Equilibrium isotherms, kinetics, and thermodynamics studies for congo red adsorption using calcium alginate beads impregnated with nano-goethite

Ecotoxicology and Environmental Safety 141 (2017) 226–234

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Equilibrium isotherms, kinetics, and thermodynamics studies for congo red adsorption using calcium alginate beads impregnated with nano-goethite

MARK

⁎

Venkata Subbaiah Munagapati, Dong-Su Kim

Department of Environmental Science and Engineering, Ewha Womans University, 11-1 Daehyun-Dong, Seodaemun-Gu, Seoul 120-750, Republic of Korea

A R T I C L E I N F O

A B S T R A C T

Keywords: Adsorption Congo red Kinetics Isotherms Thermodynamics Temperature

The present study is concerned with the batch adsorption of congo red (CR) from an aqueous solution using calcium alginate beads impregnated with nano-goethite (CABI nano-goethite) as an adsorbent. The optimum conditions for CR removal were determined by studying operational variables viz. pH, adsorbent dose, contact time, initial dye ion concentration and temperature. The CABI nano-goethite was characterized by Fourier transform infrared spectroscopy (FTIR), X- ray diffraction (XRD) and Scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS) analysis. The CR sorption data onto CABI nano-goethite were described using Langmuir, Freundlich, Dubinin-Radushkevich and Temkin isotherm models. The results show that the best fit was achieved with the Langmuir isotherm model. The maximum adsorption capacity (181.1 mg/g) of CR was occurred at pH 3.0. Kinetic studies showed that the adsorption followed a pseudo-second-order model. Desorption experiments were carried out to explore the feasibility of regenerating the adsorbent and the adsorbed CR from CABI nano-goethite. The best desorbing agent was 0.1 M NaOH with an efficiency of 94% recovery. The thermodynamic parameters ΔG°, ΔH°, and ΔS° for the CR adsorption were determined by using adsorption capacities at five different temperatures (293, 303, 313, 323 and 303 K). Results show that the adsorption process was endothermic and favoured at high temperature.

1. Introduction Synthetic dyes are widely used in different industries like paper, printing, rubber, food processing, cosmetic, leather, plastics, textile, pharmaceutical etc. as visual enhancer and also to increase the attractiveness of the product. Currently more than 10,000 dyes are commercially available owning diverse applications with a global annual production in excess of 7×105 million tons. On the other hand around 5–10% of these dyestuffs are discharged into water as wastes (Liu et al., 2014). Discharge of wastewater containing even a small amount of synthetic dyes is a serious matter of concern because of harmful effects of those dyes to the aquatic environment as well as carcinogenic and mutagenic effects to human beings. Therefore, it is important to remove the dyes from wastewater before disposal into natural water bodies. Congo red [1-naphthalene sulfonic acid, 3,30-(4,40-biphenylenebis (azo))bis(4-amino-) disodium salt] is a water soluble diazo dye. CR is widely used in printing, textile, paper, leather and plastic industries (Shu et al., 2015). It is an anionic acid dye used as a laboratory aid (pH indicator and a histological stain for amyloid) for testing free hydrochloric acid in gastric contents, in the diagnosis of amyloidosis. It has a

⁎

strong affinity to cellulose fibres and thus is employed in textile industries. It is considered as toxic due to its metabolization to benzidine (well-known human carcinogen) and its exposure causes an allergic reaction. Even a low concentration of CR dye affects the aquatic life and thus the food web. It also leads to health hazardous symptoms such as difficulties in breathing, diarrhoea, vomiting and nausea. Therefore, the removal of CR from wastewater effluents before mixing with unpolluted natural water bodies is of environmental significance. Numerous techniques for the removal of dyes from effluents have been developed, such as oxidation (Osugi et al., 2009), adsorption (Zhou et al., 2012), photocatalysis (Liu et al., 2013), coagulation-flocculation (Chafi et al., 2011), ozonation (Moussavi and Mahmoudi, 2009) and membrane filtration (Solis et al., 2012). Among all these methods, adsorption is the most popular treatment process for the removal of dye from an aqueous solution due to its simplicity in operation, high treatment efficiency without discharging any harmful by-products, easy recovery and the reusability of the adsorbent. Metal oxides in particularly, iron oxides and hydroxides (hematite, goethite, maghemite, magnetite, etc.) play significant role in different areas of environmental chemistry. The nano powder form of metal oxides/hydroxides provides a high surface area for increased adsorp-

Corresponding author. E-mail address: [email protected] (D.-S. Kim).

http://dx.doi.org/10.1016/j.ecoenv.2017.03.036 Received 16 August 2016; Received in revised form 14 March 2017; Accepted 22 March 2017 0147-6513/ © 2017 Elsevier Inc. All rights reserved.

Ecotoxicology and Environmental Safety 141 (2017) 226–234

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crystalline phase of the samples was conducted using a D/Max-2500 V/ PC Rigaku X-ray diffractometer (XRD, Japan) with Cu-Kα radiation source operating under a voltage of 40 kV and a current of 50 mA. The diffraction patterns were recorded in the 2Ɵ range of 10–90°.

tion capacity, but this small particle size also drives the need for energyintensive post-treatment filtration to recover the nanoparticles for regeneration and reuse. Such limitations have been overcome by coating, doping, or packing iron oxides and hydrous iron oxides on support materials. The use of iron oxide containing sand, zeolite, and activated carbon has been well documented (Deliyanni et al., 2013; Maji et al., 2011). Sodium alginate, a water soluble linear polysaccharide, is a natural occurring polymer composed of α-guluronate and βmannuronate residues. Alginate has some unique properties such as, its biocompatibility, hydrophilicity and it is considered to be non-toxic substance (Hassan et al., 2014). The gelling properties of sodium alginate are mainly affected by the confirmed exchange of sodium ions from the guluronic acid residues with different divalent cations (Ca2+, Sr2+, Ba2+, etc.). All divalent cations bind to the α-L-guluronic acid blocks between two different chains resulting in a 3D network (Li et al., 2013). Calcium alginate has been widely used to immobilize activated carbon (Kim et al., 2008), titania nanoparticles (Mahmoodi et al., 2011), carbon nanotubes (Li et al., 2010a, 2010b), and magnetite nanoparticles to generate different adsorbent materials to remove metals, dyes and pigments from aqueous solutions (Rocher et al., 2008). Goethite (α-FeOOH), formed under a broad range of climatic and hydrologic conditions, is one the most important iron oxides in soil and sediment. It is thermodynamically stable and would be incorporated with lots of geochemically and environmentally important oxyanions and cations in its complex matrix. Polymer supported metal oxide based composite adsorbents have recently gained considerable attention for heavy metal and dye removal due to their biocompatibility, water permeability, and ability to load large amounts of solid particles (Liu et al., 2015; Sigdel et al., 2016; Tanhaei et al., 2015). In this study CABI nano-goethite was chosen as an adsorbent for the removal of CR from aqueous solutions. In this paper, CR was selected as a dye to be removed from its aqueous solution using CABI nano-goethite. The influence of experimental parameters such as pH, adsorbent dosage, contact time, initial dye concentration and temperature was investigated. Moreover, the adsorption kinetic, isotherm and thermodynamic of dye adsorption were studied. Further, the recovery of the dye from the adsorbent was also attempted. The adsorbent was characterized by Fourier transform infrared spectroscopy, X- ray diffraction and Scanning electron microscopy/energy dispersive spectroscopy.

2.3. Preparation of calcium alginate beads impregnated with nano-goethite Nano-goethite was prepared using a method based on the report of Schwertmann and Cornell (Schwertmann and Cornell, 1996). The method was briefly described here under an 80 mL of the 0.2 M FeCl3·6H2O is added to a 500 mL borosilicate glass bottle followed by the rapid addition of 20 mL of 2.5 M KOH solution. The resulting suspension was aged for 60 h at 70 °C where a yellow brown precipitate of goethite was formed. The supernatant solution was separated and goethite suspension was washed repeatedly with double distilled water and dried at 60 °C for 24 h in an oven then fine ground. The obtained product was named as nano-goethite. The prepared nano-goethite was immobilized using alginate with calcium ion via entrapment. To a required sodium alginate solution (3 wt%), a freshly prepared nanogoethite suspension was added and stirred at 300 rpm for 4 h for uniform mixing. The alginate and nano-goethite ratio in the mixture was fixed at 3:1. The mixture was then poured into a 3% CaCl2 solution through the tip (1 mL) and these gel beads were aged in CaCl2 solution for 4 h; which lead to the formation of insoluble and stable beads. The beads were washed with double distilled water and dried in an oven at 60 °C for 24 h and stored in vacuumed desiccators for further use. 2.4. Adsorption experiments

An anionic dye congo red (Dye content≥35%, Molecular formula= C32H22N6Na2O6S2, Molecular weight=991.82, λmax=497 nm) was procured from Sigma Aldrich, Korea. All other chemicals such as sodium alginate (C6H7NaO6)n, ferric chloride (FeCl3·6H2O), potassium hydroxide (KOH), hydrochloric acid (HCl) and sodium hydroxide (NaOH) used were of analytical grade. A stock solution (1000 mg/L) was prepared by dissolving required amount of CR in double distilled water which was later diluted to desired concentrations.

Batch sorption experiments were carried out to study the various operating parameters such as pH, adsorbent dose, contact time, initial dye concentration and temperature. The pH experiments were performed by adding 30 mL of CR solution (300 mg/L) and 0.03g of adsorbent in 50 mL falcon tubes. The pH of the solutions was adjusted and controlled from 3.0 to 10.0 by adding 0.1 M HCl or NaOH. The tubes were agitated in an electronical thermostatic reciprocating shaker at 180 rpm and 298 K for 24 h. After reaching equilibrium, the adsorbent was separated from the aqueous solutions and the residual concentrations of CR were measured using an UV/Vis spectrophotometer (Optizen Pop, Korea) after appropriate dilution. The adsorbent dosage on CR sorption was performed by varying the amount of sorbent from 0.01 to 0.15g/30 mL. The effect of temperature was studied at pH 3.0 in the range of 293–333 K. Sorption kinetic experiments were carried out using different CR concentrations (100, 200 and 300 mg/L). The experimental procedure was the same as described in the pH experiments, except that the samples were collected at different time intervals to determine the attainment of sorption equilibrium. The sorption isotherm study was performed using different concentration of CR solutions i.e. from 100 to 1000 mg/L. A 0.07g of adsorbent with 30 mL of CR solutions of various initial concentrations was shaken at 180 rpm for 24 h at 298 K. All the sorption experiments were carried out in duplicate. The amount of CR adsorbed per unit mass of adsorbent was calculated using the following mass balance equation:

2.2. Characterizations

q=

Fourier transform infrared (FTIR) spectra were performed to investigate the functional groups present in CAB, CABI nano-goethite and CR-loaded CABI nano-goethite using FTIR spectroscopy (Nicolet IS10, Thermo Scientific, USA). FTIR spectra were recorded in the region of 400–4000 cm−1. For IR spectral studies, 10 mg of sample was mixed and ground with 100 mg of KBr and made into a pellet. The background absorbance was measured by using a pure KBr pellet. The morphology of CABI nano-goethite was analyzed by Scanning electron microscopy (JEOL, JSM-7600F, Japan). The samples were first sputter-coated with homogeneous gold layer and then loaded onto a copper substrate. The

where Ci and Cf (mg/L) are the initial and equilibrium concentration of CR, respectively. Vi is the initial volume and Vf is the final volume (L), which was calculated by initial volume plus the added volume of HCl or NaOH. m is the mass of the adsorbent (g).

2. Materials and methods 2.1. Chemicals

Ci Vi − Cf Vf M

(1)

2.5. Desorption and reusability studies Desorption of CR from CABI nano-goethite was investigated using different NaOH concentrations (0.1–1.0 M). Fresh adsorbent was loaded with CR by agitating mixture of 0.07 g and 30 mL (300 mg/L) 227

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of CR solution at pH 3.0 for 4 h. The mixture was filtered and the filtrate was analyzed for amount of dye adsorbed using UV/Vis spectrophotometer. The CR loaded adsorbents were washed with double distilled water to remove unadsorbed dye and dried. The desorption process was carried out by adding 30 mL of each desorption eluent with the dried adsorbents and shaken for a predetermined time and the filtrate was analyzed for amount of CR desorbed from CABI nano-goethite. The CABI nano-goethite after desorption was reused in adsorption experiment, and the process was repeated for five times. The efficiency of desorbed dye from the adsorbent was calculated by the using the following equation.

Desorption efficiency =

Amount of CR desorbed × 100 Amount of CR adsorbed

(2)

2.6. The chi-square test (χ2) The advantage of using chi-square test is for comparing all isotherms on the same abscissa and ordinate. The chi-square test (χ2) statistic is basically the sum of the squares of the differences between the experimental data obtained by calculating from models with each squared difference divided by the corresponding data obtained by calculating from models. The chi-square test has some similarity with the root mean square error and is given as:

χ2 =

⎛ (q − q ) 2 ⎞ e e, m ⎟⎟ qe, m ⎝ ⎠

∑ ⎜⎜

(3)

Fig. 1. FTIR spectra of: (A) CAB, (B) CABI nano-goethite and (C) CR-loaded CABI nanogoethite.

where qe,m is the equilibrium capacity obtained by calculating from the model (mg/g) and qe is the experimental data on the equilibrium capacity (mg/g). 3. Results and discussion 3.1. Characterization of the adsorbent The FTIR spectra of CAB, CABI nano-goethite and CR-loaded CABI nano-goethite are shown in Fig. 1. The FTIR spectrum of CAB showed (Fig. 1A) broad band at 3392 cm−1 due to O-H stretching vibration which indicates the presence of hydroxyl groups. The appearance of two bands at 1637 and 1425 cm−1 are due to (CO) asymmetric and symmetric stretching vibrations of carboxyl group and the band at 1132 cm−1 attributes to C-O-C anti symmetric stretching. The bands present at 3357 and 1608 cm−1 in Fig. 1B of CABI nano-goethite are attributed to –OH stretching and bending vibrations of absorbed water as well as asymmetrical stretching vibrations of carboxyl group. However, weak symmetrical stretching and bending vibrations of -CH and C-O-C groups are also appeared near 1420, 1086 cm−1. The absorption band at the low frequency zone of 500–700 cm−1 is assigned to the stretching vibration of the Fe-O in goethite. The interaction between the carboxylic groups of alginate and the surface hydroxyl groups of goethite may be the expected reason for attachment of alginate with goethite. On the other hand significant changes has been observed in band intensities of hydroxyl (3362 cm−1) and carboxyl (1615 cm−1) groups in the FTIR spectra of CR-loaded CABI nanogoethite (Fig. 1C) illustrated the contribution of these functional groups in CR uptake. The significant reduction in –OH band intensity indicating the important role of –OH groups in CR sorption by CABI nanogoethite. A small lower shift was observed for all band positions after CR adsorption. The Fe-O band at 723 cm−1 shifted towards lower frequency and showed decrease in intensity indicating the formation of new bond of Fe with CR. The FTIR studies confirmed the involvement of hydroxyl (–OH) and carboxyl (–C=O) groups for CR removal. The crystalline nature of the CABI nano-goethite was also analyzed by the XRD pattern. Fig. 2 shows the XRD patterns of CAB and CABI nano-goethite samples in 2θ scan range of 10–90°. The CAB do not

Fig. 2. X-ray diffraction patterns of (A) CAB and (B) CABI nano-goethite.

reveal any diffraction peak for goethite, while CABI nano-goethite reveal distinct diffraction peaks at 2θ=21.2 (110), 33.04 (130), 34.6 (021), 36.6 (111), 39.8 (121), 40.9 (140), 53.04 (221), 59.04 (151), 61.2 (002) and 64.01 (061) corresponding to the crystal planes of goethite. All the peaks in the XRD pattern can be perfectly indexed to that of a pure orthorhombic phase [space group: Pbnm(62)] of goethite with lattice constants a=4.608 Å, b=9.956 Å and c=3.021 Å (JCPDS no. 29–0713). The morphology of dry beads was observed using SEM (Fig. 3). The SEM images of CAB and CABI nano-goethite beads were nearly spherical in shape (Fig. 3A and B). The CAB had relatively smooth surfaces, whereas the CABI nano-goethite was rough. Rough surfaces are favorable for molecular diffusion and could provide a larger surface area for the adsorption contaminants. The elemental analysis of CABI nano-goethite showed in Fig. 3C, confirmed that the beads contained 228

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Fig. 3. SEM micrographs of (A) CAB, (B) CABI nano-goethite and (C) EDS of CABI nano-goethite.

SO3Na) dissolve and the sulphonate (SO3−) groups dissociate and they are converted to anionic dye ions (Dye- SO3−).

major amount of iron and calcium and very low amount of sodium. These results suggested the release of sodium ions form sodium alginate matrix into water during the process of crosslinking reaction of sodium alginate with calcium alginate.

Dye − SO3 Na⟶Dye − SO3− + Na+ H+

S − OH ⟶S − OH2+ + Dye − SO3− → S − OH2+〉 − −−〈Dye − SO3− 3.2. Effect of pH OH −

S − OH ⟶S − O− + Dye − SO3− + H2 O → S − O−〈 − −−〉Dye − SO3−

The pH of the system determines the adsorption capacity due to its influence on the surface properties of CABI nano-goethite. The effect of pH on the adsorption capacity of CR by CABI nano-goethite was studied in different pH conditions ranging from 3.0 to 10.0 (Fig. 4). The adsorption capacity of CR was decreased when the solution pH increased from 3.0 to 10.0. The maximum adsorption capacity of CR was 137.7 mg/g, at pH 3.0. Hence, all the succeeding investigations were carried out at pH 3.0. In aqueous solution, the anionic dyes (Dye-

where S denotes the surface of CABI nano-goethite. At pH 3.0 the H+ ion concentration in the system is high and surface of CABI nano-goethite acquires positive charge by absorbing H+ ions. Therefore at lower pH, high electrostatic attraction exists between the positively charged surface of CABI nano-goethite and CR leading to maximum dye adsorption. Whereas, with an increase in solution of pH the number of hydroxide ion increases and compete with anionic form 229

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the increase of unsaturation of adsorption sites through the adsorption reaction. Another reason may be due to the particle interactions, such as aggregation, resulting from high sorbent concentration. Such aggregation would lead to decrease in total active surface area of the adsorbent.

140 120 100 80

3.4. Effect of contact time

60

Contact time of adsorbent and adsorbate is of great importance of adsorption, because it depends on the nature of the system used. The effect of contact time on the adsorption of CR using CABI nano-goethite was studied at different concentrations (100, 200 and 300 mg/L). The adsorption capacity increased with increasing contact time. The rate of adsorption of CR was rapid in the beginning, proceeded at a slower rate and finally attained equilibrium at about 180 min. After this equilibrium period, the amount of adsorbed dye did not show time dependent change. This is due to the high availability of free active sites at the beginning of the adsorption process and after a specific period of time the active sites on adsorbent will be gradually occupied by adsorbate which will slow down the process of adsorption.

40 20 0 2

3

4

5

6

7

8

9

10

11

Fig. 4. Effect of pH on the adsorption of CR by CABI nano-goethite. Error bars represent ± S.D.

of CR on the adsorption site thus reducing the adsorption capacity. Moreover, negative charged surface sites on the CABI nano-goethite do not favor the adsorption of CR ions due to electrostatic repulsion. Therefore, the possible mechanisms of CR dye adsorption may be adsorbent-sorbate interactions between protonated adsorption sites of the adsorbent and negatively charge dye ions in addition to hydrogen bonding interaction (Gupta et al., 2013). A similar type of behavior was also reported for the adsorption of CR on different adsorbents (Saha et al., 2013; Torkian et al., 2012).

3.5. Kinetics of adsorption To determine the kinetics of CR adsorption on CABI nano-goethite, the kinetics experimental data were simulated using pseudo-first-order and pseudo-second-order models. The pseudo-first-order equation assumes the adsorption is of one adsorbate molecules onto one active site on the adsorbent surface while in pseudo-second-order model one adsorbate molecule is adsorbed onto two active sites. The non-linear forms of these two models are expressed by the following equations: Pseudo-first-order (Lagergren, 1898)

3.3. Effect of adsorbent dosage

qt = q1 (1 − exp(−k1 t )) The adsorbent dosage is also an important parameter as it determines the adsorption capacity of an adsorbent for a given initial concentration. The effect of adsorbent dosage (Fig. S1) was studied on both the adsorption capacity and the removal efficiency of CR onto CABI nano-goethite were studied at pH 3.0 employing adsorbent dosage within the range of 0.01–0.15 g/30 mL and initial dye concentration of 300 mg/L. From the figure it can be observed that increasing the adsorbent dosage increased the removal efficiency of CR from 34.5% up to 57.4% with the required optimum dosage of 0.07 g/30 mL. The maximum removal efficiency of CR reached 57.4% as the adsorbent dosage reached 0.07 g. Beyond the optimum dosage the removal efficiency did not change with the adsorbent dosage. The removal efficiency of CR increases with the adsorbent dosage due to the greater availability of the exchangeable sites or surface area at higher concentrations of the adsorbent. On the other hand, the adsorption capacity (q), or amount adsorbed for unit mass of the adsorbent (mg/g), decreases by increasing the adsorbent dosage (Fig. S1). The decrease in adsorption capacity with increasing adsorbent dosage is mainly due to

(4)

Pseudo-second-order (Ho and McKay, 1999)

qt =

q22 k2 t 1 + q2 k2 t

(5)

where q1 and q2 are the amount of dye sorbed at equilibrium, qt is the amount of dye sorbed at time t, k1 is the pseudo-first-order equilibrium rate constant, and k2 is the pseudo-second-order equilibrium rate constant. The resulting kinetic parameters and the correlation coefficients (R2) determined by non-linear regression are given in Table 1. The R2 values (0.9857–0.9931) for pseudo-second-order kinetic model at all the concentrations studied are higher than those for pseudo-firstorder model. Also, the theoretical qe values obtained from this model was closer and good agreement with the experimental values. It was suggested that the pseudo-second-order model is more suitable for describing the adsorption of CR onto CABI nano-goethite. The validity of pseudo-second-order kinetic model for preferential adsorption of CR dye was evaluated by the sum of squared errors (SSE)

Table 1 Kinetics parameters for the adsorption of CR onto CABI nano-goethite. Kinetic model

Parameter

Concentration (mg/L) 100

200

300

Pseudo-first-order

qe, exp (mg/g) q1, cal (mg/g) k1(L/min) R2 SSE

24 ± 0.06 18.4 ± 0.35 0.0212 ± 0.04 0.9394 0.989

53 ± 0.07 41.5 ± 0.79 0.0154 ± 0.02 0.9898 0.991

74 ± 0.04 62.7 ± 0.16 0.0271 ± 0.01 0.9755 0.998

Pseudo-second-order

q2, cal (mg/g) k2(g/mg min) R2 SSE

25 ± 0.15 0.0011 ± 0.0002 0.9857 0.106

53.1 ± 0.37 0.0004 ± 0.0001 0.9902 0.112

75.9 ± 0.54 0.0005 ± 0.0001 0.9931 0.262

230

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used to determine the mean free energy of adsorption. The non-linear form of this model is represented by following equation:

analysis and SSE was calculated using the following equation:

SSE =

∑

⎛ (qt , e − qt , m )2 ⎞ ⎜⎜ ⎟⎟ q2 ⎝ ⎠

2⎞ ⎛ ⎡ ⎛ 1 ⎞⎤ ⎟ qe = Qm exp ⎜⎜ −K ⎢RT ln ⎜1 + ⎟⎥ ⎟ ⎝ Ce ⎠ ⎦ ⎠ ⎣ ⎝

(6)

where qt,e and qt,m are the experimental adsorption capacities of CR (mg/g) at equilibrium time and the corresponding values that are obtained from the kinetic models. The lower value of SSE (Table 1) proves that preferential adsorption of CR by CABI nano-goethite was best fitted into pseudo-second-order model than pseudo-first-order model.

qe = Qm = exp(−Kε 2 )

The adsorption isotherm indicates how the adsorbing molecules distribute between the liquid phase and the solid phase when the adsorption process reaches an equilibrium state. The adsorption data was analyzed by fitting to isotherms models like Langmuir, Freundlich, Dubinin-Radushkevich and Temkin.

E=

(11)

3.6.4. Temkin isotherm The Temkin isotherm (Temkin and Pyzhev, 1940) model contains a factor that explicitly takes into the account of adsorbing speciesadsorbate interactions. By ignoring the extremely low and large value of concentrations, the model assumes that heat of adsorption of all molecules in the layer would decrease linearly rather than logarithmic with coverage. As implied in the equations, its derivation is characterized by a uniform distribution of binding energies, up to some maximum binding energy. The non-linear Temkin isotherm model is represented by the following equation:

(7)

where qe is the amount of adsorbate adsorbed at equilibrium, qm is the maximum adsorbate uptake at equilibrium, Ce is the adsorbate concentration at equilibrium and KL is the coefficient related to the affinity between the adsorbent and adsorbate. The isotherm data for CR adsorption by CABI nano-goethite was fitted with Langmuir Model (R2=0.9952). The maximum monolayer adsorption capacity (qm) and the Langmuir constant were estimated as 181.1 mg/g and 0.0061 L/mg respectively. The main aim of the present work is to enhance the adsorption capacity of the CAB. The adsorption capacity of CAB was found to be 116.6 mg/g. But after the impregnated with nano-goethite the adsorption capacity increased to 181.1 mg/g. Therefore in the remaining experiments in the present study were conducted and concentrated on CABI nano-goethite.

qe = β ln α + β ln Ce

(12)

where β is a constant related to the heat of adsorption and it is defined by the expression β=RT/b, b is the variation of the adsorption energy, R is the universal gas constant (8.314 kJ/mol K), T is the temperature (K) and α is the equilibrium binding constant corresponding to the maximum binding energy. Typical bonding energy range for ionexchange mechanism is reported to be in the range of 8–16 kJ/mol while physisorption processes are reported to have adsorption energies less than −40 kJ/mol. Very low values of b (0.06 kJ/mol) obtained in the present study indicates rather weak ionic interaction between the sorbate and the present sorbent and the dye removal seems to involve physisorption. The experimental data on the effect of an initial concentration of CR on the CABI nano-goethite were fitted to the different non-linear isotherm models and the graphical representations of these models are represented in Fig. S2. All of the constants are presented in Table 2. It was observed that the best fitted adsorption isotherms considering the correlation coefficient obtained for the isotherms were to be in the order of prediction precision: Langmuir > Temkin > Freundlich > Dubinin-Radushkevich isotherms. Comparing the high correlation coefficient (R2) and low chi-square (χ2) values suggest the Langmuir isotherm model is better fitted, followed by Temkin, Freundlich and Dubinin-Radushkevich models.

3.6.2. Freundlich isotherm The Freundlich isotherm model (Freundlich, 1906) is usually used for assuming a heterogeneous surface and a nonuniform distribution of adsorption heat over surface without a saturation of adsorption sites. The non-linear form of Freundlich isotherm model is represented by the following equation:

qe = Kf Ce1/ n

1 2K

The value of E is very useful in predicting the type of adsorption and if the value is less than 8 kJ/mol, then the adsorption is physical in nature and if it is in between 8 and 16 kJ/mol, then the adsorption is due to exchange of ions (Ghasemi et al., 2014). In the present stud, the value of E was found < 8 kJ/mol, so the adsorption process was physical in nature.

3.6.1. Langmuir isotherm Langmuir model (Langmuir, 1918) is based on the assumption that maximum adsorption corresponds to monolayer formation of adsorbate layer on the adsorbent surface. The energy of adsorption is constant and no transmigration of adsorbate occurs on the surface. The non-linear form of the Langmuir isotherm model is described by the following equation.

qm KL Ce 1 + KL Ce

(10)

where Qm is the maximum amount of the dye ion that can be sorbed onto a unit weight of sorbent, ε is the Polanyi potential which is equal to RT ln (1+1/Ce), R is the gas constant 8.314 kJ/mol K, T is the temperature (K) and Ce is the dye equilibrium concentration. The mean free energy of sorption E (kJ/mol) required to transfer one mole of dye from the infinity in the solution to the surface of CABI nano-goethite can be determined by the following equation:

3.6. Adsorption isotherms

qe =

(9)

(8)

where Kf and n are the Freundlich constants denoting relative adsorption capacity and intensity of adsorption, respectively. The value of n is an indication of the favorability of adsorption. Values of n > 1 represent favorable nature of adsorption. The present study is considered as a favorable adsorption process as the value n > 1 was observed for the sorption of CR onto CABI nano-goethite. 3.6.3. Dubinin-Radushkevich isotherm The Dubinin-Radushkevich (D-R) isotherm model (Dubinin and Radushkevich, 1947) is used to determine the adsorption type, physical or chemical. The D-R isotherm equation is more general than the Langmuir model because it does not assume a homogeneous surface or a constant sorption potential or absence of steric hindrance between sorbed and incoming particles. The D-R isotherm equation has been

3.7. Comparison of adsorption capacity (qm) with other adsorbents The maximum monolayer adsorption capacity (qm) of CABI nanogoethite for the removal of CR was compared with those of other adsorbents reported in the literature and the values are shown in Table 3. It is clear from Table 3 CABI nano-goethite has a much higher 231

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Table 2 Isotherm model parameters for the adsorption of CR onto CABI nano-goethite.

Kc =

CAe Ce

(13) (14)

Isotherm

Parameters

Values

ΔG = −RT ln Kc

Langmuir

qm (mg/g) KL (L/mg) R2 χ2

181.1 ± 2.32 0.0061 ± 0.0006 0.9952 14.3

ΔG o = ΔH o − TΔS o

Freundlich

Kf (mg/g) n R2 χ2

10.42 ± 3.501 2.431 ± 0.343 0.9541 135.8

Dubinin-Radushkevich

Qm (mg/g) K R2 χ2

133.1 ± 3.44 0.0114 ± 0.014 0.8841 231.9

Temkin

b (kJ/mol) α (L/g) R2 χ2

0.06 ± 0.002 5.42 ± 0.004 0.9863 27.3

ln Kc = −

qm (mg/g)

References

m-cell/Fe3O4/ACCs N,O-CMC-MMT CTS-MMT Magnetic composite (chitosan coated magnetic Fe3O4 particle) MWCNTs/Calcined eggshell

66.09 74.24 54.52 56.66

Zhu et al. (2011) Wang and Wang (2008) Wang and Wang (2007) Zhu et al. (2012)

136.99

Zeolites modified with DAAO Magnetic Fe3O4@graphene Iron-grafted clinoptilolite ZrO2 microspheres Hallow Zn-Fe2O4 nanospheres CABI nano-goethite

69.94 33.66 36.70 59.5 16.10 181.1

Seyahmazegi et al. (2016) Liu et al. (2014) Yao et al. (2012) Akgul (2014) Wang et al. (2014) Rahimi et al. (2011) Present study

RT

+

(15)

ΔS o R

(16)

where CAe and Ce are the equilibrium concentration in solution (mg/L) and the solid phase concentration at equilibrium (mg/L), respectively. Kc is the distribution constant of each temperature. R is the universal gas constant (8.314×10−3 kJ/mol K), and T is the absolute temperature (K). From the slope and intercept of the linear plot of ln Kc against 1/T (Fig. S4), the values of enthalpy and entropy changes could be obtained. Five ΔG° values (−0.099, −0.902, −1.673, −2.508 and −3.378 kJ/mol) at different temperatures (293, 303, 313, 323 and 333 K) are negative thereby implying that the adsorption of CR on CABI nano-goethite was spontaneous process. The values of ΔG° become more negative with increasing temperature, demonstrating the higher temperatures facilitate the adsorption of CR on CABI nano-goethite. The ΔG° values also confirmed the adsorption of CR onto CABI nanogoethite to be physical process with the physical adsorption values ranging from −20 to 0 kJ/mol while value from −80 to −400 kJ/mol describes chemical absorption (Li et al., 2010a, 2010b). The positive value of ΔHo (24.4 kJ/mol) indicates that CR adsorption onto the CABI nano-goethite is physical and endothermic reaction. An adsorption process is generally considered as physical if ΔHo < 25 kJ/mol and as chemical when ΔHo > 40 kJ/mol (Gupta et al., 2013). The positive value of ΔSo (0.083 kJ/mol K) suggests the increased randomness at the solid/solution interface during the adsorption of CR onto CABI nanogoethite (Agarwal et al., 2016).

Table 3 Comparison of the maximum monolayer adsorption capacity (qm: mg/g) of CR onto CABI nano-goethite with other reported literature. Adsorbent

ΔH o

3.9. Desorption studies The regeneration of adsorbent is important for the practical application of adsorption process and the adsorbents having regeneration ability are considered cost effective and applicable at pilot scale. The adsorbed CR onto CABI nano-goethite at the selected pH was desorbed with different NaOH concentrations and the results were shown in Fig. S5. Results showed that desorption of CR was 94%, 80%, 68%, 57% and 45% for 0.1, 0.3, 0.5, 0.7 and 1.0 M NaOH solutions respectively. The maximum percentage recovery of CR was 94% with 0.1 M NaOH solution.

adsorption capacity compared to other adsorbents. The results revealed CABI nano-goethite as a promising adsorbent for removal of CR from aqueous solutions. 3.8. Effect of temperature

3.10. Regeneration studies

Various textile dye effluents are produced at relatively high temperatures; therefore temperature can be an important factor for the real application of CABI nano-goethite. Temperature is an indicator for the adsorption nature whether it is an endothermic or exothermic process. The effect of temperature on adsorption capacity of adsorbent under optimized conditions was studied. The results are shown in Fig. S3. From this figure it is observed that adsorption of CR increases from 73.9 to 111.9 mg/g with temperature from 293 to 333 K respectively. The adsorption is an endothermic process because the adsorption capacity increased with an increase in temperature. This may be due to an increase in the mobility of the CR molecules and the number of active sites for the adsorption with increasing temperature. An increasing number of molecules may also acquire sufficient energy to undergo an interaction with active sites at the surface. Further, an increase in temperature increases the mobility of the large dye ions and reduces the swelling effect thus enabling the large dye molecule to penetrate in the adsorbent. Thermodynamic parameters were evaluated to confirm the nature of the adsorption of CR by CABI nano-goethite. The thermodynamic spontaneity and feasibility of the adsorption process was assessed by calculating the three basic thermodynamic parameters viz., Gibbs free energy (ΔGo), enthalpy change (ΔHo) and entropy change (ΔSo) using following equations.

In order to make the process more economical and feasible, the adsorbent was regenerated that leads to significant improvement in the economy of the process. Stability is generally important when the same adsorbent is reused in multiple adsorption and desorption cycles. Therefore the reusability of adsorbent was examined based on adsorption/desorption ability. This was done by performing five consecutive cycles of adsorption/desorption using 0.1 M NaOH as desorbent/eluent and the results were presented in Fig. S6. The desorption efficiency of 94% was obtained using 0.1 M NaOH as eluent in the first cycle, indicating adsorbents suitability for reuse. There is a gradual decrease in CR adsorption with an increase in the number of cycles. The small fraction of sorbed dye which was not recovered by desorption presumably present on CABI nano-goethite, as a result, adsorption capacity was gradually reduced in the subsequent cycles. After sequence of four cycles, the CR uptake capacity of the adsorbent was reduced from 85% to 76%. The desorption efficiency in all four cycles was high; > 84% recovery of CR was observed in each cycle. Therefore it can be concluded that the CABI nano-goethite could be used repeatedly in CR adsorption studies with a small loss in the total adsorption capacity. 232

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3.11. Adsorption mechanism The adsorption of CR onto CABI nano-goethite might involve the combination of electrostatic attraction and physisorption. Several factors that may influence the adsorption behavior, such as dye structure and adsorbent surface properties. The electrostatic attraction may be the principal mechanism for the adsorption behavior. The behavior of dye in low pH is more anionic nature because of dissociation of sulfonic group as shown/discussed in pH experimental result in Section 3.2. In pH experimental results there was high sorption capacity of CR in low pH conditions and the sorption capacity decreased drastically when pH increased to 10.0. This type of trend generally observed when electrostatic interactions play major role in the sorption process. The hydroxyl groups (-OH) present on the surface of CABI nano-goethite generally behaves as positive groups (–OH2+) in acidic medium because of their easily protonated nature. The experimental result showed high adsorption capacity of CR was observed at high acidic solutions (pH=3.0). The strong electrostatic interaction between the –OH2+ of CABI nano-goethite and dye anions resulted the high sorption capacity of CR on CABI nano-goethite. Therefore the process of sorption is mainly depends on electrostatic attraction forces between CR and CABI nano-goethite in low pH medium. 4. Conclusions The present study shows that the CABI nano-goethite can be used as a potential adsorbent for the removal of CR from aqueous solutions. The experimental parameters; pH of solution, contact time, adsorbent dosage, initial dye concentration and temperature were found to be effective on the adsorption process. The CABI nano-goethite was characterized by FTIR, XRD and SEM/EDS techniques. The experimental data was analyzed using Langmuir, Freundlich, DubininRadushkevich and Temkin isotherms models. The experimental data was better fitted by the Langmuir isotherm model compared to the Temkin, Freundlich and Dubinin-Radushkevich. The maximum monolayer adsorption capacity was 181.1 mg/g at pH 3.0. The kinetic studies revealed that the adsorption process followed the pseudo-second-order kinetic model. Thermodynamic parameters depicted feasible, spontaneous and endothermic nature of adsorption. Five adsorption/desorption cycles were carried out with 0.1 M NaOH as the desorbing agent without any loss of adsorbent or appreciable reduction in adsorption capacity. Acknowledgements This research was supported by the R & D Program for Society of the National Research Foundation (NRF) funded by the Ministry of Science, ICT & Future Planning (Grant No: NRF-2013M3C8A3078596). This study was supported by the R & D Center for Valuable Recycling (Global-Top Environmental Technology Development Program) funded by the Ministry of Environment (Project No: GT-11-C-01–070-0). Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at doi:10.1016/j.ecoenv.2017.03.036. References Agarwal, S., Tyagi, I., Gupta, V.K., Ghasemi, N., Shahivand, M., Ghasemi, M., 2016. Kinetics, equilibrium studies and thermodynamics of methylene blue adsorption on Ephedra strobilacea saw dust and modified using phosphoric acid and zinc chloride. J. Mol. Liq. 218, 208–218. Akgul, M., 2014. Enhancement of the anionic dye adsorption capacity of clinoptilolite by Fe3+-grafting. J. Hazard. Mater. 267, 1–8. Chafi, M., Gourich, B., Essadki, A.H., Vial, C., Fabregat, A., 2011. Comparison of electrocoagulation using iron and aluminium electrodes with chemical coagulation

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