Exploring the Valorization Potential of Microplastics: An Investigation into Their Use as Efficient Adsorbents in Wastewater Treatment

Exploring the Valorization Potential of Microplastics: An Investigation into Their Use as Efficient Adsorbents in Wastewater Treatment

• Autor
• Lucas de Carvalho Silva
• Co-autores
• Eduardo Malosti Barbosa , Camila Gabriela Pedroso Grassmann , Leonardo José Viginheski , Carla Bastos Vidal
• Resumo

•  

   Exploring the Valorization Potential of Microplastics: An Investigation into Their Use as Efficient Adsorbents in Wastewater Treatment

   

  Lucas de Carvalho Silvaa, Eduardo Malosti Barbosab, Camila Gabriela Pedroso Grassmannb, Leonardo José Viginheskic, Carla Bastos Vidalb,d

  a Federal University of Technology – Paraná (UTFPR) - Civil Engineering Graduate Program, Deputado Heitor de Alencar Furtado St., 5000, Ecoville, Curitiba, Paraná 81280-340, Brazil.

  b Federal University of Technology – Paraná (UTFPR) - Environmental Sciences Graduate Program, Deputado Heitor de Alencar Furtado St., 5000, Ecoville, Curitiba, Paraná 81280-340, Brazil.

  c Federal University of Technology – Paraná (UTFPR) – Department of Chemistry and Biology, Deputado Heitor de Alencar Furtado St., 5000, Ecoville, Curitiba, Paraná 81280-340, Brazil.

  c Federal University of Ceará  (UFC) – Department of Analytical Chemistry and Physical Chemistry, Humberto Monte  St., s/n, Campus do Pici, Bloco 938/939, Fortaleza, Ceará 60440-900, Brazil.

   

  Abstract

  Plastic, a ubiquitous material in modern society, generates waste that accumulates in the environment, leading to severe ecological and social consequences. Among these wastes, microplastics (MPs) are an increasing concern due to their persistence and potential toxicity. The valorization of MPs as adsorbents for pollutant removal emerges as a promising strategy for the circular economy and the mitigation of plastic pollution. Therefore, understanding the interaction between the adsorbate and the adsorbent is crucial, aiming to elucidate the behavior of microplastics when exposed to pollutants. This study evaluated the efficacy of Polyethylene - PE and poly (ethylene terephthalate) - PET (0.05 mm) spheres in adsorbing Congo red dye from synthetic textile effluents. The results demonstrated that for PE, the data fit the non-linearized Langmuir model better, indicating that adsorption occurred in monolayers and suggesting a homogeneous distribution of active sites on the adsorbent surface. For PET, the data fit the non-linearized Freundlich models, indicating the presence of physisorption and the formation of multilayers.. Additionally, a rapid adsorption capacity of Congo red dye by the material was observed in the initial minutes, followed by a decrease in the slope of the curve after 60 minutes, indicating the equilibrium time. The dye removal rate by PE and PET in the form of microplastics was 4 and 10% at equilibrium time, respectively, indicating relatively low adsorption efficiency. Despite the rapid adsorption capacity, additional studies are needed to understand the underlying mechanisms of the interactions between the material and the dye.

   

  Keywords: Polymer; Adsorption; Circular economy; dye removal; Congo red

   

   

   

   

   

  1. Introduction

  Microplastics (MPs) are a generic term for a variety of unique chemical compounds with a size < 5 mm, originating from different products with various polymers and morphologies (Sun et al., 2022). Previous studies have shown that different MPs are detected in aquatic environments and, due to their characteristics, can accumulate various toxins and chemical pollutants, serving as vehicles for long-distance transport. Additionally, MPs can alter microbial community structure, affect gene expression, and damage cells and tissues (Mierzejewski et al., 2023).

  However, recent studies suggest that MPs can also play a beneficial role by adsorbing pollutants such as persistent organic pollutants, pharmaceuticals and personal care products, and heavy metals due to their specific surface area, porosity, and surface chemistry. For example, MP adsorption has been explored as a potential strategy for wastewater treatment (Rodrigues et al., 2023).

  In summary, the study of adsorption on microplastics such as polyethylene (PE) and polyethylene terephthalate (PET) is essential both for the valorization of these waste materials and for understanding the dynamics between these materials and other pollutants present in the environment. This study aims to evaluate the efficacy of PE and PET spheres in adsorbing Congo red dye, a common contaminant in synthetic textile wastewater, from synthetic wastewater. The focus is understanding the interaction between the dye (adsorbate) and microplastics (adsorbents).

  2. Methodology

  A standard synthetic solution containing an initial concentration of 100 mg L-1 of Congo red dye from Dinâmica, Brazil, was employed. Successive dilutions were conducted for kinetics and adsorption equilibrium tests. The adsorbent material used was PE and PET microplastics in powder form, with a particle size of 0.05 mm.

  Batch adsorption experiments were conducted using 125 mL glass reactors. Synthetic textile effluent was combined with the adsorbent material at a dosage of 1 g L-1 in the reactors. Stirring was performed using a Novatecnica NT 145 magnetic stirrer at 195 rpm. Operational conditions included a pH of 7.0 and a temperature of 22 °C.

  In the adsorption equilibrium experiments, tests were conducted by varying the initial concentration of Congo red dye in volumes of 50 mL, with concentrations of 1, 5, 10, 25, and 50 mg L-1. These tests were carried out for 24 hours. After the experiments, the adsorption capacity of the material was calculated using Eq. (1), and adsorption isotherms (q versus Ce) were constructed.

                             (1)

  Where q represents the adsorption capacity in mg g-1, Co and Ce are the initial and equilibrium concentrations of Congo red dye, respectively, in mg L-1; V is the effective volume in liters; m is the mass of the adsorbent used in grams.

  Theoretical models of Langmuir and Freundlich, both linearized and non-linearized (as per Table 1), were applied. The Solver® tool adjusted the non-linear models by minimizing the sum of squared errors (SSE) (Eq. 02) to quantify the error.

  Table 1. Linear and Non-linear Theoretical Models of Adsorption Isotherms

  Langmuir

  Linear

  Não-Linear

  Freundlich

  Linear

  Não-Linear

  Where b is the Langmuir equilibrium constant (L mg-1); qm is the theoretical maximum capacity (mg g-1); K (L mg-1) and 1/n are parameters of the Freundlich model.

                (2)

  Where Qe and Qt are the experimental and theoretical adsorption capacities, respectively.

  In the adsorption kinetics experiments, tests were conducted varying the time from 0 to 1440 minutes under the same conditions mentioned earlier. To calculate the surface diffusion coefficient (Ds), the solid homogeneous diffusion equation (Eq. 03) was applied and solved using the numerical method of separation of variables. This method considered only the initial times of the kinetics (small times) (q?<0.3) and assumed zero external resistance, allowing the equation to be rewritten as Eq. 04.

                (3)

                 (4)

  Where r is the particle radius in cm, t is the time in minutes, q is the average adsorption capacity (mg g-1), qe is the equilibrium adsorption capacity (mg g-1), and Ds is the diffusion coefficient in cm min-1.

  The analysis of Congo red dye concentrations was performed using a UV-1800 spectrophotometer (Shimadzu) at a wavelength of 500 nm.

  3. Results and discussion

  Using the data obtained from the adsorption equilibrium experiments, non-linear isotherm graphs were created for PE (Fig. 1) and PET (Fig. 2), including both the experimental curve and the curves fitted to the theoretical Langmuir and Freundlich models.

   

  Fig. 1. Equilibrium isotherms for Congo red dye adsorption onto PE.

   

   

  Fig. 2. Equilibrium isotherms for Congo red dye adsorption onto PET.

  As observed in Fig. 1, the experimental adsorption isotherm for PE exhibited a concave curvature to the abscissa, defined as type L2 (GILES et al., 1960), indicating that adsorption occurred in monolayers. The results suggest that as the adsorbent sites are filled, it becomes increasingly difficult for the adsorbate molecules to find unoccupied sites. For PET, as shown in Fig. 2, the adsorption isotherm displayed a linear form, classified as type C1 (GILES et al., 1960), indicating that the amount adsorbed varies linearly with the adsorbate concentration. This type of isotherm is favored in porous materials with different degrees of crystallinity.

  Normalized isotherm models for PE indicate that the highest R² value (0.906) was obtained for the Langmuir model, which suggests the occurrence of chemical adsorption (FEBRIANTO et al., 2009). For PET, the Freundlich model provided the best fit with an R² value of 0.985. The Freundlich model describes equilibrium on heterogeneous surfaces, indicating physisorption and the formation of multilayers (Cooney, 1999).

  The experimental adsorption capacity for PE was 1.67 mg g?¹, while the Langmuir model fits indicated a maximum adsorption capacity (Qmax) of 1.64 mg g?¹ for the non-linear model, presenting a lower associated error (SSE = 0.172). For PET, the Freundlich model that best fit the data was the non-linear model (SSE = 0.055), indicating maximum dye adsorption of 2.74 mg g?¹, similar to the experimental Qmax of 2.74 mg g?¹.

  Based on the results of the adsorption kinetics studies, it was possible to calculate the adsorption capacities for polyethylene at different time intervals. The kinetics graphs are available in Fig. 3.

   

  Fig. 3. Congo red dye adsorption kinetics onto PE and PET adsorbents.

  According to Fig. 3, it is possible to observe that the materials exhibited rapid adsorption capacity within the initial minutes. For both materials, from 60 minutes onwards, there is a noticeable decrease in the slope of the curve, indicating that this point marks the equilibrium time. The adsorption efficiency during this equilibrium period was 4% and 10% for PE and PET, respectively, remaining constant at 90 minutes and slightly decreasing to 2.7% for PE and 9% for PET after 1440 minutes (24 h) of contact with the dye.

  The rapid adsorption observed is related to the transfer of molecules to the surface of the adsorbent and the availability of adsorption sites, marking the first phase of the process. Consequently, the adsorption process reaches equilibrium due to the saturation of the adsorbent, as described by Choi, Kim, and Kim (2008).

  It is notable that the adsorption amount (Q) for PE decreases slightly after 1440 minutes of agitation. This behavior can be attributed to the phenomenon of desorption that might occur over prolonged contact times. During extended periods of agitation, the dye molecules that are initially adsorbed onto the surface of the PE may desorb back into the solution. This desorption can be influenced by several factors, such as weaker dye-adsorbent interactions, surface saturation, and dynamic equilibrium conditions.

  For PE, the adsorption sites might become saturated at earlier stages, and the continued agitation could disturb the already adsorbed Congo red molecules. This could lead to their gradual release back into the solution, explaining the slight reduction in Q at 1440 minutes. This observation suggests that there is a balance between adsorption and desorption processes, especially over prolonged times, which is more pronounced in PE compared to PET, likely due to differences in their surface properties and affinity towards the dye."

  4. Conclusion

  For PE, the data fit the non-linearized Langmuir model better, assuming that adsorption occurred in monolayers, suggesting a homogeneous distribution of active sites on the adsorbent surface. For PET, the data fit the non-linearized Freundlich models, indicating the presence of physisorption and the formation of multilayers.

  A rapid adsorption capacity of Congo red dye by the material was observed in the initial minutes, followed by a decrease in the slope of the curve after 60 minutes, indicating the equilibrium time. The dye removal rate by PE and PET in the form of microplastics was 4% and 10% at equilibrium time, respectively, indicating relatively low adsorption efficiency. Despite the rapid adsorption capacity, additional studies are needed to understand the underlying mechanisms of the interactions between the material and the dye.

  References

  [1]  Mierzejewski, K., Kurzynska, A., Golubska, M., Calka J, Galecka I, Szabelski M, Paukszto L, Andronowska, A., Bogacka, I. New insights into the potential effects of PET microplastics on organisms via extracellular vesicle-mediated communication. Science of The Total Environment, v. 904, p. 166967, 15 dez. (2023).

  [2] Rodrigues, C., Pinheiro, G., Rezende, V., Val, L., Souza, D., Cristina, M., & Amaral, S.  Microplastics in surface water?: occurrence , ecological implications , quantification methods and remediation technologies. vol. 474. (2023).

  [3] Giles, C.H, MacEwan, T.H, Nakhwa, S.N, Smith, D. Studies in Adsorption. Part XI. A System of Classification of Solution Adsorption Isotherms, and its Use in Diagnosis of Adsorption Mechanisms and in Measurement of Specific Surface Areas of Solids. 846 (1960).

  [4]  Febrianto, J., Kosasih, A.N., Sunarso, J., Ju, Y.H., Indraswati, N., Ismadji, S. Equilibrium and kinetic studies in adsorption of heavy metals using biosorbent: A summary of recent studies. Journal of Hazardous Materials, v. 162, n. 2-3, p. 616-645, 2009.

  [5] Cooney, D. O. Adsorption Design for Wastewater Treatment. Florida: CRC Press, (1999).

  [6] Choi, K.J., Kim, S.G., Kim, S.H. Removal of antibiotics by coagulation and granular activated carbono filtration. Journal of Hazardous Materials, v. 151, p. 38-43 (2008).

   

• Palavras-chave
• Polymer, Adsorption, Circular economy, dye removal, Congo red
• Área Temática
• Adsorção aplicada ao meio ambiente