Advertisement
Browse Subject Areas
?

Click through the PLOS taxonomy to find articles in your field.

For more information about PLOS Subject Areas, click here.

  • Loading metrics

Open Access

Peer-reviewed

Research Article

Adsorption and desorption of methyl orange dye on environmentally aged polyethylene, polyethylene terephthalate and polystyrene microplastics in aquatic environment

  • Ahsan Habib,

    Roles Data curation, Formal analysis, Methodology, Software, Visualization, Writing – original draft

    Affiliation Department of Environmental Science and Technology, Jashore University of Science and Technology, Jashore, Bangladesh

    ⨯
  • Sayedunnesa Nishi,

    Roles Data curation, Investigation, Methodology, Validation

    Affiliation Department of Environmental Science and Technology, Jashore University of Science and Technology, Jashore, Bangladesh

    ⨯
  • Md. Muhaiminul Haque,

    Roles Data curation, Formal analysis, Methodology, Visualization

    Affiliation Department of Environmental Science and Technology, Jashore University of Science and Technology, Jashore, Bangladesh

    ⨯
  • Md. Tauhiduzzaman,

    Roles Data curation, Formal analysis, Resources

    Affiliation Department of Environmental Science and Technology, Jashore University of Science and Technology, Jashore, Bangladesh

    ⨯
  • Kobita Khatun,

    Roles Data curation, Formal analysis, Methodology, Visualization

    Affiliation Department of Environmental Science and Technology, Jashore University of Science and Technology, Jashore, Bangladesh

    ⨯
  • Mst Shamima Akter,

    Roles Data curation, Methodology, Resources, Validation

    Affiliation Department of Environmental Science and Technology, Jashore University of Science and Technology, Jashore, Bangladesh

    ⨯
  • Esrat Sultana,

    Roles Data curation, Formal analysis, Methodology

    Affiliation Department of Environmental Science and Technology, Jashore University of Science and Technology, Jashore, Bangladesh

    ⨯
  • Tapos Kumar Chakraborty,

    Roles Conceptualization, Methodology, Project administration, Writing – review & editing

    Affiliation Department of Environmental Science and Technology, Jashore University of Science and Technology, Jashore, Bangladesh

    ⨯
  • Samina Zaman,

    Roles Conceptualization, Methodology, Writing – review & editing

    Affiliation Department of Environmental Science and Technology, Jashore University of Science and Technology, Jashore, Bangladesh

    ⨯
  • Gopal Chandra Ghosh

    Roles Conceptualization, Funding acquisition, Methodology, Project administration, Resources, Supervision, Validation, Writing – original draft, Writing – review & editing

    gopal@just.edu.bd, gopales8@hotmail.com

    Affiliation Department of Environmental Science and Technology, Jashore University of Science and Technology, Jashore, Bangladesh

    ⨯

Abstract

This study investigated the adsorption of methyl orange (MO), an anionic dye, on environmentally aged polyethylene (PE), polyethylene terephthalate (PET), and polystyrene (PS) microplastics (MPs) to understand their interactions in aquatic environments. The MPs were characterized using Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The adsorption experiment data followed pseudo-second-order kinetics and fit well with the Langmuir and Freundlich isotherm models. The adsorption capacities of MO onto the MPs were 2.86 mg/g, 3.64 mg/g, and 3.81 mg/g for PE, PET, and PS, respectively, at a neutral pH of 7.0. The MPs combined with MO mainly through chemisorption, hydrogen bonding, and hydrophilic interaction. The optimum conditions for MO (23.41% for PE, 22.8% for PET, and 22.64% for PS) removal by the MPs were: pH 2, MO concentration of 27.5 mg/L, and MPs dose of 15.0 g/L, as determined using response surface methodology (RSM). Additionally, the presence of salt (NaCl) and humic acid (HA) competed with MO for adsorption sites on the MPs. The desorption of MO from the MPs was relatively higher in freshwater (16–30%) than in simulated seawater (12–19%). This study elucidates the interaction of MO with environmentally aged PE, PET, and PS MPs in aquatic environments, and demonstrates the transport capacity of MO dye from wastewater to freshwater, and eventually to the ocean.

Citation: Habib A, Nishi S, Haque MM, Tauhiduzzaman M, Khatun K, Akter MS, et al. (2025) Adsorption and desorption of methyl orange dye on environmentally aged polyethylene, polyethylene terephthalate and polystyrene microplastics in aquatic environment. PLoS One 20(7): e0323516. https://doi.org/10.1371/journal.pone.0323516

Editor: Amitava Mukherjee, VIT University, INDIA

Received: April 9, 2025; Accepted: June 29, 2025; Published: July 28, 2025

Copyright: © 2025 Habib et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Data Availability: We confirm that our submission includes all relevant raw data required to replicate the results presented in the study. These have been uploaded as Supporting Information files alongside the main text.

Funding: The author(s) received no specific funding for this work.

Competing interests: The authors have declared that no competing interests exist.

1. Introduction

Microplastics (MPs) are either fragments of larger plastic particles or engineered plastic particles less than 5 mm in size [1]. Over the years, MPs have emerged as contaminants of concern due to their extensive use, widespread presence, persistence, difficulty in degradation, and eco-toxicological effects [2]. Global plastic production has risen dramatically from 1.5 million tons in the 1950s to 390 million tons in 2021 [3]. The combination of continuous disposal and low recycling rates (10%) has led to significant environmental challenges [4–9]. One of these is the gradual breakdown of plastic polymers into smaller fragments, such as MPs [10,11]. Plastics are highly persistent in aquatic environments, causing harmful physical and biological consequences to organisms, including plants and animals life [12] by ingestion, entanglement, and physical damage [13–16]. During this period plastic particles undergo a range of weathering processes driven by factors such as sunlight, heat, thermal breakdown, biological activity and oxidative reactions. These processes, collectively known as environmental aging, can significantly modify the physical and chemical characteristics of materials—such as surface morphology, specific surface area, hydrophobicity, structural integrity, mechanical resilience, and elemental composition—and may influence their behavior in aquatic environments as well as their interactions with pollutants [17–19]. Toxic chemicals leaching from MPs, such as additives like plasticizers, flame retardants, and antioxidants, pose further risks to marine organisms by entering the food chain and triggering bioaccumulation and biomagnification [9,20–26].

The potential of MPs to transfer and expose pollutants to aquatic ecosystems is enhanced by their small size, large surface area, porosity, amorphous structure, strong hydrophobicity, and various functional groups [27–29]. MPs readily adsorb and interact with organic and inorganic contaminants [26,30,31] such as toxic trace elements [32], heavy metals [33], fungicides [34], antibiotics [35], dyes [22,36,37], persistent organic pollutants (POPs), polychlorinated biphenyls (PCBs), polyaromatic hydrocarbons (PAHs), bacteria, and viruses [21,38–40]. These interactions create secondary pollutants, amplifying toxic effects on ecological functions [31], and these complex contaminants may release pollutants upon entering new environments [41]. Organisms consuming such contaminants are exposed to dual toxicity risks [10,41–43].

Therefore polymers are increasingly recognized for their potential as effective adsorbent, largely due to their diverse physiochemical characteristics such as modifiable surface properties [36] which results affinity for adsorption, relative high surface area, functional groups [31,44] electronic structures [45–48]. Recent studies have shown that nonconductive polymers like polyethylene (PE), Polyethylene Terephthalate (PET) and polystyrene (PS) can exhibit substantial adsorption capacities for organic and inorganic contaminants [17,28,36,49,50]. The primary attributed to mechanisms such as van der Waals forces, hydrophobic interactions, and π–π interactions. Furthermore environmentally aged MPs surfaces led to the formation of oxygen containing functional groups and micro cracks, thereby enhancing the adsorption capacity of MPs with contaminants [49].

Among the toxic pollutants adsorbed by MPs, dyes are of critical concern, particularly in developing nations [51]. Dyes are widely used in industries such as plastics, textiles, cosmetics, paper, paints, food, ink, varnishes, and tanneries [52–54]. Globally, over 700,000 tons of dyes are produced annually, with the textile industry as the largest consumer [55]. During dyeing processes, 10–15% of dye contaminants are discharged into aquatic environments [49,56]. Alarmingly, over 50% of dyes used during industrial manufacturing do not adhere to fabrics and are released as colored effluent [57,58]. Even trace amounts of dyes significantly restrict light penetration in aquatic environments, disrupting photosynthesis and biological metabolism in organisms [59,60].

Methyl orange (MO) is an anionic, acidic, and water-soluble azo dye widely used in the textile industry [61,62]. It is also released from diverse sources such as research laboratories, food processing, paper manufacturing, printing, pharmaceuticals, plastics, leatherworking, cosmetics, and dye production [62–64]. This azo dye’s molecular structure includes aromatic and -N = N- groups, making it highly toxic, carcinogenic, and teratogenic [65–67]. MO poses severe health risks, impacting kidney and liver function, the central nervous system, brain, and reproductive systems [64,68,69].

Although studies on dye adsorption onto MPs are limited, recent investigations have explored various interactions, including malachite green on nylon MPs [70], methylene blue on PE [36], crystal violet on naturally aged MPs [17], rhodamine B on polyvinyl chloride (PVC) MPs [49], and MO with malachite green on polyamide MPs [71]. Yet there are no research has explored the interaction between specific aged polymers and anionic dyes under varying environmental conditions. Notably, the effects of key factors on adsorption and desorption dynamics. Understanding the interactions between MO and environmentally aged MPs (PE, PET, and PS) will shed light on the role of MPs as pollutant carriers and their potential ecological impacts in aquatic environment.

The main objectives of this study are: 1) to identify the role of environmentally aged MPs properties on MO adsorption, 2) to investigate the influence of controlling factors (contact time, pH, salinity, HA, adsorbent, and adsorbate doses) on MO adsorption onto the MPs, 3) to explore the adsorption performance of MO on the MPs in a freshwater environment, and 4) to evaluate the desorption efficiency of MO from MPs in both freshwater and marine environment.

2. Materials and methods

2.1 Chemicals and reagents

MO was procured from Sigma-Aldrich (MO, USA). Sodium hydroxide (NaOH) and sodium chloride (NaCl) were obtained from Merck Specialities Private Limited (Mumbai, India), while hydrochloric acid (HCl) was sourced from Merck (Darmstadt, Germany). Humic acid (HA) was acquired from Loba Chemie Private Limited (Mumbai, India). A stock solution of MO (1000 mg/L) was prepared using double-distilled water and stored at 4°C until use. All working standards were freshly prepared from the stock solution through dilution on the day of the experiments.

2.2 Preparation and characterization of adsorbent

Environmentally aged PE, PET, and PS plastics were collected from a waste dumping site located in Jashore City, Bangladesh. They were then ultrasonically washed (GT SONIC, VGT-2227QTD, Germany) with distilled water to remove adhering particles from the plastic surfaces and dried at 70°C in an oven (Labtech LDO–150F, Korea). The dried PE, PET, and PS plastics were then cut into small pieces (< 5 mm in length) and sieved to obtain a size range between 0.5 mm and 1 mm. Finally, the MPs were stored in a borosilicate glass bottle and kept in desiccators for further studies.

FTIR spectra of the prepared MPs were recorded before and after adsorption of MO in an FTIR spectrometer (NICOLET IS20, Thermo Scientific, USA) in the range of 500–4,000 cm-1 with 16 co-scans collected at 4 cm-1 resolutions, and well-equipped with attenuated total reflection (ATR) units. The obtained spectra were processed with Omnic software (OMNIC 8.2) from Nicolet Instrument Corp. (Madison, WI, USA) and identified by comparing them with the internal polymer spectra library databases (Hummel polymer sample library, HR Nicolet Sampler Library, Aldrich condensed Phase Sample Library, etc). Finally, the spectrum obtained by ATR-FTIR that matched >70% to the reference database to confirm the specific kind of polymer. The surface morphology of the MPs was observed before and after the adsorption of MO using a field emission scanning electron microscope (FE–SEM, Zeiss Sigma, Carl Zeiss, Germany) with an accelerating voltage of 10 kV. Before imaging, the samples were coated with a thin gold layer using an ion sputtering device (Hitachi E-1045, Japan) to improve image quality and avoid the buildup of local electrical charges. The pH of the zero-point charge (pHzpc) of the MPs was obtained by adding 1 g/L of MPs to 100 mL of 0.01 M NaCl solution with different pH values (pH 2–11) in Erlenmeyer flasks at room temperature for 48-hour reaction periods [72]. The final pH of each solution was determined. The difference between the final and initial pH (ΔpH) values was then plotted against the initial pH values, and the point where ΔpH is zero was taken as the pHzpc.

2.3 Adsorption experiments

All experiments were performed using 50 mL of working solution in 60 mL glass screw-cap tubes at room temperature (25 ± 2°C). MO adsorption onto MPs was conducted through batch experiments using a suspension mixer (SM-3000, Digisystem Laboratory Instruments Inc., Taiwan) at 120 rpm. Experimental conditions included varying contact times (0.5–96 hours), pH levels (pH 2–11), adsorbent doses (1–15 g/L), initial MO concentrations (5–50 mg/L), sodium chloride (NaCl) concentrations (0–16 g/L), and humic acid (HA) concentrations (0–50 mg/L) in a freshwater environment. To investigate the influence of pH on adsorption, the solution pH was adjusted using 0.1 N HCl or 0.1 N NaOH. After completing each experiment, samples were filtered using glass microfiber filter paper (GF/B, Whatman, USA) to remove adsorbent particles. The MO concentration in the filtrate was measured using a UV-visible spectrophotometer (HACH DR 3900, USA) at a wavelength of 464 nm. Duplicate experiments were conducted, and mean values were used for analysis. The amount of MO adsorption at equilibrium and the removal (R (%)) were calculated by using equations (1) and (2), respectively.

(1)(2)

Where Co and Ce are the initial and equilibrium MO concentrations (mg/L), respectively. qe is the equilibrium MO adsorption capacity (mg/g). V is the volume of solution (L) and ms is the mass of the adsorbent (g).

Adsorption kinetics experiments were conducted by adding 1 g/L of adsorbent to 50 mL of MO solution (10 mg/L concentration) at pH 7, with stirring at 120 rpm at room temperature (25 ± 2°C). Samples were collected at specified time intervals (0.5, 1, 2, 4, 8, 12, 16, 24, and 48 hours), filtered, and analyzed.

Adsorption isotherm experiments were carried out using 50 mL of MO solutions with varying initial concentrations (5, 10, 20, 30, 40, and 50 mg/L). A fixed adsorbent dose of 1 g/L was added to each tube, with solution pH maintained at 7 and room temperature (25 ± 2 °C) for equilibrium contact time.

2.4 Box-Behnken response surface methodology (BB RSM)

A three-factor, three-level Box-Behnken experimental design comprising 17 tests was employed using Stat-Ease software (Design-Expert 13.0, Stat-Ease, Inc., Minneapolis, USA) in combination with response surface modeling (RSM). This approach was used to identify the optimal conditions for maximizing the adsorption ofMO) dye onto MPs. The response variable was the MO removal efficiency, while the experimental factors—denoted as A, B, and C—represented the coded terms for the three independent test variables: pH, initial MO dye concentration, and MPs dose, respectively (S1 Table). The experimental design levels were set at −1, 0, and +1 (S1 Table). The RSM polynomial equation (Equation 3) was utilized to model and predict MO dye removal efficiency from the aqueous solution:

(3)

Where, Y is the predicted response (MO removal efficiency), β0 and βi are constant and linear coefficients, respectively. βij is the interaction coefficients, βii is the quadratic coefficients, Xi and Xj are the coded values of the process variables, ε is the error, and n is the number of variable studies.

2.5 Desorption experiments

The saturated adsorbent MPs were collected after the adsorption process reached equilibrium and subsequently dried in a drying oven at 60°C. For the desorption study, 50 mg of dried MO-loaded MPs were combined with 50 mL of freshwater and simulated seawater (details provided in the supplementary materials) at the natural pH levels of the solutions. The mixtures were agitated at ambient temperature (25 ± 2°C) and a stirring speed of 120 rpm for 48 hours using a suspension mixer. The desorption efficiency (D %) was estimated with the equation (4).

(4)

Where Co (mg/L) is the initial dye concentration and Ce (mg/L) is the dye concentration at equilibrium, Cd (mg/L) indicates the desorbed dye concentration in the desorption process. The volume of fresh or simulated seawater in the desorption study is denoted by Vd (L), and dye solution as Vi (L).

2.6 Data analysis and model fitting

To analyze the kinetic behavior of the adsorption process, Lagergren’s pseudo-first-order model [73] and Ho’s pseudo-second-order model [74] were utilized. Additionally, the diffusion mechanism and the potential rate-controlling steps were examined using the inter-particle diffusion model [75]. To further understand the interaction between the adsorbate and adsorbent, and to evaluate the nature of these interactions, two equilibrium adsorption isotherm models—Langmuir [76] and Freundlich [77]—were applied. The respective linear equations for these models are presented in Table in S2 Table.

2.7 Quality control and quality assurance

Environmentally aged PE, PET, and PS plastics were collected and prepared for batch adsorption process by soaking with de-ionized water then use ultra-sonication to remove contaminants from plastics cleaning followed by thoroughly washing with distilled water. Plastics chopped into small pieces using scissors and then dried for one days at 70°C. After that, by grinding and sifting, the PE, PET, and PS MPs particle were store in vacuum desiccator for future studies.

All the prepared solutions (stock solution and the working solution) for adsorption process were prepared with de-ionized water and the amounts of solute were weight accurately with digital weight machine. Each batch experiment was carried out in a glass screw-cap tube with a 60 mL volume capacity. Glassware and other experimental materials were cleaned and dried before use. Each clean glass tube was first filled with a specific amount of MPs, and then a layer of Teflon tape was created to cover the glass tube. Then, using a syringe, prepared working solutions of MO dye were injected into each tube. After vertical shaking, the solution was filtered by GF/ B Whatman 1 mm filters with a diameter of 47 mm was used to remove the adsorbent after each experiment to filter the samples at a specific volume. Then, absorbance was measured using a UV visible spectrometer at a particular wavelength.

To minimize the risk of contamination during sample processing, all experimental work was conducted in a clean enclosed chamber. Throughout the procedures laboratory personnel were cotton lab coats and latex gloves to further reduce external contamination sources. To ensure the accuracy of the adsorption data, blank control samples were prepared and processed during the exact same procedures as the test samples. Furthermore, all adsorption experiments were performed in triplicate under the same experimental conditions to verify reproducibility. The consistencies observed across these replicates confirm the reliability and repeatability of the results.

3. Results and discussion

3.1 Identification and characteristics of MPs

Fig 1(a) illustrates the FTIR results before and after the adsorption of MO onto PE, PET, and PS. In the FTIR spectrum of PE, a strong peak at 3434 cm ⁻ ¹ corresponds to O–H vibrations, while bands at 2919 cm ⁻ ¹ and 2850 cm ⁻ ¹ are associated with the bending vibration of C–H. Other notable peaks include a C = O vibration at 1715 cm ⁻ ¹, an aromatic ring vibration at 1471 cm ⁻ ¹, and a pronounced peak around 719 cm ⁻ ¹, attributed to the stretching vibration of C–Cl [78]. For PS, a peak at 3391 cm ⁻ ¹ corresponds to O–H stretching vibration, while CH₂ molecular vibrations are observed at 3026 cm ⁻ ¹, 2920 cm ⁻ ¹, and 2851 cm ⁻ ¹. Additionally, peaks related to the aromatic ring vibrations appear at 1600 cm ⁻ ¹, 1491 cm ⁻ ¹, 1028 cm ⁻ ¹, and 753 cm ⁻ ¹ [79]. In the FTIR spectrum of PET, O–H stretching is observed at 3429 cm ⁻ ¹, CH₂ wagging at 2969 cm ⁻ ¹, O = H stretching at 1716 cm ⁻ ¹, C–O rocking at 1264 cm ⁻ ¹, and benzene ring stretching at 873 cm ⁻ ¹ and 732 cm ⁻ ¹ [80]. These findings indicate that the MPs possess surface functional groups capable of linking and interacting with MO molecules (S3 Table).

thumbnail
Download:
Fig 1. (a) FTIR spectra of the MPs: before (PE, PET and PS) and after (PE + MO, PT + MO and PS + MO) adsorption of MO; and (b) pH of point zero charges of PE, PET and PS MPs.

https://doi.org/10.1371/journal.pone.0323516.g001

SEM images of the MPs reveal distinctly irregular profiles and relatively wrinkled surfaces, offering a large surface area for interaction with MO dye molecules (S1(a), S1(c) and S1(e) Fig). The study indicates that the surfaces of the MPs were moderately saturated with MO dye molecules following adsorption (S1(b), S1(d) and S1(f) Fig).

The ΔpH vs. initial pH curve (Fig 1(b)) reveals the pHzpc values of the MPs to be 5.2, 6.7, and 6.8 for PS, PE, and PET, respectively. These pHzpc values suggest that the MPs exhibit a positively charged surface when the pH is below the pHzpc and a negatively charged surface when the pH exceeds the pHzpc.

3.2 Effects of the different controlling factors on MO adsorption on MPs

3.2.1 Effect of contact time.

To evaluate the effect of contact time on MO adsorption by MPs, experiments were conducted with varying contact times (0.5–96 hr) under the following conditions: an initial MO dye concentration of 10 mg/L, an adsorbent dose of 1 g/L, pH 7, a stirring speed of 120 rpm, and a temperature of 25 ± 2°C. The results (Fig 2(a)) revealed three distinct phases in the adsorption process. During the initial phase (0.5–2 hr), the adsorption rate was rapid due to the high MO concentration and the abundance of active sites on the adsorbent surface. In the second phase (2–48 hr), the rate of adsorption decreased as the availability of active sites diminished. Finally, adsorption equilibrium was achieved at 48 hr, which was subsequently chosen as the optimal contact time for further experiments.

thumbnail
Download:
Fig 2. Effects of different controlling factor on MO adsorption by the MPs: (a) Contact time, (b) pH, (c) MPs doses, (d) Initial concentration, (e) NaCl concentration, and (f) HA concentration.

https://doi.org/10.1371/journal.pone.0323516.g002

Under these experimental conditions, MO removal efficiencies were recorded as 9.0% for PE, 6.3% for PET, and 8.7% for PS. Consistent with these findings, recent studies report that dye adsorption onto MPs typically reaches equilibrium within 24–72 hr, depending on the dye and plastic types.[17,28,36,49,81].

3.2.2 Effect of pH.

The adsorption of MO onto MPs across varying pH levels (pH 2–11) is depicted in Fig 2(b), demonstrating that MO adsorption is strongly influenced by the solution pH. Removal efficiency values for PE (23.40–4.83%), PET (22.80–1.23%), and PS (22.64–2.21%), as well as adsorption capacities for PE (2.34–0.48 mg/g), PET (2.28–0.12 mg/g), and PS (2.26–0.22 mg/g), consistently decreased as pH increased (pH 2–11).

Maximum removal efficiency was observed at lower pH levels (Fig 2(b)), attributed to electrostatic attraction between anionic MO dye molecules and positively charged adsorbent surfaces. Electrostatic sorption occurs when a negatively charged dye molecule (like methyl Orange) interact with a positively charged surface site on MPs [82]. Puckowski et al. reported that acidic environments significantly enhance pollutant adsorption onto MPs [83]. In contrast, as pH rises, MO removal efficiency and adsorption capacity progressively decline (Fig 2(b)) due to electrostatic repulsion between anionic MO dye molecules and negatively charged adsorbent surfaces (Fig 1(b)) [68,84].

3.2.3 Effect of adsorbent dose.

The effect of MPs dose on MO adsorption is illustrated in Fig 2(c), indicating that MO removal efficiency increased with higher MPs doses (PE: 8.82–11.17%; PET: 6.47–9.80%; PS: 7.53–10.55%) as the dose increased from 1 to 15 g/L. This improvement in removal efficiency can be attributed to the larger surface area and the greater availability of active sites on the adsorbent surface [52].

However, a higher adsorbent dose (1–15 g/L) resulted in a reduced adsorption capacity (PE: 0.88–0.07 mg/g; PET: 0.64–0.06 mg/g; PS: 0.75–0.07 mg/g), likely due to competition or overlapping of MO molecules on the MPs, such as molecular aggregation. Adsorption of organic and inorganic ions is known to depend on the adsorbent dose, with adsorption amounts decreasing as the adsorbent dose increases [17,37,85].

3.2.4 Effect of initial MO dye concentration.

The removal rate of MO decreases with increasing initial MO concentrations, as shown in Fig 2(d). Removal efficiencies for PE (8.76–3.71%), PET (6.57–3.82%), and PS (8.79–4.22%) decline as the initial concentration of MO rises. While the adsorption capacity of MPs remains constant at a fixed adsorbent dose, removal efficiency is significantly influenced by the initial concentration of MO dye in the solution. At higher MO concentrations, the saturation of active sites on the adsorbent surface leads to excess MO dye remaining unadsorbed in the solution.

The adsorption capacities of MPs (PE: 0.87–1.85 mg/g; PET: 0.65–1.91 mg/g; PS: 0.87–2.11 mg/g) increase with rising MO dye concentrations from 5 to 50 mg/L at a fixed MP dose of 1 g/L (Fig 2(d)). This trend is attributed to the greater driving force that enhances the transfer of MO dye from the liquid phase to the solid phase in aqueous solutions.

3.2.5 Effect of salinity.

NaCl, abundantly present in oceans, rivers, and lakes, plays a critical role in the adsorption of pollutants onto MPs [80]. Fig 2(e) illustrates the effect of salinity on MO adsorption by MPs, showing a gradual decrease in adsorption efficiency with increasing NaCl concentration (0–16 g/L). The removal efficiencies for PE (6.42–1.61%), PET (5.65–1.72%), and PS (6.80–1.78%) declined as NaCl concentration increased, primarily due to heightened competition between excess Na⁺ ions and MO dye molecules for adsorption sites on the MPs surface [86].

Moreover, the presence of Na⁺ ions reduces electrostatic interactions between MPs and MO by neutralizing the negative charges on the active adsorption sites of MPs [87]. Additionally, the increased cohesive energy caused by Cl⁻ ions enhances the cohesive density of MP chains, reducing the availability of free sites on the MPs and thereby diminishing the adsorption rate [88].

Electrostatic interaction is shown to be a crucial factor in the adsorption of MO onto MPs. Consistent with these findings, studies by Chen et al., Du et al., You et al., Zhong et al., and Xu have demonstrated that salinity reduces the adsorption of pollutants by MPs [28,36,49,89,90] a trend also observed in this study.

3.2.6 Effect of humic acid.

Humic acid (HA), a prominent component of natural organic matter (NOM), plays a crucial role in the transport, binding, analysis, and remediation of pollutants in natural environments due to its widespread occurrence and chemical reactivity [91]. Fig 2(f) illustrates the effect of HA concentration (0–50 mg/L) on MO dye adsorption onto MPs. A significant decline in MO adsorption was observed with increasing HA concentrations, with removal efficiencies decreasing for PE (8.66–0.96%), PET (6.53–0.79%), and PS (8.60–1.07%).

Previous research has consistently demonstrated the inhibitory effects of HA on pollutant adsorption onto MPs [46,90,92,93]. As a macromolecular organic substance, HA can alter or block the surface of MPs, forming a barrier that restricts the adsorption of MO ions at active sites [94,95].

3.2.7 Effect of the natural environment.

The objective of this experiment was to investigate the actual adsorption behavior of MO dye onto MPs in a natural freshwater environment, using Padma River water as a representative sample. The results (S2 Fig) revealed that the adsorption of MO onto MPs followed the order: PS > PET > PE. Adsorption capacities in the natural environment were recorded as PE (0.55–1.91 mg/g), PET (0.62–1.96 mg/g), and PS (0.73–2.13 mg/g), which were similar to those observed in controlled adsorption studies (PE: 0.87–1.85 mg/g; PET: 0.65–1.91 mg/g; PS: 0.87–2.11 mg/g) illustrated in Fig 2(d).

3.3 Adsorption kinetics

In this study, the adsorption behavior of MO onto MPs was analyzed using Lagergren’s pseudo-first-order, Ho’s pseudo-second-order, and intraparticle diffusion models (Fig 3). Experimental data were compared with the predictions of these adsorption kinetics models based on the correlation coefficient (R²), as summarized in Table in S4 Table. The pseudo-second-order kinetic model yielded higher R² values for MO adsorption compared to the pseudo-first-order model, indicating a better fit. The calculated adsorption capacities derived from the pseudo-second-order model (qe,cal: 1.003 mg/g for PE, 0.708 mg/g for PET, and 0.942 mg/g for PS) closely aligned with the experimental values (qe,exp: 0.878 mg/g for PE, 0.642 mg/g for PET, and 0.871 mg/g for PS) presented in S4 Table suggesting that chemisorption which indicates the chemical bonds between adsorbate molecules and the adsorbents [82] is the primary mechanism controlling the adsorption process [96,97]. The performance of these MPs compare with other adsorbents reported in Table in S5 Table.

thumbnail
Download:
Fig 3. (a) Pseudo first order, (b) Pseudo second order, (c) Intra-particle diffusion kinetics Models, (d) Langmuir, and (e) Freundlich Isotherm models for MO adsorption on the MPs (PE, PET, and PS).

https://doi.org/10.1371/journal.pone.0323516.g003

Diffusion mechanisms were further investigated using the intraparticle diffusion model. The intraparticle diffusion plot did not pass through the origin (Fig 3(c)), indicating that external diffusion, involving the aggregation of MO molecules on the adsorbent surface, was the rate-limiting step for MO adsorption onto MPs. Although internal diffusion, referring to the transport of MO molecules within the adsorbent particles, may also occur, it was not identified as the rate-limiting step in this study [28].

These results are consistent with findings from prior research, including studies on methylene blue dye adsorption on aged and virgin PE-MPs [36], crystal violet adsorption onto aged polyethylene and polypropylene MPs [17], and the adsorption of malachite green [70] and Rhodamine B [49] dyes onto MPs.

3.4 Adsorption isotherms

The equilibrium experimental data for MO adsorption onto MPs were analyzed using the Langmuir and Freundlich isotherm models (Fig 3). The adsorption isotherms and fitting results are summarized in Table in S6 Table. The findings indicate that the Langmuir isotherm provided a better fit for the adsorption of MO onto PE and PET, whereas the Freundlich isotherm better described the adsorption onto PS (S6 Table). Similar trends have been reported for chlorobenzenes and trifluralin adsorption on PE [98], rhodamine B adsorption on PET [49], and polychlorinated biphenyl adsorption on PS [99].

These results suggest that MO molecules form a monolayer with a uniform distribution on PE and PET surfaces. In contrast, the PS surface exhibits non-uniformity, with heterogeneous sites for MO adsorption. PS structure consists of a long hydrocarbon chain with a phenyl group gives PS unique properties allow interaction with other material [100]. PS holds aromatic structure with benzene rings that promote π-π interaction with dye molecules, contributing to heterogeneous adsorption sites. Additionally environmental aging can introduce oxygen containing functional groups and surface irregularities enhancing surface heterogeneity [101–103]. The Langmuir isotherm determined the maximum monolayer adsorption capacities for MO as 2.86 mg/g for PE, 3.64 mg/g for PET, and 3.81 mg/g for PS (S6 Table).

The RL values derived from the Langmuir isotherm (PE: 0.29–0.81, PET: 0.48–0.90, PS: 0.31–0.82) were between 0 and 1, confirming that MO adsorption onto MPs was favorable under the studied conditions. Similarly, the adsorption intensity (n) values (PE: 1.66, PET: 1.32, PS: 1.38) were greater than 1, and the Freundlich constant (KF) values (PE: 0.19, PET: 0.12, PS: 0.16) further demonstrated that the adsorption process was favorable for all MPs [104].

3.5 Box–Behnken experimental design and optimization of process variable by RSM

The data presented in Table in S7 Table lists the pH of the adsorbate, initial MO dye concentration, adsorbent dose, and the responses to the examined reactions. MO removal percentages ranged from 3.71–23.41% for PE, 1.23–22.8% for PET, and 2.21–22.64% for PS MPs. The highest MO removal was observed under acidic conditions (pH 2) with an adsorbent dose of 1 g/L, yielding removal efficiencies of 23.41% for PE, 22.8% for PET, and 22.64% for PS.

These results highlight the influence of solution pH on MO adsorption, indicating that the adsorbent surface acquires a net positive charge under acidic conditions. As pH decreases, the electrostatic interactions between the adsorbent surface and the acidic dye (MO) become more prominent, enhancing adsorption efficiency regression analysis was performed to develop response functions fitted to the experimental data. Equations (5), (6), and (7) represent the roles of specific variables (solution pH, MO dye concentration, and MPs dose) and their interaction effects. The adsorption of MO was significantly impacted by the interplay of these variables, as detailed in Equations (5–7):

(5)(6)(7)

The effects of these variables on MO removal efficiency (R%) for PE, PET, and PS MPs are illustrated in Fig 4. At low pH (2), adsorption is enhanced due to strong surface charge interactions, resulting in higher removal efficiencies (Runs 4, 15, 16, 17). At neutral pH (6.5), adsorption is moderate (Runs 1, 2, 5, 6, 8, 13) as electrostatic forces weaken, while at high pH (11), adsorption decreases significantly (Runs 3, 7, 9, 10, 11) due to repulsion effects.

thumbnail
Download:
Fig 4. 3D response surface model of MO dye removal efficiency: MO concentration (mg/L) and pH (a1 to a3), MPs dose (g/L) and pH (b1 to b3), and MPs dose (g/L) and MO concentration (mg/L) (c1 to c3) by PE, PET and PS, respectively.

https://doi.org/10.1371/journal.pone.0323516.g004

The interaction between pH and MO concentration further reveals adsorption efficiency variations. At low MO concentration (5 mg/L), lower pH improves adsorption slightly (Runs 1, 4, 7, 14), while at medium concentration (27.5 mg/L), acidic conditions consistently enhance adsorption (Runs 2, 5, 6, 8, 9, 13, 17). For high MO concentrations (50 mg/L), low pH (Run 15) results in significantly greater adsorption compared to high pH (Run 3).

Analysis of variance (ANOVA) was conducted to assess the significance of the second-order model (S8 Table). ANOVA results indicate that solution pH has a negative impact on MO adsorption, as MO is an anionic dye, and electrostatic attraction occurs between the positively charged MPs surface and MO dye molecules in an acidic environment. The model F-values (PE: 142.91, PET: 162.31, PS: 98.42) demonstrate that the model terms are statistically significant for PE, PET, and PS (P < 0.0001), as shown in Table in S8 Table. The P-value for “lack of fit” for all adsorbents was > 0.01, suggesting that random variations or noise may contribute to the lack of fit F-values without significantly affecting the model’s reliability.

Key model terms based on variable interactions were identified as follows: PE = A, C, AB, AC, BC, A²; PET = A, C, AB, AC, BC, A², B²; and PS = A, C, AB, AC, A². Solution pH and adsorbent dose are critical factors influencing MO adsorption, as outlined in Table in S7 Table. The coefficients of determination (R² values) for PE (0.994), PET (0.995), and PS (0.992) and adjusted R² values (PE: 0.987; PET: 0.989; PS: 0.982) indicate that the model accurately explains 99% of the response variability.

As illustrated in Fig 5, the experimental data points closely align with the model’s predictions, demonstrating a strong fit. The agreement between R² and adjusted R² values further confirms the model’s robustness. Additionally, the Adeq Precision values (signal-to-noise ratio) of PE (41.53), PET (46.01), and PS (34.44) exceed the desired threshold of 4, signifying sufficient signal strength. This model is suitable for navigating the design space.

thumbnail
Download:
Fig 5. Predicted removal efficiency versus actual removal efficiency of MO by (a) PE, (b) PET, and (c) PS MPs.

https://doi.org/10.1371/journal.pone.0323516.g005

3.6 Desorption behavior of MPs in fresh and simulated seawater

This study also explores the desorption behavior of MPs in freshwater and simulated seawater environments, as illustrated in Fig 6. During the desorption phase, the desorption rates of MO were observed to be higher in freshwater (PE: 27.48% (2.83 mg/g), PET: 30.93% (4.5 mg/g), PS: 16.79% (2.5 mg/g)) compared to seawater (PE: 19.11% (2.76 mg/g), PET: 16.75% (2.5 mg/g), PS: 12.18% (1.93 mg/g)).

thumbnail
Download:
Fig 6. Desorption of MO from (a) PE, (b) PET, and (c) PS MPs under freshwater and simulated seawater environment.

https://doi.org/10.1371/journal.pone.0323516.g006

This difference suggests that NaCl molecules in seawater compete with MO molecules for adsorption sites on the MPs, thereby influencing the balance between adsorption (the binding of MO molecules to the MPs surface) and desorption (the release of MO molecules from the MPs surface) [80]. Such processes highlight the regulatory role of NaCl in controlling adsorption and desorption mechanisms.

Consequently, MPs that have adhered to pollutants may pose greater risks to freshwater ecosystems and their associated organisms, as higher desorption rates in freshwater can enhance the mobility and bioavailability of pollutants, exacerbating their environmental impact.

4. Conclusions

This study assesses the adsorption and desorption capacity of environmentally aged PE, PET, and PS MPs for MO dye using batch experiments. Solution pH negatively impacts MO adsorption, while high concentrations of Na⁺ and HA obstruct the process. PE and PET adsorb MO through monolayer chemisorption, while PS follows multi-layer chemisorption. Adsorption aligns with the pseudo-second-order kinetic model and fits well with Langmuir and Freundlich isotherms. PS achieves the highest adsorption capacity (3.80 mg/g), followed by PET (3.64 mg/g) and PE (2.68 mg/g).

RSM results show significant agreement between experimental and predicted data, highlighting the role of pH, dye concentration, and MPs dose in MO removal. Electrostatic interactions, hydrogen bonding, and hydrophobic interactions are key factors driving adsorption. Interestingly, MO exhibited greater desorption rates in freshwater compared to seawater.

Finally, MPs serve as pathways for MO dye migration into ecosystems via adsorption and desorption. Further studies are needed to explore MPs’ interactions with pollutants and evaluate dye release dynamics in real-world environments.

4.1 Limitations and future research direction

This study provides valuable insights into the adsorption and desorption behavior of methyl orange dye on PE, PET, and PS MPs under varying environmental conditions. However, there are certain limitations such as the long term stability and reusability of the MPs as adsorbents were not assessed through cyclic sorption experiment, secondly, while seawater conditions were simulated using NaCl to represent ionic strength, real sea water contains a more complex mixture of ions which may influence sorption dynamics differently. Additionally quantitative surface characterization such as BET surface area and porosity analysis was not performed which could have provided a deeper understanding of the adsorption properties. Future research should incorporate repeated adsorption –desorption to evaluate long term stability of adsorbents and explore the effects of a variety of ionic matrix in natural sea water to improve the environmental relevance and predictive accuracy.

Supporting information

S1 Table. Independent variables, their experimental range and three levels of these variables.

https://doi.org/10.1371/journal.pone.0323516.s001

(DOCX)

S2 Table. The linear equations used in kinetics and isotherm models.

https://doi.org/10.1371/journal.pone.0323516.s002

(DOCX)

S3 Table. Major peaks with functional group of MPs after adsorption.

https://doi.org/10.1371/journal.pone.0323516.s003

(DOCX)

S4 Table. Pseudo-First, Second-Order and Intra-particle diffusion model parameter of MO Adsorption on MPs.

https://doi.org/10.1371/journal.pone.0323516.s004

(DOCX)

S5 Table. Adsorption efficiency of adsorbents (PE, PET and PS MPs) compare with other adsorbents.

https://doi.org/10.1371/journal.pone.0323516.s005

(DOCX)

S6 Table. Langmuir and Freundlich Isotherms model parameters for MO adsorption on MPs.

https://doi.org/10.1371/journal.pone.0323516.s006

(DOCX)

S7 Table. Box–Behnken design matrix for MO dye removal by PE, PET and PS MPs.

https://doi.org/10.1371/journal.pone.0323516.s007

(DOCX)

S8 Table. Analysis of variance (ANOVA) for the response surface quadratic model.

https://doi.org/10.1371/journal.pone.0323516.s008

(DOCX)

S1 Fig. SEM images before MO adsorption and after MO adsorption.

https://doi.org/10.1371/journal.pone.0323516.s009

(DOCX)

S2 Fig. MO dye adsorption on the MPs in freshwater environment.

https://doi.org/10.1371/journal.pone.0323516.s010

(DOCX)

S1 Data. MO dye adsorption and desorption on MPs.

https://doi.org/10.1371/journal.pone.0323516.s011

(XLSX)

Acknowledgments

The authors are grateful to the Department of Environmental Science and Technology, Jashore University of Science and Technology; and Asian Arsenic Network, Jashore for providing laboratory support during this study.

References

  1. 1. Thompson RC, Olsen Y, Mitchell RP, Davis A, Rowland SJ, John AWG, et al. Lost at sea: where is all the plastic? Science. 2004;304(5672):838. pmid:15131299
  2. 2. Akhbarizadeh R, Dobaradaran S, Schmidt TC, Nabipour I, Spitz J. Worldwide bottled water occurrence of emerging contaminants: A review of the recent scientific literature. J Hazard Mater. 2020;392:122271. pmid:32311916
  3. 3. PlasticEurope. Plastics – the facts 2022. [cited 2024 Jan 2. ]. Available from: https://plasticseurope.org/knowledge-hub/plastics-the-facts-2022//
  4. 4. Moura DS, Pestana CJ, Moffat CF, Hui J, Irvine JTS, Edwards C, et al. Adsorption of cyanotoxins on polypropylene and polyethylene terephthalate: microplastics as vector of eight microcystin analogues. Environ Pollut. 2022;303:119135. pmid:35283205
  5. 5. Schymanski D, Goldbeck C, Humpf H-U, Fürst P. Analysis of microplastics in water by micro-Raman spectroscopy: release of plastic particles from different packaging into mineral water. Water Res. 2018;129:154–62. pmid:29145085
  6. 6. Jahan S, Strezov V, Weldekidan H, Kumar R, Kan T, Sarkodie SA, et al. Interrelationship of microplastic pollution in sediments and oysters in a seaport environment of the eastern coast of Australia. Sci Total Environ. 2019;695:133924. pmid:31756867
  7. 7. Bergmann M, Wirzberger V, Krumpen T, Lorenz C, Primpke S, Tekman MB, et al. High quantities of microplastic in arctic deep-sea sediments from the HAUSGARTEN observatory. Environ Sci Technol. 2017;51(19):11000–10. pmid:28816440
  8. 8. Prokić MD, Radovanović TB, Gavrić JP, Faggio C. Ecotoxicological effects of microplastics: examination of biomarkers, current state and future perspectives. TrAC Trends Anal Chem. 2019;111:37–46.
  9. 9. Wright SL, Kelly FJ. Plastic and human health: a micro issue? Environ Sci Technol. 2017;51(12):6634–47. pmid:28531345
  10. 10. Andrady AL. Microplastics in the marine environment. Mar Pollut Bull. 2011;62(8):1596–605. pmid:21742351
  11. 11. Browne MA, Crump P, Niven SJ, Teuten E, Tonkin A, Galloway T, et al. Accumulation of microplastic on shorelines woldwide: sources and sinks. Environ Sci Technol. 2011;45(21):9175–9. pmid:21894925
  12. 12. Issac MN, Kandasubramanian B. Effect of microplastics in water and aquatic systems. Environ Sci Pollut Res Int. 2021;28(16):19544–62. pmid:33655475
  13. 13. Ahmed MB, Rahman MS, Alom J, Hasan MS, Johir MAH, Mondal MIH, et al. Microplastic particles in the aquatic environment: a systematic review. Sci Total Environ. 2021;775:145793. pmid:33631597
  14. 14. Jin Y, Xia J, Pan Z, Yang J, Wang W, Fu Z. Polystyrene microplastics induce microbiota dysbiosis and inflammation in the gut of adult zebrafish. Environ Pollut. 2018;235:322–9. pmid:29304465
  15. 15. Lei L, Wu S, Lu S, Liu M, Song Y, Fu Z, et al. Microplastic particles cause intestinal damage and other adverse effects in zebrafish Danio rerio and nematode Caenorhabditis elegans. Sci Total Environ. 2018;619–620:1–8. pmid:29136530
  16. 16. Zhan Z, Wang J, Peng J, Xie Q, Huang Y, Gao Y. Sorption of 3,3’,4,4’-tetrachlorobiphenyl by microplastics: a case study of polypropylene. Mar Pollut Bull. 2016;110(1):559–63. pmid:27230985
  17. 17. Du H, Ma H, Xing B. Identification of naturally weathering microplastics and their interactions with ion dyes in aquatic environments. Mar Pollut Bull. 2022;174:113186. pmid:34847418
  18. 18. Liu G, Zhu Z, Yang Y, Sun Y, Yu F, Ma J. Sorption behavior and mechanism of hydrophilic organic chemicals to virgin and aged microplastics in freshwater and seawater. Environ Pollut. 2019;246:26–33. pmid:30529938
  19. 19. Liu P, Lu K, Li J, Wu X, Qian L, Wang M, et al. Effect of aging on adsorption behavior of polystyrene microplastics for pharmaceuticals: adsorption mechanism and role of aging intermediates. J Hazard Mater. 2020;384:121193. pmid:31610348
  20. 20. Herrera A, Acosta-Dacal A, Pérez Luzardo O, Martínez I, Rapp J, Reinold S, et al. Bioaccumulation of additives and chemical contaminants from environmental microplastics in European seabass (Dicentrarchus labrax). Sci Total Environ. 2022;822:153396. pmid:35092768
  21. 21. Vivekanand AC, Mohapatra S, Tyagi VK. Microplastics in aquatic environment: challenges and perspectives. Chemosphere. 2021;282:131151. pmid:34470176
  22. 22. Wilcox C, Van Sebille E, Hardesty BD. Threat of plastic pollution to seabirds is global, pervasive, and increasing. Proc Natl Acad Sci U S A. 2015;112(38):11899–904. pmid:26324886
  23. 23. Browne MA, Niven SJ, Galloway TS, Rowland SJ, Thompson RC. Microplastic moves pollutants and additives to worms, reducing functions linked to health and biodiversity. Curr Biol. 2013;23(23):2388–92. pmid:24309271
  24. 24. Gallo F, Fossi C, Weber R, Santillo D, Sousa J, Ingram I, et al. Marine litter plastics and microplastics and their toxic chemicals components: the need for urgent preventive measures. Environ Sci Eur. 2018;30(1):13. pmid:29721401
  25. 25. Stolte A, Forster S, Gerdts G, Schubert H. Microplastic concentrations in beach sediments along the German Baltic coast. Mar Pollut Bull. 2015;99(1–2):216–29. pmid:26198261
  26. 26. Ghosh GC, Akter SM, Islam RM, Habib A, Chakraborty TK, Zaman S, et al. Microplastics contamination in commercial marine fish from the Bay of Bengal. Reg Stud Mar Sci. 2021;44:101728.
  27. 27. Kumar P, Inamura Y, Bao PN, Abeynayaka A, Dasgupta R, Abeynayaka HDL. Microplastics in freshwater environment in Asia: a systematic scientific review. Water. 2022;14(11):1737.
  28. 28. Zhong Y, Wang K, Guo C, Kou Y, Hassan A, Lu Y, et al. Competition adsorption of malachite green and rhodamine B on polyethylene and polyvinyl chloride microplastics in aqueous environment. Water Sci Technol. 2022;86(5):894–908. pmid:36358036
  29. 29. Ng KL, Obbard JP. Prevalence of microplastics in Singapore’s coastal marine environment. Mar Pollut Bull. 2006;52(7):761–7. pmid:16388828
  30. 30. Akter MS, Kumar Chakraborty T, Ghosh GC, Nice MS, Zaman S, Khan AS. Microplastics and heavy metals in freshwater fish species in the southwestern region of Bangladesh: an emerging concern for public health. ECs. 2024;10(3):100325.
  31. 31. Kinigopoulou V, Pashalidis I, Kalderis D, Anastopoulos I. Microplastics as carriers of inorganic and organic contaminants in the environment: a review of recent progress. J Mol Liq. 2022;350:118580.
  32. 32. Stabnikova O, Stabnikov V, Marinin A, Klavins M, Vaseashta A. The role of microplastics biofilm in accumulation of trace metals in aquatic environments. World J Microbiol Biotechnol. 2022;38(7):117. pmid:35597812
  33. 33. Liu S, Huang J, Zhang W, Shi L, Yi K, Yu H, et al. Microplastics as a vehicle of heavy metals in aquatic environments: a review of adsorption factors, mechanisms, and biological effects. J Environ Manage. 2022;302(Pt A):113995. pmid:34700080
  34. 34. Hai N, Liu X, Li Y, Kong F, Zhang Y, Fang S. Effects of microplastics on the adsorption and bioavailability of three strobilurin fungicides. ACS Omega. 2020;5(47):30679–86. pmid:33283116
  35. 35. Marathe NP, Bank MS. The microplastic-antibiotic resistance connection. In: Microplastic in the environment: pattern and process. Cham: Springer; 2022. p. 311–22.
  36. 36. You H, Huang B, Cao C, Liu X, Sun X, Xiao L, et al. Adsorption-desorption behavior of methylene blue onto aged polyethylene microplastics in aqueous environments. Mar Pollut Bull. 2021;167:112287. pmid:33892435
  37. 37. Anastopoulos I, Pashalidis I, Kayan B, Kalderis D. Microplastics as carriers of hydrophilic pollutants in an aqueous environment. J Mol Liq. 2022;350:118182.
  38. 38. Mato Y, Isobe T, Takada H, Kanehiro H, Ohtake C, Kaminuma T. Plastic resin pellets as a transport medium for toxic chemicals in the marine environment. Environ Sci Technol. 2001;35(2):318–24. pmid:11347604
  39. 39. Rochman CM, Browne MA, Halpern BS, Hentschel BT, Hoh E, Karapanagioti HK, et al. Policy: classify plastic waste as hazardous. Nature. 2013;494(7436):169–71. pmid:23407523
  40. 40. Rios Mendoza LM, Jones PR. Characterisation of microplastics and toxic chemicals extracted from microplastic samples from the North Pacific Gyre. Environ Chem. 2015;12(5):611.
  41. 41. Zhang L, Li Y, Wang W, Zhang W, Zuo Q, Abdelkader A, et al. The potential of microplastics as adsorbents of sodium dodecyl benzene sulfonate and chromium in an aqueous environment. Environ Res. 2021;197:111057. pmid:33757825
  42. 42. Brennecke D, Duarte B, Paiva F, Caçador I, Canning-Clode J. Microplastics as vector for heavy metal contamination from the marine environment. Estuar Coast Shelf Sci. 2016;178:189–95.
  43. 43. Farrell P, Nelson K. Trophic level transfer of microplastic: Mytilus edulis (L.) to Carcinus maenas (L.). Environ Pollut. 2013;177:1–3. pmid:23434827
  44. 44. Guo X, Pang J, Chen S, Jia H. Sorption properties of tylosin on four different microplastics. Chemosphere. 2018;209:240–5. pmid:29933160
  45. 45. Sarkar K, Deb K, Bera A, Debnath A, Saha B. Tuning of optical and electrical properties of polyaniline on flexible cellulose through Eosin Y dye interaction. Mater Res Express. 2019;6(7):075317.
  46. 46. Sarkar K, Deb K, Debnath A, Bera A, Debnath A, Saha B. Polaron localization in polyaniline through methylene blue dye interaction for tuned charge transport and optical properties. Colloid Polym Sci. 2018;296(12):1927–34.
  47. 47. Deb K, Sarkar K, Bera A, Debnath A, Saha B. Coupled polaron-electron charge transport in graphite functionalized polyaniline on cellulose: metal free flexible p-type semiconductor. Synth Met. 2018;245:96–101.
  48. 48. Das D, Das J, Debnath A, Chakraborty S, Saha B. Polyaniline-graphite on cellulose substrate: a flexible, low-cost, use-and-throw sensor for glucose concentration detection. Cellulose. 2023;30(10):6423–33.
  49. 49. Du H, Zhang Y, Jiang H, Wang H. Adsorption of rhodamine B on polyvinyl chloride, polystyrene, and polyethylene terephthalate microplastics in aqueous environments. Environ Technol Innov. 2022;27:102495.
  50. 50. Fu L, Li J, Wang G, Luan Y, Dai W. Adsorption behavior of organic pollutants on microplastics. Ecotoxicol Environ Saf. 2021;217:112207. pmid:33866287
  51. 51. Cui M-H, Sangeetha T, Gao L, Wang A-J. Efficient azo dye wastewater treatment in a hybrid anaerobic reactor with a built-in integrated bioelectrochemical system and an aerobic biofilm reactor: evaluation of the combined forms and reflux ratio. Bioresour Technol. 2019;292:122001. pmid:31444121
  52. 52. Chakraborty TK, Ghosh GC, Akter M, Adhikary K, Islam M, Ghosh P. Biosorption of reactive red 120 dye from aqueous solutions by using mahagoni (Swietenia mahagoni) wood and bark charcoal: equilibrium and kinetic studies. Pollution. 2021;7(4):905–21.
  53. 53. Chakraborty TK, Audhikary K, Ghosh GC, Rahman MS, Habib A, Islam MS, et al. Adsorption of acid and basic dye from the simulated wastewater using carbonized microplastic particles synthesized from recycled polyethylene terephthalate plastic waste bottles: an integrated approach for experimental and practical applications. AQUA. 2023;72(4):491–506.
  54. 54. Chakraborty TK, Ghosh S, Islam MS, Nice MS, Islam KR, Netema BN, et al. Removal of hazardous textile dye from simulated wastewater by municipal organic solid waste charcoal using machine learning approaches: kinetics, isotherm, and thermodynamics. Heliyon. 2023;9(8):e18856. pmid:37701407
  55. 55. Al-Zawahreh K, Barral MT, Al-Degs Y, Paradelo R. Comparison of the sorption capacity of basic, acid, direct and reactive dyes by compost in batch conditions. J Environ Manage. 2021;294:113005. pmid:34130138
  56. 56. Rao H, Qi W, Su R, He Z, Peng X. Mechanistic and conformational studies on the interaction of human serum albumin with rhodamine B by NMR, spectroscopic and molecular modeling methods. J Mol Liq. 2020;316:113889.
  57. 57. Yönten V, Sanyürek NK, Kivanç MR. A thermodynamic and kinetic approach to adsorption of methyl orange from aqueous solution using a low cost activated carbon prepared from Vitis vinifera L. Surf Interfaces. 2020;20:100529.
  58. 58. Boumya W, Khnifira M, Machrouhi A, Abdennouri M, Achak M, Qourzal S, et al. Box–Behnken design for understanding of adsorption behaviors of cationic and anionic dyes by activated carbon. Desalin Water Treat. 2021;15:17–9.
  59. 59. Chakraborty TK, Islam MS, Zaman S, Kabir AHME, Ghosh GC. Jute (Corchorus olitorius) stick charcoal as a low-cost adsorbent for the removal of methylene blue dye from aqueous solution. SN Appl Sci. 2020;2(4).
  60. 60. Mittal A, Malviya A, Kaur D, Mittal J, Kurup L. Studies on the adsorption kinetics and isotherms for the removal and recovery of Methyl Orange from wastewaters using waste materials. J Hazard Mater. 2007;148(1–2):229–40. pmid:17379402
  61. 61. Hidayat ARP, Sulistiono DO, Murwani IK, Endrawati BF, Fansuri H, Zulfa LL, et al. Linear and nonlinear isotherm, kinetic and thermodynamic behavior of methyl orange adsorption using modulated Al2O3@UiO-66 via acetic acid. J Environ Chem Eng. 2021;9(6):106675.
  62. 62. Ghosh GC, Chakraborty TK, Zaman S, Nahar MN, Kabir AHME. Removal of methyl orange dye from aqueous solution by a low-cost activated carbon prepared from mahagoni (Swietenia mahagoni) bark. Pollution. 2020;6(1):171–84.
  63. 63. Gong R, Ding Y, Li M, Yang C, Liu H, Sun Y. Utilization of powdered peanut hull as biosorbent for removal of anionic dyes from aqueous solution. Dyes Pigm. 2005;64(3):187–92.
  64. 64. Cheah W, Hosseini S, Khan MA, Chuah TG, Choong TSY. Acid modified carbon coated monolith for methyl orange adsorption. Chem Eng J. 2013;215–216:747–54.
  65. 65. Bai Y-N, Wang X-N, Zhang F, Wu J, Zhang W, Lu Y-Z, et al. High-rate anaerobic decolorization of methyl orange from synthetic azo dye wastewater in a methane-based hollow fiber membrane bioreactor. J Hazard Mater. 2020;388:121753. pmid:31806438
  66. 66. Haque MM, Haque MA, Mosharaf MK, Marcus PK. Decolorization, degradation and detoxification of carcinogenic sulfonated azo dye methyl orange by newly developed biofilm consortia. Saudi J Biol Sci. 2021;28(1):793–804. pmid:33424369
  67. 67. Wu L, Liu X, Lv G, Zhu R, Tian L, Liu M, et al. Study on the adsorption properties of methyl orange by natural one-dimensional nano-mineral materials with different structures. Sci Rep. 2021;11(1):10640. pmid:34017049
  68. 68. León G, García F, Miguel B, Bayo J. Equilibrium, kinetic and thermodynamic studies of methyl orange removal by adsorption onto granular activated carbon. Desalin Water Treat. 2016;57(36):17104–17.
  69. 69. Ozcan AS, Ozcan A. Adsorption of acid dyes from aqueous solutions onto acid-activated bentonite. J Colloid Interface Sci. 2004;276(1):39–46. pmid:15219427
  70. 70. Lin L, Tang S, Wang X, Sun X, Yu A. Adsorption of malachite green from aqueous solution by nylon microplastics: reaction mechanism and the optimum conditions by response surface methodology. Process Saf Environ Prot. 2020;140:339–47.
  71. 71. Wang K, Kou Y, Wang K, Liang S, Guo C, Wang W, et al. Comparing the adsorption of methyl orange and malachite green on similar yet distinct polyamide microplastics: uncovering hydrogen bond interactions. Chemosphere. 2023;340:139806. pmid:37574090
  72. 72. Gadekar MR, Ahammed MM. Modelling dye removal by adsorption onto water treatment residuals using combined response surface methodology-artificial neural network approach. J Environ Manage. 2019;231:241–8. pmid:30343219
  73. 73. Lagergren SK. About the theory of so-called adsorption of soluble substances. Sven Vetenskapsakad Handlingar. 1898;24:1–39.
  74. 74. Ho Y, McKay G. The kinetics of sorption of divalent metal ions onto sphagnum moss peat. Water Res. 2000;34(3):735–42.
  75. 75. Weber WJ Jr, Morris JC. Kinetics of adsorption on carbon from solution. J Sanit Engrg Div. 1963;89(2):31–59.
  76. 76. Langmuir I. The constitution and fundamental properties of solids and liquids. II. Liquids.1. J Am Chem Soc. 1917;39(9):1848–906.
  77. 77. Freundlich HMF. Over the adsorption in solution. J Phys Chem. 1906;57(385471):1100–7.
  78. 78. Luo M, Wang H, Zhang Y, Zhong Y, Wang K. Surface treatment by the Fe(III)/sulfite system for flotation separation of hazardous chlorinated plastics from the mixed waste plastics. J Hazard Mater. 2019;377:34–41. pmid:31132679
  79. 79. Zhang Y, Jiang H, Wang H, Wang C. Separation of hazardous polyvinyl chloride from waste plastics by flotation assisted with surface modification of ammonium persulfate: Process and mechanism. J Hazard Mater. 2020;389:121918. pmid:31879107
  80. 80. Liu Z, Qin Q, Hu Z, Yan L, Ieong U-I, Xu Y. Adsorption of chlorophenols on polyethylene terephthalate microplastics from aqueous environments: kinetics, mechanisms and influencing factors. Environ Pollut. 2020;265(Pt A):114926. pmid:32544662
  81. 81. Anastopoulos I, Pashalidis I. Single-use surgical face masks, as a potential source of microplastics: Do they act as pollutant carriers? J Mol Liq. 2021;326:115247. pmid:33518855
  82. 82. Agboola OD, Benson NU. Physisorption and chemisorption mechanisms influencing micro (nano) plastics-organic chemical contaminants interactions: a review. Front Environ Sci. 2021;9:678574.
  83. 83. Puckowski A, Cwięk W, Mioduszewska K, Stepnowski P, Białk-Bielińska A. Sorption of pharmaceuticals on the surface of microplastics. Chemosphere. 2021;263:127976. pmid:32835979
  84. 84. Beakou BH, El Hassani K, Houssaini MA, Belbahloul M, Oukani E, Anouar A. A novel biochar from Manihot esculenta Crantz waste: application for the removal of Malachite Green from wastewater and optimization of the adsorption process. Water Sci Technol. 2017;76(5–6):1447–56. pmid:28953471
  85. 85. Spyridakis I, Tzanakakis VA, Pashalidis I, Kalderis D, Anastopoulos I. Polyamide nylon 6 as a potential carrier of nitrate anions in aqueous environments. J Mol Liq. 2022;352:118706.
  86. 86. Torres FG, Dioses-Salinas DC, Pizarro-Ortega CI, De-la-Torre GE. Sorption of chemical contaminants on degradable and non-degradable microplastics: recent progress and research trends. Sci Total Environ. 2021;757:143875. pmid:33310573
  87. 87. Li J, Zhang K, Zhang H. Adsorption of antibiotics on microplastics. Environ Pollut. 2018;237:460–7. pmid:29510365
  88. 88. Wang F, Zhang M, Sha W, Wang Y, Hao H, Dou Y, et al. Sorption behavior and mechanisms of organic contaminants to nano and microplastics. Molecules. 2020;25(8):1827. pmid:32316227
  89. 89. Chen Y, Li J, Wang F, Yang H, Liu L. Adsorption of tetracyclines onto polyethylene microplastics: a combined study of experiment and molecular dynamics simulation. Chemosphere. 2021;265:129133. pmid:33276997
  90. 90. Xu B, Liu F, Brookes PC, Xu J. Microplastics play a minor role in tetracycline sorption in the presence of dissolved organic matter. Environ Pollut. 2018;240:87–94. pmid:29729573
  91. 91. Uyguner-Demirel CS, Bekbolet M. Significance of analytical parameters for the understanding of natural organic matter in relation to photocatalytic oxidation. Chemosphere. 2011;84(8):1009–31. pmid:21621242
  92. 92. Wu C, Zhang K, Huang X, Liu J. Sorption of pharmaceuticals and personal care products to polyethylene debris. Environ Sci Pollut Res Int. 2016;23(9):8819–26. pmid:26810664
  93. 93. Mo Q, Yang X, Wang J, Xu H, Li W, Fan Q, et al. Adsorption mechanism of two pesticides on polyethylene and polypropylene microplastics: DFT calculations and particle size effects. Environ Pollut. 2021;291:118120. pmid:34520951
  94. 94. Zuo L-Z, Li H-X, Lin L, Sun Y-X, Diao Z-H, Liu S, et al. Sorption and desorption of phenanthrene on biodegradable poly(butylene adipate co-terephtalate) microplastics. Chemosphere. 2019;215:25–32. pmid:30300808
  95. 95. Chen W, Ouyang Z-Y, Qian C, Yu H-Q. Induced structural changes of humic acid by exposure of polystyrene microplastics: a spectroscopic insight. Environ Pollut. 2018;233:1–7. pmid:29049941
  96. 96. Ezzati R. Derivation of pseudo-first-order, pseudo-second-order and modified pseudo-first-order rate equations from Langmuir and Freundlich isotherms for adsorption. Chem Eng J. 2020;392:123705.
  97. 97. Lesmana SO, Febriana N, Soetaredjo FE, Sunarso J, Ismadji S. Studies on potential applications of biomass for the separation of heavy metals from water and wastewater. Biochem Eng J. 2009;44(1):19–41.
  98. 98. Tubić A, Lončarski M, Apostolović T, Kragulj Isakovski M, Tričković J, Molnar Jazić J, et al. Adsorption mechanisms of chlorobenzenes and trifluralin on primary polyethylene microplastics in the aquatic environment. Environ Sci Pollut Res Int. 2021;28(42):59416–29. pmid:33415621
  99. 99. Llorca M, Ábalos M, Vega-Herrera A, Adrados MA, Abad E, Farré M. Adsorption and desorption behaviour of polychlorinated biphenyls onto microplastics’ surfaces in water/sediment systems. Toxics. 2020;8(3):59. pmid:32824499
  100. 100. Ahmed H, Hashim A. Tunable spectroscopic, electronic and thermal characteristics of PS/Nb5Si3/ZnS nanostructures for optics and potential nanodevices. Opt Quant Electron. 2022;55(1):9.
  101. 101. Chen Z, Yang J, Huang D, Wang S, Jiang K, Sun W, et al. Adsorption behavior of aniline pollutant on polystyrene microplastics. Chemosphere. 2023;323:138187. pmid:36806808
  102. 102. Hüffer T, Weniger A-K, Hofmann T. Sorption of organic compounds by aged polystyrene microplastic particles. Environ Pollut. 2018;236:218–25. pmid:29414343
  103. 103. Yao J, Peng Z, Chen W, Lin Q, Cheng M, Li H, et al. Surface characteristics of polystyrene microplastics mainly determine their coagulation performances. Mar Pollut Bull. 2023;186:114347. pmid:36436274
  104. 104. Liu Y, Huang Y, Zhang C, Li W, Chen C, Zhang Z, et al. Nano-FeS incorporated into stable lignin hydrogel: a novel strategy for cadmium removal from soil. Environ Pollut. 2020;264:114739. pmid:32434113