Ion imprinted sodium alginate hydrogel beads enhanced with carboxymethyl cellulose and β-cyclodextrin to improve adsorption for Cu2+

doi:10.21203/rs.3.rs-1383952/v1

Using Cu²⁺ as template ion, three sodium alginate hydrogel beads including SA, SAC, and SAB beads were prepared with sodium alginate blending with different precursors of carboxymethyl cellulose and β-cyclodextrin, respectively. The effect of pH, adsorption time, initial concentration of Cu²⁺, adsorbent dosage, and coexisting ions on the adsorption efficiency of three hydrogel beads was investigated. The results indicated that adding carboxymethyl cellulose and β-cyclodextrin into skeleton of beads increased the toughness of the beads and improved the adsorption capacity of Cu²⁺. The adsorption rate of Cu²⁺ on SA beads was conformed to the pseudo-first order kinetic equation, while the adsorption behavior of Cu²⁺ on SAC or SAB beads could be confirmed to the pseudo-second-order kinetic equation. Compared with the saturated adsorption capacity 510 mg/g of Cu²⁺ on SA, the saturated adsorption capacity of Cu²⁺ on SAB and SAC reached 817 mg/g and 822 mg/g, respectively, with their Cu²⁺ adsorption efficiency over 95% at 25°C. Thus, SAC and SAB beads could be used as adsorption material for detecting and removing Cu²⁺ from wastewater.

sodium alginate

hydrogel beads

copper ions

carboxymethyl cellulose

β-cyclodextrin

With the development of modern industry, wastewater polluted by heavy metals from the manufacture of pesticides, fertilizers and metals has become an important field of environmental treatment [1–3]. The non-degradability of heavy metals in soil and water makes them accumulate in organisms, which leads to organism damage and finally endangers human life and health [4]. Therefore, the efficient treatment of heavy metals has always been the key point of national environmental protection [5]. China has established a series of discharge standards for heavy metals in aquatic ecosystem. For example, the comprehensive sewage discharge standard GB 8978 − 1996 stipulates that the maximum acceptable emission concentration of Cu²⁺ is 0.5 mg/L. The treatment methods for heavy metals in wastewater include: chemical method [6], adsorption [7, 8], electro-dialysis [9, 10], ion exchange method [7, 11], photocatalysis methods [12, 13], and etc. With the adsorption method’s advantages such as economic, simple and easy to operate, and wide scope of application, the research and development of green adsorption materials with good performance and specific selectivity is becoming an important trend of controlling heavy metal pollution and recycling of resources [14–17].

Sodium alginate (SA) is a natural macromolecular polysaccharide extracted from brown algae and composed of α-L-glucuronic acid and β-D-mannouronic acid. It is highly water-soluble and contains a large number of free carboxyl groups that can interact with heavy metal ions. Thus, it can be used as an excellent adsorption material for heavy metals [18, 19]. And β-cyclodextrin (β-CD) with a circular structure formed by the α-1,4-glycosidic bond connecting seven D-glucopyranose basic units, contains a cavity structure that can encapsulate ions and organics, etc., and its molecular surface contains a large number of hydroxyl groups that can chelate with heavy metal ions to form complexes [20–22]. Carboxymethyl cellulose (CMC) is a renewable natural cellulose ether compound, which contains abundant hydroxyl and carboxyl groups and can form hydrogels through physical or chemical methods [23, 24].

In this paper, three kinds of sodium alginate hydrogel beads were prepared by sol-hydrogel method. Two of them were enhanced with carboxymethyl cellulose and β-cyclodextrin to improve adsorption for Cu²⁺ from wastewater. SEM and FT-IR were used to characterize and analyze the sodium alginate hydrogel beads, and the influence of different factors on adsorption performance of the hydrogel beads was also investigated. Some adsorption experiments and different adsorption kinetics models were used to analyze the adsorption mechanism of Cu²⁺ on the hydrogel beads.

2.1 Chemicals and instruments

Sodium alginate (Chemically Pure, C.P.), carboxymethyl cellulose (C.P.), β-cyclodextrin (C.P.), glutaraldehyde (25%, Biochemical Reagent, B.R.), hydrochloric acid (38%, A.R.), and sodium hydroxide (A.R.) were all purchased from Sinopharm Chemical Reagents Co., Ltd. Anhydrous copper chloride (99.99%) was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. Triple distilled water was used in the experiment.

The instruments and equipment used in this paper mainly included QUANTA FEG 250 field emission environmental scanning electron microscope (FEI Co., OR, USA), Nicolet iS5 Fourier transform infrared spectrometer (Thermo Fisher Scientific, MA, USA), Optima 8000 inductive coupled plasma (ICP) emission spectrometer (Perkin-Elmer, MA, USA), SZ-97 automatic triple water distiller (Yarong Biochemical Instrument Co., Shanghai, China), PXSJ-226 pH meter (INESA Instrument Co., Shanghai, China), AL204 electronic analytical balance (Mettler Toledo Instrument Co., Zurich, Switzerland), TL-F6 micro injection pump (Tongli Weina Co., Shenzhen, China), and HYG-A shaker (Taicang Equipment Co., Jiangsu, China).

2.2 Preparation of sodium alginate hydrogel beads

The structural stability and the adsorption effect of sodium alginate hydrogel beads on Cu²⁺ were improved by adding different precursors. The effects of concentrations of precursors and concentration of template Cu²⁺ on the pellet formation of hydrogel beads were investigated in the preliminary stage, and Cu²⁺ pre-adsorption experiments were conducted to obtain the optimal conditions for the preparation of sodium alginate hydrogel beads.

2.2.1 Preparation of SA hydrogel beads

The SA hydrogel beads were synthesized by the following procedure. SA (2.0 g) was added into 200 mL distilled water with a magnetic stirrer for 6 h at 25°C. Then, 4 mL glutaraldehyde solution in H₂O (25%, w/w) was added into above solution and reacted at 25°C for 12 h under sealing and stirring to prepare sol solution. The prepared sol solution was dropped into 200 mL 0.2 mol/L Cu²⁺ aqueous solution by a micro-injection pump at a constant rate to obtain hydrogel beads, and the hydrogel beads were stabilized by Cu²⁺ ion imprinting for 10 h. After washing the hydrogel beads with distilled water for 5 times (50 mL each time), SA hydrogel beads (abbr. as SA beads) were obtained by eluting Cu²⁺ in the hydrogel beads with 100 mL 0.5 mol/L HCl for 5 times and washing with 50 mL distilled water for 5 times. The proposed reaction was shown in Scheme 1-a.

2.2.2 Preparation of SAC hydrogel beads

The SAC hydrogel beads were synthesized by the following procedure. SA (3.0 g) and CMC (3.0 g) were added into 200 mL distilled water with a magnetic stirrer for 6 h at 25°C. After the same subsequent treatment as the Section 2.2.1, the SA + CMC hydrogel beads (abbr. as SAC beads) were obtained. The proposed reaction was shown in Scheme 1-b.

2.2.3 Preparation of SAB hydrogel beads

The SAB hydrogel beads were synthesized by the following procedure. SA (3.0 g) and β-CD (3.0 g) were added into 200 mL distilled water with a magnetic stirrer for 6 h at 25°C. After the same subsequent treatment as the Section 2.2.1, the SA + β-CD hydrogel beads (abbr. as SAB beads) were obtained. The proposed reaction was shown in Scheme 1-c.

Scheme 1. The proposed preparation reaction of SA(a), SAC (b), SAB (c) sodium alginate hydrogel beads

2.3 Structure characterization for hydrogel beads

All sodium alginate hydrogel beads were freeze-dried (–80°C) before structure characterization in order to avoid damaging the structure by drying in normal oven. Scanning electron microscopy (SEM) was used to characterize the morphology of the hydrogel beads, and Fourier transform infrared spectroscopy (FT-IR) was used to characterize the composition change of the hydrogel beads.

2.4 Adsorption Experiment

Adsorption experiment was conducted to investigate the effects of different experimental parameters on the adsorption of Cu²⁺ on the hydrogel beads. A fixed amount of the sodium alginate hydrogel beads was added into a series of Cu²⁺ solution, respectively. Then the solution was oscillated for adsorbing 12 h at 120 r/min in a shaker under different temperature. After that, the concentration of Cu²⁺ in the solution reaching equilibrium adsorption was determined by the inductive coupled plasma emission spectrometer (ICP). All the adsorption experiments were repeated for three times and the mean values were recorded. The adsorption capacity and adsorption efficiency of the sodium alginate hydrogel beads were calculated according to the following equations, respectively.

$$\text{q=}\frac{\text{（}{\text{C}}_{\text{0}}\text{–}{\text{C}}_{\text{1}}\text{）}\text{×V}}{\text{m}}$$

where q is the adsorption capacity (mg/g) of Cu²⁺ on hydrogel beads, C₀ is the initial concentration of Cu²⁺ (mg/L) in the solution, C₁ is the concentration of Cu²⁺ (mg/L) after adsorption, V is the volume of Cu²⁺ solution (L), and m is the weight of the dry hydrogel beads (g).

$$\text{Q}\text{=}\frac{{\text{C}}_{\text{0}}\text{–}{\text{C}}_{\text{1}}}{{\text{C}}_{\text{0}}}\text{×}\text{100\%}$$

where Q is the adsorption efficiency (%), C₀ is the initial concentration of Cu²⁺ (mg/L) in the solution, C₁ is the concentration of Cu²⁺ (mg/L) after adsorption.

2.4.1 Effect of pH on adsorption

The dry hydrogel beads (4 mg) were added into 50 mL Cu²⁺ solution (5 mg/L) with different pH ranging 1 − 8. In a shaker, the adsorption was carried out under 25°C at 120 r/min for 10 h to investigate the effect of pH on the adsorption.

2.4.2 Effect of hydrogel beads dosage on adsorption

The dry hydrogel beads with different weight (0.4 − 10 mg) were added into 50 mL Cu²⁺ solution (5 mg/L) with pH = 7, respectively. In a shaker, the adsorption was carried out under 25°C at 120 r/min for 10 h to investigate the effect of hydrogel beads dosage on the adsorption.

2.4.3 Effect of initial concentration on adsorption

A series of Cu²⁺ solution with different concentration ranging 1 mg/L to 700 mg/L was prepared firstly. The dry hydrogel beads (4 mg) were added into above all Cu²⁺ solutions (50 mL for each one) after adjusting pH to 7, respectively. In a shaker, the adsorption was carried out under 25°C at 120 r/min for 10 h to investigate the effect of initial concentration of Cu²⁺ on the adsorption.

2.4.4 Adsorption kinetics experiment

The dry hydrogel beads (40 mg) were added into a 50 mL Cu²⁺ solution (5 mg/L) with pH = 7. In a shaker, the adsorption was carried out under 25°C at 120 r/min. The concentration of Cu²⁺ in the adsorbing solution was measured at a certain time interval, and the adsorption effect of hydrogel beads along with different treatment time was investigated. The pseudo-first-order kinetic model and the pseudo-second-order kinetic model were used to fit the dynamic adsorption data to explore the adsorption mechanism.

The pseudo-first-order kinetic equation is:

$${\text{ln(}{\text{q}}_{\text{e}}\text{–}\text{q}}_{\text{t}}\text{)=ln}{\text{q}}_{\text{e}}\text{–}{\text{k}}_{\text{1}}\text{t}$$

And the pseudo-second-order kinetic equation is:

$$\frac{\text{t}}{{\text{q}}_{\text{t}}}\text{=}\frac{\text{1}}{{\text{k}}_{\text{2}}{\text{q}}_{\text{e}}^{\text{2}}}\text{+}\frac{\text{1}}{{\text{q}}_{\text{e}}}$$

where q_t is the adsorption capacity (mg/g) at time t, q_e is the saturated adsorption capacity at equilibrium (mg/g), t is the adsorption time (h), k₁ is the pseudo-first-order adsorption rate constant (1/h), k₂ is the pseudo-second-order adsorption rate constant [g/(mg/h)].

2.4.5 Adsorption thermodynamic experiment

For better understanding the interaction mechanism between hydrogel beads and Cu²⁺ after evaluating their adsorption capacity, both Langmuir model and Freundlich model were used to fit the experimental results. In general, the Langmuir adsorption isotherm equation is used to describe the adsorption process of monolayer adsorption. The Freundlich adsorption isotherm equation is commonly used to describe the multilayer adsorption process.

The Langmuir adsorption isotherm equation is:

$$\frac{\text{1}}{{\text{q}}_{\text{e}}}\text{=}\frac{\text{1}}{{\text{K}}_{\text{L}}{\text{q}}_{\text{m}}{\text{C}}_{\text{e}}}\text{+}\frac{\text{1}}{{\text{q}}_{\text{m}}}$$

And the Freundlich adsorption isotherm equation is:

$$\text{ln}{\text{q}}_{\text{e}}\text{=}\text{mln}{\text{C}}_{\text{e}}\text{+ln}{\text{K}}_{\text{f}}$$

where q_e is the equilibrium adsorption amount of Cu²⁺ on hydrogel beads at adsorption equilibrium (mg/g), q_m is the saturation adsorption amount of Cu²⁺ on hydrogel beads (mg/g), C_e is the adsorption equilibrium concentration of Cu²⁺ (mg/L), K_L is the parameter of Langmuir isotherm equation, which is related to the strength of adsorption capacity, and its magnitude mainly depends on the nature of adsorbent, adsorbent mass and temperature; K_f is the adsorption equilibrium constant of Freundlich isotherm equation, which indicates the adsorption amount at C per unit concentration; m is the Freundlich characteristic adsorption parameter.

2.4.6 Effect of coexisting ions on adsorption

A mixed solution of Cu²⁺ and 15 coexisting ions (Al³⁺, Ag⁺, Ba²⁺, Ca²⁺, Cd²⁺, Co²⁺, Cr³⁺, Fe²⁺, Fe³⁺, Li⁺, Mg²⁺, Mn²⁺, Ni²⁺, Pb²⁺ and Zn²⁺) was prepared with the same concentration of 5 mg/L, and 50 mL mixed solution was taken to adjust pH to 7 before 4 mg dried hydrogel beads were added. In a shaker, the adsorption was carried out under 25°C at 120 r/min for 10 h. After that, the concentration of Cu²⁺ and other coexisting ions in the adsorbing solution was measured by the ICP and the effect of coexisting ions on the adsorption of Cu²⁺ was analyzed.

The separation factors (β) were calculated according to the following equation.

$$\text{β}\text{=}\frac{\text{Q}\text{(}\text{Cu}\text{2+}\text{)}}{\text{Q}\text{(}\text{other ion)}}$$

where Q(Cu²⁺) is the adsorption efficiency of Cu²⁺ on hydrogel beads, and Q(other ions) is the adsorption efficiency of other coexisting ion on hydrogel beads.

2.5 Regeneration and recycling

The dried sodium alginate hydrogel beads (4 mg) with adsorbed Cu²⁺ were added into 50 mL HCl solution (0.5 mol/L) for desorption for 5 h, then the beads were washed with 50 mL distilled water for 5 times. The desorbed hydrogel beads were placed into 50 mL Cu²⁺ aqueous solution (5 mg/L) again for another adsorption for 10 h, and the adsorption-desorption-adsorption process was repeated to investigate the reutilization of the hydrogel beads.

3.1 Structure characteristics of sodium alginate hydrogel beads

3.1.1 SEM morphological characteristics

The SEM images of SA, SAC, and SAB sodium alginate hydrogel beads were shown in Fig. 1. Although the newly prepared sodium alginate hydrogel beads were basically spherical with diameter of 3 − 5 mm (Fig. 1-g), the freeze-dried sodium alginate hydrogel beads were ellipsoid with particle size of 1.2 − 2.0 mm, and with layered and scale-like folds on the surface (Fig. 1-a, c, e), indicating the existence of internal cavity. Figure <link rid="fig1">1</link>-b, <link rid="fig1">1</link>-d, and <link rid="fig1">1</link>-f showed that more fold structures appeared after adding CMC and β-CD than that of SA beads, leading to a significant increase in the specific surface area of the hydrogel beads, thus enhancing the adsorption capacity of Cu²⁺ by the SAC and SAB beads.

3.1.2 FT-IR spectral characteristics

The Fourier transform infrared (FT-IR) spectra of SA, SAC and SAB hydrogel beads before and after adsorption were shown in Fig. 2. The absorption peaks at 3427 cm^− 1, 3440 cm^− 1 and 3466 cm^− 1 were O − H stretching vibration peaks [25, 26]. At 3440 cm^− 1, O − H of both sodium alginate and sodium carboxymethyl cellulose produced stretching vibration peak. The broad peak of 3466 cm^− 1 was the stretching vibration peak generated by hydrogen bond between sodium alginate and hydroxyl group on cyclodextrin. The peaks of 1740 cm^− 1 and 1370 cm^− 1 were the characteristic absorption peaks of the symmetric and asymmetric stretching vibration of COO − of sodium alginate and the C = O superimposed on sodium carboxymethyl cellulose, respectively [27, 28]. The absorption peaks at 1370 cm^− 1 and 1366 cm^− 1 were related to the C − O stretching vibration and O − H bending vibration of sodium alginate and sodium carboxymethyl cellulose [29, 30]. The absorption peak of the C = O double bond at 1738 or 1740 cm^− 1 indicated that the crosslinking agent glutaraldehyde was successfully polymerized with sodium alginate. And the peak of 2904 cm^− 1 was the C − H stretching vibration of methylene of sodium alginate [31], and the absorption peak of C − H stretching vibration at 2904 cm^− 1 was weakened by the chelation of carboxylic acid group with Cu²⁺.

The absorption peaks’ wavenumber of sodium alginate hydrogel beads did not change significantly before and after adsorption, but the intensity of absorption peaks increased or decreased slightly, which indicated that the main structure of sodium alginate hydrogel beads did not change before and after adsorption of Cu²⁺, and perhaps treating Cu²⁺ by hydrogel beads existed not only physical adsorption but also chemical adsorption which is dominated by coordination reaction.

3.2 Evaluation on adsorption properties

3.2.1 Effect of pH on adsorption

The effect of pH in initial solution on the adsorption efficiency of Cu²⁺ on the three sodium alginate hydrogels was shown in Fig. 3. The adsorption efficiency of SA and SAB beads was lower than 20% at pH 1–3 while that of SAC beads reached about 40%, but the adsorption efficiency of SA and SAB beads increased sharply within pH 3–4 until more than 80% within pH 4–7, and reached the maximum values at pH = 7. However, the adsorption efficiency of SA hydrogel beads reached the maximum value at pH = 8. These might be caused by the positive charge on the adsorbent surface, which decreased with increasing of pH in solution. Considering that Cu²⁺ will generate Cu(OH)₂ precipitation under alkaline conditions, the optimal pH for the adsorption of the three hydrogel beads is confined to 7.

3.2.2 Effect of the dosage of hydrogel beads on adsorption

The influence of the dosage of SA, SAC, and SAB hydrogel beads on the adsorption was shown in Fig. 4. The initial concentration of Cu²⁺ in the solution was 5 mg/L, and the adsorption efficiency of Cu²⁺ in the solution gradually increased with the increase of the dosage of three kinds of hydrogel beads. When the dosage of hydrogel beads reached 4 mg in dry, the adsorption capacity of Cu²⁺ in the solution reached the maximum. Thus, when treating 50 mL wastewater containing Cu²⁺ with a concentration of 5 mg/L, the dosage of hydrogel beads should be used at least 4 mg in dry to achieve adsorption equilibrium, and the adsorption efficiency exceeded 95% under this condition.

3.2.3 Effect of initial concentration on adsorption

The adsorption capacity of SA, SAC, and SAB hydrogel beads was affected by the initial concentration of Cu²⁺ in the solution (Fig. 5). When the concentration of Cu²⁺ was 600 mg/L, the three kinds of hydrogel beads reached the saturated adsorption capacity. The saturated adsorption capacity of sodium alginate hydrogel beads with CMC and β-CD increased significantly with 817 mg/g for SAC beads and 822 mg/g for SAB beads, respectively, which was more than 1.5 times of that of SA beads (510 mg/g). This suggested that the hydrogel beads formed by cross-linking carboxymethyl cellulose or β-cyclodextrin with sodium argent alginate play a very important role in metal ion adsorption. More importantly, these values were much higher than the literature reported values for other polymeric hydrogels (Table 1). Thus, the synthesized hydrogel beads with strong adsorption capacity could be a good adsorbent for removing Cu²⁺.

Table 1

Saturated adsorption capacities for Cu²⁺ in comparison with previous work
Sample	Saturated adsorption capacity (mg/g)	References
Carboxymethyl cellulose/epichlorohydrin hydrogel	415	^[28]
Chitosan entrapped carboxymethylated cellulose hydrogel	45	^[32]
Cellulose/gelatin composite hydrogel	52	^[33]
Cellulose-graft-polyacrylamide/hydroxyapatite composite hydrogel	175	^[34]
Carboxymethyl cellulose/polyacrylamide composite hydrogel	227	^[35]
Nanofibrillated cellulose/polyethyleneimine composite hydrogels	175	^[36]
Carboxymethyl cellulose-graft-sodium alginate hydrogel imprinted with Cu²⁺	817	This work
β-cyclodextrin-graft-sodium alginate hydrogel imprinted with Cu²⁺	822	This work

3.2.4 Adsorption kinetic model

The adsorption kinetics curves of the three kinds of hydrogel beads at 308 K were shown in Fig. 6, the inserted image presented the effect of Cu²⁺ adsorption on the hydrogel beads (Cu²⁺ solution on the left and Cu²⁺ adsorption on the bottom right). The adsorption capacity of Cu²⁺ on the hydrogel beads increased with the increase of adsorption time firstly. The adsorption of SA beads reached equilibrium at about 500 min, SAC beads reached equilibrium at about 250 min, and SAB beads reached equilibrium at about 350 min. It could be concluded that adding CMC and β-CD would shorten the equilibrium time of sodium alginate hydrogel beads.

Table 2 showed the fitting parameters of quasi-first-order and pseudo-second-order kinetic equations at 308 K. The pseudo-first-order kinetic model for SA beads had higher correlation coefficient than those for SAC and SAB beads, and the calculated q_e of Cu²⁺ on SA beads was very close to that of experimental q_e(exptl.) in three kinds of hydrogel beads, which demonstrated that the adsorption behavior of Cu²⁺ on SA beads could be consonant with the pseudo-first-order kinetic equation, and the adsorption rate of on SA beads was positively correlated to the Cu²⁺ concentration in a certain concentration range.

Table 2

Simulation parameters of pseudo-first-order and pseudo-two-order equations for sodium alginate hydrogel beads at 308 K.
Materials	q_e(exptl.) (mg/g)	Pseudo-first-order kinetic model
Materials	q_e(exptl.) (mg/g)	q_e(mg/g)	k₁(1/h)	R²	q_e(mg/g)	k₂[g/(mg/h)]	R²
SA	59.25 ± 1.56	55.54	0.4372	0.9857	74.07	0.00648	0.9844
SAC	61.56 ± 1.75	24.79	0.3972	0.7536	94.88	0.01063	0.9894
SAB	61.64 ± 2.01	34.35	0.5858	0.9423	65.53	0.02357	0.9989
Note: exptl.= experimental.

However, the pseudo-second-order kinetic models for SAC and SAB beads had higher correlation coefficient than that for SA beads, and the calculated q_e of Cu²⁺ on SAC or SAB beads was very close to that of experimental q_e(exptl.), which demonstrated that the adsorption behavior of Cu²⁺ on SAC or SAB beads could be confirmed to the pseudo-second-order kinetic equation, and the adsorption rate of on SAC or SAB beads was positively correlated to the beads dosage and the Cu²⁺ concentration in a certain concentration range.

3.2.5 Adsorption isotherm model

The adsorption isotherm curves of the three kinds of hydrogel beads at 308 K were shown in Fig. 7. And the fitting parameters of Langmuir and Freundlich for adsorption isotherm equations at 308 K were shown in Table 3. The correlation coefficients of the Freundlich adsorption isotherm equation for the three kinds of hydrogel beads are higher than those of Langmuir isotherm equation, and all of q_m, the adsorption amounts of the three kinds of hydrogel beads calculated by the Langmuir adsorption isotherm equation, were tremendously different from the experimental q_e(exptl). The adsorption behavior of Cu²⁺ on gel spherical beads is consistent with the Freundlich adsorption isotherm equation, which indicates that the adsorption of Cu²⁺ by gel spherical beads is a multilayer adsorption process in a certain concentration range.

Table 3

Simulation parameters of Langmuir equation and Freundlich equation for adsorption isotherm curve of SA, SAC, and SAB hydrogel beads at 308 K.
Materials	q_e(exptl.) (mg/g)	Langmuir adsorption isotherm model			Freundlich adsorption isotherm model
Materials	q_e(exptl.) (mg/g)	q_m (mg/g)	K_L	R₁²	K_F	m	R₂²
SA	527.5 ± 5.7	-1365.65	-7.3519⊆10^− 3	0.6856	8.7216	0.5134	0.8453
SAC	817.3 ± 6.4	625.00	8.9888⊆10^− 2	0.5743	10.4431	0.4629	0.8642
SAB	822.6 ± 6.3	384.62	0.1566	0.7510	10.4792	0.4265	0.9210

The Freundlich adsorption isotherm equations of SAC and SAB hydrogel beads had higher correlation coefficients, and it was known that the addition of CMC and β-CD could improve the adsorption capacity of hydrogel beads.

3.2.6 The adsorptive selectivity of Cu²⁺ on hydrogel beads

For studying the adsorption selectivity of SA, SAC, and SAB hydrogel beads, Cu²⁺ was mixed with 15 coexisting ions (Al³⁺, Ag⁺, Ba²⁺, Ca²⁺, Cd²⁺, Co²⁺, Cr³⁺, Fe²⁺, Fe³⁺, Li⁺, Mg²⁺, Mn²⁺, Ni²⁺, Pb²⁺ and Zn²⁺) in the same concentration of 5 mg/L before the adsorption experiment. The adsorption efficiency of Cu²⁺ on SA, SAC, and SAB hydrogel beads were the maximum ones, reaching 85.1%, 87.0%, and 89.2%, respectively, while the adsorption efficiency of other coexisting ions on SA, SAC, and SAB hydrogel beads were only from 7.3%−26.9%, 4.8 − 28.9%, and 1.5 − 26.1%, respectively (Fig. 8). The separation factors of Cu²⁺ to other ions β(Cu²⁺/other ions) was 3.2 − 11.7 for SA beads, β(Cu²⁺/other ions) was 3.0 − 18.1 for SAC, and β(Cu²⁺/other ions) was 3.4 − 59.5 for SAB, which reflected the adsorption separation abilities of SA, SAC, and SAB hydrogel beads between Cu²⁺ and other coexisting ions. Thus, the SA, SAC, and SAB hydrogel beads prepared with Cu²⁺ as template, especially SAB beads, show better selectivity for Cu²⁺ than other coexisting ions.

3.3 Cycling and regeneration

Figure 9 showed the effect of HCl solution on the desorption and regeneration of Cu²⁺ adsorbed sodium alginate hydrogel beads. Although the adsorbent maintained its original morphology with the increase of elution time, the desorption-adsorption process caused some damage inside the adsorbent, resulting in a gradual decrease in the adsorption efficiency of Cu²⁺ on the hydrogel beads, the adsorption efficiency of Cu²⁺ on the SAC and SAB hydrogel beads remained above 88% after recycling desorption-adsorption process for six times, while the adsorption efficiency of SA hydrogel beads decreased to below 80%. All above-mentioned results proved that the hydrogel beads synthesized by our proposed method possessed high specific adsorption, selectivity, and reusability.

SA, SAC, and SAB hydrogel beads were prepared by sol-coagulated-hydrogel method with sodium alginate, carboxymethyl cellulose and β-cyclodextrin using glutaraldehyde as cross-linker and Cu²⁺ as template ion. By SEM characterization, the surface of sodium alginate hydrogel beads has lamellar folds and many cavities inside. SA, SAC, and SAB hydrogel beads have good adsorption effect on Cu²⁺ under neutral and weak alkaline conditions with higher selectivity. The adsorption rate is positively correlated with the initial concentration. The saturated adsorption capacity of SAC and SAB hydrogel beads was 817 mg/g and 822 mg/g, respectively, which was more than 1.5 times of SA hydrogel beads. The adsorption efficiency of three kinds of sodium alginate hydrogel beads for low concentration Cu²⁺ in wastewater was more than 95%, which indicated that sodium alginate hydrogel beads will have extensive application prospect as Cu²⁺ adsorption enrichment materials. After the adsorption is saturated, the copper could be recycled by electrochemical treatment or burning treatment of the absorbed beads.

Acknowledgments

The authors acknowledge the financial support of the Provincial Key Research and Development Plan in Hunan, China (2020NK2019), the National Natural Science Foundation for Young Scientists of China (22004132), and the Natural Science Foundation of Hunan Province, China (2020JJ4940). All authors appreciate the editors and the anonymous reviewers for their constructive comments and critical evaluation.

Authors’ Contributions All authors contributed to the study conception and design. YF: data curation, investigation, methodology, writing original draft. DS: investigation, project administration, writing original draft. YY: methodology, writing review and editing. XH: project administration, writing original draft. YG: formal analysis, funding acquisition. YZ: formal analysis, writing original draft. ZL: writing review and editing. LX: funding acquisition, supervision, writing review and editing. All authors read and approved the final manuscript.

Conflict of interest The authors declared that they have no conflicts of interest to this work.

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Scheme 1 is available in supplementary section.

Scheme01.png
Scheme 1. The proposed preparation reaction of SA (a), SAC (b), SAB (c) sodium alginate hydrogel beads

Ion imprinted sodium alginate hydrogel beads enhanced with carboxymethyl cellulose and β-cyclodextrin to improve adsorption for Cu2+

Status:

Version 1

Abstract

Figures

1 Introduction

2 Materials And Methods