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Article

Sorption Behavior of Organic Pollutants on Biodegradable and Nondegradable Microplastics: pH Effects

by
Maja Vujić
*,
Sanja Vasiljević
,
Jasmina Nikić
,
Branko Kordić
,
Jasmina Agbaba
and
Aleksandra Tubić
Department of Chemistry, Biochemistry and Environmental Protection, Faculty of Sciences, University of Novi Sad, Trg Dositeja Obradovića 3, 21000 Novi Sad, Serbia
*
Author to whom correspondence should be addressed.
Appl. Sci. 2023, 13(23), 12835; https://doi.org/10.3390/app132312835
Submission received: 22 September 2023 / Revised: 29 October 2023 / Accepted: 31 October 2023 / Published: 29 November 2023
(This article belongs to the Section Environmental Sciences)
Browse Figures
Versions Notes

Abstract

:
Microplastics (MPs), chlorinated phenols (CPs), polycyclic aromatic hydrocarbons (PAHs) and halogenated benzenes (HBs) are pollutants that are widely present in freshwater systems. As alternatives to conventional plastics, bioplastics are receiving a lot of attention, but there are limited data on their impact on pollutant behavior. This work therefore investigated the impact of pH on the sorption of CPs, PAHs and HBs, as some of the toxic and highly persistent pollutants, on seven different plastics using kinetic and isotherm studies. The pH of the water matrix impacted the adsorption behavior of CPs on all selected MPs, with the highest degree of adsorption occurring at pH 7 for the majority of the selected CPs. The highest adsorption affinity of CPs on the MPs, at pH 7, was obtained for 4-chlorophenol and 2,4-dichlorophenol on powdered polyethylene standard (qt = 221 μg/g), while the lowest was obtained for the adsorption of pentachlorophenol on polyethylene terephthalate (qt = 25 μg/g). On the other hand, the pH value of the water matrix did not affect the adsorption of halogenated benzenes and PAHs on MPs. The pseudo-second-order rate model fit the adsorption kinetics data of all experiments. The results obtained for the adsorption of CPs on MPs indicated a lower sorption affinity of CPs with MPs at pH 4 and pH 10 compared to pH 7. The Langmuir isotherm, at pH 7, implied that 4-chlorophenol’s adsorption affinity was not significantly influenced by the type of MPs. On the other hand, at pH 7, the adsorption of 2,4-dichlorophenol, 2,4,6-trichlorophenol and pentachlorophenol varied greatly, with powdered MP types showing the highest affinity for CP adsorption. Furthermore, the obtained adsorption isotherm results imply that electrostatic attraction, hydrogen bonds, π-π interactions and van der Waals interactions, are an integral part of adsorption mechanisms of the CPs on the MPs.

1. Introduction

With the large-scale usage of plastic, the poor management of plastic waste now represents one of the main threats to the environment, and contemporary society faces a worldwide plastic pollution crisis [1]. While reports concerning plastic pollution have been available since the 1970s, the scientific community only started to show interest in this topic at the start of the 21st century, making the presence of microscopic plastic particles in the environment a relatively recent area of research [2,3]. Based on the latest report by Plastic Europe, in 2021, global plastic production increased by 4%, which amounts to more than 390 million tons, implying that it remains in high demand on the market [4].
The wide application of plastics in industry, transport, medicine and commercial and household activities, as well as the poor management of plastic waste, has led to the fact that the residues after the degradation of these materials, better known as micro- and nano-plastics, have emerged as contaminants in the environment [1,2,3]. In addition to moves by decision-makers to limit the use of plastic in everyday human activities, as well as the efforts of the scientific and professional communities to find optimal solutions for reducing pollution and suitable substitutes for plastic materials, several hundred million tons of plastic products are still produced every year [4,5,6,7,8,9].
The research carried out so far all over the world, has shown that in all parts of the environment, one can find the remains of plastic in sizes that fall between 1 and 1000 μm; in recent times, plastics in this size range have been most often defined as microplastics (MPs). The range of polymers that are included in this group is very wide, and the effects of polypropylene (PP), polyethylene (PE), polyvinyl chloride (PVC), polyethylene terephthalate (PET) and polystyrene (PS) on the environment and in various parts of the natural world are the most investigated [5,6,7,8]. Microplastic particles are detected in different shapes, sizes, colors and densities, and possess other characteristics, depending on their origin in the environment, their age, the process they went through during their production, their application and the environmental conditions in which they occur [10]. Any change in the basic polymer during production (addition of additives, plasticizers), as well as during their lifetime in the environment, will affect the properties and behavior of MPs in the environment, their interaction with other compounds (e.g., organic and inorganic pollutants), and their potential harmful impact on living organisms and human health [5,6,9].
In water environments, MPs have the potential to become vectors for different organic and inorganic pollutants and pose a significant problem [9,11,12,13]. From the environment, MPs can sorb and accrue organic pollutants and heavy metals due to their hydrophobic properties and their large surface areas [14,15,16,17]. The interaction between MPs and organic pollutants has been extensively researched by other authors. Besides the MP properties and concentrations, there are other factors, shown in Figure 1, that influence the sorption capacity of these pollutants, such as temperature, pH, ionic strength, dissolved organic matter content, solubility and many others [17,18]. The presence and behavior of organic pollutants in water environments, particularly those that are ionizable, are often dependent on pH. Additionally, the interaction between cationic, anionic and zwitterionic molecules with the plastic surface is different depending on the MP surface [13].
In terms of different factors influencing the sorption of pollutants, pH has been highlighted as the most significant, since, in many cases, it can affect both the sorbent surface properties and the nature of the sorbate. Several studies have emphasized the impact of pH as an environmental factor in increasing or inhibiting the adsorption capacity depending on the polymer type as well as the type of pollution [19,20]. In this regard, previous research results have implied that the surfaces of polyethylene and polystyrene particles change and protonate with decreases in pH, consequently increasing the sorption of organic pollutants, such as perfluorooctane sulfonate, chlorinated phenols and others [3,21,22,23]. Moreover, variable pHs in the surrounding water may change the ionization states of pollutants and influence their sorption processes on MPs [3,21,22,23]. Among the various studies investigating the adsorption of various organic pollutants (such as pesticides or pharmaceuticals) on a number of MPs [23,24,25,26,27,28,29,30,31,32,33,34,35], pH was not always considered. However, as one of the main factors, the influence of pH on the sorption behavior of MPs is not yet fully understood and should be investigated further. Specifically, the amount of information available about how pH affects the adsorption mechanism of organic pollutants is limited.
The majority of the research on aquatic pollution caused by MPs has been focused on marine environments, as well as on the interaction between MPs and both organic and inorganic pollutants. Inland waters play a major role in transporting MPs from generation sites, such as non-point pollution sources and wastewaters, to the ocean, as well as to coastal areas and inland seas. Furthermore, the higher concentrations of microplastic particles and pollutants in inland waters could result in a significant difference in the interactions between MPs and pollutants in these water bodies [36]. Additionally, there is a wide range of studies highlighting concerns about the sorption interactions between MPs and single-solute systems, often neglecting the fact that mixtures of pollutants are more common in natural environments. The behavior of organic pollutants in mixtures can be very different compared to single-compound systems, and highly complex sorption interactions can occur with adsorbents such as MPs. Thus, only studying the sorption process in single-solute systems may not be enough to predict the interaction between organic pollutants and MPs in water environments [19,37,38,39].
Based on previous discussion and our earlier studies, we hypothesized that various MPs have different sorption capacities for specific groups of organic pollutants, and due to their different physico-chemical characteristics, the sorption behaviors of mixture solutions of selected groups of organic pollutants chlorinated phenols (CPs), polycyclic aromatic hydrocarbons (PAHs) and halogenated benzenes (HBs) on MPs can be influenced by environmental factors. Therefore, the main objective of this study was to investigate the influence of one of the most important environmental factors, pH value, on the adsorption behaviors and mechanisms of mixture solutions of selected groups of organic pollutants, CPs, PAHs and HBs, on seven types of MPs that are frequently detected in water environments.

2. Materials and Methods

2.1. Materials

In this study, the microplastic particles selected as a sorbents for the experiments were commercially available powdered standard polyethylene (PEp) supplied by Thermo Fisher Scientific (Waltham, MA, USA), granulated standard polyethylene (PEg) supplied by Sigma-Aldrich (St. Louis, MO, USA), standard polyethylene terephthalate (PET) and standard polypropylene (PP). Polylactic acid (PLA), a biodegradable microplastic, was provided by Sigma-Aldrich. Additionally, as microplastic particles are often used to increase the abrasion properties of personal care products, in this research paper, two types of powdered polyethylene extracted from different personal care products (PE_PCPs_1 and PE_PCPs_2) were used. For this purpose, two types of facial scrubs were selected based on their specifications. Isolation of the MPs from the personal care products was conducted using the isolation method provided by Lončarski et al. (2020), which included mixing the product with boiled water (100 °C) and the addition of 30% H2O2 to induce degradation of the organic matter [23,33].
In order to better understand the adsorption behavior of the selected organic pollutants on microplastics, characterization of the selected MPs was conducted. Chemical characterization of all selected all MPs, including the commercially available MPs, PLA and the particles isolated from personal care products was conducted by Fourier transform infrared spectroscopy (FTIR) (Nicolet iS20 FTIR spectrometer, Thermo Fisher Scientific, USA), and the results are presented in Figure S1. The results of the FTIR analysis were compared with the OMNIC 9, Thermo Fisher Scientific, USA, software library (Hummel Polymer Sample Library), confirming the chemical structure of all selected polymers. More details about the chemical characterization of PEp, PE_PCPs_1, PE_PCPs_2, PEg, PET, PP and PLA are given in Lončarski et al. (2020, 2021). Furthermore, the physico-chemical properties of the MPs were obtained by Brunner Emmett Teller (BET) analysis, and these results are presented in the Supplementary Materials (Table S1). The unit size of the granulated MPs (PEg, PET, PP and PLA) as provided by the supplier was 3 mm [23,24,33]. The unit sizes of the powdered MPs (Figure S2) were determined by scanning electron microscopy (TM3030, Hitachi, Tokyo, Japan) and ranged between 49.7 and 259 µm for the PEp particles and between 80 and 358 µm for the PE isolated from the two personal care products [40]. Additionally, the determination of the point of zero charge (pHPZC) was performed according to the procedure provided by Ofomaja and Ho (2008). The detailed procedure for determination of the point of zero charge is given in the Supplementary Materials [41].
HBs (1,2,3-trichlorobenzene, 1,2,4-trichlorobenzene, 1,3,5-trichlorobenzene, pentachlorobenzene, hexachlorobenzene and trifluralin), CPs (4-chlorophenol, 2,4-dichlorophenol, 2,4,6-trichlorophenol and pentachlorophenol) and PAHs (naphthalene, fluorene, fluoranthene and pyrene), manufactured by Pestanal®, Sigma–Aldrich GmbH (Steinheim, Germany), were selected as sorbate models and their physico-chemical characteristics are presented in Table S2. The selected organic pollutants differed in hydrophobicity and water solubility, suggesting that their behavior in the presence of MPs in water would also be different. Additionally, the physico-chemical characteristics of the synthetic water used in this paper are given in Tubić et al. (2019) and are presented in the Supplementary Materials (Table S3) [24].
During the experiments and for sample preparations, hexane and methanol (provided by J.T. Baker), acetic anhydride, hydrogen peroxide, CaCl2, NaHCO3 and MgSO4·7H2O (provided by Sigma-Aldrich) were used. Additionally, for pH adjustment of the synthetic water matrix, HNO3 (Fluka, Seelze, Germany) and NaOH (Sigma-Aldrich) were used. All the reagents were analytical grade and used without further purification.

2.2. Batch Adsorption Experiments

The experiments pertaining to the sorption kinetics and sorption mechanisms of the selected organic pollutants on the MPs were performed using a batch equilibrium method under laboratory conditions. The selected adsorption kinetic models were applied in order to identify the correlation between adsorption time and adsorption capacity, as well as to possibly predict the adsorption mechanisms of the selected organic pollutants on the MPs. The pseudo-first-order (PFO) kinetic, pseudo-second-order (PSO) kinetic and Elovich (E) models were used in this study. The adsorption kinetic procedures and equations for the PFO kinetic, PSO kinetic and E models are presented in our previous papers, Tubić et al. (2019, 2021) and Lončarski et al. (2021), and described in the Supplementary Materials [23,24,40]. In order to determine the adsorption mechanisms of the organic pollutants on the MPs, adsorption isotherm experiments were performed. The detailed experiment procedures are given in Tubić et al. (2019, 2021) and Lončarski et al. (2021) and are presented in the Supplementary Materials [23,24,40].

2.3. Analytical Procedure, Quality Assurance and Quality Control

The concentration of the dissolved organic carbon (DOC) was obtained using a LiquiTOC II (Elementar, Langenselbold, Germany) and the stipulated SRPS ISO 8245:2007 method. Using the 340i, WTW, SenTix®21 electrode, according to the method of SRPS H.Zi.111:1987, the pH values of the experimental samples were measured. The residual chloride concentration in the water was determined according to the SRPS ISO 9297/1:2007 method. The sulfate concentration was determined by iodometric titration of excess chromate ion with Na2S2O3 solution after proper sample precipitation by the addition of excess barium chromate. Water alkalinity was analyzed in the form of hydrogen carbonate concentration, using the volumetric method (APHA, 1982). Determination of the selected organic pollutants was performed using gas chromatography with a mass detector (GC/MSD) (Agilent 7890A/5975C, Santa Clara, CA, USA) and gas chromatograph with an electron capture detector (GC/μECD). The detailed analytical, quality assurance and quality control procedures are given in our previous papers and shown in the Supplementary Materials [23,24,33,40].

3. Results and Discussion

3.1. Determination of Point of Zero Charge

Point of zero charge (pHPZC) indicates the pH value at which the surface of the material is neutrally charged. The point of zero charge is an important parameter for explaining the adsorption behavior of the sorbent, in this case, the MPs. In order to assay the possible influence of the constituents of the water matrix on the pHPZC, the experiments were conducted in both distilled water and a synthetic water matrix.
The obtained results for the determination of the pHPZC for selected MPs in distilled water and the synthetic water matrix are shown in Figure S3. The point of zero charge obtained for the selected MPs in distilled water ranged from pH = 3.83 to pH = 8.17 in the following order: PP < PLA < PET < PEp < PEg < PE_PCPs_2 < PE_PCPs_1. Furthermore, the point of zero charge obtained for the MPs in the synthetic water matrix was in the range from pH = 4.04 to pH = 8.71, with the sequence PP < PLA < PET < PEg < PE_PCPs_2 < PEp < PE_PCPs_1. Based on the obtained results shown in Figure S3, the constituents of the water matrix had no effect on pHPZC for the selected MPs, except in the case of PEp, where the pHPZC in the distilled water was pHPZC = 4.70, while in the synthetic matrix it was pHPZC = 8.21.
Xu et al. (2018) also determined the point of zero charge of polyethylene, where its obtained value in distilled water was pHPZC = 4.30. The results obtained by Xu et al. (2018) also indicated that the constituents of the water matrix significantly affect the charge of MPs. They also implied that it is important to use same water matrix during experiments in order to prevent the uneven diffusion in solution, and ensure the accuracy of the determination [42]. Additionally, research by Fotopoulou and Karapanagioti (2012) indicated that the water matrix has a significant effect on the point of zero charge of the material [43]. On the other hand, based on the obtained results of the point of zero charge for the powdered polyethylene isolated from the cosmetic products (PE_PCPs_1 and PE_PCPs_2), it could be observed that the change in the water matrix had a weaker effect on these materials in comparison with PEp. Furthermore, the pH values at which PE_PCPs_1 and PE_PCPs_2 were neutrally charged in distilled water were lower at pHPZC = 8.11 and pHPZC = 6.14, respectively, compared to the values observed with the synthetic water matrix, which were pHPZC = 8.71 and pHPZC = 7.29, respectively. This behavior of isolated MPs in relation to pure PEp can be explained by the change in the structure of polyethylene during the production process of cosmetic products, which possibly stabilizes the surface of the polymer and significantly reduces the influence of the water matrix on the point of zero charge of PE_PCPs_1 and PE_PCPs_2 [44]. Such an effect occurring in the production of personal care products may also be the reason for a significant difference in surface charge between isolated microplastic particles (pHPZC = 8.11 and 6.14 for PE_PCPs_1 and PE_PCPs_2, respectively) and Pep (pHPZC = 4.70) in distilled water.
The pHPZC values obtained for Peg, PET and PP in distilled water were 5.06, 4.66, and 3.83, respectively, which is in accordance with the results of other authors [22,45]. By comparing the obtained results for the point of zero charge of the granulated MPs in distilled water and synthetic matrix to the results for powdered MPs, it can be noted that in the case of granular MPs (PEg, PET, PP), the influence of the water matrix on pHPZC was significantly smaller. This behavior can be attributed to the greater granulation and smooth surface of MPs, which means that the presence of salt in the synthetic matrix did not affect the pHPZC [43,46,47,48,49].
Different surface charges of the selected MPs depending on the pH value and chemical composition of the water matrix can lead to different behaviors in the environment [43]. The obtained results shown in Figure S3 imply that at pH 7.23 ± 0.06 in the synthetic matrix, the surfaces of the pure PEp, PE_PCPs_1 and PE_PCPs_2 were positively charged (pH < pHPZC). On the other hand, the pHPZC values for PP, PET, PLA and PEg (pHPZC = 4.04, 4.53, 5.28 and 5.90, respectively) indicates that at the pH of the synthetic water matrix, their surfaces were negatively charged.

3.2. pH Effect on Sorption Affinity of Organic Pollutants on MPs

In this study, the adsorption behaviors of the three selected groups of organic pollutants on MPs were obtained at three different pH values (pH 4, 7 and 10) in order to cover the widest possible range of pH values in a water environment. Figures S4–S6 show that changes in the pH value of the water matrix had the highest impact in the case of the selected CPs. On the other hand, changes in pH value had a very low impact on the adsorption behavior of the selected PAHs and HBs on MPs.
The obtained results shown in Figure S6 imply that the highest percentage of adsorption of CPs was observed at pH 7 (25 to 68%). The adsorbed percentage of 4-CP adsorbed on MPs was in the following order: PP (30%) < PE_PCPs_1 ≈ PE_PCPs_2 ≈ PLA (35–40%) < Pep ≈ PEg (48%) < PET (60%). Furthermore, the results imply an increase in the adsorption percentage of 2,4-DCP onto MPs as follows: PE_PCPs_1 (40%) < PE_PCPs_2 ≈ PEg ≈ PLA (50%) < Pep ≈ PET ≈ PP (65%). The adsorption affinity of 2,4,6-TCP and PCP on MPs was the highest with PP, PLA, PE_PCPs_2 and PEg (around 50–60%). A slightly lower adsorption percentage was determined for 2,4,6-TCP and PCP on PEp and PE_PCPs_1 (30–40%), while the lowest was found with PET, at 25–30%.
Based on the physico-chemical characteristics of the CPs shown in Table S2, or to be precise, the pKa values of the acid constants, it can be seen that at pH 4, the selected CPs used in this study were in their molecular form, while at pH 10, they were in their ionic form. Additionally, based on the experimentally determined pHPZC of the MPs at pH 4, all selected microplastic particles (PEp, PE_PCPs_1, PE_PCPs_2, PEg, PET, PP and PLA) were positively charged, while at pH 10, their surfaces were negatively charged. Therefore, at pH 4 in water, the binding percentage was low due to the impossibility of interaction between the positively charged surfaces of the MPs and CPs in their molecular form [50,51]. On the other hand, at pH 10, the surface of the MPs was negatively charged, and the investigated CPs were in an ionic, negatively charged form. Considering this fact, at pH 10, the selected CPs were repelled from the surface of the MPs and a low percentage of interactions occurred in comparison to pH 7 [50,51].
Based on the obtained results, a slight influence of the pH value on the formation of interactions between the tested benzene derivatives and MPs was observed. It was also possible to observe a significantly higher binding percentage compared to the results obtained for CPs (Figures S4 and S5). This behavior can be explained by the higher hydrophobicity of the selected benzene derivatives compared to CPs, which led to their higher affinity towards adsorption on MPs [52,53]. Based on the results shown in Figure S5, the selected HBs had the highest adsorption affinity toward the PE standard and those isolated from cosmetic preparations (PE_PCPs_1 and PE_PCPs_2). Additionally, there was an increase in the percentage of HBs that increased in hydrophobicity in the range of 55–99%. Thus, the binding percentage of HBs changed in the following order: 1,2,3-TeCB < 1,3,5-TeCB < 1,2,4-TeCB < PeCB < HeCB < TFL. The results shown in Figure S5 imply that the selected HBs had a significantly lower affinity for PLA (30–90%) in comparison to the selected nondegradable MPs. Similar results were obtained in terms of the influence of the pH value on the adsorption of PAH on the MPs. Based on the results shown in Figure S6, an insignificant influence of the pH value on the formation of interactions between the selected PAHs and MPs can be observed. Additionally, the obtained results imply a slightly higher affinity of naphthalene and fluorene toward the standard polyethylenes (both PEp and PEg) and PE_PCPs_2 compared to other types of MPs, which may be a consequence of the larger specific surface areas of PEp, PEg and PE isolated from cosmetics in comparison to the active surface area of the other selected MPs. The obtained results shown in Figure S6 imply that the adsorption affinity of the selected PAHs toward the MPs increased with hydrophobicity, and the difference in the binding percentage on MPs was less significant. Thus, naphthalene, which was less hydrophobic, had the lowest affinity towards the MPs. The percentage of naphthalene adsorption was about 20% for PET, while for PEp, PEg, PE_PCPs_1, PE_PCPs_2 and PP it was about 85%, 95%, 65%, 95% and 75%, respectively. With the increase in the hydrophobicity of the tested PAHs, an increase in the binding percentage was determined. In all four types of PE and PP, it was in the range of 95–99%, while for PET it was 65%. The lowest adsorption affinity of PAHs was shown with PLA, where the binding percentages for naphthalene, fluorene, fluoranthene and pyrene were 30%, 50%, 60% and 75%, respectively.
Since the change in pH value had the highest impact on the adsorption behavior of CPs towards MPs, in order to better understand the effect of pH on sorption kinetics with MPs, the obtained experimental data were fitted with three kinetic models: the pseudo-first-order, the pseudo-second-order and the Elovich models. Furthermore, to better understand the influence of pH value on the adsorption mechanisms of the selected CPs on MPs, adsorption isotherm experiments were conducted.

3.3. The pH Effect on Adsorption Kinetics of CPs on MPs

The results of the sorption kinetics of CPs on the PEp, PE_PCPs_1, PE_PCPs_2, PEg, PET, PP and PLA particles at different pH levels in the water matrix are shown in Figure 2. After 12–48 h, the adsorption of the selected CPs onto the MPs reached equilibrium, depending on the sorbate–sorbent combinations investigated. As shown in Figure 2, the uptake of CPs by the selected MPs and PLA increased with time, until the sorption equilibrium was achieved after 12 h in the case of powdered MPs and 48 h for granulated MPs. These results are in the range of the usually reported periods for the sorption equilibria of organic compounds on various types of MPs.
The powdered MPs had the highest uptake for all selected CPs at neutral pH, or pH 7, and the highest adsorption affinity at pH 7 was obtained for 2,4-DCP (221.0 µg/g). The other three selected CPs indicated lower adsorption affinities compared to 2,4-DCP (137.7 µg/g, 162.0 µg/g and 90.74 µg/g for 4-CP, 2,4,6-TCP and PCP, respectively). The lower adsorption affinities toward these types of MPs could be a consequence of both the physico-chemical properties of the compounds and their molecular structures. Based on the physico-chemical properties of the selected CPs, as shown in Table S2, it could be assumed that PCP would exhibit the greatest change in its percentage of adsorption on PEp due to its low solubility in water and its high hydrophobicity compared to the other tested CPs. The influence of the hydrophobicity of molecules on the adsorption of organic compounds on MPs was pointed out by Wang and Wang (2018), who concluded that compounds with higher hydrophobicity are better adsorbed on MPs. However, the amount of PCP adsorbed was the smallest, which could be a consequence of the size of the molecule and the position of chlorine atoms in the PCP molecule, which may hinder the approach of the hydroxyl group to the surface of PEp, and thus hinder its adsorption. Furthermore, this confirms the chemical structure of 2,4-DCP and 2,4,6-TCP, where with the decrease in the number of chlorine atoms, the adsorption affinity increased (qt = 221 µg/g for 2,4-DCP and qt = 162.0 µg/g for 2,4,6-TCP) [22,54]. In order to investigate the adsorption behavior of the CPs in the presence of other types of MPs in water, PET and PP were selected since they are often detected in the environment [55,56]. The highest adsorption affinity at pH 7 towards PET was obtained by 2,4-DCP (qt = 125 μg/g). The adsorption affinity of CPs to PET further decreased in the following order: 2,4,6-TCP ≈ 4-CP > PCP; the adsorbed amounts for 2,4,6-TCP and 4-CP were 67 μg/g and 25 μg/g for PCP.
The obtained results for the adsorption kinetic of the CPs on PLA implied that the highest adsorption affinity has 2,4,6-TCP and PCP, where the qt after 24 h was 110 μg/g and 100 μg/g, respectively. The lowest adsorption affinity of CPs on PLA was obtained for 4-CP, qt = 85 μg/g.
On the other hand, the obtained results shown in Figure 2 indicate that at pH 4, the adsorption affinity of all selected CPs towards MPs decreased in comparison with the data obtained at pH 7. They also imply that, regardless of the pH, the highest adsorption affinity of CPs was obtained with the powdered MPs. Additionally, at pH 4, the highest adsorbed amount was obtained for 2,4-DCP on PE isolated from personal care products (73.8–91.0 µg/g). As in the case of pH 7, at a lower pH in the water matrix (pH 4), the adsorbed amount decreased for CPs with higher hydrophobicity but larger molecule structure. Therefore, the adsorbed amount of 2,4,6-TCP and PCP on PE isolated from personal care products ranged between 47.7 and 66.9 µg/g and 55.9 and 58.8 µg/g, respectively. At pH 10, the highest adsorbed amount was obtained for 4-CP on powdered MPs (about 137 µg/g for PEp, 89.4 µg/g for PE_PCPs_1 and 87.5 µg/g for PE_PCPs_2). The adsorption affinity toward powdered MPs decreased with increases in hydrophobicity and molecular sizes in the selected CPs at pH 10.
The results shown in Figure 2 imply that the selected CPs had significantly lower affinities toward granulated types of MPs (6.04–202.4 µg/g). Furthermore, pH effects were evident in the adsorption behavior of all CPs on MPs. In terms of pH effects, the highest impact on the adsorption behavior of CPs was detected for PP, which adsorbed the lowest amounts of all the CPs at pH 4 (6.04–23.3 µg/g). The same trend was seen in the obtained results, but with much lower differences between the adsorbed amounts of CPs being detected for all selected MPs at both pH 4 and pH 10. The lowest matrix effect was observed for PLA at both pH 4 and pH 10 (17.3–45.5 µg/g).
In order to obtain more comprehensive information about the influence of pH on the adsorption kinetics of CPs on MPs, three kinetic models were used: the pseudo-first-order, the pseudo-second-order and Elovich models. The obtained modeling data are summarized in Table S5. Based on the obtained results and the highest correlation coefficient (R2 = 0.862–0.998), the PSO kinetic model fit the data better than the PFO and E models. The R2 values calculated for the PSO for the adsorption kinetics of 4-CP, 2,4-DCP and 2,4,6-TCP on the seven materials were greater than 0.900. Slightly lower R2 values were obtained for PCP, but the values were still high, ranging from 0.862 to 0.983.
Since all the correlation coefficient values for the PFO and E models within the CP experiment were higher than 0.800, the PSO kinetic model fit the data better. Figure 2 illustrates that the results for fitting the experimental data in the exponential form using the PSO model was the most appropriate. The calculated qe, k2 and R2 values for the pseudo-second order model are listed in Table S5. Based on the qe values obtained with the PSO model and the experimentally obtained qe values, it can be concluded that this kinetic model gave the best fit for the obtained data. Therefore, the PSO model can successfully be used to describe the adsorption process of the selected CPs onto MPs regardless of the pH value of the water from the beginning of the sorption process to the equilibrium stage. This means that CPs can be adsorbed to different binding sites on the investigated MPs [24]. The sorption kinetics of organic pollutants on PE can be described using the PSO model, which is in agreement with the findings of other authors [38,46,47,54]. The high values of the R2 obtained for the PSO kinetic model by other studies for the adsorption of organic compounds on MPs, indicate a possible chemisorption mechanism during the adsorption process [12,36,48,57]. The results presented in Figure 3 imply that the adsorption speed of CPs on the MPs was significantly faster at the beginning of the adsorption process and that this may be a consequence of the free active sites on the surface of MPs [46,49,58]. The obtained results of the kinetic experiments indicate that the PSO kinetic model is suitable for describing the adsorption kinetics of CPs on MPs, which is In line with the observations of other authors [59,60,61,62,63,64].

3.4. The pH Effect on Adsorption Isotherms of CPs on MPs

The Freundlich and Langmuir models were employed to describe the experimental data from the adsorption mechanism studies. The obtained parameter values of the isotherm models are presented in Table S6, while Figure 4 shows the fitting results of both the Freundlich and Langmuir isotherms in the experiment regarding the adsorption of CPs on PEp, PE_PCPs_1, PE_PCPs_2PEg, PEg, PET, PP and PLA in water with different pH values.
The presented results (Table S6 and Figure 4) indicate that the sorption of CPs can be described well with both adsorption isotherm models, based on the high R2 values obtained. The values of parameter n (<1) obtained by the Freundlich model imply a chemical adsorption process between the investigated CPs and MPs [17,24,33,40,65]. However, the better correlations with the Langmuir model indicate a monolayer coverage of the sorbate over a homogenous sorbent surface [66]. The obtained qmax values calculated using the Langmuir adsorption isotherms imply that, at pH 7, the highest adsorption capacity toward the selected MPs has 4-CP, towered selected MPs and it ranged between qmax = 33.4–205.7 µg/g. On the other hand, for the remaining CPs, 2,4-DCP, 2,4,6-TCP and PCP, the highest adsorption capacities were obtained at pH 4, and they were in the ranges qmax = 15.5–198.8 µg/g qmax = 30.4–259.9 µg/g qmax = 25–190.6 µg/g, respectively. The results for the calculated qmax values were consistent with the obtained kinetic results (Figure 3 and Table S5). Additionally, the calculated KL values for the Langmuir adsorption isotherms were between 0 and 1 for all results, implying that the adsorption process of the selected CPs on the MPs was favorable regardless of the changes in the synthetic water matrix pH [12], and with values near zero (0.003–0.300 µg/g), it can be postulated that the process would be almost irreversible.

3.5. Possible Adsorption Mechanism between the CPs and MPs

Wu et al. (2016) highlighted the significant influence of the water matrix on the adsorption behavior of pharmaceuticals whose structures contained functional groups similar to those in the structure of CPs (hydroxyl and chloride), depending on the physicochemical characteristics of the compound. They implied that the different influences of water constituents on the adsorption behavior of organic pollutants depended on the physicochemical properties of the compounds, which is in accordance with the results obtained here with the adsorption of CPs, 2,4-DCP, 2,4,6-TCP and PCP on the selected MPs [58].
The obtained results presented in Figure 4 imply a significant influence of the pH value on the adsorption affinity and adsorption capacity of the CPs on the MPs. The highest adsorption capacity of the selected CPs was obtained at pH 7. In order to obtain a better understanding of the possible adsorption mechanisms of CPs, the behavior of the active functional groups and the physico-chemical properties of the selected CPs and the MPs at the selected experimental pH levels were determined. The possible interactions between the MPs and the selected CPs are shown in Figure 5.
Based on the values of the pKa shown in Table S2, it can be seen that, at acidic and neutral pHs, the selected CPs were in their molecular form, while the surface of the MPs were positively charged. Taking into account that the pHPZC values were in the range of 4.04–8.71, it can be inferred that electrostatic interactions between sorbent and sorbate at this pH range did not occur. Furthermore, at pH 10, the selected MPs were in their ionic forms, as were the CPs 2,4-DCP, 2,4,6-TCP and PCP, resulting in a low adsorption affinity with the selected MPs due to electrostatic repulsion. Therefore, other types of interactions are responsible for the adsorption of CPs on MPs at pH 4 and pH 7, such as π-π interactions, hydrogen bonding, halogen bonding and van der Waals interactions. It is possible that the higher adsorption affinity at pH 7 in comparison with pH 4 occurred due to competition between H+ ions and the free electron pairs of phenol functional groups in the molecules of the selected CPs [67].
In general, the obtained results for the adsorption isotherms for the selected CPs on MPs imply that the dominant adsorption mechanism is chemisorption. Additionally, the obtained results also indicate the possible development of non-specific van der Waals forces and hydrophobic and π-π interactions, which has also been pointed out by other authors. The results also indicate that the adsorption mechanisms of organic compounds on MPs can be significantly influenced by the physico-chemical properties of the organic compounds and MPs, as well as the water matrix’s chemical properties, pH value and salinity [11,53,60,68].

4. Conclusions

Plastic debris has become an integral part of the environment and could serve as a source of organic pollutants. The transportation of pollutants through different environments can be affected by MPs, and MPs could also affect the way pollutants express their toxicity towards aquatic organisms and humans. The widespread use of bioplastics in everyday life is leading to an increase in their environmental impact. Understanding the interactions between different MPs and organic pollutants is crucial.
The findings in this study show that the fate and transport of specific CPs, PAHs and HBs can be significantly impacted by the presence of MPs and bioplastics in aquatic environments. Furthermore, this study performed an in-depth investigation on the adsorption properties of CPs on MPs under different environmental pH conditions. The obtained results of this study are helpful in terms of further our understanding of the interactions between MPs and CPs in different real environmental conditions. The water matrix’s pH value affected most adsorption behaviors of CPs on all the selected types of MPs, and the highest degree of adsorption occurred at pH 7. Furthermore, the adsorption of the HBs and PAHs did not depend on changes in the pH value of the water matrix. Besides the pH value of the water matrix, sorbate’s hydrophobicity was observed to have a significant impact on the adsorption affinity of the HBs and PAHs on the MPs, which is one of the factors that has to be taken into account. Therefore, the influence of different environmental parameters on the adsorption behavior of organic pollutants on the MPs is a potential direction for further research.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/app132312835/s1, Figure S1: FTIR spectra of the selected microplastics used in the experiments: (a) PEp, (b) PE_PCPs_1, (c) PE_PCPs_2, (d) PEg, (e) PET, (f) PP and (g) PLA; Figure S2: SEM micrographs of (a) PEp, (b) PE_PCP_1 and (c) PE_PCP_2 microplastic particles; Figure S3: Point of zero charge for (a) PEp, (b) PE_PCPs_1, (c) PE_PCPs_2, (d) PEg, (e) PET, (f) PP and (g) PLA in distilled water and synthetic water matrix; Figure S4: Water matrix pH effect on adsorption affinity of (a) 4-CP, (b) 2,4-DCP, (c) 2,4,6-TCP and (d) PCP on microplastics (n = 2, mean value _ SD); Figure S5: Water matrix pH effect on adsorption affinity of (a) 1,2,3-TeCB, (b) 1,3,5-TeCB, (c) 1,2,4-TeCB, (d) PeCB, (e) HeCB and (f) TFL on microplastics (n = 2, mean value _ SD); Figure S6: Water matrix pH effect on adsorption affinity of (a) naphthalene, (b) fluorene, (c) fluoranthene and (d) PCP on microplastics (n = 2, mean value _ SD); Table S1: Specific surface area and pore size of microplastics obtained by BET analysis; Table S2: Physico-chemical properties of selected organic pollutants; Table S3: Physico-chemical characteristics of the synthetic water; Table S4: Mathematical models used for modelling data obtained in kinetic and adsorption experiments; Table S5: Kinetic model parameters for selected chlorinated phenols sorption on microplastics; Table S6: Calculated parameters with different isotherm models for the adsorption of chlorinated phenols on PEp, PE_PCPs_1, PE_PCPs_2, PEg, PEg, PET, PP and PLA in water. References [69,70,71,72,73] are cited in Supplementary Materials file.

Author Contributions

Conceptualization, M.V., J.A. and A.T.; Methodology, M.V. and A.T.; Software, M.V., J.N. and B.K.; Investigation, M.V. and S.V.; Resources, J.A. and A.T.; Writing—original draft, M.V. and A.T.; Writing—review and editing, M.V. and J.N.; Visualization, M.V. and A.T.; Project administration, A.T.; Funding acquisition, J.A. and A.T. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Ministry of Science, Technological Development and Innovation of the Republic of Serbia (Grant No. 451-03-47/2023-01/200125).

Data Availability Statement

The data presented in this study are available on request from the corresponding author. The data are not publicly available due to privacy.

Conflicts of Interest

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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Applsci 13 12835 g001
Figure 1. Most important factors influencing the sorption of pollutants on MPs.
Figure 1. Most important factors influencing the sorption of pollutants on MPs.
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Figure 2. Experimental adsorption of (a) 4-CP; (b) 2,4-DCP; (c) 2,4,6-TCP and (d) PCP on PEp, PE_PCPs_1, PE_PCPs_2, PEg, PET, PP and PLA in water at different pH values (n = 2).
Figure 2. Experimental adsorption of (a) 4-CP; (b) 2,4-DCP; (c) 2,4,6-TCP and (d) PCP on PEp, PE_PCPs_1, PE_PCPs_2, PEg, PET, PP and PLA in water at different pH values (n = 2).
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Figure 3. Plots for the sorption kinetics, based on the pseudo-second order model of (a) 4-CP, (b) 2,4-DCP, (c) 2,4,6-TCP and (d) PCP on PEp, PE_PCPs_1, PE_PCPs_2, PEg, PEg, PET, PP and PLA in water at different pH values (n = 2).
Figure 3. Plots for the sorption kinetics, based on the pseudo-second order model of (a) 4-CP, (b) 2,4-DCP, (c) 2,4,6-TCP and (d) PCP on PEp, PE_PCPs_1, PE_PCPs_2, PEg, PEg, PET, PP and PLA in water at different pH values (n = 2).
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Figure 4. Adsorption isotherm plots for (a) 4-CP, (b) 2,4-DCP, (c) 2,4,6-TCP and (d) PCP adsorption on PEg, PEp, PET, PP and PLA in the synthetic water matrix at different pH values (n = 2).
Figure 4. Adsorption isotherm plots for (a) 4-CP, (b) 2,4-DCP, (c) 2,4,6-TCP and (d) PCP adsorption on PEg, PEp, PET, PP and PLA in the synthetic water matrix at different pH values (n = 2).
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Figure 5. Possible adsorption mechanism between the CPs and MPs.
Figure 5. Possible adsorption mechanism between the CPs and MPs.
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MDPI and ACS Style

Vujić, M.; Vasiljević, S.; Nikić, J.; Kordić, B.; Agbaba, J.; Tubić, A. Sorption Behavior of Organic Pollutants on Biodegradable and Nondegradable Microplastics: pH Effects. Appl. Sci. 2023, 13, 12835. https://doi.org/10.3390/app132312835

AMA Style

Vujić M, Vasiljević S, Nikić J, Kordić B, Agbaba J, Tubić A. Sorption Behavior of Organic Pollutants on Biodegradable and Nondegradable Microplastics: pH Effects. Applied Sciences. 2023; 13(23):12835. https://doi.org/10.3390/app132312835

Chicago/Turabian Style

Vujić, Maja, Sanja Vasiljević, Jasmina Nikić, Branko Kordić, Jasmina Agbaba, and Aleksandra Tubić. 2023. "Sorption Behavior of Organic Pollutants on Biodegradable and Nondegradable Microplastics: pH Effects" Applied Sciences 13, no. 23: 12835. https://doi.org/10.3390/app132312835

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