Ethylenediaminetetraacetic Dianhydride (EDTAD) Modified Coconut Frond for Removal of Pb(II) Ions: Kinetics, Isotherm and Thermodynamic

Ethylenediaminetetraacetic dianhydride modified coconut frond (ECFP) was prepared, characterized and applied as a potential adsorbent to remove Pb(II) ions from aqueous solutions. Factors influencing adsorption such as pH of the solution, adsorbent dosage, Pb(II) concentration, contact time, and temperature were investigated. The optimum conditions for adsorption of Pb(II) ions were at pH 4 and dosage of 0.02 g. Adsorption reached its equilibrium state in 30 min for all Pb(II) concentrations. Chemisorption was suggested as the rate-limiting step as the adsorption process correlated well with the pseudo-second-order kinetics model. Based on adsorption isotherm results, Langmuir model fitted the experimental data well, and the maximum adsorption capacity was 84.03 mg.g-1 at 300 K. Based on the thermodynamic study, Pb(II) adsorption occurred spontaneously with the enthalpy and entropy changes recorded were 0.0615 kJ mol-1 and 241.28 J K-1 mol-1, respectively. It was found that the nature of adsorption was endothermic as the ∆Ho value obtained was positive.


INTRODUCTION
The release of non-bioessential heavy metals in the aquatic system has resulted in a serious environmental pollution issue. Lead (Pb) is listed as one of the harmful pollutants by World Health Organization due to its high propensity for biological accumulation, non-biodegradability and very toxic even presents at a low concentration (Bai et al. 2019, Cao et al. 2019. At Pb(II) ions concentration above 0.05 mg.L -1 in drinking water, one might experience acute lead poisoning symptoms such as anaemia, hepatitis, nephritic syndrome, and encephalopathy. Excess exposure to Pb(II) ions might damage central nervous and gastrointestinal systems, liver and kidneys. For this reason, the United State Environmental Protection Agency (USEPA) had set the maximum standard concentration of Pb(II) ions in drinking water of 300 µg.L -1 (Hu & Qiu 2019). Due to these reasons, various methods such as ion exchange, chemical precipitation, membrane filtration, and adsorption have been used to remove Pb(II) ions from wastewater (Cao et al. 2019).
In Malaysia, coconut (Cocos nucifera) is the third most important industrial crop after oil palm and rubber. In 2001, about 151 000 ha of land in Malaysia were used for coconut plantation and the value reduced gradually due to competition with oil palm plantation. As a result, the coconut industry revitalizing plan was carried out for replanting coconut trees between 2008 and 2015. This consequently raised another environmental issue, which was related to the disposal of coconut waste, as large quantities of coconut fronds were produced during pruning and silvicultural practices (Njoku et al. 2014, Shafie et al. 2012. Ethylenediaminetetraacetic dianhydride (EDTAD) is a revivification of ethylenediaminetetraacetic acid (EDTA) with two additional anhydride groups on each molecule of EDTA. EDTAD is a biodegradable and powerful complexing agent due to the presence of carboxylic and amine groups (Júnior et al. 2009, Yu et al. 2008, Zhang et al. 2011. Several researchers had used EDTAD in modifying low-cost biomaterials such as neem leaf powder (Hanafiah et al. 2013), Aloe vera rind powder (Hanafiah et al. 2018), sugarcane bagasse (Júnior et al. 2009) and baker's yeast biomass (Xia et al. 2015, Yu et al. 2008, Zhang et al. 2011 to improve their adsorption performance. In this study, the adsorption behaviour of Pb(II) ions onto EDTAD modified coconut frond which included the effects of pH, dosage, concentration, contact time, and temperature were evaluated.

Materials:
Coconut fronds were collected from a coconut tree plantation located in Kelantan, Malaysia. The fronds were washed thoroughly, cut into small pieces, dried and washed with tap water to remove insoluble impurities before with deionized water. The fronds were dried in an oven at 333 K overnight before being ground and sieved by using a mechanical grinder to obtain the average particle size of 125-250 µm. The dried coconut frond powder was boiled in deionized water for 30 min to destroy microbes, washed and dried again in an oven overnight at 333 K. Ethylenediaminetetraacetic dianhydride (EDTAD) treatment was performed according to the method proposed by Hanafiah et al. (2013). Five grams of dried untreated coconut frond powder (UCFP) was added into the mixture of 0.50 g of EDTAD and 400 mL N, N-dimethylacetamide (DMAc) in a three-neck round bottom flask equipped with a condenser. The mixture was refluxed at 343 K for 24 h. The mixture was filtered and rinsed with 0.10 M NaOH thoroughly. The wet coconut frond powder was further rinsed with DMAc before finally rinsed with 4 L deionized water until the constant pH was obtained. The EDTAD treated coconut frond powder was dried in an oven at 333 K overnight and designated as ECFP.
Characterizations of ECFP: UCFP and ECFP were characterized by using an Attenuated Total Reflectance-Fourier Transform Infrared (ATR-FTIR) spectrometer (PerkinElmer, Spectrum 100, USA). Determination of pH zpc of ECFP was done according to the method reported by Hanafiah et al. (2018) with some modification. A volume of 50 mL (0.01 M) KNO 3 was placed into different 100 mL conical flasks. The initial pH of this solution (pH o ) was adjusted from 2 to 10 by adding drops of 0.10 M HCl or NaOH solutions. A weight of 0.10 g ECFP was added into each flask and shaken in a water bath shaker for 24 h at 120 stroke min -1 at 300 K. ECFP was filtered and the final pH of KNO 3 solutions (pH f ) was measured using a pH meter.

Batch adsorption experiments:
The analytical grade of 1000 mg.L -1 stock solution of Pb(II) was obtained from Merck (Germany). Working concentration solutions of Pb(II) ions were prepared by appropriate dilutions. All adsorption experiments were done using 50 mL of 10 mg.L -1 Pb(II) solutions (otherwise stated) and 0.02 g ECFP at 300 K and the shaking rate was 120 stroke min -1 . The effect of pH on adsorption of Pb(II) ions onto ECFP was done by adjusting the initial pH from 1 to 5 by adding drops of 0.10 M HCl and NaOH solutions. The effect of ECFP dosage on adsorption of Pb(II) ions was performed by varying the weight of ECFP (0.02 to 0.10 g) in Pb(II) ions solutions at pH 4. The mixtures were shaken for 90 min. In the kinetic study, the concentration of Pb(II) ions solutions was varied from 10 to 30 mg.L -1 and the solutions were shaken at different time intervals (1, 3, 6, 10, 20, 30, 60, 90 and 120 min). The pH and the ECFP dosage were fixed at 4 and 0.02 g, respectively. Similar pH and adsorbent dosage were used in isotherm study, but the concentrations of Pb(II) ions were varied (5,10,15,20,25,30,40 and 75 mg.L −1 ). The mixtures were then shaken for 90 min at 300 K. In the thermodynamic study, 5,10,15,20,25,30,40 and 75 mg.L -1 Pb(II) ions solutions were shaken with 0.02 g ECFP for 90 min at different temperatures (303, 313 and 323 K). All experiments were done in duplicate. The mixtures were then filtered, and the final concentrations were analysed using an Atomic Absorption Spectrometer (AAS, PerkinElmer PinAAcle 900T, USA). The amount of adsorbed Pb(II) ions, q e (mg.g -1 ) and the removal percentage (%) of Pb(II) ions were calculated using eqs. (1) and (2), respectively.
(AAS, PerkinElmer PinAAcle 900T, USA). The the removal percentage (%) of Pb(II) ions were c Where, Ci and Cf are initial and final concentra the weight of ECFP (g) and V is the volume of P Fig. 1

Characterizations of ECFP:
Where, Ci and Cf are initial and final concentrations of the weight of ECFP (g) and V is the volume of Pb(II) io Fig. 1 showed the FTIR s ECFP. A strong and broad band was observed at 3323 might be attributed to the presence of O-H groups from c peaks were observed at 1727, 1607, 1368, 1033, 898, C=O, -NH, C-O, C-H, and =C-H. The presence of a n attributed to the introduction of ester groups, as suggeste suggest that EDTAD was successfully introduced onto U from coconut frond (Júnior et al. 2009). After the adso peaks at 3323 and 1033 cm -1 was reduced, suggesting tha binding of Pb(II) ions during the adsorption process. Be 1593 cm -1 suggesting that a bond might be formed betwe

Characterizations of ECFP:
Where, C i and C f are initial and final concentrations of Pb(II) ions (mg.L -1 ), respectively, m is the weight of ECFP (g) and V is the volume of Pb(II) ions solutions (L). Fig. 1 showed the FTIR spectra of UCFP, ECFP and Pb(II) loaded ECFP. A strong and broad band was observed at 3323 cm -1 in the FTIR spectra of UCFP. This might be attributed to the presence of O-H groups from cellulose, hemicellulose and lignin. Other peaks were observed at 1727, 1607, 1368, 1033, 898, and 562 cm -1 suggesting the presence of C=O, -NH, C-O, C-H, and =C-H. The presence of a new peak at 1736 cm -1 in ECFP could be attributed to the introduction of ester groups, as suggested by Pereira et al. (2010). Thus, it might suggest that EDTAD was successfully introduced onto UCFP by forming an ester with -OH groups from coconut frond (Júnior et al. 2009). After the adsorption of Pb(II) ions, the intensity of the peaks at 3323 and 1033 cm -1 was reduced, suggesting that -OH and C-O-C groups took part in the binding of Pb(II) ions during the adsorption process. Besides, there was a shift of peak at 1603 to 1593 cm -1 suggesting that a bond might be formed between Pb(II) ions and -NH groups.

Characterizations of ECFP:
The pH of zero point charge or pH zpc was measured to determine the pH where the adsorbent had a zero net surface charge. Basically, at pH=pH zpc , there were balance charges on the adsorbent surface. In general, at pH < pHzpc, an adsorbent surface tended to carry positive charges, while at pH > pH zpc, the adsorbent surface was negatively charged. Therefore, the adsorption of a cationic metal is favoured when the pH was higher than pH zpc (Srivastava et al. 2015). Fig. 2 shows the plot of pH zpc of ECFP. The pH zpc of ECFP was found at 6.4, based on the intersection point of the pH o -axis. This indicated that when the pH of the solution was higher than 6.4, Pb(II) ions could have easily been attracted to the ECFP surface as more negative charges surrounded the adsorbent surface.

Effect of pH on adsorption of Pb(II) ions onto ECFP:
The pH of a solution is a critical parameter in the adsorption of metal ions onto an adsorbent. The variation of pH might influence the affinity between adsorbent and adsorbate. To study the effect of initial pH on adsorption of Pb(II) ions onto ECFP, the initial pH of Pb(II) solution was varied from 1 to 5. Fig. 3 showed that low adsorption capacity, q e was recorded at pH 1 (4.43 mg.g -1 ). This could be explained by electrostatic repulsion of positively charged Pb(II) ions with positively charged adsorbent due to protonation of functional groups on the adsorbent surface or excessive cations from sorbed H + ions on the adsorbent surface itself (Ali et al. 2011, Faghihian et al. 2005, Torres-Blancas et al. 2013, Zhang et al. 2016 The q e value increased gradually with the increasing of initial pH of Pb(II) ions, where 7.25, 14.48, 22.20 and 23.45 mg.g-1 were recorded at pH 2, 3, 4 and 5, respectively. Basically, the increase of pH could lead to the reduction of repulsive force of the adsorbent surface towards the adsorbate The pH of zero point charge or pHzpc was measured to determine the pH where adsorbent had a zero net surface charge. Basically, at pH=pHzpc, there were balance charge the adsorbent surface. In general, at pH ˂ pHzpc, an adsorbent surface tended to carry pos charges, while at pH ˃ pHzpc, the adsorbent surface was negatively charged. Therefore, adsorption of a cationic metal is favoured when the pH was higher than pHzpc (Srivastava e 2015). Fig. 2 shows the plot of pHzpc of ECFP. The pHzpc of ECFP was found at 6.4, based on intersection point of the pHo-axis. This indicated that when the pH of the solution was higher 6.4, Pb(II) ions could have easily been attracted to the ECFP surface as more negative cha surrounded the adsorbent surface.

Effect of pH on adsorption of Pb(II) ions onto ECFP:
The pH of a solution is a critical parameter in the adsorption of metal ions onto an adsorbent. The variation of pH might influence the affinity between adsorbent and adsorbate. To study the effect of initial pH on adsorption of Pb(II) ions onto ECFP, the initial pH of Pb(II) solution was varied from 1 to 5. Fig. 3 showed that low adsorption capacity, qe was recorded at pH 1 (4.43 mg.g -1 ). This could be explained by  ion (Ali et al. 2011, Bhatnagar & Sillanpää 2009, Galhoum et al. 2015, Gok 2014) and thus promoted more adsorption sites as for the adsorption of metal ions. Furthermore, the number of protons (H + ) decreased with the increase of pH and thus reduced the competition between metal ions and H + for the adsorption sites (Miraoui et al. 2015, Torab-Mostaedi et al. 2015.

Effect of adsorbent dosage on adsorption of Pb(II) ions onto ECFP:
The dosage of adsorbent was an important parameter as it strongly influenced the capacity of biosorption at a given initial concentration (Asgari et al. 2012). The dependence of adsorption of Pb(II) ions on the dosage of ECFP was shown in Fig. 4. The per cent of Pb(II) removal increased from 88.80 to 98.20 % for the adsorbent dosage of 0.02 g to 0.08 g, respectively. This could be attributed to the increase of surface area due to the increase of adsorbent weight or higher number of adsorption sites for a fixed concentration of Pb(II) (Mu et al. 2013). But, beyond dosage 0.08 g, there was no appreciable change in the percentage removal of Pb(II) because the number of Pb(II) ions became the limiting factor. The amount of Pb(II) ions adsorbed on the other hand showed the opposite trend. As more ECFP increased, the amount of Pb(II) ions adsorbed reduced. This was mainly due to the higher number of unsaturated adsorption sites when the dosage was increased, as stated in eq. (1).
Adsorption rate and kinetic models: Fig. 5 showed the effect of different concentrations on the adsorption rate of Pb(II) ions onto ECFP. Generally, the plots showed that the adsorption of Pb(II) ions involved two phases; the rapid adsorption pattern, and the slower, gradual adsorption rate until equilibrium uptake was achieved. At the beginning of the adsorption stage, a high amount of metal ions was adsorbed due to the high number of available active sites on the surface of the biosorbent. The actives sites were quickly occupied by the Pb(II) ions that led to a small difference in the change of adsorption rate after 10 min. Based on Fig. 5, it can be concluded that the amount of Pb(II) ions adsorbed increased proportionally with the initial concentration of Pb(II) ions. The amount of 10, 20 and 30 mg.L -1 Pb(II) ions adsorbed were 22.20, 35.10 and 46.15 mg.g -1 , respectively. This situation could be explained by the increase in the driving force of the concentration gradient. The increase of driving force had overcome all the mass transfer resistance of metal ions from the aqueous phase to the solid phase, and subsequently provided a higher collision probability between metal ions and the active sites (Zhu et al. 2015). Overall, adsorption of Pb(II) ions onto ECFP showed relatively rapid adsorption process as equilibrium was achieved in less than 30 min for all Pb(II) concentrations. When the concentration of Pb(II) ions was low, the ratio of Pb(II) ions to the number of available adsorption sites was also low. Therefore, the adsorption sites seemed to take up the available Pb(II) ions much quickly since there was less competition among adsorbates ions (Gupta & Bhattacharyya 2008).
The rate of the adsorption of the metal ions is the momentum of the molecules to move from the aqueous solution to the adsorbent surface. In general, the adsorption process is governed by one or more mechanisms. Basically, adsorption process consists four stages (Srivastava et al. 2015), i.e; (i) the transfer of the solute from the solution to the boundary layer that surrounds the adsorbent surface, (ii) the transportation of the solute from the boundary layer to the adsorbent surface, (iii) the transfer of solute to the intraparticle sites from the adsorbent surface and (iv) the binding of solute ions to the available sites in the internal surface of the adsorbent.
Kinetics is the utmost important information for the adsorption study. A full-scale batch adsorption process can be obtained from the kinetics study. In addition, by applying certain adsorption kinetics models to the experimental data, one would gain a better understanding of the adsorption mechanism and the potential rate-limiting step (Sadeek et al. 2015, Zhang et al. 2016. Moreover, the rate of adsorption obtained through the kinetic study may help in optimizing the reactor dimension and residence time in any particular adsorption system (Sadeek et al. 2015). In this study, the pseudo-first (Ho & McKay 1998) and pseudo-second (Ho & McKay 2000) order kinetic models were used to determine the adsorption for the adsorption of Pb(II) ions onto ECFP and the linearized equations are given as follows: kinetic models were used to determine the adsorption f and the linearized equations are given as follows: log(q e − q t ) = log q e,cal − k 1 2.303 t t q t = 1 k 2 q e,cal 2 + 1 q e,cal t Where, t is time (min), qe,cal is the calculated adsorpti concentration of the analyte at time t (mg.g -1 ), k1 is ps is pseudo-second-order rate constant (g mg -1 min -1 ).
first-order and pseudo-second-order kinetic models f Pb(II) ions onto ECFP at 300 K, respectively. The res presented in Table 1. Where, t is time (min), qe,cal is the calculated a concentration of the analyte at time t (mg.g -1 ), is pseudo-second-order rate constant (g mg -1 first-order and pseudo-second-order kinetic m Pb(II) ions onto ECFP at 300 K, respectively. presented in Table 1.

…(4)
Where, t is time (min), q e,cal is the calculated adsorption capacity at equilibrium (mg.g -1 ), q t is the concentration of the analyte at time t (mg.g -1 ), k 1 is pseudo-first-order rate constant (min -1 ) and k 2 is pseudo-second-order rate constant (g mg -1 min -1 ). Figs. 6 and 7 showed the plots of pseudo-first-order and pseudo-second-order kinetic models for the adsorption of 10, 20 and 30 mg.L -1 Pb(II) ions onto ECFP at 300 K, respectively. The results analysed from both kinetics models are presented in Table 1.
In general, both pseudo-first and pseudo-second-order plots showed relatively good linearity (R 2 > 0.85), yet the pseudo-second-order kinetics model had a better agreement with the experimental data. This could be noticed from the values of R 2 that were almost unity (0.9990 to 0.9999) for Kinetics is the utmost important information for the adsorption study. A full-scale batch adsorption process can be obtained from the kinetics study. In addition, by applying certain adsorption kinetics models to the experimental data, one would gain a better understanding of the adsorption mechanism and the potential rate-limiting step (Sadeek et al. 2015, Zhang et al. 2016).
Moreover, the rate of adsorption obtained through the kinetic study may help in optimizing the reactor dimension and residence time in any particular adsorption system (Sadeek et al. 2015). In this study, the pseudo-first ( Table 1.      (Galhoum et al. 2015, Gok 2014, Pasquier & Largitte 2016, Toor & Jin 2012, Wang et al. 2013).
Equilibrium and isotherm study: The equilibrium or adsorption isotherm describes the interaction of adsorbate with the adsorbent. It represents the distribution of different initial concentrations of solute at a constant temperature in the aqueous phase and solid phase. As adsorption isotherm provides that information, it turns as the most vital study in optimizing the adsorbent-adsorbate system, mechanistic pathways and understanding the adsorbent surface properties (Galhoum et al. 2015, Rangabhashiyam et al. 2014, Zhang et al. 2016).
To study the adsorption isotherm for the adsorption of Pb(II) ions onto ECFP, the initial concentration of Pb(II) ions was varied from 5 to 75 mg.L -1 . The mixtures were shaken for 90 min to ensure the equilibrium state. At the low concentrations of Pb(II) ions (<30 mg.L -1 ), the sharp slope could be observed as presented in Fig. 8, an indication of a high-efficiency adsorbent for the adsorption of low metal ion concentrations. As the concentration of Pb(II) ions increased to 40 mg.L -1 , the slope reduced drastically. This condition was associated with the saturation of adsorption sites due to the increase in the ratio of the number of Pb(II) ions to the number of adsorption sites.
In general, the application of suitable isotherm models onto the experimental data is important as it can provide valuable information about the distribution of adsorption sites on the adsorbent surface, adsorption characteristic and affinity of adsorbent-adsorbate in the adsorption system. Several mathematical models can be applied to the experimental data such as two-parameter isotherm models which include Langmuir (Langmuir 1916) and Freundlich (Freundlich 1906). The linearized equations for Langmuir and Freundlich models are given in eqs. (5) and (6) system. Several mathematical models can be appli parameter isotherm models which include Langmuir ( 1906 log q e = 1 n log C e +log K F Where, qe is the equilibrium adsorption capacity (m capacity (mg.g -1 ), Ce is the equilibrium concentra equilibrium constant (L mg -1 ), KF is a Freundlich con the heterogeneity of the adsorbent surface and its affi Where, q e is the equilibrium adsorption capacity (mg.g -1 ), Q max,cal is the maximum adsorption capacity (mg.g -1 ), C e is the equilibrium concentration of the adsorbate (mg.L -1 ), b is the equilibrium constant (L mg -1 ), K F is a Freundlich constant (mg.g -1 ) and n is a constant related to the heterogeneity of the adsorbent surface and its affinity for the adsorbate.
Figs. 9 and 10 showed Langmuir and Freundlich plots for adsorption of Pb(II) ions onto ECFP and the parameters for the isotherm models were listed in Table 2. In general, both isotherm models recorded high R 2 values (>0.92). Based on the agreement of experimental and theoretical values of Q max , Langmuir isotherm model seemed to have a better fitting to the experimental data compared to Freundlich isotherm model. This condition might explain the monolayer coverage of Pb(II) ions on the ECFP surface.
Adsorption thermodynamics: To study the effect of different temperature on adsorption of Pb(II) ions, solutions ranged from 0 to 75 mg.L -1 was stirred with ECFP at 303, 313 and 323 K for 90 min. Fig. 11 showed the isotherm plot for the adsorption of Pb(II) ions at 303, 313 and 323 K. In general, adsorption of Pb(II) onto ECFP was more favourable of electrons or ion exchange mechanism (Galhoum et al. 2015, Gok 2014, Pasquier & Largitte 2016, Toor & Jin 2012, Wang et al. 2013).

Equilibrium and isotherm study:
The equilibrium or adsorption isotherm describes the interaction of adsorbate with the adsorbent. It represents the distribution of different initial concentrations of solute at a constant temperature in the aqueous phase and solid phase. As adsorption isotherm provides that information, it turns as the most vital study in optimizing the adsorbent-adsorbate system, mechanistic pathways and understanding the adsorbent surface properties (Galhoum et al. 2015, Rangabhashiyam et al. 2014, Zhang et al. 2016. shaking rate: 120 stroke min -1 ; adsorbent dosage: 0.02 g; temperature: 300 K).
To study the adsorption isotherm for the adsorption of Pb(II) ions onto ECFP, the initial    Pb(II) ions on the ECFP surface.     at higher temperatures as the recorded adsorption capacities increased with increasing temperatures, which also suggested an endothermic adsorption behaviour.
The thermodynamic parameters such as enthalpy (ΔH°), entropy (ΔS°), and Gibb's free energy (ΔG°) were calculated to determine the adsorption process by using the following equations: Adsorption thermodynamics: To study the effect of different temperature on adsorption of Pb(II) ions, solutions ranged from 0 to 75 mg.L -1 was stirred with ECFP at 303, 313 and 323 K for 90 min. Fig. 11 showed the isotherm plot for the adsorption of Pb(II) ions at 303, 313 and 323 K. In general, adsorption of Pb(II) onto ECFP was more favourable at higher temperatures as the recorded adsorption capacities increased with increasing temperatures, which also suggested an endothermic adsorption behaviour. The thermodynamic parameters such as enthalpy (ΔH°), entropy (ΔS°), and Gibb's free energy (ΔG°) were calculated to determine the adsorption process by using the following equations: Adsorption thermodynamics: To study the effect of different temperature on adsorption of Pb (II) ions, solutions ranged from 0 to 75 mg.L -1 was stirred with ECFP at 303, 313 and 323 K for 90 min. Fig. 11 showed the isotherm plot for the adsorption of Pb(II) ions at 303, 313 and 323 K. In general, adsorption of Pb(II) onto ECFP was more favourable at higher temperatures as the recorded adsorption capacities increased with increasing temperatures, which also suggested an endothermic adsorption behaviour. Fig. 11: Adsorption isotherm plots of Pb(II) ions onto ECFP at 303, 313 and 323 K (volume: 50 mL; pH: 4; shaking rate: 120 stroke min -1 ; adsorbent dosage: 0.02 g).
The thermodynamic parameters such as enthalpy (ΔH°), entropy (ΔS°), and Gibb's free energy (ΔG°) were calculated to determine the adsorption process by using the following equations: Where, K c is the equilibrium constant, C ad is the concentration of Pb(II) ions adsorbed on solid at equilibrium (mmol.L -1 ), C e is the equilibrium concentration of Pb(II) ions in the solution (mmol.L -1 ), R is the gas constant (8.314 J K -1 mol -1 ) and T is the temperature in Kelvin. Fig. 12 showed the Van't Hoff plot for the adsorption of Pb(II) ions onto ECFP at 303, 313 and 323 K. The calculated values for ΔH°, ΔS°, and ΔG° were listed in Table 3. In general, all ΔG° values for the adsorption of Pb(II) at 303, 313 and 323 K were negative, an indication of spontaneous adsorption process at those temperatures. The positive ΔH° value recorded in this study indicated that adsorption of Pb(II) ions onto ECFP was endothermic, where adsorption was more favourable at a higher temperature. Meanwhile, the positive ΔS° value suggested that this process was an entropy-driven process where a higher degree of freedom of the ions was obtained at a higher temperature.

CONCLUSION
The current work revealed the potential application of ECFP as an adsorbent for removing Pb(II) ions from aqueous solutions. The amount of Pb(II) ions adsorbed was reduced in a highly acidic condition and the optimum adsorption pH range was 4 to 5. The satisfactory adsorption capacity was recorded from the Langmuir isotherm model with the q max value of 84.03 mg.g -1 being recorded. The potential functional groups responsible for adsorbing Pb(II) ions as revealed by the FTIR spectra were hydroxyl, amino, carbonyl, aromatics and ether. Adsorption process could be considered rapid due to the short time taken to reach equilibrium stage for all concentrations of Pb(II) ions. Based on the thermodynamic study, Pb(II) ions were more favoured to be adsorbed at a higher temperature, suggesting an endothermic adsorption behaviour.  The thermodynamic parameters such as enthalpy (ΔH°), entropy (ΔS°), and Gibb's free energy (ΔG°) were calculated to determine the adsorption process by using the following equations: 1 ), R is the gas constant (8.314 J K -1 mol -1 ) and T is the temperature in Kelvin. Fig. 12 showed the Van't Hoff plot for the adsorption of Pb(II) ions onto ECFP at 303, 313 and 323 K. The calculated values for ΔH°, ΔS°, and ΔG° were listed in Table 3. In general, all ΔG° values for the adsorption of Pb(II) at 303, 313 and 323 K were negative, an indication of spontaneous adsorption process at those temperatures. The positive ΔH° value recorded in this study indicated that adsorption of Pb(II) ions onto ECFP was endothermic, where adsorption was more favourable at a higher temperature. Meanwhile, the positive ΔS° value suggested that this process was an entropy-driven process where a higher degree of freedom of the ions was obtained at a higher temperature.