Synthesis of sodium polyacrylate–bentonite using in situ polymerization for Pb²⁺ removal from aqueous solutions

Abstract

Polymer/clay composites have attracted a great deal of interest because of their wide applications in environmental protection. In this study, a sodium polyacrylate–bentonite material (PAANa–Bent), as an adsorbent for heavy metal ions, is synthesized for the first time using in situ polymerization. A series of analytical methods including FTIR, zeta potential measurements, SEM, TG/DTA, DSC, a N₂ adsorption–desorption isotherm study and XRD were used to characterize PAANa–Bent. Furthermore, to evaluate the heavy metal ion adsorption capability of PAANa–Bent, batch experiments were conducted using Pb²⁺ as an adsorbate. The results demonstrate that PAANa–Bent was prepared successfully and the layered structure of bentonite was protected during polymerization. PAANa–Bent has more negative surface charges and a better dispersity than the original bentonite (O-Bent). The maximum adsorption capacity of PAANa–Bent is 70.41 mg g⁻¹ at pH = 6, t = 300 min and T = 25 °C, an increase of 113% compared with O-Bent. A kinetic study shows that the adsorption process of PAANa–Bent obeys a pseudo-second-order model and the best-fit adsorption isotherm is a Freundlich isotherm, indicating the heterogeneous nature of PAANa–Bent. Besides, desorption experiments suggest that the PAANa–Bent can be regenerated easily with 0.1 M HCl as a stripping agent and the desorption ratio was still more than 91% after four cycles. Overall, the results indicate the potential of PAANa–Bent as a low-cost and highly efficient adsorbent for Pb²⁺ removal from aqueous solutions.

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1. Introduction

Contamination with heavy metal ions is one source of environmental pollution and heavy metal ions are hazardous even at very low concentration. Heavy metal ions come from various industrial sources and are harmful to human health because they are non-biodegradable and bio-accumulate in the body.¹ For example, excessive Cu²⁺ will cause a metallic taste in the mouth, stomach ulcers and mental retardation.² Cr⁶⁺ will cause severe diarrhea, pulmonary congestion, and liver and kidney damage.³ The toxic effects of Ni include bone, nose and lung cancer, chest pains and so on.⁴ Pb²⁺ can’t be metabolized by the body and eventually is a cumulative poison, which damages the kidneys, liver, reproductive system, basic cellular processes and brain function. Pb²⁺ is especially hazardous to fetuses, babies and young children, as it impedes their normal mental and physical development.^5,6 Therefore, the elimination of heavy metals from water is very important for public health and extensive research has been carried out concerning heavy metal ion removal from contaminated water.⁷

Currently, the main methods used to remove heavy metal ions from water include chemical precipitation and filtration⁸ (hydroxides, sulfides, etc.), membrane technologies^9–11 (reverse osmosis, nanofiltration, etc.), electrolysis,^12–14 adsorption^15,16 and ion-exchange.^17,18 Among these techniques, adsorption has been proven to be an economical and effective method, owing to the flexibility in design and operation, high efficiency and low cost.^19,20 In addition, the used adsorbent can be regenerated by a simple desorption process.²¹ The main adsorbents employed for heavy metal ion adsorption include activated carbon,²² synthetic polymers²³ and silica-based adsorbents,²⁴ though these adsorbents haven’t been widely employed in most industries due to developing nations being affected by their high cost.²

In recent years, bentonite has attracted enormous attention due to its abundance, low cost,^25–27 and unique cation exchange capacity (CEC), arising from isomorphous substitution (e.g., Mg²⁺ for Al³⁺ in the octahedral sheet, or Fe³⁺ or Al³⁺ for Si⁴⁺ in the tetrahedral sheet) in the montmorillonite layers.²⁸ Bentonite is a clay-based natural material and the major component is mineral montmorillonite, a 2 : 1 layered clay consisting of an octahedral alumina (AlO₆) sheet sandwiched between two tetrahedral silica (SiO₄) sheets.^29,30 There are numerous terminal –OH groups on the crystal edges of the aluminate sheets, which provide anchoring sites for modification.³¹ Thereby, these advantages make bentonite a widely used adsorbent in the field of environmental remediation, such as in the fate and transport of heavy metal ions,^32,33 dyes^34,35 and other inorganic or organic pollutants.^36,37 However, the original bentonite always presents some drawbacks such as the aggregation of bentonite particles and low adsorption efficiency.^38,39 Therefore, some modifications have been performed to improve the dispersibility and adsorption capacity of bentonite. Recently, the main modifications of bentonite have included forming (i) pillared clays,⁴⁰ (ii) organoclays,^41,42 and (iii) polymer/clay composites.⁴³ Among all the approaches, the use of polymer/clay composites has been found to be one of the most effective methods, owing to a variety of possible polymerization methods, the controllable degree of the polymerization and the numerous functional groups present in the molecular chains.^44,45 For example, Emmanuel I. Unuabonah et al.⁴⁶ prepared a polyvinyl alcohol–clay composite used to remove Pb²⁺ from an aqueous solution; Xiaohuan Wang et al.⁴⁷ prepared a novel chitosan–poly(vinyl alcohol)/bentonite (CTS–PVA/BT) composite for adsorption of Hg(II) and the CTS–PVA/BT showed a high adsorption capacity for Hg(II); Li et al.⁴⁸ prepared a cationic-polymer/bentonite (EPI-DMA/bentonite) composite for the removal of non-ionic and anionic dyes. However these traditional polymer/clay composites are unstable and unbind, because the bonding mechanism of the polymer/clay composites is only physical adsorption between the polymer and clay.^49,50 In addition, the layered structure of bentonite will be destroyed due to polymerization of monomers in the interlayers.⁵¹ To date, there isn’t yet a report about the modification of clay using polymers, through covalent bonds, where at the same time the layered structure of bentonite is protected.

In this work, in order to prepare a layered covalently-bonded polymer–clay with a high adsorption capability for heavy metal ions, a sodium polyacrylate–bentonite (PAANa–Bent) material is synthesized for the first time using in situ polymerization on the surface of the bentonite through covalent bonds. The use of sodium acrylate can efficiently avoid the structural damage to bentonite caused by polymerization in the interlayers because sodium acrylate is an anion in water and this will stop it entering into the electronegative interlayer spaces arising from isomorphous substitution. Meanwhile, the large number of carboxyl groups provides new adsorption sites for Pb²⁺. The prepared sodium polyacrylate–bentonite is characterized using Fourier transform infrared spectroscopy (FTIR), zeta potential measurements, scanning electron microscopy,⁵¹ thermogravimetric analysis (TG/DTA), differential scanning calorimetry (DSC), N₂ adsorption/desorption experiments and X-ray diffraction (XRD). Lastly, Pb²⁺ is chosen as an adsorbate to evaluate the heavy metal ion adsorption capability of PAANa–Bent and the adsorption mechanism of Pb²⁺ with PAANa–Bent is also investigated.

2. Experimental

2.1. Materials and chemicals

The original bentonite (O-Bent) used in this study was purchased from Weifang Huawei Bentonite Co., Ltd. (Weifang, Shandong Province, China). (3-Aminopropyl)trimethoxysilane (APTS), acryloyl chloride, triethylamine, sodium acrylate and potassium persulfate (K₂S₂O₈) were all purchased from Aladdin Industrial Corporation. Pb(NO₃)₂, used as a Pb²⁺ source, was provided by Sinopharm Chemical Reagent Co., Ltd (China). All chemicals were analytical grade and used as-received. Deionized water was used in the preparation of all the solutions.

2.2. Preparation of sodium polyacrylate–bentonite

The synthetic route is reported in Fig. 1. Firstly, APTS (2 g) dissolved in 200 mL of ethyl alcohol/H₂O (3 : 1, v/v) was anchored onto bentonite (4 g) over 8 h at 60 °C and the resulting intermediate was filtered, washed with deionized water and dried at 60 °C under vacuum (denoted as NH₂–Bent). Next, 3 g of NH₂–Bent and 2.5 g of triethylamine were combined in 100 mL of CH₂Cl₂ and the mixture was cooled to 0 °C. To this solution, 7 mL of acryloyl chloride diluted in 100 mL of CH₂Cl₂ was added dropwise, under cooling in an ice bath. The reaction was allowed to warm to room temperature overnight. After being filtered and washed with CH₂Cl₂, the product was designated as CH₂=CH–Bent. The bentonite precursor with reactive carbon–carbon double bonds had been prepared. Then, 1 g of CH₂=CH–Bent and 0.8 g of K₂S₂O₈ were dispersed in 100 mL of deionized water and a 95 mL aqueous solution of sodium acrylate (5 wt%) was added dropwise. The polymerization reaction was conducted for 4 h at 60 °C under a N₂ atmosphere. The product was filtered and washed with a Soxhlet extractor to remove any sodium polyacrylate that wasn’t bonded to the surface of the bentonite. The resultant product was named PAANa–Bent, after it was vacuum dried at 60 °C and ground.

Fig. 1 Preparation of PAANa–Bent.

2.3. Characterization

2.3.1. FTIR spectroscopy. The FTIR spectra were obtained using a PE Spectrum One spectrometer at 450–4000 cm⁻¹ (32 scans of accumulation, spectral resolution of 4 cm⁻¹). The samples were mixed with KBr (5 wt%) and pressed into a pellet for measurement.

2.3.2. Zeta potential study. Zeta potentials of the O-Bent and PAANa–Bent particles in water were determined as a function of pH (2–10), adjusted with 0.1 M HCl and 0.1 M NaOH, with a Zetasizer Nanosystem (JS94H) from Shanghai POWEREACH Inc. (Shanghai China). Suspensions of different pH values were obtained by adjusting with 0.1 M HCl and 0.1 M NaOH. The suspensions were prepared by dispersing 0.05 g of O-Bent and PAANa–Bent in 40 mL of deionized water respectively. Zeta potential experiments were run in triplicate for every pH (2–10) and the average values are presented.

2.3.3. SEM measurements. The dispersity and structures of O-Bent and PAANa–Bent were examined using SEM (JSM-6700F, JEOL Ltd.). The samples were dispersed in ethyl alcohol and then the sediments were dried at 50 °C before gold coating.

2.3.4. TG/DTA and DSC measurements. The TG/DTA was carried out in an air atmosphere (100 mL min⁻¹) with a 10 °C min⁻¹ heating rate using a thermal analyzer (PerkinElmer, USA) and the temperature ranged from 30 to 800 °C. DSC was performed using a TGA/DSC1/1600HT METTLER from 30 to 800 °C at a scanning rate of 10 °C min⁻¹ under an air atmosphere.

2.3.5. N₂ adsorption–desorption isotherm study. The porosity of O-Bent and PAANa–Bent was studied using N₂ adsorption–desorption isotherms at −196 °C with a Tristar II 3020 (Micromeritics Inc., USA). Prior to the measurements, the samples were outgassed under flowing nitrogen at 150 °C for 10 h. The specific surface area (S_BET) was calculated according to the Brunauer Emmett and Teller method at P/P₀ = 0.22 and the total pore volume (V_0.99) was evaluated at P/P₀ = 0.99. The volumes of the micropores (V_micro) and mesopores (V_meso) were calculated based on a t-plot method and the Barrett, Joyner, and Halenda method from the desorption branch of the isotherm, respectively.

2.3.6. X-ray diffraction. XRD patterns were recorded using a BRUKER AXS D 8 Focus X-ray diffractometer (λ_{CuK α1} = 1.540598 Å, 40 kV, 40 mA, 2–25° scan range, step size 0.02° 2θ, 0.2 s per step).

2.4. Batch adsorption experiments

The adsorption of Pb²⁺ onto O-Bent and PAANa–Bent was studied using batch adsorption experiments. The effects of dominating experimental variables, including the contact time (t), pH and initial concentration (c_i) of Pb²⁺, were determined. The effect of the contact time was studied for a range of 30–720 min by adding 0.05 g of O-Bent or PAANa–Bent to 40 mL of a Pb²⁺ solution, at a solution concentration of 100 mg L⁻¹, pH = 6 and at a room temperature of 25 °C. The effect of pH was determined for pH values of 2–6 by adding 0.05 g of O-Bent or PAANa–Bent to 40 mL of a Pb²⁺ solution, at a solution concentration of 100 mg L⁻¹, contact time of 300 min and at a room temperature of 25 °C. The effect of the initial Pb²⁺ concentration was determined for a concentration range of 20–300 mg L⁻¹ by adding 0.05 g of O-Bent or PAANa–Bent to 40 mL of a Pb²⁺ solution, with a contact time of 300 min, pH = 6 and at a room temperature of 25 °C. The detailed batch adsorption experimental conditions are listed in Table 1.

Table 1 Batch adsorption experiment conditions (m_a, V_s, T, S representing respectively the mass of adsorbent, volume of solution, experimental temperature and rotational speed)

Adsorbent	t (min)	pH	ci (mg L^−1)	ma (g)	Vs (mL)	T (°C)	S (rpm)
PAANa–Bent	30–720	6	100	0.05	40	25	270
(O-Bent)	300	2–6	100	0.05	40	25	270
	300	6	20–300	0.05	40	25	270

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Each experiment was performed in triplicate and the mean values were computed to ensure quality assurance. All batch adsorption experiments were conducted in a temperature controlled water bath shaker (Dong Peng SHA-C). The adsorbed amount of Pb²⁺ with PAANa–Bent was calculated using the following equation:

where c_e is the equilibrium concentration of Pb²⁺ in the solution (mg L⁻¹), V is the volume of the solution (L), and m represents the adsorbent mass (g). The c_e was obtained by utilizing a PE 900T atomic absorption spectrometer to detect the concentration of Pb²⁺ in the supernatant after the Pb²⁺ aqueous solution with the adsorbent was centrifuged. The flame type is air–acetylene and the Pb²⁺ absorption wavelength is 283.3 nm.

3. Results and discussion

3.1. Characterization

3.1.1. FTIR analysis. The FTIR spectra are shown in Fig. 2. It is observed that the FTIR spectra of O-Bent, NH₂–Bent, CH₂=CH–Bent and PAANa–Bent display a typical ν_–OH (Al–OH) at 3632 cm⁻¹ and ν_–OH (H–OH) at 3428 cm⁻¹.⁵² The band at 1638 cm⁻¹ corresponds to the deformation vibration of H–OH.⁵³ The intensive band around 1040 cm⁻¹ is attributed to ν_Si–O. The band at 920 cm⁻¹ represents ν_Al–O.⁵⁴ The peak at 798 cm⁻¹ is a result of Fe–O stretching vibrations.⁵⁵ The deformation of Si–O appears at 624 cm⁻¹.⁵⁶ The bands at 524 cm⁻¹ are ascribed to the Si–O–Al and Si–O–Si bending vibrations.⁵⁷ The bands of NH₂–Bent, CH₂=CH–Bent and PAANa–Bent at 2935 cm⁻¹ correspond to the asymmetric stretching of –CH₂– after modification. The band at 3290 cm⁻¹ of CH₂=CH–Bent is caused by C=C–H stretching vibrations. The spectra of PAANa–Bent with bands at 1590 and 1409 cm⁻¹ are indicative of COO⁻ asymmetric and symmetric stretching, respectively.

Fig. 2 FTIR spectra of O-Bent, NH₂–Bent, CH₂=CH–Bent and PAANa–Bent.

3.1.2. Zeta potential analysis. Zeta potential can reflect the surface charge of samples, and the change of the zeta potential with variation of the pH is displayed in Fig. 3. As observed, the zeta potentials of both O-Bent and PAANa–Bent all decrease with the increase of pH and then rise slightly at the end. The zeta potential of O-Bent decreases from −4.9 (pH 2) to −28.5 (pH 5) and then increases slightly thereafter. The transition in the surface charge of O-Bent can be identified as due to the effect of deprotonation of terminal silanol and aluminol groups that are present along the crystal edges of the clay platelets. The zeta potential increases slightly in the pH range of 5–10 due to the thickness of the diffusion layer being compressed with the increasing concentration of electrolyte according to DLVO theory. Similarly, the zeta potential of PAANa–Bent decreases sharply from −3.6 (pH 2) to −50.1 (pH 7) because of deprotonation to form –COO⁻ groups on the surface of PAANa–Bent and increases mildly thereafter for the same reason as for O-Bent. However, the PAANa–Bent possesses more negative surface charges than O-Bent owing to the existence of plentiful –COO⁻ groups on the surface through modification.

Fig. 3 Zeta potentials of O-Bent and PAANa–Bent aqueous suspensions as a function of pH.

3.1.3. SEM analysis. The SEM images of O-Bent and PAANa–Bent are shown in Fig. 4. It can be seen that the PAANa–Bent possesses better dispersity and the average size of the lamellas is approximately 1–1.5 μm. However, a serious aggregation of the original bentonite lamellas was observed in the SEM image of O-Bent. The better dispersity and smaller lamella average size of PAANa–Bent are because the PAANa–Bent lamellas possess stronger electrostatic repulsion. The results correspond well to the zeta potential analysis. In addition, the SEM image of PAANa–Bent shows a lamellar structure, which confirms that exfoliation of the bentonite did not occur and the layered structure of bentonite was protected during the process of polymerization.

Fig. 4 SEM images of O-Bent and PAANa–Bent.

3.1.4. TG/DTA and DSC analysis. TG/DTA tests were carried out to investigate the thermal stability and the content of polymer anchored on the surface of the bentonite. The TG/DTA results of O-Bent and PAANa–Bent are presented in Fig. 5. As observed, PAANa–Bent shows a bigger mass loss and a unique exothermic peak compared to O-Bent. The curve of O-Bent reveals a two-step mass loss, occurring in the ranges of 60–180 °C and 600–700 °C. In the first step, the mass loss of 4% can be attributed to evaporation of physically adsorbed water on the surface of the bentonite (60–110 °C) and the loss of structural water in the interlayers (110–180 °C). The second step mass loss (2%) at 600–700 °C is due to dehydroxylation of the aluminosilicate layers. In the case of PAANa–Bent, the TG trace exhibits a three-step mass loss. The elimination of water from the surface and interlayers causes a 5% mass loss, occurring at 50–150 °C. The mass loss of 10% at 250–630 °C results from oxidation of the polymer. The mass loss of 2% from 600 to 700 °C is associated with the dehydroxylation. In terms of DTA measurements, the curve of PAANa–Bent shows a very large exothermic peak from 250 to 630 °C, corresponding to oxidation of the polymer. The DSC curves of O-Bent and PAANa–Bent are shown in Fig. 6. Two endothermic peaks for O-Bent are observed. The 112 °C peak is due to the evaporation of physically adsorbed water on the surface of the bentonite and the loss of structural water in the interlayers. The 640 °C peak is attributed to dehydroxylation of the aluminosilicate layers. Compared to O-Bent, the 92 °C peak of PAANa–Bent is caused by elimination of water from the surface and interlayers. The obvious exothermal peak at 395 °C is caused by oxidation of a polymer, verifying the existence of polymer on the PAANa–Bent surface. The results of the DSC are in good agreement with the results of TG/DTA.

Fig. 5 TG/DTA curves of O-Bent and PAANa–Bent.

Fig. 6 DSC curves of O-Bent and PAANa–Bent.

3.1.5. N₂ adsorption–desorption isotherm analysis. Fig. 7 shows the typical nitrogen adsorption–desorption isotherms of O-Bent and PAANa–Bent. In Fig. 7, hysteresis loops are observed in the nitrogen adsorption–desorption curves of O-Bent and PAANa–Bent. Both of them feature an isotherm of type IV according to the IUPAC classification, proving a mesoporous structure of samples. The textural properties of the two samples are presented in Table 2. It is apparent that the values for these main properties of O-Bent, including S_BET, V_0.99, V_micro and V_meso, are larger than for PAANa–Bent. The decrease in S_BET and pore volume after polymerization is attributed to the pore blocking effect. In other words, a part of the pores in PAANa–Bent is occupied by the sodium polyacrylate anchored on the surface of the bentonite. The same result was also observed by X. Wang et al.⁴⁷

Fig. 7 N₂ adsorption–desorption isotherms of O-Bent and PAANa–Bent.

Table 2 The textural properties of O-Bent and PAANa–Bent

Sample	O-Bent	PAANa–Bent
SBET (m^2 g^−1)	16.337 ± 0.042	5.891 ± 0.096
V0.99 (cm^3 g^−1)	0.063 ± 8.165 × 10^−4	0.030 ± 3.37 × 10^−4
Vmicro (cm^3 g^−1)	0.005 ± 1.715 × 10^−5	0.000
Vmeso (cm^3 g^−1)	0.062 ± 5.307 × 10^−4	0.029 ± 7.92 × 10^−5

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3.1.6. XRD analysis. The position of the 001 reflection of bentonite in XRD patterns can be used to identify the basal spacing. XRD patterns of O-Bent, NH₂–Bent and PAANa–Bent are presented in Fig. 8. Compared with O-Bent, the 001 reflection of NH₂–Bent shifts towards the left, indicating an increase from 1.38 nm to 1.71 nm for the basal spacing after amination of the bentonite. This is because the remaining APTS molecules intercalate into the gallery of bentonite and enlarge the basal spacing. A similar result was also observed by Filomena et al.⁵⁸ However, there is no significant change in the basal spacing between NH₂–Bent (1.71 nm) and PAANa–Bent (1.73 nm), demonstrating that few polymers intercalated into the bentonite gallery. The result can be attributed to the negative charge of the acrylate stopping it from entering into the electronegative interlayer spaces of the bentonite and thereby avoiding destruction of the layered structure caused by polymerization in the interlayers.

Fig. 8 XRD patterns of O-Bent, NH₂–Bent and PAANa–Bent.

3.2. Adsorption study of Pb²⁺

3.2.1. Effect of contact time and a kinetic study. The effect of the contact time on the adsorption of Pb²⁺ onto O-Bent and PAANa–Bent is presented in Fig. 9. As observed, the amount of Pb²⁺ adsorbed by PAANa–Bent and O-Bent increases rapidly in the first 300 min, beyond which there is hardly any further increase of the adsorption capacity. This is because the number of free adsorption sites on PAANa–Bent and O-Bent will become less and less with the increase of time. Therefore, 300 min is chosen as the equilibrium contact time. In addition, the kinetic data were fitted with pseudo-first-order and pseudo-second-order kinetic models. The pseudo-first-order kinetics model is based on the assumption that the adsorption rate is proportional to the number of unoccupied adsorption sites and the rate equation is

Fig. 9 Effect of contact time on the removal of Pb²⁺.

Eqn (2) was integrated with the boundaries of t = 0 to t = t and q = 0 to q = q, giving

where q_e is the amount of adsorbed Pb²⁺ at equilibrium (mg g⁻¹), q_t is the amount at a predetermined time (mg g⁻¹) and k₁ (min⁻¹) represents the rate constants.

For the pseudo-second-order model, it is considered that the adsorption rate is proportional to the square of the number of unoccupied adsorption sites, and the rate equation is

Eqn (4) was integrated with the boundaries of t = 0 to t = t and q = 0 to q = q, giving

where q_e is the amount of adsorbed Pb²⁺ at equilibrium (mg g⁻¹), q_t is the amount at a predetermined time (mg g⁻¹) and k₂ (g mg⁻¹ min⁻¹) represents the rate constants.

The values of the calculated kinetic data are shown in Fig. 10 and listed in Table 3, together with the corresponding correlation coefficients R² and experimental equilibrium amounts q^exp_e. Compared with the pseudo-first-order model, the pseudo-second-order model with higher correlation coefficients R² is considered more appropriate for depicting the adsorption processes of O-Bent and PAANa–Bent. Besides, q^exp_e and q_e obtained from the pseudo-second model show excellent agreement.

Fig. 10 Different kinetic plots for the adsorption of Pb²⁺ onto O-Bent and PAANa–Bent.

Table 3 Calculated kinetic model constants

Adsorbent	q^expe (mg g^−1)	Pseudo-first-order			Pseudo-second-order
		qe (mg g^−1)	k1 (min^−1)	R^2	qe (mg g^−1)	k2 (g mg^−1 min^−1)	R^2
O-Bent	33.07	14.43	7.28 × 10^−2	0.83	33.47	4.63 × 10^−3	0.99
PAANa–Bent	70.41	143.71	4.17 × 10^−3	0.97	70.87	1.94 × 10^−3	0.99

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3.2.2. Effect of pH. The pH of an aqueous solution has been known as the most important variable governing heavy metal adsorption onto an adsorbent. The range of pH tested is limited to 2–6 to prevent the hydrolysis of Pb²⁺ (at pH higher than 7) covering up the real cation-exchange capabilities of O-Bent and PAANa–Bent. As shown in Fig. 11, q_e increases sharply with the pH variation from 2 to 4 and there is not an obvious rise after pH 4. This is due to the fact that the surface charge of the adsorbent changes with the pH variation and the competition between H⁺ and Pb²⁺. The q_e of O-Bent increases because less H⁺ competes with Pb²⁺ in the interlayers from pH 2 to 4. However, in the case of PAANa–Bent, the surface of PAANa–Bent is protonated and more H⁺ competes with Pb²⁺ in the interlayers at low pH, as shown in Fig. 12(B). With the increasing pH, the surface of PAANa–Bent becomes more negative and less H⁺ competes with Pb²⁺ in the interlayers, causing q_e to increase as shown in Fig. 12(A). The results correspond well to the results of the zeta potential analysis (Section 3.1.2.).

Fig. 11 Effect of pH on the removal of Pb²⁺.

Fig. 12 Modeling of the adsorption of Pb²⁺ onto unprotonated (A) and protonated (B) PAANa–Bent.

3.2.3. Effect of the Pb²⁺ initial concentration. The effect of the Pb²⁺ initial concentration was studied for a range of 20–300 mg L⁻¹ and the results are presented in Fig. 13. As observed, the q_e of O-Bent increases from 13.64 mg g⁻¹ to 34.66 mg g⁻¹ and the q_e of PAANa–Bent increases from 15.31 mg g⁻¹ to 70.86 mg g⁻¹ for Pb²⁺ initial concentrations of 20–100 mg L⁻¹. At higher concentrations of 100–300 mg L⁻¹, there isn’t any evident increase in q_e. This is because adsorption is a dynamic equilibrium process and a higher initial concentration favors the adsorption of Pb²⁺, however, once the adsorbent becomes saturated with the increase of the Pb²⁺ concentration, no further increase in adsorption is observed. This trend indicates that a saturated adsorption is obtained on the surface of PAANa–Bent when the initial concentration of Pb²⁺ exceeds 100 mg L⁻¹ (c_i ≥ 100 mg L⁻¹).

Fig. 13 Effect of the Pb²⁺ initial concentration on the removal of Pb²⁺.

3.2.4. Adsorption isotherm. An adsorption isotherm can provide vital information on the capability of an adsorbent to remove hazardous species and the basic data for evaluating the adsorption process. In this regard, Langmuir (eqn (6)) and Freundlich (eqn (7)) isotherms, as the two most widely applied isotherm models, were applied to the experimental data. The Langmuir isotherm was initially used for the adsorption of gas molecules onto metal surfaces and was later successfully used to describe solid–liquid adsorption systems. The Langmuir isotherm is valid for monolayer adsorption onto a surface containing a finite number of identical adsorption sites.⁵⁹ The Freundlich isotherm is applied for multilayer adsorption of adsorbates with a heterogeneous surface and a non-uniform distribution of adsorption-heat.⁶⁰ Their linear forms are as follows:where c_e and q_e represent the equilibrium concentration of adsorbate in the solution and the equilibrium adsorption capability of the adsorbent, respectively. q_m is the maximum adsorption capability in theory. K_L is the Langmuir constant, representing the intensity of the adsorption. K_F is a partition coefficient that relates to the energy of adsorption and n is a dimensionless constant related to the adsorption intensity of the adsorbent.

The linear simulations are shown in Fig. 14 and their parameters are listed in Table 4. In the case of O-Bent, the adsorption data fit well to the Langmuir adsorption isotherm compared to the Freundlich adsorption isotherm, based on the higher correlation coefficient R². In addition, the calculated q_m (37.23 mg g⁻¹) is in agreement with the q^exp_m (33.07 mg g⁻¹). This suggests that a monolayer adsorption of Pb²⁺ takes place and that there is only one type of adsorption site in O-Bent, i.e. a cation exchange capacity in the interlayers. However, the Freundlich adsorption isotherm is more suitable for PAANa–Bent, according to the higher correlation coefficient R². This confirms heterogeneous surfaces (more than one type of binding site) of PAANa–Bent after modification. This is because there are two types of adsorption sites coexisting in PAANa–Bent, i.e. –COO⁻ groups on the surface of PAANa–Bent and a cation exchange capacity in the interlayers. The high K_F (16.80) indicates a high adsorption capability and that adsorption takes place on heterogeneous surfaces.^59,61 The value of n (1.68) lies between 1 and 10, indicating a favorable adsorption of Pb²⁺ using PAANa–Bent.¹

Fig. 14 Different isotherm plots for the adsorption of Pb²⁺ onto O-Bent and PAANa–Bent.

Table 4 Langmuir and Freundlich isotherm constants

Adsorbent	Langmuir			Freundlich
	KL (L mg^−1)	qm (mg L^−1)	R^2	KF (mg g^−1)	n (L mg^−1)	R^2
O-Bent	0.17	37.23	0.99	10.75	3.33	0.94
PAANa–Bent	0.18	94.61	0.91	16.80	1.68	0.98

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3.3. Regeneration

The reuse of adsorbents helps to minimize the cost of the adsorption process. Thus, for an efficient adsorbent, a good desorption potential is important. Desorption experiments were performed with a 0.1 M HCl solution as the stripping agent. Firstly, 0.05 g of O-Bent or PAANa–Bent was added into 40 mL of a Pb²⁺ aqueous solution (100 mg L⁻¹) and shaken for 300 min at 25 °C, pH = 6. After the first adsorption, the adsorbent was regenerated using 100 mL of 0.1 M HCl for 2 h at 25 °C. The adsorbent after elution was washed to neutral with deionized water and used for the next adsorption cycle. The adsorption–desorption cycle was repeated 4 times to assess the reusability of O-Bent and PAANa–Bent. The desorption experiments were performed in triplicate and the mean values are presented. The desorption ratio was calculated using the equation:

Desorption ratio = V_DC_D/q_e (8)

where V_D is the volume of the desorption solution; C_D is the Pb²⁺ concentration in the desorption solution; q_e is the amount of Pb²⁺ adsorbed by the adsorbent in the last adsorption.

Fig. 15 shows the performance of O-Bent and PAANa–Bent after regeneration. As observed, O-Bent shows an excellent reusability and the desorption ratios for the four cycles are all higher than 96%. The decline in the desorption ratio may be due to the fact that there is still a small amount of Pb²⁺ retained in the adsorbent after the desorption process. Compared to O-Bent, the reusability of PAANa–Bent declined slightly, however the desorption ratios for the four cycles are all more than 91%. The slight decrease may be due to more adsorption sites of PAANa–Bent to impede Pb²⁺ desorption. In general, the desorption experiments confirmed that PAANa–Bent is a recyclable adsorbent and can be easily regenerated with 0.1 M HCl.

Fig. 15 The performance of O-Bent and PAANa–Bent during multiple cycles of regeneration.

4. Conclusions

In conclusion, PAANa–Bent as a novel adsorbent was prepared successfully using in situ polymerization on the surface of bentonite. A series of analytical methods were applied to characterize the PAANa–Bent. Having the sodium polyacrylate anchored on the surface of the bentonite through covalent bonds is different from the traditional polymer/clay composites formed by physical adsorption. In addition, the layered structure of bentonite was protected from polymerization due to the selection of sodium acrylate as the monomer.

Compared to the maximum q_e (33.07 mg g⁻¹) of O-Bent, the maximum q_e of PAANa–Bent of 70.41 mg g⁻¹ had increased by approximately 113% under optimum conditions (pH = 6, t = 300 min c_i = 100 mg L⁻¹ and T = 25 °C). This indicates that modification causes a significant improvement of the Pb²⁺ adsorption capacity of bentonite.

The kinetic adsorption data of PAANa–Bent follow a pseudo-second-order model. Besides, the adsorption isotherm data of PAANa–Bent are best fitted with a Freundlich isotherm, revealing the heterogeneous nature of PAANa–Bent. The value of n lies between 1 and 10, indicating a favorable adsorption of Pb²⁺ using PAANa–Bent.

The high desorption ratio of Pb²⁺ from the PAANa–Bent suggests a good reusability.

Acknowledgements

Prof. Dr Bin Han (University of Jinan) and Dr Yue Wu (Qilu University of Technology) are acknowledged for help with the test instruments. Prof. Dr Wei (Lamar University) is acknowledged for help with the language.

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