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Li, S., Chen, F., Lin, F., Kong, Y., and Dai, H. (2018). "Adsorption performance of SiO2/CPAM composites for aqueous Ca(II)," BioRes. 13(2), 3554-3570.


Silica/cationic polyacrylamide (SiO2/CPAM) composites with a dendritic structure were prepared via in situ polymerization based on silica containing vinyl groups and acrylamide monomers. The structure and surface properties of the composites were revealed with scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier-transformed infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS). The adsorption behavior and properties of aqueous Ca(II) on the SiO2/CPAM composites were also investigated. The effect of the pH value, the initial Ca(II) concentration, adsorption time, and temperature on adsorption properties were examined. The results showed that the SiO2/CPAM composites displayed a high adsorption performance for aqueous Ca(II). The maximum adsorption capacity of the SiO2/CPAM composites for Ca(II) was 123.4 mg•g-1 at room temperature and pH 9. The adsorption behavior was in agreement with the Langmuir isotherm model. The adsorption was an endothermic and spontaneous process. Adsorption kinetics fitted well with the pseudo-second-order model. The adsorption of Ca(II) on the SiO2/CPAM composites was mainly attributed to chemical interaction, and chelation was more significant than electrostatic interaction.

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Adsorption Performance of SiO2/CPAM Composites for Aqueous Ca(II)

Shan Li,a Fa Chen,a Furong Lin,a Yong Kong,a and Hongqi Daib,*

Silica/cationic polyacrylamide (SiO2/CPAM) composites with a dendritic structure were prepared via in situ polymerization based on silica containing vinyl groups and acrylamide monomers. The structure and surface properties of the composites were revealed with scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier-transformed infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS). The adsorption behavior and properties of aqueous Ca(II) on the SiO2/CPAM composites were also investigated. The effect of the pH value, the initial Ca(II) concentration, adsorption time, and temperature on adsorption properties were examined. The results showed that the SiO2/CPAM composites displayed a high adsorption performance for aqueous Ca(II). The maximum adsorption capacity of the SiO2/CPAM composites for Ca(II) was 123.4 mg•g-1 at room temperature and pH 9. The adsorption behavior was in agreement with the Langmuir isotherm model. The adsorption was an endothermic and spontaneous process. Adsorption kinetics fitted well with the pseudo-second-order model. The adsorption of Ca(II) on the SiO2/CPAM composites was mainly attributed to chemical interaction, and chelation was more significant than electrostatic interaction.

Keywords: Silica; Cationic polyacrylamide; Composites; Adsorption; Ca(II)

Contact information: a: School of Petrochemical Engineering, Changzhou University, Changzhou, Jiangsu 213164, China; b: Jiangsu Province Key Lab of Pulp and Paper Science and Technology, Nanjing Forestry University, Nanjing, Jiangsu 210037, China; *Corresponding author:


Calcium ion (Ca(II)) is widely found in natural water bodies. It is involved in the production processes of industry and agriculture, as well as the biological activities of organisms. However, excess calcium ions have many negative consequences. In industrial production, water containing a high content of calcium ions causes incrustation scale, which decreases the heat transfer efficiency of heat exchangers. The accumulation of metal ions such as Ca(II) and Mg(II) in the closed whitewater systems of the papermaking industry has adverse effects, such as disturbing the wet-end chemical balance, affecting operation stability of the paper machine, and decreasing product quality (Gao et al. 2009; Ba and Cao 2014; Sun et al. 2014). There are many methods for Ca(II) removal from aqueous solutions, such as filtering, ion exchange, adsorption, precipitation, and complexation (Fan et al.2012; Hemalatha et al. 2017; Hettiarachchi et al. 2017). Among these, complex formation can be regarded as the simplest and most economical method.

It is also an effective method for controlling multivalent metal ions by taking advantage of the “soft-hard-acid-base” theory (Elving et al. 1987) to design a polymer that can exert complexation with Ca(II). Ye et al. (2009) reported that the modified natural polymer (named as ACS) containing carboxyl groups can control aqueous Ca(II), and the Ca(II) adsorption amount was 32.0 mg•g-1 with a removal of 81%. Sun et al. (2012) synthesized an amphoteric polymer (P(AM-DMDAAC-AA)) to regulate Ca(II) with an adsorption amount of 80.2 mg•g-1. Chen and Yu (2013) prepared a poly(acrylate-acrylamide) adsorbent resin to adsorb Ca(II) in hard water with a maximum adsorption amount of 108.0 mg•g-1. However, the adsorption performance of all known adsorbents is still not satisfactory.

Polyacrylamide (PAM) is a water-soluble polymer with amino (NH2) groups on the molecular chain. It is an ideal material used for designing and preparing water treatment agents with specific applications through the introduction of various functional monomers, such as for the removal of hazardous materials. Furthermore, it is common for PAM to be combined with inorganic materials in composites to resolve a number of practical problems (Zhao et al. 2010; Wang and Li 2015).

Silica (SiO2) or mesoporous silica can be mixed with or bonded to PAM to produce composite adsorbents or hydrogels due to its large specific surface area, high chemical stability, and ease of modification. They can be widely applied for the removal of heavy metal ions or organic substances from aqueous solution, such as lead (Ghorai et al. 2012), mercury (Saad et al. 2016), cholesterol (Clausen et al. 2014), aniline (An et al. 2009a), dyestuffs (Hadizade et al. 2017), and 2,4,6-trinitrotoluene (TNT) (An et al. 2009b). SiO2/PAM composite resin can be synthesized through an in situ sol-gel method using tetraethyl orthosilicate (TEOS) in the solution of acrylamide monomers. SiO2/PAM composite adsorbent resin has also been obtained after modification with sulfomethylation for adsorbing metal ions, such as Pb(II) and copper(II) (Li et al. 2012). SiO2/PAM composites have numerous possible applications in industrial wastewater treatment.

In this work, dendritic SiO2/cationic polyacrylamide composites (SiO2/CPAM) were fabricated by copolymerization of acrylamide and silica microspheres containing vinyl groups (vinyl-SiO2) for adsorbing the aqueous Ca(II). A number of vinyl groups on the surface of vinyl-SiO2 can be copolymerized with other vinyl monomers to form SiO2-based dendritic composites connected by SiO2 cores. Composites with such structure and components can greatly increase the adsorption for Ca(II). The influence of the pH value, initial Ca(II) concentration, adsorption time, and temperature on the adsorption properties of SiO2/CPAM were systematically investigated. The adsorption kinetics, thermodynamics, and mechanisms of Ca(II) adsorption onto SiO2/CPAM were also discussed.


Reagents and Materials

Acrylamide (AM), anhydrous ethanol, potassium persulfate (K2S2O8), sodium hydrogen sulfite (NaHSO3), ethylene diamine tetraacetic acid disodium salt (EDTA•2Na), hydrogen chloride (36.0-38.0 wt.%), chrome black T, calcium chloride, ammonium chloride, and ammonium hydroxide (25.0-28.0 wt.%) were obtained from Sinopharm Chemical Reagent Co. (Shanghai, China). 2,2-Azobis (2-methylpropionamidine dihydrochloride) (AIBA•2HCl) was purchased from Sigma-Aldrich (St. Louis, USA). Vinyltriethoxysilane (VTES) was obtained from Energy Chemical (Shanghai, China). Acryloyloxyethyl trimethyl ammonium chloride (DAC, 85.5 wt.%) was supplied by Shandong Liyuan Guosheng Chemical Co. (Jinan, China). All chemicals were used without further purification. Ultrapure water (18.2 M•cm) was used directly from a Milli-Q water system. Filtering film (50 mm/0.45 μm) was provided by Shanghai Xinya Purification Equipment Co. (Shanghai, China).

Preparation of SiO2/CPAM Composites

Preparation of vinyl-SiO2

Vinyl-SiO2 was prepared as described (Deng et al. 2009) with minor modifications. In brief, 3 mL of VTES was dispersed in 50 mL of water under vigorous magnetic stirring (300 rpm), which was followed by addition of 0.05 mL of 2 M HCl aqueous solution. The mixture was stirred for a certain time until a transparent solution was obtained. Next, 0.6 mL of ammonium hydroxide was added dropwise into the reaction mixture, and the reaction proceeded at room temperature for 2 h. The final mixture contained 5.08 wt.% VTES and 0.26 wt.% NH3•H2O. After completion of the reaction, the reaction mixture was centrifuged at 2,000 rpm for 10 min, which was followed by ultrasonic treatment in ethanol to remove impurities such as ammonia, water, and unreacted VTES. The same separation process was repeated thrice. Finally, the resulting particles were dried at room temperature.

Preparation of SiO2/CPAM composites

Vinyl-SiO2 was adopted as the substrate for in situcopolymerization of AM and DAC. In a typical process, 0.0391 g of dry vinyl-SiO2, 2.2808 g of AM (1/3 of the total mass), and 1.1427 g of DAC were mixed in water. The mixture was sonicated for 5 to 10 min and then added to a four-necked flask equipped with a condenser and mechanical agitation. To prevent oxygen during the synthesis, a flow of nitrogen gas was introduced into the mixture for 30 min. Next, 0.0015 g of EDTA and 0.0047 g of NaHSO3 were added into the flask. The remaining 2/3 AM and the initiator solution containing K2S2O8 and AIBA•2HCl (0.6 wt.% based on total monomer mass, in which AIBA•2HCl was 35.0 wt.%) were dropped slowly into the mixture at 40 °C. The temperature was controlled using a thermostat water bath. The mixture was reacted at this temperature for 2 h and then 65 °C for another 4 h; a transparent yellow solution was obtained. The crude composites were precipitated with ethanol, washed with ethanol several times, and dried at room temperature.


Scanning electron microscopy (SEM) images were recorded using a JEOL JSM-6360LA microscope (Tokyo, Japan). Transmission electron microscopy (TEM) was performed on a JEOL JEM-2100 microscope operating at an accelerating voltage of 200 kV. Fourier transform infrared (FT-IR) spectra of different samples were measured on a Shimadzu Nicolet FTIR-8400S spectrophotometer (Kyoto, Japan) using KBr pellets. X-ray photoelectron spectroscopy (XPS) studies were performed with a VG Corporation ESCALAB MKII spectrometer (London, UK) with an Al K X-ray source.

Batch Adsorption Studies

The adsorption ability of SiO2/CPAM was measured by titration. In a typical process, 0.08 g of dry SiO2/CPAM sample was dissolved with ultrapure water and then transferred into a 100 mL volumetric flask, where CaCl2 (V0c1:0.1 mol•L-1) was added. The volume of the flask was metered with ultrapure water, followed intermittent oscillation at room temperature. After 90 min, the solution was filtered with the filtering film, and 10 mL of the supernatant was placed in a 250 mL Erlenmeyer flask with 10 mL of NH3-NH4Cl buffer solution (pH = 10) and 50 mg of chrome black T solid indicator. The mixture was titrated with EDTA standard solution (c2: 0.002 mol•L-1) so that the endpoint would be indicated by the change of the mixture hue from wine-red to blue. The result was the average value of three measurements.

The adsorption capability of SiO2/CPAM for Ca(II) was calculated by Eq. 1,


where Q (mmol•g-1) is the adsorption capability of SiO2/CPAM for Ca(II), m (g) is the mass of samples, c1 (mol•L-1) and c2(mol•L-1) are the concentrations of CaCl2 and EDTA, respectively, and V0 (mL) and V (mL) are the consumed volumes of CaCl2 solution and EDTA standard solution at the endpoint, respectively. The adsorption ability was also expressed as Qm(mg•g-1Q= 40.078Q), in order to compare the results with data from the literature.

The residual Ca(II) concentration was calculated by Eq. 2,


where cr (mol•L-1) is the residual Ca(II) concentration in the sample solution and V′(mL) is the total volume of the sample solution.


Characterization of SiO2/CPAM Composites

Figure 1a shows the SEM image of vinyl-SiO2.

Fig. 1. SEM images of vinly-SiO2 (a) and SiO2/CPAM (b), and TEM of SiO2/CPAM (c)

The microparticles were complete spheres with a uniform dispersion and smooth surface, and they had an average diameter of approximately 625 nm. Figure 1b displays the SEM image of SiO2/CPAM. The micromorphology of the composites presented a distinct dendritic structure with a center of vinyl-SiO2. Also, it was demonstrated more clearly that the composites were connected by the vinyl-SiO2 microparticles from Fig. 1c, because vinyl groups on the surface of vinyl-SiOcan make them copolymerized with AM and DAC via in situ polymerization. They linked the organic molecular chains together to form a large dendritic structure.

The IR spectra of vinyl-SiOand SiO2/CPAM are shown in Fig. 2. The peaks at 3352 cm-1 and 3194 cm-1 were ascribed to asymmetrical and symmetrical stretching vibrations of NH in primary amide groups (Li et al. 2006; Pirzada et al. 2012, 2014; Pan 2015); these peaks overlapped with the 3400 to 3500 cm-1band of SiOH in the SiO2/CPAM composites. The peak at 3025 cm-1 corresponded to =CH stretching vibrations in vinyl-SiO2spheres, which was not seen in the SiO2/CPAM composites. However, new distinct CH3 and CH2 peaks were found in the SiO2/CPAM composites at 2940 cm-1 and 2866 cm-1, respectively. The strong absorbing peak at 1662 cm-1corresponded to C=O stretching vibrations, while peaks at 1611 cm1 and 1451 cm-1 corresponded to NH bending vibrations and CH2 bending vibrations in CH2N+(CH3)3, respectively. Peaks at 1128 cm-1 and 1045 cm-1 were attributed to SiOSi stretching vibrations in vinyl-SiO2. A SiC stretching vibration band appeared at 767 cm-1. These peaks confirmed the presence of vinyl-SiO2, and they were connected to AM and DAC via in situcopolymerization to form a dendritic structure.

Fig. 2. IR spectra of vinyl-SiO2, SiO2/CPAM, and SiO2/CPAM-Ca

The typical sampling depth by using XPS was 2 to 10 nm so that the surface elemental compositions of the particles could be obtained for the near surface. Figure 3 demonstrates the XPS survey spectra recorded for the SiO2/CPAM composites. Carbon, oxygen, silicon, nitrogen, and chlorine were detected by XPS on the surface of SiO2/CPAM.

Figure 3a shows Si 2p and Si 2s signals at 101.9 eV and 152.5 eV, respectively, indicating the presence of silica at the surface of the SiO2/CPAM composites (Fielding et al. 2011). The peaks at 101.6 eV, 102.0 eV, and 102.6 eV from Fig. 3c were attributed to Si(-O)1, Si(-O)2, and Si(-O)3, respectively (Alexander et al. 1999), confirming the existence of hybrid silica without Si(-O)4. Deconvolution of the XPS for C 1s electron of SiO2/CPAM (Fig. 3b) presents carbon in three different chemical states. The dominant component at 284.7 eV was related to the C atom of CC linkage, and the resolved weak component at 286.5 eV was attributed to CO linkage (Jedlicka et al. 2007; Peng et al. 2014). However, the peak at 288.0 eV corresponded to C=O bonding of NC=O (Jedlicka et al. 2007). Thus, AM residues appeared mainly on the surface of SiO2/CPAM, and the CO bonding was attributed to DAC or VTES residue (CH2CH3OSi) that was not hydrolyzed completely. It was concluded that vinyl-SiO2 was preferentially reacted with the positively charged DAC and then copolymerized with AM. The positive charges within the polymer chains made the dendritic structure of SiO2/CPAM well-dispersed in solution, reducing the aggregates and leading to excellent adsorption behavior.

Fig. 3. XPS survey spectra recorded for SiO2/CPAM (a) and C 1s (b), and Si 2p (c) XPS spectra

Effects of the pH Value and Initial Ca(II) Concentration

The pH value was closely related to the presence of Ca(II) in the solution. To investigate the effect of pH, the sorption experiments were conducted with an initial Ca(II) concentration of 5.0 mmol•L1, as shown in Fig. 4. The Ca(II) adsorption amount (Q) increased with the pH value, while the residual Ca(II) concentration decreased continuously. The excess H+ had a competitive adsorption with the positively charged Ca(II) in the lower pH values (1 to 3), resulting in a decreased adsorption of Ca(II) in the active sites of SiO2/CPAM. In addition, the surface of the silica spheres and the functional groups on the polymer chain were protonated to form SiOH, SiOH2+, or NH3+ so that electrostatic repulsion occurred between the metal ions and the protonated polymers (Kong et al. 2011), leading to the decline of the adsorption ability. The adsorption increased abruptly at pH 12 due to the formation of CaCOprecipitation in the strong alkaline condition (Sun et al. 2012). When the pH value was in the range of 8 to 11, Q had a maximum value and the adsorption was close to saturation.

Fig. 4. Effect of the pH value on the adsorption amount and residual Ca(II) concentration

The effect of the initial Ca(II) concentration was investigated while maintaining a pH value of 9, as demonstrated in Fig. 5. When the initial Ca(II) concentration was varied from 1.0 to 9.0 mmol•L-1, the adsorption amount increased rapidly, indicating the active sites on SiO2/CPAM were available. A possible reason was that increasing the concentration of metal ions can increase the effective collision odds between metal ions and adsorbents, leading to the increase of the adsorption amount (Wu et al. 2012). The residual Ca(II) concentration also increased with increment of the initial Ca(II) concentration. When the initial Ca(II) concentration was less than 9 mmol•L1, the residual Ca(II) concentration increased slightly, indicating an unsaturated adsorption process. When the initial Ca(II) concentration was more than 9 mmol•L1, the adsorption amount increased continuously until it leveled off at 13 mmol•L1, while the residual Ca(II) concentration also grew greatly. This result can be interpreted as the adsorption sites gradually becoming saturated and the electrostatic repulsion and steric hindrance both hindering the further adsorption of SiO2/CPAM for Ca(II) gradually. Therefore, the optimal initial Ca(II) concentration was 13 mmol•L1.

Fig. 5. Effect of the initial Ca(II) concentration on the adsorption amount and residual Ca(II) concentration

Fig. 6. Effect of the adsorption time on the adsorption amount and residual Ca(II) concentration

Adsorption Kinetics

The effect of the adsorption time (in the range of 20 to 180 min) was investigated while maintaining a pH value of 9 and an initial Ca(II) concentration of 5.0 mmol•L-1, as shown in Fig. 6. At the beginning, the adsorption amount increased with the adsorption time; it then reached a platform with a maximum amount of 3.08 mmol•g-1. In other words, SiO2/CPAM can adsorb 123.4 mg Ca(II) per gram when the adsorption time was equal to or more than 80 min. At the same time, the adsorption process reached equilibrium and the residual Ca(II) concentration remained constant.

To further determine the adsorption kinetics of Ca(II) on the SiO2/CPAM composites, the obtained data in Fig. 6 were fitted with the pseudo-first-order kinetic model (Eq. 3) and the pseudo-second-order kinetic model (Eq. 4) (Azizian 2004; Li et al. 2015; Mohanta and Ahmaruzzman 2018),

ln (qe – qt) = ln qe – k1t (3)


where qe (mg•g-1) and qt (mg•g-1) are the amounts of Ca(II) adsorbed at equilibrium and at time t (min), respectively, and k1(min-1) and k2 (g•mg-1•min-1) are the pseudo-first-order and pseudo-second-order rate constants, respectively.

(a) Pseudo-first-order model linear fitting

(b) Pseudo-second-order model linear fitting

Fig. 7. Pseudo-first-order model linear fitting and pseudo-second-order model linear fitting

The plots of ln(qeqt) versus t and t/qt versus t are shown in Fig. 7. The dynamic parameters and correlation coefficients were calculated by the fitting equations and are shown in Table 1. The R2 (0.9559) corresponding to the plot fitted with the pseudo-second-order kinetic model was higher than that obtained from the pseudo-first-order kinetic model (0.7163), and the calculated equilibrium adsorption amount (qe=123.4 mg•g-1) was essentially the same as the experimental value (123.4 mg•g-1), indicating that the adsorption kinetics of SiO2/CPAM for Ca(II) can be better described by the pseudo-second-order model. Furthermore, the formation of chemical bonds was the main factor affecting the pseudo-second-order adsorption, which also illustrated that the adsorption process was mainly chemisorption (Bai et al. 2011).

Table 1. Kinematic Models’ Linear Regressions and Kinetics Parameters of Adsorption of SiO2/CPAM for Ca(II)

Adsorption Thermodynamics

The relationship of qe and ce at different temperatures is shown in Fig. 8. The qe value increased continuously with ce and the system temperatures, indicating the adsorption process was endothermic and driven by entropy (Mondal et al. 2011). The viscosity of the solution decreased with the increase of temperature, which facilitated the movement of metal ions across the outer boundary and diffusion onto the molecular chains of the adsorbent (Al-Qodah et al. 2000).

Fig. 8. Adsorption isotherms at different temperatures

The equilibrium adsorption isotherm is one of the most important methods for understanding the mechanism of adsorption. The Langmuir and Freundlich thermodynamic models are widely used to examine the thermodynamic data (Xie et al. 2014; Mona and Kaushik 2015). Therefore, the adsorption data of SiO2/CPAM for Ca(II) were fitted with the Langmuir model (Tazrouti and Amrani 2009) and the Freundlich model (Freundlich and Helle 1939) as follows,


ln q= ln KF + 1/n ln C(6)

where qe (mg•g-1) and qm (mg•g-1) are the amount of Ca(II) adsorbed at equilibrium and the saturated adsorption capacity, respectively, ce (mg•L-1) is the Ca(II) concentration at equilibrium, b (L•mg-1) is the Langmuir constant, and KF (mg•g-1) and n are the Freundlich constants related to the adsorption capacity and intensity, respectively.

The plots of ce/qe versus ce and lnqe versus lnCe for Ca(II) adsorption are shown in Figs. 9 and 10. The values of qm and bwere calculated, respectively, according to the intercepts and slopes of the plots, as shown in Table 2. A linear relationship was observed at different temperatures from Fig. 9, and qm values were 341.3 mg•g-1, 432.9 mg•g-1 and 526.3 mg•g-1 at 20 °C, 40 °C, and 60 °C, respectively. Furthermore, the corresponding coefficients of determination (R2) were all higher than 0.99, indicating that the adsorption of Ca(II) fitted well with the Langmuir isotherm at different temperatures. While the adsorption data were also fitted with the Freundlich model, all the R2 values were less than 0.8 (Table 3).

Fig. 9. Fitting of equilibrium data to the Langmuir isotherm model

Table 2. Regression Parameters of Langmuir Isotherm Model for Adsorption of SiO2/CPAM for Ca(II)

Fig. 10. Fitting of equilibrium data to the Freundlich isotherm model

Hence, the adsorption of the investigated metal ions on SiO2/CPAM was monolayer and after saturation of this layer no further adsorption would take place (Alqadami et al. 2017), which matched the data of Fig. 8. In addition, the b value was higher than 0, indicating a spontaneous process; a larger b value indicated more stable products and an easier, stronger adsorption reaction (Chong and Volesky 1995). When the b value is lower than 0, that means the adsorbed product is unstable and easily desorbed (Li et al. 2011). The maximum b value was obtained at 40 °C, indicating that the adsorption was most favorable at this temperature.

Table 3. Regression Parameters of Freundlich Isotherm Model for Adsorption of SiO2/CPAM for Ca(II)

Analysis of Adsorption Mechanism

The FT-IR spectra of SiO2/CPAM before and after adsorbing Ca(II) (the latter is denoted as SiO2/CPAM-Ca) were compared to make certain the adsorption mechanism, as illustrated in Fig. 2. The bands at 3352 cm-1 and 3194 cm-1 were attributed to asymmetrical and symmetrical stretching vibrations of NH bonding in the primary amide groups in SiO2/CPAM, and they were transferred to one strong and broad peak at 3397 cm-1 with a shift towards the high band of 45 cm-1 after adsorbing Ca(II). The peak at 1611 cm-1 was ascribed to the stretching vibration of N-H bonding in SiO2/CPAM and did not appear after adsorption, indicating that the N atoms in primary amines were involved in the chelation of Ca(II) (Deng and Chen 2003; Sun and Liang 2010). The adsorption peak of C=O bonding in amino groups in SiO2/CPAM was moved from 1662 cm-1 to 1668 cm-1 (a shift of 6 cm-1) because these two peaks in the amino group were overlapped with the acyloxy group.

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Fig. 11. Adsorption mechanism between SiO2/CPAM and Ca(II)

The C=O bonding in amino groups cannot participate in chelation with metal ions, according to the reference (Deng and Chen 2003). Therefore, both the amino and acyloxy group were involved in the chelation reaction, resulting in the formation of p-π coordination of Ca(II). It was also inferred that the adsorption of SiO2/CPAM for Ca(II) was mainly based on chelation and electrostatic adsorption. The possible adsorption mechanism is presented in Fig. 11. The negative silicon hydroxyl group on the surface of SiO2 interacted with positive Ca(II), and a large number of N and O atoms on the PAM molecular chain were chelated with Ca(II).

Table 4. Comparison of Adsorption Capacity (Qm) of Ca(II) on Different Adsorbents

Comparisons with Other Adsorbents

The maximum adsorption capacity (Qm) of SiO2/CPAM at 20 °C was compared with other adsorbents for the adsorption of Ca(II) (Table 4). All adsorbents exhibited a lower adsorption amount than that recorded in this work. Compared with the direct-chain molecule, more active adsorption sites occurred on the surface of dendritic SiO2/CPAM, leading to chelation with Ca(II) easily. XPS data confirmed that AM residues were located on the molecular chain ends, while DAC residues were closer to the surface of SiO2. Thus, SiO2/CPAM was well-dispersed in solution, so that chelation with Ca(II) within the molecules grew easier and more stable and could not be influenced by the external force.


  1. The SiO2/CPAM composites were successfully synthesized with a dendritic structure extending outward from the SiO2center.
  2. High temperature was beneficial to the adsorption of Ca(II) onto SiO2/CPAM due to the formation of chelation between Ca(II) and N and O atoms on the surface of SiO2/CPAM. Adsorption of Ca(II) followed the Langmuir isothermal model well, as evidenced by a higher value of R2. The kinetic data fitted well with the pseudo-second-order dynamic model.
  3. The adsorption capacity was found to be superior to those of other reported adsorbents. The studies presented here reveal that it is a promising adsorbent to be applied for other contaminant metal ions in many fields.


The authors are grateful for the financial support of the National Natural Science Foundation of China (Grant No. 31470599), the Natural Science Foundation of Jiangsu Province (Grant No. BK20171194), and Prospective Joint Research Project of Jiangsu Province (Grant No. BY2015027-19).


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Article submitted: January 1, 2018; Peer review completed: March 5, 2018; Revised version received: March 14, 2018; Accepted: March 17, 2018; Published: March 23, 2018.

DOI: 10.15376/biores.13.2.3554-3570