Journal of Molecular Liquids 202 (2015) 52–61

Contents lists available at ScienceDirect

Journal of Molecular Liquids

j ourna l homepage: www.e lsev ie r .com/ locate /mol l iq

Synthesis of novel nano-layered double hydroxide by urea hydrolysismethod and their application in removal of chromium(VI) from aqueoussolution: Kinetic, thermodynamic and equilibrium studies

Amita Jaiswal ⁎, Raj Mani, Sushmita Banerjee, Ravindra Kumar Gautam, M.C. ChattopadhyayaEnvironmental Chemistry Research Laboratory, Department of Chemistry, University of Allahabad, Allahabad 211002, India

⁎ Corresponding author.E-mail addresses: jaiswalamita.3@gmail.com, amita_e

http://dx.doi.org/10.1016/j.molliq.2014.12.0040167-7322/© 2014 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 9 September 2014Received in revised form 1 December 2014Accepted 3 December 2014Available online 8 December 2014

Keywords:Layered double hydroxideUrea hydrolysis methodX-ray diffractionFT-IRTEMTG–DTAAdsorption isotherm

Wastewater contaminated with chromium(VI) ions was released in the aquatic environment during differ-ent industrial processes and is considered dangerous, since chromium(VI) is a powerful epithelial irritantand a human carcinogen. It is therefore required to develop new materials that can effectively used to re-move metallic species from liquid phase. In this perspective layered double hydroxides (LDHs) with Co/Bi = 3 were synthesized via urea hydrolysis method. The as-prepared LDH material was further character-ized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier-transform infrared spec-troscopy (FT-IR) and thermogravimetric analysis (TGA). The adsorption behavior of Cr (VI) was studied atdifferent initial concentrations (100–500 mg/L), contact time (15–120 min), pH (2.0–12.0), temperature(298–318 K) and adsorbent dose (0.1–0.5 g) by batch equilibrium technique. Adsorption experiments indi-cate that the maximum adsorption capacity of Cr(VI) reached to 277.7 mg/g on the optimum adsorptionconditions (dosage of 0.1 g, contact time for 120 min., pH at 7 and temperature at 283 K). The kineticdata agrees well with pseudo-second order (R2= 0.995). The adsorption isotherm study reveals that exper-imental data fitted well in both Freundlich and Redlich–Peterson model. The study of thermodynamic pa-rameters indicated that adsorption of Cr(VI) is spontaneous and exothermic in nature. Desorption studyrecommends that adsorbent can be easily regenerated by 0.1 M NaHCO3.

© 2014 Published by Elsevier B.V.

1. Introduction

Chromium is a common contaminant present in wastewater. Awide range of industries use chromium in their processes. As a re-sult, the effluents contain significant amounts of this metal [1].Chromium exhibits two oxidation states: chromium(VI) andchromium(III). At high levels, chromium(VI) is more toxic thanchromium(III) [2–6]. High contamination occurs in groundwaterdue to its significant mobility in the subsurface environment [7]. Dif-ferent techniques have been employed for removal of chromium(VI)from wastewaters. Physicochemical treatments are the most com-monly used technique as the technique is economically feasiblethan the electrochemical ones [8]. Sorption process is one of themost effective physical processes that may be used to removeCr(VI) from wastewaters. Varieties of adsorbent such as activatedcarbon [9], zeolites [10], minerals [11], cationic [12] and anionic[13] clays, have been tested for its potential in the removal ofCr(VI) from aqueous solutions. The anionic clays also known as lay-ered double hydroxides (LDHs) are one of the most commonly usedsorbent materials [14–16]. The general formula of LDHs can be

csl@rediffmail.com (A. Jaiswal).

expressed as [M(II)1 − XM(II)x(OH)2]x+An−x/n; where M(II) is a di-

valent metal ion, M(III) is a trivalent metal and A is an anion that oc-cupy the interlayer region of these crystalline materials. The layereddouble hydroxides are commonly prepared by co-precipitation ofmetallic salts in alkaline medium at constant pH 12. With the pur-pose of modifying the textural properties of the LDHs, these sub-stances can be synthesized by the homogeneous precipitationmethod [17,18] and urea hydrolysis method [19]. In the presentstudy, LDHs with Co/Bi = 3 were synthesized by the urea hydrolysismethod. Urea has some excellent features and can also be used as aprecipitation agent with the purpose of modifying the thermal andtextural properties of the sorbent and results in enhancing the sorp-tion capacity of the sorbent. The adsorbent was previously reportedby our research group for the study of sorption performance of leadion [9] from aqueous solutions.

2. Experimental

2.1. Reagents

The following reagents were used: Co(NO)3·6H2O (Thomas Baker)and Bi(NO)3·5H2O (Merck), K2Cr2O7 (analytical grade), NH2CONH2

0 10 20 30 40 50 60 70 80

200

400

600

800

1000

1200

1400

1600

1800

Two-Theta (deg)

Fig. 1. XRD spectra of homogeneously synthesized Co/Bi-LDH.

53A. Jaiswal et al. / Journal of Molecular Liquids 202 (2015) 52–61

(Merck), NaOH (Merck), HCl (Loba Chemie) and NaHCO3 (analyticalgrade).

2.2. Synthesis of the layered double hydroxides (LDHs)

The layered double hydroxides with Co/Bi = 3 were prepared byurea hydrolysis method [19]. 200 mL solution of cobalt nitrate and bis-muth nitrate was maintained in a ratio of 3:1 with the required molarconcentration of 0.24 M. The mixed metal solution was further dis-solved in urea with the molar concentration of 0.8 M. The solutionwas stirred and heated to 90 °C until a white precipitate appeared.After cooling, the resulting slurry was filtered, washed with distilledwater and dried at 70 °C. The dried powder was calcined at 450 °C in amuffle furnace for 4 h.

2.3. Characterization of the layered double hydroxides (LDHs)

The synthesized Co/Bi-LDH was characterized by X-ray diffractiontechnique for the characterization of minerals [20]. XRD was recordedon X'PERT-PRO diffractometer operated at 40 kV/30 mA, using CuKα1

radiation with a wavelength of 1.54 Å in the wide angle region from20 to 80 on 2θ scale. FTIR (Perkin Elmer FT-IR spectrometer)was carriedout to analyze functional groups, using potassium bromide (KBr) diskmethod. Transmission electron microscopy (TEM) was performedusing a FEI TEM electron microscope. Brunauer–Emmett–Teller (BET)specific surface area was measured on Micromeritics ASAP 2010 andthe thermal stability was studied by thermogravimetric analysis(TGA), under a dynamic air atmosphere at a heating rate of 10 min−1

using a Perkin Elmer TGA instrument.

2.4. Adsorption kinetics and isotherms

All synthetic solutions were prepared by diluting chromium solu-tions (1000 ± 1 mg/L) obtained by dissolving K2Cr2O7 into deionizedwater. In order to determine the optimumCr(VI) adsorption conditions,batch adsorption experiments were carried out with a certain amountof Co/Bi-LDHs and 50 mL of Cr(VI) solutions (100 mg/L) in a series ofa 100 mL conical flask. The effect of dosage (LDHs) was tested in therange of 0.1–0.5 g (natural pH, contact time for 120 min., temperatureat 298 K). The effect of contact time was studied in the time range of15–120 min (dosage of 0.1 g, natural pH, temperature at 298 K) andthe effect of pH was investigated in the range of 2.0–12.0 (dosage of0.1 g, contact time for 120 min, temperature at 298 K). Furthermore,batch equilibrium isotherm studies on LDHs were conducted at differ-ent concentrations of Cr(VI) solutions by contacting a constant mass(0.1 g) of Co/Bi-LDHs for 120min at 298K, 303 K and 318 K respectively.The adsorption capacity of Co/Bi-LDH was determined from the contactof various concentrations (100–500 mg/L) of metal solution in conicalflasks, with 0.1 g of LDHs. The mixture was shaken in a orbital shaker(Shivam, IS0 900/2000) at 120 rpm followed by centrifugation and theresidual concentration of Cr(VI) ions in the supernatant liquid was de-termined standard procedure [21,22] at 540 nm (λmax), following the1,5-diphenylcarbazide method using an UV–visible spectrophotometer(Systronic 509).

The percentage (%) removal of Cr(VI) and amount of Cr(VI)adsorbed on LDHs (qe) was calculated by Eqs. (1) and (2) respectively:

% removal ¼ 100� Co−Ce=Ce ð1Þ

qe ¼ Co−Ceð ÞV=M ð2Þ

where, qe is the amount of Cr(VI) adsorbed on adsorbent at equilibrium(mg/g), and Co and Ce are the initial and equilibrium concentrations(mg/L) of Cr(VI) in solution respectively. V is the volume of solution(L) and M is the mass of adsorbent (g).

2.5. Desorption and regeneration studies

The saturated adsorbent, which contains Cr(VI), is not safe for dis-posal because of stringent environmental constraints. Therefore it is im-portant and appropriate to propose a suitable method for theregeneration and reuse of the adsorbent in order to reduce the load onthe environment in terms of the disposal of polluted adsorbents. Afterthe Cr(VI) adsorbed at a concentration of 100 mg/L, the exhaustedLDHswere separated from the adsorption experiments followed bydry-ing at 80 °C. The dried Cr(VI)-LDH sample of 0.1 gwas placed in 50mLof0.1 M NaHCO3 under stirring condition at 298 K for 1 h. The mixturethen centrifuged at 1000 rpm for the separation of the solids from theliquid phase. The material obtained then kept for oven drying at 80 °Cfor 24 h and then reused for the sorption performance. The adsorptionand desorption experimentswere repeated for five times in order to de-termine the adsorption competency of the regenerated adsorbent.

3. Results and discussions

3.1. Characterization of layered double hydroxides (LDHs)

Fig. 1 shows the XRD spectra of the LDH with peaks at the positionsof 11.0°,13.25°, 22.25°, 26.75°, 29.0°, 33.5°, 35.75°, 38.0°, 40.25°, 44.75°,49.25°, 53.75°, 56.0°, 60.5° and 62.75°, which are in good agreementwith JCPDS file (JCPDS 38-0486, a=3.076 Å, b= c=22.80 Å). All avail-able reflections of the present XRD peaks (i.e., Scherrer's broadening)attributed to the formation of LDHs. The average crystallite size was de-termined as ca. 60.56 nm from the Scherrer's formula.

The SEM micrograph of synthesized LDH at 40,000× magnificationwas shown in Fig. 2 and it was clear that the surface of Co/Bi-LDH washomogeneous and platelet-like primary particles with well-definedhexagonal shape and well developed crystals having pores which pro-vide suitable sites for adsorption. TEM results also support the resultsof XRD analysis (Fig. 3). Textural properties of the adsorbent also playan important role in the adsorption process. The textural characteristicsof the adsorbent have been depicted in Table 1. The data generated onBET surface area analyzer shows that the prepared LDHs comprise ofwidespread distribution of porous structures. The pore size distributionof the adsorbent has been evaluated according to BJH (Barrett–Joyner–Hanlenda) method [23,24]. BET surface area and pore volume of LDHswere found to be of 70m2/g and 0.25 cm3/g respectively. These charac-teristics substantiate that urea hydrolysis based synthesis results in im-proved crystallinity of Co/Bi-LDHs as compared to that of Co/Bi-LDHsobtained by co-precipitation method [9]; this may be due to separationof nucleation and crystal growth steps during the formation of particles

Fig. 2. SEM of synthesized Co/Bi-LDH.

Table 1Textural properties of synthesized layered double hydroxides.

Textural properties Values

BET surface area (m2/g) 70Particle size (μm) 0.50Porosity (%) 70Pore volume (cm3/g) 0.0253Pore area (m2/g) 11.8

54 A. Jaiswal et al. / Journal of Molecular Liquids 202 (2015) 52–61

in urea hydrolysis method which favors materials with higher crystal-linity degree.

The FT-IR spectra for Co/Bi-LDH (Fig. 4) showed a broad and strongband in the range 3000–3500 cm−1 centered at 3400 cm−1 whichwas due to the O\H stretching vibration of the inorganic layers and in-terlayer water molecules. Another common wavenumber for LDH-likematerial is a band at 1625 cm−1 which is assigned to the bending vibra-tion of interlayer water molecules. For Co/Bi-LDH, a strong absorptionband centered at 1380 cm−1 is assigned to the N\O stretching vibra-tion. A band in the lowerwave number region corresponds to the latticevibration mode such as the translation vibrations of Co\OH at600 cm−1 and deformation vibration of OH\Co\Bi\OH at around588 cm−1 [16,17].

The TG–DTA curve of the Co-Bi/LDHs is shown in Fig. 5. Decomposi-tion of LDHs occurs in four steps. The first step in the range of 50–150 °Cis due to elimination of water from surface of Co-Bi/LDHs that results instrong exothermic peak at corresponding temperature. The second andthird steps in the ranges of 150–300 °C and 300–400 °C are mainly at-tributed to the dehydroxylation of the brucite-like octahedral layers inCo-Bi/LDHs which can be corroborated to the exothermic peaks obtain-ed at similar temperature range. Finally, the forth minor step observedabove 400 °C is due to the degradation of carbonate interlayer anionsin the Co-Bi/LDH interlayer [25].

Fig. 3. TEM of synthesized Co/Bi-LDH.

3.2. Effect of initial concentration of Cr(VI) solution

The effect of different concentration of Cr(VI) (100–500 mg/L) hasbeen investigated at 298 K. Fig. 6 shows that the adsorption of chromi-um ion decreased with the increase of equilibrium concentration [18].With increasing Cr(VI) concentration from 100 to 500mg/L, the remov-al decreased from 99.12 to 45.86%, and the adsorption capacity in-creased from 19.81 to 45.86 mg/g. The decrease in Cr(VI) removalcould be explained by the fact that all of the adsorbents had a limitednumber of active sites, which became saturated above a certain concen-tration. The increase in the adsorption capacity with increasing Cr(VI)concentration may have been due to the higher adsorption rate andthe utilization of all of the active sites available for adsorption at higherconcentrations. For an initial Cr(VI) concentration of 100 mg/L, thevalues of Cr(VI) removal percentage and adsorption capacity werefound to be 99.12% and 45.86 mg/g, respectively.

3.3. Effect of contact time

Effect of contact time on adsorption of Cr(VI) on the Co/Bi-LDHs isshown in Fig. 7. It was found that 99.76% of the chromium ion was re-movedwithin 120min. A further increase in the contact time had a neg-ligible effect on the rate of Cr(VI) adsorption. The nature of theadsorbent and the available adsorption sites affected the rate of Cr(VI)adsorption. In the initial stages of Cr(VI) adsorption, the concentrationgradient between the film and the available pore sites was large, andhence, the rate of Cr(VI) adsorption was faster. The decrease in therate of adsorption in the later stages of Cr(VI) adsorption was probablydue to the slow pore diffusion of the solute ions into the bulk of the ad-sorbent. Hence, 120minwas adopted as the equilibrium time for Cr(VI)adsorption onto LDHs.

Fig. 4. FT-IR of synthesized Co/Bi-LDH.

0 200 400 600 800 1000

6.5

7.0

7.5

8.0

5

10

15

20

25

30

Wei

ght %

Temperature ( 0C)

Heat

flow

End

o Up

(m W

)

Fig. 5. TG–DTA of synthesized Co/Bi-LDH.

0 20 40 60 80 100 120

40

50

60

70

80

90

% r

emov

al o

f Cr

(VI)

Contact Time (min.)

Fig. 7. Effect of contact time on adsorption of Cr(VI) on Co/Bi-LDH.

55A. Jaiswal et al. / Journal of Molecular Liquids 202 (2015) 52–61

3.4. Effect of pH

The initial pH of the test solution varied from2.0 to 12.0. The amountof Cr(VI) adsorbed on the adsorbent increases steadily with increasingpH in range from 2 to 7 (Fig. 8). Cr(VI) exists in various oxoanionicforms in aqueous solution depending upon the solution pH: pH N 8,CrO4

2− yellow; pH 2–6, HCrO4− and Cr2O7

2− orange-red; pH b 1,H2Cr2O7. The significance of using LDHs as an adsorbent lies in the factthat an optimum amount of Cr(VI) was adsorbed at pH 7, which is thenatural pH of all natural water bodies.

3.5. Effect of temperature

The effect of temperature on the adsorption of Cr(VI) on LDHs wasinvestigated by experiments for 100 mg/L Cr(VI) at 298 K to 318 K. Itwas observed that on increasing the temperature, percentage removalof metal ion decreased (Fig. 9). This showed that the adsorption processwas exothermic which was also confirmed by TG–DTA analysis.

3.6. Regeneration of LDHs

From Fig. 10, it is clear that approximately 57.2% adsorbed Cr(VI)was released within 1 h at 25 °C in 0.1 M NaHCO3 due to a high

100 200 300 400 50040

50

60

70

80

90

100

% r

emov

al o

f Cr

(VI)

Conc.(mg/L)

Fig. 6. Effect of concentration on adsorption of Cr(VI) on Co/Bi-LDH.

degree of affinity of carbonate and bicarbonate for LDHs [26]. Innext 24 h desorption increases slowly and the percentage reachesto approximately 99%. The desorbed LDHs sample was collectedand dried at 80 °C, using this regenerated sample at first time forconducting adsorption of Cr(VI) experiments again. From the exper-iment we found that the LDHs of the first time regeneration hadmore than 90% of original adsorption capacity (Fig. 11). This sorp-tion–regeneration cycles for Cr(VI) by the LDHs was repeated fivetimes. In Fig. 11, it is clear that the adsorption percentage continu-ously decreases with each cycle of regeneration. At the last regener-ation cycle, the removal % of Cr(VI) still more than 50% furtherimplies that synthesized LDHs possess good regeneration power.

4. Adsorption kinetics

The study of the kinetics is important for evaluating the efficiencyand mechanism of interaction between adsorbate and adsorbent.

2 4 6 8 10 12

0

20

40

60

80

100

% r

emov

al o

f Cr

(VI)

pH

Fig. 8. Effect of pH on adsorption of Cr(VI) on Co/Bi-LDH.

280 285 290 295 300 305 310 315

60

65

70

75

80

85

90

95

100

% r

emov

al o

f Cr

(VI)

temp.(K)

Fig. 9. Effect of temperature on the adsorption of Cr(VI) on Co/Bi-LDH.

Co/Bi- LDHs FIRST SECOND THIRD FOURTH FIFTH0

20

40

60

80

100

adso

rptio

n ca

paci

ty (%

)

Regenerated Sample

Fig. 11. Regeneration of LDHs to Cr(VI).

56 A. Jaiswal et al. / Journal of Molecular Liquids 202 (2015) 52–61

4.1. Pseudo-first-order model

The pseudo-first-order equation is given as

dqtdt

¼ K f qe−qtð Þ ð3Þ

where, dt is the amount of adsorbate adsorbed at time t, qe is the adsorp-tion capacity at equilibrium (mg g−1), Kf the pseudo-first-order rateconstant (min−1), and t the contact time (min). The integration ofEq. (3) with the initial condition, qt = 0 at t = 0 [27].

log qe−qtð Þ ¼ log qe−K f

2:303tð4Þ

The values of the adsorption rate constant (Kf) for Cr(VI)were deter-mined from the plot of log (qe − qt) vs t (Fig. 12).

0 5 10 15 20 25

0

20

40

60

80

100

Cr(V

I)des

orpt

ion

%

Time (h)

Fig. 10. Desorption of Cr(VI) from samples.

4.2. Pseudo-second-order model

The pseudo-second-order model is represented as [28],

dqtdt

¼ Ks qe−qtð Þ2 ð5Þ

where, Ks is the pseudo-second-order rate constant (g mg−1 min−1).Integrating Eq. (5) and noting that qt= 0 at t= 0, the following equa-tion is obtained.

qt ¼tKsqe

2

1þ tKsqeð6Þ

The initial sorption rate, h (mg g−1 min−1), at t = 0 is defined as

h ¼ Ksqe2: ð7Þ

Eq. (6) can be rearranged to obtain

qt ¼t

1hþ 1

qe

� �� t

: ð8Þ

Fig. 12. Pseudo-first order kinetic model for adsorption of Cr(VI) on Co/Bi-LDH.

Fig. 13. Pseudo-second order kinetic model for adsorption of Cr(VI) on Co/Bi-LDH.

Fig. 14. Elovich model for adsorption of Cr(VI) on Co/Bi-LDH.

57A. Jaiswal et al. / Journal of Molecular Liquids 202 (2015) 52–61

The qe is obtained from the slope of t/qt vs. t (Fig. 13) and h is obtain-ed from the intercept. Since qe is known from the slope, theKs can be de-termined from the value of h. The best fit values of h, qe and Ks alongwith correlation coefficients for the pseudo-first-order and pseudo-second-order models are shown in Table 2. The qe

exp and the qecal values

for the pseudo-first-order model and pseudo-second-order model areshown in Table 2. The qe

exp and the qecal values from the pseudo-

second-order are very close to each other, and also the calculated corre-lation coefficients, R2 are also closer to unity for pseudo-second-orderkinetics than that for the pseudo-first-order kinetics. Therefore, the ad-sorption can be approximated more appropriately by the pseudo-second-order kinetic model than the pseudo-first-order kinetic modelfor the adsorption of Cr(VI).

4.3. Elovich model

Elovich equation is also used to describe secondorder kinetic assum-ing that the actual solid surfaces are energetically heterogeneous, butthe equation does not propose any definite mechanism for adsorbate–adsorbent interaction [29]. It has extensively been accepted that thechemisorption process can be described by this semi-empirical equa-tion. The linear form of this equation is given by,

qt ¼ln aebebe

þ 1be ln t

ð9Þ

where, ae, is the initial adsorption rate (mg g−1 min−1), and the param-eter be is related to the extent of surface coverage and activation energyfor chemisorption (g mg−1). The initial adsorption rate, ae, and the de-sorption constant, be, were calculated from the intercept and slope ofthe straight-line plots of qt against ln t (Fig. 14). Table 2 lists the kineticconstants obtained from the Elovich equation and R2 values for Cr(VI)onto LDHs, which were nearly similar to those of pseudo-second ordermodel. Thus Elovich model found to be appropriate for the predictionof kinetics of adsorption of Cr(VI) onto LDHs. Further suitability of the

Table 2Kinetic parameters for adsorption of Cr(VI) onto LDHs.

Metal ion qeexp (mg g−1) Pseudo-first-order Pseudo-second-order

Kf (min−1) qecal (mg g−1) R2 h (mg g−1 min−1) Ks (

Cr(VI) 3.1425 0.078 4.281 0.983 0.263 0.0

Elovich model supported that adsorption process of Cr(VI) onto LDHsfollow pseudo-second-order.

5. Adsorption isotherms

Adsorption isotherms are important for the description of how ad-sorbate will interact with an adsorbent and are critical in optimizingthe use of adsorbent. Thus the correlation of equilibrium data using ei-ther a theoretical or empirical equation is essential for adsorption datainterpretation and prediction. Various isotherm equations like those ofLangmuir, Freundlich, Redlich–Peterson (R–P) and Temkin isothermhave been used to describe the equilibrium characteristics ofadsorption.

5.1. Langmuir isotherm

The Langmuir adsorption isotherm [30] assumes that adsorptiontakes place at specific homogeneous sites within the adsorbent andhas found successful application in many sorption processes of mono-layer adsorption. The Langmuir adsorption isotherm equation can berepresented as:

qe ¼qmKLCe

1þ KL

Ce

� � : ð10Þ

The linearized form of Eq. (10) can be written as Eq. (11)

Ce

qe¼ 1

KLqm

� �þ Ce

qm

� �ð11Þ

where Ce = equilibrium concentration (mg L−1), qe = amountadsorbed (mg g−1), and qm = monolayer adsorption capacity(L mg−1) related to the free energy of adsorption. The values of qmand KL were calculated from the slopes (1/qm) and intercepts (1/KLqm)of the linear plots of Ce/qe vs. Ce (Fig. 15).

Elovich model

g mg−1 min−1) qecal (mg g−1) R2 be (g mg−1) ae (mg g−1 min−1) R2

37 3.819 0.995 4.348 2.095 0.978

Fig. 15. Langmuir isotherm for adsorption rate of Cr(VI) on Co/Bi-LDH.

58 A. Jaiswal et al. / Journal of Molecular Liquids 202 (2015) 52–61

5.2. Freundlich isotherm

The Freundlich isotherm [31] assumes that adsorption process oc-curs on heterogeneous surfaces and the capacity of adsorption is relatedto the concentration of metal ions at the equilibrium. The Freundlichequation is expressed as follows,

qe ¼ K FCe1=n

: ð12Þ

Logarithmic form of Eq. (12) can be written as Eq. (13)

ln qe ¼ ln K F þ1n

� �ln Ce ð13Þ

where, KF and n = Freundlich constants related to adsorption capacity(mg g−1) and adsorption intensity of adsorbent, respectively. Thevalues of KF and nwere calculated from the intercepts (ln KF) and slopes(1/n) of the plots ln qe vs. ln Ce (Fig. 16).

5.3. Redlich–Peterson isotherm (R–P)

The Redlich–Peterson isotherm (R–P) [32] is a combination of Lang-muir–Freundlich model. It approaches the Freundlich model at high

Fig. 16. Freundlich isotherm for adsorption rate of Cr(VI) on Co/Bi-LDH.

concentration and is in accord with the low concentration limit of theLangmuir equation. The equation is given as:

qe ¼KRCe

1þ aRCeβ ð14Þ

where KR is the R–P isotherm constant (L g−1), aR the R–P isotherm con-stant (L mg−1) and β the exponent which lies between 0 and 1, Ce, theequilibrium liquid phase concentration (mg L−1).

For high concentration, Eq. (14) reduces to Freundlich isotherm, i.e.Eqs. (12) and (13), where, KF = KR / aR is the Freundlich constant(L mg−1), and (1/n) = (1− β) is the heterogeneity factor.

For β = 1, Eq. (14) reduces to Langmuir equation, i.e. Eqs. (10) and(11), where, KL = aR, is the Langmuir adsorption constant (L mg−1) re-lated to the energy of adsorption and qm = KR / aR signifies adsorptioncapacity (mg g−1).

For β = 0, Eq. (14) reduces to Henry's equation, i.e.

qe ¼KRCe

1þ aR: ð15Þ

The R–P isotherm incorporates three parameters and can be appliedeither in homogeneous and heterogeneous systems. Eq. (15) can beconverted to a linear form by taking logarithms of both the sides as

lnKR Ce

qe−1

� �¼ ln aR þ β lnCe: ð16Þ

A plot of ln (KRCe / qe − 1) vs. ln Ce (Fig. 17) enables the determina-tion of isotherm constants aR and β from the intercept and sloperespectively.

5.4. Temkin isotherm

Temkin and Pyzhev considered the effects of some indirect adsor-bate interactions on adsorption isotherms and suggested that becauseof these interactions the heat of adsorption of all the molecules in thelayer would decrease linearly with coverage [33]. The Temkin isothermhas been used in the following form:

qe ¼RT

b ln KTCeð Þ ð17Þ

where KT is the equilibrium binding constant (L g−1), b is the Temkinisotherm constant, R is the universal gas constant (8.314 J/mol K) and

Fig. 17. Redlich–Peterson isotherm for adsorption rate of Cr(VI) on Co/Bi-LDH.

Fig. 18. Temkin isotherm for adsorption rate of Cr(VI) on Co/Bi-LDH.

Table3

Freu

ndlic

h,Lang

muir,Re

dlich–

Peterson

andTe

mkinad

sorption

parametersforthead

sorption

ofCr(V

I)from

aque

oussolution

byLD

Hs.

Metal

Lang

muirpa

rameters

Freu

ndlic

hpa

rameters

Redlich–

Peterson

parameters

Temkinpa

rameters

KL (Lmg−

1)

q m (mgg−

1)

R2x2

APE

sKF (Lg−

1)

nR2

x2APE

sa R (L

mg−

1)

βR2

x2APE

sB 1 (J

mol

−1)

KT (Lmg−

1)

R2x2

APE

s

Cr(V

I)0.07

327

7.77

0.93

0620

.03

16.01

3.93

71.36

480.97

580.51

2.52

2.17

01.44

40.97

100.33

1.84

0.71

192.50

390.95

749

.16

19.14

59A. Jaiswal et al. / Journal of Molecular Liquids 202 (2015) 52–61

T is the absolute temperature (K). Eq. (17) can be written as the follow-ing form:

qe ¼ B1 ln KTCeð Þ þ B1 lnCe ð18Þ

where, B1 = RT/b.A plot of qe vs. ln Ce (Fig. 18) enables the determination of the B1 and

KT from the slope and intercept respectively. KT is the equilibrium bind-ing constant (L g−1) corresponding to the maximum binding energyand the constant B1 is related to the heat of adsorption (J mol−1).

The isotherm constants for all the isotherms studied, and the corre-lation coefficients, R2 with the experimental data are listed in Table 3.The correlation coefficient (R2) values were used to predict the best fitlinear equation. However in order to describe the equilibrium data ade-quately χ2 (chi-square) and average percentage errors (APEs) are oftenadopted. Chi square and APEs were calculated using Eqs. (19) and (20),respectively, and this indicated the fit between the experimental andcalculated values of adsorption capacity:

χ2 ¼X qe; exp−qe; calð Þ2

qe; expð19Þ

APE ¼

XN1

qe; exp−qe; calj j=qe; expN ð20Þ

where, qe, cal is the equilibrium capacity obtained by calculating fromthemodel (mg g−1) and qe, exp is the experimental data on the equilib-rium capacity (mg g−1). N is the group number of experiments.

In Table 3, it can be shown that Langmuir correlation coefficient (R2)and Temkin correlation coefficient (R2) are 0.9306 and 0.9572 respec-tively while APE values and the χ2 value were high for both themodel; this indicated that the difference between the experimentalvalues and calculated values was very high, which further suggestedthat the equilibriumdata not agreeswellwith the Langmuir and Temkinisotherm model. In the case of Freundlich and Redlich–Peterson iso-therm, the obtained correlation coefficient values were high and theAPE and χ2 values were very low in comparison to Langmuir andTemkin model. Thus from the calculated values of error function it hasbeen found that the isotherm data better represented by Freundlichand Redlich–Peterson isotherm model.

6. Adsorption thermodynamics

In any adsorption process, both energy and entropy considerationsmust be taken into account in order to determine what process will

Fig. 19. Thermodynamic parameters for adsorption rate of Cr(VI) on Co/Bi-LDH.

Table 5Comparative adsorption capacities of different adsorbents for Cr(VI).

S.no.

Adsorbent Adsorbent capacity (qe)mg g−1

Reference

1. Activated C-based iron containingadsorbents

68.49 [35]

2. GG/nZnO biocomposite 55.56 [36]3. Palygorskite clay 58.50 [37]4. Activated carbon from hazelnut 170 [38]5. Modified Jacobsite (MnFe2O4) 31.5 [39]6. Siderite 17 [40]7. Diatomite-supported magnetite 69.2 [41]8. Prawn shell activated carbon 100 [42]9. Nano-Al2O3 8.56 [43]10. River bed sand 1.19 [44]11. Co/Bi-LDH 277.77 Present

study

60 A. Jaiswal et al. / Journal of Molecular Liquids 202 (2015) 52–61

occur spontaneously. Values of thermodynamic parameters are the ac-tual indicators for practical application of a process. The equilibriumconstant K was calculated [34] using the following equation,

K ¼ qeCe

ð21Þ

where, qe is the amount of metal ion adsorbed at equilibrium (mg g−1)and Ce is the amount remaining in the solution at equilibrium (mg L−1).The values of Gibb's free energy (ΔG°) for the adsorption at differenttemperatures using the following Eq. (22),

ΔG� ¼ −RT lnK ð22Þ

where, ΔG° is the Gibbs free energy change, R is the universal gas con-stant and T is the absolute temperature (K). The standard free energychange is also related to standard enthalpy change (ΔH°) and the stan-dard entropy change (ΔS°) by Eq. (23),

ΔG� ¼ ΔH�−TΔS�: ð23Þ

In Eqs. (22) and (23), we obtained:

lnK ¼ ΔS�

R−ΔH�

RT: ð24Þ

Hence, from the slope and intercept of plot ln K vs 1/T (Fig. 19) thevalues ofΔH° andΔS° were obtained respectively. Values of all the ther-modynamic parameters are given in Table 4. The negative values of ΔG°indicate that the adsorption process was spontaneous in nature and thenegative value of ΔH° explicates the exothermic nature of the processand the positive value of ΔS° indicates the increase in randomness atthe solid–liquid interface.

7. Comparative study

A comparative study in terms of adsorption capacity which has beencarried out with other reported adsorbents is summarized in Table 5. In

Table 4Thermodynamic parameters for the adsorption of Cr(VI) from aqueous solution by LDHs.

Temp. ΔG° (kJ/mol) ΔH° (kJ/mol) ΔS° (kJ/mol K)

Cr(VI) Cr(VI) Cr(VI)

298 K −36.23303 K −306.33 −7877.7 27.01318 K −576.43

Table 5, it is clear that the adsorption capacity of Co/Bi-LDH is greaterthan that of other adsorbent materials, reflecting a promising futurefor this LDH utilization in Cr(VI) removal from aqueous solutions.

8. Conclusions

The LDHs synthesized by urea hydrolysis method produce homoge-neous sizes and platelet-like primary particleswithwell-defined hexag-onal shapes and good crystal quality which can be efficiently used as anadsorbent for the removal of Cr(VI) from aqueous solutions. The maxi-mum adsorption was found at pH 7. The percentage adsorption ofCr(VI) was found to be 99.12%. The process was very fast initially andmaximum adsorption was observed within 120 min. Adsorption kinet-ics was well described by the pseudo-second order kinetic TheFreundlich isotherm and Redlich–Peterson model were best fitted forthe adsorption of Cr(VI). The process found to be spontaneous and exo-thermic in nature. The adsorbent can be successfully reused for thesorption studies without significant loss of adsorption capacity.

Acknowledgments

The authors are thankful to Prof. O.N. Srivastava and Mr. DineshJaiswal, Department of Physics, Banaras Hindu University for providingthe facility for XRD and TEM. Mr. Amit Singh, Mr. Satish and HOD, De-partment of Chemistry, Banaras Hindu University for recording FT-IRspectra, TGA–DTA respectively and University Grant Commission(UGC), New Delhi [Award Letter No. F. 15/53/12 (SA-II)], India, for thefinancial support.

References

[1] L.F. de Filipps, C.K. Pallaghy, Heavymetals: sources and biological effects, in: L.C. Rai,J.P. Guar, C.J. Soeder (Eds.), Advances in Limnology Series: Algae Water Pollution, E.Scheizerbartsche Press, Stuttgart, 1994, pp. 31–77.

[2] M. Cieslak-Golonka, Toxic and mutagenic effects of chromium(VI), Polyhedron 15(1996) 3667–3918.

[3] W.J. McLay, F.P. Reinhard, Waste minimization and recovery technologies, Met. Fin-ish. 98 (1999) 817–850.

[4] S.B. Lalvani, T. Wiltowski, A. Hubner, A. Weston, Removal of hexavalent cations bynovel carbon adsorbents, Carbon 36 (1998) 1219–1226.

[5] E. Chmielewska, Adsorption of arsenate and chromate from waters onhydrophobized zeolitic media, Turk. J. Chem. 27 (2003) 639–648.

[6] A. Lu, J. Chen, J. Shi, X. Lu, J. Tang, M. Guo, One-step disposal of Cr(VI) — bearingwastewater by natural pyrrhotite, Chin. Sci. Bull. 45 (2000) 1614–1616.

[7] N.K. Lazaridis, K.A. Matis, M.Webb, Flotation of metal-loaded clay anion exchangers.Part I: the case of chromates, Chemosphere 42 (2001) 373–378.

[8] M. del Arco, D. Carriazo, C. Martin, A.M. Perez-Grueso, V. Rives, Characterization ofchromate-intercalated layered double hydroxides, Mater. Sci. Forum 514516(2006) 1541–1545.

[9] A. Jaiswal, M.C. Chattopadhyaya, Synthesis and characterization of novel Co/Bi-lay-ered double hydroxides and its adsorption performance for lead in aqueous solu-tion, Arab. J. Chem. (2013), http://dx.doi.org/10.1016/j.arabjc.2013.09.010.

[10] J. Lee, Y. Liu, X. Liu, Synthesis and adsorption characteristics of humic/MgAl layereddouble hydroxide hybrid material, J. Appl. Sci. Environ. Sanit. 5 (2010) 345–351.

61A. Jaiswal et al. / Journal of Molecular Liquids 202 (2015) 52–61

[11] A. Jaiswal, M.C. Chattopadhyaya, Interaction of Mn2+, Fe2+ and Cu2+ heavy metalions from aqueous solution by zaccagnaite, a hydrotalcite-like compound, Desalin.Water Treat. 29 (2011) 252–257, http://dx.doi.org/10.5004/dwt.2011.1197.

[12] F. Trifiro, A. Vaccari, Hydrotalcite-like anionic clays (layered double hydroxides), in:J.L. Atwood, J.E.D. Davies, D.D. MacNicol, F. Vögtle (Eds.), Solid-state SupramolecularChemistry: Two- and Three-dimensional Inorganic Networks, Comprehensive Su-pramolecular Chemistry, vol. 7, Pergamon, Oxford, UK, 1996, pp. 251–291.

[13] E. Ramos, T. Lopez, P. Bosch, M. Asomoza, R. Gomez, Thermal stability of sol–gelhydrotalcites, J. Sol–Gel Sci. Technol. 8 (1997) 437–442.

[14] L. Xuefeng, H. Wanguo, X. Jie, Sorption of Pb(II) on Mg–Fe layered double hydrox-ides, Chin. J. Chem. 27 (2009) 1981–1988.

[15] Y. Yasin, M. Mohamad, A. Saad, A. Sansui, F.H. Ahmad, Removal of lead ions fromaqueous solution using intercalated tartrate–Mg–Al layered double hydroxides,Desalin. Water Treat. (2013), http://dx.doi.org/10.1080/19443994.2013.803935.

[16] Y. Feng, D. Li, Y. Wang, D.G. Evans, X. Duan, Synthesis and characterization of a UVadsorbent-intercalated Zn–Al layered double hydroxide, Polym. Degrad. Stab. 91(2006) 789–794.

[17] L.F. Zhang, D.G. Evan, C. Forano, X. Duan, Structure and thermal evolution of Mg–Allayered double hydroxides containing interlayer organic glyphosate anion,Thermochim. Acta 424 (2004) 15–23.

[18] A. Jaiswal, M.C. Chattopadhyaya, Removal of zinc ion from industrial effluents byhydrotalcite-like compound, Desalin. Water Treat. 12 (2009) 127–132.

[19] U. Costantino, F. Marmottini, M. Nocchetti, R. Vivani, Eur. J. Inorg. Chem. 10 (1998)1439.

[20] J. Thorez, Practical Identification of ClayMinerals: A Handbook for Teachers and Stu-dents in Clay Mineralogy, Belgium State University Press, Dison-Lelotte, 1976.

[21] A.I. Vogel, Textbook of Quantitative Inorganic Analysis, third ed. ELBS, 1969.[22] N.K. Lazaridis, D.D. Asouhidou, Kinetics of sorptive removal of chromium(VI) from

aqueous solutions by calcined Mg–Al–CO3 hydrotalcite, Water Res. 37 (2003)2875–2882.

[23] D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquerol, T. Siemieniewska,Reporting physisorption data for gas solid systems with special reference to the de-termination of surface-area and porosity (Recommendations 1984), Pure Appl.Chem. 57 (1985) 603.

[24] W.W. Wang, J.B. Zhou, Z. Zhang, J.G. Yu, W.Q. Cai, Different surfactants-assisted hy-drothermal synthesis of hierarchical γ-Al2O3 and its adsorption performance forparachlorophenol, Chem. Eng. J. 233 (2013) 168–175.

[25] T. Kameda, Y. Fubasami, N. Uchiyama, T. Yoshioka, Thermochim. Acta 499 (2010)106.

[26] P.S. Braterman, Z.P. Xu, F. Yarberry, Layered double hydroxides (LDHs), in: S.M.Auerbach, K.A. Carrado, P.K. Dutta (Eds.), Handbook of Layered Materials, MarcelDekker Inc., New York, 2004, pp. 373–474.

[27] S. Lagergren, About the theory of so called adsorption of soluble substances, Ksver.2Veterskapsakad. Handl. 24 (1898).

[28] Y.S. Ho, G. McKay, Pseudo-second order model for sorption processes, ProcessBiochem. 34 (1999) 451–465.

[29] D.L. Sparks, Kinetics of Soil Chemical Processes, Academic Press, New York, 1989.210.

[30] I. Langmuir, The constitution and fundamental properties of solids and liquids. Part I.Solids, J. Am. Chem. Soc. 38 (1916) 2221–2295.

[31] H. Freundlich, Over the adsorption in solution, J. Phys. Chem. 57 (1906) 385–470.[32] E.A. Ayuso, A.G. Sanchez, Removal of cadmium from aqueous solution by

palygorskite, J. Hazard. Mater. 147 (2007) 594–600.[33] M.I. Temkin, V. Pyzhev, Kinetics of ammonia synthesis on promoted iron catalysts,

Acta Physiochim. URSS 12 (1940) 327–356.[34] J.C. Igwe, A.A. Abia, Studies on the effects of temperature and particle size on biore-

mediation of As(III) from aqueous solution using modified and unmodified coconutfiber, Glob. J. Environ. Res. 1 (2007) 22–26.

[35] W. Liu, J. Zhang, C. Zhang, L. Ren, Preparation and evaluation of activated carbonbased iron-containing adsorbents for enhanced Cr(VI) removal: mechanism study,Chem. Eng. J. 189 (2012) 295–302.

[36] T.A. Khan, M. Nazir, I. Ali, A. Kumar, Removal of chromium(VI) from aqueous solu-tion using guar gum-nano-zinc oxide biocomposite adsorbent, Arab. J. Chem.(2013), http://dx.doi.org/10.1016/j.arabjc.2013.08.019.

[37] J.H. Potogieter, S.S. Potogieter, S.S. Vermaak, P.D. Kalibantonga, Heavymetals remov-al from aqueous solution by palygorskite clay, Miner. Eng. 19 (2006) 463–470.

[38] M. Kobya, Removal of Cr(VI) from aqueous solutions by adsorption onto hazelnutshell activated carbon: kinetic and equilibrium studies, Bioresour. Technol. 91(2004) 317–321.

[39] J. Hu, I. Lo, G. Chen, Fast removal and recovery of Cr(VI) using surface-modifiedjacobsite (MnFe2O4) nanoparticle, Langmuir 21 (2005) 11173–11179.

[40] M. Erdem, F. Gur, F. Tumen, Cr(VI) reduction in aqueous solutions by siderite, J. Haz-ard. Mater. 113 (2004) 217–222.

[41] P. Yuan, D. Liu, M. Fan, D. Yang, R. Zhu, F. Ge, J. Zhu, H. He, Removal of hexavalentchromium [Cr(VI)] from aqueous solution by diatomite supported/unsupportedmagnetite nanoparticles, J. Hazard. Mater. 173 (2010) 614–621.

[42] M. Arulkumar, K. Thirumalai, P. Sathishkumar, T. Palvannan, Rapid removal of chro-mium from aqueous solution using novel prawn shell activated carbon, Chem. Eng.J. 185–186 (2012) 178–186.

[43] Y.C. Sharma, V. Srivastava, A.K. Mukherjee, Synthesis and application of Nano-Al2O3

powder for the reclamation of hexavalent chromium from aqueous solutions, J.Chem. Eng. Data 55 (2010) 2390–2398.

[44] S. Yadav, V. Srivastava, S. Banerjee, C.H. Weng, Y.C. Sharma, Adsorption characteris-tics of modified sand for the removal of hexavalent chromium ions from aqueoussolutions: kinetic, thermodynamic and equilibrium studies, Catena 100 (2013)120–127.