Amidation modified waste polystyrene foam as an efficient recyclable adsorbent for organic dyes removal

Considerable organic dye wastewater is discharged directly without treatment, which has caused a sharp increase in the types and content of existing pollutants in water bodies (Chan et al. 2016; Wang et al. 2018b; Cheng et al. 2019; Roy et al. 2019). These pollutants are often complex, highly toxic, and difficult to degrade; thus, they are gradually accumulated. Through the food chain, such pollutants cause great pollution to the ecological environment and biosphere, and restrict the sustainable development of harmonious coexistence between human and nature (Han et al. 2019; Choi et al. 2020; Dinari et al. 2020; Lei et al. 2020). Therefore, removal of organic dyes from wastewater has become important for the environment (Cho et al. 2018; Wang et al. 2020). However, cationic dye Methylene blue (MB) and anionic dye Congo red (CR) are common organic dyes (Wan et al. 2017; Ma et al. 2020).

At present, biodegradation, photocatalysis, and flocculation can effectively remove dyes in wastewater (Liu et al. 2018; Unnikrishnan et al. 2018; Wang et al. 2018a; Naushad et al. 2019; Satheshkumar et al. 2020; Sharma et al. 2020). However, these methods usually have complex process flows, high operating costs, and high energy consumption. By contrast, adsorption has become a research hotspot in recent years because of its simple operation, large processing capacity, easy design and operation, and low cost (Naushad et al. 2015; Duan et al. 2019).

At present, domestic and foreign researchers have conducted several studies on the resource utilisation of waste polystyrene foam (Dardouri et al. 2018). Polystyrene has a three-dimensional network structure, which has a certain good effect on adsorption. A novel amino modified hierarchically macro–mesoporous cross-linked polystyrene adsorbent (HP CLPS-EDA) was fabricated via the combination of colloidal crystal templating with the Friedel-Crafts (F-C) technique, and the adsorption performance of salicylic acid from aqueous solutions on HP CLPS-EDA was investigated (Zhang et al. 2018a). A novel hybrid nanomaterial was fabricated by encapsulating chitosan molecules (CS) into charged polystyrene resin (PS) through electrostatic adsorption. The resulting hybrid absorbent (PS-CS) demonstrates highly efficient sorption towards cupric ion (Liu et al. 2019b). However, research on the use of waste polystyrene adsorption is limited. A conjugated microporous polymer (CMP) was synthesised using post-consumer waste polystyrene (WPS) and activated into a resin with sulfonic acid groups (SCMP). The maximum adsorption capacity of CR for CMP and SCMP was 500 and 357 mg/g, respectively (Chaukura et al. 2017).

The removal of MB and CR from wastewater is important. To date, some researchers have prepared highly effective adsorbents for the adsorption of MB and CR (Albadarin et al. 2017; Daneshvar et al. 2017; Ahmed et al. 2020). It provides ideas for studying the adsorption materials of MB and CR. Therefore, we will synthesise a series of new dye adsorbents that are highly efficient, stable, and reusable. In this paper, PS-SD was synthesised from dopamine and carboxylated waste polystyrene, which was prepared by the Friedel-Crafts reaction of waste polystyrene foam and succinic anhydride. Waste polystyrene foam is easily soluble in dichloromethane. The reaction conditions are efficient and simple. This work will provide a novel way to recycle waste polystyrene materials into high value-added products that can be used for wastewater treatment. Using CR and MB as model pollutants, the adsorption characteristics of amide-functionalised polystyrene adsorbents are studied. The amount of adsorbent, pH value, temperature, time, and initial dye concentration (C₀) during adsorption have been well studied and optimised. The main objectives of this work are as follows: to study the feasibility of using the modified PS as an adsorbent to remove CR and MB, to determine the applicability of various isotherm models (Langmuir and Freundlich) and find the best-fitting isotherm equation, to evaluate the kinetic parameters and explain the adsorption properties, and to compare PS-SD with other adsorbents.

Waste polystyrene foam was obtained from a reagent packaging box. Dopamine hydrochloride (DA) (98%) was purchased from Aladdin, China. Other materials such as succinic anhydride and N,N''-dicyclohexylcarbodiimide (DCC) were purchased from Adamas Chemical Reagent Factory (Shanghai, China). Dichloromethane (CH₂Cl₂) was purchased from Kelong Chemical Reagent Factory (Chengdu, China). MB and CR were of analytical grade and purchased from Adamas Chemical Reagent Factory (Shanghai, China).

The carboxyl group was grafted onto waste polystyrene foam by the Friedel-Crafts reaction to obtain carboxylated polystyrene (PS-S). 0.01 mol of PS-S, 0.01 mol of dopamine, and 0.01 mol of DCC were added to a round-bottomed flask containing 35 mL of dichloromethane and stirred at room temperature for 12 h. Then 20 mL of deionised water was added, and the solid was washed with dichloromethane, ethanol, and deionised water to obtain PS-SD. The reaction scheme is shown in Figure 1.

The adsorption kinetics experiment determines the mechanism and potential rate control step of adsorption by analysing the change trend of the adsorption capacity over time. The adsorption kinetics experiments were conducted by adding 10 mg of PS-SD into a 20 mL glass bottle containing 10 mL of MB and CR solutions with a concentration of 100 mg/L. Adsorption was performed by shaking in a water bath (200 rpm) at 298.15 K. The experiment was repeated three times. The pseudo-first-order equation and pseudo-second-order equation in the diffusion model were used to fit the experimental data to nonlinear adsorption.

The resulting materials were characterised by Fourier transform infrared (FTIR, WQF-520) spectroscopy between 4,400 and 400 cm⁻¹ using the KBr pellet technique. Scanning electron microscopic (SEM) images were obtained on a field emission scanning electron microscope (THS-112). Nitrogen adsorption/desorption isotherm was measured at 77 K with Autosorb- 1 (Quantachrome, USA). The specific surface area was calculated by the Brunauer-Emmett-Teller (BET) equation using the adsorption data at relative pressure of 0.05–0.3. Thermogravimetric analysis (TGA) of PS-S and PS-SD was performed using a SDTA851 (METTLER, Switzerland) thermo-gravimetric analyser under N₂ atmosphere at a heating rate of 10 °C min⁻¹ from 40 to 700 °C.

FTIR analysis was performed to explore the surface properties of PS-S and PS-SD, and the spectrum is presented in Figure 2. In Figure 2(b), the peak at 3,450 cm⁻¹ was attributed to the stretching vibration of the O-H bond and the peak at 1,680 cm⁻¹ was assigned to stretching vibration of the C = O bond (Liu et al. 2020b). In Figure 2(c), the peaks at 3,400 and 1,640 cm⁻¹ were related to the N-H and C = O bonds in the amide group.

Nitrogen adsorption–desorption isotherms of PS-SD before and after adsorption of MB and the pore size distributions of samples are compared in Figure 3. The specific surface area was calculated from the N₂ adsorption isotherm by using the Branauer-Emmett-Teller (BET) model, and the pore size distribution was determined by using the Barrett-Joyner-Halenda (BJH) model. PS-SD showed a typical H4 hysteresis loop and the isotherm belonged to type IV (Duan et al. 2019). The adsorption capacity of PS-SD dropped sharply after MB adsorption because of the MB molecules at the adsorption sites; thus, the BET surface area of PS-SD declined from 21.63 to 5.84 m² g⁻¹ during adsorption. The result indicated that PS-SD had an abundant mesoporous structure, as further confirmed by the pore size distribution. The pore size of PS-SD ranged from 1.46 to 140 nm with a peak value at a pore diameter of 20 nm.

The thermal decomposition of PS-SD was investigated and compared with that of PS and PS-S (Figure 4). The total mass of PS-S and PS-SD gradually decreased with the increase of temperature. We have considered the weight losses of 2–6% at 200 °C insignificant, as the evaporation of solvent and/or water molecules was retained in the polymer chains. However, a big difference in mass loss occurred between PS-S and PS-SD before reaching 350 °C, which was primarily due to the volatilization of the pyrolysis of the oxygen-containing functional groups (Wang et al. 2019). The serious weight loss of PS-SD in this stage demonstrated the occurrence of several defects and oxygen-containing functional groups including amide and hydroxyl groups in PS-SD. A pure PS sample showed weight loss with its onset at approximately 350 °C. After reaching 350 °C, rapid mass loss rates of PS, PS-S, and PS-SD occurred at approximately 430 °C, which were due to decarbonisation of the polystyrene matrix. These results demonstrated that PS, PS-S, and PS-SD polymers had good thermal stability. The thermal degradation of PS-SD indicated that amidation modification had been completed.

The microstructures of the as-prepared products were characterised from their SEM images. Figure 5 shows representative SEM images of PS and PS-SD. Polystyrene had a smooth surface. After amidation modification, the surface was fluffy and porous, which was suitable for adsorption. As shown in Figure 5(c) and 5(d), after MB was adsorbed, the pores were occupied, and MB molecules were found on the surface. This result indicated that adsorption was consistent with physical adsorption and chemical adsorption.

Solution pH played an important role in adsorption because it strongly influenced the binding sites of the adsorbent. As shown in Figure 6, the adsorption capacity of CR increased and then decreased with the increase of pH. Under acidic conditions, the amino group of the modified PS was protonated to form -NH₃⁺, which combined with the anionic CR through electrostatic interaction. When the pH value was relatively low (pH < 4), H⁺ combined with the sulfonic acid group of CR, and the electronegativity of CR was weakened. The electrostatic attraction between the adsorbent and CR was reduced. The adsorption capacity of MB gradually increased with the increase of pH. Under alkaline conditions, the phenolic hydroxyl of modified PS was combined with cationic MB through hydrogen bond interaction (Wan et al. 2017; Lin et al. 2019). The adsorption of MB on the PS-SD surface generally followed an ionic interaction mechanism in which the surface charge of the adsorbent determined the type of interactions between the binding sites of the adsorbent and the adsorbate molecules. When pH = 9, the surface of the adsorbent was already positively charged, and electrostatic repulsion was observed between the adsorbent and the dye molecules. However, PS-SD still had high adsorption performance for CR at this time, indicating the occurrence of other adsorption mechanisms apart from electrostatic interaction. Under natural conditions, the adsorption effect of modified PS-SD on anionic dyes was higher than that on cationic dyes (Lin et al. 2019).

The adsorption isotherm model fitting results of the amide modified waste polystyrene PS-SD for CR and MB dyes are shown in Figures 7 and 8, respectively, and the fitting parameters are shown in Table 1. R² (0.99, 0.93) of MB and CR by Langmuir was higher than R² (0.88, 0.55) of MB and CR by Freundlich. This finding indicated that adsorption can be better described by the Langmuir adsorption isotherm model. These results showed that adsorption occurred at specific homogeneous sites within the adsorbent, indicating a monolayer coverage adsorption mechanism (Li et al. 2019). The maximum adsorption capacity of the modified PS on CR and MB can be calculated using the fitting equation as 1,880.91 and 881.62 mg/g, which were different from the actual measurement of 1,862.18 and 863.74 mg/g, respectively. Remarkably, the maximum adsorption capacities were much higher than the values obtained in previously reported adsorbents. Compared with most hydrogels, carbon materials, and other adsorbents reported in Table 2, PS-SD showed excellent MB and CR adsorption capacity.

In addition, n calculated by the Freundlich model equation was in the range of 1–10, revealing that CR and MB adsorption onto the absorbent was favorable adsorption.

The adsorption capacity and first-order and second-order kinetics of the amidated modified polystyrene PS-SD with adsorption time of 150 min are shown in Figure 9. Within 10 min of the beginning of adsorption, the amidated modified polystyrene PS-SD had a fast adsorption rate for dyes, and the adsorption capacity increased rapidly and reached saturation adsorption (Xi et al. 2020). In the initial stage of adsorption, the surface activity adsorption sites of the modified polystyrene PS-SD were abundant, and a large number of cavities appearing on the surface of the particles after amide modification were not completely occupied by dye molecules, which made dye adsorption PS-SD easy (Liu et al. 2020a). When the adsorption time continued to extend, the active adsorption sites on the surface of the adsorbent and the inner surface of the cavity were largely occupied by dye molecules (Wan et al. 2017). Similarly, as adsorption progressed, the dye concentration in the solution decreased, and the concentration driving force was not conducive for adsorption. The adsorption rate decreased, and the adsorption capacity did not increase after 30 min; the adsorption equilibrium state was basically reached (Wu et al. 2020). As shown in Figure 9(b) and 9(c), the fitting degree of the pseudo-second order kinetic model was better than that of the pseudo-first order kinetic model, which was supported by the correlation coefficient shown in Table 3. For instance, the correlation coefficient R² from the pseudo-first order kinetic model of MB and CR was only 0.64 and 0.53, which were lower than those of the pseudo-second order model (R² ≥ 0.995). In addition, the experimental values of q_e were in close agreement with the values calculated by using the pseudo-second order model, whereas the experimental values of q_e differed largely from the calculated values obtained by the pseudo-first order model. This finding implied that the pseudo-second order model was suitable to describe the kinetic behaviours of MB and CR onto PS-SD. Therefore, the PS-SD adsorption rate was susceptible to chemical adsorption, and chemical adsorption might be one of the rate control steps in adsorption process (Li et al. 2020).

The different thermodynamic parameters of adsorption could determine its spontaneity, randomness, endothermicity or exothermicity. Figure 10 shows the equilibrium adsorption capacity of MB and CR by modified polystyrene PS-SD at 298.15, 308.15, and 318.15 K. Table 4 shows that ΔH^θ was always positive during adsorption, which indicated that the adsorption of dyes was an endothermic process. The increase of temperature will increase the collision efficiency between adsorbate molecules and the adsorbent (Zhang et al. 2018b). This finding indicated that the adsorption of dye by PS-SD was a spontaneous process (ΔG^θ < 0). The positive value of ΔS^θ suggested that the randomness increased at the solid-solute interface during adsorption.

The regeneration property of amide modified PS-SD for MB and CR dyes is shown in Figure 12. Two litres of CR and MB solutions were prepared with a concentration of 100 mg/L. Then 1 g of the adsorbent was added into the corresponding volumetric flask for adsorption. The adsorbed dye molecules were desorbed on the surface of PS-SD. The adsorption-desorption cycle experiment was repeated 8 times, and then each cycle was washed with ethanol for 1 h. As the number of adsorption cycles increased, the adsorption capacity of amide-modified PS-SD for MB and CR was gradually reduced, but the reduction was small. After 8 cycles of use, the adsorption capacity of amide modified PS-SD for MB removal was 88%, and the adsorption removal for CR was 85%, indicating that the amide-modified PS-SD had good recyclability.

Adsorption from solutions is a competitive process where equilibrium is determined by many factors resulting from the dye structure, textural properties, and surface chemistry of adsorbent and the specific interaction between the adsorbent surface and adsorbate. For the PS-SD adsorbent, some factors such as the surface area of the swelling and active sites, including the −NH₃⁺ and –OH groups, played a pivotal role in the dye adsorption mechanism. To further clarify the adsorption mechanism, the FTIR spectra results were first evaluated before and after the adsorption (Figure 13). The characteristic peaks of MB is observed in the spectra of PS-SD after adsorption. The characteristic peak at around 3,380 cm⁻¹ represented the stretching vibration of O-H of hydroxyl groups exhibits a slight shift to 3,405 cm⁻¹ after MB adsorption, which suggests that the existence of hydrogen bond interactions between the PS-SD and MB molecules (Liu et al. 2019a). In addition, the characteristic peak at around 1,590 cm⁻¹ that represented the stretching vibration of C = C of PS-SD exhibited a slight shift to 1,582 cm⁻¹ after MB adsorption, which suggests that the existence of π-π stacking interaction and electrostatic interaction between the PS-SD and MB molecule. A reasonable mechanism by which dyes can be removed from a solution using the PS-SD adsorbent is illustrated in Figure 14.

PS-SD was synthesised from dopamine and carboxylated waste polystyrene, which was prepared by the Friedel-Crafts reaction of waste polystyrene foam and succinic anhydride. The synthesised material was porous with a considerable BET specific surface area. Under low pH conditions, the PS-SD material had a high removal capacity for CR dyes. The kinetic data of the two dyes followed the pseudo-second-order kinetics, and chemical adsorption played a leading role. Moreover, the isotherm data followed the Langmuir isotherm model. Compared with other adsorbents, the maximum adsorption capacity of amidated PS-SD for CR and MB can reach 1,880.91 and 881.62 mg/g, respectively, which showed high adsorption performance. The reusability, and high adsorption capacity of amidated PS-SD showed great application potential for the removal of dyes from wastewater.

The authors thank the Sichuan Youth Science and Technology Innovation Research Team Project (No.2020JDTD0018).

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