Polyethyleneimine modified activated carbon for high-efficiency adsorption of copper ion from simulated wastewater

With rapid industrial development, many pollutants containing heavy metals have been introduced into natural water bodies in recent years. Heavy metal ions cannot be biodegraded and can exist for a long time and enter the human living environment through the food chain (Abdolali et al. 2015; Yagmur Goren et al. 2022). When the level of copper ions (Cu²⁺) exceeds the normal human tolerance range, it affects the internal organs of the human body, especially the liver and gallbladder, and may lead to liver cirrhosis, ascites, and other severe diseases (da Silva Alves et al. 2021). Methods for removing Cu²⁺ include high-efficiency binders, chemical precipitation, ion exchange, and adsorption. Adsorption is a widely used method because it is economical, convenient, and easy to apply (Wang et al. 2021).

Activated carbon (AC), magnetic carbon nanotubes and graphene, and resins are often used as adsorbents to adsorb heavy metals. AC is the most widely used adsorbent in wastewater treatment, possessing the advantage of good stability, large specific surface area, and high adsorption performance (Fang et al. 2018; Shahrashoub & Bakhtiari 2021). However, the technology of removing Cu²⁺ through only by physical adsorption using AC has inherent limitations, such as limited absorption capacity, poor reusability, high ash content, uneven distribution of micropores, and an insufficient number of surface functional groups to selectively adsorb heavy metal elements (Fiorati et al. 2020; He et al. 2021). Therefore, it is necessary to modify its surface structure to improve the ability of AC to adsorb Cu²⁺. Adsorbents containing hydrazine, amine, and thioamide groups can remove metal ions from aqueous solutions by chelation. In particular, the amine group is one of the most important functional groups for the chelation of Cu²⁺ (Hernández-Morales et al. 2012).

Polyethyleneimine (PEI) is an organic macromolecule with a high cationic charge density that exhibits a high selective adsorption capacity through chelation and electrostatic interaction. PEI is suitable for treating Cu²⁺ due to the presence of a large number of amino groups and its good reactivity and ease of functionalization (Chen et al. 2018). However, PEI is difficult to recycle and can cause secondary pollution of water bodies. Furthermore, it cannot be applied alone to remove water pollution. Liu & Huang (2011) used eggshell as raw material and glutaraldehyde (GA) as the cross-linking agent, and PEI functionalized adsorbent was prepared by the cross-linking of functional groups, such as amide and aldehyde groups. As a result, the adsorption amount of chromium ion reached 160 mg/g. Chen et al. studied a magnetic gel material composed of GA polyimide PEI-modified corn cob and applied it to the adsorption of heavy metal ions in an aqueous solution. They exhibited effective Cu²⁺ and lead ion adsorption at 303 K, with maximum adsorption of 459.4 and 290 mg/g, respectively (Chen et al. 2022). Deng & Ting (2005) developed a surface-modified biomass adsorbent by modifying with PEI. The adsorption capacity of the modified material to Cu²⁺, lead ion and nickel ion were significantly higher. The adsorption capacity of Cu²⁺, lead ion and nickel ion reached 92, 204, 55 mg/g.

In this study, PEI was firmly attached to the surface of AC applying GA as a cross-linking agent. The PEI-modified AC composite adsorbent, AC@PEI, was prepared for the adsorption of Cu²⁺ in wastewater. The optimal adsorption conditions, such as pH, initial concentration, and temperature during adsorption, were studied through experiments. Furthermore, the thermodynamics, kinetics, and adsorption mechanism of the Cu²⁺ adsorption process of the modified adsorption material were studied.

NaOH, PEI, HCl CuSO₄·5H₂O, and ethylenediaminetetraacetic acid disodium salt dihydrate (EDTA) were provided by Aladdin Industries Co. (Shanghai). GA (50%) was procured from Damao Chemical Reagent Co. (Tianjin). Tris-buffered saline (TBS) buffer (0.02 mol/L) was purchased from Sempega Biotechnology Co. (Nanjing). All the chemicals employed in this study were of analytical grade and used without further purification. AC was purchased from Ningxia Burtons Activated Carbon Co. (Yinchuan).

A Fourier-transform infrared (FTIR) spectrometer (Thermo Nicolet Avatar 380 USA) was used to analyze the surface functional groups and bonding, and the data were collected over a transmission range of 4,000–500 cm⁻¹. A scanning electron microscope (SEM, JSM–7500F, Jeol, Japan) was used to analyze the morphological characteristics. The adsorption properties of the materials were analyzed using an X-ray photoelectron spectrometer (XPS, ESCALAB Xi + XPS, Thermo Fisher Scientific, USA). The surface elemental composition and valence were investigated. N₂ adsorption-desorption isotherms (ASAP2460, Micromeritics, USA) were used to determine the adsorbents specific surface areas, pore volumes, and pore size distributions. The specific surface areas of the adsorbents were calculated using the Brunauer-Emmett-Teller (BET) equation, and the pore volumes and pore size distributions were calculated using the Barrett-Joyner-Halenda (BJH) equation. ζ-potential analysis using a Malvern Zeta potentiometer was employed to analyze the adsorbents and determine their surface ζ-potentials.

Table 1 demonstrates that for AC, the specific surface area was 959.3 m²/g, the pore volume was 0.22 cm³/g, and the pore size was 2.84 nm. For AC-NaOH, the specific surface area was 1,169.9 m²/g, the pore volume was 0.26 cm³/g, and the pore size was 2.85 nm, which were slightly higher than those of AC. Compared with that of AC@PEI, the specific surface area of AC@PEI decreased sharply with an increase in the amount of loaded PEI. Furthermore, AC@PEI displayed a significant decrease in its pore size and volume. AC-NaOH@PEI-200 of specific surface area and pore volume compared to AC were reduced to 556.9 m²/g and 0.11 cm³/g, respectively, due to GA successfully cross-linking PEI to occupy a particular space in the AC pores. The impregnation and binding of PEI on the AC surface partially blocked the pores, indicating that PEI was successfully combined with AC, which is consistent with the above FTIR results (Xie et al. 2019).

Table 2 presents the ζ-potential of the samples tested at room temperature and a pH of 6.0. The ζ-potential of the AC surface in aqueous solution was −27.2 mV, whereas after loading PEI, the ζ-potential of the AC@PEI surface in aqueous solution increased to 10.4 mV. The ζ-potential in aqueous solution increased to 10.4 mV, which is because the amine group of the PEI (-NH or -NH₂) could readily adsorb cations (H⁺) and exhibited a positive charge on the surface, which led to a significant increase in the ζ-potential of the AC surface. This result demonstrates that PEI was successfully loaded on the AC surface (Chen et al. 2015). After the adsorption of Cu²⁺, the ζ-potential of the AC surface changed to 12.5 mV (Tian et al. 2015).

This study examined different loading, pH, adsorption time, temperature, and other experimental conditions. The testing procedures and results for different experimental conditions are described below.

The metal ions in a solution and the chemical state of the reactive groups on the adsorbent (degree of protonation) are influenced by the pH of the solution. By adjusting the pH, the charge distribution on the adsorbent surface can be effectively manipulated, which affects the affinity of the adsorbent for the target metal ions and thus the adsorption efficiency. An alkaline pH produces Cu(OH)₂ flocculent precipitate, which has a significant effect on the adsorption efficiency and leads to inaccurate adsorption results. Therefore, the pH range for this experiment was set to 2–6. Figure 6(b) illustrates the effect of the pH value on the removal rate of Cu²⁺. The Cu²⁺ adsorption behavior of the AC-NaOH@PEI-200 adsorbent was closely related to the pH value. The removal rate was 48% at pH = 2. The Cu²⁺ removal rate of AC@PEI increased significantly from 70 to 91% when the pH was increased from 3 to 5. The highest Cu²⁺ removal rate of AC-NaOH@PEI-200 was 93% when pH = 6 (Wang et al. 2015a, 2015b). The amine groups on the adsorbent surface were protonated at low pH by a large amount of H⁺ present in the solution, thus implying that complexation could not occur. In addition, electrostatic repulsion of Cu²⁺ occurred, which led to a decrease in the adsorption capacity. When the pH increased, the amount of amine present in the adsorbent increased the free radical content of the solution, competitive adsorption of protons and Cu²⁺ was weakened, electrostatic repulsion was reduced, and ligand group of Cu²⁺ on the surface of the adsorbent increased (Wang et al. 2015a, 2015b). By studying the effect of the initial solution pH, it was found that the optimal pH for Cu²⁺ adsorption for AC-NaOH@PEI-200 was 6 (Saleh et al. 2017).

The effect of the initial concentration of Cu²⁺ on the adsorption was investigated for the adsorption of Cu²⁺ by AC-NaOH@PEI-200 at three different temperatures of 298.15, 308.15, and 318.15 K, as illustrated in Figure 6(c). Different initial concentrations had a significant effect on the adsorption of Cu²⁺ by AC-NaOH@PEI-200. As the Cu²⁺ concentration increased from 20 to 100 mg/L, the equilibrium adsorption capacity q_e of AC-NaOH@PEI-200 increased from 18.76 to 47.8 mg/g at 298.15 K. This can be explained by the fact that a higher initial concentration of Cu²⁺ produced a higher gradient concentration of Cu²⁺, which increased the mass transfer rate and led to greater uptake of Cu²⁺, thus increasing the Cu²⁺ adsorption capacity of AC-NaOH@PEI-200. In addition, the adsorption of q_e decreased to 41.68 mg/g, when the temperature was increased to 318.15 K, indicating that the adsorption of Cu²⁺ on AC-NaOH@PEI-200 is an exothermic process.

Figure 7(c) presents the relevant equation as a curve. If the relationship between q_t and t^1/2 is linear, the adsorption is governed by an internal diffusion process. The intraparticle diffusion curve was composed of three or more phases. The initial linear part indicates transient or external surface adsorption, demonstrating that macropore diffusion achieved mass transfer in the early stages of adsorption. This suggests that the process was divided into three phases. The first stage involves the membrane diffusion of Cu²⁺ from the aqueous solution to the outer surface of the adsorbent. The second stage involves the intraparticle distribution of Cu²⁺ from the surface to the internal pores. As the value of Ki₁ was greater than Ki₂, the intraparticle distribution was a gradual process. The low concentration of Cu²⁺ and the strong repulsion between Cu²⁺ and AC-NaOH@PEI-200 led to the slow distribution of AC-NaOH@PEI-200, indicating that the model was a good fit for the adsorption process (Al-Anber & Matouq 2008; Simonin 2016).

where n is a constant representing the adsorption strength, K_F is a constant related to the adsorption capacity, and 1/n is the heterogeneity factor related to the adsorption strength and heterogeneity of the surface of the material (Jang et al. 2018).

Table 6 presents the thermodynamic parameters of the adsorption process, and Figure 8(b) presents the relationship between ln K and 1/T. Table 6 illustrates that ΔG < 0 and ΔH < 0, indicating that the adsorption of Cu²⁺ on AC-NaOH@PEI-200 was an exothermic process, similar to other findings reported in the literature. ΔS had a negative value. Before the adsorption of Cu²⁺ on AC-NaOH@PEI-200, Cu²⁺ moved freely in the solution, leading to a chaotic state. However, after the adsorption of Cu²⁺ on AC-NaOH@PEI-200, Cu²⁺ was in a stationary state, and the order of the adsorption process increased, leading to a decrease in ΔS values during adsorption, negative values of ΔS (Zhu et al. 2011). Table 6 illustrates that the value of ΔG changed from negative to positive with increasing temperature at Cu²⁺ concentrations of 90 and 100 mg/L, indicating that the adsorption of Cu²⁺ on AC-NaOH@PEI-200 was a spontaneous process at low temperatures and a nonspontaneous process at high temperatures (Tran et al. 2016; Ghosal & Gupta 2015, 2017).

In the actual environment, wastewater contains more complex heavy metal ions. Therefore, it is necessary to investigate competitive adsorption. AC-NaOH@PEI-200 adsorbent (0.1 g) was added to an aqueous solution containing 50 mg/L of Cd²⁺, Li⁺, Cu²⁺ to form a competitive adsorption experiment of Cd²⁺ and Cu²⁺ or Li⁺. The results of competitive adsorption are shown in Figure 9(b). It is shows that the presence of Li⁺ has a significant inhibitory effect on the adsorption of Cu²⁺ on AC-NaOH@PEI-200, indicating that Cu²⁺ exhibits a strong competitive adsorption ability associated with Li⁺, which may be related to the size of the metal ion radius. The smaller the ionic radius, the easier it is to adsorb on AC-NaOH@PEI-200. The ionic radius of Li⁺ is 0.076 nm, and that of Cu²⁺ is 0.077 nm, so the adsorption capacity of AC-NaOH@PEI-200 is similar for Li⁺ and Cu²⁺. The ionic radius of Cu²⁺ is less than 0.95 for Cd²⁺, and the competition coefficient of Cd²⁺ is relatively tiny in the presence of Cu²⁺. Therefore, it is challenging to remove Cd²⁺ using AC-NaOH@PEI-200 adsorbent when Cu²⁺ is also present in the aqueous solution (Naghizadeh 2015; Deng et al. 2017).

Adsorption capacity is a crucial index to evaluate the performance of materials. Some materials are listed in Table 7. Researcher studied that palm shell AC was modified by PEI as adsorbent to treat Cu²⁺ in waste water and the adsorption capacity reached 25.5 mg/g (Yin et al. 2007). Low-cost and effective AC for Cu²⁺ adsorption was prepared from walnut shell by CO₂ activation in a fluidized bed, and the adsorption capacity reached 32.2 mg/g at 40 min. (Wu et al. 2018). Biochar was modified as a high efficient and selective absorbent for Cu²⁺ by nitration and reduction and the adsorption capacity reached 17.01 mg/g (Yang & Jiang 2014). In this work, the adsorption capacity of AC-NaOH@PEI-200 could reach 47.85 mg/g. AC-NaOH@PEI-200 shows the good adsorption capacity to Cu²⁺, indicating that it has great potential in the field of rapid wastewater treatment.

In this study, GA was used to cross-link linear PEI, and the resulting of AC@PEI composites was used to adsorb and remove Cu²⁺ from wastewater. The specific surface area and pore volume of AC@PEI decreased significantly after surface loading of PEI. However, the Cu²⁺ adsorption capacity of AC-NaOH@PEI increased significantly. This indicates that the specific surface area of AC@PEI is not the decisive factor affecting the adsorption capacity. The surface ζ-potential of AC-NaOH@PEI-200 increased from −27.2 to 12.5 mV, and the surface became more positively charged. The pH value is an important factor affecting the adsorption of Cu²⁺. AC-NaOH@PEI-200 exhibited the most removal of Cu²⁺ in aqueous solutions when the pH was 6. The adsorption capacity of the untreated AC was only 20.02 mg/g, and the adsorption equilibrium required 4 h. The adsorption capacity of AC-NaOH@PEI-200 was increased to 47.8 mg/g and the adsorption equilibrium was shortened to 2 h. The adsorption isotherm data were in good agreement with the Langmuir adsorption equation, and the adsorption kinetics could be better described by a quasi-second order model, indicating that the adsorption of Cu²⁺ by AC-NaOH@PEI-200 involves monomolecular layer adsorption and that this adsorption is a chemical process. XPS analysis demonstrated that -NH₂ and -NH were mainly involved in the adsorption of Cu²⁺. Combined with the cyclic regeneration results of AC-NaOH@PEI-200, it was found that the adsorption of Cu²⁺ on AC@PEI is a nonphysical process. i.e., a strong acid (0.1 mol/L HCl) and complexing solid agent (0.1 mol/L EDTA) are adequate for the regeneration of AC-NaOH@PEI-200, and the removal of Cu²⁺ adsorbed by AC@PEI retained 84% after four regeneration cycles. AC-NaOH@PEI-200 is thus a promising adsorption material that can be commercially mass-produced via a simple modification based on commercial AC.

Ningxia Hui Autonomous Region Key R&D Project 2022ZDYF0189 This work was financially supported by the Industrial Bioprocess Key Materials and Key Technology Innovation Team (No. KJT2019004).

The authors declare that they have no known competing financial interests or personal relationships that could have influenced the work reported in this paper.

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

The authors declare there is no conflict.