Adsorption of Bisphenol-A by Eucalyptus bark/magnetite composite: Modeling the effect of some independent parameters by multiple linear regression

Abstract

In the present study, Eucalyptus camaldulensis bark/magnetite composite was used for potential application as a low-cost adsorbent for the removal Bisphenol-A. The effects of various independent parameters, contact time, initial Bisphenol-A concentration, temperature, pH, and Eucalyptus camaldulensis bark/magnetite composite dosage on adsorption were investigated. It was found that the adsorption capacity of Eucalyptus camaldulensis bark/magnetite composite increases with the increasing of Bisphenol-A concentration, temperature, and decreasing dosage of Eucalyptus camaldulensis bark/magnetite composite. The adsorption capacity was found to be 290.6 mg/g with 0.1 g Eucalyptus camaldulensis bark/magnetite composite at pH 7 and 50℃. The Freundlich isotherm model described the adsorption process better (R²= 0.998) than the Langmuir, Dubinin–Radushkevich, Jovanovic, and Vieth–Sladek isotherm models. According to multiple linear regression analysis, Eucalyptus camaldulensis bark/magnetite composite dosage is the most effective parameter on adsorption capacity at equilibrium and independent variables accounted for 79.4% of the total variability of equilibrium adsorption capacity of Eucalyptus camaldulensis bark/magnetite composite.

Keywords

magnetite Bisphenol-A adsorption multiple linear regression

Introduction

Bisphenol-A (BPA) is widely used in producing various organic chemical substances, epoxy resins, and polycarbonate plastics. Many studies have concluded that BPA is an endocrine disrupting chemical, and it is classified as a chemical that should be removed (Auriol et al., 2006; Chapin et al., 2008; Esplugas et al., 2007; Lister and Van Der Kraak, 2001; Roy et al., 2009). Studies have shown that BPA is associated with certain diseases such as cancer, diabetes, liver, kidney, and brain function disorders, sperm count reduction, early sexual maturation in females, obesity prevalence, and immunodeficiency (Chung et al., 2011; Gong et al., 2010; Kundakovich et al., 2013; Wetherill et al., 2002). Additionally, some researchers have reported that the effective removal of BPA is vital to maintaining public human health (Joseph et al., 2011).

BPA has been found in water, surface waters, ground waters, wastewaters, sediments, sewage sludges, landfill leachates, influents and effluents of wastewater treatment plants, garbage leakages, and even tap water (Bruchet et al., 2014; Focazio et al., 2008; Furhacker et al., 2004; Jardim et al., 2012; Nie et al., 2012; Seyhi et al., 2011, 2013; Shao et al., 2008; Urbatzka et al., 2007; Vethaak et al., 2005; Yamamoto et al., 2001), revealing the potential risk of public exposure to BPA. For example, the maximum concentrations of BPA were found to be 17.2 mg/L in waste landfill leachates (Yamamoto et al., 2001).

Adsorption is a widely used and effective physical method for the treatment of colored wastewater. Adsorption systems have gained prominence as treatment processes that ensure good quality effluents that are low in concentrations of dissolved organic compounds (Walker and Weatherley, 1997). It is important to develop technologies available for water treatment. Generally, these technologies should be cost-effective, highly efficient, and easy to handle (Khalid et al., 2015). The magnetic particle technology has received considerable attention in recent years to solve environmental problems (Luiz et al., 2004). The magnetite (Fe₃O₄) nanoparticles can be used as a surface coating material in adsorption processes for enhancing the surface properties of adsorbents. These nano-adsorbents have attracted substantial interest in adsorption studies because of their high surface area and highly active surface sites (Nethaji et al., 2013). Nano adsorbents can be easily collected from liquid phase by magnetic field (Khalid and Asem, 2014).

The main objective of this study is to investigate the potential use of Eucalyptus camaldulensis bark/magnetite composite (EBMC) as a novel low-cost adsorbent for removal of BPA and to evaluate efficiency of multiple linear regression (MLR) for modeling adsorption behavior.

The Eucalyptus camalduensis barks were selected as a low-cost adsorbent because of their renewable character, wide availability, and easy collection.

Material and methods

Preparation of the EMBC

In the south region of Turkey, E. camaldulensis is a common species. It is native to Australia. Large evergreen tree E. camaldulensis has 24–40 m high and its stout trunk is often short and crooked, to 2 m in diameter and crown open, widely spreading irregular. Their barks are smooth, white, gray, or buff (Little, 1983). Since the E. camaldulensis barks are free of cost from any processing industries, only the transport cost is involved for the tertiary wastewater treatment. The barks of E. camaldulensis were collected from Balcali campus of the University of Cukurova (Adana, Turkey). Upon collection, barks were crushed and sieved to 0.125 mm particle size and washed with distilled water to remove impurities.

The EBMC was prepared by co-precipitation technique. A suspension of 5 g of EBMC in a 2000 mL of solution containing 5.82 g FeCl₃·6H₂O and 3 g FeSO₄.7H₂O stirred at 70℃ for 1.5 h under N₂ atmosphere. Then a NaOH solution (5 mol/L) was added dropwise to the precipitate iron oxide. The mixtures were aged at 70℃ for 3 h, then washed with distilled water and dried at 60℃ for 24 h.

BPA

Preparation of a stock solution of BPA was achieved using distilled water. The structure of BPA is shown in Figure 1. Physical properties and toxicity of BPA are shown in Table 1. In all, 1000 mg/L stock solution of BPA was prepared in 5% methyl alcohol and 95% distilled water mixture. BPA solutions with desired concentrations were prepared by dilutions using distilled water.

Figure 1.

Chemical structure of BPA.

Table 1.

Properties and toxicity of the BPA (Chen et al., 2002; Ocampo-Pérez et al., 2012; Staples et al., 1998).

Property	Value
Molecular weight (g/mol)	228.29
Molecular size (x,y,z)	1.27 × 0.86 × 0.68
V_A^a (cm³/mol)	294
Solubility in water (g/L)	0.09
Specific gravity (g/cm³)	1.09–1.19
pK_a	9.6
Log K_ow	3.4
48-h EC₅₀ (D. manga) (mg/L)	10

Liquid molar volume at the normal boiling point.

Analyses of BPA

The BPA concentrations in solutions were determined by a high pressure liquid chromatography (HPLC) vessel (Perkin Elmer). Extraction of BPA was performed by OASIS HLB cartridges, first cartridge was conditioned with 3 mL of methanol and 3 mL of water. Then 3 mL sample was loaded to the OASIS HLB cartridge with 1 mL/min flow rate. The cartridge was washed with 3 mL of 5% methanol for removal of weak polar impurities. At the last stage, BPA was eluted by methanol with desired volume. Determination of the BPA concentrations was achieved using HPLC/UV detection device at 278 nm. Hundred percent methanol was used as mobile phase. Zorbax Eclipse XDB-C18 column (4.6 × 150 mm²) 5 µm was used. Injection volume and flow rate were 10 µL and 1.0 mL/min, respectively (Chen et al., 2008).

Statistical methods

MLR analysis is generally used in environmental studies to determine the cumulative effects of several independent variables on the dependent variable.

Mathematical expression of MLR can be represented as:

Y = a 0 + a i X i + a 2 X 2 \cdot \cdot \cdot + a n X n + ɛ

(1)

where Y is the dependent (predicted) variable, a_i (i = 0,…,n) are the regression coefficients, X_i (i = 1,…,n) are the independent variables (predictors) and ɛ shows stochastic error of the regression (Utkan et al., 2011). In this study, MLR analysis was performed to examine the cumulative effects of independent variables such as contact time, pH, temperature, BPA concentration, and adsorbent dosage on the equilibrium adsorption capacity of EBMC as a depended variable. SPSS statistics 20.0 was used for MLR analyses.

Adsorption experiments

The batch experiments were performed for the adsorption of BPA by EBMC. In all, 500 mL erlenmeyer flasks were used, each containing 200 mL BPA solution with different concentrations and with the desired weight of EBMC. The flasks were stirred at 250 rpm in a temperature–controlled orbital shaker. The effect of contact time (ranging from 5 to 70 min), pH (ranging from 1 to 12), temperature (ranging from 5 to 55℃), initial BPA concentration (ranging from 50 to 200 mg/L), and EBMC dosage (ranging from 0.1 to 0.7 g) on adsorption were studied. The samples were withdrawn at certain time intervals during the adsorption test, centrifuged for 5 min at 4000 rpm, and supernatant was used to determine the residual BPA concentration. Controlled experiments were performed without EBMC under the same conditions of adsorption experiments. BPA removal was not detected in control experiments. All experiments were carried out triplicate and average value was used for calculations.

The amount of BPA adsorbed onto EBMC at different times was calculated from equation (2):

q = \frac{(C 0 - C e) V}{W}

(2)

and BPA removal efficiency was calculated as;

BPAremovalefficiency (%) = \frac{C t - C 0}{C 0} \times 100

(3)

where q is the adsorption capacity (mg/g), C₀ and C_e are the initial and equilibrium concentrations of the BPA in the aqueous phase (mg/L), respectively, V is the volume of the aqueous phase (L), and W is the amount of EBMC (g).

Results and discussion

Effect of contact time

The correct representation of the dynamic adsorptive separation of the BPA from the aquatic phase onto EBMC depends on an accurate understanding of the equilibrium separation between the BPA and the EBMC (Allen et al., 2003). The equilibrium time gives the optimum time for the removal of the BPA from the aqueous solutions. The effect of contact time was performed with 0.1 g EBMC and 200 mg/L BPA at pH 7 and 20℃, the effect of contact time on the adsorption capacity of EBMC for BPA is shown in Figure 2. Forty minutes is necessary to reach equilibrium for the BPA adsorption onto the EBMC. The adsorption capacity was calculated 142.4 mg/g at equilibrium time. Forty minutes was used for equilibrium time for further experiments. However, the equilibrium time was controlled under varying parameters such as pH, temperature, the BPA concentration, and EBMC dosage.

Figure 2.

Equilibrium for BPA (EBMC dosage = 0.1 g, C₀ = 200 mg/L, pH = 7, T = 20℃).

Effect of pH

In this work, the effect of pH on the adsorption of the BPA onto the EBMC was performed while the initial BPA concentration, EBMC dosage, and temperature were fixed at 200 mg/L, 0.1 g, and 20℃, respectively. The pH value of the solution is one of the most important parameters that can alter the adsorption process. The pH value of the solution could change the charge density of the adsorbent surface, and the concentration of dissolved ions in the solution would affect the adsorption capacity of the adsorbent (Behzad et al., 2015). The effect of the pH on the adsorption of the BPA onto the EBMC is given in Figure 3. It was found that the pH has an important effect on the adsorption of BPA onto EBMC. The highest adsorption capacity of the EBMC was obtained at pH 7 (142.4 mg/g). It was determined that at acidic and basic pH values, the adsorption capacity significantly decreased (14.1 mg/L at pH 1, 31.2 mg/g at pH 12). Similar results have been reported in literature on the adsorption of BPA onto the ionic resins (Víctor-Ortega et al., 2016). At acidic and basic pH values, H⁺ and OH⁻ may occupy the adsorption sites of EBMC. The low adsorption capacities at low and high pH values may occur due to the competitive adsorption between H⁺, OH⁻ ions, and BPA molecules (Senthilkumaar et al., 2005). It was determined that 40 min is necessary to reach to equilibrium time for all pH values.

Figure 3.

The effect of pH on adsorption of BPA onto EBMC (EBMC dosage = 0.1 g, C₀ = 200 mg/L, T = 20℃).

Effect of temperature

The effect of temperature (ranging from 5 to 55℃) on the adsorption of the BPA onto the EBMC was performed while the initial BPA concentration, EBMC dosage, and pH were fixed at 200 mg/L, 0.1 g, and 7, respectively (Figure 4). Temperature is an important factor that indicates whether the adsorption process is exothermic or endothermic (Ghaedi et al., 2011). The adsorption capacity of the EBMC significantly increased with the increasing temperature, which indicates that the adsorption of the BPA onto the EBMC was an endothermic process. The adsorption capacities within 40 min for 5 and 50℃ were found to be 16.6 and 290.6 mg/g, respectively. The adsorption capacities for 55℃ were found to be 300.6 mg/g. There were no significant differences between adsorption capacities for 50 and 55℃. Therefore, 50℃ was selected as optimum temperature for adsorption of BPA by EBMC. Higher adsorption capacities at higher temperatures may be due to the increasing mobility of BPA molecules. Also, the number of active sites of EBMC may increase with increasing temperature (Almeida et al., 2009; Sara and Tushar, 2012; Senthilkumaar et al., 2005; Víctor-Ortega et al., 2016).

Figure 4.

Effect of temperature on the adsorption of BPA onto EBMC (EBMC dosage = 0.1 g, C₀ = 200 mg/L, pH = 7).

Thermodynamic parameters

The thermodynamic parameters such as change in standard free energy (ΔG°), enthalpy (ΔH°), and entropy (ΔS°) were estimated using the following equations:

Δ G^{\circ} = Δ H^{\circ} - T Δ S^{\circ}

(4)

\log (\frac{q e}{C e}) = \frac{Δ H^{\circ}}{2.303 R} + \frac{- Δ H^{\circ}}{2.303 RT}

(5)

where q_e is the amount of BPA adsorbed per unit mass of EBMC (mg/g) at equilibrium, C_e is equilibrium concentration (mg/L), T is temperature in K, and R is the gas constant (8.314 J/mol K) (Arias and Sen, 2009).

The thermodynamic parameters were calculated to comprehend the feasibility and nature of the adsorption of BPA onto EBMC (figure not shown). The results are presented in Table 2.

Table 2.

Thermodynamic parameters of BPA adsorption onto the EBMC at different temperatures (C₀ = 200 mg/L, EBMC dosage = 0.1 g, pH = 7).

Temperature (℃)	ΔG° (kJ/mol)	ΔH° (kJ/mol)	ΔS° (kJ/mol)
5	3599.646	55,833.06	187.89
10	2660.196
15	1720.746
20	781.296
25	−158.154
30	−1097.6
35	−2037.05
40	−2976.5
45	−3915.95
50	−4855.4
55	−5794.85

The Gibbs free energy represents the level of spontaneity of the adsorption process. High negative Gibbs free energy values indicate a more energetically favorable adsorption (Upendra, 2011). Positive ΔG° values were determined at 5 to 20℃, indicating that spontaneity is not favored at these temperatures. Generally, positive ΔG° values indicate that the adsorption reactions need some external energy source to convert reactants into products (Papita and Shamik). The negative values of ΔG° at 25 to 55℃ confirmed the thermodynamic feasibility and spontaneity of the BPA adsorption onto the EBMC at these temperatures. Therefore, over 25℃ solution temperature is required for the effectively removal of BPA by EBMC. The positive values of ΔH° verify the endothermic nature of BPA adsorption onto the EBMC. The adsorption capacity is increasing with the increasing of temperature. Therefore, a temperature level over 30℃ is required for the removal of BPA from aqueous solution with high removal efficiencies. The positive value of ΔS° indicates that the degree of randomness at the solid–liquid interface increased during BPA adsorption onto the EBMC (Sara and Tushar, 2012).

Effect of BPA concentration

The effect of the initial BPA concentration (ranging from 50 to 200 mg/L) on the adsorption of the BPA onto the EBMC was performed while the temperature, the EBMC dosage, and the pH were fixed at 50 mg/L, 0.1 g, and 7, respectively. It was determined that the adsorption capacity of the EBMC significantly increased with the increasing initial concentration, and the BPA removal efficiency increased with the decreasing initial BPA concentration. The effect of initial BPA concentration onto the adsorption capacity and removal efficiency is given in Figure 5. When the initial BPA concentration increased from 50 to 200 mg/L, the adsorption capacity of the EBMC increased from 85.2 to 290.6 mg/g, while the removal percentage of the BPA decreased from 85.2% to 72.6%.

Figure 5.

Effect of initial BPA concentration on the adsorption capacity of EBMC and BPA removal efficiency (EBMC dosage = 0.1 g, T = 50℃, pH = 7).

Effect of the EBMC dosage

The investigation of the effects of adsorbent dosage on adsorption experiments is an important stage for determination of the adsorption capacity for the studied BPA concentrations. The effect of the EBMC dosage (ranging from 0.1 to 0.7 g) on the adsorption of the BPA onto the EBMC was performed while the initial BPA concentration, temperature, and pH were fixed at 200 mg/L, 50℃, and 7, respectively. It was found from Figure 6 that the adsorption capacity of the EBMC significantly increased from 57.08 to 290.6 mg/g with the decrease of the adsorbent dosage from 0.7 to 0.1 g. The increase in the adsorption capacity with the decreasing EBMC dosage is due to the concentration gradient between the BPA concentration in the solution and in the surface of EBMC (Sara and Tushar, 2012). It was also found that the percentage of the BPA removal at equilibrium increased from 72.6% to 99.8% with the increase of the adsorbent mass from 0.1 to 0.7 g. This may be explained by the increase in availability of surface active sites with increasing doses of EBMC (Kannan and Sundaram, 2001). It was determined that 0.2 g EBMC was sufficient for removal of 200 mg/L BPA with a ∼90% efficiency.

Figure 6.

Effect of initial EBMC dosage on the adsorption capacity of EBMC and BPA removal efficiency (C₀ = 200 mg/L, T = 50℃, pH = 7).

Equilibrium isotherms

The adsorption isotherms indicate the distribution of the adsorption molecules between the liquid phase and the solid phase at the equilibrium point. The analysis for fitting the isotherm to the adsorption isotherm models is a major stage for finding the best model to be used for the design of the adsorption systems (Kumar et al., 2010). The isotherm parameters were calculated according to the non-linear regression method due to the inherent bias resulting from the linearization of the adsorption equations. Non-linear regression ensures a mathematically sensitive method for calculating the parameters of the isotherms by using the original form of the isotherm equation (Chan et al., 2012). The minimization procedure is performed to solve the adsorption isotherm equations by maximizing the correlation coefficient between the experimental data points and the theoretical model predictions with the solver add-in function of Microsoft Excel (Wong et al., 2004). The criteria for the selection of the best isotherm model are essentially based on the correlation coefficient and the average percentage errors (APE). The correlation coefficient shows the fit between the experimental data and isotherm model, while the APE indicates the fit between the experimental data and the calculated data used for plotting the isotherm curves (Subramanyam and Ashutosh, 2012). The equation of APE can be written as:

APE (%) = \frac{\sum_{i = 1}^{N} | ((q e) \exp - (q e) cal) / (q e) \exp |}{N} \times 100

(6)

where q_e is the amount of adsorbate adsorbed per unit mass of adsorbent (mg/g) at equilibrium, and N is the number of experimental data.

Langmuir isotherm

This model assumes monolayer adsorption on to the homogenous adsorbent surface. According to the Langmuir isotherm, the adsorbent reaches to the maximum adsorption capacity when a saturated monolayer occurred on the adsorbent surface (Khalid and Eric, 2015). When the adsorption reaches to the equilibrium, no further adsorption can occur due to the finite adsorption sites of the sorbent. The Langmuir isotherm can be represented as:

q e = \frac{q max K L C e}{1 + K L C e}

(7)

where q_e is the amount of adsorbate adsorbed per unit mass of adsorbent (mg/g) at equilibrium, C_e is the equilibrium concentration of the adsorbate (mg/L), q_max is the maximum adsorption capacity (mg/g), and K_L is the Langmuir constant related to the adsorption rate (L/mg) (Langmuir, 1918).

Freundlich isotherm

Freundlich improved an empirical equation applied to define the heterogeneous adsorption processes (Freundlich, 1906). The Freundlich model can be successfully applied to the multilayer adsorption processes especially for organic molecules on the molecular sieves (Foo and Hameed, 2010). The Freundlich isotherm equation can be represented as:

q e = K F C_{e}^{1 / n}

(8)

where K_F is the Freundlich isotherm constant (L/mg). 1/n represents the intensity of the surface heterogeneity and ranges between 0 and 1. A value of 1/n closer to zero indicates the intensity of the surface heterogeneity (Weber and Chakravorti, 1974).

Dubinin–Radushkevich (D-R) isotherm

D-R assumes the adsorbed possesses a multilayer character (Dabrowski, 2001). The model can be successfully applied to the high solute activities adsorption systems (Foo and Hameed, 2010). The equation of the D-R isotherm is given by the following equation:

q e = q max \exp (- B D ɛ^{2})

(9)

ɛ = RT \ln (1 + \frac{1}{C e})

(10)

where B_D (mol²k/J²) is D-R constant, ɛ is the Polanyi constant, R is the gas constant (8.31 J/mol K), and T is the absolute temperature (Bennani et al., 2009).

Jovanovic isotherm

The Jovanovic model was derived for an adsorption on a homogeneous solid surface, considering the phenomenon non-specific, without lateral interactions, and covering the surface with a monolayer of the solute. The equation of the Jovanovic isotherm is given as:

q e = q max (1 - e^{(K j C e)})

(11)

where q_max is the maximum adsorption capacity (mg/g) and K_j is the model constant (Rafael et al., 2013).

Vieth–Sladek isotherm

Vieth and Sladek developed a three parameters adsorption isotherm equation. The equation is a combination of a linear component and a non-linear one of the Langmuir type. The linear part corresponds to gas dissolved in the amorphous sites of a glassy polymer and the nonlinear part corresponds to gas trapped in microvoids of polymer (Crank, 1975).

q e = K VS C e + \frac{q max β VS C e}{1 + β VS C e}

(12)

Estimated isotherm parameters are given in Table 3. In all, 548.48 mg/g maximum adsorption capacity of EBMC was obtained from the Langmuir isotherm with a moderate correlation coefficient (0.988) and a 2.385% APE. Also the Jovanovic isotherm predicted 365.14 mg/g adsorption capacity with a moderate correlation (0.985) and 3.194% APE. In all, 349.65 mg/g adsorption capacity of EBMC was calculated from the D-R isotherm. Among the tested isotherms, the lowest correlation coefficient (0.925) was obtained from the D-R isotherm. Therefore, an inadequate fitting of the experimental results of the adsorption isotherms is obtained using the D-R. Among the tested adsorption isotherms, which predicted the maximum adsorption capacity, best representation of the experimental results is obtained using the Vieth–Sladek model. The Vieth–Sladek isotherm predicted 343.44 maximum adsorption capacity with 0.992 correlation coefficient and 1.933% APE. Therefore, 343.44 mg/g maximum adsorption capacity was selected from the Vieth–Sladek isotherm. According to Table 3, the coefficient of correlation of the Freundlich model is perfect (0.998) and the mean value of the APE was calculated at 0.477%. The value of 1/n (0.639) indicated favorable adsorption and intensity of the surface heterogeneity. The highest correlation coefficient and the lowest APE were obtained from the Freundlich isotherm. The Freundlich isotherm described the data very well. Therefore, the Freundlich isotherm was found to be most suitable isotherm for the adsorption of BPA onto EBMC (Bennani et al., 2009; Crank, 1975; Dabrowski, 2001; Khalid et al., 2012; Rafael et al., 2013; Weber and Chakravorti, 1974).

Table 3.

Model parameters estimated for the adsorption isotherms BPA by EBMC (pH = 7, T = 50℃, EBMC dosage = 0.1 g).

Isotherms	Parameters		APE, %
Langmuir	q _max	548.48	2.385
	K_L	10.86
	R²	0.988
Freundlich	1/n	0.639	0.477
	K_F	22.36
	R²	0.998
D-R	q _max	349.65	3.503
	B_D	0.0054
	R²	0.925
Jovanovic	q _max	365.14	3.194
	K_J	−0.0276
	R²	0.985
Vieth–Sladek	q _max	343.44	1.933
	K_VS	1.331
	β_VS	0.029
	R²	0.992

There are limited studies on the removal of BPA from aqueous solutions by low-cost adsorbents. Comparison of the adsorption capacities of different low-cost adsorbents for BPA under optimum experimental conditions is given in Table 4. It shows that EBMC has a large adsorption capacity compared with many other low-cost adsorbents. Adsorption capacity of activated carbon from Potato peel ranges between 133 and 444 mg/g. However, these adsorption capacities were obtained from carbonized peels under high cost methods such as different chemical and thermal conditions.

Table 4.

Adsorption capacities of various alternative adsorbents for BPA.

Adsorbent	Adsorbent capacity, mg/g	References
Bamboo	58	Goodson et al. (2004)
Almond shell	188.9	Bautista-Toled et al. (2005)
Commercial activated carbon	129.6	Bautista-Toled et al. (2005)
Hydrophobic zeolite	111.1	Wen et al. (2006)
Carbon nanotube	61	Kuo (2009)
Phenyl-functionalized mesoporous silica	351	Yong et al. (2011)
Activated rice straw	181.19	Chang et al. (2012)
Potato peel based activated carbon	133–444	Anastasia et al. (2016)
Coir pith	4.308	Zainab et al. (2015)
Coconut shell	4.159	Zainab et al. (2015)
Durian peel	4.178	Zainab et al. (2015)
Banana bunches	4.532	Zainab et al. (2015)
Coconut bunches	4.662	Zainab et al. (2015)
EBMC	343.44	This study

MLR analyses

MLR analysis was performed to model the effects of experimental conditions such as contact time, pH, temperature, initial BPA concentration, and EBMC dosage on the q_e values. Coefficients of MLR model are given in Table 5. The model obtained from MLR analysis is given below:

q e pred = - 288.665 + 1.871 X 1 + 3.681 X 2 + 5.775 X 3 + 1.093 X 4 - 446.189 X 5

where X₁ is the contact time, X₂ is pH value, X₃ is temperature, X₄ is BPA concentration, and X₅ is dosage of EBMC.

Table 5.

Coefficients of MLR model.

Model	Unstandardized coefficients	Standardized coefficients		t	Sig.
Model	B	Std. error	Beta	t	Sig.
Constant	−288.665	33.450		−8.630	.000
Contact time	1.871	.446	.192	4.199	.000
pH	3.681	1.757	.093	2.095	.039
Temperature	5.775	.323	1.106	17.907	.000
Initial BPA conc.	1.093	.119	.491	9.183	.000
EBMC dosage	−446.189	29.915	–.833	−14.915	.000

It can be seen from Table 5. EBMC dosage is the most effective parameter on adsorption capacity at equilibrium due to the highest coefficient (−446.189). The negative value showed the negative relationship between adsorption capacity and EBMC dosage. It was mentioned above the increase in adsorption capacity with decreasing EBMC dosage is due to the concentration gradient between BPA concentration in the solution and in the surface of EBMC (Sara and Tushar, 2012). Experimental q_e and MLR predicted q_e is given in Figure 7. Correlation coefficients for experimental q_e and MLR predicted q_e were calculated as 0.794. According to the MLR, independent variables (contact time, pH, temperature, and EBMC dosage) accounted for 79.4% of the total variability of q_e.

Figure 7.

Experimental q_e and MLR predicted q_e.

Cost analysis of EBMC

The literature rarely reports the cost analysis of the tested adsorbents, it is important to evaluate the potential use of the adsorbents in the industrial processes. In this study, Eucalyptus camaldulensis barks were selected as a novel low-cost adsorbent because of their renewable character and wide availability. Eucalyptus is known worldwide as a source of fiber for the pulp and paper industry (Marília et al., 2014). Moreover, the abundant amounts of barks are produced during the debarking process. A simple, cost estimation was performed for the production and use of EBMC. Approximately $200 was calculated for the production of one ton of the EMBC after considering the cost of transport, chemicals, and electrical energy. The price of the commercially activated carbon, ranges from $700 to $5000/ton depends on the quality (Saygili et al., 2015). Thus, the price of the EMBC is significantly cheaper than the commercially activated carbon.

Conclusion

The results of the present study show that EBMC can be used as an alternative low-cost adsorbent for the effective removal of BPA. The adsorption capacity of EBMC was found to increase with increasing initial BPA concentrations and temperature, and the adsorption capacity was found to increase with decreasing dosage of EBMC. Freundlich isotherm model could fit the experimental data perfectly (R²= 0.999). Vieth–Sladek predicted 343.44 maximum adsorption capacity with 0.992 correlation coefficient and 1.933% APE. The thermodynamic analysis showed that the adsorption of BPA onto EBMC was an endothermic process. MLR analysis showed that dosage of EBMC is the most effective parameter on adsorption capacity at equilibrium and independent variables accounted for 79.4% of the total variability of equilibrium adsorption capacity of EBMC. It was determined that only 0.2 g EBMC was sufficient for removal of 200 mg/L BPA with a ∼90% efficiency from 200 mL solution. The results of this study showed that EBMC is a potential low-cost alternative adsorbent for the removal BPA by adsorption process.

Footnotes

Acknowledgements

The authors would like to thank Cukurova University for their support.

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

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