Poly(acrylamide) grafted and glutaraldehyde-crosslinked alginic acid nano-magnetic adsorbent (AAMA) was prepared by selecting Cd2+ as a template ion. Scanning electron microscope (SEM), thermo-gravimetric analyzer (TGA), vibrating sample magnetometer (VSM) and infrared spectroscopy (IR) were used to characterize the morphology and structure of AAMA. The adsorption of AAMA for different metal ions was compared and the impact of various factors for adsorption of Cd2+ was systematically investigated. These results suggested that the AAMA was the aggregates of Fe3O4 nanoparticles with a diameter of about 50–100 nm and had selectivity for Cd2+ adsorption. The maximum adsorption capacity for Cd2+ is 175 mg/g at pH 5.0 and 303 K. The experimental data were well described by the Langmuir isotherm model and pseudo-second-order model. The parameters of adsorption thermodynamics concluded that the adsorption progress is spontaneous and endothermic in nature. The parameters of adsorption activation energy suggested that there is physical adsorption and chemisorption on the adsorption of metal ions. AAMA could be regenerated by EDTA and still keep 71% adsorption capacity in the fifth consecutive adsorption-regeneration cycle. Therefore, AAMA would be useful as a selective and high adsorption capacity nano-magnetic adsorbent in the removal of Cd2+ from wastewater.

  • A novel poly(acrylamide) grafted and glutaraldehyde-crosslinked alginic acid nano magnetic adsorbent was prepared and used for selective adsorption of Cd2+.

  • The maximum adsorption capacity for Cd2+ is 175 mg/g, which was more 4.79 times that for any other heavy metal ions.

  • AAMA selectively adsorbs Cd2+, which not only relies on the ion cavity, but also depends on functional groups.

Heavy metal ions in water have been a major preoccupation for many years because of toxicity to biology and the ecosystem threat. Cadmium cannot be degraded by microorganisms and can be accumulated in biology. It is considered as one of the most toxic metals. Cadmium is carcinogenic, it can cause dyspnea, lung fibrosis, testicular degeneration and chronic lung disease (Ahmaruzzaman 2011). Therefore, cadmium removal from wastewater has become a greatly significant subject today. Researchers have developed many technologies and methods for cadmium removal from wastewater including chemical precipitation, reverse osmosis, membrane separation, adsorption, and electrodialysis (Uddin 2017). Considering the efficiency and economy points of view, adsorption is one of the most widely used and promising methods for cadmium removal.

Alginic acid is extracted from brown algae, one of the most abundant natural polymers. Alginic acid has three different functional groups, including hydroxyl, ether and carboxyl. The type, source, and vegetation conditions of alginic acid have an impact on the arrangements of mer units. There are more than 200 kinds of alginate in the market. Alginic acid has been widely used in the food, pharmaceutical, and cosmetic industry due to its stabilizing properties and gelling abilities. Alginic acid, with its good coordination ability, can be applied to remove cadmium ions from effluent (Vaid et al. 2015). It is well known that alginic acid can form a three-dimensional coordination bond with cadmium ions. Moreover, the adsorption capacity of brown algae biomass biosorbent and its chemical modifiers have been investigated by many researchers. Matheickal et al. revealed the adsorptive property of heavy metal ion biosorbents using Ecklonia radiata and Durvillaea potatorum. In addition to metal ion chelation, alginic acid has advantageous physicochemical properties that affect its high attention in polymer research (Pawar & Edgar 2012). However, unsatisfying mechanical properties and water dispersion have limited its applications as an adsorbent (Mahesh et al. 2011). In order to solve the question of water dispersion, alginic acid is often chemically modified by crosslinking with suitable cross linkers or introducing amide. Nevertheless, the adsorption capacity of cross-linked alginic acid for heavy metal ions would decrease because of the hydroxyl and hydroxy groups that are located on alginic acid (Ge & Huang 2010). Therefore, in order to increase the adsorption capacity of the crosslinked alginic acid, it must be grafted with more functional groups such as thiol, carboxyl and amine (Yong et al. 2013).

Ion imprinting technology is the development of the adsorption method. The ion imprinting technique is used to prepare crosslinked metal complexed polymers, in which a regulator for cross-linking and the metal cation acts as a template. Ion-imprinted polymer has selectivity performance and great adsorption capacity for target metal ions. Recently, precipitation polymerization, bulk polymerization, surface imprinting, emulsion polymerization and suspension polymerization have been applied to prepare ion-imprinted polymer (Lenoble et al. 2015). Different metal ions have been used as template ions. Various metal ion-imprinted polymers have been used to adsorb specific metal ions, compared with a control group lacking metal ions in preparation of adsorbent, the adsorption capacity of imprinted polymers showing to be significantly enhanced. Various polymers have acted as imprinted polymers, such as chitosan, acrylamide (Zhu et al. 2017), mesoporous silica and carbon nanofiber (Mishra & Verma 2017). To our knowledge, there are no researchers that have selected alginic acid as an imprinted polymer.

Herein, acrylamide grafted and glutaraldehyde-crosslinked alginic acid nano magnetic adsorbent (AAMA) was prepared by selecting Cd2+ as a template ion. FT-IR, TGA, SEM, TEM, VSM and XRD were applied to characterize the properties of AAMA. The adsorption capacity of AAMA for Cd2+ was tested by batch experiments. Various factors such as pH, kinetics, isotherms and reuse of AAMA were investigated.

Materials

Alginic acid, acrylic amide (AA), ammonium persulfate and potassium persulfate (KPS) were obtained from China Xiya Reagent Corporation Ltd (Shang-hai, China). The metal salts (Pb(NO3)2, Cd(NO3)2, Cu(NO3)2, Co(NO3)2, Zn(NO3)2 and HgCl2 were obtained from Guoyao Chemical Reagents Corporation Ltd (Shang-hai, China). In this study, all other reagents were analytical grade and the water was deionized water.

Preparation of AAMA

Magnetic Fe3O4 was synthesized by the hydrothermal method (Guo et al. 2017).

1 g Fe3O4 was added in 210 mL distilled water with ultrasound for 15 min. Then, 3 g alginic acid and 1.5 g NaOH were added in solution. After adding 25 mL glutaraldehyde, the mixture was reacted with stirring for 2 h at 40 °C. 6.2 g acrylic amide was added into the mixed solution for 30 min under stirring. By adding 3 mol/L hydrochloric acid, the pH of the solution was adjusted to 5. After adding 0.4 g Cd(NO3)2, the mixture was heated to 60 °C with stirring for 0.5 h under a nitrogen stream. Afterwards, 0.2 g ammonium persulfate and 0.2 g KPS were mixed into the solution. The mixture was warmed to 80 °C and kept stirring for 4 h. After the reaction, the suspension was separated by the use of a magnet. The solid was washed several times with distilled water, acetone and ethanol in sequence, and dried at 65 °C for 12 h under vacuum. The obtained brown product contained the template Cd2+ and was named Cd-AAMA. After being ground, the product was immersed in 0.5 mol/L EDTA solution to remove the template Cd2+ ion. The magnetic brown solid was separated by use of a magnet and washed with distilled water, then dried at 65 °C in a vacuum. The obtained brown product was named AAMA. The possible preparation processes are given in Figure 1.

Figure 1

The synthetic route of AAMA.

Figure 1

The synthetic route of AAMA.

Close modal

Characterization of AAMA

The hysteresis loops were detected by vibrating sample magnetometer (VSM) (LDJ 9600, USA). Fourier transform infrared (FTIR) spectrograms were obtained by Perkin Elmer spectrum FTIR (Perkin Elmer, USA). SEM images were photographed by Sigma (Carl Zeiss, Germany). The thermal stability of AAMA was tested by thermogravimetric analysis (TGA) (SDTA851, Swit). The concentration of metal ions was detected by Inductively coupled plasma mass spectrometry (ICP-MS) (Agilent 7500, USA).

Adsorption experiments

Aqueous solutions of metal ions (concentration:100 mg/L) were prepared from Pb(NO3)2, Cd(NO3)2, Cu(NO3)2, Co(NO3)2, Zn(NO3)2 and HgCl2. 0.1 mol/L HCl and 0.1 mol/L NaOH were used to adjust the pH values of solution. Generally, 100 mg AAMA was added to 100 mL metal ion solutions and the mixture was subjected to ultrasound for 10 min and shocked for a certain time. The adsorption capacity q(mg/g) of AAMA was calculated as follows:
formula
(1)
where Ce and C0 (mg/L) are the equilibrium and initial concentration of metal ions, m (g) is the mass of AAMA, V (L) is the volume of aqueous solution. The isothermal and kinetic models used in the study are shown in Table 1.
Table 1

The kinetic and isothermal adsorption models used in the study

ModelLinear equationPlotNomenclatureReference
Kinetic model 
Pseudo first-order  log(qe − qt) vs. t qe and qt are the amounts of adsorbed by adsorbent at equilibrium and time t, respectively; K1 is the pseudo first-order rate constant. Ge et al. (2016)  
Pseudo second-order  t/qt vs. t K2 is the pseudo second-order rate constant Ge & Huang (2010)  
Intraparticle diffusion  qt vs t0.5 C is a constant. Guo et al. (2017)  
Isotherm model 
Freundlich  log qe vs. log Ce KF is a Freundlich constant; n is a constant depicting the adsorption intensity He et al. (2016)  
Langmuir  Ce/qe vs. Ce KL is the adsorption equilibrium constant Guo et al. (2020)  
Dubinin-Raduskevich   lnqe vs. A B is a Dubinin-Raduskevich model constant Pang et al. (2011a)  
Redlich-Petterson   qe vs Ce KRP, ap and β are Redlich-Petterson model constants and the exponent. Zhu et al. (2017)  
ModelLinear equationPlotNomenclatureReference
Kinetic model 
Pseudo first-order  log(qe − qt) vs. t qe and qt are the amounts of adsorbed by adsorbent at equilibrium and time t, respectively; K1 is the pseudo first-order rate constant. Ge et al. (2016)  
Pseudo second-order  t/qt vs. t K2 is the pseudo second-order rate constant Ge & Huang (2010)  
Intraparticle diffusion  qt vs t0.5 C is a constant. Guo et al. (2017)  
Isotherm model 
Freundlich  log qe vs. log Ce KF is a Freundlich constant; n is a constant depicting the adsorption intensity He et al. (2016)  
Langmuir  Ce/qe vs. Ce KL is the adsorption equilibrium constant Guo et al. (2020)  
Dubinin-Raduskevich   lnqe vs. A B is a Dubinin-Raduskevich model constant Pang et al. (2011a)  
Redlich-Petterson   qe vs Ce KRP, ap and β are Redlich-Petterson model constants and the exponent. Zhu et al. (2017)  

Measurement of point of zero charge (PZC)

The PZC of AAMA was tested by the salt addition method. First, AAMA (100 mg) was added in 0.01 M NaNO3 solution (100 mL). Second, NaOH (0.1 M) and HNO3 (0.1 M) were used to adjust the pH of the suspension. Third, the mixed solution was continuously stirred for 24 h. Finally, the pH values were detected and the changes of the pH (ΔpH) plotted against the initial pH. The point where the plot bisects the x-ax is was the PZC of AAMA (Mahmood et al. 2011).

Desorption and regeneration studies

The adsorption experimental conditions: Cd2+ is 200 mg/L, contact time is 180 min, 100 mg AAMA in 100 mL solution at 303 K. After magnetic separation, AAMA that had adsorbed Cd2+ was added in 50 mL of 0. 5 mol L−1EDTA and stirred for 6 h at 298 K. Afterwards, the AAMA was magnetically separated from the solution and washed several times with distilled water. AAMA was reused in a new adsorption-desorption cycle and recycled five times.

Characterization of AAMA

Figure 2 shows the SEM image of Fe3O4 and AAMA. Fe3O4 shows polyhedral morphology. AAMA has an irregular block morphology with pores inside and granular protrusions on the surface. Figure 2(c)–2(e) show the higher magnification TEM images of AAMA; it clear to observe that Fe3O4 is distributed in AAMA. Moreover, the observed lattice fringes indicate that Fe3O4 has a good crystalline structure (Figure 2(e)). The fringe spacing of 0.25 nm could be corresponded to the (311) plane of Fe3O4 (Guo et al. 2020). This clearly testified that Fe3O4 has combined with AAMA. The EDX results of AAMA are shown in Figure S1 and Table S1. It clear to observe that AAMA is rich in N, O elements, which is advantageous for the adsorption of metal ions. The Fe and O elements are also shown in Figure S1, suggesting Fe3O4 has combined with alginic acid.

Figure 2

SEM image of Fe3O4 (a) and AAMA (b), TEM image of AAMA (c-e).

Figure 2

SEM image of Fe3O4 (a) and AAMA (b), TEM image of AAMA (c-e).

Close modal

Figure 3 shows the XRD results of Fe3O4 and AAMA. The peaks of Fe3O4 are at 30.1 (220), 35.5 (311), 43.1 (400), 53.4 (422), 57.0 (511) and 62.6 (440), which agreed with the data of Fe3O4 in the JCPDS file (PDF No. 85-1436). This proved that Fe3O4 has a spinel structure. The characteristic peaks of AAMA are identified with Fe3O4 except the peaks at 27.2, indicating that the grafted alginic acid does not influence the crystal structure of Fe3O4. Moreover, the XRD results of AAMA confirmed each other with SEM diffraction fringes.

Figure 3

XRD results of Fe3O4 and AAMA.

Figure 3

XRD results of Fe3O4 and AAMA.

Close modal

To confirm the functional group in AAMA, the FTIR spectra of prepared intermediate and AAMA are shown in Figure 4. The FTIR spectra of Fe3O4, Fe3O4 + AA, Fe3O4 + AA + AM, Fe3O4 + AA + AM + Cd2+ and AAMA revealed a pointed absorbance band at 576 cm−1, corresponding to the stretching vibration of Fe-O. The peak at 1,024 cm−1 corresponds to C-O stretching vibration, indicating a crosslinking reaction between epoxy chloropropane and functional monomers (Zhu et al. 2017). The absorbance peaks of AA, Fe3O4 + AA, Fe3O4 + AA + AM, Fe3O4 + AA + AM + Cd2+ and AAMA at 1,640 cm−1, 1,409 cm−1 and 2,941 cm−1 were attributed to the stretching of C = O of acrylamide, bending vibration and stretching vibration of C-H. Peaks at 3,649 cm−1 and 1,155 cm−1 are contributing to N-H stretching vibration and the weak swing vibration of NH2, respectively (Ge et al. 2016). In addition, as the degree of crosslinking increases, the possibility of a six- or five-membered ring increases, resulting in a red-shift of the stretching vibration peak of the amino group. FTIR proved that the AAMA have successfully grafted acrylamide and alginic acid.

Figure 4

The IR spectra of Fe3O4, AA, Fe3O4 + AA, Fe3O4 + AA + AM, Fe3O4 + AA + AM + Cd2+ and AAMA.

Figure 4

The IR spectra of Fe3O4, AA, Fe3O4 + AA, Fe3O4 + AA + AM, Fe3O4 + AA + AM + Cd2+ and AAMA.

Close modal

Figure 5 shows the TGA curves of prepared intermediate and AAMA. At 50 °C to 125 °C, the Fe3O4 had a weight loss of about 3.57%, corresponding to adsorbed water volatilization (Guo et al. 2017). In addition, the Fe3O4 had a weight loss of about 3.86% at range of 570 °C to 750 °C, which corresponds to hydroxyl decomposition. Compared to Fe3O4, Fe3O4 + AA reveals an additional thermal decomposition of 28.7% related to the decomposition of the amino group and carboxyl, which are located at AA (Pandi & Viswanathan 2015). Compared to Fe3O4 + AA, Fe3O4 + AA + AM shows an additional loss of 30.2%, corresponding to the decomposition of AM. Fe3O4 + AA + AM and AAMA show higher consistency in thermal weight loss, except for the temperature range of 719–822 °C. This may be due to the fact that there are large amounts of ionic holes in AAMA, which increases the specific surface area and leads to rapid decomposition of the functional groups. Referring to the thermal decomposition of AA, it can be concluded that AAMA have been successfully prepared.

Figure 5

TGA results of Fe3O4, AA, Fe3O4 + AA, Fe3O4 + AA + AM and AAMA.

Figure 5

TGA results of Fe3O4, AA, Fe3O4 + AA, Fe3O4 + AA + AM and AAMA.

Close modal

To test the magnetic characteristics of the AAMA, the magnetic saturation value of Fe3O4 and AAMA are tested at 298 K (Figure 6). The magnetization saturation values of the Fe3O4 and AAMA are 57.55 and 12.26 emu/g, respectively. According to the IR and TGA results, AA and AM are modified on the surface of Fe3O4. Since AA and AM are not magnetic, this will mask the magnetic properties of Fe3O4 (Ma et al. 2005). VSM results showed that the AA and AM had been grafted on the surface of Fe3O4.

Figure 6

Magnetization curve of the magnetic Fe3O4 and AAMA measured at room temperature.

Figure 6

Magnetization curve of the magnetic Fe3O4 and AAMA measured at room temperature.

Close modal

Adsorption of AAMA for heavy metal ions

Table 2 shows the adsorption capacities of AAMA at pH 5 for different metal ions (Pb2+, Hg2+, Cu2+, Cd2+, Co2+ and Zn2+). The adsorption capacity of AAMA for Cd2+ was more than 4.79 times that for any other heavy metal ions, which is higher than an other adsorbent (Ge et al. 2016). The results indicate that AAMA had selective adsorption toward Cd2+. This selectivity is the reason for the reserve of holes and coordination (Figure 1).

Table 2

The adsorption capacities of AAMA for different heavy metal ions

Metal ionCd2+Zn2+Co2+Cu2+Hg2+Pb2+
Adsorption capacity (mg/g) 163 34 18 22 31 24 
Metal ionCd2+Zn2+Co2+Cu2+Hg2+Pb2+
Adsorption capacity (mg/g) 163 34 18 22 31 24 

Effect of factors for adsorption of Cd2+ by AAMA

Effect of pH

It is well recognized that the pH value of a solution plays an important part in the adsorption process. Figure 7 shows the change of the adsorption capacity of AAMA by varying the pH from 1.0 to 5.5. It can be discovered that the pH value of the solution has an intense effect on the adsorption capacity of AAMA. The adsorption capacity of AAMA for Cd2+ first increases rapidly at pH 1–5 and then decreases at pH 5–5.5. The maximum adsorption capacity of AMAA for Cd2+ at pH 5.0 achieved 165 mg/g. The other reports also showed this phenomenon (Ge et al. 2012). The adsorption capacity of AAMA is much higher than that of the composite material for Cd2+, which reaches a maximum of 100.9 mg/g at pH 5.5 (Fan et al. 2020).

Figure 7

Effect of initial pH value on the adsorption capacity adsorption (experimental conditions: metal ion concentration is 200 mg/L, contact time is 140 min, 100 mg AAMA in 100 mL solution at 303 K, Repeat time: 3).

Figure 7

Effect of initial pH value on the adsorption capacity adsorption (experimental conditions: metal ion concentration is 200 mg/L, contact time is 140 min, 100 mg AAMA in 100 mL solution at 303 K, Repeat time: 3).

Close modal

The influence of pH on the adsorption process is mainly impacted in the interaction between metal ions and functional groups. Point of zero charge (PZC) can reflect the surface charge characteristics of AAMA. When the pH value of the solution is lower than PZC, the surface charge of the adsorbent is positive, resulting in the access of metal ions becoming difficult due to repulsive forces. Because the PZC of AAMA was 4.7 (which is consistent with the zeta potential results, see Supporting information Figure S2), the surface charge of AAMA was positively charged; there was competition between metal ions and hydrogen ions on the AAMA when the pH values were below 4.7, leading to the lower adsorption capacity at solution pH below 4.7. When the pH values of the solution were higher than 4.7 the surface charge of AAMA was negative and Cd2+ was positively charged, so the adsorption capacity of AAMA increased thanks to the electrostatic attractive force between metal ions and functional groups (Lee et al. 2012). Moreover, the pH value of the solution would affect the chelation strength between functional groups and metal ions. With the increase of pH, Cd2+ began to precipitate as hydroxides. Therefore, 5.0 was selected as the optimum pH value for Cd2+ adsorption. In other studies, the adsorption capacity of Cd2+ was highest at pH 5.0 (Rahmi & Nurfatimah 2018).

Effect of AAMA dosage

Figure 8 shows the adsorption capacity of AAMA at different dosages. It can be found that the adsorption capacity of AAMA for Cd2+ slightly decreased with the increase in dosage (20–100 mg). Nevertheless, the adsorption capacity of AAMA quickly decreased in a dosage range of 100–140 mg. It might be the fact that AAMA could effectively contact with more Cd2+ in the same concentration of solution while the AAMA dosage was smaller. Mittal considered that the greater the amount added, the greater the probability of AAMA agglomeration, resulting in a decrease in the number of adsorption sites and a decrease in adsorption capacity (Mittal et al. 2015). In order to reduce the error caused by weighing as much as possible, the added amount is 100 mg in the experiment.

Figure 8

Effects of adsorbent dosage for the adsorption of Cd2+ on AAMA (experimental conditions: metal ion concentration is 200 mg/L(100 mL), pH = 5, contact time is 140 min, 303 K, repeat time: 3).

Figure 8

Effects of adsorbent dosage for the adsorption of Cd2+ on AAMA (experimental conditions: metal ion concentration is 200 mg/L(100 mL), pH = 5, contact time is 140 min, 303 K, repeat time: 3).

Close modal

Effects of temperature and adsorption time on the adsorption capacity

The effect of temperature and adsorption time on the adsorption capacity for Cd2+ are shown in Figure 9. Obviously, a rapid initial rate of Cd2+ adsorption on the AAMA was discovered during the first 70 min, contributing to approximately 85% of the total adsorption capacity of AMAA, with a gradual approach to the limiting adsorption capacity at 140 min. In its infancy, the high initial adsorption rate contributed to strong chelation, the effective mass transfer and the abundant adsorption sites available (Jing et al. 2015). The spendable adsorption sites decreased with the increase in adsorption time and the travel rate slowed down gradually due to the reduction in concentration difference. Moreover, it can also be found that the adsorption capacity of AAMA was influenced by the temperature of the solution. With the increase in temperature, the adsorption capacity of AAMA for Cd2+ reflected the phenomenon of first increasing and then decreasing. When the temperature is 303 K, the adsorption capacity of AAMA for Cd2+ is largest. This may be due to the fact that the phenomenon of thermal expansion was discovered on AAMA when the temperature rises. In the temperature range of 298–303 K, with the increase in temperature, the metal ion diffusion rate increases, causing metal ions to enter the deeper part of the AAMA, and the ions' cavity utilization increases. Thereby the adsorption capacity is increased. However, in the temperature range of 303–308 K, AAMA undergoes thermal expansion. The ion cavities were squeezed with the expansion of AAMA, making it difficult for Cd2+ to enter them. Thereby, the adsorption capacity decreased. The adsorption of Pb2+ on crosslinked chitosan (Ge et al. 2016) and the adsorption of Cd2+ on β-cyclodextrin (Zhu et al. 2017) also found analogous results. Hence, AAMA had an optimal temperature (303 K) for the adsorption of Cd2+.

Figure 9

Effects of adsorption time and temperature on the adsorption capacity (experimental conditions: metal ion concentration is 200 mg/L,pH = 5, 100 mg AAMA in 100 mL solution, repeat time: 3).

Figure 9

Effects of adsorption time and temperature on the adsorption capacity (experimental conditions: metal ion concentration is 200 mg/L,pH = 5, 100 mg AAMA in 100 mL solution, repeat time: 3).

Close modal

Adsorption kinetics

In order to investigate the adsorption kinetics of the adsorption process, the pseudo-first-order kinetic model, pseudo-second-order kinetic model and intra-particle diffusion model were employed to fit the adsorption rate data.

The pseudo-first-order kinetic model and pseudo-second-order kinetic model are shown in Figure 10. Table 3 shows the results of correlation coefficients (R2) and kinetic constants. From the point view of predicted qe and fitting R2, the pseudo-second-order model is more suitable than the pseudo-first-order model to describe the adsorption kinetics, indicating that the adsorption rate of Cd2+ is controlled by chemical progress (Pang et al. 2011a).

Table 3

Dynamic parameters for the adsorption of Cd2+ onto AAMA at different temperature

TemperaturePseudo-first-order
Pseudo-second-order
K1qeR2K2qeR2
298 K 0.0279 105.75 0.9082 4.99 × 10−4 172.71 0.9992 
303 K 0.0229 114.05 0.8816 4.13 × 10−4 184.50 0.9991 
308 K 0.0282 100.89 0.8973 5.13 × 10−4 166.67 0.9989 
TemperaturePseudo-first-order
Pseudo-second-order
K1qeR2K2qeR2
298 K 0.0279 105.75 0.9082 4.99 × 10−4 172.71 0.9992 
303 K 0.0229 114.05 0.8816 4.13 × 10−4 184.50 0.9991 
308 K 0.0282 100.89 0.8973 5.13 × 10−4 166.67 0.9989 
Figure 10

Pseudo-first-order kinetic model (a) and pseudo-second-order kinetic model (b) for the adsorption of Cd2+ onto AAMA.

Figure 10

Pseudo-first-order kinetic model (a) and pseudo-second-order kinetic model (b) for the adsorption of Cd2+ onto AAMA.

Close modal

To further explore the adsorption mechanism, the intra-particle diffusion model is used to investigate the adsorption progress. Figure 11 shows the plots of qt versus t1/2 for Cd2+. From Figure 11, the fitted linear can be divided into three stages. Table 4 shows the calculated intra-particle diffusion constants. From Table 4, it can be found that the order of adsorption rate is Kid,1> Kid,2 > Kid,3. The first stage is that Cd2+ is adsorbed in the ion cavities, which are located on the exterior surface of AAMA. When the exterior surface of AAMA reaches saturation, Cd2+ gradually enters into the interior of AAMA. As the interspace becomes smaller, Cd2+ diffusion resistance becomes larger, resulting in the decrease of diffusion rates (Kid,2). The final stage corresponds to the equilibrium period, in which the adsorption and desorption rates are equal (Li et al. 2011).

Table 4

Intra-particle diffusion model parameters for the adsorption of Cd2+ onto AAMA

TemperatureFirst linear portion
Second linear portion
Kd1C1R12Kd2C2R22
298 K 32.6754 −23.3412 0.9920 4.5665 107.9714 0.9693 
303 K 32.6549 −22.8904 0.9929 5.6895 103.2264 0.9829 
308 K 32.5933 −25.9351 0.9900 4.0529 107.5182 0.9709 
TemperatureFirst linear portion
Second linear portion
Kd1C1R12Kd2C2R22
298 K 32.6754 −23.3412 0.9920 4.5665 107.9714 0.9693 
303 K 32.6549 −22.8904 0.9929 5.6895 103.2264 0.9829 
308 K 32.5933 −25.9351 0.9900 4.0529 107.5182 0.9709 
Figure 11

Intra-particle diffusion model for the adsorption of Cd2+ onto AAMA.

Figure 11

Intra-particle diffusion model for the adsorption of Cd2+ onto AAMA.

Close modal

Effect of initial concentration on the adsorption capacity

The effect of initial Cd2+ concentration on the adsorption capacity of AMAA is shown in Figure 12. As shown in Figure 12, the adsorption capacity for Cd2+ starts with a rapid increase and then increases slowly in the late stage. The maximum adsorption capacity of AMAA for Cd2+ has reached 167 mg/g at the initial concentration of 200 mg/L. AAMA can adsorb more Cd2+ in the ion cavities with increasing initial concentration. However, the adsorption capacity of AAMA for Cd2+ has a certain critical amount of ion cavities. The ion cavities are gradually filled with Cd2+ when the initial concentration increases to a certain value. As the initial concentration of Cd2+ increases further, the adsorption capacity remains at a relatively stable value. Compared with the adsorption capacity of metal ions by alginic acid, the adsorption capacity of Cd2+ by AAMA is smaller (Vaid et al. 2015). This might be because the adsorption of Cd2+ by AAMA is dependent on ion holes and chemical adsorption, and ion holes are evenly distributed in the AAMA sphere. There is a certain steric hindrance for Cd2+ to enter AAMA, resulting in some ion holes not being used. So the adsorption capacity of AAMA is reduced compared to the adsorption capacity of alginic acid.

Figure 12

Effect of initial concentration on the adsorption capacity (experimental conditions: pH = 5, contact time is 140 min, 100 mg AAMA in 100 mL solution at 303 K, repeat time: 3).

Figure 12

Effect of initial concentration on the adsorption capacity (experimental conditions: pH = 5, contact time is 140 min, 100 mg AAMA in 100 mL solution at 303 K, repeat time: 3).

Close modal

Adsorption isotherms

The adsorption isotherms are often used to explore the adsorption mechanism at different initial concentrations. The Langmuir, Freundlich, Dubinin-Raduskevich and Redlich-Petterson model are applied to describe the adsorption isotherm behavior.

The adsorption isotherms of Cd2+ with corresponding Freundlich, Langmuir Dubinin-Raduskevich and Redlich-Petterson plots are shown in Figure 12 and Figure S3. The fitting parameters of both Freundlich and Langmuir isotherms are shown in Table 5 and Table S2. From Table 5 and Table S2, it can be found that the correlation coefficients of Langmuir were highest among the Freundlich, Dubinin-Raduskevich, and Redlich-Petterson models. This indicated that the adsorption progress is monolayer and homogeneous (Pang et al. 2011b). The forecast maximum adsorption capacity of AAMA by the Langmuir model for Cd2+ is 181.82 mg/g, which is higher than previously reported adsorbents (Table 6). In addition, the values of dimensionless constant RL ranged from 0.12 to 0.34 for Cd2+, suggesting that the adsorption of Cd2+ is considered as favorable.

Table 5

Isotherm modeling parameters for the adsorption of Cd2+ onto AAMA

AdsorbentLangmuir isotherm model
Freundlich isotherm model
q(mg/g)KLRLR2KFnR2
AAMA 181.82 0.2443 0.12–0.34 0.9990 45.03 2.84 0.8876 
AdsorbentLangmuir isotherm model
Freundlich isotherm model
q(mg/g)KLRLR2KFnR2
AAMA 181.82 0.2443 0.12–0.34 0.9990 45.03 2.84 0.8876 
Table 6

Adsorption comparison of AAMA and reported studies on adsorbents for Cd2+

AdsorbentpHT(°C)C0 (mg/L)Adsorption capacity (mg/g)References
Modified silica particle 25 150 55.56 Radi et al. (2015)  
l-arginine modified adsorbent 25 210 120.2 Guo et al. (2017)  
Inverse emulsion polymerization: 30 250 107 Zhu et al. (2017)  
AAMA 30 200 175 In this work 
AdsorbentpHT(°C)C0 (mg/L)Adsorption capacity (mg/g)References
Modified silica particle 25 150 55.56 Radi et al. (2015)  
l-arginine modified adsorbent 25 210 120.2 Guo et al. (2017)  
Inverse emulsion polymerization: 30 250 107 Zhu et al. (2017)  
AAMA 30 200 175 In this work 

Adsorption thermodynamics

Thermodynamic parameters such as free energy change (ΔG0), entropy change (ΔS0) and enthalpy change (ΔH0) can be calculated by Equations (2)–(4).
formula
(2)
formula
(3)
formula
(4)
where Ce(mg/L) and qe(mg/L) are the concentration and the adsorption capacity of AMAA under equilibrium condition. T (K) is the temperature. The fitted linear by lnKd − 1/T is shown in Figure 13. Table 7 shows the related parameters for the adsorption process. As shown in Table 7, ΔG0 has negative values. ΔH0 and ΔS0 have positive values in the range of 293–303 K, while ΔH0 and ΔS0 have negative values. The activation energy can be defined by the Arrhenius equation. The linear expression is:
formula
(5)
where K2 is the rate constant of the pseudo-second-order model, Ea is the activation energy of adsorption, T is temperature, and K0 is the temperature influence factor. The linear fitted by lnK2 − 1/T is shown in Figure 13. The activation energy of Cd2+ adsorbed on the surface of AAMA is 41.26 kJ/mol. This suggested that both chemisorption and physical adsorption are involved in the adsorption of Cd2+ by AAMA. Chemical adsorption is the main mechanism, while physical adsorption is the auxiliary (Cao et al. 2017).
Table 7

The related thermodynamic parameters for the adsorption of Cd2+ onto AAMA

T(K)ΔG0ΔH0ΔS0R2
293 −2.63 55.97 0.20 0.9945 
298 −3.63 
303 −4.63 
308 −3.35 − 30.15 − 0.087 0.9999 
313 −2.92 
318 −2.48 
T(K)ΔG0ΔH0ΔS0R2
293 −2.63 55.97 0.20 0.9945 
298 −3.63 
303 −4.63 
308 −3.35 − 30.15 − 0.087 0.9999 
313 −2.92 
318 −2.48 
Figure 13

Effect of temperature on adsorption capacity (experimental conditions: metal ion concentration is 200 mg/L, pH = 5, contact time is 140 min, 100 mg AAMA in 100 mL solution, repeat time: 3).

Figure 13

Effect of temperature on adsorption capacity (experimental conditions: metal ion concentration is 200 mg/L, pH = 5, contact time is 140 min, 100 mg AAMA in 100 mL solution, repeat time: 3).

Close modal

Adsorption mechanism

The ion cavity is very important in the adsorption mechanism of adsorption of Cd2+ by AAMA. In order to confirm the role of functional groups in the adsorption mechanism, the XPS survey spectrum before and after adsorption (named AAMA and AAMA-Cd2+) and high resolution XPS scans of N1 s, O1 s and C1 s were investigated.

The survey scans' XPS spectra of AAMA and AAMA-Cd2+ are shown in Figure 14(a). From Figure 14(a), it can be found that the elements of Fe, O, N and C were detected. After Cd2+ adsorption, a new intense peak for Cd at 400.5 eV was detected, suggesting Cd2+ was successfully adsorbed on AAMA. The high resolution O 1 s, N1 s and C1 s spectra of AAMA are shown in Figure 14(b), 14(d) and 14(f). For the AAMA before adsorption, there are two peaks for O1 s spectra, corresponding to 529.6 and 531.4 eV. The peak at 529.6 was related to amide (O = C–N). The peak at 531.4 eV can be attributed to carboxyl (COO) (He et al. 2016). The contents of the above oxygen atoms before adsorption were 32.96% and 67.04%, respectively. After Cd2+ adsorption, a new peak disappeared at 532.4 eV (Figure 14(c)). The contents of the three peaks turned to be 28.80% (529.6 eV), 8.49% (531.4 eV) and 62.71% (532.4 eV). This proved that the content of the oxygen atoms in the carboxyl group decreased after Cd2+ adsorption, while the content of carbonyl increased because of the formation of the Cd2+–O–C = O group. The C 1 s spectrum was constituted by three peaks, corresponding to a peak at 284.8 eV (C-C), a peak at 286.1 eV (amide (NH2–C = O)), and a peak at 288.1 eV (carbonyl (C = O)). The contents of the above carbon atoms before adsorption were 9.12%, 13.34%, and 77.54%, respectively, while their contents became 9.19%, 21.83%, and 68.98% after Cd2+ adsorption (Figure 14(e)). This suggested that the carbonyl group participated in the coordination reaction. The N1 s spectrum was composed of two peaks, corresponding to a peak at 399.7 eV (N-H) and a peak at 398.7 eV (C-N). The contents of the above nitrogen atoms before adsorption were 85.76% and 14.23%, respectively, while their contents became 100% (406.1 eV) after heavy metal ion adsorption (Figure 14(g)). This indicated that the amino was reacted with Cd2+. The O1 s, N1 s and C1 s peak areas of AAMA before and after Cd2+ adsorption were different. These results suggested that carboxyl and amino play a vital role in the Cd2+ adsorption progress. In other words, AAMA selectively adsorbs Cd2+, which not only relies on ion holes, but also depends on functional groups.

Figure 14

The survey scan XPS spectra (a) and high-resolution scan XPS spectra of O1 s (b) N 1 s (d) C 1 s (f). O1 s (c), N 1 s (e) and C 1 s (g) of AAMA after adsorption of Cd2+.

Figure 14

The survey scan XPS spectra (a) and high-resolution scan XPS spectra of O1 s (b) N 1 s (d) C 1 s (f). O1 s (c), N 1 s (e) and C 1 s (g) of AAMA after adsorption of Cd2+.

Close modal

Desorption and regeneration

The recyclability of the adsorbent is a very important parameter for evaluating the potential for industrial applications. The regeneration and reuse of AAMA for Cd2+ removal was evaluated. Because of the strong chelating ligand between EDTA and metal ions, it was used as the desorption solvent in this study. AAMA can be regenerated by 0.5 M EDTA. The adsorption capacity of five cycles was 175, 163, 145, 131 and 125 mg/g, respectively. and still keep about 71% adsorption capacity in the fifth cycle of regeneration (Figure 15). The slight decrease in adsorption capacity may be due to two reasons. Firstly, active sites of chemical adsorption are lost in the process of repeated use. Secondly, the specific geometry structure of AAMA and ion holes is destroyed in the process of repeated use, resulting in a decrease in the number of ion holes and a change in the spatial size of ion holes, which makes it difficult to accurately and selectively adsorb Cd2+. Other adsorbents have also found that the adsorption capacity decreases slightly with increasing recycle time. The adsorption capacity of AMAA for Cd2+ removal changed slightly, which might be due to the loss of active sites and the specific geometry structure being damaged in the adsorption/desorption/regeneration processes (Jing et al. 2015). These results suggested that AAMA has a good potential for industrial applications.

Figure 15

Cycle adsorption experiments of Cd2+ by AAMA (experimental conditions: metal ion concentration is 200 mg/L, contact time 140 min, pH = 5, 100 mg AAMA in 100 mL solution at 303 K, repeat time: 3).

Figure 15

Cycle adsorption experiments of Cd2+ by AAMA (experimental conditions: metal ion concentration is 200 mg/L, contact time 140 min, pH = 5, 100 mg AAMA in 100 mL solution at 303 K, repeat time: 3).

Close modal

According to the data exposed above, the following conclusions can be drawn:

  • (1)

    SEM, TEM, TGA, XRD, and VSM were employed to characterize AAMA. This proved that Cd2+ imprinted alginic acid and polymers magnetic adsorbent has been successfully prepared.

  • (2)

    The adsorption capacity of AAMA was investigated by the different factors. These results revealed that AAMA had optimum adsorption capacity at a temperature of 303 K, initial concentration of 200 mf/L, and pH of 5.0. The adsorption capacity of AAMA for Cd2+ was more than 4.79 times that for other metal ions, suggesting its selectivity for the adsorption of Cd2+.

  • (3)

    The experimental data were well described by the pseudo-second-order kinetic model and Langmuir isotherm model. The maximum adsorption capacity for Cd2+ can reach 175 mg/g. The parameters of adsorption thermodynamics concluded that the adsorption progress is endothermic and spontaneous in nature.

  • (4)

    The parameters of adsorption activation energy suggested that there is chemisorption and physical adsorption involved in the adsorption of AAMA selectively adsorbs Cd2+, which not only relies on ion holes, but also depends on functional groups.

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

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Supplementary data