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

Modifying environmentally harmful waste polystyrene foam as an efficient recyclable adsorbent for organic dyes is important. Amidation modified polystyrene (PS-SD) was prepared by the FriedelCrafts reaction and N,N’-dicyclohexylcarbodiimide (DCC) dehydration condensation reaction of waste polystyrene foam. PS-SD had highly efficient removal performance for organic dyes in large volume water sample solutions, and equilibrium was achieved in 0.5 h. The maximum adsorption capacities for Methylene blue (MB) and Congo red (CR) were 881.62 and 1,880.91 mg/g, respectively, at room temperature according to the Langmuir adsorption isotherm (R> 0.99). The kinetic data of the two dyes followed pseudo-second-order kinetics. The removal percentage remained high (>85%) after eight filtration-regeneration cycles. Experimental results showed that PS-SD was an excellent adsorbent for water treatment with high recyclability and long life.


INTRODUCTION
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. ; Wang et al. b; Cheng et al. ; Roy et al. ). 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. ; Choi et al. ; Dinari et al. ; Lei et al. ). Therefore, removal of organic dyes from wastewater has become important for the environment (Cho et al. ; Wang et al. ). However, cationic dye Methylene blue (MB) and anionic dye Congo red (CR) are common organic dyes (Wan et al. ; Ma et al. ).
At present, biodegradation, photocatalysis, and flocculation can effectively remove dyes in wastewater (Liu et  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. ; Duan et al. ).
At present, domestic and foreign researchers have conducted several studies on the resource utilisation of waste polystyrene foam (Dardouri et al. ). 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. a). 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. b). However, research on the use of waste polystyrene adsorption is limited. A conjugated microporous polymer (CMP) was synthesised using postconsumer 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. ).
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. ; Daneshvar et al. ; Ahmed et al. ). 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 amidefunctionalised polystyrene adsorbents are studied. The amount of adsorbent, pH value, temperature, time, and initial dye concentration (C 0 ) 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.

Materials
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 (CH 2 Cl 2 ) 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).

Preparation of modified PS-SD
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.

Effect of solution pH on adsorption
The acidity and alkalinity of the adsorption system are important factors for studying the mechanism of adsorption, which can affect the adsorption of different electrical dyes. CR and MB were selected as ionic dyes for experiments. MB and CR solutions were prepared with a concentration of 100 mg/L. The pH of MB and CR solutions was adjusted by 1.0 mol/L of NaOH and 1.0 mol/L of HCl aqueous solution to investigate the effect of pH on adsorption for MB and CR. Ten milligrams of PS-SD was added directly into a 20 mL glass bottle containing 10 mL of MB and CR solutions (100 mg/L), which was treated by ultra-sonication for 10 min and shaken at 200 rpm in a water bath at 298.15 K for 150 min. The experiment was repeated three times. The pH range of the dye solution was adjusted to 2-12. The adsorption capacity of the PS-SD for dyes can be calculated using the following formula (Wan et al. ): where C 0 is the initial dye concentration (mg/L), C e is the dye concentration in solution (mg/L) at equilibrium, V is the volume of the solution (L), and m is the mass of the adsorbent (g).

Effect of initial concentration on adsorption
The static adsorption experiment was performed in a volumetric flask, and 10 mg of the modified PS samples was added to MB (the concentration range was 50-1,000 mg/L) and CR (the concentration range was 50-2,500 mg/L) solutions. Adsorption was performed by shaking in a water bath (200 rpm) at 298.15 K. The experiment was repeated three times. According to the pre-established calibration curve, the concentration of MB and CR in the aqueous solution was analysed at 665 and 497 nm, respectively, using an ultraviolet spectrophotometer. The Langmuir adsorption isotherm model and Freundlich adsorption isotherm model were used to simulate the experimental data. The Langmuir equation is as follows (Tran et al. ): where q m (mg/g) is the maximum saturated monolayer adsorption capacity of an adsorbent, q e (mg/g) is the amount of adsorbate uptake at equilibrium, and K L (L/mg) is a constant related to the affinity between an adsorbent and adsorbate. The Freundlich equation is as follows (Qiao et al. ): where q e (mg/g) is the amount of adsorbate uptake at equilibrium, K F (mg/g)/(mg/L) n is the Freundlich constant, and n (dimensionless) is the Freundlich intensity parameter.

Effect of contact time on adsorption
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 pseudosecond-order equation in the diffusion model were used to fit the experimental data to nonlinear adsorption. The pseudo-first-order equation is as follows ( Ji et al. ): where q e and q t are the amounts of adsorbate uptake per mass of adsorbent at equilibrium and at any time t (min), and k 1 (1/min) is the rate constant of the PFO equation.
The pseudo-second-order kinetic model refers to the linear relationship between the reaction rate and the concentration of the two reactants. The rate control step of adsorption was the interaction between the adsorbate and the adsorption site on the adsorbent surface, which was expressed as the equation (Wu et al. ): where k 2 (1/min) is the rate constant of the PSO equation.

Effect of solution temperature on adsorption
Thermodynamic experiments were conducted by adding 10 mg PS-SD into a 20 mL glass bottle containing 10 mL of MB and CR solutions with concentrations of 1,000 and 2,500 mg/L, respectively. At this concentration, the maximum adsorption capacity was reached. Adsorption was performed by shaking in a water bath (200 rpm) at 298.15, 308.15, and 318.15 K. The experiment was repeated three times. The thermodynamic parameters, including Gibbs free energy change value (ΔG θ ), enthalpy change (ΔH θ ), and entropy change (ΔS θ ), were calculated using the Van't Hoff formula, which can describe the adsorption mechanism. The Van't Hoff formula is as follows (Gao et al. ): where R is the universal gas constant (8.3144 J/(mol × K)) and T is the absolute temperature in Kelvin.

Characterisation
The resulting materials were characterised by Fourier transform infrared (FTIR, WQF-520) spectroscopy between 4,400 and 400 cm À1 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 N 2 atmosphere at a heating rate of 10 C min À1 from 40 to 700 C.

RESULTS AND DISCUSSION
Characterisation of PS-SD 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 À1 was attributed to the stretching vibration of the O-H bond and the peak at 1,680 cm À1 was assigned to stretching vibration of the C ¼ O bond (Liu et al. b). In Figure 2(c), the peaks at 3,400 and 1,640 cm À1 were related to the N-H and C ¼ 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 N 2 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. ). 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 m 2 g À1 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. ). 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.

Effect of solution pH on 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 -NH 3 þ , which combined with the anionic CR through electrostatic interaction. When the pH value was relatively low (pH < 4), H þ 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 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 ¼ 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. ).

Isotherm studies
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. R 2 (0.99, 0.93) of MB and CR by Langmuir was higher than R 2 (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  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.

Kinetic studies
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. ). 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. a). 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. ). 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. ). 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 R 2 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 (R 2 ! 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. ).

Effect of solution temperature
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. b). 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.

Selective adsorption
The selective adsorption performance of PS-SD on cationic dyes (MB) and anionic dyes (CR) is shown in Figure 11. Under alkaline conditions, the UV-Vis spectra of the mixture solution before and after adsorption exhibited that the absorption peaks of MB (665 nm) almost disappeared whereas the absorption peaks of CR (497 nm) almost remained at the same intensity at the same position. PS-SD selectively adsorbed the cationic dye MB. On the contrary, PS-SD selectively adsorbed the anionic dye CR under acidic conditions. After adsorption, the colour of the mixed solution was restored to red and blue (CR and MB, respectively), indicating that the dyes were selectively adsorbed on PS-SD. The possible explanation could be that the hydroxyl groups on the surface of PS-SD were easier to ionise H þ to form O À under alkaline conditions. In this case, the MB molecule was attracted by O À to form monodentate complexes. However, the possible explanation could be that the amino groups on the surface of PS-SD were easier to combine H þ to form -NH 3 þ under acidic condition. In this case, the CR molecule was attracted by -NH 3 þ to form monodentate complexes. This formation could be expressed using the following equations:

Research on recycling
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 mechanisms
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 ÀNH 3 þ 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 À1 represented the stretching vibration of O-H of hydroxyl groups exhibits a slight shift to 3,405 cm À1 after MB adsorption, which suggests that the existence of hydrogen bond interactions between the PS-SD and MB   molecules (Liu et al. a). In addition, the characteristic peak at around 1,590 cm À1 that represented the stretching vibration of C ¼ C of PS-SD exhibited a slight shift to 1,582 cm À1 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.
CONCLUSION 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.