Introduction

Lead (Pb(II)) is one of the most toxic heavy metals discharged by a wide variety of industries such as acid battery, ceramic and glass manufacturing, metal planting and finishing, printing, tanning and production of lead additives for gasoline [1]. Indeed, Pb(II) metal ions may cause severe damage to the kidneys, liver, brain and nervous system, while a long-term exposure can induce sterility, abortion and neonatal death [2]. There are several methods for the removal of heavy metal ions from water and wastewater which include degradation [3], membrane process [4], precipitation [5] and adsorption [6]. Among these methods, adsorption is a widely accepted alternative due to high efficiency, rapidly and relatively low-cost operation [7]. The efficiency of adsorption depends on the textural properties, surface morphology, polarity and functional groups attached to the adsorbent. To date, many efforts have been made by the research community toward the development of novel adsorbents based on activated carbon [8], silicates [9], clay minerals [10], zeolite [11, 12], fly ash [13], natural biomass [14], chitin [15], chitosan [16] and synthetic polymers [17]; however these adsorbents have several drawbacks associated with high cost, low solubility, inefficient adsorption capacity and poor reusability. In this sense, the polymeric adsorbents have received great attention due to their low cost, high absorption capacity, mechanical rigidity and feasibility of regeneration. Among them, polystyrene (PS) is a thermoplastic widely used in industrial applications as packaging and construction material due to their good mechanochemical properties and high resistance [18]; however, there also are environmental concerns due to its excess accumulation and non-biodegradability. Thus, it is imperative to find a sustainable utility of PS waste. The reuse of PS waste by means of chemical functionalization is an successful alternative that allows converting it into a low-cost material with potential application as an alternative adsorbent. Although PS has excellent mechanical properties and chemical resistance, it cannot absorb individual heavy metal ions efficiently. To overcome this disadvantage, different functional groups have been introduced inside polymer chains. For example, Memon et al. [19] found that PS foam functionalized with 2,2-pyridyl is able to absorb Cd(II) metal ions with a maximum removal of 90% (Fig. 1a) and Xiong et al. [20] demonstrated that PS-2-aminothiazole shows a highly selective toward Au(III) metal ions with an adsorption capacity of 734 mg g−1 (Fig. 1b). A recent report by Bekri-Abbes et al. [21] shows that sulfonation of PS enhances functionality for Pb(II) and Cd(II) metal ions removal with a percent greater that 70% (Fig. 1c). Likewise, Bulbul Sonmez et al. [22] reported that poly(acrylamide) functionalized cross-linked PS beads are able to remove Hg(II) metal ions with an absorption capacity of 6.9 mmol g−1 (Fig. 1d) and Reddy et al. [23] found that polystyrene-supported chelating polymer resins can be used for the selective removal of Hg(II) and Pb(II) metal ions with maximum uptake capacities of 12.31 and 12.97 mg g−1, respectively, under alkaline conditions (Fig. 1e). Moreover, Saadeh et al. [24] reported that PS-based terpyridine polymer shows a high selectivity for the removal for heavy metal ions with a maximum absorption in the 60–70% range (Fig. 1f).

Fig. 1
figure 1

Absorbent materials based on polystyrene applied as adsorbents for heavy metal ions

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It should be noted that in the above reports the synthetic routes involve the use of catalysts, multi-step chemical reactions, subsequent purification steps and long reaction times. With this above analysis in mind, here we report the synthesis, characterization and adsorption properties of PS functionalized oligomers prepared by low-temperature thermocatalytic degradation of polystyrene in the presence of zeolite clinoptilonite as a degradation template and subsequent functionalization with acrylamide (AAm) for the removal of Pb(II) metal ions from aqueous solutions. The kinetic, isotherm and equilibrium data of PS functionalized oligomers (PS-AAm) suggest that the adsorption mechanism is suitable for the removal of Pb(II) metal ions which pollutes the environment. Finally, the desorption capability and the regeneration of PS-AAm were also studied by applying recyclability assay. To the best of our knowledge, the use of PS-AAm for removal of Pb(II) metal ions has not been explored.

Materials and methods

Materials and samples preparation

Styrene (S) (99%, Sigma-Aldrich, USA), zeolite clinoptilonite (Industrial Co. Mexico), THF (99% DEQ, Mexico), methanol (99%, DEQ, Mexico), decahydronaphthalene (98%, Sigma-Aldrich, USA), acrylamide (99%, Sigma-Aldrich, USA) and benzoyl peroxide (BPO) (99%, Sigma-Aldrich, USA) were used without purification. Infrared spectra (ATR-IR) were recorded on a Perkin-Elmer Spectrum One spectrophotometer equipped with a Pike MiracleTM ATR accessory with a single-reflection ZnSe ATR crystal. The mass-weighted molecular weight (Mw), the number-weighted molecular weight (Mn) and dispersity factor (D) were measured on a YL9100 GPC system with refractive index detector (YL9170) by using a PLgel 5 µm MIXED-C column (7.5 × 300 mm) as the stationary phase and THF as mobile phase at 40 °C with a flow rate of 1.0 mL min−1. Differential scanning calorimetry (DSC) was carried out in a temperature range from 15 to 300 °C under nitrogen atmosphere at a heating rate of 10 °C min−1 by using a Phoenix®DSC 204F1 (Netzsch) apparatus. Thermogravimetric analysis (TGA) was performed in a temperature range from 30 to 600 °C under nitrogen atmosphere at a heating rate of 10 °C using a Libra®TG 209F1 (Netzsch) thermal analyzer. The surface morphology was examined by field emission scanning electron microscope (FESEM) in a JEOL model JSM-6701F apparatus. Batch adsorption experiments were carried out in a Thermo Scientific Precision 270 circulating water bath. Pb(II) metal ions from aqueous solutions were analyzed by flame atomic absorption spectroscopy (FAAS) on a Perkin-Elmer PinAAcleTM 900F atomic absorption spectrophotometer (see ESI, Fig. S1).

Experimental methods

Synthesis of polystyrene (PS)

In order to obtain a virgin material free of additives, PS was synthesized by free radical polymerization from styrene monomer. A solution of 5.0 g of styrene monomer and 0.02 g of benzoyl peroxide (BPO) as radical initiator was heated and stirred at 90 °C under nitrogen atmosphere for 2 h. After the reaction was completed, the resulting PS was analyzed by gel permeation chromatography (GPC) to determine the relative molecular weight of polymer as well as the distribution of molecular weight. It is important to mention that PS is used in the degradation phase with zeolite clinoptilolite.

Thermocatalytic degradation of PS

A sample of 5.0 g of PS mixed with 2.5 g of zeolite clinoptilolite as a catalyst and 5.0 mL of decahydronaphtalene was heated and stirred at 180 °C under nitrogen atmosphere for 3 and 5 h, respectively. The reaction mixture was slowly warmed to room temperature, and the catalyst was removed with THF, and then, the solvent was evaporated. The resulting oligomers, labeled as degraded PS (DPS), were purified by precipitation in methanol and then lyophilized.

Chemically functionalization of DPS with acrylamide

A homogenous solution of DPS (0.5 g), 0.01 g benzoyl peroxide (BPO) and 10.0 mL of THF was added in a Schlenk flask. Thereafter, acrylamide monomers (3.5 g), mixed with degassed THF, were added with dropping funnel and stirred vigorously at 70 °C for 2 h under nitrogen atmosphere. The reaction mixture was slowly cooled to room temperature, and the polyacrylamide (homopolymer formed during the reaction) was removed under vacuum; the residue was taken up in a water/acetone (6:4) mixture, and the precipitated was filtered off and washed with methanol and lyophilized to give 4.0 g of PS functionalized with AAm (DPS-AAm). The yield of AAm grafted by radical reaction was gravimetrically determined as the percentage of weight increase by applying the next Eq. (1):

$$ {\text{Yield of grafting}} \% = \frac{{W_{\text{g}} - W_{\text{o}} }}{{W_{\text{o}} }} \times 100 $$
(1)

where \( W_{\text{o}} \) and \( W_{\text{g}} \) are the weights of DPS and DPS-AAm, respectively.

Batch adsorption study

Influence of contact time on adsorption

The influence of contact time on absorption was evaluated in a batch method. The procedure consists in shaking at 200 rpm for 360 min, 20.0 mL of Pb(II) metal ions in aqueous solution (40.0 mg L−1) with 11.0 mg of dried adsorbent dosage at pH of 5.60.

Likewise, the percentage of metal removal (R%) was calculated by Eq. (2) as follows:

$$ R \left( \% \right) = \frac{{C_{\text{o}} - C_{\text{e}} }}{{C_{\text{o}} }} \times 100 $$
(2)

where \( C_{\text{o}} \) and \( C_{\text{e}} \) are the concentration at initial and equilibrium states (mg L−1), respectively.

Moreover, the equilibrium amount (mg g−1) adsorbed per unit mass of adsorbent was determined using Eq. (3):

$$ q_{\text{e}} \left( {{\text{mg g}}^{ - 1} } \right) = \frac{{\left( {C_{\text{o}} - C_{\text{e}} } \right)V}}{W} $$
(3)

where \( q_{\text{e}} \) is the equilibrium amount of Pb(II) metal ions adsorbed per unit mass, V (L) is the volume of solution and W (g) is the mass of DPS-AAm.

Adsorption kinetic

The adsorption kinetic was used to evaluate the absorption of Pb(II) metal ions by DPS-AAm as a function of time and to determine whether the behavior of heavy metal ions adsorbed could be explained by using a predicted model. The adsorption data of Pb(II) metal ions removal were described with respect to Lagergren’s pseudo-first-order kinetic model [25], pseudo-second-order model [26] and intraparticle diffusion model [27]. Each model is given in Eqs. (4), (5) and (6), respectively.

$$ \log \left( {q_{\text{e}} - q_{\text{t}} } \right) = \log \left( {Q_{\text{e}} } \right) - \frac{{k_{1} }}{2.303}t $$
(4)

where \( q_{\text{e}} \) and \( q_{\text{t}} \) represent the amount of Pb2+ ions absorbed on the adsorbent (mg g−1) at equilibrium and time \( \left( t \right) \), respectively, and \( k_{1} \) (min−1) is the rate constant of the pseudo-first-order kinetic. The value of adsorption rate constant \( k_{1} \) is calculated from the straight-line plot f \( \log \left( {q_{\text{e}} - q_{\text{t}} } \right) \) against \( t \).

The pseudo-second-order model could be expressed as:

$$ \frac{t}{{q_{\text{t}} }} = \frac{1}{{k_{2} q_{\text{e}}^{2} }} + \frac{1}{{q_{\text{e}} }}t $$
(5)

where \( k_{2} \) (g mg min−1) is the rate constant for a pseudo-second-order model and the definitions of \( q_{\text{e}} \) and \( q_{\text{t}} \) remain the same. The slope and intercept of the linear plot of \( t /q_{\text{t}} \) against \( t \) give the values of \( q_{\text{e}} \) and \( k_{2} \), respectively.

Rate-limiting step in adsorption can be found out from intraparticle diffusion model [28]. The intraparticle diffusion model is expressed as:

$$ q_{\text{t}} = k_{\text{pi}} t^{1/2} + C $$
(6)

where C is the intercept and \( k_{\text{pi}} \) is the intraparticle diffusion rate constant (mg g−1 min 0.5), which can be evaluated from the slope of the linear plot of \( q_{\text{t}} \) versus \( t^{1/2} \).

Adsorption isotherm studies

The adsorption capacity of DPS-AAm as an alternative adsorbent for removing Pb(II) metal ions was described using Langmuir and Freundlich isotherm models. The experiments were carried out at 25, 35 and 45 °C for 4.2 h, initial concentration of Pb(II) metal ions in the range from 100 to 900 mg L−1, pH 5.60, shaking speed 200 rpm, and 11.0 mg of dried adsorbent dosage. Langmuir model explains adsorption process by assuming a monolayer adsorption onto a surface containing finite number of adsorption sites [29]. This linear form is given in Eq. (7) as follows:

$$ \frac{{C_{\text{e}} }}{{q_{\text{e}} }} = \frac{1}{{K_{\text{L}} Q_{\text{a}}^{0} }} + \frac{{C_{\text{e}} }}{{Q_{\text{a}}^{0} }} $$
(7)

\( Q_{\text{a}}^{0} \) (mg g−1) and \( K_{\text{L}} \) (L mg−1) are Langmuir constants related to adsorption capacity and rate of adsorption, respectively. The essential characteristics of Langmuir model can be described by dimension less separation, RL, Eq. (8):

$$ R_{\text{L}} = \frac{1}{{1 + K_{\text{L}} C_{\text{o}} }} $$
(8)

where \( C_{0} \) is the highest initial solute concentration, \( R_{\text{L}} \) values indicate whether the adsorption is unfavorable \( (R_{\text{L}} > 1) \), linear \( (R_{\text{L}} = 1) \), favorable \( ( < R_{\text{L}} 1) \) or irreversible when \( R_{\text{L}} \) value is equal to zero \( (R_{\text{L}} = 0) \).

On the other hand, the Freundlich model assumes heterogeneous energies. Its linear form is given by Eq. 9 [30]:

$$ { \log } q_{\text{e}} = \log k_{\text{f}} + \frac{1}{n}\log C_{\text{e}} $$
(9)

where \( k_{\text{f}} \) and n are Freundlich constants. Generally, n > 1 suggested favorable adsorption is physical (n > 1), chemical (n < 1) or linear (n = 1) [31].

Desorption of Pb(II) metal ions from the adsorbent

Desorption study was carried out in a similar way to that of adsorption studies after sorption metal-loaded DPS-AAm (initial concentration of metal ions 100 mg L−1, pH 5.60, shaking speed 200 rpm and temperature 37 °C) was dried, weighed and shaken with 100 mL of 0.5 M EDTA solution as regenerating agent on a laboratory shaker at 200 rpm.

Results and discussion

Thermocatalytic degradation of PS

The dispersity parameters of PS and DPS were obtained by GPC analysis in spectroscopic grade tetrahydrofuran (Table 1). The analysis of the dispersity parameters of PS obtained by free radical polymerization revealed a moderate distribution of chain length with an average of 945 repeating units. In the case of DPS prepared by thermocatalytic degradation from PS, the GPC analysis revealed a decrease in the mass-weighted molecular weight (Mw) from 99,294 to 66,987 g mol−1 during the first 3 h with an average of 638 repeating units. However, after 5 h the Mw is slightly increased due to recombination of PS oligomers with free radicals produced during the degradation process. Thus, 3 h was taken as the best time to degrade PS and produced oligomers of appropriate length to be functionalized with AAm. It is important to remark that zeolite clinoptilonite used as a catalyst during the degradation step exhibits an average pore size of 15 nm, which allows us to classify it as mesoporous material (see ESI, Fig. S2–S3). These textural properties promote the breaking of PS chains inside the channel networks as previously reported by Czégény for catalytic pyrolysis of plastic mixtures by using HZSM-5 zeolite [32].

Table 1 Dispersity parameters for PS and DPS
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Functionalization of DPS with AAm

Chemical functionalized PS oligomers (DPS-AAm) were synthesized by free-radical polymerization of DPS with AAm monomers in the presence of BPO as initiator (Scheme 1). Table 2 compares time, temperature, monomer and grafting yield obtaining in this work with those previously reported [24, 33,34,35]. Our grafting yield of AAm grafted gave a value of 40%, which is moderate in comparison with analogs macromolecules. However, it is important to remark that preparation of these macromolecules involves the use of catalyst, multi-step chemical reactions, purification steps, long reaction times and harmful high-energy radiation. Conversely, our synthetic method avoids the use of high temperatures and reduces reaction time by a factor of 12 times compared with PS–terpyridine (Fig. 1f), poly(glycidylmethacrylate)–PS (Fig. 1g), PS–triethylenetetramine (Fig. 1h) and poly(epichlorohydrin-methylacrylate) (Fig. 1i).

Scheme 1
scheme 1

Proposed chemical functionalization of DPS chains with acrylamide

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Table 2 Comparison between different routes for the chemical functionalization of PS
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Spectroscopic characterization

Infrared analysis of DPS (Fig. 2, black line) showed stretching and bending frequencies at 696, 756, 1446 and 1493 cm−1 attributed to the alkyl groups as well as the characteristic bands from 2919 to 3024 cm−1 of the phenyl rings which are present in the PS repeating units. The insert blue line shows clearly a stretching band at 1660 cm−1 with a medium intensity attributed to the amide functional groups which confirms the covalent attachment existent in the DPS-AAm. Zahedi et al. have reported similar wave number characteristics for chemical functionalization of polystyrene nanofibers. In those experimental conditions, the authors confirmed the presence of a broad stretching signal at 1660 cm−1 corresponding to the amide groups grafted to polystyrene [36].

Fig. 2
figure 2

ATR-FTIR spectra for DPS and amplification of group frequency region of DPS-AAm (inserted figure)

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TGA-DTA analysis of DPS and chemically functionalized DPS-AAm

The thermal parameters such as initial decomposition temperature (To), maximum weight loss temperature (Tmax), glass transition temperature (Tg) and other relevant thermal features for DPS and DPS-AAm are given in Table 3. Figure 3 shows the TGA curves for DPS and DPS-AAm which present a good thermal stability up to 395–396 °C. Moreover, according to TGA data, DPS-AAm is slightly more stable compared to DPS, which indicates that the incorporation of AAm into the oligomer structures enhances the thermal properties. The DTA analysis revealed that DPS-AAm has a melting temperature (Tm) and crystallinity higher than DPS oligomers. These thermal data are in good agreement with those reported by Liu et al. [37] poly(arylene ether) functionalized with alkenyl substituents. In those experimental conditions, the authors found that the addition of conjugated moieties and non-coplanar structures enhances thermal stability.

Table 3 Thermal parameters for DPS and DPS-AAm
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Fig. 3
figure 3

TGA data of DPS and DPS-AAm at 10 °C min−1 in N2 atmosphere

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DSC analysis of PS and chemically functionalized DPS-AAm

Thermal stability of DPS and DPS-AAm was also studied by DSC analysis, and their thermogram curves are given in Fig. 4. The DSC analysis shows that Tg of DPS-AAm is decreased by 3.0 °C with the increase in crystallinity in comparison with DPS due to incorporation of AAm into the DPS chains. It is important to remark that the increase in crystallinity is only observed for DPS-AAm as can be seen in Fig. 4.

Fig. 4
figure 4

DSC thermograms of the first stage heating for DPS-AAm and DPS. The heating cycles were performed at scanning rates of 10 °C min−1 under nitrogen atmosphere

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Surface morphology analysis of PS, DPS and chemically functionalized DPS-AAm

A high adsorption capacity is a characteristic that all adsorbent materials must possess for being considered as promising adsorbents for removing heavy metal ions. With the aim to enhance the absorption properties and found a sustainable use for PS waste, their degraded oligomers were functionalized with AAm. The surface morphology of the DPS was analyzed by field emission scanning electron microscopy (FESEM) before and after grafting with AAm. The analysis of FESEM images of PS shows the existence of irregular shapes and agglomerated microstructures with a length range from 0.5 to 2 μM (Fig. 5a), while for the case of DPS were obtained microstructures with a honeycomb morphology (Fig. 5b), which confirms the degradation of PS under our experimental conditions. On the contrary, when DPS is functionalized with AAm, it can be seen that the formation of microspheres agglomerates with a highly irregular surface and pores, and these pores contribute to a very large surface area. The porous and cavity-like structural arrangement enhances the chemical functionalization of DPS oligomers which are suitable for absorption of heavy metal, as previously reported by Santhi et al. [38] for removal of Cr(II) and Pd (II) metal ions using a redox polymer as an adsorbent (Fig. 5c).

Fig. 5
figure 5

Representative FESEM images of PS (a), DPS (b) and DPS-AAm (c)

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Batch adsorption studies

Influence of contact time on adsorption

Figure 6 shows the time-dependent behavior of the absorption of Pb(II) metal ions using DPS-AAm as an adsorbent. As can be seen in Fig. 6, the absorption of Pb(II) metal ions increased as the contact time increased for the chemically functionalization of DPS oligomers and the maximum adsorption capacity (qe) was found to be 33.85 mg g−1 and a percentage removal (R%) of 90.94% at the end of 360 min with the adsorbent dosage of 0.011 mg L−1 at pH 5.60. The increase in the values of qe and R% at higher time for functionalized polystyrene oligomers may be attributed to the higher yield of graft [39]. The increase in the yield of graft leads to higher surface area and more active sites in comparison with DPS (qe = 0.82 mg g−1, R = 1.32%) and PS (qe = 3.20 mg g−1, R = 5.16%), respectively, as can be seen in ESI, Table S1. The absorption equilibrium was enriched at 240 min; initially, the Pb(II) metal ions reach the boundary layer and then diffuse through the surface of the DPS-AAm containing the amide functional groups and then diffuse into the surface cavities of the absorbent which requires relatively longer contact time [40]. The leveling effect of the absorption process may be attributed to the saturation of the active sites on DSP-AAm, as can be seen in Fig. 6.

Fig. 6
figure 6

Influence of contact time on adsorption of Pb(II) metal ions using DPS-AAm as an adsorbent

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Adsorption kinetics

It is well knowledge that adsorption process could be controlled with different kinds of mechanisms, such as mass transfer, diffusion control, chemical reactions and particle diffusion. With the aim to clarify the absorption process, different adsorption models have been applied to evaluate the experimental data. For this purpose, Lagergren’s pseudo-first-order kinetic model and pseudo-second-order kinetic model were considered and fitted with the experimental data. Figure 7 shows the plot of the first-order and second-order models for adsorption of Pb(II) metal ions by the DPS-AAm, respectively. Experimental and theoretical calculated qe values and coefficients related to kinetic plots are given in Table 4. It can be seen from Table 4 that the pseudo-second-order model (Fig. 7a) provided a better fit to the experimental data in comparison with the Lagergren’s pseudo-first-order model (Fig. 7b). It is evident from the results of the pseudo-second-order model that the correlation coefficient is very high and the experimental (33.85 mg g−1) and theoretical (37.17 mg g−1) qe values are in good match. Thus, these results suggest that the adsorption of the Pb(II) metal ions by DPS-AAm follows the pseudo-second-order-type kinetic reaction.

Fig. 7
figure 7

Pseudo-first-order a and pseudo-second-order model plots b for adsorption of Pb(II) metal ions by DPS-AAm

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Table 4 Absorption kinetic rate constants of Pb(II) metal ions removal by DPS-AAm
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Nevertheless, both adsorption models cannot entirely explain the adsorption mechanism. Thus, the intraparticle diffusion model was also applied to determine the rate-limiting step in adsorption process. Figure 8 shows that the absorption of Pb(II) metal ions by DPS-AAm is multi-step process. It has three linear regions related directly to the three adsorption events: The first event is attributed to instantaneous adsorption stage, the second one is the gradual adsorption stage, and the third event represents the adsorption–desorption process between the adsorbate and active sites. The measurement of slope in the first linear portion suggests that the rate of intraparticle diffusion is carried out faster onto the adsorbent surface than the sorption process of the adsorbate on the active sites [41].

Fig. 8
figure 8

Intraparticle diffusion model plots for the adsorption of Pb(II) metal ions by DPS-AAm

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Adsorption isotherm studies

The equilibrium relationships between adsorbent and adsorbate are best explained by sorption isotherms [40]. The experimental values are fitted with Freundlich and Langmuir isotherm equations. Table 5 depicts the values of regression coefficients obtained from these models, and these were used as the fitting criteria to find out the isotherms. It was found that Pb(II) metal ions at the initial concentration of 40 mg L−1 and DPS-AAm have the linear form (Fig. 9) of the isotherm and high correlation with the Freundlich isotherm model indicating that multilayers adsorption exists and this may be due to non-uniform distribution of the heat adsorption over the DPS-AAm surface. In general, the qe of Pb(II) metal ions decreased from 199.90 to 120.52 mg g−1 as the temperature increased from 25 to 45 °C which reveals the existence of a strongly endothermic process. The Freundlich constant n value lying in the 2.48–4.03 range confirms the favorable conditions for physical adsorption [42]. The linear fit of Langmuir isotherm given in ESI, Fig. S4, has least correlation coefficient, which concludes that monolayer adsorption does not exist.

Table 5 Isotherm parameters for the adsorption of Pb(II) ions by DPS-AAm
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Fig. 9
figure 9

Freundlich isotherm plot for absorption of Pb(II) ions by DPS-AAm

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Desorption of Pb(II) metal ions from the adsorbent

A high adsorption capacity is a characteristic that all adsorbents must possess for their application in removal of toxic heavy metal ions from water. However, its regeneration ability is also an important characteristic for the reusability of the absorbent. For desorption studies, metal absorbed by DPS-AAm was washed with ultra-pure water to remove the unadsorbed loosely attached Pb(II) metal ions to the adsorbent. In order to demonstrate the reusability of the adsorbent, six consecutive cycles of adsorption and desorption of Pb(II) metal ions by DPS-AAm with 0.5 M EDTA solution as regenerating agent were performed [43]. As can be seen in Fig. 10, maximum recovery of Pb(II) metal ions of 97.28% was reached with 0.5 M EDTA solution which proves the efficiency of DPS-AAm as an adsorbent. Moreover, the desorption capacity of DPS-AAm shows a moderate change in a range of 8–10% during the consecutive adsorption and desorption cycles. Therefore, these results demonstrate that DPS-AAm has a regeneration capability higher than 90%.

Fig. 10
figure 10

Desorption profile of Pb(II) metal ions absorbed on DPS-AAm

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Conclusions

We have reported the low-temperature thermocatalytic degradation, characterization and post-functionalization of polystyrene that have been functionalized with acrylamide in good graft yield based on easy and cheap synthetic route. The functionalized polystyrene oligomers were fully characterized by infrared spectroscopy, thermal and morphological analysis. A preliminar absorption study using functionalized polystyrene oligomers as an adsorbent indicates that material has a good potential to absorb heavy metal of Pb(II) from the aqueous solutions. Equilibrium data show a maximum adsorption capacity of 33.85 mg g−1 with a percentage removal of 90.94% which fitted well with Freundlich model, and kinetic data were best described by pseudo-second-order model. The adsorption–desorption cycles reveal that functionalized polystyrene oligomers could be recommended as a promising adsorbent for Pb(II) metal ions contained in aqueous systems.