Adsorption of hexavalent chromium on a lignin-based resin: Equilibrium, thermodynamics, and kinetics

Journal of Environmental Chemical Engineering 1 (2013) 1301–1308

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Adsorption of hexavalent chromium on a lignin-based resin: Equilibrium, thermodynamics, and kinetics Feng-Bing Liang, Yan-Lei Song, Chong-Pin Huang *, Jie Zhang, Biao-Hua Chen State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China

A R T I C L E I N F O

A B S T R A C T

Article history: Received 27 February 2013 Received in revised form 18 September 2013 Accepted 27 September 2013

A novel lignin-based resin (LBR), prepared by condensation polymerization of sodium lignosulfonate with glucose under acidic conditions, was used as an adsorbent for sorption of Cr(VI). The effects of varying experimental parameters (such as pH, initial metal ion concentration, LBR dose, contact time, and temperature) on the adsorption process were studied by batch adsorption experiments. The surface properties change during adsorption of Cr(VI) was characterized by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) analysis. Isotherm modeling studies demonstrated that the Freundlich isotherm model gave a better fit to the experimental data, and the maximum adsorption capacity was 57.681 mg g1. The calculated thermodynamic parameters of the sorption (DG, DH and DS) showed that the adsorption of Cr(VI) on LBR is spontaneous and endothermic under the conditions employed. The experimental data was also tested by both pseudo-first-order kinetic and pseudo-second-order kinetic models. The adsorption process is well described by the pseudo-second-order kinetic model. A three-step removal mechanism of Cr(VI) by LBR was proposed, including: electrostatic attraction of acid chromate ion by protonated LBR, reduction of Cr(VI) to Cr(III) and bond formation of Cr(III) with the oxygen-containing functional groups. ß 2013 Published by Elsevier Ltd.

Keywords: Hexavalent chromium Lignosulfonate Adsorption Thermodynamics Kinetics

Introduction As chromium compounds have a wide range of applications in metallurgy, dyes, paints, and tanning of leather, chromium pollution has been a vital environmental issue. In wastewater, the dissolved chromium is present as either the trivalent form (Cr(III)) or as the hexavalent form (Cr(VI)). Cr(VI) is known as a health hazard to both plants and animals, because of its toxicity and carcinogenic properties, but the trivalent chromium is proved to be a dietary requirement for a number of organisms [1]. Thus, the removal of Cr(VI) from wastewaters attracts more public attention in recent years. Adsorption on ion-exchange resins is one of the most effective methods for removing chromium from wastewaters [2–6]. However, the application of ion-exchange resins has been found to be restricted, because of its expensive investment. With the depletion of fossil fuels, the cost of ionexchange resins synthesized from hydrocarbon monomers can be expected to increase in the foreseeable future. Therefore, for the environmental and economic factors, it is necessary to develop new ion-exchange resins based on renewable materials to replace conventional organic ion-exchange resins.

* Corresponding author. Tel.: +86 10 6441 2054; fax: +86 10 6441 9619. E-mail address: [email protected] (C.-P. Huang). 2213-3437/$ – see front matter ß 2013 Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.jece.2013.09.025

Lignin is one of the main constituents of lignocellulosic biomass (15–30% by weight). Recent industrial applications of lignin are mainly based on lignosulfonates (LSs), which are water-soluble anionic polyelectrolyte polymers and can be recovered from the spent pulping liquids (red or brown liquor) from sulfite pulping. The aromatic three-dimensional structure of LSs involving a large number of functional groups, such as methoxy groups, hydroxyl groups, carboxylic acid groups and sulfonate groups [7], suggests that LSs can play an important role in the formation of macromolecular materials. In recent decade, a few studies about adsorption of chromium ions on some lignin-based materials have been investigated. Zhang and coworkers [8] has investigated the adsorption of heavy metal ions on lignin obtained from black liquor—a paper industry waste material, and the maximum equilibrium adsorption capacity of Cr(III) (11.25 mg g1) was obtained. Fan and coworkers [9–11] prepared a spherical lignin-based ion exchange resin by reverse phase suspension polymerization technique with lignosulfonates as raw material and studied the adsorption capacities for Cr(III) (60.00 mg g1) and for Cr(VI) (19.50 mg g1). However, these lignin-based materials have less adsorption capacities than that of petroleum-based ion exchange resins. Herein, a novel lignin-based resin (LBR) with a high density of acidic groups was used as an adsorbent for the removal of Cr(VI), and a detailed investigation under various parameters such as pH,

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initial metal ion concentration, LBR dose, sorption time, and temperature has also been carried out by the batch experiments. The adsorption isotherms, thermodynamic and kinetic studies were evaluated from the experimental data. Furthermore, a threestep mechanism for the adsorption process was proposed. Materials and methods Synthesis of LBR In a typical procedure, sodium lignosulfonate (20 g) and Dglucose (1 g) were dissolved in 100 mL of dilute sulfuric acid (pH = 1), and then the solution was poured into a mechanically stirred autoclave. After keeping the reactor temperature constant at 463 K for 10 h, the solution was removed by vacuum filtration, and the resulting solid material (LBR) was washed repeatedly in boiling water until impurities such as sulfate ions were no longer detected in the wash water. LBR is then dried and stored for subsequent use. The test for the presence of sulfate ions can be conducted through barium sulfate gravimetric method, where barium sulfate (a precipitate) is generated by combining solutions of barium ions and sulfate salts. All the reagents, including sodium lignosulfonate, D-glucose, potassium dichromate (K2Cr2O7), sulfuric acid, hydrochloric acid, sodium hydroxide, were purchased at high purity (AR grade) from Beijing Chemical Reagent Company. Preparation of synthetic wastewater solutions A stock solution of Cr(VI) (1000 mg L1) was prepared in distilled water by dissolving the appropriate amount of potassium dichromate (K2Cr2O7). Working solutions of varying concentrations were obtained by successive dilution of this stock solution. Adsorption experiments The batch experiments were conducted in 150 mL Erlenmeyer flasks using 50 mL of Cr(VI) solutions prepared by dilution of the 1000 mg L1 stock solution. The pH value of the solutions measured by pH meter was adjusted to a predetermined value using 1 M HCl or NaOH, and the required loading dose of LBR was added to the solution. The resulting mixture was stirred for a specified amount of time in a thermostated reciprocating shaker at 150 rpm while keeping the pH constant. At the end of each experiment, a sample of the suspension was separated by a fiberglass filter (0.22 mm pore size) to remove LBR particles. The filtrate was then analyzed for residual Cr ions. The effects of varying the pH (in the range 2–8), adsorbent concentration (in the range 2–16 g L1), Cr(VI) concentration (in the range 50– 200 mg L1), contact time (in the range 10–300 min) and temperature (in the range 303–323 K) on Cr(VI) adsorption were studied. All tests were conducted in triplicate, and their mean values (the accuracy is considered to be 2.5%) were used in analyzing the data. The residual concentration of Cr ions was determined by inductively coupled plasma—atomic emission spectroscopy (ICPAES; IRIS INTREPID II, ThermoELemental, U.S.). The equilibrium adsorption capacity, qe (mg g1), was calculated according to Eq. (1): qe ¼

VðC i  C f Þ M

(1)

where V is the volume of Cr(VI) solution (L), Ci and Cf are the initial and the residual Cr(VI) ions concentration in solution (mg L1), respectively, and M is the mass of LBR (g).

Surface characterization of LBR Surface states of LBR were characterized by neutralization titration, pH of zero point charge (pHzpc), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FTIR) analysis. The ion-exchange capacity of LBR was estimated by neutralization titration using aqueous NaOH solution [12]. The pHzpc of LBR, which is the pH value when the electrical charge density on the surface of LBR is zero, was determined by the procedure described by Ramrakhiani et al. [13]. Oxidation state of chromium adsorbed onto LBR particles was examined using XPS, when Cr-laden LBR was obtained through contact with 200 mg L1 of Cr(VI) at pH 2 for 5 h. XPS spectra were collected on a VG Scientific model ESCALAB 250 (Thermo Fisher Scientific, U.S.). The calibration of the binding energy of the spectra was performed with the C1s peak of the aliphatic carbons, which is at 284.6 eV. The functional groups on the surface of LBR were analyzed using FTIR (TENSOR 27, Bruker, Germany). FTIR spectra for LBR and the Crladen LBR were obtained by using KBr pellets containing samples in the wave number range 400–4000 cm1. For preparing KBr pellets, approximately 1% mixtures of the solid sample in KBr (200 mg) were combined in the mortar. Results and discussion Characteristics of LBR LBR can be readily prepared by heating sodium lignosulfonate with glucose in acidic solution at 453–473 K. Under the experimental conditions, 5-hydroxymethylfurfural and levulinic acid were made from glucose, and desulfonation of LSs have been simultaneously employed. Once these reactants were formed, the formation of LBR took place via acetylization of desulfonated lignosulfonates with sugar derivatives [14]. Fig. 1a shows SEM image of LBR, suggesting that LBR exists as spherical beads with grain sizes greater than 1 mm. The possible model structure for LBR is also depicted in Fig. 1b, showing that three types of acid sites (carboxyl, lactones and phenolic groups) are present on the surface of LBR. The physical and chemical properties of LBR are summarized in Table 1. Effect of pH The removal of Cr(VI) by LBR at different pH values in the range from 2 to 8 was studied using the solutions with Cr(VI) concentration of 100 mg L1 at a LBR dose of 6 g L1 for 5 h. Fig. 2 shows that the adsorption of Cr(VI) onto LBR was strongly affected by the solution pH. The maximum removal was obtained at pH value below 2. With increasing pH from 1 to 7.5, the residual concentration of Cr ions (C) increases from 0 mg L1 to 88 mg L1, but the equilibrium adsorption capacity of LBR is on a declining curve from 16.67 mg g1 to 2.00 mg g1. Thus, pH 2 was chosen as the more adequate pH for the Cr(VI) sorption on LBR. Similar results were obtained for other ion-exchange resins by previous studies. For instance, the most favorable pH for Amberlite IRA 96 and Ceralite IRA 400 ion-exchange resins were 2.0 and 2.0, respectively [15,16]. The pH dependence of Cr(VI) adsorption on LBR can be related to both the type and ionic states of surface functional groups on LBR and the aqueous chemistry of chromium(VI) [17]. On the one hand, the pHzpc value of LBR is 2.96, indicating the effect of pH on the surface state of LBR. If pH < pHzpc, the surface charge of LBR was positive; If pH >pHzpc, c, charges on the surface of LBR become negative [18,19]. According to the solubility equilibrium of chromium, on the other hand, the dominant species in dilute Cr(VI) solutions at pH between 1 and 3 is the acid chromate ion species (HCrO4) [20].

[(Fig._1)TD$IG]

[(Fig._2)TD$IG]

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Fig. 2. Effect of varying pH on Cr(VI) adsorption (initial Cr(VI) concentration 100 mg L1, LBR dose 6 g L1, agitation speed 150 rpm, contact time 5 h, temperature 303 K).

Fig. 1. SEM image (a) and model depicting structural features characteristic (b) of LBR.

Thus, at lower pH (<3), the HCrO4ion can bind to positively charged protonated functional groups on the surface of LBR by electrostatic attraction, thus, facilitating high removal efficiency. Furthermore, at lower pH, the functional groups (–CH2OH, –CHO, CH25 5CH–, etc.) present on the surface of LBR can be reductive agents to reduce the Cr(VI) species into Cr(III), which requires a large number of protons as in Eq. (2): HCrO4  þ 7Hþ þ 3e ! Cr3þ þ 4H2 O

(2)

Characterization of the surface properties change during Cr(VI) adsorption The typical characteristics of LBR for Cr(VI) adsorption were studied using a solution with Cr(VI) concentration of 200 mg L1 and a LBR dose of 8 g L1 at pH 2 for 5 h. Under the experimental

Table 1 Characteristic properties of LBR. LBR Resin type Ionic form Matrix Functional groups Total exchange capacity (mmol g1) pH of zero point charge (pHzpc)

Weak acidic cation-exchange resin H+ Lignin COOH/COOR/OH 4.1 2.96

conditions, the Cr(VI) ions were found to sharply decrease until the complete removal was achieved. It is interesting to note that no Cr(III) ions have been detected in the filtrate. However, in the previous studies [21,22], Park’s group has claimed that the Cr(III) appeared in the aqueous phase and increased proportionately to the Cr(VI) depletion in the Cr(VI) biosorption on the biomass-based materials. The oxidation state of chromium adsorbed on LBR was examined by XPS. Low-resolution XPS spectrum of LBR indicates that besides C and O, no significant contributions are present from other elements, but the spectrum of the Cr-laden LBR shows that there is a new significant contribution of chromium. As shown in Fig. 3a, high-resolution spectrum of the Cr-laden LBR was collected from Cr 2p core region. Significant bands were observed at binding energies of 577.0–578.0 eV and 586.5–588.0 eV which can be attributed to the Cr(2p3/2) and Cr(2p1/2) bands, respectively, indicating that the chromium bound on LBR was only trivalent form [21,22]. Therefore, based on the above analyses and the effect of pH, it can be concluded that the adsorbed Cr(VI) is completely reduced to Cr(III) by contact with LBR, and no reduced Cr(III) ions are repelled from the surface of LBR. In order to determine the main functional groups on the surface of LBR which actively participate in the Cr(VI) adsorption, FTIR was employed. Fig. 3b shows the FTIR spectra of LBR before and after adsorption. FTIR spectrum of LBR displays a number of absorption peaks, indicating the complex structure of LBR (Fig. 1b). The broad and strong peak around 3415 cm1 was due to bounded hydroxyl (–OH) groups. The peaks observed at 2961–2855 cm1 can be assigned to C–H stretching vibration. The bands at 1697, 1451 and 1263 cm1 were attributed to C5 5O stretching of COOH groups, coupled OH in-plane deformation vibration and C(5 5O)–O stretching vibrations, respectively. The region between 1200 and 1000 cm1 represented C–O stretching of alcohols and ethers. Compared with the FT-IR spectrum of LBR, there are some changes in the spectrum of the Cr-laden LBR. The peaks at 3415, 1615, and 1099 cm1 were shifted to 3419, 1601, and 1090 cm1, respectively. The bands of 1697, 1451 and 1263 cm1 corresponding to the vibration of carboxyl groups disappeared completely after adsorption. Furthermore, a new peak at 955 cm1 attributed to the vibration of Cr–O bonds was observed [23,24]. Thus, FTIR results confirmed that all oxygen-containing functional groups, such as, hydroxyl, ether, carbonyl, and carboxyl groups, are involved in the Cr(VI) removal. Moreover, Fig. 3c shows the SEM image of LBR after Cr(VI) adsorption. There were no significant

[(Fig._3)TD$IG]

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Fig. 4. Effect of the adsorbent dose on the residual Cr concentration (a) and the adsorption capacity (b) (initial Cr(VI) concentration 50–200 mg L1; pH 2; stirring speed 150 rpm; contact time 5 h; temperature 303 K).

Fig. 3. High-resolution X-ray photoelectron spectra collected from the Cr 2p core region (a), FTIR spectra (b), and SEM image (c) for Cr(VI)-loaded LBR.

differences in the appearance between LBR before and after adsorption. Effect of LBR dose and initial Cr(VI) concentration The Cr(VI) adsorption as a function of LBR dosage was studied at 303 K. The experiments were conducted with the given LBR dose at the concentrations between 50 and 200 mg L1. As shown in Fig. 4, as expected, the residual concentration (C) decreases sharply with

increasing adsorbent loading for a given initial Cr(VI) concentration until the concentration of Cr was zero, which can be attributable to increase the number of adsorption sites with increasing of LBR dose [25]. Similarly the adsorption capacity (q) decreased with a fixed Cr(VI) concentration, when the loading of LBR mass was increased from 2 g L1 to 16 g L1, due to an increase in the ratio of adsorbent to adsorbate [26–30]. In addition, the minimum loadings of LBR required to complete removal for the Cr(VI) concentrations of 50, 100, 150 and 200 mg L1 were 4, 6, 8 and 8 g L1, respectively. It can be concluded that the optimal LBR dose can depend directly on the Cr(VI) concentration. The effect of initial Cr(VI) concentration was also investigated with different initial Cr(VI) concentrations ranging from 50 to 200 mg L1 at temperature 303 K with a fixed LBR dose. As shown in Fig. 4, when the initial Cr(VI) concentration was increased from 50 to 200 mg L1, the concentration of residual Cr ions increased, which may be explained as the given LBR beads have limited adsorption sites for a certain concentration and it would be lack of available adsorption sites at higher initial Cr(VI) concentration [26,31]. However, the amount of Cr(VI) adsorbed on LBR (q) increased. Higher initial Cr(VI) concentration provided higher mass transfer driving force to overcome all the mass-transfer resistances toward the removal of the metal ions from the aqueous phase to the solid phase, thus, facilitating higher metal sorption capacity [32].

[(Fig._5)TD$IG]

[(Fig._6)TD$IG]

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Fig. 5. Effect of varying contact time on adsorption of Cr(VI) (temperature 303 K, LBR dose 8 g L1 mL, pH 2). Fig. 6. Langmuir and Freundlich isotherms for the adsorption of Cr(VI) by LBR dose of 2 g L1, pH 2.0, contact time 5 h, temperature 303 K.

Effect of contact time The removal of Cr(VI) onto LBR as a function of contact time was investigated. The experiments were conducted at temperature 303 K with a LBR dose of 8 g L1 at pH 2. As shown in Fig. 5, the C value decreased with increasing contact time at each concentration, but the q shows an opposite tendency. It was observed that adsorption of Cr(VI) by LBR consists of two steps: an initial fast step lasting 40 min followed by a slower second step which continued until the establishment of equilibrium. In the 1st step, a major fraction of Cr(VI) was sorbed onto LBR. Besides, under experimental conditions, the equilibrium adsorption time of the Cr(VI) concentrations of 100 and 200 mg L1 was 120 and 180 min, respectively, indicating that a higher Cr(VI) concentration requires a longer contact time to establish equilibrium. Effect of temperature It is well known that temperature is one of important factors affecting the metals removal from aqueous solution. The effect of temperature (in the range from 303 K to 323 K) on Cr(VI) adsorption was studied. The experiments were conducted with Cr(VI) concentration of 100 mg L1 and a LBR dose of 2 g L1 at pH 2 for 5 h. The results show that the residual concentration decreased, but the adsorption capacity increased, when the temperature was increased from 303 to 323 K, indicating that the adsorption of Cr(VI) onto LBR is an endothermic process. Under higher temperature, new adsorption sites were developed by bond rupture on the LBR surface, thus, facilitating higher q value [26,30]. Equilibrium isotherm models Analysis of equilibrium data is one of the important steps in the evaluation of a sorption process by developing an equilibrium isotherm model. To examine the relationship between adsorption and aqueous concentration at equilibrium, two-parameter isotherm models, such as Langmuir and Freundlich isotherms, were studied. The Langmuir model, the most widely used isotherm for adsorption processes, is based on monolayer sorption onto a surface with a finite number of homogeneous sites. The Langmuir isotherm is represented in the following equation: qe ¼

qm bC e 1 þ bC e

(3)

where Ce is the residual Cr(VI) ions concentration in solution at equilibrium (mg L1), qe is the mass of chromium adsorbed on LBR at equilibrium (mg g1), qm is the maximal adsorption capacity (mg g1), and b is the Langmuir constant of the system (L mg1). The Freundlich model assumes a heterogeneous adsorbed surface and active sites with different energy, and can be described by the following form: 1=n

qe ¼ K F Ce

(4) 1

where KF is a constant related to the adsorption capacity (mg g ) and 1/n is an empirical parameter related to the adsorption intensity. As shown in Fig. 6, the experimental data were described by two types of adsorption isotherms, and the isotherm parameters were summarized in Table 2. The coefficient of determination (R2) indicates that how well data points fit a line or curve. Since R2 can be influenced by the range of the independent variable and the number of the model parameters, it is not always suitable for evaluating the goodness of fit and comparing models [33]. The use of an adjusted R2 is an attempt to take account of the phenomenon of the R2 automatically and spuriously increasing when extra explanatory variables are added to the model. Thus, in this work, adjusted R2 has been selected to give the information about the goodness of fit of a model. The adjusted R2 is defined as: Adj-R2 ¼ 1  ð1  R2 Þ

n1 n p1

(5)

Adj-R2 for the Langmuir isotherm was less than that of the Freundlich model, indicating that a better fit to the equilibrium data is given by the Freundlich isotherm model. The maximum adsorption capacity (qm) for LBR, according to the fitted Langmuir model, is 57.681 mg g1. In many of the previous studies, the values of qm for various ion-exchange resins were estimated. Fan and Zhang [11] found that the maximal adsorption capacity the adsorption of hexavalent chromium on lignosulfonates resin is 19.50 mg g1. Senthil Kumar’s group [16] has studied the removal of hexavalent chromium using Ceralite IRA 400 ion-exchange resins and found that the maximal adsorption capacity was 43.39 mg g1. Thus, LBR exhibits high adsorption capacity and can be considered a viable adsorbent for removal of chromium from aqueous solution. It is worth noting that the qm value is not a practical obtainable capacity. In general, the value of q obtained at the experimental conditions is far less than the qm value [4,11,16].

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Table 2 Summary of adsorption isotherm parameters for Cr(VI). Isotherm

Table 3 Thermodynamic parameters for the adsorption of chromium on LBR.

Unit

Information

[(Fig._7)TD$IG]

Langmuir model: nonlinear fitting mg g1 qm b L mg1 Adj-R2 Freundlich model: nonlinear fitting KF mg g1 n Adj-R2

57.681 0.288 0.613 33.605 8.631 0.988

T (K)

Kc

DG (kJ mol1)

DH (kJ mol1)

DS (J mol1 K1)

303 313 323

3.422 7.806 12.927

3.099 5.262 6.660

104.917

356.897

absolute temperature (K), and R is the gas constant (8.314 J mol1 K1). The values of DH and DS can be determined from the slope and the intercept of the plot of ln Kc versus 1/T (see Fig. 7) [34]. The values of DG, DH and DS for adsorption of chromium on LBR are given in Table 3. The values of DG are negative, suggesting that the adsorption process on LBR is spontaneous. The magnitude of the Gibbs free energy increased with increasing temperatures, indicating that better adsorption capacity of LBR for chromium(VI) can be obtained at higher temperatures. The positive value of DH confirms that the adsorption of Cr(VI) on LBR is an endothermic process. The reason why adsorption level for LBR increased with increasing temperature can be also explained by the value of DH. Compared with the DH for the sorption Cr(VI) ions on D301 and D354 anion-exchange resins (7.906 and 16.635 J mol1) [4], the higher DH value indicates that the Cr(VI) adsorption onto LBR is chemisorption. Furthermore, the positive value of DS indicates the increased randomness at the solid/liquid interface during the sorption of Cr(VI) on LBR, which reflects the fact that chromium has a good affinity for LBR. Similar trends in the values of DG, DH and DS have also been reported for removal of Cr(VI) with other adsorbents [6,35].

Fig. 7. Plot of ln b vs. 1/T for adsorption of Cr(VI) by LBR.

Kinetics studies However, the q values for LBR can reach up to 57.11 mg g1, very close to the qm value, indicating that LBR has high practical application property. Moreover, the high KF value indicates the higher affinity of LBR for Cr(VI) ions, with the fact that n lies between 1 and 10 indicating favorable adsorption.

For evaluating the adsorption kinetics of Cr(VI) ions, the pseudo-first-order and pseudo-second-order kinetic models were used to test the experimental data. The pseudo-first-order expression [36] may be represented as follows:

Thermodynamic analysis

qt ¼ qe ½1  expðk1 tÞ

Thermodynamic analysis is necessary to understand the mechanism of adsorption. In order to describe the thermodynamic behavior of the adsorption of chromium onto LBR, the changes in Gibbs free energy (DG), enthalpy (DH) and entropy (DS) were calculated according to Eqs. (6), (7) and (8): Kc ¼

C ad Ce

(6)

DG ¼ RT ln K c ln K c ¼

DS R



(7)

DH

(8)

RT

where Kc is the distribution constant, Cad is the concentration of solute adsorbed on LBR at equilibrium (mg L1), T is the

(9)

and the pseudo-second-order equation [37] is given in the following form: qt ¼

k2 q2e t 1 þ k2 qe t

(10)

where qe is the adsorption capacity at equilibrium (mg g1), qt is the adsorption capacity at time t (min), k1 is the first-order rate constant (min1), and k2 is the second-order rate constant (mg g1 min1). The fitness of the experimental data to the kinetics models was shown in Fig. 8, and the calculated rate constants, qe, and the values of Adj-R2 are listed in Table 4. Although the values of qe estimated from the both kinetic models gave slightly different values in comparison with experimental data, the pseudosecond-order model has higher correlation coefficients than the pseudo-first-order model with experimental data for moderate

Table 4 Results of Cr(VI)adsorption kinetic modeling. Concentration (mg L1)

100 200

qe(exp.)(mg g1)

12.5 25

Pseudo-first-order model

Pseudo-second-order model

k1 (min1)

qe (mg g1)

Adj-R2

k2 (g mg1 min1)

qe (mg g1)

Adj-R2

0.112 0.0513

12.288 24.130

0.925 0.953

0.0168 0.00283

12.973 26.822

0.975 0.985

[(Fig._8)TD$IG]

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(100 mg L1) and high (200 mg L1) concentrations of Cr(VI) ions. Thus, the adsorption of Cr(VI) ions on LBR can be well described by the pseudo-second-order model, indicating that the adsorption process may be governed by chemisorption. Moreover, the initial adsorption rates v0 calculated by the pseudo-first-order model are 2.827 mg g1 min1 for 100 mg L1 and 2.036 mg g1 min1 for 200 mg L1, indicating that high initial concentration of Cr(VI) may led to lower initial adsorption rate. The possible explanation may be the stronger mutual repulsion between the acid chromate ion species (HCrO4) in the boundary liquid film under the high concentration than that for the moderate concentration. Elucidation about Cr(VI) removal by LBR

Fig. 8. Plots of the pseudo-first-order (a) and the pseudo-second-order (b) adsorption kinetics of Cr(VI) onto LBR at different initial concentrations (temperature 303 K, LBR dose 8 g L1, pH 2).

According to all above-mentioned analysis, the possible mechanism of Cr(VI) removal by LBR is proposed and shown in Fig. 9. The removal of Cr(VI) by LBR undergo a series of steps, including: firstly, under lower pH conditions, the LBR surface was positively charged, facilitating the diffusion of HCrO4anions through a boundary layer surrounding the LBR particles and the adsorption of the anions onto the positively charged active sites; secondly, in the presence of large number of protons and electron-donor groups, all adsorbed Cr(VI) anions were reduced to Cr(III) on the LBR surface; finally, the coordination complexes were formed between the resulting Cr(III) species and the oxygen-containing functional groups (such as the alcohols, ethers, carbonyl, and carboxyl groups). Until now, many studies have claimed different sorption mechanisms for the Cr(VI) removal from the aqueous phase with various adsorbents. Park et al. [38] proposed the two mechanisms for direct and indirect Cr(VI) removal by the brown seaweed. Zheng et al. [39] also studied the removal mechanism of Cr(VI) by Sargassum. All these previous mechanism mentioned that part of the reduced Cr(III) ions were repelled from the surface of the biosorbents. However, the release of the reduced Cr(III) ions into the aqueous phase was not found in our study, due to the high metal-ion-binding strength for Cr(III) on LBR [15].

[(Fig._9)TD$IG]

Fig. 9. Proposed mechanism of Cr(VI) adsorption by LBR.

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Conclusions In present work, adsorption characteristics of Cr(VI) ions on a novel lignin-based resin (LBR) have been studied. Removal of Cr(VI) by LBR is an adsorbate, adsorbent, and temperature dependent process under strongly acidic conditions. The optimum pH for the Cr(VI) sorption on LBR is 2. The experimental data follow the Freundlich isotherm model more closely than the other isotherm models. The maximum Cr(VI) adsorption capacity was found to be 74.29 mg g1, indicating that LBR is an efficient sorbent for removal of Cr(VI) from aqueous solutions. The values of the thermodynamic parameters show that the adsorption process is endothermic and spontaneous. Kinetics of the removal of Cr(VI) ions on LBR was also investigated. The adsorption process can be well described by the pseudo-second-order model. Moreover, an original mechanism for Cr(VI) adsorption has also been proposed. Acknowledgements The authors are grateful for financial support from the National Science Foundation of China (No. 20806007) and the Major Project of Chinese National Programs for Fundamental Research and Development (No. 2010CB226902). References [1] D.G. Barceloux, D. Barceloux, Chromium, Clin. Toxicol. 37 (1999) 173–194. [2] S. Mustafa, K.H. Shah, A. Naeem, M. Waseem, T. Ahmad, S. Khan, Co-ion effect on Cr3+ sorption by amberlyst-15 (H+), Water Air Soil Pollut. 217 (2011) 57–65. [3] S. Mustafa, K.H. Shah, A. Naeem, T. Ahmad, M. Waseem, Counter-ion effect on the kinetics of chromium (III) sorption by Amberlyst.15 in H+, Li+, Na+, Ca++, Al+++ forms, Desalination 264 (2010) 108–114. [4] T. Shi, Z. Wang, Y. Liu, S. Jia, D. Changming, Removal of hexavalent chromium from aqueous solutions by D301, D314 and D354 anion-exchange resins, J. Hazard. Mater. 161 (2009) 900–906. [5] S.K. Sahu, P. Meshram, B.D. Pandey, V. Kumar, T.R. Mankhand, Removal of chromium(III) by cation exchange resin, Indion 790 for tannery waste treatment, Hydrometallurgy 99 (2009) 170–174. [6] S. Mustafa, K.H. Shah, A. Naeem, M. Waseem, M. Tahir, Chromium (III) removal by weak acid exchanger Amberlite IRC-50 (Na), J. Hazard. Mater. 160 (2008) 1–5. [7] J. Zakzeski, P.C.A. Bruijnincx, A.L. Jongerius, B.M. Weckhuysen, The catalytic valorization of lignin for the production of renewable chemicals, Chem. Rev. 110 (2010) 3552–3599. [8] Y. Wu, S. Zhang, X. Guo, H. Huang, Adsorption of chromium(III) on lignin, Bioresour. Technol. 99 (2008) 7709–7715. [9] J. Fan, H.Y. Zhan, M.R. Liu, Adsorption of Cr3+ and Cr2O72 on the spherical ligninbased ion exchange resin, Trans. China Pulp Pap. 20 (2005) 111–113. [10] J. Fan, H. Zhan, M. Liu, Preparation of spherical lignin-based ion exchange resin and its adsorption properties for Cr3+, Ion Exc. Adsorpt. 22 (2006) 231–236. [11] J. Fan, H.Y. Zhan, Adsorption of hexavalent chromium on lignosulphonates resin, J. Hunan Univ. Nat. Sci. 36 (2009) 59–62. [12] P.A.G. Cormack, A. Davies, N. Fontanals, Synthesis and characterization of microporous polymer microspheres with strong cation-exchange character, React. Funct. Polym. 72 (2012) 939–946. [13] L. Ramrakhiani, R. Majumder, S. Khowala, Removal of hexavalent chromium by heat inactivated fungal biomass of Termitomyces clypeatus: surface characterization and mechanism of biosorption, Chem. Eng. J. 171 (2011) 1060–1068. [14] F.B. Liang, Y.L. Song, C.P. Huang, Y.X. Li, B.H. Chen, Synthesis of novel lignin-based ion exchange resin and its utilization in heavy metals removal, Ind. Eng. Chem. Res. 52 (2013) 1267–1274.

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