Silica based inorganic–organic hybrid materials for the adsorptive removal of chromium

We employed polymer functionalized silica gel as an adsorbent for the removal of Cr(vi) from water. The chains of 2-aminoethyl methacrylate hydrochloride (AEMA·HCl) polymer were grown from the surface of silica gel via surface-initiated conventional radical polymerization and the resulting hybrid material exhibited high affinity for chromium(vi). To investigate the adsorption behavior of Cr(vi) on diverse polymer based hybrid materials, the removal capacity of (SG-AEMH) was compared with our previously reported branched polyamine functionalized mesoporous silica (MS-PEI). The adsorption capacities of polymer based materials were also compared with their respective monolayer based platforms comprising a 3-aminopropyltriethoxysilane (APTES) functionalized silica gel (SG-APTES) and mesoporous silica (MS-APTES). The polymer based systems showed excellent Cr(vi) adsorption efficiencies compared to monolayer counterparts. The structural characteristics and surface modification of these adsorbents were examined by Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and thermogravimetric analysis (TGA). The experimental data were analyzed using the Langmuir and Freundlich models. Correlation coefficients were determined by analyzing each isotherm. The kinetic data of adsorption reactions were described by pseudo-first-order and pseudo-second-order equations. Thermodynamic parameters, i.e., change in the free energy (ΔG°), the enthalpy (ΔH°), and the entropy (ΔS°), were also evaluated. The synthesized hybrid materials exhibited a high adsorption capacity for chromium ions. Furthermore, they could be regenerated and recycled effectively.

mobility in the waterbed. 4 Various methods such as chemical precipitation, membrane ltration, ion exchange, electrochemical processes, chemical coagulation and adsorption have been utilized to remove heavy metals from wastewater. 5 Among these methods, adsorption is known to be the most efficient method. A large number of natural and synthetic materials have been used for the adsorption-based removal of heavy metals from wastewater. [6][7][8][9] These materials include zeolites, clays, biosorbents, resins, activated carbon magnetic particles and silica. Simple and low cost adsorbents have been synthesized by several researchers for an effective removal of heavy metals including Cr(VI) even at low concentration. [10][11][12][13][14][15][16][17] Li et al., demonstrated the preparation of chitosan nanobers with an average diameter of 75 nm and cross linked with glutaraldehyde for the removal of Cr(VI). 18 Aboutorabi et al., employed TMU-30 based metal-organic framework (MOF) containing isonicotinate N-oxide as adsorptive sites for the adsorption of Cr(VI) from aqueous solution. 19 Recently, Dong et al., prepared the ionic liquid functionalized cellulose (ILFC) through the graing of glycidyl methacrylate onto cellulose microsphere followed by reaction with ionic liquid 1-aminopropyl-3-methyl imidazolium nitrate for the adsorptive removal of Cr(VI). 20 Table 1 gives a simple comparison of the adsorption ability of different adsorbent materials for the adsorption of Cr(VI).
Silica based porous materials are considered as promising adsorbents for water remediation due to their high surface area, well dened tunable pore size and high adsorption capacity. 21,22 Owing to their economic feasibility, high thermal and mechanical stabilities, they can be utilized as inorganic solid matrixes in the inorganic-organic hybrid materials. 23,24 Several researchers have contributed in the development of functionalized silica based adsorbents for the removal of heavy metals. [25][26][27][28][29][30][31] Fan et al., prepared the Schiff base functionalized Pb(II) imprinted silica-supported organic-inorganic hybrid adsorbent for the selective removal of Pb(II) from aqueous solution. 32 Radi et al., reported the synthesis of chelate bketoenol furan functionalized silica particles (SiNFn) for the selective adsorption of Cd(II). 33 More Recently, Qihui et al., demonstrated the fabrication of thiol functionalized silica microspheres doped with CdTe quantum dots (CQDSMs) for the efficient adsorption of Ag + . 34 The surface of silica can be tailored with different functional groups to enhance their selectivity towards specic pollutants. 35,36 Modication can be achieved via post-synthesis graing and co-condensation. 37 Post-synthesis graing offers a facile avenue to controlling surface properties of materials and facilitates the functionalization of the internal pores of porous materials, ultimately helping in developing material with optimized bulk and interfacial properties. 38 Numerous organic functional groups such as amine, thiol, carboxylate, alkyl chloride, and aromatic functional groups have been incorporated through post-synthesis graing strategy. [39][40][41][42][43][44] In case of silica based materials, the silanol groups present on the surface assist the covalent introduction of a wide range of functional groups, which act as stable and efficient chelating moieties towards a variety of metal ions. The excellent metal adsorption property of these functionalized silica materials are attributed to the presence of electron donor heteroatoms such as O, S and N in the incorporated functional groups. 45,46 The surface functionalization can be either monolayer or polymer based. The polymer based surface functionalization results in a higher surface functional group density that ultimately improves the absorption capacity of the functionalized material. Despite obvious advantages of the polymer based surface functionalization, majority of the efforts in the eld of developing materials for water remediation have been focused on monolayer based surface functionalizations. Herein, we demonstrated the potential of polymer functionalized silica based inorganic-organic hybrid materials for Cr(VI) adsorption (Scheme 1). The chains of 2-aminoethyl methacrylate hydrochloride were graed on the surface of silica gel via surface-initiated conventional radical polymerization (SI-cRP) approach. We have also compared the adsorption capacity of SG-AEMH with polyamine functionalized MCM-41 mesoporous silica (MS-PEI). 47 The APTES derived monolayer based amine functionalized silica gel (SG-APTES) and mesoporous silica (MS-APTES) were also examined and compared with polymer graed silica materials. Our results show that SG-AEMH and MS-PEI were more effective for chromium adsorption. Furthermore, the experimental data were tted to different adsorption models, and the corresponding parameters were determined. In addition, kinetic and thermodynamic analyses were performed to understand the mechanism of the adsorption processes.

Activation of silica gel surfaces: (SG)
Silica gel was activated by stirring its suspension in conc. HCl for 24 h at ambient temperature. The acid suspension was subsequently diluted with deionized water and activated silica gel was separated by centrifugation (4000 rpm, 10 min). The activated silica gel was washed with deionized water until neutral and dried under vacuum at 90 C for overnight.

Synthesis of APTES functionalized silica gel (SG-APTES)
Amine functionalized silica NPs were prepared by a previously reported method. 47 Activated silica gel (4 g) and 10% APTES solution (60 mL) were added in dry toluene and reuxed at 80 C for 24 h under inert atmosphere. The reaction mixture was cooled and silica gel was separated by centrifugation at 4000 rpm for 10 min followed by washing with toluene, acetone and methanol. The APTES functionalized silica gel was dried in a vacuum oven at 70 C for overnight.

Surface modication of silica gel with azoinitiator (SG-AZ)
The surface of silica gel was further modied with azoinitiator according to the previously reported method. 49 A solution of ACPC (4,4-azobis 4-cyanopentanoylchloride) (0.5 g) was prepared in 17 mL of dry dichloromethane, followed by the addition of dry TEA (216 mL) under inert atmosphere. This solution was injected over degassed APTES functionalized silica gel (SG-APTES 2 g) under nitrogen ow and stirred for 2.5 h at ambient temperature. The particles were separated by centrifugation (4000 rpm), followed by washing with DCM and ethanol. The particles were stored in refrigerator until further use.

Graing of poly AEMH$HCl brushes on the surface of silica gel (SG-AEMH)
AEMH$HCl monomer (2.7 g) was dissolved in 13 mL deionized water and solution was degassed for 1 h at room temperature. The monomer solution was transferred to a Schlenk containing already degassed azoinitiator coated silica gel (0.4 g). The polymerization was carried out under N 2 (gas) at 75 C for 24 h. Polymer functionalized silica gel was separated by centrifugation (4000 rpm), washed with water and dried in a vacuum oven at ambient temperature for 24 h.

Characterization
Attenuated total reection Fourier transform infrared (ATR-FTIR) spectra were recorded on Alpha Bruker, spectrometer (Germany). Transmission electron microscopic (TEM) images were obtained on FEI Tecnai G2 F20 instrument with an accelerating voltage of 200 kV. Samples were prepared by drop casting two to three drops of particle dispersions in ethanol onto a carbon coated copper TEM grid. X-ray photoelectron spectroscopy (XPS) measurements were carried out using Thermo Scientic K-Alpha. The Mg Ka (1253.6 eV) X-ray source was operated at 300 W. Pass energy of 117.40 eV was used for the survey scans. The spectra were recorded using a 60 take off angle relative to the surface normal. The UV/Vis absorption spectra were recorded using a Shimadzu UV-1800 spectrophotometer. Thermogravimetric measurements were carried out on a TGA Q50 V6.2 Build 187 thermogravimetric analyzer. Samples were heated at 10 C min À1 from ambient temperature to 800 C under nitrogen ow.

Adsorption studies
The adsorption studies were carried out by investigating the effect of different pH. The pH values were adjusted by using 0.1 M HCl and 0.1 M NaOH. Approximately, 10 mg of adsorbents were shaken at room temperature (200 rpm) with 10 mL aqueous Cr(IV) solutions of known initial concentration (40 ppm for SG-APTES and SG-AEMH, while 20 ppm for MS-APTES and MS-PEI) at optimized contact time. At the end of the adsorption period, the solutions were centrifuged and the concentration of Cr(IV) in the supernatant solutions before and aer the adsorption was determined using a calibration curve (l max 353 nm). The amount of metal adsorbed at equilibrium q e (mg g À1 ) was calculated from the following equation.
where q e is the adsorption capacity (mg g À1 ) of the adsorbent at equilibrium, C 0 and C e (mg g À1 ) are the initial and equilibrium concentrations of solute, V is the volume of the aqueous solution in liter, and W is the mass in grams of the adsorbent used.

Result and discussion
The surface functionalization of silica gel with monolayer and polymer was affirmed by FTIR spectroscopic analysis (Fig. 1). The bands at 1054 cm À1 and 791 cm À1 are characteristic of asymmetric and symmetric vibrations of Si-O-Si. The surface modication of SG with APTES was conrmed by the appearance of -NH 3 + bending vibration at 1583 cm À1 followed by the presence of NH 2 bending vibration at 1660 cm À1 and C-H (CH 2 ) stretching vibration at 2867 cm À1 and 2920 cm À1 . The C]O stretching vibration at 1724 cm À1 and N-H stretching vibration of at 3330 cm À1 further supported the immobilization of AEMH on the surface of silica gel. The successful surface modications were further established by XPS analysis (Fig. 1). The survey scan of SG-APTES showed signals at 143 and 100 eV, which correspond to the binding energies of Si 2s and Si 2p orbitals of silicon. The signal for the C 1s and O 1s orbitals of the carbon and oxygen contents can be observed at 283 and 532 eV. The presence of N 1s orbital signal at 400 eV in the XPS survey scan supported the amine functionalization of SG. In case of SG-AEMA, the XPS survey scan also showed the signal for Cl 2s (268 eV) and Cl 2p (198 eV), because the monomer used for the polymer brush growth was in its hydrochloride form. Thermogravimetric analysis was conducted to evaluate the extent of surface functionalization (Fig. 2). The pristine SG and MS exhibited a total weight loss of 9.92% and 8.38% respectively at temperatures up to 800 C, which was attributed to the weight loss by the removal of silanol groups. In the case of SG-APTES and MS-APTES, the weight loss was 19.55% and 20.87%, respectively, which was attributed to the decomposition of monolayer of APTES. By graing the polymer onto the surface of the silica materials, the weight loss rises sharply to 25.90% for SG-AEMH and 24.32% for MS-PEI.
In case of silica gel, the evaluation of surface functionalization by TEM ( Fig. 3a and f) was limited by the large variation in size and relatively thin layer of the surface-immobilized monolayer and polymer. SG forms large clusters size ranges from few micrometres to few hundred nanometres. MS samples, on the other hand, have more regular shapes with narrower size distribution ($500 nm). The mesopores of MS were also evident in the TEM images. The TEM images of MS-PEI revealed a thin layer of PEI coated on the surface of MS.

Effect of pH
The pH value of the medium controls the adsorption capacity due to its inuence on the ionic forms of the chromium ions in solutions, surface change and protonation degree of functional groups on the adsorbent.   Maximum adsorption was achieved at 20 mg for SG-AEMH (98%) and at 10 mg for MS-PEI (98%). This could be attributed to the increase in the adsorbent specic surface area and availability of more adsorption sites. 63

Effect of contact time
Adsorbent needs to show rapid uptake of pollutants for an ideal and practical adsorption process. To investigate the adsorption capacity of silica sorbents as a function of time, different adsorbents (10 mg) developed in this study were added in 10 mL of Cr(VI) solution (40 ppm for SG-APTES and SG-AEMH, while 20 ppm for MS-APTES and MS-PEI) separately and percentage removal was monitored at room temperature at 5 minutes time intervals for 30 minutes. The uptake of adsorbate increased with contact time. SG-AEMH and MS-PEI showed higher adsorption capacities at any time slot than SG-APTES and MS-APTES. All adsorbents under study exhibited maximum adsorption aer 30 min and thereaer no signicant change in removal was observed. Adsorption was 93% for SG-AEMH and 21% for SG-APTES while 98% adsorption was achieved for MS-PEI and 44% for MS-APTES. The rapid adsorption performance of adsorbents might be related to the availability of greater number of active sites in beginning but as the time increases, active surfaces become saturated with adsorbate species. It was rational to assume that the fast adsorption equilibrium was not only due to strong chelation and good affinity of the sorbents towards Cr(VI) (Fig. 6). 64

Effect of initial Cr(VI) concentration
To investigate the effect of initial concentration on the metal removal capability of adsorbents, adsorption was carried out at different initial concentrations (20,40,60,80, 100 mg L À1 ) with 10 mg of adsorbents. It was observed for all the adsorbents that an increase in Cr(VI) concentration resulted in the decrease in Cr(VI) removal capacity (Fig. 7). This trend may be attributed to the lesser number of available active sites for the adsorption against increased Cr(VI) concentration. 65

Effect of temperature
Temperature plays an important role in the process of adsorption. To study the effect of temperature on the adsorption capacity, adsorption was performed at different temperatures    For all the adsorbents, the adsorption capacity decreased with an increase in temperature. This was attributed to the fact that with increase in temperature the interaction between the metal ions and adsorbents became weak. The highest percentage removal (93% for SG-AEMH and 98% for MS-PEI while 21% for SG-APTES and 44% for MS-APTES) was observed at room temperature (Fig. 8). 66

Adsorption isotherms
The adsorption isotherm facilitates in understanding the relationship between the adsorbate and adsorbent. Langmuir, 67 Freundlich 68 isotherms were employed to express the adsorption data. The Langmuir isotherm assumes the monolayer adsorption of metal ions on the homogeneous adsorbent surface with a nite number of adsorption sites and is expressed by the following equation.
where q e is the amount of adsorbed metals ions in the sorbent (mg g À1 ), C e is the equilibrium metal ion concentration in solution (mg L À1 ), b (L mg À1 ) is the equilibrium constant related to the adsorption energy, and q max is the maximum adsorption capacity (mg g À1 ). In addition, the viability of adsorption of Cr(VI) can be expressed by using a dimensionless factor, called separation factor (R L ), which may be dened by following equation: where b is the Langmuir constant (L mg À1 ) and C 0 refers to the initial metal ions concentration (mg L À1 ). The value of R L related to the shape of isotherm indicates whether the adsorption is irreversible (R L ¼ 0), linear (R L ¼ 1) favourable (0 < R L <1) or unfavourable (R L > 1). The Freundlich isotherm is based on the assumption that the adsorbate adsorbs onto the heterogeneous adsorbent surface and is not restricted to monolayer formation. The linear form of the Freundlich isotherm is represented by the following equation: where K F is the Freundlich isotherm constant related to adsorption capacity. C e and q e are the equilibrium concentration of adsorbate in solution and on adsorbent respectively. The slope 1/n (with favorable range between 0 and 1) is the measure of surface heterogeneity and adsorption intensity, respectively. The lower the value of 1/n, the more heterogeneous is the adsorption process. Table 1 summarizes both the Langmuir and the Freundlich parameters, together with the correlation coefficients. Table 2 summarizes both the Langmuir and the Freundlich parameters, together with the correlation coefficients. It can be observed that for SG-AEMH the Langmuir model provided a good t to the experimental data with high R 2 (0.99) value compared to the Freundlich model R 2 (0. 87) In case of MS-PEI, the value of R 2 for the Freundlich isotherm model (0.98) was slightly higher than that for the Langmuir (0.97). Furthermore, the higher values of b (Langmuir constant) for SG-AEMA (0.31 L mg À1 ) and MS-PEI (0.60 L mg À1 ) indicated a stronger attraction of Cr(VI) ions on the polymer functionalized surfaces compared to the monolayer based adsorbent surfaces. The maximum adsorption capacities (q max ) for SG-AEMH (63.29) and MS-PEI (50.26) are higher than SG-APTES (10.34) and MS-APTES (34.09). The calculated values of 1/n range between 0 and 1 for all adsorbents imply that adsorption process was chemical in nature. The values of 1/n depict adsorption process is more heterogeneous for MS-PEI (0.23) than for SG-AEMH (0.30). Moreover, the calculated   value of R L is also in the required range of 0 < R L < 1 for SG-AEMH (0.13) and MS-PEI (0.07), signifying a favourable adsorption of Cr(VI). 49,[69][70][71] Adsorption kinetics Adsorption kinetic is one of the most important parameter, which represents the adsorption efficiency. It determines the adsorbate uptake rate and evaluates the equilibrium time required for the sorption isotherm. To understand the kinetic mechanism of the adsorption process, pseudo-rst-order 72 and pseudo-second-order 73 kinetics models were applied to t the kinetic data. The linear form of pseudo-rst-order kinetic equation is expressed by following equation: logðq e À q t Þ ¼ log q e À k 1 2:303 t where q e and q t are the amount of metal ions adsorbed on the adsorbent in mg g À1 at equilibrium and at time t, respectively, and k 1 is the constant of rst-order adsorption (min À1 ). The pseudo-second-order kinetic rate equation is linearly expressed as following: where k 2 is the pseudo-second-order rate constant at the equilibrium (g mg À1 min À1 ) that can be determined experimentally. The kinetics parameters and correlation coefficients were calculated from the linear plots and are listed in Table 3. The adsorption data of SG-APTES, MS-APTES, SG-AEMH and MS-PEI t the pseudo-second-order model with higher correlation coefficient (R 2 ) values. The theoretical q e values for the adsorbents were very close to the experimental q e values in the case of second-order kinetics. These results suggest that the rate limiting step involves chemisorption of the adsorbate onto the adsorbent. [74][75][76][77] Adsorption thermodynamics To evaluate the thermodynamic feasibility and spontaneous nature of the adsorption process, thermodynamic parameters including the entropy (DS ), enthalpy (DH ) and standard Gibbs free energy (DG ) were calculated. [78][79][80] The magnitude of DG was calculated from the following equation: Where K is the equilibrium constant, T is the absolute temperature (K), and R is the universal gas constant (8.314 J mol À1 K À1 ). The change in enthalpy DH and DS can be determined from the following equation: The equilibrium constant K can be calculated as expressed in eqn (9): where, K is the equilibrium constant, q e is the solid phase concentration at equilibrium (mg L À1 ) and C e is the equilibrium concentration in solution (mg L À1 ). The values of the thermodynamic parameters are given in Table 4. The negative values of DG implied that the adsorption process was feasible and spontaneous.
In addition, the negative values of DH suggested that the adsorption of Cr(VI) onto SG-AEMH, SG-APTES, MS-PEI and MS-APTES was exothermic in nature.
The positive values of DS for SG-AEMH and MS-PEI exhibited the increasing randomness at the solid-liquid interfaces during the adsorption of metal ions on the adsorbents and could be due to some structural changes in the adsorbents. While, the negative values of DS for SG-APTES and MS-APTES suggested that the randomness decreased at the solid/ solution interface as a results of Cr(VI) adsorption onto the surface of adsorbents. This implied that the adsorption process was energetically stable. [81][82][83][84] The molar entropy of adsorption is where ad is adsorption, m is molar, s is interface, and l is solution phase (liquid). While S m s can be calculated from following equation: where N s is moles of adsorbate at the interface, T is temperature and A is the total area of the adsorbent. For monolayer based surfaces (SG-APTES and MS-APTES), DS m is negative (the entropy of adsorbates at the interface is smaller than the entropy in the solution). Therefore, entropically driven adsorption is restricted. This is because, the entropy of molecules on the monolayer coated surface is much lower than in solution phase since vibrational, rotational and also translational degrees of freedom are restricted at the interface. However, the polymer decorated silica gel (SG-AEMH) and mesoporous silica (MS-PEI) showed positive entropy change upon adsorption, since molecules have more freedom to move compared to monolayer. Besides, the positive value of entropy also means that the change of amount of adsorbate as a function of entropy at the interface is larger than in the solution. Therefore, entropy driven adsorption is more favorable for polymer functionalized solid adsorbents as compared to their monolayer counterparts.

Desorption
A successful desorption process must restore the adsorbent close to its initial properties for effective reutilization. Sorbent regeneration is signicant in evaluating the competitiveness of the adsorbent system. The regeneration of adsorbents was monitored by different eluting agents (NaOH, NaNO 3 , mixture of NaOH with NaNO 3 (1 : 1)). It was observed that best desorption results for SG-AEMH (up to 70%) were obtained by using NaNO 3 and for SG-APTES (up to 74%) were obtained by using NaOH, while for MS-PEI (up to 90%) and MS-APTES (up to 88%) the best desorption results were obtained by using NaOH. The effect of pH on desorption was also explored. The maximum desorption was observed at basic conditions, due to an increase in the negative species in the media (Fig. 9).
At higher pH (pH ¼ 10), desorption was up to 20% for SG-AEMH, whereas desorption percentage was up to 75% in the case of SG-APTES at pH ¼ 12, while, desorption was up to 98% for MS-PEI and up to 91% in the case of MS-APTES at pH ¼ 12 (Fig. 10). 85,86 Regeneration/reusability The regeneration ability of the adsorbent reduces the process cost and assesses the competence of adsorption systems. To investigate the reusability, Cr(VI) loaded adsorbents were    washed with 0.1 M NaOH solution and then rinse with deionized water to neutrality and reconditioned for reuse. The results showed that a performance drop of 21% and 57% was observed in the adsorption capacity of SG-AEMH and SG-APTES between the 1st and 5th cycles, respectively. MS-PEI could be effectively reused up to sixth adsorption-desorption cycles with 56% performance loss while, a drop of 63% was observed in the adsorption capacity of MS-APTES up to sixth cycle (Fig. 11). 87,88 Conclusions In summary, silica gel was functionalized with polymer to improve the adsorption behaviour towards Cr(VI). The removal efficiency of polymer functionalized silica (SG-AEMH) was compared with the mesoporous silica tethered with a branched polymer (MS-PEI). The polymer decorated silica gel (SG-AEMH) and mesoporous silica (MS-PEI) exhibited better adsorption capacities as compared to the monolayer based SG-APTES and MS-APTES platforms. The prepared silica sorbents exhibited attractive characteristics, such as high adsorption capacity, fast adsorption kinetics, and superior regeneration performance. The adsorption process of SG-AEMH was well described with a Langmuir model while Freundlich model gave a good t for the adsorption data of MS-PEI. Pseudo-second order equation gave a better correlation for the adsorption data of SG-AEMH and MS-PEI. The thermodynamic study indicated that the adsorption processes were spontaneous and exothermic for SG-AEMH and MS-PEI based sorbents. The present study revealed that SG-AEMH and MS-PEI are promising materials for the removal of Cr(VI) ions from aqueous media and could be regenerated and reused up to ve cycles for SG-AEMH and six cycles for MS-PEI that highlight their economic viability.

Conflicts of interest
There are no conicts to declare.