Competition adsorption of malachite green and rhodamine B on polyethylene and polyvinyl chloride microplastics in aqueous environment

It is estimated that by 2050, 1.2 × 10⁹ tons of plastic waste will be released into the natural environment (You et al. 2021). Waste plastics will be further degraded or broken up by physical, chemical, and biological processes (Wang et al. 2020b; Yang et al. 2021). Plastics less than 5 mm in diameter are usually defined as micro-plastics (MPs) (Wang et al. 2019). MPs are dispersed in an aqueous environment by the movement of water (Nematollahi et al. 2021). MPs are highly susceptible to ingestion by aquatic organisms, causing a variety of adverse effects such as reduced feeding and inflammation (Wang et al. 2020a). Therefore, the environment pollution caused by MPs is a major concern for scientists and the public (Qi et al. 2021; Ding et al. 2020).

Due to their small size, large specific surface area (Fang et al. 2019), strong hydrophobicity, and the presence of different functional groups (Cui et al. 2022), MPs become carriers of various pollutants (Lv et al. 2019; Zhang et al. 2019), such as heavy metals (Zhou et al. 2019) polycyclic aromatic hydrocarbons (Fang et al. 2019), and pesticides (Wang et al. 2020c). In an aquatic environment, contaminated MPs are taken up by surrounding aquatic organisms and then transferred through the food chain (Li et al. 2021b). Contaminated MPs can release contaminants when they enter organisms, which will cause dual toxicity (Zhang et al. 2021), such as low growth rate and reproductive complications (Fu et al. 2022), or in more severe cases, the death of the organism. Therefore, the studies of the interaction between contaminants and MPs in the multicomponent systems would be an important research focus nowadays and in future.

Available studies have shown that the capacity and mechanism of the pollutant adsorption on MPs mainly depends on the structure and nature of the polyvinyl chloride (PVC), polyethylene (PE), polypropylene and triclosan (Sheng et al. 2020). Currently, PE and PVC are two commonly used plastics (Wu et al. 2020; Yu et al. 2020b; Chen et al. 2021). The adsorption capacity and mechanism of pollutants on MPs with different group may be quite different. Mo et al. found that intermolecular van der Waals forces were one of the main adsorption mechanisms for the adsorption of two pesticides by PE (Mo et al. 2021). Li et al. revealed that the adsorption of three pesticides by PE was mainly physical and controlled by intermolecular van der Waals forces and micro-porous filling mechanisms (Li et al. 2021a). Bao et al. found that the adsorption of anthracene and its hydroxyl derivatives by PVC was mainly due to electrostatic interactions and CH-π interaction forces (Bao et al. 2021a). Wu et al.'s study on the adsorption of five kinds of bisphenol A on PVC showed that electrostatic interactions and non-covalent bonding (hydrogen and halogen bonding) were present in the adsorption process (Wu et al. 2019b).

Like pesticides, dyes are a large group of contaminants that pose a significant safety risk to humans (Akpan & Hameed 2009). For instance, malachite green (MG) (Bulut et al. 2008) is a common methylated di-amino tri-phenyl-methane dye, and Rhodamine B (RhB) (Chatterjee et al. 2019) is a highly water-soluble cationic heteroanthracene dye, both of which are highly toxic and carcinogenic in nature (Tariq et al. 2020). These two dyes are widely used in the textile, paper, cotton, and dyeing industries and generate large amounts of dye effluent in the environment (Chatterjee et al. 2019). However, the studies on their sorption behavior by MPs are very limited.

There are many dyes and MPs in the water system, and many complex multi-component systems are formed. The inevitable interaction between dyes and MPs will lead to more complex environmental behavior. Although Lin et al. (2020) explored the interaction mechanism and optimal conditions for nylon plastic to adsorb MG, and You et al. explored and found that the aging PE had a better adsorption effect on methylene blue compared with the original plastic (You et al. 2021). The adsorption of single dyes on MPs is not enough to explain the real situation in the environment. Thus, at the preliminary stage, the researches on the adsorption relationship of binary component dyes on MPs will be a closer step to the real environment, so it is more meaningful to study the adsorption mechanism of adsorption of MPs with different polarities and dyes in the binary component system.

In this paper, the adsorption kinetics and isotherms of MG and RhB on PE and PVC in single system were investigated. The effect of the salinity and pH on the adsorption behavior for the dyes was studied. On the basis of these results, the competitive adsorptions of the two dyes on MPs were investigated with different ratio of dyes, and possible competitive adsorption mechanisms were proposed. This study would provide a new way for understanding the interactions between dyes and MPs in the multicomponent systems.

Malachite green (MG, A.R. grade), rhodamine B (RhB, A.R. grade), calcium chloride (CaCl₂, 96%), 0.1 mol L⁻¹ hydrochloric acid (HCl) and sodium hydroxide (NaOH) analytical standard solution were all purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. Sodium chloride (NaCl, A.R. grade) was purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). These reagents can be used without further purification. PE and PVC were purchased from Dongguan Plastic raw materials Factory in China. The octanol-water partition coefficient (Kow) of MG and RhB is shown in Table S5.

The surface morphological of MPs were characterized by scanning electron microscopy (SEM) (S-4800 Hitachi, Tokyo, Japan), Fourier transform infrared (FT-IR) (FT/IR-4100 Jasco Inc., Tokyo, Japan) was performed to identify the dominant functional groups of MPs, the hydrophobicity of MPs was measured by contact angle (JY-82B Kruss DSA), the surface potential values of PE and PVC were measured by zeta potential (ZS90, Malvern Inc., UK) and N₂ adsorption-desorption experiment was carried out to determine the surface area, the pore size distributions and the pore volumes of the MPs. The surface area was calculated using Brunauer-Emmett-Teller (BET) method and pore size distribution was analyzed using Barrett-Joiner-Halenda (BJH) model. The concentration of dye was measured by ultraviolet spectrophotometer (UV-2550, Shimadzu, Japan). (The detection limit of MG and RhB is 0.27 mg/L and 0.51 mg/L, in Fig. S2.)

The adsorptions of MG and RhB on PE and PVC were investigated. There are three parallel samples during the experiment, including a blank group without MPs. In general, 60 mg of adsorbent (PE or PVC) and 60 mL of contaminant solution containing 0.1 mol/L CaCl₂ were added to a brown glass centrifuge tube. All adsorption experiments were performed in a thermostatic vibrating chamber at a rate of 160 rpm and 25 °C. The concentration of dye was measured by a UV spectrophotometer.

As for kinetics experiments, 60 mg PE and PVC were added to a 60 mL brown glass bottle with an initial concentration of 10 mg/L MG and 5 mg/L RhB solution, respectively. The solution was taken at 0.5, 1, 3, 7, 12, 24, 35, 48, 58, 69 h after adsorption, 2.5 mL samples were collected each time, and centrifuged with a centrifuge at a speed of 10,000 rpm/min for 5 min, and the supernatant was taken. Finally, the equilibrium concentration of the two dyes was determined by a UV spectrophotometer. According to the results of the kinetics experiment, after 60 h of reaction, the adsorption reached equilibrium. Therefore, we have carried out adsorption isotherm experiments. To ensure the adsorption ratio is in the range of 20–80%, the concentration of MG solution was set as 10, 20, 30, 40, and 50 mg/L, and RhB solution was set as 2, 3, 5, 8, 10 mg/L, while the concentration of PE and PVC was maintaining at 60 mg.

To study the effect of salinity on adsorption behavior, different masses of salinity (0, 3, 6, 12, 24, 36, 48 mg) were added into a 60 mL brown glass bottle with 60 mL MG or RhB solutions and 60 mg PE or PVC. To eliminate the interference of the inner wall of the container on dye adsorption, all experiments included blank space, and only MPs and dye solutions were compared. The experimental process is consistent with the adsorption kinetics mentioned above. After 60 h, samples are collected and MPs are removed by centrifugation before analysis.

As for understanding the effect of pH on adsorption behavior, while other conditions were consistent with the adsorption kinetics experiment, different pH values (2, 4, 6, 8, and 10) were set in the experiment. The following experimental operation is consistent with the above-mentioned experiment. The adsorption behavior of MG and RhB on MPs was investigated. Three groups of MG solutions at 10 mg/L including 10 mg/L, 20 mg/L, and 40 mg/L RhB, three groups of RhB solutions at 5 mg/L including 5 mg/L, 10 mg/L, and 20 mg/L/MG were prepared in the adsorption experiment. After adding sorbents (PE and PVC), six groups of suspensions were shaken at 25 °C in dark. Samples were collected and tested after 60 h.

FT-IR spectroscopy was used to characterize the changes of functional groups on PE and PVC surfaces before and after the adsorption of MG and RhB. As shown in Fig. S1(a), the FT-IR spectroscopy of PE before adsorption of dyes shows that the peaks located at 2,920 cm⁻¹ and 2,855 cm⁻¹ are the stretching vibrations of the C-H bond. The bending vibration and vibration of the C-H bond occurred at 1,465 cm⁻¹ and 1,371 cm⁻¹, and 721 cm⁻¹, respectively (You et al. 2021; Rozman et al. 2021). However, Fig. S1(a) confirmed that no new chemical bonds were formed during the adsorption of PE, which proved that chemical adsorption is not the main factor in the adsorption process (Lin et al. 2021).

The FT-IR spectroscopy of PVC before adsorption of dyes is depicted in Fig. S1(b), the peaks located at 2,913 cm⁻¹ and 1,427 cm⁻¹ correspond to stretching and bending vibrations of -CH₂-. Out-of-plane deformation vibration of Cl-CH at 1,251 cm⁻¹ (Ma et al. 2019), and the peak of 609 cm⁻¹ was attributed to the C-Cl tensile vibration (Bao et al. 2021b; Yu et al. 2020a). It can be seen from the FT-IR spectrum of Fig. S1(b) that the peaks (1,427 cm⁻¹ and 1,251 cm⁻¹) intensities slightly decreased after PVC adsorbed MG (Tang et al. 2021a, 2021b). This is the result of the specific interaction between the chlorine groups on PVC and the groups on MG. This confirms that -Cl on PVC will form a halogen-hydrogen bond with the proton-donating functional group of MG to enhance the adsorption (Wu et al. 2019b). This halogen-hydrogen bond interaction also exists between PVC and RhB, as can be seen by the green shading in Fig. S1(b), which can be used to predict that PVC would have stronger adsorption capability than that of PE.

Langmuir and Freundlich models were used to simulate the adsorption process of MG and RhB on MPs as shown in Figure 6. The corresponding parameters of the two models were displayed in Table S4. The Langmuir model assumes that the surface of the adsorbent is uniform with monolayer adsorption of only one molecule per adsorption site (Foo & Hameed 2010). The Freundlich isotherm is the basis of an assumption regarding surface heterogeneity and can be used to model monolayer (chemical adsorption) and multi-layer adsorption (Van der Waals force adsorption) (Yang 1998). The correlation coefficients of Freundlich and Langmuir are slightly different, but as summarized in Table S4, the q_max values of the two MPs obtained by the Langmuir equation are far greater than those obtained in our experiments, and the K_L values are tiny, which does not conform to our test results (Zhang et al. 2020). Therefore, the Freundlich model is more in line with our experimental results. This phenomenon indicated that all the adsorption of dyes on MPs were multilayer adsorption, rather than molecular adsorption on a uniform surface (Mo et al. 2021). This finding is consistent with previous studies on the adsorption of five bisphenol analogs on PVC (Wu et al. 2019b). The K_F value is an indicator of adsorption capacity, and the larger the K_F value is, the larger the adsorption capacity is (Liu et al. 2020a). In general, PVC is more attractive to bind MG and RhB than PE. In addition, the values of parameter n of Freundlich models deviating significantly from 1 as shown in Table S4, the adsorption process were nonlinear, which means that the distribution of adsorption active sites on PE and PVC could be heterogeneous. The adsorption process involved several adsorption mechanisms (Karimi et al. 2019; Koopal et al. 2020), such as electrostatic adsorption, surface deposition, etc.

According to the adsorption kinetics of MG and RhB on PE and PVC in single system, the equilibrium adsorption capacities of PVC for MG and RhB are 7.68 and 2.83 mg/g, and the equilibrium adsorption capacities of PE for MG and RhB are 4.52 and 1.27 mg/g, respectively. The adsorption capacities of PVC for MG and RhB are significantly higher than those of PE. In addition, the characterization results show that PVC has more adsorption sites than PE.

In addition, by comparing Figure 9(a) and 9(b), it can be also found that the concentration of MG remains unchanged (10 mg/L). When RhB/MG ratio was 1:1, the adsorption capacity of PE and PVC for RhB/MG is 1.2/2.5 and 1.7/5.6, respectively. When the ratio was 2:1, the adsorption capacity of PE to RhB/MG is 1.7/1.7, the adsorption capacity of PVC to RhB/MG is 2.0/5.1. When the ratio was 4:1, the adsorption capacity of PE to RhB/MG is 2.6/0.8, and the adsorption capacity of PVC to RhB/MG is 3.5/4.1. By comparison, it can be found that under the condition of the same RhB/MG ratio change (from 1:1 to 4:1), the change of the adsorption ratio of the two dyes on PE is significantly greater than that on PVC. Similarly, by comparing Figure 9(c) and 9(d), the concentration of RhB remains unchanged, and the MG/RhB ratio increases from 1:1 to 4:1 under the same conditions (increasing from 1:1 to 4:1), the change of adsorption capacity of the two dyes on PE is also significantly greater than that on PVC. This is because PE adsorption for two dyes is mainly electrostatic action, and the SSA of PE is smaller than PVC, so the adsorption site of PE is less than PVC, which indicates PE is susceptible to external adsorption concentration changes. This is consistent with the results that the adsorption capacity of aged PE is higher than that of PE MPs, which attributes to the larger surface area of aged PE MPs (Lan et al. 2020). So, with the ratios of two dyes changing, the change of adsorption on PE is obvious. Meanwhile, the hydrophilicity of PVC is stronger than that of PE, it is consistent with the results that the addition of surfactants makes MPs have higher hydrophilicity, which significantly improves the adsorption of lead ions (Shen et al. 2020). In addition, the adsorption between PVC with MG and RhB will form halogen-hydrogen bonds resulting in more adsorption sites (Pengfei et al. 2018). Hence, PVC has more adsorption sites than PE, and is less affected by the change of the concentration of external adsorbed matter. Therefore, the adsorption mechanism of PE to the two dyes is mainly electrostatic action, while the adsorption of PVC to MG and RhB is controlled by electrostatic action, hydrophilic and hydrophobic, SSA and halogen hydrogen bond.

The results in Figure 9 show that there is competition between MG and RhB in the binary system. This is consistent with the the competitive adsorption between tetracycline and ciprofloxacin on montmorillonite (Wu et al. 2019a). The adsorption capacity of two dyes on PVC is greater than that on PE. This may be due to the fact that PVC has a larger SSA than PE, and the ability to form halogen-hydrogen bonds with MG and RhB, which makes PVC have more adsorption sites.

In this paper, the adsorption behaviors of MG and RhB were investigated over PE and PVC MPs in one- and two-component systems. Following a number of intermittent experiments, we observed that the pseudo-second-order kinetics and the Freundlich isothermal model can accurately explain the adsorption kinetics and isothermal process of two dyes on MPs. The results show that the adsorption processes are multilayer, and the adsorption ability of MG and RhB on PE is lower than that of PVC, and MG exhibits greater adsorption amounts on the two MPs than on RhB. Consequently, the nature of PE and PVC has an influence on the adsorption results. The adsorption of the two dyes on PE is primarily regulated by electrostatic action, while the adsorptions of RhB and MG on PVC are mainly affected by halogen-hydrogen bond and specific surface area. Furthermore, the adsorptions of MG and RhB on PE and PVC exhibited competitive interactions in the dual-dye system. The adsorption amounts of MG on PE and PVC are greater than those of RhB due to the stronger halogen bonding. MG shows a greater adsorption capacity probably because it is more hydrophilic than RhB. This study focuses on competitive adsorption behavior, which provides a new idea for studying the complex adsorption behavior of multi-component dye pollutants on MPs.

We acknowledged the financial support from the National Natural Science Foundation of China (32061133005), and the Research Council of Norway (RCN, project 320456). This work was also supported by Key Laboratory of Oil and Gas Fine Chemicals, Ministry of Education & Xinjiang Uygur Autonomous Region.

This work was supported by the National Natural Science Foundation of China (32061133005) and the Research Council of Norway (RCN, project 320456).

Yiping Zhong: Conceptualization, acquisition of data, investigation, validation, analysis and interpretation of data, drafting of the manuscript, visualization. Kangkang Wang: Study conception and design, drafting of the manuscript, data curation, validation, visualization. Changyan Guo: Data curation, validation, visualization. Yuli Kou: Investigation, validation, visualization. Afaq Hassan: Visualization. Yi Lu: Conceptualization, methodology, validation, resources, supervision. Jide Wang: Conceptualization, validation, resources, writing-review & editing, supervision, project administration, funding acquisition. Wei Wang: Data curation, validation, visualization, funding acquisition.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.