Removal of chromium(VI) from aqueous solution using treated waste

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Jan 30, 2016 - c Department of Chemistry, Jamia Millia Islamia (Central University), New .... The surface area of the waste newspaper (WNP) and treated ..... calculated the Gibbs free energy of Cr (VI) adsorption on newspapers ..... Direct Link: Abstract PDF ... [46] A. Adamson, A. Gast, Physical Chemistry of Surface, Wiley, ...

Journal of Molecular Liquids 215 (2016) 671–679

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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq

Removal of chromium(VI) from aqueous solution using treated waste newspaper as a low-cost adsorbent: Kinetic modeling and isotherm studies Mohammad Hadi Dehghani a,b, Daryoush Sanaei a,⁎, Imran Ali c, Amit Bhatnagar d a

Department of Environmental Health Engineering, School of Public Health, Tehran University of Medical Sciences, Tehran, Islamic Republic of Iran Institute for Environmental Research, Center for Solid Waste Research, Tehran, Islamic Republic of Iran c Department of Chemistry, Jamia Millia Islamia (Central University), New Delhi 110025, India d Department of Environmental Science, University of Eastern Finland, P.O. Box 1627, FI 70211, Kuopio, Finland b

a r t i c l e

i n f o

Article history: Received 6 August 2015 Received in revised form 30 November 2015 Accepted 17 December 2015 Available online xxxx Keywords: Low-cost adsorbent Chromium (VI) Adsorption isotherms Adsorption kinetics Treated waste newspaper

a b s t r a c t In the present study, treated waste newspaper (TWNP) was used to remove chromium(VI) from aqueous solution using batch experiments. The adsorption parameters optimized were: initial Cr(VI) concentration (5, 20, 50 mg/l), contact time (60 min.), adsorbent dose (3.0 g/L), and solution pH (3.0). The experimental data fitted well to Langmuir isotherm (R2 = 0. 98; maximum adsorption capacity 59.88 mg/g.) and pseudo-second-order kinetic model. The rate constant k2 varied from 0.0019 to 0.0068 at initial Cr (VI) concentration from 5 to 20 mg/L. It was observed that adsorption of Cr(VI) was pH dependent. The percentage removal of Cr(VI) was 59.88 mg/g (64% at pH 3). The results of the present study suggest that TWNP may be used as a low-cost adsorbent for the removal of chromium (VI) from aqueous solutions. © 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

1. Introduction Discharge of untreated or poorly treated wastewater containing toxic heavy metals such as Cr, Ni, Cd, Pb, Hg, Zn, Co and Cu from industrial effluents into the natural water bodies is a major environmental problem because of their high toxicity and their tendency to accumulate through the food chain [1].Chromium is one of the most notorious heavy metals released by various industries such as tanning and leather industries, manufacturing industries, catalyst and pigments, fungicides, ceramics, crafts, glass, photography, electroplating industry and corrosion control application [1,2]. Chromium forms three common oxidation states in its compounds, + 2, + 3, and + 6. The + 3 and + 6 oxidation states are the most commonly observed in chromium compounds, whereas +1, +4, and +5 states are rare. The most prominent example of toxic chromium is hexavalent chromium (Cr(VI) [2,3]. International Agency for Research on Cancer (IARC) has classified chromium (VI) in Group 1 (carcinogenic to humans) and metallic chromium and chromium (III) in Group 3 (not classifiable as to their carcinogenicity to humans [3]. Therefore, the removal of chromium (VI) from

⁎ Corresponding author. E-mail addresses: [email protected] (M.H. Dehghani), [email protected] (D. Sanaei).

wastewater is extremely important before its discharge into the aquatic system, which needs immediate attention. Conventional treatment technologies have been developed to remove Cr(VI) from water and wastewater, including reduction followed by chemical precipitation [4], ion exchange [5,6], membrane separation [7], electrocoagulation [8], nanoparticles [9], dialysis/electrodialysis [10], and adsorption/filtration [11]. Capital and operational costs often limit efficiency and the effectiveness of these methods, principally, when large volumes of effluents contain relatively low concentrations [11]. In contrast, the adsorption technique is a highly effective method because it is a simple and cost effective method for recovering and eliminating heavy metal ions from dilute solutions [15–19]. Recently, a variety of cheap materials have been examined as adsorbents for the removal of Cr (VI) from aqueous solution with the aim of finding cheaper alternatives for conventional sorbent materials such as activated carbon which is an expensive adsorbent. Some of the lowcost adsorbents include anaerobic sludge [12], apple residue [13], sawdust [14], rice Polish [15], clay [16], zeolite [17], fly ash [18], chitosan [19], waste tea [20,21], seaweeds [22], and polyaniline coated on sawdust [23] which have been used for the purpose. Papers (old newspaper, old magazine, printed papers and mixed office waste paper) are complex materials and principally consist of cellulose, which contains functional polar groups such as alcohols and ethers. These functional groups can be protonated at lower pH and,

http://dx.doi.org/10.1016/j.molliq.2015.12.057 0167-7322/© 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

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thus, bind Cr (VI) by means of electrostatic interactions. The treated newspaper pulp was employed by Chakravarty et al. to remove zinc from aqueous solutions [24]. In the present study, the attempts have been made to remove chromium (VI) ion from aqueous solution using treated waste newspaper as a low-cost adsorbent. Besides, the kinetic modeling and isotherm studies are also presented.

and pH 2 to 7. The chromium (VI) percent removal (%)was calculated as follows: % Removal of Cr ðVIÞ ¼

C i −C e  100: Ci

ð1Þ

The adsorption Cr (VI) capacity per unit mass of the TWNP was calculated according to the following expression:

2. Materials and methods 2.1. Preparation of treated waste newspaper pulp The waste newspaper was cut into piece (2 cm × 2 cm) strips using a paper shredding machine. It was treated with concentrated sodium bicarbonate solution for removing foreign materials such as grease, black ink and bleaching material (chlorine dioxide); which are usually present in the newspaper. The waste newspaper pulp was then refluxed with 5.0% Na 2HPO4 using a water condenser for 3 h to impregnate the phosphate into the cellulose matrix. After phosphorylation, the solution was cooled and passed through Whatman 40 filter paper. The characterization of the newspaper pulp is given elsewhere [25]. 2.2. Characterization of adsorbent The surface area of the waste newspaper (WNP) and treated waste newspaper (TWNP) were measured by BET method (Brunauer– Emmett–Teller nitrogen adsorption technique) by using a Brunauer, Emmett and Teller, Surface Area Analysis (Tristar 3000, Micromeritics) in the range 0.05 ≤ relative pressure (P/P0) ≤ 0.3. The pore volume of the samples were calculated from the volume of adsorbed N2 at P/P0 = 0.99. The samples were preheated at 200 °C for 4 h at vacuum condition (6.67 Pa) to clean the surface before the measurements. According to the SEM analysis of WNP and TWNP, the microstructures of newspapers consist of fibers with agglomerated fine particles filling the spaces between the fibers. The SBET values of WNP ranged from 885 to 1020 m2 g− 1. The specific surface area (BET) of TWNP (by chemical analysis confirming impregnation of phosphorous during the chemical treatment) was ranged from 1214 to 1652 m2 g−1. The pore volume (VP) values for WNP and TWNP were 0.98 and 1.01 ml g− 1, respectively. Also, the moisture content in WNP and TWNP was 7.68% and 6.82%. 2.3. Reagents In this study, the stock solution of Cr(VI) was prepared by dissolving a known quantity of potassium dichromate (K2Cr2O7) in deionized water. Stock solution was further diluted to obtain the required concentrations of Cr(VI) solutions. 1.0 N NaOH and 1.0 N HCl were used for pH value adjustments. 2.4. Batch adsorption experiments The stock solution of Cr (VI) was prepared by dissolving 0.1414 g of K2Cr2O7 in double distilled water and diluted to 100 ml. In all the batch adsorption studies, solutions of 5 to 70 mg/L concentrations were used. The required amount of the adsorbent was added to 250 mL glass stoppered conical flasks containing 100 mL of aqueous Cr (VI) ion solution. The contents of the flask were shaken in a mechanical shaker by continuous mixing with a constant agitation speed of 120 rpm at room temperature for a definite period. A known volume of the solution was removed and centrifuged for Cr (VI) analysis. The batch adsorption experiments were carried out at room temperature at different contact times (20 to 150 min), initial concentration of chromium ion (5 to 70 mg/L), TWNP dose (0.4 to 4 g/100 mL)

qe ¼

C i −C e V m

ð2Þ

where Ci and Ce are the initial and final chromium concentrations (mg/L), respectively, qe is the amount of Cr (VI) adsorbed onto TWNP (mg/g), V is the total volume of solution (L), and m is the TWNP dosage (g). The adsorption isotherm studies were carried out by varying the Cr(VI) initial concentrations from 5 to 70 mg/L at fixed volume (100 mL), TWNP dose (0.4 g), pH (3), optimum uptake time (60 min) and room temperature. The results were analyzed by Freundlich and Langmuir isotherm models. The experiments of batch kinetic adsorption by TWNP was carried out by mixing 0.4 g of TWNP with 100 mL Cr(VI) solution at three initial chromium concentrations (5, 20 and 50 mg/L), at pH 3 and contact time (0–60 min). The data was fitted to the first order, pseudo-first-order and pseudo-second-order models. All the experiments were repeated five times and average values are reported. The relative standard deviation (RSD) was found to be ±1.8% to ±2.4%. 3. Results and discussion 3.1. Characterization of TWNP The surface functional groups were ascertained by Fourier transform infrared spectroscopy (FTIR) in the Treated Waste Newspaper (TWNP) before and after Cr (VI) adsorption (Figs. 1 and 2). The FTIR spectra of WNP and TWNP were recorded in the range of 400 and 4000 cm−1 in FTIR Spectrum 400 (Perkin Elmer). Characteristic cellulose peak in the region of 1000 to 1200 cm−1 was shown in the FTIR. The 1162 cm−1 and 1111 cm−1 band in WNP related to C OC group bonds in cellulose and the band near 1318 cm−1 corresponded to CH2-wagging vibrations in the cellulose. The band near 1351 cm−1 represented the –OH vibrations. The band near 3698 cm−1 is split into two less intense peaks in TWNP due to the change in intra-molecular hydrogen bonding interactions. There was a band appearing in TWNP at 1033 cm−1 describing the aliphatic P–O stretching [26]. 3.2. Effect of solution pH on adsorption Solution pH in the adsorption is considered as one of the important adsorption characteristics that affect the adsorption behavior of metal ions in aqueous solution. The pH dependence of metal adsorption is largely related to the surface functional groups in the biosorbents and metal solution chemistry [27]. The effect of pH is shown in Fig. 3 in the pH range of 2.0–7.0. It is clear from this figure that the optimum pH required for maximum adsorption of Cr (VI) onto TWNP was 3.0. It was also observed that by increasing the pH value, a drastic decrease in Cr (VI) adsorption percentage was observed. This might be due to the weakening of the interactions between the oppositely charged TWNP and Cr(VI); leading to the reduction in sorption capacity [28]. Newspaper materials contain functional polar groups such as alcohol and ethers. At low pH, these functional polar groups were protonated and, therefore, the surface of the adsorbent becomes positively charged. Moreover, due to impregnation of phosphorous in cellulose matrix, the active functional group might be phosphate. This further corroborated

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673

Fig. 1. FTIR spectra of TWNP before Cr(VI) adsorption.

the presence of net negative charge on the surface of the cellulose matrix. HCrO− 4 is the dominant form of Cr (VI) over the range of pH 3.0, while CrO2− 4 is dominant in the range of pH N 7. The TWNP surface contains a large number of active site [−OH] which is associated with intramolecular hydrogen bond. At low pH concentration of OH− decreases, The surface charge of the adsorbent thus changes to positive charged sites [29].

3.3. Effect of contact time The effect of contact time was studied for an initial Cr (VI) concentration of 20 mg/L; TWNP dosage of 0.4 g/100 mL; solution pH of 3.0 at room temperature. Fig. 4 shows that adsorption rate of chromium (VI) increased with contact time until equilibrium was reached. The time to reach equilibrium for chromium (VI) adsorption was 60 min. The

Fig. 2. FTIR spectra of TWNP after Cr(VI) adsorption.

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Fig. 3. Adsorption profile of Cr (VI) onto TWNP at varying pHs.

rate of chromium (VI) adsorption on the TWNP was higher within the first 60 min due to the availability of plenty of sorption sites at the sorbent and a high concentration gradient. After the active sites of the adsorbent get exhausted (saturated), when equilibrium is attained, the sorption became slow in the later stages [30]. A contact time of 120 min was chosen to ensure that adsorption equilibrium is achieved in all cases.

Fig. 5. Effect of chromium concentration on Cr (VI) removal (adsorbent dose 0.4 g/100 ml, agitation speed; 120 rpm, contact time 1 h, temperature 25 °C and pH 3).

Evidently chromium (VI) removal efficiency increased with an adsorbent dose of 0.4 to 3 g/100 mL (Fig. 6), and then, it is not increased so significantly due to the occurrence of aggregation. This is likely due to the equilibrium concentration of the Cr (VI) in solution was lower in the presence of high adsorbent concentrations. An optimum dose of 0.4 g/100 ml is selected for all the experiments.

3.4. Effect of initial Cr (VI) concentration 3.6. Adsorption isotherms The experiments were done with variable initial chromium concentration (5, 20, 50, and 70 mg/L), optimized adsorbent dose (0.4 g/100 mL), contact time (1 h) and pH (3) at room temperature. The percentage of Cr (VI) ion uptake on the TWNP is presented in Fig. 5. Fig. 5 shows that by increasing initial chromium (VI) concentration, chromium removal efficiency is also increased and remained nearly constant after equilibrium time. Thus, results suggest that adsorption capacity of TWNP is dependent on the initial concentration of chromium. This can be attributed to the saturation of sorption sites on adsorbents. The initial concentration provides a significant driving force to overcome all mass transfer resistance of metal ions between aqueous and solid phases [31].

The equilibrium adsorption of Cr (VI) ions on the TWNP was analyzed using adsorption isotherms as discussed below. 3.6.1. Langmuir isotherm The experimental data were fitted to the Langmuir equation: Ce 1 Ce ¼ þ qe Q max:b Q max

where, Ce (mg/L) is the equilibrium concentration of the adsorbate, qe(mg/g) is the amount of the adsorbate adsorbed at equilibrium,

3.5. Effect of adsorbent dose The effect of adsorbent dose (0.4, 0.8, 1, 2, 3 and 4 g/L) is shown in Fig. 6. The other parameters used were pH (3) and contact time (60 min), initial concentration (5 mg/l)) and room temperature. It is apparent that the adsorbed chromium ion amount per unit weight of adsorbent (qe) decreased as the adsorbent concentration increased (Fig. 6). This result was due to the aggregates formed with increasing adsorbent dose, which reduced the effective adsorption [32]. The more amounts of adsorbent results in higher surface area and adsorption regions which causes enhanced removal of chromium (VI).

Fig. 4. Effect of contact time on Cr (VI) removal (initial concentration 20 mg/l, adsorbent dose 0.4 g/100 ml, agitation speed; 120 rpm, temperature 25 °C and pH 3).

ð3Þ

Fig. 6. Plot of effect of adsorbent concentrations on Cr (VI) adsorption by TWNP.

M.H. Dehghani et al. / Journal of Molecular Liquids 215 (2016) 671–679

Fig. 7. Langmuir isotherm plots for the removal of Cr (VI) by TWNP.

Fig. 9. Freundlich isotherm plots for the removal of Cr (VI) by TWNP.

Qmax is the maximum adsorption capacity, and b is the Langmuir equilibrium constant (l/mg) which shows quantitatively the affinity between Cr (VI) and TWNP. Fig. 7 shows Cr (VI) Langmuir adsorption isotherm plots of Cr (VI) on TWNP. A further analysis of the Langmuir model and also the affinity between Cr(VI) and TWNP adsorbent can be predicted using the Langmuir parameter b from the dimensionless constant separation factor RL, which is defined by the following relationship [33]: RL ¼

1 1 þ bC 0

ð4Þ

where, C0 is the initial Cr (VI) concentration (mg/L) and b is Langmuir isotherm constant. The value of RL indicates information as to whether the adsorption may be described as follows: RL N 1 unfavorable RL = 1 Linear 0 b RL b 1 favorable RL = 0 Irreversible. In the present study, the calculated RL value for adsorption of Cr(VI) on the TWNP adsorbent using the above expression were found to be 0.303, 0.140, and 0.058 at initial concentration range of 5–20 mg/L (Fig. 8). The calculated RL confirmed that TWNP is desirable for adsorption of chromium from wastewater under the conditions used in this study. 3.6.2. Freundlich isotherm Freundlich adsorption isotherm [34] is an empirical equation employed to describe the data for heterogeneous adsorbents. Freundlich adsorption equation takes the following general form: qA ¼ K A C A 1=n

675

ð5Þ

The linear form is as follows: LogðqA Þ ¼ Log ðK A Þ þ

  1 Log ðC A Þ n

ð6Þ

where, KA = Freundlich adsorption capacity parameter, (mg/g) · (L/mg)1/n 1/n = Freundlich adsorption intensity parameter. Freundlich isotherm (ln qe vs ln Ce) provided a satisfactory fitting of equilibrium data (Fig. 9). The parameters of the linear form of Langmuir isotherm, Freundlich isotherm and R2 values for adsorption of Cr (VI) onto TWNP are given in Table 1. 3.6.3. Temkin isotherm Temkin isotherm is the model describing the effects of indirect adsorption interaction and adsorption substances on adsorption isotherms. Temkin [50] assume that the heat of adsorption decreases linearly with increasing coverage and the adsorption is characterized by a uniform distribution of binding energies. The Temkin isotherm has a convenient linear form, which is expressed by the following equation: qe ¼ B ln AT þ B ln C e B¼

ð7Þ

RT b

ð8Þ

where, AT is Temkin isotherm equilibrium binding constant corresponding to the maximum binding energy (L/g), B is constantly related to the heat of sorption (J/mol), R is the universal gas constant (8.314 J/mol/K), T is absolute temperature at 298 K°, b is Temkin isotherm constant, which indicates the adsorption potential of the adsorbent. Both AT and B can be determined from a plot qe vs. ln Ce (Fig. 10) and the constants were determined from the intercept and slope, respectively. The related parameters are presented in Table 1. Table 1 shows the parameters of the isotherms and the correlation coefficient (R2) for the fitting of the experimental data. In this study n values are greater than unity (smaller value of 1/n) indicating a strong interaction between adsorbate [Cr (VI)] and adsorbent (TWNP).

Table 1 Parameters of linearized Langmuir, Freundlich and Temkin isotherms for adsorption of chromium (VI) onto TWNP. T(K)

Fig. 8. Plot of RL vs initial Cr (VI) concentration.

298 308 318

Langmuir isotherm

Freundlich isotherm 2

b

Q max

R

0.156 0.263 0.371

36.32 42.36 59.88

0.98 0.96 0.98

Temkin isotherm 2

1/n

KA

R

0.4256 0.512 0.691

11.84 16.63 19.02

0.95 0.94 0.96

AT

B

R2

2.14 2.52 2.88

12.90 12.33 12.13

0.96 0.95 0.94

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M.H. Dehghani et al. / Journal of Molecular Liquids 215 (2016) 671–679 Table 2 Thermodynamic parameters of Cr (VI) on TWNP. T (K)

ΔG (kJ/mol)

ΔH (kJ/mol)

ΔS (kJ/mol·K)

298 308 318

−5.577 −6.476 −7.278

19.89

0.085

change (ΔG°) and entropy change (ΔS°) were estimated by the following Eq.:

Lnk∘ ¼

ΔS∘ ΔH ∘ − R RT

ð9Þ

Fig. 10. Temkin isotherm plots for removal of Cr (VI) by TWNP.

ΔG∘ ¼ −RT ln k∘ Langmuir constants, Q max and b are found to be 59.88 mg/g and 0.156 l/mg, respectively (Table 1). The results showed that Langmuir gave the best fit for the chromium (VI) adsorption by TWNP with R2 = 0.98. 3.6.4. Thermodynamic studies To determine whether the process is spontaneous and to observe the effect of temperature on adsorption of Cr (VI) onto TWNP, thermodynamic parameters such as enthalpy (ΔH°),the Gibbs free energy

ð10Þ

where ko is the thermodynamic equilibrium constant corresponding to the temperatures of 298, 308 and 318 K that is derived from plotting a straight line of ln (qe/Ce) vs. qe (Fig. 11) and extrapolating qeto zero, R is the universal gas constant (8.314 J/mol/K), T is the absolute temperature (K). The values of ΔH° and ΔS° (Table 2) were estimated from the slope and intercept of the linear plot of lnko vs. 1/T (Fig. 12). The calculated negative ΔG° values (Table 2) at all temperatures for TWNP confirmed that the adsorption process was feasible and spontaneous in nature and the magnitude of the Gibbs free energy change increased with the rising temperature. [24] calculated that ΔG° of Cu (II) adsorption on newspaper pulp was: − 22.15 kJ/mol, − 22.98 kJ/mol and − 23.81 kJ/mol at a given temperature (303 to 323 K). [35] also calculated the Gibbs free energy of Cr (VI) adsorption on newspapers was: − 1.981, − 4.162 and − 4.375 kJ/mol for the temperature of 30, 40 and 50 °C, respectively. The sign of the positive standard entropy change (ΔS°) value described the increased randomness at the TWNP–solution interface during the adsorption of chromium by the TWNP. The positive value of ΔH° (19.89 kJ/mol) for this study indicated that the interaction between Cr (VI) ion and TWNP surface is endothermic and might attribute to the deprotonation reaction and the diffusion process. The results of the thermodynamic parameters were shown in Table 2. 3.7. Comparison with other adsorbents The adsorption capacity of Cr (VI) onto TWNP was compared with other low cost adsorbents and is listed in Table 3. The results indicated that the maximum adsorption capacity (Qmax) at 30 °C and solution pH 3.0 obtained in this study is higher (59.88 mg/g) as compared with other low-cost adsorbents and comparable to those from such as activated carbon (F400) and tire-based activated carbon adsorbents.

Fig. 11. qe vs qe=C e plots for adsorption Cr (VI) onto TWNP.

Fig. 12. Plot of ln ks versus T−1 estimation of the activation energy of Ea for the adsorption of Cr (VI) onto TWNP.

M.H. Dehghani et al. / Journal of Molecular Liquids 215 (2016) 671–679

677

Table 3 Comparison of adsorption capacities of Cr(VI) with other adsorbents. Sl. no.

Adsorbents

Qmax (mg/g)

pH

References

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

Common fig (Ficus carica) Cactus leaves Rice straw Eucalyptus bark Tire activated carbon Wool Pine needles Sugar cane bagasse Maize cob Olive CAKE Activated carbon (F400) Gulmohar's fruit shell Almond shell Hazelnut shell Ground nut she Modified oak sawdust TWNP

28.90 7.08 3.15 45 58.5 41.15 21.50 13.4 13.8 33.44 53.2 12.28 3.40 8.28 5.88 1.7 59.88

3.5 2 2 2 2 2 2 2 2 2 2 3 3 3 3 3 3

[36] [37] [38] [39] [40] [37] [37] [41] [42] [37] [40] [43] [44] [44] [44] [45] This study

Fig. 13. Lagergren's plots for the adsorption of chromium (VI) at varying Cr (VI) concentrations.

where, k2 is the rate constant of pseudo-second-order kinetics (g/mg·min). Eq. (12) became 13 on integration with the boundary conditions t = 0 to t = t and qt = 0 to qt = qt

3.8. Adsorption kinetics Removal of heavy metals by adsorption onto porous adsorbents involves a number of steps, each of which can affect the overall adsorption kinetics. These are (1) bulk solution transport, (2) external (film) resistance to transport, (3) internal (intraparticle) transport, and (4) adsorption (this step is rapid for physical adsorption) [46]. The transport steps occur in the series, so the slowest step, called the rate-limiting step, will control the rate of the removal. The most important factor in adsorption system design correlates the adsorbate uptake rate with the bulk concentration of the adsorbate, adsorbate residence time and the reactor dimensions controlled by the system's kinetics. In this study, several kinetic models are used to describe the reaction order of adsorption of Cr (VI) on TWNP. First-order rate, second-order rate, pseudo-first-order kinetic and pseudo-second-order kinetic models are used for this study [47] but only pseudo-first-order kinetic and pseudo second-order kinetic models fitted the best. 3.8.1. Pseudo first-order kinetics As early as 1898 [48], Lagergren proposed a pseudo first-order equation to describe the kinetic process of liquid–solid phase adsorption based on the adsorption capacity of the adsorbent. In this study, it was assumed that one chromium ion was adsorbed onto one sorption site of TWNP surface. The linear form of Lagergren's pseudo-first-order model is generally expressed as follows:

Log ðqe −qt Þ ¼ Logqe −

k1 t 2:303

ð11Þ

1 1 ¼ þ k2 t: ðqe −qt Þ qe

ð13Þ

Eq. (13) can be rearranged to obtain linearized Eq. (14): t 1 t ¼ þ : qt k2 qe 2 qe

ð14Þ

The change in the amount of the adsorbed Cr(VI) with time was found to fit the pseudo-second order rate Eq. (14), and the intercepts and slopes of plots t/qt versus t were used to calculate the pseudo second-order rate constants k2 and qe, respectively (Fig. 14). The kinetic parameters acquired from the fitting results of Cr (VI) adsorption onto TWNP are given in Table 4. The decreasing first-order rate constant (k1) in Table 4 favors the adsorption of Cr (VI) onto TWNP at a lower concentration. Value of the pseudo-first-order constant k1 was found to decrease generally from 0.052 to 0.007 (1/min). It can also be seen from Table 4 that the pseudo-first-order adsorption capacities qe varied between 12.3 to 1.63 mg/g. These result showed clearly that k1 and qe are dependent of initial Cr (VI) concentrations. Table 4 also shows the kinetic parameters of pseudo second-order for Cr (VI) adsorption which were calculated from the slope and intercept of the linear plot of t/qt versus t (min) (Fig. 14). In this study, the calculated qe values were closer to the experimental qe values obtained using the pseudo-second-order as compared to those of the pseudo-first order kinetic (Table 4). This indicates that Cr (VI) adsorption system onto TWNP follows pseudo-second-order kinetic model. A comparison of Figs. 13 and 14 showed that pseudo-second-order was the best model for the

where, qe is the amount of Cr(VI) adsorbed onto the treated waste newspaper (TWNP) at equilibrium (mg/g), qt is the amount of Cr(VI) adsorbed onto TWNP at time t (min), k1 is the pseudo first-order rate constant for the kinetic model (1/min). Lagergren's plots for the adsorption of Cr (VI) at varying concentrations are given in Fig. 13. 3.8.2. Pseudo-second-order model Ho and McKay [49] described pseudo-second-order model as the kinetic process of the adsorption. The kinetics rate equation based on adsorption equilibrium capacity may be represented in the following form: dqt ¼ k2 ðqe −qt Þ2 dt

ð12Þ

Fig. 14. Pseudo-second-order kinetics plots for adsorption of chromium (VI) onto TWNP at varying Cr (VI) concentration (pH 3, adsorbent dosage level 0.4 g/L).

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Table 4 Kinetic parameters for the removal of Cr (VI) by TWNP. Initial concentration (mg/l)

5 20 50

qe(exp) (mg/g)

0.754 2.612 5.283

1st order

Pseudo 1st order model

Pseudo 2nd order model

k (1/min)

R2

k1 (1/min)

qe(cal) (mg/g)

R2

k2 (g/mg.min)

qe(cal) (mg/g)

R2

0.009 0.005 0.004

0.881 0.916 0.895

0.052 0.026 0.007

12.2 3.4 1.63

0.94 0.90 0.97

0.0019 0.0068 0.002

2.61 3.95 8.79

0.99 0.99 0.99

chromium (VI) removal onto TWNP with a higher correlation coefficient (R2 = 0.99) than for pseudo-first-order (R2 = 0.90 to 0.97).

The adsorption capacity of TWNP was decreased by 8% for removing Cr (VI) from aqueous solutions, after recovery of Cr (VI) ions from the adsorbent.

4. Regeneration of TWNP Acknowledgments The recovery of Cr (VI) from the adsorbent was performed using 0.01, 0.1 and 1.0 M HCl solution. Adsorbent dose of 0.4 g was loaded with 250 ml of 20 mg/l of chromium solution. The Cr (VI) was adsorbed by TWNP and desorption studies attempted to recover Cr (IV) from metal ion loaded adsorbent for above-mentioned concentration. The results show that 72% of the adsorbed Cr (VI) was desorbed from TWNP using 0.1 M HCl. During desorption studies, the TWNP surface was completely covered by H+ ions. It is evident from Fig 15, that the regeneration of TWNP resulted in the release of Cr (VI) ions from adsorbent's surface to the solution and regenerated TWNP can be reused for Cr (VI) removal from aqueous solution. 5. Conclusions In this study, TWNP which is an unavoidable waste material is used as an inexpensive adsorbent for the removal of hexavalent chromium from aqueous solutions. The studies indicated that TWNP is an effective, low-cost adsorbent for the removal of toxic Cr (VI) from aqueous solution. The results indicated that Cr(VI) removal rate increased with decreased initial Cr (VI) concentration and pH and with increased adsorbent dosage. It was found that maximum Cr(VI) adsorption capacity could be achieved to be 59.88 mg/g (64%) with an adsorbent dosage of 3.0 g/L and contact time of 60 min with an initial Cr(VI) concentration of 5 to 70 mg/L and optimum pH of 3.0. Langmuir isotherm was found better fitted with a high correlation coefficient (R2 = 0.98) than to Freundlich model with a correlation coefficient (R2 = 0.95). The rate of adsorption of Cr (VI) on the TWNP was found to fit better with pseudo-second-order kinetic model with a good correlation coefficient. TWNP can be considered as an effective, easily available, inexpensive and natural adsorbent for removing chromium (VI) from contaminant wastewater. Also, the desorption studies showed that adsorbent can be reused. On the basis of the results obtained, it can be concluded that 72% of Cr (VI) recovery can be achieved from TWNP using 0.1 M HCl.

Fig. 15. The Recovery Cr (VI) from TWNP after adsorption.

The authors wish to thank the school of public health, Tehran University of Medical Sciences (24577-46-03-92) for the support. Appendix A

B C0 CA Ce Ci Ct K kA k1 k2 m n qA qe qe(cal) qt Q max R2 RL T V

Langmuir equilibrium constant (l/mg) Initial chromium (VI) concentrations (mg/l) The equilibrium chromium (VI) concentration (mg/l) after adsorption in Freundlich equation The equilibrium chromium (VI) concentration (mg/l) after adsorption Initial chromium (VI) concentrations (mg/l) The chromium (VI) concentration after time (mg/l) First-order rate constant, (1/min) Freundlich adsorption capacity parameter, (mg/g) (L/mg) 1/n The Pseudo first-order rate constant for the kinetic model (1/min) The rate constant of pseudo-second-order adsorption (g/mg·min) The mass of adsorbent (g) Freundlich adsorption intensity constant (dimensionless) The amount Cr (VI) adsorbed (mg/g) onto the treated waste newspaper (TWNP) at Freundlich equation The amount Cr (VI) adsorbed (mg/g) onto the treated waste newspaper (TWNP) at equilibrium The calculated value of the equilibrium adsorbate solid concentration in the solid phase (mg/g) The amount Cr (VI) adsorbed (mg/g) onto the treated waste newspaper (TWNP) at time (min) Maximum adsorption capacity of the adsorbent (mg/g) Correlation coefficient Separation factor Time (min) Volume of solution (l)

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