Recent advances in dyes uptake by microplastics in aquatic environments: Influencing factors and ecotoxicological behaviors

3.2.1 Adsorption kinetics

Valuable information about the rate and mechanisms of adsorption is provided by kinetic experiments (Gao et al., 2021). Table 2 shows the data of the adsorption kinetics of dyes onto MPs. You et al. (2021) found that 90.3 % of methylene blue (MB) can be removed from aged PE in aqueous media (You, 2021). Research has been done to investigate the kinetics of reaching equilibrium on polyethylene (PE), polyvinyl chloride (PVC), PET, and PP (Tubić, 2023). For instance, in 50 % of dye uptake studies on MPs, equilibrium was reached within 24 h. However, in a few cases, equilibrium was achieved in 8 days (Du et al., 2022; Wang, 2023; Alfaro-Núñez, 2021; Boom, 2023; Noor and Rehman, 2022). Lower molecular weight can limit their ability to enter active sites, and it causes an extended state of equilibrium; however, accessibility is not exclusively influenced by molecular weight (Tubić, 2023). Table 2 demonstrates the uptake of various dyes onto MPs following the pseudo-second-order kinetic model. This finding suggests that chemical is the primary mechanism governing the interaction between dyes and MPs (Du et al., 2022; Lach et al., 2022). It was also found that the Elovich model describes the rate control of chemical uptake on an energetically heterogeneous adsorbent as a diffusion process (Tubić, 2023). There were three steps in the uptake of MB on the aged PE MPs: a fast adsorption process, a transition portion, and a quasi-equilibrium stage (You, 2021). Furthermore, Chakraborty et al. (2023) show the application of interaparticle diffusion to examine the mechanisms involved in the adsorption of methyl orange (MO) and MB dye onto carbonized MP particles (Chakraborty, 2023).

3.2.2 Adsorption isotherms

Several isotherm studies have focused on dye adsorption onto MPs, as shown in Table 3. The concentration of dyes used in the isotherm studies varied widely, ranging from 5 mg/L to 160 mg/L. Studies have primarily examined the relationship between initial concentration and adsorption capacity of various dye compounds such as phthalocyanine)Cyan), Magenta, MG, RhB, basic yellow 28)BY28(, reactive red 120) RR120(, MO, CV, aniline, and MB. These studies have reported MPs with high uptake capacities. For example, using the Langmuir model, the powdered polyethylene (PEp) sorption capacity for Cyan was 2874.4 mg/g (Tubić, 2023). According to the value of R² > 0.95 that has been done in most of the studies, the Langmuir isotherm has shown a better fit with data than other models. The Langmuir adsorption model explains the monolayer adsorption process, where a single layer of adsorbate molecules is formed on a uniform MP surface. This model assumes that the MP surface is homogeneous and there is no significant interaction between the adsorbed molecules and the uptake sites with uniform energies (Behnamfard and Salarirad, 2009).

3.2.3 Adsorption mechanisms

Information can be obtained to discern the interaction mechanism between MPs and dyes by examining the active functional groups and their involvement in adsorption (Tubić, 2023). Fig. 3 shows the mechanisms that control or regulate the process. For example, You et al. (2021) investigated, at pH values > pH_PZC (point of zero charge), negatively charged PE MPs adsorb cationic MB through electrostatic attraction (You, 2021). Cationic dyes are soluble in water with a negative charge that can chemically bind to fiber materials such as cotton and wool (Sharma et al., 2021). In an investigation by Zhong et al. (2022), they studied the contact angles of two different MPs and found that PE MPs have a more hydrophobic surface and can absorb hydrophobic dyes (Zhong, 2022). PE and PP polymers interact with organic compounds through non-specific van der Waals forces due to non-specific functional groups (Tubić, 2023). Lin et al. (2021) explored the impacts of the solution pH and Pb²⁺ content on MG uptake by aged nylon MPs. The uptake of MG by aged nylon MPs decreased with increasing Pb²⁺ concentration (Lin et al., 2021). Pb²⁺ and aged nylon MPs may interact through surface complexation (Tang et al., 2020). Carboxyl functional groups on the MP surface have a higher affinity for the uptake of Pb²⁺ ions. These functional groups prohibit the electrostatic interaction between MG and MPs (Lin et al., 2021). Tubić et al. (2023) reported that Magenta and Cyan dyes contain aromatic rings with π-electrons that can interact with PET through π-π bonding (Tubić, 2023). Magenta and MG are classified as synthetic dyes belonging to the introductory class. They are created from organic bases and can ionize in water, producing colored cations (Sharma et al., 2021). So, the hydrophobic interactions do not play a main role in Cyan and Magenta’s uptake process. New bands were associated with Cyan dye on granulated PEg (PEg) and PVC after adsorption (Tubić, 2023). Du, Ma and Xing (2022) discovered that the adsorption process depended on the electrostatic attraction between aged PE, aged PP, and ionic dyes. Furthermore, hydrophobic interactions between the alkenyl chains, silicon-based additives, and the benzene ring in the dyes, and the hydrogen bonding between heteroatom, like nitrogen atoms in CV and oxygen atoms in oxygen-containing functional groups of aged MPs, may also influence the uptake of dyes onto MPs (Du et al., 2022). The most frequently used colorants are nitrogen-containing dyes. Some examples of these dyes include MG, CV, lemon yellow, and Congo red (Fan et al., 2021).

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Fig. 3

Mechanisms involved in the interactions between MPs and dyes.

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Different techniques are utilized to understand adsorption mechanisms from various perspectives (see Table 3). These include the methods of FTIR, scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDX), and X-ray photoelectron spectroscopy (XPS). The EDX spectrum of nylon MPs aged with MG shows an increase in elemental C and O atoms content. This indicates MPs have absorbed MG dye (Lin et al., 2020). SEM-EDX can provide information about the material’s physical structure and the elements’ distribution (Suiyi, 2024). XPS is a surface analysis technique that provides information about the adsorbent surface’s chemical composition and oxidation states. It can help identify the presence of specific elements and chemical bonds involved in the adsorption process (Wang et al., 2024; Guo et al., 2023). The FTIR spectrum was obtained after the adsorption of MG on PVC, and it displays a reduction in the peak intensities (1427 cm⁻¹ and 1251 cm⁻¹). This reduction indicates an interaction between the chlorine groups in PVC and the functional groups in MG. The presence of -Cl on PVC can lead to a halogen-hydrogen bond with proton-donating functional groups (Zhong, 2022). The FTIR spectrum after the adsorption of aniline on PS showed an increase in the characteristic peaks of N–H bond formation at 1636 cm⁻¹ and 685 cm⁻¹. The C–N and C–H bonds on the MPs indicate the adsorption of aniline (Chen, 2023). The MB uptake mechanism onto MPs is due to hydrogen bonding between the positively charged MB molecule and the oxygen atoms in the C-O or C-O-C bonds on the surface of MPs. This bonding occurs through the sharing of electrons. It leads to the uptake of MB onto the MP’s surface (Mahabeer et al., 2023).

3.2.4 Desorption efficiency

Desorption studies play a central role in understanding how adsorbates and adsorbents can be recovered (Rápó and Tonk, 2021). Desorption tests were performed to determine the stability of MPs after dye uptake (Tubić, 2023). You et al. (2021) examined the desorption efficiency of MB from aged PE MPs. They found that this process depends on the physical conditions of the environment (You, 2021). In environments such as the sea, dyes absorbed into microplastics are released into the bodies of organisms that mistakenly ingest the plastic particles. Fig. 4 represents the desorption efficiency of dyes from MPs under different conditions. Wang et al. (2023) show that the dye desorbs from MPs in the first 10 h with a faster efficiency under the simulated conditions. This phase is vital to evaluate the desorption behavior of dye in simulated gastrointestinal fluids (Wang, 2023). Du, Ma and Xing (2022) discovered that the efficiency of CV desorption from MPs (PE and PP) samples was higher in seawater than in fresh water. This might be due to NaCl in seawater, which occupies some of the adsorption sites on MPs. The competitive adsorption of NaCl with CV molecules on MPs may be responsible for the phenomenon of desorption promotion. So, the findings suggest that aquatic organisms can absorb dyes that attach to MPs through a critical exposure route depending on the organisms’ physiological conditions (Du et al., 2022). In another study, the desorption of methylene blue dye from plastic particles in distilled water was reported as 99–100 % (Arslan and Günay, 2018). In 2019, Ünlü et al. reported the 100 % desorption of two cationic dyes (RB and MG) from a fibrous plastic adsorbent (Ünlü et al., 2020). The high desorption in these studies is due to working in a distilled water. Because, like other environments (seawater and tap water), there are no ionic interferences and organic substances.

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Fig. 4

Desorption efficiency of dyes from MPs under various conditions (A, B, C, D, E, F, G: (Tubić, 2023), H: (Lin et al., 2020), I (You, 2021), J (Wang, 2023), K, L: (Du et al., 2022).

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3.3.1 Solution pH

PH level is important in capturing dyes in adsorbents like MPs. It contributes to creating binding sites on the MP surface, which facilitates the capture of the adsorbate (Mahabeer et al., 2023). Altering a solution's pH significantly impacts a compound's properties, chemical reactivity, equilibrium conditions, and toxicity (Atugoda, 2021). You et al. (2021) show a competitive uptake between excess H⁺ and MB. However, when the pH level of the solution increased, the surface charge of the aged PE MPs became negative; it weakened the electrostatic repulsive force and enhanced MB uptake onto MPs (You, 2021). Mahabeer et al. (2023) reported that increasing the medium pH led to an increase in MB dye uptake on MPs derived from waste plastic. The highest uptake was observed at a pH of 12.83, with removal rates ranging from 91.5 % to 94.9 % and adsorption capacity values around 11.4 mg/g (Mahabeer et al., 2023).

On the other hand, the lowest uptake was observed at a pH of 1.5 under the same conditions. The increase in pH leads to the addition of hydroxyl groups on the MPs' surface, which makes them more electronegative (Mahabeer et al., 2023). In 2020, Dahdouh et al.'s findings showed when the pH level is < 6, the Sulphonated Waste Poly Methyl Methacrylate (SW PMMA) becomes positively charged. This decreases uptake efficiency because a high concentration of H⁺ ions competes with cationic dyes for the active sites on the SW PMMA surface (Dahdouh et al., 2020). The uptake capacity of aniline onto MPs increased at a pH (5–7) above the pKa value (4.6). There is an inhibitory effect in primary conditions (pH = 8) and an unfavorable adsorption process because of repulsion (Chen, 2023). The pH factor affects the adsorbent in two significant ways; firstly, it changes the ionic state of the pollutant or solubility, and secondly, it alters the surface charge of MPs (Du et al., 2022).

3.3.2 Temperature

Studies have shown that higher temperatures can increase the ability of MPs to attract and release organic and inorganic pollutants (Vieira et al., 2021). Dahdouh et al. (2020) stated that the adsorption percentage of two types of dyes, MB and BY28, on SW PMMA decreases as the temperature increases. For MB, the adsorption percentage decreases from 85.27 % to 32.49 %, and BY28 decreases from 87.86 % to 35.61 % when the temperature increases from 20 °C to 60 °C (Dahdouh et al., 2020). Chakraborty et al. (2023) studied the adsorption thermodynamics of MB and MO on CMPs at different temperatures. The results suggested that the values of ΔG, ΔH, and ΔS are negative, which indicates the dye adsorption process is favorable, spontaneous, and exothermic (Chakraborty, 2023). Chen et al. (2023) studied the uptake thermodynamics of aniline on PS at various temperatures. The results showed that the values of ΔG > 0, ΔH < 0, and ΔS < 0, indicating the dye uptake process was nonspontaneous and exothermic. The adsorption capacity of MPs to aniline in high temperatures decreased (Chen, 2023). However, Mahabeer et al. (2023) further investigated the sorption thermodynamics parameters of MB onto MPs in synthetic wastewater and drew the opposite conclusion. This phenomenon can be explained by the fact that at higher temperatures, molecules possess more kinetic energy and move around more rapidly. This thermal energy allows the MB dye molecules to overcome the activation energy for chemisorption onto the MP’s surface. As a result, the uptake capacity of the MPs increases at higher temperatures due to the more vital interaction between the dye and the adsorbent surface. The increase in temperature may have caused the pores of the MP adsorbent to become larger (Mahabeer et al., 2023). Besides, Du et al. (2022) observed a promotion of RhB adsorption on PVC with increasing temperature. One possible explanation is that as the temperature increased, the PVC material became more rigid and glass-like in its structure. This change in the material's physical properties may have made it easier for RhB to adhere and spread across the surface (Du et al., 2022). Hence, the operating temperature affects the dye uptake on the MPs surface, which is an important factor in the process (Mahabeer et al., 2023).

3.3.3 Ionic strength

A gradient solution is used to simulate the salinity of water bodies (estuaries, rivers, and oceans) (Wang, 2023). The existence and concentration of various salts in wastewater influence the salinity (Mahabeer et al., 2023). As salinity levels increase in a system, the adsorption of pollutants such as MG and RhB to negatively charged surfaces of PE and PVC decreases. Because introducing Na⁺ weakens the electrostatic attraction between the contaminants and the surfaces of PE and PVC. MG and RhB are stable cationic forms in neutral environments, while PE and PVC surfaces are negatively charged at pH 6 (Zhong, 2022). Similar trends were observed for RhB adsorption on PVC, PS, and PET; adsorption gradually decreased when salinity levels increased from 0 to 3.5 % (Du et al., 2022). Wang et al. (2023) discovered that the uptake of MG onto PVC MPs increases with increasing solution salinity. MG in water is cationic and ionizes to generate positively charged colored ions. This is due to dye dimerization in the solution, and high ionic strength promotes dye gathering and enhances surface uptake onto MPs (Wang, 2023). There was a reduction in aniline uptake capacity onto MPs when NaCl concentrations increased to 500 mg/L. Because of accumulation, Na⁺ occupies adsorption sites and inhibits dye uptake onto MPs (Anastopoulos et al., 2022). Afmataj et al. (2023) showed that phosphate or nitrate ions did not affect RR120 adsorption by PN6, meaning there was no conflict between the dye and competing anions. Aniline adsorption increased with Na⁺ and Ca²⁺ in MPs, but Na⁺ had a more substantial effect than Ca²⁺ (Afmataj, 2023). MG adsorption by aged nylon MPs decreased as Pb²⁺ concentration increased, but virgin MPs were unaffected by lead ions. Because aged MPs had carboxyl groups that attracted MG. Virgin nylon MPs accumulated MG mainly through hydrogen bonding, while aged nylon MPs used electrostatic attraction and hydrogen bonding (Lin et al., 2020). The ionic strength negatively affected the adsorption of CV on PE and PP, and it reduced adsorption. There are three possible reasons for this phenomenon: (1) the added cations (like Na⁺will compete with the cationic CV molecules for the active sites on the MPs; (2) the electrolytes increase the ionic strength in the solution, which affects the diffusion rate of CV in the solution, so the electrolyte makes the CV move slower; and (3) the presence and increase of NaCl weakens the electrostatic attraction between the anionic groups on MPs and the cationic CV. These interactions result in lower uptake capacity of MPs in higher salinity (Du et al., 2022). The researchers believed that MPs have a higher potential to cause harm in freshwater environments than seawater. The reason for this is that MPs tend to absorb certain organic pollutants more easily in freshwater environments (Du et al., 2022).

3.3.4 Presence of humic acid

Natural organic matter (NOM) comprises various components, like polysaccharides, proteins, and humic substances (HSs). HSs are formed by microbial and abiotic breakdown of plants and animal residuals, and they can be found in soil, sediments, rivers, surface water, and groundwater (Sharma et al., 2021). Humic acid is a kind of NOM that has a spiral shape with a negative charge and many oxygen-functional groups. When HSs and MPs combine, they may change their environmental behaviors and form a complex that causes water pollution and harms aquatic organisms and human health (Zhang et al., 2023). HSs have molecules of various sizes that interfere with the adsorption. The giant molecules clog the pores, and the small molecules compete for the same micropores (Liu et al., 2019). Du, Ma and Xing (2022) discovered that PE-MPs and PP-MPs had lower dye adsorption when there were HSs. Without HSs, PE-MPs and PP-MPs removed 27.67 % and 41.50 % of CV, respectively. With HSs (40 mg/L), they only removed 2.91 % and 4.91 % of CV. HSs molecules had more functional groups that could bind to CV by stronger intermolecular interactions, such as hydrogen bonding, electrostatic attraction, and hydrophobic interactions. These interactions weakened the bonds between MPs and CV (Du et al., 2022). Du et al. (2022) reported that when HSs increased, MPs adsorbed less RhB. HSs molecules had many functional groups that could affect how MPs or organics adsorbed organic pollutants. HSs might also bind to RhB through hydrophobic interaction or complexation; it reduces the amount of RhB that MPs could adsorb. Moreover, HSs are a large organic molecule, so they cover the surface of MPs and could block RhB from reaching the uptake sites (Du et al., 2022).

3.4.1 Particle size and specific surface area

Aging processes create smaller-sized MPs (Liu, 2019). Lim et al. (2022) compared the affinity between MPs with different particle sizes and dye. They concluded that the adsorption of RR120 and BB9 onto MPs increased significantly with particle size (Lim et al., 2022). The PS particles were sieved to four size groups by Chen et al. (2023), and they showed that PS had the aniline adsorption capacity of 0.060 mg/g (P4 group) with a specific surface area (0.7214 m²/g). It was 3.3 times higher than that of the P1 group. MPs in groups were irregular in shape, and FTIR analysis showed that among the different groups, there was no difference in chemical structure. The higher adsorption is due to the larger specific surface area (Chen, 2023). There are more likely MPs in the natural environment with smaller sizes, it is due to various physical and chemical effects (Du et al., 2022).

3.4.2 Aging

Over time, microplastic particles undergo changes such as discoloration, cracks, even embrittlement and collapse (Mao et al., 2020). Cracks and pores with distorted texture surfaces attached to fragments and small grains were observed for PP by the researcher. These cracks show a weathering process, binding sites for the surrounding coexisting species, and increasing the force of interaction with dyes (Li et al., 2023). You et al. (2021) reported an increase in the adsorption capacity of the aged PE for MB from 0.63 mg/g to 8.12 mg/g. Hydrophilic with oxygen-containing functional groups increased adsorption capacity (Ben Slama, 2021). Similarly, Wang et al. (2023) demonstrated the effects of potassium permanganate (KMnO₄) pre-oxidation on the physicochemical characteristics of PVC MPs and their adsorption capacity for MG in wastewater treatment plants. Pre-oxidation with KMnO₄ leads to the production of MnO₂ on the surface of PVC MPs and increased their hydrophilicity and MG adsorption. The presence of MnO₂ particles on the surface of pre-oxidation PVC MPs was confirmed through XPS and SEM. The composite oxidation treatment intensified the adhesion of MnO₂ particles. The adsorption capacity of pristine PVC for MG was 2.6 mg/g, but it increased to 7.0 mg/g for single oxidation and 140.7 mg/g for composite oxidation (Wang, 2023).