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Adsorption of 2-chlorophenol onto the surface of underutilized seed of Adenopus breviflorus: A potential means of treating waste water
Journal of Environmental Chemical Engineering 4 (2016) 664–672
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Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece
Adsorption of 2-chlorophenol onto the surface of underutilized seed of Adenopus breviflorus: A potential means of treating waste water Adewale Adewuyia,b,* , Andrea Göpfertb , Omotayo Anuoluwapo Adewuyic , Thomas Wolffb a b c
Department of Chemical Sciences, Faculty of Natural Sciences, Redeemer’s University, Mowe, Ogun state, Nigeria Physical Chemistry, Technische Universität Dresden, D-01062 Dresden, Germany Enviro Africa Limited, plot 182b Kofo Abayomi Street, Victoria Island, Lagos, Nigeria
A R T I C L E I N F O
A B S T R A C T
Article history: Received 20 October 2015 Received in revised form 8 December 2015 Accepted 9 December 2015 Available online 12 December 2015
Seed of Adenopus breviflorus was prepared as an adsorbent and its adsorption capacities for 2-chlorophenol in aqueous solutions was studied. Fourier Infrared spectroscopy (FTIR), X-ray Diffraction analysis (XRD), particle size distribution, zeta potential, thermogravimetric (TG) analysis and Scanning Electron Microscopy (SEM) measurements were used to characterize the A. breviflorus seed adsorbent. The kinetics studies showed that the adsorption of 2-chlorophenol onto the adsorbent followed the pseudo-second order model while adsorption isotherm plots yielded good results for Temkin, Langmuir and Freundlich models. The adsorption capacity of 2-chlorophenol onto A. breviflorus seed increased with a decrease in pH of 2-chlorophenol solution. From the study, seed of A. breviflorus exhibited properties which suggested its potential application as adsorbent for the removal of 2-chlorophenol from polluted or waste water. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: Adenopus breviflorus Adsorption 2-Chlorophenol Isotherm SEM
1. Introduction Rapid industrialization and modern agricultural practice in Nigeria is generating environmental contaminants which are found in water resources. Some of these contaminants belong to the group of phenol and its derivatives which have been consistently considered as priority pollutants since they are toxic to plants, animals and humans even at low concentrations [1]. Water pollution is primarily associated with domestic and industrial waste. Both types of waste water pose threats to water quality which may be classified into health hazards and sanitary nuisances [2]. In most parts of Africa, people have no access to potable water and consequently, raw water from polluted rivers and streams form the major source of water. Water treatment is a serious issue in Africa and other developing nations of the world as large volumes of waste water are generated from time to time with no adequate means of treatment. Although industrialization is crucial, it has also brought several devastating environmental and human hazard over a period of years with search light on industries as the major contributor to environmental degradation and pollution in developing nations.
* Corresponding author. E-mail address: [email protected] (A. Adewuyi). http://dx.doi.org/10.1016/j.jece.2015.12.012 2213-3437/ ã 2015 Elsevier Ltd. All rights reserved.
Presently, there is increase in demand for food which has resulted in the use of large amounts of pesticides to control different agricultural pests [3]. 2-Chlorophenol is used in the manufacture of different compounds such as antiseptics, herbicides, dyes and other organic compounds [4]. Unfortunately, the presence of 2-chlorophenol and other chlorinated compounds has been detected in wastewaters; moreover, most chlorinated phenols have been found in municipal waste, agricultural run-off, leachates from polluted or contaminated site, soil, water, sediments, air, food products and body fluids [1,5–7]. Effective removal of phenol and its chlorinated derivatives from polluted and waste waters has been a problem. Chlorinated phenols are among the list of priority organic pollutants proposed by the US Environmental Protection Agency [8]. They are known to persist in many environments, because of inappropriate conditions for biodegradation [9]. Chlorophenols are carcinogenic and mutagenic; they are weak acids which are capable of penetrating human skin and are readily absorbed by gastro-intestinal tract with acute toxicity exhibiting symptoms like increased respiratory rate, vomiting and nausea [10]; thus it is important to get rid of them before they get into the environment in order to avoid the biomagnified toxicity to aquatic flora and fauna through various food chains [11]. Several methods have been used in waste water treatment some of these methods are based on ion exchange, chemical
A. Adewuyi et al. / Journal of Environmental Chemical Engineering 4 (2016) 664–672
precipitation, oxidation, reduction and reverse osmosis [12]. However, many of these methods are less effective or difficult for practical use due to their toxicity, high price, sludge disposal problem, sustainability and selectivity. To minimize this problem, there is need to find an alternative approach to waste water treatment that will be low cost, effective, less toxic and efficient especially in developing countries like Nigeria. The quest for cheap and cost effective technology for removal of pollutants from wastewater containing organic pollutants has led to the use of materials of biological origin as adsorbent. Presently, the use of agricultural waste or biomass as adsorbent for waste or polluted water treatment is of much importance. Adsorption as a process is easy to control and design, cheap, reliable and may be easily start
665
up. Over the years, there has been a growing interest in the development of adsorption as a method for the treatment of contaminated or polluted wastewater using biomass [13–18] to remove pollutants from waste water. They have been recognized as potential adsorbents for the removal of pollutant ions from aqueous solution with the view to replacing existing technologies [19]. Some of these biomasses have low adsorption capacity for organic pollutants unlike in the case for inorganic pollutants. This has pointed attention in a search for better biomass with high adsorption capacity for organic pollutants. The objective of the present study is to investigate the feasibility of using seed of Adenopus breviflorus—an underutilized plant as an adsorbent for the removal of 2-chlorophenol from
Fig. 1. (A) FTIR of Adenopus breviflorus, (B) XRD of Adenopus breviflorus, (C) TG of Adenopus breviflorus, (D) particle size distribution (PSD) and (E) zeta potential of Adenopus breviflorus.
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wastewaters. A. breviflorus is a sub-tropical plant and an annual climber which produces fruit containing fibrous vascular system. Some tribes in Nigeria believe the fruit to have some supernatural healing powers. Presently, the seed of A. breviflorus has no specific use in Nigeria; it is usually discarded as waste and as such creating waste disposal problem. Our aim is to find applications for the seed of A. breviflorus and avoid it being discarded into the environment as waste. We have previously reported the fatty acid composition of the seed oil [20]. In this study, seed of A. breviflorus was used with little pretreatment as an adsorbent for the removal of 2-chlorophenol in aqueous system. Batch experiments were carried out for kinetic studies on the removal of 2-chlorophenol from aqueous solutions. The effect of various parameters such as pH, adsorbent weight, contact time, and initial 2-chlorophenol concentration were investigated.
Shimadzu) with filtered Cu Ka radiation operated at 40 kV and 40 mA. The XRD pattern was recorded from 10 to 80 C of 2u per second with a scanning speed of 2.0000 of 2u per minute; zeta potential analyser (DT1200, Dispersion technology) was used to obtained the surface potential and particle size distribution, the seed was made into powder form and the equipment was operated at 25 C; TG analysis was carried out on DTG-60 (Shimadzu, C30574600245) under nitrogen atmosphere with steady increase in temperature; FTIR spectra were recorded on Perkin Elmer, spectrum RXI 83303. The spectra were recorded in the range of 4000–400 cm1. The surface evaluation was conducted using SEM (Carl Zeiss, DSM, 982 Gemini, 984A-ISUS, Germany); the surfaces were coated with gold during sample preparation in order to increase electrical conductivity and to improve the quality of the micrographs.
2. Material and methods
2.3. Kinetics and equilibrium experiment
2.1. Materials
Biosorption kinetics and equilibrium experiments were carried out by agitating 50 mL of 2-chlorophenol solution of known initial concentrations ranging from 100 to 400 mg/L with 0.5 g of the prepared A. breviflorus seed adsorbents. Experiments were carried out at room temperature (298 K) with a constant agitation at 180 rpm. Samples were agitated at different time intervals, filtered, and then analyzed for 2-chlorophenol concentration using UV–vis spectrophotometer (Varian Australia PTY LTD. Model: EL07093760) at a wavelength of 273 nm.
The mature seeds of A. breviflorus were obtained from the garden of University of Ibadan, Ibadan, Oyo state, Nigeria. They were identified at the herbarium unit, Botany Department University of Ibadan. All solvents and chemicals used in the study were of analytical grade and were purchased from Merck and VWR International GmbH, Darmstadt, Germany. 2.2. Preparation of adsorbent
2.4. Effect of adsorbent dosage The seeds were crushed in a grinding machine. The ground seeds were successively extracted firstly with hexane [20,21] and secondly with methanol in a soxhlet extractor for 12 h. This was further soaked in slightly acidified aqueous solution (HCl, 0.005 M), washed continuously with deionized water, filtered and air dried. This was later characterized. The structural information was obtained using X-ray diffraction (XRD-7000X-Ray diffractometer,
The effect of adsorbent dosage used on the equilibrium uptake was estimated by agitating the 2-chlorophenol solution of initial concentration of 400 mg/L with different weighed amount of adsorbents ranging from 0.1 to 1.0 g. The agitation was allowed for 180 min after which the solution was filtered and taken for UV–vis analysis.
Fig. 2. (I) SEM result of the surface of Adenopus breviflorus before treatment, (II) SEM result of the surface of Adenopus breviflorus after treatment and (III) SEM result of the surface of Adenopus breviflorus after adsorption of 2-chlorophenol.
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2.5. Effect of pH on adsorption
667
30
3. Results and discussion
25 20 qt (mg/g)
The effect of pH on the adsorption of 2-chlorophenol was evaluated by agitating 0.5 g of adsorbent with 50 mL of 2-chlorophenol solution of initial concentration 400 mg/L at different solution pH range (2.0–12.0) for 180 min this was filtered and taken for UV–vis analysis.
15 100 mg/L
10
300 mg/L 400 mg/L
3.1. Preparation of adsorbent
5
The seed was extracted with hexane to remove non polar substances such as fats while methanol and slightly acidified water were meant to remove polar compound which may get into the water system during treatment. The FTIR result revealed the presence of some functional groups at the surface of the adsorbent as presented in Fig. 1A. These functional groups include ester carbonyl (1735 cm1), alcohol (3287 cm1) and alkane (2955 cm1—CH3 group and 2872 cm1—CH2 group). The XRD revealed the structural information of the adsorbent. As presented in Fig. 1B, the X-ray diffraction pattern of the A. breviflorus seed adsorbent is similar to those of semi-crystalline materials with an amorphous broad hump [22]. The characteristic decomposition pattern, degradation, organic and inorganic content of the adsorbent was evaluated using TG; Fig. 1C revealed a loss in mass at temperature below 190 C which was accounted for as being the removal of physisorbed water. The graph also showed a sharp loss in mass within the range 200–300 C which may be attributed to predominant decomposition of hemicelluloses and other volatile matters [23]; mass loss observed in the range 300–350 C may be due to the decomposition of cellulose and loss above 350 C may be considered as being the decomposition of lignin [24]. The particle distribution was found to be monomodal (Fig. 1D) with a mean particle size distribution (PSD) of 181.49 micron. The zeta potential increased as the pH value increases as shown in Fig. 1E. The SEM results for the surface of the seed before and after treatment are presented in Fig. 2I and Fig. 2II; respectively. After the treatment, it was obvious that the pores at the surface of the seed were opened up (Fig. 2II). When the seed was used as adsorbent for 2-chlorophenol, the opened pores were filled as shown in Fig. 2III indicating that the surface of the adsorbent was covered with the 2-chlorophenol which suggests that adsorption has taken place.
0
The amount of 2-chlorophenol adsorbed on the A. breviflorus seed adsorbent was calculated from the mass balance equation as given below:
40
60 Time (min)
80
100
Fig. 3. Adsorption of 2-chlorophenol on Adenopus breviflorus at various contact time and different initial concentrations.
higher initial concentration [25]. The percentage removal of 2-chlorophenol was also found to increase with increase in time as shown in Fig. 4 which is an indication that the removal of 2-chlorophenol by A. breviflorus seed adsorbent was time dependent. The data obtained were treated with pseudo-first-order model which can be expressed as [26]: dqt ¼ K 1 ðqe qt Þ dt
ð2Þ
on integrating Eq. (2) under the boundary conditions of t = 0 to t = t and qt = 0 to qt = qt, gives a linear expression: Inðqe qt Þ ¼ Inqe K 1 t
ð3Þ
The values of k1 can be obtained from the slope of the plot of log (qe qt) versus t . Also, the data were subjected to pseudo-second-order model which can be expressed as [27]: qt ¼ K 2 q2e t
ð4Þ
On differentiating, Eq. (4) becomes: dqt ¼ K 2 ðqe qt Þ2 dt
ð5Þ
On integrating Eq. (5) for the boundary conditions t = 0 to t = t and qt = 0 to qt = qt, gives: ð6Þ
This can be linearized to become: t 1 t ¼ qt K 2 q2e qe
ð7Þ
ð1Þ
where Co and Ce are the initial and equilibrium concentration of 2-chlorophenol solution (mg/L), respectively, qe is the equilibrium 2-chlorophenol concentration on adsorbent (mg/g), V is the volume (L) of the 2-chlorophenol solution and W is the mass (g) of the A. breviflorus seed adsorbent used. The kinetics and equilibrium study of the adsorption of 2-chlorophenol on A. breviflorus seed adsorbent was achieved using different concentration range at different contact time as shown in Fig. 3. An equilibrium adsorption time of 80 min was obtained after several experiments. The adsorption of 2-chlorophenol on A. breviflorus seed adsorbent was found to increase as the concentration of 2-chlorophenol increased over the period of time studied. This can be ascribed to the increase in driving force as a result of applying
Percentage removal (%)
v w
20
1 1 ¼ K2t qe qt qe
3.2. Kinetics and equilibrium experiment
qe ¼ ðC o C e Þ
0
70 60 50 40 30 20 10 0
400 mg/L 100 mg/L 300 mg/L
0
20
40
60
80
100
Contact time (min) Fig. 4. Effect of contact time on the percentage removal of 2-chlorophenol by Adenopus breviflorus.
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h ¼ K 2 q2e
ð8Þ
where K2 is the rate constant, qe is the adsorption capacity of the adsorbent (mg/g), h is the initial sorption rate and qt is the amount of 2-chlorophenol adsorbed at time t (mg/g). The plot for the pseudo-first-order and pseudo-second-order models are shown in Fig. 5 and Fig. 6; respectively. The correlation coefficients of the models were determined and presented in Table 1. In the pseudofirst order, the difference in the logarithm value between qe and qt is inversely proportional to time where as in pseudo-second-order model the value of t/qt increases with time. The correlation coefficient (r2) values for pseudo-second-order model are higher than those of pseudo-first-order model except for concentration at 100 mg/L as shown in Table 1, suggesting that the adsorption of 2-chlorophenol on A. breviflorus seed adsorbent may be via chemisorption. The r2 values increased with increase in concentration in both models. The results reveal pseudo-second-order adsorption mechanism to be predominant, and that the overall rate of the 2-chlorophenol adsorption process appeared to be controlled by the chemisorption process [28]. The values of the initial sorption calculated were found to increase as the concentration increased just as similar observation has been reported by Adebowale et al. [28]. The effect of A. breviflorus dose (0.1–1.0 g) on the biosorption of 2-chlorophenol onto the surface of A. breviflorus was studied by contacting different dose of A. breviflorus with 2-chlorophenol solution agitated at 180 rpm for 180 min and at temperature 298 K. It was found that the adsorption of 2-chlorophenol on A. breviflorus increased as the dose of A. breviflorus was increased from 0.1 to 1.0 g. This may have been due to the availability of more active site for the 2-chlorophenol to adsorb onto since this would have offered more site for the 2-chlorophenol to interact with. 3.3. Adsorption isotherm During adsorption process, the state of equilibrium is usually described with the help of isotherm equations with parameters which expresses the surface properties and affinity of the adsorbent [29]. These isotherms are usually generated using theoretical models and among these models Langmuir and Freundlich models are the most commonly used [30]. The Langmuir model was built on the assumption that uptake of adsorbate ions takes place on a homogenous surface via monolayer sorption without any interaction among adsorbed ions according
50 40 t/qt (min g/mg)
where qe is the amount of 2-chlorophenol adsorbed at equilibrium (mg/L), qt is the amount of 2-chlorophenol adsorbed at time t (mg/g) and k2 is the rate constant of the pseudo-second order sorption (g/mg min). The initial sorption rate was also obtained as:
5
10
15
-1
20
25
30
100 mg/L 300 mg/L 400 mg/L
-1.5 -2
Fig. 5. Pseudo-first-order kinetics for adsorption of 2-chlorophenol onto Adenopus breviflorus.
10
15
20
25
30
Fig. 6. Pseudo-second-order kinetics for adsorption of 2-chlorophenol onto Adenopus breviflorus.
Table 1 Pseudo-first-order and pseudo-second-order rate constants, h and qe values for the. adsorption of 2-chlorophenol on Adenopus breviflorus. Pseudo-first-order kinetic model Co(mg/L)
qe(mg/g)
K1(1/min)
R2
h (mg g1 min1)
100 300 400
2.388 14.84 25.94
0.041 0.044 0.021
0.942 0.966 0.980
0.0575 2.3893 3.4243
Pseudo-second-order kinetic model Co(mg/L)
qe(mg/g)
K2(g/mg min)
R2
h (mg g1 min1)
100 300 400
11.36 5.81 12.82
0.0002 1.0566 0.1294
0.016 0.975 0.999
0.0280 35.7143 21.2766
to the equation [31]: 1 1 1 ¼ þ Q e Q o bQ o C e
ð9Þ
In Eq. (9) above, Qe is the amount adsorbed (mg/g). Ce is the equilibrium concentration of the 2-chlorophenol ion (mg/L), while Qo and b are maximum monolayer coverage capacity and Langmuir isotherm constants, respectively. From the expression above the essential characteristics of this model can be described in terms of an equilibrium parameter R, which is given as: R¼
1 1 þ bC o
ð10Þ
where b is Langmuir constant (L/mg) and Co is initial concentration (mg/L). The magnitude of b is largely estimated by the heat of biosorption. The higher the magnitude of b, the higher the heat of biosorption and the stronger the bond formed [19]; b was found to be 0.132 L/mg for the adsorption of 2-chlorophenol on A. breviflorus. The value of Qo (mg/g) is the monolayer capacity of the biosorbent for the 2-chlorophenol. A good fit of the Langmuir model to the experimental data indicates the homogeneity of the
8 7 6 5 4 3 2 1 0 0
Time (min)
5
Time (min)
Ce/qe (g/L)
log (qe - qt) (mg/g)
0
10 0
0.5
-0.5
100 mg/L 300 mg/L 400 mg/L
20
0
1
0
30
50
100 Ce (mg/L)
150
200
Fig. 7. Langmuir model plot for adsorption of 2-chlorophenol onto Adenopus breviflorus.
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active sites on the biosorbent surface which was reflected in the r2 (0.991) of the plot as shown in Fig. 7. The adsorption capacity of A. breviflorus seed adsorbent for 2-chlorophenol was obtained as 25.94 mg/g. It has been previously shown that when R is 0 < R < 1, then Langmuir isotherm is favored [32]; in the present study the calculated R value for the adsorbent was found to be 0.0498 which confirms the favorable uptake of 2-chlorophenol by A. breviflorus via Langmuir model. On the other hand, Freundlich model is an empirical equation built on adsorption on a heterogeneous surface. This is expressed as [33]: 1 log qe ¼ log K f þ log C e n
qe ¼
RT lnðAT C e Þ bT
ð12Þ
This can be expressed linearly as: qe ¼
RT RT ln K T þ ln C e b b
ð13Þ
In qe (mg/g)
bT is the Temkin isotherm constant, B is the constant related to sorption heat (J/mol), T is temperature, AT is the Temkin isotherm equilibrium binding constant in L/g and R is the idea gas constant which is 8.314 J/mol K. Derivation from Eqs. (12) and (13) above may be characterized by uniform distribution of binding energies by plotting qe against lnCe while the constants are obtained from the slop and intercept as shown in Fig. 9. As presented in Table 2,
5.2 5.15 5.1 5.05 5 4.95 4.9 4.85 2.42
2.44
2.46
Table 2 Isotherms constants for adsorption of 2-chlorophenol onto Adenopus breviflorus.
ð11Þ
where qe represents the amount of 2-chlorophenol adsorbed (mg/g), Ce is the equilibrium concentration of the 2-chlorophenol ion (mg/L), while Kf and n are the Freundlich constants indicating the adsorption capacity and the adsorption intensity; respectively. The plot of Inqe against InCe gave a straight line (Fig. 8) which was used to determine the values of Kf,n and R2 as presented in Table 2. In summary, isotherm plots yielded good results for both Langmuir and Freundlich models suggesting that the adsorption sites were non-uniform and nonspecific in nature [29]. Similar results have also been reported from the use of Amberlite XAD-16 resin [38] and modified chitosan [41] as shown in Table 3. It was also noticed that the R2 value of Langmuir was higher than that of Freundlich showing Langmuir monolayer capacity to be largely in conformity with its ability to take up more of the adsorbate. The adsorption capacity of A. breviflorus for 2-chlorophenol has been found better than a few adsorbent reported in Table 3 and it compared favorably with modified chitosan adsorbent [41]. The data obtained was further subjected to Temkin isotherm model to test for equilibrium description at room temperature. This isotherm assumes that the heat of adsorption decreases linearly with sorption coverage as a result of adsorbent–adsorbate interaction rather than being logarithmic as implied in the Freundlich model. The equation is expressed as [42]:
2.48
2.5 2.52 In Ce (mg/L)
2.54
2.56
2.58
2.6
Fig. 8. Freudlich model plot for adsorption of 2-chlorophenol onto Adenopus breviflorus.
669
Isotherm
Adenopus breviflorus
Langmuir Qo(mg/g) b (L/mg) R2
13.438 0.132 0.991
Freundlich Kf(mg/g (L/mg)1/n) n R2
0.978 0.562 0.969
Temkin AT (L/mg) B (J/mol) R2
0.011 24.456 0.999
the values of B and AT were calculated from the expression: B¼
RT bT
qe ¼ Bln AT þBln C e
ð14Þ
ð15Þ
B was estimated to be 24.456 J/mol while AT was 0.011 L/g. These values were used to predict the type of sorption taking place which indicated physisorption dominating chemisorption and any other sorption process. The R2 value of 0.999 showed Temkin isotherm as a better choice of explaining binding energy at the surface of the A. breviflorus seed. In the same line with the Langmuir isotherm, the Temkin isotherm further corroborate the fact that the physisorption process is nonspecific. This physisorption at the surface of A. breviflorus may have taken place as a result of the weak Van der Waals forces between the adsorbate and the adsorbent. 3.4. Diffusion processes The diffusion distribution of 2-chlorophenol on A. breviflorus may be considered in term of the total surface area of the A. breviflorus seed adsorbent which may be taken to be made up of both external and internal surfaces. Intraparticle diffusion is important in understanding this. It is usually estimated as a functional relationship which considers uptake to vary almost proportionally with the half-power of time (T0.5) rather than time (T) itself. The intraparticle diffusion model was estimated as described by Srivastava et al., [43]: qt ¼ K id T 0:5 þ C
ð16Þ
From Eq. (16), kid is the intraparticle diffusion rate constant (mg/g/min0.5), and C (mg/g) is a constant that describes the thickness of the boundary layer, i.e. the larger the value of C the greater the boundary layer effect [44]. So, a straight line plot of qt versus T0.5 is an indication that sorption process was controlled by intra-particle diffusion only; the slope of which gives the rate constant Kid. As shown in Fig. 10, a straight line was obtained from the plot which depicts macropore diffusion as rate limiting [45,46]. From the plot, it may be suggested that the biosorption process occurred by surface biosorption and intraparticle diffusion. The intraparticle diffusion rate constant (Kid) and constant (C) are given in Table 4. The intraparticle diffusion rate constant increased with increasing adsorbate concentration from 100 to 400 mg/L. The liquid film diffusion model was also employed to study the movement of the sorbate molecules from the liquid phase up to the solid phase boundary in the adsorption process. This can be
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Table 3 Comparison of adsorption of 2-chlorophenol by Adenopus breviflorus with other reported results. Material
qe (mg/g)
Isotherm
Adsorption order
Reference
Coir pith carbon Zeolite Activated carbon Mesoporous MCM-41 material Amberlite XAD-16 resin Activated carbon Rice-straw-based carbon Modified chitosan Adenopus breviflorusseed
<12 – 24.8 – 2.27 – 14.2 26.72 25.94
Freundlich Elovich Langmuir Freundlich Langmuir and Freundlich Langmuir Langmuir Langmuir and Freundlich Langmuir and Freundlich
Second Second – Second – – – Second Second
[34] [35] [36] [37] [38] [39] [40] [41] This study
perfect linear curves (Fig. 11) indicating that the diffusion of sorbate into the pores of the sorbent did not fit well for the liquid film diffusion model. Moreover, the significance of liquid film diffusion in rate determination of the sorption process showed that the intercept values were more than zero for 300 mg/L (2.021) and 400 mg/L (2.181) while in the case of 100 mg/L (1.406) it was less than zero and far from the origin.
26.5 26
qe (mg/g)
25.5 25 24.5 24
3.5. Effect of pH on adsorption
23.5 23 22.5
5.42
5.44
5.46
5.48
5.5 In Ce
5.52
5.54
5.56
5.58
Fig. 9. Tempkin model plot for adsorption of 2-chlorophenol onto Adenopus breviflorus.
30
qt (mg/g)
25 20 100 mg/L 300 mg/L 400 mg/L
15 10 5 0 0
2
4
6 T0.5(min0.5)
8
10
Fig. 10. Intraparticle diffusion kinetics of 2-chlorophenol on Adenopus breviflorus.
Table 4 Intraparticle rate parameters at different initial concentrations of 2-chlorophenol. Co(mg/L)
Kid(mg/g/min0.5)
C (mg/g)
100 300 400
0.304 0.305 0.396
0.000 12.440 22.540
The presence or ionic form of 2-chlorophenol in solution is pH dependent because it is a weak acid which has a pKa of 8.48 at 298 K. In this line, the effect of solution pH on adsorption of 2-chlorophenol was also determined. At pH greater than 8, the 2-chlorophenol dissociates to anion with the possibility of the surface of A. breviflorus also being negative which causes repulsion between the surface of the A. breviflorus and the adsorbate [25] as presented in Fig. 12. Usually, biomass used as adsorbent have certain organic functional groups at their surfaces which contribute to their ability to function in this capacity, such functional groups may include; carboxylic, ketone, alcohol, aldehydes, phenolic, ester and ether groups. The ability of these functional groups to play active role in adsorption has also been reported to depend on their solution pH [19,48]. The adsorption capacity of A. breviflorus was found to increase as the pH reduces. This observation may be attributed to the presence of positively charged sites on the surface of A. breviflorus at this low pH. Also, the magnitude of negative charge on the A. breviflorus may have reduced at low pH resulting in a reduction in the repulsion between the surface of the adsorbent and 2-chlorophenol which might have given rise to the increase in the amount of 2-chlorophenol adsorbed at the surface of A. breviflorus. 4. Conclusions The seed of A. breviflorus was analysed for its ability to adsorb 2-chlorophenol. The seed was pre-treated and characterized using FTIR, XRD, SEM, zeta potential and TG before being used for the
6 5
expressed as [47]:
where F is obtained from: F¼
qe qt
4
ð17Þ
ð18Þ
Kid is the adsorption rate constant. A linear plot of—In (1 F) versus T with zero intercept will suggest that the kinetics of the sorption process is controlled by diffusion through the liquid surrounding the solid sorbent [47]. The line plots did not yield
3 In (1-F)
lnð1 FÞ ¼ K id T
2
100 mg/L
1
300 mg/L 400 mg/L
0 -1 0
20
40
60
80
100
-2 -3
Time (min)
Fig. 11. Liquid film diffusion kinetics of 2-chlorophenol onto Adenopus breviflorus.
A. Adewuyi et al. / Journal of Environmental Chemical Engineering 4 (2016) 664–672
25 [15]
qe (mg/g)
20 15
[16]
10
[17]
5
[18]
0
[19]
0
2
4
6
pH
8
10
12
14 [20]
Fig. 12. Effect of pH on the adsorption of 2-chlorophenol on Adenopus breviflorus. [21]
adsorption process. The surface analysis of the seed revealed that the surface is heterogeneous and contained some functional groups. The isotherm plots yielded good results for Temkin, Langmuir and Freundlich models suggesting that the adsorption sites were non-uniform and nonspecific in nature. The results also indicated that the pseudo-second order equation provided a better correlation for the adsorption data when compared with that of the pseudo-first order. The results from the study presents the seed of A. breviflorus as a potential adsorbent that can be used for the removal of 2-chlorophenol from aqueous solutions.
[22]
[23] [24]
[25]
[26]
Acknowledgements [27]
The authors are grateful to International Foundation for Science for awarding a water research grant (No. W/5401-1). Authors are also grateful to Prof Fabiano Vargas Pereira for granting a research space at Department of Chemistry, Federal University of Minas Gerais, Brazil.
[28]
[29] [30]
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Contents lists available at ScienceDirect
Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece
Adsorption of 2-chlorophenol onto the surface of underutilized seed of Adenopus breviflorus: A potential means of treating waste water Adewale Adewuyia,b,* , Andrea Göpfertb , Omotayo Anuoluwapo Adewuyic , Thomas Wolffb a b c
Department of Chemical Sciences, Faculty of Natural Sciences, Redeemer’s University, Mowe, Ogun state, Nigeria Physical Chemistry, Technische Universität Dresden, D-01062 Dresden, Germany Enviro Africa Limited, plot 182b Kofo Abayomi Street, Victoria Island, Lagos, Nigeria
A R T I C L E I N F O
A B S T R A C T
Article history: Received 20 October 2015 Received in revised form 8 December 2015 Accepted 9 December 2015 Available online 12 December 2015
Seed of Adenopus breviflorus was prepared as an adsorbent and its adsorption capacities for 2-chlorophenol in aqueous solutions was studied. Fourier Infrared spectroscopy (FTIR), X-ray Diffraction analysis (XRD), particle size distribution, zeta potential, thermogravimetric (TG) analysis and Scanning Electron Microscopy (SEM) measurements were used to characterize the A. breviflorus seed adsorbent. The kinetics studies showed that the adsorption of 2-chlorophenol onto the adsorbent followed the pseudo-second order model while adsorption isotherm plots yielded good results for Temkin, Langmuir and Freundlich models. The adsorption capacity of 2-chlorophenol onto A. breviflorus seed increased with a decrease in pH of 2-chlorophenol solution. From the study, seed of A. breviflorus exhibited properties which suggested its potential application as adsorbent for the removal of 2-chlorophenol from polluted or waste water. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: Adenopus breviflorus Adsorption 2-Chlorophenol Isotherm SEM
1. Introduction Rapid industrialization and modern agricultural practice in Nigeria is generating environmental contaminants which are found in water resources. Some of these contaminants belong to the group of phenol and its derivatives which have been consistently considered as priority pollutants since they are toxic to plants, animals and humans even at low concentrations [1]. Water pollution is primarily associated with domestic and industrial waste. Both types of waste water pose threats to water quality which may be classified into health hazards and sanitary nuisances [2]. In most parts of Africa, people have no access to potable water and consequently, raw water from polluted rivers and streams form the major source of water. Water treatment is a serious issue in Africa and other developing nations of the world as large volumes of waste water are generated from time to time with no adequate means of treatment. Although industrialization is crucial, it has also brought several devastating environmental and human hazard over a period of years with search light on industries as the major contributor to environmental degradation and pollution in developing nations.
* Corresponding author. E-mail address: [email protected] (A. Adewuyi). http://dx.doi.org/10.1016/j.jece.2015.12.012 2213-3437/ ã 2015 Elsevier Ltd. All rights reserved.
Presently, there is increase in demand for food which has resulted in the use of large amounts of pesticides to control different agricultural pests [3]. 2-Chlorophenol is used in the manufacture of different compounds such as antiseptics, herbicides, dyes and other organic compounds [4]. Unfortunately, the presence of 2-chlorophenol and other chlorinated compounds has been detected in wastewaters; moreover, most chlorinated phenols have been found in municipal waste, agricultural run-off, leachates from polluted or contaminated site, soil, water, sediments, air, food products and body fluids [1,5–7]. Effective removal of phenol and its chlorinated derivatives from polluted and waste waters has been a problem. Chlorinated phenols are among the list of priority organic pollutants proposed by the US Environmental Protection Agency [8]. They are known to persist in many environments, because of inappropriate conditions for biodegradation [9]. Chlorophenols are carcinogenic and mutagenic; they are weak acids which are capable of penetrating human skin and are readily absorbed by gastro-intestinal tract with acute toxicity exhibiting symptoms like increased respiratory rate, vomiting and nausea [10]; thus it is important to get rid of them before they get into the environment in order to avoid the biomagnified toxicity to aquatic flora and fauna through various food chains [11]. Several methods have been used in waste water treatment some of these methods are based on ion exchange, chemical
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precipitation, oxidation, reduction and reverse osmosis [12]. However, many of these methods are less effective or difficult for practical use due to their toxicity, high price, sludge disposal problem, sustainability and selectivity. To minimize this problem, there is need to find an alternative approach to waste water treatment that will be low cost, effective, less toxic and efficient especially in developing countries like Nigeria. The quest for cheap and cost effective technology for removal of pollutants from wastewater containing organic pollutants has led to the use of materials of biological origin as adsorbent. Presently, the use of agricultural waste or biomass as adsorbent for waste or polluted water treatment is of much importance. Adsorption as a process is easy to control and design, cheap, reliable and may be easily start
665
up. Over the years, there has been a growing interest in the development of adsorption as a method for the treatment of contaminated or polluted wastewater using biomass [13–18] to remove pollutants from waste water. They have been recognized as potential adsorbents for the removal of pollutant ions from aqueous solution with the view to replacing existing technologies [19]. Some of these biomasses have low adsorption capacity for organic pollutants unlike in the case for inorganic pollutants. This has pointed attention in a search for better biomass with high adsorption capacity for organic pollutants. The objective of the present study is to investigate the feasibility of using seed of Adenopus breviflorus—an underutilized plant as an adsorbent for the removal of 2-chlorophenol from
Fig. 1. (A) FTIR of Adenopus breviflorus, (B) XRD of Adenopus breviflorus, (C) TG of Adenopus breviflorus, (D) particle size distribution (PSD) and (E) zeta potential of Adenopus breviflorus.
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wastewaters. A. breviflorus is a sub-tropical plant and an annual climber which produces fruit containing fibrous vascular system. Some tribes in Nigeria believe the fruit to have some supernatural healing powers. Presently, the seed of A. breviflorus has no specific use in Nigeria; it is usually discarded as waste and as such creating waste disposal problem. Our aim is to find applications for the seed of A. breviflorus and avoid it being discarded into the environment as waste. We have previously reported the fatty acid composition of the seed oil [20]. In this study, seed of A. breviflorus was used with little pretreatment as an adsorbent for the removal of 2-chlorophenol in aqueous system. Batch experiments were carried out for kinetic studies on the removal of 2-chlorophenol from aqueous solutions. The effect of various parameters such as pH, adsorbent weight, contact time, and initial 2-chlorophenol concentration were investigated.
Shimadzu) with filtered Cu Ka radiation operated at 40 kV and 40 mA. The XRD pattern was recorded from 10 to 80 C of 2u per second with a scanning speed of 2.0000 of 2u per minute; zeta potential analyser (DT1200, Dispersion technology) was used to obtained the surface potential and particle size distribution, the seed was made into powder form and the equipment was operated at 25 C; TG analysis was carried out on DTG-60 (Shimadzu, C30574600245) under nitrogen atmosphere with steady increase in temperature; FTIR spectra were recorded on Perkin Elmer, spectrum RXI 83303. The spectra were recorded in the range of 4000–400 cm1. The surface evaluation was conducted using SEM (Carl Zeiss, DSM, 982 Gemini, 984A-ISUS, Germany); the surfaces were coated with gold during sample preparation in order to increase electrical conductivity and to improve the quality of the micrographs.
2. Material and methods
2.3. Kinetics and equilibrium experiment
2.1. Materials
Biosorption kinetics and equilibrium experiments were carried out by agitating 50 mL of 2-chlorophenol solution of known initial concentrations ranging from 100 to 400 mg/L with 0.5 g of the prepared A. breviflorus seed adsorbents. Experiments were carried out at room temperature (298 K) with a constant agitation at 180 rpm. Samples were agitated at different time intervals, filtered, and then analyzed for 2-chlorophenol concentration using UV–vis spectrophotometer (Varian Australia PTY LTD. Model: EL07093760) at a wavelength of 273 nm.
The mature seeds of A. breviflorus were obtained from the garden of University of Ibadan, Ibadan, Oyo state, Nigeria. They were identified at the herbarium unit, Botany Department University of Ibadan. All solvents and chemicals used in the study were of analytical grade and were purchased from Merck and VWR International GmbH, Darmstadt, Germany. 2.2. Preparation of adsorbent
2.4. Effect of adsorbent dosage The seeds were crushed in a grinding machine. The ground seeds were successively extracted firstly with hexane [20,21] and secondly with methanol in a soxhlet extractor for 12 h. This was further soaked in slightly acidified aqueous solution (HCl, 0.005 M), washed continuously with deionized water, filtered and air dried. This was later characterized. The structural information was obtained using X-ray diffraction (XRD-7000X-Ray diffractometer,
The effect of adsorbent dosage used on the equilibrium uptake was estimated by agitating the 2-chlorophenol solution of initial concentration of 400 mg/L with different weighed amount of adsorbents ranging from 0.1 to 1.0 g. The agitation was allowed for 180 min after which the solution was filtered and taken for UV–vis analysis.
Fig. 2. (I) SEM result of the surface of Adenopus breviflorus before treatment, (II) SEM result of the surface of Adenopus breviflorus after treatment and (III) SEM result of the surface of Adenopus breviflorus after adsorption of 2-chlorophenol.
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2.5. Effect of pH on adsorption
667
30
3. Results and discussion
25 20 qt (mg/g)
The effect of pH on the adsorption of 2-chlorophenol was evaluated by agitating 0.5 g of adsorbent with 50 mL of 2-chlorophenol solution of initial concentration 400 mg/L at different solution pH range (2.0–12.0) for 180 min this was filtered and taken for UV–vis analysis.
15 100 mg/L
10
300 mg/L 400 mg/L
3.1. Preparation of adsorbent
5
The seed was extracted with hexane to remove non polar substances such as fats while methanol and slightly acidified water were meant to remove polar compound which may get into the water system during treatment. The FTIR result revealed the presence of some functional groups at the surface of the adsorbent as presented in Fig. 1A. These functional groups include ester carbonyl (1735 cm1), alcohol (3287 cm1) and alkane (2955 cm1—CH3 group and 2872 cm1—CH2 group). The XRD revealed the structural information of the adsorbent. As presented in Fig. 1B, the X-ray diffraction pattern of the A. breviflorus seed adsorbent is similar to those of semi-crystalline materials with an amorphous broad hump [22]. The characteristic decomposition pattern, degradation, organic and inorganic content of the adsorbent was evaluated using TG; Fig. 1C revealed a loss in mass at temperature below 190 C which was accounted for as being the removal of physisorbed water. The graph also showed a sharp loss in mass within the range 200–300 C which may be attributed to predominant decomposition of hemicelluloses and other volatile matters [23]; mass loss observed in the range 300–350 C may be due to the decomposition of cellulose and loss above 350 C may be considered as being the decomposition of lignin [24]. The particle distribution was found to be monomodal (Fig. 1D) with a mean particle size distribution (PSD) of 181.49 micron. The zeta potential increased as the pH value increases as shown in Fig. 1E. The SEM results for the surface of the seed before and after treatment are presented in Fig. 2I and Fig. 2II; respectively. After the treatment, it was obvious that the pores at the surface of the seed were opened up (Fig. 2II). When the seed was used as adsorbent for 2-chlorophenol, the opened pores were filled as shown in Fig. 2III indicating that the surface of the adsorbent was covered with the 2-chlorophenol which suggests that adsorption has taken place.
0
The amount of 2-chlorophenol adsorbed on the A. breviflorus seed adsorbent was calculated from the mass balance equation as given below:
40
60 Time (min)
80
100
Fig. 3. Adsorption of 2-chlorophenol on Adenopus breviflorus at various contact time and different initial concentrations.
higher initial concentration [25]. The percentage removal of 2-chlorophenol was also found to increase with increase in time as shown in Fig. 4 which is an indication that the removal of 2-chlorophenol by A. breviflorus seed adsorbent was time dependent. The data obtained were treated with pseudo-first-order model which can be expressed as [26]: dqt ¼ K 1 ðqe qt Þ dt
ð2Þ
on integrating Eq. (2) under the boundary conditions of t = 0 to t = t and qt = 0 to qt = qt, gives a linear expression: Inðqe qt Þ ¼ Inqe K 1 t
ð3Þ
The values of k1 can be obtained from the slope of the plot of log (qe qt) versus t . Also, the data were subjected to pseudo-second-order model which can be expressed as [27]: qt ¼ K 2 q2e t
ð4Þ
On differentiating, Eq. (4) becomes: dqt ¼ K 2 ðqe qt Þ2 dt
ð5Þ
On integrating Eq. (5) for the boundary conditions t = 0 to t = t and qt = 0 to qt = qt, gives: ð6Þ
This can be linearized to become: t 1 t ¼ qt K 2 q2e qe
ð7Þ
ð1Þ
where Co and Ce are the initial and equilibrium concentration of 2-chlorophenol solution (mg/L), respectively, qe is the equilibrium 2-chlorophenol concentration on adsorbent (mg/g), V is the volume (L) of the 2-chlorophenol solution and W is the mass (g) of the A. breviflorus seed adsorbent used. The kinetics and equilibrium study of the adsorption of 2-chlorophenol on A. breviflorus seed adsorbent was achieved using different concentration range at different contact time as shown in Fig. 3. An equilibrium adsorption time of 80 min was obtained after several experiments. The adsorption of 2-chlorophenol on A. breviflorus seed adsorbent was found to increase as the concentration of 2-chlorophenol increased over the period of time studied. This can be ascribed to the increase in driving force as a result of applying
Percentage removal (%)
v w
20
1 1 ¼ K2t qe qt qe
3.2. Kinetics and equilibrium experiment
qe ¼ ðC o C e Þ
0
70 60 50 40 30 20 10 0
400 mg/L 100 mg/L 300 mg/L
0
20
40
60
80
100
Contact time (min) Fig. 4. Effect of contact time on the percentage removal of 2-chlorophenol by Adenopus breviflorus.
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h ¼ K 2 q2e
ð8Þ
where K2 is the rate constant, qe is the adsorption capacity of the adsorbent (mg/g), h is the initial sorption rate and qt is the amount of 2-chlorophenol adsorbed at time t (mg/g). The plot for the pseudo-first-order and pseudo-second-order models are shown in Fig. 5 and Fig. 6; respectively. The correlation coefficients of the models were determined and presented in Table 1. In the pseudofirst order, the difference in the logarithm value between qe and qt is inversely proportional to time where as in pseudo-second-order model the value of t/qt increases with time. The correlation coefficient (r2) values for pseudo-second-order model are higher than those of pseudo-first-order model except for concentration at 100 mg/L as shown in Table 1, suggesting that the adsorption of 2-chlorophenol on A. breviflorus seed adsorbent may be via chemisorption. The r2 values increased with increase in concentration in both models. The results reveal pseudo-second-order adsorption mechanism to be predominant, and that the overall rate of the 2-chlorophenol adsorption process appeared to be controlled by the chemisorption process [28]. The values of the initial sorption calculated were found to increase as the concentration increased just as similar observation has been reported by Adebowale et al. [28]. The effect of A. breviflorus dose (0.1–1.0 g) on the biosorption of 2-chlorophenol onto the surface of A. breviflorus was studied by contacting different dose of A. breviflorus with 2-chlorophenol solution agitated at 180 rpm for 180 min and at temperature 298 K. It was found that the adsorption of 2-chlorophenol on A. breviflorus increased as the dose of A. breviflorus was increased from 0.1 to 1.0 g. This may have been due to the availability of more active site for the 2-chlorophenol to adsorb onto since this would have offered more site for the 2-chlorophenol to interact with. 3.3. Adsorption isotherm During adsorption process, the state of equilibrium is usually described with the help of isotherm equations with parameters which expresses the surface properties and affinity of the adsorbent [29]. These isotherms are usually generated using theoretical models and among these models Langmuir and Freundlich models are the most commonly used [30]. The Langmuir model was built on the assumption that uptake of adsorbate ions takes place on a homogenous surface via monolayer sorption without any interaction among adsorbed ions according
50 40 t/qt (min g/mg)
where qe is the amount of 2-chlorophenol adsorbed at equilibrium (mg/L), qt is the amount of 2-chlorophenol adsorbed at time t (mg/g) and k2 is the rate constant of the pseudo-second order sorption (g/mg min). The initial sorption rate was also obtained as:
5
10
15
-1
20
25
30
100 mg/L 300 mg/L 400 mg/L
-1.5 -2
Fig. 5. Pseudo-first-order kinetics for adsorption of 2-chlorophenol onto Adenopus breviflorus.
10
15
20
25
30
Fig. 6. Pseudo-second-order kinetics for adsorption of 2-chlorophenol onto Adenopus breviflorus.
Table 1 Pseudo-first-order and pseudo-second-order rate constants, h and qe values for the. adsorption of 2-chlorophenol on Adenopus breviflorus. Pseudo-first-order kinetic model Co(mg/L)
qe(mg/g)
K1(1/min)
R2
h (mg g1 min1)
100 300 400
2.388 14.84 25.94
0.041 0.044 0.021
0.942 0.966 0.980
0.0575 2.3893 3.4243
Pseudo-second-order kinetic model Co(mg/L)
qe(mg/g)
K2(g/mg min)
R2
h (mg g1 min1)
100 300 400
11.36 5.81 12.82
0.0002 1.0566 0.1294
0.016 0.975 0.999
0.0280 35.7143 21.2766
to the equation [31]: 1 1 1 ¼ þ Q e Q o bQ o C e
ð9Þ
In Eq. (9) above, Qe is the amount adsorbed (mg/g). Ce is the equilibrium concentration of the 2-chlorophenol ion (mg/L), while Qo and b are maximum monolayer coverage capacity and Langmuir isotherm constants, respectively. From the expression above the essential characteristics of this model can be described in terms of an equilibrium parameter R, which is given as: R¼
1 1 þ bC o
ð10Þ
where b is Langmuir constant (L/mg) and Co is initial concentration (mg/L). The magnitude of b is largely estimated by the heat of biosorption. The higher the magnitude of b, the higher the heat of biosorption and the stronger the bond formed [19]; b was found to be 0.132 L/mg for the adsorption of 2-chlorophenol on A. breviflorus. The value of Qo (mg/g) is the monolayer capacity of the biosorbent for the 2-chlorophenol. A good fit of the Langmuir model to the experimental data indicates the homogeneity of the
8 7 6 5 4 3 2 1 0 0
Time (min)
5
Time (min)
Ce/qe (g/L)
log (qe - qt) (mg/g)
0
10 0
0.5
-0.5
100 mg/L 300 mg/L 400 mg/L
20
0
1
0
30
50
100 Ce (mg/L)
150
200
Fig. 7. Langmuir model plot for adsorption of 2-chlorophenol onto Adenopus breviflorus.
A. Adewuyi et al. / Journal of Environmental Chemical Engineering 4 (2016) 664–672
active sites on the biosorbent surface which was reflected in the r2 (0.991) of the plot as shown in Fig. 7. The adsorption capacity of A. breviflorus seed adsorbent for 2-chlorophenol was obtained as 25.94 mg/g. It has been previously shown that when R is 0 < R < 1, then Langmuir isotherm is favored [32]; in the present study the calculated R value for the adsorbent was found to be 0.0498 which confirms the favorable uptake of 2-chlorophenol by A. breviflorus via Langmuir model. On the other hand, Freundlich model is an empirical equation built on adsorption on a heterogeneous surface. This is expressed as [33]: 1 log qe ¼ log K f þ log C e n
qe ¼
RT lnðAT C e Þ bT
ð12Þ
This can be expressed linearly as: qe ¼
RT RT ln K T þ ln C e b b
ð13Þ
In qe (mg/g)
bT is the Temkin isotherm constant, B is the constant related to sorption heat (J/mol), T is temperature, AT is the Temkin isotherm equilibrium binding constant in L/g and R is the idea gas constant which is 8.314 J/mol K. Derivation from Eqs. (12) and (13) above may be characterized by uniform distribution of binding energies by plotting qe against lnCe while the constants are obtained from the slop and intercept as shown in Fig. 9. As presented in Table 2,
5.2 5.15 5.1 5.05 5 4.95 4.9 4.85 2.42
2.44
2.46
Table 2 Isotherms constants for adsorption of 2-chlorophenol onto Adenopus breviflorus.
ð11Þ
where qe represents the amount of 2-chlorophenol adsorbed (mg/g), Ce is the equilibrium concentration of the 2-chlorophenol ion (mg/L), while Kf and n are the Freundlich constants indicating the adsorption capacity and the adsorption intensity; respectively. The plot of Inqe against InCe gave a straight line (Fig. 8) which was used to determine the values of Kf,n and R2 as presented in Table 2. In summary, isotherm plots yielded good results for both Langmuir and Freundlich models suggesting that the adsorption sites were non-uniform and nonspecific in nature [29]. Similar results have also been reported from the use of Amberlite XAD-16 resin [38] and modified chitosan [41] as shown in Table 3. It was also noticed that the R2 value of Langmuir was higher than that of Freundlich showing Langmuir monolayer capacity to be largely in conformity with its ability to take up more of the adsorbate. The adsorption capacity of A. breviflorus for 2-chlorophenol has been found better than a few adsorbent reported in Table 3 and it compared favorably with modified chitosan adsorbent [41]. The data obtained was further subjected to Temkin isotherm model to test for equilibrium description at room temperature. This isotherm assumes that the heat of adsorption decreases linearly with sorption coverage as a result of adsorbent–adsorbate interaction rather than being logarithmic as implied in the Freundlich model. The equation is expressed as [42]:
2.48
2.5 2.52 In Ce (mg/L)
2.54
2.56
2.58
2.6
Fig. 8. Freudlich model plot for adsorption of 2-chlorophenol onto Adenopus breviflorus.
669
Isotherm
Adenopus breviflorus
Langmuir Qo(mg/g) b (L/mg) R2
13.438 0.132 0.991
Freundlich Kf(mg/g (L/mg)1/n) n R2
0.978 0.562 0.969
Temkin AT (L/mg) B (J/mol) R2
0.011 24.456 0.999
the values of B and AT were calculated from the expression: B¼
RT bT
qe ¼ Bln AT þBln C e
ð14Þ
ð15Þ
B was estimated to be 24.456 J/mol while AT was 0.011 L/g. These values were used to predict the type of sorption taking place which indicated physisorption dominating chemisorption and any other sorption process. The R2 value of 0.999 showed Temkin isotherm as a better choice of explaining binding energy at the surface of the A. breviflorus seed. In the same line with the Langmuir isotherm, the Temkin isotherm further corroborate the fact that the physisorption process is nonspecific. This physisorption at the surface of A. breviflorus may have taken place as a result of the weak Van der Waals forces between the adsorbate and the adsorbent. 3.4. Diffusion processes The diffusion distribution of 2-chlorophenol on A. breviflorus may be considered in term of the total surface area of the A. breviflorus seed adsorbent which may be taken to be made up of both external and internal surfaces. Intraparticle diffusion is important in understanding this. It is usually estimated as a functional relationship which considers uptake to vary almost proportionally with the half-power of time (T0.5) rather than time (T) itself. The intraparticle diffusion model was estimated as described by Srivastava et al., [43]: qt ¼ K id T 0:5 þ C
ð16Þ
From Eq. (16), kid is the intraparticle diffusion rate constant (mg/g/min0.5), and C (mg/g) is a constant that describes the thickness of the boundary layer, i.e. the larger the value of C the greater the boundary layer effect [44]. So, a straight line plot of qt versus T0.5 is an indication that sorption process was controlled by intra-particle diffusion only; the slope of which gives the rate constant Kid. As shown in Fig. 10, a straight line was obtained from the plot which depicts macropore diffusion as rate limiting [45,46]. From the plot, it may be suggested that the biosorption process occurred by surface biosorption and intraparticle diffusion. The intraparticle diffusion rate constant (Kid) and constant (C) are given in Table 4. The intraparticle diffusion rate constant increased with increasing adsorbate concentration from 100 to 400 mg/L. The liquid film diffusion model was also employed to study the movement of the sorbate molecules from the liquid phase up to the solid phase boundary in the adsorption process. This can be
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Table 3 Comparison of adsorption of 2-chlorophenol by Adenopus breviflorus with other reported results. Material
qe (mg/g)
Isotherm
Adsorption order
Reference
Coir pith carbon Zeolite Activated carbon Mesoporous MCM-41 material Amberlite XAD-16 resin Activated carbon Rice-straw-based carbon Modified chitosan Adenopus breviflorusseed
<12 – 24.8 – 2.27 – 14.2 26.72 25.94
Freundlich Elovich Langmuir Freundlich Langmuir and Freundlich Langmuir Langmuir Langmuir and Freundlich Langmuir and Freundlich
Second Second – Second – – – Second Second
[34] [35] [36] [37] [38] [39] [40] [41] This study
perfect linear curves (Fig. 11) indicating that the diffusion of sorbate into the pores of the sorbent did not fit well for the liquid film diffusion model. Moreover, the significance of liquid film diffusion in rate determination of the sorption process showed that the intercept values were more than zero for 300 mg/L (2.021) and 400 mg/L (2.181) while in the case of 100 mg/L (1.406) it was less than zero and far from the origin.
26.5 26
qe (mg/g)
25.5 25 24.5 24
3.5. Effect of pH on adsorption
23.5 23 22.5
5.42
5.44
5.46
5.48
5.5 In Ce
5.52
5.54
5.56
5.58
Fig. 9. Tempkin model plot for adsorption of 2-chlorophenol onto Adenopus breviflorus.
30
qt (mg/g)
25 20 100 mg/L 300 mg/L 400 mg/L
15 10 5 0 0
2
4
6 T0.5(min0.5)
8
10
Fig. 10. Intraparticle diffusion kinetics of 2-chlorophenol on Adenopus breviflorus.
Table 4 Intraparticle rate parameters at different initial concentrations of 2-chlorophenol. Co(mg/L)
Kid(mg/g/min0.5)
C (mg/g)
100 300 400
0.304 0.305 0.396
0.000 12.440 22.540
The presence or ionic form of 2-chlorophenol in solution is pH dependent because it is a weak acid which has a pKa of 8.48 at 298 K. In this line, the effect of solution pH on adsorption of 2-chlorophenol was also determined. At pH greater than 8, the 2-chlorophenol dissociates to anion with the possibility of the surface of A. breviflorus also being negative which causes repulsion between the surface of the A. breviflorus and the adsorbate [25] as presented in Fig. 12. Usually, biomass used as adsorbent have certain organic functional groups at their surfaces which contribute to their ability to function in this capacity, such functional groups may include; carboxylic, ketone, alcohol, aldehydes, phenolic, ester and ether groups. The ability of these functional groups to play active role in adsorption has also been reported to depend on their solution pH [19,48]. The adsorption capacity of A. breviflorus was found to increase as the pH reduces. This observation may be attributed to the presence of positively charged sites on the surface of A. breviflorus at this low pH. Also, the magnitude of negative charge on the A. breviflorus may have reduced at low pH resulting in a reduction in the repulsion between the surface of the adsorbent and 2-chlorophenol which might have given rise to the increase in the amount of 2-chlorophenol adsorbed at the surface of A. breviflorus. 4. Conclusions The seed of A. breviflorus was analysed for its ability to adsorb 2-chlorophenol. The seed was pre-treated and characterized using FTIR, XRD, SEM, zeta potential and TG before being used for the
6 5
expressed as [47]:
where F is obtained from: F¼
qe qt
4
ð17Þ
ð18Þ
Kid is the adsorption rate constant. A linear plot of—In (1 F) versus T with zero intercept will suggest that the kinetics of the sorption process is controlled by diffusion through the liquid surrounding the solid sorbent [47]. The line plots did not yield
3 In (1-F)
lnð1 FÞ ¼ K id T
2
100 mg/L
1
300 mg/L 400 mg/L
0 -1 0
20
40
60
80
100
-2 -3
Time (min)
Fig. 11. Liquid film diffusion kinetics of 2-chlorophenol onto Adenopus breviflorus.
A. Adewuyi et al. / Journal of Environmental Chemical Engineering 4 (2016) 664–672
25 [15]
qe (mg/g)
20 15
[16]
10
[17]
5
[18]
0
[19]
0
2
4
6
pH
8
10
12
14 [20]
Fig. 12. Effect of pH on the adsorption of 2-chlorophenol on Adenopus breviflorus. [21]
adsorption process. The surface analysis of the seed revealed that the surface is heterogeneous and contained some functional groups. The isotherm plots yielded good results for Temkin, Langmuir and Freundlich models suggesting that the adsorption sites were non-uniform and nonspecific in nature. The results also indicated that the pseudo-second order equation provided a better correlation for the adsorption data when compared with that of the pseudo-first order. The results from the study presents the seed of A. breviflorus as a potential adsorbent that can be used for the removal of 2-chlorophenol from aqueous solutions.
[22]
[23] [24]
[25]
[26]
Acknowledgements [27]
The authors are grateful to International Foundation for Science for awarding a water research grant (No. W/5401-1). Authors are also grateful to Prof Fabiano Vargas Pereira for granting a research space at Department of Chemistry, Federal University of Minas Gerais, Brazil.
[28]
[29] [30]
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