Modeling of adsorption of toxic chromium on natural and surface modified lightweight expanded clay aggregate (LECA)

Applied Surface Science 287 (2013) 428–442

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Modeling of adsorption of toxic chromium on natural and surface modified lightweight expanded clay aggregate (LECA) Ebrahim Mohammadi Kalhori a , Kaan Yetilmezsoy b , Nihan Uygur c , Mansur Zarrabi a,∗ , Reham M. Abu Shmeis d a

Department of Environmental Health Engineering, Faculty of Health, Alborz University of Medical Sciences, P.O. Box No: 31485/561, Alborz, Karaj, Iran Department of Environmental Engineering, Faculty of Civil Engineering, Yildiz Technical University, 34220 Davutpasa, Esenler, Istanbul, Turkey c Department of Environmental Engineering, Faculty of Engineering, Adiyaman University, 02040 Altinsehir, Adiyaman, Turkey d Department of Basic Pharmaceutical Sciences, Faculty of Pharmacy, Isra University, PO Box 140753, code 11814, Amman, Jordan b

a r t i c l e

i n f o

Article history: Received 15 July 2013 Received in revised form 17 September 2013 Accepted 28 September 2013 Available online 9 October 2013 Keywords: Batch system Chromium Isotherm Kinetic Lightweight expanded clay aggregate (LECA)

a b s t r a c t Lightweight Expanded Clay Aggregate (LECA) modified with an aqueous solution of magnesium chloride MgCl2 and hydrogen peroxide H2 O2 was used to remove Cr(VI) from aqueous solutions. The adsorption properties of the used adsorbents were investigated through batch studies, Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD), X-ray Fluorescence Spectroscopy (XRF), and Fourier Transform Infrared (FTIR) spectroscopy. The effect created by magnesium chloride on the modification of the LECA surface was greater than that of hydrogen peroxide solution and showed a substantial increase in the specific surface area which has a value of 76.12 m2 /g for magnesium chloride modified LECA while the values of 53.72 m2 /g, and 11.53 m2 /g were found for hydrogen peroxide modified LECA and natural LECA, respectively. The extent of surface modification with enhanced porosity in modified LECA was apparent from the recorded SEM patterns. XRD and FTIR studies of themodified LECA surface did not show any structural distortion. The adsorption kinetics was found to follow the modified Freundlich kinetic model and the equilibrium data fitted the Sips and Dubinin-Radushkevich equations better than other models. Maximum sorption capacities were found to be 198.39, 218.29 and 236.24 mg/g for natural LECA, surface modified LECA with H2 O2 and surface modified LECA with MgCl2 , respectively. Adsorbents were found to have only a weak effect on conductivity and turbidity of aqueous solutions. Spent natural and surface modified LECA with MgCl2 was best regenerated with HCl solution, while LECA surface modified with H2 O2 was best regenerated with HNO3 concentrated solution. Thermal method showed a lower regeneration percentage for all spent adsorbents. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Environmental pollution with heavy metals has received substantial attention due to its serious threat to the human health and the natural ecosystems [1]. The term heavy metals is applied to those that have an atomic density greater than 4 g/cm3 such as Pb (II), Hg (II), Cr (VI), Cu (II), Cd (II), and Zn (II) [2,3]. The presence of heavy metals in the environment is originated mainly from untreated wastewater resulting from numerous industries, such as electroplating, galvanization, leather tanning, steel, cooling water towers, corrosion inhibitors, textile, paints, oxidative dyeing, and batteries [4,5]. Chromium is a heavy metal that naturally occurs

∗ Corresponding author. Tel.: +982634643255; fax: +982634643822. E-mail addresses: [email protected] (E.M. Kalhori), [email protected], [email protected] (K. Yetilmezsoy), [email protected] (N. Uygur), [email protected], [email protected] (M. Zarrabi), [email protected] (R.M.A. Shmeis). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.09.175

in the environment; while high levels of chromium are the consequence of industrial activities [6]. In the environment, chromium exists in two oxidation states, namely Cr(VI) and Cr(III) forms. It is reported that Cr(VI) is more toxic (about 500–1000 times) than Cr(III) and also more soluble in soil and water [7,8]. Chromium may cause many health problems such as epigastria, nausea, vomiting, severe diarrhea, internal hemorrhage, dermatitis, and liver and kidney damage [9]. Due to the high toxicity of hexavalent chromium, its maximum permitted level in aqueous environment is considered to be below 0.05 mg/L [10]. Therefore, removal of Cr(VI) from effluents before their release is also an important environmental issue. For this purpose, researchers in recent years investigated several methods for removal of chromium and other heavy metals such as: adsorption [11], electrochemical process [10], packed bed bioreactor [12], biosorption on fungi and algae [13,14], ion exchange process [15], and other methods. Adsorption has received much attention to remove metal ions, especially using activated carbon as an adsorbent. The economic issue and the need for regeneration of used adsorbents forced

E.M. Kalhori et al. / Applied Surface Science 287 (2013) 428–442

researchers to investigate other inexpensive and low-cost adsorbents for the removal of chromium [11,14]. For that reason, many researchers have been using naturally occurring materials such as clay and geomaterials for adsorption of various environmental pollutants, especially heavy metals [16–18]. Lightweight Expanded Clay Aggregate (LECA) is characterized by its low density, high porosity, natural pH and high thermal resistance (up to 1000 ◦ C). LECA has tiny pores that retain moisture that act as air pockets to facilitate floating on water. The color of LECA is dark brown and it has been used as building material for centuries. Because of its porous structure, it can adsorb and retain environmental pollutants. To the best of the authors’ knowledge, although it has been used for the removal of poly aromatic hydrocarbons, PAHs, from water and for storm water treatment [19,20], it has not yet been used for the removal of chromium. Therefore, the main aim of the present work is to evaluate LECAs for adsorption of Cr(VI) ions from the contaminated water. In addition, the surface modified LECA with MgCl2 and H2 O2 was studied in order to improve its adsorption capacity. The reason for modification of natural LECA was based on recently published works. Many researchers have been used various natural and modified adsorbent for removal of hexavalent chromium such as modified mesoporous silica materials [1], organo-montmorillonite supported iron nanoparticles [8], aminomodified titanate nanotubes [21], dodecylamine modified sodium montmorillonite [22], chitosan polymer-based adsorbent for the removal of chromium (III) [23], Fe supported montmorillonite [24], Fe-modified activated carbon prepared from Trapa natans husk [25] and so on. It has been reported that modification of naturally occurring materials with chemical reagent such as divalent metals improve the porosity and affect the functional group of adsorbent surface leading to increment in adsorption capacity [26,27]. Recently, we used the magnesium chloride and hydrogen peroxide modified pumice for removal of fluoride [27] and magnesium chloride modified pumice for removal of hexavalent chromium [28]. Therefore, in present work, the modification of LECA with magnesium chloride and hydrogen peroxide was done based on idea that these chemical will improve the LECA adsorption capacity for Cr (VI). Based on the above-mentioned facts, the present study was undertaken: (i) to investigate the adsorption capacity of natural LECA, surface modified LECA with H2 O2 , and surface modified LECA with MgCl2 in removal of hexavalent chromium from the aqueous solution, (ii) to compare the performance and effectiveness of the various studied adsorbents, (iii) to determine the effects of contact time, temperature, adsorbant dosage, initial pH of the solution, and initial concentration of Cr(VI) on the adsorption capacity of the used adsorbents, (iv) to study the applicability of various two- and three-parameter isotherm models and kinetic equations for the determination of adsorption mechanisms and characteristic parameters for the present application, and (v) to determine adsorption thermodynamics of Cr(VI) on the studied adsorbents.

429

according to standard methods for the examination of water and wastewater using diphenylcarbazide method [29]. 2.2. Adsorbent preparation In this work, the natural LECA (NL), H2 O2 modified LECA (HML) and MgCl2 modified LECA (MGML) was used for removal of chromium from a simulated solution. First, the adsorbent was washed several times with deionized water to remove any soil impurity until the effluent turbidity was reduced to below 1 NTU. After primary washing, the adsorbent was dried at 110 ◦ C for 24 h to evaporate remaining water molecules. The dried adsorbent was then pulverized and sieved to 10–30 meshes (841–2000 ␮m). Mg-modified LECA (MGML) was prepared through transferring a portion of powdered LECA to a 1 L beaker containing 2 M MgCl2 solution, and the consituents were mixed for 24 h. The liquid to solid ratio was 50. Then, the modified adsorbent was filtered using a 0.45 micron filter (Watman) and rinsed with deionized water to remove excess amount of MgCl2 , then dried at 110 ◦ C for 24 h for further experiments. H2 O2 modified adsorbent was prepared using the same method as of the Mg-modified LECA. 2.3. Batch experiment All experiments were conducted in batch mode in 250 mL polyethylene flasks. Several experimental parameters including pH (1–9), temperature (20–60 ◦ C), adsorbent mass (2–10 g/L), initial chromium concentration (10–100 mg/L), and contact time were studied. Optimized adsorption time for natural and modified LECA adsorbents were first studied by varying the contact time at room temperature, pH of 3.0, and the adsorbent mass of 6 g per liter of solution. To do this, 6 g of adsorbent was added to 1 L of solution in a polyethylene flask containing chromium ions at a concentration of 10–100 mg/L. The mixture was shaken at 200 rpm (Hanna-Hi 190 M, Singapore). Samples were taken at predetermined time intervals, filtered (0.45 ␮, Wathman), and centrifuged (Sigma-301, Germany) then the chromium concentration was measured. The removal efficiency (RE) was calculated by means of the following equation (Eq. (1)): RE =

(C0 − Ce ) × 100 C0

(1)

where RE (%) is the percentage of chromium removed at equilibrium time; C0 and Ce are the initial and equilibrium concentrations of chromium (mg/L), respectively. 2.4. Adsorbent effect on electrical conductivity (EC) and turbidity In order to measure the influence of adsorbents on EC and turbidity of water, 6 g/L of natural or modified adsorbent was poured into 250 mL of deionized water (i.e. free of chromium ions) at pH 3.0 and shaken for 200 min at 200 rpm. At a predetermined time interval, samples were taken for conductivity and turbidity measurements (Jenway, Model 4520).

2. Materials and methods 2.5. Determination of the zero point charge 2.1. Chemicals Analytical grade K2 Cr2 O7 (Merck Chemical Corp.) was used for preparation of chromium stock solution by dissolving an appropriate amount in deionized water. The solution pH was adjusted and controlled during experiments using 0.5N NaOH or H2 SO4 , and pH was measured by means of a Jenway model 3510 pH-meter. 2 M MgCl2 and 30% H2 O2 were used for adsorbent treatment. The chromium concentration was measured by means of an UV/VIS spectrophotometer at 542 nm (model 1700, Shimadzu, Japan),

The zero point charge (ZPC) was determined using 0.01 M NaCl as an electrolyte by adding 0.1N NaOH or 0.1N HCl solutions. To conduct the test, 50 mL of the electrolyte was introduced into 8 beakers and then the pH was adjusted to the required value in the range from 2 to 12. A 0.5 g of pumice was then added into each beaker and shaken for 48 h. After 48 h of agitation, the adsorbent was filtered and the final pH of the filtrate was measured. The zero point charge of pumice was determined by plotting the initial pH versus the pH after 48 h. By plotting the initial pH versus the pH after 48 h

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of agitation, the zero point charge of adsorbents was determined, which were 5.7, 6.1 and 5.9 for natural, H2 O2 modified LECA and MgCL2 modified LECA, respectively.

Table 1 Chemical composition of the natural LECA and its Mg and H2 O2 modified form, in comparison to those of reported in the literature [19]. Component

2.6. Regeneration of spent adsorbent Regeneration of the saturated natural and modified adsorbents was carried out by adding the spent adsorbent into deionized water, 1 M H2 SO4 , 1 M HCl, and 1 M HNO3 . In this regard, the adsorbents were first saturated with chromium by adding 6 g/L of natural or modified adsorbents to three conical flasks containing 100 mg/L of chromium solution. The pH was adjusted at 3.0, and the mixture was stirred at 200 rpm until equilibrium time was reached. The saturated adsorbents was separated by filtration and dried at 105 ◦ C for 24 h. The saturated-dried adsorbent was introduced into deionized water, 1 M H2 SO4 , 1 M HCl, and 1 M HNO3 with shaking at 200 rpm for more than twice equilibrium time in order to attain maximum chromium desorption. The regenerated adsorbent was washed with deionized water to remove excess H2 SO4 , HCl, and HNO3 , and it was dried at 105 ◦ C for 24 h for the adsorption test. The regenerated-dried adsorbent was then introduced to 100 mg/L chromium solution to investigate its sorption percentage after first regeneration. Beside chemical regeneration, thermal regeneration was also conducted in the scope of this work. For this purpose, the saturated adsorbents were heated at 105 ◦ C, 200 ◦ C, and 300 ◦ C for 24 h. The thermally regenerated adsorbent was then introduced to 100 mg/L chromium solution to investigate its sorption percentage after first regeneration.

SiO2 Al2 O3 MgO P2 O5 SO3 K2 O CaO TiO2 Fe2 O3 SrO Na2 O Cl− SiO2 /Al2 O3

Chemical composition (wt.%) NL

HML

MGML

Ref. [19]

61.67 18.51 3.97 0.19 0.23 3.28 3.50 0.65 6.14 0.13 1.54 – 3.33

62.64 18.08 3.62 0.22 0.24 3.31 3.52 0.69 5.83 0.13 1.50 – 3.46

57.82 16.47 4.59 0.18 0.21 2.99 3.22 0.60 6.03 0.13 1.41 1.65 3.51

62 18 3 – – 4 3 – 7 – 2 – 3.44

determined by means of an X-ray fluorescence spectroscopy (XRF) instrument (Philips-Magix Pro, Netherland). Moreover, Fourier Transform Infrared (FTIR) spectroscopy was used as a complementary technique of XRD in order to obtain a qualitative characterization of the samples. The infrared (IR) spectra of the samples were obtained in the range of 4000–450 cm−1 wave number by means of a (Bruker-VERTEX 70, Germany). Fourier Transform Infrared (FTIR) spectrometer equipped with a 4 cm−1 resolution and in the transmission mode in spectroscopic grade KBr pellets for all the powders. The samples were prepared as pressed KBr disks. 2.9. Analytical methods

2.7. Non-linear regression analysis Due to the inherent bias resulting from linearization, alternative isotherm parameter sets were determined by the non-linear regression approach. This provides a mathematically rigorous method for determining isotherm parameters using the original form of the isotherm equation. In this study, a non-linear regression analysis was conducted to determine the isotherm and kinetic constants and statistical comparison values such as determination coefficient (R2 ), standard error of the estimate (SEE), and residual sum of squares (RSS). The batch adsorption data was evaluated using the DataFit® scientific software (version 8.1.69, Copyright© 1995–2005 Oakdale Engineering, RC167) containing 298 two-dimensional (2D) and 242 three-dimensional (3D) nonlinear regression models. The non-linear regression analysis was performed based on the Levenberg-Marquardt method with double precision as conducted by Yetilmezsoy [30]. The batch adsorption data was imported directly from Microsoft® Excel which was used as ODBC (Open Database Connectivity) data source. As the regression models were solved, they were automatically sorted according to the goodness-of-fit criteria into a graphical interface. To explore the statistical significance of the predicted results, an alpha (˛) level of 0.05 (or 95% confidence) was used in the non-linear regression analysis. 2.8. Characterization of adsorbents In this study, scanning electron microscope (SEM) analysis of the LECA samples was done in a LEO 1450 VP (England) operated at an accelaration voltage of 20 kV, with Au sputtering-coated samples fixed in an Al stub. The XRD patterns of the NL and its modified counterparts (i.e. HML and MGML) were collected by means of a PHILIPS Xpert pro with Cu K␣ as radiation (1.54056A◦ ) generated at 40 kV and 40 mA. Chemical compositions of the natural LECA (NL), hydrogen peroxide modified (HML) and Mg-modified (MGML) adsorbents were

The chromium concentration was measured by means of an UV/VIS spectrophotometer at 542 nm (model 1700, Shimadzu Japan) according to standards methods for the examination of water and wastewater using diphenylcarbazide method (method 3500-Cr D) [29]. This method is only used for the measurement of hexavalent chromium. For this purpose, a stock solution of diphenylcarbazide was prepared by adding 0.25 g of 1,5diphenylcarbazide onto 50 mL acetone. For the measurement of the hexavalent chromium concentration, 20 mL of chromium solution was transferred onto 50 mL beaker and then the pH was adjusted at 1.0(±0.3) with concentrate HNO3 . About 2 mL of prepared 1,5diphenylcarbazide solution was then added to the beaker and after 10 min reaction time, the absorbance of the purple colored solution was read at 540 nm. 3. Results and discussion 3.1. Adsorbent characterization Chemical compositions of the natural LECA (NL), hydrogen peroxide modified (HML) and Mg-modified (MGML) adsorbents, which were determined by means of an X-ray fluorescence spectroscopy (XRF) instrument (Philips-Magix Pro, Netherland), is summarized in Table 1. According to the results, the natural LECA sample is a typical aluminosilicate mineral with SiO2 /Al2 O3 ratio of 3.33. The chemical composition and the Si/Al ratio of the sample are very similar to those reported in the literature [19]. The BET (nitrogen sorption isotherm—Model ASAP 2000) specific surface area for the natural material was 11.53 m2 /g, while it remarkably increased to 53.72 and 76.12 m2 /g after modification with H2 O2 and MgCl2 , respectively. The chemical analysis data revealed that by modification of the NL sample with Mg2+ solution, magnesium cations were exchanged with other mobile cations such as calcium, sodium and potassium, in which the MgO content of the MGML was increased by 0.62% and

E.M. Kalhori et al. / Applied Surface Science 287 (2013) 428–442

Fig. 1. XRD patterns of (a) the studied natural LECA (NL), (b) H2 O2 modified LECA (HML) and (c) MgCl2 modified LECA (MGML).

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E.M. Kalhori et al. / Applied Surface Science 287 (2013) 428–442

the CaO, K2 O and Na2 O contents were decreased by 0.28%, 0.29% and 0.13%, respectively. Considering this and taking into account the XRD patterns (in which the overall patterns of the three samples are very similar), it can be suggested that the dominant mechanism of modification is ion exchange. On the other hand, chemical composition of the H2 O2 modified sample remains very close to that of NL, in which it can be suggested that hydrogen peroxide influence on the chemical composition of the modified sample is minimal. As seen in Fig. 1, The XRD analysis confirmed the mineralogical composition of the sample. The X high background and a very broad peak clearly indicate the presence of amorphous phase in the samples. Apart from amorphous phase, which is typical in the natural LECA’s samples, characteristic peaks of quartz, anorthite, calcite and dolomite can be seen among the crystalline phases presented in the samples [31]. Anorthite is the calcium-rich member of the plagioclase solid solution series with the ideal formula of CaAl2 Si2 O8 [32]. As can be seen, the XRD patterns of the modified samples remained almost intact, which can be concluded that the applied modification processes didn’t influence on the overall crystalline phases of the studied LECA sample. The SEM (LEO 1450 VP, England) micrographs of the LECA samples are shown in Fig. 2. While the overall morphology of the NL and its modified counterparts are similar, however, the HML sample shows smoother edges. Furthermore, smaller particulates can be observed in the HML and MGML, which can be attributed to the abrasion due to the 24 h of shaking (mixing) with the modifier solutions. FTIR spectra of the samples are illustrated in Fig. 3. The presence of a strong broad in-plane bending and stretching vibrations at about 1066 cm−1 represent the existence of Si–O–Si (siloxane groups). Bending and stretching modes of absorbed water molecules in the NL sample appeared at 3451 and 1640 cm−1 , respectively. The corresponding peaks of HML and MGML appeared at slightly lower wave numbers of 3444 and 1639 cm−1 (for HML) and 3419 and 1635 cm−1 (for MGML), respectively. Furthermore, a band at 3600–3650 cm−1 could be attributed to the stretching mode of the hydroxyl groups [33]. The absorption band at 1640 cm−1 is assigned to the H–O–H bending vibrations of water molecules adsorbed on the NL sample. The intensity and the location of this band is slightly changed in the HML and MGML samples, but this could be attributed to water content of the samples introduced during the modification process. Changing of the intensity of the band at 1066 cm−1 can be a result of altration in the symmetry of the surface Si–O–Si vibration as a consequence of the changes that occured in the electric field near the Si groups in proximity to more positively charged groups [34]. Nevertheless, the sharp band that appeared at 461 cm−1 can be considered as the fundamental 1 , which can be easily attributed to the presence of quartz phase [35].

3.2. Influence of adsorbent on solution EC and turbidity Partial dissolution of adsorbent in the surrounding solution may have an impact on the adsorption process affecting ion uptake capacity. The hardness of an adsorbent is a major factor, hence, in order to assess its stability in aqueous solution, the monitoring of electrical conductivity (EC) and turbidity may be useful. The corresponding test has been carried out in deionized water (Fig. 4). As could be observed, EC and turbidity increased with time for both natural and modified pumice samples. Solution EC increased from 5 to 3–13 (␮s/cm) and 5–17 (␮s/cm) and 7–24 (␮s/cm) for natural, H2 O2 and MgCl2 modified LECA, respectively. This increase can be attributed to the presence of some soluble constituents from the adsorbents. Furthermore, the higher increase in EC that was

Fig. 2. SEM images of (a) the natural LECA (NL), (b) H2 O2 modified LECA (HML) and (c) MgCl2 modified LECA (MGML).

observed for the modified MgCl2 modified LECA may be attributed to the release of Mg+2 ions loaded during the modification process. Turbidity increased from 0.2 to 1.2 NTU, 0.1 to 0.17 NTU and 0.19 to 1.22 NTU for natural H2 O2 and MgCl2 modified LECA, respectively. However, it should be noted that the increase in EC and turbidity in the recorded values after 300 min contact time appeared to be very low if compared with EC and turbidity in real wastewater. Therefore, the present adsorbents may have only a limited impact on the conductivity and the turbidity of aqueous solutions. 3.3. Influence of solution temperature The temperature effect was studied in the range of 20–60 ◦ C and thermodynamic parameters were calculated and summarized in Table 2. As shown in Fig. 5a, increases in solution temperature lead to decrease in chromium sorption capacity with higher effect on NL rather than HML or MGML. By increasing the solution temperature from 20 to 60 ◦ C, chromium removal percentages were decreased

E.M. Kalhori et al. / Applied Surface Science 287 (2013) 428–442

433

Fig. 3. FTIR spectra of the natural LECA (a), H2 O2 modified LECA (b) and MgCl2 modified LECA (c).

Table 2 Thermodynamic parameters for the present work. Studied adsorbent

Enthalpy (H◦ )

Entropy (S◦ )

G◦ Absolute temperature, T (K)

NL HML MGML

−19.21 −30.40 −22.20

−58.20 −89.80 −61.52

293

303

313

323

333

−2338 −4103 −4103

−1743 −3768 −3768

−1113 −2232 −3168

−538 −1401 −2134

−65 −828 −1839

30

NL

HM L

100

M GM L

removal efficiency (%)

(a)

E.M. Kalhori et al. / Applied Surface Science 287 (2013) 428–442

Solution EC

25 20 15 10 5 0 0

50

100

150

200

250

300

350

contact time (min)

Turbidity (NTU)

(b)

10

80

8

70

6

NL

HML

MGML

NL

4

HML

MGML

2

60

0 2

3

4

5

6

7

8

9

10

Fig. 6. Effect of adsorbent mass on chromium removal efficiency (chromium concentration = 100 mg/L, contact time 130 min, pH = 3.0, 20 ◦ C temperature and 200 rpm agitation speed).

1 0.8 0.6 NL

0.4

HM L M GM L 50

100

150

200

250

300

350

contact time (min) Fig. 4. Influence of adsorbent on water sample (a) conductivity and (b) turbidity (pH 5.5, 200 rpm agitation speed, 6 g/L adsorbent).

100

NL

HM L

M GM L

98

removal efficiency (%)

12

1.2

0

constant (8.314 J/mol.K), and T is the absolute temperature (K). A plot of ln k versus 1/T gives a straight line, the values of Ea and A0 can be obtained from the slope and the intercept, respectively. In the present work, the value of Ea was observed to be 19.21 kJ/mol, 30.40 kJ/mol and 22.20 kJ/mol for NL, HML and MGML, respectively, indicating physical adsorption rather than chemical sorption for chromium adsorption onto the studied adsorbent. Thermodynamic parameters were determined using temperatures ranging from 20 to 60 ◦ C using the equilibrium constant Kd (qe /Ce ). The change in free energy (G◦ ) is calculated from the following equation (Eq. (3)): G◦ = −RT ln kd

96

(3)

where G◦ is the standard free energy (kJ/mol). The values of enthalpy H◦ (kJ/mol) and entropy S◦ (kJ/mol) corresponding to the adsorption process were calculated from the following equation (Eq. (4)):

94 92 90 88

ln Kd =

86 84 20

25

30

35

40

45

50

55

60

o

Temperature ( C) 2

NL

HML

MGML

1.8 1.6 1.4

Ln k d

14

adsorbent mass (g/L)

0

(b)

16 90

1.4

0.2

(a)

18

qe (mg/g)

434

1.2 1 0.8 0.6 0.4 0.2 0 0.0029

0.003

0.0031

0.0032

0.0033

0.0034

0.0035

1/T Fig. 5. Effect of solution temperature on chromium removal efficiency (chromium concentration = 100 mg/L, contact time 130 min, 6 g/L adsorbent, pH 3.0 and 200 rpm agitation speed).

from 94% to 86%, 97% to 89% and 97% to 92% for NL, HML and MGMP, respectively. Furthermore, The Arrhenius equation was used to evaluate the nature of the adsorption (either physical (5–40 kJ/mol) or chemical (40–800 kJ/mol): ln kd = ln A0 −

Ea RT

(2)

where A0 is the temperature independent factor called “frequency factor”, Ea is the activation energy (kJ/mol), R is the gas law

S ◦ H ◦ − R RT

(4)

The values of enthalpy (H◦ ) and entropy (S◦ ) can be calculated from the slope and the intercept of the linear plot of ln Kd versus 1/T. Fig. 5b shows the thermodynamic plots and the related values that are collected in Table 2. The values of H◦ were negative for all systems, showing that the sorption reaction was exothermic in nature. The negative S◦ value indicates a decrease in randomness at the solid/liquid interface during the sorption process. In addition, the values of standard free energy (G◦ ) were negative indicating that the sorption of metal was not thermodynamically spontaneous. The negative value of G◦ increases as the temperature increases from 293 to 333 K. 3.4. Influence of adsorbent mass Influence of adsorbent mass was studied in the range 2–10 g/L for all systems and results are shown in Fig. 6. Removal efficiency increased by increasing adsorbent mass, hence, the highest removal percentage was observed using 10 g/L adsorbent. At pH 3.0, increasing adsorbent mass from 2 to 10 g/L increased the removal efficiencies from 69% to 98.30%, 73% to 99%, and 79% to 99% for NL, HML, and MGML, respectively, in agreement with the increase in the number of active sites as a higher amount of adsorbent was used. The increase in sorption percentages leads also to an increase in adsorption capacities from 11.50 mg/g to 16.40 mg/g, 12.2 mg/g to 16.40 mg/g, and 13.20 mg/g to 16.50 mg/g for NL, HML, and MGML, respectively. The linear increase of the adsorption capacities as a function of increasing adsorbent dosage is correlated to the availability of larger number of sorption sites to adsorb chromium ions.

E.M. Kalhori et al. / Applied Surface Science 287 (2013) 428–442

(a) 100

(a)

435

100

removal efficiency (%)

removal efficiency (%)

90 90

80

70 NL

HM L

M GM L

60

1

2

3

4

5

6

7

8

80 70 60 50 40 30

Cr=10mg/L

Cr=25 mg/L

20

Cr=50 mg/L

Cr=100 mg/L

10

9

0

Solution pH

removal efficiency (%)

90

Final solution pH

(b) 100

7 6 5 4 3 NL

HM L

M GM L

1

1

2

3

4

5

20

30

40

50

60

70

80

90

100

110

120

130

contac time (min)

(b) 8

2

10

6

7

8

80 70 60 50 40 30 20

0

The pH of the solution has an important role in the adsorption process. According to the pH, the surface of the adsorbent is positively or negatively charged [11]. The pHzpc values of used adsorbent were 5.7, 6.1, and 5.9 for NL, HML, and MGML, respectively. At higher and lower values, the surface adsorbent is occupied by OH− and H+ ions, respectively. When introduced into the solution, chromium dissolves and forms negatively charged chromium groups, as discussed below. Therefore, it is expected that an acidic environment will be the best medium for chromium removal. The effect of pH on chromium adsorption was examined for pH values from 1 to 9. Fig. 7a shows the effect of solution pH on chromium adsorption and Fig. 7b shows the final solution pH. The maximum sorption of chromium on the used adsorbents occurred at acidic environment (pH 1–3) by NL, HML, and MGMP. As can be seen from Fig. 7a, the percentages of chromium removal were 98%, 94%, 87%, 83%, 69%; 99%, 97%, 93%, 97%, 76%, 99.25%, 97%, 95%, 91%, and 83% by NL, HML, and MGML, respectively, in solutions that have pH values of 1, 3, 5, 7, and 9, respectively. In acidic environment, the main chromium species are HCrO4 − , Cr2 O7 2− and H2 CrO4 , while in alkaline medium CrO4 2− predominates [16]. In an acidic solution, the adsorbent surface was protonated to a high extent. Therefore the attraction between anionic species HCrO4 − and Cr2 O7 2− and a positively charged adsorbent surface had been strongly increased. In addition, lower pH values (namely pH < 1.5) enhance the reduction of Cr(VI) to Cr(III). This might cause poor detection of Cr(VI) concentration by the diphenylcarbazide method [5]. Increasing the pH of the solution was found to decrease the extent of positive charge on adsorbent leading to weak bonding of negatively charged species on adsorbent surface. In an alkaline environment, other negative ions such as OH− should compete with the major anion, CrO4 2− ion, for the sorption sites on the adsorbent [1,16]. Higher chromium sorption was observed in an acidic environment; the final solution pH was reported and results are shown in Fig. 7b. The pH of the final solution decreased slightly during chromium sorption; depending on the initial pH. The

Cr= 100 mg/L

10

20

30

40

50

60

70

80

90

100

110

120

130

contact time (min)

(c) 110 100

removal effieincy (%)

3.5. Influence of pH

Cr=25 mg/L

Cr=50 mg/L

10

9

Initial solution pH Fig. 7. Effect of pH on chromium removal percentage in (a) NL, (b) HML and (c) MGML (chromium concentration = 100 mg/L, contact time 130 min, 20 ◦ C temperature and 200 rpm agitation speed).

Cr=10 mg/L

90 80 70 60 50 40 30 20

Cr=10 mg/L

Cr=25 mg/L

Cr=50 mg/L

Cr=100 mg/L

10

0

10

20

30

40

50

60

70

80

90

100

110

120

130

contac time (min) Fig. 8. Influence of contact time and initial chromium concentration on the removal efficiency of NL (a), HML (b), and MGML (c) (pH = 3.0, adsorbent: 6 g/L, 20 ◦ C temperature and 200 rpm agitation speed).

slight decrease in solution pH may be attributed to ion exchange between loaded H+ ions and chromium species. Moreover, since higher chromium removal occurred at acidic pH and since the real chromium samples are acidic pH in nature, there is no need to lower the pH of the solution for practical adsorption applications. 3.6. Time–concentration profile and kinetic modeling In the first part of the present study, the adsorption characteristics of Cr(VI) on NL, HML, and MGML were examined at different Cr(VI) initial concentration with a reaction time elapse. Results for a Time–concentration profile are illustrated in Fig. 8a, b, and c for NL, HML, and MGML, respectively. The results shows that Cr(VI) removal percentage was increased with an increase in reaction time and the initial Cr(VI) concentration and equilibrium was almost attained in about 130 min for all adsorbents. Therefore, a time of 130 min was adopted thereafter. Depending on the initial chromium concentration, the precentages of chromium removal were 79.6%–94%, 96%–97%, and 87%–97% by NL, HML, and MGML, respectively. In the same manner of removal percentages, chromium adsorption capacities were also increased with an increase in initial Cr(VI) concentration. Therefore, depending on the Cr(VI) concentration, adsorption capacities were observed to be in the ranges of 1.33–15.70 mg/g, 1.44–16.20 mg/g and

436

E.M. Kalhori et al. / Applied Surface Science 287 (2013) 428–442

Table 3 Adsorption isotherm constants and statistical comparison values of two-parameter models for toxic chromium adsorption on natural LECA and surface modified LECA with H2 O2 and MgCl2 . Isotherm models Langmuir qe =

Isotherm parameters

Qo ·kL ·Ce 1+kL ·Ce

qm

Freundlich qe = kf · Ce 1/n

Temkin qe =

RT bt

Natural LECA

ln(at Ce )

qD exp





198.39

218.29

236.24

0.168 0.916 23.57 2777.72

0.252 0.886 31.01 4807.49

0.245 0.892 32.78 5374.19

kf N R2 SEEa RSSb

47.56 3.26 0.828 33.85 5727.39

59.82 3.43 0.812 39.80 7921.94

59.75 3.09 0.863 36.86 6792.91

1.725

2.876

2.729

kt

−BD RT ln 1 +

1 Ce

2 

0.0645 0.910 24.51 3003.86

BD

qD R2 SEEa RSSb a b

Surface modified LECA with MgCl2

B R2 SEEa RSSb

B1 R2 SEEa RSSb Dubinin-Radushkevich qe = 

Surface modified LECA with H2 O2

0.0616 0.885 31.09 4835.35

0.0553 0.917 28.79 4143.06

1.417

0.703

0.595

168.525 0.979 11.045 487.93

198.19 0.924 25.216 3179.18

209.65 0.881 32.548 4237.45

Standard error of the estimate. Residual sum of squares.

1.45–16.20 mg/g for NL, HML, and MGML, respectively. This could be attributed to an increase in the driving force with an increasing initial solute concentration, which lowers the mass transfer resistance [36]. At a given mass of adsorbent, the active sites for sorption of solute are fixed; therefore, sorption capacity may be decreased by increasing initial solute concentration. In the present work, for a given mass of adsorbent, the sorption capacity increased by increasing chromium concentration, indicating that the internal part of the adsorbent was used also for chromium sorption. 3.7. Fitting of equilibrium data onto isotherm model 3.7.1. Two-parameter isotherm modeling An adsorption isotherm describes the relationship between the amount of adsorbate that is adsorbed on the adsorbent and the concentration of dissolved adsorbate in the liquid at equilibrium. Several two- or three-parameter models have been published in the literature to describe experimental data of adsorption isotherms. In this study, the adsorption mechanisms and characteristic parameters of the present processes were analyzed by four two-parameter isotherm models, namely, Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich, which are widely used to describe experimental results in a wide range of concentrations. Table 3 shows the used two-parameter isotherm model for the present work. The Freundlich equation was developed empirically, having no theoretical basis. It is useful for describing the sorption of ions by chemical adsorption and surface precipitation reactions. This model, which is appropriate for heterogeneous systems, is expressed by Eq. (5) as follows: qe = Kf Ce 1/n

(5)

where qe is the amount of adsorbate (mg/g), Ce is the equilibrium concentration of adsorbate (mg/L), Kf (mg1−1/n L1/n /g), and 1/n is the Freundlich constant. A high value of Kf characterizes a high affinity to the adsorbate. For favorable adsorption, the value of the Freundlich constant (n) should be in the range of 1–10.

This empirical equation assumes that the stronger binding sites are occupied first and that the binding strength decreases with the increasing degree of site occupation [37]. It is often criticised for lacking a fundamental thermodynamic basis since it does not reduce to Henry’s law at low concentrations [38]. From Table 3, the values of Freundlich constant (n) were in the range of 1–10 (3.26 for Natural LECA, 3.43 for Surface modified LECA with H2 O2 and 3.09 for Surface modified LECA with MgCl2 ) reflecting the favorable sorption of chromium on the three used adsorbents. The values of the affinity coefficient (Kf ) were 47.56, 59.75, and 59.82 for natural LECA, surface modified LECA with MgCl2 , and surface modified LECA with H2 O2 , respectively, showing higher affinity of chromium toward surface modified LECA with H2 O2 rather than others. In contrast to the Freundlich equation, the Langmuir equation was developed from a theoretical standpoint to model the adsorption of gas molecules on surfaces. The Langmuir isotherm defines the equilibrium parameters of homogenous surfaces, monolayer adsorption and distribution of adsorption sites [39]. In its formulation, binding to the surface is primarily by physical forces and implict in its derivation is the assumption that all adsorption sites have equal affinities for adsorbate molecules and that the presence of adsorbed molecules at one site does not affect the adsorption of molecules at an adjacent site [40–42]. The mathematical correlation of the Langmuir isotherm has provided a basis for development of other models, which include similarity of adsorption and ion exchange, Gaussian energy distribution, degree of surface heterogeneity and high solute concentrations range [37]. The Langmuir isotherm was later applied to the adsorption of ions from a solution on mineral surfaces. It works reasonably well for describing ions that only bind via adsorption mechanisms. Accordingly, most anions conform to the Langmuir equation. One important point to note about the Langmuir equation is that it predicts only adsorption phenomena. Hence, it only allows a finite amount of material to be retained on the surface. The maximum amount of adsorbate on an adsorbent surface is determined by the number of adsorption

E.M. Kalhori et al. / Applied Surface Science 287 (2013) 428–442 Table 4 Maximum sorption capacity of some adsorbent for hexavalent chromium adsorption.

437

the porous structure of the adsorbent. This model is expressed as follows: qe = qD exp(−BD ε2D )

(8)

Adsorbent

qmax (mg/g)

Reference

Polyaniline-Poly ethylene glycol composite Algal bloom residue derived activated carbon Dodecylamine Modified sodium montmorillonite Biopolymer chitosan coated with poly 3-methyl thiophene polymer Methylated biomass of Spirulina platensis Activated Carbon Derived from acrylonitrile-divinylbenzene copolymer Polypyrrole-Polyaniline nanofibers Thiocarbamoyl Chitosan Aluminum magnesium mixed hydroxide Waste activated carbons treated with sulfuric acid Waste activated carbons treated with nitric acid Walnut hull Montmorillonite-supported magnetite nanoparticles Montmorillonite unsupported magnetite nanoparticles Natural LECA Surface modified LECA with H2 O2 Surface modified LECA with MgCl2

68.91 155.52 23.69

[5] [9] [22]

127.62

[43]

16.70 73.90

[44] [45]

227 434.8 105.3 7.50

[46] [47] [48] [49]

10.93

[49]

98.13 15.3

[50] [51]

E=

10.6

[51]

According to Table 3, the highest value of qD was observed for surface modified LECA with MgCl2 , showing higher sorption capacity of MgCl2 surface modified adsorbent compared to other used adsorbents. This agrees with the previous finding considering Langmiur constant (qm ), where the highest value of qm was also associated with surface modified LECA with MgCl2 . The estimated constants (BD ), which are related to adsorption energy, were observed to be 1.417, 0.703 and 0.595 mol2 /kJ2 for natural LECA, surface modified LECA with MgCl2 , and surface modified LECA with H2 O2 , respectively. The mean free energy (E) is a parameter used in predicting the type of adsorption. An E value < 8 kJ/mol is an indication of physico-sorption [52]. This constant gives an idea about the mean free energies which were valued as 0.59, 0.83, and 0.92 kJ/mol for natural LECA, surface modified LECA with H2 O2 , and surface modified LECA with MgCl2 , respectively; showing physico-sorption nature of hexavalent chromium on used adsorbents. Equilibrium data were fitted onto all the investigated twoparameter isotherm models very well, However, the highest correlation coefficient, the lowest Standard Error of the Estimate (SEE), as well as the lowest Residual Sum of Squares (RSS) were observed with the Dubinin-Radushkevich isotherm model.

198.39 218.29 236.24

 qe = qD exp

(qm bCe ) (1 + bCe )

Present work Present work Present work

(6)

where qe is the equilibrium amount of adsorbate (mg/g), Ce is the equilibrium concentration of adsorbate (mg/L), qm is the maximum adsorption capacity (mg/g), and b (L/mg) is the Langmuir constant. Based on this model, maximum adsorption capacities were 198.39 mg/g, 218.29 mg/g, and 236.24 mg/g for natural LECA, surface modified LECA with MgCl2 , and surface modified LECA with H2 O2 , respectively. Maximium sorption capacities of some recently used adsorbent are shown in Table 4. Except for thiocarbamoyl chitosan adsorbent (qm = 434.80 mg/g), the present used adsorbents have higher sorption capacity compared to the other adsorbents given in Table 4. The Temkin isotherm is also available for heterogeneous adsorption of adsorbate on a surface. The non-linear form of the Temkin model is given by Eq. (7): qe =

 RT  b1

ln(kt Ce )



1 Ce

(9)

where qD is the Dubinin–Radushkevich model constant (mg/g), which is related to the degree of sorbate sorption by the sorbent surface, while εD is the Polanyi potential. The higher the value of qD , the higher the sorption capacity. The Dubinin-Radushkevich model has been chosen to estimate the apparent free energy of adsorption. The constant (BD ) is related to the mean free energy of adsorption per mole of the adsorbate (mol2 /kJ2 ) as it is transferred to the surface of the solid from the infinite distance in the solution. This energy (kJ/mol) can be calculated using the following relationship:

sites, which refer to monolayer capacity or maximum adsorption capacity. This model is described as follows (Eq. (6)): qe =



−BD RT ln 1 +

 2 

(7)

where B1 = RT/b1 , b1 is the adsorption heat (kJ/mol) and Kt is the equilibrium binding constant (L/g) corresponding to the maximum binding energy. A high value of b1 shows a fast sorption of adsorbate at initial stage. Similarly, a low value of Kt is related to weak bonding of adsorbate onto the medium. Based on this model, the order of adsorption heats were 0.0553, 0.0616, and 0.0645 kJ/mol for Surface modified LECA with MgCl2 ; Surface modified LECA with H2 O2 and Natural LECA, respectively, showing that the highest sorption of chromium at initial stage was onto natural LECA rather than others. On the other hand, the lowest value of Kt was observed for natural LECA rather than modified once, indicating a weak bonding of chromium on natural LECA. Another popular isotherm model is Dubinin-Radushkevich isotherm. This model has been used instead of Langmuir isotherm, since it is more general than the Langmuir model, as its deviations is not based on ideal assumptions such as equipotential of sorption sites, absence of steric hindrances between sorbed and incoming particles, and surface homogeneity on microscopic level [52]. This model reports that the characteristic sorption curve is related to

1



(10)

2BD

3.7.2. Three-parameter isotherm modeling In this work, the adsorption mechanisms and characteristic parameters of the present processes were also analyzed by threeparameter (Redlich–Peterson, Sips, and Khan) isotherm models for the fitting of equilibrium data. The Redlich–Peterson [53] isotherm is an empirical isotherm incorporating three parameters. It combines elements from both the Langmuir and Freundlich equations, and the mechanism of adsorption is hybrid and does not follow ideal monolayer adsorption: qe =

kRP .Ce g

1 + pe .Ce

(11)

where KRP is the Redlich-Peterson isotherm constant (L/g); pe , the Redlich–Peterson model constant (L/mg); and g is the Redlich–Peterson model exponent that lies between 0 and 1. Ce is the equilibrium liquid-phase concentration of the adsorbate (mg/L), and qe is the equilibrium adsorbate loading onto the adsorbent (mg/g). This model has some limitations at high liquid-phase g concentrations of the adsorbate, where the values of pe · Ce become much greater than 1, and consequently, Eq. (11) reduces to the Freundlich equation (Eq. (12)) as follows: qe =

kRP 1−ˇ Ce pe

(12)

438

E.M. Kalhori et al. / Applied Surface Science 287 (2013) 428–442

Table 5 Adsorption isotherm constants and statistical comparison values of three-parameter models for toxic chromium adsorption on natural LECA and surface modified LECA with H2 O2 and MgCl2 . Isotherm models

Isotherm parameters

Redlich-Peterson qe =

kRP .Ce 1+pe .Ce g

kRP pe G R2 SEEa RSSb

Khan qe =

qm bK Ce (1+bK Ce )aK

qm bK aK R2 SEEa RSSb

Sips qe =

qms .kS .Ce ms 1+ks ·Ce ms

qms mS kS R2 SEEa RSSb

a b

Natural LECA

Surface modified LECA with H2 O2

Surface modified LECA with MgCl2

34.176

60.878

76.046

0.181 0.987 0.917 26.329 2772.90

0.346 0.948 0.889 34.203 4679.58

0.535 0.874 0.907 34.03 4631.61

182.33

175.34

139.96

0.189 0.968 0.917 26.25 2757.21

0.339 0.933 0.890 34.09 4649.46

0.500 0.852 0.908 33.74 4552.49

7.839

23.059

6.509 2.846 0.0369 0.965 17.107 1170.545

5.783 0.0402 0.955 21.713 1885.796

4.539 0.111 0.928 29.917 3580.057

Standard error of the estimate. Residual sum of squares.

where KRP /pe and (1 − ˇ) represent the parameters, KF and 1/n of the Freundlich model, respectively. When the g value equals 1, Eq. (11) reduces to Langmuir model, whereas, when the g value equals zero, the equation reduces to Henry’s law. The parameters of Redlich-Peterson isotherm model for the present work are shown in Table 5. From Table 5, equilibrium data was best fitted to the Redlich-Peterson isotherm surface modified LECA with MgCl2 rather than the surface modified LECA with H2 O2 . AsThe g values approach unity, the data are preferably fitted to the Langmuir model. The Khan isotherm model for a pure solution is expressed in the following equation: qe =

qm bK Ce (1 + bK Ce )

aK

(13)

where qm and bK are the Khan model constants and aK is the Khan model exponent. For the results given in Table 5, qm values were 182.33, 175.34, and 139.96 mg/g for natural LECA, surface modified LECA with H2 O2 , and surface modified LECA with MgCl2 , respectively. The order of sorption capacities for the used adsorbents obtained from Khan isotherm model were not in accordance with Langmiur constant (qm ). The values of bK were 0.189, 0.339, and 0.500 for natural LECA, surface modified LECA with H2 O2 and surface modified LECA with MgCl2 , respectively. Recognizing the problem of the continuing increase in the adsorbed amount with an increase in concentration of the sorbate in the Freundlich equation, Sips [54] proposed an equation similar in form to the Freundlich equation, but it has a finite limit when the concentration is sufficiently high. This model predicts a monolayer sorption capacity for high sorbate concentrations and reduces to Freundlich equation for lower sorbate concentrations: qe =

qms .kS .Ce ms 1 + ks · Ce ms

(14)

where qms is the Sips maximum adsorption capacity (mg/g), KS is the Sips equilibrium constant (L/mg)m , and mS is the Sips model exponent. The values of the three isotherm models are presented in Table 5. According to the Sips isotherm model, the maximium sorption capacities were 6.509, 7.839, and 23.059 mg/g for for natural LECA, surface modified LECA with H2 O2 , and surface modified LECA with MgCl2 , respectively.

Among the tested three-parameter equations, and based on higher determination coefficient, lower Standard Error of the Estimate (SEE), and lower Residual Sum of Squares (RSS), the Sips isotherm was the best model to describe the adsorption of hexavalent chromium ion on the used adsorbents. Comparing all the used isotherm models, it seems that three-parameter isotherm model is the best to describe adsorption of hexavalent chromium on the used adsorbents. 3.8. Non-linear kinetic modeling In adsorption processes, the mechanism of adsorption (such as chemical reaction, diffusion control, and mass transfer) was determined from kinetic models. Therefore, the time-concentration profile characteristics of the sorption of chromium by natural LECA, surface modified LECA with H2 O2 , and surface modified LECA with MgCl2 , were analyzed by Ho and McKay’s pseudo-second order, Elovich, Bangham, and modified Freundlich kinetic models. The kinetic parameters are collected in Table 7. The pseudo-second order kinetic model of Ho and McKay is [55]: qt =

q2e k2 t 1 + qe k2 t

(15)

where qe and qt are the amounts (mg/g) of adsorbate at equilibrium and at a given time t (min), respectively; and k2 is the rate constant (g/mg/min). As qt /t approaches the value of zero (mg/g min), The initial adsorption rate h0 reduces to the following expression: h0 = k2 qe 2

(16)

For systems obeying pseudo-second order kinetic model, the plot of t/q against t of Eq. (15) should give a linear relationship, from which qe and k2 can be determined from the slope and intercept of the plot, and there is no need to know any parameter beforehand. From Table 6, the sorption rates given by the pseudo-first order model for natural LECA, surface modified LECA with H2 O2 and surface modified LECA with MgCl2 were found to be in the ranges of 0.0082–0.0011 (g/mg/min), 0.0089–0.0012 (g/mg/min), and 0.0103–0.0013 (g/mg/min), respectively; depending on the initial chromium concentration (10–100 mg/L). It is reported that the pseudo-second order rate constant is not a simple function of the initial solute concentration [56]. In the present work, the change in

Table 6 Comparison of kinetics constants for toxic chromium adsorption on natural LECA and surface modified LECA with H2 O2 and MgCl2 at different initial chromium concentrations. Kinetic models

Kinetic constants

Natural LECA 10 mg/L

qt =

1 ˇ

ln(˛ˇ) +

ko m 2.303V





1 ˇ

C0 C0 −qt m



ln t

=

a

25 mg/L

50 mg/L

0.0037

0.0021

K2

0.0082

ho R2 SEEa RSSb

0.230 0.851 0.693 7.199

0.707 0.876 1.646 40.65

1.756 0.924 2.394 85.98

4.319 0.931 4.532 308.11

0.293 0.891 0.579 5.039

0.897 0.914 1.299 25.292

12.471

0.789

2.588

7.01

0.0812 0.929 4.749 315.86

0.829 0.864 0.667 6.222

0.320 0.897 1.476 30.49

0.167 0.909 2.647 98.06

12.32

1.207

3.121

5.984

0.0032

0.0019

0.0011

0.0089

Surface modified LECA with MgCl2 100 mg/L 0.0012

10 mg/L 0.0103

25 mg/L 0.0042

50 mg/L 0.0023

100 mg/L 0.0013

2.093 0.912 2.509 94.47

5.018 0.922 4.556 311.39

0.346 0.893 0.558 4.673

0.996 0.925 1.139 19.445

2.321 0.910 2.385 85.34

5.436 0.896 5.049 382.40

18.295

0.974

3.182

8.892

22.351

0.086 0.931 4.417 273.19

0.841 0.883 0.603 5.096

0.339 0.915 1.255 22.04

0.178 0.912 2.441 83.42

11.66

1.189

2.951

5.626

11.197

˛

0.553

1.840

5.103

ˇ R2 SEEa RSSb

0.825 0.825 0.778 8.464

0.313 0.849 1.875 49.201

0.162 0.911 2.679 100.52

˛

1.211

3.196

6.155

K R2 SEEa RSSb

3.708 0.825 0.778 8.465

1.641 0.849 1.875 49.202

3.025 0.911 2.679 100.53

11.18 0.929 4.749 315.86

5.745 0.864 0.667 6.222

5.343 0.897 1.476 30.49

24.64 0.909 2.647 98.06

1831.9 0.931 4.417 273.19

7.565 0.882 0.603 5.097

11.973 0.915 1.255 22.04

125.93 0.912 2.441 83.419

22016.9 0.923 4.508 284.54

kf M R2 SEEa RSSb

0.034 1.707 0.969 0.328 1.509

0.050 1.910 0.981 0.671 6.309

0.083 2.389 0.988 0.989 13.686

0.103 2.614 0.991 1.697 40.329

0.058 2.059 0.981 0.248 0.864

0.081 2.331 0.989 0.475 3.157

0.107 2.675 0.987 0.985 13.581

0.133 3.016 0.989 1.711 40.973

0.076 2.320 0.976 0.271 1.029

0.101 2.632 0.987 0.493 3.406

0.126 2.967 0.983 1.072 16.091

0.146 3.209 0.986 1.918 51.491

0.0893 0.923 4.508 284.54

+ ˛ log(t)

Modified Freundlich qt = kf Co t1/m

b

10 mg/L

q2 k t e 2

Bangham log log log

100 mg/L

1+qe k2 t

Elovich qt =



50 mg/L

E.M. Kalhori et al. / Applied Surface Science 287 (2013) 428–442

Ho and McKay’s pseudo-second order

Surface modified LECA with H2 O2

25 mg/L

Standard error of the estimate. Residual sum of squares.

439

440

E.M. Kalhori et al. / Applied Surface Science 287 (2013) 428–442

the second order constant (k2 ) with initial chromium concentration was not linear and was in agreement with Azizian comments [56] on sorption kinetic models. The initial adsorption rate h0 increased by increasing the initial chromium concentration, and the highest h0 value was observed for a surface modified LECA with MgCl2 . The Elovich kinetic model is another popular model which is successfully used for a description of second order kinetics, assuming that the actual solid surfaces are energitically heterogeneous. This model is based on chemi-sorption phenomena; firstly, it was used to describe the rate of adsorption of carbon monoxide on manganese dioxide that decreases exponentially as a result of increasing the amount of gas adsorbed and expressed as [55]: qt =

1 1 ln(˛ˇ) + ln t ˇ ˇ

(17)

where ˛ is the initial rate (mg/g min) because (dqt /dt) approaches ˛ when qt approaches zero, and the parameter ˇ is related to the extent of surface coverage and activation energy of chemisorptions (g/mg). This model involves variation of energies of chemisorption as a function of the extent of surface coverage. On the other hand, this model suggests that the active sites are heterogeneous in nature and therefore, exhibit different activation energies for chemisorption [22]. The parameters of Elovich kinetic model are presented in Table 6. From this table, it can be noted that by increasing the initial chromium concentration, the constant ˛ was increased and the constant ˇ decreased. In addition, one can see from Table 6 that with an increase in initial solute concentration, the fitted data showed a higher correlation coefficient. It is reported that the constant ˛ is related to the rate of chemisorption and the constant ˇ is related to the surface coverage [57]. Therefore, increasing the initial chromium concentration will increase the rate of chemisorption. while increasing the initial chromium concentration, causes a decrease in the value of constant ˇ, hence, increasing the concentration of solute will decrease the available sorption sites on the surface for the sorbates. Kinetic data can be further used to check whether pore diffusion is the only rate-controlling step or not. If the experimental data are represented by this equation, the adsorption kinetics are limited by the following pore diffusion Bangham equation:



log log

C0 C0 − qt m



= log

 k m  o 2.303V

+ ˛ log(t)

(18)

where q and t are defined as in the pseudo second-order model, C0 is the initial chromium concentration in the solution (mg/L), V is solution volume (mL), m is the mass of adsorbent per liter of solution (g/L), and ko and ˛ are constants. From Table 6, the fitting of kinetic data onto double logarithmic plot according to the Bangham equation was perfectly linear at a higher chromium concentration (50–100 mg/L), indicating that the diffusion of chromium into the used adsorbent at higher concentations is not the only ratecontrolling step. In addition, at lower concentrations (10–25 mg/L chromium concentration), it seems that the diffusion process in the only rate-controlling mechanism. The modified Freundlich equation was originally developed by Kuo and Lotse in 1973 [58]. This model is expressed, as follows: qt = kf C0 t 1/m

parameter with an increase in the initial metal concentration leads to an increase in the solution ionic strength. From the modified Freundlich kinetic equation (Eq. (19)), one can conclude that an increase in the metal concentration will increase the surface loading rate and solution ionic strength and consequently increase the solute uptake capacity. Comparing the determined cofficients, standard error of the estimate (SEE) and residual sum of squares (RSS), the modified Freundlich kinetic model was found to be the best model for describing the time–concentration data for adsorption of hexavalent chromium on the used adsorbents with a higher determinaton cofficient, lower standard error of the estimate (SEE) and lower residual sum of squares (RSS).

(19)

where qt is the amount of adsorbed hexavalent chromium (mg/g) at time t; kf is the apparent adsorption rate constant (L/g min), C0 the initial metal concentration (mg/L), and m is the Kuo–Lotse constant. The values of kf and m are used empirically to evaluate the effect of surface loading and ionic strength on the adsorption process. From Table 6, the values of kf were increased with increases in the initial chromium concentration, which showed that increases in the metal concentration will increase the surface loading rate leading to an increase in the metal uptake rate. An increase in m

3.9. Regeneration of spent adsorbent Regeneration of spent adsorbent is a key factor in the adsorption process that should be considered while investigating the cost effectiveness of the system. In the present work, the spent adsorbents were regenerated to assess their reusability and to examine the possiblity of recovery of valuble adsorbed components. In the case of NL, the regeneration percentages after 230 min contact time were observed to be 13.7%, 83.4%, 89.4%, and 69.8%, with deionized water, H2 SO4 , HCl, and HNO3 respectively. Thermal regeneration of NL was observed to have precentages as low as 1.3% 6.2%, and 7.63% at 105 ◦ C, 200 ◦ C, and 300 ◦ C after 24 h, respectively. For HML, the regeneration percentages after 230 min contact time were observed to be 19.4%, 88.4%, 91.3%, and 92.4% with deionized water, H2 SO4 , HCl, and HNO3 , respectively. The regeneration percentages for HML were 5.3%, 6.7%, and 7.9% at 105 ◦ C, 200 ◦ C, and 300 ◦ C, respectively. In the case of MGML, the regeneration percentages were observed to be 24.63%, 93%, 89%, and 92% with deionized water, H2 SO4 , HCl, and HNO3 , respectively. Thermal regeneration percentages were 4.6%, 8.2% and 8.9% at 105 ◦ C, 200 ◦ C and 300 ◦ C for MGML, respectively. Briefly, the regeneration percentages according to the used chemicals can be arranged as follows: NL: HCl > H2 SO4 > HNO3 > Deionized Water > Thermal Regeneration. HML: HNO3 > HCl > H2 SO4 > Deionized Water > Thermal Regeneration. MGML: HCl > HNO3 > H2 SO4 > Deionized Water > Thermal Regeneration. As can be seen from the above relationships, it is well demonstrated that chemical modification is the best regeneration method for desorption of chromium from saturated adsorbent. This may be due to the ion exchange between H+ ions from an acid solution with adsorbed chromium. In addition, poor regeneration with deionized water and thermal methods reveals that adsorbed chromium ions are strongly bouded on adsorbents and consequently, partially desorbed to solution. 4. Conclusions In this work, the removal of toxic hexavalent chromium ion from an aqueous solution was studied by natural LECA, surface modified LECA with H2 O2 , and surface modified LECA with MgCl2 . The used adsorbents were characterized by Scanning Electron Microscopy (SEM), X-ray diffraction (XRD), X-ray fluorescence spectroscopy (XRF), and Fourier Transform Infrared (FTIR) spectroscopy. The modification of adsorbents with MgCl2 and H2 O2 improved the specific surface area and the metal sorption capacity. The effect of MgCl2 was higher than H2 O2 on improving both

E.M. Kalhori et al. / Applied Surface Science 287 (2013) 428–442

the specific surface area and the metal sorption capacity. The amount of toxic hexavalent chromium ion adsorbed per unit of adsorbents mass increased with increases in initial metal concentration and adsorbents dosage, while it decreased with increases in the solution pH and temperature. Equilibrium data was best fitted in Sips (two-parameter) and Dubinin-Radushkevich (threeparameter) equations compared to other models. Kinetic studies on the adsorption of Cr(VI) onto the used adsorbents revealed that the experimental data was best fitted in the modified Freundlich kinetic model. At lower metal concentrations (10–25 mg/L of chromium concentration), the diffusion process was the only rate-controlling mechanism. The studied adsorbents partially increased the solution conductivity and turbidity. Spent adsorbents were regenerated by deionized water, H2 SO4 , HCl, and HNO3 and thermal methods. Results demonstrated that an HCl solution is the best to regenerate the spent natural and surface modified LECA with MgCl2 with regeneration percentages of 89.4%, initially and 93% later. Higher regeneration for spent surface modified LECA with H2 O2 was observed using HNO3 (92.4%). Thermal regeneration and deionized water showed lower regeneration percentages compared to acidic solutions.

Conflict of interest The authors have declared no conflict of interest.

Acknowledgments This project was supported financially by Alborz University of Medical Science. The authors gratefully thank the Alborz University of Medical Science. The authors also gratefully acknowledge Assist. Prof. Dr. Louis Mazzari (Bogazici University, Department of Western Languages and Literatures) for his helpful comments on the original manuscript and language editing.

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