Adsorption of methylene blue onto citric acid treated carbonized bamboo leaves powder: Equilibrium, kinetics, thermodynamics analyses

Adsorption of methylene blue onto citric acid treated carbonized bamboo leaves powder: Equilibrium, kinetics, thermodynamics analyses

Accepted Manuscript Adsorption of methylene blue onto citric acid treated carbonized bamboo leaves powder: Equilibrium, kinetics, thermodynamics analy...

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Accepted Manuscript Adsorption of methylene blue onto citric acid treated carbonized bamboo leaves powder: Equilibrium, kinetics, thermodynamics analyses

S.K. Ghosh, A. Bandyopadhyay PII: DOI: Reference:

S0167-7322(17)33067-2 doi:10.1016/j.molliq.2017.10.086 MOLLIQ 8048

To appear in:

Journal of Molecular Liquids

Received date: Revised date: Accepted date:

10 July 2017 23 September 2017 18 October 2017

Please cite this article as: S.K. Ghosh, A. Bandyopadhyay , Adsorption of methylene blue onto citric acid treated carbonized bamboo leaves powder: Equilibrium, kinetics, thermodynamics analyses. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Molliq(2017), doi:10.1016/ j.molliq.2017.10.086

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ACCEPTED MANUSCRIPT Adsorption of Methylene Blue onto citric acid treated carbonized bamboo leaves powder: Equilibrium, Kinetics, Thermodynamics analyses

S.K.Ghosh1,2 and A. Bandyopadhyay1 1

Department of Chemical Engineering

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University of Calcutta, 92, A.P.C. Road. Kolkata 700 009. India e-mail: [email protected]

Indian Institute of Chemical Engineers. Jadavpur. Kolkata 700 032. India

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Abstract

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In this article, the adsorption of methylene blue (MB) from its aqueous solution onto citric acid treated carbonized bamboo leaves powder as an adsorbent is reported. The adsorbent was characterized by determining the density, pHpzc (6.6), particle size (32.53 µm), and was analyzed

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by SEM, BET (surface area 393.3 m2/g) and FTIR. Batch experiments were performed as a function of various operating parameters - initial concentration (100 to 400 mg/L), contact time

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(10 to 100 min), adsorbent dose (0.2 to 1.4 g/L), shaker speed (50 to 350 rpm) and temperature

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(290 to 305 K) at natural pH of 7.5. The batch data followed Temkin isotherm amongst seven isotherms and pseudo-second order kinetic model. Further, analysis of batch data upon various diffusion models indicated that film diffusion was controlling the rate though intra-particle

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diffusion was taking place. Thermodynamic parameters (ΔG0: -33.70 to -25.43 kJ/mol; ΔH0: 134.97 kJ/mol; ΔS0: 554.51 J/mol.K) revealed spontaneity of adsorption and the nature was chemisorption which also followed the activation energy (84.07 to 98.90 kJ/mol) estimated from pseudo-second order rate constant as a function of temperature. Novelties achieved were - higher removal of MB (99.97%), higher adsorption capacity (725 mg/g), final solution pH well within the safe discharge limit. Besides, the seasonal variation was used to investigate the temperature effect in a novel manner.

Key words: adsorption; carbonized bamboo leaves; methylene blue; citric acid treatment;

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ACCEPTED MANUSCRIPT 1.0 Introduction The discharge of colored wastewater from the industries can cause deleterious effect to all forms of lives if untreated. The various industries those generate colored wastewater are textile, rubber, paper, plastic and cosmetics industry. Among these industries, textile industry uses dye extensively for imparting colors to the fabric. Adequate treatment of colored wastewater containing mainly dye thus assumes considerable importance for meeting the effluent discharge

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standards of any nation. Various techniques, such as coagulation and flocculation, membrane separation, advanced oxidation and adsorption have so far been used for the removal of dyes from wastewater. Adsorption [1–3] amongst these methods seems to be the simplest in operation and is

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cost effective. Current research trends are gradually being shifted towards the low-cost adsorbents

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from the conventional commercial activated carbon as adsorbent. In these researches, methylene blue (MB) has gained considerable attention owing to its being a probe molecule used for several

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purposes.

Several research investigations have so far been carried out on the removal of methylene blue

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(MB) using various low cost adsorbents under batch operating mode using conical (Erlenmeyer) flasks in mechanical shaker, such as activated carbon prepared from dross licorice by ultrasonic

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[4], activated carbon prepared from hazelnut husk using zinc chloride [5], porous carbon prepared

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from tea waste [6], factory-rejected tea activated carbon prepared by conjunction of hydrothermal carbonization and sodium hydroxide activation processes [7], KOH-activated carbon prepared from sucrose spherical carbon [8], activated carbon prepared from fox nutshell by chemical

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activation [9], cross-linked beads of activated oil palm ash zeolite/chitosan composite [10], mesoporous-activated carbon prepared from chitosan flakes via single-step sodium hydroxide activation [11], activated carbon prepared from karanj (Pongamia pinnata) fruit hulls [12], cashew nut shell based carbons activated with zinc chloride [13], activated carbon developed from Ficus carica bast [14], activated carbons prepared from waste carpets [15], sewage sludge-derived biochar [16], polyvinyl alcohol [17], Ephedra strobilacea saw dust and modified using phosphoric acid and zinc chloride [18], chemically activated carbon spheres derived from hydro-thermally prepared poly(vinyl alcohol) microspheres [19], functionalized MgAl-layered double hydroxides [20], [email protected](Fe) core-shells [21], MoS2 based polymer composites [22], SiO2 polymer composites [23] so on forth.

Bamboo leaves are abundantly available locally that can be used after rendering some kind of processing/treatment as an adsorbent for MB dye. An attempt has therefore, been made to develop an efficient adsorbent from powdered and dried bamboo leaves by carbonization at first followed 2

ACCEPTED MANUSCRIPT by its treatment with citric acid for performing batch experiments for the removal of MB from its aqueous solution. The various parameters chosen for characterizing the batch investigation are initial adsorbate (MB) concentration, contact time, adsorbent dosage, speed of agitation and temperature. The results obtainable are verified with the various well known isotherms and kinetics. Thermodynamic parameters as well as activation energy are also exploited from the

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batch adsorption data.

2.0 Materials and Methods 2.1 Adsorbent preparation

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The bamboo leaves were collected locally (West Bengal, India), washed thoroughly to remove the

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darts, dried in an air oven at 105°C and grinded by a mixer-grinder into fine powder. The powder was screened through BS (British Standard) 120 mesh and was carbonized at 450 OC for 6 hr. The

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material was cooled to room temperature. About 25 g of the carbonized powder was mixed with 200 mL of 1 mol/L citric acid. The mixture was sonicated for 4 hours and then washed thoroughly to free the residual (superficial) citric acid, filtered, dried at 105°C for 24 hr and stored for use as

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adsorbent referred to as the citric acid treated carbonized bamboo leaves powder.

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2.2 Adsorbate preparation

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Methylene blue (C16H18ClN3S, molar mass 319.50 g/mol, a cationic basic dye) (E.Merck Limited, Mumbai, India) was used as an adsorbate without further purification. The wavelength of maximum absorption (λmax) of MB is 664 nm. A stock solution of 1000 mg/L MB was prepared

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by dissolving 1000 mg MB in 1.0 L distilled water. The experimental solution was obtained by diluting the stock solution in accurate proportions to different initial concentrations with distilled water. All other reagents used in the present study were of analytical grade.

2.3 Characterization of the citric acid treated carbonized bamboo leaves powder as adsorbent

The bulk density and the solid density of adsorbent prepared were determined following the Archimedes principle. The pH at the point of zero charge (pHPZC) was determined using the method by measuring solution pH described in the literature [24]. The values of pH of the solutions were measured by a digital pH meter (Model # 112, Electronics India, India). The citric acid treated carbonized bamboo leaves powder as adsorbent (1 gm) was added to each flask and the flasks were agitated in a mechanical shaker for 24 hours. The values of the final pH after shaking were measured for determining the pHpzc. The particle size distribution was carried out in particle size analyzer (MASTERSIZER 2000, MALVERN Instruments, USA). The surface 3

ACCEPTED MANUSCRIPT morphologies of the adsorbent prepared before and after adsorption were characterized using Scanning Electron Microscopy (SEM) (Model: Evo–18 Special Edition, Carl Zeiss, West Germany). The surface area and the mean pore size of the adsorbent prepared were measured in a BET analyzer (Model: Autosorb 1–AS1–C, Quantachr ome Instruments, USA). The FTIR spectra were developed with the help of a spectrophotometer (IR Prestige – 21, SHIMADZU, JAPAN) by

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scanning in the range of 400 and 4000 cm-1 using KBr pellet containing the sample.

2.4 Batch adsorption studies

Batch experiments were conducted for investigating into the behavior of adsorption of MB onto

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citric acid treated carbonized bamboo leaves powder in several 250 mL stoppered conical flasks.

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After adding desired quantity of adsorbent, the flasks were shaken in a mechanical shaker for a specified contact time. The conical flasks were withdrawn after definite period of contact time.

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The supernatant was decanted to separate the adsorbent and was analyzed for residual MB using a digital UV spectrophotometer (Perkin Elmer, UV-Vis Spectrophotometer, Lambda 25, USA) at

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λmax of 664 nm.

The experimental parameters investigated were – initial MB concentration, CO: 100, 200, 300,

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400 mg/L; contact time, t: 10, 20, 30, 40, 50, 60, 70, 80, 90, 100 min; adsorbent dose, W: 0.2, 0.4,

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0.6, 0.8, 1, 1.2, 1.4 g/L; shaker speed: 50, 100, 150, 200, 250, 300, 350 rpm; Temperature, T: 290, 295, 300, 305. The experiments were carried out at natural pH of MB solution of 7.5. The other experimental temperature was 305 ±1 K. The effect of temperature was investigated uniquely in a

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novel manner to simulate the environmental condition prevailing on field. This was performed by exploiting the seasonal variations of the temperature by repeating the experiments keeping all other process variables fixed. This method has recently been reported in the literature in one of our earlier investigations [25]. The temperatures before and after the experimental runs were measured and the variation was well within  1K.

The amount of MB uptake by the adsorbent prepared at any time and at equilibrium were calculated from the following expressions

qt 

Co  C t V W

and q e 

Co  C t V W

…(1) …(2)

where qt (in mg/g) is the amount adsorbed at time t, qe (in mg/g) is the amount adsorbed measured at equilibrium, C0 (in mg/L) is the initial adsorbate concentration, Ct (in mg/L) is the final 4

ACCEPTED MANUSCRIPT adsorbate concentration at any time t, Ce (in mg/L) is the equilibrium adsorbate concentration, V (in L) is the volume of MB solution, and W (in g) is the amount of adsorbent used. The percentage removal was calculated from the following expression Percentage removal, % R 

C  Cf Initial concentration  Final concentration  100 = o  100 …(3) Initial concentration Co

where Cf (in mg/L) is the final adsorbate concentration.

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3.0 Results and discussion 3.1 Characterization of the adsorbent

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The solid density and bulk density measured were 1825.7 and 968.2 kg/m3. The variation of final

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pH as a function of initial pH is shown in Figure–1a. It can be seen from the figure that the pHPZC of the adsorbent prepared from citric acid treated carbonized bamboo leaves powder was around

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6.6 (Figure–1b). It therefore, indicates that the adsorbent surface is dominated by anionic charge when the pH of the solution is above 6.6 and is ready for adsorbing cationic species (like cationic MB dye) easily. It is noteworthy that the pHpzc is nearly neutral (7.0) in the present study. The

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effluent discharge standard for pH for the textile industry under Indian condition is in the range of 5.5 to 9.0 [26]. The present investigation therefore, could ensure a safe discharge in respect of pH of the treated effluent as it does not likely to exceed the prescribed limit. The particle size

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distribution is shown in Figure–2. The measured value of the Sauter mean diameter (SMD) of the



3 Fd i i 2 Fd i i

…(4)

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SMD 

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particles was 32.53 µm. The SMD or the volume to surface mean diameter is defined as

where Fi is the number of particles in the ith group and di is the diameter of the particles in the ith group. It is widely used in mass transfer applications as in the present case of adsorptive removal of MB. The surface morphologies of the dried adsorbent before and after adsorption are shown in Figure–3a and Figure–3b respectively. Figure–3a depicts the porous surface of the adsorbent prepared; while Figure–3b depicts coverage of the surface due to uptake of MB by the adsorbent prepared. The multipoint BET surface area measured was 393.3 m2/g, while the average pore diameter was found to be 2.09 nm. The pore volume estimated was 0.1872 cc/g. The BET plot is shown in Figure–4a. Figure–4b depicts the BET isotherm showing an open ended hysteresis indicating meso-porous structure of the adsorbent. The different absorption bands at different frequencies identified from the FTIR spectra before adsorption in the prepared adsorbent are presented in the Supplementary Figure–S1. The peak identified as 3399.85 cm-1 was due to the free –OH groups (intermolecular hydrogen bonded). The absorption band at 1706.23 cm-1 observed was due to the characteristic stretching vibration for C=O in –COOH group of organic 5

ACCEPTED MANUSCRIPT aliphatic acids. The absorption bands identified in the range of 1660 – 1530 cm-1 were due to the stretching vibrations of –COO-1 anions. These absorption bands indicate the introduction of free – COOH groups onto the surface of the prepared adsorbent using citric acid. The absorption band at 1274.21 cm-1 was due to the stretching vibrations for C–O of alcohol. The absorption bands identified at 2935.09, 1472.83 and 1434.20 cm-1 were due to the stretching vibrations of C–H (alkane), bending vibrations of C–H in –CH2 (alkane) and C–H in –CH3 (alkane) respectively. Therefore, the citric acid treated adsorbent prepared had shown to have porous structure with the

3.2 Effect of contact time and initial concentration

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introduction of free –COOH groups for adsorbing MB molecules from its aqueous solution.

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The effect of contact time on the removal of MB as well as on the adsorption capacity are shown in Figure–5a and Figure–5b respectively at different values of initial concentration (CO = 100

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and 400 mg/L) and at a fixed adsorbent dose, W of 1 g/L and shaker speed of 300 rpm. It can be observed from the figures that the removal and adsorption capacity of MB were both increased with the contact time. It is further observed from the relation of contact time with the removal of

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MB that the removal of MB was increased sharply up to 40 minutes and thereafter the increase was slow and finally reached almost a constant value at 60 minutes of contact time. The trends

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were similar for both initial concentrations. It might be attributed to the fact that the adsorbent

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surface reached the equilibrium adsorbate concentration around a contact time of 60 minutes. It was therefore, considered to be the equilibrium time for the rest of the experiments. The maximum removal of MB observed was 99.87 % (for CO = 100 mg/L) (Figure–5a) and the

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maximum adsorption capacity in this part of the experiment was 397.83 mg/g (CO = 400 mg/L) (Figure–5b).

3.3 Effect of adsorbent dosage

The effect of adsorbent dose on the removal of MB as well as on the adsorption capacity are shown in Figure–6a and Figure–6b respectively at different values of initial concentrations (CO = 100 and 400 mg/L) at equilibrium time of 60 minutes and shaker speed of 300 rpm. It can be seen from Figure–6a that the removal of MB was increased rapidly and thereafter it reached almost a constant value beyond an adsorbent dose of about 1g/L. Higher adsorbent dose increased the uptake of MB dye leading to the increase in the removal of MB, but after reaching a certain adsorbent dosage of 1 g/L there might have been interferences owing to overcrowding of adsorbent particles resulting into loss of sites for adsorption and the uptake of dye became almost constant after that dosage. The optimum adsorbent dosage was therefore, considered to be 1 g/L for further studies. The adsorption capacity (Figure–6b) however, was decreased with the 6

ACCEPTED MANUSCRIPT adsorbent dose but there was no leveling off of the adsorption capacity as observed in the case of effect of dose on percentage removal. The maximum removal of MB was 99.97% (CO = 100 mg/L; W = 1.4 g/L) and the maximum adsorption capacity was 725 mg/L (CO = 400 mg/L; W = 0.2 g/L) in this part of the experiment. The removal of MB and adsorption capacity under optimum dose of 1 g/L was observed to be 99.44% and 99.44 mg/g respectively for CO = 100 mg/L, while the respective removal and adsorption capacity for CO = 400 mg/L were observed to

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be 99.34% and 397.35 mg/g. Thus the increased dose reduced the adsorption capacity at a fixed

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CO which was more than compensated resulting in enhanced % R.

3.4 Effect of speed of agitation

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The effects of shaker speed on the removal of MB as well as on the adsorption capacity are shown in Figure–7a and Figure–7b respectively at different values of initial concentration (CO = 100

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and 400 mg/L) for W = 1.0 g/L at equilibrium time of 60 minutes. The trend is similar to that in the case of contact time on removal of MB and adsorption capacity. It is observed from the figures that the removals of MB and adsorption capacity were both increased with the shaker

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speed. It is further observed from Figure–7a (effect of shaker speed with removal of MB) that the removal of MB was increased sharply up to shaker speed of 300 rpm and thereafter the increase

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was slow and finally reached almost a constant value at 350 rpm. It might be attributed to the fact

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that the reduction of liquid side resistance was ceased at around a speed of agitation of 300 rpm. Therefore, the optimum speed of agitation was considered to be 300 rpm for the rest of the experiments. The maximum removal of MB observed was 99.87 % (for CO = 100 mg/L) and

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maximum adsorption capacity was 397.72 mg/g (CO = 400 mg/L) in this part of the experiment at shaker speed of 350 rpm.

3.5 Effect of contact time on final solution pH Adsorptive treatment is in general offered to final polishing stage of the effluent treatment system in industries. Hence, the final solution pH plays an important role keeping in view that it can introduce additional consumption of chemicals and energy, if the final solution pH is outside the domain of the stipulated value of effluent discharge standard. In the present study, the effect of contact time on the final solution pH was measured for CO = 100 and 200 mg/L (with W = 1.0 g/L) along with a case where only adsorbent (W = 1.0 g/L) was added to distilled water of equal volume to that of the volume of the synthetic solution to compare the physical situation. These experimental results are presented in Figure–8. It can be seen from the figure that the solution pH was decreased with time in all the cases. However, the solution pH was decreased rapidly with time for a case where only adsorbent was present. Since the pHPZC of the adsorbent was 6.6 7

ACCEPTED MANUSCRIPT (slightly acidic than neutral pH), the solution pH became lower than 7.0. Further, from the FTIR spectra it was observed that there were additional –COOH groups which might lead to lowering of the pH value below the initial pH of adsorbent at time t > 0 due to the generation of –COO-1 ions followed by the release of H+ ions. For the other cases where adsorbent was added to the MB dye solution having the natural pH of 7.5 showed similar trends in the variation of pH with the contact time. It can be seen from the figure that the solution pH was reduced from 7.5 with contact time to 6.43 for CO = 100 mg/L, and 6.71 for CO = 200 mg/L owing to the generation of –COO-1 ions

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leading to the release of H+ ions and in these solutions the adsorption of MB was taking place with time. The solution pH with time was marginally higher for higher values of C O (200 g/L)

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than for lower CO value (100 mg/L). This clearly demonstrates that the final solution pH might

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not be outside the domain of the effluent discharge standards for pH under Indian condition [26]. It was further concluded that treating effluents with higher values of CO could be safer in this

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regard and in that it could be applicable globally.

4.0 Adsorption Isotherms

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The adsorption of MB onto citric acid treated carbonized bamboo leaves powder was verified by examining the batch experimental data on linearized forms of seven isotherms such as Langmuir

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[27], Freundlich [28], Temkin [29], Halsey [30], Dubinin–Radushkevich (D-R) [31], Sips [32],

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1 1 1 1   q e k L q m Ce q m ln q e  ln k f 

qe =

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and Redlich-Peterson (R-P) [33] as given below

1 ln Ce nF

RT RT ln (K T ) + ln (Ce ) bT bT

log q e =

1 1 log K H  log Ce nH nH

ln(qe )  ln(q DR )  βε 2 

where ε = RT ln 1+ 

…(5)

…(6)

…(7)

…(8)

…(9)

1  1  E Ce  2β ,

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ACCEPTED MANUSCRIPT  qe  1 ln  ln Ce  ln KS =  qm  qe  n s

…(10)

  C ln  K R e  1  g ln Ce  ln a R qe  

…(11)

The estimated values of the different isotherm parameters according to the linearized equations of the isotherms achieved through data fitting are presented in Table–1. It can be seen from the table

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that the maximum adsorption capacity based on Langmuir adsorption isotherm was 526.32 mg/g. The results of the statistical errors including the correlation coefficient (r) and coefficient of

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determination (r2) for each of the isotherm models are compared in Table–2. It is observed from the table that values of r (0.998) and r2 (0.996) were highest for the Temkin isotherm model.

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Furthermore, Temkin model offered lowest values of the overall errors. The detailed descriptions of statistical analyses for other isotherm models are presented in Supplementary Material. It can

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be further seen from Table–2 that the calculated value of the maximum adsorption capacity for Temkin isotherm model was 397.76 mg/g which is closest amongst others to a value of 397.87

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mg/g observed experimentally. Therefore, the adsorption of MB onto citric acid treated carbonized bamboo leaves powder could be described by the Temkin isotherm which is

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5.0 Adsorption kinetics

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chemisorption in nature.

Similar to the isotherm, the kinetics as well as the diffusive transport for the MB adsorption onto

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the adsorbent prepared was verified by examining the batch experimental data fitting into the Lagergren [34] pseudo-first-order, Ho and McKay [35] pseudo-second-order as well as Boyd’s [36] film or boundary layer diffusion, and Weber and Morris [37] intra-particle diffusion. This data fitting was performed by using the following linearized equations log(q e  q t )  log q e 

k1 t 2.303

…(12)

t 1 1   t 2 q t k 2qe qe

…(13)

q t  k id t 0.5  I

…(14)

BT  0.498  ln[1  Ft ]

…(15)

The calculated values of rate constants, diffusion parameters and statistical parameters are presented in Table–3. The values of r and r2 were highest for the pseudo-second order model 9

ACCEPTED MANUSCRIPT as compared with the pseudo-first order model. The calculated values of k1 were ranging between 0.054 to 0.142 min-1 while the calculated values of k2 were ranging between 0.003 to 0.015 g/mg.min at 305 K. The pseudo-second order model is applicable when the adsorption is characterized by the second order kinetics with regard to the available sites on the surface of the adsorbent rather than the concentration of the adsorbate in the bulk solution [38]. In contrast, pseudo-first order model can not reflect the true picture of an order but approximates to the first

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order rate law [39]. Under these considerations, the adsorption of MB onto citric acid treated carbonized bamboo leaves powder could be governed by the pseudo-second order kinetics better

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than the pseudo-first order kinetics.

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The intra-particle diffusion and Boyd’s film diffusion model are applied for exploring the diffusion mechanism of a specific adsorbate-adsorbent pair in solution since neither of the

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pseudo-first order or the pseudo-second order kinetic model could serve this purpose. The diffusion of the solute can be divided into three stages appearing at the plot of the intra-particle diffusion into three linear segments (often described as multi-linearity) – (i) the first linear

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segment as the external diffusion, (ii) the second linear segment as the gradual surface adsorption commonly known as the intra-particle diffusion, and (iii) finally, the third linear segment as the

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slow equilibrium stage. The effect of square root of contact time on the adsorption capacity is

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shown in Figure–9a depicting the multi-linearity of adsorption of MB onto citric acid treated carbonized bamboo leaves powder studied. In this figure, all the three segments could be visible for all values of CO ranging from 100 to 400 mg/L. The first external diffusion was seen

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completed within the 30 minutes for all the cases while the intra-particle diffusion was completed within the next 30 minutes before the onset of the equilibrium at 60 minutes. The equilibrium time of 60 minutes could also be established from this observation as was elucidated in Section 3.2. The values of the first kid were varying from 1.22 to 4.94 mg/g min0.5, while the values of the second kid were varied from 0.41 to 2.39 mg/g min0.5 respectively (Table–4). The third segment was practically horizontal and thus there were no such values. The diffusion can be controlled by the intra-particle diffusion only if the linear plots of intra-particle diffusion are passing through the origin. But in the present case, none of the linear segments passed through the origin due to the presence of perceptible non-zero intercepts indicating that the intra-particle diffusion was not controlling the rate in the adsorption of MB onto citric acid treated carbonized bamboo leaves powder. In the case of the Boyd’s model, the diffusion is controlled by the intra-particle diffusion if the linear plot of “Bt against t” is passing through the origin; otherwise, the film diffusion controls the 10

ACCEPTED MANUSCRIPT rate. The particle diffusion is defined by the diffusion of adsorbate molecules into the pores of the adsorbent particles while the film diffusion is defined by the diffusion of adsorbate molecules to the external surface of the adsorbent [40, 41]. The plot of Bt against t is shown in Figure–9b and lines were not passing through the origin indicating that the rates were controlled by the film diffusion.

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6.0 Effect of Temperature The influence of the operating temperature assumes considerable importance for estimating the thermodynamic parameters on adsorption at equilibrium. In country like India, there are many

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places (such as Kolkata, India) where the temperature varies at a wide range from about 10 OC in

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winter to 40 OC in summer in a year. In this light, studying the influence of temperature on the adsorption in the present work is important for the purpose of consideration of functioning of the

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treatment method. It is already detailed in Section 2.4 that the effect of temperature was uniquely investigated by exploiting the seasonal variation of temperatures. The effect of temperature was

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investigated for concentration of MB of 100 mg/L, 200 mg/L, 300 mg/L and 400 mg/L. In the present study, the change in standard Gibbs free energy (G0), standard enthalpy change

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(H0) and standard entropy change (S0) for dilute solution were estimated with reasonable

expression

…(16)

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KC  k L .MMB

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accuracy [42] from the thermodynamic equilibrium constant, K C utilizing the following

In Eq (16), kL is the Langmuir constant and MMB is the molecular weight of MB dye molecule. The consideration of using the Langmuir constant for determining the value of KC avoiding the consideration of non-dimensionalization of the thermodynamic equilibrium constant has classically been elucidated by Liu [42]. The approach for non-dimensionalization of KC has shown departure from the thermodynamic behavior of adsorption rather than on introducing a dimension onto KC using the Langmuir constant as theorized by Liu [42] in Eq (16). The values of ΔG0 at various temperatures were determined from the following equation

ΔG0   RT ln K C

…(17)

The values of ΔH0 and ΔS0 were determined from the Van’t Hoff’s linear relationship as follows ln K C  

H0 S0  RT R

...(18)

The values of the average ΔH0 and ΔS0 were calculated from the slope and the intercept of the

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ACCEPTED MANUSCRIPT linear plot of ln K C as a function of

1 (Figure–10a). The calculated values of ΔG0, ΔH0 and ΔS0 T

at different temperatures are shown in Table–5. The value of ΔG0 was increasingly negative with temperature for all values of CO indicating the feasibility and spontaneity of the adsorption of MB on adsorbent prepared. The positive value of ΔH0 ensured that the adsorption process was endothermic in nature and the positive value of ΔS0 indicated the affinity of MB onto the adsorbent prepared. Further, the process was endothermic due to the values of TΔS0 was greater

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than ΔH0 making ΔG0 negative and thus the adsorption of MB onto the adsorbent prepared was found favorable at higher temperature. The values obtainable for ΔG0 and ΔH0 could help in

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describing the mechanism of adsorption process [43, 44]. The values of ΔG0 ranging between –20

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and 0.0 kJ/mol would be indicative of physisorption, while chemisorption would predominate if its values fall in the range of –80 to –20 kJ/mol. In the present study, the values of ΔG0 were well

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within the range of –80 to –20 kJ/mol (i.e., from –33.70 to –25.43 kJ/mol) indicating chemisorption could be the predominant mechanism for the adsorption of MB onto citric acid treated carbonized bamboo leaves powder. Furthermore, the values of ΔH0 have an important role

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to play in elucidating the mechanism of the adsorption studied. Three regimes have been reported in the literature [44] based on the values of ΔH0 for describing the adsorption mechanism, such as (i) for values of ΔH0 ranging between 0 kJ/mol and 20 kJ/mol, interaction due to van der Waals

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forces predominates, (ii) for values of ΔH0 in the range of 20 to 80 kJ/mol, interaction due to

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physisorption like electrostatic interactions contributes, and (iii) for values of ΔH0 in the range of 80 to 450 kJ/mol, interaction due to chemisorption occurs. In the present study, the observed

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value of ΔH0 was 134.97 kJ/mol and was falling in the range of 80 to 450 kJ/mol, and here also the adsorption of MB onto the adsorbent prepared could be described predominantly by chemisorption. This observation agreed excellently well with the findings of the isotherm discussed previously in Section 3.2.2. From the values of KC, it is evident that the system performed better at higher temperature, i.e., the removal efficiency in summer would be higher than in winter in the presented system.

With the rise in temperature from 290 to 305 K, the calculated values of k2 (pseudo-second order rate constant) were varied from 0.0021 to 0.0155 g/mg.min at CO = 100 mg/L, 0.0039 to 0.0125 g/mg.min at CO = 200 mg/L, 0.0029 to 0.007 g/mg.min at CO = 300 mg/L and 0.0016 to 0.0033 g/mg.min at CO = 400 mg/L. The increase in the value of k2 with the rise in temperature was indicating faster and more favourable adsorption of MB at higher temperature. The well known Arrhenius equation was used for calculating the activation energy, Ea, as follows

12

ACCEPTED MANUSCRIPT E 1 ln(k 2 )  –  a   lnA  R T

…(19)

The activation energies for four values of CO (100, 200, 300 and 400 mg/L) were calculated from the slopes (using R = 1.987 cal/mol.K) of the straight lines drawn ( ln(k 2 ) as a function of (1/T) in Figure–10b) and was observed to vary from 84.07 to 98.90 kJ/mol. It was reported in the literature [45, 46] that relatively higher values of Ea in the range of 14.3 to 191 kcal/mol indicated

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for chemisorption. The observed values of Ea in the present investigation were falling within this domain and hence the adsorption of MB onto the citric acid treated carbonized bamboo leaves

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powder could be characterized by the chemisorption. This observation agreed excellently well

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with the findings of isotherm and thermodynamics discussed previously.

7.0 Mechanism of adsorption of MB onto the adsorbent prepared

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The various absorption bands at different frequencies identified from the FTIR spectra after adsorption were further analyzed (Supplementary Figure–S2) and compared with the FTIR

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spectra before adsorption. After adsorption of MB, the characteristic absorption bands appeared at different frequencies corresponding to MB molecule were at 1596.75 cm-1 for the aromatic ring C=C bending vibration, 1382.72 cm-1 for the C–N stretching vibration and 1319.63 cm-1 for –CH3

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group stretching vibration [47, 48]. This demonstrates that MB molecules adsorbed onto the

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adsorbent surface positively [49]. Moreover, the peak appeared at 3402.12 cm-1 was assigned to the stretching vibration of free –OH groups (intermolecular hydrogen bonded) which was

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broadened and its intensity decreased compared to that observed in the FTIR spectra before adsorption. It indicates that the electrostatic attraction between the surface –OH group of the adsorbent and cationic groups or nitrogen atoms of MB was playing a role for MB adsorption. Peak identified at 1226.04 cm-1 was assigned to the C–O of alcohol showing a significant shift from 1274.21 cm-1 appeared before adsorption indicating anchoring of MB molecule on to the surface of the adsorbent due to adsorption. It further implies that MB could have interacted through hydrogen bonding with –OH group. The absorption bands appeared at 1657.21, 1555.23 and 1536.72 cm-1 were due to the stretching vibrations of –COO-1 and the intensities were also decreased significantly. This indicates that MB molecule could have adsorbed through electrostatic attraction between the cationic groups of MB and –COO-1 anion. The peak appeared at 1708.07 cm-1 was due to the stretching vibration of C=O in –COOH (of aliphatic acid) and the intensity was decreased compared to that appeared in the FTIR spectra before adsorption at 1708.07 cm-1 showing the existence of electron acceptor-donor interaction between the carbonyl groups on the surface of the adsorbent and MB molecule [50]. The acidic functional group in the 13

ACCEPTED MANUSCRIPT case of citric acid treated adsorbent is –COOH group and its pKa value is about 3. Therefore, the non-ionic form of the carboxyl group i.e., –COOH, was dominant in solution at pH < 3 and under this condition, the adsorption of cationic MB molecule would be small owing to the absence of electrostatic interaction [51]. In contrast, at pH > 3, the anionic form of the carboxyl group i.e., – COO-, would become dominant in solution resulting into increased MB adsorption. In the present study, the solution pH was around 7.5 (natural) and was greater than 3, thus the available sites

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were favorable for adsorption of MB. As a result, the hydrogen bonding, electrostatic and electron acceptor-donor interactions between the citric acid treated adsorbent prepared and MB molecule

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could be responsible for the adsorption studied.

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8.0 Novelties of the present investigation

Since the equilibrium time was appreciably lower (60 minutes), the adsorbent could be used for

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practical purposes suitably. The pH of the treated solution was well within the discharge standard for pH. Thus, no additional chemicals and costs would be necessary as required in many other investigations reported. Very high removal efficiency (99.97%) leading to almost colorless liquid

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was obtained in the presented system that is what is required as per the statutory requirement. The observed maximum adsorption capacity was 725 mg/g and was higher than many of those

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reported in the literature (Table–6). The seasonal variation was used to study the effect of

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temperature in adsorption to simulate the real situation. This would help in predicting the performance data realistically from experiment to scale up. It is further proposed to perform such experiments at various seasons for simulating the field condition realistically to obtain the

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influence of temperature and to avoid energy consumption leading to emission of CO2, a green house gas. Since the performance was observed better at higher temperature i.e., the system could be better operated in warmer climatic conditions (or in summer) rather than in colder or hilly region of the country.

9.0 Conclusion

The adsorption of methylene blue (MB) from its aqueous solution onto citric acid treated bamboo leaves powder as an adsorbent was reported in this article. The adsorbent was characterized by determining the solid as well as bulk density, pHpzc, particle size, and was analyzed by SEM, BET and FTIR. The measured values of solid density, bulk density, pHpzc, particle size (SMD), BET surface area were1825 kg/m3, 968.2 kg/m3, 6.6, 32.53 µm and 393.3 m2/g respectively. Batch experiments were performed as a function of various operating parameters such as - initial adsorbate (MB) concentration (100 to 400 mg/L), contact time (10 to 100 min), adsorbent dose (0.2 to 1.4 g/L), shaker speed (50 to 350 rpm) and temperature (290 to 305 K) at natural pH of 14

ACCEPTED MANUSCRIPT 7.5. The batch experimental data followed Temkin isotherm amongst seven well known isotherms (Langmuir, Freundlich, Temkin, Halsey, D-R, Sips, R-P) examined. The experimental results further followed pseudo-second order kinetic model. Though intra-particle diffusion was present, the rate was controlled by the film diffusion. The thermodynamic parameters (ΔG0: -33.70 to 25.43 kJ/mol; ΔH0: 134.97 kJ/mol; ΔS0: 554.51J/mol.K) revealed spontaneity of adsorption and the nature was chemisorption which was also followed from the activation energy (84.07 to 98.90

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kJ/mol) estimated from the pseudo-second order rate constant as a function of temperature. Novelties achieved were - higher removal of MB (99.97%), higher adsorption capacity (725 mg/g), shorter equilibrium time than others (60 minutes), and final solution pH well within the

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safe discharge limit. Besides, the seasonal variation was used to investigate the temperature effect

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in a novel manner. The mechanistic behavior revealed that the hydrogen bonding, electrostatic and electron acceptor-donor interactions between the citric acid treated adsorbent prepared and

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MB molecule could be responsible for the adsorption studied.

Acknowledgements: Authors also acknowledge the Centre for Research in Nanoscience &

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Nanotechnology University of Calcutta, for carrying out the SEM analyses; Indian Association for the Cultivation of Science, for carrying out the BET surface area and micro-pore analysis; and the Post Graduate Department of Chemistry, Rashbehari Siksha Prangan, University of Calcutta for carrying out the

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FTIR Analyses.

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The authors have declared no conflict of interest.

Appendix A. Supplementary material Supplementary material associated with this article can be found in Supplementary data file.

15

ACCEPTED MANUSCRIPT Nomenclatures A

Pre-exponential factor in Arrhenius equation, dimensionless

aR

Redlich–Peterson isotherm constant (L/mg)

ARE

Average relative error, 100 n

bT

n



q

e,calc

i=1

 q e,meas 

q e,meas

i

Temkin constant related to the heat of sorption indicating 1/bT as the adsorption potential of the adsorbent, J/mol Equilibrium concentration of MB, mg/L

Cf

Final concentration, mg/L

CO

Initial concentration of MB, mg/L

Ct

Concentration of MB at any time t, mg/L

di

Diameter of the particles in the ith group, µm

E

Sorption energy, kJ/mol

Ea

Activation energy, kJ/mol

Fi

Number of particles in the ith group

F(t)

Fractional attainment of adsorption equilibrium  

g

Redlich–Peterson isotherm exponent

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SC

RI

PT

Ce

Hybrid fractional error function, 100

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HYBRID

D

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 qt   , dimensionless  qe 

n

 n  p  i=1

q

e,calc

 q e,meas 

2

q e,meas i

Intercept in Weber-Morris intra-particle diffusion model, mg/g

k1

Pseudo-first order adsorption kinetic rate constant, min-1

k2

Pseudo-second order adsorption kinetic rate constant, g/mg.L.min

KC

Thermodynamic equilibrium constant, dimensionless

kf

Freundlich isotherm constant, (mol/g). (mol/L)-1/n

kid

Rate constant [intra-particle diffusion], mg/g.(min)0.5

KH

Halsey constant, dimensionless

kL

Langmuir constant, L/mg

KR

Redlich–Peterson isotherm constant (L/g)

KS

Sips isotherm model constant (L/g)

KT

Temkin isotherm constant, L/gm

MMB

Molecular weight of MB, 319.5 g/mol

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I

MPSD Marquardt’s percent standard deviation, 100

n  q  1   e,calc  q e,meas       n  p  i=1  q e,meas  

2

i

16

n

Number of data points

nF

Freundlich constant, dimensionless

nH

Halsey constant, dimensionless

ns

Sips isotherm model exponent

P

Pressure at which gas is adsorbed, kPa

Po

Saturated vapor pressure, kPa

p

Number of parameters

qDR

Theoretical monolayer sorption capacity, mg/g

qe

Amount of MB adsorbed measured at equilibrium, mg/g

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or qe,meas

Amount of MB adsorbed calculated at equilibrium, mg/g

qm

Maximum amount of MB adsorbed, mg/g

qt

Amount of MB adsorbed at time t, mg/g

r

Correlation coefficient, dimensionless

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qe,cal

2

Coefficient of determination, dimensionless

R

Universal gas constant, 8.314 J/mol K, 1.987 cal/mol.K

%R

Percentage of MB dye removed, dimensionless

SAE

Sum of the absolute errors,

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r

n

e,calc

 q e,meas i

D

q i=1

Sum of the squares of the errors,

n

 (q

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SSE

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ACCEPTED MANUSCRIPT

e,calc

 q e,meas )i2

i=1

Contact time, min

T

Absolute temperature, K

V

Volume of dye solution, L

Vm

Volume of gas adsorbed under pressure P, mL

W

Amount of adsorbent used, g

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t

Greek Letters ΔG0

Gibbs free energy change, kJ/mol

ΔH0

Enthalpy change, kJ/mol

ΔS0

Entropy change, J/mol.K

β

Mean energy of sorption per molecule of sorbate related to the average energy of sorption as the dye molecule is transferred from the bulk of the system to the surface of the solid adsorbent, mol2/J2

χ2

Chi-square,

 (q e,calc  q e,meas ) 2     q e,meas i=1    i n

17

ACCEPTED MANUSCRIPT ε

Dubinin–Radushkevich isotherm constant

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Figure Captions pH at point of zero charge (pHPZC) of citric acid treated carbonized bamboo leaves powder

Figure–2.

Particle size distribution of the prepared adsorbent

Figure–3.

SEM images of the adsorbent (a) before adsorption, (b) after adsorption.

Figure–4.

(a) BET plot for determining BET surface area; (b) BET isotherm of the adsorbent showing open ended hysteresis

Figure–5.

Effect of contact time on (a) removal of MB, and (b) adsorption capacity

Figure–6.

Effect of adsorbent dose on (a) removal of MB, and (b) adsorption capacity

Figure–7.

Effect of shaker speed on (a) removal of MB, and (b) adsorption capacity

Figure–8.

Effect of contact time on pH of the post treated (adsorption) solution

Figure–9.

(a) Weber- Morris plot for investigating intra-particle diffusion; (b) Boyd’s plot of Bt vs contact time

Figure–10.

Determining (a) thermodynamic parameters, and (b) the activation energy

Table1. Table–2. Table3.

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Table Captions

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SC

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Figure–1.

Estimated values of different isotherm parameters Comparison of correlation coefficient (r), coefficient of determination (r2) and other estimated errors for different isotherms Calculated values of the rate constants of adsorption kinetics and statistical parameters

Table4.

Calculated values of diffusion parameters and statistical parameters

Table–5.

Thermodynamic parameters for adsorption of MB onto the prepared adsorbent

Table–6.

Maximum adsorption capacity of different adsorbents for adsorption of MB dye

22

ACCEPTED MANUSCRIPT

14

(a)

1.5

0.0

8 6

(b)

1.0 0.5

-0.5

PT

Final pH

10

pH = [ pH i - pH f]

12

Difference in pH,

2.0

-1.0

4 2

-2.0

2

4

6

8

10

12

1

2

Initial pH

3

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-1.5

4

5

6

7

8

9

10

11

12

SC

Initial pH

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D

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Figure–1. pH at point of zero charge (pHPZC) of citric acid treated carbonized bamboo leaves powder

23

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SC

RI

PT

ACCEPTED MANUSCRIPT

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D

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Figure–2. Particle size distribution of prepared adsorbent

24

ACCEPTED MANUSCRIPT

(b)

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RI

PT

(a)

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Figure–3. SEM images of the adsorbent (a) before adsorption, (b) after adsorption.

25

ACCEPTED MANUSCRIPT

120

0.08 0.06 0.04

100 80 60 40

N2 Adsorption;

20

Mass of adsorbent = 0.0244 g

0

0.05

0.10

0.15

0.20

0.25

0.30

0.0

0.2

0.4

0.6

0.8

1.0

P/Po

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P/Po

0.35

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-0.02 0.00

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0.02 0.00

(b)

140

0.10

1/(Vm((Po/P)-1))

BET surface area = 393.3 m2/g

PT

0.12

160

(a)

RI

0.14

Equation: y = a + b*x; Adj. r2 = 0.99664 Value Standard Error Intercept -0.00599 0.00144 Slope 0.50042 0.00918 Linear fit of 1/(Vm((Po/P)-1))

Volume [cc/g] STP

0.16

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Figure–4. (a) BET plot for determining BET surface area; (b) BET isotherm of the adsorbent showing open ended hysteresis

26

ACCEPTED MANUSCRIPT

425

(a)

(b) 400

96 95

Adsorbent dose = 1 g/L Shaker speed = 300 rpm Symbol Co, mg/L

94

100 400 20

40

60

80

RI

100

100

0

20

40

60

80

100

Contact time, min

D

Contact time, min

100 400

90

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0

350

Adsorbent dose = 1 g/L Shaker speed = 300 rpm Symbol Co, mg/L

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97

375

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98

Adsorption capacity, mg/g

99

Removal of MB, %

PT

100

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Figure–5. Effect of contact time on (a) removal of MB, and (b) adsorption capacity

27

ACCEPTED MANUSCRIPT

100

(a)

(b)

700

Contact time, t = 60 min Shaker speed = 300 rpm Symbol CO, mg/L

70 60

Contact time, t = 60 min Shaker speed = 300 rpm Symbol CO, mg/L

50

100 400

40

0.4

0.6

0.8

1.0

400 300 200

1.2

SC

0.2

500

100

30

100 400

PT

80

600

RI

Adsorption capacity, mg/g

Removal of MB, %

90

1.4

0.2

0.6

0.8

1.0

1.2

1.4

Adsorbent dose, g/L

NU

Adsorbent dose, g/L

0.4

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Figure–6. Effect of adsorbent dose on (a) removal of MB, and (b) adsorption capacity

28

ACCEPTED MANUSCRIPT

100

420

(a)

(b)

400

98

92 Contact time, t = 60 min Adsorbent dose = 1 g/L Symbol CO, mg/L

90 88

100 400

PT

94

Contact time, t = 60 min Adsorbent dose = 1 g/L Symbol CO, mg/L

360 340

100

100 400

RI

96

SC

Adsorption capacity, mg/g

Removal of MB, %

380

90 80

50

100

150

200

250

300

50

100

150

200

250

300

350

Shaker speed, rpm

MA

Shaker speed, rpm

350

NU

86

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Figure–7. Effect of shaker speed on (a) removal of MB, and (b) adsorption capacity

29

ACCEPTED MANUSCRIPT

12 11

9

100 200 0

RI

8 7

SC

Final solution pH

10

PT

Initial (Natural) pH = 7.5 Contact time, t = 60 min Shaker speed = 300 rpm Adsorbent dose = 1g/L Symbol CO, mg/L

6

4 3 20

40

MA

0

NU

5

60

80

100

Contact time, min

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Figure–8. Effect of contact time on pH of the post treated (adsorption) solution

30

ACCEPTED MANUSCRIPT

100

16

(a)

94 200

100 200 300 400

12

PT

198 196

CO= 200 mg/L

194 300 298 296 294 292 290 288

RI

8

SC

B.t

Adsorption Capacity, mg/g

CO= 100 mg/L

96

(b)

Adsorbent dose = 1 g/L Shaker speed = 300 rpm Symbol CO, mg/L

98

CO= 300 mg/L

4

390 385

CO= 400 mg/L

380 375 4

6

8

Square root of time,

10

12

MA

2

NU

395

min0.5

0

0

10

20

30

40

50

60

70

80

Contact time, min

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Figure–9. (a) Weber- Morris plot for investigating intra-particle diffusion; (b) Boyd’s plot of Bt vs contact time

31

ACCEPTED MANUSCRIPT

-4

(b)

(a)

14

-5

8

Intercept = 66.69537 Slope = -16234.17587 Adj. r square = 0.95821

-6

-7

6 0.00330

0.00335

0.00340

0.00345

0.00330

0.00335

0.00340

0.00345

1/T

NU

1/T

Symbol CO, mg/L 100 200 300 400

SC

-8

PT

10

RI

ln(k2)

ln(KC)

12

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Figure–10. Determining (a) the thermodynamic parameters, and (b) the activation energy

32

ACCEPTED MANUSCRIPT Table1.Estimated values of different isotherm parameters Isotherm parameters and constants determined

Langmuir

qm: 526.32; kL: 1.9

Freundlich

kf: 300.04; nF:2.110

Temkin

bT: 24.58; kT: 20.57

Halsey

nH: 2.110; KH: 169187

Dubinin-Radushkevich

qDR: 384.69; E: 3.244

Sips

qmax: 481.9; Ks: 2.088; n: 1.003

R-P

KR: 792; aR: 1.194;g: 1.534

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SC

RI

PT

Type of Isotherms

33

ACCEPTED MANUSCRIPT

Table–2. Comparison of correlation coefficient (r), coefficient of determination (r2) and other estimated errors for different isotherms Isotherm Models

r

r2

χ2

SSE

SAE

ARE

HYBRID

MPSD

Exp.

Cal.

qmax

qmax

0.994 0.989

6.19

1863.29

51.49

4.31

309.84

10.78

526.32

Freundlich

0.971 0.942

11.59

2768.72

3.19

0.76

579.35

17.46

433.09

Temkin

0.998 0.996

1.17

212.19

1.42

0.29

58.31

6.00

397.76

Halsey

0.971 0.942 5304.1

614330

847.20

D-R

0.994 0.988

4.45

1540.5

3.69

Sips

0.995 0.990

2.48

539.76

0.72

R-P

0.993 0.986

15.63

5245.8

RI

202.33 265202.8 497.73 397.83 780.40

SC

0.16

NU

0.18 0.92

222.54

8.11

368.70

247.73

11.24

394.60

1562.6

21.79

349.42

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1.02

PT

Langmuir

34

ACCEPTED MANUSCRIPT

Table3. Calculated values of the rate constants of adsorption kinetics and statistical parameters

Pseudo second order kinetics

Pseudo first order kinetics

min-1

qmax

qmax

99.87

8.14

χ2

r

r2

1.00

1

χ2

K2,

Exp.

Cal.

gm/mg.min

qmax

qmax

0.015

99.867

100.60

200 0.142 199.71 85.86 0.917 0.841 3224

0.013

199.709 200.80

1.00

0.999 0.052

300 0.117 299.11 93.37 0.861 0.741

0.007

299.109 300.30

1.00

0.999

400 0.079 397.83 68.39 0.978 0.957

0.003

0.979 0.958

397.83

401.61 0.999 0.999

NU MA D TE

100 0.054

r

PT

Cal.

r2

RI

Exp.

SC

K1,

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ACCEPTED MANUSCRIPT

Table4. Calculated values of diffusion parameters and statistical parameters

C0;

Intra-particle Diffusion Model Kid, mg/g.min0.5

mg/L Line segment

0.9701 0.9912 0.8784

0.9413 0.9824 0.7717

1st

189.25 193.05 199.70

0.9204 0.8972 0.7899

0.8471 0.8049 0.6239

2nd

283.94 288.16 299.09

0.9904 0.9763 0.7899

0.9808 0.9532 0.6239

362.10 378.32 396.75

0.996 0.9373 0.7899

0.992 0.8792 0.6239

RI

3

1st

2.01 1.34 -

2nd

MA

3rd 1st

4.94 2.39 -

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2nd 3rd

NU

3rd

400

91.50 96.25 98.98

PT

1.22 0.41 1.48 0.83 -

rd

300

r2

1st 2nd

200

r

SC

100

Intercept

36

ACCEPTED MANUSCRIPT

Table–5. Thermodynamic parameters for adsorption of MB onto the prepared adsorbent

590314.3

-33.70

300

349122.5

-31.83

295

137258.1

-29.01

290

38121.5

-25.43

ΔS0 (J/mol.K)

134.97

554.51

AC CE P

TE

D

MA

NU

305

ΔH0 (kJ/mol)

PT

ΔG0 (kJ/mol )

RI

KC, L/mol

SC

Temp, K

37

ACCEPTED MANUSCRIPT

Table–6. Maximum adsorption capacity of different adsorbents for adsorption of MB dye *Adsorption capacity, mg/g

References

Activated carbon prepared from dross licorice by ultrasonic

82.9

[1]

Activated carbon prepared from hazelnut husk using zinc chloride

476.2

[2]

402.25

[3]

487.4

[4]

KOH-activated carbon prepared from sucrose spherical carbon

704.2

[5]

Activated carbon prepared from Fox nutshell by chemical activation

968.74

[6]

Cross-linked beads of zeolite/chitosan composite

151.51

[7]

Mesoporous-activated carbon prepared from chitosan flakes via single-step sodium hydroxide activation

143.53 (50 °C)

[8]

Activated carbon prepared from karanj (Pongamia pinnata) fruit hulls

154.8

[9]

Cashew nut shell based carbons activated with zinc chloride

476

[10]

47.62

[11]

oil

palm

ash

AC CE P

TE

D

activated

MA

NU

SC

Factory-rejected tea activated carbon prepared by conjunction of hydrothermal carbonization and sodium hydroxide activation processes

RI

Porous carbon prepared from tea waste

PT

Adsorbent

Activated carbon developed from Ficus carica bast Activated carbons prepared from waste carpets treated with H3PO4

[12]

Solid waste: H3PO4 (1:1)

403.2

Solid waste: H3PO4 (1:2)

704.2

Solid waste: H3PO4 (1:3)

769.2

Sewage sludge-derived biochar

28.82

[13]

Poly vinyl alcohol

123.3

[17] 38

ACCEPTED MANUSCRIPT Ephedra strobilacea char

31.152

E. strobilacea char modified using phosphoric acid

21.929

E. strobilacea char zinc chloride

37.037

Activated carbon spheres derived from

602.4

[21] [22]

244.03

RI

MoS2 composite through the self-polymerization of levodopa (MoS2-PDOPA)

221

PT

[email protected](Fe) core-shells

SC

688.85

[23]

725

Present study

NU

Citric acid treated carbonized bamboo leaves powder

[19]

925.9 (45OC)

hydrothermally-prepared poly(vinyl alcohol) microspheres

Functionalization SiO2 composites

[18]

AC CE P

TE

D

MA

[* values are at room temperature unless otherwise specified]

39

ACCEPTED MANUSCRIPT

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SC

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Graphical Abstract

40

ACCEPTED MANUSCRIPT Research Highlights



Citric acid treated carbonized bamboo leaves powder used as adsorbent for Methylene Blue Temperature effect investigated in a novel manner exploiting seasonal variation



Observed higher removal efficiency (99.97%) and adsorption capacity (725 mg/g)



Achieved relatively shorter equilibrium time of 60 minutes than others



Isotherm, kinetics and thermodynamics agreed chemisorption

AC CE P

TE

D

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NU

SC

RI

PT



41