New nanoporous resin (NPR), based on organic xerogel compounds, was prepared at 150 °C by sol–gel method from pyrogallol and formaldehyde mixtures in water using perchloric acid as catalyst. NPR was characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction and nitrogen porosimetry. The metal uptake characteristics were explored using well-established and effective parameters including pH, contact time, initial metal ion concentration and temperature. Optimum adsorptions of Cr(VI) and Cd(II) were observed at pH 3 and 4, respectively. Langmuir model gave the better fit for Cd(II) whereas for Cr(VI), Freundlich model was better than the other models to fit the experimental data. Kinetic studies revealed that the adsorption of Cd(II) was very fast compared to Cr(VI), and its data are well fitted by the pseudo-second-order kinetic model. The thermodynamic properties, i.e. ΔG°, ΔH° and ΔS°, showed that adsorption of Cr(VI) and Cd(II) onto NPR was endothermic, spontaneous and feasible in the temperature range of 27–55 °C.

INTRODUCTION

There are various methods for removing heavy metals from waste water including chemical precipitation, membrane filtration, ion exchange, liquid extraction or electrodialysis (Sitting 1981; Patterson 1985). These methods are not widely used due to their high cost and low feasibility for small-scale industries (Sohail et al. 1999). In contrast, adsorption technique is by far the most versatile and widely used. Sorbents which have been studied for adsorption of metal ions include activated carbon, fly ash, crab shell, coconut shell, zeolite, manganese oxides and resins (ion exchange and chelating).

During the past decade, several metal-retaining resins containing a variety of complexion or chelating ligands have been reported to efficiently remove heavy metals (Genc et al. 2002; Lebrun et al. 2007). Synthetic polymers containing amino, thio, oxo, carboxyl, phosphoryl and other groups have been developed. In particular, the amino/carboxyl group on an adsorbent has been found to be one of the most effective chelate functional groups for adsorption or removal of heavy metal ions from an aqueous solution (Rivas & Castro 2003; Kas goz 2006; Chen et al. 2007). It has also been reported that the amine groups can provide reactive sites for specific adsorption of various metal ions (Jin & Bai 2002;,Li & Bai 2005; Yang et al. 2010).

An efficient sorbent with both high capacity and fast rate adsorption should have functional groups and large surface area (Huang et al. 2011). Unfortunately, most current inorganic sorbents rarely have both at the same time. On the contrary, organic polymer, polyphenylenediamine, holds a large amount of polyfunctional groups (amino and imino groups) and can effectively adsorb heavy metal ions, whereas their small specific surface area and low adsorption rate limit their application. Therefore, new sorbents with both polyfunctional groups and high surface area are still required. More recently, the development of hybrid sorbents has opened up new opportunities for their application in deep removal of heavy metals from water (Pan et al. 2009; Zhao et al. 2011). Polymer-layered silicate nanocomposites (Pavlidou & Papaspyrides 2008) have attracted both academic and industrial attention because they exhibit dramatic improvement in properties at very low filler contents. The hybrid polymers were synthesized from the ring-opening polymerization of pyromellitic acid dianhydride and phenylaminomethyl trimethoxysilane (Liu et al. 2010).

The aim of this work is to assess the uptake of Cr(VI) and Cd(II) from aqueous solution onto nanoporous resin (NPR). In the first step, synthesis by sol–gel method of NPR is reported and structural and morphological properties investigated. In the second step, the efficiency of NPR for the adsorption of Cr(VI) and Cd(II) from aqueous solutions is studied.

MATERIALS AND METHODS

Preparation and characterization of the NPR

The preparation of NPR structure has been done in two steps. In the first one, organic xerogels were prepared by mixing formaldehyde (F) with dissolved pyrogallol (P) in water (W) solution and using perchloric acid as catalyst. The stoichiometric P/F and P/W molar ratios were 1/3 and 1/6, respectively. The wet gel was formed in a few seconds. In the second step, the obtained product was dried in a humid atmosphere at 50 °C for 2 weeks. To obtain a structured xerogel, the wet gel was transferred into an incubator and dried at 150 °C at a heating rate of 10 °C/day. The drying temperature was then maintained for 2 days, and finally the sample was cooled (El Mir et al. 2007). Figure 1 exhibits the reaction of the polymerization of pyrogallol and formaldehyde.

Figure 1

Reaction of the polymerization of pyrogallol and formaldehyde.

Figure 1

Reaction of the polymerization of pyrogallol and formaldehyde.

The synthesized product was characterized using a JEOL JSM-6300 scanning electron microscope (SEM) and a JEM-200CX transmission electron microscope (TEM). The specimens for TEM were prepared by putting the as-grown products in ethyl alcohol and immersing them in an ultrasonic bath for 15 min, then dropping a few drops of the resulting suspension containing the synthesized materials onto a TEM grid. The X-ray diffraction (XRD) patterns of NPR were carried out by a Bruker D5005 diffractometer, using Cu K radiation (λ = 1.5418 Å). The nitrogen adsorption–desorption isotherm of NPR was recorded by using Micrometrics ASAP2020 equipment.

Adsorption experiments

The stock solutions of cadmium and chromium were prepared by dissolving cadmium nitrate and potassium dichromate in distilled water separately. The test solutions containing single cadmium and chromium ions were prepared by diluting 1 g/L stock metal ion solution. The initial metal ion concentration ranged from 20 to 140 mg/L. The pH of each solution was adjusted to the required value with HCl or NaOH before mixing the adsorbent. Adsorption experiments were carried out in an Erlenmeyer flask by taking 10 mg of NPR in 25 mL of metal solution at the desired temperature (25 ± 1 °C) and pH. The flasks were agitated on a shaker for 12 hours, which is more than ample time for adsorption equilibrium. The amount of metal adsorbed was determined by the difference between the initial metal ion concentration and the final one after equilibrium was reached. The residual Cd(II) and Cr(VI) concentrations were measured by SPECTRO GENESIS inductively coupled plasma-atomic emission spectrometry.

The results are given as a unit of adsorbed and unadsorbed metal ion concentration per gram of adsorbent in solution at equilibrium and are calculated by Equation (1) 
formula
1
where qe is the adsorbed metal ion quantity per gram of adsorbent at equilibrium (mg/g), m is the weight of adsorbent (g), C0 the initial metal concentration (mg/L), Ce the metal concentration at equilibrium (mg/L) and V is the working solution volume (L). The removal percentage was calculated by Equation (2) 
formula
2

RESULTS AND DISCUSSION

Adsorbent characterizations

Figure 2 exhibits the XRD patterns of the extracted product as prepared after heat drying at 150 °C in natural atmosphere. According to this diffractogram, the sample is partly amorphous, due to the presence of a small band centered at around 25°, corresponding to (0 0 2) hkl plan, the most intensive diffraction peak of crystalline graphite phase.

Figure 2

XRD patterns of NPR.

Figure 2

XRD patterns of NPR.

The adsorption–desorption isotherm of the sample is of type I (Figure 3) in the Brunauer, Emmett and Teller (BET) classification, and characteristic of microporous solids. The microporous specific surface area is 562 m2/g determined by the conventional BET method, with micropore volume of about 0.27 cm3/g. The mean micropore size determined from the BET surface area and the pore volume in the approximation of cylindrical pores is close to 2 nm.

Figure 3

Cryogenic N2 adsorption–desorption isotherm of NPR.

Figure 3

Cryogenic N2 adsorption–desorption isotherm of NPR.

Figure 4 displays three SEM micrographs of NPR samples; particles with 1–5 μm in diameter appear to coagulate together leaving little space between them. The surface area and pore volume of this carbon indicate that these particles are essentially microporous. These results are consistent with porosity measurements.

Figure 4

SEM micrographs of NPR.

Figure 4

SEM micrographs of NPR.

TEM micrographs of the NPR in Figure 5 present nanospheres inside microparticles. It is clearly shown that the microparticles consist of a series of spherical nanoparticles with diameters in the range of 10 nm. These particles are arranged in a three-dimensional network. The TEM data confirm that the interconnected solid nanoparticles comprise an open-celled network with continuous nanodimension porosity. These observations are also consistent with porosity measurements.

Figure 5

TEM micrographs of NPR.

Figure 5

TEM micrographs of NPR.

Effect of pH

In this study the initial concentration of Cd(II) is fixed at 50 mg/L, whereas for Cr(VI) it is fixed at 45 mg/L. The mass of NPR used in this study is 10 mg. Figure 6 shows the effect of initial pH on the removal of Cd(II) and Cr(VI) using NPR. This figure shows that the cadmium adsorption by the NPR increases with increasing pH and reaches three maxima of 70.56, 68.06 and 83.175 mg/g at pH 4, pH 7 and pH 11, respectively. The low cadmium sorption at low pH value may be explained on the basis of active sites being protonated, resulting in a competition between H+ and M2+ for occupancy of the binding sites (Tobin et al. 1984). However, for pH values between 7 and 9, lower adsorption capacity was observed; this might be due to the precipitation and lower polarity of cadmium ions at higher pH values. At pH 11 the precipitation reaction contributed to the removal of Cd(II) and the high value of qe is not solely due to the adsorption. For this reason the pH of the solution should be lower than the critical pH of hydroxide precipitation (9.03 for Cd(II)) when considering the adsorption efficiency (Dong et al. 2010).

Figure 6

Effect of initial pH on the removal of Cd(II) and Cr(VI) using NPR.

Figure 6

Effect of initial pH on the removal of Cd(II) and Cr(VI) using NPR.

Figure 6 shows that the adsorption of Cr(VI) decreases with increase in pH from 3 to 10. The maximum removal is 117.91 mg/g and it occurred at initial pH 3. The chromate may be represented in various forms, such as H2CrO4, HCrO4, CrO42−, Cr2O7 and Cr2O72− in the solution phase as a function of pH and concentration. In the pH range from 2 to 7, HCrO4 and Cr2O7 are predominant. In this pH range the amounts of Cr(VI) ion can be taken by one active site of the resin in acidic condition and it is double that in alkali condition. At pH < 3, the decrease in adsorption capacity may be attributed to the competitive adsorption between Cl anions (Mao et al. 2012). For pH > 7, only CrO42− ions exist in the solution throughout the experimental concentration range. At alkaline pH values, the sorption trend can likely be ascribed to the effect of competitive binding between CrO42− and OH for the binding sites on the surface of the resins. At higher pH, an excess of OH can compete effectively with CrO42− for the bonding sites, resulting in a lower level of Cr(VI) ion uptake (Edebali & Pehlivan 2010).

Effects of contact time

In this study, 100 mg of NPR was added to each 150 mL of chromium and cadmium solutions and the pH of each solution fixed at 6.5. The initial concentrations of Cd(II) and Cr(VI) were fixed at 22.34 and 17.2 mg/L, respectively. Figure 7 shows the effect of the contact time on the sorption of Cr(VI) and Cd(II) by NPR. As can be seen from this figure, with the beginning of adsorption, the uptake of Cd(II) increased quickly, and after only 10 min the process of adsorption reached equilibrium. After this equilibrium period, the amount of adsorbed metal ions did not significantly change with time whereas for Cr(VI), the process of adsorption was very slow and reached equilibrium after 600 min.

Figure 7

Effect of contact time for Cr(VI) and Cd(II) removal onto NPR.

Figure 7

Effect of contact time for Cr(VI) and Cd(II) removal onto NPR.

Kinetic models

The integrated linear form of the pseudo-first-order equation can be expressed as follows (Lagergren 1998): 
formula
3

where qe (mg/g) and qt (mg/g) are the adsorption capacity at equilibrium and at time t (min), respectively, K1 (1/min) is the rate constant of pseudo-first-order adsorption. The straight line plots of ln (qeqt) against t (Figure 8) were used to determine the rate constant, K1, qe, and correlation coefficient R2 values of the metal ions.

Figure 8

Pseudo-first-order kinetic plots for Cr(VI) and Cd(II) adsorption onto NPR.

Figure 8

Pseudo-first-order kinetic plots for Cr(VI) and Cd(II) adsorption onto NPR.

The integrated linear form of the pseudo-second-order equation can be expressed as follows: 
formula
4
where K2 (g/(mg/min)) is the rate constant of pseudo-second-order adsorption. The equilibrium adsorption amount (qe) and the pseudo-second-order rate parameters (K2) are calculated from the slope and intercept of plot t/qt versus t (Figure 9). Table 1 gives the kinetic parameters obtained from pseudo-first-order and pseudo-second-order kinetic models for Cd(II) and Cr(VI) adsorption on NPR.
Table 1

Adsorption kinetic model rate constants for Cd(II) and Cr(VI) adsorption on nanoporous resin

Ion qe, exp (mg/g) Pseudo-first-order Pseudo-second-order 
k1 (1/min) qe1, cal (mg/g) R2 k2 (g/mg/min) qe2, cal (mg/g) R2 
Cr(VI) 22.473 3.3 × 10−3 15.31 0.978 8.42 × 10−4 21.93 0.994 
Cd(II) 57.125 2.1 × 10−3 2.58 0.104 1.75 × 10−2 56.818 0.999 
Ion qe, exp (mg/g) Pseudo-first-order Pseudo-second-order 
k1 (1/min) qe1, cal (mg/g) R2 k2 (g/mg/min) qe2, cal (mg/g) R2 
Cr(VI) 22.473 3.3 × 10−3 15.31 0.978 8.42 × 10−4 21.93 0.994 
Cd(II) 57.125 2.1 × 10−3 2.58 0.104 1.75 × 10−2 56.818 0.999 
Figure 9

Pseudo-second-order kinetic plots for Cr(VI) and Cd(II) adsorption onto NPR.

Figure 9

Pseudo-second-order kinetic plots for Cr(VI) and Cd(II) adsorption onto NPR.

It can be concluded from the R2 values in Table 1 that the sorption mechanism of Cd(II) and Cr(VI) does not follow the pseudo-first-order kinetic model. Moreover, the experimental values of qe, exp are not in good agreement with the theoretical values calculated (qe1, cal) from Equation (3). Therefore, the pseudo-first-order model is not suitable for modeling the sorption of Cd(II) and Cr(VI) by NPR. However, for the pseudo-second-order, the R2 value is 0.99 and the theoretical qe2, cal values were closer to the experimental qe, exp values (Table 1). Based on these results, it can be concluded that the pseudo-second-order kinetic model provided a good correlation for the adsorption of Cd(II) and Cr(VI) by NPR in contrast to the pseudo-first-order model.

Adsorption isotherms

Cadmium and chromium adsorption isotherms are obtained by varying the initial concentration of each metal (20–140 mg/L) at room temperature. The plot of the cadmium and chromium adsorption capacity against their equilibrium concentration is shown in Figure 10. The value of qe increases sharply at low equilibrium concentrations whereas at higher values of Ce, the increase of qe is slowed down.

Figure 10

Adsorption isotherm of Cr(VI) and Cd(II) by NPR.

Figure 10

Adsorption isotherm of Cr(VI) and Cd(II) by NPR.

Three models were used to fit the experimental data: Langmuir isotherm, Freundlich isotherm and Temkin isotherm.

Langmuir isotherm

The linear form of the Langmuir isotherm model is described as 
formula
5
where KL is the Langmuir constant related to the energy of adsorption and qm is the maximum adsorption capacity (mg/g) (Boparai et al. 2011). The slope and intercept of plots of Ce/qe versus Ce were used to calculate qm and KL and the values of these parameters are given in Table 2.
Table 2

Langmuir, Freundlich and Temkin isotherm model parameters and correlation coefficients for adsorption of Cd(II) and Cr(VI) on nanoporous resin

Metal Langmuir Freundlich Temkin 
qm (mg/g) KL (L/mg) R2 KF n R2 bT KT R2 
Cd(II) 109.90 0.225 0.994 41.39 0.883 13.353 18.098 0.909 
Cr(VI) 188.68 0.023 0.856 12.88 1.96 0.978 37.452 0.288 0.874 
Metal Langmuir Freundlich Temkin 
qm (mg/g) KL (L/mg) R2 KF n R2 bT KT R2 
Cd(II) 109.90 0.225 0.994 41.39 0.883 13.353 18.098 0.909 
Cr(VI) 188.68 0.023 0.856 12.88 1.96 0.978 37.452 0.288 0.874 

Freundlich isotherm

The linear form of the Freundlich equation is expressed as 
formula
6

where KF and n are Freundlich isotherm constants related to adsorption capacity and adsorption intensity, respectively, and Ce is the equilibrium concentration (mg/L). The Freundlich isotherm constants KF and n are determined from the intercept and slope of a plot of log qe versus log Ce, and they are given in Table 2. The obtained values of n are greater than unity indicating chemisorptions (Yang 1998). Isotherms with n > 1 are classified as L-type isotherms reflecting a high affinity between adsorbate and adsorbent and are indicative of chemisorptions (Yang 1998).

Temkin isotherm

The linear form of Temkin isotherm model is given by the equation 
formula
7
where bT is the Temkin constant related to the heat of sorption (J/mol) and KT is the Temkin isotherm constant (L/g) (Boparai et al. 2011). KT and bT were determined from the intercept and slope of a plot of qe versus ln Ce (Table 2).

The estimated adsorption constants with corresponding correlation coefficients (R2) are given in Table 2. For cadmium the value of correlation coefficients obtained from each model indicated that the Langmuir model is better than the Freundlich and Temkin models to fit the experimental data, which confirms that the adsorption is a monolayer, the adsorption of each molecule has an equal activation energy and the adsorbate–adsorbate interaction can be negligible. Thus, it is clear that the adsorption occurs on a homogeneous surface. The maximum adsorption capacity of Cd(II), calculated using the Langmuir model, is 109.9 mg/g at room temperature. For chromium, the correlation coefficients obtained from each model indicated that the Freundlich model was better than the Langmuir and Temkin models to fit the experimental data.

Thermodynamic parameters

The standard Gibbs free energy ΔGo (kJ/mol) was calculated using the following equation: 
formula
8
where k is the thermodynamic equilibrium constant, or the thermodynamic distribution coefficient, and it can be defined as 
formula
9
where ae is the activity of metal ion in solution at equilibrium; as is the activity of adsorbed metal ion; Cs is the surface concentration of metal ion (mmol/g) in the adsorbent; Ce is the metal ion concentration in solution at equilibrium (mmol/mL); γe represents the activity coefficient of the metal ion in solution; and γs is the activity coefficient of the adsorbed metal ion. As the metal ion concentration in the solution declines to zero, k can be obtained by plotting ln (Cs/Ce) versus Cs and extrapolating Cs to zero (Chiron et al. 2003; Tu et al. 2012). The points of the obtained straight line are fitted by least-squares analysis. The intercept at the vertical axis yields the values of k. The obtained values of k and ΔG° at different temperatures are given in Table 3.
Table 3

Thermodynamic parameters for adsorption of heavy metals on nanoporous resin

Ion T (K) ΔG° (kJ/mol) ΔS° (J/mol/K) ΔH° (kJ/mol) 
Cd(II) 300 5.751 −4.364 53.1 1.158 
313 6.828 −4.999 
328 8.547 −5.851 
Cr(VI) 300 1.3157 −0.684 108.4561 3.189 
313 2.1312 −1.969 
328 3.9162 −3.722 
Ion T (K) ΔG° (kJ/mol) ΔS° (J/mol/K) ΔH° (kJ/mol) 
Cd(II) 300 5.751 −4.364 53.1 1.158 
313 6.828 −4.999 
328 8.547 −5.851 
Cr(VI) 300 1.3157 −0.684 108.4561 3.189 
313 2.1312 −1.969 
328 3.9162 −3.722 
The average standard enthalpy change (ΔH°) and entropy change (ΔS°) of metal ion adsorption onto NPR were calculated by the following equation: 
formula
10
where ΔH° and ΔS° were calculated from the slope and the intercept in the plot of ln (k) against 1/T, respectively. These results are shown in Figure 11. Table 3 presents the obtained values of ΔH° and ΔS° at different temperature for metal ion adsorption process on NPR.
Figure 11

Plot of ln k versus 1/T for the estimation of thermodynamic parameters for adsorption of Cr(VI) and Cd(II) on NPR.

Figure 11

Plot of ln k versus 1/T for the estimation of thermodynamic parameters for adsorption of Cr(VI) and Cd(II) on NPR.

The thermodynamic equilibrium constant k increased with temperature indicating that the adsorption was endothermic. Negative values of ΔG° for the two metal ions indicate spontaneous adsorption and the degree of spontaneity of the reaction increases with increasing temperature. The values of standard enthalpy change for Cd(II) and Cr(VI) are positive. This suggests that the adsorption of Cd(II) and Cr(VI) by NPR is endothermic, which is supported by the increasing of adsorption with temperature for the two elements. The positive standard entropy change of Cd(II) and Cr(VI) reflects the affinity of the NPR toward the two metal ions (Yang 1998).

Comparison of Cd and Cr adsorption capacity among different adsorbents

The adsorption capacity of NPR for the removal of Cd(II) and Cr(VI) has been compared to various adsorbents reported in the literature and their adsorption capacities are given in Table 4. A comparison between our work and the reported data from the literature shows that NPR is a more efficient and promising adsorbent for Cd(II) and Cr(VI) removal than other adsorbents. Therefore, it can be safely concluded that NPR has a considerable potential for the removal of Cd(II) and Cr(VI) from water and wastewater.

Table 4

Comparison of adsorption capacities of heavy metals for various adsorbents

Adsorbent qe (mg/g) T (K) pH Reference 
Cd(II) 
 Oxidized nanoporous AC 31.4 297  Xiao & Thomas (2004)  
 Acid-treated AC 15.2 298 6.0 Jin et al. (2013)  
 CSTU resin 120  Monier & Abdel-Latif (2012)  
 D152 resin 378 298 5.59 Xiong & Yao (2009)  
 NPR 113.08 313 6.5 This work 
Cr(VI) 
 Amberlite IRA96 resin 33.38 298 Edebali & Pehlivan (2010)  
 Lignin-based resin (LBR) 57.68  Liang et al. (2013)  
 PS-EDTA magnetic resin 250 298 Mao et al. (2012)  
 Raw PS-EDTA resin 123.05 298 Mao et al. (2012)  
 KIP210 resin 102 308 Yang et al. (2014)  
 NPR 233.99 313 6.5 This work 
Adsorbent qe (mg/g) T (K) pH Reference 
Cd(II) 
 Oxidized nanoporous AC 31.4 297  Xiao & Thomas (2004)  
 Acid-treated AC 15.2 298 6.0 Jin et al. (2013)  
 CSTU resin 120  Monier & Abdel-Latif (2012)  
 D152 resin 378 298 5.59 Xiong & Yao (2009)  
 NPR 113.08 313 6.5 This work 
Cr(VI) 
 Amberlite IRA96 resin 33.38 298 Edebali & Pehlivan (2010)  
 Lignin-based resin (LBR) 57.68  Liang et al. (2013)  
 PS-EDTA magnetic resin 250 298 Mao et al. (2012)  
 Raw PS-EDTA resin 123.05 298 Mao et al. (2012)  
 KIP210 resin 102 308 Yang et al. (2014)  
 NPR 233.99 313 6.5 This work 

CONCLUSION

Nanometer-scale resin particles with diameters in the range of 10 nm aggregated in micrometer-scale particles have been synthesized by a new protocol of sol–gel method. The NPR monolith obtained from the polymerization of pyrogallol and formaldehyde in water using perchloric acid as catalyst, presents some particular behaviors. Structural and textural analysis indicated an amorphous microporous phase with a specific surface area, micropore volume and pore size of about 562 m2/g, 0.27 cm3/g and 2 nm, respectively. This NPR was identified as a potential and highly efficient nanoporous structure for the removal of Cr(VI) and Cd(II) from water; the adsorption depends strongly on different parameters like pH and temperature. Kinetic studies revealed that the equilibrium was reached within 10 min for Cd(II) and within 600 min for Cr(VI) and the pseudo-second-order kinetic model provides the best correlation with the experimental data compared to the pseudo-first-order model. The maximum adsorption capacity of Cr(VI) and Cd(II) were found to be 233.99 and 113.08 mg/g, respectively, under pH of 6.5, and temperature of 40 °C. The Langmuir model yields a better fit than the Freundlich and Temkin models for Cd(II) adsorption on NPR, whereas the Freundlich model yields a better fit than the Langmuir and Temkin models for Cr(VI) adsorption under the investigated temperatures. From the thermodynamic studies, the adsorption process was spontaneous and endothermic. These results provide the enhancement of Cr(VI) and Cd(II) uptake from aqueous solutions by NPR which is considered as adsorbent for removing metals from water and wastewater.

ACKNOWLEDGEMENT

This work was supported by the National Plan, for Sciences, Technology and Innovation, at Al-Imam Mohammed Ibn Saud Islamic University, College of Sciences, Kingdom of Saudi Arabia.

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