Preparation of the sulfur-enriched carbon material from waste plastic and application for removal of Cr(VI) from wastewater

Chao Lv; Xiangwang Zeng

doi:10.25259/AJC_931_2025

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Original Article

ARTICLE IN PRESS

doi:

10.25259/AJC_931_2025

Preparation of the sulfur-enriched carbon material from waste plastic and application for removal of Cr(VI) from wastewater

Chao Lv^a,b,*, Xiangwang Zeng^c

aInstitute of Resources and Environment Engineering, Shanxi University, Taiyuan, Shanxi, China

bSchool of Coal Engineering, Shanxi Datong University, Datong, Shanxi, China

cCollege of Chemistry and Chemical Engineering, Henan Polytechnic University, Jiaozuo, China.

* Corresponding author: E-mail address: lvchao0711@126.com (C. Lv)

Received: 2025-08-25, Accepted: 2025-12-27, Epub ahead of print: 2026-03-17,

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Abstract

In this work, the low-density polyethylene (LDPE) waste plastic is utilized to synthesize the sulfur-enriched porous carbon material (SPCM) through an initial sulfuric acid-assisted solvothermal treatment, followed by a two-stage activation process for Cr(VI) wastewater removal. Chemical properties of SPCM are characterized, indicating that SPCM has abundant sulfur/oxygen-containing functional groups. Cr(VI) removal amount is 250.4 mg.g^-1 at 298 K, based on Langmuir model calculation. The coexisting ions minimally impact the effectiveness of Cr(VI) removal. Cycling experiment demonstrates that SPCM has good cycling stability. The removal mechanism analysis indicates that SPCM achieves the synergistic effect of physical adsorption, electrostatic attraction and chemical reduction for Cr(Ⅵ) removal through developed pores and surface sulfur/oxygen functional groups. The SPCM exhibits considerable application potential in lithium-ion batteries.

Keywords

High-value utilization

Lithium-ion batteries

Porous carbon material

Removal mechanism

Waste plastic

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1. Introduction

It has been reported that by 2050 the world will generate about 3.4 billion tons of waste plastic per year due to the rapid increase in the manufacture of plastics and the widespread use of single-use plastics [1]. The concern is that the microplastics released from waste plastic have been found both in the depths of the oceans and in human bodies [2]. Therefore, recycling plastic waste is of particular importance to prevent ecological damage [3]. In recent years, China has implemented waste import bans and established plastic recycling programs, while countries in Europe and the United States are in the process of developing and preparing to implement similar recycling initiatives [4]. The effective recycling of waste plastic remains a global challenge that requires coordinated efforts across nations.

Traditional waste plastic management methods, such as landfilling and incineration, are unsustainable and cause significant environmental harm. Besides, scholars generally used chemical recycling strategies such as hydrothermal synthesis, solvent heating, pyrolysis, and other chemical recycling strategies to extract high-value-added products from waste plastic [5]. For example, the polyvinyl chloride waste plastic is converted into an efficient adsorbent by hydrothermal treatment and sulfonation to remove heavy metals from wastewater [6]. In addition, researchers have prepared porous carbon from polyethylene terephthalate plastic waste for capturing the global warming CF₄ gas [7]. The above results show that the waste plastic can be converted into high-value products to solve problems such as waste disposal and environmental pollution. Among many waste plastics, the polyethylene waste plastic is considered an excellent carbon precursor due to its high carbon content and low cost. However, these olefin plastics are difficult to directly carbonize to form carbon materials. Hence, it is crucial to explore efficient approaches for converting olefin-based plastics, such as polyethylene, into valuable carbon materials to address this environmental challenge. For instance, a two-step approach involving pre-carbonization/oxidation followed by chemical activation has been employed to convert hybrid waste plastics into porous carbon materials for CO₂ adsorption and energy storage capabilities [8]. Moreover, the carbon fiber membranes with active pores are successfully prepared from waste polypropylene nonwovens by a series of processes, including programmed sulfonation and KOH activation [9]. Therefore, waste plastic can be converted into high-value carbon material through a two-step process, including pretreatment and activation, which also realizes the high-value utilization of waste plastic.

The Cr(VI) is usually characterized by high toxicity, causing serious ecological pollution and posing a great threat to organisms, which is also carcinogenic [10]. Many techniques are used for Cr(VI) removal, including electrocoagulation, precipitation, ultrafiltration, reverse osmosis, and adsorption [11]. Among them, adsorption has become a fundamental tool for addressing challenges related to Cr(VI) pollution control because of its affordable cost and simple operational process [12]. According to the available studies, adsorption and reduction processes demonstrate a prominent synergy‌ in Cr(VI) removal, with both mechanisms typically occurring simultaneously [13]. Commonly used adsorbents include hydrotalcites, carbonaceous materials, organic complexes, and metal-organic skeletons [14]. However, low-cost and highly efficient adsorbents are a key focus in the development of adsorption-based remediation technology. Therefore, preparing porous carbon material from waste plastic for Cr(VI) removal is a promising approach.

However, surface functional groups of porous carbon material prepared from waste plastic are not available for Cr(VI) removal, with poor removal performance, which needs to be improved. The optimization of porous carbon material properties through heteroatom incorporation (e.g., nitrogen, oxygen, and sulfur) serves as an effective approach to improve Cr(VI) removal capacity [15]. Studies have demonstrated that optimizing the integration of sulfur atoms within the porous carbon matrix improves both spin density and chemical activity of the material’s structural framework through polarization of sulfur’s lone pair electrons [16]. This enhancement significantly improves interaction between Cr(VI) and sulfur-modified carbon matrices. Furthermore, sulfur-containing functional groups act as the effective active sites within the carbon structure, thereby facilitating Cr(VI) removal. Zhuang et al. prepared porous carbon with sulfur modified by Na₂S₂O₄ for the removal of Cr(VI), exhibiting excellent removal performance [17]. Zhao et al. prepared the potassium bicarbonate-modified pyrite/porous biochar composite for the removal of Cr(VI), exhibiting that sulfur-containing functional groups significantly contribute to effective removal of Cr(VI) [18]. Therefore, sulfur-containing substances modified porous carbon material represent a promising approach for enhancing the removal efficiency of Cr(VI).

Herein, low-density polyethylene (LDPE) waste plastic is transformed into the sulfur-enriched porous carbon material (SPCM) through an initial sulfuric acid-assisted solvent-heating treatment, followed by a two-stage activation process. The physicochemical properties of SPCM are investigated. Besides, the capability of SPCM used in lithium-ion batteries has also been evaluated. A simple method is proposed in this study for the preparation of SPCM using waste plastic for Cr(VI) removal, which is a promising strategy to treat waste with waste.

2. Materials and Methods

2.1. Material

LDPE is purchased from Guangdong Dongguan Feihong Co., Ltd. The potassium dichromate (K₂Cr₂O₇) and potassium acetate are obtained from Tianjin Comio Chemical Reagent Co., Ltd., respectively.

2.2. Preparation of SPCM

Preparation process of SPCM has been presented in Figure 1. 1 g LDPE powder (0.1 mm) is introduced into the 100 mL stainless steel autoclave equipped with a Teflon lining, and subsequently 10 mL sulfuric acid (18.4 mol L^-1) is added. The mixture solution is well stirred and then heated to 110°C (5°C/min), maintained for 10 h. Following the gradual cooling of the autoclave to ambient temperature, the residue is taken from the autoclave, which is filtered and washed using deionized water several times. Subsequently, the sample is dried at 80°C for 10 h using an electric oven, which is named S-LDPE. Then, 10g S-LDPE is mixed with 10 g potassium acetate, and subsequently evenly mixed. The 10g mixture is heated to 800 °C in a resistance tube furnace for 2 h under a nitrogen atmosphere (200 mL/min) with a heating rate of 5°C/min. The remaining residue obtained from the resistance tube furnace is referred to as SPCM. Analytical techniques used to characterize SPCM are detailed in the supplementary information.

Supplementary Information

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2.3. Cr(VI) removal process

An adsorption kinetics study was conducted by mixing 0.1 g SPCM with 100 mL Cr(VI) solution (100 mg L^-1, pH = 2) in the 250 mL volumetric flasks at 298K. The mixture solution was stirred at 180 rpm in the constant temperature shaking incubator. The solution was sampled at different times and filtered for Cr(VI) concentration detection using the 0.45 mm filter membrane. The remaining Cr(VI) amount was detected by a UV-VIS spectrophotometer and the standard colorimetric 1,5-diphenylcarbazide method (540 nm). Pseudo-first order, Pseudo-second order, Intraparticle diffusion, and Elovich model were used to investigate Cr(VI) removal process (Table S1) [19].

Table S1

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The experimental procedure for the adsorption isotherm follows a methodology comparable to that of an adsorption kinetics study. The 0.75 mg SPCM and 100 mL Cr(VI) solution is mixed and then stirred for 24 h in the constant temperature shaking incubator with a rotational speed of 180 rpm until adsorption equilibrium [20]. Cr(VI) concentration used in the adsorption isotherm study is 50∼250 mg L^-1 with an adsorption temperature of 298-318K. After the adsorption equilibrium, the supernatant is filtered to detect Cr(VI) concentration. Langmuir, Freundlich, and Temkin adsorption isotherm models are used to analyze the Cr(VI) removal process (Table S2). The adsorption amount (q_t, mg/g) at a certain time interval and the adsorption equilibrium amount (q_e, mg g^-1) of Cr(VI) are obtained by the following equation 1-2 [21].

Table S2

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(1)

q_{t} = \frac{V (C_{0} - C_{t})}{M}

(2)

q_{e} = \frac{V (C_{0} - C_{e})}{M}

C₀ and C_t refer to initial Cr(VI) concentration and Cr(VI) concentration over time (mg L^-1), respectively. C_e refers to Cr(VI) concentration at adsorption equilibrium. M and V are the quality of SPCM (g) and solution volume (L).

Influence of pH (2, 3, 4, 5, 6, 7) on Cr(VI) removal is analyzed (SPCM: 0.25g, solution volume:100 mL, and Cr(VI) concentration: 100 mg L^-1). Initial pH is adjusted using the 0.1 M HNO₃ or KOH. Influence of coexisting ions (Cl^-, SO₄^2-, NO₃^-, Na⁺, Ca²⁺, and Mg²⁺) on the removal efficiency of Cr(VI) is also analyzed to analyze the anti-ion interference capability of the SPCM in the impurity ion solution. The SPCM is added into the mixed solutions formed by coexisting ions and Cr(VI), with a total volume of 100 mL and a pH of 2.0. Cr(VI) concentration is set at 100 mg L^-1 with an adsorption time of 24 h. After Cr(VI) adsorption, the mixed solution is filtered to detect Cr(VI) concentration. SPCM after Cr(VI) adsorption is suspended in 1 L 0.3 mol L^-1 NaOH solution in the desorption experiment. Finally, the mixture solution is stirred for 24 h, filtered, and dried (Temperature: 80˚C and time: 24 h) for the next Cr(VI) adsorption cycle. All experiments are carried out three times to obtain reliable experimental data. Electrochemical measurements of the SPCM are added in the supplementary information.

3. Results and Discussion

3.1. SEM analysis of SPCM

Microstructure analysis of SPCM has been shown in Figure 2, which reveals the surface microstructure of the SPCM. As Figures 2(a-c) show, it can be observed that the surface of the SPCM appears loose and porous structure under the etching effect of potassium acetate. The energy dispersive spectrometer (EDS) result indicates that the S element is successfully loaded onto SPCM, demonstrating that SPCM has sulfur-containing functional groups (Figure 2d).

(a-c) SEM images and (d) EDS analysis of SPCM.

3.2. Pore structure analysis of the SPCM

Figure 3(a) shows the pore structure of the SPCM. The adsorption-desorption curve type of the SPCM corresponds to the type I isotherm. The nitrogen adsorption amount sharply increases at P/Po<0.1, and reaches a saturation as the relative pressure continues to increase. The existence of the hysteresis loop suggests that the pore of SPCM has a certain degree of irregularity, demonstrating the existence of a mesoporous structure. The surface area of SPCM is 1423 m² g^-1. As Figure 3(a) shows, the pore structure of SPCM mainly consists of micropores with an average pore size of 0.82 nm. The reason for the formation of such developed pores might be that the organic potassium salts, like potassium acetate, interact with the carbon skeleton through an etching process during the preparation process. This process tends to form the dispersed microporous structure, which ultimately contributes to the enhancement of the specific surface area of SPCM.

(a) Pore structure analysis and analysis of the pore structure of the inset figure and (b) FT-IR analysis of SPCM.

3.3. Fourier transform infrared spectroscopy analysis of SPCM

As Figure 3(b) shows, the stretching vibration of the -OH group is located at 3410 cm^-1 [22]. Peak at 1623 cm^-1 might be attributed to the presence of C=C or C=O group. The characteristic peaks at 1145 cm^-1 and 671 cm^-1 are the C-O/C=S groups and C-S group, respectively [16]. This result demonstrates the successful incorporation of a sulfur atom on SPCM. The SPCM has an abundant functional group, contributing to subsequent Cr(VI) removal. After Cr(VI) removal, the intensity of sulfur-and oxygen-containing functional groups of SPCM decreases. Additionally, a new peak at 582 cm^-1 corresponds to the Cr-O group, indicating that SPCM involves Cr(VI) removal by surface complexation.

3.4. X-ray photoelectron spectroscopy analysis of SPCM

Figure 4 shows XPS analysis of SPCM prior to and following Cr(VI) removal, analyzing alterations in surface chemical states of the SPMC and the evolution of associated elemental species prior to and following Cr(VI) removal. The spectra show the peaks of C 1s (284.8 eV), O 1s (531.8 eV), and S 2p (163.1 eV), with relative content of C, O, and S being 90.03%, 8.85%, and 1.12%, respectively (Figure 4(a)). XPS analysis proves the successful synthesis of SPCM, which is consistent with EDS analysis. The full spectrum of the SPCM after removal of Cr(VI) shows a characteristic peak of Cr 2p (577.62 eV), indicating that SPCM can effectively capture Cr(VI). Proportions of C, O, S, and Cr elements in the SPCM after removal of Cr(VI) have changed to 88.48%, 10.45%, 0.48%, and 0.58%, respectively (Table S3). The observed changes in relative percentages of C, O, and S prior to and following Cr(VI) removal by SPCM suggest that surface functional groups may participate in corresponding redox reactions during the Cr(VI) removal process. Besides, the relative content of the S element decreases after Cr(VI) removal, demonstrating that sulfur-containing surface functional group participates in removing Cr(VI) (Table S3).

Table S3

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(a) XPS full spectra and (b) high-resolution spectra of C 1s, (c) O 1s, (d) S 2p of SPCM prior to and following the Cr(VI) removal process.

Figure 4(b) shows XPS high-resolution spectra of C 1s prior to and following Cr(VI) removal. The peak positions at 284.80, 285.35, 288.29, and 291.11 eV correspond to C=C/C-C/C-H, C-OH, C=O, and O-C=O groups, respectively. As Tables S3 and S4 shown, relative proportions of C-OH and O-C=O groups decrease from 39.93% and 6.79% to 11.26% and 6.44% after Cr(VI) removal, respectively. While proportions of C=C/C-C/C-H and C=O groups increase from 43.75% and 9.53% to 68.29% and 14.01%, respectively. This might be because the reducing groups (C-OH/O-C=O groups) on SPCM can convert Cr(VI) into Cr(III), while being oxidized to C=O and C-H groups.

Figure 4(c) shows XPS fine spectra of O 1s prior to and following removal of Cr(VI). Before Cr(VI) removal, the peaks at 532.25 eV, 533.94 eV and 535.77 eV are C-O, C=O and O-C=O groups, respectively [23]. Relative percentages of these functional groups have undergone noticeable changes after Cr(VI) removal (Table S3 and Table S4). The corresponding proportion of C-O and O-C=O groups decreases by 20.79% and 0.77%, respectively. However, the C=O group proportion increases from 22.41% to 23.29%. This analysis result indicates that C-O and O-C=O groups undergo oxidation to form C=O and C-H groups. While Cr(VI) undergoes reduction to form Cr(III). Additionally, Cr(III) would chelate with C=O groups. Furthermore, the Cr-O band appears at a peak of 531.14 eV, proving that Cr(VI) is adsorbed onto SPCM [24]. The change in binding energy from 533.94 and 535.77 eV to 533.95 and 535.27 eV might be due to intermolecular hydrogen bonds (C=O/O-H...O=Cr(VI)) or electrostatic interactions [25].

Table S4

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Figure 4(d) shows the XPS deconvolution of the S 2p. Peaks observed at 168.51 and 170.29 eV are -SOx (x = 2-4) groups [26]. While the signals at 164.12 and 165.29 eV correspond to C-SH and C=S bonds, respectively [26,27]. This result further verifies that SPCM has an abundant sulfur-containing functional group. The proportions of C-SH and C=S groups decrease by 5.86% and 2.91% after Cr(VI) removal, respectively. This phenomenon may be attributed to the conversion of adsorbed Cr(VI) to Cr(III) facilitated by the C-SH group, which undergoes oxidation to form the C=S group, and the C=S group subsequently forms a chelate complex with Cr(III).

3.5. Influence of pH

As Figure 5(a) shows, the pH value exhibits a notable influence on Cr(VI) removal owing to the pH value determining Cr(VI) speciation in aqueous solution. Besides, it can influence surface charge distribution and polarity of functional groups on SPCM. Cr(VI) removal amount reaches the optimum with a removal amount of 98 mg g^-1 at pH =2 (Figure 5a). However, Cr(VI) removal amount decreases from 198 to 20.8 mg g^-1 as the pH level rises from 2 to 7. The point of zero charge (pHpzc) of SPCM is 4.8 (Figure S1). The surface of the SPCM shows positive charge characteristics at pH < 4.8. The reason is that the -OH₂⁺, -COOH₂⁺, and -C=SH⁺ groups appear owing to protonation. The Cr(VI) predominantly occurs as HCrO₄^- or Cr₂O₇^2- species at pH of 2.0-6.8. Therefore, electrostatic attraction occurs between SPCM and HCrO₄^-/Cr₂O₇^2-, which significantly promotes Cr(VI) removal. Meanwhile, the abundant oxygen- and sulfur-containing groups of SPCM can specifically bind to the HCrO₄^-/Cr₂O₇^2- by the hydrogen bonding interactions (S-H/C=O/O-H...O=Cr(VI)). When the pH value exceeds 4.8, the surface charge characteristic of SPCM transitions is negative, resulting in electrostatic repulsion with Cr(VI) and significantly reducing its removal amount.

Figure S1

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(a) Influence of pH value on Cr(VI) removal, (b-d) adsorption isotherms at 298 K-318 fitting Cr(VI) adsorption data, (e) effect of adsorption time on Cr(VI) removal, and (f) Cr(VI) adsorption data fitting pseudo-second-order model.

3.6. Adsorption isotherms analysis

Figures 5(b-d) show fitting Cr(VI) adsorption isotherms. Fitting results of adsorption isotherm models for corresponding adsorption data have been shown in Table 1. As Figure 5(b) displayed, Cr(VI) removal amount gradually increases with increasing Cr(VI) concentration at each temperature (298 K-318 K). However, when the dosage of the SPCM is constant, competition adsorption for the limited active sites of SPCM becomes more intense as Cr(VI) concentration increases, resulting in a slow reduction in Cr(VI) removal amount until the adsorption equilibrium. In addition, Cr(VI) removal amount shows an upward trend at 298 K-318 K, indicating that the increase in temperature can enhance the Cr(VI) removal amount of SPCM [28]. Correlation coefficient (R²) of the Langmuir model consistently exceeds that of the Freundlich and Temkin models under different temperature conditions. This result indicates the presence of monolayer adsorption on SPCM. Cr(VI) removal process by SPCM may simultaneously involve both physical adsorption and chemical adsorption. The 1/n adsorption intensity values for SPCM are all below 1, which indicates that Cr(VI) can easily penetrate multiple layers of adsorption and be captured on SPCM, based on the Freundlich model analysis [29]. R_L value of removal Cr(Ⅵ) on SPCM is 0.191-0.665 at different temperatures. The R_L value is less than 1, proving the feasibility of Cr(VI) removal by SPCM.

Table 1. Results of fitting adsorption isotherm equations for Cr(VI) removal data.

Model	Parameter		Temperature (K)
Model	Parameter	298 K	308 K	318 K
Langmuir	q_m (mg g^-1)	250.4	296.3	330.3
	K_L (L mg^-1)	0.0169	0.0126	0.0101
	R²	0.850	0.910	0.914
Freundlich	K_F (L mg^-1)	32.923	25.763	23.214
	n	3.007	2.521	2.332
	R²	0.721	0.812	0.820
Temkin	k_T (L g^-1)	57.236	72.336	82.660
	B_T	0.1469	0.0948	0.0786
	R²	0.799	0.889	0.903

Cr(VI) removal amount of comparable adsorbents has been listed in Table S5. As shown in Table S5, SPCM has a relatively large Cr(Ⅵ) removal amount, indicating that SPCM is a significant potential adsorbent for treating wastewater containing Cr(VI).

Table S5

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3.7. Adsorption kinetics

Adsorption kinetic models are employed to analyze the Cr(VI) removal process. Cr(VI) removal performance of SPCM is dynamically monitored at different adsorption times. The Cr(VI) removal process exhibits an overall biphasic trend, characterized by a rapid adsorption phase during the initial 0-60 min, followed by an equilibrium phase from 60 min until near saturation (Figure 5e). However, the Cr(VI) removal amount doesn’t show a significant further increase with prolonged adsorption time owing to the adsorption equilibrium.

The pseudo-first-order and pseudo-second-order models, along with the intraparticle diffusion model and the Elovich model, are employed to determine which kinetic model most accurately represents Cr(Ⅵ) adsorption behavior [30]. Table 2 presents adsorption kinetic fitting parameters obtained through fitting with the adsorption kinetic models. Correlation coefficient (R²) derived from the pseudo-second-order model is larger than that obtained from the pseudo-first-order model (Table 2). The above analysis proves that the pseudo-second-order model could better describe the Cr(VI) removal process (Figure 5f). Rate-controlling step of Cr(Ⅵ) adsorption on SPCM is further analyzed through application of the intra-particle diffusion model, which can be divided into three different stages.

Table 2. Fitting parameters of adsorption kinetic models fitting Cr(VI) adsorption data.

Model	Parameter	Result
Pseudo-first order	q_e (mg g^-1)	157.2
	k₁ (1/min)	0.0756
	R²	0.919
Pseudo-second order	q_e (mg g^-1)	168.4
	k₂ (g mg^-1 min^-1)	0.00078
	R²	0.974
	K_d1	7.65
	C_d1	80.44
	R_d1²	0.945
	K_d2	1.84
Intraparticle diffusion model	C_d2	125.37
	R_d2²	0.902
	K_d3	1.06
	C_d3	139.79
	R_d3²	0.864
	α (mg g^-1 min^-1)	7.712
Elovich model	β (g mg^-1)	15.11
	R²	0.965

As can be seen from Table 2, the rate constant K_d1 is relatively high in the first stage (within 60 min), which is attributed to the membrane diffusion mechanism of Cr(VI) on SPCM, and the transfer of Cr(VI) from the solution to the boundary layer of SPCM. The K_d2 in the second stage (60-240 min) is relatively stable, which could be associated with the chemical reduction of Cr(VI). The third stage (240-1440 min) is a dynamic equilibrium, and the active sites gradually become saturated (Figure S2). Meanwhile, the fact that the internal diffusion line does not intersect the origin reveals that Cr(VI) diffusion into the interior of particles is not the only Cr(VI) removal mechanism. Error analysis of the fitted adsorption kinetic models is presented in Table S6. The results indicate that the model fits are statistically acceptable.

Figure S2

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Table S6

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The fitting analysis of the Elovich kinetic model indicates that the removal process of Cr(VI) by SPCM involves multiple mechanisms coexisting with chemical interactions.

3.8. Adsorption thermodynamics analysis

Figure S3 presents the relationship between ln(qe/Ce) and 1/T. It can be concluded that at 298K, 308K, and 318K, the ∆G values are -2.82 kJ mol^-1, -3.42 kJ mol^-1, and -3.99 kJ mol^-1, respectively. This result demonstrates that the Cr(VI) removal process is spontaneous owing to negative ∆G values [31]. Additionally, as the temperature increases, the ∆G values show a decreasing trend, suggesting that increasing in temperature improves Cr(VI) removal. Meanwhile, the reaction enthalpy (∆H) is 14.64 kJ/mol. The positive ∆H value demonstrates that Cr(VI) removal involves an endothermic reaction, further confirming that an increase in temperature contributes to Cr(VI) removal [32]. Moreover, the entropy (∆S) is 58.60 J mol^-1 K^-1. The increase in entropy suggests that the disorder at the solid-liquid interface has increased during the Cr(VI) removal process.

Figure S3

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3.9. Influence of coexisting ions

The presence of multiple ions may have a certain influence on Cr(VI) removal in actual wastewater. Therefore, a binary coexistence ion system is constructed to systematically investigate the interference degree of typical components such as Cl^-, SO₄^2-, NO₃^-, Na⁺, Ca²⁺, and Mg²⁺ on Cr(VI) removal. Figure S4 shows the influence of coexisting ions on Cr(VI) removal. The analysis result proves that the Cr(VI) removal amount of SPCM is 178.2 mg/g without adding the coexisting ions. When the Cl^-, SO₄^2-, NO₃^-, Na⁺, Ca²⁺, and Mg²⁺ ions are added, the decrease in Cr(VI) removal amount is relatively small. This result is also consistent with the Hard-Soft interaction theory [33]. However, the Cr(VI) removal amount of SPCM decreases to 152.4 mg g^-1 as the concentration of SO₄²⁻ reaches 100 mg L^-1. This might be because the anionic form of Cr(VI) has certain similarities with SO₄^2- in terms of ionic radius and charge characteristics, leading to competitive adsorption between HCrO₄^-/Cr₂O₇^2- and SO₄^2- on the SPCM. The above results indicate that SPCM can still maintain a good removal efficiency on Cr(VI) under complex aqueous solution conditions.

Figure S4

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3.10. Reusability of the SPCM

A good adsorbent should have certain reusability. Therefore, the experiment of removing Cr(VI) is conducted four times on the SPCM to investigate its reusability. The Cr(VI) removal capacity of SPCM gradually declines in the following cycles after undergoing adsorption-desorption cycle experiments (Figure S5). This might be owing to a gradual reduction in the content of adsorption sites and the deactivation of active sites during the adsorption-desorption process. Besides, some HCrO₄^-/Cr₂O₇^2- have strong binding ability with the active sites, which are not completely desorbed by the eluent and remain in the SPCM. It can continuously accumulate during the adsorption-desorption cycle experiments. Moreover, Cr(VI) removal amount remains 35 mg g^-1, indicating that SPCM has certain cyclic stability.

Figure S5

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3.11. Removal mechanism

Firstly, the surface area of SPCM is 1423.21 m² g^-1, indicating that SPCM can effectively adsorb Cr(VI) in wastewater through its well-developed pore structure. Secondly, the FT-IR and XPS characterization analyses demonstrate that the SPCM contains abundant surface functional groups (Figures 3b and 4). After participating in the reaction for removing Cr(VI), the intensity and relative percentage of the sulfur/oxygen-containing functional groups have undergone significant changes, indicating that the Cr(VI) removal process involves electrostatic adsorption, chelating, or hydrogen bonding interactions of surface sulfur and oxygen functional groups (C-SH, C=S, -COOH, etc.). Meanwhile, the C-O group proportion decreases from 39.93% to 11.26%, while the C=O group proportion increases from 9.53% to 14.01%, demonstrating that some C-O groups are oxidized to C=O groups by Cr(VI). This might be due to the role of the C-O group as an electron donor. Therefore, the C-O group may serve as active sites for both adsorption and reduction processes in SPCM. Therefore, adsorption and chemical reduction simultaneously occur.

As Figure 6(a) shows, two sub-peaks at 577.8 eV and 587.3 eV correspond to Cr 2p3/2 and Cr 2p1/2, respectively. Further, the two sub-peaks can be distinguished as Cr(III) 2p3/2 (577.88 eV), Cr(VI) 2p3/2 (580.90 eV), Cr(III) 2p1/2 (587.41 eV), and Cr(VI) 2p1/2 (589.95 eV), respectively [27]. Tables S3 and S4 show the binding energies and corresponding percentages of Cr, C, S, and O prior to and following Cr(VI) removal. This analysis suggests that corresponding proportions of Cr(III) and Cr(VI) are 77.82% and 22.18%, respectively, indicating that during the removal process of Cr(VI), a significant portion is transformed into Cr(III). The adsorption kinetics experiment analysis indicates that the solution contains 4.02 mg L^-1 of Cr(III) after adsorption.

(a) XPS spectrum of Cr 2p, and (b) schematic illustration of Cr(VI) removal mechanism by SPCM.

Finally, it could be inferred that a portion of Cr(VI) is captured by SPCM through micropore adsorption, electrostatic interaction, hydrogen bonding or chelation. Subsequently, a portion of Cr(VI) undergoes transformation into Cr(III) through interaction with SPCM, indicating that reduction is one of Cr(VI) removal mechanisms. The schematic illustration of Cr(VI) removal mechanism is presented in Figure 6(b).

3.12. Energy storage of the SPCM

The first three galvanostatic charge/discharge (GCD) of SPCM have been shown in Figure S6(a). Figure S6(a) shows a distinct voltage plateau between 0.5 and 1.0 V, which corresponds to the formation of the solid electrolyte interphase (SEI) film. Figure S6(b) shows the cyclic voltammetry (CV) curves of the SPCM in the voltage range of 0.01-3 V. As Figure S6(b) shows, a broad reduction peak is observed in the first cathodic scan between 0.5 and 1.0 V, which is related to the formation of an irreversible SEI film. Figure S6(c) shows the rate performance of SPCM at 50-2000 mA g^-1. Specific capacity of SPCM at 5-2000 mA g^-1 is 488-86 mAh g^-1. Specific capacity of SPCM is comparable to the first charge reversible capacity, with the capacity retention of 84.49% when current density returns to 50 mA g^-1, indicating that SPCM shows good reversibility. SPCM exhibits a large specific capacity for energy storage used in lithium-ion batteries. The reason is that the layered structure of SPCM provides additional free space and pores to accommodate volume changes because of the large surface area, which provides favorable and efficient channels for the lithium-ions/electrons transportation. SPCM has rich sulfur/oxygen-containing functional groups, which improve the surface wettability of SPCM. The formed defect site contributes to the adsorption of lithium ions. Besides, the improved electrochemical performance of SPCM can be primarily attributed to the synergy of its distinctive structural characteristics and sulfur doping, which contributes to enhancing electrochemical activity for lithium storage. SPCM also offers sufficient available space or active sites for lithium-ion storage. Therefore, the SPCM shows promising potential in energy storage based on the above analysis. The cycling performance of the SPCM at 1000 mA⋅g⁻¹ is shown in Figure S6(d). After 500 cycles, the SPCM retains a discharge capacity of 151 mAh⋅g⁻¹. The coulombic efficiency of SPCM remains above 96%, which further demonstrates the excellent long-term cycling stability of the SPCM. As shown in Figure S6(e), the Nyquist plots of SPCM exhibit a semicircular feature in the high-frequency region and a steep linear slope in the low-frequency region. The galvanostatic intermittent titration technique (GITT) test is used to explore the changing trend of the SPCM (Figure S6f). In the GITT curve, the rapid initial potential drop is related to the additional active sites and defects brought by sulfur doping, which facilitate the rapid insertion of Li⁺. The introduction of sulfur doping into SPCM generates structural defects, which facilitate the formation of rapid diffusion pathways for Li⁺, thereby enhancing the kinetics of Li⁺ insertion and extraction.

Figure S6

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4. Conclusions

In summary, this work provides a feasible preparation method of the SPCM using LDPE as a carbon source by solvent heat and potassium acetate activation for Cr (VI) removal. The SPCM has sulfur/oxygen-containing functional groups and abundant pore structure, exhibiting a Cr (VI) removal amount of 250.4 mg g^-1. The adsorption kinetics and isotherms models are used to describe the Cr (VI) removal process. The SPCM has strong anti-interference performance and excellent cyclic stability in the Cr (VI) removal process. The Cr (VI) removal mechanism is analyzed and investigated. Besides, SPCM could act as a promising anode material for lithium-ion batteries, showing promising potential in energy storage.

Acknowledgment

The authors would like to express their gratitude to the Specialized Research Fund for Fundamental Research Program of Shanxi Province (202403021211071) for financial support.

CRediT authorship contribution statement

Lv Chao: Conceptualization, Methodology, Supervision, Writing - review & editing. Xiangwang Zeng: Conceptualization, Formal analysis, Investigation, Writing-original draft.

Declaration of competing interest

The authors affirm that there are no known financial competing interests or personal relationships that could potentially influence the work presented in this paper.

Declaration of generative AI and AI-assisted technologies in the writing process

The authors confirm that there was no use of artificial intelligence (AI)-assisted technology for assisting in the writing or editing of the manuscript, and no images were manipulated using AI.

Supplementary data

Supplementary material to this article can be found online at https://dx.doi.org/10.25259/AJC_931_2025.

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Show Sections

[1] Browning S., Beymer-Farris B., Seay J.R.. Addressing the challenges associated with plastic waste disposal and management in developing countries. Current Opinion in Chemical Engineering. 2021;32:100682. https://doi.org/10.1016/j.coche.2021.100682
[Google Scholar]

[2] Ragusa A., Svelato A., Santacroce C., Catalano P., Notarstefano V., Carnevali O., Papa F., Rongioletti M.C.A., Baiocco F., Draghi S., D’Amore E., Rinaldo D., Matta M., Giorgini E.. Plasticenta: First evidence of microplastics in human placenta. Environment International. 2021;146:106274. https://doi.org/10.1016/j.envint.2020.106274
[Google Scholar]

[3] Cheng S., Tao C., Li H.. Preparation of carbon nanotubes from organic solid wastes: A review. Journal of Analytical and Applied Pyrolysis. 2025;192:107240. https://doi.org/10.1016/j.jaap.2025.107240
[Google Scholar]

[4] Matthews C., Moran F., Jaiswal A.K.. A review on European Union’s strategy for plastics in a circular economy and its impact on food safety. Journal of Cleaner Production. 2021;283:125263. https://doi.org/10.1016/j.jclepro.2020.125263
[Google Scholar]

[5] Liu X., Tian K., Chen Z., Wei W., Xu B., Ni B.J.. Online TG-FTIR-MS analysis of the catalytic pyrolysis of polyethylene and polyvinyl chloride microplastics. Journal of Hazardous Materials. 2023;441:129881. https://doi.org/10.1016/j.jhazmat.2022.129881
[Google Scholar]

[6] Xu X., Zhu D., Wang X., Deng L., Fan X., Ding Z., Zhang A., Xue G., Liu Y., Xuan W., Li X., Makinia J.. Transformation of polyvinyl chloride (PVC) into a versatile and efficient adsorbent of Cu(II) cations and Cr(VI) anions through hydrothermal treatment and sulfonation. Journal of Hazardous Materials. 2022;423:126973. https://doi.org/10.1016/j.jhazmat.2021.126973
[Google Scholar]

[7] Fan Z., Liu J., Liu H., Liu L., She Y., Wen X., Wang H., Hu G., Niu R., Gong J.. Synergism of solar-driven interfacial evaporation and photo-Fenton Cr(VI) reduction by sustainable Bi-MOF-based evaporator from waste polyester. Journal of Energy Chemistry. 2024;94:527-540. https://doi.org/10.1016/j.jechem.2024.03.011
[Google Scholar]

[8] Cheng L.X., Zhang L., Chen X.Y., Zhang Z.J.. Efficient conversion of waste polyvinyl chloride into nanoporous carbon incorporated with MnOx exhibiting superior electrochemical performance for supercapacitor application. Electrochimica Acta. 2015;176:197-206. https://doi.org/10.1016/j.electacta.2015.07.007
[Google Scholar]

[9] Liu Q., Yang M., Deng Y., Zhang M., Liu C., Hu F., Jian X., Chen Y.. Flexible carbon fiber membrane derived from polypropylene for symmetric quasi-solid-state supercapacitors. Journal of Power Sources. 2024;602:234336. https://doi.org/10.1016/j.jpowsour.2024.234336
[Google Scholar]

[10] Nezamzadeh-Ejhieh A., Shahanshahi M.. Modification of clinoptilolite nano-particles with hexadecylpyridynium bromide surfactant as an active component of Cr(VI) selective electrode. Journal of Industrial and Engineering Chemistry. 2013;19:2026-2033. https://doi.org/10.1016/j.jiec.2013.03.018
[Google Scholar]

[11] Jayaraman T., Raja S.A., Priya A., Jagannathan M., Ashokkumar M.. Synthesis of a visible-light active V₂O₅-g-C₃N₄ heterojunction as an efficient photocatalytic and photoelectrochemical material. New Journal of Chemistry. 2015;39:1367-1374. https://doi.org/10.1039/C4NJ01807A
[Google Scholar]

[12] Mohamed Khalith S.B., Rishabb Anirud R., Ramalingam R., Karuppannan S.K., Dowlath M.J.H., Pandion K., Ravindran B., WoongChang S., Ovi D., Arasu M.V., Ignacimuthu S., Al-Dhabi N.A., Chandrasekaran M., Arunachalam K.D.. Synthesis and characterization of magnetite carbon nanocomposite from agro waste as chromium adsorbent for effluent treatment. Environmental Research. 2021;202:111669. https://doi.org/10.1016/j.envres.2021.111669
[Google Scholar]

[13] Cheng S., Zeng X., Liu P.. One-step synthesis of magnetic N-doped carbon nanotubes derived from waste plastics for effective Cr(Ⅵ) removal. Arabian Journal of Chemistry. 2024;17:105956. https://doi.org/10.1016/j.arabjc.2024.105956
[Google Scholar]

[14] Luo Y., Hu Z., Lei X., Wang Y., Guo X.. Fluorescent magnetic chitosan-based hydrogel incorporating amino-functionalized Fe₃O₄ and cellulose nanofibers modified with carbon dots for adsorption and detection of Cr (VI) Colloids and Surfaces A: Physicochemical and Engineering Aspects. 2023;658:130673. https://doi.org/10.1016/j.colsurfa.2022.130673
[Google Scholar]

[15] Zhang L., Tang Y., Zhang Y., Sun W., Yang Z., Li J., Liang S., Zhou J.. Novel Fe-Mn modified self-nitrogen biochar composite: Synergistic adsorption-reduction mechanisms for enhanced Cr(VI) removal in aquatic systems. Separation and Purification Technology. 2025;374:133703. https://doi.org/10.1016/j.seppur.2025.133703
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[16] Wang L., Sha Y., Bi C., Lin D., Liu Y., Dai L., Wang X., Lin H., Jrar J.A., Hou J.. One-step microwave method synthesis of N, S-codoped carbon with the frustrated Lewis pairs for Cr (VI) removal. Journal of Water Process Engineering. 2025;69:106757. https://doi.org/10.1016/j.jwpe.2024.106757
[Google Scholar]

[17] Zhuang M., Wang H., Qi L., Cui L., Quan G., Yan J.. Production of activated biochar via a self-blowing strategy-supported sulfidated nanoscale zerovalent iron with enhanced reactivity and stability for Cr(VI) reduction. Journal of Cleaner Production. 2021;315:128108. https://doi.org/10.1016/j.jclepro.2021.128108
[Google Scholar]

[18] Zhao L., Li C., Li H., Shu Z., Luo Y., Yang H., Chen Q., Xu W., Zhang W., Tan X.. Efficient Cr(VI) removal by pyrite/porous biochar: Critical role of potassium salt and sulphur. Environmental Pollution (Barking, Essex : 1987). 2024;346:123641. https://doi.org/10.1016/j.envpol.2024.123641
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[19] Cheng S., Liu Y., Xing B., Qin X., Zhang C., Xia H.. Lead and cadmium clean removal from wastewater by sustainable biochar derived from poplar saw dust. Journal of Cleaner Production. 2021;314:128074. https://doi.org/10.1016/j.jclepro.2021.128074
[Google Scholar]

[20] Cheng S., Meng W., Xing B., Shi C., Wang Q., Xia D., Nie Y., Yi G., Zhang C., Xia H.. Efficient removal of heavy metals from aqueous solutions by Mg/Fe bimetallic oxide-modified biochar: Experiments and DFT investigations. Journal of Cleaner Production. 2023;403:136821. https://doi.org/10.1016/j.jclepro.2023.136821
[Google Scholar]

[21] Cheng S., Zhao S., Guo H., Xing B., Liu Y., Zhang C., Ma M.. High-efficiency removal of lead/cadmium from wastewater by MgO modified biochar derived from crofton weed. Bioresource Technology. 2022;343:126081. https://doi.org/10.1016/j.biortech.2021.126081
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[22] Ghahremani P., Vakili M.H., Nezamzadeh-Ejhieh A.. Optimization of Pb(II) removal by a novel modified silica aerogel using Quince seed mucilage with response surface methodology. Journal of Environmental Chemical Engineering. 2021;9:106648. https://doi.org/10.1016/j.jece.2021.106648
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[23] Li S.Y., Teng H.J., Guo J.Z., Wang Y.X., Li B.. Enhanced removal of Cr(VI) by nitrogen-doped hydrochar prepared from bamboo and ammonium chloride. Bioresource Technology. 2021;342:126028. https://doi.org/10.1016/j.biortech.2021.126028
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[24] Li J., Cheng R., Chen J., Lan J., Li S., Zhou M., Zeng T., Hou H.. Microscopic mechanism about the selective adsorption of Cr(VI) from salt solution on nitrogen-doped carbon aerogel microsphere pyrolysis products. The Science of the Total Environment. 2021;798:149331. https://doi.org/10.1016/j.scitotenv.2021.149331
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[25] Guo J.Z., Xu H., Chen L., Li B.. A pyridinium functionalization chitosan for efficient elimination of methyl orange and Cr(VI) Bioresource Technology. 2022;365:128112. https://doi.org/10.1016/j.biortech.2022.128112
[Google Scholar]

[26] Li H., Li N., Zuo P., Qu S., Shen W.. Efficient adsorption-reduction synergistic effects of sulfur, nitrogen, and oxygen heteroatom co-doped porous carbon spheres for chromium(VI) removal. Colloids and Surfaces A: Physicochemical and Engineering Aspects. 2021;618:126502. https://doi.org/10.1016/j.colsurfa.2021.126502
[Google Scholar]

[27] Chen F., Zhang M., Ma L., Ren J., Ma P., Li B., Wu N., Song Z., Huang L.. Nitrogen and sulfur codoped micro-mesoporous carbon sheets derived from natural biomass for synergistic removal of chromium(VI): Adsorption behavior and computing mechanism. The Science of the Total Environment. 2020;730:138930. https://doi.org/10.1016/j.scitotenv.2020.138930
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[28] Eshraghi F., Nezamzadeh-Ejhieh A.. EDTA-functionalized clinoptilolite nanoparticles as an effective adsorbent for Pb(II) removal. Environmental Science and Pollution Research International. 2018;25:14043-14056. https://doi.org/10.1007/s11356-018-1461-0
[Google Scholar]

[29] Nasiri-Ardali M., Nezamzadeh-Ejhieh A.. A comprehensive study on the kinetics and thermodynamic aspects of batch and column removal of Pb(II) by the clinoptilolite–glycine adsorbent. Materials Chemistry and Physics. 2020;240:122142. https://doi.org/10.1016/j.matchemphys.2019.122142
[Google Scholar]

[30] Salmani-Mobarakeh J., Nezamzadeh-Ejhieh A.. A kinetic and thermodynamic study of Cd(II) removal by hexylamine-clinoptilolite nanoparticles composite. Desalination and Water Treatment. 2018;116:158-169. https://doi.org/10.5004/dwt.2018.22604
[Google Scholar]

[31] Shafiof M.S., Nezamzadeh-Ejhieh A.. A comprehensive study on the removal of Cd(II) from aqueous solution on a novel pentetic acid-clinoptilolite nanoparticles adsorbent: Experimental design, kinetic and thermodynamic aspects. Solid State Sciences. 2020;99:106071. https://doi.org/10.1016/j.solidstatesciences.2019.106071
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[Google Scholar]

[33] Mehrali-Afjani M., Nezamzadeh-Ejhieh A.. Efficient solid amino acid–clinoptilolite nanoparticles adsorbent for Mn(II) removal: A comprehensive study on designing the experiments, thermodynamic and kinetic aspects. Solid State Sciences. 2020;101:106124. https://doi.org/10.1016/j.solidstatesciences.2020.106124
[Google Scholar]

Preparation of the sulfur-enriched carbon material from waste plastic and application for removal of Cr(VI) from wastewater

Abstract

Keywords

High-value utilization

Lithium-ion batteries

Porous carbon material

Removal mechanism

Waste plastic

1. Introduction

2. Materials and Methods

2.1. Material

2.2. Preparation of SPCM

2.3. Cr(VI) removal process

3. Results and Discussion

3.1. SEM analysis of SPCM

3.2. Pore structure analysis of the SPCM

3.3. Fourier transform infrared spectroscopy analysis of SPCM

3.4. X-ray photoelectron spectroscopy analysis of SPCM

3.5. Influence of pH

3.6. Adsorption isotherms analysis

3.7. Adsorption kinetics

3.8. Adsorption thermodynamics analysis

3.9. Influence of coexisting ions

3.10. Reusability of the SPCM

3.11. Removal mechanism

3.12. Energy storage of the SPCM

4. Conclusions

Acknowledgment

CRediT authorship contribution statement

Declaration of competing interest

Declaration of generative AI and AI-assisted technologies in the writing process

Supplementary data

References

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