Use of banana trunk waste as activated carbon in scavenging methylene blue dye: Kinetic, thermodynamic, and isotherm studies

Accepted Manuscript Use of banana trunk waste as activated carbon in scavenging methylene blue dye: Kinetic, thermodynamic, and isotherm studies

Mohammed Danish, Tanweer Ahmad, Shahnaz Majeed, Mehraj Ahmad, Lou Ziyang, Zhou Pin, S.M. Shakeel Iqubal PII: DOI: Reference:

S2589-014X(18)30070-7 doi:10.1016/j.biteb.2018.07.007 BITEB 61

To appear in:

Bioresource Technology Reports

Received date: Revised date: Accepted date:

12 June 2018 28 July 2018 28 July 2018

Please cite this article as: Mohammed Danish, Tanweer Ahmad, Shahnaz Majeed, Mehraj Ahmad, Lou Ziyang, Zhou Pin, S.M. Shakeel Iqubal , Use of banana trunk waste as activated carbon in scavenging methylene blue dye: Kinetic, thermodynamic, and isotherm studies. Biteb (2018), doi:10.1016/j.biteb.2018.07.007

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ACCEPTED MANUSCRIPT Use of banana trunk waste as activated carbon in scavenging methylene blue dye: Kinetic, thermodynamic, and isotherm studies Mohammed Danisha*, Tanweer Ahmadb , Shahnaz Majeedc, Mehraj Ahmadd, Lou Ziyange, Zhou Pine, S. M. Shakeel Iqubalf a

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Bioengineering Technology Section, Universiti Kuala Lumpur Malaysian Institute of Chemical and Bioengineering Technology (MICET), Lot 1988, Kawasan Perindustrian Bandar Vendor, Taboh Naning, 78000 Alor Gajah, Melaka, Malaysia.

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Department of Chemistry, College of Natural and Computational Science, Madda Walabu University, Bale Robe, Ethiopia

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Faculty of Pharmacy and Health Sciences, Universiti Kuala Lumpur, Royal College of Medicine, Perak 30450, Malaysia d

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School of Food Science & Bioengineering, Zhejiang Gongshang University, 18 Xuezheng Street Xiasha, Hangzhou Zhejiang 310018, China e

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Department of Basic Science (Chemistry), Ibn Sina National College, Jeddah (Saudi Arabia)

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f

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School of Environmental Science and Engineering, 800 Dongchuan Road, Shanghai Jiao Tong University, Shanghai 200240 China

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ACCEPTED MANUSCRIPT Abstract Activated carbon (BTAC) derived from the banana trunk was utilized in this experiment to scavenge the methylene blue (MB) dye from synthetic dye solution. The optimized large surface area (1173.16 m2/g) BTAC was prepared with phosphoric acid (6.96 mol/L) chemical activation.

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The maximum adsorption capacity (qmax) of BTAC was experimentally observed to be 166.51

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mg/g at 25 °C, and adsorbent dose of 1.5 g/L for 250 ppm dye concentration. The MB dye

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adsorption onto BTAC reached equilibrium within 20 min of contact time. The kinetic data shown equilibrium achieved quickly (20 min) and followed a linear equation of the pseudo-

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second-order kinetic model. The adsorption of dye molecules stacking in three layers on the

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surface of the BTAC, after each layer formation the rate of adsorption slow down, as evident

spontaneous and exothermic. Activated

carbon;

Banana

trunk;

Isotherm;

Kinetics;

Methylene

blue;

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Keywords:

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from the intraparticle diffusion study. The thermodynamic study shown that adsorption was

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Thermodynamics.

*Author for Correspondence:

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Dr. Mohammed Danish, Bioengineering Technology Section, Universiti Kuala Lumpur Malaysian Institute of Chemical and Bioengineering Technology (MICET), Universiti Kuala Lumpur, Lot 1988, Kawasan Perindustrian Bandar Vendor, Taboh Naning, 78000 Alor Gajah, Melaka, Malaysia E-mail: [email protected], [email protected]

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KFM ΔG KD-R EA Ε Ɵ

qm HA & HB W & Wb U Ce qe kn k1

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B T R A

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B

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KT

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N

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RL KF

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KL

Interpretation the maximum uptake of MB dye corresponding to the sites saturation through Langmuir isotherm model (mg/g) Constant used in Langmuir model, it corresponds to the energy of adsorption (L/g). Dimensionless separation factor Constant used in the Freundlich isotherm model, it corresponds to adsorption capacity. Constant used Freundlich isotherm model, it corresponds to adsorption intensity Temkin isotherm constant, it expressed the equilibrium binding constant (L/mg) Temkin constant for the heat of adsorption (~RT/b) Heat of adsorption constant Temperature in Kelvin scale (K) Universal gas constant (8.314 J/K/mol) Interaction parameter which can be positive or negative values Frumkin constant related to adsorption equilibrium. Gibbs free energy change Dubinin and Radushkevich constant correspond to the adsorption energy. Mean free energy of adsorption Polanyi potential ~ RTln(1+1/Ce) Fractional occupation calculated by the ratio of adsorption capacity at equilibrium and theoretical monolayer saturation capacity. Maximum monolayer adsorption capacity determined through D-R isotherm Harkins-Jura isotherm constant for multilayer adsorption Constant parameters for Smith isotherm equation Fractional attainment of equilibrium the equilibrium concentration of MB dye (mg/L). the amount of MB dye uptake at equilibrium (mg/g). Net first order reversible rate constant(min-1) Rate of adsorption of the first-order reversible kinetic model(min-1).

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Abbreviations qmax

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List of abbreviations

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k-1

Rate of desorption of first-order reversible kinetic model (min-1). Equilibrium constant Rate of adsorption for the pseudo-secondorder kinetic model (min-1). Initial sorption rate for pseudo-second order kinetic model (mg/g/min) Weber-Morris adsorption capacity in the first phase of adsorption (mg/g) Weber-Morris adsorption rate constant in the first phase of adsorption (mg/g/min.1/2) Boundary layer in the first phase of adsorption in the first phase Weber-Morris adsorption capacity in the second phase of adsorption (mg/g) Weber-Morris adsorption rate constant in the second phase of adsorption (mg/g/min.1/2) Boundary layer in the first phase of adsorption in the second phase Weber-Morris adsorption capacity in the third phase of adsorption (mg/g) Weber-Morris adsorption rate constant in the third phase of adsorption (mg/g/min.1/2) Boundary layer in the first phase of adsorption in the third phase

Kc k2 h qt1

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Kd1

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C1 qt2

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Kd2 C2

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qt3

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Kd3

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C3

1. Introduction

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Water soluble dye molecules are considered to be a hazardous organic compound for the aquatic

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environment. They are used in textile and other industries like cosmetic, pharmaceutical, rubber, plastic, food, and paper industry (Rafatullah et al. 2010). Artificial dyes are also toxic to various microorganisms which can either cause inhibition or destruction to their catalytic and functional capabilities. The presence of various dye molecules in aquifer reduces light penetration which directly affects the photosynthesis process of aquatic flora. The wastewaters discharged after dyeing processes showed high biological oxygen demand (BOD), high chemical oxygen demand (COD), visible pollutants, and large amounts of dissolved solids. Some dyes are mutagenic, 4

ACCEPTED MANUSCRIPT carcinogenic and teratogenic (Bhattacharyya and Sharma 2005; Kaur et al. 2015). Water streams contaminated with artificial dyes can cause dermatitis, allergy, irritation of the skin, which in turn can cause the mutation in the nucleotides of deoxyribonucleic acid (DNA), that leads to the growth of cancer cells in humans and animals (Veliev et al. 2006). The presence of dye makes

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the water objectionable for drinking. Therefore, it is in dire need to scavenge dye from

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wastewater to prevent the continuous environmental pollution and to protect the water quality for

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flora and fauna.

Methylene blue (MB) has a wide range of applications especially as a coloring agent for hair,

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paper, cotton, wools, and coating paper. Chemical studies showed that MB dye molecule is a

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heterocyclic aromatic compound. The chemical properties of the MB dye molecules have been intensively studied due to its strong binding with solids, and quite often it serves as a model

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compound for the separation of colored bodies and colorless organic compounds from various treatment

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contaminated mixtures (Rafatullah et al. 2010). Many advanced techniques and

methods were employed for the separation/mineralization of dye molecules from wastewater

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such as photo-catalytic (Houas et al. 2001), Fenton process and electrochemical combined

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treatments (Lahkimi et al. 2007), adsorption process (Danish et al. 2013), electrochemical degradation (Panizza et al. 2007), flocculation (Han et al. 2002), chemical coagulation and

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ozonation (Sarasa et al. 1998), nano-filtration and ultrafiltration (Fersi and Dhahbi 2008), cloud point extraction (Bezerra et al. 2005), and reverse osmosis (Al-Bastaki 2004; Nataraj et al. 2009). Adsorption is a surface phenomenon which occurs at the surface, and the adsorbate is remained stuck at the surface of the adsorbent without any change in chemical composition or structure. So, it has the advantage to be recovered the adsorbate species in its original form through desorption. The adsorption methods can be widely applied to recover the specific chemical

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ACCEPTED MANUSCRIPT species from wastewater or aqueous solutions if we use highly selective adsorbents. The low cost, sustainable, selective, and eco-friendly adsorbents could be proved to be an effective and advanced technique to recover dyes from wastewater. Contemporary researchers are showing great interest in developing new adsorbent materials with excellent properties, diverse

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compositions, and selective functionalities. The biomass waste derived activated carbons (AC)

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are the most promising materials which can serve as excellent adsorbent materials. Although

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commercial activated carbon (mostly derived from coal mining dust) is a widely used adsorbent to remove various dye molecules, it is too expensive to be used in small-scale industries.

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Literature survey shows that in last decades so many low-cost alternative adsorbents are

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proposed that includes coconut shells (Dwivedi et al. 2008), Acacia mangium wood (Danish et al. 2011), rice husk (Sahu et al. 2009), pomegranate shell (Ghaedi et al. 2012), bamboo (Hameed

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et al. 2007), banana waste (Ahmad and Danish 2018), oil palm waste (Rafatullah et al. 2012) and

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date stone (Danish et al. 2014). Methylene blue dye removal through biochar from oak wood (Babaei et al. 2016), pine wood, pig manure, and cardboard (Lonappan et al. 2016) were also

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tried.

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Banana is the second most commonly cultivated fruit in Malaysia. According to the statistical report of the department of agriculture Malaysia, the banana tree plantation cover around 30,320

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hectares and produced 321,810 metric tonnes of fresh banana fruits in the year 2016. Whereas, the world production of banana in 2016 was 144 million metric tonnes (Ahmad and Danish 2018). Banana plant can produce fruit once in a lifetime, so after fruit harvesting, the whole plant is discarded as waste. It was estimated that near 100 kg of waste generated for every tonne of banana fruit production. If this much huge waste can be transformed into useful material, will be a great help to banana grower farmers. Keeping these in view, the activated carbon formation

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ACCEPTED MANUSCRIPT from banana waste will benefit the farmers in two possible ways: it will increase the revenue as well as reduce the volume of the waste significantly. At the same time, it could support the environment by fixing the carbon as a solid framework. Otherwise, these waste dumping disposal in water and humid conditions could lead to methane and carbon monoxide gas

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formation or their burning could produce a large quantity of CO2 gas; all these gases are

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responsible for global warming.

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The current study was designed to assess the scavenging behavior of banana trunk activated carbon (BTAC) against aqueous methylene blue (MB) dye. For the first time kinetic,

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thermodynamic, and isotherm study of MB dye adsorption onto the novel prepared banana trunk

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waste activated carbon (BTAC) was studied. The effect of adsorbent dosage, initial MB dye concentration, contact time, and solution temperature was evaluated for the scavenging of MB

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dye through BTAC adsorbent. The thermodynamic parameters of the scavenging behavior of

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BTAC and MB dye were calculated, and the kinetic rate of MB adsorption onto BTAC was

also been investigated.

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established through various models. The mechanism of scavenging behavior and isotherms has

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2. Materials and methods Preparation of BTAC

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The banana trunk was collected from a banana plantation farm at Tampin, Negeri Sembilan, Malaysia. The banana trunk was cut into small pieces (dimension, 3 cm x 3 cm x 4 cm) and washed thoroughly several times with water to remove sand and soil residue from the surface. In a subsequent step, the washed pieces of the banana trunk were set for drying at 105 °C for 24 h in a forced draft oven. The moisture content of the representative sample of the banana trunk was taken in triplicate. Nearly 10 g of banana trunk pieces were taken in glass Petri dishes and kept

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ACCEPTED MANUSCRIPT for drying at 105 °C till constant weight was obtained. After drying, the dried banana trunk pieces were grounded and sieved into a uniform particle size of 0.5 μm. For activated carbon preparation, 100 g of dried banana trunk powder was mixed with 5.09 mol/L of phosphoric acid (H3PO4) (optimized activating agents for methylene blue removal

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study, Danish et al. 2018) and kept for complete soaking of the activating agent into the banana

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trunk powder. The soaking was performed at room temperature. Then, the mixture was placed in

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the muffle furnace at 845 °C for 50 min in a loosely covered silica crucible. After the activation process, the muffle furnace was left for 12 h to bring the temperature down. The washed BTAC

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product was dried at 105 °C for 24 h inside a hot air oven. Finally, the dried BTAC product was

Methylene blue dye stock solution

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kept in tight lid glass bottles for further analysis and application studies.

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Methylene blue dye was procured from Scharlau’s chemicals, 1.000±0.005 g of dye was dissolved in 1 L standard flask to prepare 1000 ppm stock solution. Later this stock solution was

Adsorbent dosage optimization experiment

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2.3

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used to prepare a desired concentration dye solution for further studies.

Initially, the experiments were performed to optimize the adsorbent dosage (g/L) for maximum

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removal of MB dye through BTAC adsorbent. Randomly selected adsorbent dosage of 0.3, 0.5, 0.7, 1.0, 1.5, 1.7, 2.0 g/L were used for finding the optimum dosage for maximum removal percentage against 50 ppm MB dye solution. The experiments required three conical flasks for each selected dosage, to find the deviation in the reproducibility of the dye adsorption. The BTAC adsorbent was weighed on butter papers using analytical balance (Sartorius Quas) that was capable of weighing accurately in milligrams. Butter paper was preferred during the

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ACCEPTED MANUSCRIPT activated carbon weight measurement due to its non-sticky surface that minimizes the mass loss and prevents the powder from being scattered around. For 30 ml of 50 ppm MB dye, the weight of the BTAC adsorbent was accurately measured using the following relationship (Eq.1). dosage( g / L)  MB dye vol.(mL) 1000

(1)

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Adsorbent weight 

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The precise weight of BTAC adsorbent was transferred into conical flasks, containing 30 ml of

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100 ppm MB solution. The mixture was thoroughly mixed and set for 1 h continuous agitation. After that, BTAC was filtered through the Whatman filter paper No. 42, and residual MB dye

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concentration was measured through UV-VIS spectrophotometer (Perkin Elmer, Lambda 35 model). The UV-VIS spectrophotometer was calibrated before the actual measurement was

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conducted. The standard solution of MB dye of various concentrations such as 0.01 ppm, 0.03

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ppm, 0.05 ppm, 0.07 ppm, and 0.10 ppm was prepared carefully. The calibration curve was

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prepared between the standard solution concentration and their corresponding absorbance as shown in Supplementary Fig S1. The regression coefficient (R2) of the calibration curve was

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calculated to be 0.995 with the intercept at 0.00. This high correlation coefficient (R2) value gave

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the highly precise results of residual MB dye concentration. Kinetic measurements

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Thirty-nine conical flasks were prepared with a fixed dosage of 1.5 g/L (45 mg of BTAC for 30 mL of 100 ppm MB dye solution) BTAC adsorbent that was determined through dosage experiment. Each conical flask after mixing the MB dye solution was agitated for a selected time interval of 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, and 30 min. The filtered residual MB dye solution was measured through the UV-VIS spectrophotometer as stated in the previous section. Each time interval was repeated three times to find the deviation in the measured value. 2.5

Thermodynamic measurements 9

ACCEPTED MANUSCRIPT Thermodynamic parameters of MB dye adsorption onto BTAC adsorbent were measured for three dye concentrations viz., 50, 100, and 150 ppm at 30, 40, 50, and 60 °C. For each concentration of MB dye solution, 1.5 g/L adsorbent dosage was used. The temperature of the dye and adsorbent mixture inside a conical flask was maintained by immersing the conical flask

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inside the water bath shaker for a fixed time of 1 h. After the fixed time of agitation inside the

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water bath at a constant selected temperature, the residual MB dye solution was filtered and kept

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in a vial at room temperature (25 °C). Later residual concentration of the MB dye was measured through the UV-VIS spectrophotometer as mentioned in section 2.3.

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2.6 Adsorption isotherm study

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Isotherm study of MB dye adsorption onto BTAC was conducted with five different concentrations of dye solutions viz., 50, 100, 150, 200, and 250 ppm using a fixed dosage of 1.5

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g/L adsorbent. The maximum deviation in the dilution of stock solution (1000 ppm) to the

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selected concentration was ±10 ppm. These deviations in the dye concentration were considered during the calculations to get the correct values of isotherm model constants and adsorption

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capacity. Isotherm study was conducted at room temperature only. The amount of MB adsorbed

(Ci  Ce )V m

(2)

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qe 

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per unit mass of adsorbents (q) at equilibrium was calculated using the following equation:

The amount of MB adsorbed per unit mass of adsorbents at the time t, was calculated through the following equation:

qt 

(Ci  Ct )V m

(3)

The percentage MB removal has been calculated by the following equation (4):

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ACCEPTED MANUSCRIPT %MB removal 

(Ci  Ce ) 100 Ci

(4)

where, Ci, Ce and Ct (mg/L), = Initial, equilibrium, and after time-t concentration of MB dye,

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respectively.

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V = Solution volume of MB dye (units in L)

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m = Amount of adsorbent (units in g)

Effect of adsorbent dosage and contact time on MB adsorption

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3.1

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3. Results and discussion

The adsorption percentage of MB onto different dosage of BTAC adsorbent with 100 ppm of

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initial dye concentration (as shown in Supplementary Fig. S2). The adsorbent dosage increased

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from 0.3 g/L to 1.5 g/L, the adsorption percentage increased with regular increment in the dosage and reached 99.98%. Further increase in BTAC dosage up to 2.0 g/L didn’t change the

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adsorption percentage anymore. So BTAC dosage of 1.5 g/L was chosen as maximum removal

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for 100 ppm MB dye solution for further studies. At an optimum dosage of BTAC adsorbent, the equilibrium contact time was established through

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the plot of percent adsorption vs. time curve (as shown in Supplementary Fig. S3). The curve showed that MB dye adsorption was rapid and 99.53% dye was scavenged in the initial 10 min of BTAC and MB dye interaction. At equilibrium, the MB dye removal percentage was found to be 99.87%.

3.2

Kinetic studies

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ACCEPTED MANUSCRIPT The adsorption of MB dye vs. time data was tested against various available kinetic models. The aim of the kinetic studies was to explore the possible mechanism of sorption of MB dye onto BTAC adsorbent. The well established kinetic models such as reversible first order reaction, pseudo-first-order, and pseudo-second-order models were tested in this study. The conformity

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between predicted model values and experimental data was expressed by the regression

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coefficient (R2). The higher regression coefficient value with the available kinetic experimental

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data is considered as an applicable model.

A first-order reversible kinetic model was used to explore the possibility of the applicability of

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the MB dye adsorption data. The simplified reversible kinetic model is given below: (5)

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ln(1  U )  knt

where, U represents the fractional attainment of equilibrium; kn is net rate constant, and t is a

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time in min.

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The plot between ln(1-U) vs. t, should give a straight line that must pass through the origin for a

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perfect first order reversible kinetic reaction between adsorbent and adsorbate. We applied MB dye adsorption data to calculate U ( as shown in the supplementary data file), then plot ln(1-U)

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vs. t. The representative of the plot is shown in Fig. 1(a). It was observed that the line was not a

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straight line and the regression coefficient (R2) was found to be 0.856, which suggested that MB dye adsorption was not followed the first-order reversible kinetic model. Constants kn, k1, and k-1 were calculated and reported in Table 1.

Table 1: Kinetic models parameters for the adsorption of MB onto BTAC. Kinetic parameters

Values

qe (mg/g) (for 100 ppm initial conc of MB dye)

66.57

First-order-reversible reaction kinetics model 12

ACCEPTED MANUSCRIPT kn (min-1) k1(min-1) k-1(min-1) Kc R2

0.319 0.047 0.271 0.175 0.856

Pseudo-First order reaction kinetics model

qe,cal (mg/g) k1(min-1) R2

31.19 0.067 0.961

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67.57 0.056 256.4 0.999

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qe,cal (mg/g) k2(min-1) h (mg/g/min) R2

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Pseudo-Second order reaction kinetics model

Weber-Morris model

101.068 14.694 35.354 0.9856 65.414 0.6082 62.694 0.8237 66.517 0.1358 65.91 0.7019

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qt1(mg/g) Kd1(mg/g/min.1/2) C1 R12 qt2(mg/g) Kd2(mg/g/min.1/2) C2 R22 qt3(mg/g) Kd3 (mg/g/min.1/2) C3 R32

Pseudo-first order kinetic model was applied to the MB dye adsorption data; the following

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equation was used for model calculation.

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log(qe  qt )  log qe  (

k1' )t 2.303

(6)

where, qt and qe (mg/g ) = adsorption capacity at time, t and equilibrium, respectively k1’ (1/min) = rate constant for pseudo-first-order adsorption. The representative of the plot is shown in Fig. 1 (b). The regression coefficient, R2 for this model is 0.961 and not close to the ideal correlation coefficient value. Constant k value was calculated and mentioned in Table 1.

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ACCEPTED MANUSCRIPT The pseudo-second-order kinetics model (Ho and Mckay 1999) can be expressed as:

t 1 1   t qt h qe

(7)

where;

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k2 (g/ (mg.min)) = Rate constant of the pseudo-second-order reactions

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qe and qt (mg/g) = The adsorption capacities at equilibrium and at time t, respectively

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h = Initial sorption rate as qt/t→0 h  k2  qe2

The plot of t/qt vs. t of eq. (7) is shown in Fig. 1 (c) and give a linear relationship, from which qe,

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k2, and h can be calculated. The values for qe, k2, and h were calculated and are summarized in

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Table 1. The regression coefficient (R2) value for this pseudo-second-order model is 0.999 which is very close to 1.00, the ideal value of the regression coefficient. Therefore, it can be

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concluded that the sorption of MB onto BTAC follows the pseudo-second-order kinetic model

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perfectly.

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Fig. 1

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Intraparticle diffusion of the MB dye molecules was calculated through Weber-Morris model (the model is represented in Eq. 8).

qt  kd  t1/2  c

(8)

where, qt (mg/g) = The adsorption capacities at time t; kd (mg/g/min1/2)= diffusion rate constant, t= time, and c (mg/g)=boundary thickness.

It can be seen from Fig. 1 (d), the plot shows three different slopes. Initial slope was sharp increasing with high diffusion rate constant value (kd=14.694 mg/g/min1/2), when the BTAC 15

ACCEPTED MANUSCRIPT adsorbent surface and pores were freely available for the MB dye molecules, so thickness boundary (35.35 mg/g) was also less. In the second stage, the diffusion rate decreased to 0.6082 mg/g/min1/2, and boundary thickness increased to 62.69 mg/g. In the third stage, the diffusion rate further decreased to 0.1358 mg/g/min1/2, and boundary thickness increased to 65.90 mg/g. It

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is clear from the results that as the adsorption of MB dye progresses with time the diffusion rate

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decreased due to pores and surface spaces were occupied the dye molecules. Consequently, an

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increase in boundary thickness at the surface of BTAC adsorbent was observed. The Weber-

3.3

Effect of temperature on MB dye sorption

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Morris model parameters with correlation coefficients (R2) are reported in Table 1.

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Adsorbate solution temperature is an extremely significant factor during adsorption processes, so the experiments were conducted to observe it for MB dye adsorption onto BTAC. The experiments were

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designed at selected temperatures such as 303.15 K, 313.15 K, 323.15 K, and 333.15 K, for different

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initial concentrations (50 ppm, 100 ppm, and 150 ppm) of MB dye. The percentage sorption of MB

dye onto BTAC adsorbent was calculated against various temperature and initial concentrations

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to explore the nature of the adsorbate-adsorbent interaction. With the help of Fig. 2 (a), the effect

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of temperatures on three different initial concentrations of MB dye was represented. It was observed that with the rise in temperature from 303.15 K to 333.15 K, the adsorption percentage

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was marginally decreased for each higher initial concentrations of MB dye. At a higher initial concentration of MB dye and higher temperature among the selected one, the effect of temperature was observed to be more. This behavior can be explained based on the kinetic movements of the adsorbate (MB dye) molecules and adsorbent particles (BTAC). Since at higher concentration of MB dye molecules, the more molecules adsorb and desorb at a higher temperature, so net adsorption was observed to be lower. Further, this thermal interaction was used to calculate the thermodynamic parameters. 16

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3.4

Fig. 2

Thermodynamic studies

The thermodynamic calculations for MB dye sorption onto BTAC was conducted using Von’t Hoff relation and standard Gibbs free energy change (ΔG°), standard enthalpy change (ΔH°), and standard entropy change (ΔS°) relationship (as given in Eq. 9 and 10). The ΔH° and ΔS° were

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ACCEPTED MANUSCRIPT evaluated through the slope and intercept of Fig. 2 (b) plots, respectively. The ΔG° was estimated by using ΔH° and ΔS° (calculated from the ln (Kc) vs. 1/T plot) at different temperatures in Eq. 10.

S  H  1 ln Kc   ( ) R R T

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(9) (10)

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Go  H o  T S o

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where, Kc is the thermodynamic equilibrium constant (calculated through the ratio of adsorbed concentration to residue MB dye concentration, at equilibrium), T is the temperature in Kelvin

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scale (K), and R represents the universal molar gas constant (8.314 J/mol/K).

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Fig. 2 (b) indicated that the rate of sorption decreased marginally along with the rise in temperature from 303.15 K to 333.15 K. The linearity of the Vont Hoff plots were observed to be

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0.9892, 0.993, and 0.9872 for 50 ppm, 100 ppm, and 150 ppm initial concentration of MB dye,

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respectively. The thermodynamic parameters for MB adsorption onto BTAC is reported in Table 2. The result showed that the value of ΔG° for MB dye sorption onto BTAC at different

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temperature and initial concentrations were varied between -18.313 kJ/mol and -19.719 kJ/mol.

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The ΔG°<0 indicated that MB dye sorption onto BTAC was thermodynamically favorable and spontaneous. Since the ΔG° values fall within the range of 0 and -20 kJ/mol, that indicated the

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electronic interaction between adsorption sites in the adsorbent (BTAC) and the adsorbate molecules (MB dye) (Al-Othman et al. 2013). Meanwhile, ΔG° values ranging from -80 and 400 kJ/mol revealed that the sorption involving sharing or transferring of charge from the surface of the adsorbent to the adsorbate to form a coordinate bond.

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ACCEPTED MANUSCRIPT Table 2: Thermodynamics parameters for MB dye adsorption onto BTAC ΔH° ΔS° -1 (kJ mol ) (kJ mol-1 K-1) -24.571 -0.0157

-0.0088

-15.600

0.0089

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-22.039

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Concentration Temperature ΔG° (ppm) (°C) (kJ mol-1) 50 30 -19.719 40 -19.639 50 -19.482 60 -19.324 100 30 -19.358 40 -19.270 50 -19.182 60 -19.093 150 30 -18.313 40 -18.403 50 -18.492 60 -18.582

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At a fixed temperature, when we compared the ΔG° values of different initial concentrations. It was observed increasing trends, which indicated the decrease in the spontaneity of sorption. The

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increase in ΔG° values with the rise of temperature indicated that the better sorption of MB dye

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onto BTAC could be found at lower temperatures. The overall ΔG° obtained in this study was

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indicating for physisorption of MB dye molecules onto the BTAC surface because of ΔG° lies between -20 kJ/mol to 0.00 kJ/mol in all cases studied here (Mahmoodi 2011).

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The standard enthalpy and entropy changes in the selected range of temperature (303.15 K to

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333.15 K~30-60°C) were found to be -24.571 kJ/mol and -0.0157 kJ/ mol/K, respectively for 50 ppm initial MB dye concentration. For 100 ppm MB dye solution the value for ΔH° is -22.039 kJ/mol while the value for ΔS° obtain was -0.0088 kJ/mol/K. Lastly, for 150 ppm MB dye solution, the standard enthalpy and entropy change in the range of 30-60°C temperature were found to be -15.600 kJ/mol and 0.0089 kJ/mol/K. The negative values of the ΔH° indicated that the adsorption was exothermic (ΔH° <0) (Mortimer 2008). It can be inferred from the obtained data, at a fixed initial concentration the increased in temperature decreased the spontaneity of the

19

ACCEPTED MANUSCRIPT MB dye adsorption onto BTAC surface except at higher initial concentration (150 ppm). Because, at low initial MB dye concentrations more heat evolved (for 50 ppm ΔH° = -24.571 kJ/mol and

for 100 ppm ΔH° = -22.039 kJ/mol) compared to higher initial MB dye

concentration (150 ppm, ΔH° = -15.600 kJ/mol). Negative standard entropy change (ΔS°) can be

T

explained as the decrease in randomness, and positive ΔS° values indicated the increase in

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randomness in the system. We found negative ΔS° for 50 ppm and 100 ppm MB dye solution but

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positive ΔS° values for 150 ppm solution. The reason for the increase in randomness in the system for higher concentration was the replacement of some of the small molecules from the

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surface of adsorbent due to a large crowd of adsorbate molecules approaches to the surface.

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Some authors considered even displacement of water molecules from the surface of the adsorbent added more translational entropy than it lost due to the settling of adsorbate molecules

Adsorption isotherm models

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3.5

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system (Malkoc and Nuhoglu 2007).

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on the surface of adsorbent; this phenomenon also caused an increase in randomness in the

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The adsorption isotherm identifies the distribution of concentration variation of adsorbate molecules between solid (adsorbent) and liquid phases at a constant temperature. The isotherms

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successfully represent the dynamic adsorption behavior of adsorbate. At equilibrium, it can be expressed as the amount of adsorbate molecules adjusted at the surface of per unit mass adsorbent. All available adsorption isotherm models are composed of certain constant values which assigned to specific surface behavior and used to compare the adsorption properties of the adsorbents against pollutants (Dursun et al. 2005). To identify the sorption mechanism of the MB dye onto the BTAC, different initial concentrations of MB dye was keep in contact with the

20

ACCEPTED MANUSCRIPT BTAC adsorbent (1.5 g/L) and their respective equilibrium concentrations values were applied to the linearized form of Langmuir, Freundlich, Dubinin-Raduskevich, Temkin, Frumkin, HarkinsJura, and Smith isotherms models. The adsorption isotherm constants and regression coefficient (R2) were calculated through respective linearized isotherm equations as summarized in Table 3.

Freundlich isotherm 1 ln(qe )  ln( K f )   ln(Ce ) N

mg/g L/g

M

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KD-R qm EA R2

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CE

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Frumkin isotherm  1 ln[( ) ]  ln( K FM )  2  a  1   Ce Harkins-Jura isotherm 1 HB 1  [  log(Ce )] 2 qe H A H A Smith isotherm qe  Wb  W  ln(1  Ce )

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qmax KL RL R2 Kf N R2

Dubinin-Raduskevich isotherm ln(qe )  ln(qm )  K D  R   2

Temkin isotherm qe  B  ln( KT )  B  ln(Ce)

Units

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Langmuir isotherm Ce C 1   e qe qmax  K L qmax

Models constants

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Isotherms

KT B B R2 A ln(KFM) ΔG R2

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Table 3: MB dye adsorption onto BTAC isotherm model constants and their correlation coefficients. Values

227.272 11.000 18.15-3.64x10-4 0.9635 377.851 1.765 0.9926

mol2/kJ2 mg/g kJ/mol

2.00x10-11 199.877 5.00 0.9863

L/g

116.531 46.207 54.546 0.9608

J/mol

kJ/mol

-0.905 4.3344 -10.924 0.6385

HA HB R2

1428.571 -0.71429 0.8190

Wb W R2

40.378 487.94 0.9565 21

ACCEPTED MANUSCRIPT 3.5.1 Langmuir isotherm model for monolayer adsorption The Langmuir isotherm is based on the assumption that the adsorbent surface has uniformly distributed homogenous surface adsorption sites available for adsorbate molecules. It also assumes that during adsorption the energy is uniformly distributed between the adsorbate surface

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and adsorbent molecules for monolayer adsorption at a fixed temperature.

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From Fig. 3 (a), the scattered data points of Ce/qe vs. Ce plot has been shown. The best fit line

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with their equation is also represented there. Five gradual increasing initial concentrations (50, 100, 150, 200, and 250 ppm) of MB dye were considered for this plot. The slope and intercept of

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the best fit line were employed to determine the Langmuir model parameters such as qmax and

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KL. The values of isotherm constants qmax and KL were found to be 227.27 mg/g and 11.00 L/g, respectively. The nature of the MB dye adsorption on BTAC was also tested through

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dimensionless constant commonly known as equilibrium parameter or separation factor, RL. The

1 1  K LCo

(11)

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RL 

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separation factor was calculated by the following relation (Eq. 11):

CE

where C0 is the initial MB dye highest concentration (mg/L), KL is the Langmuir isotherm model constant corresponds to the adsorption energy (L/g). The RL value is an essential feature of the

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Langmuir isotherm to explore whether the adsorption is “favorable (0˂RL˂1)”, “unfavorable (RL˃1)”, “linear (RL=1)” or “irreversible (RL=0)”. For MB dye adsorption on BTAC, the RL values were found to be 0.001815, 0.000908, 0.000606, 0.000454, and 0.000364 while initial concentrations of MB dye were 50, 100, 150, 200, and 250 mg/L at 303.15 K, respectively. These RL values indicated that adsorption of MB dye on BTAC was favorable. Table 3 contains all the calculated parameters of the Langmuir isotherm model. The regression coefficient (R2) of

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ACCEPTED MANUSCRIPT this model was found to be 0.9635, this regression coefficient shows the isotherm data deviated from the linearity. Therefore, adsorption of MB dye onto BTAC did not seem to be a monolayer. 3.5.2 Freundlich model This isotherm assumes that the energy is not uniformly distributed at the surface site of the

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adsorbent and the adsorbate molecules, rather energy is exponentially varied in adsorbent surface

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and the adsorbate molecules. The Freundlich isotherm model was developed by assuming the

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non-uniform distribution of adsorption energy at the heterogeneous surface of the adsorbent. The experimental data was applied to the plot of ln(Ce) vs. ln(qe), and the extent of linear relationship

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was established through the regression coefficient (R2). The linearized Freundlich model is

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shown in Table 3. The Freundlich isotherm helps to explore the possibility of multilayer adsorption characteristics of the MB dye onto BTAC.

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The experimental data plot of ln(Ce) vs. ln(qe) is shown in Fig. 3 (b), the experimental data

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points are in good agreement with the linearity of the plot. The regression coefficient (R2) was found to be 0.9926, which seems to be the best fit for the linearity test of the Freundlich isotherm

PT

model. The sorption of MB dye onto the BTAC was a multilayer. The Freundlich constant

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related to adsorption (Kf) for MB dye has very high value 377.85, and Freundlich constant related to the intensity of adsorption was also high (1.765). This model better explains the

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adsorption characteristics of the MB dye onto BTAC.

3.5.3 Dubinin-Radushkevich (D-R) adsorption isotherm model Dubinin-Radushkevich isotherm also considering the concept of heterogeneity of the adsorbent surface as in Freundlich isotherm and assume that the sorption sites are not identical (Misra

23

ACCEPTED MANUSCRIPT 1969). The linearized equation of isotherm is represented in Table 3. The Polyani potential (Ɛ) was calculated using the following relation (Eq. 12).

  RT ln(1 

1 ) Ce

(12)

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The Dubinin-Radushkevich isotherm constant (KD-R) can be used to the mean free energy of

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adsorption (EA) per molecule of adsorbate when the adsorbate molecules move from infinity to

1 2K DR

(13)

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EA 

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the surface of the adsorbent. It can be calculated by the given below equation (Eq. 13).

The Dubinin-Radushkevich isotherm constants and its related mean free energy of adsorption for

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MB dye adsorption onto BTAC are reported in Table 3. The linearized D-R plot is shown in Fig.

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3 (c), the correlation coefficient (R2) for the linear plot was found to be 0.9863, which is in very

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close agreement with the perfect linear (R2=1). After Freundlich isotherm, this isotherm model was more closely followed by the MB dye adsorption data. According to D-R isotherm model

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predicted maximum adsorption capacity (qm=199.87 mg/g) of the BTAC adsorbent was also very

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close to the experimentally calculated adsorption capacity (qe=166.50 mg/g). 3.5.4 Temkin adsorption isotherm model

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Temkin isotherm is modeled by assuming that the decrease in the heat of adsorption followed the linear trend rather than a logarithmic curve. The non-linearized form of Temkin isotherm can be mathematically expressed by the following equation (Eq. 14). qe 

RT ln( KT  Ce ) b

(14)

B

RT b

(15)

24

ACCEPTED MANUSCRIPT where, B is constant dealing with the heat involved during adsorption. The heat of adsorption constant (b) can be calculated through the slope of qe vs. ln Ce plot as shown in Fig. 3 (d). The calculated values of the Temkin isotherm constants and its related parameters are summarized in Table 3. The correlation coefficient of the model for MB dye adsorption data was found to be

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0.9608, so the calculated values are near to the actual heat involved in the adsorption.

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3.5.5 Frumkin adsorption isotherm model

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The Frumkin isotherm assumes the homogeneous surface of the adsorbent with uniform interaction with adsorbent molecules. This isotherm used a constant term ‘a’ which stands for the

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interaction of adsorbate molecules with the adsorbed layer. The positive value of ‘a’ indicated

AN

that during adsorption the energy increased due to the lateral interaction of the adsorbate molecule with the surface of the adsorbent. The negative value of ‘a’ pointed that the repulsive

M

force was operated between the incoming adsorbate molecules and the molecules settled on the

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surface of the adsorbent. The linear form of Frumkin isotherm with its parameter values are shown in Table 3. The fractional occupation (θ) of adsorbate at the surface of adsorbent was

PT

calculated through the ratio of experimental adsorption capacity to the monolayer adsorption

CE

capacity (calculated through D-R isotherm) for each initial concentration of MB dye. From the experimental data of MB dye adsorption on BTAC, it was observed that Frumkin isotherm

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model gave poor linearity with a correlation coefficient (R2) 0.6385 as shown in Fig. 3 (e). The isotherm constant parameter ‘a’ was having negative values that indicated that there was repulsion between the incoming MB dye molecules with the molecules settled on the surface. The repulsion between the bounded and incoming adsorbate molecule is not true because the experimental data verified multilayer adsorption (Freundlich isotherm) characteristics. Therefore,

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ACCEPTED MANUSCRIPT the other parameters of the isotherm model could not be explained because experimental data was not following this model. 3.5.6 Harkins-Jura adsorption isotherm model This isotherm explains the multilayer adsorption on a heterogeneous surface with a range of pore

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distribution. The linearized Harkins-Jura adsorption isotherm in terms of 1/qe2 and log(Ce) was

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used to apply on the adsorption data of MB dye on BTAC adsorbent. The plot is shown in Fig. 3

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(f). The HA represents for multilayer adsorption isotherm constant and explains the existence of heterogeneous pores, and HB represents the isotherm constant. The relevant isotherm parameters

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with their values are reported in Table 3. The correlation coefficient (R2) of the model was found

3.5.7 Smith adsorption isotherm model

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to be 0.8190, which indicated that the obtained MB dye adsorption data do not follow this model.

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This isotherm model assumes the adsorbent has a heterogeneous surface, and the adsorbed

ED

molecules at the surface of adsorbent divided into bounded and condense fractions. The bounded fraction of the adsorbate at the inner or outer surface of the adsorbent facilitates the condensation

PT

of the incoming adsorbate molecules (Smith 1947). The condensed fraction of the adsorbate

CE

molecules forms in layers, each condense fractions have more than one layers. The linearized form of the Smith isotherm with their constants values are reported in Table 3. The plot between

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qe and ln(1-Ce) is shown in Fig. 3 (g), the correlation coefficient value was found to be 0.9565, which again confirms the heterogeneous surface of BTAC adsorbent and adsorption of MB dye was a multilayer. The heterogeneous surface of the BTAC adsorbent can be in field emission scanning electron microscopic (FESEM) image as in Fig. 3 (h). The various size of the diameter of the pores varies between 5.71 μm to 20.22 μm.

26

AC

CE

PT

ED

M

AN

US

CR

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T

ACCEPTED MANUSCRIPT

Fig. 3

27

ACCEPTED MANUSCRIPT 3.6 Mechanism and comparison of MB dye adsorption Possible functional groups in the BTAC and MB dye, and changes in the surface of BTAC after MB dye adsorbed was identified through FTIR spectra (as shown in Supplementary Fig. S4). Based on the FTIR spectrum information the functional groups on the surface of BTAC was

T

observed at wavenumbers 3443 cm-1 (-OH groups, a broad peak), 2366 cm-1 (-CN groups, weak

IP

peak), 1637 cm-1 (-C=C-COO- groups, medium peak), 1592 cm-1 (-C=C-O-S groups, medium

CR

peaks), 1415 cm-1 (heterocyclic ring skeleton, weak peak). In MB dye the peaks were observed at wavenumbers 3433 cm-1 (-OH groups) due to presence of moisture in the MB dye, and at

US

wavenumber 2357 cm-1 (-C=N-C groups, sharp and strong peak),1645 cm-1 (-C=N- group, weak

AN

peak), and 1550 cm-1 (-S-C-C-N-, heterocyclic ring, medium peak) was due to presence of functional groups in the MB dye molecules. After MB dye adsorption on to the surface of the

M

BTAC, the BTAC-MB dye complex was also analyzed through the FTIR, and spectrum shown

ED

some peaks of similar functional groups get stronger, like peak at wavenumber 3443 cm-1 getting strong and broad, in BTAC the weak peak was observed at 2366 cm-1, but after MB dye

PT

adsorption this peak shifted 2363 cm-1 with medium strength. Similarly, the peaks in the BTAC

CE

surface at wavenumber 1637 cm-1, shifted to 1644 cm-1, with increased peak strengths. These changes in the functional groups peak position and strength in the BTAC-MB dye complex gave

AC

the evidence of MB dye adsorption onto the BTAC surface. The interaction sites in the BTAC and MB dye macromolecule was identified and drawn in Fig. 4. The negatively charged functional groups such as -CN-, -O-S-, and -COO- at the surface of BTAC interact with positively charged site =N+(CH3)2 of the MB dye molecules. The adsorption of MB dye on BTAC was a multilayer as established through the isotherm and intraparticle diffusion models. The possible BTAC surface macromolecule interaction with dye molecules are presented in Fig.

28

ACCEPTED MANUSCRIPT 4. The first layer of adsorption was quick as indicated by the first diffusion rate (Kd1=14.694 mg/g/min1/2) due to negatively charged functional groups (-COO-) on the BTAC and positively charged site (=N+(CH3)2) of MB dye molecules. The second layer of adsorption was due to induced dipole formed due to the approach of incoming dye molecules but the diffusion rate of

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stacking of molecules reduced to 0.6082 mg/g/min1/2, and in the third layer it further reduced and

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reached 0.1358 mg/g/min1/2. As the layers increased on the surface of BTAC the induced dipole

CR

effect decreased, that was observed through the diffusion rate experiments.

The adsorption of the methylene blue onto various adsorbents were tried. The biochars were

US

prepared from pine wood, pig manure, and paper waste for MB dye removal. The maximum

AN

adsorption capacity was reported to be 3.99 mg/g (pine wood biochar), 16.30 mg/g (pig manure biochar), and 1.66 mg/g (paper waste biochar) (Lonappan et al. 2016). Oak wood was activated

M

to prepare the biochar for MB dye removal from aqueous solution. The maximum adsorption

ED

capacity of oak wood biochar through Langmuir isotherm was reported to be 97.55 mg/g at 50 °C (Babaei et al. 2016). On comparing adsorption capacities of these biomass-derived

PT

adsorbents, BTAC was found to be more efficient with maximum adsorption capacity 199.87

AC

dose 1.5 g/L.

CE

mg/g calculated through Dubinin-Radushkevich (D-R) isotherm model at 25 °C and adsorbent

29

ACCEPTED MANUSCRIPT CH2

H2C

CN-

OH

OH O

S-

BTAC adsorbent surface functional groups

COOH +

S

CH3

US

MB dye molecules

CH3

Cl

CR

N

N

IP

CH3

H3C

Second layer of molecules

Third layer of molecules

N

Cl H3C

S

N

N

H3C

CH3

N

MB dye molecules

CH3

SH

AN

N

CH3

M

H3C

T

N

CH3

N

Cl

CH3 H3C

CH3

N

SH

N H3C

H2C

OH

N

N H3C

S

First layer of molecules

CN-

PT

CH2

ED

H3C

CH3

N

OH

CE

H3C O

N

Cl

CH3

S-

AC

BTAC adsorbent surface functional groups COO H

+

CH3

N N S H3C

CH3

N CH3

Fig. 4

30

ACCEPTED MANUSCRIPT 4. Conclusions The present study is dedicated to kinetic, thermodynamics, and isotherm behavior of MB dye adsorption onto BTAC adsorbent. The following conclusion was drawn from the study: (1) Percentage MB removal increased with an increase in time and achieves equilibrium after

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20 min of contact time.

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(2) The kinetics data of MB dye adsorption on BTAC was found to follow the pseudo-

CR

second-order kinetic model with a correlation coefficient (R² > 0.99) close to absolute linearity (R2=1.00).

US

(3) The Weber-Morris intraparticle diffusion model was also applied to the kinetic data to

AN

estimate the diffusion rate constants and boundary thickness after each layer of adsorption. The results revealed that three layers of adsorbate formed on the surface of

M

the adsorbent for each subsequent layers the diffusion rate was observed to be decreased,

ED

and boundary thickness increased.

(4) The thermodynamic parameters are shown adsorption was spontaneous and decrease with

PT

increasing temperature (exothermic). The thermodynamic parameter ΔG°, ΔH°, and ΔS°

CE

indicated that the adsorption process was spontaneous and exothermic. (5) The isotherm data were tested against the Langmuir, Freundlich, Dubinin-Radushkevich,

AC

Temkin, Frumkin, Harkins-Jura, and Smith isotherm models to verify the isotherm characteristics. The isotherm data followed the Dubinin-Radushkevich model closely, and the model predicted qmax was 199.87 mg/g; whereas, the maximum adsorption capacity of BTAC calculated through experimental values with 250 ppm initial MB dye concentration was 166.51 mg/g.

31

ACCEPTED MANUSCRIPT In view of the experimental findings, this study recommended that the banana trunk chemically activated carbon (BTAC) can be used effectively in the scavenging of MB dye from aqueous solution. Acknowledgment

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This research did not receive any specific grant from funding agencies in the public, commercial,

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or not-for-profit sectors. However, the authors acknowledge the Universiti Kuala Lumpur

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Malaysian Institute of Chemical and Bioengineering Technology (MICET) for providing

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research facilities for this research.

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Figure Caption:

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Fig. 1: (a) First order reversible kinetic model plot, (b) Pseudo-first order kinetic model plot, (c) Pseudo-second-order kinetic model, (d)Weber-Morris diffusiopn model plot, for MB dye adsoptiopn onto BTAC.

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Fig. 2: (a) Effect of temperature on MB dye adsorption onto BTAC, (b) Vont Hoff plot for MB dye adsorption onto BTAC. Fig. 3: (a) Langmuir plot, (b) Freundlich plot, (c) D-R isotherm plot, (d) Temkin isotherm plot, (e) Frumkin isotherm plot, (f) Harkins-Jura isotherm plot, (g) Smith isotherm plot, for MB dye adsorption on BTAC (h) FESEM image of BTAC. Fig. 4: Schematic representation of mechanism of MB dye molecules adsorption on the macromolecular surface of BTAC. Different layers of dye molecules are also represented here.

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Highlights Banana Trunk activated carbon has been tested against methylene blue dye. Kinetics, thermodynamics, and isotherm of the adsorption was established. Mechanism of the MB dye was established. Pseudo second order kinetic rate was verified for MB dye on BTAC. The maximum MB dye removal efficiency of BTAC was 166.51 mg/g.

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