Preparation of microgel/sodium alginate composite granular hydrogels and their Cu²⁺ adsorption properties

Abstract

Acrylamide/2-acrylamido-2-methylpropane sulfonic acid (AMPS)-based microgels containing hydroxymethyl groups and sodium alginate (SA) were used to prepare microgel/SA composite spherical granular hydrogels, which contained the SA network and a microgel network. Their preparation and Cu²⁺ adsorption properties were investigated. Acetone was used as solvent for dehydration to remove some of the water before drying, which was important for the spherical morphology of dry granular hydrogels. The composite hydrogel had a comprehensive excellent performance. The Cu²⁺ adsorption capacity decreased as the microgel/SA ratio increased, which was caused by the decrease of the ionic dominating adsorption groups. However, the composite hydrogel was stable in acid conditions without damage. The composite hydrogel had a faster swelling rate than that of the SA hydrogel. The low density and heterogeneous surface, such as micro cracks, micro particles and a large number of microgel particles with a diameter of about 100 nm, were conducive to increase the adsorption rate of the composite hydrogel. As the Cu²⁺ concentration increased from 5 to 35 mmol L⁻¹, the Cu²⁺ adsorption capacity of the composite hydrogels with a microgel/SA mass ratio of 2.4 increased from 0.11 to 0.73 mmol g⁻¹. The Langmuir and Freundlich isotherm adsorption equations were used to fit the adsorption of the microgel/SA hydrogel with different Cu²⁺ concentrations. The fitting results showed that the Freundlich isotherm equation had a good fitting effect. The pH value of Cu²⁺ solutions also affects the adsorption capacity. The hydrogel also has the ability to adsorb other metal ions.

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1. Introduction

With the development of industries such as electroplating, mining, and batteries, heavy metal ions, such as Pb(II), Cu(II), Cr(III)/Cr(VI), and Zn(II), contained in waste water directly or indirectly flow into streams, lakes, rivers or oceans and cause water pollution.¹ Many techniques, such as chemical precipitation, ion exchange, reverse osmosis, membrane filtration, electrochemical treatment and adsorption, have been used to remove heavy metal ions from waste water.² The adsorption technique has been considered as an effective method because it has many advantages such as wide application ranges, simple operational conditions and easy regeneration of the adsorbents.^3,4 Hydrogels have many potential advantages as adsorbents, such as hydrophilic properties, swelling and insoluble properties, contain ion exchange and chelation adsorption groups and easily diffuse in the solutes.^5,6 Traditional adsorbents can adsorb heavy metals into pores or onto a large surface area, whereas hydrogels adsorb metal ions through the functional groups (i.e., –COO⁻, –SO₃²⁻, –SH, –NH₂ and –OH) in their network structures and possess relatively high adsorption capacities and adsorption rates.⁷

Sodium alginate (SA) is one kind of polysaccharide which is composed of α-L-guluronic acid (G units) and β-D-mannuronic acid (M units) by a β-1,4-glycosidic bond and contains many –COO⁻ and –OH groups. Alginate can adsorb significant amounts of toxic divalent metals, such as Pb²⁺, Cd²⁺, and Cu²⁺.^8,9 Spherical granular sodium alginate hydrogels can easily be prepared by crosslinking of bivalent or trivalent cations such as Ca²⁺. SA is abundant, renewable, nontoxic, water-soluble, biodegradable, and biocompatible. However, the poor mechanical strength, easy decomposition by bacteria in waste water and partial dissolution in the regeneration process by strong acid all limit the application of ionic crosslinking SA hydrogels.

Improving the hydrogel mechanical strength is a challenge for researchers. Many methods have been developed. A double network was used to improve the hydrogel mechanical strength.¹⁰ The sacrificial bond theory was used to explain the reasons for the high mechanical strength of the hydrogel with a double network structure.^11,12 A hybrid hydrogel of acrylamide and alginate with an interpenetrating network had an obvious improved mechanical strength compared with the single network hydrogel.¹³ So, adding another crosslinking network with a high mechanical strength is beneficial to improve the SA hydrogel properties and can also reduce loss in the acid regeneration process. Microgels^14–20 were also used as multifunctional crosslinkers to improve the hydrogel mechanical strength. The strength of a hydrogel with a double network structure can also be improved by microgels.²¹ The microgel particles can also enforce the hydrogel swelling rate.^22–25 When the microgel content was high, the microgel crosslinked hydrogel had a fast swelling rate and high mechanical strength.²⁶ Since nanocomposite hydrogels were reported,²⁷ many novel composite hydrogels with excellent mechanical properties have also been developed by the mixing of nanoparticles, such as nano-spherulites (CNS)²⁸ and layered double hydroxide (LDH)²⁹ with polymers.

Compared with the traditional bulk hydrogels, granular hydrogels from direct preparation can reduce the amount of energy needed to dry, smash, and granulate them. Much research effort was made to produce a granular hydrogel in a polymerization reaction such as a one-step fabrication method.⁷ It was necessary to simplify the preparation process and improve the uniformity of particles for the preparation of granular hydrogels. Microgel dispersions can easily mix with SA solutions because of their flow characteristics. So, the combination of the SA network and the microgel crosslinked network is expected to bring their respective advantages.

In this paper, microgel/SA composite spherical granular hydrogels were prepared by SA and acrylamide (AM)/2-acrylamido-2-methylpropane sulfonic acid (AMPS) based microgels containing hydroxymethyl groups. The effects of the microgel/SA ratio on the granular hydrogel preparation and Cu²⁺ adsorption properties were investigated. The Cu²⁺ adsorption mechanism of a microgel/SA composite hydrogel was also fitted by the Langmuir and Freundlich isotherm equations.

2. Experimental section

2.1. Materials

Acrylamide (AM, 98%) was produced by Dia-Nitrix Co (Japan). 2-Acrylamido-2-methylpropane sulfonic acid (AMPS, 99%), hydroxymethyl acrylamide (NMA, 98%) and N,N′-methylenebisacrylamide (NMBA, 99%) were purchased from Shandong Lianmeng Chemical Group Co. (China), Shandong Zibo Xinye Chemical Co. (China) and Sinopharm Chemical Reagent Co. (China), respectively. Cyclohexane (analytical grade reagent) was produced by Tianjin Damao Chemical Co. (China). Sodium alginate (SA) was produced by Tianjin Tianli Chemical Reagent Co. (China). Octylphenol ethoxylate (molecular weight 647), sorbitan monolaurate (molecular weight 182), ammonium persulfate (analytical grade reagent), sodium bisulfite (analytical grade reagent), Cu(NO₃)₂·3H₂O (analytical grade reagent), Pb(NO₃)₂ (analytical grade reagent) and acetone (analytical grade reagent) were all purchased locally. Distilled water was used in all the experiments.

2.2. Preparation of microgels containing hydroxymethyl groups

Microgels were prepared by inverse emulsion polymerization. AM solutions (42 g AM in 70 mL water), AMPS (5 g), NMA (5 g), NMBA (0.004 g), sorbitan monolaurate (11 g) and octylphenol ethoxylate (2.5 g) were added into a 500 mL round-bottomed three-neck flask with a reflux condenser, a mechanical stirrer, and a thermometer. After stirred for 20 min, cyclohexane (120 mL) was added into the flask. After being bubbled by nitrogen for 20 min, the system was initiated by the redox initiator of ammonium persulfate (0.02 g in 1 mL water) and sodium bisulfite (0.04 g in 1 mL water) at 30 °C. A nitrogen atmosphere was preserved throughout the polymerization. After 3 h of reaction, the microgel emulsions were obtained.

2.3. Preparation of microgel/SA composite hydrogels

The microgel emulsions were precipitated by acetone for further use. The SA solution with 2% mass content and the CaCl₂ solution with 4% mass content were also made up for further use. The designed quantity of SA, precipitated microgel and water were all added to a beaker. After being fully stirred by a magnetic stirrer, the mixed liquid was dropped into the beaker with the CaCl₂ solution by a syringe with a 20 mm stainless steel needle and was left overnight to stabilize. The spherical granular hydrogels were formed by Ca²⁺ crosslinking. Then, they were filtered and washed several times with water to remove the residual Ca²⁺. The as-prepared spherical granular hydrogels were placed in acetone to remove part of the water. Finally, they were placed in a square enamel plate for natural drying. The pure SA spherical granular hydrogel was also prepared by the same procedure as described above.

2.4. Measurements of the swelling properties

The swelling experiments were performed by immersing the hydrogel in a large excess of water at room temperature. The hydrogel was weighed until a constant weight was reached, changing the water several times. The swelling ratio was calculated as follows.

Q_e = (W_e − W_d)/W_d (1)

where W_e and W_d are the weight of the equilibrium swollen hydrogel and the corresponding dried hydrogel, respectively.

2.5. Images of the granular hydrogels

A digital camera was used to acquire images of the macroscopic granular hydrogels. The morphologies of the microgel composite hydrogel were observed by using a JEOL JSM-7600F scanning electron microscope (SEM) after sputter-coating with platinum.

2.6. Cu²⁺ adsorption measurements

A 752C ultraviolet spectrophotometer (Shanghai No. 3 analytical instrument factory, China) was used to measure the Cu²⁺ adsorption capacity. Static adsorption was performed by immersing hydrogels of about 0.4 g in 150 mL of Cu(NO₃)₂ solution with a certain concentration at 25 °C. The accuracy of Cu²⁺ concentration was acquired from the absorbance and a standard curve. The adsorption capacity was calculated by eqn (2).where q is the Cu²⁺ adsorption capacity, and c₀ and c are the Cu²⁺ concentration before and after adsorption, respectively. V₀ is the initial volume of the Cu²⁺ solution, and m is the dried hydrogel weight.

The adsorption capacity at time t after immersion in the Cu²⁺ solution was calculated by eqn (3).

where q_t and c_t are the Cu²⁺ adsorption capacity and the Cu²⁺ concentration, respectively, at time t after immersion in the Cu²⁺ solution; c₀, V₀ and m have the same meaning as described above.

The time dependencies of the swelling ratio were represented as .

2.7. Measurements of the granular hydrogel density

The density of the granular hydrogels was calculated by the following equation.where m_h and V are the weight and the hydrogel volume, respectively. The hydrogel of about 0.4 g was accurately weighed by an electronic analytical balance to acquire the m_h.

Volume V was estimated by the following equation.

where N and d are the number and average diameter of the granular hydrogels, respectively. The number N of the particles was counted one by one. About 15 hydrogel particles were randomly selected to acquire the average diameter d of granular hydrogels by a digital Vernier caliper.

3. Results and discussion

3.1. Preparation of microgel/SA composite spherical granular hydrogels

When the SA solution with a 2% mass content was dropped into the 4% CaCl₂ solution, wet spherical granular hydrogels were easily obtained. Acetone was important for preparing the dried spherical granular hydrogels. When the as-prepared wet spherical granular hydrogel was directly placed in the square enamel plate for natural drying, the dried hydrogel was not spherical, but flat in shape, as shown in Fig. 1(b), because the hydrogel had a poor mechanical strength when the water content was high. When acetone was used for removing part of the water, the water content of the hydrogel decreased and a spherical granular hydrogel with a diameter of about 1.14 mm was obtained after natural drying. When the wet spherical granular hydrogel was treated with acetone for the microgel/SA composite hydrogel, a dried spherical granular hydrogel with a diameter of about 1.52 mm was also acquired, as shown in Fig. 1(c). As shown in Fig. 1, the appearance of the hydrogels is significantly different. The dried SA hydrogel is pale yellow, but the microgel/SA hydrogel has a white appearance because SA solutions were yellow and microgel dispersions were white. The microgel/SA hydrogel was dark green after adsorption caused by the color of Cu²⁺ ions.

Fig. 1 The images of hydrogels. (a) SA spherical particle, 1.14 mm, (b) SA flat particle, Φ 2.8 mm × 0.2 mm, (c) microgel/SA (mass ratio 5.7) composite spherical particle, 1.52 mm, (d–f) microgel/SA (mass ratio 3.7) composite hydrogel in 1 M HCl solution, original state and swollen state in 1 M HCl solution, respectively, (g and h) before compression and after compression (about 70 kPa), respectively, for the swollen microgel/SA hydrogel with a mass ratio 3.7 in Cu(NO₃)₂ solution (9.7 mmol L⁻¹).

When the dry microgel/SA composite granular hydrogel was immersed in 1 M hydrochloric acid solution for several days, the hydrogel was not soluble and maintained a spherical morphology. The hydrogel particles swelled very significantly in the acid solution, as shown in Fig. 1(d)–(f). The swelling ratio in different pH solutions is shown in Table 1. Concentrated hydrochloric acid was used to regulate the pH value. As the pH increases, the swelling ratio increases. However, the swelling ratio increases slightly from pH = 2 to pH = 6 because of the large amide non electrolytes. The swelling results indicate that the microgel/SA composite hydrogel is stable in hydrochloric acid solution. Microgels containing hydroxymethyl groups can act as a crosslinker in the natural drying acrylamide based hydrogel.^16,17 As shown in Scheme 1, the dried composite hydrogel contains the SA network crosslinked by Ca²⁺ and the microgel network by the crosslinking of hydroxymethyl groups and amide groups. Although the pure SA network can be destroyed by the hydrochloric acid solution with a high concentration, the microgel network is stable in the hydrochloric acid solution. So, the combination of microgels and SA can improve the stability of the SA hydrogel in a strong acid condition.

Table 1 Swelling ratio of hydrogel in HCl solutions (microgel/SA mass = 3.7)

1 M solution	pH = 2 solution	pH = 4 solution	pH = 6 solution
4.3	6.0	6.1	6.3

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Scheme 1 Schematic diagram of microgel/SA composite hydrogel formation.

As shown in Fig. 1(g) and (h), the swollen microgel/SA composite granular hydrogel in Cu(NO₃)₂ solution could sustain high stress, which was not destroyed after compression (about 70 kPa). The high mechanical strength of the hydrogel is beneficial as an adsorbent for industrial applications.

3.2. Effects of the mass ratio of microgel/SA on the Cu²⁺ adsorption capacity

As shown in Fig. 2, the Cu²⁺ adsorption capacity decreases as the mass ratio of microgel/SA increases. This indicates that the adsorption capacity of the composite hydrogel is better than that of acrylamide based hydrogels and lower than that of pure SA hydrogels. Two influencing factors, the swelling ratio and functional groups, were investigated. As shown in Table 2, the swelling ratio in distilled water slightly decreases as the mass ratio of microgel/SA increases. When the microgel content was high, the hydrogel had a high crosslinking density.²⁶ Microgels contain hydroxymethyl groups as the crosslinking functional groups. As the microgel content increases, the crosslinking density increases. In addition, microgels also contain a large number of nonionic amino groups. The increase of crosslinking density and the number of nonionic groups in the hydrogel leads to a reduction of the water adsorption. However, in Cu²⁺ solution with a concentration of 9.7 mmol L⁻¹, the swelling ratio of the microgel/SA composite hydrogel is significantly higher than that of the SA hydrogel, which is due to the small effect of the nonionic groups. However, the increase of the swelling ratio in the Cu²⁺ solution does not lead to an increase of the Cu²⁺ adsorption capacity. This indicates that the effect of the high swelling ratio caused by the structural factors in the Cu²⁺ solution on the hydrogel adsorption capacity is not the main factor. Adsorption groups have a dominant role.

Fig. 2 Cu²⁺ adsorption capacity under different mass ratios of microgel/SA (9.7 mmol L⁻¹ of Cu²⁺ concentration).

Table 2 Swelling ratio of microgel/SA hydrogels in different solutions

Mass ratio of microgel/SA	Swelling ratio in distilled water, g g^−1	Swelling ratio in Cu(NO3)2 solution (9.7 mmol L^−1), g g^−1
0	8.2	0.42
1.2	7.9	3.1
2.4	7.6	5.0
3.7	7.4	5.7
5.7	6.2	5.5

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The microgel/SA composite hydrogel contains many adsorption groups of Cu²⁺, such as carboxyl groups and hydroxyl groups from SA, and sulfonic groups and amide groups from AM/AMPS based microgels. For sodium alginate hydrogel crosslinked by Ca²⁺, the carboxyl groups of the mannuronate and guluronate residues were the dominant functional groups that can adsorb heavy metals caused by ion exchange, and hydroxyl groups only had metal-binding affinities.⁹ After heavy metal ions enter the network, they chelate with the functional groups and a chemical adsorption process occurs.⁷ In addition to the ionic activity of metal cations, the electrostatic attraction between the anions of hydrogels and the cations of the solutions plays a major role for ion exchange.³⁰ The sulfonic acid groups from AMPS also are anionic and have an electrostatic attraction with heavy metal cations, and also have the adsorption capability of heavy metals the same as carboxyl groups. Nonionic –NH₂ groups also have a coordination effect, which also can adsorb heavy metal ions by chelation.

The microgel molar ratio of –NH₂ from the AM monomer and –SO₃H groups from the AMPS monomer is about 24. The number of –NH₂ groups is much larger than that of –SO₃H groups in the microgels. The molar ratio of hydroxyl groups and carboxyl groups is about 2 for SA. As the mass ratio of microgel/SA decreases, the increase of –SO₃H groups from AMPS is less than the decrease of carboxyl groups from SA. The overall result is a decrease of the dominant ionic groups. However, the number of –NH₂ groups has an obvious increase. As shown in Fig. 2, when the mass ratio of microgel/SA is less than 2.4, the Cu²⁺ adsorption capacity has a sharp decrease as the microgel content increases. It also shows that the anionic groups have the dominate role for Cu²⁺ adsorption instead of –NH₂ groups. As shown in Fig. 2, when the mass ratio of microgel/SA is greater than 2.4, the Cu²⁺ adsorption capacity reduces slowly as the mass ratio increases. This shows that the effect of SA decreases and the effect of microgel increases. As the mass ratio of microgel/SA is greater than 3.7, the Cu²⁺ adsorption capacity is almost stable, which is about 0.2 mmol g⁻¹. According to the amount of AMPS in the microgel, the calculated maximum adsorption capacity caused by the sulfonic acid group is about 0.09 mmol g⁻¹, which is lower than that of the microgel/SA composite hydrogels. This indicates that –NH₂ groups also have a Cu²⁺ adsorptive action in the microgel/SA composite hydrogels.

3.3. The Cu²⁺ adsorption rate of microgel/SA composite hydrogels

As shown in Fig. 3, the microgel/SA composite hydrogel has a larger adsorption rate than that of the SA hydrogel. In addition, the effect of Cu²⁺ concentration on the adsorption rate of the composite hydrogel is very small. For adsorbents, the kinetics of metal sorption is influenced by three steps: the transport of metal ions from the bulk solution to the hydrodynamic boundary layer, the transport through the layer and the diffusion of metal ions into the hydrogel itself. For hydrogel adsorbents, the third step of the metal ions diffusing into the hydrogel is the rate limiting procedure.³¹ Hydrogels can swell in water. Cu²⁺ ions easily diffuse into the hydrogel with the water. So, the adsorption rate of microgel/SA hydrogel is influenced by the swelling rate. The swelling rate of the hydrogel depends on the characteristic dimension. As the characteristic dimension decreases, the swelling rate increases. The characteristic dimension is related to the microstructure, such as effective interconnected pores of microns in addition to the particle size.³²

Fig. 3 Cu²⁺ adsorption rate of a microgel/SA composite hydrogel (mass ratio of 5.7), 1–9.7 mmol L⁻¹ of Cu²⁺ concentration, 2–35 mmol L⁻¹ of Cu²⁺ concentration.

As shown in Fig. 1, the SA hydrogel has a smaller particle size than that of the microgel/SA composite hydrogel. However, the SA hydrogel has a lower adsorbing rate than that of the microgel/SA composite hydrogel, as shown in Fig. 3. This indicates that the characteristic dimension of the microgel/SA composite hydrogel is lower than that of the SA hydrogel in spite of the large microgel/SA composite hydrogel particle size. As shown in Fig. 4(a), the surface of the SA hydrogel is homogeneous and porous. Although the traditional SA hydrogel also has a porous structure, it is different from the porous hydrogel using forming technology. The pores on the surface are random closed to semi open cells, which are not connected and do not extend to the hydrogel inside. Only the interconnected pores provide the effective porosity for improving the hydrogel swelling rate. The capillary effect is helpful to increase the swelling rate when the interconnected open cells exist in the hydrogel.³³ As shown in Fig. 4(b), the surface of the microgel/SA composite hydrogel is not homogeneous. Many cracks of several microns in width and a small amount of micron microgel particles locate on the hydrogel surface. As shown in Fig. 4(c), large numbers of microgel particles of about 100 nm diameter exist in the hydrogel and pores between the microgel and hydrogel matrix are also observed. As shown in Fig. 4(b) and (c), the microgel particles are obviously linked together. When the hydrogel has interpenetrating channels, the swelling rate can be improved obviously.³⁴ Micro cracks can reduce the characteristic dimension of diffusion. Microgel particles themselves have a large swelling rate. So, micro cracks and the microgel aggregation structure are all beneficial to increase the water absorption rate.

Fig. 4 SEM images of hydrogels with different microgel/SA mass ratios: (a) 0, (b) 5.7 and (c) 2.4.

The density of the hydrogel was also measured. The SA hydrogel had a density of 1.474 × 10³ kg m⁻³. The density of the microgel/SA hydrogel was 1.048 × 10³ kg m⁻³. It is also shown that SA hydrogel is more compact than the microgel/SA composite. The loose character of the microgel/SA hydrogel makes it easier to diffuse within inner of the particle. It also results in a larger adsorption rate.

3.4. Effects of Cu²⁺ concentration on the adsorption capacity

As the Cu²⁺ concentration increases from 5 to 35 mmol L⁻¹, the Cu²⁺ adsorption capacity increases from 0.11 to 0.73 mmol g⁻¹. In the range from 5 to 10 mmol L⁻¹ of the Cu²⁺ concentration, the increase of the Cu²⁺ adsorption capacity is more obvious and the curve has a larger slope. The corresponding equilibrium concentration of the adsorption capacity is shown in Fig. 5. In order to investigate the adsorption mechanism, the adsorption isotherm equation was used to fit the adsorption of the microgel/SA hydrogel.

Fig. 5 Cu²⁺ adsorption capacity under different Cu²⁺ concentrations (microgel/SA = 2.4).

The phenomena of the liquid–solid adsorption are because the interaction between the molecules of solid and liquid is larger than that of liquid and liquid. So far, the mechanism of the liquid–solid adsorption is not as complete as that of the gas–liquid adsorption.³⁰ The investigation of the liquid–solid adsorption is still in the exploratory stage. The nature of the adsorbing groups is the determining factor in the adsorption for hydrogels. The Langmuir and Freundlich adsorption isotherm equations are always used to treat the adsorption results. So the two equations were used to fit the Cu²⁺ adsorption results.

The Langmuir isotherm equation:

where q and q_m are the actual equilibrium adsorption capacity and maximum adsorption capacity, respectively, c is the equilibrium solute concentration, and k_l is the Langmuir equation parameter. The linear form of the Langmuir equation is as follows:

The Freundlich isotherm equation is:

where q and c are the adsorption capacity and solute concentration, respectively and k_f is the Freundlich equation parameter. The n can be considered as a measure of adsorption intensity. The linear form of the Freundlich isotherm equation is as follows:

The linear fitting results of the Langmuir and the Freundlich isotherm equations are shown in Fig. 6. The fitting results are shown in Table 3. The correlation coefficient of the Freundlich equation fitting is 0.98778. However, the correlation coefficient of the Langmuir isotherm equation is only 0.50884. The standard deviation of the Freundlich equation is obviously far lower than that of the Langmuir equation. As shown in Table 3, the fitting results show that the Freundlich equation has a better fitting result than that of the Langmuir equation. The Freundlich model is usually applied in a strictly empirical and theoretical interest in terms of adsorption onto an energetically heterogeneous surface.³¹ However, the Langmuir model assumes a monolayer adsorption process on a homogeneous surface, which is suitably used in cases where all binding sites exhibit uniform behavior. The composite hydrogel contains many adsorbing groups, such as carboxyl groups from the G units and M units of SA, sulfonic acid groups and amino groups of microgels, which introduces heterogeneity onto the surface and is in violation of the basic assumption of the Langmuir isotherm.

Fig. 6 Fitting results: (a) the Langmuir isotherm equation and (b) the Freundlich isotherm equation (the c is equilibrium Cu²⁺ concentration).

Table 3 Fitting results of the Langmuir and Freundlich isotherm equations

Equation	Fitting equation	Correlation coefficient	Standard deviation
Langmuir	y = 0.16393x + 39.9833	0.50884	3.29478
Freundlich	y = 1.01848x − 1.66278	0.98778	0.05511

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3.5. Effects of pH on the adsorption capacity

As shown in Fig. 7, the pH value of Cu²⁺ solutions affects the adsorption capacity. When the pH is 1.0, the adsorption is very low. As the pH value increases from 3.0 to 5.0, the hydrogel has a high adsorption capacity. The adsorption has the highest value when the pH value is about 4.2. The hydrogel contains carboxyl and sulfonic acid groups. When the pH value is low, the hydrogen ions occupy part of the adsorption sites, which reduces the adsorption capacity.

Fig. 7 Cu²⁺ adsorption capacity under different pH values (microgel/SA = 2.4 and 9.7 mmol L⁻¹ of Cu²⁺ concentration).

3.6. Effects of Pb²⁺ on the Cu²⁺ adsorption ability

The Cu²⁺ adsorption capacity in a Cu²⁺ and Pb²⁺ mixed solution was also measured. As shown in Fig. 8, the experiments showed that the Cu²⁺ adsorption capacity was only equivalent to 20% of the Cu²⁺ adsorption capacity in pure Cu²⁺ solutions. This indicates that the hydrogel has the ability to adsorb other metal ions.

Fig. 8 Cu²⁺ adsorption capacity (microgel/SA = 2.4), 1 – in Cu²⁺ solutions (9.7 mol L⁻¹), 2 – in Cu²⁺ (9.7 mol L⁻¹) and Pb²⁺ (9.7 mol L⁻¹) mixed solutions.

4. Conclusions

Spherical granular composite microgel/SA hydrogels with comprehensive excellent performances in the adsorption capacity, adsorption rate and stability in a strong acid were prepared by SA and AM/AMPS based microgels.

Dehydration of part of the water is important for a spherical morphology of the dry granular hydrogels. The composite hydrogel contains an SA network and a microgel network. The microgel network causes the stability of the hydrogel in strong acidic conditions. The swelling ratio of the composite hydrogel in Cu²⁺ solutions is higher than that of the SA hydrogel caused by the nonionic groups. The effect of the adsorption group on the hydrogel adsorption capacity is more than that of the swelling ratio caused by structural factors. The Cu²⁺ adsorption capacity decreases as the microgel/SA ratio increases, which is mainly caused by the decrease of the ionic dominant adsorption groups. As the mass ratio of microgel/SA becomes greater than 3.7, the effect of the microgel increases and the Cu²⁺ adsorption capacity is almost stable. The –NH₂ groups also have a Cu²⁺ adsorptive action in the microgel/SA composite hydrogel.

The composite hydrogel has a low density and heterogeneous surface such as micro cracks, micro particles and a large number of microgel particles with a diameter of about 100 nm, which leads to the faster adsorbing rate. The correlation coefficient of the Freundlich equation fitting is 0.98778. The fitting results show that the Freundlich isotherm equation has better results than that of the Langmuir isotherm equation.

The adsorption capacity has a high value when the pH value is 3–5. The hydrogel also has the ability to adsorb Pb²⁺ ions.

With the idea that in a combination of SA and microgel each plays their respective advantages, it is favorable to develop hydrogel adsorbents with a high adsorption capacity, fast swelling rate and high mechanical properties according to changing the adsorbing functional groups for heavy metal ions and the crosslinking of the microgel network.

Acknowledgements

This study was supported by the Shandong Provincial Natural Science Foundation (ZR2012BM013). Thanks for the support of the Key Laboratory of Special Functional Aggregated Materials, Ministry of Education, Shandong University.

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