Introduction

Iran is in the top ten world metal producers in terms of copper production (Mollai et al. 2009). Demand for nickel and copper in the Iran market is progressively growing, while primary resources are being depleted. Nickel is used in combination with other metals to form alloys used for coins, jewelry, stainless steel, electroplating, to color ceramics, and in battery production (Schaumlöffel 2012). Copper, also, is used as a conductor of heat and electricity, a building material, and a constituent of various metal alloys and catalysts (Agrawal and Sahu 2010).

However, there are some health risks associated with these valuable metals. An increase in the intake of Cu2+ can cause health problems such as vomiting (Fukui et al. 1994), hypotension (López et al. 2004), melena (Oldenquist and Salem 1999), gastrointestinal distress (Knobeloch et al. 1994), and kidney failure (Kayaalt et al. 2010), while high Ni2+ intake can lead to nasopharynx (Metzgeroth et al. 2007), lung and dermatological diseases (Samitz and Katz 1975; Xu et al. 2011), birth effects (Brodziak-DopieraŁa et al. 2011), embolism (Szabó et al. 1985), and chronic bronchitis (Tessier and Pascal 2006). Therefore, it is essential to control the concentration of these heavy metals in the drinking water as well as, from the economical point of view, recover as much as possible of the copper and the nickel (Giannopoulou and Panias 2007).

To reduce heavy metal pollution problem and also recovery of these valuable metals, some traditional methods can be used including oxidation-reduction (Amara et al. 2008; Gupta et al. 2012; Saleh and Gupta 2012a, 2011; Saleh et al. 2011), filtration (Molinari et al. 2008), electrochemical treatment (Akbal and Camc 2011; Gupta et al. 2007a), solvent extraction (Ramachandra Reddy and Neela Priya 2005), reverse osmosis (Keane 1998), membrane separation (Pośpiech and Walkowiak 2007), and adsorption (Ghaee et al. 2012; Gupta et al. 2009, 2010, 2007b, 2011a, b, c; Saleh and Gupta 2012b). Because of high operation cost and low metal ion retention capacity of these methods, our low-cost adsorbents are an effective and economical method to adsorb and recover valuable Ni2+ and Cu2+ (Uğuzdoğan et al. 2010).

Recently, the adsorption process with strong affinity and high loading capacity for targeted metal ions has developed much attention, which leads to modification of hydrogels (Demirbilek and Özdemir Dinç 2012). Hydrogels are loosely cross-linked hydrophilic polymers that can absorb, swell, and retain aqueous solution up to hundred times of their own weight (Barati et al. 2010). Some attempts have been made to modify the properties of hydrogels, for example, the incorporation of nano or micro particles of inorganic materials such as montmorillonite (Hua et al. 2010), kaolin (Pourjavadi et al. 2008), mica (Limparyoon et al. 2011), bentonite (Shirsath et al. 2011), laponite (Li et al. 2009), attapulgite (Zhang et al. 2007a, b), and zeolites (Bhardwaj et al. 2012) into polymer networks have been recently investigated.

Although many works have focused on the improvement of the swelling ability (Zhang et al. 2009), gel strength (Xu et al. 2010), mechanical and thermal stability (Zhang et al. 2007a, b) of composite hydrogel, only few literature have been focused on the adsorption and recovery capacity of heavy metal ions into the composite hydrogels (Yan et al. 2012; NüketTirtom et al. 2012; Hou et al. 2012).

Therefore, on the basis of our previous work on the preparation and ability to remove Pb2+ and Cd2+ by poly(acrylamide-co-acrylic acid) modified with porous materials (e.g., zeolites) (Zendehdel et al. 2011a), the adsorption capacity of the poly(methacrylamide-co-acrylic acid)/montmorillonite nanocomposites for Cu2+ and Ni2+ was studied in this paper. The effects of pH value and temperature of the Cu2+ and Ni2+ solutions, contact time with solution, montmorillonite content (weight percent), and the initial concentration of Cu2+ and Ni2+ solutions (Ci) were investigated. The adsorbed Cu2+ and Ni2+ ions can be desorbed from this adsorbent by a small amount of inorganic acid obtaining thus a high concentration solution of corresponding ions by which Ni2+ and Cu2+ can be enriched and recycled.

Material and methods

Chemicals

Methacrylamide (MAAm, Merck, Germany) was used as purchased. Acrylic acid (AA, Merck, Germany) was distilled under reduced pressure before use. Potassium hydroxide and N, N′-methylene bisacrylamide (Bis) were supplied by Merck, Germany. Ammonium persulfate (APS, analytical grade, Merck, Germany) was recrystallized from water. Natural montmorillonite (MMT) modified with a quaternary ammonium salt, dehydrogenated tallow dimethyl ammonium chloride (Cloisite® 15A with the average particle size of 75 nm measured by particle size analyzer) was supplied by Southern Clay Products Co., USA. Other agents were all analytical grade, and all solutions were prepared with distilled water.

Preparation of poly(MAAm-co-AA)/MMT nanocomposite hydrogels

The optimum molar ratio of initial materials for preparation of poly(MAAm-co-AA) considered at prior work in our laboratory (Zendehdel et al. 2010) but with small modifications. A series of poly(MAAm-co-AA)/MMT nanocomposite from MAAm, neutralized AA, and MMT was prepared using the one-step method via in situ intercalated polymerization (Zendehdel et al. 2011b). Typical preparation conditions for samples are as follows:

At first, potassium acrylate (KAA) was prepared by partially neutralizing a diluted acrylic acid solution with a predetermined amount of 50 wt% aqueous potassium hydroxide solution, which was added dropwise in an ice bath. Then, 10 g MAAm was dissolved in a mixing solution which consisted of 19 g of KAA, 2 g of Bis, and 55 ml of distilled water in a 250-ml four-neck flask equipped with a stirrer, a condenser, a thermometer, and a nitrogen line. Then, 0.62 g of MMT (ultrasound for 1 h before use for getting nano dispersed suspension) was slowly added to stirring monomer solution. The obtained suspension was mixed by a 300-rpm magnetic stirrer for 30 min. After removal of oxygen, this suspension was heated to 60 °C gradually, while 0.2 g of APS was introduced to initiate MAAm and KAA to generate macro-radicals. The temperature was kept at 70 °C for 3 h to complete the polymerization reaction. Then, the resulting product was washed several times with hydro ethanol solution and soaked in distilled water for 24 h. After that, the swelled hydrogels were taken out of the water and dried at 70 °C to a constant weight. The dried product was milled and screened. All samples used had a particle size in the range of 60–80 mesh.

Characterization

FTIR spectra of poly(MAAm-co-AA) hydrogel and poly(MAAm-co-AA)/MMT nanocomposite hydrogel were recorded (Perkin-Elmer, Spectrum 100) using KBr pellets. XRD patterns were obtained using a Philips X'Pert PRO Alpha-1diffractometer (Cu K radiation, λ = 1.54056 A°, 40 kV, 40 mA) scanning from 5° to 80° at 4°/min. The morphology of the fractured specimens was observed on a XL30 SEM instrument (Philips Company, Netherlands) at an acceleration voltage of 30 kV after sputter coating with gold under vacuum.TGA was performed using a Perkin-Elmer (TGA 4000) Thermal Analyzer in air at a heating rate of 15 °C/min in the temperature range of 25–600 °C.

Adsorption experiment

Adsorption experiments were done in batch equilibrium mode. All experiments were done by mixing 100 ml of aqueous solution of known concentration of Cu2+ and Ni2+ ions with 0.1 g of synthetic nanocomposites. The mixtures were shaken in an orbital shaker (THZ-98A) with 300 rpm at 298 K for a given contact time (5 to 180 min). The pH values of initial solutions were adjusted with 0.1 mol L−1 HCl or NaOH solutions by using a pH meter (Metrohm-744). The solution is filtered using a Whatman #40 filter paper. The filtrate was analyzed using inductively coupled plasma optical emission spectrometry Perkin Elmer Optima™ series using wavelength 324.75 and 231.60 nm for Cu2+ and Ni2+, respectively (Futalan et al. 2011).

For adsorption studies, 25 up to 700 mg L−1 were chosen as the initial concentration of Cu2+ and Ni2+ single and binary solutions. The effects of pH were studied in the pH range of 2–11 at an initial concentration of 25 or 100 mg L−1 for both ions and contact time of 60 min. The adsorption capacity was calculated according to the following equation:

$$ {q_e}=\frac{{\left( {{C_i}-{C_e}} \right)V}}{m} $$
(1)

Where q e is the adsorption capacity, the amount of solute sorbed at equilibrium (milligrams per gram), C i and C e are the initial and final or equilibrium Cu2+ or Ni2+ concentration (milligrams per liter), m is the mass of adsorbent used, and V is the volume of Cu2+ or Ni2+ solution (milliliters). All the batch isotherm tests were replicated, and the experimental blanks were run in parallel. Check standards and blanks were run. Standard sources were used for instrument calibration and standard verification. All the experiments were done in triplicate, and average values are reported.

Desorption and regeneration

Desorption experiments were done in batch equilibrium mode by dispersing used (loaded) adsorbent (adsorbent dose in adsorption experiment = 100 mg/ml; temperature = 298 K; contact time = 60 min; pH = 6.8; MMT content = 3 wt%) into two different volumes of 1 mol L−1 HCl solution (10 and 20 ml) and agitating at 298 K for 90 min in the orbital shaker. Then, the desorbed amount of Cu2+ and Ni2+ was obtained. In this study, we used 0.1 mol L−1 NaOH as the regenerating agent to investigate the regeneration ability of developing nanocomposite hydrogel adsorbent.

Enrichment and recovery

A 100-mg synthetic nanocomposite was contacted with 100 ml of Cu2+ or Ni2+ solution at 25 mg L−1 and agitated for 90 min at 25 °C to reach the equilibrium. Then, the used adsorbent was separated using filtration and contacted again with another 100 ml of fresh solution at 25 mg L−1 to reach the adsorption equilibrium. A similar procedure was repeated until the percentage of adsorption A<95 % (Zheng et al. 2011). Using 1 mol L−1 HCl solution to elute and recover the Cu2+ or Ni2+ which retained on the adsorbent.

Results and discussion

FT-IR spectral analysis

The FT-IR spectra of poly(MAAm-co-AA) and poly(MAAm-co-AA)/montmorillonite nanocomposite hydrogel containing 3 wt% montmorillonite are shown in Fig. 1a and b, respectively. In Fig. 1a, a strong and broad absorption band has been observed at 3,406 cm−1 due to N-H groups of methacrylamide, and the peak at 1,666 cm−1 represents the carbonyl group. There is a shoulder at 1,715 cm−1 that corresponds to the carbonyl group of polyacrylate. Furthermore, one can see the two bonds at 1,388 and 1,537 cm−1 related to COO− group in polyacrylate and the peaks at 1,475 and 2,891 cm−1 which represent the C-N and C-H bonding, respectively (Ni et al. 2005).

Fig. 1
figure 1

FTIR spectra of a poly(MAAm-co-AA) and b poly(MAAm-co-AA)/MMT

Full size image

In (b) spectrum, the peaks of carbonyl shifted to around 1,653 cm−1 and the intensity of the peaks at 3,504 cm−1 corresponding to the reduced N-H and shifted to a lower frequency. This may be due to the formation of hydrogen bonding between COOH, NH2 of the copolymer and hydroxyl group of Al-OH and Si-OH in montmorillonite. In addition, one can see two bands at about 1,022 cm−1 due to Si-O and 418 cm−1 related to Al-OH in the nanocomposite.

XRD analysis

The XRD patterns of montmorillonite and poly(MAAm-co-AA)/montmorillonite nanocomposite hydrogel are plotted in Fig. 2. The XRD pattern of montmorillonite shows a strong peak at 2θ = 2.39° which corresponds to a basal spacing of 30.1 A°. The absence of diffractions peaks for poly(MAAm-co-AA)/montmorillonite suggests that the clay layers have been exfoliated and dispersed in the polymer matrix to form a nanocomposite structure (Al et al. 2008; Liu et al. 2007). Hence, a broad peak observed at 2θ = 20° is due to poly(MAAm-co-AA) (Li et al. 2005).

Fig. 2
figure 2

X-ray diffraction patterns of a MMT and b poly(MAAm-co-AA)/MMT

Full size image

SEM analysis

The micrographs of cross-linked poly(MAAm-co-AA) and poly(MAAm-co-AA)/montmorillonite nanocomposite containing 1 and 3 wt% of montmorillonite are depicted in Fig. 3a, b, and c, respectively. It is clearly observed that the poly(MAAm-co-AA) hydrogel shows a porous surface with interconnected pore, which interspersed on the surface. This surface morphology is beneficial for water intake, making the ions easily and quickly accessible to the reactive functional groups distributed within the polymeric networks. In (b) and (c) micrographs, it can be observed that the organo montmorillonite nano particles are more finely dispersed in the polymer matrix.

Fig. 3
figure 3

Scanning electron micrographs of a cross section of poly(MAAm-co-AA) and surface of poly(MAAm-co-AA)/MMT b 1 wt% of MMT, c 3 wt% of MMT

Full size image

Thermogravimetric analysis

The TGA of poly(MAAm-co-PA) hydrogel and poly(MAAm-co-PA)/montmorillonite nanocomposite hydrogel are shown in Fig. 4a and b, respectively. The initial decomposition temperature (T id) was found to be 252 and 297 °C and the final decomposition temperature (T fd) was observed as 609 and 615 °C for poly(MAAm-co-PA) hydrogel and poly(MAAm-co-PA)/montmorillonite nanocomposite hydrogel, respectively. The incorporation of montmorillonite in poly(MAAm-co-PA) matrix increased the thermal stability of the formulation, i.e., poly(MAAm-co-PA)/montmorillonite nanocomposite hydrogel. This can be attributed to the high thermal stability of the clay minerals and to the interaction between the clay mineral particles and the polymer matrix (Li et al. 2005). Further, by comparison between Fig. 4a and b, it is observed that maximum loss in cross-linked polymer occurred after first stage of decomposition where temperature range was usually corresponded to the de-polymerization process. Since the proposed nanocomposite hydrogel is intended to be used for the recovery of Cu2+ and Ni2+ ions from the aqueous solution, and the removal process was normally carried out at room temperature, the nanocomposite polymer was totally stable in the room temperature, which varied between 27 and 32 °C.

Fig. 4
figure 4

TGA thermograms of a poly(MAAm-co-AA) and b poly(MAAm-co-AA)/MMT (3 wt% of MMT)

Full size image

Adsorption capacity

Figure 5 shows the effect of initial concentration of Cu2+ and Ni2+ ions on the adsorption capacity, respectively (pH = 6.8, temperature = 25 °C, contact time = 60 min, and amount of MMT = 3 wt%). Results show that the adsorption capacity increases with increasing the initial concentration of both ions up to 500 mg L−1. This is attributed to the increase in Cu2+ or Ni2+ ion concentration resulting in an increase in the diffusion of ions into the polymeric network as a result of an increase in the driving force of the concentration gradient. However, by increasing the initial concentration of both ions from 500 to 700 mg L−1, the amount of ions adsorbed remains nearly constant due to saturation of the adsorption sites at the adsorbents. However, even with very high initial concentration of 700 mg L−1 of Cu2+ and Ni2+ solutions, nanocomposite can adsorb about 29 mg g−1 of Cu2+ and 21 mg g−1 of Ni2+.

Fig. 5
figure 5

Effect of Cu2+ and Ni2+ initial concentration on adsorption capacity of nanocomposite hydrogel (contact time = 60 min; pH = 6.8; temperature = 298 K; adsorbent dose = 100 mg/100 ml; MMT content = 3 wt%)

Full size image

The effect of contact time on the adsorption capacity was investigated at an initial Cu2+ or Ni2+ concentrations of 25 and 100 mg L−1 as shown in Fig. 6 (pH = 6.8, temperature = 25 °C, and amount of MMT = 3 wt%). The adsorption increases with time up to 60 min for Cu2+ and up to 30 min for Ni2+ and then saturates. Thus, it appears that the adsorption is rapid, and the time to reach equilibrium is 60 min for Cu2+ and 30 min for Ni2+ ions. This rapid adsorption rate is consistent with the expected results derived from SEM analysis, which revealed their porous microstructure.

Fig. 6
figure 6

Influences of contact time on the adsorption capacity of nanocomposite hydrogel for metal ions with different amount of C i (pH = 6.8; temperature = 298 K; adsorbent dose = 100 mg/100 ml; MMT content = 3 wt%)

Full size image

Figure 7 shows the adsorption capacity of Cu2+ and Ni2+ ions at different MMT levels in as-prepared nanocomposite hydrogel. The result shows that with increasing of MMT content from 1 to 3 wt% (C i = 100 mg L−1, pH = 6.8, temperature = 25 °C, and contact time = 60 min), the adsorption of Cu2+ and Ni2+ increased. According to the FT-IR analysis, Fig. 1b, –OH bond of MMT participated in the copolymerization and the formation of the exfoliated nanostructure in poly(MAAm-co-AA)/MMT, which may improve the polymeric network, and hence enhance the adsorption capacity. But any further increase in MMT content of nanocomposite hydrogel had a reverse effect on the adsorption of both metal ions, and the adsorption capacity decreased. This is maybe because of –OH bond of hydrated montmorillonite, interaction between MMT with AA which became intensive. This means that more chemical and physical cross-link bonds were formed, and elasticity of polymer chains decreased. Consequently, the osmotic pressure between polymeric network and external solution for water permeation decreased which results in the lower adsorption capacity of poly(MAAm-co-AA)/MMT nanocomposite. This is in line with finding of Wang and co-worker who studied the adsorption performance of chitosan-g-poly(acrylic acid)/montmorillonite nanocomposite hydrogels for methylene blue (MB) and concluded a similar result that an appropriate MMT content was critical for improving the adsorption rate for MB (Wang et al. 2008).

Fig. 7
figure 7

Effect of MMT content on adsorption capacity of nanocomposite hydrogel (C i = 100 mg L−1; pH = 6.8; temperature = 298 K; contact time = 60 min, and adsorbent dose = 100 mg/100 ml)

Full size image

Adsorption kinetics

The study of adsorption kinetics identifies what type of adsorption mechanism occurs. These models also describe the solute uptake rate, and evidently this rate controls the residence time of adsorbate uptake at the nanocomposite hydrogel–solution interface. To analyze Cu2+ and Ni2+ adsorption on the nanocomposite hydrogel, nonlinear form of pseudo-first-order (Langergren 1898) and pseudo-second-order (Ho et al. 1999) kinetic models was utilized and expressed as follows:

Pseudo-first-order model,

$$ \log \left( {{q_e}-{q_t}} \right)=\log {q_e}-\frac{{{k_1}t}}{2.303 } $$
(2)

Pseudo-second-order model,

$$ \frac{t}{{{q_t}}}=\frac{1}{{{k_2}q_e^2}}+\frac{t}{{{q_e}}} $$
(3)

Where k 1 (grams per gram per minute) is the pseudo-first-order kinetic constant; k 2 (grams per milligram per minute) is the pseudo-second-order kinetic constant; q e is the amount of solute sorbed at equilibrium (milligrams per gram), while q t is the amount of solute sorbed at time t (milligrams per gram) before reaching equilibrium.

The conformity between experimental data and the model-predicted values was expressed by the correlation coefficients (R 2, values close or equal to 1). A relatively high R 2 value indicates that the model successfully describes the kinetics of Cu2+ and Ni2+ adsorption.

Table 1 shows the kinetic constants as well as the correlation coefficients, R 2 (C i = 25 and 100 mg L−1, temperature = 298 K, pH = 6.8, the amount of MMT = 3 wt%). Pseudo-second-order equation has the highest correlation coefficient for both metal ions (R 2 > 0.98). Table 1 data indicate that the q e th values for the second-order model are closer to q e exp values (3.67 and 2.82 mg g−1 for the Cu2+ and Ni2+, respectively) in comparison to first-order values. So, it can be concluded that Cu2+ and Ni2+ sorption onto the nanocomposite hydrogel seems to be more pseudo-second order, which means that chemisorption is the rate-controlling mechanism for adsorption of Cu2+ and Ni2+ onto our adsorbents.

Table 1 Parameters of kinetic models of Cu2+ and Ni2+ adsorption onto nanocomposite hydrogel
Full size table

Adsorption isotherms

Adsorption isotherm studies were carried out by fitting our data (see Fig. 8a, b) to the Langmuir, Freundlich, and Temkin isotherms. Adsorption isotherms describe how ions interact with adsorbent materials and therefore are critical in optimizing the use of adsorbents. The Langmuir isotherm is valid for monolayer sorption due to a surface of a finite number of identical sites and expressed in the linear form as Eq. 4.

Fig. 8
figure 8

Adsorption isotherms of a Cu2+ and b Ni2+ on nanocomposite hydrogel at different temperatures (contact time = 60 min; pH = 6.8; adsorbent dose = 100 mg/100 ml; MMT content = 3 wt%)

Full size image
$$ \frac{1}{{{q_e}}}=\frac{1}{{b{q_{\max }}{C_e}}}+\frac{1}{{{q_{\max }}}} $$
(4)

Where q e is the amount of adsorbed ions at equilibrium (millimoles per gram), q max is the monolayer adsorption capacity of the adsorbent (millimoles per gram), C e is the equilibrium concentration of ion solution (moles per liter), and b is the Langmuir adsorption constant (liters per mole) and is related to the free energy of adsorption. The Freundlich isotherm describes the heterogeneous surface energies by multilayer adsorption and is expressed in a linear form as Eq. 5.

$$ \log {q_e}=\log {K_F}+\frac{1}{n}\log {C_e} $$
(5)

Where K f includes adsorption extent (milligrams per gram), and n is an empirical parameter related to the intensity of adsorption, which varies with the nonlinearity between solution concentration and adsorption. The Temkin isotherm (Tempkin and Pyzhev 1940) is the early model describing the adsorption of metal ions onto adsorbents. The isotherm contains a factor that explicitly takes into account the adsorbent–adsorbate interaction and is based on the assumption that the free energy of sorption is a function of the surface coverage. The Temkin isotherm in the linear form is expressed as follows:

$$ {q_e}=\frac{RT }{{{b_T}}}\ln {A_T}+\left( {\frac{RT }{{{b_T}}}} \right)\ln {C_e} $$
(6)

Where A T is the equilibrium binding constant corresponding to the maximum binding energy (liters per milligram), b T is the Temkin isotherm constant, T is the temperature (Kelvin) and R is the ideal gas constant (8.314 J mol−1 K−1; Gupta et al. 2010)

The isotherm constants and correlation coefficient R 2 were calculated and summarized in Table 2. On comparing the linear regression values, it is concluded that the Langmuir and Freundlich isotherms are capable of representing the data more satisfactorily (R 2 = 0.98–0.99) than the Temkin isotherm (R 2 = 0.95–0.98). Both Cu2+ and Ni2+ have high correlation value (R 2 > 0.99) with Freundlich isotherm that the 1/n values were between 0 and 1 indicating the adsorption of Cu2+and Ni2+ ions onto as-prepared nanocomposite was favorable at studied conditions. The extent of adsorption of Cu2+ and Ni2+ by different other adsorbent, collected from the literature, along with the results of the present study is included in Table 3. The data in the table indicate that synthesized nanocomposite hydrogel in current study has comparable adsorption capacity compared to many adsorbents.

Table 2 Estimated adsorption isotherm parameters for Cu2+ and Ni2+ adsorption
Full size table
Table 3 Comparison of Cu2+ and Ni2+ adsorption on poly(MMAm-co-AA)/MMT with other adsorbents
Full size table

Cu2+ may coexist with Ni2+ in industrial wastewater, and thus, investigation of the adsorption capacities of these coexisting ions in a binary system is important. For this binary system, the original Langmuir and Freundlich isotherm equations were applied to determine the effect of presence of another metal ion on the isotherm constants. In Table 4, the isotherm constant values for Cu-Ni system are reported. For Cu-Ni system, Cu2+ best fits Freundlich (R 2 = 0.9989), while Ni2+ has a better correlation coefficient value for Langmuir (R 2 = 0.9975). Figure 9 shows the experimental data and models for binary system of Cu2+ and Ni2+.

Table 4 Estimated adsorption isotherm parameters for Cu2+ and Ni2+ in (Cu-Ni) binary metal systems
Full size table
Fig. 9
figure 9

Comparison of experimental isotherm data of binary systems of Cu-Ni against the theoretical non-linear plots generated using the Langmuir and Freundlich models (contact time = 60 min; pH = 6.8; temperature = 298 K; adsorbent dose = 100 mg/100 ml; MMT content = 3 wt%)

Full size image

Effect of temperature on Cu2+ and Ni2+ adsorption

The adsorption behavior of nanocomposite hydrogel is affected by the temperature of the adsorption medium in many ways (C i = 100 mg L−1, pH = 6.8, and amount of MMT = 3 wt%). With increasing the temperature, the adsorption of heavy metal ions increased (Fig. 10a, b). The increase in the temperature of the adsorption medium is usually accompanied by the enhancement of the solution intake rate due to increased kinetic energy of solvent molecules. The activation energy of the solution intake process (swelling process) was determined by fitting the experimental data to the Arrhenius equation given below:

Fig. 10
figure 10

Effect of temperature on the adsorption capacity of nanocomposite hydrogel for a Cu2+ and b Ni2+ (Ci = 100 mg L−1; pH = 6.8; adsorbent dose = 100 mg/100 ml; MMT content = 3 wt%)

Full size image
$$ \ln k=ln(A)-\left( {\frac{{{E_a}}}{RT }} \right) $$
(7)

Where E a (kilojoules per mole) is the apparent activation energy for the adsorption process, k the rate constant (minutes), A the Arrhenius constant, R the gas constant (8.314 J mol−1 K−1), and T (Kelvin) is the solution temperature. The activation energies for the nanocomposite hydrogel, as determined from the slope of the linear plot (see Fig. 11) between the logarithm of k (the rate constant of pseudo-first-order kinetic equation (k 1; Table 5), were applied, and 1/T has been found to be 9.108 and 5.782 kJ mol−1 for Cu2+ and Ni2+, respectively. It was obtained from this result that the adsorption involved physisorption and the process controlled by a diffusion process (Zou et al. 2006; Argun 2008).

Fig. 11
figure 11

Plot of pseudo-first order kinetic constant (ln k 1) vs. temperature (1/T). The activation energies of the adsorption process are determined from this graph

Full size image
Table 5 Parameters of kinetic models of Cu2+ and Ni2+ adsorption onto nanocomposite hydrogel in different solution temperature
Full size table

Effect of pH on Cu2+ and Ni2+ adsorption

Figure 12 shows the effect of equilibrium pH on the amount of Cu2+ and Ni2+ adsorbed on as-prepared adsorbent (C i = 100 mg L−1, temperature = 298 K, pH = 6.8, contact time = 60 min, and amount of MMT = 3 wt%). The adsorption capacity of both metal ions strongly depends on the solution pH. It can be seen from Fig. 12 that the adsorption capacity increased from 2.3 to 14.5 mg g−1 when pH of solution media increased from 2.1 to 6.8, but further increment of pH from 6.8 to 11.5 decreased the adsorption capacity of both metal ions. This is because when the pH of the medium is low (pH = 2), the carboxylic groups of poly(potassium acrylate) are almost in an undissociated state and, therefore, these polymeric chains are closely packed through hydrogen bonding. When pH of the adsorbing medium increases from pH = 2, the carboxylic groups dissociate into COO− and result in relaxation of the networks by breaking the hydrogen bonds. Also, in this situation, more mobile ions (K+ ions) exist in the hydrogel which may make the ion-exchange reaction between Cu2+ or Ni2+ ions and exchangeable cations existing in the composite happen very easily. This in turn increases the diffusion of water molecules into the network, thereby increasing the adsorbing capacity. When the pH of the adsorbing medium is greater than 7, where the number of carboxylate anions reaches the maximum level, no more repulsion could be expected in the cross-linked hydrogel. This causes expulsion of water molecules from within the hydrogel into the media due to the weakening of the hydrogen bonds formed between water molecules and cross-linked hydrogel chains. The formation of Cu(OH)2 or Ni(OH)2 precipitation is another cause of the decrease in adsorbing capacity when the pH value of adsorbing medium is in the alkaline range (Zheng et al. 2010).

Fig. 12
figure 12

Effects of pH on adsorption capacity of nanocomposite hydrogel for Cu+2 and Ni+2 (contact time = 60 min; temperature = 298 K; adsorbent dose = 100 mg/100 ml; MMT content = 3 wt%)

Full size image

Desorption and regeneration study

Desorption studies are necessary for optimization of the regeneration of the nanocomposite, so that it can be used again to adsorb metal ions as well as for the recovery process of the Cu2+ and Ni2+ ions from the saturated nanocomposite hydrogel According to Wang et al. (2009), for desorption of Cu2+ and Ni2+, the best performance was obtained using 1 mol L−1 HCl solution with desorption ratios of 96 and 98 %, respectively.

Reusing ability of an adsorbent by a regeneration process is an important parameter. To test the reusability of the adsorbent, adsorption–desorption cycles were repeated five times using the same adsorbent. Figure 13 shows the adsorption capacities of the adsorbents for Cu2+ and Ni2+ ions over five successive adsorption–desorption cycles. The results show that the adsorption capacity of the adsorbents slightly decreased with an increase in adsorption–desorption cycles. As shown in Fig. 13, more than 90 and 92 % of the initial adsorption capacities were obtained after fifth cycle of desorption for Cu2+ and Ni2+ ions, respectively. Therefore, poly(MAAm-co-AA)/MMT nanocomposite hydrogel is a good reusable adsorbent and could be successfully utilized for the recovery of heavy metal ions from water and wastewater.

Fig. 13
figure 13

Regeneration studies of poly(MAAm-co-AA)/MMT after five cycles

Full size image

Conclusion

Poly(methacrylamide-co-acrylic acid)/montmorillonite nanocomposite hydrogel was synthesized, characterized, and was used for the adsorption of Cu2+ and Ni2+ from aqueous solution. The as-prepared adsorbent not only possesses good chemical stability but also shows good affinity to Cu2+ and Ni2+ ions, with the maximum adsorption capacity of 29 and 21 mg g−1, respectively. Some important factors affecting the adsorption such as contact time, initial metal concentration, temperature, and pH were studied. The equilibrium data of Cu2+ were best fitted the Freundlich isotherm, while those of Ni2+ followed the Langmuir isotherm. The kinetic data indicated that the adsorption kinetics followed the pseudo-second-order kinetic model. The high adsorption capacity and average desorption efficiency during the consecutive five-time adsorption–desorption processes of poly(MAAm-co-AA)/MMT nanocomposites implied that the nanocomposites possess the potential of regeneration and reuse. From the results above, it could be concluded that poly(MAAm-co-AA)/MMT nanocomposite hydrogel is an quite effective adsorbent for the removal and recovery of Cu2+ and Ni2+ from aqueous solution.

In summary, the proposed method is technically effective. How to prepare the nanocomposite hydrogel adsorbent and how to operate it optimally based on the effect of operation conditions on the adsorption process have been discussed. Furthermore, the proposed method is economically effective as well. The suggested adsorbent is relatively cheap to make and also provides a better chance of recovering the heavy metals adsorbed.