Removal of Bisphenol A from Aqueous Solution by ... - CiteSeerX

Water Air Soil Pollut (2014) 225:2148 DOI 10.1007/s11270-014-2148-x

Removal of Bisphenol A from Aqueous Solution by Activated Carbon Derived from Oil Palm Empty Fruit Bunch Riry Wirasnita & Tony Hadibarata & Abdull Rahim Mohd Yusoff & Zulkifli Yusop

Received: 10 June 2014 / Accepted: 4 September 2014 # Springer International Publishing Switzerland 2014

Abstract The potential of the activated carbon prepared from the empty fruit bunch of oil palm wastes to remove bisphenol A (BPA) from aqueous media was investigated. The experiments were performed by varying the contact time, activated carbon dose, initial BPA concentration, and pH of the solution. The Langmuir, Freundlich, and Temkin isotherm models were employed to discuss the adsorption behavior. The equilibrium data were perfectly represented by the Langmuir isotherm with R2 of 0.9985. The maximum monolayer adsorption capacity of the activated carbon was found to be 41.98 mg/g. Kinetic studies indicated that the adsorption process followed the pseudo-second-order kinetic with a rate constant of 0.3×10−3/min. The activated carbon was characterized by means of Fourier transform infrared spectrometry, Brunauer–Emmett–Teller, and field emission scanning electron microscopy analyses. The results of the present study indicate that the activated carbon prepared from the empty fruit bunch is a promising candidate as a low-cost bio-adsorbent for the removal of BPA from aqueous solution.

R. Wirasnita : T. Hadibarata (*) : A. R. M. Yusoff : Z. Yusop Institute of Environmental and Water Resource Management (IPASA), Faculty of Civil Engineering, Universiti Teknologi Malaysia, 81310 Skudai, Johor Bahru, Johor, Malaysia e-mail: [email protected] A. R. M. Yusoff Department of Chemistry, Faculty of Science, Universiti Teknologi Malaysia, 81310 UTM Skudai, Johor Bahru, Malaysia

Keywords Activated carbon . Adsorption isotherm . Bisphenol A . Kinetic study . Oil palm empty fruit bunch

1 Introduction Persistent organic pollutants (POPs) such as biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), and bisphenol A (BPA) have been found in the ecosystem and humans because of their highly bioaccumulative nature, their persistency in the environment, and are known toxins, mutagens, and carcinogens. This also makes them barely bioavailable to microorganisms and not easily removed by biodegradation techniques (Choi et al. 2005; Hadibarata et al. 2012; Hadibarata and Kristanti 2013; Takekuma et al. 2004). 2,2-Bis (4hydroxyphenyl) propane (CAS Registry No. 80-05-7) generally known as Bisphenol A, is considered to be one of the most frequently detected compounds of new emerging pollutants in aquatic environment and in wastewater with a relatively high concentration owing to its widespread application and high production (Choi et al. 2005). The major sources of the BPA found in environmental water are the discharge of municipal effluent and industrial wastewater (Arnold et al. 2013). Bisphenol A is one of the endocrine disruptor compounds (EDCs) that are generally used as monomers in the polymer industry, such as in the production of epoxy resins, polycarbonates, phenol resins, polyacrylates, and polyesters (Kang et al. 2006). As an EDC, BPA may block or mimic hormones and disrupt normal bodily functions if absorbed into the body which

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may pose health risks to humans and aquatic organisms (Erler and Novak 2010; Oehlmann et al. 2008). To curtail the negative effects, BPA must be eliminated from environmental water. The molecular structure and the physical-chemical characteristics of BPA are as shown in Fig. 1 and Table 1. Adsorption onto activated carbon is considered as an efficient method for controlling organic pollutants in water, but the overlying cost of commercial activated carbon has fuelled the search for alternative low-cost precursors for the preparation of activated carbon. A large number of researchers have studied the preparation of activated carbon from agricultural by-products such as hazelnut shell (Doğan et al. 2008), durian shell (Chandra et al. 2007), almond shell (Bautista-Toledo et al. 2005), olive bagasse (Demiral et al. 2008), palm shell (Adinata et al. 2007), rattan sawdust (Hameed and Rahman 2008), coconut husk (Tan et al. 2008), and coconut shell (Cazetta et al. 2011). As one of the world’s leading palm oil exporters and producers, Malaysian palm oil mills generate an abundance of biomass waste. From 1 ton of fresh fruit bunch (FFB), palm oil mill produces 14 % fibers, 7 % shell, and 23 % empty fruit bunch (EFB). These biomass wastes are traditionally used as fuels to generate energy and are burned in incinerators to be used as a fertilizer (Piarpuzán et al. 2011). However, a large amount of biomass waste is still leftover from plantation and causes environmental problems (Abdullah et al. 2011). As the major agricultural by-product of palm oil extraction, oil palm EFB waste is proposed as a potential precursor for the production of activated carbon. This will give several advantages such as solving environmental problems caused by the disposal of abundant EFB wastes; reducing the product cost of activated carbon; and eliminating harmful pollutants, in this case bisphenol A. Activated carbon can generally be prepared by physical activation and chemical activation (Ioannidou and Zabaniotou 2007). Physical activation involves carbonization followed by activation using carbon dioxide or steam. Chemical activation involves treatment with dehydrating reagents such as NaOH, KOH, H2SO2, K2CO3, ZnCl2, and H3PO4 (Gerçel and Gerçel 2007; Hameed and Rahman

Fig. 1 Molecular structure of BPA

Water Air Soil Pollut (2014) 225:2148 Table 1 Physical-chemical characteristics of bisphenol A Molecular formula

C15H16O2

Appearance

White solid

Molecular weight

228.29 g/mol

Melting point

158 to 159 °C

Boiling point

220 °C (4 mmHg)

Aqueous solubility (15 °C)

120–300 ppm (21.5 °C)

2008; Hayashi et al. 2000; Kılıç et al. 2012). Chemical activation is an efficient method to produce activated carbons with high surface area and high distribution of porosity. Several studies have reported the removal of bisphenol A by adsorption onto several adsorbents (Bautista-Toledo et al. 2005; Gong et al. 2009; Nakanishi et al. 2002; Tsai et al. 2006; Zhou et al. 2011). However, so far, there is no report describing the preparation of activated carbons from oil palm EFB waste employing ZnCl2 as chemical agent and evaluating its potential in the removal of bisphenol A. The chemical and textural characterizations of the raw EFB and the prepared-activated carbon were also reported in this study. Thus, this study aimed to evaluate the adsorption process of bisphenol A onto oil palm EFBderived activated carbon. The effects of initial BPA concentration, EFB-activated carbon mass, contact time, and pH on the adsorption of BPA have been carried out. Equilibrium and kinetic studies have also been investigated to study the adsorption mechanism of BPA molecules onto the EFB-activated carbon.

2 Materials and Methods 2.1 Materials The waste of raw oil palm EFB used in the experiment was obtained from Telok Sengat Palm Oil Mill, Johor, Malaysia. Zinc chloride was supplied by Qrec™, Malaysia, and Bisphenol A was supplied by Aldrich, USA. Other chemicals used were analytical reagent grade. A stock solution of BPA was prepared by dissolving 100 mg of BPA granules in 5 mL ethanol, followed by dilution with distilled water in a 500-mL volumetric flask. The series of 5, 20, 40, 60, 80, and 100 mg/L of BPA standard solution was prepared from this stock solution.

Water Air Soil Pollut (2014) 225:2148

2.2 Experimental Methods 2.2.1 Precursor and Activated Carbon Preparation Raw oil palm EFB was washed several times using tap water to remove dirt then dried under ambient atmosphere for 24 h after that in the oven at 105 °C overnight. The dried EFB samples were impregnated with 10 % ZnCl2 solution for 24 h. The solution was then decanted off and the impregnated EFB was dried in the oven at 105 °C before being carbonized in a furnace. Carbonization was performed in a horizontal furnace, under nitrogen gas at 500 °C for 1 h; the N2 flow rate was 2.5 L/min. The activated carbon was washed with 10 % HCl solution to remove residual impurities then washed with distilled water until the pH of the washed water became neutral (±7), dried at 105 °C for 3 h, ground until fine, and stored in a container at room temperature. 2.2.2 Activated Carbon Characterization The surface area of the raw oil palm EFB and EFBderived activated carbon were determined using the Brunauer–Emmett–Teller (BET) method based on N2 adsorption at 77 K using a Micromeritics instrument. The physical-chemical properties of oil palm EFB was determined by following the ASTM E871, ASTM E1755, and ASTM E872 standard methods. The surface morphology of the oil palm EFB and the EFB-activated carbon were investigated using field emission scanning electron microscope (FESEM). Prior to FESEM analysis, it was necessary to coat the sample surface with gold using splutter coater so the image can be derived. The functional groups on the surface of the oil palm EFB and EFB-activated carbon were analyzed using Fourier transform infrared spectrometry (FTIR). The activated carbon was mixed with KBr powder at 100:1 weight ratio then pressed using hydraulic press into KBr pellet disk. The spectrum was scanned in the frequency range of 4000 to 450 cm−1. 2.2.3 Adsorption Test Adsorption tests were carried out in a batch experiment. The batch experiments were performed in 100-ml conical flasks by introducing 50 mL BPA solution of various concentrations. Activated carbon (50 mg) was added to each flask. The flasks were shaken at

Page 3 of 12, 2148

150 rpm using a mechanical shaker at room temperature (±27 °C). The speed was kept constant throughout the experiment for each run to ensure equal mixing. The adsorption kinetics was evaluated over the contact time ranging from 0–96 h. The effect of BPA concentration was studied using an initial BPA concentration of 20–100 mg/L. The effect of the initial pH on the adsorption process was investigated over the pH range of 2–11. The initial pH was adjusted with 0.1 mol/L HCl or 0.1 mol/L NaOH. The influence of the activated carbon loading on BPA adsorption was investigated over the adsorbent concentration range of 0.5–3 g/L. The experiment was performed in duplicate and other parameters were the same. The flask containing the sample was withdrawn from the shaker at the predetermined time, the contents were filtered using AdvantecTM filter paper, and the residual concentration of the sample was measured by acquisition of the UV/Visible adsorption spectrum at the maximum wavelength of 277 nm. The amount adsorbed at equilibrium, qe (mg/g), was calculated by: qe ¼

ðC o − C e ÞV W

ð1Þ

Where, Co and Ce are the liquid-phase concentrations of BPA at initial and equilibrium (mg/L), respectively. V is the volume of BPA solution (L) and W is the mass of adsorbent (g) used (Doğan et al. 2008).

3 Results and Discussion 3.1 Characterization Study The specific surface area of raw oil palm EFB and EFBactivated carbon were analyzed based on nitrogen adsorption at 77 K. The initial surface area of the raw EFB sample was 4.29 m2/g. After conversion of the raw oil palm EFB to activated carbon, the surface area increased slightly to 86.62 m2/g due to opening of the restricted pores. Physical-chemical properties of oil palm EFB was described in Table 2. Figure 2a, b shows the FESEM analysis of the surface morphology of the oil palm EFB before and after carbonization, observed at a magnification of ×1000. As shown in Fig. 2a, the surface of raw EFB was rough with a very few pores. After treatment, the oval-shaped pores were developed on the surface of the EFB-activated carbon (Fig. 2b). The

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Table 2 Physical-chemical properties of oil palm EFB (Abdullah et al. 2011; Piarpuzán et al. 2011) Component

Literature Measured Method values

Cellulose

46.77 %

–

–

Hemicellulose

17.92 %

–

–

Lignin

4.15 %

–

–

49.07 %

48.73 %

EDAX analysis

Oxygen

38.29 %

44.71 %

EDAX analysis

Hydrogen

6.48 %

–

–

Nitrogen

0.7 %

–

–

Silicon

na

6.57 %

EDAX analysis

Potassium

2.00 %

–

–

Moisture

7.95 %

10.51 %

ASTM E871

Ash

5.36 %

4.11 %

ASTM E1755

Volatile matter 83.86 %

85.73 %

ASTM E872

Fixed carbon

28.90 %

by difference

Elemental analysis Carbon

Proximate analysis

Surface area

10.78 % na

2

4.29 m /g The single point BET

na not available

FESEM analysis of the surface of EFB-activated carbon clearly showed that the EFB-activated carbon contained small cavities with average size of 3–5 μm. The development of the porosity of the EFB-activated carbon was influenced by the burning temperature of the pyrolysis process and the activation with zinc chloride. ZnCl2 acted as dehydrating agent which induces the charring and restricts the formation of tar (Hayashi et al. 2000). FTIR was employed to determine the surface functional groups responsible for BPA uptake by EFBactivated carbon. Figure 3 shows the FTIR spectrum of oil palm EFB and EFB-activated carbon. The FTIR spectrum presented in Fig. 3a shows various absorption peaks. The broad and strong band at 3420.84 cm−1 is assigned to the stretching vibration of the (–OH) hydroxyl group. The absorption peaks at 2930–2850 cm−1 are attributed to C–H stretching vibration of the –CH3 group. The small peak at 1740–1700 cm−1 is attributed to C═O and C–O stretching of carbonyl, esters, and carboxylic acid. The peaks between 1260 and 1000 cm−1 are ascribed to either C–O stretching and Si–O as a yield of silica containing minerals (Al-Qodah and Shawabkah 2009). It was in agreement with Si atom detected by EDAX analysis (Table 2). The peaks at

1637.74 cm−1 and 1466.49 cm−1 are assigned to amide groups. The peak at 858.31 cm−1 is ascribed to the outof-plane C–H bending (Socrates 2004). The FTIR spectrum of EFB-activated carbon is presented in Fig. 3b. After raw EFB converted to activated carbon, the strong band was observed at 1586.74 cm−1, which may be due to the intense stretching of conjugated C═C in aromatic ring or oxygen-aromatic bonding in aromatic ether. The band located at 3349.89 cm−1, which is attributed to the –OH stretching vibration was weaker than –OH vibration in Fig. 3a, probably due to the loss of hydroxyl groups and elimination of volatile molecules from raw EFB. The peak at 1373.68 cm−1 is ascribed to C–H bending vibrations. The intensity of the band located at 1740–1700 cm−1 ascribed to C═O stretching was also weakened and reduction of complex band at 1260–1000 cm−1 was observed which include the reduction of carboxylic acids, alcohol, and ester. The surface functional groups of EFB-activated carbon may be the potential active sites for interaction with BPA (Bouchelta et al. 2008). Besides using ZnCl2, oil palm EFB samples were also carbonized with several chemical agents such as K2CO3, NaOH, H2SO4, H3PO4 and without chemical agents in one step carbonization process. But according to the result as shown in Table 3, only chemical activation using ZnCl2 gives the satisfied result to remove BPA; this is probably because only ZnCl2 can develop micro pores and create active sites on EFB-activated carbon surfaces, therefore adsorption of BPA using carbon activated with ZnCl2 was chosen (Hayashi et al. 2000). 3.2 Effect of Contact Time on BPA Adsorption Figure 4 shows a plot of the percentage BPA removal and adsorption capacity of the activated carbon versus the contact time. The adsorption generally increased with increasing contact time of BPA with the activated carbon. From the figure, it is seen that increasing contact time tends to increase the percent removal and adsorption capacity. Over 96 h of adsorption time observed that optimum adsorption equilibrium was reached at 48 h, with a percentage BPA removal of 96.1 % and its adsorption capacity of 19.3 mg/g. The adsorption process was initiated with rapid adsorption within the first 18 h due to the availability of readily accessible sites, followed by slower adsorption within the period of 18– 48 h. In the slower adsorption, the accessible sites are


Page 5 of 12, 2148

Fig. 2 Morphology structure of a raw oil palm EFB and b oil palm EFB-activated carbon

decreased and BPA has to transport deeper. Finally, a plateau was reached after 48 h, indicating that the activated carbon was saturated (Demiral et al. 2008; Hameed and Rahman 2008). UV spectra of BPA adsorption after contact with EFB-activated carbon for 48 h was shown in Fig. 5. Despite its good adsorption capacity, EFB-activated carbon has a disadvantage. The EFB-activated carbon takes a longer adsorption time to reach equilibrium. The same equilibrium time was reached on removal of BPA with sugi chip and sawdust (Nakanishi et al. 2002) but it was longer than the adsorption time of esterified carboxyl cotton achieved by Gong et al. (2009).

Furthermore, it should be noted that there was a slight decrease on the percentage removal of BPA after optimal adsorption was achieved which was caused by the release of some BPA molecules from adsorbent surface. 3.3 Effect of Initial Bisphenol A Concentration The initial concentration of the BPA solution was varied from 20–100 mg/L. As can be seen in Fig. 6a, the percentage removal of BPA decreased from 94 to 42.4 % as the initial BPA concentration increased, whereas the adsorption capacity increased from 19.8 to 40.8 mg/g with an increase in the initial BPA

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(a)

Fig. 3 FTIR spectrum of: a oil palm EFB b EFB-activated carbon

58.0

485.06

57 2066.10

56

771.41

3840.61

55

3734.30

858.31

897.13

712.61

468.17

611.46

54 53 52 51 %T

50 1738.15

49

1319.69

1717.31

1376.43 1249.73

2851.28

48

1162.38

47 1637.74

2920.62

46

1112.53 1054.28

1466.49

45

1035.30

3420.84

44 43.0 4000.0

(b)

3600

3200

2800

2400

2000

1800

cm-1

1600

1400

1200

1000

800

600

450.0

67.00 66.5

504.98

668.22

66.0 65.5

3780.64

812.11

65.0 64.5

596.94

573.27

64.0

528.39

550.28 477.51

63.5 63.0 62.5 62.0

%T

61.5 3349.89

61.0

2923.71

60.5 60.0

1373.68

59.5 1206.39

59.0 58.5

1586.74

58.0 57.5 57.00 4000.0

3600

concentration. With increasing initial concentration of BPA, the driving force of mass transfer became more dominant, thereby enhancing the driving force to get over mass transfer resistance between BPA in solution and activated carbon, resulting in higher adsorption capacity (Tan et al. 2008; Tsai et al. 2006). This phenomenon was also observed in the studies by Tan et al.

3200

2800

2400

2000

1800

cm-1

1600

1400

1200

1000

800

600

450.0

(2008) when initial methylene blue concentrations was increased from 50 to 500 mg/l, and Tsai et al. (2006) when initial bisphenol A concentrations was increased from 60 to 100 mg/l. However, at higher initial concentration, the ratio of adsorbate to adsorbent is low thus decreasing removal percentage.

Table 3 Adsorption of BPA onto various activated carbons prepared from oil palm EFB with different chemical agents (C0 =50 mg/L; adsorbent conc.= 2 g/L; time, 4 h) Sample

qe (mg/g)

Removal (%)

EFB-carbon

0

0

K2CO3-activated carbon

0

0

ZnCl2-activated carbon

21.6

87.35

H2SO4-activated carbon

0.39

1.6

H3PO4-activated carbon

0

0

NaOH-activated carbon

0

0

Fig. 4 Effect of contact time on BPA adsorption (C0 =20 mg/L; adsorbent conc.=1 g/L; initial pH=normal)


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Fig. 5 UV spectra of BPA adsorption after contacted with EFB-activated carbon for 48 h

3.4 Effect of Adsorbent Dose The effect of the adsorbent loading on the adsorption of BPA was evaluated by varying the loading of activated carbon concentrations from 0.5 to 3 g/L. The results show that the adsorption efficiency is dependent on the amount of activated carbon added; increasing the activated carbon loading may increase the percentage removal of BPA but decrease the adsorption capacity. The maximum removal of BPA was at adsorbent loading of 1 and 1.5 g/L, further loading of adsorbent will not give any considerable change of removal. The adsorption capacity for the loading of 1.5 g/L was 11.3 mg/g, which is less than achieved with an adsorbent loading of 1 g/L (16.3 mg/g); thus, a minimum loading of 1 g/L adsorbent was more effective for removal of BPA from a 50 mL aliquot of 20 mg/L BPA (Fig. 6b). The similar trend was also observed by Ghaedi et al. (2013), Senthil Kumar et al. (2010) and Han et al. (2011). This decrease of qe was caused by the split in the concentration gradient between adsorbates in the solution and in the adsorbent surface (Senthil Kumar et al. 2010).

above pH 10 on mesoporous silicon dioxide (meso-SiO2) prepared by Fan et al. (2011), plausibly because at very basic pH, BPA is deprotonated to bisphenolate anion (Bautista-Toledo et al. 2005). Consequently, the adsorption capacity and percentage BPA removal achieved with the activated carbon was reduced due to repulsive interaction between the bisphenolate anion and the negativecharged activated carbon affected by oxygen-containing functional groups (Tsai et al. 2006). 3.6 Adsorption Isotherm Isothermal study is a basic technique for determining the nature of adsorption between the activated carbon and BPA. It indicates the distribution of BPA in the equilibrium phase. Langmuir, Freundlich, and Temkin models are the most commonly used models for evaluating the experimentally acquired adsorption isotherms. The Langmuir equation is applicable to homogeneous sorption whereas the Freundlich equation is applicable to heterogeneous sorption (Doğan et al. 2008). Adsorption isotherms of BPA were represented by the following Langmuir, Freundlich, and Temkin isotherm models:

3.5 Effect of Initial pH The effect of the initial pH on the removal of BPA is shown in Fig. 6c. The pH of the solution generally did not have a significant effect on the removal of BPA. In excess of 80 %, BPA removal was achieved over the entire pH range from 2 to 9. It has been reported that above pH 11, the percentage removal of BPA declined to below 70 %. The declining of BPA adsorption was also observed at

Langmuir equation

Ce 1 Ce ¼ þ K L qm qe qm

ð2Þ

1 Freundlich equation lnqe ¼ lnK F þ lnC e n

ð3Þ

Temkin equation qe ¼ B lnA þ BlnC e

ð4Þ

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0.9985 as shown in Fig. 7a and Table 4. The Freundlich and Temkin isotherm models generate a correlation factor (R2) of 0.9843 and 0.9911, respectively, with a less linear relationship (Fig. 7b, c) than was achieved with the Langmuir isotherm model. By comparing the correlation factors of the Langmuir, Freundlich, and Temkin isotherm models (Table 4), it is evident that the Langmuir model provides a better fit for explaining the adsorption of BPA on the activated carbon. The Langmuir isotherm suggests that EFB-activated carbon has a homogeneous surface where each potentially active site of EFB-activated carbon can accommodate one molecule of BPA (Chandra et al. 2007). The Langmuir

Fig. 6 a Effect of initial BPA concentration (adsorbent conc.=1 g/ L; time, 48 h; initial pH=normal). b Effect of adsorbent dose on BPA adsorption (C0 =20 mg/L; time, 24 h; initial pH=normal). c Effect of initial pH on BPA adsorption (C0 =20 mg/L; adsorbent conc.=1 g/L; time, 48 h)

where Ce (mg/L) is the concentration of BPA solution at equilibrium, qe (mg/g) is the amount of sorbed BPA at equilibrium, qm is the maximum sorption capacity, KL is the Langmuir constant, KF and n are the Freundlich constants, A is the Temkin equilibrium binding constant (L/g), and B is the Temkin constant related to heat of sorption (J mol−1) (Hameed and Rahman 2008). For the Langmuir isotherm model, the Ce/qe was plotted to Ce then the slope will be equal to 1/qm and intercept will be equal to 1/(KLqm). The plot of Ce/qe vs. Ce gives a linear curve with the correlation factor (R2) of

Fig. 7 Langmuir model (a), Freundlich model (b), and Temkin model (c) for adsorption of BPA onto EFB-activated carbon


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Table 4 The adsorption isotherm parameters of BPA adsorption Adsorption isotherm model

Parameter

Langmuir isotherm

almond shell activated carbons have higher adsorption capacity of BPA; however, Sorbo-Norit and Merck are commercial activated carbons and the almond shell activated carbon contact time was 7 days, longer than the adsorption time of EFB-activated carbon in this study.

qm (mg/g)

41.98

KL (L/mg)

0.51

R2

0.9985

3.7 Adsorption Kinetics

KF (mg/g) (L/mg)1/n

20.66

n

5.56

R2

0.9843

Evaluation of the reaction kinetics is important for determining the rate of adsorbates uptake during the adsorption. The BPA sorption kinetics was investigated by examining the influence of contact time on BPA removal for 96 h. The kinetics of the adsorption of BPA approached equilibrium within ca. 48 h as illustrated in Fig. 4. The conditions of kinetic procedure were identical to equilibrium procedure. The aqueous samples were taken at interval time and the concentrations of residual BPA were measured. The amount of adsorption at t time, qt (mg/g), was calculated by:

Freundlich isotherm

Temkin isotherm A (L/g)

54.56 −1

B (J mol )

5.16

R2

0.991

isotherm model can describes the capacity of BPA to be adsorbed by activated carbon. From the data in Table 4, it can be seen that the EFB-activated carbon has a maximum adsorption capacity of 41.98 mg/g to form monolayer coverage. As compared with adsorption capacity of other adsorbents in Table 5, the EFB-activated carbon has a relatively good adsorption capacity. BautistaToledo et al. (2005) reported Sorbo-Norit, Merck and

Table 5 Comparison of BPA adsorption capacity onto several adsorbents Sample

Adsorption capacity (mg/g)

qt ¼

ðC o −C t ÞV W

ð4Þ

Where Co (mg/L) is the initial of liquid-phase concentrations of BPA and Ct (mg/L) is the liquid-phase concentrations of BPA at time (t). The kinetics data for BPA adsorption were treated with the following pseudo-first-order, pseudo-secondorder models, and intraparticle-diffusion: pseudo–first–order model lnðqe −qt Þ ¼ lnqe − k 1 t

ð5Þ

Reference

pseudo–second–order model EFB-activated carbon

41.98

This study

Almond shell activated carbon Sorbo-Norit, 3-A-7472

188.9

Bautista-Toledo et al.

129.6


Merck, K27350518015

263.1


Sugi chip

11.5

Nakanishi et al.

Sugi sawdust

12.1

Nakanishi et al.

Hinoki sawdust

18.0

Nakanishi et al.

Esterified carboxyl cotton Unmodified base peat

87.72

Gong et al.

15.97

Zhou et al.

Modified fibric peat

29.15

Zhou et al.

Andesite

0.53

Tsai et al.

Diatomaceous earth

0.73

Tsai et al.

Graphene

182

Xu et al.

Meso-SiO2

353.4

Fan et al.

t 1 t ¼ þ 2 qt k 2 qe qe

intraparticle diffusion qt ¼ k dif t 1=2 þ C

ð6Þ

ð7Þ

where qe and qt (mg/g) respectively indicate the amount of BPA adsorbed at equilibrium and at time t (min); k1 is the pseudo-first-order rate constant and k2 is the pseudo-second-order rate constant; kdif is the intraparticle diffusion rate constant (mg/g min1/2), and C is the intercept which indicates the boundary layer thickness (Ghaedi et al. 2013; Rebitanim et al. 2012). Table 6 shows kinetics parameters calculated from the experimental data. By plotting ln (qe −qt) vs. t for the pseudo-first-order model and t/qt vs. t for pseudosecond-order models, the correlation coefficients, kinetics constant and qe values can be calculated. Based on

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Table 6 The kinetic parameters of BPA adsorption

3.8 Adsorption Mechanism

Kinetic model

Parameter

qe experiment (mg/g)

19.28

Pseudo-first-order qe calculated (mg/g)

4.98

K1

0.0007

R2

0.1384

Pseudo-second-order qe calculated (mg/g)

18.94

K2

0.0003

R2

0.9957

Adsorption is a complex process and involves several interactions such as electrostatic or non-electrostatic interaction. Although a specific adsorption mechanism occurs between the adsorbent and adsorbate, it still needs to be identified clearly. The experimental data from equilibrium isotherm and kinetic studies are useful in understanding adsorption mechanism. For a liquid– solid adsorption process, the steps of BPA adsorption into activated carbon may be assumed as follows (Doğan et al. 2004):

Intraparticle-diffusion C

8.001

kdif

0.1724

R2

0.7468

the data in Table 6, the experimental qe values (qe,exp) of 19.28 mg/g was closer to the calculated value (qe,cal) of 18.94 mg/g for the pseudo-second-order model. The R2 value of 0.9957 for the pseudo-second-order model indicated a better fit than the pseudo-first order model for which R2 was 0.1384. It is thus suggested that the adsorption of BPA followed the pseudo-second-order model, from which it is proposed that the overall rate of the BPA adsorption mechanism may be a ratelimiting chemisorption (Ho and McKay 1999). Pseudo-second-order kinetic was also reported by Doğan et al. (2004) for adsorption of methylene blue onto perlite and Demiral et al. (2008) for adsorption of Cr(IV) onto olive bagasse-derived activated carbon, Diffusion mechanism was identified with intraparticle diffusion model. Intraparticle diffusion is the sole ratelimiting step. By applying Eq. (7), adsorption process is controlled by the intraparticle diffusion if the plot of qt vs. t1/2 gives a linear relationship. Then, kdif and C values can be calculated from this equation. According to Table 6, R2 value is 0.7468, poor coefficient correlation indicates that the diffusion process is not solely controlled by intraparticle diffusion. The qt vs. t1/2 plot is a multi-linear plot which explains that there are two or more steps in the adsorption process. This result is in agreement with the result reported by Zhou et al. (2011), the adsorption process may be determined in three steps which includes rapid external surface sorption, gradual interior surface sorption, and final equilibrium step where the adsorption rate starts to slow down.

1. BPA molecules transport from the bulk of solution to the external carbon surface. 2. BPA molecules diffuse through the boundary layer to the surface pores of activated carbon. The boundary layer resistance reduces by the rate of adsorption and the increase of contact time. 3. BPA molecules adsorb at the active sites of carbon surface which have homogenous surface energy as BPA adsorption shows a good correlation with Langmuir theory. Each active site accommodates one BPA; once the carbon surface becomes saturated, no further adsorption can take place. Interaction is chemisorption in which BPA attaches by chemical bonding such as π–π interaction of C═C double bonds or benzene rings between π electrons of BPA and activated carbon, hydrogen bonding between the oxygen-containing groups (e.g. hydroxyl or carboxyl groups) in both BPA, and activated carbon and electron donor–acceptor complexes. (Bautista-Toledo et al. 2005; Xu et al. 2012).

4 Conclusion Activated carbons produced from oil palm EFB wastes treated with zinc chloride at 500 °C are described in this study. BPA was effectively removed from aqueous media based on the good adsorption of the EFB-activated carbon. The adsorption time was 48 h with 96.1 % removal of BPA. The isotherm data showed better correlation with the Langmuir isotherm model than the Freundlich and Temkin isotherm models. The best R2 value obtained from the kinetic model was 0.9957, indicating that the adsorption process obeyed the pseudo-second-order model for the entire adsorption


period. The removal of BPA was found to be favorable, and EFB-activated carbon is deemed a prospective low-cost alternative to commercial absorbents. Acknowledgments The authors would like to acknowledge the Ministry of Education Malaysia for providing LRGS Grant on Water Security entitled Protection of Drinking Water: Source Abstraction and Treatment (203/PKT/ 6720006)(R.J130000.7809.4L810).

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