GelatinâSilica-Based Hybrid Materials as Efficient Candidates for

Article pubs.acs.org/IECR

Gelatin−Silica-Based Hybrid Materials as Efficient Candidates for Removal of Chromium(Vi) from Aqueous Solutions Samjeet S. Thakur and Ghanshyam S. Chauhan* Department of Chemistry, Himachal Pradesh University, Shimla, India 171005 ABSTRACT: Removal of Cr(VI) from water bodies is an environmental concern of utmost priority. It requires adsorbents that can efficiently operate under real conditions. In view of this, two new mesoporous hybrid materials were synthesized from gelatin and silica in the presence of tetraethylorthosilicate and sodium dodecyl sulfate via a sol−gel method. The as-synthesized hybrid material had a surface area of 394.86 m2 g−1. It was subjected to calcination at 550 °C, and the resultant hybrid material had a surface area of 427.79 m2 g−1. These materials were used as adsorbents of Cr(VI) from its aqueous solutions, and the maximum adsorption capacities of 92.44 and 94.47 mg g−1, for the as-synthesized and the calcined materials, were observed. The adsorption was spontaneous as signified by the negative values of Gibbs free energy. Application of the experimental data to different kinetic models and adsorption isotherms reveals that the adsorption process follows a second-order kinetic model and Freundlich isotherm.

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INTRODUCTION Discharge of heavy metal ions, such as chromium and mercury, in water bodies has become a major environmental concern due to their serious adverse effects on the human body.1 Cr(III) and Cr(VI) are pollutants of major environmental concern as these are stable in the environment for a long time. Cr(VI) is considerably more toxic than Cr(III) as it has carcinogenic, mutagenic, and toxic effects.2 It also inhibits plant growth and imparts changes in plant morphology.3 Effluents from the anthropogenic activities such as dyeing, electroplating, leather tanning, or mining are the main sources of Cr(VI).4 Chromium exists in different oxidation states ranging from −2 to +6. In aqueous solution, Cr(VI) exists in different ionic forms such as chromate (CrO42−), dichromate (Cr2O72−), or hydrochromate (HCrO4−). The proportion of Cr(VI) ions in solution is pH-dependent as CrO42− ions dominate in the neutral and basic pH while Cr2O72− ions are the dominant species at low pH. Such pH-dependency of different ionic species in aqueous solution makes Cr(VI) separation also pH-dependent. Cr(VI) is a strong oxidizing agent, and on reaction with organic substrates present in water it gets reduced to Cr(III).5 A number of processes including adsorption, chemical precipitation, electrochemical methods, ion exchange, or membrane separation have been reported for the separation of Cr(VI) from its aqueous solution. However, the adsorption process is convenient as well as effective. Adsorption of Cr(VI) by a number of materials has been reported in the literature.6−20 Chauhan et al.21−24 have reported well-designed biopolymer-based hydrogels, synthesized by grafting of polymers or cross-linking reactions, as adsorbents of Cr(VI). Mesoporous hybrid materials are good candidates to replace costly wastewater treatment materials used in ion-exchange, electroflotation, membrane separation, reverse osmosis, electrodialysis, or solvent extraction processes.25−27 There are reports in the literature on the use of silica-based composites for Cr(VI) adsorption, but none of these has gelatin as one of the components.28,29 Gelatin is an easily available biopolymer that has good chemical, physical, and biological stability. It acts as an © 2014 American Chemical Society

efficient adsorbent of ionic species as the functional groups present such as OH, NH2, NH3+, and COO− act as the binding sites through polar or ionic interactions. At low pH it bears net positive charge on the surface. It has electrostatic interactions with the negatively charged silica surface resulting in the formation of a biocompatible silica−gelatin nanohybrid material. The Cr(VI) ion uptake has been reported to result from the weak forces which facilitate an efficient binding process with silica-based hybrid material.30 Due to the repulsive electrostatic interactions, Cr(VI) anions are poorly adsorbed at higher pH than at the low pH. At low pH, COOH, NH2, and OH groups of gelatin get protonated and the high adsorption capacity of Cr(VI) ions can be expected by the electrostatic attraction between the negatively charged dichromate anion and the protonated COOH, NH2, and OH groups. In continuation of our earlier research, in the present work we report new biocompatible gelatin and silica-based mesoporous biohybrid materials for the adsorption of chromate ions from their aqueous solutions. Effects of operating conditions such as contact time, pH, temperature, and initial Cr(VI) concentration on the adsorption capacity were investigated. Adsorption isotherm, kinetic, and thermodynamic models were used to understand the mechanism and kinetics of the adsorption process.

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EXPERIMENTAL SECTION Materials. Gelatin bacteriological (Glaxo India Ltd., Mumbai, India), tetraethoxysilane (TEOS, Sigma-Aldrich, Munich, Germany), 99.5% silicon dioxide (Himedia Laboratories Pvt. Ltd., Mumbai, India), sodium dodecyl sulfate suprapure, nhexane (SD Fine Chemicals Ltd., Mumbai, India), hydrochloric acid (RANKEM, Faridabad, India), sodium hydroxide, hydrochloric acid, disodium hydrogen orthophosphate anhydrous

Received: Revised: Accepted: Published: 4838

June 25, 2013 February 25, 2014 March 1, 2014 March 3, 2014 dx.doi.org/10.1021/ie401997g | Ind. Eng. Chem. Res. 2014, 53, 4838−4849

Industrial & Engineering Chemistry Research

Article

Figure 1. FTIR spectra of the (a) as-synthesized hybrid material (H1) and its calcined derivative (H11) and (b) Cr(VI)-loaded H11.

solution. The contents were stirred in a chemical reactor for 30 min at room temperature. In the second stage, 140 mL of n-hexane, 50 mL of double distilled water, 16.0 g of HCl (5% by weight), and sodium dodecyl sulfate (0.15 g) were thoroughly mixed to obtain an oil in water (o/w) emulsion. To the resulting emulsion was added 2.5 g each of gelatin and SiO2. The contents were again stirred for 30 min. The contents from both stages were mixed together to initiate the hydrolytic polycondensation process to obtain gelatin-based solid spheres. The mixture was aged at 37 °C for 120 h and thereafter precipitated from the solution. After exhaustive washing with acetone, methanol, and water, the separated particles

(Na2HPO4), potassium dichromate, sodium dihydrogen orthophosphate dehydrate (NaH2PO4·2H2O) (SD Fine Chemicals), and reagent Cr-1, reagent Cr-2 (Merck, Schuchardt, Germany), all of analytical grade, were used as received. Synthesis of Hybrid Materials. Silica−gelatin-based mesoporous hybrid materials were synthesized via a two-stage sol−gel method by modifying an earlier reported protocol.31 In the first stage, 10 g of tetraethoxysilane was dissolved in 20 mL of n-hexane and that was followed by the addition of 0.1 mL of 5% hydrochloric acid (by weight). HCl acts as catalyst for the hydrolyzation, and an increase in the amount of HCl also reduces the interfacial tension without any effect on the viscosity of the 4839

dx.doi.org/10.1021/ie401997g | Ind. Eng. Chem. Res. 2014, 53, 4838−4849


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Figure 2. TGA curves of hybrid materials ((a) H1 and (b) H11).

were dried in a vacuum oven at 100 °C for 50 h. A part of the synthesized sample was calcined at 550 °C for 8 h. The assynthesized hybrid material and its calcined form were designated as H1 and H11, respectively. Characterization of Mesoporous Hybrid Materials. The synthesized materials were characterized by Fourier transform infrared (FTIR) spectroscopy, thermogravimetric/differential thermal analysis (TGA/DTA), Brunauer−Emmett−Teller (BET) analysis, and scanning electron microscopy−energydispersive X-ray spectrometry (SEM-EDX). FTIR spectra were recorded on a Nicolete 5700 intrument in transmittance mode in KBr. TGA/DTA was carried out in a TGA/DTA 6300, SII EXSTAR thermal analyzer. The thermal investigation was carried by heating samples from 50 to 700 °C in a platinum crucible in air atmosphere, heating at a rate of 10 °C/min. Surface morphology

of the samples was observed by scanning electron microscopy (Model Leica Cambridge Stereoscan 440 SEM), and EDX spectra were recorded on SEM QUANTA 250 D9393. The weight percents of carbon, nitrogen, oxygen, and silicon were analyzed from the EDAX data. Surface area and pore size of the hybrid materials were analyzed using a BET surface area analyzer with SMART SORB 92/93. The sample was degassed at 100 °C before the measurement. Batch Adsorption Experiments and Optimization of Adsorption Parameters. A stock solution of Cr(VI) was prepared by dissolving 1.4143 g of K2Cr2O7 in 500 mL of double distilled deionized water. All of the batch experiments were carried out with 50 mL of Cr(VI) aqueous solutions at 150 rpm. The effect of variation of different adsorption parameters was studied as a function of time (60−720 min), temperature 4840



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(25−45 °C), pH (2.0−9.2), and initial Cr(VI) concentration (10−50 ppm) using 50 mL of stock solution of Cr(VI) and 100 mg of H1 or H11. The pH of the solution was measured with a pH meter (Eutech 20). The concentration of the unadsorbed Cr(VI) ions was determined at 540 nm by using a Photolab 6600 UV−visible spectrophotometer. Adsorption isotherms were obtained by plotting the equilibrium adsorption capacity (qe) versus the equilibrium concentration of the residual Cr(VI) in the solution (Ce). The qe was calculated from the following relationship: qe =

Co − Ce V W

Table 1. Surface Area Data of the Hybrid Materials samples

surface area (m2 g−1)

pore size (nm)

H1 H11

426.62 452.53

7.647 7.208

(1) −1

where qe is the equilibrium adsorption capacity (mg g ) and Co and Ce are the initial and equilibrium liquid phase Cr(VI) concentrations (ppm), respectively. V is the liquid phase volume (L), and W is the amount of H1 or H11 (g). The percent uptake (Pu) was calculated from the following equation: Pu =

Co − Ce × 100 Co

(2)

To investigate the kinetic characteristics of the adsorption process, 0.1 g of H1 or H11 was added to 50 mL of Cr(VI) solution with concentration (50 ppm), and the samples were agitated for a definite time period (30, 60, 120, 240, 360, and 720 min). At the optimum conditions so evaluated, the maximum adsorption capacity of the materials was evaluated by using the same sample repeatedly in multiple cycles. Desorption Experiments. The adsorption−desorption cycles were repeated consecutively six times to determine the reusability of the hybrid materials. After adsorption experiments (volume, 50 mL; hybrid materials (H1 or H11), 0.1 g; initial concentration, 50 ppm; pH value, 4.0; contact time, 720 min; temperature, 35 °C; agitation speed, 150 rpm), the hybrid materials (H1 or H11) adsorbed with Cr(VI) ions were separated from the solution by filtration, then added to 20 mL of the stripping solution (0.1N NaOH), and stirred at 150 rpm for 30 min at 35 °C, and the final Cr(VI) concentration was determined. After each adsorption−desorption cycle, hybrid materials (H1 or H11) were washed with the double distilled deionized water and used in the subsequent cycle. The desorption ratio of Cr(VI) ions from hybrid materials was calculated from the following relationship: desorption ratio =

Cdes × 100 Cads

Figure 3. Wide-angle XRD patterns of hybrid materials ((H1 and H11).

The slope and intercept of eqs 4 and 5 give the value of k1 and qe and k2 and qe by plotting log(qe − qt) vs t and t/qt versus t, respectively. The linear form of the Elovich equation is given as34 1 1 qt = ln(αβ) − ln(t ) β β (6) Constants α and β were calculated from the slope and intercept of the plot qt versus ln(t). The linear form of the intraparticle diffusion model is represented as35 log R = log K id + a log(t )

where the constants a and kid were calculated from the slope and intercept of the plot log R versus log(t). For the evaluation of the adsorption capacity, as a function of Cr(VI) ion concentration, equilibrium isotherm models, viz., Langmuir, Freundlich, and Redlich−Peterson, were tested.36−38 The Langmuir adsorption isotherm assumes that surface is uniform with all of the adsorption sites having equal adsorbate affinity.39 The Langmuir isotherm may be represented as 1 1 1 1 = · + qe KLqm Ce qm (8)

(3)

where Cdes is the amount of Cr(VI) ions desorbed to the desorption medium and Cads is the amount of Cr(VI) ions adsorbed onto the adsorbent. Kinetic and Isotherm Modeling. Four different models were used to evaluate the kinetics and mechanism of the adsorption process. The following linear form of the pseudo-firstorder equation was used:32 log(qe − qt ) = log(qe) −

k1 t 2.303

The slope and intercept of the plot 1/qe versus 1/Ce yield the values of KL and qm. The Freundlich adsorption isotherm illustrates adsorption as a heterogeneous or multilayer phenomenon. In linear form it can be represented as 1 log qe = log KF + log Ce (9) n Freundlich isotherm constants, n and KF, were calculated from the slope and intercept of the plot of log qe vs log Ce yield. The Redlich−Peterson is a three-parameter isotherm and has the features of the Langmuir and the Freundlich isotherms, and g (0 < g < 1) characterizes the isotherm. It shows a linear dependence on ion concentration in the numerator and an exponential function in the denominator depicted as follows:

(4)

The linear form of the pseudo-second-order equation is given as33 t 1 1 = + t qt qe k 2qe 2

(7)

(5) 4841



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Figure 4. SEM-EDX of hybrid materials ((a, c) H1 and (b, d) H11).

nonspontaneous.40 The Gibbs free energy change, −ΔG°, is an indication of the spontaneity of a chemical reaction. The equilibrium constant Kc is given by the relation

Table 2. EDAX Analysis of the Hybrid Materials H1

H11

element

wt %

at. %

wt %

at. %

O Si total

53.74 25.09 78.83

78.99 21.01 100.00

57.96 30.08 88.04

77.18 22.82 100.00

qe =

ACe 1 + BCe g

Kc =

(12)

where Ce and Ce° (both in mg L−1) are the equilibrium concentrations for Cr(VI) ions on the H1 or H11 and in the solution, respectively. ΔG°, ΔH°, and ΔS° are related to the adsorption equilibrium constant Kc, which can be calculated from the following equation:

(10)

When g = 0, it follows the Langmuir isotherm, and for g = 1, it follows the Freundlich isotherm and its linear form is presented as: ⎛ C ⎞ ln⎜⎜A e − 1⎟⎟ = g ln Ce + ln B ⎝ qe ⎠

Ce Co

ΔG° = −RT ln Kc

(13)

where ΔG° is the standard free energy change (J/mol), R is the universal gas constant (8.314 J/(mol K)), and T is the absolute temperature (K).

(11)

ln Kc = −

The isotherm constants, A, B, and g, were calculated from eq 12. A general trial and error procedure was used to determine the correlation coefficient, r2, for a series of values of A for the linear regression of ln(Ce) on ln[A(Ce/qe) − 1], so that the best value of A was obtained which yields the maximum “optimized” value of r2. Evaluation of Thermodynamic Parameters. Thermodynamic considerations of an adsorption process are very important to conclude whether the process is spontaneous or

ΔH ° ΔS° + RT R

(14)

The values of ΔH° and ΔS° can be calculated from the slope and intercept of a plot of lnKc versus 1/T.

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RESULTS AND DISCUSSION A new hybrid material, H1, was synthesized from gelatin and silica using TEOS as the coupling agent via a two-stage sol−gel method. It was subjected to calcination at 550 °C, thus 4842



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Figure 5. Effect of (a) pH (initial concentration, 50 mg/L; contact time, 12 h; shaking rate, 150 rpm; 35 °C), (b) temperature (initial concentration, 50 mg/L; shaking, 150 rpm; pH 4.0; 35 °C), (c) contact time (initial concentration, 50 mg/L; shaking, 150 rpm; pH 4.0; 35 °C), and (d) concentration (contact time, 12 h; shaking, 150 rpm; pH 4.0; 35 °C) on the uptake of Cr6+.

Characterization of Hybrid Materials. The FTIR spectrum of pure gelatin shows absorbances at 3600, 2925, 1740, and 1103 cm−1 due to the stretching vibrations of NH, C−H, CO, and COC stretching mode, respectively.41 The differences in the spectra of the hybrid materials on comparison with the pure gelatin confirm formation of the gelatin−silica-based hybrid materials. The FTIR spectrum of H1 has some important bands at 3436, 1638, 1085, 789, and 455 cm−1 due to the stretching mode of NH, CO of amide, COC, or SiOC and SiO, respectively, whereas the calcined hybrid material (H11) has corresponding peaks at 3480, 1639, 1076, 802, and 466 cm−1, Figure 1. The broad peaks of the SiOC band are observed due to SiO and CO bonds located between 870 and 1350 cm−1.42 Therefore, vibration bands near 1065−1100 cm−1 are for the SiOC asymmetric

Table 3. Conductivity Measurements of Chromium(VI) after Adsorption at Different pH conductivity of the Cr(VI) ion in solution in hybrid material pH

as-synthesized (H1)

calcined (H11)

2.0 4.0 5.0 7.0 9.2 pure water

2.69 mS 809 μS 1.20 mS 1.98 mS 8.01 mS

2.46 mS 743 μS 1.18 mS 1.89 mS 7.86 mS 373 μS

generating a new functional hybrid material, H11. Calcination was used to remove the moisture or unbound material and to improve the surface properties of the hybrid material H1. 4843



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It means that most of the available Cr(VI) ions were adsorbed on the hybrid materials at this pH. A similar trend in the pH-dependency of the Cr(VI) removal has been reported for the polymeric materials at pH 4.0,43 while other researchers reported the optimum pH other than 4.0 for the gelatin-based adsorbents.14,17,24,41 The effect of temperature was studied at five different temperatures, 25−45 °C, and at 50 ppm and pH 4.0. The adsorption capacity (mg g−1) increased linearly from 10.58 to 11.17 for H1 and from 10.66 to 11.29 for H11 with the rise in temperature from 25 to 35 °C (Figure 5b). The effect of the variation of contact time on the Cr(VI) ion adsorption is presented in Figure 5c and that of Cr(VI) ion concentration in Figure 5d. Evaluation of Thermodynamic Parameters. A plot of ln Kc versus 1/T for the calculation of ΔG°, ΔH, and ΔS° is presented in Figure 6. The results show that the adsorption

stretching mode (Figure 1). The absorption bands due to the amide CO stretching present in the FTIR spectrum of H11 confirms that chemically bonded gelatin is present in the hybrid material (Figure 1a). After adsorption of Cr(VI) ions on H11, the absorption bands due to the stretching mode of NH, CH, amide CO, COC, or SiOC groups along with the bending mode of CH and SiO bonds shifted to 3422, 2927, 1658, 1089, 800, 658, and 465 cm−1, respectively. An additional band also appears at 563 cm−1 due to the stretching mode of CrO (Figure 1b). Full thermal degradation of pure gelatin takes place in three stages.41 The hybrid materials exhibited different decomposition behavior with degradation taking place in four stages (Figure 2). The presence of silica in the hybrid material makes them more hydrophobic in nature. From Figure 2 it is implied that the mechanism of thermal degradation is almost the same for both hybrid materials against thermal decomposition. But, the total weight loss for H1 is 21.4% while it is just 5.6% after calcination for H11 when heated to 700 °C. Both the samples lost the adsorbed moisture in the first stage up to 100 °C, and thereafter it is only the unbounded gelatin component that is mainly lost from the hybrids above 500 °C. From 100 to 500 °C, H1 lost 15% more weight than H11. It implies that calcination improves thermal stability of the hybrid material. These observations can be used as the guiding principle to synthesize tailored hybrid materials with effective bonding and thermal stability. BET surface area characterization of the hybrid materials was carried out by N2 physiosorption at liquid nitrogen temperature. The results are presented in Table 1. The surface area and the average pore size were found to be 426.62 m2 g−1 and 7.647 nm, and 452.53 m2 g−1 and 7.208 nm for H1 and H11, respectively. There from it follows that the calcination process has a positive effect on the surface properties of the hybrid materials with a decrease in the pore size and an increase in the surface area. X-ray diffraction patterns are presented in Figure 3. It is evident there from that both materials have almost the same pattern but H11 has more intense peaks and hence is more crystalline than that of its precursor, H1. SEM images are presented in Figure 4. The particles are of spherical nature. The actual size of particles is in the range of 100−300 nm, but these agglomerate to form larger particles. After calcination the particles became bigger in size and smoother on the surface than that of the precursor (Figure 4a). The interior of the particles is highly porous, rough, and interconnected. The corresponding EDX graphs of H1 and H11 are presented along with their SEM images. The compositional analysis of the samples from the EDX data with the SEM images is presented in Table 2. The Si/O ratio was found to be ∼0.47 and 0.52, for H1 and H11, respectively. EDAX/EDS also has peaks for carbon and nitrogen which confirm that chemically bonded gelatin was also present in the calcined sample, H11 (Figure 4b). Effect of Different Operational Parameters on Cr(VI) Adsorption. The effect of pH on Cr(VI) adsorption is presented in Figure 5a. The adsorption of Cr(VI) increased with pH from 2.0 to 4.0. At pH 4.0 a high adsorption capacity of 15.75 and 15.94 mg g−1 was obtained for H1 and H11, respectively. The adsorption capacity decreased drastically with an increase of pH from 4.0 to 9.2. This indicates that the adsorption of the hybrid material is pH-dependent. Moreover, from the conductivity measurements given in Table 3, it was observed that the conductivity value of the solution after the adsorption study at pH 4.0 was the minimum compared to those studied at other pH values.

Figure 6. Thermodynamic plot of ln Kc as a function of the reciprocal of temperature (1/T) for the adsorption of Cr(VI) by hybrid materials (H1 and calcined H11).

process was of an endothermic nature as ΔH° obtained was positive. Since the adsorbents (H1 and H11) are porous in nature, the possibility of diffusion of Cr(VI) ions into the interior of the particles cannot be ruled out. Thus, an increase in temperature is favorable for Cr(VI) ions transport within the pores of the adsorbents. An increase in the adsorption with the rise of temperature is mainly due to an increase in the number of the adsorption sites generated because of breaking of some internal bonds near the active sites of adsorbent. The values of thermodynamic parameters are presented in Table 4. The negative Table 4. Thermodynamic Parameters for Adsorption at Different Temperatures sample

temp (K)

−ΔG° (kJ/mol)

ΔH° (kJ/mol)

−ΔS° (kJ/mol/K)

H1

298 303 308 298 303 308

0.769 0.670 0.545 0.735 0.622 0.493

7.427

22.33

7.926

24.12

H11

values of free energy of adsorption (−ΔG°) indicate the spontaneity of the process, while the negative values of the standard entropy (ΔS°) indicate a favorable adsorption process. 4844



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Figure 7. Plot of different kinetic models: (a) pseudo-second-order model, (b) Elovich equation model, and (c) intraparticle diffusion model, and (d) their comparison with experimental data for H1 and H11.

Kinetic Studies. Four kinetic models, pseudo-first-order, pseudo-second-order, Elovich equation, and intraparticle diffusion model, were applied to examine the adsorption kinetics (Figure 7). Values of qe and k calculated from the slope and intercept of the plot, respectively, are presented in Table 5. The pseudo-second-order model best fits the experimental data, compared to the other models, with correlation values (R2) quite close to unity and theoretical values of calculated equilibrium adsorption capacities (qt) closer to the experimental values (Figure 7d). Thus, it is suggested that the adsorption kinetics of

Cr(VI) can be best described by the pseudo-second-order model, as also reported elsewhere.12,44−46 The transport of Cr(VI) ions by diffusion process involves bulk diffusion, external mass transfer resistance, and intraparticle diffusion into the interiors of the pores of hybrid materials. The rate constants of intraparticle diffusion (kid) were calculated (Table 5). The correlation value favors the intraparticle diffusion process as the rate-limiting step. Higher values of kid illustrate an improved bonding between Cr(VI) ions and the hybrid material. The nearly linear curve was obtained due to the different extent of adsorption in the initial 4845



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Figure 8. Plot of different adsorption isotherm models: (a) Langmuir isotherm, (b) Freundlich isotherm, (c) Redlich−Peterson isotherm, and (d) comparison of the Langmuir isotherm, Freundlich isotherm, and the Redlich−Peterson isotherm with experimental data for H1 and calcined H11.

of 10−11 cm2 s−1 is a sign of intraparticle diffusion as the ratedetermining step. In the present work, the values of δ obtained for H1 and H11 (1.30 × 10−18 and 8.60 × 10−19 cm2 s−1 at 35 °C, respectively) was almost 10−18 cm2 s−1, which was 7 orders of magnitude less. Thus, intraparticle diffusion was not only the rate-controlling step but also the boundary layer mechanism might be involved in the adsorption process. Isotherm Studies. The adsorption isotherms Langmuir, Freundlich, or Redlich−Peterson were evaluated, and the equations were used to model the adsorption capacities obtained as a function of the concentration of the adsorbed Cr(VI) ions and residual Cr(VI) ions in solution. The values of the isotherm constants and maximum adsorption capacities (qmax) were calculated from the slope and intercept of Figure 8 and are

and final stages of the experiment, which is attributed to the intraparticle diffusion effects. However, it is clear from Figure 7c that the intraparticle diffusion was not the only rate-controlling step because it did not pass through the origin. The intraparticle diffusion plot deviates from the origin, and that signifies the effect of external resistance. Similar results have also been reported in the literature.47 This was further supported by calculating the intraparticle diffusion coefficient (δ) using the following equation:48 δ=

0.03r 2 t1/2

(15)

where r (cm) is the average radius of the sorbent particle and t1/2 (min) is the time for half of the adsorption; a δ value of the order 4846



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concentration of Cr(VI) at equilibrium. In addition, it can be stated that the theoretical value of qe calculated from the Freundlich isotherm is quite close to the experimental values. Apart from the participation of different charged amino acid groups on the surface of the gelatin, adsorption also takes place in the interior of the hybrid material. As a consequence of this the adsorption of Cr(VI) ions on these hybrid materials is of a heterogeneous nature. Thus, the Freundlich isotherm provides a more rational description of the adsorption by the gelatin−silicabased hybrid materials because it accounts for different types of binding sites and their interactions, surface heterogeneity, and the power of the biosorbent surface. Li et al.49 have also reported that the biosorption of Cr(VI) by Synechococcus sp. obeys the Freundlich model. Reusability and Maximum Adsorption Capacity Studies. Desorption experiments were carried out by 0.1 M NaOH as the weakly bonded Cr(VI) ions were completely desorbed in the basic solution. The regenerated hybrid materials were reused for six adsorption−desorption cycles, and the results are illustrated in Figure 9a. The adsorption−desorption results indicated that Cr(VI) ions were removed from the surface of the hybrid materials at pH 9.2 and 35 °C. The adsorption capacity depends on the characteristics of the adsorbent as well as the conditions used for adsorption. In the present study hybrid materials were subjected to six feeds of Cr(VI) ions solution of 50 ppm, at 35 °C with initial pH 4.0. Results are presented in Figure 9b. Pu values of 88.692% (22.173 mg g−1) and 89.708% (22.427 mg g−1) were observed with, after the six feeds, the maximum adsorption capacities (MACs) of 92.44 and 94.47 mg g−1 were obtained for H1 and H11, respectively. For the sake of comparison the maximum adsorption capacities of some other adsorbents have been studied and tabulated (Table 7). The results obtained in the present study are higher than the adsorbents reported in the literature and reviewed here.13,20,50−58 Since MAC depends on the operational parameters such as the initial concentration used, hence the operational parameters are also cited here.

Table 5. Comparison of Rate Constants and Correlation Coefficients for Different Kinetic Models Applied kinetic model applied pseudo-first-order

pseudo-second-order

Elovich equation

intraparticle diffusion model

kinetic parameter

H1

H11

qe k1 R2 qe k2 R2 α β R2 a

14.73 5.07 × 10−3 0.644 21.24 1.184 × 10−3 0.998 3.87 0.377 0.956 0.158

15.11 4.84 × 10−3 0.670 21.64 1.176 × 10−3 0.998 3.85 0.379 0.967 0.153

kid R2

29.36 0.934

30.86 0.948

Table 6. Comparison of Isotherm Constants and Correlation Coefficients for Different Isotherm Models Applied adsorption isotherms Langmuir

Freundlich

Redlich−Peterson

isotherm constants

H1

H11

qm KL R2 n KF R2 A B g R2

22.173 9.176 × 10−2 0.984 1.49 2.20465 0.999 6.75 2.17 0.402 0.990

22.427 9.889 × 10−2 0.989 1.53 2.41413 0.998 5.03 1.21 0.471 0.993

presented in Table 6. The higher value of R2 for the Freundlich isotherm indicated that the Freundlich model best fits the adsorption process taking place on the heterogeneous surfaces, as in the present case, and adsorption capacity is related to the

Figure 9. (a) Reusability and (b) maximum adsorption capacity. 4847



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Table 7. Comparison of the Maximum Adsorption Capacity of Some Adsorbents Reported in Literature sample no. 1 2 3 4 5 6 7 8 9 10 11 12 13

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adsorbent maize bran spent tea dust coffee dust rice husk ash (RHA) modified RHA coconut coir eucalyptus bark Neem saw dust pine needles PANi/PEG composite polyaniline/zeolite nanocomposite guar gum−nano-zinc oxide clay minerals natural halloysite nanotubes hybrid material (H1) hybrid material (H11)

pH

contact time (h)

adsorbent dose (g/L)

2.0 4.0

3 3

20 3

200 100

2.0

3

40

10

2.0 2.0 2.0 2.0 5.0 2.0

0.5 3 2 2 0.5 0.1

6.25 5 2 8 1 5

200 250 100 100 50 100

7.0 4.6 5.0 4.0

0.83 4 0.5 12

2 2 3 2

30 50 100 50

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; ghanshyamschauhan@ gmail.com. Tel.: +911772830944; +919418003399 (M), Fax: +911772830775. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS CSIR, UGC-SAP, and DST-FIST, Department of Chemistry, Himachal Pradesh, University, Shimla, HP, India are acknowledged for providing research facilities.

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max adsorption capacity (mg/g) 312.52 44.9 39.0 0.49 0.84 26.8 45 58.82 21.50 68.97 25 55.56 10.6−13.9 37.25 92.44 94.47

ref 13 20 50 51 52 53 54 55 56 47 57 58 present work

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CONCLUSION The present study was designed to synthesize a new gelatin− silica-based adsorbent material by sol−gel process. It was further functionalized by calcination to enhance its surface properties. These hybrid materials were found to be effective adsorbents for adsorption of Cr(VI) from its aqueous solutions. Adsorption was most effective in the acidic pH 4.0 and at 35 °C. The results obtained exhibit that the adsorption process was feasible and of an endothermic nature. Experimental data show good agreement with the pseudo-second-order model and Freundlich adsorption isotherm. The materials exhibited a high maximum retention capacity and good reusability for up to six cycles; hence, these are good candidates for water purification technologies.

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concn of adsorbate Cr(VI) (mg/L)

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