Journal of Molecular Liquids 215 (2016) 671–679

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Journal of Molecular Liquids

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Removal of chromium(VI) from aqueous solution using treated wastenewspaper as a low-cost adsorbent: Kinetic modeling andisotherm 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 Iranb Institute for Environmental Research, Center for Solid Waste Research, Tehran, Islamic Republic of Iranc Department of Chemistry, Jamia Millia Islamia (Central University), New Delhi 110025, Indiad Department of Environmental Science, University of Eastern Finland, P.O. Box 1627, FI 70211, Kuopio, Finland

⁎ Corresponding author.E-mail addresses: hdehghani@tums.ac.ir (M.H. Dehgh

(D. Sanaei).

http://dx.doi.org/10.1016/j.molliq.2015.12.0570167-7322/© 2016 The Authors. Published by Elsevier B.V

a b s t r a c t

a r t i c l e i n f o

Article history:Received 6 August 2015Received in revised form 30 November 2015Accepted 17 December 2015Available online xxxx

In the present study, treated waste newspaper (TWNP) was used to remove chromium(VI) from aqueous solu-tion 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 fittedwell to Langmuir isotherm (R2 = 0. 98; maximum adsorption capacity 59.88 mg/g.) and pseudo-second-orderkinetic model. The rate constant k2 varied from 0.0019 to 0.0068 at initial Cr (VI) concentration from 5 to20 mg/L. It was observed that adsorption of Cr(VI) was pH dependent. The percentage removal of Cr(VI) was59.88 mg/g (64% at pH 3). The results of the present study suggest that TWNPmay be used as a low-cost adsor-bent 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/).

Keywords:Low-cost adsorbentChromium (VI)Adsorption isothermsAdsorption kineticsTreated waste newspaper

1. Introduction

Discharge of untreated or poorly treated wastewater containingtoxic heavy metals such as Cr, Ni, Cd, Pb, Hg, Zn, Co and Cu from indus-trial effluents into the natural water bodies is a major environmentalproblembecause of their high toxicity and their tendency to accumulatethrough the food chain [1].Chromium is one of the most notoriousheavymetals released by various industries such as tanning and leatherindustries, manufacturing industries, catalyst and pigments, fungicides,ceramics, crafts, glass, photography, electroplating industry and corro-sion control application [1,2]. Chromium forms three common oxida-tion states in its compounds, +2, +3, and +6. The +3 and +6oxidation states are the most commonly observed in chromium com-pounds, whereas +1, +4, and +5 states are rare. The most prominentexample of toxic chromium is hexavalent chromium (Cr(VI) [2,3]. Inter-national Agency for Research on Cancer (IARC) has classified chromium(VI) in Group 1 (carcinogenic to humans) and metallic chromium andchromium (III) in Group 3 (not classifiable as to their carcinogenicityto humans [3]. Therefore, the removal of chromium (VI) from

ani), daryss2572@gmail.com

. This is an open access article under

wastewater is extremely important before its discharge into the aquaticsystem, which needs immediate attention. Conventional treatmenttechnologies have been developed to remove Cr(VI) from water andwastewater, including reduction followed by chemical precipitation[4], ion exchange [5,6], membrane separation [7], electrocoagulation[8], nanoparticles [9], dialysis/electrodialysis [10], and adsorption/filtra-tion [11]. Capital and operational costs often limit efficiency and the ef-fectiveness of these methods, principally, when large volumes ofeffluents contain relatively low concentrations [11].

In contrast, the adsorption technique is a highly effectivemethod be-cause it is a simple and cost effective method for recovering and elimi-nating heavy metal ions from dilute solutions [15–19].

Recently, a variety of cheapmaterials have been examined as adsor-bents for the removal of Cr (VI) from aqueous solution with the aim offinding cheaper alternatives for conventional sorbent materials suchas activated carbon which is an expensive adsorbent. Some of the low-cost adsorbents include anaerobic sludge [12], apple residue [13], saw-dust [14], rice Polish [15], clay [16], zeolite [17], fly ash [18], chitosan[19], waste tea [20,21], seaweeds [22], and polyaniline coated on saw-dust [23] which have been used for the purpose.

Papers (old newspaper, old magazine, printed papers and mixedofficewaste paper) are complexmaterials and principally consist of cel-lulose, which contains functional polar groups such as alcohols andethers. These functional groups can be protonated at lower pH and,

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

thus, bind Cr (VI) by means of electrostatic interactions. The treatednewspaper pulp was employed by Chakravarty et al. to remove zincfrom aqueous solutions [24].

In the present study, the attempts have been made to remove chro-mium (VI) ion from aqueous solution using treated waste newspaperas a low-cost adsorbent. Besides, the kinetic modeling and isothermstudies are also presented.

2. Materials and methods

2.1. Preparation of treated waste newspaper pulp

The waste newspaper was cut into piece (2 cm × 2 cm) strips usinga paper shredding machine. It was treated with concentrated sodiumbicarbonate solution for removing foreign materials such as grease,black ink and bleaching material (chlorine dioxide); which areusually present in the newspaper. The waste newspaper pulp wasthen refluxed with 5.0% Na2HPO4 using a water condenser for 3 hto impregnate the phosphate into the cellulose matrix. After phos-phorylation, the solution was cooled and passed through Whatman40 filter paper. The characterization of the newspaper pulp is givenelsewhere [25].

2.2. Characterization of adsorbent

The surface area of the waste newspaper (WNP) and treatedwaste 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 volumeof the samples were calculated from the volume of adsorbed N2 atP/P0 = 0.99. The samples were preheated at 200 °C for 4 h at vacuumcondition (6.67 Pa) to clean the surface before the measurements. Ac-cording to the SEM analysis of WNP and TWNP, the microstructures ofnewspapers consist of fibers with agglomerated fine particles fillingthe spaces between the fibers. The SBET values of WNP ranged from885 to 1020 m2 g−1. The specific surface area (BET) of TWNP (bychemical analysis confirming impregnation of phosphorous duringthe chemical treatment) was ranged from 1214 to 1652 m2 g−1.The pore volume (VP) values for WNP and TWNP were 0.98 and1.01 ml g−1, respectively. Also, the moisture content in WNP andTWNP was 7.68% and 6.82%.

2.3. Reagents

In this study, the stock solution of Cr(VI) was prepared by dissolvinga known quantity of potassium dichromate (K2Cr2O7) in deionizedwater. Stock solutionwas further diluted to obtain the required concen-trations of Cr(VI) solutions. 1.0 N NaOH and 1.0 N HCl were used for pHvalue adjustments.

2.4. Batch adsorption experiments

The stock solution of Cr (VI) was prepared by dissolving 0.1414 gof K2Cr2O7 in double distilled water and diluted to 100 ml. In all thebatch adsorption studies, solutions of 5 to 70 mg/L concentrationswere used. The required amount of the adsorbent was added to250 mL glass stoppered conical flasks containing 100 mL of aqueousCr (VI) ion solution. The contents of the flask were shaken in a me-chanical shaker by continuous mixing with a constant agitationspeed of 120 rpm at room temperature for a definite period. Aknown volume of the solution was removed and centrifuged for Cr(VI) analysis.

The batch adsorption experiments were carried out at room tem-perature at different contact times (20 to 150 min), initial concentra-tion of chromium ion (5 to 70 mg/L), TWNP dose (0.4 to 4 g/100 mL)

and pH 2 to 7. The chromium (VI) percent removal (%)was calculatedas follows:

% Removal of Cr VIð Þ ¼ Ci−Ce

Ci� 100: ð1Þ

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

qe ¼Ci−Ce

m� V 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 dos-age (g).

The adsorption isotherm studies were carried out by varyingthe 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 andLangmuir isotherm models. The experiments of batch kinetic adsorp-tion by TWNP was carried out by mixing 0.4 g of TWNP with 100 mLCr(VI) solution at three initial chromium concentrations (5, 20 and50 mg/L), at pH 3 and contact time (0–60 min). The data was fitted tothe first order, pseudo-first-order and pseudo-second-order models.All the experiments were repeated five times and average values arereported. 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 groupswere ascertained by Fourier transforminfrared spectroscopy (FTIR) in the Treated Waste Newspaper (TWNP)before and after Cr (VI) adsorption (Figs. 1 and 2). The FTIR spectra ofWNP and TWNP were recorded in the range of 400 and 4000 cm−1 inFTIR Spectrum 400 (Perkin Elmer). Characteristic cellulose peak in theregion 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 celluloseand the band near 1318 cm−1 corresponded to CH2-wagging vibrationsin the cellulose. The band near 1351 cm−1 represented the –OH vibra-tions. The band near 3698 cm−1 is split into two less intense peaks inTWNP due to the change in intra-molecular hydrogen bonding interac-tions. There was a band appearing in TWNP at 1033 cm−1 describingthe aliphatic P–O stretching [26].

3.2. Effect of solution pH on adsorption

Solution pH in the adsorption is considered as one of the importantadsorption characteristics that affect the adsorption behavior of metalions in aqueous solution. The pH dependence of metal adsorption islargely related to the surface functional groups in the biosorbents andmetal solution chemistry [27].

The effect of pH is shown in Fig. 3 in the pH range of 2.0–7.0. Itis clear from this figure that the optimum pH required for maximumadsorption of Cr (VI) onto TWNP was 3.0. It was also observed that byincreasing the pH value, a drastic decrease in Cr (VI) adsorption per-centage was observed. This might be due to theweakening of the inter-actions between the oppositely charged TWNP and Cr(VI); leading tothe reduction in sorption capacity [28].

Newspapermaterials contain functional polar groups such as alcoholand ethers. At low pH, these functional polar groups were protonatedand, therefore, the surface of the adsorbent becomes positively charged.Moreover, due to impregnation of phosphorous in cellulose matrix, theactive functional group might be phosphate. This further corroborated

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

673M.H. Dehghani et al. / Journal of Molecular Liquids 215 (2016) 671–679

the presence of net negative charge on the surface of the cellulose ma-trix. HCrO4

− is the dominant form of Cr (VI) over the range of pH 3.0,while CrO4

2− is dominant in the range of pH N 7. The TWNP surface con-tains a large number of active site [−OH]which is associatedwith intra-molecular hydrogen bond. At low pH concentration of OH− decreases,The surface charge of the adsorbent thus changes to positive chargedsites [29].

Fig. 2. FTIR spectra of TWNP

3.3. Effect of contact time

The effect of contact timewas studied for an initial Cr (VI) concentra-tion of 20 mg/L; TWNP dosage of 0.4 g/100 mL; solution pH of 3.0 atroom temperature. Fig. 4 shows that adsorption rate of chromium (VI)increased with contact time until equilibrium was reached. The timeto reach equilibrium for chromium (VI) adsorption was 60 min. The

after Cr(VI) adsorption.

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

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

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rate of chromium (VI) adsorption on the TWNP was higher within thefirst 60 min due to the availability of plenty of sorption sites at thesorbent and a high concentration gradient. After the active sites of theadsorbent get exhausted (saturated), when equilibrium is attained,the sorption became slow in the later stages [30]. A contact time of120 min was chosen to ensure that adsorption equilibrium is achievedin all cases.

3.4. Effect of initial Cr (VI) concentration

The experiments were done with variable initial chromium con-centration (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 inFig. 5. Fig. 5 shows that by increasing initial chromium (VI) concen-tration, chromium removal efficiency is also increased and remainednearly constant after equilibrium time. Thus, results suggest that ad-sorption capacity of TWNP is dependent on the initial concentrationof chromium. This can be attributed to the saturation of sorptionsites on adsorbents. The initial concentration provides a significantdriving force to overcome all mass transfer resistance of metal ionsbetween aqueous and solid phases [31].

3.5. Effect of adsorbent dose

The effect of adsorbent dose (0.4, 0.8, 1, 2, 3 and 4 g/L) is shown inFig. 6. The other parameters used were pH (3) and contact time(60min), initial concentration (5mg/l)) and room temperature. It is ap-parent that the adsorbed chromium ion amount per unit weight ofadsorbent (qe) decreased as the adsorbent concentration increased(Fig. 6). This result was due to the aggregates formed with increasingadsorbent dose, which reduced the effective adsorption [32].

The more amounts of adsorbent results in higher surface area andadsorption regions which causes enhanced removal of chromium (VI).

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

Evidently chromium (VI) removal efficiency increased with an adsor-bent dose of 0.4 to 3 g/100 mL (Fig. 6), and then, it is not increased sosignificantly due to the occurrence of aggregation. This is likely due tothe equilibrium concentration of the Cr (VI) in solution was lower inthe presence of high adsorbent concentrations. An optimum dose of0.4 g/100 ml is selected for all the experiments.

3.6. Adsorption isotherms

The equilibrium adsorption of Cr (VI) ions on the TWNP was ana-lyzed using adsorption isotherms as discussed below.

3.6.1. Langmuir isothermThe experimental data were fitted to the Langmuir equation:

Ceqe

¼ 1Q max:b

þ CeQ max

ð3Þ

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

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

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.

675M.H. Dehghani et al. / Journal of Molecular Liquids 215 (2016) 671–679

Qmax is the maximum adsorption capacity, and b is the Langmuir equi-librium constant (l/mg) which shows quantitatively the affinity be-tween Cr (VI) and TWNP. Fig. 7 shows Cr (VI) Langmuir adsorptionisotherm plots of Cr (VI) on TWNP.

A further analysis of the Langmuir model and also the affinity be-tween Cr(VI) and TWNP adsorbent can be predicted using the Langmuirparameter b from the dimensionless constant separation factor RL,which is defined by the following relationship [33]:

RL ¼ 11þ bC0

ð4Þ

where, C0 is the initial Cr (VI) concentration (mg/L) and b is Langmuirisotherm constant. The value of RL indicates information as to whetherthe adsorption may be described as follows:

RL N 1 unfavorableRL = 1 Linear0 b RL b 1 favorableRL = 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 be0.303, 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 adsorp-tion of chromium from wastewater under the conditions used in thisstudy.

3.6.2. Freundlich isothermFreundlich adsorption isotherm [34] is an empirical equation

employed to describe the data for heterogeneous adsorbents. Freundlichadsorption equation takes the following general form:

qA ¼ KA CA1=n ð5Þ

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

The linear form is as follows:

Log qAð Þ ¼ Log KAð Þ þ 1n

� �Log CAð Þ 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 Langmuirisotherm, Freundlich isotherm and R2 values for adsorption of Cr (VI)onto TWNP are given in Table 1.

3.6.3. Temkin isothermTemkin isotherm is the model describing the effects of indirect

adsorption interaction and adsorption substances on adsorption iso-therms. Temkin [50] assume that the heat of adsorption decreaseslinearly with increasing coverage and the adsorption is characterizedby a uniform distribution of binding energies. The Temkin isothermhas a convenient linear form, which is expressed by the followingequation:

qe ¼ B ln AT þ B ln Ce ð7Þ

B ¼ RTb

ð8Þ

where, AT is Temkin isotherm equilibriumbinding constant correspond-ing to the maximum binding energy (L/g), B is constantly related to theheat of sorption (J/mol), R is the universal gas constant (8.314 J/mol/K), Tis absolute temperature at 298 K°, b is Temkin isotherm constant, whichindicates the adsorption potential of the adsorbent. Both AT and B can bedetermined from a plot qe vs. ln Ce (Fig. 10) and the constants were de-termined from the intercept and slope, respectively. The related param-eters are presented in Table 1.

Table 1 shows the parameters of the isotherms and the correlationcoefficient (R2) for the fitting of the experimental data. In this study nvalues are greater than unity (smaller value of 1/n) indicating a stronginteraction between adsorbate [Cr (VI)] and adsorbent (TWNP).

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

T(K) Langmuir isotherm Freundlich isotherm Temkin isotherm

b Qmax R2 1/n KA R2 AT B R2

298 0.156 36.32 0.98 0.4256 11.84 0.95 2.14 12.90 0.96308 0.263 42.36 0.96 0.512 16.63 0.94 2.52 12.33 0.95318 0.371 59.88 0.98 0.691 19.02 0.96 2.88 12.13 0.94

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

Table 2Thermodynamic parameters of Cr (VI) on TWNP.

T (K) ΔG (kJ/mol) ΔH (kJ/mol) ΔS (kJ/mol·K)

298 −5.577 19.89 0.085308 −6.476318 −7.278

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

Langmuir constants, Qmax and b are found to be 59.88 mg/g and0.156 l/mg, respectively (Table 1). The results showed that Langmuirgave the best fit for the chromium (VI) adsorption by TWNP withR2 = 0.98.

3.6.4. Thermodynamic studiesTo determine whether the process is spontaneous and to observe

the effect of temperature on adsorption of Cr (VI) onto TWNP, thermo-dynamic parameters such as enthalpy (ΔH°),the Gibbs free energy

Fig. 11. qe vs qe=Ceplots for adsorption Cr (VI) onto TWNP.

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

Lnk∘ ¼ ΔS∘

R−

ΔH∘

RTð9Þ

ΔG∘ ¼ −RT ln k∘ ð10Þ

where ko is the thermodynamic equilibrium constant correspondingto the temperatures of 298, 308 and 318 K that is derived fromplotting a straight line of ln (qe/Ce) vs. qe (Fig. 11) and extrapolatingqeto zero, R is the universal gas constant (8.314 J/mol/K), T is theabsolute temperature (K). The values of ΔH° and ΔS° (Table 2) wereestimated 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 forTWNP confirmed that the adsorption process was feasible and sponta-neous in nature and the magnitude of the Gibbs free energy change in-creased with the rising temperature. [24] calculated that ΔG° of Cu (II)adsorption on newspaper pulp was: −22.15 kJ/mol, −22.98 kJ/moland −23.81 kJ/mol at a given temperature (303 to 323 K). [35] alsocalculated the Gibbs free energy of Cr (VI) adsorption on newspaperswas: −1.981, −4.162 and −4.375 kJ/mol for the temperature of30, 40 and 50 °C, respectively. The sign of the positive standard entro-py change (ΔS°) value described the increased randomness at theTWNP–solution interface during the adsorption of chromium by theTWNP.

The positive value ofΔH° (19.89 kJ/mol) for this study indicated thatthe interaction between Cr (VI) ion and TWNP surface is endothermicandmight attribute to the deprotonation reaction and the diffusion pro-cess. The results of the thermodynamic parameters were shown inTable 2.

3.7. Comparison with other adsorbents

The adsorption capacity of Cr (VI) onto TWNP was compared withother low cost adsorbents and is listed in Table 3. The results indicatedthat the maximum adsorption capacity (Qmax) at 30 °C and solutionpH 3.0 obtained in this study is higher (59.88 mg/g) as compared withother low-cost adsorbents and comparable to those from such as acti-vated carbon (F400) and tire-based activated carbon adsorbents.

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

Table 3Comparison of adsorption capacities of Cr(VI) with other adsorbents.

Sl. no. Adsorbents Qmax (mg/g) pH References

1. Common fig (Ficus carica) 28.90 3.5 [36]2. Cactus leaves 7.08 2 [37]3. Rice straw 3.15 2 [38]4. Eucalyptus bark 45 2 [39]5. Tire activated carbon 58.5 2 [40]6. Wool 41.15 2 [37]7. Pine needles 21.50 2 [37]8. Sugar cane bagasse 13.4 2 [41]9. Maize cob 13.8 2 [42]10. Olive CAKE 33.44 2 [37]11. Activated carbon (F400) 53.2 2 [40]12. Gulmohar's fruit shell 12.28 3 [43]13. Almond shell 3.40 3 [44]14. Hazelnut shell 8.28 3 [44]15. Ground nut she 5.88 3 [44]16. Modified oak sawdust 1.7 3 [45]17. TWNP 59.88 3 This study

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

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

677M.H. Dehghani et al. / Journal of Molecular Liquids 215 (2016) 671–679

3.8. Adsorption kinetics

Removal of heavy metals by adsorption onto porous adsorbentsinvolves a number of steps, each of which can affect the overall ad-sorption 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 therate-limiting step, will control the rate of the removal. The most im-portant factor in adsorption system design correlates the adsorbateuptake rate with the bulk concentration of the adsorbate, adsorbateresidence time and the reactor dimensions controlled by the system'skinetics.

In this study, several kineticmodels are used to describe the reactionorder of adsorption of Cr (VI) on TWNP. First-order rate, second-orderrate, pseudo-first-order kinetic and pseudo-second-order kineticmodels are used for this study [47] but only pseudo-first-order kineticand pseudo second-order kinetic models fitted the best.

3.8.1. Pseudo first-order kineticsAs early as 1898 [48], Lagergren proposed a pseudo first-order equa-

tion to describe the kinetic process of liquid–solid phase adsorptionbased on the adsorption capacity of the adsorbent. In this study, it wasassumed that one chromium ion was adsorbed onto one sorption siteof TWNP surface. The linear form of Lagergren's pseudo-first-ordermodel is generally expressed as follows:

Log qe−qtð Þ ¼ Logqe−k1t

2:303ð11Þ

where, qe is the amount of Cr(VI) adsorbed onto the treated wastenewspaper (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 rateconstant for the kinetic model (1/min).

Lagergren's plots for the adsorption of Cr (VI) at varying concentra-tions are given in Fig. 13.

3.8.2. Pseudo-second-order modelHo and McKay [49] described pseudo-second-order model as the

kinetic process of the adsorption. The kinetics rate equation based onadsorption equilibrium capacity may be represented in the followingform:

dqtdt

¼ k2 qe−qtð Þ2 ð12Þ

where, k2 is the rate constant of pseudo-second-order kinetics(g/mg·min).

Eq. (12) became 13 on integration with the boundary conditionst = 0 to t = t and qt = 0 to qt = qt

1qe−qtð Þ ¼

1qe

þ k2t: ð13Þ

Eq. (13) can be rearranged to obtain linearized Eq. (14):

tqt

¼ 1k2qe2

þ tqe

: ð14Þ

The change in the amount of the adsorbed Cr(VI) with time wasfound to fit the pseudo-second order rate Eq. (14), and the interceptsand slopes of plots t/qt versus t were used to calculate the pseudosecond-order rate constants k2 and qe, respectively (Fig. 14).

The kinetic parameters acquired from thefitting results of Cr (VI) ad-sorption onto TWNP are given in Table 4. The decreasing first-order rateconstant (k1) in Table 4 favors the adsorption of Cr (VI) onto TWNP at alower concentration. Value of the pseudo-first-order constant k1 wasfound to decrease generally from 0.052 to 0.007 (1/min). It can also beseen from Table 4 that the pseudo-first-order adsorption capacities qevaried between 12.3 to 1.63 mg/g. These result showed clearly that k1and qe are dependent of initial Cr (VI) concentrations. Table 4 alsoshows the kinetic parameters of pseudo second-order for Cr (VI) ad-sorption which were calculated from the slope and intercept of thelinear plot of t/qt versus t (min) (Fig. 14). In this study, the calculatedqe values were closer to the experimental qe values obtained using thepseudo-second-order as compared to those of the pseudo-first order ki-netic (Table 4). This indicates that Cr (VI) adsorption systemonto TWNPfollows pseudo-second-order kinetic model. A comparison of Figs. 13and 14 showed that pseudo-second-order was the best model for the

BCC

CCCKkkkmnq

q

q

q

QRRT

Table 4Kinetic parameters for the removal of Cr (VI) by TWNP.

Initial concentration (mg/l) qe(exp) (mg/g) 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

5 0.754 0.009 0.881 0.052 12.2 0.94 0.0019 2.61 0.9920 2.612 0.005 0.916 0.026 3.4 0.90 0.0068 3.95 0.9950 5.283 0.004 0.895 0.007 1.63 0.97 0.002 8.79 0.99

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

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

4. Regeneration of TWNP

The recovery of Cr (VI) from the adsorbent was performed using0.01, 0.1 and 1.0 M HCl solution. Adsorbent dose of 0.4 g was loadedwith 250ml of 20mg/l of chromium solution. The Cr (VI) was adsorbedby TWNP and desorption studies attempted to recover Cr (IV) frommetal ion loaded adsorbent for above-mentioned concentration. The re-sults show that 72% of the adsorbed Cr (VI) was desorbed from TWNPusing 0.1 M HCl. During desorption studies, the TWNP surface wascompletely covered by H+ ions. It is evident from Fig 15, that the regen-eration of TWNP resulted in the release of Cr (VI) ions from adsorbent'ssurface 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 usedas an inexpensive adsorbent for the removal of hexavalent chromiumfrom aqueous solutions. The studies indicated that TWNP is an effective,low-cost adsorbent for the removal of toxic Cr (VI) from aqueous solu-tion. The results indicated that Cr(VI) removal rate increased with de-creased initial Cr (VI) concentration and pH and with increasedadsorbent dosage. It was found that maximumCr(VI) adsorption capac-ity could be achieved to be 59.88 mg/g (64%) with an adsorbent dosageof 3.0 g/L and contact time of 60minwith an initial Cr(VI) concentrationof 5 to 70 mg/L and optimum pH of 3.0. Langmuir isotherm was foundbetter fitted with a high correlation coefficient (R2 = 0.98) than toFreundlich model with a correlation coefficient (R2 = 0.95). The rateof adsorption of Cr (VI) on the TWNP was found to fit better withpseudo-second-order kinetic model with a good correlation coefficient.TWNP can be considered as an effective, easily available, inexpensiveand natural adsorbent for removing chromium (VI) from contaminantwastewater.

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

Acknowledgments

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

Appendix A

Langmuir equilibrium constant (l/mg)

0 Initial chromium (VI) concentrations (mg/l) A The equilibrium chromium (VI) concentration (mg/l) after adsorption in

Freundlich equation

e The equilibrium chromium (VI) concentration (mg/l) after adsorption i Initial chromium (VI) concentrations (mg/l) t The chromium (VI) concentration after time (mg/l)

First-order rate constant, (1/min)

A Freundlich adsorption capacity parameter, (mg/g) (L/mg) 1/n 1 The Pseudo first-order rate constant for the kinetic model (1/min) 2 The rate constant of pseudo-second-order adsorption (g/mg·min)

The mass of adsorbent (g)

Freundlich adsorption intensity constant (dimensionless)

A

The amount Cr (VI) adsorbed (mg/g) onto the treated waste newspaper(TWNP) at Freundlich equation

e

The amount Cr (VI) adsorbed (mg/g) onto the treated waste newspaper(TWNP) at equilibrium

e(cal)

The calculated value of the equilibrium adsorbate solid concentration inthe solid phase (mg/g)

t

The amount Cr (VI) adsorbed (mg/g) onto the treated waste newspaper(TWNP) at time (min)

max

Maximum adsorption capacity of the adsorbent (mg/g) 2 Correlation coefficient L Separation factor

Time (min)

Volume of solution (l) V

References

[1] P.C. Grevatt, Toxicological Review of Hexavalent Chromium. Support of SummaryInformation on the Integrated Risk Information System (IRIS), US EnvironmentalProtection Agency, Washington DC, US, 1998.

[2] M.H. Dehghani, M. Mohammadtaher, A.K. Bajpai, B. Heibati, I. Tyagi, M. Asif, S.Agarwal, V.K. Gupta, Removal of Noxious Cr (VI) Ions Using Single-walled CarbonNanotubes and Multi-walled Carbon Nanotubes, 279 (2015) (2015) 344–352.

[3] Organization, W.H., I.W.G.o.t.E.o.C.R.t. Humans, IARC Monographs on the Evaluationof Carcinogenic Risks to Humans: Schistosomes, Liver Flukes and Helicobacter pylori,International Agency for Research on Cancer, 1994.

[4] D. Park, et al., How to study Cr (VI) biosorption: use of fermentation waste fordetoxifying Cr (VI) in aqueous solution, Chem. Eng. J. 136 (2) (2008) 173–179.

[5] D. Petruzzelli, R. Passino, G. Tiravanti, Ion exchange process for chromium removaland recovery from tannery wastes, Ind. Eng. Chem. Res. 34 (8) (1995) 2612–2617.

[6] S. Rengaraj, K.-H. Yeon, S.-H. Moon, Removal of chromium from water and waste-water by ion exchange resins, J. Hazard. Mater. 87 (1) (2001) 273–287.

[7] C.A. Kozlowski, W. Walkowiak, Removal of chromium (VI) from aqueous solutionsby polymer inclusion membranes, Water Res. 36 (19) (2002) 4870–4876.

[8] F. Akbal, S. Camcı, Copper, chromium and nickel removal from metal plating waste-water by electrocoagulation, Desalination 269 (1) (2011) 214–222.

[9] A. Assadi, M.H. Dehghani, N. Rastkari, S. Nasseri, A.H. Mahvi, Photocatalytic reduc-tion of hexavalent chromium in aqueous solution with zinc oxide nanoparticlesand hydrogen peroxide, Environ. Prot. Eng. 38 (4) (2012) 5–16.

[10] T. Mohammadi, et al., Modeling of metal ion removal from wastewater by electro-dialysis, Sep. Purif. Technol. 41 (1) (2005) 73–82.

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

[11] S. Babel, T.A. Kurniawan, Low-cost adsorbents for heavy metals uptake from con-taminated water: a review, J. Hazard. Mater. 97 (1) (2003) 219–243.

[12] M. Ulmanu, et al., Removal of Copper and Cadmium Ions From Diluted AqueousSolutions by low Cost and Waste Material Adsorbents, Water Air Soil Pollut. 142(1–4) (2003) 357–373.

[13] S.H. Lee, et al., Removal of heavy metals from aqueous solution by apple residues,Process Biochem. 33 (2) (1998) 205–211.

[14] S.S. Shukla, et al., Removal of nickel from aqueous solutions by sawdust, J. Hazard.Mater. 121 (1) (2005) 243–246.

[15] K. Singh, R. Rastogi, S. Hasan, Removal of cadmium from wastewater using agricul-tural waste ‘rice Polish’, J. Hazard. Mater. 121 (1) (2005) 51–58.

[16] H. Farrah,W.F. Pickering, The sorption of lead and cadmium species by clayminerals,Aust. J. Chem. 30 (7) (1977) 1417–1422.

[17] A. El-Kamash, A. Zaki, M.A. El Geleel, Modeling batch kinetics and thermodynamicsof zinc and cadmium ions removal from waste solutions using synthetic zeolite A,J. Hazard. Mater. 127 (1) (2005) 211–220.

[18] Z. Al-Qodah, Biosorption of heavy metal ions from aqueous solutions by activatedsludge, Desalination 196 (1) (2006) 164–176.

[19] I. Jha, L. Iyengar, A.P. Rao, Removal of cadmium using chitosan, J. Environ. Eng. 114(4) (1988) 962–974.

[20] Y. Orhan, H. Büyükgüngör, The removal of heavy metals by using agriculturalwastes, Water Sci. Technol. 28 (2) (1993) 247–255.

[21] S. Ahluwalia, D. Goyal, Removal of heavy metals by waste tea leaves from aqueoussolution, Eng. Life Sci. 5 (2) (2005) 158–162.

[22] A.C.A. Da Costa, F.P. De França, Cadmium uptake by biosorbent seaweeds: adsorp-tion isotherms and some process conditions, Sep. Sci. Technol. 31 (17) (1996)2373–2393.

[23] M. Mansour, M. Ossman, H. Farag, Removal of Cd (II) ion from waste water by ad-sorption onto polyaniline coated on sawdust, Desalination 272 (1) (2011) 301–305.

[24] S. Chakravarty, et al., Removal of copper from aqueous solution using newspaperpulp as an adsorbent, J. Hazard. Mater. 159 (2) (2008) 396–403.

[25] S. Chakravarty, et al., Adsorption of zinc from aqueous solution using chemicallytreated newspaper pulp, Bioresour. Technol. 98 (16) (2007) 3136–3141.

[26] S. Majumdar, B. Adhikari, Polyvinyl alcohol-cellulose composite: a taste sensingmaterial, Bull. Mater. Sci. 28 (7) (2005) 703–712.

[27] P. Chen, et al., Interaction of hydrogen with metal nitrides and imides, Nature 420(6913) (2002) 302–304.

[28] S.S. Baral, S.N. Das, P. Rath, Hexavalent chromium removal from aqueous solution byadsorption on treated sawdust, Biochem. Eng. J. 31 (3) (2006) 216–222.

[29] E. Demirbas, et al., Adsorption Kinetics for the Removal of Chromium (VI) FromAqueous Solutions on the Activated Carbons Prepared From Agricultural Wastes,Water SA 30 (4) (2004) 533–539.

[30] A. Verma, S. Chakraborty, J. Basu, Adsorption study of hexavalent chromium usingtamarind hull-based adsorbents, Sep. Purif. Technol. 50 (3) (2006) 336–341.

[31] G. Dönmez, Z. Aksu, Removal of chromium (VI) from saline wastewaters byDunaliella species, Process Biochem. 38 (5) (2002) 751–762.

[32] A. Esposito, et al., Biosorption of heavy metals by Sphaerotilus natans: an equilibriumstudy at different pH and biomass concentrations, Hydrometallurgy 60 (2) (2001)129–141.

[33] T. Weber, R. Chakraborti, J. Am Inst Chem Eng, 20, 228. Direct Link: Abstract PDF(1066 K) References Web of Science® Times Cited, 1974 522.

[34] H. Freundlich, Over the adsorption in solution, J. Phys. Chem. 57 (385471) (1906)1100–1107.

[35] X.S. Wang, Z.Z. Li, Removal of Cr (VI) from aqueous solution by newspapers, Desali-nation 249 (1) (2009) 175–181.

[36] V.K. Gupta, et al., Remediation of noxious chromium (VI) utilizing acrylic acidgrafted lignocellulosic adsorbent, J. Mol. Liq. 177 (2013) 343–352.

[37] M.Dakiky, et al., Selective adsorption of chromium(VI) in industrialwastewater usinglow-cost abundantly available adsorbents, Adv. Environ. Res. 6 (4) (2002) 533–540.

[38] H. Gao, et al., Characterization of Cr (VI) removal from aqueous solutions by a sur-plus agricultural waste–rice straw, J. Hazard. Mater. 150 (2) (2008) 446–452.

[39] V. Sarin, K.K. Pant, Removal of chromium from industrial waste by using eucalyptusbark, Bioresour. Technol. 97 (1) (2006) 15–20.

[40] M. Kobya, Adsorption, kinetic and equilibrium studies of Cr (VI) by hazelnut shellactivated carbon, Adsorpt. Sci. Technol. 22 (1) (2004) 51–64.

[41] D. Sharma, C. Forster, A preliminary examination into the adsorption of hexavalentchromium using low-cost adsorbents, Bioresour. Technol. 47 (3) (1994) 257–264.

[42] H.S. Altundogan, et al., The use of sulphuric acid-carbonization products of sugarbeet pulp in Cr (VI) removal, J. Hazard. Mater. 144 (1) (2007) 255–264.

[43] A.G.D. Prasad, M.A. Abdullah, Biosorption of Cr (VI) from synthetic wastewater usingthe fruit shell of gulmohar (Delonix regia): application to electroplating wastewater,Bioresources 5 (2) (2010) 838–853.

[44] E. Pehlivan, T. Altun, Biosorption of chromium (VI) ion from aqueous solutions usingwalnut, hazelnut and almond shell, J. Hazard. Mater. 155 (1) (2008) 378–384.

[45] M.E. Argun, et al., Heavy metal adsorption by modified oak sawdust: thermody-namics and kinetics, J. Hazard. Mater. 141 (1) (2007) 77–85.

[46] A. Adamson, A. Gast, Physical Chemistry of Surface, Wiley, N. Y., 1997 368.[47] Y.-S. Ho, Review of second-order models for adsorption systems, J. Hazard. Mater.

136 (3) (2006) 681–689.[48] S. Lagergren, About the theory of so-called adsorption of soluble substances, Kungl.

Sven. Vetenskapsakademiens Handl. 24 (4) (1898) 1–39.[49] Y. Ho, G. McKay, A comparison of chemisorption kinetic models applied to pollutant

removal on various sorbents, Process Saf. Environ. Prot. 76 (4) (1998) 332–340.