Journal of Environmental Management 155 (2015) 24e30
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Journal of Environmental Management journal homepage: www.elsevier.com/locate/jenvman
A novel magnetic adsorbent based on waste litchi peels for removing Pb(II) from aqueous solution Ruixue Jiang a, b, Jiyu Tian a, Hao Zheng a, Jinqiu Qi a, Shujuan Sun a, Xiaochen Li a, * a b
Water Conservancy and Civil Engineering College, Shandong Agricultural University, Tai'an, Shandong 271018, PR China Environmental Science and Engineering College, Hohai University, Nanjing, Jiangsu 210098, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 December 2014 Received in revised form 3 March 2015 Accepted 5 March 2015 Available online
A new magnetic bioadsorbent, magnetic litchi peel (MLP), was synthesized by coating powdered litchi peel with Fe3O4, and was used for removing Pb(II) from aqueous solutions. The influencing factors, adsorption isotherms, kinetics, and thermodynamics of Pb(II) adsorption by MLP were investigated using batch assays. Optimum Pb(II) adsorption by MLP was achieved using a contact time of 120 min, an adsorbent dose of 5 g/L, and pH of 6.0. The adsorption equilibrium data conformed to the Langmuir isotherm model, yielding a maximum Pb(II) adsorption capacity of 78.74 mg/g. The adsorption kinetics for Pb(II) adsorption by MLP followed a pseudo-second-order model. The thermodynamic results suggested that Pb(II) adsorption by MLP was spontaneous and exothermic. Additionally, the magnetic adsorbent was easily and rapidly separated out of solution under an external magnetic field. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Litchi peel Magnetic bioadsorbent Pb(II) Adsorption Separation
1. Introduction Heavy metal pollution is a serious environmental problem that is continually escalating with the rapid advancement of technology. Heavy metals pose a significant threat to the environment and public health because of their toxic effects and tendency toward bioaccumulation in the food chain, and their persistence in nature (Bulgariu and Bulgariu, 2012). Routine techniques such as ion exchange, precipitation, electrochemical processes, and/or membrane processes are commonly applied to the treatment of industrial effluents. However, the application of such processes is sometimes restricted owing to technical or economic constraints (Volesky, 2007). In the search for new technologies for removing toxic metals from wastewater, bioadsorption based on the metalbinding capacities of various biomaterials has gained prominence as a promising approach. A wide variety of functional groups (such as hydroxyl, carboxyl, carbonyl, amine, amide, alcoholic, phenolic, thiol, and phosphate groups) found in certain waste biomaterials have shown high affinity for heavy metal ions, with the consequent generation of
* Corresponding author. Water Conservancy and Civil Engineering College, Shandong Agricultural University, Daizong Road No. 61, Tai'an, Shandong 271018, PR China. E-mail address: [email protected] (X. Li). http://dx.doi.org/10.1016/j.jenvman.2015.03.009 0301-4797/© 2015 Elsevier Ltd. All rights reserved.
complexes and/or chelates (Volesky, 2007). Intensive research efforts have led to the development of effective bioadsorbents based on waste biomaterials and agricultural wastes (Nguyen et al., 2013). For instance, algae, bacteria, fungi, yeasts (Witek-Krowiak and Harikishore Kumar Reddy, 2013), and agricultural wastes (Kong et al., 2014; Lasheen et al., 2012; Liao et al., 2012; Wang et al., 2013) have been utilized as prospective bioadsorbents. The major advantages of such biomaterials over conventional materials include their low cost, high efficiency, and minimal requirement for chemicals. This class of bioadsorbents has been successfully applied to the removal of toxic heavy metal ions from aqueous solutions (Areco et al., 2013; Chiban et al., 2012; Jiang et al., 2014; Reddy et al., 2011; Weng et al., 2014; Witek-Krowiak, 2012). Litchi (Litchi chinensis Sonn.) is a popular subtropical fruit with an annual production of about 1.5 million ton in P.R. China (Yang et al., 2005). Litchi peel accounts for more than 15% of the litchi fresh weight, resulting in a significant generation of solid waste from litchi production (Nagle et al., 2011). Litchi peels possess various functional groups such as hydroxyl, carboxyl, and amide groups (Song et al., 1999); thus, litchi peel may be exploited as an adsorbent for removing heavy metal ions from aqueous solution. To date, however, this possibility has not been explored. Although several raw and chemically modified types of agrowaste have been successfully employed as low-cost and highefficiency bioadsorbents for the removal of various pollutants
R. Jiang et al. / Journal of Environmental Management 155 (2015) 24e30
from aqueous solution (Bulgariu and Bulgariu, 2012; Liao et al., 2012), separation of these bioadsorbents from water is a nontrivial task due to their low specific gravity and tendency to agglomerate. This tendency is particularly pronounced for bioadsorbents dispersed as fine particles in aqueous solution (Dong et al., 2014). These issues have substantially limited their widespread application in batch and column modes (Chang and Juang, 2004). Magnetic separation is a convenient technique in which separation of solid particles from a suspension can be achieved by the application of an external magnetic field. This technique has recently gained prominence in water treatment processes and is now recognized as a prospective technique for resolving the aforementioned issues (Tang and Lo, 2013). However, to the best of our knowledge, little work has been done on the preparation of magnetic adsorbents based directly on agrowastes for application to the removal of heavy metals from aqueous solution. Therefore, in this work, Fe3O4 magnetic nanocrystals are prepared and then appended to litchi peels to develop a new magnetic bioadsorbent, magnetic litchi peel (MLP), with the objective to address the issues related to the separation of bioadsorbents based on agrowastes in adsorption processes. The adsorption capacity of MLP for Pb(II) is investigated by assessing the impact of contact time, pH, and adsorbent dose using batch assays. Equilibrium isotherms, kinetics, and thermodynamics for the adsorption of Pb(II) on MLP are also determined in order to evaluate the adsorption process. 2. Material and methods 2.1. Preparation of magnetic bioadsorbents based on litchi peels coated with Fe3O4 Fe3O4 magnetic nanoparticles were prepared by coprecipitating Fe2＋ and Fe3＋ ions in an ammonia solution under ultrasonic enhancement (Numerical Control Ultrasonic Cleaners, Model KQ5200DE, Working frequency 40 KHz, Kunshan). In a typical run, a cosolution of FeCl3$6H2O (0.04 M) and FeSO4$7H2O (0.02 M) was prepared in 200 mL deionized water at pH 7.0. Aqueous ammonia solution (25e28 wt%) was dropwise added to this solution under ultrasonic enhancement until the pH of the solution became 9.2. The ultrasonic enhancement treatment was conducted for 60 min. The obtained magnetite was washed immediately with deionized water until the pH became 7.0. The obtained magnetite was then washed twice with ethanol. Finally, the magnetite nanoparticles were dried at 70 C. Dry litchi peels were pulverized to a powder using a grinder, and then sieved through a 60 mesh sieve. A 30 g sample of the litchi peel powder was dispersed in 400 mL deionized water and homogenized by rapid stirring for 30 min. Subsequently, 20 g Fe3O4 magnetic nanoparticle powder was added to the mixture solution with rapid stirring for 30 min to maintain homogeneous mixing. Finally, 7.5 g Na5P3O10 (TPP) was added to the mixed solution, which was then stirred for 60 min. The particles were separated from the solution by applying an external magnetic field. The resultant particles were washed thrice with ethanol and deionized water, respectively, and then dried at 70 C. The final dried powders are denoted as MLP, and were kept in a polyethylene bottle for further experiments. Fig. 1 shows a schematic of the integral steps in the synthesis of MLP.
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Fig. 1. Schematic of the magnetic litchi peel (MLP) synthesis protocol.
evaluated using the BrunauereEmmetteTeller (BET, Model 3H2000PS1/2, Beishide Instrument-ST Co., Ltd., Beijing) nitrogen sorption technique. Fourier transform infrared spectroscopic (FTIR) studies were carried out to identify the functional groups of MLP in the range of 4000 to 400 cm1 frequency (PerkineElmer, 2000 FT-IR, Japan). The crystalline structure of the nanoparticles was characterized by X-ray diffraction (XRD) of the dried samples using a D/Max-2400 X-ray diffractometer equipped with a Cu Ka monochromatic radiation source (l ¼ 1.54187 Å). The pHpzc (defined below) of the MLP samples was determined as follows: 50 mL of 0.01 M NaCl solution was added to an Erlenmeyer flask, which was then stoppered. The pH was adjusted to within the range 2.0e12.0 using 0.1 M HCl and 0.1 M NaOH solutions. Subsequently, 0.15 g of the prepared MLP was added, and the final pH was measured after agitating the solution for 48 h at 25 C. The pHpzc is the point where the pHfinal versus pHinitial plot crosses the line of pHinitial ¼ pHfinal.
2.3. Adsorption experiments A series of polyethylene centrifuge tubes (100 mL) covered with Teflon sheets were used in the batch experiments. The effects of the contact time, pH value, and adsorbent dose were investigated using an initial Pb(II) concentration of 50 mg/L. The effect of a particular parameter was studied by progressive variation of the selected parameter while keeping the other two parameters constant. After the adsorption experiments, the solutions were centrifuged and then filtered through a cellulose acetate membrane (0.45 mm) filter. The concentration of Pb(II) in the filtrates was determined via atomic absorption spectroscopy at 283.3 nm. The adsorption efficiency (A %) of the adsorbent for Pb(II) was calculated from Eq. (1):
2.2. Characterization of magnetic litchi peel
A% ¼ ðC0 Ce Þ=C0 100% The magnetic properties of MLP were assessed at room temperature using a vibrating sample magnetometer (VSM, Model 7410, Lake Shore Co. Ltd., USA). The surface area of MLP was
(1)
where C0 and Ce are the initial and equilibrium Pb(II) concentrations (mg/L), respectively.
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2.4. Kinetics studies A series of polyethylene centrifuge tubes (100 mL) containing 50 mL Pb(II) aqueous solutions (50 mg/L) were kept in a thermostatic shaking water bath (25 C). As described above, the solution pH was adjusted to 6.0 using 0.1 M HCl and 0.1 M NaOH solutions. A 0.25 g sample of the prepared MLP powder was accurately weighed and added to each tube, these tubes were then mechanically agitated at 220 rpm. At given time intervals (0e180 min), the solutions were centrifuged and filtered through a 0.45 mm cellulose acetate membrane filter. The filtrates were then analyzed for Pb(II) using atomic absorption spectroscopy. The pseudo-second-order model was employed to discuss the kinetics of Pb(II) adsorption on MLP. 2.5. Isothermal studies Solutions with Pb(II) concentrations of 50e240 mg/L were prepared by adequately diluting the stock solutions of Pb(II). The pH was adjusted to 6.0 using 0.1 M HCl and 0.1 M NaOH solutions. A 0.25 g sample of the prepared MLP powder was added to 50 mL of each metal solution, and the solutions were then agitated for 3 h at 25 C. Subsequently, the suspensions were filtered, and the filtrates were analyzed for Pb(II). Langmuir and Freundlich isotherms were plotted using standard linear equations, and the two corresponding parameters were calculated from the respective graphs. 2.6. Thermodynamics studies A series of polyethylene centrifuge tubes (100 mL) containing 50 mL Pb(II) solutions (50 mg/L) were kept in a thermostatic shaking water bath; the temperature values of the bath were set at 288, 298, 308, 318, and 328 K for the respective samples. The pH of the solutions was adjusted to 6.0 using 0.1 M HCl and 0.1 M NaOH solutions, and the tubes were agitated mechanically at 220 rpm. At set intervals, the solutions were centrifuged and filtered, and the filtrates were analyzed for Pb(II). The standard free energy (DG ), enthalpy changes (DH ), and entropy changes (DS ) were calculated to assess the feasibility of the adsorption process. 2.7. Separation experiment An accurately weighed 0.1 g sample of LP powder was placed in 20 mL deionized water in a 20 mL quartz cuvette and homogenized. The tubes were then statically settled, and the time taken for the LP sample to completely precipitate from the homogenized solution unaided was recorded. For comparison, an analogous experiment was simultaneously conducted using MLP instead of LP. Additionally, the same settling processes were also monitored for MLP under the application of an external magnetic field. 3. Results and discussion 3.1. Characterization of MLP The FT-IR spectra of LP and MLP (Fig. 2a) indicated that the hydroxyl, amido, and carbonyl groups were abundantly present in the adsorbent based on the intensity of the absorption peaks. The broad vibration around 3000e3500 cm1 is indicative of the presence of hydroxyl groups on the surface of the LP and MLP samples (Witek-Krowiak and Harikishore Kumar Reddy, 2013). The peaks at 2300 cm1 and 2920 cm1 are attributed to the stretching vibrations of RNH and RNH2 in MLP and LP (Ma et al., 2007). The peaks at 1500e1650 cm1 are assigned as the stretching vibrations of the carboxyl group (eCOOH). The band at 1055 cm1 can be
Fig. 2. Characterization of litchi peel (LP) and magnetic litchi peel (MLP). (a, FT-IR spectra of LP and MLP. b, XRD pattern of Fe3O4 nanoparticles. c, Room-temperature magnetization curves of Fe3O4 nanoparticles and magnetic litchi peel).
assigned to the CeOH stretching vibration of the alcoholic and carboxylic acid groups. From the FTIR spectral analysis, it is evident that the litchi peel possesses a significant number of functional groups for binding heavy metal ions; moreover, the magnetization treatment of LP with Fe3O4 nanoparticles did not change the types and quantity of the active groups on the surface of LP.
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The X-ray diffraction (XRD) pattern of pure Fe3O4 is shown in Fig. 2b The XRD peaks could be indexed to the cubic spinel phase of magnetite (JCPDS No. 89-43191). The diffraction peaks located at 30.18 , 35.65 , 43.36 , 53.62 , 57.01, and 62.83 are assigned to the reflection planes of magnetite Fe3O4 (Shen et al., 2013). The litchi peel sample was amorphous and thus did not provide quantitative peaks for the assessment of the crystallization modes of the MLP adsorbent. The saturation magnetization of the MLP and Fe3O4 nanoparticles, prepared as mentioned above, is shown in Fig. 2c, which shows zero axial S-type curves for the MLP and Fe3O4 nanoparticles, indicating the superparamagnetic nature of these materials. The respective saturation magnetization intensities of the Fe3O4 nanoparticles and MLP were 61.6 and 20.5 A m2 kg1. These results indicate that the litchi peel was successfully magnetized when coated with Fe3O4 nanoparticles in the presence of TPP. The decrease in the magnetization of MLP relative to that of the Fe3O4 nanoparticles may be attributed to the reduced ferrite content in the litchi peels and Fe3O4 composite (MLP).
3.2. Determination of influencing factors 3.2.1. Effect of initial solution pH The pH is a significant parameter that can affect not only the forms of the compounds distributed in water but also the dissociation of the groups on the surface of the biosorbent (Reddy et al., 2011). The effects of pH on the adsorption of Pb(II) by MLP were studied over the pH range 3.0e8.0, while maintaining all other parameters constant (Fig. 3a). As observed in Fig. 3a, the adsorption of Pb(II) by MLP was highly pH dependent. The Pb(II)-adsorption efficiency of MLP increased sharply on increasing the pH from 3.0 to 6.0 in the initial stage, whereas no further increase in the Pb(II) adsorption efficiency of MLP could be detected in the higher pH range 6.0e8.0.
27
The observed influence of the pH may be attributed to the fact that at lower pH values, there is a higher concentration of Hþ; these protons compete strongly with Pb(II) for the active sites on the surface of MLP, resulting in the reduction of Pb(II) binding on the adsorbent surface. However, with increasing pH, more functional groups become available for Pb(II) ligation due to the reduction in the Hþ concentration; consequently, Pb(II) adsorption is enhanced. At pH values higher than 7.0, insoluble lead hydroxide starts precipitating from the aqueous solution; however, this condition is often not desirable given that metal precipitation may result in the incorrect deduction of the true Pb(II) adsorption capacity (Shafaghat et al., 2012). The pHpzc (the pH point of zero charge) is the point at which the surface acidic functional groups no longer contribute to the pH of the solution. Adsorption of cations will be favorable at pH values higher than pHzpc, whereas the adsorption of anions will be favorable at pH values lower than pHpzc (Nomanbhay and Palanisamy, 2005). The pHpzc of MLP was found to be around 5.0 (Fig. 3b). Thus, it could be concluded that the optimum pH for Pb(II) adsorption on MLP was 6.0. Similar observations have been reported for the adsorption of Pb(II) by bagasse flyash (Gupta and Imran, 2004) and chemically modified sugarcane bagasse (Karnitz et al., 2007).
3.2.2. Effect of contact time The effect of contact time on the adsorption of Pb(II) on MLP was evaluated at pH 6.0 by treating the sample at a shaking speed of 220 rpm at 25 C (Fig. 3c). As observed from Fig. 3c, the efficiency of Pb(II) adsorption by MLP increased from 88% to 93% as the contact time increased from 5 to 120 min. No further increase in the Pb(II) adsorption efficiency was observed as the contact time was increased from 120 to 180 min. Thus, it is deduced that adsorption equilibrium could be achieved within 120 min.
Fig. 3. Effects of operating parameters on Pb(II) adsorption by MLP (Basic conditions: volume of the medium: 50 mL, initial concentration 50 mg/L, temperature: 25 C.). (a, Effect of initial solution pH on Pb(II) adsorption, adsorbent dose: 0.25 g; contact time: 3 h. b, The pHpzc of MLP. c, Effect of contact time on Pb(II) adsorption, adsorbent dose: 0.25 g; pH: 6.0. d, Effect of adsorbent dose on Pb(II) adsorption, pH: 6.0; contact time: 3 h).
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3.2.3. Effect of adsorbent dose To determine the maximum Pb(II) adsorption efficiency of MLP, the dose of MLP was varied from 1.0 to 20.0 g/L while maintaining the other parameters constant. Fig. 3d shows the profile for Pb(II) adsorption on MLP as a function of the adsorbent dose. The Pb(II) adsorption efficiency of MLP increased markedly as the MLP dosage increased, and the maximum adsorption efficiency of 94.23% was achieved at a dosage of 5 g/L. The increased adsorption efficiencies with increasing adsorbent dose may be attributable to the higher surface area and the greater availability of active sites. However, the Pb(II) adsorption efficiency of MLP decreased slightly on further increasing the adsorbent dosage. This indicated that higher adsorbent dose can lead to aggregation of adsorbent, resulting in a decrease of the total surface area of adsorbent. Similar trends have been documented for the adsorption of heavy metals by modified orange peels (Lasheen et al., 2012). Thus, a dose of 5 g/L was selected as the optimum adsorbent dose for Pb(II) adsorption.
mechanism, and the adsorption process is mainly dominated by chemisorption. 3.4. Adsorption isotherms The Langmuir and Freundlich isotherm models were utilized to evaluate the Pb(II) adsorption capacity of MLP. The Langmuir isotherm model is commonly applied to adsorption on a completely homogenous surface (Gode and Pehlivan, 2005). This model assumes a uniform energy for adsorption on the substrate surface, and that maximum adsorption depends on the saturation level of the monolayer. The Langmuir model can be expressed as follows:
Ce 1 Ce þ ¼ qe qmax b qmax
(3)
The adsorption kinetics can provide valuable insight into the reaction pathways and mechanisms of adsorption processes. In this study, pseudo-second-order kinetic models were applied to investigate the kinetics of Pb(II) adsorption by MLP. The pseudo-second-order kinetics equation based on equilibrium adsorption can be expressed as Eq. (2) (Ho and Mckay, 1998):
where qe is the amount adsorbed (mg/g), Ce is the equilibrium concentration of the adsorbate (mg/L), qmax is the monolayer capacity of the adsorbent (mg/g), and b is the Langmuir isotherm constant (L/mg). According to Eq. (3), the values of b and qmax can be obtained from the intercept [1/(qmax$b)] and slope [1/qmax]. The Freundlich isotherm is regarded as one of the earliest empirical equations, and is shown to be consistent with exponential distribution of the active centers (Ho, 2005; Veliev et al., 2006). This model assumes that the adsorption of metal ions occurs on a heterogeneous surface (Daneshvar et al., 2002). The Freundlich equation can be expressed as follows:
t 1 1 ¼ þ t qt k2 q2 qecal ecal
1 log qe ¼ log KF þ log Ce n
3.3. Kinetics studies
(2)
where, t (min) is the adsorption time, k2 (g/(mg min)) is the pseudo-second-order rate constant, qe (mg/g) is the equilibrium Pb(II) adsorption capacity of MLP, and qt (mg/g) is the adsorption capacity at a time t. The equilibrium adsorption capacity qe and the rate constant k2 can be calculated from the slope (1/qe) and the intercept (1/(k2$q2e )) of the plot of t/qt against time t. As observed in Fig. 4, the kinetics of Pb(II) adsorption by MLP shows perfect correlation with the pseudo-second-order model, with a correlation coefficient (R2) of 0.9999. Furthermore, the theoretical value of qe(cal.) obtained from the pseudo-second-order kinetic model was 9.46 mg/g, which was in close agreement with the experimental value (qe(exp.)) of (9.39 mg/g). These results suggest that Pb(II) adsorption on MLP follows a pseudo-second-order
(4)
where KF and 1/n are the Freundlich isotherm constants for metal ion adsorption. Using Eq. (4), KF and n can be calculated from the intercept (log KF) and slope (1/n) of the plot of log qe versus log Ce. The corresponding parameters of the two isotherm models obtained in the present study are listed in Table 1. As shown in Table 1, the relatively high values of the correlation coefficient (R2) for the Langmuir and Freundlich models indicate that the equilibrium data for MLP was in good agreement with both models. Additionally, the value of 1/n obtained from the Freundlich isotherm falls within the range 0e1.0, indicating favorable adsorption (Stephen Inbaraj and Sulochana, 2004). Based on the Langmuir isotherm, the maximum Pb(II) adsorption capacity of MLP was 78.74 mg/g, which was much higher than that reported for other bioadsorbents (e.g., 7.97 mg/g for modified banana peels (Annadurai et al., 2003), 6.366 mg/g for untreated sugarcane bagasse, and 7.297 mg/g for modified sugarcane bagasse (MartinLara et al., 2010)). These results demonstrate the potential of MLP as a viable bioadsorbent for the removal of heavy metals from aqueous solutions. 3.5. Thermodynamic studies Study of the adsorption thermodynamics is used to identify the adsorption mechanisms (Wang et al., 2010). Thermodynamic parameters such as DG , DH , and DS were calculated from the experimental data using the Van't Hoff equation (Eq. (5)):
Table 1 Isotherm constants for the adsorption of Pb(II) onto the magnetic litchi peels (MLP). Metal Fig. 4. Pseudo-second-order kinetics of Pb(II) adsorption by MLP (conditions: volume of the medium: 50 mL; initial concentration 50 mg/L; dose of MLP adsorbent: 0.25 g; temperature: 25 C; pH: 6.0).
Pb(II)
Langmuir isotherm constants
Feundlich isotherm constants
qmax (mg/g)
b (L/mg)
R2
b
KF
n
R2
78.74
0.073
0.99
0.21
2.25
1.44
0.98
R. Jiang et al. / Journal of Environmental Management 155 (2015) 24e30 Table 2 Thermodynamic parameters for the adsorption of Pb(II) onto the magnetic litchi peels (MLP). Temperature (K)
DG (kJ mol1)
DH (kJ mol1)
DS (J mol1 K1)
288 298 308 318 328
7.13 7.19 7.24 7.30 7.35
5.56
5.44
DG DH DS ¼ þ ln Kd ¼ RT RT R
(5)
T is the temperature in K, R is the ideal gas constant (8.314 J/ (mol K)), and Kd is the thermodynamic equilibrium constant, which is expressed as follows:
Kd ¼
Cad;c Ce
(6)
where Cad,c is the concentration of metal ions on the adsorbent at equilibrium and Ce is the equilibrium concentration of heavy metals in the aqueous solution (mg/L). The values of DH and DS can be
29
calculated from the slope and intercept of a plot of lnKd versus 1/T, respectively (Liang et al., 2010); the values of DG at different temperatures were calculated from Eq. (5). As shown in Table 2, the negative values of DG at various temperatures indicated the spontaneous nature of Pb(II) adsorption by MLP. In addition, the decrease in DG with increased temperature might be attributed to fewer available active sites on the surface of MLP or the increased mobility of Pb(II) in the aqueous solution at higher temperatures (Singh and Pant, 2004). The negative value of DH suggests that the adsorption process is exothermic, and the positive value of DS confirmed an increase in randomness at the solidesolution interface during the Pb(II) adsorption process. The data are comparable to prior thermodynamic results obtained for the adsorption of As(III) on activated alumina (Singh and Pant, 2004).
3.6. Separation of solideliquid The solideliquid separation process is of significance for the practical implementation of an adsorption system. Sedimentation is possibly the simplest solideliquid separation process. Fig. 5 illustrates the separation of bioadsorbents from homogenized LP or
Fig. 5. Analysis of LP and MLP separation from a homogenized solution. (a, The separation process of LP from homogenized solutions only under the action of gravity. b, The separation process of MLP from homogenized solutions only under the action of gravity. c, The separation process of MLP from homogenized solutions with the application of an external magnetic field).
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MLP solutions, with and without the application of an external magnetic field. In the absence of an external magnetic field, the time required for the complete sedimentation of LP and MLP to the bottom of the tubes was 130 5400 and 120 700, respectively. However, in the presence of an external magnetic field, the time required for complete separation of MLP was dramatically reduced to 3000 67. This finding clearly suggests that the novel magnetic bioadsorbent based on litchi peels coated with Fe3O4 can be separated from solutions more easily and rapidly in an external magnetic field than untreated bioadsorbents. These results are thus of great significance for the reuse and further treatment of exhausted bioadsorbents. 4. Conclusion A novel magnetic adsorbent, magnetic litchi peel (MLP), was successfully synthesized from litchi peels and Fe3O4. MLP exhibited high adsorption efficiency for Pb(II), achieving maximal adsorption at 120 min and pH 6.0, by using an adsorbent dosage of 5 g/L. Pb(II) adsorption on MLP followed the Langmuir as well as Freundlich isotherm models, yielding a maximum adsorption capacity of 78.74 mg/g. The kinetics of Pb(II) adsorption on MLP followed a pseudo-second-order model. The thermodynamic results suggest that the Pb(II) adsorption process on MLP is spontaneous and exothermic. Furthermore, in an external magnetic field, MLP could be rapidly and easily separated from homogenized solutions. Acknowledgments This study was funded by the National Natural Science Foundation of China (Grant No.: 51208173) and the Shandong Province Natural Science Foundation of China (No. ZR2014EEM005). References Annadurai, G., Juang, R.S., Lee, D.J., 2003. Adsorption of heavy metals from water using banana and orange peels. Water Sci. Technol. 47, 185e190. Areco, M.M., Saleh-Medina, L., Trinelli, M.A., Marco-Brown, J.L., Afonso, M.S., 2013. Adsorption of Cu(II), Zn(II), Cd(II) and Pb(II) by dead Avena fatua biomass and the effect of these metals on their growth. Colloid. Surf. B 110, 305e312. Bulgariu, D., Bulgariu, L., 2012. Equilibrium and kinetics studies of heavy metal ions biosorption on green algae waste biomass. Bioresour. Technol. 103, 489e493. Chang, M.Y., Juang, R.S., 2004. Adsorption of tannic acid, humic acid, and dyes from water using the composite of chitosan and activated clay. J. Colloid Interface Sci. 278, 18e25. Chiban, M., Soudani, A., Sinan, F., Persin, M., 2012. Wastewater treatment by batch adsorption method onto micro-particles of dried Withania frutescens plant as a new adsorbent. J. Environ. Manage. 95, 61e65. Daneshvar, N., Salari, D., Aber, S., 2002. Chromium adsorption and Cr(VI) reduction to trivalent chromium in aqueous solutions by soya cake. J. Hazard. Mater. 94, 49e61. Dong, C.L., Chen, W., Liu, C., Liu, Y., Liu, H.C., 2014. Synthesis of magnetic chitosan nanoparticle and its adsorption property for humic acid from aqueous solution. Colloid. Surf. A 446, 179e189. Gode, F., Pehlivan, E., 2005. Adsorption of Cr(III) ions by Turkish brown coals. Fuel Process. Technol. 86, 875e884. Gupta, V.K., Imran, A., 2004. Removal of lead and chromium from wastewater using bagasse fly ash-a sugar industry waste. J. Colloid Interface Sci. 271, 321e328. Ho, Y.S., 2005. Effect of pH on lead removal from water using tree fern as the sorbent. Bioresour. Technol. 96, 1292e1296. Ho, Y.S., Mckay, G., 1998. Sorption of dye from aqueous solution by peat. Chem. Eng.
J. 70, 115e124. Jiang, W.J., Cai, Q., Xu, W., Yang, M.W., Cai, Y., Dionysiou, D.D., O'Shea, K.E., 2014. Cr(VI) adsorption and reduction by humic acid coated on magnetite. Environ. Sci. Technol. 48 (14), 8078e8085. Karnitz Jr., O., Gurgel, L.V.A., De Melo, J.C.P., 2007. Adsorption of heavy metal ion from aqueous single metal solution by chemically modified sugarcane bagasse. Bioresour. Technol. 98, 1291e1297. Kong, Z.L., Li, X.C., Tian, J.Y., Yang, J.L., Sun, S.J., 2014. Comparative study on the adsorption capacity of raw and modified litchi pericarp for removing Cu(II) from solutions. J. Environ. Manage. 134, 109e116. Lasheen, M.R., Ammar, N.S., Ibrahim, H.S., 2012. Adsorption/desorption of Cd(II), Cu(II) and Pb(II) using chemically modified orange peel: equilibrium and kinetic studies. Solid State Sci. 14, 202e210. Liang, S., Guo, X.Y., Feng, N.C., Tian, Q.H., 2010. Isotherms, kinetics and thermodynamic studies of adsorption of Cu2þ fromaqueous solutions by Mg2þ/Kþ type orange peel adsorbents. J. Hazard. Mater. 174, 756e762. Liao, P., Ismael, Z., Zhang, W.B., Yuan, S.H., 2012. Adsorption of dyes from aqueous solutions by microwave modified bamboo charcoal. Chem. Eng. J. 196, 339e348. Ma, W., Ya, F.Q., Han, M., Wang, R., 2007. Characteristics of equilibrium, kinetics studies for adsorption of fluoride on magnetic-chitosan particle. J. Hazard. Mater. 143, 296e302. Martin-Lara, M.A., Rico, I.L.R., Vicente, I.C.A., García, G.B., Hoces, M.C., 2010. Modification of the sorptive characteristics of sugarcane bagasse for removing lead from aqueous solutions. Desalination 256, 58e63. Nagle, M., Habasimbi, K., Mahayothee, B., Haewsungcharern, M., Janjai, S., Müller, J., 2011. Fruit processing residues as an alternative fuel for drying in northern Thailand. Fuel 90, 818e823. Nguyen, T.A.H., Ngo, H.H., Guo, W.S., Zhang, J., Liang, S., Yue, Q.Y., Li, Q., Nguyen, T.V., 2013. Applicability of agricultural waste and by-products for adsorptive removal of heavy metals from wastewater. Bioresour. Technol. 148, 574e585. Nomanbhay, S.M., Palanisamy, K., 2005. Removal of heavy metal from industrial wastewater using chitosan coated oil palm shell charcoal. Electron. J. Biotechnol. 8, 43e53. Reddy, D., Ramana, D.K.V., Seshaiah, K., Reddy, A.V.R., 2011. Biosorption of Ni(II) from aqueous phase by Moringa oleifera bark, a low cost biosorbent. Desalination 268, 150e157. Shafaghat, A., Salimi, F., Valiei, M., Salehzadeh, J., Shafaghat, M., 2012. Removal of heavy metals (Pb2þ, Cu2þ and Cr3þ) from aqueous solutions using five plants materials. Afr. J. Biotechnol. 11, 852e855. Shen, P., Jiang, W., Wang, F.H., Chen, M.D., Ma, P.C.H., Li, F.S.H., 2013. Preparation and characterization of Fe3O4@TiO2 shellon polystyrene beads. J. Polym. Res. 20, 1e6. Singh, T.S., Pant, K.K., 2004. Equilibrium, kinetics and thermodynamic studies for adsorption of As(III) on activated alumina. Sep. Purif. Technol. 36, 139e147. Song, G.Q., Bu, X.Z., Gu, L.Q., Chen, R., 1999. GC-MS analysis of components in Litchi Pericarp. Acta Sci. Nat. U. N. 38, 48e51. Stephen Inbaraj, B., Sulochana, N., 2004. Carbonised jackfruit peel as an adsorbent for the removal of Cd(II) from aqueous solution. Bioresour. Technol. 94, 49e52. Tang, S.C.N., Lo, I.M.C., 2013. Magnetic nanoparticles: essential factors for sustainable environmental applications. Water Res. 47, 2613e2632. € Veliev, E.V., Oztürk, T., Veli, S., Fatullayev, A.G., 2006. Application of diffussion model for adsorption of azo reactrive dye on pumice. Pol. J. Environ. Stud. 15, 347e353. Volesky, B., 2007. Biosorption and me. Water Res. 41, 4017e4029. Wang, J., Peng, S.C., Wan, Z.Q., Yue, Z.B., Wu, J., Chen, T.H., 2013. Feasibility of anaerobic digested corn stover as biosorbent for heavy metal. Bioresour. Technol. 132, 453e456. Wang, Z.Y., Yu, X.D., Pan, B., Xing, B.S., 2010. Norfloxacin sorption and its thermodynamics on surface-modified carbon nanotubes. Environ. Sci. Technol. 44, 978e984. Weng, C.H., Lin, Y.T., Hong, D.Y., Sharma, Y.C., Chen, S.C., Tripathi, K., 2014. Effective removal of copper ions from aqueous solution using base treated black tea waste. Ecol. Eng. 67, 127e133. Witek-Krowiak, A., 2012. Analysis of temperature-dependent biosorption of Cu2þ ions on sunflower hulls: kinetics, equilibrium and mechanism of the process. Chem. Eng. J. 19, 13e20. Witek-Krowiak, A., Harikishore Kumar Reddy, D., 2013. Removal of microelemental Cr(III) and Cu(II) by using soybean meal waste eunusual isotherms and insights of binding mechanism. Bioresour. Technol. 127, 350e357. Yang, B., Zhao, M.M., Liu, Y., Li, B.Z., 2005. Characterization of litchi pericarp polysaccharide. Nat. Prod. Res. Dev. 17, 685e687.
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Journal of Environmental Management journal homepage: www.elsevier.com/locate/jenvman
A novel magnetic adsorbent based on waste litchi peels for removing Pb(II) from aqueous solution Ruixue Jiang a, b, Jiyu Tian a, Hao Zheng a, Jinqiu Qi a, Shujuan Sun a, Xiaochen Li a, * a b
Water Conservancy and Civil Engineering College, Shandong Agricultural University, Tai'an, Shandong 271018, PR China Environmental Science and Engineering College, Hohai University, Nanjing, Jiangsu 210098, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 December 2014 Received in revised form 3 March 2015 Accepted 5 March 2015 Available online
A new magnetic bioadsorbent, magnetic litchi peel (MLP), was synthesized by coating powdered litchi peel with Fe3O4, and was used for removing Pb(II) from aqueous solutions. The influencing factors, adsorption isotherms, kinetics, and thermodynamics of Pb(II) adsorption by MLP were investigated using batch assays. Optimum Pb(II) adsorption by MLP was achieved using a contact time of 120 min, an adsorbent dose of 5 g/L, and pH of 6.0. The adsorption equilibrium data conformed to the Langmuir isotherm model, yielding a maximum Pb(II) adsorption capacity of 78.74 mg/g. The adsorption kinetics for Pb(II) adsorption by MLP followed a pseudo-second-order model. The thermodynamic results suggested that Pb(II) adsorption by MLP was spontaneous and exothermic. Additionally, the magnetic adsorbent was easily and rapidly separated out of solution under an external magnetic field. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Litchi peel Magnetic bioadsorbent Pb(II) Adsorption Separation
1. Introduction Heavy metal pollution is a serious environmental problem that is continually escalating with the rapid advancement of technology. Heavy metals pose a significant threat to the environment and public health because of their toxic effects and tendency toward bioaccumulation in the food chain, and their persistence in nature (Bulgariu and Bulgariu, 2012). Routine techniques such as ion exchange, precipitation, electrochemical processes, and/or membrane processes are commonly applied to the treatment of industrial effluents. However, the application of such processes is sometimes restricted owing to technical or economic constraints (Volesky, 2007). In the search for new technologies for removing toxic metals from wastewater, bioadsorption based on the metalbinding capacities of various biomaterials has gained prominence as a promising approach. A wide variety of functional groups (such as hydroxyl, carboxyl, carbonyl, amine, amide, alcoholic, phenolic, thiol, and phosphate groups) found in certain waste biomaterials have shown high affinity for heavy metal ions, with the consequent generation of
* Corresponding author. Water Conservancy and Civil Engineering College, Shandong Agricultural University, Daizong Road No. 61, Tai'an, Shandong 271018, PR China. E-mail address: [email protected] (X. Li). http://dx.doi.org/10.1016/j.jenvman.2015.03.009 0301-4797/© 2015 Elsevier Ltd. All rights reserved.
complexes and/or chelates (Volesky, 2007). Intensive research efforts have led to the development of effective bioadsorbents based on waste biomaterials and agricultural wastes (Nguyen et al., 2013). For instance, algae, bacteria, fungi, yeasts (Witek-Krowiak and Harikishore Kumar Reddy, 2013), and agricultural wastes (Kong et al., 2014; Lasheen et al., 2012; Liao et al., 2012; Wang et al., 2013) have been utilized as prospective bioadsorbents. The major advantages of such biomaterials over conventional materials include their low cost, high efficiency, and minimal requirement for chemicals. This class of bioadsorbents has been successfully applied to the removal of toxic heavy metal ions from aqueous solutions (Areco et al., 2013; Chiban et al., 2012; Jiang et al., 2014; Reddy et al., 2011; Weng et al., 2014; Witek-Krowiak, 2012). Litchi (Litchi chinensis Sonn.) is a popular subtropical fruit with an annual production of about 1.5 million ton in P.R. China (Yang et al., 2005). Litchi peel accounts for more than 15% of the litchi fresh weight, resulting in a significant generation of solid waste from litchi production (Nagle et al., 2011). Litchi peels possess various functional groups such as hydroxyl, carboxyl, and amide groups (Song et al., 1999); thus, litchi peel may be exploited as an adsorbent for removing heavy metal ions from aqueous solution. To date, however, this possibility has not been explored. Although several raw and chemically modified types of agrowaste have been successfully employed as low-cost and highefficiency bioadsorbents for the removal of various pollutants
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from aqueous solution (Bulgariu and Bulgariu, 2012; Liao et al., 2012), separation of these bioadsorbents from water is a nontrivial task due to their low specific gravity and tendency to agglomerate. This tendency is particularly pronounced for bioadsorbents dispersed as fine particles in aqueous solution (Dong et al., 2014). These issues have substantially limited their widespread application in batch and column modes (Chang and Juang, 2004). Magnetic separation is a convenient technique in which separation of solid particles from a suspension can be achieved by the application of an external magnetic field. This technique has recently gained prominence in water treatment processes and is now recognized as a prospective technique for resolving the aforementioned issues (Tang and Lo, 2013). However, to the best of our knowledge, little work has been done on the preparation of magnetic adsorbents based directly on agrowastes for application to the removal of heavy metals from aqueous solution. Therefore, in this work, Fe3O4 magnetic nanocrystals are prepared and then appended to litchi peels to develop a new magnetic bioadsorbent, magnetic litchi peel (MLP), with the objective to address the issues related to the separation of bioadsorbents based on agrowastes in adsorption processes. The adsorption capacity of MLP for Pb(II) is investigated by assessing the impact of contact time, pH, and adsorbent dose using batch assays. Equilibrium isotherms, kinetics, and thermodynamics for the adsorption of Pb(II) on MLP are also determined in order to evaluate the adsorption process. 2. Material and methods 2.1. Preparation of magnetic bioadsorbents based on litchi peels coated with Fe3O4 Fe3O4 magnetic nanoparticles were prepared by coprecipitating Fe2＋ and Fe3＋ ions in an ammonia solution under ultrasonic enhancement (Numerical Control Ultrasonic Cleaners, Model KQ5200DE, Working frequency 40 KHz, Kunshan). In a typical run, a cosolution of FeCl3$6H2O (0.04 M) and FeSO4$7H2O (0.02 M) was prepared in 200 mL deionized water at pH 7.0. Aqueous ammonia solution (25e28 wt%) was dropwise added to this solution under ultrasonic enhancement until the pH of the solution became 9.2. The ultrasonic enhancement treatment was conducted for 60 min. The obtained magnetite was washed immediately with deionized water until the pH became 7.0. The obtained magnetite was then washed twice with ethanol. Finally, the magnetite nanoparticles were dried at 70 C. Dry litchi peels were pulverized to a powder using a grinder, and then sieved through a 60 mesh sieve. A 30 g sample of the litchi peel powder was dispersed in 400 mL deionized water and homogenized by rapid stirring for 30 min. Subsequently, 20 g Fe3O4 magnetic nanoparticle powder was added to the mixture solution with rapid stirring for 30 min to maintain homogeneous mixing. Finally, 7.5 g Na5P3O10 (TPP) was added to the mixed solution, which was then stirred for 60 min. The particles were separated from the solution by applying an external magnetic field. The resultant particles were washed thrice with ethanol and deionized water, respectively, and then dried at 70 C. The final dried powders are denoted as MLP, and were kept in a polyethylene bottle for further experiments. Fig. 1 shows a schematic of the integral steps in the synthesis of MLP.
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Fig. 1. Schematic of the magnetic litchi peel (MLP) synthesis protocol.
evaluated using the BrunauereEmmetteTeller (BET, Model 3H2000PS1/2, Beishide Instrument-ST Co., Ltd., Beijing) nitrogen sorption technique. Fourier transform infrared spectroscopic (FTIR) studies were carried out to identify the functional groups of MLP in the range of 4000 to 400 cm1 frequency (PerkineElmer, 2000 FT-IR, Japan). The crystalline structure of the nanoparticles was characterized by X-ray diffraction (XRD) of the dried samples using a D/Max-2400 X-ray diffractometer equipped with a Cu Ka monochromatic radiation source (l ¼ 1.54187 Å). The pHpzc (defined below) of the MLP samples was determined as follows: 50 mL of 0.01 M NaCl solution was added to an Erlenmeyer flask, which was then stoppered. The pH was adjusted to within the range 2.0e12.0 using 0.1 M HCl and 0.1 M NaOH solutions. Subsequently, 0.15 g of the prepared MLP was added, and the final pH was measured after agitating the solution for 48 h at 25 C. The pHpzc is the point where the pHfinal versus pHinitial plot crosses the line of pHinitial ¼ pHfinal.
2.3. Adsorption experiments A series of polyethylene centrifuge tubes (100 mL) covered with Teflon sheets were used in the batch experiments. The effects of the contact time, pH value, and adsorbent dose were investigated using an initial Pb(II) concentration of 50 mg/L. The effect of a particular parameter was studied by progressive variation of the selected parameter while keeping the other two parameters constant. After the adsorption experiments, the solutions were centrifuged and then filtered through a cellulose acetate membrane (0.45 mm) filter. The concentration of Pb(II) in the filtrates was determined via atomic absorption spectroscopy at 283.3 nm. The adsorption efficiency (A %) of the adsorbent for Pb(II) was calculated from Eq. (1):
2.2. Characterization of magnetic litchi peel
A% ¼ ðC0 Ce Þ=C0 100% The magnetic properties of MLP were assessed at room temperature using a vibrating sample magnetometer (VSM, Model 7410, Lake Shore Co. Ltd., USA). The surface area of MLP was
(1)
where C0 and Ce are the initial and equilibrium Pb(II) concentrations (mg/L), respectively.
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2.4. Kinetics studies A series of polyethylene centrifuge tubes (100 mL) containing 50 mL Pb(II) aqueous solutions (50 mg/L) were kept in a thermostatic shaking water bath (25 C). As described above, the solution pH was adjusted to 6.0 using 0.1 M HCl and 0.1 M NaOH solutions. A 0.25 g sample of the prepared MLP powder was accurately weighed and added to each tube, these tubes were then mechanically agitated at 220 rpm. At given time intervals (0e180 min), the solutions were centrifuged and filtered through a 0.45 mm cellulose acetate membrane filter. The filtrates were then analyzed for Pb(II) using atomic absorption spectroscopy. The pseudo-second-order model was employed to discuss the kinetics of Pb(II) adsorption on MLP. 2.5. Isothermal studies Solutions with Pb(II) concentrations of 50e240 mg/L were prepared by adequately diluting the stock solutions of Pb(II). The pH was adjusted to 6.0 using 0.1 M HCl and 0.1 M NaOH solutions. A 0.25 g sample of the prepared MLP powder was added to 50 mL of each metal solution, and the solutions were then agitated for 3 h at 25 C. Subsequently, the suspensions were filtered, and the filtrates were analyzed for Pb(II). Langmuir and Freundlich isotherms were plotted using standard linear equations, and the two corresponding parameters were calculated from the respective graphs. 2.6. Thermodynamics studies A series of polyethylene centrifuge tubes (100 mL) containing 50 mL Pb(II) solutions (50 mg/L) were kept in a thermostatic shaking water bath; the temperature values of the bath were set at 288, 298, 308, 318, and 328 K for the respective samples. The pH of the solutions was adjusted to 6.0 using 0.1 M HCl and 0.1 M NaOH solutions, and the tubes were agitated mechanically at 220 rpm. At set intervals, the solutions were centrifuged and filtered, and the filtrates were analyzed for Pb(II). The standard free energy (DG ), enthalpy changes (DH ), and entropy changes (DS ) were calculated to assess the feasibility of the adsorption process. 2.7. Separation experiment An accurately weighed 0.1 g sample of LP powder was placed in 20 mL deionized water in a 20 mL quartz cuvette and homogenized. The tubes were then statically settled, and the time taken for the LP sample to completely precipitate from the homogenized solution unaided was recorded. For comparison, an analogous experiment was simultaneously conducted using MLP instead of LP. Additionally, the same settling processes were also monitored for MLP under the application of an external magnetic field. 3. Results and discussion 3.1. Characterization of MLP The FT-IR spectra of LP and MLP (Fig. 2a) indicated that the hydroxyl, amido, and carbonyl groups were abundantly present in the adsorbent based on the intensity of the absorption peaks. The broad vibration around 3000e3500 cm1 is indicative of the presence of hydroxyl groups on the surface of the LP and MLP samples (Witek-Krowiak and Harikishore Kumar Reddy, 2013). The peaks at 2300 cm1 and 2920 cm1 are attributed to the stretching vibrations of RNH and RNH2 in MLP and LP (Ma et al., 2007). The peaks at 1500e1650 cm1 are assigned as the stretching vibrations of the carboxyl group (eCOOH). The band at 1055 cm1 can be
Fig. 2. Characterization of litchi peel (LP) and magnetic litchi peel (MLP). (a, FT-IR spectra of LP and MLP. b, XRD pattern of Fe3O4 nanoparticles. c, Room-temperature magnetization curves of Fe3O4 nanoparticles and magnetic litchi peel).
assigned to the CeOH stretching vibration of the alcoholic and carboxylic acid groups. From the FTIR spectral analysis, it is evident that the litchi peel possesses a significant number of functional groups for binding heavy metal ions; moreover, the magnetization treatment of LP with Fe3O4 nanoparticles did not change the types and quantity of the active groups on the surface of LP.
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The X-ray diffraction (XRD) pattern of pure Fe3O4 is shown in Fig. 2b The XRD peaks could be indexed to the cubic spinel phase of magnetite (JCPDS No. 89-43191). The diffraction peaks located at 30.18 , 35.65 , 43.36 , 53.62 , 57.01, and 62.83 are assigned to the reflection planes of magnetite Fe3O4 (Shen et al., 2013). The litchi peel sample was amorphous and thus did not provide quantitative peaks for the assessment of the crystallization modes of the MLP adsorbent. The saturation magnetization of the MLP and Fe3O4 nanoparticles, prepared as mentioned above, is shown in Fig. 2c, which shows zero axial S-type curves for the MLP and Fe3O4 nanoparticles, indicating the superparamagnetic nature of these materials. The respective saturation magnetization intensities of the Fe3O4 nanoparticles and MLP were 61.6 and 20.5 A m2 kg1. These results indicate that the litchi peel was successfully magnetized when coated with Fe3O4 nanoparticles in the presence of TPP. The decrease in the magnetization of MLP relative to that of the Fe3O4 nanoparticles may be attributed to the reduced ferrite content in the litchi peels and Fe3O4 composite (MLP).
3.2. Determination of influencing factors 3.2.1. Effect of initial solution pH The pH is a significant parameter that can affect not only the forms of the compounds distributed in water but also the dissociation of the groups on the surface of the biosorbent (Reddy et al., 2011). The effects of pH on the adsorption of Pb(II) by MLP were studied over the pH range 3.0e8.0, while maintaining all other parameters constant (Fig. 3a). As observed in Fig. 3a, the adsorption of Pb(II) by MLP was highly pH dependent. The Pb(II)-adsorption efficiency of MLP increased sharply on increasing the pH from 3.0 to 6.0 in the initial stage, whereas no further increase in the Pb(II) adsorption efficiency of MLP could be detected in the higher pH range 6.0e8.0.
27
The observed influence of the pH may be attributed to the fact that at lower pH values, there is a higher concentration of Hþ; these protons compete strongly with Pb(II) for the active sites on the surface of MLP, resulting in the reduction of Pb(II) binding on the adsorbent surface. However, with increasing pH, more functional groups become available for Pb(II) ligation due to the reduction in the Hþ concentration; consequently, Pb(II) adsorption is enhanced. At pH values higher than 7.0, insoluble lead hydroxide starts precipitating from the aqueous solution; however, this condition is often not desirable given that metal precipitation may result in the incorrect deduction of the true Pb(II) adsorption capacity (Shafaghat et al., 2012). The pHpzc (the pH point of zero charge) is the point at which the surface acidic functional groups no longer contribute to the pH of the solution. Adsorption of cations will be favorable at pH values higher than pHzpc, whereas the adsorption of anions will be favorable at pH values lower than pHpzc (Nomanbhay and Palanisamy, 2005). The pHpzc of MLP was found to be around 5.0 (Fig. 3b). Thus, it could be concluded that the optimum pH for Pb(II) adsorption on MLP was 6.0. Similar observations have been reported for the adsorption of Pb(II) by bagasse flyash (Gupta and Imran, 2004) and chemically modified sugarcane bagasse (Karnitz et al., 2007).
3.2.2. Effect of contact time The effect of contact time on the adsorption of Pb(II) on MLP was evaluated at pH 6.0 by treating the sample at a shaking speed of 220 rpm at 25 C (Fig. 3c). As observed from Fig. 3c, the efficiency of Pb(II) adsorption by MLP increased from 88% to 93% as the contact time increased from 5 to 120 min. No further increase in the Pb(II) adsorption efficiency was observed as the contact time was increased from 120 to 180 min. Thus, it is deduced that adsorption equilibrium could be achieved within 120 min.
Fig. 3. Effects of operating parameters on Pb(II) adsorption by MLP (Basic conditions: volume of the medium: 50 mL, initial concentration 50 mg/L, temperature: 25 C.). (a, Effect of initial solution pH on Pb(II) adsorption, adsorbent dose: 0.25 g; contact time: 3 h. b, The pHpzc of MLP. c, Effect of contact time on Pb(II) adsorption, adsorbent dose: 0.25 g; pH: 6.0. d, Effect of adsorbent dose on Pb(II) adsorption, pH: 6.0; contact time: 3 h).
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3.2.3. Effect of adsorbent dose To determine the maximum Pb(II) adsorption efficiency of MLP, the dose of MLP was varied from 1.0 to 20.0 g/L while maintaining the other parameters constant. Fig. 3d shows the profile for Pb(II) adsorption on MLP as a function of the adsorbent dose. The Pb(II) adsorption efficiency of MLP increased markedly as the MLP dosage increased, and the maximum adsorption efficiency of 94.23% was achieved at a dosage of 5 g/L. The increased adsorption efficiencies with increasing adsorbent dose may be attributable to the higher surface area and the greater availability of active sites. However, the Pb(II) adsorption efficiency of MLP decreased slightly on further increasing the adsorbent dosage. This indicated that higher adsorbent dose can lead to aggregation of adsorbent, resulting in a decrease of the total surface area of adsorbent. Similar trends have been documented for the adsorption of heavy metals by modified orange peels (Lasheen et al., 2012). Thus, a dose of 5 g/L was selected as the optimum adsorbent dose for Pb(II) adsorption.
mechanism, and the adsorption process is mainly dominated by chemisorption. 3.4. Adsorption isotherms The Langmuir and Freundlich isotherm models were utilized to evaluate the Pb(II) adsorption capacity of MLP. The Langmuir isotherm model is commonly applied to adsorption on a completely homogenous surface (Gode and Pehlivan, 2005). This model assumes a uniform energy for adsorption on the substrate surface, and that maximum adsorption depends on the saturation level of the monolayer. The Langmuir model can be expressed as follows:
Ce 1 Ce þ ¼ qe qmax b qmax
(3)
The adsorption kinetics can provide valuable insight into the reaction pathways and mechanisms of adsorption processes. In this study, pseudo-second-order kinetic models were applied to investigate the kinetics of Pb(II) adsorption by MLP. The pseudo-second-order kinetics equation based on equilibrium adsorption can be expressed as Eq. (2) (Ho and Mckay, 1998):
where qe is the amount adsorbed (mg/g), Ce is the equilibrium concentration of the adsorbate (mg/L), qmax is the monolayer capacity of the adsorbent (mg/g), and b is the Langmuir isotherm constant (L/mg). According to Eq. (3), the values of b and qmax can be obtained from the intercept [1/(qmax$b)] and slope [1/qmax]. The Freundlich isotherm is regarded as one of the earliest empirical equations, and is shown to be consistent with exponential distribution of the active centers (Ho, 2005; Veliev et al., 2006). This model assumes that the adsorption of metal ions occurs on a heterogeneous surface (Daneshvar et al., 2002). The Freundlich equation can be expressed as follows:
t 1 1 ¼ þ t qt k2 q2 qecal ecal
1 log qe ¼ log KF þ log Ce n
3.3. Kinetics studies
(2)
where, t (min) is the adsorption time, k2 (g/(mg min)) is the pseudo-second-order rate constant, qe (mg/g) is the equilibrium Pb(II) adsorption capacity of MLP, and qt (mg/g) is the adsorption capacity at a time t. The equilibrium adsorption capacity qe and the rate constant k2 can be calculated from the slope (1/qe) and the intercept (1/(k2$q2e )) of the plot of t/qt against time t. As observed in Fig. 4, the kinetics of Pb(II) adsorption by MLP shows perfect correlation with the pseudo-second-order model, with a correlation coefficient (R2) of 0.9999. Furthermore, the theoretical value of qe(cal.) obtained from the pseudo-second-order kinetic model was 9.46 mg/g, which was in close agreement with the experimental value (qe(exp.)) of (9.39 mg/g). These results suggest that Pb(II) adsorption on MLP follows a pseudo-second-order
(4)
where KF and 1/n are the Freundlich isotherm constants for metal ion adsorption. Using Eq. (4), KF and n can be calculated from the intercept (log KF) and slope (1/n) of the plot of log qe versus log Ce. The corresponding parameters of the two isotherm models obtained in the present study are listed in Table 1. As shown in Table 1, the relatively high values of the correlation coefficient (R2) for the Langmuir and Freundlich models indicate that the equilibrium data for MLP was in good agreement with both models. Additionally, the value of 1/n obtained from the Freundlich isotherm falls within the range 0e1.0, indicating favorable adsorption (Stephen Inbaraj and Sulochana, 2004). Based on the Langmuir isotherm, the maximum Pb(II) adsorption capacity of MLP was 78.74 mg/g, which was much higher than that reported for other bioadsorbents (e.g., 7.97 mg/g for modified banana peels (Annadurai et al., 2003), 6.366 mg/g for untreated sugarcane bagasse, and 7.297 mg/g for modified sugarcane bagasse (MartinLara et al., 2010)). These results demonstrate the potential of MLP as a viable bioadsorbent for the removal of heavy metals from aqueous solutions. 3.5. Thermodynamic studies Study of the adsorption thermodynamics is used to identify the adsorption mechanisms (Wang et al., 2010). Thermodynamic parameters such as DG , DH , and DS were calculated from the experimental data using the Van't Hoff equation (Eq. (5)):
Table 1 Isotherm constants for the adsorption of Pb(II) onto the magnetic litchi peels (MLP). Metal Fig. 4. Pseudo-second-order kinetics of Pb(II) adsorption by MLP (conditions: volume of the medium: 50 mL; initial concentration 50 mg/L; dose of MLP adsorbent: 0.25 g; temperature: 25 C; pH: 6.0).
Pb(II)
Langmuir isotherm constants
Feundlich isotherm constants
qmax (mg/g)
b (L/mg)
R2
b
KF
n
R2
78.74
0.073
0.99
0.21
2.25
1.44
0.98
R. Jiang et al. / Journal of Environmental Management 155 (2015) 24e30 Table 2 Thermodynamic parameters for the adsorption of Pb(II) onto the magnetic litchi peels (MLP). Temperature (K)
DG (kJ mol1)
DH (kJ mol1)
DS (J mol1 K1)
288 298 308 318 328
7.13 7.19 7.24 7.30 7.35
5.56
5.44
DG DH DS ¼ þ ln Kd ¼ RT RT R
(5)
T is the temperature in K, R is the ideal gas constant (8.314 J/ (mol K)), and Kd is the thermodynamic equilibrium constant, which is expressed as follows:
Kd ¼
Cad;c Ce
(6)
where Cad,c is the concentration of metal ions on the adsorbent at equilibrium and Ce is the equilibrium concentration of heavy metals in the aqueous solution (mg/L). The values of DH and DS can be
29
calculated from the slope and intercept of a plot of lnKd versus 1/T, respectively (Liang et al., 2010); the values of DG at different temperatures were calculated from Eq. (5). As shown in Table 2, the negative values of DG at various temperatures indicated the spontaneous nature of Pb(II) adsorption by MLP. In addition, the decrease in DG with increased temperature might be attributed to fewer available active sites on the surface of MLP or the increased mobility of Pb(II) in the aqueous solution at higher temperatures (Singh and Pant, 2004). The negative value of DH suggests that the adsorption process is exothermic, and the positive value of DS confirmed an increase in randomness at the solidesolution interface during the Pb(II) adsorption process. The data are comparable to prior thermodynamic results obtained for the adsorption of As(III) on activated alumina (Singh and Pant, 2004).
3.6. Separation of solideliquid The solideliquid separation process is of significance for the practical implementation of an adsorption system. Sedimentation is possibly the simplest solideliquid separation process. Fig. 5 illustrates the separation of bioadsorbents from homogenized LP or
Fig. 5. Analysis of LP and MLP separation from a homogenized solution. (a, The separation process of LP from homogenized solutions only under the action of gravity. b, The separation process of MLP from homogenized solutions only under the action of gravity. c, The separation process of MLP from homogenized solutions with the application of an external magnetic field).
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R. Jiang et al. / Journal of Environmental Management 155 (2015) 24e30
MLP solutions, with and without the application of an external magnetic field. In the absence of an external magnetic field, the time required for the complete sedimentation of LP and MLP to the bottom of the tubes was 130 5400 and 120 700, respectively. However, in the presence of an external magnetic field, the time required for complete separation of MLP was dramatically reduced to 3000 67. This finding clearly suggests that the novel magnetic bioadsorbent based on litchi peels coated with Fe3O4 can be separated from solutions more easily and rapidly in an external magnetic field than untreated bioadsorbents. These results are thus of great significance for the reuse and further treatment of exhausted bioadsorbents. 4. Conclusion A novel magnetic adsorbent, magnetic litchi peel (MLP), was successfully synthesized from litchi peels and Fe3O4. MLP exhibited high adsorption efficiency for Pb(II), achieving maximal adsorption at 120 min and pH 6.0, by using an adsorbent dosage of 5 g/L. Pb(II) adsorption on MLP followed the Langmuir as well as Freundlich isotherm models, yielding a maximum adsorption capacity of 78.74 mg/g. The kinetics of Pb(II) adsorption on MLP followed a pseudo-second-order model. The thermodynamic results suggest that the Pb(II) adsorption process on MLP is spontaneous and exothermic. Furthermore, in an external magnetic field, MLP could be rapidly and easily separated from homogenized solutions. Acknowledgments This study was funded by the National Natural Science Foundation of China (Grant No.: 51208173) and the Shandong Province Natural Science Foundation of China (No. ZR2014EEM005). References Annadurai, G., Juang, R.S., Lee, D.J., 2003. Adsorption of heavy metals from water using banana and orange peels. Water Sci. Technol. 47, 185e190. Areco, M.M., Saleh-Medina, L., Trinelli, M.A., Marco-Brown, J.L., Afonso, M.S., 2013. Adsorption of Cu(II), Zn(II), Cd(II) and Pb(II) by dead Avena fatua biomass and the effect of these metals on their growth. Colloid. Surf. B 110, 305e312. Bulgariu, D., Bulgariu, L., 2012. Equilibrium and kinetics studies of heavy metal ions biosorption on green algae waste biomass. Bioresour. Technol. 103, 489e493. Chang, M.Y., Juang, R.S., 2004. Adsorption of tannic acid, humic acid, and dyes from water using the composite of chitosan and activated clay. J. Colloid Interface Sci. 278, 18e25. Chiban, M., Soudani, A., Sinan, F., Persin, M., 2012. Wastewater treatment by batch adsorption method onto micro-particles of dried Withania frutescens plant as a new adsorbent. J. Environ. Manage. 95, 61e65. Daneshvar, N., Salari, D., Aber, S., 2002. Chromium adsorption and Cr(VI) reduction to trivalent chromium in aqueous solutions by soya cake. J. Hazard. Mater. 94, 49e61. Dong, C.L., Chen, W., Liu, C., Liu, Y., Liu, H.C., 2014. Synthesis of magnetic chitosan nanoparticle and its adsorption property for humic acid from aqueous solution. Colloid. Surf. A 446, 179e189. Gode, F., Pehlivan, E., 2005. Adsorption of Cr(III) ions by Turkish brown coals. Fuel Process. Technol. 86, 875e884. Gupta, V.K., Imran, A., 2004. Removal of lead and chromium from wastewater using bagasse fly ash-a sugar industry waste. J. Colloid Interface Sci. 271, 321e328. Ho, Y.S., 2005. Effect of pH on lead removal from water using tree fern as the sorbent. Bioresour. Technol. 96, 1292e1296. Ho, Y.S., Mckay, G., 1998. Sorption of dye from aqueous solution by peat. Chem. Eng.
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