Breadnut peel as a highly effective low-cost biosorbent for methylene blue: Equilibrium, thermodynamic and kinetic studies

Linda B.L. Lim; Namal Priyantha; D.T.B. Tennakoon; Hei Ing Chieng; Muhammad Khairud Dahri; Montri Suklueng

doi:10.1016/j.arabjc.2013.12.018

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Original article

10 (

2_suppl

); S3216-S3228

doi:

10.1016/j.arabjc.2013.12.018

Breadnut peel as a highly effective low-cost biosorbent for methylene blue: Equilibrium, thermodynamic and kinetic studies

Linda B.L. Lim^{^a,⁎}, Namal Priyantha^{^b}, D.T.B. Tennakoon^{^a}, Hei Ing Chieng^{^a}, Muhammad Khairud Dahri^{^a}, Montri Suklueng^{^c}

a Department of Chemistry, Faculty of Science, Universiti Brunei Darussalam, Jalan Tungku Link, Gadong, Brunei Darussalam

b Department of Chemistry, Faculty of Science, University of Peradeniya, Peradeniya, Sri Lanka

c Energy Research Group, Universiti Brunei Darussalam, Jalan Tungku Link, Gadong, Brunei Darussalam

⁎Corresponding author. Tel.: +673 8748010; fax: +673 2461502. linda.lim@ubd.edu.bn (Linda B.L. Lim)

Received: 2013-2-4, Accepted: 2013-12-22, Published: 2013-05-01

Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsgibility of King Saud University.

Abstract

This work reports the potential use of peel of breadnut, Artocarpus camansi, as an effective low-cost biosorbent for the removal of methylene blue (MB). Oven dried A. camansi peel (ACP), which had a point of zero charge at pH = 4.8, showed maximum biosorption capacity which was far superior to most literature reported fruit biomasses, including samples that have been activated. Isotherm studies on biosorption of MB onto ACP gave a maximum biosorption capacity of 409 mg g⁻¹. The Langmuir model was found to give the best fit among various isotherm models investigated and error analyses performed. Kinetics studies were fast with 50% dye being removed in less than 8 min from a 50 mg L⁻¹ dye solution and further, kinetics followed the pseudo second order. Thermodynamic studies indicated that the biosorption process was both spontaneous and exothermic. Fourier transform infrared (FT-IR) of ACP before and after MB adsorption was investigated. It can be concluded that oven dried breadnut peel is a highly promising low-cost biosorbent with great potential for the removal of MB.

Keywords

Breadnut peel

Methylene blue

Thermodynamics

Kinetics

Adsorption isotherms

Low-cost biosorbent

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1 Introduction

Ever since the first synthetic dye, Mauveine, first appeared in the market, more and more dyes are being synthesized over the decades. Today there are over 1 × 10⁵ types of dyes available and more than 7 × 10⁵ tonnes of dyestuff are annually produced to provide for the needs of industries, such as textile, cosmetics, food and paper (Al-Degs et al., 2008). With this comes an inevitable increase in the amount of dyes being disposed into wastewater or natural ecosystem, thus resulting in serious pollution to the environment. Several methods have been developed over the years to remove these toxic dyes from wastewater (Forgacs et al., 2004). Among the methods developed, one of the best ways to remove toxic dyes is by adsorption using suitable adsorbents, such as granulated or powdered activated carbon (Walker and Weatherley, 1997; Ho and McKay, 2003; Jain et al., 2003; Ahmad and Hameed, 2010 ). However, due to the high cost in the production of good quality activated carbon, attention is being turned to the use of low-cost biosorbents as viable alternatives (Gupta and Suhas, 2009; Patel, 2012; Waseem et al., 2014 ).

This research focuses on the use of peel of Artocarpus camansi Blanco (ACP) to remove methylene blue (MB). A. camansi is commonly known as breadnut and locally known as “kemangsi” in Brunei Darussalam. It belongs to the genus Artocarpus of the Moraceae family. In Brunei Darussalam, the more popular Artocarpus fruits are the jackfruit (Artocarpus heterophyllus), tarap (Artocarpus odoratissimus), cempedak (Artocarpus champeden), breadfruit (Artocarpus altilis) and nanchem, which is a hybrid of jackfruit and cempedak. Studies of some of the nutritional values of locally grown Artocarpus fruits have been reported (Lim et al., 2011; Tang et al., 2013). Unlike these fruits, A. camansi is less popular among the Bruneians.

In Brunei Darussalam, A. camansi is usually grown in home backyards and sold in the local open markets. The edible seeds are mainly cooked as a vegetable dish or they can be boiled or roasted and eaten as nuts. The peel and core are inedible and discarded as waste. To date, there have been limited studies on the use of Artocarpus waste as low-cost biosorbents. These reports include the use of jackfruit peel, in both its non-activated and activated forms, for the removal of dyes (Inbaraj and Sulochana, 2006; Prahas et al., 2008; Hameed, 2009; Foo and Hameed, 2012a,c ) and heavy metals (Inbaraj and Sulochana, 2004), and jackfruit leaf for the adsorption of dyes (Uddin et al., 2009a,b; Saha et al., 2012 ). Skin of breadfruit, tarap and nanchem and also core of tarap have recently been reported for their use as biosorbents to remove heavy metals Cd(II) and Cu(II) and toxic dyes (Lim et al., 2012; Lim et al., 2013a; Lim et al., 2014; Priyantha et al., 2013 ).

In this study, the aim was to explore the possible use of breadnut peel as a novel low-cost biosorbent for MB adsorption. The effects of various experimental parameters, such as contact time, medium pH, thermodynamics and kinetics were investigated.

2 Materials and methods

2.1

2.1 Materials

The fruit samples were purchased from the local open markets in the Brunei-Muara District of Brunei Darussalam. The peel, which was separated from the rest of the fruit, was dried in an oven at 80 °C until a constant mass was obtained. Other than oven drying, there was no pre-treatment of samples by other physical or chemical means prior to adsorption studies. The dried sample was then powdered and sieved to obtain the fraction with particle sizes from 355 to 850 μm. To ensure uniformity, the sieved sample was thoroughly mixed prior to using them for all the experiments in this study. All experiments were carried out at 298 K, unless otherwise stated, in duplicate/triplicate.

2.2

2.2 Characterization of ACP

Dried ACP sample was analysed for its fibre, fat and protein contents. The point of zero charge (pH_pzc) was determined by using 0.1 M KNO₃ solution and adjusted to the required pH (2, 4, 6, 8, 10 and 12) by the addition of 0.1 M HNO₃ and 0.1 M NaOH. The KNO₃ solution (25.0 mL) at different pHs was added into a flask filled with pre-weighed ACP (0.05 g) and agitated on an orbital shaker at 250 rpm for 2 h. Graph of ΔpH against pH_i (initial pH) was plotted. The point of zero charge of ACP is the point that passes through the x-axis. ΔpH was calculated from the difference between initial pH and final pH of KNO₃ solution.

2.3

2.3 Chemicals and reagents

A 1000 mg L⁻¹ stock solution of MB (Sigma–Aldrich Corporation) was prepared by dissolving the dye in distilled water. This stock solution was used to prepare a series of dye concentrations ranging from 10 to 1000 mg L⁻¹. Solutions of different pH were prepared using NaOH and HNO₃, both of which were purchased from Fluka. All chemicals were used without further purification.

2.4

2.4 Instrumentation

UV–Vis spectrophotometer (Shimadzu/Model UV-1601PC) was used for measurement of absorbance of MB solutions at $λ_{max} = 664.4 nm$ . FT-IR spectrophotometer (Shimadzu Model IRPrestige-21) was used for absorbance measurements of solids. Ashing was carried out using Thermolyne 1400 Furnace. Fats, fibre and protein were analysed using GerhardT Soxtherm multistate/SX PC, GerhardT Fibretherm FT12 and GerhardT VAP 50S Nitrogen Protein Analyzer, respectively. C, H, N and S were analysed in the Elemental Analysis Laboratory at the National University of Singapore. Elements present in ACP were determined using X-ray fluorescene (XRF) PANalytical Axios^max instrument. Morphological characteristics of the adsorbent surface were carried out using Tescan Vega XMU Scanning Electron Microscope (SEM). SPI-MODULE™ Sputter Coater was used to coat the biosorbent.

2.5

2.5 Optimization of parameters

2.5.1

2.5.1 Effect of contact time

The effects of contact (shaking and settling) time were carried out using ACP:solution in the ratio of 1:500 for 10 mg L⁻¹ MB solution. The shaker was set at 250 rpm at ambient temperature for different shaking times from 30 to 240 min. To determine the optimum shaking time, each solution was filtered after shaking, and the absorbance of the filtrate was determined using UV–Vis spectrophotometer at the appropriate $λ_{max}$ value for the determination of the remaining dye content. To determine the optimum settling time, the sample was shaken in dye solution in the same sample:solution ratio of 1:500 at 250 rpm at ambient temperature for optimum shaking time. The contents were allowed to settle for different time periods up to 240 min. Each filtrate was then analysed as described earlier.

2.5.2

2.5.2 Effect of pH

The effect of medium pH was carried out by shaking, for an optimized shaking and settling time, the sample in each dye solution (10 mg L⁻¹) which was adjusted to a range from pH 2.0 to 10.0. Each solution was then filtered and the filtrate was analysed by UV–Vis spectrophotometer at the appropriate $λ_{max}$ value.

2.6

2.6 Thermodynamic studies

ACP (0.05 g) was mixed with 100 mg L⁻¹ MB dye solution (25.0 mL) and shaken using a water-bath shaker with temperature adjusted to 298, 314, 324, 334 and 344 K at 250 rpm using predetermined shaking and settling times. The solution was then filtered and the filtrate was analysed using UV–Vis spectrophotometer. Thermodynamic parameters, such as Gibbs free energy (ΔG°), enthalpy (ΔH°) and entropy (ΔS°) were calculated to investigate the sorption process.

2.7

2.7 Effect of ionic strength

ACP (0.05 g) was added to MB dye solution (100 mg L⁻¹) and diluted using KNO₃ solution of concentration ranging from 0.001 to 1.0 M. The solutions were shaken to the optimum shaking and settling times at room temperature on an orbital shaker at 250 rpm. The ACP/MB dye mixture was separated and the filtrate was analysed using UV–Vis spectrophotometer at the appropriate $λ_{max}$ value.

2.8

2.8 Isotherm analysis

Each biosorbent in 1:500 solid/solution ratio was treated with a series of MB concentrations (0–1000 mg L⁻¹) and shaken at 250 rpm to the optimum shaking and allowed to settle for the optimum time period. The solution was filtered, and the filtrate was analysed using UV–Vis spectrophotometer at the $λ_{max}$ of 664.4 nm.

2.9

2.9 Kinetic studies of biosorption

Each biosorbent (0.05 g) was mixed with 50, 500, 1000 mg L⁻¹ of MB solution (25.0 mL), and the suspension was stirred at 250 rpm at ambient temperature as soon as mixing commenced. Samples were withdrawn at every one minute intervals until the equilibrium was reached. Each solution was filtered, and the filtrate was analysed using UV–Vis spectrophotometer at the appropriate $λ_{max}$ value.

3 Results and discussion

3.1

3.1 General characterization of ACP sample

Analysis of ACP for its fibre, fat and protein contents indicates that approximately 21% of ACP is comprised of fibre (Table 1). The high fibre content could prove to be of advantage in adsorption as fibres have been reported for use as low-cost biosorbents (Ofomaja, 2007a; Hameed et al., 2008a; Mahmoud et al., 2012 ). From CHNS analysis, carbon was found to be approximately 41%.

Table 1 General characterization of ACP.

Characterization	Results (%)
Fat	4.0
Fibre	21.2
Protein	7.0
Ash	9.2

CHNS analyses
C	41.1
H	5.2
N	2.0
S	<0.5

3.1.1

3.1.1 Analysis of elements by XRF

XRF shows that ACP contains the elements Mg, Al, Si, P, S, Cl, K, Ca, Fe, Zn, Rb and Ru. Of these, K and Ca were the two major elements present (Table 2). From Fig. 1, it can be seen that adsorption of MB on ACP significantly reduces both K and Ca i.e., K reduced from 28.6% to 0.6% and Ca was reduced from 17.9% to 8.5%, indicating that the sites occupied by both K⁺ and Ca²⁺ were probably being replaced by MB during the adsorption process.

Table 2 Characterisation of elements of ACP by XRF.

Elements	Result (%)
O	28.7
Mg	2.1
Al	0.1
Si	4.1
P	2.9
S	1.5
Cl	1.9
K	28.6
Ca	17.9
Fe	3.0
Zn	8.3
Rb	0.1
Ru	0.8

XRF spectra showing K of ACP before and after adsorption with MB.

3.1.2

3.1.2 Point zero charge

It can be seen from Fig. 2 that the pH_pzc of ACP was at approximately 4.8 suggesting that the surface charge is zero at pH = 4.8. This indicates that at pH lower than pH_pzc the surface of adsorbent will be positively charged, and it is negatively charged if the pH is made greater than the pH_pzc. The pH_pzc of peel of another Artocarpus genus, jackfruit, was reported to be 3.9 (Uddin et al., 2009a), which is lower than that of ACP.

The point of zero charge of ACP using 0.1 M KNO3 solution.

3.1.3

3.1.3 SEM analysis of ACP

Fig. 3A and B (i) show the SEM images of ACP before adsorption with MB and Fig. 3B(ii) shows ACP after adsorption with 1000 mg L⁻¹ MB. From Fig. 3A it can be seen that the ACP surface consists of strands of fibres which are randomly arranged. Such arrangement is similar to what was reported for carded cotton fibres (Kaewprasit et al., 1998) and could enhance the accessibility to the dye which can easily diffuse through the fibres. Apart from these fibres, the surface is also very rough and uneven in nature, as seen in Fig. 3B (i), with pores and cavities which provide a large surface area for the adsorption of MB. This can be confirmed with Fig. 3B(ii) which shows that on treatment with MB, the surface roughness changed significantly and the pores are packed with MB after adsorption. The surface becomes smoother due to decrease in surface heterogeneity and adsorption of MB. Hence, it can be seen that both the fibrous and porous surface of ACP could be the reason for its high adsorption capacity for MB as compared to most low-cost biosorbents reported (Table 9).

SEM showing (i) fibrous nature of ACP at 238× magnification (ii) scale showing the width of fibrous strands.

SEM showing (i) rough, porous surface nature of ACP at 1000× magnification before treatment with MB (ii) after adsorption with MB at 1000× magnification.

3.1.4

3.1.4 Specific surface area of ACP

The specific surface area, S, can be calculated using the following equation:

(1)

S = \frac{q_{m} \times A_{MB} \times N_{A} \times 10^{- 20}}{M_{r}}

where q_m is the maximum adsorption capacity (mg g⁻¹), A_MB is the occupied surface area of one molecule of MB which is taken as 197.2 Å² (Graham, 1955), N_A is Avogadro’s number (6.02 × 10²³ mol⁻¹) and M_r is the molecular weight of MB (319.85 g mol⁻¹). The specific surface area was calculated to be 1519 m² g⁻¹. This is higher than phosphoric acid activated jackfruit peel, another Artocarpus genus, which was reported to have a surface area of 1261 m² g⁻¹ (Prahas et al., 2008). A larger surface area will enhance the biosorption efficiency of the biosorbent.

3.2

3.2 Optimization of parameters

3.2.1

3.2.1 Effect of contact time

Investigation of the effect of shaking time on the extent of removal of MB shows rapid removal of the dye at initial stages of contact with the system, reaching equilibrium within 60 min (Fig. 4). The time taken to reach equilibrium was faster compared to other biosorbents which generally required an optimum shaking time of more than 120 min. Fast equilibrium adsorption was also reported for jackfruit peel, another species from the Artocarpus genus (Table 3). A rapid uptake of adsorbate by an adsorbent is especially important when applied to wastewater treatment by means of adsorption which signifies the efficacy of an adsorbent to be used in wastewater treatment.

Effect of shaking time on biosorption of MB onto ACP.

Table 3 Comparison of the shaking time of selected biosorbents.

Adsorbent	Shaking time (min)	References
Artocarpus camansi peel	60	This work
Jackfruit peel	180	Hameed (2009)
Jackfruit leaf	300	Uddin et al. (2009b)
Pineapple stem	90	Hameed et al. (2009)
Coconut bunch waste	300	Hameed et al. (2008c)
Lotus leaf	240	Han et al. (2011)
Pomelo skin	315	Hameed et al. (2008b)
Tea waste	720	Nasuha et al. (2010)

After optimization of shaking time, the time taken for the system to rest was investigated. Unlike in homogenous systems, the adsorbates in liquid/solid heterogeneous systems usually undergo various mass transfer steps, some of which could be relatively slow. Therefore, it is important to optimize settling time to ensure that the equilibrium is reached. ACP was found to reach equilibrium within 1 h (Fig. 5). Hence, in order to ensure that the system reaches full equilibrium, all experiments for adsorption of MB on ACP were carried out with 2 h shaking at 250 rpm followed by 1 h settling time.

Effect of settling time on biosorption of MB onto ACP.

3.2.2

3.2.2 Effect of pH

One of the important parameters in biosorption is the effect of pH. In this study, the effect of pH on adsorption of MB on ACP was carried out from pH 2 to 10. At ambient pH, under the conditions employed, 90% MB was adsorbed by ACP (Fig. 6). As the pH decreases from pH 5 to 2, there was a steady decrease in the amount of MB being adsorbed. The effect was the greatest at pH 2 where a reduction of 40% MB was observed. This effect was also reported for other low-cost biosorbents such as jackfruit leaf (Uddin et al., 2009b), cedar sawdust (Hamdaoui, 2006) and kenaf fibre char (Mahmoud et al., 2012). MB is a cationic dye and therefore, as the pH decreases, the surface of the ACP becomes more positively charged which results in an increase in the electrostatic repulsion between the positively charged adsorbate and the positively charged MB cations. At ambient pH or pH > pH_pzc the surface of ACP is expected to be negatively charged, thus favouring adsorption of cationic MB. According to Fig. 6, there was little effect on the amount of MB being adsorbed from ambient pH to pH 10. Hence, the ambient pH was chosen for all the other experiments in this study.

Effect of medium pH on the amount of MB being removed by ACP.

3.3

3.3 Thermodynamic parameters

In order to determine whether the biosorption process occurs spontaneously, the energy and entropy factors were considered. By plotting ln K_c vs 1/T (Fig. 7), the change in Gibbs free energy (ΔG°), enthalpy (ΔH°) and entropy (ΔS°) for the biosorption process were determined using Eqs. (2) and (3).

(2)

Δ G^{°} = - RT \ln K_{c}

where ΔG° is the Gibbs free energy (kJ mol⁻¹), R is the universal gas constant (8.314 J mol⁻¹ K⁻¹), T is the absolute temperature (K) and K_c is the equilibrium constant (C_s/C_e). C_s is the concentration of the dye on adsorbent (mg L⁻¹) and C_e is the equilibrium concentration of MB (mg L⁻¹). The enthalpy (ΔH°) and entropy (ΔS°) values can be calculated from the Van’t Hoff equation:

(3)

\ln K_{c} = - \frac{Δ H^{°}}{RT} + \frac{Δ S^{°}}{R}

Enthalpy and entropy values of MB dye were obtained from the slope of (−ΔH°/R) and intercept (ΔS°/R).

Van’t Hoff plot of ln Kc vs 1/T for the biosorption of MB onto ACP at different temperatures.

From Table 4, the Gibbs free energy change, ΔG°, was found to be −6.25 kJ mol⁻¹ at 298 K which indicates a favourable biosorption process that was spontaneous and exothermic (ΔH° = −25.44 kJ mol⁻¹) over the temperature range from 298 to 344 K. The positive entropy change within the same temperature range (ΔS° = 64.57 J mol⁻¹ K⁻¹) suggests that there would be an affinity of the ACP towards MB. It also suggests a random and disorderly increase at the biosorbent–adsorbate interface with possible structural changes. It is further observed that there is an increase in the negative value of ΔG° as the temperature increases. This implies that the biosorption process favours lower temperature and this is further confirmed by the adsorption capacity, q_max, which decreases as the temperature increases. The same effect has also been previously reported (Ho and McKay, 1998; Han et al., 2011).

Table 4 Thermodynamic parameters for the adsorption of MB onto ACP at different temperatures.

T (K)	ΔG° (kJ mol⁻¹)	ΔS° (J mol⁻¹ K⁻¹)	ΔH° (kJ mol⁻¹)	q_max (mg g⁻¹)
298	−6.25	64.57	−25.44	409.1
314	−4.99			362.1
324	−4.65			300.9
334	−3.82			311.4
344	−3.25			310.2

3.4

3.4 Effect of ionic strength

Generally, there is a decrease in the removal of MB by ACP when the ionic strength was increased (Fig. 8). A slight decrease in the extent of removal of MB was observed when the concentration changed from 0 to 0.001 M KNO₃ solution. However, from 0.001 to 0.01 M, there was a drastic reduction of approximately 50% followed by a gradual decrease of about 16% from 0.1 to 1.0 M KNO₃ solution. This implies that ionic strength has strong influence on the adsorption of MB on ACP. A decrease in the adsorption indicates that there is electrostatic force of attraction between ACP and MB. As the ionic strength increases, there is a competition between the K⁺ ions and MB for the sites available for the sorption process and a reduction in adsorption was thus observed (Hamdaoui et al., 2008).

Effect of ionic strength on the removal of MB by ACP.

3.5

3.5 Adsorption isotherm and modelling

Adsorption isotherms, carried out using initial dye concentrations within the range from 0 to 1000 mg L⁻¹, were indicative of the completion of a monolayer by the levelling of the isotherm adsorption, thus preventing further adsorption leading to multilayer coverage (Fig. 9). From the Langmuir isotherm model (Langmuir, 1916), five different linearized Langmuir models (Table 5) were used to determine the best fit. Based on the regression coefficient, R², and the error analyses (Table 6), it was found that Langmuir I with a plot of C_e/q_e vs C_e gave the best fit (Table 7). The maximum adsorption capacity, q_max, of MB on ACP was found to be 1.28 mmol g⁻¹ (409 mg g⁻¹). The dimensionless constant separation factor, R_L, which can be calculated from Eq. (4), for adsorption of MB onto ACP was 0.076, which indicates that the adsorption behaviour was favourable for the dye (R_L < 1).

(4)

R_{L} = \frac{1}{1 + {KC}_{0}}

where C₀ is the highest initial concentration of dye (mg L⁻¹), K is the Langmuir constant (L mmol⁻¹). The value R_L indicates the type of the isotherm where R_L > 1 is unfavourable, R_L = 1 is linear, 0 < R_L < 1 is favourable or R_L = 0 is irreversible.

Table 5 Different isotherm models.

Isotherm model	Non-linear	Linear	Plot
Langmuir (Type I)		$\frac{C_{e}}{q_{e}} = \frac{1}{K_{L} q_{max}} + \frac{C_{e}}{q_{max}}$	$\frac{C_{e}}{q_{e}} vs C_{e}$
Langmuir (Type II)		$\frac{1}{q_{e}} = \frac{1}{q_{max}} + \frac{1}{K_{L} q_{max} C_{e}}$	$\frac{1}{q_{e}} vs \frac{1}{C_{e}}$
Langmuir (Type III)	$q_{e} = \frac{K_{L} q_{max} G_{e}}{1 + K_{L} G_{e}}$	$q_{e} = - (\frac{q_{e}}{K_{L} C_{e}}) + q_{max}$	$q_{e} vs \frac{q_{e}}{C_{e}}$
Langmuir (Type IV)		$\frac{q_{e}}{C_{e}} = - K_{L} q_{e} + K_{L} q_{max}$	$\frac{q_{e}}{C_{e}} vs q_{e}$
Langmuir (Type V)		$\frac{1}{C_{e}} = \frac{K_{L} q_{max}}{q_{e}} - K_{L}$	$\frac{1}{C_{e}} vs \frac{1}{q_{e}}$

Freundlich	$q_{e} = K_{F} C_{e}^{\frac{1}{n}}$	$\ln q_{e} = \frac{1}{n} \ln C_{e} + \ln K_{F}$	ln q_e vs ln C_e
Temkin	$q_{e} = \frac{RT}{b_{T}} \ln A_{T} C_{e}$	$q_{e} = \frac{RT}{b_{T}} \ln A_{T} + (\frac{RT}{b_{T}}) \ln C_{e}$	q_e vs ln C_e
Dubinin–Radushkevich (D–R)	$q_{e} = q_{max} \exp (- B ε^{2})$	ln q_e = ln q_max − Bε²	ln q_e vs ε²
	$ε = RT \ln [1 + \frac{1}{C_{e}}]$
Sips	$q_{e} = \frac{q_{max} K_{s} C_{e}^{1 / n}}{1 + K_{s} C_{e}^{1 / n}}$	$\ln (\frac{q_{e}}{q_{max} - q_{e}}) = 1 / n \ln C_{e} + \ln K_{s}$	$\ln (\frac{q_{e}}{q_{max} - q_{e}}) vs \ln C_{e}$

Table 6 Error analyses used.

Error Function	Abbreviation	Expression
Average relative error	ARE	$\frac{100}{n} \sum_{i = 1}^{n} {\|\frac{q_{e,meas} - q_{e,calc}}{q_{e,meas}}\|}_{i}$
Sum square error	EERSQ	$\sum_{i = 1}^{n} {(q_{e,calc} - q_{e,meas})}_{i}^{2}$
Sum of absolute error	EABS	$\sum_{i = 1}^{n} \| q_{e,meas} - q_{e,calc} \|$
Hybrid fractional error function	HYBRID	$\frac{100}{n - p} \sum_{i = 1}^{n} {[\frac{{(q_{e,meas} - q_{e,calc})}^{2}}{q_{e,meas}}]}_{i}$

Table 7 Parameters for Langmuir I–V for adsorption of MB on ACP.

Langmuir	R²	q_max	K_L	ARE	EERSQ	EABS	HYBRID
I	0.9341	1.2790	0.0121	27.87	0.1860	1.1379	1.8873
II	0.9838	−0.7240	−0.0141	314.03	955.75	40.54	7339.21
III	0.3439	0.9182	0.0219	87.87	5.0787	6.53	44.61
IV	0.3439	1.9200	0.0075	29.03	0.4327	1.69	3.74
V	0.9838	−0.6098	−0.0164	118.89	49.36	14.79	385.91

Apart from the Langmuir isotherm model, four other isotherm models (Freundlich (Freundlich, 1906), Temkin (Temkin, 1940), Dubinin–Radushkevich (Dubinin and Radushkevich, 1947) and Sips (Sips, 1948)) were also used to analyse the adsorption isotherm. The non-linear and linearized equations are shown in Table 5 and the results obtained from all the isotherm models are shown in Table 8 and Fig. 10. The validity of these models was analysed using four different error analyses, ARE, ERRSQ, EABS and HYBRID, which have been commonly used in equilibrium isotherm studies (Ho et al., 2002; Demirbas et al., 2008; Foo and Hameed, 2010; Chan et al., 2012 ).

Table 8 Parameters for different adsorption isotherm models.

Isotherm	Parameter			R²	ARE	EERSQ	EABS	HYBRID
Langmuir	q_max (mmol g⁻¹)	K_L (L mmol⁻¹)
	1.279	0.0121		0.9341	24.78	0.19	1.14	1.89

Freundlich	K_F (mmol g⁻¹)	n
	0.0193	1.286		0.9102	40.46	2.14	3.31	14.47

Temkin	A_T (L mmol⁻¹)	b_T (J mol⁻¹)
	0.3331	11241.4		0.9334	177.40	1.01	2.29	3.56

Dubinin–Radushkevich	q_max (mmol g⁻¹)	B (mol² J⁻²)
	0.2721	−1.00 × 10⁻⁶		0.4230	2555.77	21.70	10.63	4.55

Sips	q_max (mmol g⁻¹)	K_S (L mmol⁻¹)	n
	1.06	0.0283	1.372	0.9741	191.21	0.44	2.70	32.90

Comparison of experimental and calculated adsorption isotherm of MB onto ACP according to the Langmuir, Freundlich, Sips and Temkin isotherm models.

Of all the models employed, the Dubinin–Radushkevich isotherm model, which does not assume a homogenous surface or a constant biosorption potential as the Langmuir model does, is the least fitting with very low R² of 0.423. Errors are found to be very high and therefore the simulated isotherm is not included in Fig. 8. The Temkin isotherm model takes into account the interaction between the adsorbate and adsorbent while the Sips isotherm model is a combination of both the Langmuir and Freundlich models. Even though both the Sips and Temkin isotherm models give good R² (0.9741 and 0.9334 respectively), error analyses show that these models would not be suitable to describe the adsorption isotherm of MB on ACP. The Freundlich isotherm model, which assumes a heterogeneous surface with non-uniform distribution of heat of adsorption over the surface, also did not correlate to the experimental data. Hence, of all the various isotherm models used, only the Langmuir isotherm model, which assumes a monolayer adsorption that takes place at specific homogenous sites, is the most suitable, which gives a good R² (0.9341) with the least errors.

Since all experiments were carried out at ambient pH = 6 and the pH_pzc of ACP was 4.8, this implies that under the experimental conditions employed, the surface of ACP is negatively charged. This will therefore enhance electrostatic force of attraction between the ACP surface and the cationic MB dye thereby increasing adsorption capacity. This is also further confirmed by the effect of medium pH where the adsorption capacity of ACP increased by 50% from pH 2 to ambient pH. An increase in the electrostatic force of attraction could also contribute to the observation that thermodynamic studies indicate the reaction to be spontaneous and favourable.

In this study, except for oven drying of ACP at 80 °C, no other pre-treatment of the biosorbent prior to adsorption was necessary. Many low-cost biosorbents had to be pre-treated by either acid or base treatment, or by formation of activated carbon using high temperature or microwave technology in order to enhance their adsorption capacity. For example, as seen from Table 9, adsorption of MB with oven dried jackfruit peel gave a q_max of 286 mg g⁻¹ and the adsorption capacity was increased to 400 mg g⁻¹ when the jackfruit peel was microwave activated. A high q_max (409 mg g⁻¹) for adsorption of MB by ACP obtained in this study is indicative that ACP exhibits far superior biosorption capacity than most other biomasses reported (Table 9). There is a great possibility that this adsorption capacity could be further enhanced by activation of ACP.

Table 9 Comparison of maximum biosorption capacity for the removal of MB on selected biosorbents.

Adsorbent	q_max (mg g⁻¹)	References
Artocarpus camansi peel	409	This work
Jackfruit peel:
Oven dried	286	(Hameed, 2009)
Microwave induced NaOH activation	400	(Foo and Hameed, 2012a)
Phosphoric acid activation	280	(Prahas et al., 2008)
Jackfruit leaf powder	326	(Uddin et al., 2009b)
Pineapple stem	119	(Hameed et al., 2009)

Coconut husk:
Activated carbon	435	(Tan et al., 2008)
Microwave modified	418	(Foo and Hameed, 2012b)
Cedar sawdust	142	(Hamdaoui, 2006)
Peat	111	(Lim et al., 2013b)

Oil palm fibre:
Oven dried	223	(Ofomaja, 2007a)
KOH–CO₂ activated carbon	204	(Hameed et al., 2008a)
Microwave activated	313	(Foo and Hameed, 2011a)
HCl activated	672	(Ofomaja, 2007b)

Rejected tea:
Oven dried	147	(Nasuha et al., 2010)
NaOH modified	242	(Nasuha and Hameed, 2011)
Tea waste	85.2	(Uddin et al., 2009c)

Bamboo charcoal:
Thermal activation	26.5	(Liao et al., 2012)
Microwave	35.3	(Liao et al., 2012)
Base activated	454	(Hameed et al., 2007)

Cotton stalk:
Oven dried	147	(Deng et al., 2011)
Sulphuric acid treated	557	(Deng et al., 2011)
Phosphoric acid treated	222	(Deng et al., 2011)
Leaves of Solanum tuberosum	52.6	(Gupta et al., 2016)
Stem of Solanum tuberosum	41.6	(Gupta et al., 2016)
Activated banana peel	19.7	(Amela et al., 2012)

Pomelo skin:
Oven dried	345	(Hameed et al., 2008b)
Activated	501	(Foo and Hameed, 2011b)

3.6

3.6 Kinetic modelling

Fast biosorption of MB onto ACP was observed when kinetics studies were carried out using 50, 500 and 1000 mg L⁻¹ dye concentrations (Fig. 11). Kinetic studies with 50 and 500 mg L⁻¹ MB showed the time taken for 50% removal of the dye to be less than 8 and 40 min respectively. In order to consider the pseudo order kinetics of a chemical reaction, it is vital that the concentration of one of the reactants be present in large excess. Hence, the kinetic studies were carried out using 1000 mg L⁻¹ dye concentration.

Adsorption kinetics of MB on ACP at different dye concentrations.

In order to investigate the mechanism of adsorption of MB on ACP and to find the best model to represent the experimental data, four kinetic models were used i.e., pseudo first order, pseudo second order, Weber Morris intraparticle diffusion and Elovich models and their equations are given in Table 10.

Table 10 Kinetic models used for the adsorption of MB onto ACP.

Kinetic model	Non-linear	Linear	Plot	References
Pseudo first order	$\frac{d q_{t}}{d t} = k_{1} (q_{e} - q_{t})$	$\log (q_{e} - q_{t}) = \log (q_{e}) - \frac{k_{1}}{2.303}$	$\log (q_{e} - q_{t}) vs t$	Lagergren (1898)
Pseudo second order	$\frac{d q_{t}}{d t} = k_{2} {(q_{e} - q_{t})}^{2}$	$\frac{t}{q_{t}} = \frac{1}{k_{2} q_{e}^{2}} + \frac{1}{q_{e}} t$	$\frac{t}{q_{t}} vs t$	Ho and McKay (1999)
Intraparticle diffusion	q_t = k₃t^1/2	q_t = k₃t^1/2 + c	$q_{t} vs t^{1 / 2}$	Weber and Morris (1963)
Elovich	$\frac{d q_{t}}{d t} = α e^{- β q_{t}}$	$q_{t} = \frac{1}{β} \ln (α β) + \frac{1}{β} \ln t$	$q_{t} vs \ln t$	Chien and Clayton (1980)

With pseudo first order kinetics, even though the R² improves with increasing dye concentration, a comparison of the calculated amount of MB adsorbed vs the actual experiment data shows that as the concentration increases, pseudo first order kinetics deviates from experimental data and therefore it cannot be used as a model in this study (Fig. 12). A similar observation was also obtained for the Weber Morris intraparticle diffusion kinetics model (Fig. 14). Of the four models, the Elovich model was the most unsuitable, where the simulated kinetics for all three different dye concentrations did not correlate with the actual data obtained from experiments (Fig. 15). Under the experimental conditions employed, adsorption of MB on ACP is found to obey the pseudo second order kinetics. From Table 11, it can be seen that the pseudo second order kinetics model gives very good R² value close to 1 with least errors for all the three different dye concentrations. Fig. 13 shows that both the experimental and calculated data correlate very well for all the three different concentrations used in this investigation. Hence, it is concluded that the sorption of MB on ACP follows the pseudo second order kinetics.

Comparison of experimental and calculated Pseudo first order kinetic models for biosorption of MB onto ACP at different dye concentrations.

Comparison of experimental and calculated Pseudo second order kinetic models for biosorption of MB onto ACP at different dye concentrations.

Comparison of experimental and calculated Weber Morris Intraparticle Diffusion models for biosorption of MB onto ACP at different dye concentrations.

Comparison of experimental and calculated Elovich models for biosorption of MB onto ACP at different dye concentrations.

Table 11 Parameters of different kinetic models for adsorption of MB on ACP.

Pseudo 1st order	q_e (mmol g⁻¹)	k₁ (min⁻¹)	R²	ARE	EERSQ	EABS	HYBRID
50 ppm	0.0407	0.0544	0.8512	54.30	0.0279	1.0107	1.4432
500 ppm	0.5833	0.0431	0.9370	9.00	0.0289	0.8149	0.2888
1000 ppm	0.5164	0.0332	0.9798	45.22	0.9472	5.7852	7.2732

Pseudo 2nd order	q_e (mmol g⁻¹)	k₂ (g mmol⁻¹ min⁻¹)	R²	ARE	EERSQ	EABS	HYBRID
50 ppm	0.0741	2.0741	0.9969	6.40	0.0004	0.1031	0.0261
500 ppm	0.6707	0.0697	0.9918	5.81	0.0113	0.5249	0.1178
1000 ppm	0.7467	0.0969	0.9929	7.15	0.0189	0.7001	0.1503

Intraparticle diffusion	c	k₃ (mmol g⁻¹ min^−0.5)	R²	ARE	EERSQ	EABS	HYBRID
50 ppm	0.025	0.0056	0.7133	48.58	0.0249	0.9028	1.2837
500 ppm	0.0413	0.0566	0.923	18.82	0.1139	1.7738	1.0399
1000 ppm	0.1261	0.0597	0.9238	34.06	0.6299	4.5393	4.4836

Elovich	α (mmol g⁻¹)	β (mmol g⁻¹)	R²	ARE	EERSQ	EABS	HYBRID
50 ppm	7.7519	0.0815	0.9595	280.8	1.0629	5.7498	48.037
500 ppm	70.922	0.0312	0.9140	57.82	1.702	6.8404	12.489
1000 ppm	7.2939	0.1555	0.9750	16.82	0.1373	2.1252	1.1393

3.7

3.7 FT-IR of ACP

It can be seen from Fig. 16 that upon adsorption of MB by ACP, the broad OH and NH bands at 3395 cm⁻¹ was shifted to 3384 cm⁻¹. This is indicative of the binding of MB with hydroxyl and amino groups. There is a shift in the C⚌O of carbonyl peak from 1737 to 1734 cm⁻¹ as well as a more significant shift for the strong C⚌O peak of carboxylic acid from 1621 to 1602 cm⁻¹. This indicates that carboxyl groups are likely to be the main group to be involved in binding with MB. Since the biosorption was carried out at ambient pH which was higher than pH_pzc, the COOH groups will be deprotonated to form carboxylate ions which would attract the cationic MB.

FT-IR spectra of ACP before (top) and after (bottom) adsorption with 1000 mg L−1 MB.

4 Conclusion

In this work, the peel from A. camansi was successfully utilized as a low-cost biosorbent for the removal of methylene blue (MB). When compared to most reported biosorbents, the optimum shaking time for this system to reach equilibrium is relatively short. Thermodynamics studies show that the biosorption process is favourable and spontaneous with maximum adsorption capacity at 298 K, and heating is not required to enhance its absorption capacity. The adsorption isotherms, corresponding to the Langmuir model, gave maximum adsorption capacities (q_max) which are far superior to most biosorbents. This is particularly attractive in real life applications in that unlike for other biosorbents which have to be activated in order to enhance biosorption capacity, A. camansi peel requires little processing, only oven drying, with no pre-treatment of sample. This will be particularly attractive since it would be more economical compared to many biosorbents. The kinetic modelling of MB dye was rapid and follows the pseudo second order kinetic model. Fast kinetics coupled with a high q_max make A. camansi peel attractive and great advantage in wastewater treatment as it would be cost effective and energy saving. The adsorption capacity of this biosorbent could be further enhanced through chemical modification, such as heat and/or chemical treatment.

Acknowledgements

The authors would like to thank the Government of Negara Brunei Darussalam and the Universiti Brunei Darussalam (UBD) for their financial support in carrying out this research. The authors are also grateful to the Energy research group and the Department of Biology at UBD for the use of XRF and SEM.

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Show Sections

[1] Ahmad A.A., Hameed B.H., . Fixed-bed adsorption of reactive azo dye onto granular activated carbon prepared from waste. J. Hazard. Mater.. 2010;175:298-303.
[Google Scholar]

[2] Al-Degs Y.S., El-Barghouthi M.I., El-Sheikh A.H., Walker G.M., . Effect of solution pH, ionic strength, and temperature on adsorption behavior of reactive dyes on activated carbon. Dyes Pigm.. 2008;77:16-23.
[Google Scholar]

[3] Amela K., Hassen M.A., Kerroum D., . Isotherm and kinetics study of biosorption of cationic dye onto banana peel. Energy Procedia. 2012;19:286-295.
[Google Scholar]

[4] Chan L.S., Cheung W.H., Allen S.J., McKay G., . Error analysis of adsorption isotherm models for acid dyes onto bamboo derived activated carbon. Chin. J. Chem. Eng.. 2012;20:535-542.
[Google Scholar]

[5] Chien S.H., Clayton W.R., . Application of Elovich equation to the kinetics of phosphate release and sorption on soils. Soil Sci. Soc. Am. J.. 1980;44:265-268.
[Google Scholar]

[6] Demirbas E., Kobya M., Konukman A.E.S., . Error analysis of equilibrium studies for the almond shell activated carbon adsorption of Cr(VI) from aqueous solutions. J. Hazard. Mater.. 2008;154:787-794.
[Google Scholar]

[7] Deng H., Lu J., Li G., Zhang G., Wang X., . Adsorption of methylene blue on adsorbent materials produced from cotton stalk. Chem. Eng. J.. 2011;172:326-334.
[Google Scholar]

[8] Dubinin M.M., Radushkevich L.V., . The equation of the characteristic curve of the activated charcoal. Proc. Acad. Sci. USSR Phys. Chem. Sec.. 1947;55:331-337.
[Google Scholar]

[9] Foo K.Y., Hameed B.H., . Insights into the modeling of adsorption isotherm systems. Chem. Eng. J.. 2010;156:2-10.
[Google Scholar]

[10] Foo K.Y., Hameed B.H., . Microwave-assisted preparation of oil palm fiber activated carbon for methylene blue adsorption. Chem. Eng. J.. 2011;166:792-795.
[Google Scholar]

[11] Foo K.Y., Hameed B.H., . Microwave assisted preparation of activated carbon from pomelo skin for the removal of anionic and cationic dyes. Chem. Eng. J.. 2011;173:385-390.
[Google Scholar]

[12] Foo K.Y., Hameed B.H., . Potential of jackfruit peel as precursor for activated carbon prepared by microwave induced NaOH activation. Bioresour. Technol.. 2012;112:143-150.
[Google Scholar]

[13] Foo K.Y., Hameed B.H., . Coconut husk derived activated carbon via microwave induced activation: effects of activation agents, preparation parameters and adsorption performance. Chem. Eng. J.. 2012;184:57-65.
[Google Scholar]

[14] Foo K.Y., Hameed B.H., . A cost effective method for regeneration of durian shell and jackfruit peel activated carbons by microwave irradiation. Chem. Eng. J.. 2012;193:404-409.
[Google Scholar]

[15] Forgacs E., Cserhati T., Oros G., . Removal of synthetic dyes from wastewaters: a review. Environ. Int.. 2004;30:953-971.
[Google Scholar]

[16] Freundlich H.M.F., . Uber die adsorption in losungen (adsorption in solution) Z. Phys. Chem.. 1906;57:384-470.
[Google Scholar]

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Breadnut peel as a highly effective low-cost biosorbent for methylene blue: Equilibrium, thermodynamic and kinetic studies

Abstract

Keywords

Breadnut peel

Methylene blue

Thermodynamics

Kinetics

Adsorption isotherms

Low-cost biosorbent

1 Introduction

2 Materials and methods

2.1 Materials

2.2 Characterization of ACP

2.3 Chemicals and reagents

2.4 Instrumentation

2.5 Optimization of parameters

2.5.1 Effect of contact time

2.5.2 Effect of pH

2.6 Thermodynamic studies

2.7 Effect of ionic strength

2.8 Isotherm analysis

2.9 Kinetic studies of biosorption

3 Results and discussion

3.1 General characterization of ACP sample

3.1.1 Analysis of elements by XRF

3.1.2 Point zero charge

3.1.3 SEM analysis of ACP

3.1.4 Specific surface area of ACP

3.2 Optimization of parameters

3.2.1 Effect of contact time

3.2.2 Effect of pH

3.3 Thermodynamic parameters

3.4 Effect of ionic strength

3.5 Adsorption isotherm and modelling

3.6 Kinetic modelling

3.7 FT-IR of ACP

4 Conclusion

Acknowledgements

References

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