Adsorption of Aniline on Cross-Linked Starch Sulfate from Aqueous

Sep 16, 2009 - Page 1 ... A new environment friendly adsorbent, cross-linked starch sulfate ... experimental data, and the pseudo-second-order kinetic...
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Ind. Eng. Chem. Res. 2009, 48, 10657–10663

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Adsorption of Aniline on Cross-Linked Starch Sulfate from Aqueous Solution Lei Guo,* Guiying Li, Junshen Liu, Ping Yin, and Qian Li School of Chemistry and Materials Science, Ludong UniVersity, Yantai 264025, China

A new environment friendly adsorbent, cross-linked starch sulfate (CSS), was prepared and used to adsorb aniline from aqueous solution. Different adsorption parameters like initial pH, adsorption time, equilibrium aniline concentration, and temperature were thoroughly studied, and the kinetic, equilibrium, and thermodynamics of the adsorption process were further investigated. It showed that CSS can effectively remove aniline from the solution. The adsorption capacity is highly dependent on the amount of sulfate groups on adsorbents and the initial pH. The pseudo-first-order and pseudo-second-order kinetic models were applied to test the experimental data, and the pseudo-second-order kinetic model provided a better correlation of the experimental data in comparison with the pseudo-first-order model. The maximum removal efficiency of aniline is 75.29% using 1 g L-1 of CSS3 dose. Langmuir, Freundlich, Dubinin-Radushkevich, and Sips models have been applied to study the adsorption equilibrium, and the equilibrium adsorption data were well described by the Sips isotherms. The Scatchard plot analysis was used to evaluate the binding sites of CSS. The adsorption of aniline on CSS is exothermic in nature. Introduction Aniline is a kind of important organic chemical raw material, which can be widely used for producing methylene diphenyl diisocyanate (MDI), dyestuff, organic pigment, rubber assistant, pesticide, and medicine, etc. Excessive application of aniline gives aniline the chance to join the food chain. Aniline has been detected in drinking water and has also been found in surface water.1 However, aniline has been classified as very toxic in humans, with a probable oral lethal dose in humans at 50-500 mg per kg body weight (mg/kg).2 Because of its dramatically negative environmental impacts once released into receiving waters, several techniques have been proposed for the removal of aniline from industrial effluents, such as biological treatments,3,4 oxidation processes,5,6 electrochemical techniques,7 irradiation treatment,8 and adsorption procedures.9-11 Among these techniques, adsorption is generally preferred for the removal of aniline due to its high efficiency, easy handling, and availability of different adsorbents. Compared with traditional adsorbents such as the silica gels, polymeric materials,12-14 organobentonites,15 and the activated carbons,16-21 the starch-based materials are viewed as a more attractive alternative due to its low-cost and environment friendly characters. Starch-based materials are widely used to adsorb a wide range of toxic derivatives, in particular heavy metals and dyes,22-25 from aqueous solution. However, there are few papers to study the adsorption between starch derivatives, especially starch sulfate, and aniline. Therefore, the aim of this article is to prepare cross-linked starch sulfate (CSS) and study the adsorption between this modified starch and aniline. Batch experiments were conducted to study the main parameters such as initial pH, treatment time, initial aniline concentration, and temperature. The adsorption mechanism is explained according to the studies of the kinetic, equilibrium, and thermodynamics of the adsorption process. Experimental Section Corn starch (Zhucheng Xingmao Corn Developing Co., Ltd., food-grade) was dried at 105 °C before it was used. Aniline * To whom correspondence should be addressed. Tel.: +86-5356672176. Fax: +86-535-6697667. E-mail: [email protected].

(analytical reagent grade) was used to prepare the adsorbate solution. Sodium bisulfite, sodium nitrite, and all other commercial chemicals were of analytical reagent grade and were used without further purification. All solutions and standards were prepared using deionized water. Cross-linked starch was prepared using epichlorohydrin as a cross-linker according to the method described in previous work.26 Cross-linked starch sulfate was prepared by cross-linked starch with trisulfonated sodium amine (N(SO3Na)3) according to Cui’s method.27 A certain amount of sodium bisulfite was dissolved in distilled water in a stirred glass kettle. Then the aqueous solution of sodium nitrite was added dropwise to the kettle at 90 °C and reacted for 1.5 h. In this way, the sulfating agent trisulfonated sodium amine (N(SO3Na)3) was obtained. After the pH of the sulfating agent solution was adjusted to 9.0, a preweighed amount of cross-linked corn starch was added to this stirred glass kettle. The reaction was allowed to proceed for 4 h at 40 °C. At the end of reaction time, the product was washed three times with deionized water and one time with ethanol. The product was then dried at 50 °C in vacuum for 24 h. Three samples with different quantity of sulfate groups, named CSS1, CSS2, and CSS3, were prepared to be used as adsorbents. Sulfur content of adsorbents was determined by elemental analysis (PE 2400II, Perkin-Elmer), and the surface properties of adsorbents were determined using a surface area and porosity analyzer (ASAP2020). Adsorption experiments were carried out by batch methods. The desired dose of CSS was added to 50 mL of aqueous aniline solution in a series of 100 mL glass-stoppered Erlenmeyer flasks. The suspension was stirred on a magnetic stirrer at a uniform speed of 120 rpm in a constant temperature bath. After a certain adsorption time, the suspension was filtered through a 0.45 µm nylon membrane and the concentration of aniline in the aqueous phase was determined using a Shimadzu UV-vis spectrophotometer (UV-2550, Japan) at the maximum wavelength of 280 nm. The initial pH value of aniline solution was adjusted by adding either 0.1 M HCl solution or 0.1 M NaOH solution before adding the adsorbent. The adsorption capacity was calculated from the following expression:

10.1021/ie9010782 CCC: $40.75  2009 American Chemical Society Published on Web 09/16/2009

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Q)

(Ci - Ct)V m

Table 1. Elemental Analysis Results of Cross-Linked Starch Sulfates

(1)

where Q is the adsorption capacity of the adsorbent (mg g-1), Ci and Ct (mg L-1) are the initial and terminal concentrations of aniline in the adsorption solution, respectively, and V (L) and m (g) are the volume of the adsorption solution and the dose of the adsorbent, respectively. Results and Discussion Characterization of Adsorbents. Elemental analysis results of the adsorbents are shown in Table 1. The sulfur content of adsorbents is in the order of CSS1 < CSS2 < CSS3, which means the content of active sulfate groups changes in the same order. The surface properties of adsorbents were determined using a surface area and porosity analyzer, and the results are shown in the Table 2. The data of surface properties have no direct relation to the sulfur content of adsorbents. Effect of pH on the Adsorption Process. In wastewater treatment, pH is an important factor that is probably influencing the removal of pollutants in a given process. Figure 1 shows the effect of pH on the adsorption of aniline on CSS in the pH ranges of 2.0-10.0. As the pH values increase from 2 to 10, the adsorption capacities of aniline for all the three samples increase sharply, reach a maximum value at about pH 4 and then decrease with the pH increasing from 4 to 10. It can also be seen that the adsorption capacities increase with the increase of the sulfur content of the CSS. The effect of pH on the adsorption process can be explained with the protonation of the functional groups on the adsorbents as well as adsorbates. A potential mechanism of the adsorption of aniline on CSS is shown in eqs 2, 3, and 4.

found (%) samples CSS1 CSS2 CSS3

C

H

N

S

O

29.71 27.05 23.31

4.15 3.71 3.21

0.37 0.36 0.39

13.09 15.58 18.82

52.68 53.3 54.27

Table 2. Surface Area and Pore Parameters for Cross-Linked Starch Sulfates parameters Surface Area single point surface area at p/p0) 0.20 (m2 g-1) BET surface area (m2 g-1) Pore Volume BJH adsorption cumulative volume of pores between 1.7000 and 300.0000 nm diameter (103 cm3 g-1) Pore Size adsorption average pore width (4 V/A by BJH) (nm)

CSS1

CSS2

CSS3

0.31

0.40

0.32

0.42

0.25

0.35

0.65

0.58

0.71

7.58

5.61

7.32

linking and then sulfating, which makes most of the active groups on the particle surface, and this makes the rate-limiting step rapid. The adsorption kinetics within wastewater treatment is significant, as they provide valuable insights in adsorption system design. A number of kinetic models have been described in an attempt to find a suitable mechanism explanation for solid/ liquid adsorption systems, in which Lagergren’s first-order31 and Ho’s pseudo-second-order kinetic models32 are the two most widely applied. Then, the two kinetic models were used in this study to analyze the adsorption kinetic data. The linear forms of pseudo-first-order equation33 and pseudosecond-order equation32 are generally expressed as log(Qe - Q) ) log Qe t 1 t ) + 2 Q Q K2Qe e

At low pH values, the high concentration of the H+ in solution makes the sulfate groups exist in the form of -SO3H, which prevents the adsorption of aniline onto CSS, as shown in eq 2. As the pH increases from 2.0 to 4.6, the active sites become ionized, as shown in eq 3, and the cationic aniline becomes adsorbed due to the better valence forces between aniline and CSS. The optimal pH can be well explained according to the pKa of aniline (pKa ) 4.6). When the pH values increase beyond 4.6, the amine groups of aniline are less positive and therefore the adsorption capacities decrease. Similar findings were reported for the adsorption of some aromatic compounds.28-30 Adsorption Kinetics Studies. Adsorption capacities of aniline were measured as a function of time, as shown in Figure 2. The adsorption was very fast from the beginning to 40 min. With a further increase of time, the adsorption gradually approached to equilibrium within 60 min in all the cases. It also can be seen that the equilibrium time for the different sample is almost the same although the quantity of active sulfate groups is different. According to the chemisorption steps,22 transporting within the adsorbent particle is considered as the rate-limiting step. The adsorbents were prepared by first cross-

K1 t 2.303

(5) (6)

According to eq 5, the adsorption rate constant (K1) can be determined experimentally by plotting of log(Qe - Q) versus t. Linear plots of t/Q versus t curves (eq 6) were used to determine the rate constant (K2) and equilibrium adsorption capacity (Qe). Figure 3 shows pseudo-first-order and pseudo-second-order plots

Figure 1. Effect of pH on the adsorption capacities of aniline on CSS (Conditions: Ci, 100 mg L-1; adsorption time, 60 min; temperature, 293 K; dose of CSS, 50 mg).

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Figure 2. Effect of the adsorption time on the adsorption capacities of aniline (Conditions: Ci, 100 mg L-1; temperature, 293 K; pH, 4.0; dose of CSS, 50 mg).

for the adsorption of aniline on CSS. The rate constants (K1, K2), equilibrium adsorption capacities (Qe1, Qe2), and the correlation coefficient (r2) for each system were calculated according to the linear least-squares method and are given in Table 3 along with equilibrium adsorption capacities (Qe-exp) from the experiments. The values of the correlation coefficient indicate a better fit of the pseudo-second-order model with the experimental data compared with the pseudo-first-order model. Moreover, as shown in Table 3, the equilibrium adsorption capacities calculated from the pseudo-second-order kinetic model fitting are more coherent with the experiments (Qe-exp) than the pseudofirst-order kinetic model fitting. The pseudo-second order equation is usually applicable for chemisorption processes.33 In many cases, the pseudo-second-order kinetic model provided better results for the adsorption of organic substances from aqueous solution.28,34-37 The good applicability of the pseudosecond-order model means that the adsorption process is chemisorption involving valence forces through sharing or exchange of electrons between aniline and CSS, as shown in eq 4. Adsorption Equilibrium Studies. The effect of equilibrium aniline concentration on the adsorption capacity is shown in Figure 4. The higher the equilibrium aniline concentration is, the higher the adsorption capacities. The influence of initial aniline concentration is also found to be significant. A high equilibrium aniline concentration increases the difference in concentration between the bulk liquid and the adsorption site, which increases the rate of removal and the removal efficiency

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at equilibrium. The removal efficiency of aniline can be reckoned according to Figure 4. The maximum removal of aniline is observed up to 75.29% at the initial concentration of 120 mg L-1 using 1 g L-1 of the CSS3 dose. The equilibrium adsorption isotherms express the specific relation between the concentration of adsorbate and its degree of accumulation onto the adsorbent surface at constant temperature. Langmuir38 and Freundlich39 models are widely used to describe the equilibrium of an adsorption process between an adsorbent and an organic adsorbate.40-42 The Langmuir model is based on monolayer adsorption on equi-energetic active surface sites, while the Freundlich model relies on heterogeneous adsorption. The Dubinin-Radushkevich isotherm43 is more general than the Langmuir isotherm, because it does not assume a homogeneous surface or constant adsorption potential. The Dubinin-Radushkevich isotherm model was often applied to determine the nature of the adsorption process as physical or chemical. By combining the Langmuir and Freundlich isotherm models, Sips proposed an empirical isotherm equation containing three parameters.44 In order to better understand the adsorption mechanism of anilineonCSS,theLangmuir,Freundlich,Dubinin-Radushkevich, and Sips models have been applied in the present study. The Langmuir,38 Freundlich,39 Dubinin-Radushkevich,43 and Sips44 isotherms can be, respectively, described as QmbCe 1 + bCe

(7)

Qe ) KFCe1/n

(8)

Qe )

Qe ) Qm exp(-KDRε2) and ε ) RT ln(1 + 1/Ce′) (9) Qe )

Qm(bCe)1/n 1 + (bCe)1/n

(10)

The isotherm constants were determined using the nonlinear regression analysis, as shown in Figure 4. The different isotherm parameters along with correlation coefficient values (R2) are given in Table 4. In the present study, high values of the R2 value indicate that the Sips isotherm model best fits the adsorption equilibrium data. At low aniline concentrations, it effectively reduces to the Freundlich isotherm, while at high aniline concentrations it predicts a monolayer adsorption capacity characteristic of the Langmuir isotherm. Compared with the Qm values from the Sips model fitting, adsorption capacities

Figure 3. Pseudo-first-order (A) and pseudo-second-order (B) plots for adsorption of aniline on CSS (Conditions: Ci, 100 mg L-1; temperature, 293 K; pH, 4.0; dose of CSS, 50 mg).

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Table 3. Pseudo-First- and Second-Order Kinetic Parameters for the Adsorption of Aniline on CSSa pseudo-first-order

pseudo-second-order

sample

Qe-exp (mg g-1)

Qe1 (mg g-1)

K1 (min-1)

r2

Qe2 (mg g-1)

K2 (103 g mg-1 min-1)

r2

CSS1 CSS2 CSS3

53.17 65.17 74.20

154.52 67.72 111.17

0.107 0.099 0.106

0.9094 0.8236 0.8897

60.98 71.94 81.97

1.42 1.79 1.57

0.9934 0.9925 0.9934

a

Conditions: Ci, 100 mg L-1; temperature, 293 K; pH, 4.0; dose of CSS, 50 mg.

increase with the increase of the sulfur content of the CSS, which also indicates that the adsorption capacity depends on the contents of active groups on CSS. The Sips model exponent n indicates surface heterogeneity, and for a highly heterogeneous system, the deviation of the n value from unity will be higher. The value of n (0.3-0.4) indicates that the CSS surface is heterogeneous in nature. The surface heterogeneity of CSS for the adsorption of aniline can further be proven by the Scatchard plot analysis.45 The Scatchard equation is one of the widely used transformations of the classical Langmuir equation, and it is represented as follows45 Qe ) b(Qm - Qe) Ce

sites in the CSS are heterogeneous with respect to the affinity for aniline and indicates that the binding sites could be classified into two groups with specific binding properties. This surface heterogeneity of CSS may be explained by steric hindrance. There are three different hydroxyls (positions 2, 3, and 6, Figure 6) on the molecule of starch. All of them have the possibility to produce a sulfate group. The valence forces are different between the sulfate groups on different hydroxyls and aniline due to the different steric hindrance. Considering the similarity of positions 2 and 3, two types of binding site may, respectively, belong to the sulfate groups on positions 2 or 3 and on position 6.

(11)

Qe/Ce versus Qe plot of aniline is given in Figure 5. The Scatchard plot is not a single straight line but consists of two linear parts with different slopes. This suggests that the binding

Figure 5. Scatchard plots for the aniline adsorption on CSS (Conditions: adsorption time, 60 min; temperature, 293 K; pH, 4.0; dose of CSS, 50 mg).

Figure 4. Effect of the equilibrium aniline concentration on the adsorption capacities and adsorption equilibrium isotherms (Conditions: adsorption time, 60 min; temperature, 293 K; pH, 4.0; dose of CSS, 50 mg). Table 4. Adsorption Isotherm Model Constants and Correlation Coefficients for the Adsorption of Aniline on CSSa Figure 6. Molecule structure of amylose.

sample isotherms Langmuir Freundlich DubininRadushkevich

Sips

parameters Qm (mg g-1) b (L mg-1) R2 KF n R2 Qm (mg g-1) KDR (109 mol2 J-2) E (kJ mol-1) R2 Qm (mg g-1) b (L mg-1) n R2

CSS1

CSS2

CSS3

141.46 0.0099 0.9159 3.69 1.57 0.8769 553.97

167.47 0.015 0.9347 6.47 1.67 0.8907 656.36

160.48 0.028 0.9180 12.83 2.03 0.8567 548.32

7.43 8.20 0.8919 74.59 0.027 0.32 0.9899

6.74 8.61 0.9065 96.69 0.036 0.37 0.9966

5.39 9.63 0.8744 108.09 0.052 0.35 0.9915

a Conditions: adsorption time, 60 min; temperature, 293 K; pH, 4.0; dose of CSS, 50 mg.

Table 5. Thermodynamic Parameters for the Adsorption of Aniline on CSSa sample

T (K)

Ka0 (L g-1)

∆G0 (kJ mol-1)

∆H0 (kJ mol-1)

∆S0 (J mol-1 K-1)

CSS1

293 303 313 323 293 303 313 323 293 303 313 323

0.336 0.321 0.299 0.261 0.668 0.612 0.565 0.510 1.49 1.33 1.19 1.09

2.60 2.91 3.22 3.53 0.97 1.24 1.51 1.78 -0.97 -0.72 -0.47 -0.22

-6.47

-30.97

-6.99

-27.16

-8.26

-24.90

CSS2

CSS3

a

Conditions: adsorption time, 60 min; pH, 4.0; dose of CSS, 50 mg.

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0

∆H and ∆S were obtained from the slope and intercept of the plots of ln Ka0 versus 1/T (Figure 7). Table 5 shows the calculated values of the thermodynamic parameters. The negative values of ∆H0 and the increase in ∆G with the increase temperature show that the adsorption process is more favorable at low temperature. Conclusions

Figure 7. The plots of ln Ka0 versus 1/T for the adsorption of aniline on CSS (Conditions: adsorption time, 60 min; pH, 4.0; dose of CSS, 50 mg).

Furthermore, the mean energy of adsorption (E) can be gained from the Dubinin-Radushkevich model fitting, which can be calculated from the equation46 E)

1

(12)

√2KDR

The calculated results of E are also given in Table 4. The mean energy of adsorption means the free energy for the transfer of 1 mol of aniline from infinity to the surface of the adsorbent. The magnitude of the mean energy of adsorption is often used to estimate the type of adsorption process. According to the experimental results, the value of E was calculated to be 8-10 kJ mol-1, which is within the energy range of chemsorption involving valence forces, 8-16 kJ mol-1.47 Adsorption Thermodynamic Studies. The adsorption equilibrium can be regarded as the distribution equilibrium of adsorbate between adsorbents and the solution of adsorbate. Then the apparent equilibrium constant (Ka) and the standard thermodynamic equilibrium constant (Ka0) of the adsorption process are defined as Ka )

Qe Ce

Ka0 lim Ka lim Cef0

Cef0

(13) Qe Ce

(14)

Ka0 can be obtained from Ka using the activity instead of concentration. The concentration is infinitely close to the activity at infinite dilution. Then the value of Ka0 can be found by calculating Ka at different equilibrium concentrations of aniline and extrapolating to zero. Experiments were conducted at different equilibrium concentrations and different temperatures, and the values of Ka0 at four different temperatures are depicted in Table 5. It can be found that the value of Ka0 decreases with the temperature increase from 293 to 323 K. Thermodynamic parameters such as change in Gibbs free energy (∆G0), enthalpy (∆H0), and entropy (∆S0) were determined using the following equations:48 ln Ka0 ) -

∆S0 ∆H0 + RT R

∆G0 ) ∆H0 - T∆S0

(15) (16)

The present study shows that cross-linked starch sulfate (CSS) is an effective adsorbent for the removal of aniline from aqueous solutions. The maximum removal efficiency of aniline is observed up to 75.29% using 1 g L-1 of CSS3 dose. The adsorption capacity can be controlled by adjusting the amount of sulfate groups on adsorbents, which is proportional to the adsorption capacity. The adsorption capacities of aniline on CSS first increase and then decrease with the pH increasing from 2 to 10, and the optimal adsorption effect is achieved at pH ) 4.6. The adsorption of aniline on CSS is rapid, and the adsorption processes reach equilibrium in about 60 min. The adsorption process follows the pseudo-second-order model well. The magnitudes of the mean energy of adsorption calculated from the Dubinin-Radushkevich model fitting are in accordance with the chemisorption process. The equilibrium adsorption isotherm data can be well represented by the Sips isotherm equation. The maximum adsorption capacity of CSS3 from the Sips isotherm fitting is 108.09 mg g-1. Results of Scatchard analysis further indicate the presence of two types of binding site for aniline on CSS. Thermodynamic calculations indicate that the complete adsorption process is exothermic in nature, and therefore, the aniline removal decreases with increasing temperature. Acknowledgment The authors acknowledge the research grants provided by LudongUniversity,inChina(ContractGrantNumbersLY20062901 and LY20062902). The authors also greatly appreciate the support provided by the Project of Innovation Team Building of Ludong University(Grant No. 08-CXB001). Nomenclature b ) the thermodynamic equilibrium constant in Langmuir’s adsorption model (L mg-1) Ci ) the initial concentrations of aniline in the adsorption solution (mg L-1) Ct ) the terminal concentrations of aniline in the adsorption solution (mg L-1) Ce ) the equilibrium aniline concentration (mg L-1) Ce′ ) the equilibrium aniline concentration (mol L-1) E ) the mean energy of adsorption (kJ mol-1) K1 ) the pseudo-first-order rate constant (min-1) K2 ) the pseudo-second-order rate constant (g mg-1 min-1) Ka ) the apparent equilibrium constant (L g-1) Ka0 ) the standard thermodynamic equilibrium constant (L g-1) KDR ) the activity coefficient related to mean adsorption energy (mol2 J-2) KF ) the Freundlich constant related to adsorption capacity m ) the dose of the adsorbent (mg) n ) the Freundlich constants related to adsorption intensity Q ) the adsorption capacity of the adsorbent (mg g-1) Qe ) the equilibrium adsorption capacity (mg g-1) Qe1 ) the equilibrium adsorption capacity calculated from pseudofirst-order kinetic model fitting (mg g-1)

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Qe2 ) the equilibrium adsorption capacity calculated from pseudosecond-order kinetic model fitting (mg g-1) Qe-exp ) the equilibrium adsorption capacity from the experiments (mg g-1) Qm ) the maximum adsorption capacity (mg g-1) R ) the gas constant, 8.3145 J mol-1 K-1 t ) the adsorption time (min) T ) temperature in Kelvin (K) V ) the volume of the adsorption solution (mL) ∆G0 ) Gibbs free energy change (kJ mol-1) ∆H0 ) enthalpy change (kJ mol-1) ∆S0 ) entropy change (J mol-1 K-1) ε ) the Polanyi potential (J mol-1)

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ReceiVed for reView July 5, 2009 ReVised manuscript receiVed August 6, 2009 Accepted September 4, 2009 IE9010782