The highly efficient simultaneous removal of Pb²⁺ and methylene blue induced by the release of endogenous active sites of montmorillonite

The combined pollution-containing heavy metals and non-biodegradable organic contaminants (HMs-NOCs) are widespread in chemical wastewater such as those in electroplating, coking, dye, and pharmaceutical production. The high mobility and persistence of combined pollution in aquatic environments pose serious threats to both ecology and health (Liu et al. 2017a, 2017b). Since the contaminants are high in toxicity and difficult to degrade, it is difficult for traditional wastewater treatment facilities to implement their simultaneous removal. In comparison to ion exchange, chemical oxidation, flocculation/coagulation, electrochemistry and electrocoagulation, adsorption is considered one of best choices to ensure the synchronous removal of multiple pollutants in wastewater due to low cost, convenient operation and efficient removal rate (Zhang & Hou 2008; Uddin 2017). For multiple pollution systems, it is crucial that the novel adsorption material is applicable on a wide range of pH and characterized by high adsorption capacities and strong selectivity (Jawed et al. 2020).

Montmorillonite is a sandwich-structured clay mineral with high ion exchange capacity, adjustable interlayer spacing, and changeable interlayer environment, which provide nanoscale reaction sites for HMs-NOCs adsorption (Miao et al. 2021). However, the limited interlayer domain of montmorillonite is an unfavorable factor for organic contaminants adsorption (Rathnayake et al. 2015; Zhu et al. 2016). Besides, the inherent periodic structure of montmorillonite increases the energy barrier for the internal active units such as Si-O tetrahedron and Al-O octahedron to undergo chemical reactions with contaminants. Chitosan is a natural adsorbent for anionic organic pollutants and electronegative heavy metal complexes (Pauletto et al. 2020). The downside to chitosan, however, is that its chemical stability is greatly affected by pH and is not easily recovered from solution (dos Santos et al. 2019; Soares et al. 2019). For this reason, carbon-based composite environmental materials are widely used and developed (Roosen et al. 2014; Qiu et al. 2019). Based on these qualities, if the internal active units of montmorillonite and chitosan were to be combined to fabricate a novel adsorptive material, their shortcomings can be dampened, thus benefiting for the synchronous adsorption of both positively and negatively charged contaminants (Zhou et al. 2014; Sun et al. 2016; Kang et al. 2018).

Intercalation assembly and potential-induced lay-by-lay assembly are the key preparation technologies of montmorillonite-chitosan based adsorptive materials. Intercalation assembly takes advantage of amino protonation to develop chitosan molecules intercalated into the montmorillonite interlayer, converting the hydrophilic interlayer environment into an oleophilic environment (Wang & Wang 2007). However, the limited interlayer domain leads to low adsorption capacity of organic contaminants. Through potential-induced lay-by-lay assembly, electropositive chitosan and electronegative montmorillonite can be cross-linked to form 3D network structure, benefiting for microsphere adsorbent fabrication (Chen et al. 2019a, 2019b). It is noteworthy that there is still obvious occupation of active reaction sites on the binding surface between chitosan and mont-morillonite, which is not conducive to their synergistic adsorption performance. Highly ordered mesoporous structure is beneficial to improving the adsorption capacity of target contaminants onto fabricated material due to appropriate pore size, pore volume and large specific surface area (Yang et al. 2017). The potential induced layer-by-layer assembly between chitosan and montmorillonite is a key factor to the pore volume, pore diameter and specific surface area. Functional groups on the surface of microspheres can provide reactivity points for coordination complexation of heavy metal ions and cationic pollutants (Liu et al. 2016, 2018; Meng et al. 2016). However, the selective adsorption mechanism of pollutants by the active units such as Si-O tetrahedron and Al-O octahedron is still unclear.

The aim of this study is to use a high-intensity ultrasound method to release the endogenous active units of montmorillonite and fabricate the carbon@chitosan@montmorilonite adsorbent by lay-by-lay assembly for the simultaneous adsorption of Pb²⁺ and methylene blue (MB). Batch adsorption experiments were carried out to investigate the adsorption competition behavior of Pb²⁺ and MB. The theoretical calculations were used to investigate the change in electron density, differential charge density, bond lengths and bond angles, molecular orbital distribution and adsorption binding energy during the adsorption reaction, revealing the effect of electron transfer direction on the competitive mechanism of binary pollutants.

All experimental chemicals are of analytical grade and used without further purification. Montmorillonite is purchased from Tianjin Damao Technology Co., Ltd. Chitosan, glacial acetic acid, methylene blue (MB), lead nitrate, hydrochloric acid and sodium hydroxide were purchased from Sinopharm Group Chemical Reagent Co., Ltd (Shenyang, China).

A montmorillonite suspension (20wt%) was fabricated by an ultrasonic cell disrupter (1,800 W and 20 kHz) with 80% amplitude for 10 min. Subsequently, the suspension was centrifuged at 12,000 r/min for 10 min to produce the homogeneous supernatant, in the form of water-soluble montomorillonite nanosheets (Ding et al. 2021; Zhang et al. 2021; Zhao et al. 2022).

The preparation strategy of carbon@chitosan@montmorillonite microspheres (C@CS@Mt) is shown in details in Figure S1 (Section 4 of Supplementary Information). In the preparation of C@CS@Mt, carbon microspheres were spherical hard templates with electronegativity, while chitosan acted as a crosslinking agent with electropositivity. Water-soluble montmorillonite nanosheets were electronegative. Through layer by layer assembly method induced by electrostatic adsorption, C@CS@Mt was successfully prepared. The specific preparation procedure of microspheres are given as follows: carbon microspheres are prepared by a hydrothermal synthesis method and used as a hard template for microsphere fabrication (Wang et al. 2021; Zhong et al. 2021; Zhao et al. 2022). Two grams carbon microspheres are added to 100 mL chitosan solution (0.8 wt%) and stirred magnetically for 5 min to prepare a solution containing yolk-shell structured carbon@chitosan microspheres. The montmorillonite nanosheet suspension was subsequently added into this solution dropwise; the C@CS@Mt product was rinsed with deionized water and collected through freeze drying.

The morphology of C@CS@Mt samples was analyzed using scanning electron microscopy (SEM, FEI Quanta25, USA) at an accelerating voltage of 20 kV and transmission electron microscopy (TEM, Tecnai G2 F20 FEI, USA) at an accelerating voltage of 20 kV. The specific surface area of C@CS@Mt samples was characterized by Brunauer-Emmett-Teller (BET, V-Sorb 2800P, GAIC, China). Fourier transform infrared spectrometer (FT-IR, Nicolet iS50, Thermo Scientific, USA) was used to investigate the difference in the functional groups of C@CS@Mt samples. The concentrations of Pb²⁺ and MB were determined by inductively coupled plasma optical emission spectrometer (ICP-OES, PE8300, USA) and ultraviolet visible spectrophotometer (UV-2600, Shimadzu, Japan) at a wavelength of 617 nm, respectively.

The single adsorption experiments are carried out using different concentrations of Pb²⁺ or MB to determine their maximum adsorption capacities. Then, the effect of initial pH, reaction temperature and time of their adsorption behavior were also investigated under the optimal concentration of Pb²⁺ or MB solution. Subsequently, the alternative adsorption experiments between Pb²⁺ and MB were carried out to investigate their competitiveness to active adsorption points.

The alternative adsorption experiments between Pb²⁺ and MB were carried out at the condition of pH = 6, reaction temperature 30 °C, reaction time of 4 h and 400 mg/L of contaminant concentration. The adsorption time of adsorbent in Pb²⁺ solution was 0.5 h, 1.0 h, 1.5 h, 2.0 h, 2.5 h, 3.0 h, and 3.5 h, while the corresponding reaction time of adsorbent in MB solution was 3.5 h, 3.0 h, 2.5 h, 2.0 h, 1.5 h, 1.0 h, and 0.5 h.

Frontier orbital theory and density function theory were adopted to investigate the adsorption mechanism of Pb²⁺ and MB onto C@CS@Mt. The generalized gradient approximation (GGA) with the function of Perdew-Burke-Ernzerhof (PBE) is used to deal with the exchange correlation function to calculate the electron density, differential charge density, bond lengths and bond angles, the molecular orbital distribution and adsorption binding energy. In the following theoretical calculations, Si-O tetrahedron rings and Al-O octahedron rings were selected as active adsorption units (Figure S6, Section 4 of Supplementary Information).

FT-IR patterns (Figure S2, Section 4 of Supplementary Information) indicated that the stretching vibration absorption peak of -NH₂ appeared at 1,655 cm⁻¹, indicating that chitosan was successfully coated on carbon microspheres. The stretching vibration absorption peaks of Si-O and Al-O appeared at 1,012 cm⁻¹ and 802 cm⁻¹ (Goldstein et al. 2008), respectively, implying that silicon/oxygen tetrahedron and aluminum/oxygen octahedron in montmorillonite nanosheet were also successfully implanted on the surface of carbon@chitosan microspheres.

BET results (Table 1) indicated that there was a remarkable increase from 1.67 m²/g to 122.31 m²/g in the specific surface area in the layer-by-layer assembly of microspheres, improving their adsorption capacity. Meanwhile, incrementally the pore volume became larger significantly. When the montmorillonite nanosheets were assembled on the surface of microspheres, their pore volume and pore diameter were increased by 37 times and 2 times, respectively, indicating that montmorillonite nanosheets have good hole-making ability.

In single adsorption experiments of Pb²⁺ or MB, the effect of pH on C@CS@Mt adsorption behavior in the range between 2 and 6 was investigated. Figure 3(b) showed that the adsorption capacity of Pb²⁺ onto C@CS@Mt increased significantly from 223.44 mg·g⁻¹ to 884.19 mg·g⁻¹, indicating that Pb²⁺ adsorption was in a dominant position in the competitive adsorption between H⁺ and Pb²⁺ when pH increased from 2 to 6. Another phenomenon was also found that the adsorption capacity of MB onto C@CS@Mt was 356.21 mg·g⁻¹ when pH = 2. With the increase of pH from 2 to 6, the adsorption capacity of MB decreased by 30 mg·g⁻¹. It was worth noting that the adsorption capacities of Pb²⁺ and MB onto C@CS@Mt were more than 879 mg·g⁻¹ and 325 mg·g⁻¹ under weak acidic conditions in the range of pH from 5 to 6, respectively, indicating that it was a potential activated carbon substitute in the field of industrial wastewater treatment. As shown in Table 2, the absolute adsorption capacity of C@CS@Mt was significantly higher than that of other adsorbents. Therefore, it was demonstrated to be an excellent alternative to activated carbon.

Five temperature levels were selected to investigate their effect on the adsorption capacities of Pb²⁺ and MB onto C@CS@Mt (Figure 3(c)). When the reaction temperature increased from 10 °C to 40 °C, the adsorption capacity of Pb²⁺ and MB increased from 599 mg·g⁻¹ to 859 mg·g⁻¹ and from 289 mg·g⁻¹ to 332 mg·g⁻¹, respectively. It was seen that the adsorption reaction of Pb²⁺ and MB was endothermic. However, when the reaction temperature was higher than 40 °C, there was a pronounced reduction in their adsorption capacities onto C@CS@Mt, implying that the desorption rate of Pb²⁺ and MB was greater than their adsorption rate at higher temperature.

Six levels were selected to investigate the relationship between the reaction time and the adsorption capacities of Pb²⁺ and MB onto microsphere adsorbent (Figure 3(d)). The adsorption capacities of Pb²⁺ and MB exceeded 725 mg·g⁻¹ and 225 mg·g⁻¹, respectively, when the reaction time was less than 1 h, indicating that fast and efficient adsorption had occured. When the reaction time was in the range from 1 h to 6 h, the adsorption capacities of Pb²⁺ and MB increased by 127.26 mg·g⁻¹ and 77.38 mg·g⁻¹, while the percentage of their adsorption increments only were 17.48% and 31.14%. Therefore, in this investigation, C@CS@Mt was more effective against Pb²⁺ than MB.

A Langmuir adsorption model and Freundlich adsorption model were used to investigate the adsorption behavior of Pb²⁺ and MB onto C@CS@Mt. Their linear fitting formulas were given in Section 1 of Supplementary Information. Table S1 and Figure S3 (Section 4 of Supplementary Information) indicated that MB adsorption was better fit with the Langmuir adsorption model due to the higher R² value, indicating physisorption was in a dominant position. Pb²⁺ adsorption was well described by Freundlich adsorption model, implying chemisorption was predominant. The nonlinear models of pseudo-first-order kinetic and pseudo-second-order kinetic were used to investigate the physicochemical process of adsorption reaction and their model formulas were given in Section 2 of Supplementary Information. The comparative analysis of R², theoretical and actual adsorption capacity (Table S2 and Figure S4, Section 4 of Supplementary Information) identified that MB adsorption was well described by pseudo-first-order kinetics model, while a pseudo-second-order kinetics model was more suitable for Pb²⁺ adsorption. Therefore, it can be inferred that MB adsorption was caused by physisorption and Pb²⁺ adsorption depended on chemisorption, consistent with their adsorption kinetics results.

The thermodynamic model formula was given in Section 3 of Supplementary Information. The literature indicated that the value of enthalpy (ΔH) was an important parameter to distinguish physisorption from chemisorption. When ΔH was in the range from 2.1 kJ/mol to 20.9 kJ/mol, physisorption was the driving factor. When ΔH was in the range between 80 kJ/mol to 200 kJ/mol, chemisorption was the driving force. The thermodynamic parameter ΔH of Pb²⁺ adsorption and MB adsorption were 55.1 kJ/mol and 25.91 kJ/mol (Table S3, Section 4 of Supplementary Information), respectively. It can be inferred that both chemisorption and physisorption existed in Pb²⁺ adsorption, in which chemisorption was in a predominant position. MB adsorption was mainly driven by physisorption. The adsorption is a physical and chemical reaction between adsorbent and adsorbate by virtue of ion exchange, bonding effects, electrostatic forces, π-conjugated effect, van der Waals forces and hydrogen bonds (Miao et al. 2021). In the view of chemical compositions, ion exchange, electrostatic forces and van der Waals forces are the main role in the adsorption reaction. In short, Pb²⁺ adsorption was mainly caused by ion exchange and electrostatic forces, while MB adsorption originated from electrostatics and van der Waals forces. However, the simultaneous adsorption mechanisms of two pollutants is still unclear, and it is necessary to carry out molecular theoretical calculation to reveal the competitive mechanism.

In our preliminary experiments, four adsorbents like carbon microspheres, carbon@chitosan microspheres (CS), montmorillonite (Mt) and C@CS@Mt were selected to carry out the adsorption of MB and Pb²⁺ to evaluate their adsorption performance. The adsorption capacities of C@CS@Mt towards MB and Pb²⁺ were 8.85, 5.53, 1.40 times and 13.35, 10.30, 6.53 times higher than that of carbon microspheres, C@CS, Mt (Figure S8 of Supporting Information). Obviously, the significant increase in adsorption capacity of C@CS@Mt originated from the endogenous active unit (Si-O tetrahedron and Al-O octahedron) of montmorillonite located on the outermost layer of adsorbent. Therefore, we focused on the interaction between adsorbents and the endogenous active units of montmorillonite.

Differential charge density image indicated that the charge transfer between MB and Al-O octahedron rings was not observed (Figure S7c, Section 4 of Supplementary Information), implying that electrostatic attraction was the driving force of adsorption reaction. Furthermore, there was obvious charge transfer from the Si-O bond located at the upper right side of Si-O tetrahedron to the left side benzene ring of MB (Figure S7d, Section 4 of Supplementary Information), which was ascribed to cation-π interaction (Maiyelvaganan et al. 2019; Yi et al. 2020). In binary adsorption reaction, the charge transfer in Si-O + Pb/MB system was stronger than that of Si-O + MB/Pb system (Figure 5(b)), indicating that the groups on the surface of Si-O tetrahedron rings could more easily accept electrons from Pb, sharing lone pairs to produce a coordination effect. Compared Al-O + Pb/MB adsorption system and Al-O + MB/Pb adsorption system, it was found that there was almost no charge transfer between Pb and Al-O + MB system when MB was preferentially combined with Al-O octahedron rings, implying that the combination force between Al-O octahedron rings and MB was stronger due to cation-π interaction.

In adsorption reaction, the adsorbent combined with adsorbate to become a new composite system, existing in the lowest energy configuration structure. Figure 5(c) indicated that Al-O octahedron rings presented obvious bending and folding when Al-O + Pb complex was bound to MB. The distance from O(1) to O(2) decreased from 5.749 Å to 5.437 Å, but the angle of Al (1)-O(3)-O(4) increases significantly from 99.459° to 131.075°. When Al-O + MB complex was bound to Pb, there was no obvious bending or folding. Although there was a slight increase in the angle of Al (2)-O(5)-Al(3) from 99.459° to 102.088°, the distance from O(6) to O(7) decreased remarkably from 5.749 Å to 4.711 Å. The similar changes were also observed in the combination reaction of Si-O tetrahedron ring bound to Pb and MB (Lan et al. 2019). It was known from bond lengths and bond angles that Pb adsorption caused a significant change in bond angle, whereas MB adsorption had little effect on the structure of active adsorption units.

Highest occupied molecular orbitals (HOMOs) and Lowest unoccupied molecular orbitals (LUMOs) describe the ability of active sites in molecules to donate and accept electrons. Figure S7 (Section 4 of Supplementary Information) indicated that HOMOs were mainly located on O atoms of Al-O octahedron rings in Al-O + Pb complex, whereas LUMOs were distributed on Pb atoms, suggesting that the electron transfer direction was from Al-O octahedron to Pb ion. Unlike Al-O + Pb complex, the electron transfer direction was from Pb to Si-O tetrahedron in Si-O + Pb complex. In Si-O + MB/Pb reaction system (Figure 5(d)), HOMOs were mainly distributed on Pb and Si-O tetrahedron, whereas LUMOs were mainly located on benzene ring of MB. It was obvious that both Pb and Si-O tetrahedron donated electrons to MB, which was ascribed to cation-π interaction. Furthermore, Si and O atoms of Si-O tetrahedron, Al and O atoms of Al-O octahedron could provide coordination electron pairs for Pb and coordination sites for MB, benefiting the co-adsorption of multi-contaminants. However, when Al-O octahedron rings preferentially react with MB to form Al-O + MB complex, driven by electrostatic forces, the coordination sites of Al-O octahedron tend to saturate, in order that there was no more active sites for Pb adsorption (Figure 5(d) and 5(e)).

The adsorption binding energies of Si-O tetrahedron rings towards Pb/MB and MB/Pb were 35.5332 ev and 34.7908 ev lower than that of Al-O octahedron rings (Table S4, Section 4 of Supplementary Information), implying that Si-O tetrahedron was more prone to bind with target contaminants. In comparison to Al-O + Pb/MB and Si-O + Pb/MB, the adsorption binding energies of Al-O + MB/Pb and Si-O + MB/Pb increased by 2.2805 eV and 3.0229 eV, respectively (Table 3). Pb atoms donated electrons to Al-O + MB complex and Si-O + MB complex, producing chemical reactions and causing an increasing in the instability of the multisystem (Yao et al. 2015; Khorram et al. 2017).

The layer-by-layer assembly was successfully used to prepare C@CS@Mt microsphere adsorbent from carbon microspheres, chitosan and montmorillonite nanosheets. BET results indicated that montmorillonite nanosheets were beneficial to improving the specific surface area, pore volume and pore diameter of microspheres due to good hole-making ability. The maximum adsorption capacities of Pb²⁺ and MB onto C@CS@Mt were 884.19 mg·g⁻¹ and 326.21 mg·g⁻¹, respectively, which was significantly better than activated carbon for wastewater treatment. The fitting calculation of adsorption isotherms, kinetics and thermodynamics indicated that chemisorption was dominant in Pb²⁺ adsorption, while physisorption was the driving factor of MB adsorption. The theoretical calculations indicated that Pb was preferentially adsorbed by Si-O tetrahedron and Al-O octahedron due to donating electrons. Furthermore, Si-O tetrahedron rings were more prone to react with cationic pollutants than Al-O octahedron rings due to the higher absolute value of adsorption binding energy.

This work was financially supported by the Liaoning revitalization talents program (No. XLYC1807045).

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

The authors declare there is no conflict.