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Regeneration of magnetic biochar derived from eucalyptus leaf residue for lead(II) removal
Bioresource Technology xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Short Communication
Regeneration of magnetic biochar derived from eucalyptus leaf residue for lead(II) removal Sheng-ye Wang a, Yan-kui Tang a,⇑, Cheng Chen a, Jin-tao Wu a, Zhining Huang a, Ya-yuan Mo a, Kai-xuan Zhang a, Ji-bo Chen b a b
College of Environmental Science and Engineering, Guangxi University, Nanning 530004, China The Environmental Protection Department of the Guangxi Zhuang Autonomous Region, Nanning 530028, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
EDTA-2Na was effective for Pb(II)
desorption and adsorbent regeneration. Leaching amount of Fe ions was around 1 mg/g in each regeneration cycle. Higher SBET and pore volume were found in regenerated adsorbents. Pb(II) removal efficiency maintains in certain level after six regeneration cycles. Regenerated adsorbents showed good magnetic separation performance.
a r t i c l e
i n f o
Article history: Received 15 February 2015 Received in revised form 26 March 2015 Accepted 29 March 2015 Available online xxxx Keywords: Magnetic biochar Lead adsorption Adsorbent regeneration Separation
a b s t r a c t Regeneration of Pb-loaded magnetic biochar prepared with eucalypts leaf residue was studied by using EDTA-2Na as a regenerant. The desorption efficiency was found to be 84.1% in 120 min with iron leaching amount of 1.1 mg g 1. Higher SBET and pore volume were observed in regenerated magnetic biochars and no significant band shifts occurred in FTIR spectra during 6 regeneration cycles. The decrease of Pb(II) adsorption capacity (from 52.4 to 41.5 mg g 1) was only found in the first regeneration cycle. Magnetic separation performance of adsorbents was not significantly affected by multiple regeneration cycles. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction A large amount of Pb-contained wastewater is inevitably discharged from mining-related industries yearly in China, as it is one of the world’s largest producer and consumer of Pb (Zhang et al., 2012b). Many people, especially children (Cheng and Hu, 2010; Wang and Zhang, 2006), in several parts have suffered from ⇑ Corresponding author. Tel.: +86 13977187116; fax: +86 0771 3273440. E-mail address: [email protected] (Y.-k. Tang).
elevated blood lead levels (Yang et al., 2014). Therefore, treatment of Pb-contaminated wastewater has been one of the major tasks in China. In the previous study, magnetic biochar prepared with eucalyptus leaf residue (MELRC) has been successfully applied for chromium-contained wastewater treatment (Wang et al., 2014), and its high removal capacity for Pb(II) was also found in later studies. Compared to other conventional adsorbents for adsorption-based treatment techniques, magnetic biochar stands out to be a potential adsorbent because it not only remains remarkable adsorption
http://dx.doi.org/10.1016/j.biortech.2015.03.139 0960-8524/Ó 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Wang, S.-y., et al. Regeneration of magnetic biochar derived from eucalyptus leaf residue for lead(II) removal. Bioresour. Technol. (2015), http://dx.doi.org/10.1016/j.biortech.2015.03.139
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S.-y. Wang et al. / Bioresource Technology xxx (2015) xxx–xxx
capacity of biochar (Oliveira et al., 2002), but also possesses magnetism that makes it possible for the fast separation by an external magnetic field (Ren et al., 2015). Reuse of adsorbents and recovery of metals are big challenges and have been major research focuses worldwide for many years (Cao et al., 2010; Nassar, 2010; Zhang et al., 2012a). A convenient and economical regeneration method can reduce not only the cost of transportation but also the amount of wasted metal-loaded adsorbents. In the case of magnetic biochars, which are composed of both magnetic particles (such as ferriferous oxides (Wang et al., 2012)) and biochar, regeneration is much more necessary to properly valuate their economic efficiency and technical feasibility for application due to the extra production cost caused by the magnetic modification (Han et al., 2014). On the other hand, the regeneration process may easily lead to the reduction of magnetism because of the loss of magnetic materials, which may negatively affect magnetic separation performance of the regenerated adsorbents. Although it has been reported earlier that Pb(II) in aqueous solution can be effectively removed by magnetic biochar (Yan et al., 2014), few researches have studied the regeneration of used magnetic biochar and its effect on the magnetic separation performance. The aim of this work was therefore to study the regeneration of Pb-loaded MELRC. In addition, the variations of physico-chemical properties, adsorption capacity and magnetic separation performance of the regenerated MELRC were also investigated. 2. Methods 2.1. Materials Eucalyptus (Eucalyptus urophylla) leaves were obtained from the campus of Guangxi University in Nanning, China. After being washed, the leaves were crushed and dried at 40 °C for further use. NdFeB magnets (6 cm 6 cm 1.5 cm) with a magnetic filed of 0.5 T were purchased from Shenzhen Dongsheng Magn. Mater. Co., Ltd., China.
2.4. Adsorbent characterization The functional groups of MELRC were analyzed using Fourier transform infrared spectroscopy attenuated total reflectance (FTIR-ATR, Thermo Nicolet Corporation, USA). The specific surface areas (SBET) of original and regenerated MELRC were measured from the N2 adsorption isotherms by a surface analyzer (NOVA 4200e, USA). Samples were outgassed at 300 °C for 12 h before the N2 adsorption. The micro- and total pore volumes (Vmi, Vt) were obtained from the amount of N2 adsorbed at a pt of 0.1 and 0.95, respectively. Mesopore volume (Vme) was the volume of N2 adsorbed (Vt) at p/p0 = 0.95 minus the Vmi at p/p0 = 0.10 (Zhao et al., 2007). Hysteresis measurements were carried out on a Vibrating Sample Magnetometer (VSM, Lakeshore 7410, USA). Before the measurement, samples were meshed and 100–160 mesh adsorbents were used. The point of zero charge (pHPZC) of the adsorbents were measured by using 0.1 mol L 1 KNO3 aqueous solutions at pH 2, 5, 7, 9 and 11 adjusted by 0.1 mol L 1 HNO3 or 0.1 mol L 1 NaOH aqueous solutions. Ten milliliter of these solutions was added into conical flasks with 0.02 g of the samples and the mixtures were shaken for 48 h and the pHs of supernatants were measured. 2.5. Adsorption experiments Batch adsorption studies were carried out by adding a desired weight of magnetic eucalyptus leaf residue biochar (MELRC) into 100 mL conical flasks containing Pb(II) solution with a concentration of 100 mg L 1. After agitating at 150 rpm in a mechanical shaker for a desired time, the conical flasks were withdrawn and the mixtures were filtrated through 0.45 lm pore size nylon membrane filters. The Pb(II) concentrations were determined using Atomic Absorption Spectroscopy (AAS, AA-7000 Shimadzu, Japan). 2.6. Separation experiments
All chemicals, purchased from Sinopharm Chemical Reagent Beijing Co., Ltd., China, are either AR or GR-grades. Pb(II) aqueous solutions were prepared by diluting the stock solutions of Pb (NO3)2 (1000 mg L 1) to desired concentration with deionized water. Different solution pHs were adjusted by HCl (0.1 mol L 1) and NaOH (0.1 mol L 1).
The separation of MELRC from liquid was studied by batch experiments. One hundred milliliters of deionized water was taken into plastic cups (U = 4.5 cm). Then a desired weight of MELRC was added into each cup. After stirring the solution for about 10 min with an agitator at a speed of 60 rpm, the cups were then put on the magnets. When the separation process is done, 20 mL of the water sample 2 cm above the bottom in each cup was collected by an injector through a pipe for turbidity measurement using a turbid meter (HACH 2100 N, USA).
2.3. Preparation of adsorbent
2.7. Adsorbent regeneration
The magnetic biochar was prepared according to reported procedure (Wang et al., 2014). Briefly, the eucalyptus leaf residue obtained from eucalyptus leaves after essential oil extraction was carbonized at 400 °C. Thereafter the product was mixed with ZnCl2 and the combination was exposed to 700 °C for 2 h. Then the product was washed with HCl (0.1 mol L 1) followed by deionized water until the pH was 7.0 ± 0.5. After that, the biochar was dried and sieved (100–160 mesh). The biochar was mixed with FeCl3 and FeSO4 solutions and stirred at room temperature for 20 min. Thereafter the solution pH was adjusted to 10–11 by NaOH. After being stirred for 1 h, the suspension was boiled for 1 h, filtered and washed with deionized water and ethanol. The magnetic biochar was then dried at 70 °C for 12 h in a hot air oven.
Before the desorption process, MELRC was loaded with Pb(II) in the following condition: pH-5, Pb(II) concentration-800 mg L 1, adsorbent dose-20 g L 1 solution, contact time-2 h and temperature-25 °C. The Pb-loaded sample was separated by filtration, washed with deionized water and dried. Batch desorption studies were carried out by adding a desired weight of Pb-loaded MELRC into 150 mL conical flasks containing regenerant solution. After agitating at 150 rpm and 25 °C in a mechanical shaker for a desired time, the conical flasks were withdrawn and the mixtures were filtrated through quantitative filter paper. The filter cake was collected, washed by deionized water until neutral and dried for the next adsorption–desorption cycle. The filtrate was then filtrated through 0.45 lm pore size nylon membrane filter for detection of metal ions.
2.2. Chemicals
Please cite this article in press as: Wang, S.-y., et al. Regeneration of magnetic biochar derived from eucalyptus leaf residue for lead(II) removal. Bioresour. Technol. (2015), http://dx.doi.org/10.1016/j.biortech.2015.03.139
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2.8. Statistical analysis
an earlier report (Tristão et al., 2015). It was thus concluded that EDTA-2Na can be an effective regenerant in present work.
All experiments were replicated at least twice. Average values and standard deviations (SD) were calculated. Data were tested for statistical significance with one-way analysis of variance (ANOVA). A value of p < 0.05 was considered significant.
3. Results and discussion 3.1. Pb(II) removal by MELRC The effects of solution pH, adsorbent dose and temperature on Pb(II) removal and adsorption isotherms and kinetic have been explained in details in Supporting information.
3.2. Regeneration and reuse 3.2.1. Choice of the regenerants Experiments were conducted to find an effective agent for Pbloaded MELRC regeneration with a high desorption efficiency of Pb(II) and a small leaching amount of iron ions. Five types of solutions, including 0.1 mol L 1 HCl, acetic acid (HAc), EDTA-2Na and deionized water, were selected as regenerants and twenty-five milliliters of each were added into conical flasks with 0.25 g Pbloaded MELRC. Fig. 1 shows that there were few Pb(II) ions eluted by deionized water, illustrating that Pb(II) ions were firmly adsorbed onto MELRC. Among all these selected regenerants, EDTA-2Na was the most efficient with a desorption efficiency of 91.1%. It is contributed to the high value of the conditional formation constant of the complex Pb-EDTA, which is 3.55 1011 when the pH is 5.0 (Deng et al., 2007). Moreover, compared with HCl, regeneration with EDTA-2Na solution leached less iron from MELRC, helping maintain the magnetism of MELRC. Strong iron leaching from Fe3O4 shaken with HCl solution was also found by
Fig. 1. Desorption performance of different regenerants for Pb-loaded MELRC.
3.2.2. Desorption time and solid/liquid ratio The effect of contact time on desorption of Pb(II) was studied with a solid–liquid ratio of 10 g L 1 using 0.1 mol L 1 EDTA-2Na as the regenerant. The results are shown in Fig. S4. It was clear that both desorption efficiency of Pb(II) and leaching amount of iron increased with increasing time. Most of Pb(II), about 84%, was desorbed during the first 120 min and for the rest 360 min, the desorption efficiency only increased by 3.3%. However, the leaching amount of iron sustainably increased throughout the whole process (1.11 mg L 1 at 120 min and 2.35 mg L 1 at 480 min). Thus, the contact time chosen was 120 min. Desorption studies with a various solid/liquid (the mass of Pbloaded adsorbent/the volume of regenerant solution) ratios were also measured. The experiments were carried out as mentioned in previous section, keeping the contact time of 120 min. The results are shown in Fig. S5, a maximum percentage of desorption (95.2%) was found at the lowest solid/liquid ratio of 2 g L 1. Further increase in solid/liquid ratio decreased the desorption efficiency. However, lower solid/liquid ratio leads to more EDTA-2Na solution use to regenerate the same amount of adsorbent, which will enhance the cost and, on the other hand, more desorption liquor with a lower Pb(II) content is generated again. This is not favorable for recovery of Pb(II). Therefore, take desorption efficiency and Pb(II) concentration into consideration, a solid/liquid ratio of 10 g L 1 is suitable, which means that treating 5 L of the wastewater containing around 100 mg L 1 Pb(II) generates only 1 L of leach liquor containing more than 350 mg L 1 Pb(II).
3.2.3. Multiple regeneration cycles Multi-regeneration study was carried out through adsorption– desorption cycles. For adsorption step, Pb-loaded MELRC was prepared with the dose of 2.0 g L 1, contact time of 120 min, Pb(II) concentration of 108 mg L 1 and solution pH of 5.01. In the case of desorption step, as-prepared Pb-loaded MELRC was desorbed by 0.1 mol L 1 EDTA-2Na with a solid/liquid ratio of 10 g L 1 and contact time of 120 min. As shown in Table 1, the SBET, Vmi, Vme, Vt and pHPZC of the magnetic biochar increased significantly during the first regeneration cycle and continued to increase slightly in further cycles. These increases might be due to the dissolution of ash and the loss of iron oxides, which were dispersed on the surface of MELRC. Fig. S6 shows the leaching amount of iron in each cycle. Loss of iron was not found during adsorption, while it was only 1.1 mg g 1 during desorption, indicating that the loss of the magnetic particles for MELRC regeneration was not a concern. Moreover, FTIR-ATR spectra of original and regenerated MELRC (shown in Fig. S7) showed that active groups on MELRC, such as C@C (1537.63 cm 1), C@O (1108.24 cm 1) and Fe–O (562.31 cm 1), were not affected by regeneration process.
Table 1 Physico–chemical characteristics of original and regenerated MELRC. Properties
Original
Cycle 1
Cycle 2
Cycle 3
Cycle 4
Cycle 5
Cycle 6
SBET (m2 g 1) Vmi (cm3 g 1) Vme (cm3 g 1) Vt (cm3 g 1) Ms. (emu g 1) pHPZC
501.53 0.20 0.17 0.37 16.12 7.11 ± 0.02
543.90 0.23 0.27 0.50 15.83 7.18 ± 0.04
555.97 0.23 0.28 0.51 15.57 7.15 ± 0.03
557.73 0.24 0.28 0.52 15.11 7.16 ± 0.02
569.22 0.24 0.28 0.52 14.71 7.16 ± 0.02
576.66 0.24 0.28 0.52 13.21 7.19 ± 0.00
595.08 0.24 0.29 0.53 13.05 7.13 ± 0.06
Please cite this article in press as: Wang, S.-y., et al. Regeneration of magnetic biochar derived from eucalyptus leaf residue for lead(II) removal. Bioresour. Technol. (2015), http://dx.doi.org/10.1016/j.biortech.2015.03.139
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this effect became ignorable after 20 min. For instance, within the same separation time of 1 min, the turbidity of supernatant containing original MELRC was 23.3 NTU, while that of supernatant containing regenerated (6 times) MELRC was 43.3 NTU and the two turbidities became very close (2.2 and 3.1 NTU, respectively) in 20 min. A previous study showed that the turbidity of supernatant with a same content of original MELRC was about 80 NTU after separating by gravity in 20 min (Wang et al., 2014), which is far more than those of supernatant containing regenerated MELRCs in this study. To conclude, the small amount of dissolved iron did not affect significantly the magnetic separation of the regenerated MELRC. 4. Conclusions
Fig. 2. Removal efficiency of Pb(II) by regenerated MELRC.
By using EDTA-2Na as a regenerant, desorption of Pb(II) and regeneration of the Pb-loaded magnetic biochar can be achieved simultaneously with little loss of iron. A reduction in Pb(II) adsorption capacity was observed only in the first regeneration cycle and further cycles do not affect the Pb(II) removal efficiency. Meanwhile, the regenerated adsorbents still present a good adsorption and magnetic separation performance. In conclusion, the regeneration process for the magnetic biochar is relatively simple, time saving and can be carried out in situ, suggesting that reuse of this adsorbent is technically and economically feasible. Acknowledgements The authors would like to acknowledge financial support provided by the National Natural Science Foundation of China (No. 51168001) and the Special Fund from the Central Government of China for Heavy Metal Pollution Prevention (No. 20140944). Appendix A. Supplementary data
Fig. 3. Magnetic separation performance of original and regenerated MELRC.
Fig. 2 compares the Pb(II) removal performance of MELRCs with different regeneration cycles. Adsorption capacity of first-regenerated MELRC decreased from 52.4 mg g 1 to 41.5 mg g 1. This diminution is due to the effect of the prior regeneration process, which solubilized some parts of MELRC, changed superficial structures of MELRC and subsequently led to loss or blockage of adsorption sites (Lodeiro et al., 2006). An increased pHPZC of the first-regenerated MELRC also does not favor the Pb(II) adsorption (Anoon Krishnan and Anirudhan, 2003). Statistical analysis showed that Pb(II) removal maintained almost the same level during the further five cycles (p-value >0.05), indicating that there were no irreversible binding sites on the surface of MELRC (Khodaverdiloo et al., 2012). The results confirm that MELRC is a reusable adsorbent for Pb(II) removal. The experiments of magnetic separation were carried out with the MELRC content of 2 g L 1 and different separation time (1, 2, 5, 10 and 20 min). Although the saturation magnetization (Ms) of original MELRC was higher than that of regenerated ones (shown in Table 1), the turbidity of supernatant containing first-regenerated MELRC after magnetic separation was smaller than that of supernatant containing original MELRC (shown in Fig. 3). This can be explained as that original MELRC contained more tiny particles, which are hardly separated by a low magnetic field (Yavuz et al., 2006). Those tiny particles were lost during adsorption and desorption processes. Regeneration cycles (more than twice) negatively affected the separation performance in the first 5 min, but
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.biortech.2015.03. 139. References Anoon Krishna, N.K., Anirudhan, T., 2003. Removal of cadmium (II) from aqueous solutions by steam-activated sulphurised carbon prepared from sugar-cane bagasse pith: kinetics and equilibrium studies. Water SA 29 (2), 147–156. Cao, J., Tan, Y., Che, Y., Xin, H., 2010. Novel complex gel beads composed of hydrolyzed polyacrylamide and chitosan: an effective adsorbent for the removal of heavy metal from aqueous solution. Bioresour. Technol. 101 (7), 2558–2561. Cheng, H., Hu, Y., 2010. Lead (Pb) isotopic fingerprinting and its applications in lead pollution studies in China: a review. Environ. Pollut. 158 (5), 1134–1146. Deng, L., Su, Y., Su, H., Wang, X., Zhu, X., 2007. Sorption and desorption of lead (II) from wastewater by green algae Cladophora fascicularis. J. Hazard. Mater. 143 (1), 220–225. Han, Z., Sani, B., Mrozik, W., Obst, M., Beckingham, B., Karapanagioti, H.K., Werner, D., 2014. Magnetite impregnation effects on the sorbent properties of activated carbons and biochars. Water Res. 70, 394–403. Khodaverdiloo, H., Rahmanian, M., Rezapour, S., Dashtaki, S.G., Hadi, H., Han, F., 2012. Effect of wetting–drying cycles on redistribution of lead in some semiarid zone soils spiked with a lead salt. Pedosphere 22 (3), 304–313. Lodeiro, P., Herrero, R., Sastre de Vicente, M., 2006. Batch desorption studies and multiple sorption–regeneration cycles in a fixed-bed column for Cd (II) elimination by protonated Sargassum muticum. J. Hazard. Mater. 137 (3), 1649–1655. Nassar, N.N., 2010. Rapid removal and recovery of Pb (II) from wastewater by magnetic nanoadsorbents. J. Hazard. Mater. 184 (1), 538–546. Oliveira, L.C., Rios, R.V., Fabris, J.D., Garg, V., Sapag, K., Lago, R.M., 2002. Activated carbon/iron oxide magnetic composites for the adsorption of contaminants in water. Carbon 40 (12), 2177–2183. Ren, J., Li, N., Li, L., An, J.-K., Zhao, L., Ren, N.-Q., 2015. Granulation and ferric oxides loading enable biochar derived from cotton stalk to remove phosphate from water. Bioresour. Technol. 178, 119–125.
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Please cite this article in press as: Wang, S.-y., et al. Regeneration of magnetic biochar derived from eucalyptus leaf residue for lead(II) removal. Bioresour. Technol. (2015), http://dx.doi.org/10.1016/j.biortech.2015.03.139
Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Short Communication
Regeneration of magnetic biochar derived from eucalyptus leaf residue for lead(II) removal Sheng-ye Wang a, Yan-kui Tang a,⇑, Cheng Chen a, Jin-tao Wu a, Zhining Huang a, Ya-yuan Mo a, Kai-xuan Zhang a, Ji-bo Chen b a b
College of Environmental Science and Engineering, Guangxi University, Nanning 530004, China The Environmental Protection Department of the Guangxi Zhuang Autonomous Region, Nanning 530028, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
EDTA-2Na was effective for Pb(II)
desorption and adsorbent regeneration. Leaching amount of Fe ions was around 1 mg/g in each regeneration cycle. Higher SBET and pore volume were found in regenerated adsorbents. Pb(II) removal efficiency maintains in certain level after six regeneration cycles. Regenerated adsorbents showed good magnetic separation performance.
a r t i c l e
i n f o
Article history: Received 15 February 2015 Received in revised form 26 March 2015 Accepted 29 March 2015 Available online xxxx Keywords: Magnetic biochar Lead adsorption Adsorbent regeneration Separation
a b s t r a c t Regeneration of Pb-loaded magnetic biochar prepared with eucalypts leaf residue was studied by using EDTA-2Na as a regenerant. The desorption efficiency was found to be 84.1% in 120 min with iron leaching amount of 1.1 mg g 1. Higher SBET and pore volume were observed in regenerated magnetic biochars and no significant band shifts occurred in FTIR spectra during 6 regeneration cycles. The decrease of Pb(II) adsorption capacity (from 52.4 to 41.5 mg g 1) was only found in the first regeneration cycle. Magnetic separation performance of adsorbents was not significantly affected by multiple regeneration cycles. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction A large amount of Pb-contained wastewater is inevitably discharged from mining-related industries yearly in China, as it is one of the world’s largest producer and consumer of Pb (Zhang et al., 2012b). Many people, especially children (Cheng and Hu, 2010; Wang and Zhang, 2006), in several parts have suffered from ⇑ Corresponding author. Tel.: +86 13977187116; fax: +86 0771 3273440. E-mail address: [email protected] (Y.-k. Tang).
elevated blood lead levels (Yang et al., 2014). Therefore, treatment of Pb-contaminated wastewater has been one of the major tasks in China. In the previous study, magnetic biochar prepared with eucalyptus leaf residue (MELRC) has been successfully applied for chromium-contained wastewater treatment (Wang et al., 2014), and its high removal capacity for Pb(II) was also found in later studies. Compared to other conventional adsorbents for adsorption-based treatment techniques, magnetic biochar stands out to be a potential adsorbent because it not only remains remarkable adsorption
http://dx.doi.org/10.1016/j.biortech.2015.03.139 0960-8524/Ó 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Wang, S.-y., et al. Regeneration of magnetic biochar derived from eucalyptus leaf residue for lead(II) removal. Bioresour. Technol. (2015), http://dx.doi.org/10.1016/j.biortech.2015.03.139
2
S.-y. Wang et al. / Bioresource Technology xxx (2015) xxx–xxx
capacity of biochar (Oliveira et al., 2002), but also possesses magnetism that makes it possible for the fast separation by an external magnetic field (Ren et al., 2015). Reuse of adsorbents and recovery of metals are big challenges and have been major research focuses worldwide for many years (Cao et al., 2010; Nassar, 2010; Zhang et al., 2012a). A convenient and economical regeneration method can reduce not only the cost of transportation but also the amount of wasted metal-loaded adsorbents. In the case of magnetic biochars, which are composed of both magnetic particles (such as ferriferous oxides (Wang et al., 2012)) and biochar, regeneration is much more necessary to properly valuate their economic efficiency and technical feasibility for application due to the extra production cost caused by the magnetic modification (Han et al., 2014). On the other hand, the regeneration process may easily lead to the reduction of magnetism because of the loss of magnetic materials, which may negatively affect magnetic separation performance of the regenerated adsorbents. Although it has been reported earlier that Pb(II) in aqueous solution can be effectively removed by magnetic biochar (Yan et al., 2014), few researches have studied the regeneration of used magnetic biochar and its effect on the magnetic separation performance. The aim of this work was therefore to study the regeneration of Pb-loaded MELRC. In addition, the variations of physico-chemical properties, adsorption capacity and magnetic separation performance of the regenerated MELRC were also investigated. 2. Methods 2.1. Materials Eucalyptus (Eucalyptus urophylla) leaves were obtained from the campus of Guangxi University in Nanning, China. After being washed, the leaves were crushed and dried at 40 °C for further use. NdFeB magnets (6 cm 6 cm 1.5 cm) with a magnetic filed of 0.5 T were purchased from Shenzhen Dongsheng Magn. Mater. Co., Ltd., China.
2.4. Adsorbent characterization The functional groups of MELRC were analyzed using Fourier transform infrared spectroscopy attenuated total reflectance (FTIR-ATR, Thermo Nicolet Corporation, USA). The specific surface areas (SBET) of original and regenerated MELRC were measured from the N2 adsorption isotherms by a surface analyzer (NOVA 4200e, USA). Samples were outgassed at 300 °C for 12 h before the N2 adsorption. The micro- and total pore volumes (Vmi, Vt) were obtained from the amount of N2 adsorbed at a pt of 0.1 and 0.95, respectively. Mesopore volume (Vme) was the volume of N2 adsorbed (Vt) at p/p0 = 0.95 minus the Vmi at p/p0 = 0.10 (Zhao et al., 2007). Hysteresis measurements were carried out on a Vibrating Sample Magnetometer (VSM, Lakeshore 7410, USA). Before the measurement, samples were meshed and 100–160 mesh adsorbents were used. The point of zero charge (pHPZC) of the adsorbents were measured by using 0.1 mol L 1 KNO3 aqueous solutions at pH 2, 5, 7, 9 and 11 adjusted by 0.1 mol L 1 HNO3 or 0.1 mol L 1 NaOH aqueous solutions. Ten milliliter of these solutions was added into conical flasks with 0.02 g of the samples and the mixtures were shaken for 48 h and the pHs of supernatants were measured. 2.5. Adsorption experiments Batch adsorption studies were carried out by adding a desired weight of magnetic eucalyptus leaf residue biochar (MELRC) into 100 mL conical flasks containing Pb(II) solution with a concentration of 100 mg L 1. After agitating at 150 rpm in a mechanical shaker for a desired time, the conical flasks were withdrawn and the mixtures were filtrated through 0.45 lm pore size nylon membrane filters. The Pb(II) concentrations were determined using Atomic Absorption Spectroscopy (AAS, AA-7000 Shimadzu, Japan). 2.6. Separation experiments
All chemicals, purchased from Sinopharm Chemical Reagent Beijing Co., Ltd., China, are either AR or GR-grades. Pb(II) aqueous solutions were prepared by diluting the stock solutions of Pb (NO3)2 (1000 mg L 1) to desired concentration with deionized water. Different solution pHs were adjusted by HCl (0.1 mol L 1) and NaOH (0.1 mol L 1).
The separation of MELRC from liquid was studied by batch experiments. One hundred milliliters of deionized water was taken into plastic cups (U = 4.5 cm). Then a desired weight of MELRC was added into each cup. After stirring the solution for about 10 min with an agitator at a speed of 60 rpm, the cups were then put on the magnets. When the separation process is done, 20 mL of the water sample 2 cm above the bottom in each cup was collected by an injector through a pipe for turbidity measurement using a turbid meter (HACH 2100 N, USA).
2.3. Preparation of adsorbent
2.7. Adsorbent regeneration
The magnetic biochar was prepared according to reported procedure (Wang et al., 2014). Briefly, the eucalyptus leaf residue obtained from eucalyptus leaves after essential oil extraction was carbonized at 400 °C. Thereafter the product was mixed with ZnCl2 and the combination was exposed to 700 °C for 2 h. Then the product was washed with HCl (0.1 mol L 1) followed by deionized water until the pH was 7.0 ± 0.5. After that, the biochar was dried and sieved (100–160 mesh). The biochar was mixed with FeCl3 and FeSO4 solutions and stirred at room temperature for 20 min. Thereafter the solution pH was adjusted to 10–11 by NaOH. After being stirred for 1 h, the suspension was boiled for 1 h, filtered and washed with deionized water and ethanol. The magnetic biochar was then dried at 70 °C for 12 h in a hot air oven.
Before the desorption process, MELRC was loaded with Pb(II) in the following condition: pH-5, Pb(II) concentration-800 mg L 1, adsorbent dose-20 g L 1 solution, contact time-2 h and temperature-25 °C. The Pb-loaded sample was separated by filtration, washed with deionized water and dried. Batch desorption studies were carried out by adding a desired weight of Pb-loaded MELRC into 150 mL conical flasks containing regenerant solution. After agitating at 150 rpm and 25 °C in a mechanical shaker for a desired time, the conical flasks were withdrawn and the mixtures were filtrated through quantitative filter paper. The filter cake was collected, washed by deionized water until neutral and dried for the next adsorption–desorption cycle. The filtrate was then filtrated through 0.45 lm pore size nylon membrane filter for detection of metal ions.
2.2. Chemicals
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2.8. Statistical analysis
an earlier report (Tristão et al., 2015). It was thus concluded that EDTA-2Na can be an effective regenerant in present work.
All experiments were replicated at least twice. Average values and standard deviations (SD) were calculated. Data were tested for statistical significance with one-way analysis of variance (ANOVA). A value of p < 0.05 was considered significant.
3. Results and discussion 3.1. Pb(II) removal by MELRC The effects of solution pH, adsorbent dose and temperature on Pb(II) removal and adsorption isotherms and kinetic have been explained in details in Supporting information.
3.2. Regeneration and reuse 3.2.1. Choice of the regenerants Experiments were conducted to find an effective agent for Pbloaded MELRC regeneration with a high desorption efficiency of Pb(II) and a small leaching amount of iron ions. Five types of solutions, including 0.1 mol L 1 HCl, acetic acid (HAc), EDTA-2Na and deionized water, were selected as regenerants and twenty-five milliliters of each were added into conical flasks with 0.25 g Pbloaded MELRC. Fig. 1 shows that there were few Pb(II) ions eluted by deionized water, illustrating that Pb(II) ions were firmly adsorbed onto MELRC. Among all these selected regenerants, EDTA-2Na was the most efficient with a desorption efficiency of 91.1%. It is contributed to the high value of the conditional formation constant of the complex Pb-EDTA, which is 3.55 1011 when the pH is 5.0 (Deng et al., 2007). Moreover, compared with HCl, regeneration with EDTA-2Na solution leached less iron from MELRC, helping maintain the magnetism of MELRC. Strong iron leaching from Fe3O4 shaken with HCl solution was also found by
Fig. 1. Desorption performance of different regenerants for Pb-loaded MELRC.
3.2.2. Desorption time and solid/liquid ratio The effect of contact time on desorption of Pb(II) was studied with a solid–liquid ratio of 10 g L 1 using 0.1 mol L 1 EDTA-2Na as the regenerant. The results are shown in Fig. S4. It was clear that both desorption efficiency of Pb(II) and leaching amount of iron increased with increasing time. Most of Pb(II), about 84%, was desorbed during the first 120 min and for the rest 360 min, the desorption efficiency only increased by 3.3%. However, the leaching amount of iron sustainably increased throughout the whole process (1.11 mg L 1 at 120 min and 2.35 mg L 1 at 480 min). Thus, the contact time chosen was 120 min. Desorption studies with a various solid/liquid (the mass of Pbloaded adsorbent/the volume of regenerant solution) ratios were also measured. The experiments were carried out as mentioned in previous section, keeping the contact time of 120 min. The results are shown in Fig. S5, a maximum percentage of desorption (95.2%) was found at the lowest solid/liquid ratio of 2 g L 1. Further increase in solid/liquid ratio decreased the desorption efficiency. However, lower solid/liquid ratio leads to more EDTA-2Na solution use to regenerate the same amount of adsorbent, which will enhance the cost and, on the other hand, more desorption liquor with a lower Pb(II) content is generated again. This is not favorable for recovery of Pb(II). Therefore, take desorption efficiency and Pb(II) concentration into consideration, a solid/liquid ratio of 10 g L 1 is suitable, which means that treating 5 L of the wastewater containing around 100 mg L 1 Pb(II) generates only 1 L of leach liquor containing more than 350 mg L 1 Pb(II).
3.2.3. Multiple regeneration cycles Multi-regeneration study was carried out through adsorption– desorption cycles. For adsorption step, Pb-loaded MELRC was prepared with the dose of 2.0 g L 1, contact time of 120 min, Pb(II) concentration of 108 mg L 1 and solution pH of 5.01. In the case of desorption step, as-prepared Pb-loaded MELRC was desorbed by 0.1 mol L 1 EDTA-2Na with a solid/liquid ratio of 10 g L 1 and contact time of 120 min. As shown in Table 1, the SBET, Vmi, Vme, Vt and pHPZC of the magnetic biochar increased significantly during the first regeneration cycle and continued to increase slightly in further cycles. These increases might be due to the dissolution of ash and the loss of iron oxides, which were dispersed on the surface of MELRC. Fig. S6 shows the leaching amount of iron in each cycle. Loss of iron was not found during adsorption, while it was only 1.1 mg g 1 during desorption, indicating that the loss of the magnetic particles for MELRC regeneration was not a concern. Moreover, FTIR-ATR spectra of original and regenerated MELRC (shown in Fig. S7) showed that active groups on MELRC, such as C@C (1537.63 cm 1), C@O (1108.24 cm 1) and Fe–O (562.31 cm 1), were not affected by regeneration process.
Table 1 Physico–chemical characteristics of original and regenerated MELRC. Properties
Original
Cycle 1
Cycle 2
Cycle 3
Cycle 4
Cycle 5
Cycle 6
SBET (m2 g 1) Vmi (cm3 g 1) Vme (cm3 g 1) Vt (cm3 g 1) Ms. (emu g 1) pHPZC
501.53 0.20 0.17 0.37 16.12 7.11 ± 0.02
543.90 0.23 0.27 0.50 15.83 7.18 ± 0.04
555.97 0.23 0.28 0.51 15.57 7.15 ± 0.03
557.73 0.24 0.28 0.52 15.11 7.16 ± 0.02
569.22 0.24 0.28 0.52 14.71 7.16 ± 0.02
576.66 0.24 0.28 0.52 13.21 7.19 ± 0.00
595.08 0.24 0.29 0.53 13.05 7.13 ± 0.06
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this effect became ignorable after 20 min. For instance, within the same separation time of 1 min, the turbidity of supernatant containing original MELRC was 23.3 NTU, while that of supernatant containing regenerated (6 times) MELRC was 43.3 NTU and the two turbidities became very close (2.2 and 3.1 NTU, respectively) in 20 min. A previous study showed that the turbidity of supernatant with a same content of original MELRC was about 80 NTU after separating by gravity in 20 min (Wang et al., 2014), which is far more than those of supernatant containing regenerated MELRCs in this study. To conclude, the small amount of dissolved iron did not affect significantly the magnetic separation of the regenerated MELRC. 4. Conclusions
Fig. 2. Removal efficiency of Pb(II) by regenerated MELRC.
By using EDTA-2Na as a regenerant, desorption of Pb(II) and regeneration of the Pb-loaded magnetic biochar can be achieved simultaneously with little loss of iron. A reduction in Pb(II) adsorption capacity was observed only in the first regeneration cycle and further cycles do not affect the Pb(II) removal efficiency. Meanwhile, the regenerated adsorbents still present a good adsorption and magnetic separation performance. In conclusion, the regeneration process for the magnetic biochar is relatively simple, time saving and can be carried out in situ, suggesting that reuse of this adsorbent is technically and economically feasible. Acknowledgements The authors would like to acknowledge financial support provided by the National Natural Science Foundation of China (No. 51168001) and the Special Fund from the Central Government of China for Heavy Metal Pollution Prevention (No. 20140944). Appendix A. Supplementary data
Fig. 3. Magnetic separation performance of original and regenerated MELRC.
Fig. 2 compares the Pb(II) removal performance of MELRCs with different regeneration cycles. Adsorption capacity of first-regenerated MELRC decreased from 52.4 mg g 1 to 41.5 mg g 1. This diminution is due to the effect of the prior regeneration process, which solubilized some parts of MELRC, changed superficial structures of MELRC and subsequently led to loss or blockage of adsorption sites (Lodeiro et al., 2006). An increased pHPZC of the first-regenerated MELRC also does not favor the Pb(II) adsorption (Anoon Krishnan and Anirudhan, 2003). Statistical analysis showed that Pb(II) removal maintained almost the same level during the further five cycles (p-value >0.05), indicating that there were no irreversible binding sites on the surface of MELRC (Khodaverdiloo et al., 2012). The results confirm that MELRC is a reusable adsorbent for Pb(II) removal. The experiments of magnetic separation were carried out with the MELRC content of 2 g L 1 and different separation time (1, 2, 5, 10 and 20 min). Although the saturation magnetization (Ms) of original MELRC was higher than that of regenerated ones (shown in Table 1), the turbidity of supernatant containing first-regenerated MELRC after magnetic separation was smaller than that of supernatant containing original MELRC (shown in Fig. 3). This can be explained as that original MELRC contained more tiny particles, which are hardly separated by a low magnetic field (Yavuz et al., 2006). Those tiny particles were lost during adsorption and desorption processes. Regeneration cycles (more than twice) negatively affected the separation performance in the first 5 min, but
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Please cite this article in press as: Wang, S.-y., et al. Regeneration of magnetic biochar derived from eucalyptus leaf residue for lead(II) removal. Bioresour. Technol. (2015), http://dx.doi.org/10.1016/j.biortech.2015.03.139