Cadmium(II) Capture Using Amino Functionalized Hydrogel with Double Network Interpenetrating Structure: Adsorption Behavior Study

: Heavy metal pollution caused by the indiscriminate disposal of toxic heavy metal wastewater has become one of the serious water environmental issues. In this study, a novel NH 2 -PAA/Alginate hydrogel with double network interpenetrating structure was constructed with alginate, acrylic acid, and other raw materials. Characterized by SEM, this hydrogel shows a three-dimensional porous structure, which would be useful in adsorption process for its high diffusion coefficient. The results of adsorption experimental show that the NH 2 -PAA/Alginate possessed the well adsorption capacity when pH above 3.5, the maximum adsorption capacity calculated by Langmuir was 176.5 mg/g, the adsorption equilibrium can be achieved within 150 min. In addition, the NH 2 -PAA/Alginate has good recycling ability and stability. The results of XPS analysis reveal that the Cd(II) exchanged with Ca(II) and then coordinated with amino and hydroxyl groups in NH 2 -PAA/Alginate. The NH 2 -PAA/Alginate hydrogel can deal with all kinds of heavy metal ions and is a potential material for heavy metal adsorption.


Introduction
Water plays a vital role in the development of human society. While the increasing worldwide contamination of water systems has become one of the key environmental facing humanity [1]. According to "The Analysis Report of China's Industrial Effluent Treatment in 2019", the industrial wastewater discharge accounts for 23.55% of the total wastewater discharge in China in 2017, which is 18.16 billion tons. The nondegradable and toxic heavy metal ions from the industrial effluent, such as cadmium ion, can accumulate in the body along with the biological chain, causing a serious of damage to human health. For example, damage central nervous system, liver and kidneys [2]. Therefore, the removal of such toxic metal ions from industrial effluent is becoming a crucial issue. Various treatment technologies, such as chemical precipitation [3], solvent extraction, reverse osmosis [4], ion-exchange [5], filtration [6], electrodialysis [7], have been employed to remove heavy metal ions from industrial effluent. While adsorption technology is regarded as one of the most promising technologies owning to its low cost and easy operate.
Still, there are many problems in practical application of adsorbent. For example, the granular adsorption such as activated carbon and chelating resin, have a large number of micropores and mesopores due to their high specific surface area [8], the slow internal diffusion rates of these adsorbents dominated by Knudsen diffusion seriously limit their overall kinetics and the equilibrium times commonly require some hours [9]. In addition, the granular materials are easily subject to blocking and burial of adsorption sites, resulting in partial loss of adsorption performance [10]. Although the nano-adsorbent possess the high surface energy which can accelerate the adsorption process [11], the difficult to separate for the nanoscale could result in the increase of operation cost and secondary pollution of the environment [12]. Magnetic nanoadsorbents solve the problem of nano material recovery [13], while the magnetic particles such as iron, cobalt, nickel particles are easy to fall off from the nanoadsorbent in the adsorption process [14]. Additionally, the inefficient regeneration of magnetic nano-adsorbents under acidic conditions greatly limits their practical applications [15]. Therefore, it is of great significance to find a kind of adsorption material with high adsorption performance, easy separation and regeneration.
Polymer hydrogel is a material with characteristics of solid and liquid [16]. The diffusion coefficient of molecules in three-dimensional networks of hydrogel is close to that of water [17], although they possess macroscopical geometric structure. Sun group reported an extremely stretchable and tough dual network (DN) hydrogels by combining weak and strong crosslinks in 2012 [18]. The combination of relatively high stiffness, high toughness and recoverability of stiffness and toughness, along with an easy method of synthesis, make these materials ideal candidates for further investigation. Therefore, double network hydrogel has attracted much attention in many fields such as drug delivery [19], tissue engineering [20], environmental remediation [21].
In this study, a novel amino-group functionalized NH2-PAA/Alginate DN hydrogel adsorption was designed and prepared using simple free-radical polymerization and modification. The physico-chemical properties of NH2-PAA/Alginate hydrogel were characterized using SEM, FTIR and XPS analyses. The influence of initial pH of the solution, ionic strength, adsorbate concentration, adsorption time and temperature on NH2-PAA/Alginate hydrogel were studied, and the adsorption performance of NH2-PAA/Alginate hydrogel for Cd(II) was also evaluated. Furthermore, the adsorption mechanism was also discussed in detail.

Materials
Sodium alginate (viscosity 200 ± 20 mPa.s) was purchased from Tianjin Guangfu Fine Chemical Research Institute; Acrylic acid (analysis pure) was purchased from Shandong Qilu pharmaceuticals company and distilled under reduced pressure before use. Ammonium persulfate and triethylenetetramine were analytically pure and purchased from Sinopharm Chemical Reagent Co., Ltd; N,N-Methylenebisacrylamide, sodium bisulfite, calcium chloride, epichlorohydrin, analytically pure, purchased from Shandong West Asia Chemical Industry Co., Ltd and used as received. All aqueous solutions were prepared with deionized water.

Preparation of hydrogel
NH2-PAA/Alginate hydrogel was prepared using radical copolymerization and surface functional modification. As depicted in Fig. 1, 1.2 g acrylic acid and 0.3 g sodium alginate were added to 6 ml deionized water under stirring. Then the mixed solution, 0.025 g ammonium persulfate (initiator), 0.020 g sodium bisulfite (redox initiation system) and 0.025 g N,N-Methylenebisacrylamide (cross-linking agent) were added into a glass reactor. After ultrasonic under nitrogen atmosphere, the sealed reactor was placed in a water bath at 40℃ for 2 h to complete gelation process. The synthesized hydrogel was freeze-dried for 24 h and immersed in 5 wt% CaCl2 solution for 2 h to cross-link the alginate network and got the PAA/Alginate hydrogel.
Next, after an immersion into 6 mL of dimethyl sulfoxide solution containing 0.1 g NaOH and 0.4 mL epichlorohydrin for 1 h at 60℃, the PAA/Alginate was further immersed into 6 mL of deionized water containing sodium hydroxide (20 wt%) and triethylenetetramine (0.6 mL) for 6 h at 60℃ to obtain NH2-PAA/Alginate, which was washed with deionized water to remove unreacted reagents.

Characterization
The chemical structure of the samples was analyzed by Fourier transform infrared spectroscopy (FTIR, Nicolet 5700, Thermo Electron Scientific Instruments Corp. American). After the sample was ground into power with KBr and pressed into pellet, the test was analyzed in the range of 4000~500 cm -1 . The morphological features of freeze-dried NH2-PAA/Alginate examined by scanning electron microscope (SEM, MIRA3, Tescan Ltd., Czech Republic). The thermal stability of NH2-PAA/Alginate xerogel was analyzed by thermogravimetric analysis (TGA, TG/DTA7300, Hitachi, Ltd., Japan) from room temperature to 600℃ under a nitrogen atmosphere at a heating rate of 10℃/min. The surface chemistry of NH2-PAA/Alginate before and after adsorbing Cd(II) was determined by X-ray photoelectron spectroscope (ESCALAB 250XI, Thermo Electron Scientific Instruments Corp. American). The compression experiment was implemented by electronic universal material testing machine (AG-Xplus, Shimadzu Ltd., Japan) with compression rate of 2mm/min. The zero charge point of the hydrogels was measured by pH drift method. The 1 wt% xerogels were added into a series of solution with different pH and 0.01M NaCl. After 12 h at 25℃, the pH of the solution was measured again, and the intersection of curve with X axis of pH vs. pH was the zero charge point of NH2-PAA/Alginate. The swelling experiment was performed by immersing the hydrogels in an excess of water to reach swelling equilibrium at 25℃.

Batch adsorption experiments
The batch adsorption experiments were conducted using 1g/L NH2-PAA/Alginate dry gel. Analytical grade Cd(NO3)2 were employed to prepare the Cd(II) stock solutions.

Data analysis
All the adsorption experiments were repeated twice. The equilibrium adsorption capacity qe (mg·g -1 ), adsorption efficiency E (%), distribution adsorption coefficient Kd (L·g -1 ), and the swelling rate (SR) were calculated according to the following formula: where V is the volume of the solution, m is the weight of the dry NH2-PAA/Alginate,

Model Fitting
Kinetic models including pseudo-first-order, pseudo-second-order model and intraparticle diffusion model were employed to fit the experimental data, and the mathematical equations were expressed as follow: where qt (mg·g -1 ) is adsorption capacity at t (min), k1 (L·min -1 ), k2 (g·(mg·min) -1 ) and kp (min -1 ) are the corresponding adsorption rate constants.
Isothermal adsorption model including Langmuir, Freundlich, and Temkin model were used to fit the adsorption isotherms. The correlation coefficients (R 2 ) were used for comparing the model applicability. The parameters of isothermal adsorption were calculated by the following equations: where qmax (mg·g -1 ) is the maximum adsorption, KL (L·mg -1 ), KF (L·mg -1 ) and KLF (L·mg -1 ) are the isotherm adsorption constant, respectively, nF is the adsorption intensity, the AT (L·g -1 ) is the Temkin isotherm equilibrium binding constant and bT (J·mol -1 ) is the constant related to heat of sorption.

Characterization of hydrogel
The freeze-dried NH2-PAA/Alginate aerogel was used to characterize the internal structure of SEM, and the result is shown in Fig. 2a. As shown by SEM image, the NH2-PAA/Alginate exhibited a three-dimensional porous structure, and such a morphological feature would improve the exposure of the adsorption sites and the diffusion of metal ions [22]. The bulk NH2-PAA/Alginate (inset image in Fig. 2a) could be cut into different shape in need as well as easy separation. The compressive test of NH2-PAA/Alginate was investigated with gradually increasing strain (ε) (Fig. 2b). It can be seen that the NH2-PAA/Alginate achieved a compression stress of 724 Kpa when the load deformation was 36%. After the immediate second cycle, the compression stress was up to 1.3 Mpa at ε=51%, meanwhile, the compression stress of second test is not as high as that of first, illustrating that the hysteresis was caused by unzipping the network of ionic cross-links [23]. Significantly, the guluronic acid in alginate chains can form ionic crosslinks through Ca(II), and the electrostatic interaction may enable synergistic energy dissipation [24].    Fig. 3b shows the integral results from the thermogravimetric analysis (TGA) and the differential thermogravimetric data (DTG) analysis of NH2-PAA/Alginate. The peak in DTG curve represents the temperature where the degradation rate is maximum for degradation stage in the whole process [27]. It can be seen that there are two pyrolysis stages of the NH2-PAA/Alginate, the first thermal degradation process occurred in the temperature range 200~300℃, which is attributed to the degradation of the amino, carboxyl and hydroxyl groups, as volatile gases were released [28]. The second stage occurred in the range 300~450℃, and is attributed to the depolymerization of polymer and formation of a carbonaceous residue.
The swelling behavior of hydrogels is shown in Fig. 3c. It can be seen that the SR of PAA/Alginate was about 85% at equilibrium state. After amino modification, the hydrophilic ability of NH2-PAA/Alginate increases, and the swelling rate raised to 90%, while the hydrogel still maintains good mechanical property. Moreover, the metal ions have faster diffusion rate in the hydrogel with the higher water content [29]. The result of the zero charge point experiment is shown in Fig. 3d. The zero charge point of NH2-PAA/Alginate is 8.2, higher than that of PAA/Alginate (zero charge point is 5.2), which is due to the introduction of amine groups.

Effect of environmental conditions
In order to investigate the influence of environmental conditions on the adsorption process, the adsorption experiments of Cd(II) on NH2-PAA/Alginate under different pH, different ion species (Na, Mg, Ca) and ionic strength were studied. The pH of bulk solution precipitation (pHBSP) for Cd(II) with initial concentration of 50 mg/L was 8.51, and no precipitation occurred at pH below pHBSP. As shown in Fig. 4, the adsorption capacity of Cd(II) onto NH2-PAA/Alginate at pH 1.5 could be negligible. The adsorption capacity sharply increased with increasing pH at range of 2~3.5, and finally retained about 48% at pH above 4. Generally, the solution pH is lower than the zero charge point (the value of NH2-PAA/Alginate is 8.2), the surface of adsorption is positively charged for the protonation reaction, which is unfavorable for adsorption process due to the electrostatic repulsion towards positively charged metal ions [30].
while, the adsorption capacity of NH2-PAA/Alginate could reach to adsorption saturation at pH above 4, indicating that the adsorption process of Cd(II) on NH2-PAA/Alginate was dominated by chemical adsorption rather than electrostatic interactions [31]. In the range of pH 2.0 ~ 3.5, the protonation reaction will compete with the heavy metal ion adsorption, and the adsorption process can be described as a liquid-solid interface ion exchange process [32]:  (5) where Sur is the surface of NH2-PAA/Alginate, the overline labels refer to species on NH2-PAA/Alginate, Keq is the thermodynamic equilibrium constant. The logKd vs. pH was plotted in Fig. 4c. The slope of the linear plot is close to 1, suggesting that the adsorption process involved with the exchange of one protons to aqueous phase when forming one Cd(II) complex. In fact, the Cd(II) also exchanges Ca(II) in NH2-PAA/Alginate in the adsorption process, which could be illustrated from the analysis of the elements in XPS before and after gel adsorption (Table 1). Alkali metal and alkaline-earth metal ions are usually abundantly present in the actual industrial effluent. Although these ions are nontoxic and harmless, the competition adsorption between these ions and Cd(II) was observed. Fig. 4d shows the effects of Na(I), Mg(II) and Ca(II) on Cd(II) uptake by NH2-PAA/Alginate. It can be seen that all ions in solution had small effect on Cd(II) adsorption. The increasing the concentration of Na(I), the adsorption efficiency of Cd(II) decreased, which could mainly cause by the electrostatic repulsion [33]. While the uptake of Cd(II) increase when increasing of Mg(II) and Ca(II) concentration, this phenomena could be illustrated the formation of ionic atmosphere at higher ion concentration and reduced impact on Cd(II) adsorption.

Adsorption kinetics
Adsorption kinetics is important in the prediction of the adsorption rate, which is highly demanded for adsorbents in practical application. The effect of time on the Cd(II) adsorption by the NH2-PAA/Alginate is presented in Fig. 5. The adsorption of Cd(II) on the NH2-PAA/Alginate increased rapidly within 100 min and reached adsorption equilibrium in approximately 150 min. For bulk material-based adsorbent, the adsorption rate is faster than that of most common granular adsorbents. Obviously, the fast adsorption rate of NH2-PAA/Alginate should owe to the three-dimensional network, porous structure and its excellent water penetration which is highly accessible to Cd(II) [34].
In order to analyze the adsorption rate of Cd(II) on NH2-PAA/Alginate, the pseudo-first-order, pseudo-second-order model and intraparticle diffusion model were employed to fit the experimental data. The fitted kinetic curves are shown in Fig. 5, and the fitted kinetic parameters are summarized in Table 2. The fitting results show that the correlation coefficients were 0.9383 for pseudo-first-order and 0.9539 for pseudo-second-order, respectively. Obviously, the pseudo-second-order model gave the more significant goodness-of-fit, implying that the rate-limiting step could involve the chemical adsorption.
Generally, the adsorption process follows three steps of external diffusion, intraparticle diffusion and adsorption before reaching equilibrium. To understand the dominating step of adsorption, the intraparticle diffusion model could be employed to describe adsorption process. Two set of line segments were observed in the intraparticle diffusion model curves. According to this model, if intraparticle diffusion is the ratecontrolling step, the first line with larger slope should pass through the origin [35].

Adsorption isotherms
Dynamic adsorption model The adsorption capacity of an adsorbent is usually evaluated by isothermal adsorption experiment, The isothermal adsorption data of NH2-PAA/Alginate with different concentrations of Cd(II) at 25℃ are shown in Fig. 6. Obviously, the adsorption capacity increased with increasing Cd(II) concentrations, and finally approached the adsorption equilibrium at about 130 mg/g. To further study on the adsorption behavior, the Langmuir, Freundlich, Temkin and Langmuir-Freundlich isotherms were used to describe the adsorption of Cd(II) on NH2-PAA/Alginate. The fitting curve of four models are shown in Fig. 6a and b and the corresponding parameters are listed in Table   3. The fitting results show that the correlation coefficient (R 2 =0.9808) for Langmuir-

Selective adsorption and recyclability test
In   [22], in addition, the mass-to-charge ratio, stability constant, chemical speciation, size of hydrated, and chelation ability of these bivalent heavy metal ions also lead to the difference of adsorption capacity [9].

Adsorption mechanism
The adsorption mechanism between adsorbent and heavy metal ions plays an important role for adsorption design. In the previous discussion, a preliminarily conclusion could be inferred that the Cd(II) adsorption by NH2-PAA/Alginate was dominated by ion exchange and chemical adsorption. To further explore the adsorption mechanism, the XPS was used to analyze the chemical state of element in NH2-PAA/Alginate and Cd-NH2-PAA/Alginate. The XPS survey scan spectra (Fig. 8a)  in the adsorption process [37]. This result also can be inferred by XPS element analysis in Table 1, the content of Ca elements decreased from 1.6% to 0.7%, while the content of Cd increased to 1.16%.
The high-resolution XPS spectra show that the binding energies of N1s had significant changed before and after adsorbing Cd(II) (Fig. 8c). There is mainly one typical N1s peak at 399.75 eV in -NH2 groups of NH2-PAA/Alginate, while after the adsorption of Cd(II), the characteristic peak shifted to higher positions at 400.56 eV, which indicates the formation of coordination bond between amino group and Cd(II).
This can be interpreted that the Cd atom is able to accept isolated electron pair at N atom to form new compounds, and reducing the electron densities of the N atom [38].

Conclusions
In this study, a new hydrogel with double network interpenetrating structure was well designed and prepared via simple radical polymerization and amino modification.      Time-dependent Cd(II) sorption on hydrogel, the data are ttings to the Pseudo-rst-order, Pseudo-secondorder models and intraparticle diffusion model (C0=50 mg/L, T = 25, pH = 6.0).