MPs was added to 40 mL of deionized water, and three parallel samples were prepared. These samples were shaken at 120 rpm in a large-capacity shaker for 3 days to ensure sufficient interaction. Following this, varying amounts of Fe₃O₄ nanoparticles (20 mg, 30 mg, 40 mg, 50 mg, and 60 mg) were added to the samples for adsorption experiments, with the mixtures stirred at 100 Hz for 10 min. The MPs were then recovered by applying a magnetic field, which separated them from the solution.

As shown in Figure 10, the recovery efficiency of MPs increased with the mass of Fe₃O₄ nanoparticles, ranging from 85.2% at 20 mg to 99.3% at 60 mg. These results clearly demonstrate that increasing the amount of Fe₃O₄ nanoparticles significantly enhances recovery efficiency. The large surface area, surface modifications, and magnetic properties of Fe₃O₄ facilitate efficient interaction with MPs, promoting their effective recovery. Moreover, the magnetic properties of Fe₃O₄ enable easy and rapid separation of adsorbed MPs using a magnet, further demonstrating the practicality of this approach in real-world applications. Compared to other recovery methods reported in the literature, Fe₃O₄ nanoparticles provide highly competitive recovery efficiency. For example, studies on Fe₃O₄-based MP removal have shown removal rates exceeding 80% under optimal conditions [19,20]. The combination of high adsorption efficiency and the simplicity of magnetic separation underscores Fe₃O₄ nanoparticles as an effective and scalable solution for MP recovery.

The increase in recovery efficiency with a higher mass of Fe₃O₄ can be attributed to the nanoparticles’ large surface area and superparamagnetic properties, which provide more adsorption sites and enhance their contact with MPs. However, as seen in Figure 10, when the mass of Fe₃O₄ is lower (e.g., 20 mg), the adsorption capacity is limited, resulting in a significant decrease in the removal rate. This suggests that, in practical applications, the amount of Fe₃O₄ should be optimized based on the concentration of MPs to maximize recovery efficiency. Compared to membrane technologies, which also demonstrate high removal efficiency, such as membrane bioreactors combined with activated sludge achieving up to 99.5% removal [43], Fe₃O₄-based recovery offers a simpler, more cost-effective alternative.

3.3.2. The Effect of Different Adsorption Capacities of MPs on Fe₃O₄ Adsorption

In the next stage of the experiments, Fe₃O₄ nanoparticles were tested in Pb-contaminated wastewater to evaluate whether the adsorption of Pb by MPs would impact the efficiency of Fe₃O₄ in recovering these MPs. In this setup, 40 mg of crushed MPs was added to 40 mL of Pb-contaminated solutions at three different concentrations of Pb (10, 30, and 50 mg/L), and UV–visible spectrophotometry confirmed that the Pb concentrations decreased significantly, nearing zero. The adsorption capacities of the MPs were calculated as 10, 30, and 50 mg/g, respectively.

Subsequently, 60 mg of Fe₃O₄ nanoparticles were introduced into each sample to assess the recovery efficiency of Fe₃O₄ under varying adsorption capacities of MPs. Figure 11 shows the relationship between the removal rate of MPs and their adsorption capacity. At low adsorption capacities (0 mg/g), the recovery rate of MPs remains high at 99.3%. However, as the adsorption capacity of MPs increases, a gradual decline in the recovery rate is observed, with the rate decreasing to 92.5% when the adsorption capacity reaches 50 mg/g. This decline in efficiency suggests that as more Pb ions are adsorbed in the MPs, they begin to compete for the active adsorption sites on Fe₃O₄ nanoparticles, reducing the ability of Fe₃O₄ to effectively adsorb the MPs. Additionally, the surface properties of Pb-adsorbed MPs may change, weakening their interaction with Fe₃O₄ nanoparticles and contributing to the reduced recovery rate.

These findings indicate that the adsorption of Pb onto MPs negatively affects the interaction between Fe₃O₄ nanoparticles and MPs. This points to the need for optimization in real-world applications, where MPs may carry significant loads of heavy metals. Adjustments to Fe₃O₄ dosage and other recovery parameters may help counteract this decline in efficiency and maintain high removal rates. For example, pre-screening water to capture larger MPs before Fe₃O₄ adsorption could enhance the overall recovery efficiency by reducing the burden on Fe₃O₄ in heavy metal-laden environments [44].

Overall, Fe₃O₄ nanoparticles remain a promising solution for scalable and cost-effective MP removal. The combination of high recovery efficiency (over 85%) and the ease of magnetic separation makes Fe₃O₄ an attractive alternative to more complex and expensive technologies like membrane filtration. However, further research into the interaction between heavy metal adsorption and MP recovery is essential to optimize this approach across varying environmental conditions and contamination levels.

4. Conclusions

This study investigated the pollution characteristics of crushed PVC MPs, providing insights into their environmental and health risks when carrying harmful substances. SEM-EDS analysis revealed that crushed MPs exhibit a significantly larger adsorption capacity compared to primary MPs, attributed to their increased surface area and more adsorption sites, including oxygen-containing functional groups. This study confirmed the critical influence of MP size, MP quantity, pH, salinity, and biofilm formation on their adsorption capacity. Smaller MPs demonstrated higher adsorption due to their larger surface area, while elevated pH enhanced adsorption efficiency. Conversely, increased salinity reduced adsorption, likely due to competition between ions for adsorption sites. Increasing MP concentrations improved Pb removal efficiency but higher MP quantities led to a decrease in maximum adsorption capacity, highlighting a trade-off between removal efficiency and adsorption capacity. The formation of biofilms on MPs further increased their adsorption capacity, highlighting the potential for biofilm-coated MPs to act as vectors for transporting harmful substances through aquatic ecosystems, exacerbating ecological risks. Adsorption kinetics were best described by the pseudo-second-order and intra-particle diffusion models, indicating chemical adsorption and boundary layer diffusion as dominant processes. Isothermal adsorption followed a multi-layer mechanism, best characterized by the Freundlich model, with SEM confirming heterogeneous surface morphology. Additionally, Fe₃O₄ nanoparticles demonstrated a high recovery efficiency of 99.3% for MPs, emphasizing their potential for environmental remediation, but they experienced reduced performance when Pb was adsorbed. These findings provide critical evidence for understanding how crushed MPs interact with environmental pollutants and contribute to ecological risks. The results underscore the importance of addressing the pollution risks associated with crushed MPs in aquatic environments, offering a theoretical foundation for evaluating and mitigating their impact.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/w16233541/s1, Table S1 The adsorption results parameters for heavy metal Pb by MPs of different materials; Table S2 Adsorption kinetic and isotherm models used in this study; Table S3 adsorption kinetics fitting parameters of heavy metal Pb on MPs; Table S4 Adsorption kinetics parameters on crushed MPs in different background solutions; Table S5 Adsorption isotherms parameters on crushed MPs in different background solutions.

Author Contributions

S.L.: Conceived and designed the experiments; Performed the experiments; Analyzed and interpreted the data; Contributed data; Wrote the paper. L.C.: Conceived and designed the experiments; Analyzed and interpreted the data; Contributed reagents materials, analysis tools or data; Wrote the paper. Q.L.: Contributed reagent materials and analysis tools. S.S.: Performed the experiments. J.B.: Contributed reagent materials and analysis tools. X.Z.: Supervision and project administration. Y.G.: Conceived and designed the experiments. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the project of Zhoushan Science and Technology Bureau (grant no. 2020C11246).

Data Availability Statement

The datasets generated and analyzed in this current study are available from the corresponding author upon reasonable request.

Conflicts of Interest

The authors declare no conflict of interests.

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Figure 1. (a) SEM image of primary MPs (magnification: 10.00KX); (b) SEM image and corresponding elemental composition (EDS) bar chart of crushed MPs; (magnification: 10.00K X) (c) SEM image and corresponding elemental composition (EDS) bar chart of Pb-adsorbed MPs (magnification: 10.00K X).

Figure 2. The adsorption of Pb on primary MPs and crushed MPs with time.

Figure 3. Adsorption curves of crushed MPs with different particle sizes in Pb solution. Experimental conditions: [adsorbents] = 1 g/L; [Pb] = 30 mg/L.

Figure 4. Adsorption characteristics of crushed MPs in Pb solutions: adsorption curves (a), adsorption kinetics (b), and intra-particle diffusion model (c). Experimental conditions: [adsorbents] = 1 g/L; [MP size] = 0.2 mm.

Figure 5. Adsorption isotherm of heavy metal Pb solution on MPs. Experimental conditions: [adsorbent] = (0.1 g/L, 0.2 g/L, 0.3 g/L); [MP size] = 0.2 mm.

Figure 6. Effect of the pH (a) and salinity (b) on the adsorption of Pb on crushed PVC. Experimental conditions: [adsorbents] = 0.3 g/L; [MP size] = 0.2 mm.

Figure 7. Adsorption curves of crushed MPs with different concentrations in Pb solution. Experimental conditions: [MP size] = 0.2 mm; [Pb] = 30 mg/L.

Figure 8. Biofilm formation on the surface of MPs (magnification: 350X).

Figure 9. Effect of biofilm-coated MPs and non-coated MPs on Pb adsorption in crushed PVC. Experimental conditions: [adsorbent] = 1 g/L; [Pb] = 30 mg/L.

Figure 10. The effect of Fe₃O₄ mass on the recovery efficiency of MPs.

Figure 11. The effect of adsorption capacity of MPs on their recovery efficiency by Fe₃O₄ nanoparticles in Pb-contaminated wastewater.

Table 1. Table of experimental conditions and settings.

Experimental Influencing Factors	MP Type	MP Size	Initial Pb Concentration	The Number of MPs	pH	Salinity
MP type	Primary PVC MPs and crushed PVC MPs	0.5 mm	30 mg/L	1 g/L	7	0
MP size	Crushed PVC MPs	1, 0.5 and 0.2 mm	30 mg/L	1 g/L	7	0
Number of MPs	Crushed PVC MPs	0.2 mm	30 mg/L	0.1, 0.5, 1, 1.5 and 2 g/L	7	0
pH	Crushed PVC MPs	0.2 mm	30 mg/L	0.3 g/L	4, 5, 6, 7 and 8	0
Salinity	Crushed PVC MPs	0.2 mm	30 mg/L	0.3 g/L	7	0, 1% and 3.5%
Presence of biofilms on MPs	Biofilm-coated and non-coated MPs	1, 0.5 and 0.2 mm	30 mg/L	1 g/L	7	0

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Li, S.; Cao, L.; Liu, Q.; Sui, S.; Bian, J.; Zhao, X.; Gao, Y. Enhancing Pb Adsorption on Crushed Microplastics: Insights into the Environmental Remediation. Water 2024, 16, 3541. https://doi.org/10.3390/w16233541

AMA Style

Li S, Cao L, Liu Q, Sui S, Bian J, Zhao X, Gao Y. Enhancing Pb Adsorption on Crushed Microplastics: Insights into the Environmental Remediation. Water. 2024; 16(23):3541. https://doi.org/10.3390/w16233541

Chicago/Turabian Style

Li, Sen, Lu Cao, Qiyuan Liu, Shuting Sui, Jiayin Bian, Xizeng Zhao, and Yun Gao. 2024. "Enhancing Pb Adsorption on Crushed Microplastics: Insights into the Environmental Remediation" Water 16, no. 23: 3541. https://doi.org/10.3390/w16233541

APA Style

Li, S., Cao, L., Liu, Q., Sui, S., Bian, J., Zhao, X., & Gao, Y. (2024). Enhancing Pb Adsorption on Crushed Microplastics: Insights into the Environmental Remediation. Water, 16(23), 3541. https://doi.org/10.3390/w16233541

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Enhancing Pb Adsorption on Crushed Microplastics: Insights into the Environmental Remediation

Abstract

1. Introduction