Adsorption potential of macroporous Amberlyst-15 for Cd(II) removal from aqueous solutions

The macroporous ion exchange resins are unique and most suitable for the adsorption of heavy metal ions due to their porous three-dimensional structures and large specific surface areas. In the current investigation, a macroporous sulphonic acid cation exchange resin Amberlyst-15 was implemented for the adsorption of Cd (II) using batch adsorption technique to evaluate its removal efficiency. The characterization of resin surface was performed by several techniques: Scanning Electron Microscopy/Energy dispersive x-ray Spectroscopy (SEM/EDS), Thermogravimetric analysis (TGA), Fourier-transform infrared spectroscopy (FT-IR), Brunauer–Emmett–Teller (BET) surface area and Point of zero charge (PZC). The effects of various experimental parameters such as time, temperature, concentration, pH and dosage amount were examined in detail. The optimum pH for maximum uptake of Cd(II) onto the Amberlyst.15 was observed at pH 3 showing the efficient working of resin under highly acidic conditions. The results also proved that Amberlyst-15 showed tremendous adsorption potential toward Cd(II) removal; 99.95% removal within 30 min reaction time and 2.01 mmol g−1 maximum adsorption capacity at 323 K. The adsorption data was well described by Langmuir adsorption isotherm and pseudo second order models. The thermodynamic parameters revealed that the adsorption was endothermic, spontaneous and feasible process with increased randomness at resin surface. The free energy of adsorption (E) (13–15 kJ mol−1) determined from Dubinin-Radushkevitch (D-R) model proved the ion exchange reaction mechanism for Cd(II) adsorption. The experimental results reported herein validate that Amberlyst.15 resin is a promising adsorbent for the enhanced removal of Cd(II) and other toxic metals from contaminated water and waste effluents.


Introduction
Nowadays, water pollution is one of the important environmental issues which arise due to the uncontrolled and untreated discharge of industrial effluents containing heavy metals and organic dyes into the ecosystem [1]. The heavy metal ions are highly stable and non-biodegradable in nature, that's why cannot be removed from water and soil easily [2]. Humans exposure to heavy metals mainly by inhalation, food and drinking water can cause many health hazards which include hypertension, damaging of lungs and liver, kidney failure, muscles pain, high blood pressure, permanent head damage, diarrhea, heartburn, food allergies, gastrointestinal infection, immature growth and abnormal development [3,4]. Therefore, the removal of heavy metal ions from aqueous solutions becomes compulsory and challenging owing to their acute toxic effects. Many conventional techniques are available for removal of heavy metal ions which includes solvent extraction, precipitation, photocatalyrtic degradation, coagulation, reverse osmosis, membrane separation, biological degradation, ion exchange and adsorption [5][6][7]. Among all these described techniques, ion exchange is most promising and versatile because of numerous advantages such as high removal efficiency, more selectively, easy operation and designing, regeneration and recovery of both adsorbate and adsorbent, no sludge formation and production of secondary pollutants. Several works have been performed on synthesis and application of different types of ion exchangers; including inorganic, organic, hybrid, and surfactant and nanoparticles supported exchangers for the removal of heavy metal ions from water and wastewater solutions [4,[8][9][10].
Further, in ion exchange technology, organic ion exchange resins have been considered as suitable adsorbents due to their fast reaction rate, excellent exchange capacity, easy separation and regeneration of adsorbent, high removal efficiency even at low concentration and environment friendly nature [11,12]. Among these organic ion exchange resins, macroporous class of reins is unique due to their exceptionally high specific surface areas and more active sites, and their application for the removal of toxic metals has proved to be a great advancement in the field of ion exchange technology. They are gaining recently more interest in many environmental applications due to their greater thermal stability, more resistance to oxidation and osmotic shocks. For that reason, macroporus ion exchange resins have been extensively practiced for the adsorption of heavy metal ions, and results have shown the outstanding performance of macroporous resins D113, D151, D152, D155, NKC-9 and D72 for the removal of Dy(III), Gd(III), Ce(III), Pr(III), Co(II), Ni(II) and Pb(II) ions from aqueous solutions [13][14][15][16][17][18][19][20][21][22]. In this vicinity, some reports are also available which proved that macroporous resins have also shown better performance than gel structure or microporus type of resins because they offer comparatively faster kinetics, higher adsorption capacity and better mechanical stability [23,24].
Amberlyst-15, one of the earliest synthesized macroporous cation exchange resin, in the past, was mostly utilized as a heterogeneous catalyst in the synthesis of many organic compounds [25]. However, it is also compulsory to have a sound knowledge and detailed investigation to determine the true potential of this resin for the adsorption of heavy metal ions. In our previous investigations, we investigated its potential for the adsorption of Cr(III) independently and in mixed metal system, and results showed tremendous performance of the resin [26]. Cd(II) is one of the heavy metal ions that can cause negative impacts on human health and environment when its concentration exceeds than the recommended limit (0.003 mg.l −1 ) in surface and ground water. Therefore, its removal from aqueous solutions is compulsory to protect the environment and life on the planet [27,28].
In the current research, the adsorption potential of Amberlyst.15 for Cd(II) was investigated under the effect of reaction time, concentration, temperature, resin dosage and pH in order to optimize the adsorption efficiency of resin. The present study will provide a theoretical base and in-depth knowledge for the successful removal of Cd(II) from aqueous solutions through ion exchange technology.

Materials and methods
Amberlyst-15, CdCl 2 .6H 2 O, NaOH, HCl, NaNO 3 were obtained from BDH company (UK). All the glassware were washed thoroughly with HNO 3 (10%) and finally rinsed with distilled water prior to utilization. For stalk solution, an appropriate amount of CdCl 2 .6H 2 O was dissolved in 1 L of doubly distilled water and then working solutions of various concentrations were prepared by simple dilution method. The calibration of pH meter was made by using buffer solutions of pH 2 and 12 before performing the adsorption experiments. The pH adjustment of solutions was done by using dilute solutions of NaOH and HCl.

Characterization of resin
Surface area analyzer (Quanta chrome NOVA 2200e, USA) was used for the calculation of surface area of amberlyst-15. For degassing purpose, sample was initially heated at 125°C for 1 h and then analyzed for determination of surface area. The surface area and pore size distribution (PSD) were calculated by employing Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) models. The surface morphology of the sample was investigated using scanning electron microscope (JEOL JSM 5910 SEM, Japan) and semiquantitative analysis was performed using energy dispersive x-ray spectroscopy (INCA200 EDS, UK) coupled with SEM. The secondary electron SEM images (SEI) of amberlyst-15 were taken before and after the adsorption of Cd(II). FTIR analysis was carried out for the determination of vibrational frequency changes in functional groups of adsorbents before and after the adsorption of Cd(II) using a Perkin Elmer Spectrum Two model spectrometer. The spectrometer equipped with a LiTaO 3 detector coexists with a standard optical system to collect data in the 8300-350 cm −1 spectral range at a best resolution of 0.5 cm −1 . Prior to the analysis, samples were prepared by mixing with KBr in the 1:3 ratio and spectra was recorded in the range 4000-400 cm −1 . Thermogravimetric analyzer (Perkin Elmer, USA) was used to elucidate the physiochemical characteristics of the resin. The point of zero charge (PZC) was determined by well-known salt addition method [29].

Adsorption experiments
To study the contact time, 520 mg l −1 of Cd(II) solution was taken in a double walled glass cell and pH was maintained at 3. Then predetermined adsorbent dosage (0.1 g) was poured into the solution at constant stirring with an agitation speed of 120 rpm. A series of samples were extracted from the reaction mixture with different time intervals ranging from 1 to 120 min at different temperatures 293, 303 and 313 K. For dosage study, dose range was from 0.1 to 5 g at 293 K while keeping the other parameters constant. For pH study, temperature was kept constant at 293 K, and pH was adjusted in the range of 3-5 using concentration range of 2.68 to 1480 mg l −1 and adsorbent dosage of 0.5 g. To investigate the temperature and concentration effect, initial concentration of Cd(II) was ranged from 2.68 to 1480 mg l −1 with pH 3 and 0.5 g of dosage at 293-323 K. A temperaturecontrolled water shaker bath was used for constant stirring of the samples with an agitation speed of 120 rpm for 2 h. After filtration, filtered suspensions were analyzed by atomic absorption spectrometry (AAS 700 Perkin Elmer, USA) to determine Cd(II) ions concentration. All the experiments were conducted in triplicate and the results were obtained from their mean values. The following equation was employed to determine the amount of metal adsorbed per unite mass of adsorbent: where C o and C e represents the concentrations at initial and equilibrium respectively, (mol/l) for Cd(II) ions, and m is the adsorbent mass in gram where as V L correspond to volume of metal ions solution.

Surface area and porosity
The surface area and pore size distribution of macroporous resin Amberlyst.15 were calculated under N 2 gas adsorption/desorption isotherm at 77 K. The BET surface area of the adsorbent was estimated as 105.67 m 2 g −1 with an average pore volume of 0.55 cc/g [30]. Similarly, from the plot for pore size distribution (figure 1(a)), it was found that large numbers of particles have a pore diameter 10 nm.

Thermogravimetric analysis
Variable weight losses were observed to occur at different temperature from the thermogravimetric analysis for Amberlyst-15 ( figure 1(b)). The first 19% weight loss was noted at ∼120°C may be due to the dehydration of physically adsorbed water in the matrix. Similarly, 8% weight loss detected in 200°C-300°C temperature range can be attributed to the exclusion of interstitial water. Another weight loss in temperature range of 400°C-600°C was 23% which can be attributed to Amberlyst-15 decomposition at high temperatures [31].

PZC determination
The PZC is an important tool in optimizing the experimental conditions in adsorption and plays a vital role in characterization of adsorbent. This describes the pH at which the electrical charge density on the surface of adsorbent becomes zero. When pH is lower than PZC, then cations adsorption will be favored due to the positively charged surface. Conversely, when pH is greater than PZC, it will suit the anions adsorption due to negatively charged surface [32]. The Amberlyst-15 PZC was estimated using salt addition method as proposed elsewhere [33]. A plot of ΔpH (pH i -pH f ) versus pH i is mentioned in figure 1(c) which clearly shows the PZC of Amberlyst-15 at pH 2 where ΔpH=0. This value of PZC estimated in the present investigation agrees well with the values reported in literature [34,35].

Surface morphology
The surface morphology of Amberlyst-15 adsorbent in pure form and after metal ions Cd(II) adsorption was investigated using SEM-EDS and results are shown in figures 2(a) and (b). Amberlyst-15 particles were not observed to possess a uniform size and most of them were seen to be cracked on their surface which can contribute to the enhancement of adsorption process. Furthermore, it is clear from the SEM images that average particle size of the adsorbent beads was ranging from 0.5 to 0.8 mm in diameter. The corresponding EDS spectra of Amberlyst-15 before and after the adsorption of Cd(II) is shown in figure 5. It can be seen from EDS spectra of figure 3(a) that pure Amberlyt-15 contains only C, S and O which are the core elements of the resin; however, after the Cd adsorption new peaks appeared due to the adsorption of cadmium on the surface( figure 3(b)). Figure 4 shows FTIR spectra of the samples before and after Cd(II) adsorption. It can be observed from figure 4(a) that IR spectra of Amberlyst-15 consists of numerous peaks suggesting a multi-functional structure of the resin. A broad peak and weak peak observed at 3410 and 2928 cm −1 is assignable to the O-H and C-H stretching vibrations, respectively [36,37]. The bands observed at 1650 and 1500 cm -1 belongs to stretching vibrations of PS and DVB aromatic rings in Amberlyst-15 and two bands at 1011 and 1147 cm −1 may be assigned to C-C bond and to SO 3 − group stretching vibrations, respectively. Similarly, other peaks appeared less than 1000 cm − can be assigned to SO 3 − functional site in Amberlyst-15 responsible of ion exchange reaction [38].

FTIR analysis
The intensities and position of various bands were also observed to change significantly (figure 4(b), for example, in case of pure Amberlyst, the band at 3410 cm −1 becomes more intense and also shifts to 3373 cm −1 after the adsorption which suggested the exchange of Cd(II) with H + ions. Similarly, the peaks at1650 and 1147 cm −1 after Cd(II) adsorption shift to 1645 and 1163 cm −1 , respectively which can be attributed to the Cd(II) ions attachment at SO 3 − active sites.

Effect of time and temperature
The kinetic studies of metal ions adsorption onto Amberlyst-15 were investigated at pH 5 and temperature range of 293-313 K. The equilibrium data is graphically shown in figure 5. It shows the endothermic nature of adsorption process. It is obvious from the figure that initially the rate of adsorption is very fast due to the quick transportation of metal ions from solution phase to the solid phase. This behavior is probably the result of extensive availability of binding sites on adsorbent surface, which drives fast diffusion of Cd(II) ions among the particles of resin in the beginning of reaction [39]. However, after some time, the rate decreases gradually due to the occupation of active sites and attains equilibrium in 60 and 30 min at 293 and 313 K respectively. As evident from the figure, a negligible effect of temperature was observed for Cd(II) uptake at 293 and 303 K, however, the effect of temperature was significant at 313 K. Similar behavior of temperature effect on Cd(II) adsorption was also observed elsewhere [40,41].

Application of kinetic models
The kinetic studies of ion exchange adsorption are necessary to develop and design an ion exchange system for treatment of contaminated water on pilot and industrial scale [2]. The adsorption kinetic data for Cd(II) onto Amberlyst-15 were analyzed by pseudo-first order, pseudo-second order models [5,42]. The linear form of pseudo-first order model can be given as: In q 3 e t e 1

( ) ( )
In equation (3), k 1 (m −1 ) represents the rate constant for pseudo-first order while q e and q t are the amount of metal ions adsorbed (mol/g) at equilibrium and time t respectively. The q e and k 1 values were determined from the intercept and slop respectively by linear plotting of ln(q e -q t ) verses t ( figure 6(a)). Similarly, linear form of pseudo-second order model is given by the equation as:   In equation (4), k 2 represents the pseudo-second order rate constant (g/mg.min) while q e and q t are the amount of Cd(II) adsorbed at equilibrium and time t, respectively. The k 2 and q e values were assessed from the intercept and slope respectively from linear plotting of t/q t verses t ( figure 6(b)).
The kinetic parameters obtained from both kinetic models are summarized in table 1. It is obvious from table that pseudo-first order model could not give satisfactory results due to low correlation coefficient (R 2 ) values and inconsistency of q e.cal with the q exp . Conversely, the R 2 values obtained from pseudo-second order are very high, closer to linearity. Moreover, the q e values calculated from pseudo-second order were in close agreement to the experimental values. Therefore, on the basis of above findings, conclusion can be made that pseudo-second order model describes better the adsorption of Cd(II) ions onto Amberlyst-15 [43].

Effect of adsorbent dosage
Amount of adsorbent plays an important role in the adsorption process by providing a greater number of active sites. The effect of adsorbent dose on Cd(II) adsorption is shown in figure 7 which shows that initially the adsorption rate was very fast and percentage removal of metal ions was increased sharply by increasing the adsorbent dosage. The percentage removal of Cd(II) was increased from 90.76 to 99.5% with increasing the Amberlyst.15 dosage from 0.1 to 5 g. On the other hand, the adsorption in mol per gram was observed to decrease. This behavior can be attributed to the fact that at higher adsorbent dosage the numbers of active sites available for the adsorption process are abundant and surface area of adsorbent is greater [44]. However, the percentage removal becomes almost constant when the adsorbent amount exceeds than 0.5 g. The decrease in total adsorption may be due to the un-occupation of binding sites available on the surface of adsorbent and deficiency of metal ions for more effective saturation of binding sites. Furthermore, at higher adsorbent dosage, solid particles can create electrostatic repulsions or block some active sites from adsorbate species which can possibly stop the attraction between absorbing species and active sites of adsorbent [45,46].

Effect of concentration
The adsorption isotherms at different temperatures (293-323 K) are presented in figure 8 which reflects the effect of concentration and temperature on adsorption of Cd(II) by Amberlyst-15. The adsorption capacity of Amberlyst-15 was observed to increase with increasing temperature and concentration of metal ion solution until an equilibrium is obtained which clearly implies the endothermic nature of ion exchange adsorption process.
The maximum adsorption capacity for Cd(II) adsorption was obtained 2.01 mmol g −1 at 323 K. The increased absorption capacity with increasing initial concentration may be due to the result of increasing driving force provided by concentration gradient responsible of effective collision between metal ions and adsorbent surface. The positive effect of temperature might be due to the fact that the mobility of Cd(II) ions was efficient and the number of binding sites on the Amberlyst.15 were increased at a higher temperature. The increase in temperature of the system can also results in enhancement of the thickness of boundary and hence an increase in adsorption. Moreover, an increase in temperature may also cause an enlargement in the pore diameter of the particles which results in an enhanced adsorption [47,48].

Effect of pH
pH is one of the most important parameters which affects the dissociation of active sites as well as the hydrolysis, complexation, solution chemistry and precipitation of metal ions. The effect of pH on removal of Cd(II) onto Amberlyst−15 was investigated and results are illustrated in figure 9.
It can be predicted from the figure that adsorption was strongly controlled by the pH of the metal ion solution and resin have showed highly efficient adsorption potential even at highly acidic conditions. The maximum adsorption of Cd(II) was occurred at pH 3 which selected as optimum pH for further adsorption experiments. However, beyond pH 3, adsorption was observed to decrease from pH 4-7. This unusual behavior may be due to the replacement of H + ions from the sulfonate groups of the resin by metal hydroxyl ions [49]. Similarly, it was also assumed that at high pH, presence of OH − ions in alkaline medium affect the Cd(OH) + hydrolysis as well as Cd(OH) 2 complexation and hence adsorption decreased due to precipitation formation. Several reactions can occur between adsorbing metal ions and the groups present on the surface of the adsorbent which can lead to the formation of some complexes, such as, ion-pair or coordinative surface complexes depending upon the interaction between the adsorbing metal ions and the surface-active groups. In case of Cd(II), the following reactions are expected to occur in aqueous media:  It can be concluded from equation (7) that the recommended mechanism for the exchange of Cd(II) is coordinative and inner-sphere complexation [50].

Application of adsorption isotherm models
Adsorption isotherms are mathematical relationships which describe the distribution of adsorbate between aqueous and solid phases at a specific or constant temperature. The shapes of isotherms provide many assumptions regarding the nature of the adsorption, interaction among adsorbate and adsorbent, surface coverage and heterogeneity of the adsorbent surface [51]. Different adsorption isotherm models such as Langmuir, Freundlich and Dubinin-Radushkevitch in the present work were employed to evaluate the equilibrium data; however, the isotherm that yields higher regression values (R 2 ) and have a minimum deviation between the calculated and experimental adsorption capacities are discussed here. Langmuir model describes the monolayer adsorption on uniform surface of adsorbent due to the availability of finite number of adsorption sites. Accordingly, when a particular adsorption sites are filled then no further adsorption can take place at that sites and thus the surface of adsorbent is eventually reached at saturation value where maximum adsorption occurs. The Langmuir isotherm was applied in the present work to elucidate the adsorption of Cd(II) onto Amberlyst-15. The magnitudes of q m and K b were calculated from the slope and intercept of linear plots between C e /q e verses C e, shown in figure 10, respectively by using the equation given below: where C e , q m and K b represents the equilibrium concentration (mol/l), maximum adsorption capacity(mol/g) and binding energy constant (L/g), respectively. The Langmuir parameters are listed in table 2 which indicates that q m and K b increases with an increase in temperature. The enhancement in q m with increasing temperature can be associated to the availability of more active sites, whereas an increase in K b values points that metal ions are more firmly adsorbed on the surface of resin [52]. Moreover, an important dimensionless parameter (R L ) was utilized to judge the favorability of metal ion adsorption on the resin from equation: In equation (9), C o and K b represents the initial metal ions concentration (mol/L) and Langmuir constant, respectively. The R L values were also calculated by using equation ( where q e and q m represents the theoretical adsorption capacity and maximum adsorption capacity, respectively, β is related to mean energy of sorption (E) and ε is the Polanyi potential associated with maximum adsorption capacity which can be described in the from: where R, T and C e represents general gas constant (J.mol −1 .k −1 ), absolute temperature (K) and metal ion concentration(mol l −1 ), respectively. The q e versus ε 2 linear plots at different temperatures are mentioned in figure 11 where the intercepts and slopes of the plots were used to calculate the q m and β values listed in table 2. Similarly, the equation (12) was used to calculate mean free energy value as: When the values of E are less than 8 kJ mol −1 indicate the physical adsorption and greater than 16 kJ mol −1 represents chemical adsorption while in the range 8-16 kJ mol −1 ion exchange removal mechanism [5]. It is clear from the table 2 that E values (13-15 kJ mol −1 ) for all the temperatures obtained from the above equation resides in the range 8-16 kJ mol −1 which confirms the ion exchange reaction mechanism of Cd(II) removal onto Amberlyst-15. A small deviation between the experimental and calculated q m values may be due to the basic mathematical principles which are involved in the formulation of this model and shape of the isotherm.

Thermodynamic evaluation
In the present investigations, different thermodynamic parameters such as Gibbs free energy (ΔG), enthalpy change (ΔH) and entropy change (ΔS) were determined using equations (13) and (14) given as  The ΔH and ΔS values were computed from the slope and intercept of linear plots of lnK b verses 1/T as mentioned in figure 12 and thermodynamic parameters are tabulated in table 3. The ΔG values are observed to increase from −27 to −32 kJ mol −1 with the rise in temperature from 293 to 323 K indicating the exchange reaction was spontaneous and feasible in nature. The positive value of ΔH suggested the endothermic nature of adsorption process whereas the positive value of ΔS refers the randomness at the solid solution interface.

Comparison of amberlyst.15 performance
The monolayer adsorption capacity of Amberlyst.15 obtained in this work was compared with previously published results for Cd(II) adsorption by using different commercially available synthetic cation exchange resins. The comparative analysis given in table 4 has proved that maximum adsorption capacity of Amberlyst.15 was higher than those of other reported resins. This also confirmed that Amberlyst.15 has a greatest adsorption potential due to large surface area and porous structure and thus could be used as a promising adsorbent for Cd(II) removal from aqueous solutions.

Conclusions
The present investigation demonstrated that Amberlyst-15 can be used as an efficient and potential adsorbent for removal of Cd (II) from the aqueous solutions at domestic and pilot scale. SEM-EDS, FT-IR, TGA and BET analysis revealed that Amberlyst-15 has an amorphous and highly porous structure and also confirmed the successful exchange of Cd(II) on the surface of resin. Furthermore, Amberlyst 15 showed features of high adsorption capacity and fast kinetics for removing Cd(II). According to our optimization, Amberlyst 15 can remove 99.95% of Cd(II) in 30 min equilibrium time at temperature of 323 K, pH 3, stirring rate 120 rpm with dosage of the resin 0.5 g. The maximum monolayer adsorption capacity (2.01 mmol g −1 ) of resin used in the present investigation was comparable to the reported one in literature for Cd(II) adsorption on different commercially available ion exchange resins. Expressing such high removal efficiency and convenient operation, Amberlyst.15 can be implemented as a novel and potential adsorbent for the removal of Cd(II) in water and waste water treatment systems.